Effect of the Reynolds number and the basic design parameters on the isothermal flow field of low-swirl combustors

Fluid dynamic design of combustors is commonly aimed to set up swirling flows through sudden expansions to stabilise the flame. Fuel-air mixing promoted by the swirling motion also contributes to increase the reaction rate and the efficiency of the combustion process, and, consequently, to reduce pollutant emissions. Most of the experimental studies available in the scientific literature investigate the aerodynamics of high-swirl combustors. In such combustors the expansion of the flow in the combustion chamber produces a central recirculation zone (CRZ) which provides a stable anchoring to the flame. By comparison, few studies analyse the structure of low-swirling flows, in which the tangential component of velocity is not strong enough to induce a CRZ. However, many combustion systems for heat and power generation are currently designed to operate with low-swirling or unswirled flames and show a stable combustion over a wide range of thermal loads. This study deals with isothermal low-swirling flows generated by axial swirlers and aims at providing a comprehensive insight of the flow field by considering the effect of the Reynolds number and some relevant design parameters of the swirler-combustion chamber assembly. The paper collects, rearranges, and compares velocity measurements reported in the open literature on combustors of similar geometry and swirl number. The literature data sets are expanded with original measurements performed by the authors on a laboratory combustor characterised by a high expansion ratio. Although the considered data sets are quite limited, the comparison between velocity measurements shows quite clear data trends and motivates additional systematic tests to be performed on different configurations of the same baseline design.