NON FLOW AND FLOW PROCESSES CLASS NOTES FOR ENGINEERING

 

NON FLOW AND FLOW

PROCESSES CLASS NOTES

FOR ENGINEERING

Both non flow & flow processes play

an important role in thermodynamics.

These are part of various thermodynamic

cycles. Cycles are power producing &

power consuming.

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 Processes

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.

Pure Substance

(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.

Specific Heats

(i) Specific heat at constant volume is equal to the rate of change of specific internal energy with respect to temperature. Mathematically

          C v = (∂u/∂T)v=C

(ii) Specific heat at constant pressure is equal to the rate of change of specific enthalpy with respect to temperature.

          C p = ∂h/dT