Experimental Characterization of the Reaction Zone in an Ultra-Compact Combustor

Abstract

Significant benefits can be obtained with respect to engine thrust-to-weight ratio and specific fuel consumption if the length, weight, and pressure drop of the combustor can be reduced. The ultra-compact combustor (UCC) has the potential to aid the realization of these benefits by integrating neighboring components such as the compressor exit diffuser and the turbine inlet guide vanes (IGV) within the combustor using a systems-level engineering approach. The UCC presented here utilizes a trapped-vortex cavity. This combustor design has been shown to exhibit larger turn-down ratios, higher flame stability, shorter flame lengths, and acceptable NOx emissions when compared to conventional richburn, quick-quench, lean-burn combustors. The axial distance required to complete combustion within the mainstream dictates a minimum combustor length for obtaining acceptable levels of combustion efficiency. Hence, characterization of the reaction zone within a UCC is required to optimize the length. In this study OH* chemiluminescence imaging is used to assess the characteristics of the reaction zone via windows in the side and top of the combustor. CO, NOx, and total hydrocarbon (THC) emissions indices obtained with gas-sample probes at the exit of the combustor as well as computed combustion efficiencies are provided as a reference for the OH* chemiluminescence. Configurations with no turning vanes (CDF and CDF-2), with standard vanes (CDF-2SV), and with radialvane-cavity (RVC) vanes (CDF-2RV) were used. The first study shows that the CDF-2 configuration has similar combustion efficiencies compared to that of the previously studied CDF configuration between 0.6 1.1 but has a higher peak OH* intensity and higher window exit intensity than that of the CDF configuration. The second study shows that the addition of standard vanes to the UCC decreases the peak and exit OH* intensities and lowers the exit temperature peak to 30% height, while the addition of the RVC vane tends to increase the peak and exit OH* intensities and raise the exit temperature peak to 50% height. Nomenclature CDF = Combined diffuser-flameholder CH* = Electronically excited CH radicals CO = Carbon monoxide D = Cavity depth EICO = Emissions index of carbon monoxide EINO x = Emissions index of the oxides of nitrogen EITHC = Emissions index of total unburned hydrocarbons = Stoichiometric fuel-air ratio of JP-8 H = Height of combustor exit = Summation of normalized OH* intensity from the top window IGV = Inlet guide vane ITB = Inter-turbine burner L t = Axial length of turbine inlet vane ̇ = Mass flow of fuel ̇ = Total mass flow of air into the trapped-vortex cavity NO x = Oxides of nitrogen OH* = Electronically excited OH radicals OGV = Outlet guide vane RVC = Radial-vane-cavity T = Temperature T avg = Average temperature THC = Total unburned hydrocarbons TVC = Trapped-vortex combustor UCC = Ultra-compact combustor W = Width of cavity x = Downstream distance from CDF y = Vertical distance from top of cavity z = Spanwise distance from top window edge Greek Φ...