In some ejector designs, a converging-diverging nozzle is utilized to accomodate the expanding jet.They can operate with either incompressible or compressible fluids as the primary (driving) and secondary (driven) flows.The figure also defines the subscripts used later for primary (1), secondary (2), etc.The primary fluid is passed through a nozzle where the pressure energy is converted into kinetic energy.
The two streams mix in the mixing tube, leading to pressure recovery. Further static pressure is recovered in a narrow-angle diffuser downstream of the mixing tube. However, their simplicity and lack of moving parts make them worthy of consideration, particularly where a high-pressure stream of fluid is already available. Ejector Design Software Series Of EngineeringThe most comprehensive source of design information for ejectors can be found in a series of Engineering Sciences Data Unit (ESDU) data items, Nos. These are available on subscription as part of the ESDU Internal Flow series. Liquid-Liquid Jet Pumps Three key parameters for a jet pump are the pressure ratio, defined by. For high Reynolds number applications (above 2 10 5 ), values of 0.05,0.1,0.15 and 0.2 can be assumed for a well-designed jet pump. Alternatively, either graphical (e.g., ESDU 85032) or equation solving methods can be used. Ejector Design Software Trial And ErrorFor a particular application (i.e., given primary and secondary flowspressures), an optimum value of R can be found by trial and error. Once the ratios have been determined, the primary nozzle can be sized from. A parallel section at the nozzle outlet (see Figure 1 ) is not critical to performance, but can improve the mechanical strength of the design. Entry to the mixing tube needs to avoid large secondary flow losses: either a converging conical section or a bell-mouth entry should be used. A mixing tube length of 7-10 (mixing tube) diameters is recommended. To reduce downstream pressure losses, the flow needs to be expanded downstream of the mixing tube to lessen flow velocities to a reasonable level. As this may involve a large area ratio, a narrow angle diffuser is required (typically 2-3 half angle). When operating at low suction pressures and high flow ratios, cavitation can prove to be a problem with jet pumps. ESDU 85032. Gas-Gas Ejectors For small pressure differences, gas-gas ejectors can be treated like liquid jet pumps. However, for higher pressure ratios, compressibility effects need to be taken into account. Flow in the nozzle becomes independent of secondary pressure, and is given by.
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