1. A three-dimensional computational model of H2–air premixed combustion in non-circular micro-channels for a thermo-photovoltaic (TPV) application (completed)
Wall temperature uniformity and enhancement in a micro combustor for thermo photovoltaic (TPV) applications have attracted considerable attention from researchers in recent years because of their direct impact on efficiency and feasibility of desired energy conversion. In this regard, numerous experimental and numerical studies in micro-combustion application have been conducted and reported. However, most previous studies have been focused on geometrical configurations limited to planar and circular channels. It is therefore of interest to investigate the impact of different channel geometries on wall temperature distribution and energy conversion efficiency.
This project addressed flow and flame behavior in a micro-combustor. By utilizing the well-established computational fluid dynamics (CFD) approach, the effect of geometrical parameters on the flow behavior and wall temperature was examined and evaluated. In order to improve the productive capability of the computational model, several steady state Reynolds Average Numerical Simulation (RANS) turbulence models alongside with different reaction rate formulations were investigated. The results indicated that Reynolds Stress Model (RSM) with Eddy Dissipation Concept (EDC) provide the best quantitative prediction. The developed model was further applied to investigate the effect of inlet velocity on flame structure and outer wall temperature. Furthermore, the effect of reactor cross sections, including circular, square, rectangular, triangular and trapezoidal, on the wall temperature was also evaluated. The results showed that the wall temperature is increased with an increase in the inlet velocity. Trapezoidal and triangular cross-sections were found to have better performance in terms of Figure of Merit (FoM), a parameter used in this study to gage thermal and hydraulic performance of a micro-combustor.
2. Investigation of energy conversion and flame stability in a curved micro-combustor for thermo-photovoltaic (TPV) applications (Completed)
Energy conversion efficiency of a thermo photo voltaic (TPV) system strongly depends on the wall temperature and its uniformity. Therefore, improving heat transfer characteristics of these systems has been a focus of many recent studies. This study explores the effect of curvature on heat transfer, overall energy conversion, flame stability and emission levels for a circular micro-combustor with a sudden expansion (backward facing step). Uniformity index and figure of merit (FoM) have been defined to facilitate the analysis of computational results from a three-dimensional turbulent reaction model for curved micro-combustor. The results indicate an enhancement of 110 K in the outer wall temperature and a 7.84% maximum increase in overall energy conversion efficiency for curved channels relative to the straight channels. The flame structure in curved channels, however, is found to be more susceptible to instability such as flame flashback. The model indicates a maximum increase of about 2.5 m/s in the lower limit of the inlet mixture velocity for safe combustion operation in curved ducts.
3. Numerical investigation of flame structure and blowout limit for lean premixed turbulent methane-air flames under high pressure conditions (Completed)
With the forecast of rise in the energy usage, it is imminent that the concentration of toxic emissions such as carbon monoxide (CO) and oxides of nitrogen (NOx) in the ecosystem will increase. One of several industrial techniques to mitigate the emission levels is lean premixed combustion. However, this combustion mode often leads to the problem of Lean Blowout (LBO), which is not well understood. The present study attempts to devise an effective and computation-friendly industrial tool to predict the behavior of lean flames near extinction in a combustion chamber and estimate the lean blowout limits under high temperature and high pressure conditions. Utilizing a tabulated chemistry approach in combination with Reynolds Averaged Numerical Simulation (RANS) turbulence model, extensive validation is performed comparing plain and reacting flow simulation results with the experimental data of laboratory-scale burner at Paul Scherrer Institute (PSI). A modified Flamelet Generated Manifold (FGM) combustion model in conjunction with Reynolds Stress Model (RSM) turbulence model was found to give an accurate prediction of the flow and temperature field inside the combustor. Using this model, the study explores the impact of operational parameters, such as pressure, preheat temperature, turbulence intensity at the inlet and inlet bulk velocity on flame position, temperature, emissions and blowout limits for lean premixed methane-air flames. The combustion model was further applied to the extinguishing flames to study the flame stability limit, which is a very important criterion for an efficient combustor design. The results show that the modified FGM model can reproduce the flame stability curve within 20% of the experimental limit.