Numerical study on non-uniform heat transfer deterioration of supercritical RP-3 aviation kerosene in a horizontal tube

The convective heat transfer of supercritical-pressure RP-3(Rocket Propellant 3)aviation kerosene in a horizontal circular tube has been numerically studied,focusing mainly on the non-uniform heat transfer deterioration along the circumferential direction.The governing equations of mass,momentum and energy have been solved using the pressure-based segregated solver based on the finite volume method.The re-normalization group(RNG)k-εturbulence model with an enhanced wall treatment was selected.Considering the heat conduction in the solid wall,the mechanism of heat transfer deterioration and the buoyancy effect on deteriorated heat transfer were discussed.The evolution of secondary flow was analyzed.Effects of the outer-wall heat flux,mass flux,pressure and tube thermal conductivity on heat transfer were investigated.Moreover,the buoyancy criterion and the heat transfer correlation were obtained.Results indicate that the poor flow performance of near-wall fluid causes the pseudo-film boiling,further leads to the heat transfer deterioration.The strong buoyancy has an effect of enhancing the heat transfer at the bottom of tube,and weakening the heat transfer at the top of tube,which results in the non-uniform inner-wall temperature and heat flux distributions.Decreasing the ratio of outer-wall heat flux and mass flux,increasing the pressure could weaken the heat transfer difference along the circumferential direction,while the effect of thermal conductivity of tube on the circumferential parameters distributions is more complicated.When the buoyancy criterion of(Grq/Grth)max≤0.8 is satisfied,the effect of buoyancy could be ignored.The new correlations work well for non-uniform heat transfer predictions.

Deterioration in Heat Transfer of Endothermal Hydrocarbon Fuel

Numerical studies under supercritical pressure are carried out to study the heat transfer characteristics in a single-root coolant channel of the active regenerative cooling system of the scramjet engine, using actual physical properties of pentane. The relationships between wall temperature and inlet temperature, mass flow rate, wall heat flux, inlet pressure, as well as center stream temperature are obtained. The results suggest that the heat transfer deterioration occurs when the fuel temperature approaches the pseudo-critical temperature, and the wall temperature increases rapidly and heat transfer coefficient decreases sharply. The decrease of wall heat flux, as well as the increase of mass flow rate and inlet pressure makes the starting point of the heat transfer deterioration and the peak point of the wall temperature move backward. The wall temperature increment induced by heat transfer deterioration decreases, which could reduce the severity of the heat transfer deterioration. The relational expression of the heat transfer deterioration critical heat flux derives from the relationship of the mass flow rate and the inlet pressure.

Experimental Investigation on Heat Transfer Characteristics of Water in Inclined Downward Tube of a Supercritical Pressure CFB Boiler

In this study, experiments have been performed for an investigation on heat transfer of water in an inclined downward tube with an inner diameter of 20 mm and an inclined angle of 45° from the horizon, with the range of pressure from 11.5 to 28 MPa, mass flux from 450 to 1550 kg/(m2 s), and heat flux from 50 to 585 k W/m2. Based on the experimental data, the temperature distribution in the tube wall was derived. The heat transfer characteristics of inclined downward flow were compared with that of vertical downward flow. The effects of heat flux on wall temperature were analyzed and the corresponding empirical correlations were presented. The results show that heat transfer characteristics of water in the inclined downward tube are not uniform along the circumference from the top surface to the bottom surface. An increase in heat flux exacerbates the non-uniformity. At subcritical pressures, both dry-out and departure from nucleate boiling(DNB) occur at the top surface of the inclined downward tube; inversely, only dry-out takes place on the bottom surface of the inclined downward tube and in the vertical downward tube. At near-critical pressures, DNB and dry-out occur in the comparing tubes with greater possibility. At supercritical pressures, heat transfer gets enhanced in the pseudo-critical enthalpy region; in the high enthalpy region, the top surface temperature of the inclined downward tube decreases obviously.

Numerical Investigation on Convective Heat Transfer of Supercritical Carbon Dioxide in a Mini Tube Considering Entrance Effect

There are more and more researches on heat transfer characteristics and prediction of supercritical CO_(2).The method of adding adiabatic section before and after heating section is usually adopted in these researches to ensure that the fluid entering the heating section is no longer affected by boundary layer,but the appropriate length range of adiabatic section and the influence of entrance effect are not discussed.However,some studies show that the entrance effect would affect the heat transfer in mini tubes.This paper uses the commercial CFD code FLUENT 19.0 to numerically study the heat transfer of supercritical CO_(2) in a mini tube under different working conditions(such as Re_(in),P_(in),q_(w) and flow direction)and the lengths of the adiabatic section(l_(as)/d).The entrance effects on heat transfer is more pronounced when Re_(in) is within the transition state and wall heat flux is relatively high,the resulting heat transfer deterioration causes T_(w,x) and h_(w,x) to rise sharply.As the adiabatic section increases,the location at which the heat exchange deteriorates moves to the entrance of the heating section and eventually leaves.The buoyancy effect and flow acceleration effect caused by the sharp change of physical properties are analyzed,and the dimensionless velocity distribution at the inlet of the heating section in different adiabatic sections is compared.It is proved that the entrance effect has an influence on the convection heat transfer of supercritical CO_(2) in mini tubes.The interaction reflected by wall shear stress between boundary layer development and drastic changes in physical properties is the cause of heat transfer deterioration.