combined convection and radiation.pdf

PRODUCT AND TEACHING MANUAL

HT14 COMBINED CONVECTION AND RADIATION HT14

ISSUE 6 MARCH 2005

THIS INSTRUCTION MANUAL SHOULD BE USED IN CONJUNCTION WITH THE PRODUCT MANUAL SUPPLIED WITH THE HT10X 'HEAT TRANSFER SERVICE UNIT' OR THE HT10CX ‘COMPUTER COMPATIBLE HEAT TRANSFER SERVICE UNIT’

ARMFIELD LIMITED

HT14 1

INTRODUCTION

1-1

2

DESCRIPTION

2-1

2.1 Overview 2.2 Baseplate 2.3 Heated Cylinder 2.4 Heating Element 2.5 Cylindrical Duct 2.6 Anemometer 2.7 Fan 2.8 Throttle Plate 2.9 Thermocouple T9 2.10 Thermocouple T10 OPERATIONAL PROCEDURES

2-6 2-6 2-6 2-6 2-6 2-7 2-7 2-7 2-7 2-7 3-1

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 4

Setting the Heater Voltage Measuring the Power to the Heater Measuring the Temperatures Setting the Air Velocity Measuring the Air Velocity Connections to the I/O Data Port (HT10X Only) Connections to the USB Port (HT10XC Only) Using a Chart Recorder (Optional, HT10X Only) ROUTINE MAINTENANCE

3-1 3-1 3-2 3-2 3-2 3-3 3-4 3-5 4-1

5

LABORATORY TEACHING EXERCISES

5-1

3

5.1 5.2 5.3 5.4 5.5 5.6 5.7 6

Nomenclature Table of Physical Properties for Air at Atmospheric Pressure Exercise A: Combined Heat Transfer under Natural Convection Exercise B: Dominant Heat Transfer Coefficient Exercise C: Effect of Forced Convection on Heat Transfer Exercise D: Variation of Local Heat Transfer Coefficient Exercise E: Project Work APPENDIX A: INSTALLATION GUIDE

5-2 5-3 5-4 5-11 5-17 5-24 5-29 1

HT15

HT11

armfield

HT12

HT10X HEAT TRANSFER SERVICE UNIT

MAINS

VOLTAGE CONTROL

/O PORT

V/A/W/m /Lux/M/sec/L/min 2

R

MANUAL

L

Ua

V

Fw

REMOTE INSTRUMENTATION

TEMPERATURE ░C T11

T12

T1

T7

T6

T2

T10

T3

T9 ZERO

R

0V

T1

L

Ua

T4 T8

Fw

T5

TEMPERATURE OUTPUT 10mV/░C (6V MAX)

T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12

0V

HT10X

HT16

HT14

HT13

HT17

1

INTRODUCTION

The Armfield 'Combined Convection and Radiation' accessory HT14 has been designed to demonstrate heat transfer from a solid surface to its surroundings. A hot surface loses heat (heat is transferred) to its surroundings by the combined modes of convection and radiation. In practice these modes are difficult to isolate and therefore an analysis of the combined effects at varying surface temperature and air velocity past the surface provides a meaningful teaching exercise. The heated surface studied is a horizontal cylinder which can be operated in free convection or forced convection when located in the stream of moving air. Measurement of the surface temperature of the uniformly heated cylinder and the electrical power supplied to it allows the combined effects of radiation and convection to be compared with theoretical values. The dominance of convection at lower surface temperatures and the dominance of radiation at higher surface temperatures can be demonstrated as can the increase in heat transfer due to forced convection.

1-1

2 2.1

DESCRIPTION Front view of HT14

9

10

8

11 T10

7

12

6

13

5

T9

14

4

15 3 16

2

1

17

2-1

2.2

Plan View of HT14

18

1

3

15

17

2

7

16

2-2

2.3

Schematic diagram of HT14 combined convection and radiation

T10 θ = 90°

T10

Ua Heated zone Ua

Anemometer T9

Throttle plate

2-3

2.4

HT10X Console Diagrams

A

B

C

D

armfield MAINS

E

HT10X HEAT TRANSFER SERVICE UNIT 2

V/A/W /m /Lux/M/sec/L/min

VOLTAGE CONTROL MANUAL

R

F

L Ua

V

P

/O PORT

Fw

REMOTE

TEMPERATURE °C

INSTRUMENTATION

T12

T1

T11

T2

T10

T3

T9 ZERO

R

L

Ua

O

T4 T8

Fw

T5 T7

T6

TEMPERATURE OUTPUT 10mV/°C (6V MAX)

0V

T1

T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12

0V

G

N

H

H

M

L

K

J

Q

R

OUTPUT 1 A.C. ~ 4A MAX

O/P 3

X OUTPUT 2 A.C. ~ 1A MAX

R.C.C.B

O/P 1

MAINS INPUT A.C.~

O/P 2

OUTPUT 3 24V D.C. 10A MAX

W

V

U

2-4

T

S

2.5

HT10XC Console Diagrams

A

B

C

D

E

O

Y

J

P G Z

N Q

M

K

L

?

V

R

W

S

2-5

Refer to the drawings on pages 2-1 to 2-5. 2.6 Overview The 'Combined Convection and Radiation' accessory comprises a centrifugal fan (15) with a vertical outlet duct (4 and 6) at the top of which is mounted a heated, horizontal cylinder (7). The mounting arrangement for the cylinder is designed to minimise loss of heat by conduction to the wall of the duct allowing the combined effects of convection (free or forced) and radiation to be measured. A thermocouple attached to the wall of the heated cylinder provides a measurement of the surface temperature from which heat transfer calculations can be performed. 2.7 Baseplate The accessory is mounted on a PVC baseplate (1) which stands on the bench top alongside the HT10X/HT10XC. 2.8 Heated Cylinder The heated cylinder (7) has an outside diameter of 10 mm, a heated length of 70 mm and is internally heated throughout its length by an electric heating element which is operated at low voltage for increased operator safety. The surface of the cylinder is coated with heat resistant paint which provides a consistent emissivity close to unity. The heated cylinder is mounted in such a way that the body can be rotated to allow the position of the thermocouple to be varied and the temperature distribution around the surface of the cylinder to be determined. An insulated cover allows the hot cylinder to be rotated and a locking screw (9) allows any position to be retained. The position of the thermocouple on the heated cylinder is indicated by a dot on the end of the insulated cover. An index mark (10) on the side of the boss shows the datum position for the thermocouple. The maximum surface temperature of the cylinder is in excess of 600°C when operated in free convection at full heater power. However, to preserve the life of the heating element the maximum temperature should be limited to 500°C in normal use. The heated cylinder is mounted horizontally at the top of a cylindrical duct which is attached to the outlet of a centrifugal fan. The inside diameter of the duct is 70 mm.

2.9 Heating Element The heating element is rated to produce 100 Watts nominally at 24 VDC into the cylinder. The power supplied to the element (and hence the heated cylinder) can be varied and measured on the HT10X/HT10XC. The electrical connections to the cylinder incorporate temperature resistant insulation with plug connection (11) to the variable 24 Volt DC supply socket marked OUTPUT 3 on HT10X, or OUTPUT 2 on HT10XC. 2.10 Cylindrical Duct The cylindrical duct is fabricated in two parts (4 and 6) with a rotating vane type anemometer (5) mounted between the two sections to allow the velocity of the air approaching the heated cylinder to be measured.

