Numerical study of trapped vortex combustors characteristics in small ramjets
Date of Issue2016
School of Mechanical and Aerospace Engineering
Small-scale ramjets are promising air-breathing engines for high-speed propulsion in military, due to their high specific impulse, low aerodynamic drag and good compatibility with operating systems. The challenges of designing a miniature ramjet combustor mainly stem from insufficient space and short flow residence time for good mixing and burning, and difficulties in flame stabilization. Impossible utilization of relatively large air-blast fuel injectors in a small combustor provides more difficulties for the design. The trapped vortex combustor (TVC) concept, locking a pilot flame inside a cavity instead of exposing it to the main stream, provides a simple but effective solution for flame stabilization. In this thesis, a TVC designed for a small ramjet operating at Mach 4.0 is numerically investigated by using ANSYS FLUENT v14.5 and a large eddy simulation (LES) code from Stanford. The results show that the optimal cavity size that traps a vortex inside can be determined by calculating the drag coefficient. Because the designed small combustor does not have bypass air directed into the cavity, injecting all the fuel directly in the optimal cavity always leads to an overly fuel-rich condition. However, when the fuel is injected in the upstream with a proper fuel/air momentum flux ratio, part of fuel goes into the cavity and the rest directly flows over it. Hence an approximately stoichiometric cavity and a hot cavity flame can be obtained. Under some circumstances, the TVC may operate in a high-spinning motion. For example when it is installed in a spin-stabilized ramjet projectile, in which the spinning rate can be as high as 30,000 rpm. Numerical methods were used to investigate the effects of spinning motion on the TVC. The results show that the Coriolis effects dominate the flow in the cavity when it is subject to a high spinning motion. Multiple secondary vortices and strong three-dimensional flows are observed in the cavity, so that the mixing is improved and a hotter cavity flame is generated. The centrifugal force effects, however, generate a short recirculation zone in the main combustor and concentrate the fuel stream on the combustor axis, which leads to deteriorate fuel-air mixing.