1 7 th International Conference on Multiphase Flow, ICMF 2010, Tampa, FL, May 30 – June 4, 2010 EXPERIMENTAL STUDY AND FLOW VISUALIZATION OF GAS ENTRAINMENT IN DOWNWARD FLOW Jose M. Lopez 1,* , Dana V. Danciu 2 , Marco J. Da Silva 2,** , Uwe Hampel 2 and Ram Mohan 3 1 Mechanical Engineering Department, Universidad de Carabobo. Valencia, Carabobo 2005. Venezuela 2 Forschungszentrum Dresden Rossendorf, Dresden 01314. Germany 3 Mechanical Engineering Department, The University of Tulsa, Tulsa, OK 74014. U.S.A ABSTRACT Downward bubble flow due to the gas entrainment under a falling film was experimentally investigated using a wire- mesh sensor (WMS) and a high speed video camera. The flow pattern was identified using the flow visualization analysis of the WMS. This analysis was carried out in a 2-D (x, y), pseudo 2-D (x, t and y, t), and pseudo 3-D (x, y, t) dimensions. During the experiments the observed flow pattern was bubble flow. Good agreement was found by comparing high-speed video images with the images obtained with the WMS. Additionally, gas void fraction time series and bubble size distribution are presented. Results show that images obtained from the high speed camera, WMS and experimental observations are in good agreement. Also, it shows that different approaches for flow pattern recognition can be used. Moreover, results show the influence of the superficial liquid velocity on the gas void fraction and bubble size. Keywords: gas entrainment, flow visualization, flow pattern, wire-mesh sensor, gas void fraction INTRODUCTION The gas entrainment due to a plunging jet is a common phenomenon that can be found in nature, such as waterfalls and breaking waves in the ocean, as well as in many industrial processes, i.e. nuclear engineering, crude oil extraction, oil separation process, chemical processing, and environmental applications. Over the years, several researchers have studied this phenomenon trying to understand its mechanism [2], [3], [4], [5], [6]. However, the measurement techniques used to study this particular two-phase flow system, usually represent a constraint on the investigation of such complex phenomena. Most studies have been focused on the influence of different parameters of the gas entrainment, such as nozzle distance from the impact point and impact velocity [1], [4], [9], [10]. Because of the complexity of their measurement, parameters, such as bubble size, gas void fraction and flow visualization on the cross section of the flow are less accounted for in the literature. In most cases, non-intrusive measuring techniques are used [3]. A very important feature of these devices is that they do not generate perturbations on the flow field. Although, non-intrusive techniques are usually unable to give detailed information of the cross section of the flow. Moreover, flow image processing is of vital importance to understand the physical mechanism of the entrainment phenomenon. Over the years, high speed video cameras have been used to capture many fluid flow phenomena (especially two-phase flow). This technique of measurement has the limitation of capturing only one plane of measurements, usually the plane that is closer to the wall. Moreover, special attention has to be taken during data processing in order to properly scale the images, when these processing techniques are used. The wire-mesh sensor (WMS) device has been used in several cases to study different scenarios of two-phase upward flow [1], [12], [15]. Even though the WMS is an intrusive instrument, it has been proven that its intrusive effect is negligible, especially in upward flow and at high liquid flow rates [12]. The data obtained from the WMS contains information of the cross sectional area of the flow, which makes the device a revolutionary measurement instrument in two- phase flow systems. In the present study, the WMS is used along with a high- speed video camera to study the wall jet air entrainment. The WMS is capable of gathering information about a series of important variables in the wall jet air entrainment process, such as gas void fraction time series, bubble size, bubble size distribution, 2-D (x, y) flow visualization, pseudo 2-D (x, t), (y, t) flow visualization, and a pseudo 3-D (x, y, t) flow visualization. The images obtained with the WMS are compared with the images obtained from the high speed camera. Good agreement found in this comparison demonstrates the capabilities of the WMS as a flow visualization technique. The data analysis clearly shows the effect of the superficial liquid velocity on the gas entrainment process. Furthermore, the visualization process is very useful _________________________________________________________ *Corresponding author. Current address: 800 South Tucker Drive Tulsa, OK 74104-3189. USA. Off. L127. E-mail: jose-mendoza@utulsa.edu **Current address: Department of Electrical Engineering, Federal University of Technology – Parana, Brazil