Test: Convective Heat Transfer: One Dimensional Level - 2 - Mechanical Engineering MCQ

Common Data for Question No. 3 & 4
Air at 25°C approaches a 0.9 m long by 0.6 m wide flat plate with an approach velocity 4.5 m/sec. The plate is heated to a surface temperature of 135°C.
It is given that at a temperature of 80°C, the thermo-physical properties of air are
v = 21.07 x 10^-6m²/s
k = 0.0304 W/m-°C
Pr = 0.692
Make calculations for

The total rate of heat transfer from plate to the air is _________

Air moving at 0.3 m/s blows over the top of a chest-type freezer. The top of the freezer measures 0.9 m by 1.5 m and is poorly insulted so th a t th e su rface rem ains at 10 ° C . If the temperature of air is 30° C, make calculations for the maximum heat transfer rate by forced convection at the top of the freezer. At a temperature of 20°C, the thermophysical properties of air are 
v = 15.06 x 10^-6 m²/s
k = 0.0259 W/m-°C 
Pr = 0.703 

The oil pan of an IC engine approximates a flat plate 0.3 m wide by 0.45 m long and protrudes below the framework of the automobile. The engine oil is at 95°C  and the ambient air
temperature is 35°C. If the automobile runs at 36 km/hr, make calculations for the rate of heat transfer (in W) from the oil-pan surface. Assume negligible resistance to conduction through the oil pan.
At the mean film temperature,

The thermo-physical properties of air are
v = 18.46 x 10^-6m²/s
k = 0.0293 W/m-°C
Pr = 0.695
Predict your answer correct upto two decimal places.

Air flows through a 10 cm internal diameter tube at the rate of 75 kg/hr. Measurements indicate that at a particular section in the tube, the pressure and mixing cup temperature of air are 1.5 bar and 325 K respectively while the tube wall temperature is 375 K. The heat transfer rate (in W) for one meter length in the region is ________. The general nondimensional correlations for turbulent flow in the tube is
Nu = 0.023 Re^0.8 Pr^0.4
where the fluid properties are evaluated at the bulk temperature.
For air at t_f = 325 K and p 1.5 bar, the thermo-physical properties of air are 
μ = 1.967 x 10^-5kg/ms
k = 0.02792 W/mK 
and Pr = 0.713 

A square channel with a side 10 mm and length 1.5 m carries water with a velocity of 5 m/s. measurements indicate that lengthwise mean temperature of water is 30°C whilst the inner surface of channel is at 80°C. Calculate the convective coefficient of heat transfer (in kW/m²- K) from th e channel wall to the water. Use the correlation

where the thermo-physical properties pertain to those at the mean bulk temperature of water. Pr_w corresponds to the value of Prandtl number at the channel surface temperature and equivalent diameter is the reference dimension.
The physical properties of water at 30°C are 
p = 995.07 kg/m³ 
c_P = 4174 J/kgK
k = 0.6172 W/mK 
μ = 2.88 kg/m- hr 
v = 0.805 x 10^-6m²/s
P_r = 5.42  
At wall:
Temperature t_w = 80°C and Pr_w = 2.21 
Predict your answer correct upto 2 decimal places.

For a particular car engine, the underside of the crankcase can be idealized as a flat plate measuring 80 cm x 20 cm. The car runs at 80 km/hr and the crankcase is cooled by the air flowing past it at the same speed. Make calcu lations for the loss of heat ( in W ) from the crank case surface (t_s = 75°C) to the ambient air (t_∞ = 25°C). Due to road induced vibrations, the boundary layer becomes turbulent from the leading edge itself.
At the mean film temperature of

the thermo-physical properties of air are
k = 2.824 x 10^-2 W/mK
v = 17.95 x 10^-6m²/s 
and Pr = 0.698
Given; Nu = 0.036(Re_L)^0.8 x (Pr)^0.33

The average Nusselt number in laminar natural convection from a vertical wall at 180°C with still air at 20°C is found to be 48. If the wall temperature becomes 30°C, all other parameter remaining same, the average Nusselt number will be