The numerical aerodynamic investigation of swirling inlet flow in a vaporizer tube micro-gas turbine combustor

Abstract

A combustor was designed for a 200N micro-gas turbine for the model aircraft industry using the NREC design method. Multiple designs resulted which varied in terms of annular area split configuration, hole area splits and relative hole positions. In a previous study two likely preferable designs were selected using a devised scoring method. For this study, the effect of inlet (diffuser outlet) swirl on the internal aerodynamics of the two combustor designs previously chosen was investigated using a RANS CFD analysis. For each of the two designs a set of varying flow angles was applied at the inlet to the simulation domain. The effect on the establishment of the primary zone features is of specific interest; however, the effects and consequences of the swirl throughout the combustor were investigated. Some of the results such as mass flow splits and pressure drop are already quantitative in nature, however, the evaluation of the quality of the recirculation zone, mixing and outlet plane flow are of a more qualitative nature. A scoring system was previously devised in order to apply a quantitative value to the qualitative aspects of the flow, such as Recirculation zone (Rz), Outlet and Mixing, which are initially analysed subjectively. For each feature, the designs were subjectively evaluated relative to each other and given a rating/score. This scoring methodology for ranking different combustor designs proved to be an effective method for evaluating the effect of inlet swirl on the flow features and behaviour of the chosen combustor designs and thus provide an indication of the likely performance changes to be expected. The methodology was able to indicate which of the two top designs was the better option when considering inlet swirl, however the potential for improvement was revealed when considering scoring in a global context. This study suggests that for this engine, the inlet swirl could allow for the removal of NGV before the turbine since the flow is fairly well conditioned and “pre-turned” due to the swirling flow progressing to the outlet of the combustor. The removal of the traditional NGV allows for a reduction in NGV pressure losses which compensates for the increased combustor pressure loss experienced due to increased inlet swirl.