Experimental Measurement And Kinetic Modeling Study Of Ignition Delay Time For Endothermic Hydrocarbon Fuels

Endothermic hydrocarbon fuels are designed to solve thermal management in advanced aircraft due to their good endothermic capacity and the ability to provide extra chemical heat sinks by undergoing endothermic reactions.Prior to combustion in the combustion chamber,firstly the fuel exchanges heat in the cooling channel and then experiences thermal decomposition to produce smaller hydrocarbon species(such as ethane,ethylene)and hydrogen with good combustion performance.So it is a mixture of the pyrolysis products and unreacted fuel(marked as cracked fuel)together into the combustion chamber that undergoes combustion.Compared with conventional fuels,cracked fuels include not only unreacted fuels,but also stable products and free radicals.The complex diversity of products and the difficulty in capturing the free radicals make it difficult to measure the ignition delay time of cracked fuels.In this paper,with n-decane selected as a surrogate of endothermic hydrocarbon fuel,ignition delay times of n-decane,cracking gas and the mixture of these two(marked as stable cracked n-decane)were measured by shock tube.In addition,theoretical study of ignition delay time for cracked n-decane was further investigated based on these.Based on electrically heated tube experiments,components of cracking gas and cracked n-decane at different conversions(17.61%,37.97%,62.15% and 81.04%)were determined.The ignition delay times of n-decane,cracking gas and stable cracked n-decane in the temperature range of 1296-1915 K,pressure of 0.1-0.2 MPa,equivalent ratio of 0.5-2.0 were measured by shock tube.Both n-decane and cracking gas are more favorable for ignition under fuel-lean/higher pressure,which is also in good agreement with the prediction by mechanisms.Furthermore,ignition-delay correlations were obtained for them under the current conditions.For cracked n-decane,the ignition delay time of cracking gas is larger than that of n-decane.Thermal cracking could improve the ignitability at T > 1480 K for x = 37.97%,x = 17.61% and at T < 1480 K for x = 62.15%,to a limited degree.On the basis of the simulation of n-decane pyrolysis,the effect of pyrolysis pressure and pyrolysis conversion,equivalent ratio and pressure,and free radicals on the ignition delay time of cracked n-decane were investigated theoretically.It was found that the ignition delay time of cracked n-decane decreased with the increase of the conversions.The ignition delay time of cracked n-decane at pyrolysis pressure of 5 MPa was shorter than that of 3 MPa.Fuel-lean/higher pressure are beneficial to ignite.The presence of free radicals significantly reduces the ignition delay time of cracked n-decane.Sensitivity analysis,free radical concentration and ROP analysis were conducted to analyze the reasons.