Measurement of the laminar burning velocity using the confined and unconfined spherical flame methods - A comparative analysis

This work investigates the consistency of experimentally determining the laminar burning velocity by using two different approaches: the unconfined flame method, based on Schlieren photography of the flame front evolution, and the confined flame method, based on pressure rise monitoring during isochoric combustion. Radiation corrected laminar burning velocity values were derived by the unconfined flame method using linear as well as non-linear stretch relations. Those values were then used as reference for evaluating the accuracy of the confined flame method. Laminar burning velocity obtained by the latter method were found to depend strongly on the modeled relation between the burned mass fraction x to the pressure P. Thus, we compare several x-P relations and show that the popular linear x-P relation overestimates the burning velocity by up to 20%. However, by “right parametrization” of a more detailed analytical x-P relation, closer results to those obtained by the unconfined flame method may be achieved. The favorable usable pressure data range for the confined flame method was also considered and found to be in the range of (10% < x < 55%). Moreover, it was found that stretch effects are not totally negligible during the confined flame propagation, even for low Markstein length-mixtures. In summary, we suggest the analytical confined flame method as we define it in this work, to be a cost-effective and less labor-intense alternative to the unconfined flame method for applications where accuracy can be compromised.