Experimental and Numerical Investigation of Confined Jets in Hot Co-Flow

Abstract

Sequential staged combustion with an expansion turbine between both stages is an efficient way of extending the low emission regime of gas turbines towards very low loads. The dominating combustion regime in the second stage is auto-ignition. A confined natural gas jet in hot vitiated co-flow is investigated to obtain deeper insights in the parameters effecting auto-ignition. A generic pressurized combustion experiment is presented. Optical measurement techniques are applied to determine lift-off height and air excess ratio of the flame in the ignition region. Oxygen content of the co-flow, momentum flux ratio and pressure are varied in the experiments. Cold flow measurements are used to analyze the mixing behavior for different momentum flux ratios. Tendencies observed in the experiments are successfully simulated by a numerical method wherein the flow-, mixture- and temperature-fields are acquired using a non-reacting Realizable k-ε RANS simulation in Fluent. Mixture-PDFs obtained from water-channel measurements are used to take mixture-fluctuations into account. In a post-processing step the combustion-process is calculated with unsteady flamelet equations evaluated in Matlab. By using a progress variable approach with tabulated chemistry only two partial differential equations need to be solved. Hence the computational cost is low. With this study a low-cost numerical model for auto-ignition is demonstrated and the effect of temperature gradients in the co-flow on self-ignition is highlighted.