Study On The Mechanism And Influences Of Heat Transfer Enhancement With Hydrocarbon Fuel Pyrolysis

The hypersonic weapon is an important characteristic of the national defense characteristics,and the regenerative cooling technology is vital to the airbreathing hypersonic flight.The hydrocarbon fuel is used both as the coolant and propellant,decreasing the wall temperatures of the combustion chamber and nozzle with convective heat transfer and increasing its cooling capacity with endothermic pyrolysis.However,calculation conditions like pressure,conversion rate,and fuel type are strictly required so far in regenerative cooling simulation,and it is difficult to obtain the heat transfer characteristics with the influences of endothermic reactions.Besides,the refined predictive capacity of hydrocarbon fuel pyrolysis is promising for the development of regenerative cooling technology relating to low concentration species,such as additives or coking inhibition.Therefore,it is necessary to develop a more universal calculation method for the reactive flow,to accurately predict the pyrolysis and heat transfer in a wider range of operational conditions and fuel conversion rates,to obtain the needed heat transfer law,and to analyze the influences of the operational condition change on fuel pyrolysis and heat transfer.The following research has been carried out based on these problems.According to the poor applicability of pyrolysis kinetics so far in regenerative cooling simulations,this dissertation focuses on the accuracy of the traditional 1-D flow reactor model used to develop the chemical mechanisms.With the experimental and numerical investigations in this dissertation,the causes of the pyrolysis forecast deviations using the traditional chemical analysis method are studied.Results turn out that the uneven radial temperature distributions and the turbulence–chemistry interactions are the critical factors for the accuracy of the simulation model,which are the major causes of the predictive deviations.On the other hand,by comparing the results of simulations and experiments,the detailed mechanisms are proven to apply to the reactive heat transfer investigations of n-decane up to 80 % conversion rates at atmospheric and supercritical pressures.Besides,the effects of systematic error in the calculation model on kinetic mechanism development are discussed,which provides the rationalities for pyrolysis mechanism selections.The effects of fuel pyrolysis on heat transfer coefficient are experimentally and numerically investigated with different fuels.The pyrolysis and heat transfer characteristics of n-decane,ethylbenzene,and methylcyclohexane are studied with experiments,and results show that the heat transfer coefficient increases with pyrolysis.The corresponding numerical simulations are also performed,whose results have good agreements with the experimental ones,and the spatial distributions of different parameters are obtained.By analyzing the validated numerical results,the heat transfer enhancements show close relations with the local chemical heat absorption capacity.Then a novel thermal coefficient is proposed to describe the total heat absorption capacity of fuel,and its variations are like the heat transfer coefficient in different chemistry.Further quantitative analysis of the pyrolysis heat transfer is carried out based on the previous qualitative studies.The predictive capacities of the proposed heat transfer correlations in literature so far have been evaluated at different conversion rates,and it is necessary to develop a novel one for reactive flow at a high conversion rate.By introducing the previously developed thermal coefficient,a new dimensionless parameter is proposed with the dimensional analysis method to describe the contribution of pyrolysis to convective heat transfer.An obvious linear regulation is observed between the logarithmic value of Nusselt number enhancement and the proposed dimensionless parameter throughout the pyrolysis.Then the factor in this linear function is fitted with experimental results and the least square method,and a novel heat transfer correlation is developed.By analyzing the maximum heat transfer enhancement from fuels with different heat absorption capacities,continuously increasing the fuel chemical endotherm might bring a smaller benefit of Nusselt number augmentation,and the extreme value is close to 1.35.The pyrolysis and heat transfer of aviation fuel RP-3 with complicated compositions are experimentally and numerically investigated based on the previous studies for pure substances.By comparing the results,the predictions of wall temperature,outlet fluid temperature,and mass fraction of gaseous products are reliable.The competition of free radicals between different surrogate components is discussed during the initial stages of pyrolysis,and the contributions of endotherm are compared.Multiple effects of operational condition changes on pyrolysis are thoroughly analyzed,and the major factors among them are also discussed.It turns out that fuel concentration,reaction time,and average reaction temperature have different influence intensities in different chemical stages.