Numerical analysis of fuel—air mixing in a two-dimensional trapped vortex combustor

Abstract

The concept of trapped vortex combustion is emerging as a viable method of burning fuel effectively in aero-gas turbine engines. In a two-dimensional trapped vortex combustor (TVC), fuel-air mixing is characterized by using the parameter intensity of segregation (I s ). When fuel and oxidizer are present (with some macro-mixing already having taken place), the parameter quantifies the extent of local micro-mixing within the domain. The regions of maximum fuel-oxidizer mixedness are identified to be the vortex edge and dump shear layer of the cavity. These two regions are observed to be zones of very good fuel-air mixing with higher levels of turbulence. Increasing aspect ratio (L/D) and the mainstream velocity (V ms ) increases mainstream air entrainment into cavity, resulting in improved fuel-air distribution and mixing. However, an optimal L/D = 1.2 and V ms = 40 m/s results in stable vortex being trapped with better fuel distribution throughout the cavity.While non-optimal L/D and V ms may result in improved mainstream air entrainment, it is found to disrupt the trapped vortex, which is not conducive for combustion. Hence, for a given L/D, there is an optimal momentum ratio between the cavity injections and mainstream flow, which results in a single large vortex being trapped that can roll up the fuel-air injected and distribute them across the cavity, required for stable combustion in a TVC.

Numerical Modeling of an Axisymmetric Trapped Vortex Combustor

“…As a result, a 3D simulation with cavity injections similar to that of an experimental TVC is essential for realistic validation studies. However, comparison of the axis-symmetric cold flow simulation results ( [3]) with experimental results ( [7]) revealed that the direction of rotation of vortex predicted by the standard k-" model is opposite to that of the experiment particularly for low aspect ratio cases ( [3]). So far, no 3D validation studies have been reported to the best of our knowledge to assess the ability of numerical model in predicting the complex flow structure prevailing inside the TVC.…”

“…Though the vortex is insensitive to the mainstream condition, it should entrain certain amount of mainstream air in order to enhance the interactions between the cavity and the mainstream flow ( [10]). Various experimental studies have been conducted by the researchers to study the performance parameters of the trapped vortex combustor ( [1,2,10]); in addition to these, numerical studies have also been reported by various researchers ( [3,4,[6][7][8][9]12]). In these numerical studies, in order to predict the flow and temperature characteristics, different numerical models have been employed.…”

“…In these numerical studies, in order to predict the flow and temperature characteristics, different numerical models have been employed. For example, in the numerical study of Katta and Roquemore ( [3]), standard k-" turbulence model has been used to predict the flow features in an axisymmetric geometry ( [7]), which is similar to an internal cavity trapped vortex combustor with no cavity injections (passive cavity). It should be noted that the flow features of a TVC with a passive cavity will differ significantly from that of an active cavity because the cavity injection momentum in the later case also plays a major role in deciding the flow structure in it ( [4,6,7,9,10]).…”

“…In the aft-wall injection strategy adopted by Hsu et al 1995 ([1]), both the primary and secondary air jets are maintained at the same momentum and as a result, a streamwise vortex is observed in the cavity for a wide range of mainstream flow conditions. In contrast, a counter streamwise vortex can be established within the cavity while injecting the secondary air jet at a higher momentum than the primary jet and the mainstream flow ( [6,7,9,11]). It should be noted that the counter rotating vortex possesses the advantages of better mixing, and enhanced mainstream entrainment over the clock wise vortex ( [7]).…”

“…In contrast, a counter streamwise vortex can be established within the cavity while injecting the secondary air jet at a higher momentum than the primary jet and the mainstream flow ( [6,7,9,11]). It should be noted that the counter rotating vortex possesses the advantages of better mixing, and enhanced mainstream entrainment over the clock wise vortex ( [7]). However, for the same MFR, this counter rotating vortex will not be stable over a wide operating range as compared to the stream wise vortex ( [7]), which may be attributed to the inadequate momentum of the secondary jet.…”