Computational Study of Pulsejet-Driven Pressure Gain Combustors at High-Pressure

Abstract

This study explores computationally the performance characteristics of a pulsejet-driven pressure-gain combustor concept at high pressure conditions. The combustor consists of a pulsejet and an ejector, both housed within a shroud. The pulsejet-ejector-shroud (PES) combination forms an annular combustor across which a total pressure rise can be achieved. The computations carried out to date have demonstrated an overall pressure rise of 1.2% while achieving an overall temperature ratio commensurate with modern gas turbines. A combustor design capable of achieving even a small pressure gain rather than the usual 4%-8% loss found in conventional combustors is highly desirable since it would produce a significant improvement in engine specific fuel consumption. Computed NOx emissions for this type of combustor are also presented, and are shown to be comparable to those of conventional gas turbine engines at corresponding compressor pressure ratios. An analysis of the differences in the operation of a pulsejet at atmospheric and high-pressure conditions is also presented. The computations utilize a reduced combustion mechanism for Jet-A fuel and include a novel model for the pulsejet's reed valve. The results presented here are only preliminary, since neither the ejector nor the pulsejet itself have been optimized. With the numerical platform developed here, future optimization studies can be pursued. Nomenclaturef = pulsejet operating frequency Lpj = pulsejet length mp = primary (pulsejet) massflow rate mT = Total massflow rate (pulsejet + bypass) p = pressure p0 = inlet total pressure pback = back pressure pinj = fuel injection pressure T = temperature T0 = inlet total temperature t = t i m e tefi = end of fuel injection time tsfi = start of fuel injection time tsvc = start of valve closing time tsvo = start of valve opening time tvfc = valve fully closed time tvfo = valve fully open time x = axial coordinate Φ = equivalence ratio

Effect of Fuel Injection and Mixing Characteristics on Pulse-Combustor Performance at High-Pressure

“…Figure 1a shows experimental and computational head-end pressure ratio variation with time at atmospheric conditions, originally presented in Ref. 6. The computed amplitude of the pressure oscillations was in reasonable agreement with experiments, but the computed frequency of oscillation was approximately 10%.…”

“…23, in Ref. 6). Calculations were also performed with the baseline combustor and the reverse valve configuration at two equivalence ratios (Φ = 0.57 and Φ = 0.64).…”

“…In contrast, the combustion front for the atmospheric case was mostly constrained to the interior of the vortex (see ref. 6). This type of combustion mode appears to be more efficient in coupling with the acoustic field of the pulse-combustor.…”

“…Figure 1b shows typical computed pressures, obtained in Ref. 6, for a pulse-combustor operating at highpressure conditions. Results are presented for two equivalence ratios (Φ = 0.7, and Φ = 1.1).…”

“…However, practical aerospace applications of pressure-gain combustion systems necessitates operation at high-pressure conditions. Recently, we presented an initial study 6 that analyzed the differences in the operation of pulsecombustors at atmospheric (p0 = 1 bar; T0 = 298 K) and high-pressure conditions (p0 = 10 bar; T0 = 550 K). The results showed that the higher air temperature, and to a lesser extent, the higher pressure resulted in faster kinetics which significantly affected the combustion process, and increased the operating frequency of the pulse-combustor.…”