In this article we will discuss about:- 1. Thermodynamic Properties 2. Thermodynamic Process 3. Thermodynamic Cycle 4. Thermodynamics Examples.
Every system has certain characteristics such as Pressure, Volume, Temperature, Density, Internal Energy. Enthalpy etc. by which its physical condition may be described. Such characteristics are called as Properties of the system.
Properties may be of two types:
1. Intensive Properties:
They are independent of total mass in the system e.g., Pressure, Temperature, Density.
2. Extensive Properties:
These are dependent on the total mass in the system. When the mass of the system changes, the values of the properties will change proportionately, e.g., Volume, Weight, Energy, Enthalpy, Entropy.
Properties may also be classified as:
1. Primary properties (P, V, T)
2. Secondary or derived properties (Internal energy u, Enthalpy h and Entropy s)
The properties of unit mass are known as specific properties for example specific volume, specific internal energy etc. Thus all the specific properties indicate the values for unit mass and they are independent of total mass, so all the specific properties are intensive properties.
When all the properties of a system have definite values (such as P, V, T) then the system is said to exist at a Definite state and the properties of the state are said to be State functions or point functions. Any operation in which one or more properties of the system changes, is called Change of state, and the series of states passed during a change of state from state (1) to state (2) is called the path of change of state. The properties of the states and substances are called as Path functions.
When the path is completely specified, then the change of state is called a Process.
A Process is defined as the transformation of the system from one fixed state to another fixed state.
When any one of the properties changes, the working substance or system is said to have undergone a process.
One way of classifying a thermodynamic process is:
(A) Reversible Process:
The system will undergo a reversible process in three different conditions:
(i) Thermally Reversible Process:
When the system has uniform temperature throughout the process and is in equilibrium with the surroundings, then the system is said to be in thermal equilibrium. In other words temperature gradient in the system does not exist.
Let us consider an example of working substance undergoing a process, during which it receives heat from the source.
When the working substance retraces its path in the reverse direction, then the process is called Reversible process. In the forward direction consider a point A where the temperature is T. At that moment the temperature of the source will be (T + ∆T), so that the heat transfer takes place from source to working substance.
In the reverse direction, the working substance will have to reject heat. At point A the working substance at a temperature of T has to transfer heat to source at (T + ∆T), that is not possible and the process cannot be retraced back and the process will not be reversible. To have the process reversible, the temperature gradient must be zero i.e., ∆T = 0. In a thermally reversible process, therefore, temperature gradient does not exists between the surroundings (source) and the system.
(ii) Mechanically Reversible Process:
A system is said to be mechanically reversible, when there are no mechanical losses either internally or externally.
Let us consider a simple piston-cylinder arrangement. Position 1 represents Inner Dead Centre (IDC) and position 2 represents Outer Dead Centre position (ODC). Let space between the piston and cylinder cover be filled with a gas at a pressure of P, volume V and temperature T. And the pressure on the other side of piston be atmospheric Patm. When the piston is released, then because of pressure potential, the piston starts moving towards RHS direction — till ODC and after that piston moves towards LHS direction while the crank is rotating.
If there are no mechanical losses the piston will reach to its original position. This type of process is called as Mechanically Reversible Process.
In actual practice this does not exist. Mechanical losses are mainly frictional losses. If the process is mechanically reversible, then the friction between the moving parts must be zero.
We can have other example as a tennis ball released from a certain height, then it will start bouncing ultimately it comes to rest. If there would not have been any mechanical losses, at the point of contact, then it would have been bouncing continuously. In practice this does not happen.
(iii) Chemically Reversible Process:
The process is called chemically reversible process, when there are no chemical reactions during the process.
A system will be in thermodynamic equilibrium, when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium simultaneously.
A system in thermodynamic equilibrium is incapable of any spontaneous change and it is in complete balance with its surroundings.
(B) Irreversible Process:
When the process is not thermally reversible, mechanically reversible and chemically reversible, then it is called an Irreversible process.
A process will be thermally irreversible, when the heat transfer with finite temperature difference takes place.
A process will be mechanically irreversible, when the friction exists during the process.
A process will chemically irreversible, when chemical reactions take place during the process.
The various thermodynamic processes which are used in engineering practice are:
1. Constant Pressure Process or Isobaric Process:
A process during which pressure remains constant is called a constant pressure process. The law for the process is (P = C) and is represented on the PV-diagram by means of a horizontal line 1-2.
2. Constant Volume Process or Isochoric Process:
A process during which volume remains constant is called a constant volume process. The law for the process is V = C and is represented by means of a vertical line 3-4 on the PV-diagram.
3. Constant Temperature or Isothermal Process:
A process during which the temperature remains constant is called a constant temperature on isothermal, process. The law for the process is PV = C and is represented by the curve 5-6 on the PV-diagram. Also the law of the process may be given as T = C.
4. Adiabatic or Isentropic Process:
A process during which heat exchange with the surroundings is zero, then it is called an adiabatic process. A reversible adiabatic process is called as an Isentropic process, during which entropy remains constant (S = C). The law for the adiabatic process is PVγ = C. Irreversible adiabatic process is called a Throttling Process, during which enthalpy remain constant. It is given by law h = constant.
5. General Law Process or Polytrophic Process:
The law for the process is PVn = C. In actual practice neither we get isothermal process, nor we get adiabatic process, but what we get is the process in between these two and is known as polytropic process.
It is defined as the series of state changes such that the final state is identical with the initial state.
When the system undergoes the series of processes such that the end states are same, the system is said to have undergone a thermodynamic cycle.
Series of thermodynamic processes, the end states of which are identical, is called a thermodynamic cycle. In a thermodynamic cycle, chemical composition of the working fluid during the process does not change. Thus all the properties in the initial and final states remain unchanged.
For example- water that circulates through the steam power plant. There is a change of phase during the process, but the end states are same.