Heat Transfer And Hydrodynamic Characteristics Of Hydrocarbons At Trans-and Supercritical Pressures

The thermal protection of scramjet has been considered as the bottleneck that restricts its development.The severe thermal environment is very intense and it brings great challenges to the materials and the flight safety.Active regenerative cooling technology applied in scramjet has been considered a promising thermal protection method owing to its great advantages of strong heat absorption,simple structure,high combustion efficiency and recyclable.This technology uses kerosene to flow through the mini-tube on the heating surface as the refrigerant.In this progress,mal-distribution and flow instability are easy to occur in the channels,which seriously affects the heat transfer effect.By summarized the researches on the flow instability,it is found that the static instability characteristics and heat transfer process for hydrocarbons still need further investigations.In this paper,the static instability and heat transfer characteristics of hydrocarbons are studied experimentally in single tube under trans-and supercritical pressures.Firstly,using cyclohexane as working fluid to investigate the effect of flow direction on the onset of flow instability.In order to provide theoretical basis for further research on the coupling characteristics of static instability and heat transfer.Upflow,downflow and horizontal flow were used to study the hydrodynamic characteristics.The experimental results show that the supercritical fluid has hydrodynamic characteristics in both vertical and horizontal tubes;the horizontal tube is more prone to static instability and it is caused by redial buoyancy.The buoyancy affects the thermal boundary layer and significant affects the heat transfer process.The correlations based on dimensionless numbers were determined to predict the onset of static flow instabilities for both vertical and horizontal tubes,which indicate that the heat absorption capacity,tube geometric structure and buoyancy are strongly correlated.Secondly,selected the horizontal tube as test tube.Cyclohexane,pentane and their binary mixtures are used to study the hydrodynamic and heat transfer characteristics under trans-and supercritical pressures.The experimental results show that the regularities of mixtures on hydrodynamic characteristics are consistent with pure substance.With the increase of the proportion of more volatile substances,static instability is more likely to occur;the heat transfer process is the same for subcritical and supercritical pressure,but the extreme value of heat transfer coefficient is different.The occurrence of extreme value corresponds to the onset of flow instability.The heat transfer performance for mixtures was always weaker than for pure substances owing to the mass transfer resistance.Finally,the visualized method is used to verify the similarities and differences between pure substance and mixtures.The visualization results showed that the flow pattern and evolution of mixtures were similar to those of the pure substances,and the flow patterns were between those of the pure substances because the physical properties were between those of the pure substances.The change of flow pattern proved that the hydrodynamic characteristics had a coupling relationship with heat transfer characteristics.Correlations for predicting the static unstable area of hydrodynamic characteristics for mixtures were correlated for both subcritical and supercritical pressures.