Effect of varying composition on temperature reconstructions obtained from refractive index measurements in flames, Combust. Flame 128

Optical methods (such as holographic interferometry, speckle photography, speckle shearing interferometry, moire ́ deflectometry, rainbow Schlieren deflectometry and Talbot interferometry, and so forth) have the potential for accurately measuring the entire temperature field associated with multidimensional flames, which may be difficult to do using other techniques. These interferometric or deflectometric techniques first determine the refractive index in flames, and thereafter infer the temperature distribution. The relationship between the refractive index and temperature is obtained by using a state equation and the Gladstone–Dale relation. However, a potential source of error arises since the local composition of the flame being studied is usually unknown. In a previous investigation, we examined the occurrence of this error by assuming the local flame composition to be the same as that of air at the local temperature. This was examined through one–dimensional simulations of counterflow flames. We found that while calculating the temperature from the measured refractive index this assumption could lead to significant errors for some flames and to minimal errors in other flames. This investigation quantifies those errors in the context of two–dimensional flames in both planar and axisymmetric geometries. It is found that the refractive index values of a mixture are nearly identical with those of the refractive index of air for partially premixed flames (PPFs). The maximum error lies in the range of 6.3 to 10.7 % for one–dimensional (1–D) counterflow PPFs, and between 6.1 to 8.0 % fo