Longitudinally Stratified Combustion in Wave Rotors

Abstract

A wave rotor may be used as a pressure-gain combustor, effecting wave compression and expansion, and intermittent confined combustion, to enhance gas-turbine engine performance. It will be more compact than an equivalent pressure-exchange wave-rotor system, but will have similar thermodynamic and mechanical characteristics. Because the allowable turbine blade temperature limits overall fuel-air ratio to subftammable values, premixed stratification techniques are necessary to burn hydrocarbon fuels in small engines with compressor discharge temperatures well below autoignition conditions. One-dimensional, nonsteady numerical simulations of stratified-charge combustion are performed using an eddy-diffusivity turbulence model and a simple reaction model incorporating a flammability limit temperature. For good combustion efficiency, a stratification strategy is developed that concentrates fuel at the leading and trailing edges of the inlet port. Rotor and exhaust temperature profiles and performance predictions are presented at three representative operating conditions of the engine: full design load, 40% load, and idle. The results indicate that peak local gas temperatures will cause excessive temperatures in the rotor housing unless additional cooling methods are used. The rotor temperature will be acceptable, but the pattern factor presented to the turbine may be of concern, depending on exhaust duct design and duct-rotor interaction.

Review of Recent Developments in Wave Rotor Combustion Technology

“…A reaction progress variable was used for uniform mixtures, and multiple species were represented for stratified mixtures, with a oneor two-step reaction. For deflagration modes, mixing-controlled reaction was combined with a simple eddy diffusivity model, ignition-temperature kinetics, and a total-energy-based flammability limit [37]. The performance of detonative and deflagrative cycles was studied by combined gas dynamic and system simulation.…”

“…7). This provides a richer mixture for reliable ignition, lowering NO x emissions in the leaner main mixture and was modeled for both deflagration [37] and detonation [40] modes. For further benefits from radial partitioning of the rotor channel [72], the rotor channels include circumferential liners that extend a relatively short distance into the length of the channel, as shown in Fig.…”

“…Detonation, although fast, requires particular fuels in limited ranges of stoichiometry and channel width, and its concentrated pressure and momentum may complicate both integration with turbomachinery and rotor thermomechanical design for cyclic peak loads and temperatures. The deflagration combustion mode in a wave rotor has been studied both theoretically [37,41,42] and experimentally [38], and the detonation mode theoretically [40,43], anticipating transferability of PDE research. Fast deflagrations may be more desirable than detonations for gas turbine applications and plausibly attainable due to the high inlet turbulence levels and pressure-wave acceleration of the flame.…”

“…Nalim [37] also developed fuel stratification strategies in longitudinal and radial [72] directions of rotor channels for improving combustion, emissions and leakage impact. Longitudinal stratification can be arranged by partitioning the inlet duct and modulating a continuous-flow fuel injector to control fuel-air mixture ratio in each segment, similar to ABB, and by direct pilot fuel injection (see Fig.…”

“…Compared with pressure-exchange topping cycles and other methods that increase overall pressure ratio, judicious mixture stratification and deflagration in the wave rotor combustor allow lower combustion temperatures. With rapid combustion and gasdynamic quenching, residence time at peak temperature is also relatively short, limiting nitrogen oxide (NO x ) emissions [37,38].…”