Fanno Flow Subsonic branch

For converging nozzle feeding, increasing the tube length results in increasing the exit Mach number (normally denoted herein as $M_2$ ). Once the Mach number reaches maximum ($M=1$ ), no further increase of the exit Mach number can be achieved. In this process, the mass flow rate decreases.

Figure 9.7: The development properties in of converging nozzle

It is worth noting that entrance Mach number is reduced (as some might explain it to reduce the flow rate). The entrance temperature increases as can be seen from Figure (9.7). The velocity therefore must decrease because the loss of the enthalpy (stagnation temperature) is ``used.'' The density decrease because $\rho = {P \over R T}$ and when pressure is remains almost constant the density decreases. Thus, the mass flow rate must decrease. These results are applicable to the converging nozzle.

In the case of the converging-diverging feeding nozzle, increase of the dimensionless friction, $\fld$ , results in a similar flow pattern as in the converging nozzle. Once the flow becomes choked a different flow pattern emerges.