SRI SHANMUGHA COLLEGE OF ENGINEERING AND TECHNOLOGY

Pullipalayam, Morur (Po), Sankari (Tk), Salem (Dt).

DEPARTMENT OF MECHANICAL ENGINEERING

Engineering Thermodynamics-QUESTION BANK

UNIT-I BASIC CONCEPT AND FIRST LAW

1. What is the zeroth law of thermodynamics?

When a body A is in thermal equilibrium with a body B, and also separately with a body C, then B and C will be in thermal equilibrium with each other.

2. How can a closed system and its surroundings interact? What is the effect of such interactions on the system?

A closed system and its surroundings can interact in two ways: (a) by work transfer, and (b) heat transfer. These may be called energy interactions and these bring about changes in the properties of the system.

3. When is work said to be done by a system?

Work said to be done by a system if the sole effect on things external to the system can be reduced to the raising of a weight.

4. What are positive and negative work interactions?

When work said to be done by a system, it is arbitrarily taken to be positive, and when work is done on a system, it is taken to be negative.

5. What is displacement work?

Work is said to be done when a force moves through a distance In a piston cylinder arrangement (Fig 1) if the part of boundary of system undergoes a displacement under the action of pressure, then work done W is force (i.e. product of pressure and area) x distance moved by the piston in the direction

of force.               

The piston moves out wards because of the expansion of system i.e. gas. It can be represented on p-V diagram as shown in Fig 2 W.D; W = force x distance moved = (p x A) x distance moved = pA. dx = p (Adx) Adx small volume dV swept by piston

                       

Total work done by gas during expansion process is  6. What do you understand by point and path functions? What are exact and inexact differentials?

If for a given state, there is a definite value for each property then the property is known as a point function. The differentials of point function are exact differentials. For e.g. pressure, temperature, volume, entropy and enthalpy etc. These types of thermodynamic properties only depend on end states.

Properties which are not thermodynamic are called path functions. For e.g. work and heat. These are dependent on the path taken from one state to the other but are independent of other states. Path functions are inexact differentials.

7. Define state, property and process. Explain the concept of cyclic process. OR Define thermodynamic state, path, process and cycles State of a system indicates the specific condition of a system.

Property/Properties are those observable characteristics of system which can be used for defining it.

Process: The mode in which the change of state of a system takes place is termed as process.

Cyclic process: It refers to a typical sequence of processes in such a fashion that the initial and final states are identical.

8. Define heat and work. Heat may be termed as the energy interaction across the system boundary which occurs due to temperature difference only. Work is done when the point of application of a force moves in the direction of force. OR Work is defined as the energy transferred (without the transfer of mass) across theboundary of a system because of an intensive property difference other than temperature that exists between the system and surroundings.

9. Define the terms: system, boundary, surroundings and universe.

System: A thermodynamic system represents a specified quantity of matter under consideration to analyze a problem, so as to study the change in properties of the specified quantity of matter due to exchange of energy-in the form of heat and work. Boundary: The thermodynamic system and surroundings are separated by an envelope is called boundary of system. Surroundings: is defined as everything outside the system. Universe: System and surroundings when put together result in universe. Universe = System + Surroundings.

10.  Define thermodynamic equilibrium Discuss its various aspects also

If in a system, the temperature and pressure at all parts is same, also there is no velocity gradient and chemical equilibrium is also attained then the system is in thermodynamic equilibrium .For acute of thermodynamic equilibrium system should always be in mechanical, thermal and chemical equilibrium.

11. Define a quasi-static process. Quasi static means ‘almost static”. It refers to a process in which a thermodynamic equilibrium state passes through a process in such a way that every state in the process is in thermodynamic equilibrium.

12. Differentiate between extensive and intensive properties

                 13. How does control volume differ from a system? Let us consider an open system shown in Fig. The fixed region in space through which the mass flow takes place in known as control volume and, its surface is called control surface.

14. In what respect heat and work interactions dissimilar?

1. Heat is low grade energy, where as work is high grade energy. 2. In a stable system, there is no work transfer, but for heat interaction, system needs not to be stable. 3. Heat is the energy interaction due to temperature difference only but work is because of reasons other than temperature difference.

15. In what respect heat and work interactions similar? 1. Heat and work are path functions i.e. they are not properties of system, hence their differentials are not exact. 2. Heat and work are both transient phenomena and system does not process heat or work. 3. Heat and work represent energy crossing the system boundary and hence they are a boundary phenomenon.

