EXPERIMENTAL INVESTIGATION OF LIQUID HOLDUP IN HORIZONTAL TWO- PHASE ANNULAR FLOW USING CONSTANT ELECTRIC CURRENT METHOD (CECM)

SEMINAR NASIONAL ke-8 Tahun 213 : Rekayasa Teknologi Industri dan Informasi EXPERIMENTAL INVESTIGATION OF LIQUID HOLDUP IN HORIZONTAL TWO- PHASE ANNULAR FLOW USING CONSTANT ELECTRIC CURRENT METHOD (CECM) Andriyanto Setyawan, Anam Bahrul, Indarto, Deendarlianto Department of Mechanical and Industrial Engineering, Gadjah Mada University, Yogyakarta 55281 e-mail: andriyanto@polban.ac.id ABSTRACT Gas-liquid two-phase annular flow is commonly found in a wide range of industrial applications, including natural gas wellbore pipelines, nuclear reactors, and geothermal power plants, in which the mixtures from several wells could be gathered by pipelines to a separator near the power plant. It is characterized by liquid film flowing on the wall and a gas core containing liquid droplets. Liquid hold up is one of key parameter in such flow. Therefore, it is useful to observe its behavior for analyzing other parameters in horizontal annular flow. The investigation of horizontal air-water annular flow was conducted experimentally in 16 mm and 26 mm ID pipes using flush-mounted CECM sensors and was visualized using a video camera. The superficial velocities of air and water were varied from 12 to 4 m/s and.5 to.2 m/s, respectively, and its effects on the liquid holdup were observed. Under the predetermined conditions of the experiment, the mean liquid hold up was in the range of.4 to.15, indicating the gas dominant flow. The measured holdup is in accordance with those obtained by Hamersma and Hart (1987), Luninski et al. (1983), and Spedding and Chen (1984). Keywords: Annular flow, liquid hold up, wave velocity, cross correlation, CECM INTRODUCTION Gas-liquid two-phase annular flow is commonly found in a wide range of industrial applications, including natural gas wellbore pipelines, nuclear reactors, and geothermal power plants, in which the mixtures from several wells could be gathered by pipelines to a separator near the power plant. The importance of annular flow in geothermal application has been studied by Bilicki and Kestin (198), Karamarakar and Cheng (198), Shulyupin (1996), Chermoshentseva and Shulyupin (22), and Palsson et al. (26). This flow regime is quite complex, for both vertical and horizontal orientation, and it is characterized by liquid film on the wall and a gas core containing liquid droplets. For horizontal orientation, annular flow is characterized by the asymmetric distribution of liquid film with thicker liquid flows along the bottom of a tube than on the top, although the degree of asymmetry is dependent on the mass flow rates of liquid and vapor. The effect of gravity-induced drainage increases the thickness of the liquid film on the bottom surface while reducing it on the top surface. Similarly, the drops concentration will be higher in the bottom part than in the top of the pipe. Considerable research has been carried out over decades on horizontal annular flow. However, theoretical modeling of horizontal annular flow is generally less successful than in those of vertical flow. As a result, many important questions remain unanswered. Perhaps the most significant issue associated with horizontal annular flow is the mechanism by which the liquid film forms on the walls of the conduit, especially on the upper surface of pipe. Several models have been proposed, and the most credible and important among these are secondary flow, entrainment and redeposition of droplets, wave spreading, and pumping action due to disturbance wave. The secondary flow mechanism (Pletcher & McManus, 1965) assumes that the circumferential variation of the film thickness and disturbance waves produces gas-liquid interfacial roughness gradient around the circumference of the tube. As a result, a two-vortex secondary flow in the gas phase normal to the tube axis is created, which