2-6

A guard (8) covering the outlet from the vertical duct prevents inadvertent contact with the heated cylinder or the hot wall of the duct when the accessory is in use or cooling down following operation. 2.11 Anemometer The lead from the anemometer (13) connects directly to the socket marked Ua on the HT10X to provide readings of air velocity directly in units of metres/sec. The operating range of the anemometer is 0 - 10 metres/sec. 2.12 Fan The centrifugal fan (15) is mounted at the base of the cylindrical duct (4). In normal operation the maximum air velocity is approximately 8 metres/sec when the fan is operated from a 50 Hz electrical supply (-A version). A thermal switch (2) protects the fan against overcurrent, in the event of a fault condition, and allows the fan to be switched off for free convection demonstrations. 2.13 Throttle Plate A variable throttle plate (17) at the inlet to the fan allows the velocity of the air through the outlet duct to be varied by adjusting the screw (16) at the centre of the front of the plate. The centrifugal fan is mains operated and obtains its supply from a mains outlet (OUTPUT 1) at the rear of the service unit. The connecting lead (18) is connected to this socket on the HT10X/HT10XC. 2.14 Thermocouple T9 Thermocouple T9 is fitted in the wall of the duct, upstream of the anemometer to measure the temperature of the air upstream of the heated cylinder. This thermocouple is fitted with a miniature plug (14) for direct connection to the HT10X/HT10XC service unit. The resolution of the temperature reading is 0.1°C. 2.15 Thermocouple T10 Thermocouple T10 is attached to the wall of the heated cylinder to indicate the surface temperature of the cylinder mid way along the cylinder. This type K thermocouple is fitted with a standard plug (12) for direct connection to the HT10X/HT10XC service unit. The resolution of the temperature reading is 1°C.

2-7

2-8

3

OPERATIONAL PROCEDURES

Refer to the diagrams on pages Refer to the Connection to Installation Guide (Appendix A) for details of connections between the HT14/HT10X/HT10XC and HT10X/HT10XC to the mains electrical supply. 3.1

Setting the Heater Voltage

When operating the equipment manually, using the front panel controls, ensure that the selector switch (B) is set to the MANUAL position. This allows the voltage supplied to the heater to be adjusted using the multi-turn potentiometer (C) marked HEATER VOLTAGE. The selector switch is only set to the REMOTE position if the voltage is to be controlled from an external signal via the 50 way I/O Port connector (F). The range of the output voltage is continuously adjustable from 0 Volts to 24 Volts DC using the multi-turn potentiometer. Ensure that the clamp on the side of the knob is released before turning the knob. Note: The voltage to the heater should not be set to maximum when operating the cylinder in natural convection (no air flow). To preserve the life of the heating element the voltage should be limited to ensure that the surface temperature of the cylinder (T10) does not exceed 500°C. 3.2 Measuring the Power to the Heater While adjusting the heater voltage the actual voltage supplied to the heater can be monitored by setting the top measurement selector switch (E) to position V. The reading is displayed directly in Volts on the top panel meter (D). The current drawn by the heater in the accessory can be monitored by setting the top measurement selector switch (E) to position I. The reading is displayed directly in Amps on the top panel meter (D). As the electrical supply to the heater is Direct Current the power supplied to the heater is simply obtained from the product of the Voltage and Current, ie. Heater Power Q = Voltage V x Current I eg.

If V = 15.0 Volts and I = 2.00 Amps then Q = 2 x 15 = 30.0 Watts

Note: To allow operation to the elevated temperatures necessary for the demonstrations it is not practicable to protect the heated cylinder with a thermostat. It is therefore important to supervise the accessory when it is operated.

3-1

Under no circumstances should the heater be operated with the top of the duct covered. If temperature T10 is within the normal range of temperature but the display shows no current when voltage is applied to the heating element check the following: Check that the thermal breaker O/P3 (R) at the rear of the service unit is latched pressed in). Check that the heater lead (11) on the HT14 is connected to the socket marked O/P3 (S) at the rear of the service unit. 3.3 Measuring the Temperatures To monitor either of the two thermocouples T9 or T10 installed on the HT14 simply set the temperature selector switch (G) to the required position and read the corresponding value on the lower panel meter. Temperature T9 is indicated with a resolution of one decimal place. Temperature T10 is indicated with a resolution of no decimal places. 3.4 Setting the Air Velocity When the fan is running the flow of air and therefore the velocity of the air through the outlet duct can be reduced by moving the throttle plate towards the fan inlet. The adjusting screw (16) should be turned anti-clockwise to reduce the air velocity. 3.5 Measuring the Air Velocity To monitor the air velocity set the top measurement selector switch (23) to position Ua. The reading is displayed on the top panel meter directly in units of metres/sec with a resolution of 0.1 m/s.

3-2

3.6

Connections to the I/O Data Port (HT10X Only)

To allow access to the measurement signals in applications other than when using an Armfield IFD5, the connections to the 50 way connector (F) are listed below for information:PIN NO

CHANNEL NO

Analog Inputs (0-5 V dc): 1 Ch 0 signal

2 3 4 5 6 7 8 9 10 11 12 13 14 15-21

Ch 0 return Ch 1 signal Ch 1 return Ch 2 signal Ch 2 return Ch 3 signal Ch 3 return Ch 4 signal Ch 4 return Ch 5 signal Ch 5 return Ch 6 signal Ch 6 return Not used

Analog Outputs (0-5 V dc): 22 DAC0 signal 23 24-25

SIGNAL FUNCTION Temperatures T9 and T10 via analog switch Temperature T9 (0 - 200°C) Temperature T10 (0 - 600°C) Note: T1-8 and T11-12 not used with HT14 Voltage V (0 - 24 Volts DC) Current to heater I (0 - 10 Amps) Not used with HT14 Not used with HT14 Velocity of air Ua (0 - 10 m/s) Not used with HT14

Voltage output V (Remote operation of 0 - 24 Volts)

DAC0 ground Not used

Digital Inputs ( 0/5 V dc): 26-37) Not used Digital Outputs (0/5 V dc): 38 Ch 0 39 Ch 1 40 Ch 2 41 Ch 3 42 Digital ground 43 Ch 4 44-46 Not used 47 Digital ground 48-50 Not used

Analog switch Analog switch Analog switch Analog switch Inhibit analog switch

3-3

3.7

Connections to the USB Port (HT10XC Only)

The HT11/HT11C includes Windows™-compatible software for full remote operation of the equipment and data logging of all output signals. However, users may prefer to write their own software for control and data logging, and for the convenience of those wishing to do so, Armfield has provided additional USB drivers allowing operation of the equipment via the USB socket on the HT10XC console. The relevant channel numbers for the HT11 are as follows: CHANNEL NO

SIGNAL FUNCTION

Analog Outputs (0-5 V dc exported from socket): Ch 0 signal Temperatures T1 to T8 via analog switch Temperatures T1 - T8 (0 - 133°C) Note: Temperatures T9 - T10 not used Ch 0 return Ch 1 signal Not used on HT11 Ch 1 return Ch 2 signal Current to heater I (0 - 10 Amps) Ch 2 return Ch 3 signal Not used on HT11 Ch 3 return Ch 4 signal Not used on HT11 Ch 4 return Ch 5 signal Not used on HT11 Ch 5 return Ch 6 signal Flowrate Fw (0 - 1.5 l/min)- only available with SFT2 Ch 6 return Not used Analog Inputs (0-5 V dc input to socket): DAC0 signal Voltage output V (Remote operation of 0-24 Volts) DAC0 ground Not used Digital Outputs ( 0-5 V dc): Not used Digital Inputs (0-5 V dc): Ch 0 Analog switch Ch 1 Analog switch Ch 2 Analog switch Ch 3 Analog switch Digital ground Ch 4 Inhibit analog switch Not used Digital ground Not used

3-4

3.8 Using a Chart Recorder (Optional, HT10X Only) A chart recorder provides a means of monitoring the transient changes when adjustments are made to the accessory and assists in deciding when the readings have stabilised. Each of the temperature measurements is available as a voltage signal suitable for connection to a suitable chart recorder. The signals are accessible via a row of banana sockets (I) at the bottom of the service unit front panel. Each thermocouple is conditioned to produce an output signal of 10 mV per °C. For example the Voltage output is 1.0 V dc at 100°C. The signal is obtained by connecting the chart recorder between the appropriate red channel socket, e.g. T10 and the black 0V common connection (H).

