SCHOOL of ENGINEERING  

BENG2 APPLIED THERMODYNAMICS (THER 205)

Heat Transfer : Convection

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1. Calculate the surface heat transfer coefficient of water flowing through a 20 mm diameter tube at 2.5 m/s at a mean bulk temperature of 50°C. [11.45 kW/m²K]

2. Air at 1 atmosphere, 250°C flows at 12 m/s through a thin metal tube 25 mm in diameter. For the outside of the tube h =18.7 W/m²K. The surroundings are at 15°C. Calculate the overall heat transfer coefficient and hence find the heat transfer rate per unit length.
[12.8 W/m²K, 236 W/m]

3. For a smooth pipe of 25mm bore and in which water flows at a mean velocity of 3 m/s with a bulk mean temperature of 40°C, estimate the surface heat transfer coefficient between pipe wall and water using the following equations:-
Reynolds Analogy
Prandtl-Taylor analogy
Colburn modification [26.7, 10.5, 10.1 kW/m²K]

4. Air at 1 atmosphere, 38°C flows past a 25 mm diameter cylinder 45.5 m/s. The cylinder surface is at 149°C. Calculate the heat transfer rate per unit length.
For a cylinder in cross flow:
(properties at film temperature) where:

[1570 W/m]

5. Calculate the heat loss by natural convection per meter length from a horizontal pipe of 150 mm diameter, the surface of which is at 300°C. The room air temperature is 17°C. It has been shown that for a horizontal cylinder:-

(where the properties are evaluated at the surface temperature).

Take the coefficient of cubical expansion (b ) as 1/T, where T is the absolute temperature of the room air. [940W/m]

Note: Fluid property values can be found from HAYWOOD pp. 32-34