Dimensionless parameters determining the effect of dilution on ignition delay of syngas and hydrocarbon fuels

Abstract

This study explored the prediction of the effect of dilution on the ignition delay of syngas and hydrocarbon fuels from dimensionless parameters. In this regard, the dilution effect was categorized into two sub-effects. The first is a non-chemical effect, which mainly represents the effect of decreasing the fuel-air concentration and the effect of heat-release damping. The second is the chemical effect induced by participation of the diluent in the reaction pathway. Then, we derived the determining parameters of each sub-effect based on chemical kinetics. The non-chemical effect was found to be related to the normalized sensitivity of the ignition delay to the total concentration, n˜; the ratio of the molar constant volume specific heat of the diluent and the fuel-air mixture, cv,D*; and the average Zel'dovich number during the ignition delay, Z˜em, which represents the effect of temperature rise during the ignition delay on the reaction rate. Meanwhile, the extent of the chemical effect was found to be related to the sensitivity of the ignition delay to the reaction rate coefficient of the three-body reactions, Sτ0−kj, multiplied by the sensitivity of the reaction rate coefficients to the third body concentration, Skj−[M]j, and the ratio of the collision efficiency of the diluent and the fuel-air mixture, ηD,j*. Based on this relationship, we investigated the correlation between the extent of dilution effect and the aforementioned dimensionless parameters, n˜,cv,D*,Z˜em,Sτ0−kj,Skj−[M]j,andηD,j*, under the temperature range of 600–1400 K, pressure of 1–80 bar, and equivalence ratio of 0.5–2 with 13 different syngas or hydrocarbon fuels. Consequently, a strong correlation was found between the dilution effect and the dimensionless parameters, with the deterministic coefficient over 0.95. Furthermore, we suggested a proper form of the empirical correlation for the ignition delay that can cover a wide range of diluent concentrations, considering the extent of the chemical effect appropriately. This study presents the first dimensionless analysis for the dilution effect on ignition delay, developing a general framework for prediction of the effect based on the thermochemical characteristics of a fuel-air mixture.