Experimental Results To evaluate the flame surface fluctuations, the flame surface is defined as the area of surface of revolution [2], given by: where dx and dy are function the points (x,y), which lies on the flame front curve. Therefore it is necessary to identify the flame front curve in the each image recorded by the CCD camera and for that the following procedure was developed. IN+ Abstract The acoustic transfer function of impinging flames is defined as the ratio between the heat released rate fluctuations and the relative velocity fluctuation. The heat released rate cannot be measure directly but it can be estimated from the surface fluctuation of the flame. In this paper, image processing techniques are used to segment the flame borders and estimate its global area. This is done by assuming that flames are axis- symmetric which makes possible to model the flame surface by a revolution object. Results with real images of Impinging flames obtained with a CCD camera are presented. RecPad th edition of the Portuguese Conference on Pattern Recognition, UTAD University, Vila Real city, October 29th J. G. Merícia 1, J.M. Sanches 2, E. C. Fernandes 1 1 Laboratory of Thermofluids, Combustion and Energy Systems - IN+ 2 Institute for Systems and Robotics Instituto Superior Técnico Lisboa, Portugal Introduction Flame jet impingement is widely used in many industrial processes where enhanced convective heat transfer rates due to direct contact are required, as cooking devices and heating of glass products [9]. The corresponding flame configurations are prone to strong acoustic emission, derivated by the flame instabilities [3]. One passive way of improving stability consists in modifying the global balance between acoustic source/sink processes, by redesigning the burner [5]. In this context, it is important to determine the flame transfer function, which is the ratio of the heat release rate perturbation to velocity perturbation in the frequency domain: Previous researches on steady flames have shown that the global heat release is proportional to the flame surface area. Therefore can be replaced by the relative flame surface area fluctuations [2]. Finally, this work describes the method created based on Tomographic Image Reconstruction followed by a snake algorithm to recognize the flame front an impinging flame in images recorded by a High Speed CCD camera. Conclusion The method to determine the flame surface area based on Image Tomographic Reconstruction followed by snake algorithm to recognize the flame front achieved good results and the snake algorithm was able to identified the flame front boundaries, as expected. Acknowledgments: Authors gratefully acknowledge the support of the European Commission within 6th Framework Programme, through the Marie Curie RTN Project "AETHER", Contract No: MRTN-CT [1] D. P. Correia. Development and Implementation of Tomographic Reconstruction Techniques for the Diagnostic of Combustion Flows. PhD thesis, Technical University of Lisbon, [2] S. Ducruix, D. Durox, and S. Candel. Theoretical and experimental determinations of the transfer function of a laminar premixed flame. Proceedings of the Combustion Institute, 28(1):765 – 773, [3] E. Fernandes and R. Leandro. Modeling and experimental validation of unsteady impinging flames. Combustion and Flame, 146(4):674 – 686, [4] A. K. Jain. Fundamentals of Digital Image Processing. Prentice Hall, [5] T. Schuller, D. Durox, and S. Candel. Self-induced combustion oscillations of laminar premixed flames stabilized on annular burners. Combustion and Flame, 135(4):525 – 537, [6] R. H. Tourin. Spectroscopic gas temperature measurement. Elsevier Publishing Company, pages 61–62, [7] C. Xu and J. L. Prince. Gradient vector flow: a new external force for snakes. In Proc. IEEE Computer Society Conf Computer Vision and Pattern Recognition, pages 66–71, [8] C. Xu and J. L. Prince. Snakes, shapes, and gradient vector flow. 7(3):359–369, [9] Y. Zhang and K. Bray. Characterization of impinging jet flames. Combustion and Flame, 116(4):671 – 674, 1999 Generation of a binary mask. Record of Images with a resolution of 352 x 256 pixels and a sampling rate of ten times the frequency of the excitation signal used to impose the velocity perturbations U‘. Application of a Tomographic Image Reconstruction, based on Abel's integral inversion [1, 6]. Application of a threshold, creating a binary image. Application of a snake algorithm, which is a computer-generated curve that moves within images to find object boundaries, under the influence of internal forces coming from within the curve itself and external forces computed from the image data [7,8]. Application of the mask, finding the flame front and using it to find a polynomial curve, which can be used to calculate the flame surface area. Calculation of mean image, using a set of 100 images and application of an adaptive Bayesian filter, reducing the noise [4]. Experimental Setup Application of Tomographic Image Reconstruction and Snake Algorithm to Recognize a Wrinkled Flame Front