Researches On Turbulent Heat Transfer Mechanism And Thermal Cracking Of Supercritical Pressure Fluids

Based on the regenerative cooling technology applied in supersonic combustion engine,the convective heat transfer and thermal cracking of supercritical pressure fluid,especially supercritical pressure hydrocarbon fuel,were studied by numerical simulation and experimental study.This work is aimed to provide basic theoretical support for the thermal protection system and regenerative cooling technology of the hypersonic aircrafts.The convective heat transfer of supercritical pressure CO₂ in a vertical pipe was studied by direct numerical simulation.Due to the strong buoyancy near the wall for upward flows,the radial gradient of the axial velocity becomes smaller in the core area and the axial velocity gradually becomes flat,which then changes to M-shaped.Reynolds stress reduced to zero and then becomes negative.The production and turbulent kinetic energy first decrease to a minimum due to Reynolds shear stress and axial velocity,then gradually recover.The convective heat transfer coefficient basically depends on the magnitude of turbulent kinetic energy.The relaminarizetion reduces both the turbulent kinetic energy and the convective heat transfer coefficient.The transient vorticity field can reveal this process intuitively.The flow and heat transfer instability of supercritical pressure n-decane in a vertical pipe were investigated and analylized.Seven stability stages were observed.The main reason for the unstable stage is the transition of turbulence and dramatically variable thermal properties.The former is closely related to turbulence and the latter belongs to Helmholz-type oscillation in nonlinear dynamics.Different stability stages have different oscillation frequency,sound and amplitude characteristics.Buoyancy is in close relationship with Helmholz-type density wave oscillation.Improving the pressure,increasing the inlet temperature or using downward flow will effectively enhance the stability of the system.The convective heat transfer in a vertical pipe of two kinds of engineering fuels EHF-? and EHF-? were experimentally studied.For relatively low inlet Re,the buoyancy becomes significant especially at high heat fluxes,so that relamilarization and heat transfer deterioration in upward flows were observed,with wall temperature soaring and producing a peak.The threshold of Bo*was found to be 3×10^-7 for the EHF-? fuel.The EHF-? fuel leads to heat transfer deterioration earlier than EHF-?,but the maximum wall temperature was lower than EHF-?.The heat transfer correlation was obtained according to the experimental data of EHF-? and EHF-? with strong buoyancy.The thermal cracking of EHF-? fuel under supercritical pressure was studied.Thermal cracking mainly produces alkanes,alkenes,naphthenes and aromatic hydrocarbons.The main factors affecting the conversion are reaction temperature and residence time.The products were found to be of three types.A proportional product distribution model was developed at low conversions according to experimental data.The thermal cracking generates extra chemical heat sink to enhance convective heat transfer as well as generates bubble layer to weaken heat transfer.Therefore,the overall effect is that the thermal cracking enhances convective heat transfer to some extent.