Abstract
The validity and accuracy of performance measurements for pulse detonation engines depend on the ability to accurately measure thrust and fuel mass flow rates during system operation. Experimental tests have revealed that when fuel mass flow rates are calculated by conventional mass metering methods, incorrect values of the aggregate fuel mass in the combustor will often be calculated due to inaccurate assumptions regarding the spatial fuel distribution. The difficulty in predicting the actual fuel distribution affects the ability to achieve reliable detonations for successful operation and introduces inaccuracies directly into the performance calculations. Tunable diode laser and absorption spectroscopy techniques have been applied to provide time-resolved fuel mass fraction measurements and improve the fidelity of the specific impulse calculations. Results show that stratified fuel distributions that begin near stoichiometric at the forward end of the combustor and gradually become fuel lean near the combustor exit produce substantially higher specific impulse values than axially uniform fuel distributions with the same amount of aggregate fuel due to the ability to reliably detonate while operating at an overall lean condition. Axially uniform fuel distributions at the same average equivalence ratio demonstrated lower detonability and accordingly had lower thrust and specific impulse values. Nomenclature c = local sonic velocity F = thrust force Ispf = fuel-based specific impulse _m = mass flow rate p = pressure T = temperature u = axial velocity Vdet = detonation velocity x = axial position = detonation cell size = equivalence ratio Subscripts a = air f = fuel ifuel = initiator fuel pfuel = primary fuel O2 = initiator auxiliary oxygen I.