BENG2 APPLIED
THERMODYNAMICS (THER 205)
Heat Transfer 2
Click on [answer] to see solution.
1. For water at 50°C flowing in a 28mm ID pipe at a flow rate of 10.2 kg/s, find the Prandtl number (Pr) and Reynolds number (Re) [3.53, 8.526 x 105]
2. If the surface heat transfer coefficient is correlated by an equation:-
Nu = 0.023 Re 0.8 Pr 0.4, find the surface heat transfer
coefficient using the results from Q.l. [48.584 kW/m2K]
3. For air flowing over a flat plate the surface heat transfer coefficient
is correlated by the equation: Nu = 0.332 Pr 0.333 Re 0.5
.
If the air temperature is 20°C and it flows over a plate 30 cm long and l00 cm
width, with a velocity of 8 m/s, determine the heat transfer rate to the air
when the average surface temperature of the plate is 80°C. [180W]
4. The average air/surface heat transfer coefficient for a motor car radiator is 12.5 W/m2K. If the engine needs to dissipate 18 kW of heat find the required total surface area. Assume the average water temperature is 82°C and ambient air is at 15°C. At idle, the surface heat transfer coefficient value drops to 2.1 W/m2K and the heat dissipation drops to 3 kW. At what air temperature is the engine likely to boil? [21.5 m2, 33.5°C]
5. A steel pipe (l =49W/mK) 20mm ID, and 30mm OD carries a fluid at 180°C. The inside and outside surface heat transfer coefficients are 945W/m2K and 70W/m2K respectively. If the surrounding air is at 25°C, determine the total heat transfer rate from the fluid to the air per meter length of pipe. [913 W/m]
6. Central heating radiators have a surface emissivity of 0.92. Their height is 0.8 m and the surface heat transfer coefficient under conditions of natural convection is given by Nu = 0.378 Gr 0.25 . Assuming that the coefficient of cubical expansion (b ) = 1/T, and the surroundings are at 21°C (for both convection and radiation), determine the length of radiator required to dissipate 1 kW. Assume a radiator surface temperature of 70°C and average air temperature of 40°C. [1.3m]