Part II Combustion. Summary. F.A. Williams, T. Takeno, Y. Nakamura and V. Nayagam

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1 Part II Combustion F.A. Williams, T. Takeno, Y. Nakamura and V. Nayagam Summary Combustion, which involves exothermically chemically reacting flows, is complicated in that it includes both physical processes, such as heat and mass transfer, and chemical processes of many different types. The topic is a broad one with applications in areas that encompass power-production and safety issues. The safety-related subjects of fire and explosions are addressed in Part I. The present section focuses on more fundamental aspects of combustion. Because combustion is a complicated phenomenon, it is important as a first step to identify the most dominant controlling processes. Dimensional analysis is a good way to investigate the dominant physics and chemistry. Since there are many nondimensional groups in combustion, it is challenging to pick out the few that are most important. Once this is done, there is appreciable simplification which then makes combustion a good candidate for application of scale modeling. The scalemodeling concepts help to simplify the problem and to expose the most important aspects, thereby affording a useful first step, prior to more detailed experimental and numerical work. Since there have been many fundamental combustion studies presented at the ISSM symposia, stringent criteria had to be enforced in selecting papers for inclusion in the present volume. Papers were excluded unless they were both highly contributing to the development of fundamental combustion science and strongly related to scale modeling. This required the omission of some excellent papers and finally resulted in the selection of only 12 papers for publication. Although most of these papers are experimentally based, some theoretical and numerical studies also are included. Figure 1 summarizes the number of papers in this area presented at the different symposia. That number is seen to have reached a maximum, in both absolute value and percentage, around ISSM-III and ISSM-IV, reflecting a trend in the fraction of scale modeling research devoted to fundamentals of combustion. Figure 2 summarizes the acceptance ratios for papers in fundamental combustion. Except for ISSM-II, in which 50% of the papers were both excellent and highly relevant, the acceptance ratio remains around 20%. The overall decrease with time in the K. Saito (ed.), Progress in Scale Modeling, C Springer Science+Business Media B.V

2 164 F.A. Williams et al. Fig. 1 A summary of the number of papers Fig. 2 Acceptance ratio fraction of papers in this area that are both excellent and relevant may be attributed to the increasing capabilities and precision of direct numerical simulation. Since those capabilities, however, remain deficient for three-dimensional turbulent flows with ignition and extinction, for multiphase combustion and for other comparably complex combustion processes, further contributions of scale modeling to advances in these fundamental combustion areas may be anticipated in the future. Papers Selected from the Second Symposium 1. F.A. Williams. Modeling of Combustion Phenomena. This paper reviews a wide variety of modeling and scaling questions in combustion. It emphasizes the relevance of partial modeling, in which certain groups of possibly lesser

