Effects of axial distance on flow behaviours of continuous jet impinging on a flat surface using a finite confined nozzle

Abstract

The velocity fields and impinging pressure characteristics of confined jet impinges on a flat surface subject to the influence of various axial stages of a flat plate were experimentally examined. The instantaneous flow patterns, time-averaged flow patterns and velocity characteristics, velocity histories, and impingement pressure distributions were obtained using flow visualisation, particle image velocimeter (PIV), hot-wire anemometer measurement, and pressure detection techniques, respectively. From the analysis of the flow patterns, three characteristics of flow regimes are identified – laminar vortical wake, transition, and turbulent vortical wake – in the domain of Reynolds number and jet-to-wall distance. In laminar vortical wake mode, a ‘fat' pair of counter-rotating vortices appears between the confining surface of the nozzle and the flat plate. This results in a large region of turbulent intensities due to the entrainment of the jet flow. In transition mode, shear-layer vortices appear on the shear layer of the bifurcated jet. In turbulent vortical wake mode, the large momentum of jet fluids impinges the flat plate and deflects radially. The counter-rotating vortices and shear-layer vortices no longer exist in the wake of the bifurcated jet. The large eddies quickly break up into small eddies as the momentum of the jet-dominated flow. No obvious peak value of oscillating frequency was detected in this flow mode along the shear layers of the bifurcated impinging jet. The momentum of the fluid structures is converted into stagnation pressure on the flat plate when a strong jet impinges on it; therefore, the impingement pressure is large, consequently. The impingement pressure in the mode of the turbulent vortical wake was significantly greater than that in the modes of laminar vortical wake and transition.

KEYWORDS:

• Impinging jet
• flow visualisation
• turbulent properties
• impingement pressure
• PIV

Nomenclature

d	=	exit diameter of jet
P∞	=	ambient pressure
Rej	=	jet Reynolds number (= ujd/ν)
t	=	evolving time
v	=	instantaneous axial velocity
vj	=	jet exit velocity in y direction
u′	=	root-mean-square value of radial velocity
x	=	Cartesian coordinate in radial direction
y	=	Cartesian coordinate in axial direction
H	=	distance between jet exit and flat plate
ΔP	=	impingement pressure (=P − P∞)
Φ	=	power spectrum density function
ν	=	kinematic viscosity of jet fluid

Acknowledgments

The authors would like to acknowledge Professor Rong Fung Huang for his great support and insightful advice at the Thermal Science and Fluid Mechanics Laboratory.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

There is no data set associated with this manuscript.