Hypersonic cruise vehicles flying over Mach 5 in the atmosphere encounter heating issues at high-temperature due to friction with air and combustion during flight. This challenge can be resolved using a regenerative cooling system that uses hydrocarbon aviation fuel as a coolant. After cooling, the heated fuel enters the scramjet combustor through the injector. In this process, some phenomena, such as cavitation, arises in the injector, and in severe cases, this can reduce the fuel flow rate and affect not only injection and spray characteristics but also fuel control in scramjet engines. Therefore, it is essential to understand the internal flow characteristics of high-temperature fuels in injectors to achieve their precise control. In this study, the flow characteristics of orifice were analyzed using computational fluid dynamics(CFD), based on the conditions adopted in a previous experimental study wherein the temperature was increased up to the critical point.
To analyze the flow characteristics of the injector, thermophysical properties of the fuel in the exposed conditions are required. However, the thermophysical property data of hydrocarbon aviation fuels is defined only in limited conditions. To overcome this limitation, the thermophysical properties were predicted using the Redlich-Kwong-Peng-Robinson Equation of State (RK-PR EoS) and various transport property prediction methods in the operating condition of the regenerative cooling system.
The CFD results obtained using the predicted thermophysical property data adequately reproduced the hydraulic characteristics of experimental data. The trends of discharge coefficient and injection pressure, which changed above the boiling point of the fuel, occurred in both the experiment and CFD. Moreover, the injector’s exit pressure, which could not be obtained through the experiment, was calculated. The exit pressure remained equivalent to the ambient pressure when the fuel injection temperature was below the boiling point. However, above the boiling point, the exit pressure changed to the saturated vapor pressure at the fuel injection temperature. Furthermore, with the application of exit pressure, the discharge coefficient did not show any changes, even over the boiling point.
These results show that injection and saturation vapor pressures of high-temperature fuels affect the hydraulic characteristics of the injectors and should, therefore, be considered in when designing fuel control systems for scramjet engines. Therefore, the obtained data in this study is expected to be useful in developing precise fuel control systems in scramjets for hypersonic cruise vehicles.