drives the liquid up along the wall. Other experiments have also shown the existence of such flows, including those of Jayanti et al. (199), Paras et al. (1991), and Flores et al. (1995). However, the role of these flows in liquid film circumferential distribution is still debated. Entrainment and redeposition mechanism, proposed by Russell & Lamb (1965), suggests that the drained liquid film on the upper wall is continuously replenished by impacting liquid droplets from the vapor core. The entrainment of droplets from the bottom to the top of the tube is created by the variation in the film thickness. Wave spreading mechanism (Butterworth 1971), suggests that when a disturbance wave travels through the tube, it brings the liquid film in front of the wave up the tube walls, thus maintaining the film on the top of the tube. The idea is that the disturbance waves travel faster along the bottom of the tube than along the top. This will create a plowing effect that drives liquid film upward immediately in front of the wave. SEKOLAH TINGGI TEKNOLOGI NASIONAL, 14 Desember 213 M 166

SEMINAR NASIONAL ke-8 Tahun 213 : Rekayasa Teknologi Industri dan Informasi Pumping action mechanism due to a disturbance wave (Fukano & Ousaka 1989), states that the gas flow over a disturbance wave will produce a circumferential pressure gradient caused by the variation of the wave height. The mechanisms proposed by the former researchers by which the liquid film forms on the all surface of inner walls of the conduit, especially on the upper surface of pipe, are still debatable. This paper was, therefore, prepared to contribute new fundamental data of liquid holdup for horizontal annular flow. RESEARCH METHODOLOGY The measurements of liquid holdup were carried out in the air-water horizontal flow rig shown schematically in Figure 1. The test section is a 1 m long acrylic tube of 26 and 16 mm ID to facilitate visual observation. Air enters the test section at one end from a compressed air supply. Water is injected through a porous tube wall section. The film thickness was measured at a distance of 5.5 m from the porous wall injector, thus giving a developing length of 2 tube diameters. In view of the fact that water entered through a porous wall section, it was felt that this length was sufficient for the flow to be fully developed. Flow Meter Valve Water tank By-Pass valve Supply pump Mixer Flow Meter Valve Air regulator Kompressor Compressor x High impedance amplifier Figure 1. Experimental setup. Separator The air and water flow rate to the test section was measured by rotameter bank. Constant-electric current method (CECM) probes were used to measure the liquid holdup at two positions simultaneously (Figure 1). The probes were set in acrylic resin blocks bored out to the same inner diameter as the test section tube. 1 PC ADC + - Constant current power supply Constant current source Ground Lamp screen High Speed Camera Camera Processor Circulating pump Air Water In the CECM, the constant electric current is applied from a pair of electrodes, which will be referred to as the power electrodes in the present paper. The locations of the sensor electrodes for detecting the time fluctuating hold-up are separated from the power electrodes. The distance between the power electrodes must be considerably large and the sensor electrodes are installed in between the power electrodes. The axial distance between a pair of sensor electrode is arbitrary, a few millimeters for the measurement of a local value in some cases and several hundreds of millimeters in other cases for a space averaged value in a considerably long axial length. The CECM detects the liquid holdup based on the conductivity difference of two-phase flow components. The combined conductivity could be converted to liquid volume fraction in the conduit as electric voltage. The voltage drop at the sensor electrodes are fed into high input impedance