1. Define the term thermal engineering.
Thermal engineering is the science that deals with the energy transfer to practical applications such as energy transfer power generation, refrigeration, gas compression and its effect on the properties of working substance.
2. What is the difference between the classical and the statistical approaches to thermodynamics?
The properties of matter such as pressure, velocity, position and energy of the individual molecule at a given instant or at a particular time are studied. This approach is known as statistical thermodynamics.
Instead of studying parameters at molecular level, the behaviour of the total system in terms of properties such as pressure, volume, temperature etc. are studied. These properties at every instant can easily be measured called classical thermodynamics.
3. What is meant by control volume and control surface?
Control volume is a fixed region in space chosen for the thermodynamic study of mass and energy balances for flowing systems. Therefore, it is a properly selected region in the space. In control volume, both mass and energy can be exchanged with surrounding through its boundary.
iqro Control volume usually encloses a device which involves the mass flow such as a compressor, turbine or nozzle. Flow through these devices is best studied by selecting the region within the device as the control volume. Both mass and energy can cross the boundary of a control volume called control surface.
4. Using Kundsen number define continuum.
A continuous homogenous medium is called continuum. Continuum is based on the macroscopic approach. The continuum idealization allows us to treat the properties as point functions and assumes the properties varying continually in space with no jump discontinuities. This idealization is valid as long as the size of the system is large relative to the space between molecules.
A dimensionless parameter known as Knudsen number is given by
where
Kn = λ / L
λ = Mean free path and
L = Characteristic length.
It describes the degree of departure from continuum. If Kn is less than 0.01, the fluid is a continuum.
If Kn > 0.01, the concept of continuum does not hold good. Beyond this critical oh range of Knudsen number, the flows are known as
(iv) slip flow (0.01 < Kn < 0.1),
(v) transition flow (0.1 < Kn < 10) and
(vi) free-molecule flow (Kn > 10).
5. What is meant by thermodynamics system? How do you classify it?
Thermodynamic system is defined as any space or matter or group of matter where the energy transfer or energy conversions are studied. It may be classified into three types.
(a) Open system
(b) Closed system
(c) Isolated system.
6. Distinguish between 'macroscopic energy' and microscopic energy'.
Microscopic energy: The energy contained in myriads of molecules is called microscopic energy.
Macroscopic energy: The energy contained in the total system is called macroscopic energy.
7. What is meant by closed system? Give an example.
When a system has only heat transfer and work transfer but there is no mass transfer, it is called closed system.
Example: Piston and cylinder arrangement.
8. Define an open system. Give an example.
When a system has heat, work and mass transfers, it is called open system.
Example: Air compressor.
9. Distinguish between open and closed systems.
10. Define an isolated system:
Isolated system is not affected by surroundings. There is no heat, work and mass transfer takes place. In this system, total energy remains constant.
Example: Entire Universe
11. Should the automobile radiator be analysed as a closed system or as an open system? Explain.
The automobile radiator should be analysed as a closed system because the impermeable boundaries contains the liquid coolant. Also, it exchanges only heat but no matter exchange.
12. Define a thermodynamic system. Classify the following systems as open/closed/ isolated: (a) Mixture of ice and water in a metal container (b) A wind mill.
Thermodynamic system is defined as the any space or matter or group of matter where the energy transfer or energy conversions are studied.
(a) Mixture of ice and water in a metal container is a closed system
(b) A wind mill is an open system.
13. Define: specific heat capacity at constant pressure and specific heat capacity at constant volume.
Specific heat capacity at constant pressure is defined as the amount of heat energy required raising or lowering the temperature of unit mass of the substance through one degree when the pressure is kept constant. It is denoted by Cp.
Specific heat capacity at constant volume is defined as the amount of heat energy. required in raising or lowering the temperature of unit mass of the substance through one degree when the volume is kept constant.
14. What are surroundings and boundary?
Any other matter outside of the system boundary is called surroundings.
System and surroundings are separated by an imaging line is called boundary.
15. What is meant by thermodynamic property?
Thermodynamic property is any characteristic of a substance which is used to identify the state of the system and it can be measured when the system remains in an equilibrium state.
16. How do you classify the property?
1. Intensive or intrinsic property and
2. Extensive and extrinsic property.
17. Define intensive and extensive properties.
Or
What are intensive and extensive properties?