3-5

3-6

4

ROUTINE MAINTENANCE

To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular servicing/maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment. In addition to regular maintenance the following notes should be observed:1.

The HT10X service unit should be disconnected from the electrical supply when not in use.

2.

The HT14 accessory should be disconnected from the service unit when not in use.

3.

The HT14 accessory should be covered to prevent build up of dust on the heat transfer surface when not in use. The cylindrical rod should not be cleaned to avoid damage to the black heat transfer coating. Any cover MUST be removed before connecting the HT14 to the HT10X service unit. If the heated cylinder is powered with the outlet of the duct covered then the equipment will be damaged and the cover could catch fire.

4-1

5

LABORATORY TEACHING EXERCISES

Nomenclature

5-2

Table of Physical Properties for Air at Atmospheric Pressure

5-3

Exercise A: Determination of the Combined Heat Transfer from a Horizontal Cylinder in Natural Convection

5-4

Exercise B: Comparison of Heat Transfer by Radiation and by Convection and Investigation of the Dominance of the Respective Heat Transfer Coefficients

5-11

Exercise C: Effect of Forced Convection on Heat Transfer at Varying Air Velocities and Surface Temperatures

5-17

Exercise D: Variation of Local Heat Transfer Coefficient around a Cylinder under Forced Convection

5-24

Exercise D: Project Work

5-29

5-1

5.1

Nomenclature

Name Voltage to heated cylinder Current to heated cylinder Power supplied to heated cylinder Diameter of heated cylinder Heated length of cylinder Heat transfer area Air velocity in duct (free stream velocity) Corrected air velocity (due to blockage) Heat loss due to natural convection Heat loss due to forced convection Heat loss due to radiation Total heat loss from cylinder Heat transfer coefficient for natural convection Heat transfer coefficient for forced convection Heat transfer coefficient for radiation Stefan Boltzmann constant Emmisivity of cylinder Area factor (geometric factor) Dynamic viscosity of air Thermal conductivity of air Reynolds number (local) Nusselt number (local) Prandtl number Angular position of thermocouple (measured from the stagnation point) Surface temperature of heated cylinder Surface temperature of heated cylinder Temperature of ambient air/surroundings Temperature of ambient air/surroundings Film temperature of air Subscripts: d m f

Symbol V I Qin D L As Ua Uc Qc Qf Qr Qtot Hc

SI unit V A W m m m2 ms-1 ms-1 W W W W Wm-2K-1

Hf

Wm-2K-1

Hr σ (= 56.7 x 10-9) ξ F ν k Re Nu Pr θ

Wm-2K-1 Wm-2k-4 Dimensionless Dimensionless m2s-1 Wm-1K-1 Dimensionless Dimensionless Dimensionless Degrees

T10 Ts (=T10 + 273) T9 Ta (=T9 + 273) Tfilm

°C K °C K K

diameter mean (average) film

5-2

5.2

Table of Physical Properties for Air at Atmospheric Pressure Tfilm (K)

ν (m2s-1)

k (Wm-1K-1)

Pr (Dimensionless)

300 350 400 450 500 550 600

1.684 x10-5 2.076 x10-5 2.590 x10-5 3.171 x10-5 3.790 x10-5 4.434 x10-5 5.134 x10-5

0.02624 0.03003 0.03365 0.03707 0.04038 0.04360 0.04659

0.708 0.697 0.689 0.683 0.680 0.680 0.680

5-3

HT14 COMBINED CONVECTION AND RADIATION

5.3

Exercise A: Combined Heat Transfer under Natural Convection

Objective To determine the combined heat transfer (Qradiation + Qconvection) from a horizontal cylinder in natural convection over a wide range of power inputs and corresponding surface temperatures. To demonstrate the relationship between power input and surface temperature in free convection. Method By measuring the temperature on the surface of a horizontal cylinder subjected to heat loss by radiation and natural convection in combination then comparing the results obtained with those obtained from a theoretical analysis Equipment Required HT14 Combined Convection and Radiation Accessory HT10X Heat Transfer Service Unit or HT10XC Computer Compatible Heat Transfer Service Unit Optional Equipment Chart recorder with voltage input (1V = 100°C) or PC running Windows™ 98 or later Equipment Set Up Before proceeding with the exercise ensure that the equipment has been prepared as follows:Locate the HT14 Combined Convection and Radiation accessory alongside the HT10X/HT10XC Heat Transfer Service Unit on a suitable bench. Ensure that the horizontal cylinder is located at the top of the metal duct with the thermocouple located on the side of the cylinder (the cylinder can be rotated by releasing the thumb screw on the top of the mounting arrangement. Ensure that the thumb screw is securely tightened after adjustment). Connect the thermocouple attached to the heated cylinder to socket T10 on the front of the service unit. Connect the thermocouple located in the vertical duct to socket T9 on the service unit. Open the throttle plate at the front of the fan to allow air to enter the fan casing but do not connect the mains lead from the fan to the socket on the

5-4


service unit (the fan will not be used for this exercise). Turn the adjusting knob anticlockwise to open the throttle plate. T10 T10

θ = 90° Ua = 0

Heated zone

Anemometer T9

Throttle plate

Set the VOLTAGE CONTROL potentiometer to minimum (anticlockwise) and the selector switch to MANUAL then connect the power lead from the heated cylinder on HT14 to the socket marked O/P 3 (HT10X) or OUTPUT 2 (HT10XC) at the rear of the service unit. Ensure that the service unit is connected to an electrical supply. Theory/Background If a surface, at a temperature above that of its surroundings, is located in stationary air at the same temperature as the surroundings then heat will be transferred from the surface to the air and surroundings. This transfer of heat will be a combination of natural convection to the air (air heated by contact with the surface becomes less dense and rises) and radiation to the surroundings. A horizontal cylinder is used in this exercise to provide a simple shape from which the heat transfer can be calculated. Note: Heat loss due to conduction is minimised by the design of the equipment and measurements mid way along the heated section of the cylinder can be assumed to be unaffected by conduction at the ends of the cylinder. Heat loss by conduction would normally be included in the analysis of a real application.

5-5


In the case of natural (free) convection the Nusselt number Nu depends on the Grashof and Prandtl numbers and the heat transfer correlation can be expressed in the form: Nu = f(Gr, Pr) and the Rayleigh number Ra = (Gr Pr) The following theoretical analysis uses an empirical relationship for the heat transfer due to natural convection proposed by VT Morgan in the paper "The Overall Convective Heat Transfer from Smooth Circular Cylinders" published in TF Irvine and JP Hartnett (eds.), Advances in Heat Transfer vol. 16, Academic, New York, 1975, pp 199-269. If

Ts = Surface temperature of cylinder D = Diameter of cylinder L = Heated length of cylinder Ta = Ambient temperature of air

(K) (m) (m) (K)

Heat transfer area (surface area)

As = (πDL)

(m2)

Heat loss due to natural convection

Qc = Hcm As (Ts - Ta)

(W)

Heat loss due to radiation

Qr = Hrm As (Ts - Ta)

(W)

Total heat loss from the cylinder

Qtot = Qc + Qr

(W)

The average heat transfer coefficient for radiation Hrm can be calculated using the following relationship: Hr m

(Ts = σξF

− Ta 4 ) (Ts − Ta ) 4

(Wm-2K-1)

where: σ = Stefan Boltzmann constant σ = 56.7 x 10-9 ξ = Emmisivity of surface (Dimensionless) F = 1 = View factor (Dimensionless)

