Characteristics And Mechanism Of Convective Heat Transfer To Supercritical Pressure RP-3 In Mini-channels

Active cooling which utilizes the onboard hydrocarbon fuel as coolant has been considered to be the most effective thermal protection method for gas turbine engines and scramjets.During the cooling process,the fuel flows through mini-channels(dh<3 mm)and carries heat away under supercritical pressures.The drastic,nonlinear variations of thermophysical properties in the vicinity of the pseudo-critical temperature(Tpc)would have a considerable effect on the flow and heat transfer behaviors.A better understanding on the flow and heat transfer characteristics as well as mechanism of supercritical pressure hydrocarbon fuels and providing accurate methods for predicting the cooling process and heat transfer coefficients are of great significance for the successful application of the active cooling technology.In the thesis,flow and heat transfer of supercritical pressure hydrocarbon fuels inside mini-channels has been studied using both experimental and numerical methods.The supercritical pressure hydrocarbon fuel test facility was constructed and convective heat transfer characteristics of supercritical pressure aviation kerosene RP-3 in vertical tubes(din=1-2 mm)were experimentally investigated.The effects of heat flux,pressure,and flow direction on heat transfer characteristics under large and small mass flux conditions were studied,respectively.It was found that the buoyancy and thermal acceleration effects could be ignored under large mass flux conditions,and the heat transfer regime was mainly influenced by variations of thermophysical properties.Heat transfer enhancement(HTE)was observed when the bulk fluid temperature approached Tpc,and HTE weakened as the heat flux or operating pressure increased.Under small mass flux conditions,the thermal acceleration effect could be ignored,and the heat transfer regime was determined by the combined result of buoyancy and property variations.Heat transfer deterioration(HTD)was observed in the entrance region in upward runs,and HTD was more severe with the increase of heat flux.With the increase of operating pressure,HTD first aggravated and then weakened.The second HTD occurred when the wall temperature exceeded Tpc at low-pressure conditions.Heat transfer correlations were developed for the buoyancy-free and buoyancy-influenced conditions,respectively.An in-house code for simulating flow and heat transfer of supercritical pressure fluids was developed within the framework of OpenFOAM.Numerical investigation on convective heat transfer to aviation kerosene RP-3 and n-decane flowing in vertical tubes(din=1-2 mm)was carried out.The accuracy of a variety of turbulence models in predicting heat transfer of the entrance region cases,property variations influenced cases,and buoyancy influenced cases were carefully examined.There was significant difference in the numerical results obtained using different near-wall treatments and turbulent heat flux closure methods.The MK-HNT k-?-kt-?t model performed best in reproducing the experimental data,and the mechanisms of different heat transfer regimes were studied based on the thermal-flow fields obtained by this model.The results indicated that HTD in the entrance region under large qw/G conditions could be attributed to the increase in the thickness of the conductive sub-layer.In the trans-pseudo-critical region(Tb<Tpc<Tw),the turbulent heat flux increased drastically when the peak of isobaric heat capacity moved into the buffer layer,resulting in the enhancement of heat transfer.The buoyancy effect caused by the significant density gradient in the buffer layer led to relaminarization of the turbulent boundary layer(TBL).As a result,the production of turbulent heat flux was impeded and HTD occurred.The recovery of heat transfer downstream of HTD could be attributed to the drastic variation of specific heat in the buffer layer and the increase in turbulent shear stress after the further deformation of TBL.Flow resistance and convective heat transfer characteristics of supercritical pressure aviation kerosene RP-3 in a horizontal rectangular channel(dh=2mm)were experimentally investigated.The adiabatic friction factors at a fluid temperature range across Tpc under different pressures were obtained,and the effects of free convection and property variation on heat transfer in the laminar and transition flow regimes were studied.The transition Reynolds number range in the rectangular channel was determined as 1700-3200.The adiabatic friction factor at the laminar flow regime was larger than that calculated by the Shah and London correlation and the Troniewski and Ulbrich correlation.At the turbulent flow regime,the influence of surface roughness on adiabatic friction factor could be reasonably predicted using the Kandlikar correlation.The effect of buoyancy-induced secondary flow on heat transfer was non-negligible and there was an obvious difference in circumferential wall temperatures.The buoyancy effect enhanced fluid mixing and improved heat transfer in the laminar flow regime.In the transition flow regime,both the buoyancy and property variation effects were considerable.Heat transfer correlations were developed for the laminar and transition flow regimes in a rectangular channel,respectively.The computational code was expanded to deal with the conjugate heat transfer(CHT)between supercritical pressure hydrocarbon fuel and the surrounding solid structure.Flow and heat transfer behaviours as well as mechanism of supercritical pressure aviation kerosene RP-3 in an asymmetrically heated rectangular channel were studied.The results indicated that there were complex coupling mechanisms between the secondary flow and heat transfer.Due to the considerable fluid density gradient in the cross-section resulted from heating,the secondary flow patterns changed a lot along the flow direction.The developing secondary flow also influenced the heat transfer process.At the center of heated wall,the secondary flow drove hot fluid away from the wall.The thickness of thermal boundary layer increased,and the turbulent thermal diffusivity decreased,resulting in deteriorated heat transfer.In the vicinity of the opposite wall,the secondary flow drove cold fluid from the core region towards channel corner.At its downwash location,the thickness of thermal boundary layer decreased,and the turbulent thermal diffusivity increased,resulting in enhancement of heat transfer.At a fixed total amount of fuel working as coolant,with the increase of the channel aspect ratio,the thermal energy transferred through the lateral wall increased and that through the opposite wall decreased;besides,the intensity of secondary flow also decreased and thermal stratification was more severe.With the increase of solid thermal conductivity,the difference in thermal energy transferred through each wall decreased.The wall heat flux,temperature distributions,and heat sink utilization were more uniform.As a result,the spatial distribution of secondary flow was nearly symmetric,and the intensity of secondary flow decreased.