EXP 1-1 Experimental Set Up

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

Front Page

Introduction

Review of experimental studies

Description

Experimental Set Up

Measurement Quantities and Techniques

Data Quality and Accuracy

Measurement Data and Results

Experimental Setup

The PSA spray was investigated experimentally using PDA and HSV. The atomizer sprayed water into cross-flowing air at varying flow velocities, $u_{cf}$. The tests were conducted in 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 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 conducted using a small-scale, low-speed wind tunnel. It is an open-loop type wind tunnel with a closed test section in blowdown arrangement (Figure 5 and Figure 6). The measurement equipment is stationary (fixed to the floor of the laboratory), and the whole wind tunnel, including the atomizer (components 3–8 that are fixed together to form the compact tunnel body), can be moved with the traversing system in all three directions to position the atomizer vis a vis the selected measurement volume. The whole moveable wind tunnel was connected to the stationary fan (1) through a flexible compensator (2). This wind tunnel setup (which is rather unusual and the other way around in most similar experiments) 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 were 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 system, i.e. to the measurement volume of the PDA (Figure 5b). 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 the velocity character further improves in a contraction (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), 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 atomizer was mounted on the top of section (7) using a holder. The discharge orifice of the atomizer was positioned 35 mm below the top surface of the section. The atomizer main axis was 150 mm away from the section inlet and crossed the main section axis. Positioning of the atomizer relative to the section (7) is shown in Figure 10. The atomizer holder with the atomizer body which protrude into the section space can be, for simulation purposes, considered as a cylinder with a diameter of 45 mm and 27 mm long with its axis identical to the main atomizer axis, see Figure 10, detail A.
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 . The design is patented .

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  Figure 5a: Wind tunnel layout

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  Figure 5b: Linear 3D computer-controlled system for traversing the wind tunnel, part T1 in Figure 5a (taken from ). The main axis of the wind tunnel test section is identical to the Y-axis; the air flows to the right. The main axis of the atomizer and spraying direction are vertical and identical to the Z-axis.

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  Figure 6a: Photo of wind tunnel

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  Figure 6b: Detail of supporting system, part T2 in Figure 5a (taken from )

Atomizer used in the 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. The atomizer cut-out with its internal cavity is illustrated in Figure 7. The external dimensions of the atomizer are a diameter of 9 mm and a length of 8 mm. The external dimensions of the atomizer holder are a diameter of 45 mm and a length of 27 mm. Their dimensions and fitting in the test section are shown in Figure 10. 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 atomizer (17) by a gear pump (13) through filters (12), flow rate, temperature and pressure sensors (14, 15, and 16, 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 ±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 atomizer 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 are described using a Cartesian coordinate system, as depicted in Figure 5b and Figure 9. The main axis of the atomizer 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 direction of the Y-axis in flow studies for the flow direction. 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 atomizer discharge orifice.
The coordinate system reported in the raw PDA measurement files (txt files) differs, for technical reasons, 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

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Contributed by: Ondrej Cejpek, Milan Maly, Ondrej Hajek, Jan Jedelsky — Brno University of Technology

Front Page

Introduction

Review of experimental studies

Description

Experimental Set Up

Measurement Quantities and Techniques

Data Quality and Accuracy

Measurement Data and Results

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