amplifier, so that the constant current source is not disturbed by the presence of sensor electrodes. If the film thickness is very thin, the electric resistance becomes large while the electric current is kept always constant in the CECM. As a result, the voltage drop becomes large. The thinner the liquid film thickness, the larger the output voltage. This means that the sensitivity to the film thickness variation is higher in the case of the thinner film thickness. Therefore, the CECM is better to be used in the liquid film flow, such as annular flow. The range of liquid and gas superficial velocities for this experiment are.5 to.2 m/s and 12 to 4 m/s, respectively. Under the combinations of gas and liquid superficial velocities, the flow regimes observed in this research are annular and transition from wavy to annular if plotted in Mandhane map (Figure 3). Nonconductive duct Amplifier 5 mm 1 mm C D Amplifier Figure 2. CECM for measuring liquid holdup Figure 3. Experimental matrix in Mandhane map. RESULTS AND DISCUSSION Flow transition The flow transition from stratified to annular flow is interested to be investigated. Pipe diameter is SEKOLAH TINGGI TEKNOLOGI NASIONAL, 14 Desember 213 M 167

SEMINAR NASIONAL ke-8 Tahun 213 : Rekayasa Teknologi Industri dan Informasi one of important factor in such flow transition. To investigate the transition, visual observations were carried out, i.e. observing the flow topology for the identical gas and liquid superficial velocity for different pipe diameters, 16 mm and 26 mm. From visual observation, it is found that the flow pattern at transition are different, even observed at the same gas and liquid superficial velocity, 12 m/s and.5 m/s, respectively. Figure 4 shows the comparison of flow pattern observed at 16 mm and 26 mm pipes. then the two waves coalesce and usually continue with the speed of the faster wave. This phenomenon is called wave coalescence. In the other hand, the break of a large wave into smaller waves is also observed in this experiment. This phenomenon is called wave breakup. The coalescence and breakup of wave is illustrated in Figure 6. air disturbance wave interface ripple FLOW a. Figure 4. Disturbance and ripple waves. FLOW b. Figure 3. Flow comparison for a. 16 mm pipe, b. 26 mm pipe. As shown in Figure 3a, an annular flow pattern has been fully formed for pipe diameter of 16 mm. In the other hand, annular flow pattern has not been fully formed at pipe diameter of 26 mm for the same gas and liquid superficial velocity. In this case, the liquid covers only the lower half of the inner pipe wall, while the upper part of the inner wall of pipe remains dry. Figure 3, therefore, shows the importance of pipe diameter on the flow pattern. Disturbance wave and ripple wave The existence of disturbance wave and ripple wave in annular flow are observed in this experiment. Figure 4 shows such phenomena compared to the visual observation using video camera. The ripple wave could be captured by CECM sensor as well as large disturbance wave. The wave is identified when a liquid wave with high amplitude flows through the sensor. Disturbance waves are large amplitude roll waves that are responsible for the entrainment of liquid droplets into the gas core. The base liquid film under these waves is generally much higher than under ripple waves. Ripples are the low amplitude surface waves which create interfacial roughness and are responsible for the pressure drop. Although the waves play an important role in the interfacial dynamics, not enough is known about how they form and how they they influence other aspects like droplet entrainment, gas and liquid turbulence, and interfacial shear (Rodriguez, 29). Wave coalescence and breakup The disturbance waves tend to move with constant velocity and if faster wave overtakes a slower wave, Wave development and entrainment Other phenomena observed in this experiment are wave development and