This set of Heat Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Correlations For Forced Convection”.

1. The convective heat transfer coefficient in laminar flow over a flat plate

a) Increases with distance

b) Increases if a higher viscosity fluid is used

c) Increases if a denser fluid is used

d) Decreases with increase in free stream velocity

View Answer

Explanation: It mostly increases if a denser fluid is used.

2. For laminar flow over a flat plate, the average value of a Nusselt number is prescribed by the relation

Nu = 0.664 (Re) ^{0.5 }(Pr)^{ 0.33}

Which of the following is then a false statement?

a) Density has to be increased four times

b) Plate length has to be decreased four times

c) Specific heat has to be increased four times

d) Dynamic viscosity has to be decreased sixteen times

View Answer

Explanation: The dynamic viscosity has an inverse relation to 1/6 power. To double the convective heat transfer coefficient, the dynamic viscosity has to be decreased 64 times.

3. For turbulent flow over a flat plate, the average value of Nusselt number is prescribed by the relation

Nu = 0.664 (Re) ^{0.5 }(Pr)^{ 0.33}

Which of the following is then a false statement?

The average heat transfer coefficient increases as

a) 1/5 power of plate length

b) 2/3 power of thermal conductivity

c) 1/3 power of specific heat

d) 4/5 power of a free stream velocity

View Answer

Explanation: The average heat transfer coefficient reduces with length as 1/5th power of the length.

4. A nuclear reactor with its core constructed of parallel vertical plates 2.25 m high and 1.5 m wide has been designed on free convection heating of liquid bismuth. Metallurgical considerations limit the maximum surface temperature of the plate to 975 degree Celsius and the lowest allowable temperature of bismuth is 325 degree Celsius. Estimate the maximum possible heat dissipation from both sides of each plate. The appropriate correlation for the convection coefficient is

Nu = 0.13 (Gr Pr) ^{1/3}

a) 143 MW

b) 153 MW

c) 163 MW

d) 173 MV

View Answer

Explanation: Q = 2 h A d t = 153 MW.

5. Consider the above problem, find the value of Grashoff number

a) 101.3 * 10 ^{12}

b) 102.3 * 10 ^{12}

c) 103.3 * 10 ^{12}

d) 104.3 * 10 ^{12}

View Answer

Explanation: Grashof number = l

^{3 }p

^{2 }β g d t/µ

^{2}.

6. A thin walled duct of 0.5 m diameter has been laid in an atmosphere of quiescent air at 15 degree Celsius and conveys a particular gas at 205 degree Celsius. Base your calculations on one meter length of the duct, estimate the convective coefficient of heat transfer

a) 5.086 W/m^{2} K

b) 6.086 W/m^{2} K

c) 7.086 W/m^{2} K

d) 8.086 W/m^{2} K

View Answer

Explanation: h = 1.37 (d t/l)

^{0.25}= 5.086 W/m

^{2}K.

7. Free correction modulus is given by

a) p ^{2} β g c _{P}/µ

b) p ^{2} β g c _{P}/k

c) p ^{2} β g c _{P}/µ k

d) p ^{2} β g c _{P}

View Answer

Explanation: It contains only fluid properties and is called the free convection modulus.

8. The free convection coefficient is given by

h = C _{1} d t ^{m}/l ^{1 – 3m}

The value of exponent for laminar flow is

a) 0.5

b) 0.6

c) 0.7

d) 0.8

View Answer

Explanation: For laminar flow h = C

_{1}(d t/l)

^{0.25}.

9. For inclined plates we multiply Grashoff number with

a) Cos 2 α

b) Sin 2 α

c) Sin α

d) Cos α

View Answer

Explanation: It should be multiplied with cos α, as α is angle with the horizontal.

10. The free convection coefficient is given by

h = C _{1} d t ^{m}/l ^{1 – 3m}

The value of exponent for turbulent flow is

a) 0.43

b) 0.33

c) 0.23

d) 0.13

View Answer

Explanation: For turbulent flow h = C (d t).

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