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.
Highlights
Jet flows have been widely studied in the past due to their canonical flow configuration, easy set-up and importance in many industrial applications and natural phenomena [1]
The use of impinging jet flows is especially widespread since relatively high convective heat transfer coefficients can be achieved
A disadvantage of impinging jet cooling is the fast decay of local Nusselt numbers away from the stagnation point which can be alleviated by either using arrays of jets or coaxial jets that are known to result in a more uniform Nusselt number distribution [6,7]
Summary
Jet flows have been widely studied in the past due to their canonical flow configuration, easy set-up and importance in many industrial applications (e.g., turbine blade and electronic equipment cooling) and natural phenomena (e.g., volcano eruptions, deep sea vents) [1]. Where Uo and Ui denote the mean jet exit velocities of the outer and inner jet, respectively Note that recent experiments by Raizner et al [19] showed that jet pulsation increased the primary vortex shedding frequency, leading to enhanced heat transfer at small stand-off distance, H/D (= 2), where H denotes the distance between the jet exit and the impingement plate, and D is the jet diameter.
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