# Multiple choice question for engineering

## Set 1

1. Assumptions made for calculation of logarithmic mean temperature difference are

(i) Constant overall heat transfer coefficient

(ii) The kinetic and potential energy changes are negligible

(iii) There is no conduction of heat along the tubes of heat exchanger

Identify the correct statements

a) 1, 2 and 3

b) 1 and 3

c) 1 and 2

d) 2 and 3

### View Answer

2. A cold fluid (specific heat 2.95 k J/kg K) at 10 kg/min is to be heated from 25 degree Celsius to 55 degree Celsius in a heat exchanger. The task is accomplished by extracting heat from hot water (specific heat 4.186 k J /kg K) available at mass flow rate 5 kg/min and inlet temperature 85 degree Celsius. Identify the type of arrangement of the heat exchanger

a) Concentric tubes

b) Parallel flow

c) Counter flow

d) Shell and tubes

### View Answer

_{h }c

_{h }(t

_{h 1}– t

_{h 2}) = m

_{c }c

_{c }(t

_{c 2}– t

_{c 1}).

3. In a food processing plant, a brine solution is heated from – 12 degree Celsius to – 65 degree Celsius in a double pipe parallel flow heat exchanger by water entering at 35 degree Celsius and leaving at 20.5 degree Celsius. Let the rate of flow is 9 kg/min. Estimate the area of heat exchanger for an overall heat transfer coefficient of 860 W/m^{2} K. For water c _{P} = 4.186 * 10 ^{3} J/kg K

a) 1. 293 m^{2}

b) 0.293 m^{2}

c) 7. 293 m^{2}

d) 8. 293 m^{2}

### View Answer

_{P }d t = 9104.5 J/s. A = Q/ U α

_{m}.

4. Exhaust gases (c _{P }= 1.12 k J/kg K) flowing through a tubular heat exchanger at the rate of 1200 kg/hr are cooled from 400 degree Celsius to 120 degree Celsius. This cooling is affected by water (c _{P }= 4.18 k J/kg K) that enters the system at 10 degree Celsius at the rate of 1500 kg/hr. If the overall heat transfer coefficient is 500 k J/m^{2} hr degree, what heat exchanger area is required to handle the load for parallel flow arrangement?

a) 7.547 m^{2}

b) 6.547 m^{2}

c) 5.547 m^{2}

d) 4.547 m^{2}

### View Answer

_{h }c

_{h }(t

_{h 1}– t

_{h 2}) = m

_{c }c

_{c }(t

_{c 2}– t

_{c 1}).

5. A steam condenser is transferring 250 k W of thermal energy at a condensing temperature of 65 degree Celsius. The cooling water enters the condenser at 20 degree Celsius with a flow rate of 7500 kg/hr. Calculate the log mean temperature difference

a) 28.25 degree Celsius

b) 29.25 degree Celsius

c) 30.25 degree Celsius

d) 31.25 degree Celsius

### View Answer

_{c }c

_{c }(t

_{c 2}– t

_{c 1}) and log mean temperature difference = α

_{1 }– α

_{2 }/ log (α

_{1}/α

_{2}).

6. Consider the above problem, find what error would be introduced if the arithmetic mean temperature difference is used rather than the log-mean temperature difference? Take overall heat transfer coefficient for the condenser surface as 1250 W/m^{2} K

a) 7.61%

b) 7.71%

c) 7.81%

d) 7.91%

### View Answer

_{1 }+ α

_{2}/2. Error = 7.08 – 6.52/7.08 = 7.91%.

7. For what value of end temperature difference ratio, is the arithmetic mean temperature difference 5% higher than the log-mean temperature difference?

a) 2.4

b) 2.3

c) 2.2

d) 2.1

### View Answer

_{1}/ α

_{2}= 2.2.

8. A company is heating a gas by passing it through a pipe with steam condensing on the outside. What percentage change in length would be needed if it is proposed to triple the heating capacity?

a) 200%

b) 400%

c) 600%

d) 800%

### View Answer

_{1}= U

_{1 }A

_{1 }α

_{ 1}and new capacity, Q

_{2}= U

_{2 }A

_{2 }α

_{ 2}. According to the given condition, U

_{2 }A

_{2 }α

_{ 2}= 3 U

_{1 }A

_{1 }α

_{ 1}.

