# Thermal Engineering Questions and Answers – Heat Transfer by Convection

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This set of Thermal Engineering online test focuses on “Heat Transfer by Convection”.

1. Hot air at 50°C. If the forced heat transfer convection is 75 W/m2K, the heat gain rate gain rate by the plate through an area of 2m2 will be _____
a) 15KW
b) 22.5KJ/sec
c) 7.5KJ/sec
d) 25KJ/sec

Explanation: Rate of heat transfer
Q = Q = hA∆T
Q = 75×2×(150-50)
Q = 15 kW.

2. Nusselt number for fully developed turbulent flow in a pipe is given by Nu=C Rea Prb. the values of a and b are?
a) a=0.5 and b=0.33 for heating and cooling
b) a=0.5 and b=0.4 for heating and b=0.3 for cooling
c) a=0.8 and b=0.4 for heating and b=0.3 for cooling
d) a=0.8 and b=0.3 for heating and b=0.4 for cooling

Explanation: For fully developed turbulent flow in a pipe, Nusselt number
Nu=0.023 Re0.8 Prn
Where, n= 0.4 for heating
And n = 0.3 for cooling.

3. Which of the following non-dimensional numbers is used for transition from laminar flow to turbulent flow in the free convection?
a) Reynolds number
b) Grashoff number
c) Peclet number
d) Rayleigh number

Explanation: Grashoff’s number plays similar role to that of Reynolds number in forced convection. Its value indicates whether the fluid flow of natural convection is laminar or turbulent. The critical value of Grashoff number indicates that the transition from laminar to turbulent flow.

4. Which non-dimensional numbers relates the thermal boundary layer and hydrodynamic boundary layer?
a) Rayleigh number
b) Peclet number
c) Grashoff number
d) Prandtl number

Explanation: Prandtl number is a dimensionless number that relates the thermal boundary layer and hydrodynamic boundary layer.
$$\frac{δ}{(δ)t} = \frac{hydrodynamic \, boundary \, layer}{thermodynamic \, boundary \, layer}$$ = Pr1/3.

5. The laminar boundary layer occurs when a cold fluid over a hot plate. In which of the following positions, the temperature gradient assumes zero value?
a) At bottom of boundary layer
b) In mid free stream of fluid
c) At top of boundary layer
d) At the junction of laminar and turbulent boundary layer

Explanation: The value of temperature gradient dT/dy decreases with the distance from plate and at the top of boundary layer it becomes zero as the temperature equals to free stream temperature of the fluid.

6. The characteristic length for computing Grashoff number of horizontal cylinder is ___________
a) The length of the cylinder
b) The diameter of the cylinder
c) The perimeter of the cylinder
d) The radius of the cylinder

Explanation: The diameter becomes the characteristics length in case of free convection. Whereas length is for forced convection.

7. In laminar developing flow through a pipe with constant wall temperature, the magnitude of the pipe wall inner surface convective heat transfer coefficient shall be maximum at the:
a) Middle length of flow
b) Beginning of flow
c) End of flow
d) One third of the length of flow

Explanation: In laminar flow through pipe nusselt number constant for constant wall temperature. Therefore wall inner surface convection heat transfer coefficient will be constant for the laminar flow in pipe.

8. For a fluid with Prandtl number Pr>1, momentum boundary layer thickness?
a) Decreases rapidly compared to the thermal boundary layer thickness
b) And thermal boundary layer thickness increase at the same rate
c) Increases rapidly as compared to thermal boundary layer thickness
d) And thermal boundary layer thickness decrease at the same rate

Explanation: $$\frac{δ}{(δ)t}$$ = (Pr)1/3
For Pr>1 , δ>(δ)t
Therefore momentum boundary layer thickness increases rapidly compared to the thermal boundary layer thickness.

9. Water (Prandtl number ≈6) flows over a flat plate which is heated over the entire length. Which one of the following relationship between the hydrodynamic boundary layer thickness (δ) and the thermal boundary layer thickness (δ)t is true?
a) (δ)t > δ
b) (δ)t < δ
c) (δ)t = δ
d) Cannot be predicted

Explanation: $$\frac{δ}{(δ)t}$$ = (Pr)1/3
Since Prandtl number Pr = 6 > 1, therefore from above equation,
(δ)t < δ.

10. If qw =2500x where x is in m and in the direction of flow (x=0 at the inlet), the bulk mean temperature of the water leaving the pipe in °C is ________
a) 71
b) 76
c) 79
d) 81

Explanation: qw×πDL = mcp(t2-t1)
5000×π×0.05×3=0.01×4.18×103(t2-20)
t2=76°C
At the outlet flux can be calculated by,
Qw = h(ts-t1)
5000=1000(ts-76)
ts=81°C.

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