entrainment, as shown in Figure 5. The transport of liquid film in the pipe wall could be traced from the holdup signal. Figure 5 shows the change of wave height measured by sensor 1 and 2. The peak of the wave when sensed by sensor 2 is higher than those of sensor 1. It means that the wave grows and the phenomenon is called wave development. The reduction of wave height when it is sensed by sensor 1 and then sensor 2 is a phenomenon called entrainment, in which a portion of liquid in the wave crest is entrained when high velocity of gas flows and shear the gas-liquid interface at wave crest. Figure 5. Wave development and entrainment. Figure 6. Wave coalescence and break up. Wave velocity The signal sensed by the downstream sensor (sensor 2) is delayed by several milliseconds compared to those of sensor 1, depends on the SEKOLAH TINGGI TEKNOLOGI NASIONAL, 14 Desember 213 M 168

Wave frequency Wave Velocity [m/s] SEMINAR NASIONAL ke-8 Tahun 213 : Rekayasa Teknologi Industri dan Informasi velocity of the wave. If the time delay and the distance between the sensors are known, then the wave velocity could be calculated. To determine the time delay, a cross correlation function is used. Figure 7 shows the result of cross-correlation function of holdup signal sensed by sensor 1 and 2 for gas superficial velocity, J G, of 4 m/s and liquid superficial velocity, J L, of.5 m/s. From Figure 7, the cross correlation shows that time lag for the holdup signal sensed by sensor 1 and 2 is.48 s. The wave velocity increases with the increasing of gas superficial velocity. It could be described as follows: at the higher the air velocity, the force that shear the gas-liquid interface is also higher, resulting in higher liquid film flowing in the pipe. number decreases with the increasing of liquid superficial velocity. This is different from the results of this work, in which the wave number increases with the increase of liquid superficial velocity. 6 5 4 3 2 1 This work: JL [m/s].5.1.2 1 2 3 4 5 6 7 J G [m/s] Fukano et al. (1983), D=26mm, JL=.2m/s JL=.1m/s JL=.6m/s Figure 8. Comparison of wave velocity obtained from this work and those obtained by Fukano et al. (1983) and Paras and Karabelas (1991). 25 2 15 JL=.5 m/s JL=1. m/s JL=.2 m/s Paras, JL=.6 m/s Paras JL=.9 m/s Paras JL=.2 m/s 1 5 Figure 7. Cross-correlation function of holdup signal J G = 4 m/s and J L =.5 m/s. The experiment of Jayanti et al. (199) with 32 mm ID pipe showed that the wave velocity ranged from 1.9 to 4.5 m/s for liquid superficial velocity of.8.145 m/s and gas superficial velocity of 14 26 m/s. Using 5.8 mm ID pipe, Paras and Karabelas (1991) showed that the wave velocity was in the range of 1.6 to 3.6 m/s for liquid superficial velocity of.2.6 m/s and gas superficial velocity of 31 66 m/s. Figure 8 shows the comparison of wave velocity obtained from this work and those obtained by Jayanti et al. (199) and Paras and Karabelas (1991). Schubring and Shedd (28) have reported that the wave velocity for horizontal annular flow is 2.4 to 6 m/s for their experiment with 26.3 mm ID pipe using liquid superficial velocity of.4 to.39 m/s and 32 to 91 m/s. For the smaller pipe (8.8 and 15.1 mm), the wave velocities will be higher. Wave frequency/wave number The wave frequency or wave number could be determined from the frequency corresponding to the largest peak of power spectral density function. From Figure 9, it is shown that wave frequency increases with increasing of gas superficial velocity. Paras and Karabelas (1991) also stated that the higher gas superficial velocity, the higher the wave number. However, they showed that the wave 1 2 3 4 5 6 7 Figure 9. Wave frequency vs gas superficial velocity. The effect of diameter on the wave frequency has also been observed in this experiment. The pipe diameter has a significant effect on the wave number, as could be seen in Figure 1. It is shown that the smaller pipe gives the larger wave