Coupling Mechanism Of Hydrocarbon Fuel Pyrolysis And Heat Transfer In Active Cooling Channels

As modern advanced aircraft develops,hypersonic vehicles will encounter serious thermal protection problems.Regenerative cooling with endothermic hydrocarbon fuel（EHF）is one of the most promising thermal management technologies for advanced aircrafts to remove the waste heat from the various high-temperature components.Improved understandings of pyrolysis,heat transfer and coking of supercritical EHFs are key issues for practical design and application of regenerative cooling panel.In this paper,a series of experiments and computational fluid dynamics（CFD）simulations were conducted to provide new insights of the coupling relationship among flow,pyrolysis,heat transfer and coking.In addition,effects of near-wall pyrolysis,geometry structure and non-uniform heating on heat transfer and coking behavior were systematically analyzed to develop a novel and highly efficient structures for cooling panels of advanced engines.The supercritical pyrolysis of EHFs was experimentally investigated in a stainless steel mini-tubular reactor heated by a fluidized sand bath system.A simplified molecular reaction kinetic model（16 species and 10 reactions）was proposed with the concept of equivalent volume based on experimental results.The developed model can well predict the pyrolysis in an electrically heated tube with significant improvements,showing the conversion prediction errors less than 15%and the fuel and wall temperature deviations less than 20K.CFD simulations have been conducted in horizontal tubes to get detailed information on the coupling mechanism of heat transfer and pyrolysis.The heat transfer rate can be enhanced by pyrolysis at relative low heat flux（below 400 k W/m²）.When the heat flux increases from 426 to 758 k W/m²,Nu_bdecreases from 108.8 to 73.4.The possible reasons may be attributed to the weakened near-wall turbulence by the increased fluid viscosity in presence of secondary products,as well as the increased thermal boundary layer effect attributed to the high near-wall heat absorption and huge radial property gradient at high heat flux.Typically,the regenerative cooling channels outside the combustor are designed as rectangular structure with millimeter scale.The convective heat transfer of EHFs with pyrolysis under supercritical conditions（3.5 MPa,700 ^oC）was experimentally and numerically investigated in horizontal rectangular channels with different aspect ratios（λ=1～5）.Results showed that the heat transfer was non-uniform in the cross-section with obvious temperature difference.At topwall,increasingλwas beneficial for heat transfer and the average h_c rose from 4480 to 4709 W/m²K withλvarying from 3.2 to5,because of a relative lower heat flux and higher Q_endo in bulk fluid.At sidewall and corner,local heat transfer deterioration occured and wall temperature rose about 3-4 K,indicating that there was an optimum aspect ratio（3<λ<5）to balance effects of heat transfer area and pyrolytic reactions.This may be ascribed to a thicker boundary layer at the corner for increased fluid viscosity with secondary reactions,as well as the increased thermal resistances attributed to dramatical variations of thermophysical properties and considerable decrease of chemical heat absorption near sidewall region.In terms of the working condition in actual engine,the heat transfer and pyrolytic depositions of supercritical EHFs were experimentally studied under different non-uniform heating（q=44.1～674.8 k W/m²）.At low conversion,the heat transfer can be enhanced by secondary flow with uniform heating.When the conversion was higher than 30%,the impacts of secondary flow gradually reduced.With the increase of local maximum heat flux,variations of thermophysical properties caused by high-degreed pyrolysis in hot side significantly increased the thermal resistances in the viscos sublayer.Meanwhile,those variations also led to a decrease in turbulent kinetic energy and an increase in the distance between the heated wall and the center of turbulent core field,thus increasing the thermal resistances in core turbulent field.As a result,the average h_c gradually decreased and the maximum wall temperature sharply rosed,leading to a serious coking problem.