Validation of laser‐induced thermal acoustics for chemically reacting H 2 /air free jets

The scramjet engine is a promising propulsion concept for future space transportation systems. To enhance the engine efficiency, a strong understanding of combustion mechanisms is essential. Laser‐induced thermal acoustics (LITA) – a seedless, non‐intrusive measurement technique – is an auspicious technique for the investigation of supersonic combustion. Here, we present two validation cases and first LITA measurements in a chemically reacting hydrogen/air free jet. The first validation case is the investigation of premixed methane/air and hydrogen/air flames, produced with a commercially available flat flame burner. The comparison with coherent anti‐Stokes Raman spectroscopy measurements resulted in a deviation between the two measurement techniques smaller than the uncertainty of the techniques. The second validation case is the investigation of hot, supersonic air/air free jets at total temperatures of T t  = 1050 K and T t  = 1300 K. The comparison with computational fluid dynamics predictions shows a reasonable agreement with a maximum deviation of 13.9 % for the free jet with a temperature of T t  = 1300 K and a good agreement with a maximum deviation of 4.4 % for the free jet with a temperature of T t  = 1050 K. The LITA measurements in a chemically reacting hydrogen/air free jet showed a good reproduction of the flame position determined from an OH ∗ image. For the presented investigations, we used a homodyne, resonant LITA setup with a pulsed interrogation beam. Copyright © 2016 John Wiley & Sons, Ltd.