Laminar burning properties of lean CH4-H2-air mixtures at high pressure

Experimental evaluation of laminar burning characteristics of gaseous fuels has been carried out, analyzing spherical expanding flames. Tests were performed within the framework of the DHARMA (Device for Hydrogen-Air Reaction Mode Analysis) initiative at Istituto Motori - CNR. Based on a high-pressure, constant-volume bomb, the project is aimed at populating a systematic database on the burning properties of CH4, H2 and other species of interest, in conditions typical of i.c. engines and gas turbines. High-speed, high-resolution shadowgraph is used to record the flame growth and to infer laminar burning parameters. Details are given of the experimental apparatus and of the data analysis. Experimental results are presented for the combustion in air of CH4-H2 mixtures, with a H2 percentage of 0% and 20% (vol.). All the tests were performed in the same conditions of initial pressure (0.6 MPa) and temperature (298 K). The lean part of the flammable range was scanned, varying the (global) equivalence ratio between 1.0 and the lower flammable limit. Data analysis yielded the (unstretched) laminar burning velocity and the Markstein length. The variation of these parameters was reported as a function of the equivalence ratio: the positive effect of H2 addition to CH4 was verified and quantified, in terms of decreased LFL and increased burning velocity.