Bubble Volume Fraction Using Electrical Capacitance Tomography and High Speed Photography

Bubble volume fraction using Electrical Capacitance Tomography and High Speed Photography

By

Madhavi  V. Sardeshpande and Vivek V. Ranade

Industrial Flow modeling group, National Chemical Laboratory, Pune – 411 008, INDIA

mv.sardeshpande@ncl.res.in, vv.ranade@ncl.res.in

Abstract

A two-phase flow is one of the most common flows in nature as well as in industrial applications; it covers gas-solid, liquid-liquid, solid-liquid and gas-liquid flows. Among these, the gas-liquid flows can be encountered in wide variety of industrial applications including boilers, distillation towers, chemical reactors, oil pipelines, nuclear reactors, etc. The measurement of two-phase flow parameters such as flow regime and void fraction is considerably important and plays an important role in operational safety, process control and reliability of variety of processes (Dong, F. et al., 2003). In process industries, void fraction is one of the most important parameter to characterize the hydrodynamic behavior of two phase dispersion system in a bubble column. The void fraction is a dimensionless quantity and is often termed as “holdup or fraction” in two-phase flows. It is defined as the ratio of the volume of that phase to the total volume of the pipe (Corneliussen, S. et al., 2005) or can be defined as the fraction occupied by the gas phase in the total volume of a two or three phase mixture in a bubble column (Tang, C., 2006). Electrical Capacitance Tomography (ECT) technique is new and upcoming technology in the area of measurement of phase hold up distribution. ECT technique gives qualitative data for phase hold up distribution not the quantitative information. Therefore, it was thought desirable to validate this new technique with conventional photography technique for void fraction measurement. Therefore, an attempt was made here to design and construct an experimental test rig of laboratory scale bubble column for two-phase air-water bubble flow by using in-house facilities. This laboratory scale equipment is used to generate the phenomenon of void fraction in an ECT sensor (i.e. co-current bubble column) and its subsequent measurements. Series of experiments will be performed for the analysis of physical parameters of two-phase flow.

Experimental set up and techniques

Bubble column experimental set up was made up of Perspex of ID-48mm and height 1.2m. Two exactly similar set up assembled on test rig where (i) one is for ECT sensor/electrodes mounted on Perspex section (0.4m in length) (ii) similar transparent Perspex section  fabricated for flow visualization (for high speed photography). Both the columns were connected to each other by diameter of SS tube. The ECT sensor consists of a ring of 12-electrodes (which are separated from each other by small gap) on its outer periphery. It is connected with the data acquisition system which is then connected to computer by the in- built ECT software. A sieve plate sparger with multiple equally spaced holes has been installed at the bottom of the column with total 13 and 21 (1 mm diameter each) holes on its surface to generate uniform bubbles. A schematic of an experimental test setup is shown in Figure 1.

Electrical Capacitance Tomography

Tomography by definition refers to the process of exploring various characteristics of multi-component systems using measurement of change in internal properties of fluid and solid; depending on electrical properties of the components that is being analyzed and background conditions during measurement. Electrical capacitance tomography makes the use of capacitance measurements, section by section, using pairs of electrodes from an 8 or 12 electrode ring assembly. It provides subsequent reconstruction of the obtained data in the form of concentration profile images by various image reconstruction techniques. It can be used to investigate the internal dynamic behavior by presenting cross-sectional distribution of materials, i.e. tomographic images. The overall voidage of the contents of ECT sensor can be calculated by using the normalized pixel values in the reconstructed ECT image (Donthi, S.S., 2004).

High speed photography

High speed NR4-S1 camera was used for the current experiments with resolution 1024×1024 @1000fps and pixel size not more than 14µm. Bubble volume fraction will be measured for different sieve plate sparger designs as well as different gas flow rates. Based on the knowledge of volumetric void fraction, this current study also calculate the void fraction by using the photographic technique

	Figure 1: A schematic of an experimental test setup

The comparative study of tomography and photography would be useful to get quantitative information regarding void fraction in two phase flow. It will also be useful for two phase flow boiling applications where boiling tubes are opaque and there ECT technique will play a promising technology in order to get bubble volume fraction/vapor quality.  

Key words: two phase flow, ECT, void fraction, flow regimes

References:

Corneliussen, S., J.P. Couput, E. Dahl, E. Dykesteen, K.E. Frøysa, E. Malde, H. Moestue, P. Moksnes, L. Scheers and H. Tunheim, 2005. Handbook of Multiphase Metering. The Norwegian Society for Oil and Gas Measurement.

Dong, F., Z.X. Jiang, X.T. Qiao and L.A. Xu, 2003. Application of Electrical Resistance Tomography to Two-Phase Pipe Flow Parameters Measurement. Flow Measurement and Instrumentation, 14: 183-192.

Donthi, S.S., 2004. Capacitance Based Tomography for Industrial Applications. M.Tech. Credit Seminar Report, Electronic Systems Group, EE Department IIT Bombay, pp: 1-18.

Tang, C. and T.J. Heindel, 2006. Estimating Gas Holdup via Pressure Difference Measurements in a Cocurrent Bubble Column. International Journal of Multiphase Flow, 32: 850-863.

Ismail I, Shafquet A and Karsiti M. N. Void Fraction Estimation by Using Electrical Capacitance Tomography and Differential Pressure in an Air-Water Co-Current Bubble Column Australian Journal of Basic and Applied Sciences, 5(11): 1533-1541, 2011