Study On Heat And Mass Transfers Of Hydrocarbon Fuel With High Conversion Under Supercritical Conditions

Thermal protection technology is one of the most important technologies to develop scramjet engine.To protect the engine from damage,regenerative cooling with hydrocarbon fuel is usually adopted.Besides the sensible heat capacity,extra heat sink from chemical reactions of endothermic fuels is also used to provide thermal protection.In order to meet the cooling requirement of the hypersonic vehicle,the heat sink of hydrocarbon fuel with high conversion needs to be further utilized.However,in the region with high fuel conversion of the cooling channel,the secondary endothermic reaction and secondary exothermic reaction between the pyrolytic products make the convective heat transfer mechanism more complicated,which brings significant difficulties to study the heat and mass transfer characteristics of hydrocarbon fuels with high fuel conversion under supercritical conditions.Specific to heat and mass transfer characteristics of hydrocarbon fuels with high fuel conversion under supercritical conditions,and the influence of volumetric force and channel geometric parameters on the heat and mass transfer of supercritical reacting flow,this thesis has conducted the following investigations:Numerical model is established to study the heat and mass transfer characteristics of hydrocarbon fuel with high conversion under supercritical conditions.After model validation,the influence of chemical reactions on heat and mass transfers of hydrocarbon fuel with high fuel conversion is analyzed.Heat transfer is enhanced with conversion of hydrocarbon fuel at mild pyrolysis.Heat transfer coefficient increases.As the conversion of hydrocarbon fuel with high fuel conversion increases,fuel concentration decreases and heat absorbing capacity of pyrolysis drops.Meanwhile,the chemical reaction rate of secondary exothermic reactions is higher than that of secondary endothermic reactions,which results in a decline of heat transfer.Finally,a heat transfer deterioration region,which is induced by the secondary exothermic reactions,is generated.Compared with the starting point of heat transfer deterioration,the maximum decrease of heat transfer coefficient is 5.17%.Meanwhile,the secondary chemical reactions produce large amount of coking precursors,which leads to easily accumulate on channel wall.In addition,with the increasing pressure and heating rate,and decreasing inlet mass flow rate,the heat transfer deterioration becomes more serious.The heat transfer deterioration region moved along the direction opposite to the mass flow at high pressure.Meanwhile,with the increasing pressure,heating rate and inlet mass flow rate,surface coking precursors easily accumulates on the channel wall.The effects of volume force on the heat and mass transfer characteristics of hyarocarbon fuel with high fuel conversion under supercritical conditions are analyzed during the engine operation.The buoyancy makes thermal stratification and the nonuniformity of fuel conversion reduces in the downward flow.The wall temperature decreases,and the heat transfer coefficient increases.Moreover,the buoyancy can efficiently inhibit surface coking precursors accumulation on cooling channels.Compared with no buoyancy,the maimum difference of heat transfer coefficient and coking precursor concentration is 7.68% and 3.29%,respectively.However,in the upward flow,the buoyancy makes thermal stratification and the nonuniformity of fuel conversion rises.The wall temperature increases,and the heat transfer coefficient decreases.Meanwhile,the surface coking precursors easily accumulate on cooling channels.Compared with no buoyancy,the maimum difference of heat transfer coefficient and coking precursor concentrarion is 11.74% and 6.15%,respectively.In addition,the effect of buoyancy becomes weaker with the rising heating rate and inlet fuel conversion.Buoyancy will cause secondary flow at the cross sections of asymmetrically heated horizontal rectangular cooling channels.The secondary flow enhances heat and mass transfers between near wall region and core flow field,which can effectively improve the utilization level of fuel heat absorbing capacity and inhibit surface coking precursors accumulation on cooling channels.The intensity of secondary flow makes the flow,heat and mass transfer characteristics of the supercritical reacting flow in the rectangular cooling channel under different heating direction significantly different.In the bottom heated case,the strongest secondary flow results in highest fuel heat absorbing capacity and hardest surface coking precursor accumulation.However,in the top heated case,the weakest secondary flow results in lowest fuel heat-absorbing capacity and easiest surface coking precursors accumulation.Compared with no buoyancy,the maximum increase of heat transfer coefficient is 13.24%,5.39% and 11.68%,respectively.The maximum decrease of coking precursor concentration is 32.63%,19.64% and 4.32%,respectively.In addition,as the operating pressure increases,the buoyancy effect becomes stronger duo to fuel pyrolysis,which is distinctly different from that of non-cracking zone.As hydrocarbon fuel with high fuel conversion increases fuel velocity and inertial force,the influence of buoyancy on the heat transfer deterioration caused by the secondary exothermic reaction becomes weak.The secondary flow induced by centrifugal force enhances the fluid mixing in the curved cooling channel,and so it improves heat and mass transfers between near-wall region and core flowfield.Compared with the straight cooling channel,the curved cooling channel has higher fuel conversion,thinner thermal boundary layer and lager heat transfer coefficient,but less concentration of coking precursors in near-wall region.The vortex structure can inhibit surface coking precursors accumulation on curved channels.In the outlet of the curved section,fuel conversion increases by 15.69%,heat transfer coefficient increases by around 80%,and benzene concentration decreases by around 40%.Based on the studies above,the influence of cooling channel geometric parameters on the flow,heat and mass transfer characteristics of hydrocarbon fuel with high fuel conversion under supercritical conditions is analyzed.The change of channel aspect ratio makes heat and mss transfer characteristics of hydrocarbon fuel wih low fuel converison different from those of hydrocarbon fuel with high fuel conversion.The large aspect ratio makes the thermal stratification and the nonuniformity of fuel conversion of hydrocarbon fuel wih low fuel converison decrease,which improves fuel heat absorbing capacity,reduces the maximum wall temperature,and inhibits surface coking precursors accumulation.Therefore,increasing the aspect ratio of the cooling channel is beneficial to the heat and mass transfers of hydrocarbon fuel with mild pyrolysis.However,as the aspect ratio of the cooling channel increases,the thermal stratification and the nonuniformity of fuel conversion of hydrocarbon fuel wih high fuel converison increase,resulting in the decreasing heat absorbing capacity and the inceasing wall temperature.The heat transfer deterioration caused by the secondary exothermic reaction becomes more serious,and the heat transfer deterioration region becomes larger.Surface coking precursors easily accumulates on the channel wall.Therefore,decreasing the aspect ratio of the cooling channel is beneficial to the heat and mass transfers of hydrocarbon fuel with high fuel conversion.