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Show NUMERICAL STUDY IN THE PREHEAT ZONE OF TURBULE~T PREMIXED FLAMELET Hsiao-Kang Ma and Jiu-Yeong Law Department of Mechanical Engineering National Taiwan University Taipei, Taiwan, 10764, R.O.C. ABSTRACT The governing equations for heat coadadion. convection aDd radiation in a turbulent premixed namelet can be developed and solved for radiative transfer characteristics of absorbing. emitting and scattering properties. The preheat ZODe ofturbukDt premixed flamdet is assumed a DOD-gray optical aDd participating medium.. The analytical solutions for tbe above equations by changing parameten of condactioD(N J ). coavection(N 2). the optical thiclmess(to) and the single scattering albedo(coo) •••• etc.. are studied in detaiL The participating medium in preheat zone could be regarded as gas aad soot mediums. The toUl emissivity. absorption coefticieDts in gas mediam are calculated bued upon wide band modds due to the radiative properties vary so rapidly across tbe spectrum. The Drude-Loreatz dispened modd is used to study the complex refractive iDdex (m = n - ik) of soot which have to be known in solved the total emissivity. absorption coefficients of soot medium. I. INTRODUCTION The study of heat transfer mechanism of flamelet has always been one of the major focuses of research in the field of combustion. Two fonns of products of combustion may release radiation energy. One of them is gas radiation such as H20, CO2 and CO, ... etc. Gas radiation is not continuous, but only limited to release under specific wavelength. This is because that gas radiation is the result of energy level changes of molecules and atoms, or the release or absorption during vibration or rotation. Most gas radiation is located in the visible and Infrared. Its radiation extent is related to pressure, path length and gas temperature, ... etc. The gas radiation generated by combustion primarily comes from the radiation of H20 and CO2 . It distribution mainly concentrates in the infrared( 1-1 0 J.1117), and occupies a very small proportion in the visible. Therefore, it is called non-luminous. The other fonn consists of particles of incomplete combustion, called soot . Soot radiation is continuous. Its thermal radiation extent is related to the quantity, size and optical properties of soot The radiation range of soot includes the ultraviolet and the visible, with a continuous distribution on the spectrum. Therefore it is called luminous. The above two fonns of combustion products are the primary mechanism of flame radiation control. K. Y. Lee, Z. Y. Zhong and C. L. Tien [1] considered radiation blockage as an important flame radiation feature. The radiation blockage effects resulted from the soot between the flame and fuel phases can be observed from two simple models: (1) flame sheet with infinitely small thickness, and (2) flame layer with specific thickness. Studies have shown that soot plays a major role in the radiation changes of one dimensional flame. At the end of this paper, the numerical results are compared with the experimental data ofP~ PP, POM and PS. And, S. L. Chen., H. K. Ma and D. Y. Chen [2] assumed that an infinite number of soot were generated from the fuel surface during the combustion process, and that the soot played the role of powerful absorbents toward flame radiation. They established a simple flamelet model under this physical mechanism to explain this type of blockage effect. They also assumed the absorption-emission process dominated by soot to be 1 |
