GATE CH topic:heat-transfer PYQs & Answers

Questions 1–10 of 57. Imported answers require independent review unless marked reviewed.

A body is placed inside a perfectly black enclosure and is allowed to reach thermal equilibrium. According to the Kirchhoff’s identity, which one of the following is equal to the emissivity of the body at a given wavelength?

Your answerChoose one option.

absorptivity of the body at the same wavelength

reflectivity of the body at the same wavelength

Stefan-Boltzmann constant

zero

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In a double pipe cocurrent heat exchanger, operating at steady state, oil (specific heat capacity = 2100 J kg

°C

) entering at 7 kg s

at 100 °C is used to heat water (specific heat capacity = 4200 J kg

°C

) flowing at 3.5 kg s

from 20 °C to 50 °C. If the overall heat transfer coefficient is 291 W m

°C

, the required heat transfer area (in m

) is ____ (rounded off to one decimal place).

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A pipe with outer diameter of 0.02 m, carries hot water. The temperature at the outer surface of the pipe is 70 °C. This pipe is covered with two concentric layers of insulation, each having a thickness of 0.01 m. The first layer (adjacent to the pipe) is made up of felt material (thermal conductivity k_f = 0.12 W m⁻¹ °C⁻¹), and the next layer is made up of asbestos (thermal conductivity k_as = 0.15 W m⁻¹ °C⁻¹). The insulated pipe is exposed to ambient air at 20 °C. The convection heat transfer coefficient at the outer insulation layer is 3 W m⁻² °C⁻¹. At steady state, neglecting heat transfer due to radiation, the heat loss at the outer insulation layer (in W m⁻¹) is _____ (rounded off to two decimal places).

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In industrial heat exchanger design, the overall heat transfer coefficient

is estimated from the equation

where

and

are the convective heat transfer coefficients on the inner and outer side of the tube, respectively. This is valid for (i) tube of (ii) thermal conductivity.

Which one of the following is the CORRECT option to fill in the gaps (i) and (ii)?

Your answerChoose one option.

(i) thick-walled, (ii) high

(i) thin-walled, (ii) high

(i) thin-walled, (ii) low

(i) thick-walled, (ii) low

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Which of the following statements regarding multiple effect evaporators is/are TRUE?

Your answerSelect all correct options. You can choose more than one.

The pressure of the effect with fresh steam is the lowest for both forward feed and backward feed.

Backward feed is preferred over forward feed for cold feed.

Backward feed is preferred over forward feed for highly viscous concentrated product.

The temperature of the effect with fresh steam is the lowest for both forward feed and backward feed.

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A hot plate is placed in contact with a cold plate of a different thermal conductivity as shown in the figure. The initial temperature (at time

) of the hot plate and cold plate are

and

, respectively. Assume perfect contact between the plates.

Diagram for Chemical Engineering question 38

Which one of the following is an appropriate boundary condition at the surface S for solving the unsteady state, one-dimensional heat conduction equations for the hot plate and cold plate for

Your answerChoose one option.

Temperature at S is same for both the plates

Gradient of temperature at S is same for both the plates

Gradient of temperature vanishes at S

Temperature at S is the average of

and

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Water flowing at 70 kg/min is heated from 25 °C to 65 °C in a counter-flow double pipe heat exchanger using hot oil. The oil enters at 110 °C and exits at 65 °C. If the overall heat transfer coefficient is 300 W/(m

K), the heat exchanger area is ____ m

(rounded off to 1 decimal place).

GIVEN: Specific heat of water and oil are 4.2 kJ/(kg °C) and 2 kJ/(kg °C), respectively.

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Consider laminar flow of water over a wide flat plate maintained at a uniform temperature of 50 °C as shown in the figure. The freestream velocity and temperature of water are 1.0 m/s (parallel to the plate) and 20 °C, respectively. The distance

from the leading edge, at which the thermal boundary layer thickness

m, is ____ m (rounded off to 1 decimal place).

Diagram for Chemical Engineering question 52

GIVEN: kinematic viscosity:

Prandtl number:

velocity boundary layer thickness:

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An electrical wire of 2 mm diameter and 5 m length is insulated with a plastic layer of thickness 2 mm and thermal conductivity

W/(m K). It is exposed to ambient air at 30 °C. For a current of 5 A, the potential drop across the wire is 2 V. The air-side heat transfer coefficient is 20 W/(m

K). Neglecting the thermal resistance of the wire, the steady state temperature at the wire-insulation interface is ____ °C (rounded off to 1 decimal place).

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If the Prandtl number

, which of the following statements is/are correct?

Your answerSelect all correct options. You can choose more than one.

The moment diffusivity is much larger than the thermal diffusivity.

The thickness of the momentum boundary layer is much smaller than that of the thermal boundary layer.

The thickness of the momentum boundary layer is much larger than that of the thermal boundary layer.

The momentum diffusivity is much smaller than the thermal diffusivity.

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GATE CH topic:heat-transfer PYQs

How to practice topic:heat-transfer

Question 13

Question 52

Question 53

Question 17

Question 30

Question 38

Question 51

Question 52

Question 53

Question 27