Newtons law of convection

Question

Question 1: State Newton’s law of convection?

Question 2: Write brief notes on:

a) Natural convection.
b) Forced convection.

Question 3: Air at 20^oC at a pressure of 1 bar is flowing over a flat plate at a velocity of 3 m/s. If the plate is maintained at 60^o C, compute the heat transfer per unit width of the plate. Suppose the length of plate all along the flow of air is 2m.

Question 4: In a surface condenser, water flows via staggered tubes while the air is passed in cross flow over the tubes. The temperature and velocity of air are 30^oC and 8 m/s correspondingly. The longitudinal and transverse pitches are 22 mm and 20 mm correspondingly. The tube outside diameter is 18 mm and the tube surface temperature is 90^oC. Compute the heat transfer coefficient.

Question 5: For a specific engine, the underside of the crank case can be idealized as a flat plate measuring 80 cm x 20 cm. The engine runs at 80 km/hr and the crank case is cooled by air flowing past it at the same speed. Compute the loss of heat from the crank case surface of temperature 75^o C to the ambient temperature of 25^o C. Suppose the boundary layer becomes turbulent from the leading edge itself.

Question 6: Atmospheric air at 275 K and a free stream velocity of 20 m/s flows over a flat plate of 1.5 m long that is maintained at a uniform temperature of 325 K. Compute the average heat transfer coefficient over the region where the boundary layer is laminar, the average heat transfer coefficient over the whole length of the plate and the total heat transfer rate from the plate to the air over the length 1.5 m and width 1 m. Suppose that the transition occurs at R_e= 2x10⁵.

Question 7: Air at 25^o C flows over a flat plate at a speed of 7 m/s and heated to 85^o C. Compute the local heat transfer coefficient at a distance of 20 cm.

Question 8: Air at 25^oC at the atmospheric pressure is flowing over a flat plate at 3 m/s. If the plate is 1 m wide and the temperature T_w = 75^o C. Compute the following at a location of 1 m from the leading edge

a) Hydrodynamic boundary layer thickness.
b) Local friction coefficient.
c) Thermal boundary layer thickness
d) Local heat transfer coefficient.

Question 9: Atmospheric air at 300 K with  a velocity of 2.5 m/s flows over a flat plate of length 2 m and width w = 1m maintained at uniform temperature of 400 K. Compute the local heat transfer coefficient at 1 m length and the average heat transfer coefficient from L = 0 to L = 2 m. As well find out the heat transfer.

Question 10: Air at a pressure of 8 kN/m² and a temperature of 250^oC flows over a flat plate of 0.3 m wide and 1 m long at a velocity of 8 m/s. If the plate is to be maintained at a temperature of 78^oC, estimate the rate of heat to be removed constantly from the plate.