| The regenerative cooling technology,which uses an endothermic hydrocarbon fuel as the cooling agent,is a promising approach for thermal protection of scramjet engines.During a cooling process,the fuel is circulated in micro cooling channels surrounding the combustion chamber prior to its injection and combustion.The operation pressure is generally higher than the critical pressure of fuel,consequently leading to fluid flows and heat transfer at supercritical pressures.In supercritical-pressure heat transfer,as the fuel temperature increases,the phase transition phenomenon vanishes,but drastic variations of thermophysical properties still remain,particularly in the trans-critical region.Moreover,pyrolytic chemical reactions occur in the high-temperature region,which convert part of the heat into chemical energy and thus effectively improve the cooling process.The strong variations of thermophysical properties and the endothermic chemical reactions make a regenerative cooling process very complex.Therefore,fundamental studies of the turbulent heat transfer and endothemric fuel pyrolysis of a hydrocarbon fuel at supercritical pressures become very important for design and optimization of the regenerative cooling system in scramjet engines.In this paper,a mathematical model has been developed for numerical simulations of turbulent heat transfer of aviation kerosene RP-3 at supercritical pressures,with consideration of detailed pyrolytic chemical reactions.The physical process is closely related to the regenerative cooling of scramjet engines.The accuracy and reliability of the model are fully validated through comparisons with a series of experimental data.Based on the validated model,detailed numerical studies are conducted and summarized as follows:First,numerical studies of fluid flows,heat transfer,and fuel pyrolysis of aviation kerosene RP-3 in a mini cooling tube at supercritical pressures are carried out.The effects of pyrolytic chemical reactions,inlet flow velocity,and surface heat flux on the supercritical-pressure heat transfer are examined in detail.Results indicate that significant chemical reactions occur once the fuel temperature reaches around 800 K.The endothermic pyrolysis can effectively improve the fuel’s heat-absorbing capacity and thus drastically reduces the fuel’s temperature or increases the surface heat flux.At the inlet section,as temperature increases in the near wall region,the fluid viscosity decreases drastically,resulting in flow transition from laminar to turbulent and consequently leading to enhanced heat transfer.In the trans-critical region,fuel pyrolysis causes dramatic variations of thermophysical properties.The decreased fluid density then leads to the significantly increased flow velocity.Heat transfer is improved at a low surface heat flux.Decreasing the inlet flow velocity and increasing the surface heat flux can both lead to the increased fuel temperature and thus the increased fuel thermal decomposition.In the high temperature region,endothermic fuel pyrolysis plays a very important role in the heat transfer process,and more than 50%of the heat is converted into chemical energy through the pyrolytic chemical reactions.Second,turbulent heat transfer of the aviation kerosene RP-3 in ribbed cooling tubes at supercritical pressures are numerically simulated.The effects of rib height and solid thermal conductivity on heat transfer are studied.Results show that ribs can effectively improve the heat transfer process.At the entrance,because of the low Reynolds number and weak heat transfer,the effect of ribs on heat transfer enhancement is particularly significant.The main reasons are that the ribs change the flowfield in the near-wall region,resulting in the low-temperature fluid impinging on the wall and a thin thermal boundary layer;in the meantime,the generation of radial velocity further improves heat exchange between the wall and the central fluid.Increasing the rib height can improve heat transfer,but it also increases pressure loss.A thermal performance factor is applied to evaluate the overall effect.It is found that an optimal rib height exists for maximizing the overall thermal performance.The solid thermal conductivity only influences temperature distribution in solid region.As the solid thermal conductivity increases,the thermal resistance in the solid region decreases and the temperature at the exterior surface is reduced.Third,numerical simulations of supercritical-pressure fluid flows and heat transfer of aviation kerosene in horizontal cooling tubes are carried out,under the operating condition of electric heating.The buoyancy effect is analyzed at different inlet flow velocities and heating currents.The effect of two heating mothods,electric heating and equivalent wall heat flux,on the heat transfer process is first compared.Results show negligible differences.Under a buoyancy force,the low-temperature fluid with high density sinks to the bottom of the cooling tube,thus improving heat transfer and resulting in lower temperature on the bottom surface than that on the top surface.Compared with cases calculated without consideration of buoyancy effect,temperature on the bottom surface is significantly reduced,particularly in the inlet section,in which heat transfer deterioration is effectively suppressed.The wall temperature on top surface also shows a significant decrease in the entrance region,caused by heat flux redistribution in the solid region.Under the buoyancy effect,the lower temperature in the bottom region causes a downward heat flux in the solid material,which consequently results in the decreased heat flux on the top surface and reduced surface temperature.Increasing inlet velocity and decreasing heating current render the buoyancy effect weaker.Both formulations of Gr/Re2.7 and Grq/Grth fail to correctly identify the buoyancy effect in fluid flows and heat transfer of the aviation kerosene RP-3 in horizontal tubes at supercritical pressures.Further studies in this area are still needed. |
PDF Full Text Request