| OCR Text |
Show where R is the exit radius of the burner nozzle. Experiments have shown that the . swirl number is the significant similarity criterion of swirling jets produced by geometnc~lly similar swirl generators (Beer & Chigier, 1972). The calculation of swirl number uSIng equations (1-3) requires accurate measurements of velocity and of static pressure distributions to be made in the cross-section of the swirling jet. The designer may n.ot always have access to experimental data and there is interest in determining the sW.lfl number directly from air register design data. The angular momentum and the velOCIty term in the expression for the axial momentum can be predicted with rea~onable accuracy. However, it is more difficult to predict the value of the static pressure Integral because it undergoes changes along the flow in the swirl generator, depending upon t.he geometry of the burner nozzle. When the swirl number is calculated from input velo~Ity distributions in the swirl generator rather than from the velocity distributions in the Jet, the static pressure term can be omitted and the swirl number can be given with good approximation as (Beer & Chigier, 1972): , G, S = G' R It (4) where G; = foR pu u 21tr dr (5) In the burner tested, combustion air was split into three separate air streams, each with a different swirl number. Therefore, it was imperative to assign modified swirl numbers to the flow as a whole, rather than referring to the individual swirl numbers associated with the three streams of the burner. For a burner with three swirling combustion air streams, the modified swirl number is given by (Lawn, 1987): G~p + G~ + G~t So =-~-----:.,,--~ rT (Gxp + Gxs + Gxt) (6) where Gcpp, Gcps and Gcpt are the axial fluxes of the angular momentums for the primary, secondary and tertiary air, respectively. Similarly, Gxp, Gxs and Gxt are the axial fluxes of the linear momentums of the primary, secondary and tertiary air, respectively (Lawn, 1987). By expressing the axial fluxes of the angular momentums for the primary, secondary and tertiary air in terms of the swirl numbers and the axial fluxes of the linear momentums, it is possible to obtain an expression for the Overall Swirl Number (So) of the flow: (7) The above expression was used to assign Overall Swirl Numbers to the flames associated with each test. Experimental The diagnostics employed were a suction pyrometer, a 5-hole pitot-meter and an in situ laser-based single particle counter (Sharifi, 1996). The specifications of each instrument are reported elsewhere along with the guidelines used in their calibration and application. The operating principles of the suction pyrometer and the 5-hole pitot-meter are well understood and discussed extensively (Marsh, 1951; Mullikin, 1941; Mullikin & Osborn, 1941; Schack, 1939; Becker, 1993; Sharifi, 1996; Yoon, 1982). The in situ laser-based single particle counter, referred to as the PCSV -P, (Particle Counter Sizer Velocimeter Probe, Insitec, Inc.), is relatively new and is therefore discussed briefly. |
