Thermal and fluid flow analysis of swirling flameless combustion

Abstract

Flameless combustion is a novel combustion mode that is also to achieve ultra-low emissions of NOx and CO while producing a uniform temperature distribution and a stable combustion. In this work, a newly designed Internally Preheated Swirling Flameless Combustor (IPSFC) has been developed at the High Speed Reacting Flow Laboratory (HiREF), Faculty of Mechanical Engineering, Universiti Teknologi Malaysia (UTM) to achieve high performance combustion and low emission. The study examines the sequential development of a low emission swirling flameless vortex combustor operating from a thermal load of 7 kW to 15 kW. The swirling flameless combustor has been modified to include preheating where the fresh air passes through a helical tube that is fixed inside the combustion chamber before being injected into the flameless combustor for some experiments. The objective of this study is to investigate in detail the role of air inlet geometry with and without air preheating on the performance of the swirling flameless combustion without the addition of diluted gas. Investigation on the effect of multiple air–fuel injection configuration found that the case of SFR42 to be the best configuration for optimum flameless combustion performance. SFR42 is a swirling combustor with 4 inlets of tangential air and 12 inlets axial air with 11 inlets coaxial fuel. The lowest NOx and CO emissions are observed at the equivalence ratio of 0.8 with the value of 4 ppm and 24 ppm, respectively. In general temperature uniformity which is an important characteristic of flameless combustion is observed to vary from 0.03 to 0.06 at the different equivalence ratio. This work also demonstrated the achievement of swirling flameless combustion with and without preheated tangential air. Overall, preheated air has contributed to the increase of 5% thermal efficiency compared to the non-preheated case at the expense of 4 ppm maximum increment of NOx emission. In this thesis some simulation study is also performed to investigate the detail flow field inside the swirl combustor. The numerical investigation confirms the experimental finding on the outstanding performance of SFR42 configuration. It is found that in this configuration the bulk swirling motion was produced in the combustor for good mixing between fuel and oxidizer which in turn lead to complete combustion at low peak temperature. This results in the combustion process with low emission.