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Ejectors are flow devices that provide low back-pressure to a duct by passing a gas jet past the duct and taking advantage of the low pressure region this creates (See figure). A common usage of this device is for paint spray guns. I outlined the contraction relevant to my question in red.

diagram of an ejector

My understanding is that the static pressure of the jet is what provides the low back-pressure. If this is correct, I do not know why there would need to be a contraction/venturi assembly downstream of the suction manifold. A hypothesis is that because the jet is likely supersonic, this is needed to diffuse/slow down the discharge gases.

I would also appreciate any insights into what advantages/disadvantages an ejector has compared to the type of suction device used in carburetors. This comparison was actually the motivation for the question because an ejector looks very similar to a carburetor venturi.

Despite the above, I will specify my question: What function does the venturi like assembly in an ejector serve?

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    $\begingroup$ It's not a venturi in the normal sense. The straight-sided mixing chamber has increasing static pressure along it's length as the the two fluids mix. It is a design feature that can be set using a simple algorithm that considers the densities, pressures, and mass ratios of the two fluids. It simultaneously meters the eductant and mixes the fluids. It usually needs to be designed to work decently over a range of pressures, for instance in a jet pump. $\endgroup$
    – Phil Sweet
    Commented Dec 17, 2022 at 12:12

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