Research On Mechanism And Characteristics Of Unstable Combustion

Unstable combustion has a great impact on the performance of the propulsion system whichWorks in the steady state. However, under the control of certain acoustic conditions, unstablecombustion has many advantages over conventional combustion, such as high combustion efficiency,low pollution emissions, improving the working frequency of PDE. In order to explore acoustics andthe relationship between the combustion and acoustics of the unstable combustion, succeed in findingthe technical measures of restraining unstable combustion in propulsion system which Works in thesteady state, a lots of tests were implemented on.The results were gained in the paper as follow:1、The experimental system and model of a valveless pulsejet of the Schmidt-type were designed.The numerical simulation and analysis about the processes of the flow and unsteady combustion withcombustible mixture was made. The test results show that:(1) the variation of pressure in thecombustion chamber and inlet flow.(2) The phenomenon would not appear backflow in the outletwhen pulsejet was stamping, and the outlet would have been at a high temperature state.2、Some experiments investigation were performed on the valveless pulsejet of the Schmidt-typeand Helmholtz-type which could produce pulse combustion with Intermittent air and continuouslyfuel supply and directly making use of the resonant acoustic structure. Experimental results show that:(1) Mixing air and fuel has an affect on combustion zone; the pulsejet can produce pulse combustionwhen combustion zone is located in the head of it.(2) Installation of an evaporator in the combustionchamber can be used to provide energy for heating combustible mixture and evaporation of fuel,which is more prone to pulse combustion.3、The operational characteristics of the valveless pulsejet, such as frequency and pressure weretheoretically and experimentally investigated. The relations of the structure of the combustor, Fuelequivalence ratio, inlet pressure with Flame front pressure and frequency were obtained. The resultsindicated that:(1) The pressure fluctuation over time in accordance with the sine curve, therelationship between pulsation frequency and the length and average speed of sound of pulsejet.(2)Combustion process could drive the pressure pulsation when cyclical changes in the flame front andpressure fluctuation in phase.(3) Burning devices installed to strengthen the combustion chamber sothat the pulse frequency can be increased, but the combustion is not stable enough, the pressurefluctuations increases.(4) Within a certain range, pressure with the increase of fuel equivalence ratio,pulse combustion will be appear hopping phenomena when fuel equivalence ratio is relatively large,Increased inlet pressure, fundamental frequency of the pulse combustion is increased, while reducing the weight of high-order harmonic.4、The model of a Two-Stage Pulse Detonation Engine were designed. The resonant cavity has agrate impact on detonation knock frequency. A numerical simulation about mixed gas compositionand distribution in the resonant cavity was made. In the equivalence ratio of2.5~3, Combustion willproduce more of C, H light molecular compounds in the pre-combustion chamber, then combustionproducts burn in the resonant cavity, if the trigger shock strength is large enough, it will be able totrigger the detonation wave.