The Thermal Characteristics Study Of Valveless Self-excited Pulse Combustor Of Bend Tailpipe Helmholtz-type

Pulse combustion has many advantages, such as high combustion efficiency,high heat transfer coefficient, low pollutant emissions and self-aspiration so on.However, pulse combustion mechanism is very complex and the combustorcomponents are significantly coupling with each other and it has high requirementson operating conditions and structure design for realizing stable pulse combustion,so pulse combustors are usually designed through trial and error. Conventional pulsecombustors of Helmholtz type with mechanical valves have poor operationalreliability, short durability and low heat load adjustment scope due to valves’performance, which limits the development of large-scale pulse combustion devices.Meanwhile, experimental and theoretical researches on pulse combustion mainlylimit to pulse combustors with straight tailpipes. It’s necessary to adapt bendtailpipes for improving pulse combustors’ design and utilizing space efficiently. Anexperimental system of a valveless self-excited pulse combustor of Helmholtz typewith bend tailpipes was established. The thermal characteristics of the pulsecombustor, including the frequency characteristics, operating characteristics, heattransfer characteristics and combustion characteristics, were experimentally andtheoretically investigated. The content and results are as follows:An experimental system of a valveless self-excited pulse combustor ofHelmholtz type with bend tailpipes was established. Mechanical valves werecanceled for continuous air and fuel supply. Different bend tailpipe structures weredesigned. Pulse combustion was produced by the coupling between the acousticstructure of the combustor and combustion heat release and self-adjusted for the heatload of the combustor can be achieved. The experimental results show that theHelmholtz type pulse combustor with a bend tailpipe can produce stable, largeamplitude and low frequency pulse combustion.Pressure characteristics of the combustor were experimentally investigated. Forstable operation, pressure oscillation waves in combustion chamber approximatesine curves and average and amplitude of pressure decrease along tailpipe. Thepressure amplitude in combustion chamber decreases as the angle of tailpipe and islarger if the elbow located at exit of tailpipe than at entrance of tailpipe. If thestructure of combustor is constant, pressure amplitude increases as heat load andexcess air ratio. The NO_xemission characteristics of the combustor wereexperimentally investigated. NO_xemission at the exit of tailpipe increases as theheat load initially and reaches maximum when heat load reaches a certain value, after that decreases as heat load. NO_xemission decreases as excess air ratio. TheNO_xformation mechanism in this experiment is mainly thermal-nitrogen oxides.Combustion temperature and residence time of the flue gas in high temperature zoneinfluence NO_xemission significantly. Generally, there is a reasonable range of heatload and among which NO_xemission becomes low due to short residence time of theflue gas in high temperature zone although heat load and combustion temperatureare very high. It’s instructively significant to reduce NO_xemission for Helmholtztype pulse combustors.Based on fluid network theory, a theoretical model of the valveless self-excitedpulse combustor of Helmholtz type with bend Tailpipe was established usinganalogy circuit method. The model took those factors which might affect thecombustion stability into account. The factors include the structure parameters ofthe combustor, the heat transfer and resistance in the tailpipe. The theoreticalexpression of operation frequency for the pulse combustor was derived. For stableoperation, frequency decreases as the angle of tailpipe and is lower if the elbowlocated at exit of tailpipe than at entrance of tailpipe. If the structure of combustor isconstant, frequency decreases as heat load and excess air ratio. Theoretical andexperimental results agree well with each other. The phenomena of frequencyhopping in pulse combustor with different tailpipe structures occurred when heatload or excess air ratio changed. Increasing excess air ratio can reduce frequencyhopping effectively when combustor operates at high heat load. The structure oftailpipe also influences frequency hopping in pulse combustor significantly. Thetransaction of frequency hopping becomes longer as tailpipe’s angle. Frequencyhopping is more obvious and its transaction is longer if the elbow located at entranceof tailpipe than at exit of tailpipe.The heat transfer characteristics of the pulse combustor were investigated.Results show that heat transfer coefficient of tailpipe is about2-5times as high asstable flow at the same Re and increases as pressure amplitude and frequency. Heattransfer coefficients of bend tailpipes are higher than that of straight tailpipes. Thepulsating flow and heat transfer characteristics of90bend tailpipe pulse combustorwere numerical simulated and obtained by Fluent. The reasons of the enhancementin heat transfer of bend tailpipe were explained.The combustion characteristics of the pulse combustor were numericalsimulated and obtained by Fluent. The possible ignition sources of pulse combustionwere predicted, and the effects of the operating parameters on the pulse combustionwere analyzed. The pressure amplitude in combustion chamber increases as excessair ratio and mass flow rate at the entrance. The temperature in combustion chamber decrease as excess air ratio and increases as heat load. The simulation results andexperimental results have consistent trends.