Temperature measurements in a radially symmetric flame using holographic interferometry

Abstract

Holographic interferometry was used to measure the temperature distribution of a lean, laminar, radially symmetric, propane-air flame (φ = 0.55). The following steps were involved: A plane wave interferogram of the flame was created using double-exposure holographic interferometry; a scanning microphotometer was used to measure the position of the fringes in the interferogram; and the path-integrated refractive index distribution in the flame was calculated from the fringe position measurements. The temperature distribution in the flame was then calculated from the refractive index distribution assuming pressure and molecular weight of the gas mixture to be the same throughout the flame. In addition, a temperature profile of the flame was made using a thermocouple in order to obtain results for comparison with those obtained using holographic interferometry. Several findings resulted from this study. Frist, in the region of the flame where the thermocouple was relatively nonperturbing, the interferometrically determined burned gas temperature was withn 50K (3.5%) of that measured by the thermocouple. Second, the refractive index of the reference field must be known to within 2% of the true value to achieve that 50K accuracy. Third, the position of each fringe minimum should be known to within 1% of the local fringe spacing in regions where the fringes are widely spaced; however, the temperature distribution is less sensitive to fringe position errors when the fringes are closely spaced.