RICHARD G. CUNNINGHAM
4.3
INTRODUCTION______________________________________________________
The jet pump transfers energy from a liquid or gas primary fluid to a secondary fluid.
The latter may be a liquid, a gas, a two-phase gas-in-liquid mixture, or solid particles transported in a gas or a liquid. Examples of all these combinations have been reported in the technical literature. Reference 1, the major bibliography in this field, contains over 400 abstracts. Although the terms “ejector” and “eductor” are also applied, the term "jet pump” will be used here. The jet pump offers significant advantages over mechanical pumps: no moving parts for improved reliability, adaptability to installation in remote or hazardous environments, simplicity, and low cost. The primary drawback is efficiency: both frictional losses and unavoidable mixing losses are incurred. Neverthe-less, careful design can produce pumps with efficiencies on the order of 30—40%. The jet pump in Figure 1 is typical of liquid-jet pumps and low Mach-number gas-jet/gas pumps.
Compressible-flow pumps, for example, steam-jet ejectors, employ converging-diverging nozzles for full expansion of the jet.
LIQUID-JET PUMP THEORY FOR THREE SECONDARY-FLOW TYPES _________
The liquid-jet pump model is based on conservation equations for energy, momentum, and mass. Real-fluid losses are accounted for by friction-loss coefficients (K). The primary or motive fluid is a liquid of density r1. In the following derivation, the secondary/pumped fluid can be a second liquid of density r1 or r2, or a gas-in-liquid bubbly mixture, or a gas. These
three jet pump flow regimes are referred to as liquid-jet liquid (LJL), liquid-jet gas liquid
(LJGL), and liquid-jet gas (LJG). Equations (1), (3), (5), and (7) below apply to all three.
Assumptions:
a. The primary and secondary streams enter the mixing throat with uniform velocity
distributions, and the mixed flows leave the throat and diffuser with a uniform velocity profile.
b. The gas phase—if present—undergoes isothermal compression in the throat and diffuser.
c. All two-phase flows at the throat entry and exit consist of homogeneous bubble mixtures of a gas in a continuous liquid.
d. Heat transfer from the gas to the liquid is negligible—the liquid temperature remains constant.
e. Change in solubility of the gas in the liquid from pressure Ps to Pd is negligible.
f. Vapor evolution from and condensation to the liquid are negligibly small.
