Analysis and measurement of relative humidity effects on ejector performance

Pumping in ejectors is a measure of how much the secondary fluid is entrained by the high momentum primary flow and is one of the crucial performance indices. In the fuel recirculation system of fuel cells, the entrained secondary fluid contains water vapor due to the over-supplied hydrogen, making the secondary fluid humid. In such cases, the relative humidity of the secondary fluid alters the fluid properties of the mixed flow. This study examines such effects of relative humidity on ejector. A new analytical model has been developed, and series of measurements has been conducted to validate the model’s prediction. The present results demonstrate that higher relative humidity of the secondary fluid reduces pumping in an ejector. During the mixing process of the two fluids, condensation releases heat, increasing the ejector operating temperature. Thus, secondary flow density is decreased, and pumping is lowered.