NON FLOW AND FLOW
PROCESSES CLASS NOTES
Both non flow & flow processes play
an important role in thermodynamics.
These are part of various thermodynamic
cycles. Cycles are power producing &
1. Non Flow Processes
The boundary of a non-flow process can be fixed, moving or imaginary.
These are compression and expansion processes on gases in a cylinder with complete leak proof. In these there is only energy transfer with zero mass transfer.
These non flow processes can be the followings:
(i) constant pressure process
While heating at constant pressure
dU = δq—p dv
In an isobaric compression, heat is stored in the form of enthalpy.
( note the use of δ and d in the equation).
δ is used with heat supplied or rejected
d for change in a quantity such as volume or internal energy
(ii) constant volume process
δq = dU since δW =0
During a iso-choric (Constant volume) process, work done is zero and the total energy changes into internal energy.
(iii) constant temperature process
δq=δW since dU =0.
(iv) reversible adiabatic process
During a reversible adiabatic process, there is no friction.
No heat is supplied or rejected. Therefore δW = dU since δQ=0
A process with no heat gain or heat loss is an adiabatic process.
(v) poly-tropic process
During a poly-tropic process, δQ =δW –work lost in friction
p V n = C is a mathematical form of a poly-tropic process in which no parameter is constant.
(vi) constant internal energy process
When gravity, magnetic, electrical, motion and capillary effects are negligible. Then the total energy (E) is equal to the internal energy (U).
Flow process is one in which there is energy and mass transfer across the boundary of the system.
All flow systems are OPEN systems since energy and work cross their boundaries.
Following are the flow processes.
(i) from inlet of compressor to its outlet in a refrigeration system
work Done in a poly-tropic compression (Open system)
W=∫–v dp = n(p2v2–p1v1)/(n-1)
work Done in a poly-tropic compression (closed system)
W=∫p dv =(p2v2–p1v1)/(n-1)
Work done in an open system is n times the work done in a closed system
(ii) Through a nozzle
(iii) flow across a turbine
(iv) Flow in a pipe
Flow processes are of two types.
Steady Flow System
When properties are constant with respect to time it is called a steady flow systems. All the experimental data is recorded under steady flow conditions. Steady flow energy equation is a heat balance for the system.
Non Steady Flow System
When properties vary with respect to time it is called a non-steady flow systems. Nothing useful can be found under unsteady flow conditions.
(i) A pure substance which is homogeneous in composition and in chemical aggregates. It has same properties in all directions or at every point.
(ii) A pure substance state is defined by two independent properties provided the system is in equilibrium. Provided gravity, magnetic, electrical , motion and capillary effects are negligible.