# Heat and Mass Transfer MCQ – Set 02

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## Q1. Fourier’s law of heat conduction is (where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow, and k = Thermal conductivity of the body)

(A) k. A. (dT/dx)
(B) k. A. (dx/dT)
(C) k. (dT/dx)
(D) k. (dx/dT)

## Q2. Thermal conductivity of water _ with rise in temperature.

(A) Remain same
(B) Decreases
(C) Increases
(D) May increase or decrease depending upon temperature

## Q3. Fouling factor is used

(A) In heat exchanger design as a safety factor
(B) In case of Newtonian fluids
(C) When a liquid exchanges heat with a gas
(D) None of the above

## Q4. In case of liquids and gases, the heat transfer takes place according to

(A) Conduction
(B) Convection
(D) None of these

## Q5. A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be

(A) k₁ k₂
(B) (k₁ + k₂)
(C) (k₁ + k₂)/ k₁ k₂
(D) 2 k₁ k₂/ (k₁ + k₂)

## Q6. When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by

(A) Conduction
(B) Convection
(D) Conduction and convection

(A) Liquids
(B) Energy
(C) Temperature
(D) Entropy

## Q8. The time constant of a thermocouple is

(A) The time taken to attain the final temperature to be measured
(B) The time taken to attain 50% of the value of initial temperature difference
(C) The time taken to attain 63.2% of the value of initial temperature difference
(D) Determined by the time taken to reach 100°C from 0°C

(A) Aluminium
(B) Steel
(C) Brass
(D) Copper

## Q10. A grey body is one whose absorptivity

(A) Varies with temperature
(B) Varies with wavelength of the incident ray
(C) Is equal to its emissivity
(D) Does not vary with temperature and. wavelength of the incident ray

## Q11. The heat transfer by conduction through a thick sphere is given by

(A) Q = 2πkr1 r2 (T1 – T2)/ (r2 – r1)
(B) Q = 4πkr1 r2 (T1 – T2)/ (r2 – r1)
(C) Q = 6πkr1 r2 (T1 – T2)/ (r2 – r1)
(D) Q = 8πkr1 r2 (T1 – T2)/ (r2 – r1)

## Q12. The thickness of thermal and hydrodynamic boundary layer is equal if Prandtl number is

(A) Equal to one
(B) Greater than one
(C) Less than one
(D) Equal to Nusselt number

## Q13. The natural convection air cooled condensers are used in

(A) Domestic refrigerators
(B) Water coolers
(C) Room air conditioners
(D) All of these

## Q14. The logarithmic mean temperature difference ™ is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)

(A) tm = (Δt1 – Δt2)/ loge (Δt1/Δt2)
(B) tm = loge (Δt1/Δt2)/ (Δt1 – Δt2)
(C) tm = tm = (Δt1 – Δt2) loge (Δt1/Δt2)
(D) tm = loge (Δt1 – Δt2)/ Δt1/Δt2

• Q15. When absorptivity (α) = 1, reflectivity (ρ) = 0 and transmissivity (τ) = 0, then the body is said to be a

(A) Black body
(B) Grey body
(C) Opaque body
(D) White body

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