Flow Distribution Characteristics Of Supercritical Cyclohexane In Parallel Horizontal Channels

Active cooling technology is proposed by scholars to solve the thermal protection problem of scramjet.Regenerative active cooling technology use hydrocarbon fuel as cooling medium with a multichannel structure design,and was proved to be one of the most promising thermal protection technologies.The coolant is under supercritical condition,including high temperature and pressure,huge heat flux load,complex phase-change process and its complex physical and chemical properties in regenerative active cooling system.Those characteristics finally led to flow distribution in multichannel deeply relate to heat transfer efficiency,thermal stress distribution and other factors.Therefore,it’s necessary to analysis the flow distribution characteristics thoroughly,for regenerative cooling technology.Based on this background,experimental study on flow distribution triggered by flow excursion in heated parallel tubes has been carried out and cyclohexane was chosen to be the coolant.Experiment results can provide theoretical guidance for the structure design of regenerative active cooling technology of scramjet engines eventually.Firstly,change the mass perturbation direction and system parameters to figure out the onset of flow excursion characteristics.Next,the hydrodynamic multivalue characteristic curve was used to analyze flow excursion in parallel tubes.Flow excursion is considered as the static flow instability in parallel channels.The experimental results show that TypeⅠflow excursion occurs as mass flow decrease,while TypeⅡflow excursion occurs as mass flow increase.A hysteresis phenomenon exists during the flow distribution process because of the different paths of TypeⅠand TypeⅡexcursions.The outlet fluid temperature at TypeⅠand TypeⅡflow excursion is close to the start and the end of boundary of the pseudo-boiling interval.The flow excursion instability boundary was predicted based on experiment data using two dimensionless parameters.Finally,uneven flow distribution triggered by flow excursion in parallel channels has been investigated experimentally.Effect of inlet fluid temperature,system pressure,heat flux value and heat flux ratio of parallel tubes(Q_ratio=Q₁/Q₂)on uneven flow distribution degree was discussed deeply.The results indicate that flow excursion occurring in a single tube can trigger the flow maldistribution between parallel tubes.A higher inlet fluid temperature is favorable for a uniform flow distribution.As parallel tubes are heated uniformly,the effect of system pressure on the flow distribution is negligible and the relationship between the heat flux and flow distribution is nonmonotonic.While as parallel tubes under non-uniform heating condition,uneven flow distribution degree decreased with higher pressure.And increase heating ratio can enhance uneven flow distribution degree.