3D Numerical and Experimental Analysis of Turbulence Energy Spectrum in Swirl Jet.

Mostafa, Nabil H.

doi:10.21608/bfemu.2021.143047

3D Numerical and Experimental Analysis of Turbulence Energy Spectrum in Swirl Jet.

Document Type : Research Studies

Author

Nabil H. Mostafa

Mechanical Power Department, College of Engineering Zagazig University, Zagazig, 44519, Egypt.

10.21608/bfemu.2021.143047

Abstract

An experimental analysis and numerical computation of a turbulent jet subjected to Vortex Generating Jets, VGJ, placed at the exit of the main jet nozzle to generate swirl jet were conducted. The main jet had a R_en about 11300, and the VGJ to main jet momentum ratio were 0.055 or 0 078. The activation of VG) was tangential (α=0°), (α =45") and (α =90). In Swirl Sets, the dynamics of the flow field structure were computed using a three dimensional Navier-Stokes code. The governing equations are discretized on 3D structured grid using an upwind difference scheme. The grid was building as 3D geometry and similar to swirl jet real nozzle. The use of the multiblock structured grid system and the arbitrary interfaces between the VGJS hexahedral grids and main jet hexahedral grids facilitate building a real 3D swirl jet grid geometry with changing injection angle. The macroscopic behavior of the jet evolution is discussed with the turbulence kinetic energy and its dissipation rate The experimental velocity vectors and Total Turbulent Kinetic Energy, TTKE, contours obtained with a four-wire hot-wire velocity probe The wavelet energy density is used to measure the intermittency and associated energy content The theoretical and experimental results show that the injection deviates the boundary layer flow inside the nozzle into a number of sections equal to the number of VGJs. Each section characterized by its own vortex. The vortices, which are generated from VGJs inside the nozzle tube, are growing outside the nozzle at the free zone. VGJs enhance the jet-spreading angle over unexcited jet. The swirl jet has higher TTKE, than the baseline jet. The VGJs increased and spreaded the turbulent kinetic energy in the main (swirl) jet. The swirl jet with tangential injection and higher momentum injection ratio (α= 0, mr = 0.078) gives the highest TTKE. For swirl jet the higher TTKE region is located at a cylinder layer of diameter 0.8 of nozzle diameter and moving horizontally in a section from Z/D=2 to Z/D=3 according to momentum ratio. Wavelete analysis presents that tangential swirl jet have a stronger effect on increasing the turbulent kinetic energy at the jet border, while, swirl jet at α=45° has higher power level at axial location. Further downstream, the total wavenumber energy is distributed to a wide range of frequencies and wavenumbers. This indicates that a wide range of turbulence structures associated with the VGJs injection affects the velocity fluctuations and enhances mixing between turbulent jet and surrounding fluid. There is a good matching between the experimental and computational results.