Laminar flame properties correlations for H2/C3H8 mixtures at high temperature and pressure conditions

Abstract

Laminar burning velocity (SL) and flame thickness correlations for H2, C3H8 and their mixtures at autoignition conditions for the study of ignition regime were developed. The thermodynamic conditions covered by the correlations are 750–1100 K for the unburned temperature and 5–60 atm for the pressure, typical of weak/mixed ignition regime occurrence. The data for fitting the correlation were generated using a kinetic model with a detailed reaction mechanism. A new definition of energy mixture fraction as a weighting factor in Le Chatelier's mixing rule for H2/C3H8 was proposed to SL calculations. The expression for the flame thickness calculation was developed considering the correction by the radical diffusion from the reactive zone to the preheating zone. Experimental tests and Schlieren image recording inside the combustion chamber from a Rapid Compression Machine were carried out to evaluate the Sankaran Prediction criteria (Sap) capability to predict the ignition regime. Average absolute relative residual values of 4.36% and 0.91% and the total points with an accuracy of 10% were 92.57% and 99.43% evaluating the test data group in the proposed correlations were obtained for H2 and C3H8, respectively. The new version of energy mixture fraction allows an increase in the amount of data with an accuracy of 10% from 36.4%, 34.3% and 19.2% to 96.4%, 96.6% and 75.6 % for a content of 75%, 80% and 95 % of H2 in volume, respectively. The Sap values are in agreement with observations in the RCM, where deflagrative flame fronts were identified prior to volumetric autoignition, confirming the weak/mixed ignition regime. The proposed formulations are suitable to be used in the Sap calculations and ignition regime prediction as well as to determinate the SL and flame thickness at autoignition conditions.