DESIGN AND FABRICATION OF CONDUCTIVITY PROBE FOR MEASURING LOCAL VOID FRACTION IN TWO-PHASE FLOW

Transcription

1 DESIGN AND FABRICATION OF CONDUCTIVITY PROBE FOR MEASURING LOCAL VOID FRACTION IN TWO-PHASE FLOW H.Q.,DUONG, V.T.,NGUYEN, N.H.,BUI School of Nuclear Engineering and Environmental Physics, Hanoi University of Science and Technology, 406-C10, No 1 Dai Co Viet Road, Ha Noi, Vietnam quanduong92@gmail.com Abstract: The working principle of local two-phase void fraction sensor is based on thedifference in the electrical conductivity between liquid and gas. Gas-liquid interface passing through the sensor tip creates a voltage signal with different level which depends upon the contact of sensor tip with gas or liquid phase. This paper presents some progresses on the design and fabrication of the electrical conductivity probe using conductivity method at Hanoi University of Science and Technology. From the result of experiments, the validity of electrical conductivity prope and electrical circuit is confirmed. Keywords: gas-liquid phase, void fraction, electrical conductivity method. I. INTRODUCTION In the nuclear power plants, the two-phase flow phenomena occur in many components and systems such as pressurizer, heat exchanger and core reactor of BWR reactor. Two phase flow are very important for both fuel performance and safety analysis of nuclear power plan.particularly,the accurate prediction of void fraction in the subchannel under two phase flow condition is essentialto model the behavior of fuel assemblies. The slowing down of fast neutron, thermal conductivity, negative reactivity coefficient and capacity of nuclear power plant will be affected by local void fraction. Various void fraction measurement methods have been established, such as gamma ray absorption technique, impedance method, optical method, electrical conductivity method and so on. In this study, a two phase void fraction probe is designed and fabricated by electrical conductivity method. This method was firstly developed by Neal (1963) for measurements in mercury-nitrogen flow and Nassos (1963) for measurements in air water mixture. In addition, this method was reported in a lot of science paper, which is performed by Malnes (1966), Neal and Bankoff (1963), Serizawa (1975), Yun et al (1992), Neal and Bankoff and Serizawa have used two probes in series to measure bubble size and velocity. Most of such previous measurements have performed for air-water flow in cylindrical tube geometry. However, the two phase flow in the real reactor is affected by complicate geometrical and boiling effects. In this paper, benchmarking tests are performed for a cylindrical visual vessel. The vessel has a diameter of 20 cm and height of 30 cm. The void fraction is measured by using the electrical conductivity method. The test facility measured void distribution at different air flow rates and void fraction measurement at the same position. 1

2 II. DESIGN AND FABRICATION OF CONDUCTIVITY PROBE 1. Principle of conductivity method The principle measurement of two phases void fraction by the electrical conductivity probe is based onbetween the resistance of water and bubble. The bubble can be considered as electrically insulated so it has resistance, whereas the water as electrically conducted so resistance of bubble is higher than resistance of water. When conductivity probe contacts with continuous liquid phase and vapour phase will be created feedback resistancein signal processing. After that, a microcontroller is received signal from signal processing and the microcontroller will be programmed to calculate the accurate local void fraction. Such a signal shows a nearly immediate response to water contact with the probe tip but a delayed response to vapour contact, due to the required dewetting time of probe tip. 2. Probe design The probe design isvery important. The good signal or bad signal is depended on probe. If we want good signal, the probe will have to get some parameters as good conductivity, rigidity of wire. In this study, we used type K thermocouple a. Characteristic of type K thermocouple. Type K thermocouple is made by alloy of chromel. This thermocouple is more intrinsically resistant to corrosion. Moreover, it has non-oxidation and high melting temperature.chromel is an alloyof approximately 90 percent nickel and 10 percent chromium. Table 1: Characteristic of type K thermocouple Physical properties Chromel Electrical resistivity Melting point (μωm) 1420 C b. The requirement for probe design The requirements for probe design probe used for measurement in experimental system: The probe can not be deformed and damaged in high temperature Probe vibration isn t permitted in two phase flow condition The probe has to be sharpened and shorted in order to minimize dewetting time The probe has high conductivity The schematic configuration of designed probe is showed in table 2. Figure 1 shows the electrical conductivity probe. Table 2: Design parameters of probe Diameter of type K thermocouple d = 0.2 mm Conductive head length of probe l 0.3 mm Diameter of probe was sharpened d 30 µm 2

3 Figure 1: Electrical conductivity probe. Figure 2: Output signal at amplification circuit. Figure3: Output signal at signal processing circuit. 3