The properties which are independent on the mass of the system are called intensive properties.
Example: Pressure, temperature, specific volume etc.,
The properties which are dependent on the mass of the system are called extensive properties.
Example: Total energy, total volume, weight etc.,
18. Classify the following properties intensive or extensive or neither.
(a) Pressure (b) Temperature (c) Volume (d) Internal energy (e) Volume per mole (f) Mass (g) Enthalpy per unit mass.
(i) Pressure - Intensive property
(ii) Temperature - Intensive property
(iii) Volume - Extensive property
(iv) Internal energy - Extensive property
(v) Volume per mole - Intensive property
(vi) Mass - Intensive property
(vii) Enthalpy per unit mass - Intensive property
19. Differentiate Intensive and Extensive properties.
20. What do you understand by equilibrium of a system?
When a system remains in equilibrium state, it should not undergo any changes on its own accord.
21. When a system is said to be in "Thermodynamic Equilibrium"?
When a system is in thermodynamic equilibrium, it should satisfy the following three conditions.
(a) Mechanical equilibrium - Pressure remains constant
(b) Thermal equilibrium - Temperature remain constant
(c) Chemical equilibrium - There is no chemical reaction.
22. Define Zeroth law and first law of thermodynamics.
Zeroth law of thermodynamics:
Zeroth law of thermodynamics states that when two systems are separately in thermal equilibrium with a third systems, then they themselves are in thermal equilibrium with each other.
First law of thermodynamics:
First law of thermodynamics may be stated, "both heat and work are mutually convertible".
23. State the significance of Zeroth law of thermodynamics.
Zeroth law of thermodynamics signifies that temperature is a fundamental indicator of thermal equilibrium. If two bodies of the same temperature come in contact with each other then there is no net exchange of heat and those two bodies are said to be in thermal equilibrium.
24. Is it correct to say 'total heat' or 'heat content' of a closed system?
Yes. The total heat or heat content of a closed system is also called enthalpy.
25. Define heat.
Heat is defined as the energy crossing the boundary of a system due to the temperature difference between system and surrounding.
26. Define thermodynamic definition of work.
It is the work done by the system when the energy transferred across the boundary of the system. It is mainly due to intensive property difference between the system and surrounding.
27. Enlist the similarities between work and heat.
Both heat and work are directional quantities because both have magnitude and direction in relation to whether the energy is entering or leaving the system.
(i) Both are boundary phenomenon because they are only recognized when the energy crosses the systems boundaries.
(ii) According to the second law of thermodynamics statements, a system possesses energy.
(iii) Both heat and work are associated with a process as the system follows a path from one state to another state. Both have path functions and inexact differentials.
28. Distinguish between heat and temperature.
Temperature (T):
It is defined as a measure of velocity of fluid particles. It is a property which is used to determine the degree of hotness or coldness or the level of heat intensity of a body.
Heat:
Heat is defined as the energy crossing the boundary of a system due to the temperature difference between system and surrounding. It is usually represented by Q and expressed in Joule or kJ.
Heat exists only due to the heat transfer. For transfer of heat, there should be temperature difference between two systems.
29. Distinguish between stored energies and interaction energies.
Internal energy of a gas is the energy stored in a gas due to its molecular interactions.
Energy which is involving with system in thermodynamic process such as work and heat is called interaction energy.
30. What is the convention for positive and negative works?
(i) If work is done by the system, W is positive work
(ii) If work is done on the system, W is negative work
31. State the first law for a closed system undergoing a process and a cycle.
First law for a closed system undergoing a process:
First law of thermodynamics may be stated, "both heat and work are mutually convertible".
According to the law of conservation energy, "energy may be neither created nor destroyed but it can be transferred from one form to another form". In general, for any thermodynamic systems, the first law of thermodynamics can be written in the form of following equation.
Heat transfer = Work done + Change in internal energy
Q = W + ΔU
First law for a closed system undergoing a cycle:
First law of thermodynamics states that when system undergoes a cyclic process net heat transfer is equal to work transfer.
32. Show that energy of an isolated system is always constant.
For any isolated system, there is no heat, work and mass transfer.
Q = W = 0
According to the first law of thermodynamics,
Q = W + ΔU
⸫ ΔU = 0
U1 = U2
33. State first law of thermodynamics and any two of its corollaries.
For first law of thermodynamics, refer Q22 on Page 1.189.
