ET QB UNIT 1.pdf

P.T.Lee CHENGALVARAYA NAICKER COLLEGE
OF ENGINEERING & TECHNOLOGY, OOVERY.
Vallal P.T.Lee Chengalvaraya Nagar, Oovery, Veliyur Post, Kanchipuram, 631 502.
DEPARTMENT OF MECHANICAL ENGINEERING
ME3391 ENGINEERING THERMODYNAMICS
UNIT I BASICS, ZEROTH AND FIRST LAW
Review of Basics – Thermodynamic systems, Properties and
processes Thermodynamic Equilibrium - Displacement work
- P-V diagram. Thermal equilibrium - Zeroth law – Concept
of temperature and Temperature Scales. First law –
application to closed and open systems – steady and
unsteady flow processes.
PART-A
1. Define thermodynamic system.
A thermodynamic system is defined as a quantity of matter or a region in space, on
which the analysis of theproblem is concentrated.
2. Name the different types of system.
1. Closed system (only energy transfer and no mass transfer)
2. Open system (Both energy and mass transfer)
3. Isolated system (No mass and energy transfer)
3. Should the automobile radiator be analyzed as a closed system or as an
open system? Explain.
Automobile radiator system is analyzed as closed system. In this no mass (water) cross the
boundary.
4. Define thermodynamic equilibrium.
If a system is in Mechanical, Thermal and Chemical Equilibrium then the system is in
Thermodynamically equilibrium. (or) If the system is isolated from its surrounding there will
be no change in the macroscopic property, then the system is said to exist in a state of
thermodynamic equilibrium.
5. What do you mean by quasi-static process?
Infinite slowness is the characteristic feature of a quasi-static process. A quasi-static
process is that a success on of equilibrium states. A quasi-static process is also called as
reversible process.

6. Differentiate between point function and path function.
7. Name and explain the two types of properties.
The two types of properties are intensive property and extensive property.
Intensive Property: It is independent of the mass of the system.
Example: pressure, temperature, specific volume, specific energy density.
Extensive Property: It is dependent on the mass of the system.
Example: Volume, energy.
If the mass is increased the values of the extensive properties also Increase.
8. What is a steady flow process?
Steady flow means that the rates of flow of mass and energy across the control surface
are constant.
9. Prove that for an isolated system, there is no change in internal energy.
In isolated system there is no interaction between the system and the surroundings. There
is no mass transfer andenergy transfer. According to first law of thermodynamics as
dQ = dU + dW; dU = dQ –dW; dQ = 0, dW = 0, Therefore dU = 0 by integrating the above
equation U = constant, therefore the internal energy is constant for isolated system.
10. Indicate the practical application of steady flow energy equation.
1. Turbine, 2. Nozzle, 3. Condenser, 4. Compressor.
11. Define system.
It is defined as the quantity of the matter or a region in space upon which we focus
attention to study its property.
12. Define cycle.
It is defined as a series of state changes such that the final state is identical with the
initial state.

13. Explain Mechanical equilibrium.
If the forces are balanced between the system and surroundings are called Mechanical
equilibrium
14. Explain Chemical equilibrium.
If there is no chemical reaction or transfer of matter form one part of the system to
another is called Chemicalequilibrium
15. Explain Thermal equilibrium.
If the temperature difference between the system and surroundings is zero then it is in
ThermalEquilibrium.
16. Define Zeroth law of Thermodynamics.
When two systems are separately in thermal equilibrium with a third system then they
themselves is in thermalequilibrium with each other.
17. What are the limitations of first law of thermodynamics?
1. According to first law of thermodynamics heat and work are mutually convertible during
any cycle of a closedsystem. But this law does not specify the possible conditions under
which the heat is converted into work.
2. According to the first law of thermodynamics it is impossible to transfer heat from lower
temperature to highertemperature.
3. It does not give any information regarding change of state or whether the process is
possible or not.
The law does not specify the direction of heat and work.
18. What is perpetual motion machine of first kind?
It is defined as a machine, which produces work energy without consuming an equivalent
of energy from other source. It is impossible to obtain in actual practice, because no
machine can produce energy of its own without consuming any other form of energy.
19. Define: Specific heat capacity at constant pressure.
It is defined as the amount of heat energy required to raise or lower the temperature of
unit mass of the substancethrough one degree when the pressure kept constant.
It is denoted by Cp.
20. Define: Specific heat capacity at constant volume.
It is defined as the amount of heat energy required to raise or lower the temperature of
unit mass of the substancethrough one degree when volume kept constant.

