Experimental characterization of burning velocities of premixed methane-air and hydrogen-air mixtures in a constant volume combustion bomb at moderate pressure and temperature

Burning velocities of methane-air and hydrogen-air mixtures are investigated in a spherical constant volume combustion bomb varying the initial conditions of pressure, temperature and fuel/air equivalence ratios. Present work describes a method to determine burning velocities based on the use of a combustion bomb, in which the temporal pressure evolution is registered. A two-zone combustion model is used to analyse the experimental pressure trace and compute the thermodynamic variables that cannot be directly measured, with the mass burning rate and the associated burning velocity as the model results. The main interest of using a constant volume combustion bomb is the possibility of reaching high values of pressure and temperature, similar to the case of the combustion process in reciprocating internal combustion engines. The values obtained for the burning velocity are presented in the form of correlations as power laws of pressure and temperature, with exponents that depend on the equivalence ratio. A particular attention is paid to the validation of the burning velocity of methane-air at ambient values (300 K and 0.1 MPa), for which many experimental results are reported in literature. The first set of results include the combustion of methane-air mixtures at an initial temperature of 300 K, initial pressures of 0.05, 0.1 and 0.15 MPa, and equivalence ratios of 0.7-1.2. The ensemble of obtained burning velocities is in the range of laminar flame regime (excluding the cellular regime), covering the initial range of engine conditions (320-480 K, 0.1-0.7 MPa). These results are in a good agreement with data obtained by other authors. A second set of results refers to stoichiometric hydrogen-air mixtures at temperatures of 320-650 K and pressures of 0.1-1.6 MPa. Due to the particular characteristics of hydrogen, since it is almost impossible to have purely laminar combustion at stoichiometry when the pressure grows, the obtained values of the burning velocities are in the cellular regime. A discussion of the comparison of the present results with other authors' results is included, with considerations about the experimental and numerical approaches.