Nozzle And Diffuser

Contents 1) Nozzles 2) The flow of steam through nozzles3) Effect of friction4) Types of nozzles5) Nozzle efficiency6) Critical pressure for max discharge through

nozzle 7) General relationship of area and velocity8) Velocity in case of enthalpy9) Super saturated flow 10)Diffusers 11)Classification of diffusers

A nozzle converts high pressure, low speed flow to low

pressure, high speed flow

Nozzles

The flow of steam through nozzles

The flow of steam through nozzles may be regarded as adiabatic expansion. The expansion of steam through the nozzle is not a free expansion and the steam is not throttled because it has very high speed at the end of expansion. The enthalpy of steam decreases as expansion takes place. The pressure energy of steam is changed into kinetic energy.There is also a phenomenon known as super saturation in flow of steam through nozzles. This is due to the time lag in the condensation of steam during expansion. This supersaturated flow effects the mass and condition of steam discharged. Therefore, the flow of steam through a nozzle may be regarded as either (i)adiabatic and reversible flow (ii)adiabatic flow modified by friction (iii)a supersaturated .

Effect of frictionDue to friction caused to the steam by the nozzle, the steam is reheated and the expansion is not isentropic. Thus , the effect of friction is:(i) To reduce the enthalpy drop.(ii) To reduce the final velocity of steam.(iii) To increase the final dryness fraction and

entropy.(iv) To increase the specific volume of fluid due

to increase in dryness fraction.(v) To decrease the mass flow rate due to

increased specific volume and reduced velocity.

Types of nozzles There are three types of nozzles:(i)Converging nozzle(ii)Diverging nozzle(iii) Converging-diverging nozzle

Nozzle efficiencyNozzle efficiency is defined as the ratio of actual enthalpy drop to that due to isentropic expansion. In other words , it is ratio of actual gain in kinetic energy to that due to isentropic expansion. i.e., Efficiency= (h₁-h₂) actual (h₁-h₂s) isentropicThe efficiency of nozzle depends on following parameters:

(i) The material of which nozzle is made.(ii) Smoothness of the nozzle.(iii) Size and shape of nozzle.(iv) The angle of nozzle divergence.(v) Fluid velocity

Critical pressure for max discharge through nozzle.

The pressure at which the is min and the discharge per unit area a max is termed as the critical pressure. As we know, the smallest cross-sectional area of the nozzle is know as the throat. So, if P2 in N\m2 be the pressure at throat where the area A2 the by equation , the mass flow rate per unit area is given by

m\A2=

(n/n-1)(P1/V1)[(P2/P1)^2/n-(P2/P1)^(n+1)/n

General relationship of Area and Velocity According to continuity equation , the mass flow

rate at any section remains constant. So, for steady flow of fluid

m=A1 C1/ V1 =A2 C2 /V2 mV = AC where A=area of cross section C= uniform velocity across section m=mass rate flow in kg of steam per

second V=specific volume flowing through the

section

Velocity in case of enthalpyWhen no heat is supplied q=0 to the

apparatus ie no mechanical work is done by apparatus w=0 and gain in kinetic energy between any sections of nozzle is equal to loss of enthalpy.

C2= 2*1000(h1 – h2) m/s

=44.72 h1 – h2

Super saturated flowThe increase in measured discharge to the

theoretically calculated discharge is due to the time lag in the condensation of steam and thus the steam remains dry instead of wet this phenomenon is called super saturation

Causes behind super saturation1) Absence of tiny dust particles2) High velocity of steam3) Evaporation of small water droplets into

space that already saturated

Diffusers

A diffuser converts

high speed, low

pressure flow to low speed, high

pressure flow

Classification of Diffusers Diffusers may be classified as annular , radial

,subsonic or super sonic etc. Diffusers may be classified as divergent and

convergent also