26. Distinguish between ‘Macroscopic energy’ and ‘Microscopic energy’.
Statistical Thermodynamics is microscopic approach in which, the matter is assumed to be
made of numerous individual molecules. Hence, it can be regarded as a branch of
statistical mechanics dealing with the average behavior of a large number of molecules.
Classical thermodynamics is macroscopic approach. Here, the matter is considered to be
a continuum without any concernto its atomic structure.
27. Show that the energy of an isolated system remains constant.
A system which does not exchange energy with its surroundings through work and heat
interactions is called an isolatedsystem. That is for an isolated system dW = 0 and dQ=0.
The first law of thermodynamics gives dE = dQ – dW
Hence, for an isolated system, the first law of thermodynamics reduces to dE = 0 or E2 =
E1. In other words, the energy ofan isolated thermodynamic system remains constant.
21. Define the term enthalpy?
The Combination of internal energy and flow energy is known as enthalpy of the system.
It may also be definedas the total heat of the substance.
Mathematically, enthalpy (H) = U + pv KJ)
Where, U – internal energyp – Pressure v – Volume
In terms of Cp & T → H = m Cp (T2-T1) KJ
22. 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 theenergy possessed by a gas at a given temperature.
23. What is meant by thermodynamic 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
surroundings.
24. What is meant by reversible and irreversible process?
A process is said to be reversible, it should trace the same path in the reverse direction
when the process is reversed. It is possible only when the system passes through a
continuous series of equilibrium state
25. Why does free expansion have zero work transfer?
In free expansion there is no external force acting on the gas so that the energy given
to the gas can be utilized to produce heat and to overcome the repulsions between the
gases which does not happen in free expansion therefore there is no work transfer

28. What are the conditions for steady flow process?
 No properties within the control volume change with time. That ismcv = constant
Ecv = constant
 No properties change at the boundaries with time. Thus, the fluid properties at an inlet or
exit will remain the sameduring the whole process. They can be different at different
opens.
The heat and work interactions between a steady-flow system and its surroundings do not
change with time.
29.Define Zeroth law and first law thermodynamics.
Zeroth law of thermodynamics states when two systems are separately in
thermalequilibrium with a third system then they themselves are in thermal equilibrium with
each other.
First law of thermodynamics states that when system undergoes a cyclic process
net heat transfer is equal to work transfer. ɸQ=ɸw
PART-B
1. A gas of mass 1.5 kg undergoes a quasi-static expansion, which follows a relationship
p=a+bV, where ‘a’ and ‘b’ are constants. The initial and final pressures are 1000 kPa
and 200 kPa respectively and the corresponding volumes are 0.2 m3 and 1.2 m3. The
specific internal energy of the gas is given by the relation U = (1.5pV – 85) kJ/kg, where
p is in kPa and V is in m3. Calculate the net heat transfer and the maximum internal
energy of the gas attained during expansion.
Refer: “P.K NAG Engineering Thermodynamics for similar problems”
2. A piston – cylinder device contains 0.15 kg of air initially at 2 MPa and 3500C. The air is
first expanded isothermally to 500 kPa, then compressed polytropically with a
polytrophic exponent of 1.2 to the initial pressure, and finally compressed at the
constant pressure to the initial state. Determine the boundary work for each process and
the network of the cycle.

3. A gas undergoes a thermodynamic cycle consisting of the following processes:
(i) Process 1-2: Constant Pressure P1=1.4 bar, V1=0.028 m3, W1-2=10.5 kJ.
(ii) Process 2-3: Compression with pV=constant, U3=U2.
(iii) Process 3-1: Constant volume,
U1-U3= - 26.4 kJ.There are no
significant changes in KE and
PE
1. Sketch the cycle on a p-V diagram.
2. Calculate the network for the cycle in kJ.
3. Calculate the heat transfer for process 1-2.
4. Show that Qcycle=Wcycle.
4. A three cycle operating with nitrogen as the working substance has constant
temperature compression at 340C with initial pressure 100kPa. Then the gas undergoes
a constant volume heating and then polytropic expansion with 1.35 as index of
compression. The isothermal compression requires - 67 kJ/kg of work. Determine: (i) p,v
and T around the cycle (ii) Heat in and out (iii) Network. For nitrogen gas Cv = 0.731
kJ/kgK.
5. (i) Air enters the compressor of a gas-turbine plant at ambient conditions of 100 kPa and
250C with a low velocity and exists at 1 MPa and 3470C with a velocity of 90 m/s. The
compressor is cooled at the rate of 1500 kJ/min, and the power input to the compressor
is 250 kW. Determine the mass flow rate of air through the compressor. Assume
Cp=1.005 kJ/kg K.
(ii) Derive steady flow energy equation.
6. In a gas turbine installation air is heated inside heat exchanger up to 750OC from
ambient temperature of 27OC. Hot air then enters into gas turbine with the velocity of 50
m/sec and leaves at 600OC. Air leaving the turbine enters a nozzle at 60 m/sec velocity
and leaves nozzle at temperature of 500OC. For unit mass flow rate of air, determine the
following assumptions adiabatic expansion in turbine and nozzle, (i) heat transfer to air
in heat exchanger (ii) power output from turbine (iii) velocity at exit of nozzle. Take Cp
for air as 1.005 kJ/kgK.