9. A steam condenser is transferring 250 k W of thermal energy at a condensing temperature of 65 degree Celsius. The cooling water enters the condenser at 20 degree Celsius with a flow rate of 7500 kg/hr. If overall heat transfer coefficient for the condenser surface is 1250 W/m^{2} K, what surface area is required to handle this load?

a) 4.08 m^{2}

b) 5.08 m^{2}

c) 6.08 m^{2}

d) 7.08 m^{2}

### View Answer

_{m}. So, A = 7.08 m

^{2}.

10. Exhaust gases (c _{P }= 1.12 k J/kg K) flowing through a tubular heat exchanger at the rate of 1200 kg/hr are cooled from 400 degree Celsius to 120 degree Celsius. This cooling is affected by water (c _{P }= 4.18 k J/kg K) that enters the system at 10 degree Celsius at the rate of 1500 kg/hr. If the overall heat transfer coefficient is 500 k J/m^{2} hr degree, what heat exchanger area is required to handle the load for counter flow arrangement?

a) 2.758 m^{2}

b) 3.758 m^{2}

c) 4.758 m^{2}

d) 5.758 m^{2}

### View Answer

_{h }c

_{h }(t

_{h 1}– t

_{h 2}) = m

_{c }c

_{c }(t

_{c 2}– t

_{c 1}).

## Set 2

1. Which is a full size structure employed in the actual engineering design?

a) Proton

b) Prototype

c) Electron

d) Neutron

### View Answer

2. Which term refers to the theory and art of predicting prototype conditions from model observations?

a) Nusselt number

b) Dimensional homogeneity

c) Thermal boundary layer

d) Similitude

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3. The results obtained from experiments on models can be applied to prototype only if a complete similarity exists between the model and prototype and for that the two systems may be

(i) Geometrically similar

(ii) Kinematically similar

(iii) Dynamically similar

Identify the correct statements

a) 1 and 2

b) 2 and 3

c) 1, 2 and 3

d) 1 and 3

### View Answer

4. Geometrically similarity prescribes that the ratio of the corresponding linear dimensions of the two systems are

a) Unity

b) Same

c) Never same

d) May be twice

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5. Thermal similarity refers to the comparison of two systems made on the basis of their

a) Temperature

b) Specific heat

c) Heat flux

d) Length

### View Answer

6. The comparison of two systems made on the basic of their temperature, specific heat and heat flus is known as

a) Dynamic similarity

b) Kinematic similarity

c) Thermal similarity

d) Geometrical similarity

### View Answer

7. The similarity of masses and forces of the corresponding particles of flow is known as

a) Kinematic similarity

b) Dynamic similarity

c) Geometrical similarity

d) Thermal similarity

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8. The similarity of motion is known as

a) Thermal similarity

b) Dynamic similarity

c) Geometrical similarity

d) Kinematic similarity

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9. The similarity of shape and form is known as

a) Geometrical similarity

b) Thermal similarity

c) Geometrical similarity

d) Kinematic similarity

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10. A similarity in thermal quantities is achieved when

a) Nusselt number is same for both the fields

b) Nusselt number is different for both the fields

c) Prandtl number is same for both the fields

d) Prandtl number is different for both the fields

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## Set 3

1. The literature of heat transfer generally recognizes distinct modes of heat transfer. How many modes are there?

a) One

b) Two

c) Three

d) Four

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2. Consider system A at uniform temperature t and system B at another uniform temperature T (t > T). Let the two systems be brought into contact and be thermally insulated from their surroundings but not from each other. Energy will flow from system A to system B because of

a) Temperature difference

b) Energy difference

c) Mass difference

d) Volumetric difference

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3. An oil cooler in a high performance engine has an outside surface area 0.12 m^{2} and a surface temperature of 65 degree Celsius. At any intermediate time air moves over the surface of the cooler at a temperature of 30 degree Celsius and gives rise to a surface coefficient equal to 45.4 W/ m ^{2} K. Find out the heat transfer rate?

a) 238.43 W

b) 190.68 W

c) 543.67 W

d) 675.98 W

### View Answer

_{2 }– T

_{1}) A h = 0.12 (65-30) 45.4 = 190.68 W.

