RP-1583 -- Assessment of Burning Velocity Test Methods PDF

The objective of this research project is to critically evaluate two different burning velocity test methods, namely the closed vessel (or bomb) method and the vertical-tube method to determine their precision and accuracy and potential for test method simplification and cost reduction without sacrificing quality. The target refrigerants are mildly flammable R-32 and R-32/134a (60/40 wt%) mixture, which have the maximum burning velocities ranging from 2 to 7 cm s^-1. We have measured their burning velocities by the vertical-tube method and the schlieren photography method. To evaluate the precision of the two test methods, we have determined the intrinsic (gravity and conduction free) burning velocities of these refrigerants by the spherical-vessel method in microgravity (micro-g). The maximum burning velocity (S_u,max) of R-32/air mixture was 6.1, 6.2, and 6.5 cm s^-1 by the schlieren method, the vertical-tube method with a 40-mm diameter tube, and the micro-g experiment, respectively. These values were agreed within the error of 10% with one another. As for R-32/134a/air mixtures, the schlieren method provided Su,max as 1.9 cm s^-1 for 60/40 wt% mixture, which was agreed with that obtained by the micro-g experiment (1.8 cm s^-1). On the other hand, this mixture was found to be non-propagatable in the vertical tube. The lowest measurable "S_u,max" value of R-32/134a/air mixture by the vertical-tube method was 3.5 cm s^-1 for the corresponding mixing ratio of 80/20 wt%, which was by 11% lower than the S_u,max obtained in micro-g (3.9 cm s^-1). The maximum value below 4 cm s^-1 obtained by the current vertical-tube method will not be taken as a credible Su,max value since such slow flame propagations do not depend on flammability properties of the gas mixture but depend on the tube diameter and result from buoyancy force. As for simplification and cost reduction of the test method, we have proposed the way to 40% cost reduction of the schlieren method by replacing a high-speed video camera with an ordinary camcorder. The difference of measured Su between the original and modified methods was within 0.2 cm s^-1.