Internal flow and air core dynamics in Simplex and Spill-return pressure-swirl atomizers

Internal flow and air core dynamics in Simplex and Spill-return pressure-swirl atomizers MALÝ, M.; JEDELSKÝ, J.; SLÁMA, J.; JANÁČKOVÁ, L.; SAPÍK, M.; WIGLEY, G.; JÍCHA, M. International journal of heat and mass transfer 2018, vol. 123, August 2018, pp. 805-814 ISSN: 0017-9310 DOI: https://doi.org/10.1016/j.ijheatmasstransfer.2018.02.090 Accepted manuscript © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license (http://creativecommons.org/licenses/by-nc-nd/4.0/), doi: https://doi.org/ 10.1016/j.fuel.2013.03.053 Final version available from https://www.sciencedirect.com/science/article/pii/S0017931017356879 dspace.vutbr.cz Internal flow and air core dynamics in Simplex and Spill-return pressureswirl atomizers Milan Malý*1, Jan Jedelský1, Jaroslav Sláma2, Lada Janáčková1, Marcel Sapík1, Graham Wigley3, Miroslav Jícha1 1 Faculty of Mechanical Engineering, Brno University of Technology, Czech Republic 2 Provyko s.r.o, Czech Republic 3 Loughborough University, United Kingdom *Corresponding author: [email protected] Abstract Spill-return (SR) atomizers enhance the construction of Simplex atomizers by addition of a passage in the rear wall of the swirl chamber through which the liquid can be spilled away. It allows to discharge the liquid always at a high pressure and to spray well over a wide flow rate range. The spray characteristics of pressure-swirl atomizers are strongly linked to the internal flow, and the air-core dynamics affect the spray stability. The SR atomizers are rarely studied and their internal flow is not studied at all. Therefore, in this paper, the Simplex and SR atomizers with a central SR orifice were examined comparatively. Transparent polymethyl methacrylate (PMMA) models of both atomizers scaled 10:1 were manufactured for the visualization and velocity measurements of the flow inside the swirl chamber. The atomizers were examined by means of high-speed imaging, laser-Doppler anemometry and computational fluid dynamics tools. The experimental and numerical results were analysed and compared in terms of the spray cone angle (SCA), discharge coefficient (CD), and the morphology and temporal stability of the air core. The internal flow characteristics between the original and the model were matched using the Reynolds, Swirl and Froude numbers. The test conditions were limited to inlet Reynolds numbers from 750 to 1750. The results show that the addition of the spill passage strongly affects the internal flow even if the spillline is closed. The air core in the Simplex atomizer is fully developed and stable for all flow regimes. The SR atomizer behaved differently; with closed spill-line (spill-to-feed ratio, SFR=0), the air core does not form at all; therefore the spray is unstable. The reason is that the liquid, contained in the spillline, is drained back into the swirl chamber due to a recirculation zone found inside the spill-line. Increasing the SFR stabilizes the internal flow, and the spray becomes stable if SFR > 0.15. The air core begins to form for SFR > 0.4. The results suggest that the axially positioned spill orifice is inappropriate and its placing off-axis would improve the spray stability. The results of the 2D numerical simulation matched closely with the experiments in terms of SCA, CD, velocity profiles, and air core morphology which proved its prediction capabilities. Keywords Internal flow dynamics, Pressure-swirl, transparent nozzle, CFD 1 Introduction Pressure-swirl (PS) atomizers are used in many applications where a large surface area of droplets is needed, or a surface must be coated with a liquid, e.g. combustion, fire suspension or air conditioning. PS atomizers are easy to manufacture, reliable and provide good atomization quality. They convert the pressure energy of the pumped liquid into kinetic and surface energy of the resulting droplets. The liquid is injected via tangential ports into a swirl chamber where it gains a swirl motion under which it leaves the exit orifice as a conical liquid sheet. The centrifugal motion of the swirling liquid creates a lowpressure zone in the centre of the swirl chamber and generates an air core along the centreline. The flow inside the atomizer is rather complex; it is two-phase with secondary flow effects. There is a strong link between internal flow conditions and the resulting spray characteristics. However, not all aspects of the internal flow are well understood. A drawback of the Simplex atomizer is that the droplet size depends on the inlet pressure, hence on the liquid flow rate. The flow rate varies as the square root of the injection pressure. Thus, doubling the flow rate demands a fourfold increase in injection pressure, which means that the range of applicable flow rates is limited and thus the turn-down ratio (defined as ratio of 2 maximum liquid flow rate to minimum liquid flow rate which fulfils the requirement of atomization quality) is usually low [1]. This disadvantage can be eliminated using a SR atomizer which is basically a Simplex type with a passage added in the rear wall of the swirl chamber, see Figure 1. When the spillline is closed, the atomizer operates as a standard Simplex type. When a low injection flow rate is required, the liquid is spilled away through the spill orifice while the inlet pressure and the swirl momentum remain high, and the atomization quality remains. However, increasing spilled flow rate causes reduction in the axial momentum of discharged liquid which consequently leads to change in the spray cone angle (SCA), as the SCA is determined by the ratio of the swirl momentum to the axial momentum. Another drawback is the requirement for increased pump power and complicated for flow metering. For these reasons, the interest in SR atomizers for aircraft combustors declined, however, if the aromatic content of gas turbine fuels rises, gum formation in the small sized atomizers could pose serious problems of the atomizer blockage [2, 3]. The SR atomizers are virtually free of this defect as they have no small passages. Beside the aircraft combustors, the SR atomizer were used in stationary gas turbines [4] and industrial burners [5]. However, the above-mentioned advantages of SR atomizers are crucial in special applications that require a fine spray at very low flow rate, e.g. decontamination devices [6], or for atomization of waste fuels and liquids containing impurities where large dimensions of flow cross-sections are necessary to prevent the atomizer from clogging, or in applications where pneumatic atomizers are not allowed but the wide regulation range is required. The studied spill-return atomizer is originally used in a combustion chamber of small turbojet aircraft engine manufactured by PBS Velká Bíteš, a.s., Czech Republic. Before the advent of computational fluid dynamics, a number of authors attempted to describe the internal flow of Simplex atomizer by relatively simple analytical approaches. One of the first was presented by Taylor [7] who focused on an inviscid analysis using Bernoulli’s equation and the principle of maximal flow. Taylor derived an equation for the discharge coefficient (CD) and the spray cone angle (SCA) solely dependent on the atomizer constant k = 2·Ap/(π·do·ds), where Ap is the total area of the inlet ports, do, and ds are defined in Figure 1. Similar results were found independently by other authors, and these works have been compared and reviewed by Chinn [8, 9]. Results obtained by the inviscid theory are not generally in good agreement with experiments. However, findings from the inviscid theory may be used as a basis for design improvements. The experimental correlations for CD were found to be more complex than the inviscid theory predicted. Rizk and Lefebvre [10] derived a semi-empirical correlation where, besides the constant k, the ratio ds/do had a strong influence. Jones [11] found a weak dependence of CD on the length of the swirl chamber and exit orifice, and liquid viscosity. Ballester [12] added a dependence on the inlet pressure. Benjamin [13] followed the work of Jones [11] and found inverse trends for some parameters. Wimmer and Brenn [14] theoretically uncovered a relatively strong effect of the liquid viscosity on CD, which was later experimentally confirmed by Maly et al. [15]. The internal flow characteristics, especially the air core stability, were investigated by a few authors. Halder [16] investigated the air core shape in 21 different transparent atomizers at various inlet mass flow rates of water. Two limiting values of Reynolds number (Re) were conducted for the inception of the air core for each atomizer. Below the lower limit, the air core was not formed at all, while above the upper limit, it was always found to be stable. He observed that the limiting Re decreases with an increase in do/ds and a decrease in Ap/ds. The stable air core had a cylindrical shape, and for large Re values, it was almost constant in diameter. For Re values close to the limiting value, the diameter of the air core increased sharply with increasing Re. A similar concept of limiting values of Re was introduced by Lee et al. [17]. In this experimental work, a transparent atomizer with diesel and kerosene used over a range of inlet pressures and temperatures. They deduced that the air core stability was a function of Re related to the exit orifice, Reo. It was stable for Re > 3300; at lower values it became unstable until for Reo below 2400, where there was no air core at all due to insufficient centrifugal forces, and the spray fluctuated strongly. Kim et al. [18] investigated the influence of diameter and length of the swirl