Thermodynamics

5–4 ■ SOME STEADY-FLOW ENGINEERING DEVICESMany engineering devices operate essentially under the same conditionsfor long periods of time. The components of a steam power plant (turbines,compressors, heat exchangers, and pumps), for example, operate nonstop formonths before the system is shut down for maintenance (Fig. 5–24). Therefore,these devices can be conveniently analyzed as steady-flow devices.In this section, some common steady-flow devices are described, and thethermodynamic aspects of the flow through them are analyzed. The conservationof mass and the conservation of energy principles for these devicesare illustrated with examples.1 Nozzles and DiffusersNozzles and diffusers are commonly utilized in jet engines, rockets, spacecraft,and even garden hoses. A nozzle is a device that increases the velocityof a fluid at the expense of pressure. A diffuser is a device that increasesthe pressure of a fluid by slowing it down. That is, nozzles and diffusersperform opposite tasks. The cross-sectional area of a nozzle decreases in theflow direction for subsonic flows and increases for supersonic flows. Thereverse is true for diffusers.The rate of heat transfer between the fluid flowing through a nozzle or adiffuser and the surroundings is usually very small (Q . 0) since the fluid hashigh velocities, and thus it does not spend enough time in the device for anysignificant heat transfer to take place. Nozzles and diffusers typically involveno work (Ẇ 0) and any change in potential energy is negligible (pe 0).But nozzles and diffusers usually involve very high velocities, and as a fluidpasses through a nozzle or diffuser, it experiences large changes in its velocity(Fig. 5–25). Therefore, the kinetic energy changes must be accounted for inanalyzing the flow through these devices (ke 0).Chapter 5 | 233V 1m/sINTERACTIVETUTORIALSEE TUTORIAL CH. 5, SEC. 4 ON THE DVD.V 2m/s∆kekJ/kg0 45 150 67 1100 110 1200 205 1500 502 1FIGURE 5–23At very high velocities, even smallchanges in velocities can causesignificant changes in the kineticenergy of the fluid.Cold EndDrive Flange5-StageLow PressureCompressor(LPC)LPC BleedAir Collector14-StageHigh PressureCompressorCombustorFuel SystemManifolds2-StageHigh PressureTurbine5-StageLow PressureTurbineHot EndDrive FlangeFIGURE 5–24A modern land-based gas turbine used for electric power production. This is a General ElectricLM5000 turbine. It has a length of 6.2 m, it weighs 12.5 tons, and produces 55.2 MW at 3600 rpmwith steam injection.Courtesy of GE Power Systems