Heat Release Effect On Wall Flux In Supersonic Non-Premixed Combustion
Shaowu Pan
National Lab For Computational Fluid Dynamics
Beihang University, Beijing, China
Advised By Professor Chun-Hian Lee, (Co-Advised By Assistant Professor Zhenxun Gao)
Reference
Zhenxun Gao, Chongwen Jiang, Shaowu Pan, Chun-Hian Lee. "Combustion heat-release effects on supersonic compressible turbulent boundary layers". AIAA Journal (2015): 1-20.
Zhenxun Gao, Chongwen Jiang, Shaowu Pan, Chun-Hian Lee. "Combustion heat-release effects on supersonic compressible turbulent boundary layers". AIAA Journal (2015): 1-20.
Overview
In the scramjet engine, one of the most common approaches is to inject fuel on a backward step in order to achieve supersonic diffusion combustion (see figure on the right). Previous studies showed that this method can reduce frictional drag on the wall, but also a lot of heat release within the boundary layer, which might further worsen the thermal environment inside the combustion chamber. Although there are a lot of researches about heat release effect on boundary layer structure or turbulent transport, studies focused on heat transfer are rare.
In the scramjet engine, one of the most common approaches is to inject fuel on a backward step in order to achieve supersonic diffusion combustion (see figure on the right). Previous studies showed that this method can reduce frictional drag on the wall, but also a lot of heat release within the boundary layer, which might further worsen the thermal environment inside the combustion chamber. Although there are a lot of researches about heat release effect on boundary layer structure or turbulent transport, studies focused on heat transfer are rare.
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In the current study, theoretical analysis based on Stalker’s theory and Reynolds-Averaged-Navier-Stokes(RANS) method are conduct as two independent approaches to study the heat release effect on wall flux, i.e., pressure, drag and heat transfer in a hydrogen-air non-premixed combust- ion. The freestream Mach number for air is 2.44 and that for hydrogen is 1.00. Initial temperature for air is 1270 K and that for hydrogen is 254 K. Pressure for both jets is 0.0922 MPa. Theoretical results are compared with previous literature. One case for code validation and three cases are presented: combustion, only mixing, zero hydrogen injection. This numerical simulation strategy is also validated through compar- ison with experiment. (see figure on the left)
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By extracting the underlying assumptions in Stalker's proof for approximate formula for drag reduction in hydrogen-air combustion, a new formula for prediction of heat transfer is proposed through proper Reynolds analogy developed by Stalker. In current paper, theoretical analysis show that heat release would result in increasing heat transfer moderately (see figure on the right), compared with increase in heat release itself. By analyzing the results of heat transfer under two conditions - the combustion and 'mixing only' through theoretical approach, it is found that there is a decrease in heat transfer in the above two conditions compared with zero fuel injected. It is supposed to be mainly caused by the film-cooling effect of injections of cold hydrogen. In the cases considered, high thermal conductivity of hydrogen doesn't overweight the effect of low temperature. Further studies are need for a detailed investigation. Moreover, a comparison step forward by theoretical approach with zero fuel injected to the boundary layer show drag had been greatly reduced as what has been expected in previous literature. And numerical results matched very well a distance off the injection position. (see figure on the right)
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The results above from theoretical approach are qualitatively consistent with the numerical simulation (see figure above). What's more, the underlying velocity profile and temperature profile are extracted and studied. Dimensionless temperature profile is calculated through analytic method. (see figure above). By both analytic and numeric approaches, it is found that the heat release effect on velocity profile tends to laminarize the flow (see figure on the left), which is possibly caused by decrease in local Reynolds number, due to comparatively low density caused by high temperature in combustible boundary layer. Last, although the theory obtained from Stalker’s formula for heat transfer transfer does not match with the numerical comparison very well (see figure above), numerical simulations and theoretical analysis come to an agreement on the qualitative relationship between combustion and mixing, which suggests Stalker's modelling of heat release effect might be reasonable.
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