chamber on the air core stability. Atomizers with a ratio of swirl chamber height to its diameter hs/ds higher than 1.27 demonstrated an unstable air core. The authors [18] described the unstable air core as having a rotating and double helical structure. Moon [19] found a limiting value of the swirl number S0 = 0.6, which ensured a stable air core. The same limiting value of S0 was also proposed by Park [20] for swirling jets. SR atomizers have rarely been studied, and their internal flow has not been documented so far to the best of our knowledge. Especially the effect of the spill orifice arrangement on the internal flow is 3 not at all clear. The liquid spill can be realized by a single axial orifice, by several off-axis orifices, or by an annular slot [21]. The simplest designs use a single, axially placed spill orifice but the problems with spray stability were reported [21-23], especially under operating regimes with a closed spill-line. The former approaches to study the internal flow were mostly experimental [24, 25] and analytical [8, 9, 26]. The application of CFD has greatly simplified design process of the atomizer due to increase in computing performance in recent years. In 1997, Yule and Chinn [27] conducted one of the first numerical studies using a 2D simulation. They assumed a laminar flow even for Re = 50,000; an internal air-core was captured by the Volume of Fluid (VOF) method. They reported the difference between numerical calculations and the experiment to be less than 3%. Similarly a 2D laminar setup was used by Amini [28] and Mandal [29]; both authors reported a close match with experimental data. Summer [30] compared 2D and 3D simulations with a laminar solution and found only a small differences between them. Madsen [31] tested laminar and turbulent k-ε models together with a Large Eddy Simulation (LES). The turbulent model overestimated the turbulent viscosity; the air-core was not formed at all. The laminar model was comparable to the LES predictions. Various models to capture the liquid–air interface were investigated by Baharanchi [32]. A geometrical reconstruction scheme was found to be an optimal method for capturing the air core. While there are some papers providing CFD simulations of Simplex atomizers, no numerical simulation of SR atomizer were found. Due to the lack of published results on SR atomization, the present study investigated experimentally and numerically the internal flow of SR atomizer. Firstly the work examines the possibility to predict the atomizer characteristics such as CD and SCA, and the velocity field in the swirl chamber, using a relatively simple 2D simulation. The main focus is to elucidate on the spray fluctuations, reported in our previous works [22, 33], and to determine their source. Furthermore, the internal flow characteristics are to be compared with a Simplex atomizer. 2 Atomizer geometries and liquid properties The experiments were performed using both Simplex and SR atomizer designs. In order to examine the internal flow, the atomizers were manufactured as transparent copies. Due to the small dimensions of the original atomizers (see Figure 1), it was impossible to manufacture them and to examine their flows directly. To solve this issue, the transparent versions were designed as ten times scaled copies. The scaled atomizers have a modular construction (Figure 2, right). The assembly consists of three parts, each made from PMMA. The bottom part contains the swirl chamber with the exit orifice, the central one forms the tangential inlet ports, while the top part is a plain wall, in the case of Simplex atomizer or, contains the spill orifice in the case of the SR atomizer. This modular construction allows for each part to be replaced by another one of a different geometry or shape. The surfaces of each part were ground and polished to achieve the transparency sufficient for optical access. Figure 1. A sketch of the original SR atomizer with the main dimensions in millimetres. The Simplex atomizer has the same geometry and size, but the spill-line orifice is missing. The transparent atomizer has the same shape, and all dimensions are 10 times larger. 