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Experimental Setup

The PSA spray was investigated experimentally using PDA and 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). An HSC documented the complexity of the liquid discharge, formation and break-up of the liquid film, and the spray morphology.

Wind tunnel facility

The controlled experiment with cross-flowing air was provided using a small-scale, low-speed wind tunnel. It is an open-loop type wind tunnel with a closed test section in a blowdown arrangement (Figure 5 and Figure 6). The tunnel components (3–8) are fixed together and allow positioning of the tunnel body in all three axes. This wind tunnel setup allows measurement techniques to be rigidly installed, and additional measurement errors associated with positioning or vibrations of the measurement components are thus eliminated. Two systems were used for traversing the tunnel. The first one was a linear traverse system (T1) (ISEL AUSTRIA GMBH & CO. KG, Austria). The second supporting system (T2), equipped with screw jack MSZ-5-A-SL-Tr-1804-1-H0200-SRO-VS-BF (ZIMM GMBH, Austria) and synchronised with T1 unit to move the tunnel in a vertical direction (Z axis), was used. The T2 unit contains a rail support HGR25R rail, HGW25SC flange support (HIWIN S.R.O, Czech Republic) for movement in the horizontal plane (X, Y-axis). Four stabilisers are incorporated within T2 to stabilise the wind tunnel structure. The T1 and T2 units were joined to a 3D computer-controlled system for automatic positioning relative to the measurement volume of the PDA (Figure 5 right). A direct-driven radial low-pressure suction fan (1) RFC 355-15/3-3-L-Z (ALTEKO S.R.O, Czech Republic) was used for airflow generation. A fabric compensator (2) type B4.0 (KDMM s. r. o, Slovak Republic) connected the movable structure with the static fan (1). The air flows through an inlet diffuser (3), which connects the compensator (2) with a settling chamber (4). A wire mesh and three honeycombs were inserted into chamber (4) to laminarise the flow and reduce lateral and transversal velocity fluctuations. The flow accelerates, and velocity character further improves in a confusor (5) with a contraction ratio of 9. The air then flows via a channel (6) to the test section (7), where the spray measurement takes place. Each side of the test section is equipped with float glass windows. The front window is perpendicular to the transmitting optics of the PDA system and HSC. The test section has a rectangular shape with an internal cross-sectional area of 200 × 200 mm and a length of 400 mm.
The walls are built from float glass which is transparent for visible light wavelengths. It ensures smooth and planar walls that are parallel/perpendicular to each other. The construction of the test section, which holds the glass windows (Figure 1, right), was provided from the outside. So no ledges or surface imperfections are present. It is composed of four 25 × 25 mm steal square rods with grooves for glass support and mounted on two steel frames. The windows are fixed in grooves of approximately the same width as the glass plates to ensure that the windows are parallel.
The atomiser was mounted on the top of section (7) with its discharge orifice positioned 35 mm below the top surface of the section. The atomiser main axis was 150 mm away from the section inlet and crossed the main section axis, as shown in Figure 1, right. The part of the atomiser body which protrudes into the section space can be, for simulation purposes, considered as a cylinder with a diameter of 45 mm and 20 mm long with its axis identical to the main atomiser axis.
The flow velocity in the test section is controlled via the frequency shifter SV040iG5A-4, 4 kW (LS Industrial Systems Co., Ltd., Czech Republic) and can be set in the range from 0 to 40 m/s. The wind tunnel ends with an exit diffuser (8) connected to an exhaust pipe.
The design, construction, used components and testing of the tunnel are described in detail in the master thesis ^[1]. The design is patented ^[2].

Figure 5a': Wind tunnel design
Figure 5b: Linear 3D computer-controlled traverse system(taken from ^[1])

Figure 6a': Wind tunnel outline
Figure 6b: Detail of supporting system (taken from ^[1])

Atomiser under test and its supply system

The PSA features a hemispherically shaped swirl chamber and two tangentially arranged inlet channels with rectangular cross-sections for liquid supply, as illustrated in Figure 7. The pressurised working liquid (non-treated tap water was used here) is forced by these inlet ports to swirling flow and then discharged through a circular discharge orifice into the space of the test section.
The water was supplied from a storage tank (11) to the atomiser (17) by a gear pump (13) through filters (12), flow rate, temperature and pressure sensors (14, 15, and 17, respectively). The flow rate was controlled by the pump speed, see Figure 8. The water flow rate was measured by Coriolis mass flow meter Mass 2100 Di3 fitted with the Mass 6000 transmitter (Siemens AG, GE) (14) with an accuracy of ±0.1% from the actual flow rate. The water with a temperature of 20 ±2°C was continuously sprayed at constant inlet over-pressure $p_{in}$ = 0.5 MPa into untreated air in the tunnel section. The air velocity was set to the nominal velocity of 0, 8, 16 and 32 m/s. The air had a temperature of 21°C ±2°C, pressure of 980 kPa and humidity between 20 and 30%. The uncertainty of temperature sensing (PR-13, OMEGA Engineering, Inc., US) was 0.2° C. The static over-pressure of water at the atomiser inlet was measured by a piezo-resistive sensor DMP 331i (BD SENSORS s.r.o, CZ) (16) mounted close to the nozzle inlet with a measurement uncertainty of ±1.5 kPa. The discharged liquid was not reused, fresh water was used for each test.

Figure 7: Atomizer design with main dimensions in mm

Figure 8: Atomizer supply system

The object orientation and coordinate system

The orientation of the experiment and of the results reported here and in ^[3] are described using a Cartesian coordinate system, as depicted in Figure 5b and Figure 9. The main axis of the atomiser is vertically positioned and identical to the Z-axis, with a downstream-oriented discharge orifice, the downstream direction of the liquid discharge and spray flow (positive Z-axis direction).
The main axis of the wind tunnel test section is identical to the Y-axis of the coordinate system; the flow direction is on the right-hand side from the camera view, which corresponds to the positive direction of the Y-axis. It agrees with the common practice of using the positive part of the Y-axis in flow studies. The X-axis lies in the horizontal plane, and it is perpendicular to the airflow.
So the XY-plane is horizontal, XZ-plane is vertical and perpendicular to the flow, and YZ-plane is vertical and parallel to the flow. The origin of the coordinate system is placed in the centre of the exit cross-section of the atomiser discharge orifice.
The coordinate system reported in the raw PDA measurement files (txt files) differs from the above-described laboratory coordinate system in this way:

the Y-axis has reversed signs (Y = –20 mm in the raw data files corresponds to Y = 20 mm in the presented or processed results)
the Z-axis is shifted by –10 mm (raw data at Z = 0, 5 and 10 mm corresponds to Z = 10, 15 and 20 mm in the presented results).

References

↑ ^1.0 ^1.1 ^1.2 CEJPEK and Ondřej, University of Technology, 2020
↑ J. JEDELSKÝ, O. CEJPEK, and M. MALÝ, (2023 )
↑ O. Cejpek, M. Maly, J. Slama, M. M. Avulapati, and J. Jedelsky, Continuum Mechanics and Thermodynamics 34 (6), 1497 (2022)