Thermodynamic process

                 A thermodynamic process may be defined as the progress of a thermodynamic system proceeding from an initial state to a final state.   The series of states the substance or system experiences as it progresses through the process is called the path of the process.
               Typically, a thermodynamic process can be characterised, according to what system property e.g. temperature, pressure, or volume, etc., are held fixed.   Furthermore, it is useful to group these properities into pairs, in which the variable held constant is one member of the pair.   The six most common thermodynamic processes are shown below:

An isobaric process occurs at constant pressure...ΔP= 0 An isochoric process, or isometric process, occurs at constant volume...ΔV= 0 An isothermal process occurs at a constant temperature...ΔT= 0 An isentropic process occurs at a constant entropy...ΔS= 0 ( Adiabetic and reversible) An isenthalpic process occurs at a constant enthalpy...Δh= 0 An adiabatic process occurs without loss or gain of heat...ΔQ= 0 A cyclic process occurs with same initial and final states A polytropic process has the relationship pV k = constant

There are a number of thermodynamic process types encountered by engineers including non-flow, steady flow, semi-flow and unsteady flow.   These are described as follows:

• Non flow processes are those involving no flow of matter across the system boundaries
• Steady flow processes involve fluid entering and leaving the system control volume these flows do not change with time and the internal energy of the control is also fixed in the time period under consideration
• A Semi-flow process involves fluid flow into a control volume which may be rigid charging a gas bottle or flexible -blowing up a balloon
• An unsteady flow process is one with a variable internal energy- i.e changing liquid level in a boiler.

 Reversible /Irreversible Process

           If the substance or system passes through a continuous series of equilibrium states in progressing through the process as it receives or rejects energy it is referred to as a reversible process.  The path of this theoretical process is generally shown on diagrams as a full line.  If the process is reversed, in the thermodynamic sense, it would leave no trace of itself.
           In the real world there are no reversible processes..All processes are irreversible and are shown on diagrams as broken lines...Factors which make a process irreversible include friction, unrestrained gas expansion, heat transfer across finite temperature difference, mixing,chemical reactions etc. etc...

P-V diagrams