4 Due to the ten times model scale it is necessary to match the flow of the original and scaled atomizers so the relevant dimensionless numbers must be considered. Re is defined as the ratio of inertial force to the viscous force. In the case of the swirl atomizer, the most common definition of Re is related to the inlet ports [34] as: ν p d p wRe = (1) where wp is the mean velocity in the inlet ports, calculated as a volumetric flow rate divided by the total cross-section of inlet ports, ν is the liquid kinematic viscosity, and dp is the hydraulic diameter of the inlet ports: ) p b p h( p b p h p d+= 2 (2) , for dimensions, see figure 1. The Re values for the scaled model must match those of the original to keep the same internal flow character. The Swirl number S0 is useful in determining the ratio of the angular momentum to the axial momentum. It can be calculated as a function of the internal geometry [34]: 0 S Rr A op π = (3) where R is a radius of flow entry to the swirl chamber and Ap is the total cross-section of the inlet ports. It is obvious that the swirl numbers for the original and scaled atomizers are identical. The Froude number (Fr) shows the effect of gravity in comparison with the energy of the bulk flow and is calculated as: 22 2( ) Q Fr r r rg o oa o π = − (4) where Q is the volume flow rate and roa is the radius of the air core in the exit orifice. To minimize the effect of gravity, it is necessary to keep Fr >> 1, as in the original atomizer case. The Froude number for the lowest pressure used was 6.9 thus the effect of the gravity was small. Spray related dimensionless numbers, such as Weber number and Ohnesorge number differ between the original and scaled atomizers by an order of magnitude thus the spray parameters were not investigated except for the spray cone angle, SCA, close to the exit orifice. Table 1 lists the experimental flow regimes with their dimensionless numbers. The operating regimes were derived from those used in previous study [22]. The main control parameter was the inlet pressure of the original atomizers and consequently its mass flow rate, from which the Re was calculated. The SR atomizer was evaluated with both the closed spill-line to simulate the maximum injection rate and various spill-to-feed (SFR) regimes. Kerosene-type Jet A-1 representing the commonly used fuel was used in both the original and modelled atomizer. However, the refractive index of kerosene differs from the refractive index of the PMMA by about 0.05 at 660 nm wavelength at 25 °C which disturbs the optical measurement close to the internal surfaces of the transparent model. A liquid with a refractive index very close to the PMMA should be used to reduce the optical distortions. For this purpose, several different liquids and mixtures were evaluated to determine the most suitable. Paracymene (p-cymene or 1-Methyl-4-(propan-2-yl)benzene) was chosen. It is a colourless, transparent organic compound with a refractive index different from Plexiglas by less than 0.001 at 660 nm wavelength and at 25 °C. It also has a relatively low aggressiveness to PMMA; however, after a few hours of measurement, it did cause cracks in those parts where increased internal stresses may be anticipated, i.e. in the vicinity of bolts and threads; thus, it was only used for high-speed imaging. The physical properties of Jet A-1 are σ = 0.029 kg/s2, μl = 0.0016 kg/(m·s), ρl = 795 kg/m3 and p-cymene: σ = 0.028 kg/s2, μl = 8×104 kg/(m·s), ρl = 850 kg/m3. Similarly designed test benches were used for testing of both the original and scaled atomizers, see Figure 2. The test liquids were supplied to the atomizer (8) from a fuel tank (1) via a filter (2) by a gear pump or a centrifugal pump (3) for the original and the scaled atomizer respectively. The mass flow was regulated by varying the pump speed. The fuel flowing through the inlet line was metered by the Coriolis mass flow meter Mass 2100 Di3 fitted with the Mass 6000 transmitter (Siemens AG, GE) (4) 5 with an accuracy ±0.1% of the actual flow rate. Static inlet over-pressure was measured by a piezoresistive pressure sensor DMP 331i (BD SENSORS s.r.o., CZ) (7). The uncertainty in the pressure sensing was 0.05 kPa and 2 kPa for the scaled and the original atomizer respectively as different sensors were used in each case. The inlet line was also equipped with a temperature sensor PR-13 made by OMEGA Engineering, INC., USA with an error of 0.2 °C. The spill-line had a piezo-resistive pressure sensor DMP 331i (BD SENSORS s.r.o., CZ) (9), a ball valve (11) and a positive displacement flow meter KOBOLD DOM-S05 with accuracy ±1% of the actual flow rate (KOBOLD Messring GmbH, GE) (10). The calculated uncertainty of CD at Re = 1021was 0.14 % and 0.25 % for original and scaled Simplex atomizer respectively. The atomized liquid was captured by a collection chamber and routed back into the fuel tank. Fuel mist and vapours were ventilated by a fan. The atomizer was mounted to a CNC positioning system with a positional error less than 0.1 mm. Figure 2 Left: Schematic layout of liquid supply. Right: A Schematic of the scaled transparent atomizer Table 1 Operating flow regimes, kerosene, S0 = 3.87 Original atomizer Scaled atomizer Re Δp ml CD Fr Δp ml CD Fr [–] [MPa] [kg/h] [–] [–] [kPa] [kg/h] [–] [–] Simplex 755 0.5 5.41 0.387 137 5 53.8 0.378 6.9 Simplex 1021 1 7.31 0.369 293 10 73.1 0.366 9.3 Simplex 1252 1.5 8.97 0.365 359 15 88.2 0.362 11.4 SR 1075 0.5 7.7 0.542 308 5 69.4 0.483 9.8 SR 1431 1 10.25 0.519 411 10 93.4 0.466 13.0 SR 1731 1.5 12.4 0.510 497 15 110.0 0.454 15.7 SR, SFR 0.4 1676 1 12.0 0.378 481 10 103 0.3 15 (2) (3) (7) (5) (6) (1) (8) (4) (11) (9) (10) (12) 6 Figure 3. High-speed visualization, p-cymene, 1 MPa, Simplex, cross-section a, b and c placed 2.5, 8 and 13 mm from the top of the swirl chamber 3 Experimental and numerical setups Following subchapters document the setups of the experimental approach using a high-speed camera and laser-Doppler anemometry (LDA) and the CFD simulations. 