GEK Wiki / Ejectors
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Page history last edited by Daniel Chisholm 13 years, 2 months ago

return to Practical Engineering


There are a large number of terms floating around for various types of fluid-fluid pumps ("jet pumps" and other designs). We're generally calling these "ejectors" (ejector does seem appropriate, at least according to wikipedia) but I'd like to try to clarify the terms. There may be a way to formally define the terms based on: the "jet" pressure, the "incoming" pressure, and the "mixed" (outgoing) pressure and the main underlying principle of operation: Bernuolli, Momentum Transfer (Entrainment), Coanda Effect...




"An aspirator, also called an eductor-jet pump or filter pump, is a device that produces vacuum by means of the Venturi effect. In an aspirator, fluid (liquid or gaseous) flows through a tube which then narrows. When the tube narrows, the fluid's speed increases, and because of the Venturi effect [Bernuolli? - bk], its pressure decreases. Vacuum is taken from this point."



"Eductors (also known as jet pumps ejectors, and Venturi pumps) are the most efficient way to pump or move many types of liquids and gases in the petrochemical, process, and power industries." -


Transport of grain/solids.



"An injector, ejector, steam ejector or steam injector is a pump-like device that uses the Venturi effect of a converging-diverging nozzle to convert the pressure energy of a motive fluid to velocity energy which creates a low pressure zone that draws in and entrains a suction fluid. After passing through the throat of the injector, the mixed fluid expands and the velocity is reduced which results in recompressing the mixed fluids by converting velocity energy back into pressure energy. The motive fluid may be a liquid, steam or any other gas. The entrained suction fluid may be a gas, a liquid, a slurry, or a dust-laden gas stream."


Injectors were used to refill boilers in steam engines with water while under pressure by using the steam itself to drive the water into the boiler [citation needed]


Here is one example of a steam jet ejector (called an exhauster, but an ejector all the same).



Jet Pump


Air Amplifier

De Laval Nozzle

"Shocks form because information about conditions downstream of a point of sonic or supersonic flow cannot propagate back upstream past the sonic point.

The behaviour of a fluid changes radically as it starts to move above the speed of sound (in that fluid), ie. when the Mach number is greater than 1. For example, in subsonic flow, a stream tube in an accelerating flow contracts. But in a supersonic flow, a stream tube in an accelerating flow expands. To interpret this in another way, consider steady flow in a tube that has a sudden expansion: the tube's cross section suddenly widens, so the cross-sectional area increases.

In subsonic flow, the fluid speed drops after the expansion (as expected). In supersonic flow, the fluid speed increases. This sounds like a contradiction, but it isn't: the mass flux is conserved but because supersonic flow allows the density to change, the volume flux is not constant. This effect is utilized in De Laval nozzles."




Venturi Effect

"The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is balanced by a drop in pressure or a pressure gradient force. An equation for the drop in pressure due to venturi effect may be derived from a combination of Bernoulli's principle and the equation of continuity."



"The Coandă effect is the tendency of a fluid jet to stay attached to an adjacent curved surface that is very well shaped."






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