A novel combustor design, referred to as a stagnation-point reverse-flow (SPRF) combustor, was recently developed to overcome the stability issues encountered with most lean premixed combustion systems. The SPRF combustor is able to operate stably at very lean fuel-air mixtures with low NOx emissions. The reverse flow configuration causes the flow to stagnate and hot products to reverse and leave the combustor. The highly turbulent stagnation zone and internal recirculation of hot product gases facilitates robust flame stabilization in the SPRF combustor at very lean conditions over a range of loadings. Various optical diagnostic techniques are employed to investigate the flame characteristics of a SPRF combustor operating with premixed natural gas and air at atmospheric pressure. These include simultaneous planar laser-induced fluorescence imaging of OH radicals and chemiluminescence imaging, and spontaneous Raman scattering. The results indicate that the combustor has two stabilization regions, with the primary region downstream of the injector where there are low average velocities and high turbulence levels where most of the heat release occurs. High turbulence levels in the shear layer lead to increased product recirculation levels, elevating the reaction rates and thereby enhancing the combustor stability. The effect of product entrainment on the chemical time scales and the flame structure is quantified using simple reactor models. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zone regime throughout the combustor. The flame tends to become more flameletlike, however, for increasing distance from the injector.
The performance of dry, low NOx gas turbines, which employ lean premixed (or partially premixed) combustors, is often limited by static and dynamic combustor stability, power density limitations and expensive premixing hardware. To overcome these issues, a novel design, referred to as the Stagnation Point Reverse Flow (SPRF) combustor, has recently been developed. Various optical diagnostic techniques are employed here to elucidate the combustion processes in this novel combustor. These include simultaneous planar laser-induced fluorescence (PLIF) imaging of OH radicals and chemiluminescence imaging, and separate experiments with particle image velocimetry and elastic laser sheet scattering from liquid particles seeded into the fuel. The SPRF combustor achieves internal exhaust gas recirculation and efficient mixing, which eliminates local peaks in temperature. This results in low NOx emissions, limited by flame zone (prompt) production, for both premixed and non-premixed modes of operation. The flame is anchored in a region of reduced velocity and high turbulent intensities, which promotes mixing of hot products into the reactants, thus enabling stable operation of the combustor even at very lean equivalence ratios. Also, the flame structure and flow characteristics were found to remain invariant at high loadings, i.e., mass flow rates. Combustion in the non-premixed mode of operation is found to be similar to the premixed case, with the OH PLIF measurements indicating that nonpremixed flame burns at an equivalence ratio that is close to the overall combustor equivalence ratio. Similarities in emission levels between premixed and non-premixed modes are thus attributable to efficient fuel-air mixing in the nonpremixed mode, and entrainment of hot products into the reactant stream before burning occurs.
The flowfield of a novel combustor design that can operate stably even at high flowrates and very lean conditions is studied. This Stagnation Point Reverse Flow (SPRF) combustor consists of a central injector at the single open end of a cylindrical chamber, with the injector inlet area much less than the open area of the combustor through which the exhaust products leave. Thus the flowfield can be characterized as a confined jet in an opposed flow. Experiments with Particle Image Velocimetry (PIV) as well as computations employing Large Eddy Simulations (LES) have been used to characterize the nonreacting and reacting flowfields within the combustor for premixed and nonpremixed modes of operation. Both nonreacting and reacting cases exhibit a "stagnation" region with local average and high fluctuating velocities. The reacting flows exhibit higher mean and fluctuating velocities than the nonreacting flow. The nonreacting flow stagnates earlier than the reacting flow due to the effects of gas expansion in the reacting flow case. Consequently, the jet decay rates are higher for the reacting flows. The high shear between the forward and reverse flows causes significant recirculation, resulting in enhanced entrainment and mixing of the returning hot product gases into the incoming reactant jet. Comparison of the instantaneous flowfields reveals that the reacting jets exhibit significant lateral motion and distortion compared to the nonreacting case. This parallels the large increase in fluctuating velocities and turbulence intensities that coincide witht the region of high heat release. Nonpremixed and premixed reacting flowfields at the same fuel and air mass flow rates are found to be very similar except in the near field region of the jet, due partly to the lack of heat release there in the nonpremixed case.
The Stagnation Point Reverse Flow (SPRF) combustor has been shown to operate stably while producing ultra-low NOx emissions over a range of loadings and equivalence ratios in both gas and liquid fueled operation. In nonpremixed gaseous operation, low NOx levels have been attributed to initial shielding of fuel from hot products allowing internal premixing of fuel and air to nearly the global equivalence ratio before burning. Various optical diagnostic techniques, such as chemiluminescence imaging and laser scattering, are employed to elucidate the combustion processes of this novel combustor in liquid-fueled operation. While the overall flow features are similar for both gas and liquid fuels, the combustion characteristics and NOx performance are strongly controlled by fuel dispersion and evaporation in liquid operation. Here, fuel dispersion is controlled by varying the placement of the fuel injector, which is centrally located within the annular air inlet tube. When the liquid injector is located in plane with the exit of the air annulus, the fuel remains initially shielded from the high temperature return products (similar to the gaseous case) producing a highly lifted flame. On the other hand, injecting the liquid upstream produces a more well-dispersed fuel pattern at the reactant inlet. This leads to a reduction of the equivalence ratio in the fuel consuming reaction zones. Hence the NOx emissions were found to be lower for this less shielded injector configuration over the range of global equivalence ratios and loadings investigated. Thus it is conjectured that the added delay caused by fuel evaporation before mixing and combustion can occur, changes the optimal shielding required for liquid-fueled operation.
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