Two-phase Flow In Small Channels Research Articles

A New Contactless Cross-Correlation Velocity Measurement System for Gas–Liquid Two-Phase Flow

Based on the principle of Contactless Conductivity Detection (CCD), a new contactless cross-correlation velocity measurement system with a three-electrode construction is developed in this work and applied to the contactless velocity measurement of gas–liquid two-phase flow in small channels. To achieve a compact design and to reduce the influence of the slug/bubble deformation and the relative position change on the velocity measurement, an electrode of the upstream sensor is reused as an electrode of the downstream sensor. Meanwhile, a switching unit is introduced to ensure the independence and consistency of the upstream sensor and the downstream sensor. To further improve the synchronization of the upstream sensor and the downstream sensor, fast switching and time compensation are also introduced. Finally, with the obtained upstream and downstream conductance signals, the velocity measurement is achieved by the principle of cross-correlation velocity measurement. To test the measurement performance of the developed system, experiments are carried out on a prototype with a small channel of 2.5 mm. The experimental results show that the compact design (three-electrode construction) is successful, and its measurement performance is satisfactory. The velocity range for the bubble flow is 0.312–0.816 m/s, and the maximum relative error of the flow rate measurement is 4.54%. The velocity range for the slug flow is 0.161 m/s–1.250 m/s, and the maximum relative error of the flow rate measurement is 3.70%.

Response Characteristics of Contactless Impedance Detection (CID) Sensor on Slug Flow in Small Channels: The Investigation on Slug Separation Distance.

In recent years, CID sensors have displayed great development potential in parameter measurement of gas-liquid two-phase flow in small channels. However, the fundamental/mechanism research on the response characteristics of CID sensors is relatively insufficient. This work focuses on the investigation of the influence of separation distance between slugs on the impedance (real part, imaginary part and amplitude) response characteristics of slug flow in small channels. Experiments were carried out with the CID sensors in four small channels with inner pipe diameters of 1.96 mm, 2.48 mm, 3.02 mm and 3.54 mm, respectively. The experimental results show that for a CID sensor, the slug separation distance has significant influence on the impedance response characteristics. There is a critical value of slug separation distance. When the slug separation distance is larger than the critical value, the impedance response characteristics of each slug can be considered independent of each other, i.e., there is no interaction between the slugs. When the slug separation distance is less than the critical value, the impedance response characteristics show obvious interaction between the slugs. It is indicated that the ratios of the critical values to the pipe inner diameters are approximate 100.

A method for void fraction measurement of bubble/slug flow in small channels based on contactless impedance detection.

The flow parameter measurement of the gas-liquid two-phase flow in small channels is very crucial and challenging in both academia and industry. Conventional techniques based on radiations, optics, acoustics, or electrics most lose their superiorities in the scenario with small channels due to the spatial limitation and the online and contactless measurement requirements. In addition, the conductive characteristic of the two-phase flow is equivalent to an impedance rather than a resistance due to the existence of multi-phases. The equivalent impedance information of the two-phase flow, especially the imaginary part, is promising to provide more flowing details but has seldom been detected or analyzed. In this paper, a method for the void fraction measurement of bubble/slug flow in small channels is proposed. The method implements void fraction measurement in a contactless way, based on the acquisition of the total impedance information of the gas-liquid two-phase flow. First, a new contactless impedance detection sensor is designed, based on the simulated inductor technique and the analogphase sensitive demodulation technique, to obtain the complete equivalent impedance information of the two-phase fluid. Then, based on the flow pattern identification result, the void fraction measurement model is developed, which is a fusion of the relationships between the void fraction and the real part/the imaginary part of the equivalent impedance information, respectively. Experimental results on prototypes with different inner diameters (2.48, 3.64, and 4.52mm, respectively) validate the effectiveness of the proposed void fraction method. The maximum void fraction measurement biases are within 5.0%.

New method for bubble/slug velocity measurement in small channels.