7. Air flows steadily at the rate of 0.4 kg/s though an air compressor, entering at 6 m/s with
a pressure of 1 bar and specific volume of 0.85 m3/kg and leaving at 4.5 m/s with a
pressure of 6.9 bar and a specific volume of
0.16 m3/kg. The internal energy of air leaving is 88 kJ/kg greater than that of air
entering. Cooling water in a jacket surrounding the cylinder absorbs heat from the air at
the rate of 59 kW. Calculate the power required to drive the compressor and the ratio of
inlet and outlet cross sectional area.
8. Derive the steady flow energy equation and stating the assumptions made.
Refer: “P.K NAG & R.K.RAJPUT. Engineering Thermodynamics for derivation”
9. A fluid system, contained in a piston and cylinder machine, passes through a complete
cycle of four processes. The sum of all heat transferred during a cycle is – 340 kJ. The
system completes 200 cycles per min. Complete the following table showing the method
for each item, and compute the net rate of work output in kW.
Process Q (kJ/min) W (kJ/min) ΔE (kJ/min)
1—2 0 4340 —
2—3 42000 0 —
3—4 – 4200 — – 73200
4—1 — — —
Refer: R.K.RAJPUT.
10. 0.2 m3 of air at 4 bar and 130°C is contained in a system. A reversible adiabatic
expansion takes place till the pressure falls to 1.02 bar. The gas is then heated at
constant pressure till enthalpy increases by 72.5 kJ. Calculate :
(i) The work done ;
(ii) The index of expansion, if the above processes are replaced by a single reversible
polytropic process giving the same work between the same initial and final states.
Take Cp = 1 kJ/kg K, Cv = 0.714 kJ/kg K.
Refer: R.K.RAJPUT.
11. 0.1 m3 of an ideal gas at 300 K and 1 bar is compressed adiabatically to 8 bar. It is
then cooled at constant volume and further expanded isothermally so as to reach the

Condition from where it started. Calculate :
(i) Pressure at the end of constant volume cooling.
(ii) Change in internal energy during constant volume process.
(iii) Net work done and heat transferred during the cycle. Assume
Cp = 14.3 kJ/kg K and Cv = 10.2 kJ/kg K.
Refer: R.K.RAJPUT.
12. Air at a temperature of 20°C passes through a heat exchanger at a velocity of 40 m/s
where its temperature is raised to 820°C. It then enters a turbine with same velocity of 40
m/s and expands till the temperature falls to 620°C. On leaving the turbine, the air is taken
at a velocity of 55 m/s to a nozzle where it expands until the temperature has fallen to
510°C. If the air flow rate is 2.5 kg/s, calculate :
(i) Rate of heat transfer to the air in the heat exchanger ;
(ii) The power output from the turbine assuming no heat loss ;
(iii) The velocity at exit from the nozzle, assuming no heat loss.
Take the enthalpy of air as h = cpt, where cp is the specific heat equal to 1.005 kJ/kg°C
and t the temperature.
Refer: R.K.RAJPUT.
13. At the inlet to a certain nozzle the enthalpy of fluid passing is 2800 kJ/kg, and the
velocity is 50 m/s. At the discharge end the enthalpy is 2600 kJ/kg. The nozzle is
horizontal and there is negligible heat loss from it.
(i) Find the velocity at exit of the nozzle.
(ii) If the inlet area is 900 cm2 and the specific volume at inlet is 0.187 m3/kg, find the
mass flow rate.
(iii) If the specific volume at the nozzle exit is 0.498 m3/kg, find the exit area of nozzle.
Refer: R.K.RAJPUT.
14. 12 kg of air per minute is delivered by a centrifugal air compressor. The inlet and outlet
conditions of air are C1 = 12 m/s, P1 = 1 bar, v1 = 0.5 m3/kg and C2 = 90 m/s, P2 = 8 bar,
v2 = 0.14 m3/kg. The increase in enthalpy of air passing through the compressor is 150
kJ/kg and heat loss to the surroundings is 700 kJ/min.
Find : (i) Motor power required to drive the compressor ;
(ii) Ratio of inlet to outlet pipe diameter.
Assume that inlet and discharge lines are at the same level.
Refer: R.K.RAJPUT.
15 . The working fluid, in a steady flow process flows at a rate of 220 kg/min. The fluid
rejects 100 kJ/s passing through the system. The conditions of the fluid at inlet and outlet

are given as : C1 = 320 m/s, P1 = 6.0 bar, u1 = 2000 kJ/kg, v1 = 0.36 m3/kg and C2 = 140
m/s, P2 = 1.2 bar, u2 = 1400 kJ/kg, v2 = 1.3 m3/kg. The suffix 1 indicates the condition at
inlet and 2 indicates at outlet of the system.
Determine the power capacity of the system in MW.
The change in potential energy may be neglected
Refer: R.K.RAJPUT.

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