EFFECT OF FRICTION ON EXPANSION OF STEAM

The exit velocity of steam for a given pressure drop is reduced due to the following reasons:

1. Friction between nozzle surface and steam
2. Internal fluid friction in the steam
3. Shock losses

Most of these losses occur beyond the throat in the divergent section of the nozzle as the length and the velocity of steam is much higher there. The effects of these friction losses are as follows:

1. The expansion does not remain isentropic and the enthalpy drop is reduced, resulting in lower exit velocity.
2. The final dryness fraction of steam is increased as a part of kinetic energy gets converted into heat due to friction and is absorbed by steam, which increases the enthalpy.
3. The specific volume of steam is increased due to frictional reheating.

The effect of friction of steam flow through a nozzle is shown in h–s (Mollier) diagram in Fig. 6.4. The point A represents the initial condition of steam and the point E represents the throat of a convergent–divergent nozzle. AB represents the isentropic expansion without friction, and AC represents the expansion with friction. Isentropic enthalpy drop is (h_A − h_C) and actual enthalpy drop with friction is (h_A − h_c). If the actual enthalpy drop as percentage of theoretical enthalpy drop is known, then point C can be located. However, expansion must end at same pressure as at B. The horizontal line drawn through C to cut the back pressure line p_b at point D represents the final condition of steam. The h–s diagram shows that the dryness fraction of steam at point D is greater than that at point B and the specific volume of steam also increases.

Most of the friction occurs in the divergent part of the nozzle and actual expansion is represented by the line AED. AE represents the expansion in the convergent part, whereas ED represents in the divergent part of the nozzle. Lines AEB and AED represent expansion of steam for initially saturated steam. Likewise, A₁E₁B₁ and A₁E₁D₁ are for initially superheated steam.

Figure 6.4 Effect of friction on expansion of steam in a convergent–divergent nozzle