Convection

FREE CONVECTION
1) Water is heated in a tank by dipping a plate of 20x40 cm in size. The temperature of plate surface
is maintained at 100ºC. Assuming the temperature of surrounding water as 300C, find the heat lost
from the plate assuming 20cm side of the plate is in vertical plane.
2) Calculate the rate of free convection, heat loss from a 30cm diameter sphere maintained at 100ºC
exposed to atmosphere at 20ºC.
3) A long pipe 15cm outer diameter with a surface temperature of 80ºC passes through a water tank.
The water is at 40ºC. Estimate the heat exchanger rate/unit length of the tube.
4) A plate 0.3 m x 0.4 m whose one surface is insulated and the other surface is maintained at
temperature 1100C. The plate is exposed to quiescent atmosphere at 300C. Calculate the rate of heat
transfer for the following cases,
i) when the plate is vertical
ii) when the pate is horizontal if, a) Upper surface heated
b) Lower surface heated
5) A block 10 cm x 10 cm x 10 cm in size is suspended in still air at 100C, with one of the surface in
horizontal position. All the surfaces of the block are maintained at 1500C. Determine the total heat
transfer from the block.
6) A pipe of outer diameter 10 cm carries cold air. The outer surface of the pipe is maintained at 5 0C
and surrounding air temperature is 250C. Calculate the heat gain by the duct assuming 1 m length of
the pipe is exposed to air at i) vertical position ii) horizontal position.
7) Considering the body of a man as vertical cylinder of 300 mm diameter and 170 cm height,
calculate the heat dissipated by the body in one day. Take the body temperature as 360C and
atmosphere temperature as 140C.
8) Calculate the heat transfer from a 60 W incandescent bulb at 1150C to ambient air at 250C.
Assume the bulb as a sphere of 50 mm diameter. Also calculate the percentage of power lost by free
convection if 5% of heat is lost in visible region and the remainder is thermal radiation.
9) A 500 W cylindrical immersion heater (3cm diameter 20 cm long) is placed vertically in stagnant
water at 300C. Calculate the average surface temperature of the plate
FORCED CONVECTION
1) Air at standard conditions of 760 mm of Hg and at 200C flows over a flat plate at 3 m/s. The plate is
50 cm x 25 cm. Find the heat lost per hour if air flow is parallel to 50 cm side of the plate. If 25 cm side
is kept parallel to air flow, what will be the effect on heat transfer? Temperature of the plate is 1000C.
2) Water flowing at the rate of 0.6 kg/min is heated from 200C to 600C while passing through a tube of
3 cm diameter. Find the length of the tube required, if its surface temperature is maintained at 1200C.
3) A rectangular duct of 30 mm x 50 mm size carries water at a rate of 2 kg/s. Determine the length of
the tube required to heat water from 300C to 500C, if the wall temperature is maintained at 900C.
4) Lubricating oil at a temperature of 600C enters a 1 cm diameter tube with a velocity of 2.5 m/s. The
tube surface is maintained at 300C. Calculate the length of the tube required to cool the oil to 450C.
Assume that the oil has following average properties: Density = 865 kg/m3, absolute viscosity =
7.78 x 10-3 kg/m sec, thermal conductivity = 0.12 W/mK and CP = 1.6 kJ/kgK.
5) Air at a temperature of 27ºC is moving at a velocity of 0.3 m/sec passed a 100 W incandescent
bulb. The bulb may be treated as a sphere of 50mm diameter with its surface at a temperature of
213ºC. Estimate the heat transfer coefficient and compute the percentage of power lost due to
convection, if 5.5% of energy is emitted in visible wavelength range and remaining is thermal
radiation.
6) The surface temperature of a thin plate located parallel to air stream is 900C. The free stream
velocity is 60 m/s and air temperature is 300C. The plate is 30 cm wide and 45 cm long in the direction
of air stream. Assuming that transitional Reynolds number as 5 x 105, determine
i) Rate of heat transfer from the laminar portion of the plate. ii) Rate of heat transfer from the
turbulent portion of the plate. iii) What would be the error in rate of heat transfer if the boundary layer
is assumed to be turbulent from the leading edge itself?
7) Air at 250C and atmospheric pressure flows across a heated cylinder of diameter of 7.5 cm. If the
velocity of air flow is 1.2 m/s and the cylinder surface is maintained at temperature 950C, compute the
rate of heat transfer.
8) Water at 600C enters a 1.5 cm diameter and 3 m long tube with a velocity of 1 m/s. The tube wall is
maintained at a constant temperature of 900C. Calculate the heat transfer coefficient and amount of
heat transferred if the exit water temperature is 700C.

Convection

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