4. Unit of rate of heat transfer is

a) Joule

b) Newton

c) Pascal

d) Watt

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5. Convective heat transfer coefficient doesn’t depend on

a) Surface area

b) Space

c) Time

d) Orientation of solid surface

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6. The rate equation used to describe the mechanism of convection is called Newton’s law of cooling. So rate of heat flow by convection doesn’t depend on

a) Convective heat transfer coefficient

b) Surface area through which heat flows

c) Time

d) Temperature potential difference

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7. How many types of convection process are there?

a) One

b) Three

c) Four

d) Two

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8. Thermal conductivity is maximum for which substance

a) Silver

b) Ice

c) Aluminum

d) Diamond

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9. A radiator in a domestic heating system operates at a surface temperature of 60 degree Celsius. Calculate the heat flux at the surface of the radiator if it behaves as a black body

a) 697.2 W/m^{2}

b) 786.9 W/m^{2}

c) 324.7 W/m^{2}

d) 592.1 W/m^{2}

### View Answer

^{-8}(273+60)

^{4}= 697.2.

10. Which of the following is an example of forced convection?

a) Chilling effect of cold wind on a warm body

b) Flow of water in condenser tubes

c) Cooling of billets in the atmosphere

d) Heat exchange on cold and warm pipes

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## Set 4

1. Newton-Rikhman law is given by

a) Q = h A (t _{s} – t_{ f})

b) Q = 2 h A (t _{s} – t_{ f})

c) Q = 3 h A (t _{s} – t_{ f})

d) Q = 4 h A (t _{s} – t_{ f})

### View Answer

2. The value of film coefficient is dependent upon

(i) Boundary layer configuration

(ii) Geometry and orientation of the surface

(iii) Surface conditions

a) 1 and 2

b) 2 and 3

c) 1 and 2

d) 1, 2 and 3

### View Answer

3. The convection coefficients for boiling and condensation lie in the range

a) 5000-12500 W/m^{ 2 }K

b) 2500-100000 W/m^{ 2 }K

c) 2500-5000 W/m^{ 2 }K

d) 2500-12500 W/m^{ 2 }K

### View Answer

4. Forced air flows over a convection heat exchanger in a room heater, resulting in a convective heat transfer coefficient 1.136 k W/m^{2 }K. The surface temperature of heat exchanger may be considered constant at 65 degree Celsius, and the air is at 20 degree Celsius. Determine the heat exchanger surface area required for 8.8 k W of heating

a) 0.272 m^{2}

b) 0.472 m^{2}

c) 0.172 m^{2}

d) 0.672 m^{2}

### View Answer

_{s }– t

_{f}). So. A = 0.172 m

^{2}.

5. A region of fluid motion near a plate in which temperature gradient exist is

a) Thermal boundary layer

b) Diathermia boundary layer

c) Turbulent flow

d) Laminar flow

### View Answer

6. Thermo-physical properties of the fluid are represented by

(i) Density

(ii) Viscosity

(iii) Specific heat

(iv) Thermal conductivity

Identify the correct option

a) 1 and 2

b) 1, 2, 3 and 4

c) 2, 3 and 4

d) 1, 2 and 3

### View Answer

7. A motor cycle cylinder consists of ten fins, each 150 mm outside diameter and 75 mm inside diameter. The average fin temperature is 500 degree Celsius and the surrounding air is at 20 degree Celsius temperature. Make calculations for the rate of heat dissipation from the cylinder fins by convection when motor cycle is stationary and convective coefficient is 6 W/m^{2} K

a) 432.2 W

b) 532.2 W

c) 632.2 W

d) 763.2 W

### View Answer

Explanation: A = 0.265 m^{2} and Q = (6) (0.265) (500 – 20) = 763.2 W.

8. Consider the above problem, make calculations for the rate of heat dissipation from the cylinder fins by convection when motor cycle is moving at 60 km/hr and convective coefficient is 75 W/m^{2} K

a) 9640 W

b) 9540 W

c) 9440 W

d) 9340 W

### View Answer

^{2}and Q = (75) (0.265) (500 – 20) = 9540 W.

9. The temperature profile at a particular location on a surface is prescribed by the identity

(t _{s} – t) / (t _{s} – t_{ infinity}) = (1/2) (y/0.0075) ^{3 }+ (3/2) (y/0.0075)

If thermal conductivity of air is stated to be 0.03 W/m K, determine the value of convective heat transfer coefficient

a) 4 W/m^{2 }K

b) 5 W/m^{2 }K

c) 6 W/m^{2 }K

d) 7 W/m^{2 }K

### View Answer

_{s}– t

_{ infinity}) [d t/d y]

_{ y = 0}.

