Heat Transfer MCQs – Conduction Through a Composite Wall
A composite wall generally consists of
a) One homogenous layer
b) Multiple heterogeneous layers
c) One heterogeneous layer
d) Multiple homogenous layers
Explanation: Walls of houses where bricks are given a layer of plaster on either side.
Three metal walls of the same thickness and cross sectional area have thermal conductivities k, 2k and 3k respectively. The temperature drop across the walls (for same heat transfer) will be in the ratio
d) Given data is insufficient
Explanation: As, δ1 = δ2 = δ3 and cross sectional areas are same i.e. temperature drop varies inversely with thermal conductivity.
A composite wall is made of two layers of thickness δ1 and δ2 having thermal conductivities k and 2k and equal surface area normal to the direction of heat flow. The outer surface of composite wall are at 100 degree Celsius and 200 degree Celsius. The minimum surface temperature at the junction is 150 degree Celsius. What will be the ratio of wall thickness?
Explanation: Q = k 1 A 1 d t 1 / δ1 = k 2 A 2 d t 2 / δ2 Also areas are same.
Let us say thermal conductivity of a wall is governed by the relation k = k0 (1 + α t). In that case the temperature at the mid-plane of the heat conducting wall would be
a) Av. of the temperature at the wall faces
b) More than average of the temperature at the wall faces
c) Less than average of the temperature at the wall faces
d) Depends upon the temperature difference between the wall faces
Explanation: k0 is thermal conductivity at 0 degree Celsius. Here β is positive so it is more than average of the temperature at the wall faces.
Heat is transferred from a hot fluid to a cold one through a plane wall of thickness (δ), surface area (A) and thermal conductivity (k). The thermal resistance is
a) 1/A (1/h1 + δ/k + 1/h2)
b) A (1/h1 + δ/k + 1/h2)
c) 1/A (h1 + δ/k + h2)
d) A (h1 + δ/k + 1/h2)
Explanation: Net thermal resistance will be summation of resistance through plane wall and from left side and right side of the wall.
Find the heat flow rate through the composite wall as shown in figure. Assume one dimensional flow and take
k 1 = 150 W/m degree
k 2 = 30 W/m degree
k 3 = 65 W/m degree
k 4 = 50 W/m degree
AB = 3 cm, BC = 8 cm and CD = 5 cm. The distance between middle horizontal line from the top is 3 cm and from the bottom is 7 cm
a) 1173.88 W
b) 1273.88 W
c) 1373.88 W
d) 1473.88 W
Explanation: Q = d t/ R T. R T = R 1 + R e q + R 2 = 0.02 + 0.01469 + 0.1 = 0.2669 degree/W.
A pipe carrying steam at 215.75 degree Celsius enters a room and some heat is gained by surrounding at 27.95 degree Celsius. The major effect of heat loss to surroundings will be due to
d) Both conduction and convection
Explanation: As there is temperature difference so radiation suits well.
Radiation cannot be affected through vacuum or space devoid of any matter”. True or false
Explanation: It can be affected only by air between molecules and vacuum of any matter.
A composite slab has two layers having thermal conductivities in the ratio of 1:2. If the thickness is the same for each layer then the equivalent thermal conductivity of the slab would be
Explanation: 2(1) (2)/1+2 = 4/3.
A composite wall of a furnace has two layers of equal thickness having thermal conductivities in the ratio 2:3. What is the ratio of the temperature drop across the two layers?
d) log e 2 : log e 3
Explanation: We know that temperature is inversely proportional to thermal conductivity, so the ratio is 2:3.