4 III. SIGNAL PROCESSING In order to measure void fraction, The signal from conductivity probe must be amplified by bridge amplifier circuit to obtain maximum output voltage. And the raw output signals produced from bridge amplifier circuit will be became rectangular forms by triggering circuit. The results of void fraction will be showed on computer by RS232 connection with signal processing. Figure 4 shows the flow chart of signal processing from probes to the computer software. Conductivity probe Bridge Amplifier Circuit Triggering Circuit Connection with PC Figure 4: Flowchart of signal treatment. 1. Bridge amplifier circuit In order to maximize the voltage of signal output for resistance difference between water and bubble, a bridge amplifier circuit is designed and showed in Figure 5. Due to the feedback resistance difference of op-amp, so the gain factor and voltage of output is different. The output voltage from circuit is given by: Where: V : output voltage (V) R : feedback resistance (Ω) R, R : Fixed resist (Ω) V =[ ( ) ( ) ]. E (1) Figure 5: Sketch of bridge amplifier circuit. 4

5 2. Triggering circuit To calculate localvoid fraction from measured cutoff data, the cutoff level which is boundary between water and bubble is determined. The triggering circuit is designed for the transformation of raw output signals produced from bridge amplifier circuit into rectangular forms. This circuit is used in the consistent calculation of local void fraction. But this method has a disadvantage of losing raw data produced from bridge amplifier circuit, so it can be created deviation for result. Figure 6: Sketch of triggering circuit. Figure 7: Signal at input of triggering circuit. 5

6 Figure 8: Signal at output of triggering circuit. The received signal will be entered two CCP modules of microcontroller and a calculation local void fraction is programmed by microcontroller. In this research, pic16f877a microcontroller is used to calculate. The local void fraction is given by: ε = (2) in which: t : pulse width t : T - t T: period of pulse Conductivity probe t t Figure 9: A scheme of the conductivity probe measurement. 6

7 IV. IDENTIFICATION OF THE PIERCING EFFECTIN THE SIGNAL Figure 10 shows signals acquired at various distance X from the bubble center. R is radial bubble. When piercing occurs in the central area that mean < 0.88, full amplitude signals with a plateau are obtain. The probe tip gets enough time to dry completely. When piercing occurs close to the bubble edge that mean > 0.88, full dewetting is not obtained. As a result, incomplete signals are obtained: These signal have smaller amplitudes, no plateau, and are usually bell-shaped. When piercing occurs futher from the center, the rising flanks get a longer duration. In the region with approximately > 0.88, the falling flanks also get a longer duration. As a result, away from the center, the criterion for the bubble-water transition becomes much more important for accurate void fraction measurements. Figure 10: Signal shape as a function of piercing position V. RESULTS AND DISCUSSION 1. Experimental apparatus The experimentis depicted in Figure 11. The bubble sensor was studied in a cylindrical glass vessel with diameter of 20 cm and height of 30 cm. The bubble will be created by a bubble generator. A bubble generator was placed in the bottom of the water tank. The size of bubbles can be changed by adjusting a shut-off valve. The sensor was inserted into the vessel from the top. After that, a signal of sensor go to signal processing and the result of measurement will be displayed on the computer. Measured parameters in experiment arethe local void fraction. The conductivity probe was inserted into the vessel from the top. Size of bubble is changed by valve and showed in Figure 12. The signal from conductivity probe and measurement of local void fraction will be processed and calculated in signal processor. Figure 13 shows the signal processor. Figure 14 shows real experimental system. 7

8 Figure 11. The schematic of experimental setup. 1. Computer 2. Electronic circuits 3. Sensor 4. Vessel 5. Valve 6. Bubble generator 8

9 Figure 12: Valve Figure 13: Signal processor. Figure 14: Experimental system. 2. Local void fraction measurement 2.1Void distribution at different air flow rates In this measurement, local void fraction ismeasured along direction to bubble and each point is increased z = 0.5 cm. Measured data is visualized by chart in figure 15 and figure 16. Figure 15 shows the variation of local void fraction uses needle of 1/8 inch, Figure 16 shows the variation of local void fraction uses needle of 1/2 inch and change of air flow rate. 9

