A Fractal Study of a Premixed V-Shaped H2/Air Flame

Abstract

The flamelet surface of six turbulent H2/air V-shaped flames are studied by laser tomography. The equivalence ratio varies from (0.4-0.43), the turbulent Reynolds number. Re, from (20-40), and the turbulence intensity, I, from (3 3%-5%). These flames are within the wrinkled laminar flame regime. The tomographic records (240 for each case) are analyzed by digital image processing techniques. Fractal analysis has been applied to the flame front length recorded. The influence of the turbulent flow field characteristics, the equivalence ratio, the downstream distance dependence on the fractal dimension, D, and the outer cutoff £0 have been evaluated. Both the fractal dimension, D, and the outer cutoff, ε0, have been found to evolve with the downstream distance. The maximum value found, D = 2.2, is well below the expected constant value determined by Sreenivasan, D = 2.37 for the case of turbulent, non-reacting interface. The outer cutoff is found to follow the evolution of the integral length scale and to be independent of the equivalence ratio. Gouldin's fractal model has been compared with a direct measurement of the flame front element length. Measurement and model are found to be in good agreement when the experimental fractal dimension is still used as an input. The fractal dimension, however, far downstream is well below the expected area value of D = 2.37. The usefulness of fractal modeling on such a developing flame is not yet established. For oblique flame fronts developing with the downstream distance, the model needs significant experimental input to be able to predict accurately the flame surface area.