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Show CXH., + [ - + "- W -> .vCO + 'V H20 \2 AJ ' 2 CO + - O, -> CO, 2 " (D (2) The advantage of using the artificial specie CxHy over any combmation of reduced sets of hydrocarbons (CH4, C:H4, C2H6, etc.) is that the atomic mass balance of carbon and hydrogen is not altered. The specific lower calorific value (LCV) and the carbon and hydrogen fraction of CxHy may be calculated from the real gas composition. Thus, flue gas oxygen concentrations and total energy release from the flame may be predicted accurately. The eddy-break-up model for a 2-step reaction mechanism. Basis for the E B U model is the observation that regions in flames are either fuel rich or oxygen rich. Consider a fuel rich region, into which oxygen penetrates from the environment and assume that the oxygen is concentrated in fluid eddies. Under these cucumstances, the mixing of the oxygen with the fuel m ay be related to the life time of these oxygen containing eddies. This concept reduces the problem of determining the local mixing rate to that of determining the life tune of fluid eddies. However, the eddy life time is not a constant quantity. The variety of eddy life times form a distribution with properties which are dependent on the local circumstances. In the E B U model it is however assumed that a characteristic life time exists which is proportional to the ratio of k; e, where k is the turbulent kinetic energy and e its dissipation rate. Thus, the rate of combustion of oxygen is modelled as the product of the oxyen concentration and the characteristic life time "kg R-EBU.O, - Amixp £ kr°. m s (3) where Amix is the proportional constant and Y 0 2 denotes the mass fraction of molecular oxygen. The term k Amuc£has the dimension of time and is related to the characteristic life time of the fluid eddies. Visser and Weber (1989) compared this time with the Taylor time scale 4k sand the time scale of mean size eddies C k/e = 0A6k/e. A sensitivity study of computations of semi-industrial swirling coal flames indicated that to obtain good flame predictions inside the internal recirculation zone (IRZ) generated by the swul a value around 0.5 - 0.7 should be used for Amix. The present study applies a value of 0.6 for Amix. The corresponding reaction rates for C,Hy and CO may be written as: REBU.CxHy ~ AmixP , YCxHy REBU. CO ~ ^rnixP 7 MTO (4) (5) and in terms of the oxygen consumption rate of C KHy and C O the equations read Wn EBU.Oi(ctHt) mu/ REBU.02[CO) ~ ^rnixP U 2(1) o, C'2(2) U2 ,, lC,H, }CO^CO 'CO (6) (7) where ut and W[ denote the stoichiometric factors given in formulae (1) and (2) respectively the molecular weigth of the species considered. The indices 1 and 2 at the stoichiometric factors for the oxygen refer to the corresponding reaction number. According to the above 2- step reaction scheme the required oxygen consumption of both CVH„ and C O reads: REBU.02.req ~ R EBUo2(cxH)) + REBU.02(CO) (8) Consequently, the following local stoichiometry may be obtained: A, = °2.(l) °2 UC,H)^C,H> lC,H„ U0 W0 Ul (2) U2 °co^rco (9) 'CO At fuel rich conditions (A < 1) the following inequality will be observed: R-EBU.Oi.req - REBU.02 0") and the reaction rates of the two combustibles have to be limited by multiplying equations (6) and (7) with o lEBU.Q2 R^fE BU.02.req such that R EBU.02.req fuel-rich = R EBU.O-, (12) Obviously, the factor Cr becomes 1 for fuel-lean conditions. Following the above scheme multi-steps reaction schemes consuming one, or even more and different oxidising species, may be adopted to the E B U model widi no inconsistency in the combustion rate expressions for each reaction considered. Finite reaction rates model. To enhance reaction rate predictions near extinction at lower flame temperatures Arrhenius laws are introduced to the combustion model to limit the maximum reaction rate due to (Bockhorn and Lutz, 1984;Kjaldman, 1993). (13) * Kff =™HREBU-RArrh) Although the majority of E B U combustion models compares the rates such that the slowest rate is chosen to determine the local reaction rate (eq. 13), the present study considers the ^ two rates due to their physical meanings (Philipp, 1991) as a JL 4 |