(Wm-2K-4)

The average heat transfer coefficient for natural convection Hcm can be calculated using the following relationship: Tfilm = β=

(Ts + Ta ) 2

1 Tfilm

GrD =

(K)

(K-1)

gβ(Ts − Ta )D 3 ν2

5-6


RaD = (GrD Pr)

therefore:

gβ(Ts − Ta )D 3 Ra D = Pr ν2 Num = c (RaD)n table below:) Hc m =

(kNu m ) D

(From Morgan, where c and n are obtained from the

(Wm-2K-1)

where: Ra = Rayleigh number Gr = Grashof number Num = Nusselt number (average) Pr = Prandtl number g = Acceleration due to gravity = 9.81 β = Volume expansion coefficient ν = Dynamic viscosity of air k = Thermal conductivity of air

(Dimensionless) (Dimensionless) (Dimensionless) (Dimensionless) (ms-2) (K-1 (m2s-1) (Wm-1K-1)

Note: k, Pr, and ν are physical properties of the air taken at the film temperature Tfilm. (These may be obtained from the table in the HT14 Teaching Manual.) The actual power supplied to the heated cylinder Qin = V I (W) Table listing constant c and exponent n for natural convection on a horizontal cylinder (Source - Morgan): RaD 10-9 to 10-2 10-2 to 102 102 to 104 104 to 107 107 to 1012

c 0.675 1.02 0.850 0.480 0.125

n 0.058 0.148 0.188 0.250 0.333

Alternatively a simplified equation may be used to calculate the heat transfer coefficient for free convection from the publication "Heat Transmission" WH McAdams, 3rd ed., McGraw-Hill, New York, 1959 Hc m

⎛ Ts − Ta ⎞ = 1.32⎜ ⎟ ⎝ D ⎠

0.25

(Wm-2K-1)

The value for Hcm should be calculated using both the original and simplified equations and the values compared. 5-7


Procedure Refer to the Operational Procedures section on page 3-1 if you need details of the instrumentation and how to operate it. Switch on the front mains switch. (If the panel meters do not illuminate check the RCD and any circuit breakers at the rear of the service unit; all switches at the rear should be up.) If operating manually, set the selector switch to MANUAL. If operating remotely from a PC, set the selector switch to REMOTE and run the HT14 or HT14C software (also used for the HT14 if using HT10XC). Set the heater voltage to 5 volts: If operating manually, adjust the voltage control potentiometer to give a reading of 5 volts on the top panel meter with the selector switch set to position V. If operating remotely from a PC, use the heater control box on the mimic diagram screen to adjust the percentage of full scale until the Voltage display box reads 5V. Allow the HT14 to stabilise. If operating manually, monitor the temperatures using the lower selector switch/meter. If operating remotely, monitor the temperatures on the software display screen. When the temperatures are stable record the following. If operating the accessory manually from the console then values must be noted down by hand from the front panel display, using the selector switch to select each required value in turn. If operating remotely from a PC, values may be recorded by selecting the icon: T9, T10, V, I. Set the Heater Voltage to 8 Volts using the same method as before. Allow the HT14 to stabilise then repeat the above readings. Set the Heater Voltage to 12 Volts. Allow the HT14 to stabilise then repeat the above readings. Set the Heater Voltage to 15 Volts. Allow the HT14 to stabilise then repeat the above readings. Set the Heater Voltage to 20 Volts.

5-8


Allow the HT14 to stabilise then repeat the above readings. Note: Do not set the heater voltage in excess of 20 Volts when operating the cylinder in natural convection (no forced airflow). The life of the heating element will be considerably reduced if operated at excessive temperature. Results and Calculations For this exercise the raw data is tabulated under the following headings: Heater Voltage Heater Current Upstream air temperature Surface temperature of cylinder

V I T9 T10

Volts Amps (°C) (°C)

You should also estimate and record the experimental errors for these measurements. For this exercise the following constants are applicable: Diameter of cylinder Heated length of cylinder Emissivity of surface Stefan Boltzmann constant

D = 0.01 L = 0.07 ξ = 0.95 σ = 56.7 x 10-9

(m) (m) (Wm-2k-4)

For each set of readings the derived results are tabulated under the following headings: Heat flow (Power to heater) Heat transfer area (surface area) Heat transfer coefficient (natural convection) Heat transfer coefficient (radiation) Heat transferred by natural convection Heat transferred by radiation Total heat transferred

Qin As Hcm Hrm Qc Qr Qtot

= = = = = = =

(Watts) (m2) (Wm-2K-1) (Wm-2K-1) (W) (W) (W)

Estimate the cumulative influence of the experimental errors on your calculated values for As, Hcm, Hrm, Qc, Qr, Qtot and Qin and measured values for T9, T10, L and D. Compare the theoretical values for Qtot with the measured values for Qin and explain any differences in values. Compare the calculated heat transferred due to Convection Qc and radiation Qr Compare the value for Hcm obtained using the simplified and full empirical equations and comment on any difference.

5-9


Plot a graph of surface temperature T10 against power input Qin and observe the relationship. Observe that the heat transferred from the cylinder to the surroundings increases with the difference between the surface temperature of the cylinder and the temperature of the surroundings. Conclusions You have demonstrated how heat transfer from a heated surface to its surroundings is a combination of heat loss due to natural convection and heat loss due to radiation (the effect of conduction must also be included where relevant) when the surface is located in stationary air. For equilibrium, heat input to a surface must equal the heat transferred from the surface to its surroundings. Since heat transfer from a surface increases with difference in temperature between the surface and its surroundings, increased heat input to a surface results in an increase in the temperature of the surface. The calculation of the heat transfer coefficient Hcm for natural convection involves the use of empirical equations which are specifically related to heat transfer from a horizontal cylinder. Empirical equations are available for other classical shapes which will allow a theoretical analysis to be performed. The effect of moving air (forced convection) will be investigated in exercise HT14C. Note: Exercise HT14B should be carried out on the completion of this exercise.

5-10


5.4

Exercise B: Dominant Heat Transfer Coefficient

Objective To compare the contribution of heat transfer by convection with heat transfer by radiation and from the measurements to show the domination of the convective heat transfer coefficient Hc at low surface temperatures and the domination of the radiation heat transfer coefficient Hr at high surface temperatures Method By measuring the temperature on the surface of a horizontal cylinder subjected to heat loss by radiation and natural convection in combination then comparing the contribution by convection and radiation. Note: If results are available from exercise HT14A then this exercise can be completed using those results. Refer to the Theory section of this exercise followed by the Results and Calculations. The following instructions apply if results are not available: Equipment Required HT14 Combined Convection and Radiation Accessory HT10X Heat Transfer Service Unit or HT10XC Computer Compatible Heat Transfer Service Unit Optional Equipment Chart recorder with voltage input (1V = 100°C) or PC running Windows™ 98 or later Equipment Set Up Before proceeding with the exercise ensure that the equipment has been prepared as follows:Locate the HT14 Combined Convection and Radiation accessory alongside the HT10X/HT10XC Heat Transfer Service Unit on a suitable bench. Ensure that the horizontal cylinder is located at the top of the metal duct with the thermocouple located on the side of the cylinder. (The cylinder can be rotated by releasing the thumb screw on the top of the mounting arrangement. Ensure that the thumb screw is securely tightened after adjustment.) Connect the thermocouple attached to the heated cylinder to socket T10 on the front of the service unit. Connect the thermocouple located in the vertical duct to socket T9 on the service unit.

