EFFECTS OF NOZZLE FRICTION

The effects of nozzle friction are

1. Reduction in enthalpy drop
2. Reheating of steam i.e improving the quality of vapour at the exit
3. Reduction in exit velocity
4. Increase in specific volume
5. Decrease in mass flow rate

NOZZLE EFFICIENCY

Consider a convergent-divergent nozzle, the (h-S) diagram with friction is as shown in fig above. It is assumed that the friction losses in convergent portion are negligible and the whole loss occurs in the divergent portion.

It represents the pressure at inlet to the nozzle, pt is the throat pressure which is equal to critical pressure and pe is the exit pressure. The nozzle efficiency can be given as Nozzle efficiency, $η=\dfrac{h_1-h_e}{h_1-h_e}$

NOZZLE OR VELOCITY COEFFICIENT, CC :

It is defined as the ratio of actual exit velocity to the ideal exit velocity. Accordingly,

$$C_c=\dfrac{\text{Actual velocity}}{\text{Ideal velocity}} \\ C_c=\sqrt{\dfrac{\text{Actual enthalpy drop}}{\text{Isentropic enthalpy drop}}}=\sqrt{\dfrac{h_1-h_e}{h_1-h_e}} \\ C_c=\sqrt{\text{Nozzle efficiency},η_n}$$