# Heat Transfer MCQ’s – Turbulent Flow

1 - Question

Which of the following is true for turbulent flow?
a) G r P r > 108
b) G r P r > 109
c) G r P r > 103
d) G r P r > 1015
Explanation: The product G r P r is often referred to as Rayleigh number, and its value sets the criterion of turbulent character of flow.

2 - Question

Mc Adam relation is given by
a) Nu = 0.023 (Re) 0.8 (Pr) 0.4
b) Nu = 0.023 (Re) 0.8 (Pr) 0.3
c) Nu = 0.023 (Re) 0.8 (Pr) 0.2
d) Nu = 0.023 (Re) 0.8 (Pr) 0.1
Explanation: Here, n = 0.4 if the fluid is being heated.

3 - Question

Investigate the effect of following condition on the average value of heat transfer coefficient in flow through a tube Two fold increases in flow velocity by varying mass flow rate It may be presumed that there is no change in the temperature of the liquid and the tube wall, and that the flow through the tube is turbulent in character
a) 64.1% decrease
b) 64.1% increase
c) 74.1%decrease
d) 74.1% increases
Explanation: h 2/h 1 = (V 2/V 1) 0.8 = .0741 = 74.1%

4 - Question

Consider the above problem for two fold increase in the diameter of the tube, the flow velocity is maintained constant by change in the rate of liquid flow
a) 15%
b) 14%
c) 13%
d) 12%
Explanation: h 2/h 1 = (d 1/d 2) 0.2.

5 - Question

Calculate the rate of heat loss from a human body which may be considered as a vertical cylinder 30 cm in diameter, and 175 cm high while standing in a 30 km/hr wind at 15 degree Celsius. Human has a surface temperature of 35 degree Celsius
a) 588.86 W
b) 688.86 W
c) 788.86 W
d) 888.86 W
Explanation: Q = h A d t, h = Nu k/d.

6 - Question

A heat treat steel plate measures 3 m by 1 m and is initially at 30 degree Celsius. It is cooled by blowing air parallel to 1 m edge at 9 km/hr. If the air is at 10 degree Celsius. Estimate the convection heat transfer from both sides of the plate
a) 477.7 W
b) 547.7 W
c) 647.7 W
d) 747.6 W
Explanation: Q = h A d t, h = Nu k/d.

7 - Question

Consider the above problem, find the value of Reynolds number
a) 166003
b) 177003
c) 188003
d) 199003
Explanation: Re = p V l/µ = 16603

8 - Question

The oil pan of 1.6 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 degree Celsius and the ambient air temperature is 35 degree Celsius. If the automobile runs at 36 km/hr, make calculations for the rate of heat transfer from the oil-pan surface. Assume negligible resistance to conduction through the oil pan
a) 190.37 W
b) 180.37 W
c) 170.37 W
d) 160.37 W
Explanation: Q = h A d t, where h = (Nu) (k)/l and Nu = 0.664 (Re) 0.5 (Pr).33.

9 - Question

Air flows through a 10 cm internal diameter tube at the rate of 75 kg/hr. Measurements indicate that at a particular point in the tube, the pressure and temperature of air are 1.5 bar and 325 K whilst the tube wall temperature is 375 K. Find heat transfer rate from one meter length in the region of this point The general non-dimensional correlation 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
a) 147.35 W
b) 157.35 W
c) 167.35 W
d) 177.35 W
Explanation: Q = h A d t, where h = (Nu) (k)/l = 177.35 W.

10 - Question

Consider the above problem, find the value of Nusselt number
a) 40.46
b) 50.56
c) 60.66
d) 70.76