3.1 Experimental setup The experiments were performed on the cold test bench at room temperature. A Photron SA-Z high-speed camera was used to document the spatial and temporal behaviour of the air core. The atomizer was illuminated by a background light using an LED panel. Three records were acquired at each operating regime; the first was a general image showing the whole atomizer while the other two observed the exit orifice and the top of the swirl chamber in close up, see Figure 3. The camera frame rate was 4,000 and 20,000 fps for the general image; the resolution was 1024 × 1024 px, and the shutter speed was set to 20 μs. The close-up records used a frame rate of 28,000 fps, resolution 768 × 904 px, and a shutter time of 10 μs. Mean and RMS images were calculated for each regime. The air core dimensions were captured by MATLAB code based on the Canny edge detector. The air core fluctuations were analysed using the Fast Fourier Transform (FFT) in the cross-section b. The FFT was applied to the time-resolved air core surface captured by the Canny edge detector. Another FFT was used on the average pixel intensities over a rectangle 3 × 3 px placed near the air core boundary to verify the previous FFT approach. The air core dimensions were measured at three cross-sections (a, b and c) over the swirl chamber and one cross-section at the tip of the exit orifice. The LDA, a FlowExplorer (Dantec Dynamics A/S), was employed for the point-wise measurement of the velocity of individual particles inside the transparent atomizer. The swirl velocity component was measured in three cross-sections across the swirl chamber (see Figure 3 right). The axial distances from the top of the swirl chamber were 2.5, 8 and 13 mm for cross-sections a, b and c respectively, and 50, 38 and 25 measurement points were taken on each cross-section. The distance between two surrounding points was 0.25 mm. The LDA was configured in the backscatter mode. A built-in, diode-pumped solid-state laser generated a beam with 660 nm wavelength. The beam was split into two parallel beams with the power of 30 mW each. One of the beams was shifted by 80 MHz. A converging transmitting/receiving lens with 150 mm focal length was used to form an ellipsoidal measurement volume with the size of app. 0.1 × 0.1 × 0.8 mm. Dantec BSA P80 signal processor was used to process the measured signal. BSA flow software v5.20 was used to control the data acquisition and the following setting was used: Photomultiplier sensitivity 700 V, signal gain 20 dB, velocity centre 2.4 m/s, velocity span 4.8 m/s. The measurement was limited to 10,000 samples acquired or a 10-second acquisition duration at each measured point. A repeatability error based on three consequent measurements was less than 4%. The measuring volume position relative to the LDA positioning system had to be corrected due to the different refractive index of the atomizer body and the liquid as [35]: ) 1 11 ( 2 1 1 2 −+ = Sn R n n R S , (5) 7 where S1 is the virtual distance of measurement volume from the atomizer wall, S2 is the real distance of measurement volume, R is the diameter of the swirl chamber at measurement plane, n1 and n2 are the refractive indexes of PMMA and kerosene respectively. The measured velocity was multiplied by correction coefficient kvel based on the simplified approach from [35] as: R S n n vel k2 )1(1 2 1−+= . (6) The correction factor reached the maximum of 1.04 for kerosene at the atomizer axis. In positions close to the air-core, the raw velocity data were filtered since the strong noise was generated by the reflection from the air core surface. The filtration process seeks for the Gaussian distribution in the velocity histogram, and the mean velocity was calculated only from the data which satisfied the Gaussian distribution. The flow tracer particles were SL75 e-spheres with a mean diameter of 45 μm. Their Stokes number, based on the swirl velocity and diameter of the swirl chamber, was less than 0.01 for each regime, which ensured a sufficiently small flow traceability error. 