5-11


Set the VOLTAGE CONTROL potentiometer to minimum (anticlockwise) and the selector switch to MANUAL then connect the power lead from the heated cylinder on HT14 to the socket marked O/P 3 (HT10X) or OUTPUT 2 (HT10XC) at the rear of the service unit. T10 θ = 90°

T10

Ua = 0

Heated zone

Anemometer T9

Throttle plate

Ensure that the service unit is connected to an electrical supply. Theory/Background When a horizontal cylinder, with its surface at a temperature above that of its surroundings, is located in stationary air then heat loss from the cylinder will be a combination of natural convection to the air (air surrounding the cylinder becomes less dense and rises when it is heated) and radiation to the surroundings. Note: Heat loss due to conduction is minimised by the design of the equipment and measurements mid way along the heated section of the cylinder can be assumed to be unaffected by conduction at the ends of the cylinder. Heat loss by conduction would normally be included in the analysis of a real application. The following theoretical analysis uses an empirical relationship for the heat transfer due to natural convection proposed by WH McAdams in the publication "Heat Transmission", third edition, McGraw-Hill, New York, 1959. Total heat loss from the cylinder

Qtot = Qc + Qr where:

Heat loss due to natural convection

Qc = Hc As (Ts - Ta) and

5-12



Qr = Hr As (Ts - Ta)

Heat transfer area (surface area)

As = (π d L)

The heat transfer coefficients Hcm and Hrm can be calculated using the following relationships: HC m = 1.32 Hr m = σξF

(Ts − Ta )0.25 d

(Ts

(simplified empirical equation from McAdams)

− Ta 4 ) (Ts − Ta ) 4

where: σ = Stefan Boltzmann constant (σ = 56.7 x 10-9 Wm-2k-4) ξ = Emmisivity of surface Ts = Surface temperature of cylinder (K) Ta = Ambient temperature (K) Actual power supplied to the heated cylinder Qin = V I (Watts) Procedure Refer to the Operational Procedures section on page 3-1 if you need details of the instrumentation and how to operate it. Switch on the front mains switch. (If the panel meters do not illuminate check the RCD and any circuit breakers at the rear of the service unit; all switches at the rear should be up.) If operating manually, set the selector switch to MANUAL. If operating remotely from a PC, set the selector switch to REMOTE and run the HT14 or HT14C software (also used for the HT14 if using HT10XC). Set the heater voltage to 5 volts: If operating manually, adjust the voltage control potentiometer to give a reading of 5 volts on the top panel meter with the selector switch set to position V. If operating remotely from a PC, use the heater control box on the mimic diagram screen to adjust the percentage of full scale until the Voltage display box reads 5V. Allow the HT14 to stabilise. If operating manually, monitor the temperatures using the lower selector switch/meter. If operating remotely, monitor the temperatures on the software display screen.

5-13


When the temperatures are stable record the following. If operating the accessory manually from the console then values must be noted down by hand from the front panel display, using the selector switch to select each required value in turn. If operating remotely from a PC, values may be recorded by selecting the icon: T9, T10, V, I. Set the Heater Voltage to 8 Volts using the same method as before. Allow the HT14 to stabilise then repeat the above readings. Set the Heater Voltage to 12 Volts. Allow the HT14 to stabilise then repeat the above readings. Set the Heater Voltage to 15 Volts. Allow the HT14 to stabilise then repeat the above readings. Set the Heater Voltage to 20 Volts. Allow the HT14 to stabilise then repeat the above readings. Note: Do not set the heater voltage in excess of 20 Volts when operating the cylinder in natural convection (no forced airflow). The life of the heating element will be considerably reduced if operated at excessive temperature. Results and Calculations For this exercise the raw data is tabulated under the following headings: Heater Voltage Heater Current Upstream air temperature Surface temperature of cylinder

V I T9 T10

Volts Amps (°C) (°C)

You should also estimate and record the experimental errors for these measurements. For this exercise the following constants are applicable: Diameter of cylinder Heated length of cylinder

d = 0.01 L = 0.07

(m) (m)

For each set of readings the derived results are tabulated under the following headings: Heat flow (Power to heater) Heat transfer area (surface area)

Qin As

5-14

= =

(Watts) (m2)


Heat transfer coefficient (natural convection) Heat transfer coefficient (radiation) Heat transferred by natural convection Heat transferred by radiation Total heat transferred

Hcm Hrm Qc Qr Qtot

= = = = =

(Wm-2K-1) (Wm-2K-1) (W) (W) (W)

Estimate the cumulative influence of the experimental errors on your calculated values for As, Hcm, Hrm, Qc, Qr, Qtot and Qin and measured values for T9, T10, L and D. Compare the calculated heat transfer due to convection Qc with the calculated heat transfer due to radiation Qr by plotting graphs of Hcm and Hrm against the temperature of the surface Ts (= T10 +273). Your graph should be similar to the diagram below:

Hc Hr

Radiation

Convection

0 T10

Observe that at low surface temperatures (typically less than 230°C) the heat transfer coefficient Hcm due to natural convection is greater than the heat transfer coefficient Hrm due to radiation. Conversely, at high surface temperatures (typically greater than 230°C) the heat transfer coefficient Hcm due to natural convection is less than the heat transfer coefficient Hrm due to radiation and as the temperatures exceeds 400°C the effect of radiation becomes dominant. Conclusions You have demonstrated how the contribution of natural convection and radiation towards the heat loss from a hot surface varies with the temperature difference between the hot surface and the ambient air/surroundings (the effect of conduction must also be included where relevant). Convection is

5-15


more dominant when the temperature difference is small. radiation is more dominant when the temperature difference is large. The effect of moving air (forced convection) will be investigated in exercise HT14C. Note: Exercise HT14C should be carried out on the completion of this exercise.

5-16


5.5

Exercise C: Effect of Forced Convection on Heat Transfer

Objective To determine the effect of forced convection on heat transfer from the surface of a cylinder at varying air velocities and surface temperatures. To demonstrate the relationship between air velocity and surface temperature for a cylinder subjected to forced convection. Method By measuring the temperature on the surface of a horizontal cylinder subjected to heat loss by radiation and forced convection in combination then comparing the results with those obtained from a theoretical analysis. Equipment Required HT14 Combined Convection and Radiation Accessory HT10X Heat Transfer Service Unit or HT10XC Computer Compatible Heat Transfer Service Unit Optional Equipment Chart recorder with voltage input (1V = 100°C) or PC running Windows™ 98 or later Equipment Set Up Before proceeding with the exercise ensure that the equipment has been prepared as follows:Locate the HT14 Combined Convection and Radiation accessory alongside the HT10X/HT10XC Heat Transfer Service Unit on a suitable bench. Ensure that the horizontal cylinder is located at the top of the metal duct with the thermocouple located on the side of the cylinder (the cylinder can be rotated by releasing the thumb screw on the top of the mounting arrangement. Ensure that the thumb screw is securely tightened after adjustment). Connect the thermocouple attached to the heated cylinder to socket T10 on the front of the service unit. Connect the thermocouple located in the vertical duct to socket T9 on the service unit. Connect the mains lead from the fan (terminated at the connection box alongside the fan) to the socket marked Output 1 at the rear of the HT10X/HT10XC service unit. Close the throttle plate at the front of the fan by turning the adjusting knob clockwise.

5-17


T10 θ = 90°

T10

Ua Heated zone Ua

Anemometer T9

Throttle plate

Connect the lead from the anemometer in the vertical duct to the socket marked Ua on the front of the HT10X/HT10XC service unit. Set the VOLTAGE CONTROL potentiometer to minimum (anticlockwise) and the selector switch to MANUAL then connect the power lead from the heated cylinder on HT14 to the socket marked O/P 3 (HT10X) or OUTPUT 2 (HT10XC) at the rear of the service unit. Ensure that the service unit is connected to an electrical supply. Theory/Background In free/natural convection the heat transfer rate from a surface is limited by the small movements of air which are generated by changes in the density of the air as the air is heated by the surface. In forced convection the air movement can be greatly increased resulting in improved heat transfer rate from a surface. Therefore a surface subjected to forced convection will have a lower surface temperature than the same surface subjected to free convection, for the same power input. If a surface, at a temperature above that of its surroundings, is located in moving air at the same temperature as the surroundings then heat will be transferred from the surface to the air and the surroundings. This transfer of heat will be a combination of forced convection to the air (heat is transferred to the air passing the surface) and radiation to the surroundings. A horizontal cylinder is used in this exercise to provide a simple shape from which the heat transfer can be calculated.

