Optimum fan pressure ratio for bypass engines with separate or mixed exhaust streams

The optimum fan pressure ratio is determined both numerically and analytically for separate-stream as well as mixed-stream bypass engines. The results are applicable to civil and military engines. The optimumfan pressure ratio is shown to be predominantly a function of the speciŽ c thrust and a weak function of the bypass ratio. Two simple, explicit equations, one for each type of engine, have been derived that specify the optimum fan pressure ratio. The predictions compare very well against numerical optimization performed by a specialist computer package employing iterative and advanced search techniques and real gas properties. The analytical relations accelerate the optimization process and offer physical insight. It has been shown that the optimum fan pressure ratio achieves the conditionVjc /Vjh = ´KE in a separate-streamengine and the conditionV163/V63 ¼ ´KE in a mixedstream engine. The condition V163 /V63 ¼ ´KE applies even under situations when signiŽ cant departures from the normally assumed condition p016/p06 ¼ 1 can occur.