Simulation Study On The Spray Atomization And Combustion Of Biodiesel Stirling Engine

The Stirling engine has excellent high-efficiency and low-emission characteristics,low vibration and noise,and wide energy adaptability,especially as underwater power,which has spacious application outlooks.In this paper,the spray characteristics and Oxy-fule combustion performance of biodiesel Stirling engine injection were studied by numerical simulation.The simulation model of the injection,spray,ignition and combustion of a certain Stirling engine burning biodiesel was established,and the chemical reaction mechanism of biodiesel was simplified by decoupling method.Compared with the experimental results of spray and combustion,the simulation model established in this paper has sufficient prediction accuracy.The effects of ejection ratio,back pressure,injection pressure and load on biodiesel spray characteristics were studied.Increasing the ejection ratio can promote the diffusion of the spray droplets into the circumferential direction and the radial direction,thus increasing spray cone angle and spatial distribution of fuel mixture,which improves the mixing quality of oil and gas.Increasing the back pressure inhibits the movement of the droplet group,thereby reducing the penetration distance of the spray oil and reducing the distribution range of the spray droplets;and an excessive back pressure will cause an increase in the SDM of spray.With the increase of injection pressure and the load,the spray penetration distance and spray cone angle is increased,the spray particle size is reduced,the space of the spray droplets in the combustion chamber can be uniformly distributed,and the atomization can be improved.The effects of starting conditions on the start-up of the biodiesel Stirling engine and the initial flame propagation were studied.The effect of equivalence ratio on the ignition delay is mainly reflected in the NTC region.The larger the equivalence ratio,the lower the ignition delay time;with the increase of ejection ratio,the CO₂ inert effect is enhanced,and the ignition delay time is bigger;compared with the cold start,the shutdown start has a larger ignition delay time.But considering the fact that it takes less time for heating rod to heat the mixture till ignition occur,the ignition is faster when the machine is stopped and restarted.Under the cold start condition,the flame propagation speed changes sharply with the equivalence ratio,and when the machine is stopped and restarted,the flame propagation speed becomes softer with the change of the equivalence ratio.It is beneficial to reduce the noise during the engine startup phase due to the low sensitivity of the flame propagation speed to the unburned mixture temperature and the equivalence ratio under high CO₂ concentration environment.The effects of fuel injection quantity,injection pressure,back pressure,excess oxygen coefficient,ejection ratio and gas temperature on biodiesel Stirling engine combustion characteristics were studied.The results show that the higher the load,the high temperature region in the combustion chamber becomes larger,but the maximum temperature change is not obvious,the airflow velocity in the combustion chamber is obviously increased,the convection of the combustion chamber is enhanced,which is beneficial to improve the thermal efficiency;with the increase of ejection ratio,the combustion chamber temperature is lowered,which improves temperature distribution uniformity,enhances heat mass transfer and reduces heat load of flame shield;with the increase of back pressure,the burning becomes more fully,oxygen demand is reduced,thermal efficiency improves;increasing excess oxygen coefficient greatly promotes the reaction reactivity in the combustion chamber.Excessive excess oxygen coefficient will cause the temperature in the combustion chamber to rise sharply,and the combustion to become unstable,which is extremely damaging to the equipment.Increasing the temperature of the recirculated gas can improve the uniformity of the temperature distribution in the combustion chamber.For the Stirling engine fueled biodiesel,by increasing the injection ratio to 7.4 and controlling the recirculation gas to823K at back pressure of 2.0 MPa and an excess oxygen coefficient of1.08.A low temperature uniform combustion can be achieved.Finally,the effects of load,injection pressure,ejector ratio and recirculating gas temperature on the thermoacoustic instability of Stirling engines burning biodiesel were investigated.As the load increases,the resonant frequency of the Stirling engine's thermoacoustic instability increases and the pressure amplitude increases.As the injection pressure increases,the thermoacoustic unstable resonant frequency increases and the pressure amplitude increases,but there is a threshold for its effect on combustion.Increasing the ejector ratio increases the mass flow rate of the recirculated gas,changes the concentration of each component of the gas,and thus changing the heat release rate of the fuel swirl combustion,destroying the heat release rate of the combustion and the sound wave.Coupling changes the frequency of thermoacoustic vibration and reduces the amplitude of thermoacoustic vibration.Increasing the temperature of the recirculated gas reduces unstable combustion,thereby reducing the intensity of thermoacoustic instability,and suppressing thermoacoustic vibration.