5-18


Note: Heat loss due to conduction is minimised by the design of the equipment and measurements mid way along the heated section of the cylinder can be assumed to be unaffected by conduction at the ends of the cylinder. Heat loss by conduction would normally be included in the analysis of a real application. Total heat loss from the cylinder

Qtot = Qfm + Qrm where:

Heat loss due to forced convection

Qf = Hfm A (Ts - Ta) and


Qr = Hrm A (Ts - Ta)

Heat transfer area

A = (π D L)

The heat transfer coefficients Hfm due to forced convection and Hrm due to radiation can be calculated using the following relationships:

Hr m = σξF

(Ts

− Ta 4 ) (Ts − Ta ) 4

(Wm-2K-1)

where: σ = Stefan Boltzmann constant (σ = 56.7 x 10-9 Wm-2k-4) ξ = Emmisivity of surface Ts = Surface temperature of cylinder (K) Ta = Ambient temperature (K) Hf m =

k Nu m D

(Wm-2K-1)

where: k = conductivity of the air (Wm-1K-1) D = diameter of the cylinder (m) Num = Average Nusselt number (Dimensionless) An empirical formula can be used to calculate the value for Num as follows: Nu m = 0.3 +

(0.62 Re

Pr 0.33 ) ⎛ ⎛ Re ⎞ ⎜1 + ⎜ ⎟ 0.25 ⎜ ⎛ ⎛ 0.4 ⎞ 0.66 ⎞ ⎝ ⎝ 282000 ⎠ ⎜1 + ⎜ ⎟ ⎜ ⎝ Pr ⎟⎠ ⎟ ⎝ ⎠ 0.5

5-19

0.5

⎞ ⎟ dimensionless ⎟ ⎠


From SW Churchill and M Bernstein "A Correlating Equation for Forced Convection from Gases and Liquids to a Circular cylinder in crossflow". Journal of Heat Transfer, 99:300-306 (1977). where Re = Reynolds number = Uc D / ν (dimensionless) Pr = Prandtl number for air (dimensionless) Uc = Corrected air velocity (m/s) Corrected air velocity Uc = 1.22 Ua (m/s) (The cylinder causes a blockage in the duct resulting in a local increase in the air velocity.) Values for k, ν and Pr depend on the temperature of the air and can be found using the table included in the HT14 teaching manual. The actual power supplied to the heated cylinder Qin = V I (W) Procedure Refer to the Operational Procedures section on page 3-1 if you need details of the instrumentation and how to operate it. Switch on the front mains switch. (If the panel meters do not illuminate check the RCD and any circuit breakers at the rear of the service unit; all switches at the rear should be up.) If operating manually, set the selector switch to MANUAL. If operating remotely from a PC, set the selector switch to REMOTE and run the HT14 or HT14C software (also used for the HT14 if using HT10XC). If operating using the console: Set the upper selector switch on HT10X to position Ua to indicate the air velocity in the duct. If operating from a PC: Monitor the air velocity on the software mimic diagram Start the centrifugal fan by pressing the switch on the connection box. Open the throttle plate on the front of the fan by rotating the knob at the centre to give a reading of 0.5 m/s on the upper panel meter. Set the heater voltage to 20 volts: If operating manually, adjust the voltage control potentiometer to give a reading of 20 volts on the top panel meter with the selector switch set to position V.

5-20


If operating remotely from a PC, use the heater control box on the mimic diagram screen to adjust the percentage of full scale until the Voltage display box reads 20V. Allow the HT14 to stabilise. If operating manually, monitor the temperatures using the lower selector switch/meter. If operating remotely, monitor the temperatures on the software display screen. When the temperatures are stable record the following. If operating the accessory manually from the console then values must be noted down by hand from the front panel display, using the selector switch to select each required value in turn. If operating remotely from a PC, values may be recorded by selecting the icon: Ua, T9, T10, V, I. Adjust the throttle plate to give a velocity of 1.0 m/s. Allow the HT14 to stabilise then repeat the above readings. Repeat the above procedure changing the air velocity in steps of 1.0 m/s until the air velocity is 7.0 m/s. Results and Calculations For this exercise the raw data is tabulated under the following headings: Heater Voltage Heater Current Upstream air temperature Surface temperature of cylinder Air velocity in the duct

V I T9 T10 Ua

Volts Amps (°C) (°C) (m/s)

You should also estimate and record the experimental errors for these measurements. For this exercise the following constants are applicable: Diameter of cylinder d = 0.01 Length of cylinder L = 0.07

(m) (m)

For each set of readings the derived results are tabulated under the following headings: Heat flow (Power to heater) Qin Heat transfer area As Corrected air velocity Uc Heat transfer coefficient (forced convection) Hfm Heat transfer coefficient (radiation) Hr m Heat transfer by natural convection Qf

5-21

= = = = = =

(Watts) (m2) (m/s) (Wm-2K-1) (Wm-2K-1) (W)


Heat transfer by radiation Total heat transferred

Qr Qtot

= =

(W) (W)

Estimate the cumulative influence of the experimental errors on your calculated values for Uc, As, Hfm, Hrm, Qf, Qr, Qtot and Qin and measured values for Ua, T9, T10, L and x. Compare the theoretical value obtained for Qtot with the measured value for Qin and explain any difference in the two values. Compare the calculated heat transfer due to forced convection Qf and radiation Qr. Plot a graph of surface temperature T10 against corrected air velocity Uc. Your graph should be similar to the diagram below:

T10

Uc

Observe that the surface temperature of the cylinder reduces as the air velocity increases for a fixed heat input Qin. Observe that the surface temperature reduces more rapidly at low air velocities and reduces more slowly at high air velocities. Conclusions You have demonstrated how the heat transfer from a heated surface to its surroundings is a combination of forced convection and radiation (the effect of conduction must also be included where relevant) when the surface is located in a moving air steam. For equilibrium, heat input to a surface must equal the heat transferred from the surface to its surroundings. Since heat transfer from a surface increases with the velocity of the air, increased air velocity past a surface results in a decrease in the temperature of the surface.

5-22


The calculation of the heat transfer coefficient Hfm for forced convection involves the use of empirical equations which are specifically related to heat transfer from a horizontal cylinder. Empirical equations are available for other classical shapes which will allow a theoretical analysis to be performed. The effect of air velocity forced convection on surface temperature. Note: Exercise HT14D should be carried out on the completion of this exercise.

5-23


5.6

Exercise D: Variation of Local Heat Transfer Coefficient

Objective To demonstrate that the local heat transfer coefficient varies around the circumference of a horizontal cylinder when subjected to forced convection. Method By subjecting a horizontal cylinder to constant conditions of forced convection and radiation (constant power input) then rotating the cylinder to measure the local differences in temperature over the surface of the cylinder (positioning thermocouple T10 at different angular positions to produce a temperature profile). Equipment Required HT14 Combined Convection and Radiation Accessory HT10X Heat Transfer Service Unit or HT10XC Computer Compatible Heat Transfer Service Unit Optional Equipment Chart recorder with voltage input (1V = 100°C) or PC running Windows™ 98 or later Equipment Set Up Before proceeding with the exercise ensure that the equipment has been prepared as follows:Locate the HT14 Combined Convection and Radiation accessory alongside the HT10X/HT10XC Heat Transfer Service Unit on a suitable bench. Ensure that the horizontal cylinder is located at the top of the metal duct with the thermocouple located on the underside of the cylinder - at the bottom of the cylinder corresponding to the stagnation point on the cylinder, i.e. θ = 0 degrees (the cylinder can be rotated by releasing the thumb screw on the top of the mounting arrangement. Ensure that the thumb screw is securely tightened after adjustment). Connect the thermocouple attached to the heated cylinder to socket T10 on the front of the service unit. Connect the thermocouple located in the vertical duct to socket T9 on the service unit. Connect the mains lead from the fan (terminated at the connection box alongside the fan) to the socket marked Output 1 at the rear of the HT10X/HT10XC service unit.

