Experimental and Numerical Studies on the Ejector for Re-Circulating of Hydrogen and Oxygen to Fuel Cell Stack

Fuel cells use hydrogen as an important resource. Water (Steam) and electricity are generated by the supplied hydrogen and oxygen to fuel cell stack, whereas not all of the supplied hydrogen is reacted in the stack. The remained hydrogen in exhaust gas is detrimental to the efficiency of the fuel cell system. Fuel cell systems adopt a hydrogen re-circulating system to improve their efficiencies; the wasted hydrogen in an exhaust stream of a stack is fed back to the inlet. Blowers and ejectors are used in the re-circulating systems. The ejector types have the advantages of better energy efficiency and compactness although the blower types are superior in controllability. The ejector types are operated by supplied hydrogen gas. When the hydrogen is supplied though the ejector, it pass through a nozzle and the flow changed to supersonic flow. Supersonic flow evacuates the neighbor of the nozzle, and the induced vacuum pulls the re-circulated gas to the ejector to mix with pure hydrogen gas to fuel cell stack. In this study, the feasibility of the ejector type recirculation system has been investigated both numerically and experimentally.