Abstract

In this study, an underexpanded radial jet issuing from a small gap between two circular tubes facing each other is investigated numerically. Radial jet is formed, for example, downstream of high-pressure valves in piping system and of poppet valves in engines, and causes many industrial problems such as the noise generation and the fatigue failure of structure. In this study, the jet issuing from a small gap between two tubes with same diameter is numerically simulated. The flow field is assumed to be axisymmetric against the central axis of tubes and to be symmetric against the intermediate plane between the exits of two tubes. The axisymmetric Euler equations are solved using symmetric TVD (Total Variation Diminishing) scheme. The effects of nozzle pressure ratio and of diameter of circular tubes on the structure and the behavior of jets are examined. Typical cell structure of underexpanded jet appears in radial jet and the length of cell becomes smaller in downstream region because the jet spreads radially like a disc. The length and width of first cell are larger with higher nozzle pressure ratio. Many vortices are generated one after another near the jet boundary and move downstream, which cause the oscillation of jet. Outside of jet, two types of density waves are observed. One of them propagates toward the nozzle (toward the upstream region) and the other propagates in opposite direction. Focusing on the pressure change caused by the former waves, which is related to well-known screech, dominant frequency obtained by FFT analysis was found to become lower with higher pressure ratio and smaller diameter of tube.

Highlights

  • Supersonic jet is used in various industrial scenes, such as an assist gas to remove molten material in laser processing [1], a cooling jet in glass tempering process [2] and so on

  • Typical cell structure of underexpanded jet appears in radial jet and the length of cell becomes smaller in downstream region because the jet spreads radially like a disc

  • The present study focuses on the underexpanded jet which is radially discharged into the atmosphere

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Summary

Introduction

Supersonic jet is used in various industrial scenes, such as an assist gas to remove molten material in laser processing [1], a cooling jet in glass tempering process [2] and so on. Two-dimensional rectangular jets and axisymmetric jets have been studied by many researchers concerning jet structure, oscillation phenomena, noise generation and so on [6]-[11]. The radial jet issuing from a small gap between two circular tubes facing each other is numerically investigated and the flow characteristics are shown, such as the jet structure and the relation between oscillation phenomena and noise emission. The wall jet of impinging jet has similar structure when axisymmetric supersonic jet impinges on a flat plate. In such flow field, the oscillation of jet and the noise generation cause some problems in industrial point of view [12]

Numerical Scheme
Pressure and Density Contours
Cell Length
Frequency of Jet Oscillation
Directivity of Density Waves
Conclusions
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