Engineering Questions with Answers - Multiple Choice Questions
MCQs on Laminar Film Condensation
1 - Question
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
View Answer
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.
2 - Question
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
View Answer
Answer: a
Explanation: The film thickness increases as the fourth root of the distance down the surface.
3 - Question
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
View Answer
Answer: b
Explanation: Mass flow rate = mean flow velocity * flow area * density.
4 - Question
View Answer
5 - Question
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
View Answer
Answer: a
Explanation: The rate of condensation heat transfer is higher at the upper end of the plate than at the lower end.
6 - Question
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
View Answer
Answer: b
Explanation: The average heat transfer coefficient is 4/3 of the local heat transfer coefficient at the lower edge of the plate.
7 - Question
. 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
View Answer
Answer: d
Explanation: h 1 = 0.943 [k 3 p 2 g h f g/δ l (t sat – t s)] 0.25, h2 = 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.
8 - Question
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
View Answer
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.
9 - Question
. The critical Reynolds number for transition from laminar to turbulent film condensation is
a) 2000
b) 1900
c) 1800
d) 1700
View Answer
Answer: c
Explanation: This should be 1800 for perfect transition from laminar to turbulent film condensation.
10 - Question
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
View Answer
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.