Based on the C4D technique and cross correlation velocity measurement technique, a new method for bubble/slug velocity measurement of the gas-liquid two-phase flow in small channels is proposed. A new C4D sensor, which is suitable for the parameter measurement of the gas-liquid two-phase flow in small channels, is developed by introducing the principle of capacitive reactance elimination. With two new C4D sensors, a bubble/slug velocity measurement system is developed, and the bubble/slug velocity is determined by the cross correlation velocity measurement technique. To verify the effectiveness of the proposed bubble/slug velocity measurement method, three prototypes of bubble/slug velocity measurement systems with different diameters (1.82 mm, 2.65 mm, and 2.96 mm, respectively) were established, and the bubble/slug measurement experiments were carried out. The research results show that the capacitive reactance elimination is an effective way to overcome the unfavorable influence of the coupled capacitances on measurement results. The experimental results indicate that the proposed method can successfully realize the bubble/slug velocity measurement in small channels, and the velocity measurement accuracy is satisfactory. For the three prototypes of the bubble/slug velocity measurement system, the maximum relative errors of the bubble/slug velocity measurement are all less than 5%.

Optimized Condition for Pairing of Different Friction Factor and Viscosity Equations for the Frictional Pressure Drop of R22 and R290

Accurate prediction of the friction factor and consequently the pressure drop of two-phase flow in small channels is still an issue. Many correlations exist for the determination of the viscosity and the friction factor that appear in the frictional pressure drop and their combination often determined the degree of disagreements between the experimental data and predicted outcomes. Demands for environmentally friendly refrigerants have further posed a challenge to find compatible alternatives with as good a performance as the current coolants. Despite the many available correlations developed to date, many more are studied in effort to reduce the discrepancies. This paper presents the outcomes of a study comparing the optimized conditions when three different viscosity equations are paired with eight different friction factor correlations to minimize the frictional pressure drop. The approach used multi-objective genetic algorithm (MOGA) to assist in selecting the best pairing. Comparison is then completed with available experimental data. The study showed that the Blasius friction factor paired with the Dukler viscosity produced the least percentage difference for R22, while when paired with the McAdams viscosity produced a lower difference for R290, an environmentally friendly refrigerant being considered to replace R22.

A New Contactless Bubble/Slug Velocity Measurement Method of Gas–Liquid Two-Phase Flow in Small Channels

A new contactless bubble/slug velocity measurement method of the gas–liquid two-phase flow in small channels is proposed by extending the contactless conductivity detection technique to the contactless impedance detection (CID) technique. Two kinds of CID sensors with different constructions (tubular construction and radial construction) are developed to obtain the total impedance information (the real part, the imaginary part, and the amplitude) of the gas–liquid two-phase flow. Two bubble/slug velocity measurement systems with different constructions, the tubular three-electrode velocity measurement system and the radial four-electrode velocity measurement system, are developed. With the obtained total impedance information, three initial estimation values of the bubble/slug velocity are determined by the principle of the cross correlation velocity measurement. Then, with the real-time flow pattern identification result implemented by the fuzzy C-means, a corresponding velocity measurement model [which is the linear combination of the three initial estimation values of the bubble/slug velocity and is developed by the penalty function method and the Davidon–Fletcher–Powell algorithm] is applied to obtain the final bubble/slug velocity. Two groups of the bubble/slug velocity measurement system prototypes with three different inner diameters are developed. The bubble/slug velocity measurement experiments are carried out. The experimental results show that the proposed method is effective. For the two bubble/slug velocity measurement systems with different constructions, the maximum relative errors of the bubble/slug velocity measurement are all less than 4.5%. The research results also indicate that making full use of the total impedance information is an effective approach to improve the accuracy of the velocity measurement.