5-24


Close the throttle plate at the front of the fan by turning the adjusting knob clockwise. T10

Ua

T10

θ = 0° = 90° = 180°

Heated zone Ua

Anemometer T9

Throttle plate

Connect the lead from the anemometer in the vertical duct to the socket marked Ua on the front of the HT10X/HT10XC service unit. Set the VOLTAGE CONTROL potentiometer to minimum (anticlockwise) and the selector switch to MANUAL then connect the power lead from the heated cylinder on HT14 to the socket marked O/P 3 (HT10X) or OUTPUT 2 (HT10XC) at the rear of the service unit. Ensure that the service unit is connected to an electrical supply. Theory/Background The purpose of this exercise is to demonstrate that the temperature profile around the circumference of the cylinder is not constant, i.e. the heat transfer coefficient varies according to the position on the surface of the cylinder. The cylinder is rotated to present the thermocouple at different angular positions and, after allowing the temperature profile of the cylinder to stabilise, the temperature is recorded at each angular position. Note: Due to the relatively small size of the cylinder, the mounting arrangement of the thermocouple and the variable contact between the heating element and the inside wall of the cylinder, it is not possible to obtained values which will compare with classical results for variation of local heat transfer coefficient around a circular cylinder. However, a variation in temperature can be shown.

5-25


D

Uc

θ

Ua

Local Nusselt number Nux

500

400

300 Increasing Velocity Ua

200

100

0 0

30

60

90

120

150

180

θ Angle measured from the stagnation point, degrees

The actual power supplied to the heated cylinder Qin = V I (W) Procedure Refer to the Operational Procedures in the HT14 teaching manual if you need details of the instrumentation and how to operate it. Switch on the front Mains switch (if the panel meters do not illuminate check the RCD and circuit breakers at the rear of the service unit, all switches at the rear should be up). If operating manually, set the selector switch to MANUAL. If operating remotely from a PC, set the selector switch to REMOTE and run the HT14 or HT14C software (also used for the HT14 if using HT10XC). If operating using the console: Set the upper selector switch on HT10X to position Ua to indicate the air velocity in the duct. If operating from a PC: Monitor the air velocity on the software mimic diagram Start the centrifugal fan by pressing the switch on the connection box. Open the throttle plate on the front of the fan by rotating the knob at the centre to give a reading of 1.0 m/s on the upper panel meter or software display. Allow the HT14 to stabilise. If operating manually, monitor the temperatures using the lower selector switch/meter. If operating remotely, monitor the temperatures on the software display screen.

5-26


When the temperatures are stable record the following. If operating the accessory manually from the console then values must be noted down by hand from the front panel display, using the selector switch to select each required value in turn. If operating remotely from a PC, values may be recorded by selecting the icon: Position Ua, T9, T10, V, I. Rotate the cylinder by 30 degrees. Allow the HT14 to stabilise then repeat the above readings. Continue to rotate the cylinder in steps of 30 degrees until the thermocouple is located on the top of the cylinder, i.e. θ = 180 degrees. Adjust the throttle plate to give an air velocity of 5 m/s then repeat the above procedure. After taking readings set the Voltage Control to zero before switching off the fan. Results and Calculations For this exercise the raw data is tabulated under the following headings: Heater Voltage Heater Current I Air velocity in the duct Upstream air temperature Surface temperature of cylinder Angular position of thermocouple

V (Volts) (Amps) Ua (m/s) T9 (°C) T10 (°C) θ (degrees)

You should also estimate and record the experimental errors for these measurements. Plot graphs of surface temperature T10 versus angular position from the stagnation point θ for each setting of the air velocity Ua. Observe that the temperature varies over the surface of the cylinder. Since the heat transfer coefficient is related to the surface temperature (demonstrated in the previous exercises) the local heat transfer coefficient must vary over the surface of the cylinder.

Conclusions The heat transfer coefficient Hfm used in the calculation of heat transfer from a surface is an average value for the surface. The local heat transfer coefficient varies according to the location on the surface.

5-27


Note: Exercise HT14E should be carried out on the completion of this exercise.

5-28


5.7

Exercise E: Project Work

PROJECT WORK Designing a model to demonstrate heat transfer by combination of convection and radiation to its surroundings An interesting project for students who have completed the previous training exercises is to build and test a heat transfer model of their own design. The HT10X/HT10XC service unit provides the necessary power supplies and instrumentation to operate such a model. Provided that the model is constructed with the following principles it can be connected directly to the HT10X/HT10XC service unit for evaluation: The heater must operate from a 24 VDC electrical supply at a maximum current of 9 Amps (216 Watts maximum). The size of the model must be restricted to allow a sufficiently high surface temperature to be achieved with this power input. The heater should be arranged to provide uniform heating over the entire surface of the model. A model to demonstrate free convection combined with radiation is to be recommended as the model can be simply located on the bench and not involve modifications to the centrifugal fan/duct on the HT14. If it is required to build a model to demonstrate forced convection with radiation then the model should include a simple extension to the top of the existing duct. The existing heated cylinder should be removed to prevent disturbance to the air flow. Alternatively, an independent source of moving air could be provided. Any heated model will become extremely hot in operation and must be adequately protected to prevent injury without impeding the convection and radiation of heat from the surface of the model. The thermocouples must be type K and terminated with a miniature thermocouple plug where temperatures are less than 200°C or a standard thermocouple plug where temperatures will exceed 200°C (maximum operating temperature 600°C). To provide a reliable measurement of the surface temperature, the thermocouple must be attached rigidly to the surface or buried just beneath the surface in a hole or a trough. A mineral insulated thermocouple is recommended as the thermocouple must remain isolated from the metalwork of the heater. The thermocouple used to measure the surface temperature of the model should be located at the centre of the surface to minimise edge effects. Typical projects might include: Combined natural convection and radiation from flat plates: Horizontal - heated surface facing upwards Horizontal - heated surface facing downwards Vertical

5-29


Effect of rod cross section: Solid cylindrical rods of the same material but different diameters. The rod should be heated internally along its entire length. Effect of shape: Solid surfaces having different shapes of cross section, eg. rectangular section, square section. The section should be heated internally along its entire length. Where appropriate, any of the exercises HT14A, HT14B, HT14C or HT14D might be applied to the project model constructed by the student.

5-30


6

APPENDIX A: INSTALLATION GUIDE

The HT14 heat transfer accessory is designed for use with the HT10X ‘Heat Transfer Service Unit’ or the HT10XC ‘Computer Compatible Service Unit’. This installation guide assumes that the HT10X or HT10XC service unit to be used with the HT14 accessory has already been installed as described in the product manual supplied with the service unit.

