Researches On In-tube Convection Heat Transfer And Thermal Cracking Of Supercritical Pressure Fluids

The supercritical pressure fluids are used as coolant to protect the high temperaturewall by oonvective heat transfer or thermal cracking in transpiration cooling technologyfor liquid rocked truster and regenerative cooling technology for scramjet engine ofhypersonic vehicle. The purpose of this dissertation is to study convection heat transferand thermal cracking of supercritical pressure fluids in vertical small tubes byexperimental mesurements, theoretical analyses and numerical simulations.For the convection heat transfer of supercritical pressure CO₂in a vertical microtube with inner diameter of99.2μm, the experimental results show that flowacceleration due to pressure drop are comparable to that induced by heating. The heattransfer is impaired when the flow acceleration is strong for the low inlet Renoldsnumber （Rein≤2600） and high heat fluxes. An empirical heat transfer correlation isdeveloped based on the measured results. Numerical simulations of convection heattransfer using various Reynolds number models show that the low Renolds numbersAKN and LB models can predict the heat impairment due to flow acceleration butover-respond to the effect of flow acceleraton.The experimental results for convection heat transfer of supercritical pressuren-decane in a vertical small tube with inner diameter of0.95mm show that convectionheat transfer mainly is influenced due to the thermal properties variations and the effectsof buoyancy and flow acceleration are insignificant. The experimental results for the2mm inner diameter tube show that for low inlet Reynolds numbers （Rein≤4000） and highheat fluxes, the heat transfer is impaired for upward flow and improved for downwardflow due to buoyancy effect. An empirical correlation is developed to predict heattransfer of supercritical pressure n-decane based on the measured data. The numericalresults show that heat transfer is impaired because of the turbulence productionreduction due to the strong influences of buoyance. The experimental results for thermalcracking of supercritical pressure n-decane in a small tube show that the reaction ismainly affected by the reaction temperature and the residence time. The conversion ofn-decane is increased with the increase of the reaction temperature and the residencetime. For higher reaction pressure, the conversions are increased due to the increase of the residence time. A global chemical kinetics model for mildly thermal cracking（conversion of n-decane is less than15%） of supercritical pressure n-decane isdeveloped based on the experimental data. An improved reaction model is developed topredict thermal cracking of n-decane with higher conversion （conversion of n-decane isless than25%）.The numerical model incorporating convective heat transfer and thermal crackingfor supercritical pressure n-decane is developed and validated by the experimental data.