A tomographic study of measurements in a V-shaped H2air flame and a lagrangian interpretation of the turbulent flame brush evolution

Abstract Flamelet structures and the evolution of flame brush thickness of H 2 air V-shaped flames are studied by laser tomography. The flames have a lean equivalence ratio of φ = 0.2. The flow velocity is 5 m/s and the turbulence intensity at the flame holder generated by a grid varies from 3.5% to 5%. The flames are within the wrinkled laminar flame regime. A sheet of pulsed laser light (Ruby rod) of 75 × 80 mm illuminates an oil aerosol seeded in the H 2 air flow, and the instantaneous flame structures are obtained when the oil droplets evaporate on passage through the flame front. The flames are imaged onto a CCD camera. The camera is interfaced with a matrix memory of 512 × 512 pixels with a dynamic range of 256 gray scales, and the tomographic records are analyzed by a digital image processing technique. The gray level corresponding to the flame front is determined by a filtering and thresholding operation. The first and second moments and correlations of flame front positions are deduced from the measurement of 240 instantaneous, statistically independent flames. The results show that the turbulence characteristics and equivalence ratio have an important effect on the shape and the dynamics of the flame. In particular, the growth of turbulence flame brush, δ T , is linear for a distance that can be correlated with the Lagrangian time scale of the turbulence. Furthermore, the slope of this evolution is proportional to the turbulence intensity at the flame holder. The linear growth is consistent with results observed by others for wrinkled laminar flames. The changes in the local flame surface area with downstream distance are estimated by the length of the flamelet segment within a vertical window of 10 pixels. The flame area increases are found to be accompanied by increases in the mean flame angle as shown by the width of the burned zone. These results confirm that the wrinkled laminar flame model is valid for the flames studied here. The flame front fluctuation results are interpreted using a Lagrangian diffusion model developed by Karlovitz. From this model, the value of the rms displacement of a flame element is calculated based on the approach flow turbulence characteristics (in comparison with the propagation speed of the flame fronts). Model predictions and experimental results are in good agreement. This indicates that the concept of a diffusion process for describing flame movement is important for study of premixed turbulent flame propagation.