Experiment 4: HT14 - Combined Convection and Radiation Experiments Exercise A – Heat Transfer at Various Temperatures 1. 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 convection heat transfer coefficient, hc, at low surface temperatures and the domination of the radiation heat transfer coefficient, hr, at high surface temperatures.
2. 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.
3. Equipment Required • • • •
HT10X Heat Transfer Service Unit HT14 Combined Convection and Radiation Accessory IFD3 PC Interface Console IBM Compatible PC
4. Theory/Background When a horizontal cylinder, with its surface at a temperature above that of its surroundings, is located in stationary air, the 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. Heat loss due to conduction is minimized 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
Heat loss due to natural convection:
Qc = hnc As (Ts - Ta)
Heat loss due to radiation:
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:
(Ts − Ta ) hnc = 1.32 D where:
0.25
hr
(Ts = σε
4
− Ta 4 )
(Ts − Ta )
s = Stefan Boltzmann constant ε = Emissivity of surface (dimensionless) Ts = Surface temperature of cylinder (K) Ta = Ambient temperature (K)
Actual power supplied to the heated cylinder Qin = V I (Watts)
5. Operating Procedure a) Run the HT14 software and select Experiment B – Investigating heat transfer at varying temperatures. b) Switch power on to the IFD3 interface console (note a green illuminates whilst power is switched on). c) 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 except for the output socket 1). d) Watch the 'IFD Channel History' window and allow the HT14 to stabilize. e) When the temperatures are stable, click Sample Now in the software so that the following will be recorded: T9, T10, V, I, Qa. f) Set the heater voltage to 5 volts. g) Watch the IFD Channel History and allow the HT14 to stabilize. Take a data sample with the software. h) Repeat steps (f) and (g) setting the heater voltage to 8, 12, 15, and 20 volts. Make sure the temperatures are stabilized before taking a data sample and moving to the next voltage. i) Once experimental procedure is completed set the mimic diagram Voltage Control to zero. 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.
6. Results and Calculations The HT14 software logs and calculates the following data: Heater Voltage: Heater Current: Heater Flow: Upstream air temperature: Temperature of heated cylinder: Air velocity in the duct:
V I Qin T9 T10 Ua
Volts Amps Watts °C °C (m/s
For this exercise the following constants are applicable: Diameter of cylinder: Heated length of cylinder: Emissivity of surface: Stefan Boltzmann constant:
D L ε σ
= 0.01 m = 0.07 m = 0.95 = 56.7 x 10-9 W/(m2-K4)
For each set of readings the HT14 software calculates the following results: 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: Note:
Qin As hnc hr Qc Qr Qtot
Watts m2 W/(m2-K) W/(m2-K) W W W
To view all data columns (marked Hidden), all the questions in the Walk Through presentation in the software will have to be answered.
Questions for Discussion: Observe that at low surface temperatures (typically less than 230 °C) the heat transfer coefficient hnc due to natural convection is greater than the heat transfer coefficient hr due to radiation. Conversely, at high surface temperatures (typically greater than 230° C) the heat transfer coefficient hnc due to natural convection is less than the heat transfer coefficient hr due to radiation and as the temperatures exceeds 400°C the effect of radiation becomes dominant. a)
Plot the heat transfer coefficients, hnc and hr, against the temperature of the surface Ts (= T10 +273) similar to those produced in the software. b) Plot the percentage of the total heat transferred by natural convection and radiation on the same graph. Discuss the trends seen. c) Compare the calculated heat transfer due to convection Qc with the calculated heat transfer due to radiation Qr. d) You should also estimate and record the experimental errors for these measurements and calculations using the note-taking feature.