10. Air at 20 degree Celsius flows over a flat surface maintained at 80 degree Celsius. The local heat flow at a point was measured as 1250 W/m^{2} .Take thermal conductivity of air as 0.028 W/m K, calculate the temperature at a distance 0.5 mm from the surface

a) 57.682 degree celsius

b) 67.682 degree celsius

c) 77.682 degree celsius

d) 87.682 degree celsius

### View Answer

_{y = 0}(0.0005) = 57.682 degree celsius.

## Set 5

1. Which quantity signifies the ratio of temperature gradient at the surface to a reference temperature gradient?

a) Reynolds number

b) Nusselt number

c) Fourier number

d) Stanton number

### View Answer

Explanation: It is given by h l/k.

2. The determination of value of Nusselt number or the convective film coefficient forms a basis for the computation of heat transfer by convection. Towards that end, following approaches have been suggested

(i) Non-dimensional analysis and experimental correlations

(ii) Hydrodynamic concept of velocity boundary layer

(iii) Reynolds similarity between the mechanism of fluid friction in the boundary layer and the transfer of heat by convection

Identify the correct one

a) 1, 2 and 3

b) 1 and 2

c) 2 and 3

d) 1 and 3

### View Answer

3. Nusselt number is given by

a) h l/k

b) 2 h l/k

c) 3 h l/k

d) 4 h l/k

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4. The temperature profile at a particular location in a thermal boundary layer is prescribed by n expression of the form

t (y) = A- B y + C y ^{2}

Where, A, B and Care constants. What is the value of heat transfer coefficient?

a) B k/ (t _{s} – t_{ infinity})

b) 2 B k/ (t _{s} – t_{ infinity})

c) 3 B k/ (t _{s} – t_{ infinity})

d) 4 B k/ (t _{s} – t_{ infinity})

### View Answer

_{s}– t

_{ infinity}) [d t/d y]

_{ y = 0}and (d t/d y)

_{y = 0 }= – B.

5. The temperature profile at a particular location on a surface is prescribed by the identity

(t _{s} – t) / (t _{s} – t_{ infinity}) = sin (π y/0.015)

If thermal conductivity of air is stated to be 0.03 W/m K, determine the value of convective heat transfer coefficient

a) 6.48 W/m^{ 2} K

b) 6.38 W/m^{ 2} K

c) 6.28 W/m^{ 2} K

d) 6.18 W/m^{ 2} K

### View Answer

_{s}– t

_{ infinity}) [d t/d y]

_{ y = 0}. Therefore h = – k/ (t

_{s}– t

_{ infinity}) [π (t

_{s}– t

_{ infinity})/0.015].

6. Air at 20 degree Celsius flows over a flat plate maintained at 75 degree Celsius. Measurements shows that temperature at a distance of 0.5 mm from the surface of plate is 50 degree Celsius. Presuming thermal conductivity of air is 0.0266 W/m K, estimate the value of local heat transfer coefficient

a) 23.18 W/m^{ 2} K

b) 24.18 W/m^{ 2} K

c) 25.18 W/m^{ 2} K

d) 26.18 W/m^{ 2} K

### View Answer

Explanation: h = – k/ (t _{s} – t_{ infinity}) [d t/d y]_{ y = 0} and d t/d y = – 50 * 10 ^{3} degree Celsius/m.

7. Air at 20 degree Celsius flows over a flat surface maintained at 80 degree Celsius. Estimate the value of local heat transfer coefficient if the local heat flow at a point was measured as 1250 W/m^{2}. Take thermal conductivity of air as 0.028 W/m K

a) 23.83 W/m^{ 2} K

b) 22.83 W/m^{ 2} K

c) 21.83 W/m^{ 2} K

d) 20.83 W/m^{ 2} K

### View Answer

_{s}– t

_{ infinity}).

8. Consider the above problem, calculate the temperature gradient at the surface

a) – 44636 degree Celsius/m

b) – 34636 degree Celsius/m

c) – 24636 degree Celsius/m

d) – 14636 degree Celsius/m

### View Answer

_{y = 0}= – h (t

_{s}– t

_{ infinity})/k.

9. At the interface of solid body, heat flows by conduction and is given by

a) A (t _{s }– t _{infinity})

b) h A (t _{s }– t _{infinity})

c) h (t _{s }– t _{infinity})

d) h A

### View Answer

_{ y = 0}.

10. For a given value of Nusselt number, the convective surface coefficient h is directly proportional to

a) Length

b) Mass

c) Thermal conductivity

d) Density