Density Measurements Of Endothermic Hydrocarbon Fuel Using Random Temperature Signal Cross-correlation

Hypersonic flight has a qualitative improvement in speed,but it also brings some technical problems,for example,in the process of flight,huge aerodynamic and thermal loads will be generated on the surface of the aircraft,which will adversely affect the material endurance and aircraft structure.In order to ensure the stability and durability,effective cooling of the aircraft is required.Regenerative active cooling technology,which uses the fuel carried by the aircraft itself as the coolant and does not need to increase the weight of the aircraft,has received extensive attention in recent years.Endothermic hydrocarbon fuel is easy to be decomposed into small molecules at high temperature,which not only increases the heat absorption of fuel,but also is conducive to combustion,and has become the best fuel for hypersonic aircraft propulsion and cooling.In order to use this kind of fuel more safely,the acquisition of its physical parameters becomes a prerequisite for the optimization design of cooling structure.Density is one of the basic thermophysical parameters,and its accurate measurement under supercritical pressure has an important influence on the research of convective heat transfer characteristics and the calculation of other physical quantities.It is difficult for the traditional densitometer to accurately measure the density of a new supercritical endothermic hydrocarbon fuel in real time.A new densimeter,which is based on the mass conservation equation and cross-correlation method,was designed and constructed.Two K-type thermocouples with the same thermoelectric characteristics were installed in the upstream and downstream of the test tube with a distance of 148 mm as the cross-correlation sensor.The delay time was found using the cross-correlation between upstream and downstream signal which were measured by the thermocouples.So the mean flow velocity could be calculated by the delay time and distance.With the flow velocity and mass flow rate known,the density could be determined based on the mass conservation equation.The n-heptane,n-decane and the binary mixture of n-octane and n-heptane with the mass ratio of 1:1 were tested for the system calibration.According to error analysis,the maximum relative deviations were below ±1.2%,and the absolute average relative deviations were within 0.7%.Besides,the uncertainty of this method was 1.83%.The mainly restricted condition of the density measurement method,is the effect of Reynolds number on flow velocity correction factor K.Multiple independent and equal precision measurements were conducted at room temperature to acquire the correction factor.After averaging,K=0.677 can be obtained.On the basis of calibration experiments,densities of endothermic hydrocarbon fuel EHF and 851-2 were measured at the temperature range of(302.0-529.6)K and pressures of(3.0-5.0)MPa.Under this temperature and pressure,the density of the two basically decreases with the increase of temperature and increases with the increase of pressure,and the effect of pressure is not obvious.At room temperature,the density of 851-2 is slightly higher than EHF.The application of this method provides a new thinking of the density on-line measurement of endothermic hydrocarbon fuel under supercritical pressures.