Pressure-swirl spray in a low-turbulence cross-flow

Abstract

Pressure-swirl atomisers (PSA) produce fine spray and are used in many industrial, chemical and agricultural applications of sprays in flowing environments. The study examines spray from a small low-PSA exposed to low-turbulence cross-flowing air. The PSA spray was investigated experimentally using phase Doppler anemometry (PDA) and high-speed visualisation (HSV). The atomiser sprayed water into cross-flowing air at varying flow velocities. The tests were provided at a newly developed wind tunnel facility in the Spray laboratory at Brno University of Technology. PDA results contain information on the size and velocity of individual droplets in multiple positions of the developed spray (after the liquid break up is completed). A high-speed camera (HSC) documented the complexity of the liquid discharge, the formation and break-up of the liquid film, and the spray morphology. The data is relevant to CFD engineers and scientists involved in modelling as they can highlight the crucial phenomena to be considered in numerical simulations of the disperse two-phase flow case. The case allows us to study 1) Liquid discharge and sheet formation, the primary break-up of the liquid sheet, 2) secondary break-up and spray formation and 3) the Interaction of the sprayed liquid with surrounding air: gas–liquid mixing, droplet collisions, droplet clustering and droplet reposition.

Figure 1a: Spray image form high-speed imaging
Figure 1b: Optical measurement using PDA (taken from ^[1])

References

↑ CEJPEK Ondřej, University of Technology, 2020

Nomenclature

Symbol	Description
A	cross-section
AT	arrival time to the measurement volume
Bo	Bond number
c	droplet concentration
$C_{D}$	discharge coefficient
D	mean droplet diameter
d	diameter
$D_{10}$	arithmetic mean diameter
$D_{20}$	surface mean diameter
$D_{32}$	Sauter mean diameter
F	force acting on a liquid element
Fr	Froude number
G	gravitational acceleration
K	nozzle dimension constant
L	characteristic distance
$l_{b}$	break-up distance
LDA1	velocity in Z-direction
LDA4	velocity in Y-direction
n	wave number
Oh	Ohnesorge number
p	pressure
Q	flow rate
q	liquid-to-air momentum ratio
r	radius
Re	Reynolds number
S	swirl number
Stk	Stokes number
SCA	spray cone angle
t	time
TT	transit time through the measurement volume
Tu	turbulence intensity
u	velocity
U12	phase shift between photomultipliers 1 and 2
U13	phase shift between photomultipliers 1 and 3
w	swirl component of the velocity
We	Weber number
X, Y, Z	Cartesian coordinates

Greek symbols
$\Delta v$	difference between the gas and droplet velocity
$\eta _{n}$	nozzle efficiency
$\mu$	dynamic viscosity
ρ	liquid density
σ	surface tension

Indices
a	aerodynamic
ac	air core
c	swirl chamber
cf	cross-flow
Cr	critical
D	droplet
g	gas
i	index number of a droplet
in	atomiser inlet (inlet ports)
l	liquid
m	inertia
n	total number of droplets
o	exit orifice
p	pressure
r	relative
v0.1, v0.5, v0.9	volumetric fractions 0.1, 0.5 and 0.9 of the total droplet volume
µ	related to dynamic viscosity
σ	related to surface tension
$\tau$	liquid film thickness

Abbreviations
AC	air core
fps	frames per second
GT	gas turbine
HSC	high-speed camera
HSV	high-speed vizualization
LDA	laser Doppler anemometry,
PDA	phase Doppler anemometry
PSA	pressure-swirl atomiser
RSF	relative diameter span factor