Analysis of the dynamic characteristics of air-water two-phase flow in small channel based on multi-scale normalized Benford probability distribution

In gas and liquid two-phase flow, different flow patterns show their unique nonlinear dynamic characteristics due to the complex interaction between them. To further reveal the underlying dynamic characteristics of this physical phenomenon, this paper presents a new model for the analysis of small channel gas-liquid two-phase flow pattern signals based on the multi-scale normalized Benford probability distribution (MSNBPD) analysis. The first focus area is the difference between the characteristics of a multi-scale approach to Benford’s probability distribution and Benford’s Law (BL) distribution in a typical nonlinear system. Once the terms are defined, the authors show how BL could be applied in a multi-scale based flow-pattern based method to analyze differential pressure signals of gas-liquid two-phase flow in small channels to reveal the underlying dynamic characteristics of different gas-liquid two-phase flow patterns, and their sensitivity to scale changes. In addition, the stability of different flow patterns was revealed by the variable scale range calculation. Flow patterns could then be distinguished based on Euclidean distance and the variable scale range difference of differential pressure signals. This paper also compares the present method with other methods that have been used in flow identification. The results show that this new method can well distinguish nonlinear systems with different dynamic mechanisms. This method can also reveal the underlying dynamic characteristics of a gas-liquid two-phase flow of micro and small channels. The two characteristic values (Euclidean distance and variable scale range difference) proved to be a good distinction for two-phase flow patterns than other time and frequency methods, with several possible universal applications. However, the identification results of micro and small channels were better than that of regular pipe, which shows that this method is more suitable for the identification of micro and small channel flow.

A New Void Fraction Measurement Method for Gas-Liquid Two-Phase Flow in Small Channels.

Based on a laser diode, a 12 × 6 photodiode array sensor, and machine learning techniques, a new void fraction measurement method for gas-liquid two-phase flow in small channels is proposed. To overcome the influence of flow pattern on the void fraction measurement, the flow pattern of the two-phase flow is firstly identified by Fisher Discriminant Analysis (FDA). Then, according to the identification result, a relevant void fraction measurement model which is developed by Support Vector Machine (SVM) is selected to implement the void fraction measurement. A void fraction measurement system for the two-phase flow is developed and experiments are carried out in four different small channels. Four typical flow patterns (including bubble flow, slug flow, stratified flow and annular flow) are investigated. The experimental results show that the development of the measurement system is successful. The proposed void fraction measurement method is effective and the void fraction measurement accuracy is satisfactory. Compared with the conventional laser measurement systems using standard laser sources, the developed measurement system has the advantages of low cost and simple structure. Compared with the conventional void fraction measurement methods, the proposed method overcomes the influence of flow pattern on the void fraction measurement. This work also provides a good example of using low-cost laser diode as a competent replacement of the expensive standard laser source and hence implementing the parameter measurement of gas-liquid two-phase flow. The research results can be a useful reference for other researchers’ works.

Two-Phase Flow in Small Channels and the Implications for PEM Fuel Cell Operation

A significant challenge in designing robust PEM fuel cells is managing the flux of water from the humidified reactant streams and the electrochemical reaction. The problem of water management is especially challenging under cold conditions because of the reduced water carrying capacity of the reactant streams as well as the potential for water freezing within the fuel cell assembly which can lead to degradation of the bulk materials and surface treatments of the various fuel cell components. Effective water management begins in the gas flow channels where the product water is removed from the cell. Because of the small length scales associated with the gas flow channels, capillary forces are important to the behavior of liquid surfaces in the channel. This implies that the cross- sectional geometry and surface chemistry of the gas flow channels are important to the behavior of liquid surfaces in the channel and, subsequently, to the behavior of two-phase flow. Recent research efforts have focused on investigating the role of capillarity on the behavior of two-phase flow in systems where capillary forces are important The results of this research are dramatic. The presence of corners in a small channel can result in a completely different pressure drop and flow morphology from that found in circular cross sections. In addition, a change in the channel wall wetting (hydrophilic or hydrophobic) can also result in unique pressure drops and flow morphologies.

Two-Phase Flow in Small Channels and the Implications for PEM Fuel Cell Operation

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