3.2 Numerical setup Conservation of mass (continuity) and conservation of momentum (Navier–Stokes) equations were solved numerically using Ansys Fluent 17.2. The flow simulation was conducted as a transient 2D axisymmetric model. A Volume of Fluid (VOF) model with the geo-reconstruct scheme was used to capture the boundary of the air core. The 3D inlet boundary condition was set to conserve the mass flow rate in the radial direction and ensure the same angular momentum in the tangential direction. The pressure outlet boundary condition was applied on the outer boundaries with no-slip conditions applied on the wall boundaries. A laminar flow was assumed due to the low Re values inside the inlet ports, and also because inside the swirl chamber, the radial forces of the swirl tend to laminarise the flow [34]. The simulations were performed for both the original and scaled atomizers. The SR atomizer was simulated including a 4-mm long part of the spill-line geometry (see Figure 10 in section 4.3). The spill flow in the regime with SFR = 0.4 was set as a negative liquid source across the entire spill-line. It was not possible to set the pressure boundary condition to the spill-line wall as the solution was very unstable. The all quad structured mesh with an average skewness of 0.058 and an average aspect ratio of 1.18 was created (Figure 4), and the mesh independence test was carried out for four different element base sizes in terms of CD, SCA and the air core diameter (da) at the end of the exit orifice (do) in a dimensionless form as da/do (see Table 2). There was a significant difference between the meshes of 11,684 and 22,669 elements. This difference decreased with further increase in the number of elements, and the mesh with 46,765 elements was chosen as a good compromise between the accuracy and the calculation speed. Two sizes of an outflow area, which is an artificial area downstream of the atomizer outlet, were also tested. A calculation of four times larger outflow area revealed the same results as the original one, see results for meshes with 68,610 and 46,765 elements. Table 2. Mesh independence test Number of elements CD [–] da/do [–] SCA [deg] 11,684 0.392 0.655 58 22,669 0.365 0.707 58 46,765 0.358 0.710 57 68,610* 0.359 0.710 57 90,684 0.356 0.711 56 *The base size of the elements was the same as in the case of 46,765 elements. The outflow area was four times larger. 8 Figure 4. Left: Numerical domain and its mesh. Right: Typical results obtained with the wavy surface of the air core, phase distribution: 1 = air, 0 = liquid. 4 Results and discussion The air core shape and stability play a key role in the formation of the liquid sheet at the discharge orifice. A description of the air core dynamics is based on high-speed image records and numerical simulations. The discharge parameters are discussed in terms of CD and SCA. The measured swirl velocity profiles served for consequent validation of the numerical simulations. 4.1 Air core shape and spray cone angle In a comparative manner, the high-speed records with both kerosene and p-cymene as the working liquid are shown in Figure 5. For the kerosene image, there are darker regions towards the edge of the swirl chamber. This is caused by light refraction at the swirl chamber wall. It is not evident in the atomizer centre due to the small relative curvature. This is solved using the liquid with the same refractive index as the atomizer body which can be seen for the results of p-cymene. The air core was fully developed in the case of all the Simplex atomizers. It was cylindrically shaped and increased in its diameter inside the exit orifice; such behaviour was also described by other authors [16, 17, 28]. The dimensionless diameter of the air core in the exit orifice was da/do = 0.72 ± 0.02 for all the inlet pressures and both liquids with no evident correlations to Re. Inside the swirl chamber, da/do = 0.47 ± 0.03 and it was also almost independent of Re. Both findings are in accordance with other authors [16, 36, 37] who reported the independent air core size for high Re regimes, while Halder and Som [16] found a slightly increasing air core diameter with Re. Instabilities, in the form of air core fluctuations, both in the axial and radial direction (Figure 6), were observed at the top of the swirl chamber. These fluctuations are linked with the wavy structure on the air core surface. The frequency of the surface waves f = 32 ± 4 Hz was estimated using the FFT analysis of images for the Simplex atomizer with p-cymene at Re = 1021. A similar analysis was reported by Sumer et al. [30] who used a similarly sized atomizer, but with the velocity in the inlet ports approximately ten times higher; they found wave frequencies of f = 273 Hz. Chinn et al. in [38] studied the surface waves on the air core and described three distinctive types of surface waves: helical striations, stationary waves and random ripples. They noted that the stationary waves were responsible for changes in the liquid sheet thickness. The same phenomenon was also evident in our records. The helical striations, which are caused by finite number of the inlet ports, were not observed here.