A tandem ejector pumping system has been applied to an intermittent blowdown tunnel for cascade testing to achieve sub-atmospheric exit pressures and extend the operating range substantially. The ejectors are run from the same supply as the tunnel itself, but because they are only used at low Reynolds numbers when the cascade mass flow is small the overall running costs are kept low. A conventional one-dimensional ejector theory is developed in a new way for such an application as this, where the driving mass flow needs to be known for constant Mach number of the driven stream (the cascade exit Mach number). Several ejector geometries were tested in various configurations in a one-tenth scale model before the prototype ejectors were developed. It is demonstrated that by suitable grouping of terms it is possible to correlate both model and prototype ejector performance, and that this performance can be predicted sufficiently accurately by the theoretical model to justify its use as a design tool.
The method of operating two ejectors in tandem depends on the interaction of the exit stream of the first (forming the driven stream of the second) and the driver stream of the second. This is not immediately obvious, and is discussed fully in the light of the achieved performance.