Study On The Influence Of Combustion-preheating Vitiation On Hypersonic Aero-propulsion Performance

Working flow medium should be preheated to flight condition enthalpy in ground testing of the integrated aero-propulsion performance of an air-breathing hypersonic ve-hicle. Owing to its low cost, efficient operation and particular capability of true flight enthalpy simulation, a combustion wind tunnel meets the demands of current large-scale experiments and hence is considered one of the main facilities in this research field. However, in the preheating process, the combustion products such as H2O and CO2 form part of the test gas and cause its composition to differ from that of real air, which is known as "vitiation" or "contamination". Vitiation would result in the dif-ference of the physical and chemical properties of test gas from real flight conditions and, as a result, there would exist some uncertainty in applying the ground-test perfor-mance to true flight. Moreover, not all the freestream flow parameters of true flight can be duplicated in tests in a combustion wind tunnel. Previous studies of vitiation problems often focused on the effect on chemical reaction mechanisms and the scram-jet combustor performance, while the "pure" gas-dynamic influence of vitiation was scarcely mentioned. In the present study, the combustion-preheating vitiation effect on hypersonic aerodynamic characteristics and its mechanism are investigated by the-oretical analysis, numerical simulation and experiment measurement. Then, multiple freestream-parameter matching schemes are compared and accessed in their capability of reliably reproduce the aerodynamic compression characteristics of simple configu-rations, which are introduced to characterize different type and strength of local aero-dynamic compression induced by flow around a hypersonic vehicle. A new type of matching scheme, which is helpful for simulating the aerodynamic performance of a hypersonic vehicle, is proposed. To clarify the difference of propulsion performance between combustion wind tunnel test and true flight, the scramjet performance in typ-ical vitiated flows and matching schemes is investigated. Based on this, a conversion method of ground-predicted propulsion performance to flight, which may improve the reliability of applying the combustion wind tunnel test, is suggested.Firstly, the vitiation effect in aerodynamic test is evaluated. By numerically sim-ulating the flow around typical models, the vitiation effect on viscous flow is clarified. It is found that the contamination make little difference in the hypersonic momentum boundary layer as well as the friction coefficient and its impact on the thermal boundary layer and the wall heat flux is also weak. As for its influence on the thermodynamic parameters, the nozzle flow and aerodynamic compression process are discussed. For nozzle flow, it appears that vitiation decreases the testing flow Mach number and in- creases the static temperature, compared to clean air case, for the same facility. For the aerodynamics of the test model, vitiation tends to weaken the compression strength, and hence causes the post-wave temperature and pressure as well as the aerodynamic coefficient to be lower than that in clean air flow. As a comprehensive embodiment of vitiation effect on aerodynamics, its influence on the startability and performance of a hypersonic inlet is numerically investigated. It is found that both H2O and CO2 vitia-tion components are conducive to its starting by numerical study of the axisymmetric scramjet inlet flow, while, for conditions far from the starting limits, the vitiated air simulation may reproduce the inlet performance fairly well. This suggests that when the incoming flow condition approaches its starting limits, test results of an hypersonic inlet in a combustion wind tunnel may not be completely relied on and need to be care-fully reconsidered. A further analysis of the mechanism of vitiation effect reveals that it is mainly the increase in the heat capacity or equivalently the decrease in specific heat ratio that account for the aforementioned results.Secondly, on the basis of sufficient understanding of vitiation effect on aerody-namic characteristics and its mechanism, efforts are made to improve the reliability of aerodynamic testing in a combustion wind tunnel by adjusting the freestream parameters to match in vitiated flow. For this purpose, a rapid theoretical approach is developed to assess multiple freestream-parameter matching schemes based on simple compression configurations and basic aerodynamic laws. By comparing aerodynamic characteris-tics of oblique shock and isentropic compression configurations in different matching schemes, it is found that for shock and weak isentropic compression, the deviation of nondimensional aerodynamic parameters in the A/-matched schemes is less than 2%. Synthesizing the post-wave thermodynamic parameters, the hoPM and TPM matching schemes are more acceptable. However, vitiation effect would be more pronounced along with the enhancement of the isentropic compression, and hence the difference of aerodynamic characteristics between vitiated and clean air would be much larger. To re-produce the aerodynamic characteristics closer to real flight, a new scheme is proposed, which relaxes restrictions on the matching of Mach number and simultaneously matches the total pressure and dynamic pressure. In this matching scheme, the freestream Mach number is automatically reduced to some extent, which would weaken the vitiation ef-fect induced by lower specific heat ratio. Calculation reveals that the Po^-matched scheme is beneficial for better reproducing the nondimensional aerodynamic parame-ters in strong compression, for either the isentropic or the oblique shock case. Consider-ing both the aerodynamic characteristics and the post-wave thermodynamic parameters (which may serve as the entrance condition of the combustor), the hoP0Q matching scheme is suggested.Lastly, two typical sets of freestream-parameter matching schemes, of which one simulates total enthalpy, dynamic pressure and Mach number (known as h0QM) and the other simulates static temperature, static pressure, Mach number (known as TPM), are compared based on quasi-one-dimensional numerical simulation with chemical reac-tion, in terms of reproducing reliable scramjet propulsion performance. It is found that for the hydrogen-vitiated flow, it is easier to select the TPM matching scheme to better reproduce the combustor pressure distribution in real flight for identical fuel equivalence ratio if no local thermal-choking-induced shock structure is evident in the combustor. If thermal choking is likely to occur, the h0QM matching scheme could reproduce the flow pattern more reliably. For wind tunnels with hydrocarbons burnt for preheating, the incoming flow parameters as well as the engine performance appear nearly the same in the two matching schemes. In this case, the vitiation effect is aggravated so that it is more difficult to reliably reproduce the flow pattern by adjusting the freestream pa-rameters, especially when the fuel equivalence ratio goes higher. For the thrust of the engine, with hydrogen vitiation and identical fuel equivalence ratio, h0QM matching scheme delivers an overestimated thrust, in contrast to the decent agreement of TPM matching scheme with clean air. Good agreement of the engine thrust with that in clean air could be achieved by adjusting the fuel mass flow rate for both the two matching schemes. Further analysis reveals that the disparity of the ratio of combustion heat re-lease to the incoming flow total enthalpy (defined as combustion heat release ratio) in the two matching schemes contributes heavily to the aforementioned differences. To locate the correlation of the engine performance in obtain in wind tunnels adopting different matching schemes and using varying fuels burnt for preheating, the combustion-induced thrust is normalized based on the heat release ratio. It turns out that the combustion-induced thrust gains in unit heat release ratio for varying combinations of matching schemes and preheating fuels are almost at the same level with that in clean air. This indicates that the reliability of conversion of ground-predicted propulsion performance to flight may be enhanced based on specific-combustion-heat-release-ratio thrust gain.