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Show 10SI DE JET H • 0·015 • Uo • 80 . /H( TO • 1100 ~ O"TSIDE JE I H • 0010 • U • SO . /H( T • SOD ~ 1 YiH • IS COO"OUR I rrTERVAI. • 5. OxlO-7 I --------------------------------------------~~ -----------lIH· 50---------_ _ 1 Fig. 6 - Surface and contour plots of turbulent heat flux distribution for 2-D coaxial jets. transfer rates occur in a narrow zone which begins at the no z z 1 e exit and extends downstream approximately 30 diameters. SIMULATED COMBUSTOR FLOWS - The numerical model is especially suited to the prediction of wall-bounded flows with recirculation. It is used here to evaluate the importance of relative velocities between two co-flowing jets entering a mixing chamber. The flow configuration shown in Fig. 2(b) is central to many industrial processes which require large volumes of drying air or the reduction of primary combustion NOx and CO emissions. If the chemical reactions are sufficiently fast, this flow model also provides first-order estimates of combustor performance. The primary stream enters the chamber through a 0.05 m s lot wi th a ve loci ty of 30 m/sec and temperature of 1700 K. The secondary mass flow rate is held constant and its temperature set to 500 K. The chamber walls are assumed adiabatic. The influence of the secondary stream inlet conditions are demonstrated by comparing the velocity, temperature, and heat flux distributions resulting from design choices of inlet velocity and injection height: Case 1. Secondary air injection velocity of 20 m/sec and injection height of 0.04 m. 114 Case 2. Secondary air injection velocity of 40 m/sec and injection height of 0.02 m. Note that the total system mass flow rate is the same for both cases. Figure 7 presents a comparison of the velocity fields for Case 1 and 2. The major flow features are similar in character, with a large recirculation region near the wall just downstream of the inlet slots, and a small recirculation zone in the downstream corner at the contraction. There are, however, substantial differences between the two cases. The upstream recirculation zone of Case 2 (high ve loci ty secondary air) extends approximately 40% further along the wall and has a markedly lower velocity region on the chamber centerline at approximately 0.30 m. These differences result directly from the increased secondary jet momentum flux and larger initial flow expansion area. o 12 o 06 o 04 o 00 INSI DE JET : H • O. OS " U • 10 "/SEC T • 1100 l OUTSIDE JET : H · O.O~O " UT •• 2500 0 "l' SEC -0 04 ~~~_L~~~~_L~-L~Li~~_L~_L~_L~~ -0 . 04 o 06 0 . 16 0 . 26 o 36 o 46 o 56 Fig. 7a - Velocity field for 2-D coaxial jets expanding into a cavity. Slower out8ide jet. I"SlnE JET : OUTSIDE JET : H • O. OS " H • 0·020 " U • 10 "/SEC ~ : ~gO"'SEC o 12 o 06 o 04 o 00 -0 04 ~~~_L~~~~_L~~~~~~_L~~~~~~~ - 0 04 o 06 o 16 0 . 26 0 . 36 o 46 o 56 Fig. 7b - Velocity field for 2-D coaxial jets expanding into a cavity. Faster outside jet. The temperature contours and surface plots shown in Fig. 8 and Fig. 9 also illustrate important differences. When compared to Case 1, Case 2 exhibits greater transport of thermal energy away from the centerline and higher tempera tures near the wall. Furthermore, Case 2 shows a more gradual variation in temperature across the chamber exhaust, indicating a higher mixing efficiency. The numerical simulations also serve to highlight regions of the chamber flow where design choices directly affect the control of pollutants. To reduce NOx production, system temperatures and/or fluid transit times must be minimized. Unfortunately, lower temperatures and decreased fluid residence times also lower the CO-to-C02 conversion rate. With this in mind, consider the design choices presented by Cases 1 and 2. As illustrated by Fig. 8 and Fig. 9, the gas temperature in the major recirculation zones |