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# MCQs on Laminar Film Condensation

For laminar film condensation on a vertical plate, the velocity distribution at a distance δ from the top edge is given by

a) p g (δ y – y/2)/σ

b) p g (δ y – y ^{2})/σ

c) p g (δ y – y ^{2}/2)/σ

d) p g (δ – y ^{2}/2)/σ

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Answer: c

Explanation: An equation for the velocity distribution as a function of some distance from the wall surface can be set up by considering the equilibrium between the gravity and viscous forces on an elementary volume of the liquid film.

For laminar film condensation on a vertical plate, the film thickness is given by

a) [4 k δ (t sat – t s) x/p 2 g h f g] 0.25

b) [4 k δ (t sat – t s) x/p 2 g h f g] 0.5

c) [4 k δ (t sat – t s) x/p 2 g h f g]

d) [4 k δ (t sat – t s) x/p 2 g h f g] 1.5

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Answer: a

Explanation: The film thickness increases as the fourth root of the distance down the surface.

For laminar film condensation on a vertical plate, the mass flow rate of the condensate per unit depth of the film at any position x is given by

a) p g δ ^{3}/ 3

b) p g δ ^{3}/ 3 σ

c) p g δ ^{2}/ 3 σ

d) p g δ ^{2}/ 3

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Answer: b

Explanation: Mass flow rate = mean flow velocity * flow area * density.

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For laminar film condensation on a vertical plate, the local heat transfer coefficient at the lower edge of the plate is given by

a) [k ^{3} p^{ 2} g h _{f g }/4 δ l (t _{sat} – t _{s})] ^{0.25}

b) [k ^{3} p^{ 2} g h _{f g }/4 δ l (t _{sat} – t _{s})] ^{0.5}

c) [k ^{3} p^{ 2} g h _{f g }/4 δ l (t _{sat} – t _{s})] ^{0.1}

d) [k ^{3} p^{ 2} g h _{f g }/4 δ l (t _{sat} – t _{s})] ^{0.125}

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Answer: a

Explanation: The rate of condensation heat transfer is higher at the upper end of the plate than at the lower end.

Mark the wrong statement with respect to laminar flow condensation on a vertical plate

a) The rate of condensation heat transfer is maximum at the upper edge of the plate and progressively decreases as the lower edge is approached

b) The average heat transfer coefficient is two third of the local heat transfer coefficient at the lower edge of the plate

c) At a definite point on the heat transfer, the film coefficient is directly proportional to thermal conductivity and inversely proportional to thickness of film at the point

d) The film thickness increases as the fourth root of the distance down the upper edge

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Answer: b

Explanation: The average heat transfer coefficient is 4/3 of the local heat transfer coefficient at the lower edge of the plate.

. A plate condenser of dimensions l * b has been designed to be kept with side l in the vertical position. However due to oversight during erection and installation, it was fixed with side b vertical. How would this affect the heat transfer? Assume laminar conditions and same thermos-physical properties and take b = l/2

a) The condenser should be installed with shorter side horizontal

b) The condenser should be installed with longer side horizontal

c) The condenser should be installed with longer side vertical

d) The condenser should be installed with shorter side vertical

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Answer: d

Explanation: h _{1} = 0.943 [k ^{3} p ^{2} g h _{f g}/δ l (t _{sat }– t _{s})] ^{0.25}, h_{2} = 0.943 [k ^{3} p ^{2} g h _{f g}/δ b (t _{sat }– t _{s})] ^{0.25}. So, h _{1}/h _{2 }= 0.8409.

Determine the length of a 25 cm outer diameter tube if the condensate formed on the surface of the tube is to be same whether it is kept vertical or horizontal

a) 61.5 cm

b) 71.5 cm

c) 81.5 cm

d) 91.5 cm

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Answer: b

Explanation: h v = 0.943 [k 3 p 2 g h f g/δ l (t sat – t s)] 0.25, h H = 0.943 [k 3 p 2 g h f g/δ d (t sat – t s)] 0.25, l/d = 2.86.

. The critical Reynolds number for transition from laminar to turbulent film condensation is

a) 2000

b) 1900

c) 1800

d) 1700

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Answer: c

Explanation: This should be 1800 for perfect transition from laminar to turbulent film condensation.

Which of the following is a wrong statement in the context of a convective heat transfer coefficient in laminar film condensation?

The heat transfer coefficient varies as

a) – ¼ power of acceleration due to gravity

b) ½ power of density of liquid

c) ¼ power of enthalpy of evaporation

d) – ½ power of dynamic viscosity

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Answer: a

Explanation: h = 0.943 [k ^{3} p ^{2} g h _{f g}/δ l (t _{sat }– t _{s})] ^{0.25}. Further the heat transfer coefficient varies as ¼ power of acceleration due to gravity.