An application of speckle-based background oriented schlieren for optical calorimetry

The aim of the present study is to validate an experimental method for the measurement of in-fluid enthalpy variation by using speckle-based background-oriented schlieren (SBOS). The method is applied to solving the millijoule thermal-energy transfer to fluid induced by an electric discharge in the air. An intensified camera is used to match the electric discharge exposure time constraint. Speckle is used as the reference pattern, as it allows easier access to high-density patterns for subcentimetric imaging. Geometric optic principles are used to extract the line-of-sight integrated index of refraction. The local density field is then computed using the Gladstone-Dale equation through filtered back projection. The method is first validated in close experimental conditions provided by the density measurement of a millimeter-dimension CO2 gaseous laminar jet. An analysis of the jet provides useful insights into the noise sources and uncertainties in the method, which is then transposed to the discharge. The results of enthalpy measurements are discussed.