Heat Transfer MCQ’s – Thermal Boundary Layer
1 - Question
Which field is set up when a fluid flows past a heated or cold surface?
c) Massa) Energy
Explanation: The temperature field encompasses a very small region of fluid.
2 - Question
The zone or thin layer wherein the temperature field exists is called the
a) Single boundary layer
b) Multi boundary layer
c) Hydrodynamic boundary layer
d) Thermal boundary layer
Explanation: It is known as the thermal boundary layer. The temperature gradient results due to heat exchange between the plate and the fluid.
3 - Question
The thickness of thermal boundary layer is arbitrarily defined as the distance from the plate surface at which
a) t S – t/t S – t INFINITY = 0.34
b) t S – t/t S – t INFINITY = 0.10
c) t S – t/t S – t INFINITY = 0.99
d) t S – t/t S – t INFINITY = 0.87
Explanation: It should be 0.99. The convection of energy reduces the outward conduction in the fluid and consequently the temperature gradient decreases away from the surface.
4 - Question
The convection of energy reduces the outward conduction in the fluid so temperature gradient
d) Becomes twice
Explanation: The temperature gradient is infinite at the leading edge and approaches zero as the layer develops downstream.
5 - Question
The velocity profile of the hydrodynamic boundary layer is dependent upon
Explanation: It depends upon viscosity as the fluid velocity varies from zero at the solid surface to the velocity of free stream flow at a certain distance away from the solid surface.
6 - Question
Consider an elemental control volume for mass balance continuity equation. Which one of the following is correct?
a) Value of 1 is 2 v + d u/d x (d y)
b) Value of 2 is u + d u/d y (d x)
c) Value of 1 is v + d u/d x (d y)
d) Value of 2 is 2 u + d v/d x (d x)
Explanation: The flow velocity changes in the direction of x axis and the rate of change is d u/d x.
7 - Question
Which is true for two dimensional boundary layer?
a) d u/d x – d v/d y = 1
b) d u/d x – d v/d y = 0
c) d u/d x + d v/d y = 1
d) d u/d x + d v/d y = 0
Explanation: This a two dimensional general equation. This equation also represents the continuity equation.
8 - Question
The differential energy equation for flow past a flat plate is given by
a) u d t/d x + v d t/d y = µ/p c (d u/d y) 2 + k d 2 t/p c d y 2
b) u d t/d x + v d t/d y = µ/p c (d u/d y) + k d 2 t/p c d y 2
c) u d t/d x + v d t/d y = k d 2 t/p c d y 2 + µ/p c (d u/d y) -2
d) u d t/d x + v d t/d y = µ/p c (d u/d y) -1 + k d 2 t/p c d y 2
Explanation: Here heat generation due to viscous effects is not neglected.
9 - Question
The assumptions for thermal boundary layer are (i) Steady compressible flow (ii) Negligible body forces, viscous heating and conduction in the flow direction (iii) Constant fluid properties evaluated at the film temperature Identify the correct option
a) i and iii
b) i, ii and iii
c) ii and iii
d) i and ii
Explanation: It is steady incompressible flow. The temperature of the fluid changes from a minimum at the plate surface to the temperature of the mainstream at a certain distance from the surface.
10 - Question
The relationship between the thermal and hydrodynamic boundary layer thickness is governed by the
a) Peclet number
b) Prandtl number
c) Stanton number
d) Fourier number
Explanation: It is governed by the Prandtl number, which is indicative of the relative ability of the fluid to diffuse momentum and internal energy by molecular mechanisms.