Tomonori Mukunoki 1,*, Masaru Umezawa 1, Konstantinos Zarogoulidis 1,2, Koichi Hishida Introduction

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1 Development of a planar velocity measurement technique for gas-solid particles in liquid flows using the characteristics of scattered light on phase surfaces Tomonori Mukunoki 1,*, Masaru Umezawa 1, Konstantinos Zarogoulidis 1,2, Koichi Hishida 1 1: Department of System Design Engineering, Keio University, Yokohama, Japan 2: Department of Mechanical Engineering, Imperial College London, London, United Kingdom * correspondent author: mukunoki@tfe.sd.keio.ac.jp Abstract A new non-intrusive measurement technique for the measurement of the liquid and bubble velocity distributions of a three-phase mixture flows where the bubble and tracer size is comparable is presented. The arrangement aims for simplicity and employs a single-camera arrangement that receives light from a Light Emitting Diode (LED) and a continuous-wave laser. Due to the light scattering characteristics, continuous light illumination and constant exposure, the resulting images contain black streaks due to the shadow of the tracer particles and white streaks due to the glare points generated by the bubble-scattered light. Interrogation of the streaks provides the velocity vector information for both the gaseous and liquid phase. The bubble size information can also be obtained via the glare point separation for each bubble. The technique was successfully applied into a microbubble-mixture transiently flowing in a container with velocity vectors exhibiting the expected behavior with an error margin of less than 2% for the gaseous phase and less than 3% for the liquid phase, and in a device that uses swirling motion to achieve bubble void-fraction separation from the liquid mixture. 1. Introduction Bubbly flows manifest in many industrial applications and processes found in power production and chemical plants; typical examples would be water cooling environments, gas purification systems, etc. As such, capable and preferably non-intrusive instrumentation is required for the successful characterization and continuous improvement of such flows. In the past, a variety of such non-intrusive instrumentation has been proposed (and reviewed, for example, in Azzopardi, 1979). Recent approaches include the phase-doppler instrument (Bachalo 1984), the time-shift technique (Albrecht, 1993) which also uses a phase-doppler instrument, interferometric imaging (Maeda et al., 2000) and glare point velocimetry (Dehaeck et al., 2005). Additionally, imaging approaches are still popular (e.g. Tassin and Nikitopoulos 1995; Lelouvetel et al., 2014), as they allow for the better large bubble size determination where the assumption of the presence of perfectly spherical bubbles cannot hold. All these techniques exhibit strengths and weaknesses and their applicability depends on setup complexity, the number of velocity vector components that must be simultaneously measured and size measurement accuracy. All the above methods are effective, but unless combined with a secondary measurement technique, such as Particle Image Velocimetry (PIV), they provide information only for the gaseous phase. There is, however, the need to analyze the gaseous phase in conjunction to the liquid phase to determine the interaction between the two phases and quantities such as the exchange of turbulent kinetic energy (Lelouvetel 2011). Using photographic techniques when the bubble size is much larger compared to the tracer particle size is efficient, because the bubble projections are easily discernable; however, when the size between the bubbles and tracer particles is comparable, the separation is not straightforward (Fig. 1). In the current contribution, we describe a new non-intrusive planar technique for the measurement of the gaseous and liquid phases that employs a simple single-camera experimental arrangement for the investigation of liquid flows with high accuracy for the resolved bubble size and velocity distributions. Due to the presence of three phases in the mixture under consideration, the technique relies on the scattering properties of light on the surface of each discrete phase

2 Fig. 1 Bubble and tracer identification is difficult when they are of the same size 2. Measurement Principle and Implementation The proposed experimental technique is illustrated in Fig. 2. The liquid transporting bubbles and tracer particles is introduced to a transparent test section and is illuminated by two separate light sources. The presence of the tracer particles is solely for the determination of the liquid phase velocity, as is the case with typical measurement techniques such as PIV or Particle Tracking Velocimetry (PTV). A CCD camera is placed in the forward-scattering angle of a LED that is illuminating the mixture and at a separate nonforward scattering angle of a continuous wave laser that is less than 90 in relation to the CCD camera array. To reduce light aberrations due to the presence of the walls, flat-windows are recommended so that the light enters and exits the test section in normal planes. Fig. 2 The measurement concept Due to the LED light, the tracer particle and bubble projections produce shadows in the resulting camera image. The LED light is covered with semi-transparent paper, to diffuse the light and reduce effects such as diffraction due to the imaged particles and bubbles and is afterwards expanded via a pair of spherical lens lenses, so that the desired measurement area can be successfully illuminated. In typical shadow-imaging techniques, when the size of the bubbles is comparatively larger than the tracer particles, it would be relatively easy to discriminate between the phases and subsequently derive the bubble size and velocity distribution of both the gaseous and liquid phases. However, in the case where the bubble size is comparable to that of the tracer particle size, the two phases would be very difficult to discriminate. Fortunately, the properties of the laser light that is scattered by the bubbles can be utilized to advantage; the laser light is responsible for the formation of bright glare points on the surface of the bubbles, as described by van de Hulst (1991) and illustrated in Fig. 3, which brightens substantially the irradiation contribution of the - 2 -

