1 Numerical Study of the Mixing of Co-Axial Jets

This paper presents a numerical study of the effect of the turbulence intensity in the turbulent mixing of coaxial confined jets. The objective of this work is to study the effect of the initial levels of turbulence of each coaxial jet in the process of turbulent mixing for diameter ratios less than 2, which is the case of very low bypass turbofan engines. The computational method was tested using the available experimental data, and revealed good capabilities for the simulation of this kind of flows. The effect of the velocity ratio of the two streams is presented for velocity ratios of the two streams of 0.3, 1.5, and 6, and turbulence intensity ratios between 10-5 and 105. The results revealed that for relatively high turbulence intensities of the outer jet a vortex is always present for velocity ratios of 6 or larger, and this is independent of the turbulence intensity ratio. When the turbulence intensity of the inner jet is kept constant and the ratio of turbulence intensities is increased, the recirculation zone decreases in size, its center moves further upstream until it disappears completely. The existence of a favorable pressure gradient as a similar effect, revealing that this is an additional parameter to be taken into consideration together with the velocity ratio and turbulence intensity ratio as far as the recirculation zone is concerned. Nomenclature Cµ, C1,, C2 = dimensionaless model constant Cp = dimensionless pressure coefficient 2 1 2/1 m p U ppC ρ d0 = diameter of the test section, mm kext = initial turbulent kinetic energy of the outer jet kint = initial turbulent kinetic energy of the inner jet p = wall static pressure, N/m2 p1 = wall static pressure at the first station, N/m2 r = radial coordinate, dimensionless r0 = inner radius of the test section, mm Re = Reynods number Ui = mean velocity of the inner (core) stream, m/s Uo = mean velocity of the outer (annular) stream, m/s Um = mass averaged velocity, m/s x = axial (streamwise) coordinate, dimensionless Greek symbols ρ = air density, kg/m3 λ = velocity ratio (outer to inner), dimensionless θ = int / kkext, dimensionles