Thermal Science & Engineering - MCQ from AMIE Exams (Summer 2017)

Choose the correct answers (1 x 20)

1. In the absence of an unbalanced force within the system itself and also between the system and the surroundings, the system is said to be in

(a) Thermal equilibrium

(b) Mechanical equilibrium

(d) Phase equilibrium

2. First law is based on

(a) Law of conservation of mass

(b) Law of conservation of energy

(d) None of the above

3. When a liquid flows through a tube with subcooled or saturated boiling, the process is called

(a) Pool boiling

(b) Bulk boiling

(d) None of the above

4. A system comprising a single phase is called a

(a) Closed system

(b) Open system

(d) Homogeneous system

(e) Heterogeneous system

5. Throttling process is

(a) A reversible and constant entropy process

(b) A reversible and constant enthalpy process

(d) Reversible and isothermal process

6. In a psychrometric chart, the vertical scale shows

(a) Wet-bulb temperature

(b) Dry-bulb temperature

(d) Specific humidity

7. A 10-tonne refrigeration system consumes 10 k W of electrical energy. Its COP will be

(a) 10

(b) 3.5

(d) 35

8. Which one of the following statements is correct?

(a) The heat and work are boundary phenomena

(b) The heat and work represent the energy crossing the boundary of the system

(d) All of the above

9. For the same maximum pressure and temperature

(a) Otto cycle is more efficient than diesel cycle

(b) Diesel cycle is more efficient than than Otto cycle

(d) Dual cycle is less efficient than Otto and diesel cycles

10. A hollow sphere with uniform temperature and a small hole behaves very nearly as

(a) White body

(b) Opaque body

(d) Gray body

11. The emissivity of a black body is

(a) 0

(b) 0.5

(d) 1

12. Which one of the following is an intensive property?

(a) Internal energy

(b) Enthalpy

(d) Volume

13. The triple point of water is

(a) 273 K

(b) 273.16 K

(d) None of the above

14. For evaporators and condensers, under the given conditions, LMTD for counterflow will be

(a) Greater than parallel flow

(b) Equal to parallel flow

(d) None of the above

15. In free convection heat transfer, the transition from laminar to turbulent flow is governed by the critical value of the

(a) Reynold’s number

(b) Grashoff's number

(d) Prandtl number, Grashoff's number

16. For a closed system, the difference between the heat added to and the work done by the system is equal to the change in

(a) Enthalpy

(b) Internal energy

(d) None of the above

17. In a reversible adiabatic compression

(a) Pressure remains constant

(b) Temperature remains constant

(d) Cooling takes place

18. The average value of thermal conductivity for water at 20 °C saturate is about 0.51

(a) True

(b) False

19. The critical radius of insulation of a spherical cell is

(a) h/k

(b) 2h/k

(d) 2k/h

20. Heat transfer by convection is described by

(a) Fick’s law

(b) Fourier's law

(d) Stefan-Doltzman law

Answers

1. (b)

Thermal equilibrium. The temperature of the system does not change with time and has the same value at all points of the system.
Mechanical equilibrium. There are no unbalanced forces within the system or between the surroundings. The pressure in the system is the same at all points and does not change with respect to time.
Chemical equilibrium. No chemical reaction takes place in the system and the chemical composition which is the same throughout the system does not vary with time.

2. (b) In the early part of the nineteenth century the scientists developed the concept of energy and

the hypothesis that it can be neither created nor destroyed; this came to be known as the law of the

conservation of energy. The first law of thermodynamics is merely one statement of this general

law/principle with particular reference to heat energy and mechanical energy i.e., work.

3. (a) Pool boiling is a heat transfer mechanism carrying a phase transition from liquid to vapour. Pool boiling is defined as boiling from a heated surface submerged in a large volume of stagnant liquid. This liquid may be at its boiling point, in which case the term saturated pool boiling is employed, or below its boiling point when the term subcooled pool boiling is used.

