Thermo 5.4 - Nozzles and Diffusers - Two Solved Example Problems

In this segment, we discuss the nozzles (goal is to increase velocity) and diffusers (goal is to decrease velocity), which are steady flow engineering devices. We also solve 2 example problems. The first problem is for steam entering the nozzle as a superheated vapor and exiting as saturated water and saturated vapor mixture and requires visiting the steam tables (A-5 and A-6 in Appendix 1 Cengel). The second problem is for air going through an adiabatic nozzle and requires visiting the air tables (A-17E in Appendix 2 Cengel). This material is based upon work supported by the National Science Foundation under Grant No. 2019664. Any opinions, findings, and conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.

[@brittone2571]

In the first example: when we found the enthalpy exiting the nozzle, why do we compare it to the initial pressure 200 kpa instead of 500 kpa?

[@michaeltan6115]

if pressure is force/area, why is the nozzle at p2 less than p1? shouldn't it be greater because it has a smaller area?

[@CollegeFluidMechanics]

Good question, Michael. From the conservation of mass, V2>V1 (I think we agree there). It is harder to explain/grasp the pressure relationship. As velocity is larger in section 2, the enthalpy of the exit is smaller than the inlet. You need to replace specific enthalpy is equal to internal energy+pressure*specific volume (h=u+Pv). As T drops in a nozzle, internal energy drops more than P*v, requiring higher pressure. The explanation is much more simple if you took Fluid Mechanics class. Bernoulli's equation requires the pressure to be smaller for higher velocity.