Optimal excitation mechanism for combustion enhancement of supersonic shear layers with pulsed jets

• DOI: 10.1016/j.ijhydene.2020.06.217
• Publication Date: 2020-07-23T05:52:26Z
• Authors: Chengcheng Liu; Zi'ang Wang; Bin Yu; Bin Zhang; Hong Liu
• Keywords: 02 engineering and technology; 0204 chemical engineering; 7. Clean energy

Abstract Supersonic shear layers (convection Mach number = 0.6) with flow direction excitation are studied by a canonical Large Eddy Simulation (LES) method with quasi-laminar turbulent reaction model. By modulating velocity of central jet through ideal square wave under four excitation periods (τ = 20, 40, 60, and 80 μs), the present study aims to explore the optimal excitation mechanism for combustion enhancement. As the excitation period increases, combustion efficiency tends to increase and then decrease. The maximum combustion efficiency is achieved at τ = 40 μ s , which is nearly four times that in the no excitation scenario. The analysis of mixing characteristics of the nonreacting cases reveals that mixing enhancement mainly explains combustion gain of different excitation modes. From the limiting formation of vortex, the size of the vortex core has fully developed to reach axial contact at τ = 40 μ s . Any further increase in excitation intervals implies a larger vortex spacing, which leads to poor fluid mixing while lower excitation case forms continuous small vortices with less mixing region. The comparison of the circulation decomposition of the main vortex and trailing wakes indicates the pinch-off characteristic of the limiting formation of vortical structure. On the basis of these results, an optimal forcing criterion is proposed according to the limiting vortex formation with dimensionless excitation time scale τ ∗ ≈ 1.64 . This study provides a novel perspective for the optimal combustion enhancement of supersonic shear layers from the aspect of vorticity dynamics.