4. (d)

A homogeneous thermodynamic system is defined as one whose chemical composition and physical properties are the same in all parts of the system, or change continuously from one point to another. a homogeneous phase is a single phase of matter where everything is in solution. Filtered Apple juice that is yellow but transparent is a homogeneous phase.
A heterogeneous system is denned as one consisting of two or more homogeneous bodies. The homogeneous bodies of a heterogeneous system are referred to as phases. Each phase is separated from other phases by interfaces or boundaries, and in passing over such a boundary the chemical composition of the substance or its physical properties abruptly change. An example of a heterogeneous system is water with ice floating in it. Orange juice with pulp floating in it is heterogeneous. The pulp is a different phase (solid) and is suspended in the liquid phase. neither phase, solid or liquid, are homogeneous.

5. (c) A throttling process is defined as a process in which there is no change in enthalpy from state one to state two, h₁ = h₂; no work is done, W = 0; and the process is adiabatic Q = 0.

6. (b) Boundaries of the psychrometric chart are a dry-bulb temperature scale on the horizontal axis, a humidity ratio (moisture content) scale on the vertical axis, and an upper curved boundary which represents saturated air or 100-per cent moisture-holding capacity. The chart shows other important moist air properties as diagrammed in Figure 2: wet-bulb temperature; enthalpy; dewpoint or saturation temperature; relative humidity; and specific volume.

7. (b) $COP = \frac{{{Q_1}}}{{{W_R}}} = \frac{{35kW}}{{10kW}} = 3.5$

The Coefficient Of Performance (COP) is a performance rating that tells us how effective a heat pump or air conditioner is at transferring heat versus the amount of electrical power it consumes.
Higher COPs equate to higher efficiency, lower energy (power) consumption and thus lower operating costs. The COP usually exceeds 1, especially in heat pumps, because, instead of just converting work to heat (which, if 100% efficient, would be a COP of 1), it pumps additional heat from a heat source to where the heat is required. Most air conditioners have COP of 2.3 to 3.5.

8. (c) Let us consider two processes as described by paths A and B on the P-V diagram as shown in the figure. We know that the area projected on the volume axis on a PV diagram will give work transfer. On the above diagram, it is clear that the area inscribed by path A is different from the area inscribed by path B i.e. work transfer depends upon the path. so it is a path function. and hence not a property of the system.

Similarly, let us consider two paths A and C on T-s diagram. we know that area projected on the entropy axis on the T-s diagram gives energy transfer. On the above diagram, it is clear that the area inscribed by path A is different from the area inscribed by path C i.e. energy transfer depends upon the path. so it is a path function. and hence not a property of the system.

9. (b) Diesel engines are more efficient than Otto engines because the latter must operate at lower compression ratios to avoid knocking.

10. (c) The hole has the attributes of a nearly perfect thermal black body.

11. (d) Real materials emit energy at a fraction - called the emissivity - of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity of ε = 1.

12. (c)

In thermodynamics, a distinction is made between extensive and intensive properties.
The properties whose value does not depend on the quantity of matter present in the system are known as intensive properties. Temperature and pressure belong to intensive properties.
It is a bulk property that does not depend on the size of the matter or that of the system. It is a physical property of that system.
As both mass and volume are extensive properties they cancel out each other and density becomes intensive property.

13. (b) In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. the triple point of water (273.16 K, 0.01 °C, or 32.018 °F).

14. (b)

15. (d) In free convection, "h" is the function of Pr number and Gr number. In forced convection "h" is the function of Pr number and Re number.

16. (b) The first law of thermodynamics states that the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system. In equation form, the first law of thermodynamics is ΔU = ΔQ - ΔW

17. (c) According to the first law of thermodynamics, Internal Energy dU = q + w

For an adiabatic process, q = 0, therefore the change in internal energy will be equal to the work done by the system. This indicates that the work must be done at the expense of internal energy since no heat is being supplied from the surroundings. Thus, we can conclude that in an adiabatic process, the quantity which remains constant is the total heat of the system.

18. (a)

19. (d) The thickness up to which heat flow increases and after which heat flow decreases is termed critical thickness. In the case of cylinders and spheres, it is called the critical radius.

20. (c) Newton's law of cooling applies to convective heat transfer; it does not apply to thermal radiation. Newton's law of cooling states that the rate at which an object cools is proportional to the difference in temperature between the object and the object’s surroundings. Simply put, a glass of hot water will cool down faster in a cold room than in a hot room.

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Thermal Science & Engineering - MCQ from AMIE Exams (Summer 2017)

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