| Currently,air-breathing pulse detonation engine(PDE)has been widely studied and proven to have a great potential for practical applications.However,some technical still remain involving the design of intermittent valve intake system,rapid formation of the mixed gas,rapid-low-resistance trigger of the detonation wave,and so forth.Under this background,it is significant importance to further enhance the performance of PDE with regards to the inlet valve,the mixing chamber,and deflagration to detonation transition(DDT).In this thesis,a series of work has been focused on the two-phase air-breathing PDE by means of experimental and numerical simulation studies,as follows:(1)The air-breathing PDE with an inner diameter of 80mm was designed and processed,including the inlet,the central cone-scales valve(CCSV),the central cone-blunt valve(CCBV),the spray atomization device,the mixing chamber and its internal components,the ignition chamber as well as the intensifying combustion setting.(2)The experimental platform of air-breathing PDE was constructed,and the force status of the multi-cycle air-breathing PDE process was analyzed to prove that the direct measurement method of piezoelectric force sensor could accurately measure the average thrust of air-breathing PDE.The experimental research on air-breathing PDE spray atomization device and the comparison experimental investigation on the air-breathing PDE of the central cone-scales valve(CCSV-PDE)and the central cone-blunt valve(CCBV-PDE)were carried out.The results showed that the micro-atomization nozzle could achieve good atomization effect under liquid pressure by its unique internal eddy current blade,which satisfied the requirements of air-breathing PDE for spray atomization device.The CCSV-PDE could form a fully enclosed thrust wall in the course of the working process.On one hand,it played a good anti-backflow effect,which was able to prevent the combustion products reverse flow from the intake valve,and thus reduced the thrust loss.On the other hand,it also made the compressed wave completely reflected in DDT process,which increased the peak pressure of the thrust wall and made the thrust wall pressure curve with a wider time domain waveform,and thus improved the theoretical thrust of air-breathing PDE.Under the combined effect of these two aspects,the CCSV significantly improved the effective thrust of air-breathing PDE,that is,the effective thrust of the CCSV-PDE was 2.05 times higher than that of the CCBV-PDE at the operating conditions of 10Hz.(3)The conservation element and the solution element method(CE/SE method)with unstructured triangular mesh was deduced and applied to study the internal flow field of the CCSV-PDE.The results showed that the calculated detonation wave was steep,and its pressure peak and pressure curve were in good agreement with the experimental ones.The calculated pressure cloud at different moments showed that the initial two-phase mixture was ignited and produced compression wave propagating in the upstream and downstream directions.Increasing the pressure near the thrust wall,would result in the close of CCSV and finally formed a fully enclosed thrust wall.When the detonation wave came out of the detonation tube,the series of expansion wave passed into the detonation tube,and gradually reduced the pressure in the air-breathing PDE.When the tube pressure is lower than the total pressure of the inflow,the CCSV opened and the inflow was then flowed into the air-breathing PDE.The calculated process was consistent with the actual experimental process.When the droplet size was between 50-100μm;with the increase of the droplet size,the peak pressure and propagation velocity of the detonation wave would decrease,while the ignition-detonation time and distance would increase.When the oxygen mass fraction was between 30%-40%,with the increase of the oxygen mass fraction,the peak pressure and propagation velocity of the detonation wave would increase,while the ignition-detonation time and distance would decrease.(4)The experiments of the CCSV-PDE were carried out to study the effect of the length-diameter ratio and the internal components of the mixing chamber on the detonation characteristics.The peak pressure and propagation velocity of the detonation wave of the CCSV-PDE increased firstly and then decreased,and the ignition-detonation time of the CCSV-PDE decreased with the increase of the length-diameter ratio of the mixing chamber.The detonation effect reached to the most optimal performance with the length-diameter ratio of the mixing chamber of 2.5.Since the distance between the internal components and the micro-atomization nozzle outlet was too short,the mixing chamber with the length-diameter ratio of 1.25 weakened the atomization effect of the micro-atomization nozzle itself and was not conducive to the realization of the DDT process after it was introduced into the internal components of a small-tube evaporator or a mixing device.Regardless of its successful detonation,the mixing chamber with a length-diameter ratio of 2.5 similarly weakened the detonation effect of the CCSV-PDE due to the heat loss of the small-tube evaporator and the mixing device,which was greater than the improvement in fuel evaporation and blending role.In theory,the increase in length-diameter ratio of the mixing chamber increased the theoretical thrust of the CCSV-PDE,however,it would also inevitably cause the friction between the inner flow on the inner walls,leading to the increase in the flow resistance.The mixing chamber with a length-diameter ratio of 2.5 and without the internal element could acquire the largest effective thrust.(5)A series of experiments were carried out to introduce the influence of the coefficient of oxygen content,the equivalent ratio and the operating frequency on the CCSV-PDE.The results showed that the detonation effect of the CCSV-PDE was improved remarkably after the introduction of oxygen in the groove of the ignition chamber.When the coefficient of oxygen content(β)was 7%,the detonation wave pressure could reach to a 2.41 times higher pressure to the pure air conditions,and the detonation wave propagation speed was also increased by 35.52%.With the increase of β value,the thrust wall peak pressure,the detonation wave peak pressure,the detonation wave propagation velocity,the theoretical thrust and the effective thrust of the CCSV-PDE increased,whereas the ignition-detonation time reduced.The CCSV-PDE would form a continuous combustion when β was greater than the critical value.The introduction of oxygen in the groove of the ignition chamber would form a relatively high oxygen content in the vicinity of the ignition head,which reduced the ignition temperature of the fuel mixture and caused the mixture to be ignited earlier.Meanwhile,the increase in the oxygen content also increased the speed,strength and heat transfer efficiency of the combustion,and shortened the time and distance of the DDT.When the coefficient of oxygen content was 7%,the CCSV-PDE has been successfully detonated(seven circular annular obstacles only);the peak pressure and propagation velocity of the detonation wave were found as high as 1.885MPa and 1366.8m·s-1,,respectively.When the equivalence ratio was between 0.97 and 1.26,the peak pressure and the propagation velocity of the detonation wave as well as the ignition-detonation time greatly increased with the increase in the equivalence ratio,whereas the effective thrust exhibited an initial raise and a subsequent decline.Moreover,in the range of 8-18Hz,the effective thrust increases almost in a linear way with the increase in operating frequency,while the ignition-detonation time would be shortened.In addition,the average thrust for single waveform is rather comparable,within a narrow range of 5.2-5.5N.In this thesis,the inlet valve,the mixing chamber,and how to rapid-low-resistance realize DDT of the air-breathing PDE were studied.The research was significant importance for the improvement of the performance of PDE,so as to realize the engineering application. |
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