Numerical Study On Supercritical-Pressure Heat Transfer And Its Dynamic Response Of Hydrocarbon Fuel With Pyrolysis

Scramjet engine is the ideal propulsion system for hypersonic flight,and it is also an extremely challenging research topic for the development of hypersonic vehicles.As the increase of flight Mach number,the crucial issue needs to be solved urgently is the cooling problem of combustion chamber.The regenerative cooling technology,using hydrocarbon fuel as coolant is one of the most effective approach to ensure reliability and durability of the engine.This technology protects engine mainly through the convective heat transfer and pyrolytic heat-absorption of fuel.The operation pressure in cooling tubes during cooling process is usually higher than the critical value of fuel,causing a series of complicated physical and chemical changes.Therefore,prediction of the fluid flow,heat transfer and endothermic pyrolysis characteristics of hydrocarbon fuel at supercritical pressures accurately plays an important role in the design and optimization of scramjet engine cooling systems.In this paper,a high efficient numerical model for simulating supercritical-pressure heat transfer with consideration of pyrolytic reaction of hydrocarbon fuel,which is closely relevant to the regenerative cooling of scramjet engines,is established.The reliability of this numerical model is fully validated.Based on this,numerical studies on a number of interesting phenomena in supercritical-pressure heat transfer processes,including heat transfer enhancement,buoyancy effect,and dynamic responding behavior are conducted.The main research contents of this paper are summarized as follows:(1)To predict efficiently the heat transfer characteristics of the hydrocarbon fuel flowing inside engine cooling channel with endothermic pyrolysis,a fast algorithm based on a one-step thermal cracking mechanism is established for simulating the endothermic pyrolysis and supercritical-pressure heat transfer phenomena of hydrocarbon fuel.First,the three-dimensional look-up table method is created to acquire the thermophysical properties of cracked mixtures.Then,the equivalent one species algorithm is established through the optimization of species transport equation solving processes,and finally only one species transport equation needs to be solved directly.The computational efficiency and accuracy of the fast algorithm are examined by comparing with the available experimental and numerical results.Results indicate that the computational efficiency is improved by about 20 times,with the computational accuracy being equivalent to that of the original algorithm,which solves fully species transport equations.Simulation of the endothermic pyrolysis and heat transfer processes of n-decane flowing inside a three-dimensional rectangular cooling channel under supercritical pressures demonstrates the accuracy of the proposed algorithm and its applicability in practice.(2)Numerical investigations on flow and heat transfer of hydrocarbon fuel flowing inside corrugated tubes with endothermic pyrolysis at supercritical pressures are conducted.The influences and their mechanisms of corrugation on heat transfer,species transport,endothermic pyrolysis and heat sink utilization are analyzed through comparison with smooth tube.Moreover,the thermal performance is evaluated,and the influence of corrugation height is further studied.Results reveal that corrugated tubes can significantly improve the heat transfer ability,and lead to more uniform distributions of species concentration and pyrolytic heat-absorbing rate in radial direction.With the increase of corrugation height the heat transfer enhancement is more significant,while the pressure drop increase simultaneously.An optimum corrugation height exists for achieving the best overall thermal performance.Increase of corrugation height can improve heat sink utilization efficiency,and the heat sink waste of smooth tube can be eliminated efficiently.The velocity fluctuation induced by corrugation is the main mechanism to improve the heat/mass transfer ability and heat sink utilization efficiency.(3)Numerical studies on pyrolysis and heat transfer processes of hydrocarbon fuel in horizontal rectangular cooling tubes at supercritical pressures are performed,focusing mainly on the buoyancy effects on heat transfer and heat flux distribution of cooling tubes at different positions in scramjet combustor.Results indicate that the secondary flow caused by buoyancy leads to significant differences in the heat transfer and heat flux distribution characteristics of cooling tubes at different positions in the combustor.The heat transfer capacity increases at inner surface of the heated wall for cooling tubes on the upper,left and right sides of combustor,compared with the cases calculated without buoyancy effect.Accordingly,the wall temperature decreases and heat flux increases,and simultaneously fuel conversion rate decreases in the near-wall region.For cooling tubes on the bottom of combustor,secondary flow reduces the heat transfer ability at inner surface of heated wall.As a result,the wall temperature and heat flux decrease.The effects of wall heat flux and inlet velocity are analyzed at last.Results demonstrate that buoyancy effect become weaker as the increasing of inlet velocity and decreasing of wall heat flux.(4)Dynamic responding behaviors in flow and heat transfer of hydrocarbon fuel at supercritical pressures are numerically investigated,focusing on the effects of a number of key influential parameters,including the wall heat flux,inlet velocity,cooling tube length,and outlet pressure,on the dynamic responding characteristics.The effects of pyrolytic reaction are further examined.Results indicate that the dynamic responding process is dictated by two fundamental mechanisms:the initial thermoacoustic oscillation,which is caused by strong fluid thermal expansion,and the subsequent unsteady convection.The thermoacoustic oscillating magnitude increases as the wall heat flux and cooling tube length are increased,but it decreases as the inlet flow velocity is increased.The increase of cooling tube length also reduces the oscillating frequency of the thermoacoustie wave.However,the outlet pressure has negligible effect on thermoacoustic oscillation.Moreover,the second-stage unsteady convective process and the total responding time increase as the cooling tube length is increased or the inlet flow velocity is decreased.The pyrolytic reaction,causing obvious flow acceleration and pressure drop,occurs after thermoacoustic oscillation process at lower inlet temperature,and thus has no effect on thermoacoustic oscillation.