OCR Text |
Show GREEK SYMBOLS a quarl angle for conical quarls ~ wave number o azimuthal coordinate p density Q angular velocity ~ stream function ~o stream function containing mo A COMPREHENSIVE STUDY on the near field aerodynamics of swirl burners--the IFRF NFA program--is being carried out in order to develop a burner design tool for use by industry. The program is based on utilizing several analytic solutions to the equations of motion for swirling flow to interpret well-controlled experimental results and using the experimental results to extend the analytic solutions into a semi-empirical burner design tool. In Figure 1, a typical swirl burner geometry and flow characteristics which are known to be important are shown. Specifically, the model developed should be capable of quantitatively assessing the influence on the flow field of the quarl geometry, the diameter of centrally-located fuel pipes, the furnace diameter, the furnace exit diameter, swirl parameters such as swirl strength and profile and, finally, the influence of combustion. pnmayo ... ___ rfvfrSf lIow boundary luro QlcIOI vrloclty liM) - - rKorc:uiDtoon zone boIIldary , : 0 __ spotlOl d.strobuhon of normohud streomllfl4! wc:ondary QIr fl6lc:tion .1,. ~ X_ U, ~ ~'-~-~ / '1 7777T~~~;?I ' ~ ( --', ",errot ( '- _ _ .1 •• _1 ,.c:n:IAat.", - - - - Fig. 1 - Schematic of swirl burner with essential design variables and typical type II flow pattern Since swirling flows are quite complex, a fact which is borne out by the general lack of understanding about the influence of the above parameters despite the many theoretical and experimental studies conducted in this area, a theoretical framework was used which served to suggest similarity parameters, interpret experimental results and predict directly some of the features of the flow field. The program was divided up into three separate phases: • phase 1 Flow visualization and hot wire measurements in isothermal swirling flows; • phase 2 LDV and gas tracer measurements in isothermal swirling flows; • phase 3 LDV and gas tracer measurements in swirling flows with combustion, with phase 3 being currently in progress. During the various phases of the program, the effect of the following swirl and fluid parameters-- • swirl intensity (0-2), • swirl profile, • Reynolds number 20,000-200,000, and the geometrical parameters-- • quarl length and angle (L/A=0.S-3, ar200-900), • furnace diameter (D f /A=I-oo), • furnace exit diameter (Df f /A=I-0.3), • central blockage ratio (a2/al=0-0.7) on the flow in and downstream of the quarl have been studied. THEORETICAL BACKGROUND Below, a brief description of the theoretical model upon which the experimental program was based in given. A more complete description can be found in [1, 2, 3] if further information is required. THEORETICAL INTRODUCTION - The IFRF experimental study on swirling flows and the interactions between swirling and nonswirling flows with application-to-burner design is strongly based on several direct analytic solutions to the equations of motion. In order to obtain these solutions, the following assumptions must be made: • inviscid, steady, incompressible flow; • axisymmetric, cylindrical or quasi-cylindrical flow; • initial conditions of solid body rotation and uniform axial velocity. Despite these assumptions, the analytic solutions have been shown to give considerable insight into some of the phenomena that are important for swirling flow burner operation, even under conditions where the above assumptions are not strictly satisfied. Although at first glance the analytic solutions might appear complex, in actuality they are quite easily used in conjunction with a table of the Bessel functions. For a swirling flow subject to the assumptions given above and initially in solid body rotation, i.e., the tangential velocity varies linearly with radius, W=Qr, and a uniform axial velocity, Uo ' the equations of motion become [4]: '/ ~ 'i ~ 1 all' 2Q2r2 4Q2 --+-----=------'1' a i ar 2 r ar U 0 U 2 (1) o This equation may be solved exactly subject to two different assumptions to yield two different solutions. Firs t, if the flow is contained by a cylindrical boundary or boundaries in an annular flow, equation (1), may be solved directly to yield [4] 'l'=1/2Uor2 + DrJ l {(k2-S2)l/2r}exp(iSx) + ErY I {(k2-s2)1/2r}exp(iax) (2) where in the standard notation, J 1 and Y 1 denote Bessel functions of the first and second kind. 90 |