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Importance of thermodynamic concepts
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- Mangosuthu University Of Technology (MUT)
lecture notes about the important concepts of thermodynamics
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Important Thermodynamic ConceptsThermodynamics is the study of the relationship of heat and mechanical energy and the conversion
of one into the other.
System: Is a specifically identified mass of material separated from its surrounding by a real or
imaginary boundary
Boundary
System
Surroundings
Working Fluid: The mass within the boundary of a system can be liquid, vapour or gas.
Open System/Steady Flow System: Mass and energy can flow across the boundary.
Closed System/ Non Flow System: No mass can cross the boundary, but energy can cross the
boundary. The boundary limits are flexible.
Properties: Is a quantifiable characteristic of a system
Thermodynamic State: Is the condition in which we find the system thermodynamic properties at any
given time.
Process: When a system changes from one state to another.
Cycle: Consists of a sequence of processes in which the working fluid returns to its original
thermodynamic state.
Thermal Equilibrium: There is no difference in temperature between the system and the surrounding.
Mechanical Equilibrium: There is no pressure difference between the system and surrounding.
Phase Equilibrium: Has more than one phase present, with the amount of substance in each phase
not changing with time.
Heat: Is the transfer of energy due to a temperature gradient between the system and the
surroundings.
Work: Is the transfer of energy via mechanical, electrical or magnetic means.
Internal Energy: Is due to the motion, position and chemical bonding configuration of the individual
molecules of the working fluid.
Reversibility: A process is reversible when the system and its surroundings can be returned to its the
original state, without any net effect on the surroundings.
Irreversibility: A process is irreversible, if the system has friction and has a finite driving force.
,Heat Engine: Is a system which operates in a cyclic manner to produce work from a supply of heat.
Enthalpy: Is mathematically defined as a function of the internal energy and the flow work (PV), with
the units Joules.
Entropy: Quantifies reversibility and irreversibility of a process and the maximum work that can be
obtained from the system.
Reversibility
For a system to reversible the following criteria has to be adhered to:
The process must be frictionless; there must be no internal and mechanical friction.
The pressure difference between the fluid and the surrounding during the process must be very
small.
The difference in temperature between the fluid and its surroundings during the process must be
very small.
Thus, no process in practice is truly reversible. However, in practical process internal reversibility may
be observed. Internal reversibility is when the working fluid is constantly in an equilibrium state and
the process path is traceable to the initial state. A reversible process is drawn as a solid line on a
property diagram, while an irreversible process is drawn as dotted line on a property diagram.
Work
The work done during a reversible process is given by the area under the curve on a p-v diagram. A
reversible cycle plotted on a p-v diagram forms a closed figure, the area of which represents the net-
work of the cycle. Consider a piston, with the fluid contained in the cylinder. The force exerted on the
fluid by the piston is PA, where A is the cross sectional area of the piston. If the piston moves a distance
dl due to the force exerted, then the work done for a reversible process is;
𝑑𝑊 = −(𝑃𝐴)𝑑𝑙 = −𝑃𝑑𝑉
For mass, m:
𝑊 = −𝑚 𝑃𝑑𝑉
A process from right to left on the p-v diagram is one in which there is work input into the fluid, thus
work is positive. A process from left to right is one in which there is work output from the fluid, thus
the work is negative.
First Law of Thermodynamics
The 1st law of thermodynamics state, “When a system undergoes a thermodynamic cycle then the net
heat supplied to the system from its surroundings plus the net-work input to the system from its
surrounding must equal to zero.”
That is: Σ𝑄 + Σ𝑊 = 0
,Non Flow Equation
A non flow process is when there is no flow of fluid into or out of the system. In any non-flow process,
there will be either heat supplied or heat rejected but not both. Similarly there will either be work
input or work output, but not both. The only energy possessed by the fluid is internal energy, hence
the energy equation is,
𝑈 −𝑈 =𝑄+𝑊
Steady Flow Energy Equation (S.F.E.E)
Ina steady flow system there is mass flow into and out of the system, thus the S.F.E.E applies to an
open system. The rate of mass flow of the fluid at any section is the same at any other section, thus
the mass entering is equal to the mass exiting the system. Consider the open system shown below,
where the boundary of the system is the control surface and the system enclosed is the control
volume.
Datum
Source: https://www.academia.edu/37525271/Chapter_6_Energy_Conservation_in_Steady_Flow
For the system above:
The rate of heat supply 𝑄̇ and the rate of work input 𝑊̇ are constant as the working fluid moves
through the system.
The flow energy required to enter the system is given by, (𝑃 𝐴 ) ∗ 𝑙 which is equivalent to 𝑝 𝑣 ,
where v is the specific volume of the fluid. Similarly the flow energy required to exit the system is
𝑝 𝑣 .v
The energy stored in the mass of fluid entering the control volume is a sum of the internal energy,
kinetic energy and potential energy, i.e. 𝑚 𝑢 + + 𝑧 𝑔 . Similarly, the energy contained by the
mass of fluid exiting the system is 𝑚 𝑢 + +𝑧 𝑔
Thus, the energy entering the system consists of the energy stored in the fluid, the inlet flow energy,
the heat supplied and the work input. Since there is a steady flow of fluid into and out of the system
, and a steady rate of heat supply and work input, the energy entering the system is equal to the energy
leaving the system. Thus, the S.F.E.E is,
𝐶 𝐶
𝑚̇ 𝑢 + +𝑍 𝑔+𝑃 𝑣 + 𝑄̇ + 𝑊̇ = 𝑚̇ 𝑢 + +𝑍 𝑔+𝑃 𝑣
2 2
But h = u + Pv
𝐶 𝐶
𝑚̇ ℎ + + 𝑍 𝑔 + 𝑄̇ + 𝑊̇ = 𝑚̇ ℎ + +𝑍 𝑔
2 2
Mass flow rate;
𝐶𝐴
𝑚̇ = = 𝜌𝐶𝐴
𝑣
Continuity of mass equation;
𝐶𝐴 𝐶 𝐴
𝑚̇ = =
𝑣 𝑣
Sign Convention
The following sign convention is used in this course.
Heat supplied to the system, Q , is positive
Work input to the system, W, is positive
Work done by the system, W, is negative
Heat rejected by the system, Q, is negative
Units
Pressure = kPa
Temperature = K
Volume = m3
Specific volume = m3/kg
Tutorial 1
Air flows steadily at the rate of 0.4 kg/s through an air compressor, entering at 6 m/s with a pressure
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 20.16 m3/kg. The specific internal energy of the air leaving is 88 kJ/kg greater than that of
the 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 inlet and outlet pipe
cross-sectional areas.
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