number. Schubring and Shedd (211) reported that for pipe diameter 26.3 mm, the wave frequency ranges from 1 to 15 for the same range of gas superficial velocity. However, when the gas velocity is increased to 7 m/s, the wave number could reach 4. For pipe diameter of 15.1 mm and the same range of gas superficial velocity, the wave number ranges from 15-3, similar to those obtained from this work. Liquid holdup The liquid holdup obtained from the experiment with 26 mm pipe is compared to the result of previous researches. The comparison is done for 3 variations of liquid superficial velocity.5,.1, and.2 m/s. The comparisons are presented in Figure 11, 12, and 13. As could be seen in Figure 11, the holdup obtained from this work is smaller than those obtained by Bestion et al. (1985) and Fukano and Ousaka (1988). This work is close to those obtained by Spedding SEKOLAH TINGGI TEKNOLOGI NASIONAL, 14 Desember 213 M 169

SEMINAR NASIONAL ke-8 Tahun 213 : Rekayasa Teknologi Industri dan Informasi and Chen (1984), Luninski et al. (1983) and Hamersma and Hart (1987). For two other liquid superficial velocities, the correlations show the similar result, as shown in Figure 12 and 13. Compared to the results of Bestion et al. (1985) and Fukano and Ousaka (1988), the results of this work are smaller. However, this work has a good agreement with the correlations of Spedding and Chen (1984), Chisholm (1973), Luninski et al. (1983) and Hamersma and Hart (1987). velocity of.5 m/s and pipe diameter of 16 mm, the liquid holdup ranges from.38 to.79. For 26 mm pipe, the liquid holdup ranges from.11 to.41. Therefore, for the larger diameter, the liquid holdup will be smaller. If the liquid superficial velocity is increased to.1 m/s, the maximum liquid holdup for 16 mm and 26 mm pipes are.11 and.6, respectively. If the liquid superficial velocity is further increased to.2 m/s, the maximum liquid holdup are.15 and.9 for pipe diameter of 16 and 26 mm, respectively. From the detail observation of Figure 14, it is shown that the liquid superficial velocity affects the liquid holdup significantly. For both diameters observed, the effect of liquid superficial velocity is very clear at low gas superficial velocity for 16 mm pipe. However, for 26 mm pipe the strong correlation of liquid holdup and liquid superficial velocity could be found in all range of gas superficial velocity..2.15.1 This work JL=.2m/s Fukano & Ousaka, 1988 Spedding & Chen, 1984 Bestion et al., 1985 Chisholm, 1973 Hamersma & Hart, 1987 This work (2).5 Figure 1. Effects of diameter and J G on the wave number..2.15.1.5 This work JL=.5 m/s Fukano & Ousaka, 1988, JL=.6m/s Spedding&Chen, 1984 Bestion et al., 1985 Chisholm, 1973 Hamersma & Hart, 1987 1 2 J G [m/s] 3 4 5 Figure 11. Comparison of liquid holdup for liquid superficial velocity.5 m/s..2.15.1.5 This work JL=.1m/s Fukano & Ousaka, 1988 Spedding & Chen, 1984 Luninski et al., 1983 Bestion et al., 1985 Chisholm, 1973 Hamersma & Hart, 1987 This work (2) 1 2 J G [m/s] 3 4 5 Figure 12. Comparison of liquid holdup for liquid superficial velocity.1 m/s. The effect of diameter and gas superficial velocity on the liquid holdup of horizontal annular flow is presented in Figure 14. For liquid superficial 1 2 J G [m/s] 3 4 5 Figure 13. Comparison of liquid holdup for liquid superficial velocity.2 m/s..1.8.6.4.2.12.1.8.6.4.2.16.14.12.1.8.6.4.2 JL =.5 m/s 16 mm 26 mm 1 2 3 4 5 1 2 3 4 5 JL =.1 m/s JL =.2 m/s 16 mm 26 mm 16 mm 26 mm 1 2 3 4 5 Figure 14. The effect of diameter and J G on the liquid holdup. CONCLUSION Experiment of air-water horizontal annular flow have been carried out using 16 and 26 mm pipe and SEKOLAH TINGGI TEKNOLOGI NASIONAL, 14 Desember 213 M 17

