Evaluation of mass transfer coefficient and hydrogen concentration in supersonic flow by using catalytic reaction

The authors propose a new simple method to evaluate hydrogen concentrations in a hydrogen/air supersonic mixing layer without the need for costly apparatus. Catalytic reaction occurs on an electrically heated platinum wire in the supersonic flow of a hydrogen/air mixture. By adopting the technique of a constant-temperature hot-wire anemometer, the heat transfer coefficient and the catalytic heat release rate are measured. A series of experiments with different platinum wire temperatures shows that the platinum wire temperature does not affect the catalytic heat release rate, implying that the rate of mass transfer from the flow to the platinum wire surface is the controlling factor. This means that the catalytic heat release rate gives the mass transfer coefficient of the controlling species, which is hydrogen/oxygen in lean/rich mixtures. It is found that the effect of hydrogen concentration on the ratio of heat and mass transfer coefficients is very weak, suggesting that the mass transfer coefficient, is obtained with reasonable accuracy from the heat transfer coefficient by assuming the equivalent spatial distributions of heat and mass transfer. Based on this result, a method to translate the catalytic heat release rate into the hydrogen concentration of the flow is proposed. To prove the accuracy of this method, hydrogen concentrations of hydrogen/air premixed supersonic flows were measured successfully. Finally, one example applying this method to an actual supersonic mixing layer is presented.