SASURIE COLLEGE OF ENGINEERING, VIJAYAMANGALAM DEPARTMENT OF MECHANICAL ENGINEERING HEAT AND MASS TRANSFER ASSIGNMENT: 2 (SET-I)
DATE:
1. Air at 30°C, flows over a flat plate at a velocity of 4 m/s and plate is maintained at a uniform
temperature of 90°C. If the transition occurs at a critical Reynolds number of 5 x 10 5, calculate the thickness at which the boundary layer changes from laminar to turbulent. At that location, find the following. following. (i). Hydrodynamic boundary boundary layer thickness, thickness, (ii). Thermal boundary boundary layer thickness, (iii). Local heat transfer coefficient, (iv). Average heat transfer coefficient, (v) Heat transfer from both sides for unit width of the plate, (vi) Mass flow rate & (vii) The skin friction coefficient. [FLAT PLATE] [Heat and Mass transfer, Author: S.Senthil, Page No.2.50]
2. Air at 20°C and at atmosphere pressure at a velocity of 4.5 m/s past a flat plate with a sharp
leading edge. The entire plate surface is maintained at a temperature of 60°C. Assuming that the transition occurs at a critical Reynolds number of 5 x 10 5, find the distance from the leading edge at which the flow is the boundary boundary layer changes from laminar to turbulen turbulent. t. At the location, location, calculate the following. (i). Hydrodynamic boundary layer thickness, (ii). Thermal boundary layer thickness, (iii). Local heat transfer coefficient, (iv). Average heat transfer coefficient, (v) Heat transfer from both sides for unit width of the plate, (vi) Mass flow rate & (vii) The skin friction coefficient. Assuming cubic velocity profile and approximate method. [FLAT PLATE] [Heat and Mass transfer, Author: R.K. Rajput, Page No.422]
3. A vertica verticall plate measur measuring ing 180 mm x 180 180 mm and at 50°C 50°C is expose exposed d to atmosp atmospher heree at 10°C. 10°C. Compare the free convection heat transfer from this plate with that which would result due to a forced convection over the plate at a velocity equal to twice the maximum velocity which would occur in free convection boundary layer. [FREE CONVECTION] [Heat and Mass transfer, Author: R.K. Rajput, Page No.512]
4. A horizontal horizontal tubula tubularr 1-1 condenser condenser is used used to condense condense saturate saturated d steam at 80°C. 80°C. The condenser condenser is a shell and tube one with brass tubes (k=110 W/m°C) of 1.59 cm OD and 1.34 cm ID. Steam is outside tubes and cooling water enters the tubes at 20°C with a velocity of 1.4 m/s and leaves at 40°C. If the rate of cooling water supply is 55000 kg/h and the latent heat of condensation of steam at 80°C is 2304 kJ/kg, calculate: (i) The number of tubes, (ii) The length of each tube. For calculating the tube side heat transfer coefficient use the Dittus-Boelter equation and for the shell side heat transfer coefficient, the average value may be taken as 10760 W/m 2K. Data: Properties of water at 30°C. k = 0.659 W/mK; ρ = 979. 8 kg/m 3; c p = 4.180 kJ/kg K; µ = 0.4044 x 10 -3 Pa S. [FLOW OVER CYLINDER AND SPHERE] [Heat and Mass transfer, Author: R.K. Rajput, Page No.473]
5. In a glass making process, a plate glass 0.5 m x 2mm and 3 mm in thickness is cooled by blowing
hot air with velocity 1 m/s in direction parallel to plate, such that the rate of cooling is slow. The initial glass plate temperature is 425°C and hot air temperature is 200°C. Estimate: (i). Initial rate of cooling in °C/min, (ii). Time required for cooling from 425°C to 375°C. Assume properties of glass as: ρ = 2500 kg/m 3; c p = 0.76 kJ/kg K; Assume that air flow takes place on both sides of plate. [FLAT PLATE] [Engineering Heat and Mass transfer, Author: Mahesh M.Rathore, Page No.536]
SASURIE COLLEGE OF ENGINEERING, VIJAYAMANGALAM DEPARTMENT OF MECHANICAL ENGINEERING HEAT AND MASS TRANSFER ASSIGNMENT: 2 (SET-II)
DATE:
1. Air at 20°C and at a pressure of 1 bar is flowing over a flat plate at a velocity of 3 m/s. If the plate is 280mm wide and at 56°C calculate the following at x = 280mm. (i). Boundary layer thickness, (ii). Local friction coefficient, (iii). Average friction coefficient, (iv). Thickness of the thermal boundary layer, (v) Local convective heat transfer coefficient, (vi). Average convective heat transfer coefficient, (vii) Rate of heat transfer by convection & (viii) Total drag force on the plate. [FLAT PLATE] [Heat and Mass transfer, Author: S.Senthil, Page No.S.74]