16.  Is it possible that W 0 even if dV = 0? If so, give an example. Yes, it is possible in the case of paddle work.

It is clear from the Fig. that a paddle wheel is fitted with pulley shaft which in turn can be made to rotate by lowering the weight. Due to this lowering of weight paddle wheel gets related and work is being done on the system (fluid) which gets stirred up. Now the volume of system remains same (constant i.e. dV = 0) At the same time, weight W be displaced by a small distance dx then small work done is dW = W.dx = mg dx, which is not equal to zero.

17. Discuss whether (PdV + VdP) represents the system properties or not? The condition for the property of a system is that the quantity which represents it should be an exact differential. The quantity (PdV + VdP) d (PV) Now its integration

i.e. is a exact differential.

18. How enthalpy is related to the internal energy? Sum of internal energy and pressure volume product i.e. (U + pV) is known as enthalpy. Mathematically, H = U + pV------ 1 Where H = Enthalpy U = Internal energy p = Pressure V=Volume

The right side of equation (1) has each of quantities as properties, so enthalpy is also a property. For unit mass, h = u + pv h = specific enthalpy u = specific internal energy v= specific volume.

19. Define Internal energy. Is it a function of state or process? Energy possessed by a substance because of molecular arrangement and motion molecules is known as Internal energy. It is solely dependent on the initial and final states the system that means it is a point function and hence a property.

20. How is the property of system classified? Explain with examples. Classification of property of a system is a follows: 1. Extensive properties: The properties which are dependent on the mass a system are known as extensive or extrinsic properties. Volume, mass, internal energy, enthalpy and entropy are the examples of extensive properties. - 2. Intensive properties: The properties which are independent of the mass of system are known as intrinsic properties. Intensive properties are independent of the size of system.

21. Differentiate between open system, close a system and isolated system

Open system: A system in which the transfer of mass well as energy takes place, is known as an open system. Air compressor and a balloon containing air are known as an open system. Closed system: A system in which the mass remains constant (i.e. no transfer of mass) but transfer of energy takes place is known as a closed system. A steel flask with hot water in it is an example of a closed system. Isolated system: A system in which neither the mass nor energy transfer takes place is known as an isolated system. A steel flask with outer casing insulated from the surroundings is an example of an isolated system.

22. Discuss whether the following properties represent the system properties or not? (a) PdV (b) VdP

(a) PdV: In this quantity, p is a function of V and moreover, these are connected by a line on a pV diagram. Thus, the integration $ PdV can only be found out if relationship between P and V is clearly defined. Thus PdV is not an exact differential and hence, not a property of system. (b) VdP: This quantity is not an exact differential because of the same reason explained above, so it is also not a property of system.23. Define heat. Is it the property of system?

Heat is a type of energy which is transferred beyond the boundary of a system , because of the difference in temperature between system and surroundings. No heat is not a property of system because it is not dependent on the end states, but depends upon the path followed to achieve a particular state.

24. Differentiate between high grade and low grade energy. Giving examples.

Energy which can be completely transformed in the shaft work without any loss is known as high grade energy. Mechanical and electrical energy are the examples of high grade energy. Energy which cannot be completely converted into shaft work/mechanical work is-known as low grade energy. Heat energy and nuclear energy are the examples of low grade energy.

25. What is an irreversible process? Give an example of irreversible process. A process, which involves heat transfer due to finite difference of temperature between the system and surroundings, is known as an irreversible process. Fluid friction between the molecules because of turbulence due to high speeds is an example of an irreversible process

26. Differentiate between universal gas constant and characteristic gas constant Characteristic gas constant From the characteristic gas equation, pV= mRT R = pV/mT P = pressure V = volume m = mass T = temperature

R = Characteristic gas constant. Units of R are J/kg K and R for air is 287 J/kg K Value of gas constant R is different for different gases Universal gas constant: The product of characteristic gas constant and molecularweight of an ideal gas is known as universal gas constant.

Rmol=M.R Also Rmol (for air) = 8314- J/kg.mol °K. Where M = Molecular weight of an ideal gas R = Characteristic gas constant

27. How do you state flow and non flow work mathematically? Non flow work wnf = ∫ pdV Flow work wf= − ∫ Vdp

28. State the first law of thermodynamics. First law of thermodynamics states that, “If a system passes through a cycle of processes so that it returns to its initial state the sum of heat and work effects will be zero”

29. What is mechanical equivalent of heat?

Joule conducted a number of experiments involving various types of work interactions and found out that

Where J is known as Joule’s equivalent or mechanical equivalent of heat.

30. Represent the following processes on P-V and T-S diagrams:

(I) Constant pressure process (ii) Adiabatic process.

     

31. Represent the following processes on P-V and T-S diagram.

(i) Constant volume process (ii) Constant temperature process.                          