SEMINAR NASIONAL ke-8 Tahun 213 : Rekayasa Teknologi Industri dan Informasi both the transition of wavy-annular and fully developed annular flow have been successfully established. The common phenomena of annular flow such as ripple waves, disturbance waves, gas core, gasliquid interface, and asymmetric liquid film due to gravity effect could be observed both visually and using liquid holdup signal. The wave velocity and wave number increase with the increasing of gas superficial velocity. In addition, the liquid holdup increases with the increasing of liquid superficial velocity and decreasing of gas superficial velocity. REFERENCES 1. Bilicki Z. and J. Kestin, Flow in geothermal wells: Part IV. Transition criteria for two-phase flow patterns, Report No. GEOFL/6 US Department of Energy, December 198. 2. Butterworth, An analysis of film flow and its application to condensation in a horizontal tube. Int. J. Multiphase Flow, Vol. 1, pp. 671-682, 1974. 3. Chermoshentseva A. and A. Shulyupin, Annular-mist flows of steam-water geothermal mixture, Proceedings, Twenty-Seventh Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 28-3, 22 4. Flores, A.G., K.E. Crowe, and P. Griffith, Gasphase secondary flow in horizontal, stratified and annular two-phase flow, Int. J. Multiphase Flow Vol. 21. No. 2, 1995. 5. Fukano, T. and A. Ousaka, "Distribution of film thickness in horizontal and near-horizontal gasliquid annular flows," Int. J. Multiphase Flow, 15, No.3, pp.43-419, (1989). 6. Fukano, T. and A. Ousaka, A., Air-water twophase annular flow in near-horizontal tubes, JSME International Journal, Series II, Vol. 31, No. 3, 1988. 7. Fukano, T., Measurement of time varying thickness of liquid film flowing with high speed gas flow by CECM, Nuc. Eng. & Design 184, 63 377, 1998. 8. Hamersma, P. J. and J. Hart (1987). A Pressure Drop Correlation for Gas/Liquid Pipe Flow with a Small Liquid Holdup. Chem. Eng. Sci. 42: 1187-1196. 9. Jayanti, Hewitt, White, Time-dependent behavior of the liquid film in horizontal annular flow, Int. J. Multiphase Flow Vol. 16, No. 6, pp. 197-1116, 199. 1. Karamarakar, M. and P. Cheng. A Theoretical Assessment of James' Method for the Determination of Geothermal Wellbore Discharge Characteristics. Report for US Department of Energy under Contract W-7466-ENG-48, November198. 11. Pàlsson, H., E.S. Bergthòrsson, O.P. Pàlsson, Estimation and validation of models two phase flow from geothermal wells. 1th International Symposium on District Heating and Cooling September 3-5, 26 12. Paras and Karabelas, "Properties of the liquid layer in horizontal annular flow," Int. J. Multiphase Flow, 17, No.4, pp.439-454, 1991. 13. Pletcher, R. H. & McManus, H. N. The fluid dynamics of three-dimensional liquid films with free surface shear: a finite difference approach. In Proc. 9th Mid-Western Mechanics Conf., Wisc, 1965. 14. Rodriguez, J.M., Numerical simulation of twophase annular flow, Thesis for Doctor of Philosophy, Faculty of Rensselaer Polytechnic Institute, 29. 15. Russell and D.E. Lamb, "Flow mechanism of two-phase annular flow," Can. J. Chem. Eng., 17, No.43, pp.237-245, 1965. 16. Schubring, T.A. Shedd, A model for pressure loss, film thickness, and entrained fraction for gas liquid annular flow, International Journal of Heat and Fluid Flow 32 (211) 73 739. 17. Schubring, T.A. Shedd, Wave behavior in horizontal annular air water flow, International Journal of Multiphase Flow 34 (28) 636 646. 18. Shedd, T.A., 21. Characteristics of the liquid film in horizontal two-phase flow, Thesis for Doctor of Phil. in Mech. Eng. the University of Illinois at Urbana-Champaign. 19. Shulyupin A., Some aspects of steam-water flow simulation in geothermal wells, Proceedings, Twenty-First Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, January 22-24, I996. 2. Spedding, P.L and J.J.J. Chen, 1984. Holdup in two-phase flow. International Journal of Multiphase Flow 1(3): 37-339. 21. Weidong, Fangde, Rongxian, Lixing, Experimental study on the characteristics of liquid layer and disturbance waves in horizontal annular flow, Journal of Thermal Science, Vol. 8, No. 4, 1999, pp. 235-241. SEKOLAH TINGGI TEKNOLOGI NASIONAL, 14 Desember 213 M 171