Concept of process - Thermodynamics:
A process can be defined as change in the state or condition of a substance or working medium. For instance, heating or cooling of thermodynamic medium, compression or expansion of gas, flow of fluid from one location to another. In thermodynamics there are 2 types of process; Flow process and Non flow process.
Flow Process: The processes in open system permit transfer of mass to and from the system. Such process is called as flow process. The mass enters the system and leaves when energy is exchanged energy. For example I.C. Engine, Boilers.
Non-Flow Process: The process taking place in closed system where there is no transfer of mass across the boundary is called as non flow process. In this type of process the energy in the form of heat and work cross boundary of system.
In steady flow fluid flow at uniform rate and flow parameter do not change with time. For example if absorption of heat work output, gas flow etc. occur at uniform rate (Not changing with time), the flow will be called as steady flow. But if these vary throughout cycle with time, the flow will be called as non steady flow process for example, flow of gas or flow of heat in an engine but if long interval of time is chosen as criteria for these flows, engine will be known to be operating under non - flow condition.