2. A hot plate 1.2m wide, 0.35m height ad a 115°C is exposed to the ambient still air at 25°C. Calculate the following. (i), Maximum velocity at 180mm from the leading edge of the plate, (ii). The boundary layer thickness at 180mm from the leading edge of the plate, (iii). Local heat transfer coefficient at 180mm from the leading edge of the plate, (iv) Average heat transfer coefficient over the surface of the plate, (v). Total mass flow through the boundary, ( vi). Heat loss from the plate & (vii) Rise in temperature of the air passing through the boundary. Use approximate solution. [FREE CONVECTION] [Heat and Mass transfer, Author: S.Senthil, Page No.2.204]
3. Air at 2 bar pressure and 60°C is heated as it flows through a tube of diameter 25mm at a velocity of 15 m/s. If the wall temperature is maintained at 100°C, find the heat transfer per unit length of the tube. How much would be the bulk temperature increase over one meter length of the tube. [INTERNAL FLOW] [Heat and Mass transfer, Author: S.Senthil, Page No.2.146]
4. 3.8 kg of oil per second is heated from 20°C to 40°C by passing through a circular annulus with a velocity of 0.3 m/s. The hot gases at 400°C are passed through the inside tube of 100mm diameter and are cooled to 100°C. Find the length if the pipe required for the above heat transfer process assuming the gas is flowing in opposite direction to the oil? Take the following properties of oil and gases at mean temperature: For Oil, k = 0.2 W/m°C; ρ = 800 kg/m 3; c p = 3.350 kJ/kg K; v = 8 x 10 -6 m2/s. For Gases, k = 0.035 W/m°C; ρ = 0.8 kg/m 3; c p = 1.050 kJ/kg K; v = 32.88 x 10 -6 m2/s. [FLOW OVER CYLINDER AND SPHERE] [Heat and Mass transfer, Author: R.K. Rajput, Page No.478]
5. Water entering at 10°C is heated to 40°C in the tube of 0.02m ID at a mass flow rate of 0.01 kg/s.
The outside of the tube is covered with an insulated electric heating element that produces a uniform heat flux of 15000 W/m 2 over the surface. Neglecting any entrance effect, determine: (i). Reynolds number, (ii). The heat transfer coefficient. (iii). The length of pipe needed for a 30°C increase in average temperature, (iv) The inner tube surface temperature at the outlet, (v) The friction factor, (vi) The pressure drop in the pipe,(vii) The pumping power required if the pump if 50% efficient. [INTERNAL FLOW] [Engineering Heat and Mass transfer, Author: Mahesh M.Rathore, Page No.574]
SASURIE COLLEGE OF ENGINEERING, VIJAYAMANGALAM DEPARTMENT OF MECHANICAL ENGINEERING HEAT AND MASS TRANSFER ASSIGNMENT: 2 (SET-III)
DATE:
1. Air flow through a long rectangular of 300 mm height x 800 mm width air-conditioning duct maintains the outer duct surface temperature at 20°C. If the duct is uninsulated and exposed to air at 40°C. Calculate the heat gained by the duct. Assuming duct to be horizontal. [FREE CONVECTION] [Heat and Mass transfer, Author: S.Senthil, Page No.2.174]
2. A copper bus bar 25 mm diameter is cooled by air (in cross- flow) at 30°C and flowing past the
bus bar with a velocity of 2.5 m/s. If the surface temperature of the bar is not to exceed 85°C and resistivity of copper is 0.0175 x 10 -6 ohm-m3/m, calculate the following: (i). The heat transfer coefficient from the surface to the air, (ii). The permissible current intensity for the bus bar.
The following empirical correlations may be applicable for a single cylinder placed in cross-flow: (For 10 < Re <10 3 , Nu = 0.44 (Re) 0.5 & For 103 < Re <105 , Nu = 0.22 (Re)0.6) The thermo physical properties are evaluated at t ∞ (30°C) and are give as: k = 0.02673 W/m°C; v = 16.00 x 10-6 m2/s. [FLOW OVER CYLINDER AND SPHERE] [Heat and Mass transfer, Author: R.K. Rajput, Page No.481]
3. Consider an electrical heated square plate (60 cm x 60 cm) with one of its surface thermally and
other surface dissipating heat by convection into atmospheric air at 30°C. The heat flux over the surface of the plate is uniform and results in a mean temperature of 50°C. The plate is inclined at an angle of 50° from vertical. Determine the heat loss from the plate for the following cases: (a) heated surface facing up: (b) heated surface facing down. [FREE CONVECTION] [Engineering Heat and Mass transfer, Author: Mahesh M.Rathore, Page No.653]
4. An air stream at 0°C is flowing along a heated plate at 90°C at a speed of 75 m/s. The plate is
45cm long and 60 cm wide. Assuming the transition of boundary layer takes place at Re cr = 5 x 105. Calculate the average value of friction coefficient and heat transfer coefficient for full length of the plate. Also calculate the heat dissipation from the plate. [FLAT PLATE] [Engineering Heat and Mass transfer, Author: Mahesh M.Rathore, Page No.33]
5. Assuming a man as a cylinder of 40 cm diameter and 1.72 m high with a surface temperature of
37°C. Calculate the heat lost from its body, while standing in wind flowing at 20 km/hr at 17°C. Use relation Nu D = 0.027 ReD0.805 Pr 1/3. [FLOW OVER CYLINDER AND SPHERE] [Engineering Heat and Mass transfer, Author: Mahesh M.Rathore, Page No.543]