Appendix A-1


9

10

8

11 T10

7

12

6

13

5

T9

14

4

15 3 16

2

1

17

Appendix A-2


18

1

3

15

17

2

7

Appendix A-3

16


T10 θ = 90°

T10

Ua Heated zone Ua

Anemometer T9

Throttle plate

Appendix A-4


A

B

C

D

armfield MAINS

E

HT10X HEAT TRANSFER SERVICE UNIT 2

V/A/W /m /Lux/M/sec/L/min

VOLTAGE CONTROL MANUAL

R

F

L Ua

V

P

/O PORT

Fw

REMOTE

TEMPERATURE °C

INSTRUMENTATION

T12

T1

T11

T2

T10

T3

T9 ZERO

R

L

Ua

O

T4 T8

Fw

T5 T7

T6

TEMPERATURE OUTPUT 10mV/°C (6V MAX)

0V

T1

T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12

0V

G

N

H

H

M

L

K

J

Q

R

OUTPUT 1 A.C. ~ 4A MAX

O/P 3

X OUTPUT 2 A.C. ~ 1A MAX

R.C.C.B

O/P 1

O/P 2

MAINS INPUT A.C.~

OUTPUT 3 24V D.C. 10A MAX

W

V

U

T

Appendix A-5

S


A

B

C

D

E

O

Y

J

P G Z

N Q

M

K

L

?

V

R

W

S

Appendix A-6


Refer to the drawings on pages Appendix A-2 to Appendix A-6. The 'Combined Convection and Radiation' accessory is supplied with the section of duct removed from the fan for convenience in shipping. Carefully remove the packing then place the PVC baseplate, incorporating the centrifugal fan, on a suitable bench with the throttle plate on the fan inlet facing forwards. Remove the three fixing screws from the PVC flange at the fan outlet. Locate the lower duct section (4) on the outlet of the centrifugal fan (5) with the anemometer cable (13) exiting to the right hand side. Carefully align the PVC flanges on the duct and the fan outlet, and then fix the duct to the fan outlet by replacing the three fixing screws. Locate the upper duct section (6) on the lower duct section with the boss for the heater assembly facing to the right hand side. Insert the upper duct section into the body of the anemometer (5) taking care not to damage the blades of the impeller. Carefully align the fixing holes then fix the upper duct to the anemometer body by replacing the three fixing screws. Carefully insert the heater assembly into the boss on the side of the upper duct section ensuring that the tip of the heated cylinder (7) is located in the hole on the opposite side of the duct. Rotate the heater assembly until the black dot on the insulated cover is aligned with the index mark (10) on the boss then tighten the securing screw (9) to retain the heater assembly. Switch the mains switch (A) on the front of the console to the off, 0, position. Connect the miniature thermocouple plug from temperature sensor T9 in the duct wall to the socket marked T9 (K) on the front of the service unit. Connect the standard thermocouple plug from temperature sensor T10 attached to the wall of the heated cylinder to the socket marked T10 (K) on the front of the service unit. Set the Heater Voltage potentiometer (C) on the front of the service unit to zero (release the clamp and turn the adjusting knob fully anticlockwise). Set the selector switch (B) to the MANUAL position. If using HT10X:

Connect the power lead (11) from the heated cylinder inside the duct of the HT14 to the variable DC outlet socket marked OUTPUT 3 (S) at the rear of the service unit.

If using HT10XC:

Connect the power lead (11) from the heated cylinder inside the duct of the HT14 to the variable DC outlet socket marked OUTPUT 2 (S) at the rear of the service unit.

Appendix A-7


Connect the electrical lead from the anemometer (13) to the socket marked Ua (M) on the front of the service unit. The centrifugal fan is mains operated but derives its supply from the HT10X/HT10XC service unit. Before connecting the fan for the first time ensure that the label on the cable attached to the fan agrees with the local electrical supply and agrees with the label attached to the mains input lead on the HT10X/HT10XC service unit. Connect the power lead (18) from the fan connection box (3) on the HT14 to the mains outlet socket marked OUTPUT 1 (Q) at the rear of the service unit. If using HT10X:

An I/O Data Port connector (F) on the right hand side of the service unit front panel allows the voltage signals from each of the measurements to be connected to the USB port of a suitable PC using an Armfield IFD5 interface device.

If using HT10XC:

An integral USB port (F) on the right hand side of the service unit front panel allows the voltage signals from each of the measurements to be connected directly to the USB port of a suitable PC.

This USB interface together with the appropriate Windows based software is available as an optional addition to the HT10X, and is included as part of the HT10XC. The Windows based software covers the entire range of heat transfer accessories HT11 to HT17, with the HT11, HT12 and HT14 accessories being useable with the HT11C, HT12C and HT14C software respectively. The operation of the software is described in the help text included as part of the software. Ensure that the mains electrical supply to the service unit has been connected and switched on as described in the service unit product manual. Ensure that the RCD (V) at the rear of the service unit is in the ON (up) position. If using HT10X:

Ensure that the two miniature circuit breakers O/P1 (U) and O/P2 (T) at the rear of the service unit are in the ON (up) position. Ensure that the thermal circuit breaker O/P3 (R) at the rear of the service unit is latched in the ON position (pressed in).

Set the mains on/off switch (A) on the service unit front panel to the ON position. Observe that both digital panel meters (D, J) are illuminated (as no power is supplied to the accessory at this stage the temperature display should indicate approximately ambient temperature).

Appendix A-8


Set the temperature selector switch (G) to position T10 to indicate the surface temperature of the heated cylinder. Check that the temperature indicated is approximately ambient temperature. Set the top measurement selector switch (E) to position V to indicate the voltage supplied to the element inside the heated cylinder. Adjust the Heater Voltage potentiometer (C) to give a reading of approximately 10 Volts on the display (ensure that the clamp on the side of the knob is released before turning the knob). Check that the reading of temperature T10 gradually increases. Check that temperature T10 stabilises in free convection (no flow of air through the duct). Check that the temperature T9 indicates ambient temperature inside the duct by setting the temperature selector switch. Turn on the centrifugal fan by pressing the switch (2) on the connection box. The actual air velocity can be read directly on the top panel meter (D) with the selector switch (E) set to position Ua. Adjust the throttle plate (17) on the fan inlet to give an air velocity of 4.5 metres/sec by rotating the adjusting screw (16). Observe that the surface temperature of the heated cylinder (T10) falls in forced convection (air flowing past the cylinder). Turn off the power to the heater by setting the potentiometer to zero. If the accessory is to be operated using the Armfield software then check the operation of the software as follows: Ensure that the software has been installed on a suitable PC as described in the service unit product manual. Connect the PC to the interface device or USB socket on the console. Check that the red and green USB indicator lights illuminate. Set the selector switch (B) on the console to the ‘REMOTE’ position. Run the HT14C software (also used with the HT14 accessory). Check that the software displays ‘IFD OK’ in the bottom right-hand corner of the software window. Select the ‘Power On’ switch in the software to set the console from Standby to On. Check that the readings from the temperature sensors displayed on the mimic diagram screen are comparable with those obtained using the console (as the sensor outputs can be individually calibrated within the software the readings may not match exactly, but they should be very close). Set the heater voltage to 10 volts using the software: either use the up and down arrows beside the heater control box, or type a value into the box. The

Appendix A-9


control box is calibrated as a percentage of full range. The actually heater voltage and current are displayed next to the control box on the software screen. Check that the temperature T10 (displayed on the software screen) gradually rises. Set the heater setting back to 0. Set the service unit back to standby by selecting the Power On switch on the software screen. The software may now be exited. If the accessory is not to be used immediately then the PC may be shut down. Always switch on the service unit and connect it to the PC before running the software for an experiment or demonstration. Allow the cylinder to cool then press the fan switch (2) to turn off the cooling air. Set the mains switch on the service unit to off (0) if the accessory is not to be used immediately.

The basic operation of the 'Combined Convection and Radiation' accessory and 'Heat Transfer Service Unit' has been confirmed. Refer to the section Operational Procedures in the teaching manual for further information.

Appendix A-10

combined convection and radiation.pdf

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