3 Combustion 165 importance are purposely ignored. This paper was chosen because of its breadth of coverage and perspective. 2. S. Tabejamaat and T. Niioka. Numerical Study of the Effect of Model Scaling on Mixing and Flame Development in a Wake Flow Field. This addresses the combustion of hydrogen injected into a supersonic air stream from the downstream side of a bluff body. It applies numerical methods to identify the mechanism of flame holding. This paper was chosen because of the importance of its establishment of the mechanism of flame stabilization and because it considers the effects of model scaling, pointing out the scales that are most relevant. 3. T. Takeno and K.N.C. Bray. Molecular Diffusion Time and Mass Consumption Rate in Flames. This paper presents dimensional-analysis arguments for describing how non-uniform flow can affect combustion rates in flames. It addresses both premixed and non-premixed flames in flame-let regimes. It was chosen because of its contribution to understanding of the processes involved. 4. L.T. Yap, M. Pourkashanian, L. Howard, A. Williams, and R.A. Yetter. Nitric- Oxide Emissions Scaling of Buoyancy-Dominated Oxygen-Enriched Methane Turbulent-Jet Diffusion Flames. This paper contains detailed experimental and theoretical investigations of emissions of nitric oxide from turbulent flames to which oxygen has been added for the purpose of reducing harmful emissions. It develops and validates a scaling law for emissions based on a Froude number. This paper was chosen because of its discovery and elucidation of the most significant underlying mechanisms and its identification of the relevant scaling that occurs. 5. I.S. Wichman and B. Ramadan. Scaling Analysis of Diffusion Flame Attachment and Liftoff. This paper presents a theoretical analysis of the attachment and liftoff of a flame downstream from a splitter plate separating fuel and oxidizer flows. It identifies the different regimes that occur and the existence of triple flames under suitable conditions. It was selected because of its contribution to understanding and its identification of the relevant non-dimensional parameters needed in scaling. Papers Selected from the Third Symposium 1. V. Nayagam, A.J. Marchese and K. Sacksteder. Microgravity Droplet Combustion: An Inverse Scale Modeling Problem. This paper addresses droplet combustion questions for which there is some advantage in studying a scale model that is larger than the prototype. It identifies different regimes of droplet combustion and the relevant non-dimensional parameters in the regimes. This paper was chosen because of its insights and unconventional approach to considerations of scale modeling. 2. P.B. Sunderland, D.L. Urban, and V. Nayagam. Scaling of Gas-Jet Flame Lengths in Elevated Gravity. This paper considers gas-jet diffusion flames

4 166 F.A. Williams et al. employing both theoretical and computational methods. It develops scaling laws for flame dimensions and tests the predictions against available data obtained in experiments in which gravity levels are effectively elevated by use of centrifuges in which the combustion occurs. This paper, which is a rewritten version that also includes results in a similar paper presented at ISSM-IV, was selected because of the notable advances in understanding of scaling of flame heights that it develops. 3. Y. Nakamura, H. Ban, K. Saito and T. Takeno. Structure of Micro (Millimeter Size) Diffusion Flames. This paper presents detailed numerical and experimental results on the structure of very small methane-air laminar diffusion flames. It was chosen because of its emphasis on the relevant non-dimensional scaling parameters and its clarifications of the combustion processes that occur under these atypical conditions. Papers Selected from the Fourth Symposium 1. J.S. T ien. Some Partial Scaling Considerations in Microgravity Combustion Problems. Based on considerations of conservation equations for various microgravity combustion processes, this paper identifies similarity parameters and tests them against computational and experimental results. It emphasizes the importance of radiant energy transfer under microgravity conditions. It was selected because of its contributions to our understanding of the relevant non-dimensional parameters under microgravity conditions. 2. J. Baker, M. Calvert, and K. Saito. Scale Modeling of Magnetocombustion Phenomena. This paper analyzes heights of laminar diffusion flames in nonuniform magnetic fields, employing both theoretical and experimental methods. It identifies relevant non-dimensional parameters and tests them experimentally. It was chosen because of its novel approach to this relatively unusual combustion scenario and its strong appeal to scale modeling. 3. G.T. Linteris and I. Rafferty. Scale Model Flames for Determining the Heat Release Rate from Burning Polymers. This paper investigates the utility of flame size for assessing heat release rates of burning polymers by testing six different materials in apparatuses of different sizes. It established the relevance of Froude numbers for correlations of the results. This paper was chosen because it is a new application of scale modeling in combustion that falls in the mainstream of the subject and that appropriately advances our knowledge and understanding of the processes involved. Papers Selected from the Fifth Symposium 1. K.H. Chuah, H. Gotoda, and G. Kushida. Numerical Simulations of Methane Diffusion Flame with Burner Rotation. This paper presents numerical solutions

5 Combustion 167 of the conservation equations for combustion of methane diffusion flames on rotating burners. It exhibits pulsating flames and clarifies their mechanisms. This paper was chosen because it increases our knowledge of influences of rotation on such flames and addresses scaling laws for pulsation frequencies that can be tested by scale modeling.