Heat Transfer Multiple Choice Questions & Answers (MCQs) on “Significance Of Dimensionless Groups”.
1. Ratio of inertia force to viscous force is known as
a) Grashof number
b) Reynolds number
c) Peclet number
d) Stanton number
Answer: b
Clarification: It is given by p l v/µ. It is the indicative of the relative importance of inertial and viscous effects in a fluid motion.
2. The ratio of heat flow rate by convection to flow rate by conduction is known as
a) Stanton number
b) Graetz number
c) Fourier number
d) Peclet number
Answer: d
Clarification: It is given by I V/α. It is a function of Reynolds number and Prandtl number.
3. The ratio of heat capacity of fluid flowing through the pipe per unit of length to the conductivity of pipe material is known as
a) Graetz number
b) Reynolds number
c) Peclet number
d) Fourier number
Answer: a
Clarification: It is given by Pe (π d/4 l). It represents the ratio of the heat capacity of fluid flowing through the pipe per unit of length to the conductivity of pipe material.
4. Identify the correct statement
a) Peclet number = (Graetz number) (Fourier number)
b) Peclet number = (Stanton number) (Fourier number)
c) Peclet number = (Reynolds number) (Prandtl number)
d) Peclet number = (Graetz number) (Reynolds number)
Answer: c
Clarification: Peclet number is a function of Reynolds number and Prandtl number. Peclet number = p c l V/ k = p l V/µ (µ c/k) = (Re) (Pr).
5. What is the value of Prandtl number for highly viscous oils?
a) 100-1000
b) 0-100
c) 10-100
d) 100-10000
Answer: d
Clarification: It indicates the rapid diffusion of momentum by viscous action compared to the diffusion of energy.
6. What is the value of Prandtl number for liquid metals?
a) 0.003-0.01
b) 0.01-0.1
c) 0.1-0.5
d) 0.5-0.95
Answer: a
Clarification: It indicates more rapid diffusion of energy compared to the momentum diffusion rate.
7. The product of buoyant force and inertia force to the square of the viscous force is known as
a) Stanton number
b) Grashof number
c) Fourier number
d) Peclet number
Answer: b
Clarification: It indicates the relative strength of the buoyant to the viscous forces. It represents the ratio of the product of buoyant and inertia forces to the square of the viscous forces.
8. The ratio of heat transfer coefficient to the flow of heat per unit temperature rise due to the velocity of the fluid is known as
a) Fourier number
b) Grashof number
c) Peclet number
d) Stanton number
Answer: d
Clarification: Stanton number can be used in correlating forced convection data. This becomes obvious when we observe the velocity V contained in the expression of Stanton number. It is the ratio of heat transfer coefficient to the flow of heat per unit temperature rise due to the velocity of the fluid.
9. Which number indicates the relative ability of the fluid to diffuse momentum and internal energy by molecular mechanisms?
a) Nusselt number
b) Prandtl number
c) Peclet number
d) Stanton number
Answer: b
Clarification: It is the ratio of kinematic viscosity to thermal diffusivity of the fluid.
10. Which number establishes the relation between convective film coefficient, thermal conductivity of the fluid and a significant length parameter?
a) Nusselt number
b) Stanton number
c) Peclet number
d) Fourier number
Answer: a
Clarification: The Nusselt number may be interpreted as the ratio of temperature gradient to an overall reference temperature gradient.