This set of tricky Heat Transfer Questions & Answers focuses on “Turbulent Film Condensation”.

1. Which parameter is responsible for the commencement of the turbulent flow?

a) Fourier number

b) Reynolds number

c) Stanton number

d) Nusselt number

View Answer

Explanation: The character of condensate film can range from laminar to highly turbulent.

2. The transition from laminar to turbulent flow occurs at a critical Reynolds number of

a) 1800

b) 2200

c) 2600

d) 3000

View Answer

Explanation: For turbulent film condensation on vertical surfaces, Kirk bride has suggested the correlation for the average heat transfer coefficient which is valid for Reynolds number greater than 1800.

3. A condenser is to be designed to condense 225.0 kg of steam per hour at a pressure of 0.15 bar. A square array of 400 tubes, each of 6 mm in diameter, is available for the task. If the tube surface temperature is to be maintained at 26 degree Celsius, make calculations for the length of the tube

a) 4.353 m

b) 3.353 m

c) 2.353 m

d) 1.353 m

View Answer

Explanation: t

_{sat }= 54 degree Celsius and h

_{f g}= 2373000 J/kg, t

_{f}= 40 degree Celsius and h

_{v}= 0.725 [k

^{3}g p

^{2}h

_{f g}/l δ (t

_{sat }– t

_{s})]

^{0.25}= 5202.34 W/m

^{2 }K. Total S.A of 400 tubes = 7.536l and heat flow rate = m h

_{f g}.

4. The outer surface of a vertical tube is 1.25 m long and outer diameter is 50 mm is exposed to saturated steam at atmospheric pressure. If the tube surface is maintained at 80 degree Celsius by the flow of cooling water through it, determine the rate of heat transfer to the coolant

a) 47648 W

b) 12345 W

c) 19879 W

d) 97123 W

View Answer

Explanation: h

_{v}= 0.943 [k

^{3}p

^{2}g h

_{f g}/δ l (t

_{sat }– t

_{s})]

^{0.25}= 5046.8 W/m

^{2}k and heat flow rate = 19879 W.

5. Consider the above problem, find the rate at which steam is condensed at the tube surface

a) 7.7 * 10 ^{-3}

b) 8.7 * 10 ^{-3}

c) 9.7 * 10 ^{-3}

d) 10.7 * 10 ^{-3}

View Answer

Explanation: Steam condensation rate = 198789/2285 * 10

^{3}= 8.7 * 10

^{-3}.

6. What is the value of characteristics length in turbulent film condensation?

a) δ

b) 3 δ

c) 2 δ

d) 4 δ

View Answer

Explanation: d = 4 A/P = 4 b δ/b = 4 δ. Where δ is length of the plate.

7. What is the value of Reynolds number in terms of mass flow rate?

a) 4 m/δ b

b) 2 m/δ b

c) 3 m/δ b

d) m /δ b

View Answer

Explanation: R = V 4 δ p/δ = 4 m/δ b. Where δ is the length of the plate and b id width of the plate.

8. Kirk bride criterion is given by

a) h = 0.0057 (Re)^{ 0.4} [k ^{3 }p^{ 2} g/δ^{ 2}]^{ 1/3}

b) h = 0.0067 (Re)^{ 0.4} [k ^{3 }p^{ 2} g/δ^{ 2}]^{ 1/3}

c) h = 0.0077 (Re)^{ 0.4} [k ^{3 }p^{ 2} g/δ^{ 2}]^{ 1/3}

d) h = 0.0087 (Re)^{ 0.4} [k ^{3 }p^{ 2} g/δ^{ 2}]^{ 1/3}

View Answer

Explanation: This is valid for Re greater than 1800.

9. The relation between relative effectiveness of horizontal and vertical tubes as condensing surfaces is given by

a) h _{h}/h _{v }= 0.768 (l/d)

b) h _{h}/h _{v }= 0.768 (l/d) ^{0.25}

c) h _{h}/h _{v }= 0.768 (l/d) ^{0.5}

d) h _{h}/h _{v }= 0.768 (l/d) ^{1.25}

View Answer

Explanation: h

_{h}/h

_{v }= 0.725/0.943 [(l/d)

^{0.25}].

10. For n-tubes in a vertical column of the tube bank pattern, identify the correct statement

a) D _{e} = n_{ }D

b) D _{e} = n^{ 2}_{ }D

c) D _{e} = (1/n)_{ }D

d) D _{e} =_{ }D

View Answer

Explanation: It should be n times the diameter of a single tube in the bank.

**Sanfoundry Global Education & Learning Series – Heat Transfer.**

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