Effect of Swirl on Mixing in Underexpanded Supersonic Airflow

Abstract

This study examines the effect of imparting swirl to underexpanded nozzle airflow on supersonic mixing under matched mass flow conditions. A convergent nozzle with swirling capabilities is used to generate the underexpanded airflow. Fuel is injected coaxially at the nozzle throat. Nonreacting conditions are considered, wherein fuel is simulated by mixtures of helium, argon, and krypton inert gases. It was found that the positive effect of swirl on supersonic mixing is noticeable, even at the low degree of swirl examined here. Better mixing was observed between the supersonic and subsonic parts of the diamond-shock air flowfield at no fuel injection as well as between the supersonic air and the subsonic injected fuel. The compressible mixing-layer thickness was found to increase slightly with swirl. However, when normalized to its incompressible counterpart, the compressible thickness was observed to decrease with swirl, because the former increases more with swirl than the latter. Decreasing the air-fuel relative Mach number and/or density ratio was found to deteriorate mixing. On the other hand, decreasing the compressibility of the injected fuel (i.e., injecting fuel at lower subsonic Mach numbers) improves mixing. Nomenclature a = speed of sound D = nozzle-exit diameter (11 mm) DR = air-fuel density ratio at injection plane M rel = air-fuel relative Mach number at injection plane S = swirl number v = velocity x = fuel-mixture fraction z = axial coordinate Subscripts a = axial t = tangential

Mixing Enhancement in a Scramjet Combustor Using Fuel Jet Injection Swirl.

“…Schlieren image with vertical knife edge (conical extension cap removed) [27] Abdelhafez and Gupta (2011) also investigated flow characteristics of a coaxial jet using a convergent nozzle [43]. Helium was used in the core jet and air was used in the annular jet.…”

“…Schematic of the Supersonic nozzle assembly[43] Shock structure of underexpanded nozzle airflow in presence of coaxial injection with no fuel injection[43] Effect of fuel injection on shock structure of unexpanded nozzle flow a) no fuel injection, b) fuel injection[43] …”