3 bubbles in the resulting image. The glare points appear at almost all angles of observation, but their number and ease of analyzing them varies greatly depending on the angle (van de Hulst, 1957). Generally, for airwater combinations, two glare points corresponding to the reflection and first-order refraction rays will appear from between An observation angle at 45 provides measurement simplicity and adequate glare point clarity when bubbles are involved. The glare point intensity and separation distance are directly proportional to the bubble size ( ~ d, see e.g. Semidetnov and Tropea, 2004) from which they are created, with the intensity being proportional to the 2 square of the bubble size ( ~d ). The small distance between the glare points in smaller bubbles and their intense illumination leads to the glare points not being discernible in the final image, given typical camera resolutions, and the two glare points contribute together to the high brightness of the bubble streak. Fig. 3 The glare point generation process As the tracer particles are not transparent, glare points will not be formed due to their presence, so their only contribution is the darkening of the image area their projection occupies. Following this double illumination and, for a given exposure time, black and white streaks are created in the image for the tracers and the bubbles, respectively, due to their movement in the exposure time interval. The resulting images therefore contain direct information for the gaseous phase via the white bubble streaks and the liquid phase is resolved accordingly by the solid tracer black streaks accordingly. The streaks are created through exposure for a constant time interval for each captured frame and therefore their length is proportional to the exposure time and the velocity of the tracer or bubble. Figure 4 presents a typical acquired image, with the insets demonstrating the streaks produced by each phase. Fig. 4 A typical acquired image Figure 5 illustrates the algorithm used for the discrimination of the phases from the original acquired images that follows Tani et al. (2002). Initially, and after background noise reduction, the single captured image containing the information for both phases is separated into two images one containing only the bubble streaks and the other containing only the tracer streaks. Using the streak information and displacement, Particle Tracking Velocimetry can be used to determine the particle and bubble velocity vectors

4 Bubble Recorded camera image Solid tracer Bubble image Phase discrimination Tracer image Detection of particle position by streak PTV Bubble velocity Liquid velocity Fig. 4 The algorithm for the phase discrimination from a single image. The exposure timing chart is illustrated in Fig. 6. Each image is exposed for time Δt and consequent images at time difference t 2 - t 1. The length of each streak is estimated via local dynamic thresholding and, provided its length L pt and thickness L p, an initial estimate of its velocity can be determined. Using the estimated velocity, a search window is set in the possible new location ΔL of the streak in the second image and the streak is subsequently identified by using cross correlation with higher accuracy. Using the positions of the streak in both images and the time difference between the subsequent exposures, the velocity vectors can be estimated for both image pairings, hence providing for both the liquid velocity information from the black streak images and the bubble velocity information from the white streak images. 3. Experimental validation Fig. 5 Timing charts for the phase discrimination from a single image. The experimental technique was tested using a simple arrangement that is illustrated in Fig. 7. Water containing generated air microbubbles with a mean diameter of 40µm and tracers with a manufacturer

5 provided size distribution close to a size of approximately 15µm (density 1.15kg/m 3 ) was injected into the test section via an upper inlet and measured before settling. The test section was chamfered at the point of laser light entrance so that light distortion would be minimized. The LED light used had a wavelength of 433nm while the continuous laser light emanated at 632.8nm. A CCD camera (12bit, pixel) was positioned at 45 in relation to the laser light illumination and at 0 in relation to the LED light. The imaged area size was 9.6mm 8mm, at 30mm downstream of the inlet and bubbles could be discriminated at ±1mm from the focal plane. Close to the settling of the mixture, 150 images were obtained at the transient state of the flow at a rate of 16fps with each exposed for 10ms. Fig. 7 Experimental test section used for validation of the technique Figure 8 illustrates the instantaneous vector fields for the bubble and liquid phases for the same pair of consequent recordings. It is apparent that the buoyancy-driven upwards flowing bubble motion and the recirculating liquid motion due to the still-settling liquid could be clearly distinguished. Tab. 1 indicates the bubble streaks successfully identified as rising in the imaged area, along with the tracer streaks sinking. The erroneous vectors were less than 3% for the tracers and 2% for the bubbles. Fig. 8 Instantaneous resolved bubble (left) and liquid (right) velocity vector field Tab. 1 Successful streaks identified Streaks Measured Total Percentage Microbubbles (rising) % Tracers (sinking) % Additionally, the bubble size distribution could be evaluated from the size of the bubble streaks in the images, as shown in Fig. 9. Although the uncertainty of size estimation via the glare point separation is higher than using, for example, the out-of-focus interferometric imaging suggested by Maeda et al. (2000), - 5 -

