Coaxial Circular Jets—A Review

Abstract

This review article focuses on the near-field flow characteristics of coaxial circular jets that, despite their common usage in combustion processes, are still not well understood. In particular, changes in outer to inner jet velocity ratios, ru, absolute jet exit velocities and the nozzle dimensions and geometry have a profound effect on the near-field flow that is characterized by shear as well as wake instabilities. This review starts by presenting the set of equations governing the flow field and, in particular, the importance of the Reynolds stress distributions on the static pressure distribution is emphasized. Next, the literature that has led to the current stage of knowledge on coaxial jet flows is presented. Based on this literature review, several regions in the near-field (based on ru) are identified in which the inner mixing layer is either governed by shear or wake instabilities. The latter become dominant when ru≈1. For coaxial jets issued into a quiescent surrounding, shear instabilities of the annular (outer) jet are always present and ultimately govern the flow field in the far-field. We briefly discuss the effect of nozzle geometry by comparing the flow field in studies that used a blockage disk to those that employed thick inner nozzle lip thickness. Similarities and differences are discussed. While impinging coaxial jets have not been investigated much, we argue in this review that the rich flow dynamics in the near-field of the coaxial jet might be put to an advantage in fine-tuning coaxial jets impinging onto surfaces for specific heat and mass transfer applications. Several open questions are discussed at the end of this review.

Experimental study of turbulent bubbly jet. Part 1. Simultaneous measurement of three-dimensional velocity fields of bubbles and water

“…The characteristics of the ZEF and ZFE of the bubbly jet were observed with the variation of the streamwise velocity along the centreline, as shown in figure 12( c ) (van Hout et al. 2021). The streamwise velocity was normalized by the initial velocity of the nozzle outlet.…”

“…Due to the locations of the needles in the bubble generator, the jet velocity profile at the nozzle exit resembles a coaxial circular jet (van Hout et al. 2021).…”

Comparative analysis between methane and hydrogen regarding ignition and combustion in diffusive mode

“…For comparing the behavior of the two diffusion flames, one must keep as much as possible flow parameters at the same values. According to van Hoot et al [6], in the case of coaxial jets, the main flow parameters are:…”

Turbulent Superstructures in Inert Jets and Diffusion Jet Flames

“…It is known that jet flow dynamics depend essentially on initial conditions in inert and chemically reacting gases [10][11][12]. Nevertheless, jets issuing from tubes and holes have been much less studied.…”

“…Almost no experimental data are available for such jets as far as velocity fluctuations at low Reynolds numbers are concerned. At the same time, long tubes are used in burners [11,13].…”