Design and simulation of gas burner ejectors

Ignition within gas burner ejectors can lead to off design conditions and has significant influence on the burner behavior.Thus ignition in the ejector should be prevented.In the present study the influence of combustion reactions on the performance of gas burner injectors is investigated.To investigate if ignition is possible,simulated ignition delay times,using a detailed reaction mechanism,are compared to predicted mean residence times of the gas in the ejector.Gas burner ejectors are designed using one dimensional analytic equations,based on energy and momentum conservation equations and conventional isentropic equations.1D results are compared to 2D computational fluid dynamics(CFD)simulations,to take into account non-ideal mixing effects along the ejector.Results are validated with experiments with air at room temperature.1D results show very good agreement not only with CFD simulations for the case of non-reactive flows,but also with performed experiments.It is shown that the assumption of ideal mixing along the ejector and thus the comparison of the ignition delay time to the gas mean residence time,to predict ignition in the ejector,is not valid.Ignition in the ejector is possible,even if the ignition delay time is more than thirty times higher than the mean residence time.In addition to that,it is shown,that ignition and the choice of reaction mechanism have significant influence on the predicted gas burner ejector performance.Thus,the accurate prediction of ignition delay time and the use of a detailed reaction kinetic are mandatory to correctly predict the burner ejector behavior.

Non-isothermal effectiveness factors in thermo-chemical char conversion

Modeling heterogeneous combustion and gasification at large or industry scale is important in ongoing research and development activities.These simulations rely on comprehensive,accurate,and efficient solid conversion models.Pore diffusion is an important sub-process of gas-solid reactions and is often approximated by effectiveness factors.Analytic expressions for isothermal effectiveness factors are usually used,due to the numerical effort of determining non-isothermal ones.The consequences of this simplification are evaluated by determining non-isothermal effectiveness factors for typical combustion conditions for the reactions of carbon with O_(2),CO_(2),H_(2)O,and H_(2).The results show that exothermal reaction rates are under-estimated,while endothermal are over-estimated at low and intermediate temperatures for Thiele moduli between 0.1 and 100.In addition,the reaction zone relocates towards the outer particle layers under these conditions.Cross-sensitivity effects between the four reactions are neglected in this study.A reasonable approximation is the superposition of the individual reactions,because the oxidation reaction is dominant under typical combustion conditions.