6 the similarity in size between the bubbles and tracers can be observed. Fig. 9 Bubble and tracer size probability distribution 4. Application to Void Fraction Controlling Apparatus Bubble and liquid phase measurements in a bubble void fraction controlling device, shown in Fig. 10, were also conducted. The device introduces the mixture with the liquid phase carrying bubbles and tracers from the top section inlet with swirl. Due to the swirl, a cyclone is created that separates the bubbles depending on their size; the large bubbles exit the device through the top section outlet and for the smaller ones through the lower section outlet. For the experiments, air bubbles and tracers in water of mean diameter of 40µm and 20µm, respectively, were introduced to the test section at a rate of approximately 400ml/min. The mixture was illuminated using the same LED light arrangement with the verification experiment (433nm) in conjunction to a continuous wave laser (10mW at 476nm). A CCD camera (12bit, pixel) was placed at 45 in relation to the forward-propagation direction of the laser light. The top starting point of the measurement area was located at the entrance of outlet 2, whose diameter was 7mm, and out of which liquid and bubbles of larger size exit the device. The measurement area size was 11mm 11mm and the exposure time for each image was set to 10ms, captured at 20fps. To avoid excessive influence of the tangential velocity component, the swirl number was low, and was measured by Laser Doppler Velocimetry (LDV) to be 0.055, at 10mm upstream of outlet 2. Fig. 10 Schematic of the bubble void controlling device concept Figure 11 shows instantaneous resolved velocity distributions for the bubble and liquid velocities of the - 6 -

7 mixture. The expected movement of both phases towards the entrance of the outlet was clearly captured in these instantaneous images, with the flows contracting towards the rim of the outlet. Fig. 11 Instantaneous bubble and liquid velocity upstream of the internal outlet The mean velocity fields for both phases extracted from a series of 100 images can be seen in Fig. 12. Although the movement of the phases is similar, the intensity differs with the gaseous phase moving more!! rapidly towards the outlet. Figure 13, illustrates the vector difference V b Vl between the bubble and liquid average ensemble mean velocity. Overall, the bubbles could move by up to two times faster, compared to the tracers, but the difference is intensified towards the entrance of the inner outlet. 5. Conclusion Fig. 12 Mean bubble and liquid velocity upstream of the internal outlet A new technique that aims for simple non-intrusive, single-camera measurements of three-phase flows has been introduced. The technique is aimed at flows where the bubble size is comparable to the tracer size, where traditional photographic techniques are hindered by the similarity of the projections between the two phases. The technique has successfully described qualitatively and qualitatively the flow field of two different flow arrangements with and without swirl and measured the velocity distribution of the liquid and gaseous phases to acceptable amounts of error. Further validation with a conventional measurement technique is needed however, to establish the validity and accuracy of the technique and enforce confidence in its extracted results

8 !! Fig. 13 The difference in the velocity components between the bubble and liquid phase ( V b V ) Acknowledgements The authors gratefully acknowledge the Erasmus Mundus Build on Euro-Asian Mobility (BEAM) for the financial support of K. Zarogoulidis during his stay at Keio University. References Albrecht HE, Borys M, Hübner W (1993) Generalized Theory for the Simultaneous Measurement of Particle Size and Velocity using laser doppler and laser two-focus methods. Part Part Syst Charact 10: Azzopardi BJ (1979) Measurement of Drop Sizes. Int J Heat Mass Transfer 22: Dehaeck S, van Beeck JPAJ, Riethmuller ML (2005) Extended glare point velocimetry and sizing for bubbly flows. Exp Fluids 39: Lelouvetel J, Nakagawa M, Sato Y, Hishida K (2011) Effect of bubbles on turbulent kinetic energy transport in downward flow measured by time-resolved PTV. Exp Fluids 40: Lelouvetel J, Tanaka T, Sato Y, Hishida K (2014) Transport mechanisms of the turbulent energy cascade in upward/downward bubbly flows. J Fluid Mech 741: Maeda M, Kawaguchi T, Hishida K (2000) Novel interferometric measurement of size and velocity distributions of spherical particles in fluid flows. Meas Sci Tech 11:L13 L18 Moriya G, Yasui R, Hishida K (2012) Time series volumetric velocity measurement in aneurysm models by shadow imaged sereo streak PTV. 16 th Int Symp Appl Laser Tech Fluid Mech, Lisbon Semidetnov N, Tropea C (2004) Conversion relationships for multidimensional particle sizing techniques. Meas Sci Tech 15: Tassin AL, Nikitopoulos DE (1995) Non-intrusive measurements of bubble size and velocity. Exp Fluids 19: Tani N, Kondo H, Mori M, Hishida K, Maeda M (2002) Development of fiberscope PIV system by controlling diode laser illumination. Exp Fluids 33: van de Hulst HC (1957) Light scattering by small particles. Wiley, New York van de Hulst HC, Wang RT (1991) Glare points, Appl Opt 30: l - 8 -