Research On The Start-Up Proccesse Of Scramjet Combustor

The Scramjet (Supersonic Combustion Ramjet) propulsion, utilizing the oxygen of air instead of carrying oxdizer itself, performes higher performance and lower operation cost than that of rocket engine at high flight Mach number, will show its potential in aero-space transportation system, cruise missile or commercial transportation.The start-up (igniton) problem of the scramjet combustor at low Mach number is one of the toughest problems. When flying at low Mach number (Ma4), the static temperature of supersonic flow in combustor is lower than the temperature of fuel self-ignition. Some addition technique should be imported to facilitate the igntion. Ignition technique and fuel atomization (for liquid fuel) are two parts than can’t be avoided when facing the igntion probblem at low Mach number and also the subjects this paper focus on.Experimental research on ignition technique at low Mach number is performed on directly-connected pulse facility. The experimental inflow parameters are as following:935K of total temperature,0.8Mpa of total pressure, Mach number2.1at the nozzle exit and stable experimental time of200ms-250ms. Experiments covering cold flow experiments, fuel mixing in cold flow, five methods on combustor ignition, lean blowout limit and rich operation limit experiments for scramjet and fuel injection scheme optimization have been performed.Characteristic of cold flow in combustor has been studied by employing experimental and numerical techniques, results from numerical simulation and high speed schlieren measurement indicates that the shock waves is stronger in the combutor configuration with the cavity which with D(depth)=18mm and L/D(length/depth)=10.8, obvious boundary layer seperation is located on the bottom wall near the ramp of cavity and induces biggest resistance. The coldflow resistance is related to the L/D ratio of cavity, resicatnce of combustor with L/D=7is only half of that with L/D=10.8and10. Fuel injection penetration at wall injection scheme is lower than5mm. Fuel injected from cavity floor and upstream of cavity most distributes in cavity. Air throttle downstream the cavity improves the penetration and mixing efficiency of the fuel injected from top wall, but has little influence on the fuel injected from bottom wall.Combustor configuration with cavity (D=18,L/D=10.8) is employed for test model. Igniton at Mach4achived with the assistant of self-igniton of pilot hydrogen, igniter combined pilot hydrogen and air throttel combined pilot hydrogen. Pilot hydrogen injected from cavity floor at mass flow rate of0.43g/s to12.68g/s will be self-ignited, and then ignite ehtylene, if the mass flow rate exceedes17.27g/s, self igniton won’t occur. The ignition method by igniter and pilot hydrogen is the most reliable igniton technique, with igniter power no less than20kW, hydrogen equivalence ratio higher than0.05, hydrogen injection time longer than50ms and igniton time longer than20ms, ethylene ignition could succeed. When air throttle and pilot hydrogen flame were employed, to reliaze ethylene igniton the throttle should be locateed at875mm from isolator entrance, the mass flow rate of throttle should not be less than10%of engine inflow mass flow rate, and the hydrogen injection pressure should be5.0Mpa at lest.Lean blow-out limits are similar with fuel injected from cavity floor and uptream the cavity. But rich operation limits showes obvious differences, when fuel injected from cavity floor, the rich operation limit of engine is at eqivalence ratio of0.471, when injected upstream the cavity, the limit is at equivalence0.327. The parameter of cavity have strong effect on igniton performance, experimental results indicate that cavity with L/D=7performes worse igniton characteristic than L/D=10and10.8. fule injection schemes research offered a equivalence ratio distribution rule that fuel injection befor the exit of cavity should be less than equivalence ratio0.2which could avoid unstart of intake, and fuel injected downsteam the cavity should insure that will be ignited, ortherwise there is no comtribution to the engine’s performance. A relative optimiazed injection scheme obtained by experimetal method, that ethylene injected from5different locations, achived total equivalence ratio0.85and1401.1N thrust for the combustor.Igniton and flame propagation processes have beed record by high speed photograph. Records show that when hydrogen injected from cavity floor at high mass flow rate, the majority will enter the core flow directly and no self-igniton will occure. When employing air throttle for igniton, the throttle time should be shut before the pressure in combustor spread to the isolator entrance.Areated-liquied injection is an efficient injection scheme for liquid fuel aomization. Injection at gas-liquid ratio1.36%could achive SMD smaller than60μm which is much smaller than that of pure liquid injection. Higher gas-liquid ratio and smaller injector diameter is beneficial to reduce droplet diameter. The smallest SMD of40μm achived at gas-liquid ratio15.15%with injector diameter lmm. also the mean velocity of the droplets are increased by the aerated-liquid jet which shows the possibility to increase the injection penetration in supersonic flow.