      32.For generation of steam in a boiler requires 2500 kJ/kg of heat and in the condenser 1800 kJ/kg of heat is rejected to cooling water, using first law of thermodynamics, determine steam flow rate. Ans. h1 = 2500 kJ/kg, h2 = 1800 kJ/kg Steam flow rate = h1 — h2 = 2500 — 1800 = 700 kJ/kg.

 33. What is an isobaric process?

In an isobaric process, both volume and temperature change, where as pressure is kept constant. The volume and temperature increases when heat is supplied to system and decreases when heat is rejected by the system.

 

34. Represent isothermal and adiabatic processes on p-v chart.

     

35. Why a hyperbolic process is similar to constant temperature process in the case of gases.

The plot of a rectangular hyperbola is represented by pV = C. Since the law of an isothermal process is also the same i.e. pV = C, therefore it follows that in the case of gases, the isothermal and hyperbolic processes are same. Hence the equations derived for work, heat and change in internal energy for on isothermal and hyperbolic process are also same.

36. What are flow and non flow processes?

Flow processes: The processes occurring in open system which permit the transfer of mass as well as energy across its boundaries are known as flow processes. Non-flow processes: The processes occurring in closed system which does not permitthe transfers of mass across their boundaries are known as non-flow processes.

 37. Classify flow processes.

The flow processes may be classified as 1. Steady flow processes 2. Unsteady flow processes Steady flow process: A process is said to be steady flow process, if the conditions in the control volume remains unchanged with time. Unsteady flow process: A process is said to be an unsteady flow process If the conditions in control volume change with time.

38. What are the assumptions for a steady flow?

The assumptions for a steady flow process are: (i) Rate of mass in flow is same as rate of mass outflow i.e. the mass of fluid remains constant within the system. (ii) No change in chemical composition of system and hence no change in chemical energy is involved. (iii) Both at entry and exit, the fluid is uniform in composition, state and velocity (iv) Rate at which heat work transverse the boundaries are constant. State of fluid at any point is same at all times

39. What is throttling?

It is an irreversible steady flow expansion process in which fluid is allowed to move through a restricted passage (such as porous plug, sudden reduction in x-section of flow or a partially opened valve). 40. State some applications of throttling process.

Applications of throttling process are: 1. Liquification of gases. 2. To determine dryness fraction of steam. 3 Expansion in a capillary tube in refrigerators.

41. What are the limitations of work?

The limitations of are as follows: 1. It is only applicable to steady flow open systems. 2 The system undergoes reversible quasistatic process 3. The changes in K.E and P.E. are negligible.

42. Differentiate between reversible adiabatic process and the polytropic process.

A reversible process in which there is no heat transfer between the system and surroundings i e Q = 0 is known as reversible adiabatic process and follows the law = adiabatic index If these, is a heat transfer between the system and surroundings, the process is called a polytropic process and it follows the law pVn =C, n= polytropic exponent.

43. State Joule’s law.

Joule’s law states that the internal energy of a perfect gas is the function of absolute temperature only. Therefore, U = f (T).

 44. Write steady flow energy equation for a steam turbine.

It is assumed in the turbine: It is insulated so that no heat enters or leaves the system i.e. flow of fluid through the turbine is adiabatic.Therefore, q = 0 ……….. (1) Changes in potential energy and kinetic energy are negligible i.e.

…….(2)Applying steady flow energy equation per unit mass

Using equation (1) and (2) h1 = h2 + w w = (h1 — h2)This shows that work is done by the system due to decrease in enthalpy of system.

45. What are adiabatic and diathermic walls? A wall which is impermeable to the flow of heat is an adiabatic wall, whereas a wall which permits the flow of heat is a diathermic wall.

46. State gas laws.

1. Boyle’s law

Boyle's law says that, at constant temperature, the product of an ideal gas's pressure and volume is always constant.

2. Charles’s law

Charles's law says that, for an ideal gas at constant pressure, the volume is directly proportional to the absolute temperature.

3. Avogadro’s law

Avogadro's law states that, the volume occupied by an ideal gas is proportional to the number of moles (or molecules) present in the container.

47. Define Ideal gas and Real gas.

An Ideal gas (Perfect gas) is a theoretical gas composed of a set of randomly-moving, non-interacting point particles. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is amenable to analysis under statistical mechanics.

Real gases – as opposed to a perfect or ideal gas – exhibit properties that cannot be explained entirely using the ideal gas law. To understand the behaviour of real gases, the following must be taken into account:

Compressibility effects. Variable specific heat capacity. Van der Waals forces. Non-equilibrium thermodynamic effects. Issues with molecular dissociation and elementary reactions with variable composition.