Corollaries of first law of thermodynamics
Corollary I:
There exists a property of a closed system such that a change in its value is equal to the difference between heat supplied and work done during any change of state.
Corollary II:
The internal energy of a closed system remains unchanged if the system is isolated from its surroundings.
Corollary III:
A perpetual motion machine of first kind (PMM-1) is impossible.
34. List the limitations of First Law of Thermodynamics.
(1) First law of thermodynamics does not specify the direction of flow of heat and work. i.e., whether the heat flows from hot body to cold body or from cold body to hot body.
(2) Both heat and work are mutually convertible. The work can be converted fully into heat energy but heat cannot be converted fully into mechanical work. It violates the foresaid statements. A machine which violates the first law of thermodynamics is known as perpetual Motion machine (PMM-1) of the first kind which is impossible.
PMM-1 is a machine which delivers work continuously without any input. Thus, the machine violates first law of thermodynamics.
35. What is a PMM1? Why is it impossible?
PMM of the first kind delivers work continuously without any input. It violates first law of thermodynamics. It is impossible to construct an engine working with this principle.
36. Prove that for an isolated system, there is no change in internal energy.
For any isolated system, there is no heat, work and mass transfer.
Q = W = 0
According to the first law of thermodynamics,
Q = W + ΔU
⸫ ΔU = 0
37. Determine the molecular volume of any perfect gas at 600 N/m2 and 30°C. Universal gas constant may be taken as 8314 J/kg mole-K.
Given data:
p = 600 N/m2
T = 30°C = 30 + 273 = 303 K
R = 8314 J/kmol K
To find:
Molecular volume, V
Solution:
Ideal gas equation,
pV = mRT
600 × V = 1 × 8314 × 303
V = 4198.57 m3/kmol Ans.
38. Why does free expansion have zero work transfer?
What be In free expansion, there is no external force acting on the gas so that the energy given to the gas can be utilised to produce heat and overcome the repulsions between gases which does not happen in free expansion. So, there is no work transfer.
39. An insulated rigid vessel is divided into two parts by a membrane. One part of the vessel contains air at 10 MPa and other part is fully evacuated. The membrane ruptures and the air fills the entire vessel. Is there any heat and /or work transfer during this process? Justify your answer.
For rigid vessel and unrestrained expansion, Change in volume, dV = 0
Work transfer, W = ∫ pdV = 0
For insulated vessel, heat transfer, Q = 0
According to the first law of thermodynamics, the sum of work transfer is equal to the sum of heat transfer.
⸫ W = Q = 0
40. Define the term process.
Process is defined as the change of state undergone by a gas due to energy flow.
41. Distinguish between the terms 'state' and 'process' of thermodynamics.
42. Define cycle with one example each.
When a system undergoes a series of processes and return to its initial condition, it is known as cycle.
Example: Air standard cycle, vapour power cycles etc.
43. What is meant by open and closed cycle?
In a closed cycle, the same working substance will be recirculated again and again. In an open cycle, the same working substance will be exhausted to the surrounding after expansion.
44. What is meant by reversible and irreversible process?
A process is said to the reversible, it should trace the same path in the reverse direction when the process is reversed and it is possible only when the system passes through a continuous series of equilibrium state. If a system does not pass through continuous equilibrium state, then the process is said to be irreversible.
45. What are point and path functions?
The quantity which is independent on the process or path followed by the system is known as point function. Example: Pressure, volume, temperature etc.
The quantity which is dependent on the process or path followed by the system is known as path function. Example: Heat transfer, work transfer.
46. Differentiate between point function and path function.
47. What is Quasi- static process?
The process is said to be quasi-static if it proceeds infinitesimally slow and follows continuous series of equilibrium states. Therefore, the quasi-static process may be a reversible process.
48. Define the term enthalpy.
The combination of internal energy and flow energy is known as enthalpy of the system.
Mathematically, enthalpy (H) = U + pV
where
U = Internal energy in kJ
p = Pressure in kPa
V = Volume in m3
In terms of Cp and T,
Enthalpy, H = mCp (T2 - T1)
where
H = Enthalpy in kJ
Cp = Specific heat in kJ/kgK
T1 and T2 = Temperatures in K.
49. Define the term internal energy.
Internal energy of a gas is the energy stored in a gas due to its molecular interactions. It is also defined as the energy possessed by a gas at a given temperature.