10 Void fraction Distance (cm) Figure 15: The variation of local void fraction uses needle of 1/8 inch. Void fraction Distance (cm) Void fraction in opening valve 1/4 Void fraction in opening up valve Figure 16: The variation of local void fraction uses needle of 1/2 inch and change of air flow rate. Figure 15 and Figure 16 show the trend that the local void fractionis decreased after increasing distance between probe and bubble generator. Local void fractionaffected by turbulent flow so it is decreased. As said in section IV, we distance of conductivity probe and bubble generator approximately z = 0, when is the minimum so that void fraction is the maximum. In the case, we increase distance of conductivity probe and bubble generator, when the bubbles will be affected by turbulent flow so velocity profile is also changed, so that is decreased and void fraction is decreased. Figure 16 shows decrease of local void fraction when air flow rate increase. 10

11 2.2 Local void fraction measurement at the same position The local void fraction is defined as the fraction of bubble elapsed time to the total measuring time at a local position. For statistical treatments of local void fraction, the total measuring should be sufficiently long. Figure 17 shows typical changes of local void fraction with the total measuring time. The local void fraction is nearly constant after 60 minutes. In this study, the total measuring time for each position is set to about 60 minutes, which is long enough to satisfy the repeatability of void fraction Void fraction Local void fraction ε=0.07 Local void fraction ε=0.11 Local void fraction ε= Thời gian (s) Figure 17: Typical changes of local void fraction with the total measuring time. VI. CONCLUSIONS A local two-phase void fraction using conductance method were designed, fabricated and tested in this study. From the result of measurements, the validityof electrical conductivity probe and electrical circuit is confirmed.probe benchmarking work will be performed in the near future. 11

12 REFERENCES 1. B.J Yun, K.H. Kim, G.C Park, and C.H. Chung, (1992), A Study on the Measurement of Local Void Fraction, KAERI, Korea. 2. Byoung Jo YUN, Goon Cherl PARK, Chul Hwa SONG, Moon Ki CHUNG, Measurements of Local Two-Phase Flow Parameters in a Boiling Flow Channel, Department of Nuclear Engineering, Korea Atomic Energy Research Institute, Korea. 3. M. S. ROCHA and J. R. SIMOES-MOREIRA, 2008, Void Fraction Measurement and Signal Analysis from Multiple-Electrode Impedance Sensors, Mechanical Engineering Department, Brazil. 4. J. Enrique Juniá, Wouter K.Harteveld, Robert F.Mudde, and Harrie E.A Van den Akker, (2004), On the accuracy of the void fraction measurements using optical probes in bubbly flows, Universitat Jaume I, Delfi University of Technology, Spain, Netherlands. 5. Jir ˇ í Vejrazˇ ka, Marek Vecer, Sandra Orvalho, Philippe Sechet, Marek C. Ruzicka, and Alain Cartellier, (2010), Measurement accuracy of a mono-fiber optical probe in a bubbly flow, Institute of Chemical Process Fundamentals, Czech Academy of Sciences, VSB- Technical University of Ostrava, Laboratarie des Ecoulements Geophysiqueset Industriels, Czech Republic, France. NGHIÊN CỨU THIẾT KẾ CHẾ TẠO THIẾT BỊ ĐO TỶ SỐ BỌT KHÍ- NƯỚC BẰNG PHƯƠNG PHÁP ĐỘ DẪN ĐIỆN DƯƠNG HỒNG QUÂN,NGUYỄN VĂN THÁI, BÙI NGỌC HÀ Viện Kỹ thuật Hạt nhân và Vật lý Môi trường, Đại học Bách Khoa Hà Nội *406-C10, Số 1 Đại Cồ Việt, Hai Bà Trưng, Hà nội quanduong92@gmail.com Tóm tắt: Nguyên lý của việc đo dòng hai pha bằng phương pháp độ dẫn điện dựa trên sự khác nhau về điện trở của nước và bọt khí Trong dòng chảy pha hơi-nước, pha hơi có điện trở khá cao và pha nước có điện trở thấp hơn. Đầu cảm biến tiếp xúc với pha hơi và pha nước sẽ làm xuất hiện một điện trở hồi tiếp trong mạch xử lý tín hiệu, dựa vào sự khác nhau về điện trở của pha hơi và pha nước mạch xử lý tín hiệu sẽ thu được tín hiệu ở lối ra, khi đó bắt đầu tạo xung và hiện trên oscilloscope. Kết quả của quá trình thiết kế cho thấy dựa vào tín hiệu đo được và qua các mạch điện tử sẽ tính toán được tỷ số bọt khí trong nước và khi sử dụng hai cảm biến có thể đo được tốc độ của bọt khí. Kết quả của thực nghiệm cho thấy việc sử dụng phương pháp dẫn điện và mạch điện tử để đo sẽ cho kết quả chính xác. Từ khóa:pha hơi-nước, tỷ số bọt khí-nước, phương pháp dẫn điện. 12

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