The flow characteristics of a confined square jet with mixing tabs

Abstract

The flow characteristics of a confined square jet with mixing tabs have been determined by measurements obtained using a two-component laser Doppler anemometer at a Reynolds number of 1.026 × 105 (based on the exit hydraulic diameter, DH = 60 mm, and bulk mean velocity, Ur, of the stream at 1.71 m/s). Both tabs of rectangular and triangular shapes are considered with the same height-breadth ratio ( h/b = 1.35) and with their apex leaning downstream. Altogether four tabs have been used, with one tab each located at the centre of each side wall at the exit plane. Each tab is found to produce a dominant pair of counter-rotating streamwise vortices. The combined effects of the four tabs bifurcate the jet into four ‘fingers’, resulting in a significant increase in entrainment at the downstream locations. The strength of the streamwise vorticity generated by the rectangular tabs is some 30 per cent higher than the triangular ones and decays faster with downstream distance. This appears due to a larger tab surface area which creates a larger pressure differential across the rectangular tab than the triangular tab. The region of high turbulent kinetic energy is found firstly at the locations where the streamwise vortices stretch the normal vortices and subsequently at locations where the streamwise vortices break down, resulting in significant mixing enhancement. Finally, the effects of the so-called secondary tabs have also been examined and are found to enhance the mixing further. The orientation of the secondary tabs is, however, crucial for the mixing enhancement to occur.