50. What is meant by thermodynamic work?
It is the work done by the system when the energy is transferred across the boundary of the system. It is mainly due to intensive property difference between system and surrounding.
51. Prove that the difference in specific heat capacities equal to Cp - Cv = R.
Consider a gas heated at constant pressure,
So, heat supplied, Q = mCp(T2 - T1)
Work done, W = p (V2 - V1) = mR (T2 - T1)
Change in internal energy, ΔU = mCv (T2 - T1)
According to the first law of thermodynamics,
Q = W + ΔU
So, mCp(T2 - T1) = mR (T2 - T1) + mCv (T2 - T1)
Cp = R + Cv.
Cp - Cv = R
52. Sketch the isothermal expansion on p-V diagram and state the properties that remain constant.
The following properties remain constant
1. Temperature
2. Internal energy
3. Enthalpy.
53. What is a steady flow process?
In any system, if the rate of flow of working fluid is constant with respect to time, the system is known as steady flow system.
54. What are the conditions for steady flow process?
(i) The streams of material crossing the control surface must not change their state or flow rate with time.
(ii) Each point within the control volume must not change its state with time or only the cyclic state variation occurs.
(iii) The heat and work transfer rates must not change with time or the mean rates in the case of cyclic behaviour must not change.
55. Define flow energy.
The energy associated with fluid that enters or leaves a control volume is called flow energy.
56. Write down the equation of first law for a steady flow process.
where m = Mass represented in kg/s
A1 and A2 = Areas of cross section at entry and exit in m2
v1 = Specific volume of the working substance entering the system in m3/kg
C1 = Velocity of the working substance entering the system in m/s
h1 = Specific enthalpy energy of the working substance entering the system in J/kg
z1 = Height above the datum level for inlet in m
v2, C2, h2 and z2 are corresponding values for the working substance leaving the system.
Q = Heat supplied to the system in J/kg
W = Work delivered by the system in J/kg
57. Identify any four reasons for irreversibility in a process.
(a) Lack of equilibrium
(b) Heat transfer through a finite temperature difference
(c) Lack of pressure equilibrium within the interior of the system
(d) Free expansion
(e) Dissipative effects.
58. Give the energy equation applicable for an adiabatic nozzle and an adiabatic turbine.
Nozzle is a device which increases the velocity of kinetic energy of the working substance at the constant pressure drop.
In this system,
(1) There is no work done by the system (W = 0)
(2) There is no heat transfer taking place (Q = 0)
(3) There is no potential energy (z1 = z2)
Applying the steady flow energy equation to this system, it can be written as
Turbine is a device which converts the potential energy of working fluid into mechanical work. The turbine is fully insulated. Therefore, there is no heat transfer (Q = 0).
In the turbine, the expansion of working fluid is treated as reversible adiabatic or isentropic. The change in potential and kinetic energy is negligible. Therefore, z1 = z2 and C1 = C2.
Applying the SFEE to the above system, it can be written as
h1 = h2 + W
Work output, W = h1 - h2
59. What are the two main assumptions made in filling process?
1. Work done is zero. (⸫ W = 0)
2. Kinetic energy and potential energy are negligible.
60. Write down unsteady flow energy equation for filling the tank.
Q = m2 u2 – m1 u1 - (m2 - m1) hi
where m2 = Mass entering into the tank
u2 = Internal energy at entry
hi = Flow energy
m1 and u2 are the corresponding parameters at exit.
61. An insulated rigid tank with zero heat capacity contains mi kg of at pi bar and T1°C. it is filled with air from a large reservoir at pr and Tr°C. what will be the mass added?
Mass added, m = m2 – m1
where m1 = p1V1/RT1 and m2 = p2V2/RT2
62. A tank of volume v1 m3 contains an electric heating coil but it is thermally insulated. The tank is equipped with a release valve which allows the gas to escape when the pressure reaches p2 bar. The pressure inside the tank is p1 bar and T1°C and gas is then heated until the temperature in the tank is T3°C. Write down the principle of conservation energy after the release valve opens.
U1 + Q1-2 + Uincoming + (W)incoming = U2
where U1 = Internal energy inside the tank
Q1-2 = Heat transfer
U incoming = Internal energy of gas at entry
Wincoming = Work input
U2 = Internal energy after heating.
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