CO2 Two-phase Flow Research Articles

Real-Time Imaging and Holdup Measurement of Carbon Dioxide Under CCS Conditions Using Electrical Capacitance Tomography

This paper presented a method for real-time cross-sectional imaging and holdup measurement of gas-liquid two-phase carbon dioxide (CO2) flow using electrical capacitance tomography (ECT). A high-pressure ECT sensor with 12 electrodes was constructed and a dedicated digital ECT system with a data acquisition rate of 757 frames/s was developed for capacitance measurement. Three widely used image reconstruction algorithms were compared for tomographic imaging and phase holdup measurement. Experiments were carried out on a DN25 laboratorial scale CO2 two-phase flow rig at a pressure of 6 MPa for the gaseous mass flowrates from 0 to 430 kg/h and liquid mass flowrates at 515, 1100, and 1900 kg/h. The experimental results show that the cross-sectional distribution of two-phase CO2 flow can be monitored using the ECT system, which matches well with the images captured by a high-speed imaging system. Compared with the reference gas holdup obtained by the flowmeters in the single phase gaseous and liquid loops, the absolute accuracy of the gas holdup measurement can reach 6%, indicating that the developed system is promising for real-time monitoring of carbon dioxide in carbon capture and storage transportation pipelines.

Gas-liquid two-phase flow in a square microchannel with chemical mass transfer: Flow pattern, void fraction and frictional pressure drop

The dynamics of the gas-liquid two-phase flow are of great significance for the heat and mass transfer performance in a microreactor. In this paper, the flow pattern, void fraction and frictional pressure drop of gas-liquid two-phase flow of CO2 and MEA/[Bmim][BF4] aqueous solution accompanied by the chemical reaction in a 400 μm square microchannel were investigated experimentally by a high-speed camera and pressure sensor. Three kinds of flow regimes were observed: slug-bubbly flow, slug flow and slug-annular flow. The void fraction and frictional pressure drop in the slug flow regime were mainly investigated, and the applicability of models without consideration of the mass transfer proposed in the literature was assessed. Considering the effect of chemical absorption, two correlations were proposed with the Hatta number for predicting the two-phase void fraction and Chisholm parameter C, which showed good prediction performance.

One-dimensional model of a stratified thermal storage tank with supercritical coiled heat exchanger

This paper presents a semi-transient model of the thermal stratification in a hot water storage tank used as a gas cooler in a transcritical CO2 heat pump system. It includes two different kinds of coiled heat exchangers. The first heat exchanger model is based on the analytical solution of the 1D flow in a pipe with constant thermodynamic properties and negligible pressure loss. The second coil model is discretized along the flow direction, the properties are updated along the coil and the velocity, enthalpy and pressure field are calculated. Both heat exchanger models are steady state and the axial diffusion heat transfer in the storage tank is transient. The novelty here is that the second exchanger model accounts for supercritical and two-phase CO2 flow in the pipe immersed in the reservoir. This can thus be considered as a generalisation of TRNSYS Type 534. A numerical comparison is carried out between the actual model and the TRNSYS Type 534 from the TESS library to assess its validity. A validation of the CO2 heat exchanger is also carried out using experimental and numerical results of a tube-in-tube heat exchanger available in the literature. Finally, numerical results where the tank is used as a transcritical heat pump gas cooler were produced.

Mass flow measurement of gas-liquid two-phase CO2 in CCS transportation pipelines using Coriolis flowmeters

Carbon Capture and Storage (CCS) is a promising technology that stops the release of CO2 from industrial processes such as electrical power generation. Accurate measurement of CO2 flows in a CCS system where CO2 flow is a gas, liquid, or gas-liquid two-phase mixture is essential for the fiscal purpose and potential leakage detection. This paper presents a novel method based on Coriolis mass flowmeters in conjunction with least squares support vector machine (LSSVM) models to measure gas-liquid two-phase CO2 flow under CCS conditions. The method uses a classifier to identify the flow pattern and individual LSSVM models for the metering of CO2 mass flowrate and prediction of gas volume fraction of CO2, respectively. Experimental work was undertaken on a multiphase CO2 flow test facility. Performance comparisons between the general LSSVM and flow pattern based LSSVM models are conducted. Results demonstrate that Coriolis mass flowmeters with the LSSVM model incorporating flow pattern identification algorithms perform significantly better than those using the general LSSVM model. The mass flowrate measurement of gas-liquid CO2 is found to yield errors less than ±2% on the horizontal pipeline and ±1.5% on the vertical pipeline, respectively, over flowrates from 250kg/h to 3200kg/h. The error in the estimation of CO2 gas volume fraction is within ±10% over the same range of flow rates.

Pore-scale modeling of wettability effects on CO2–brine displacement during geological storage

Abstract Wetting properties of reservoir rocks and caprocks can vary significantly, and they strongly influence geological storage of carbon dioxide in deep saline aquifers, during which CO2 is supposed to displace the resident brine and to become permanently trapped. Fundamental understanding of the effect of wettability on CO2–brine displacement is thus important for improving storage efficiency and security. In this study, we investigate the influence of wetting properties on two-phase flow of CO2 and brine at the pore scale. A numerical model based on the phase field method is implemented to simulate the two-phase flow of CO2–brine in a realistic pore geometry. Our focus is to study the pore-scale fluid-fluid displacement mechanisms under different wetting conditions and to quantify the effect of wettability on macroscopic parameters such as residual brine saturation, capillary pressure, relative permeability, and specific interfacial area. Our simulation results confirm that both the trapped wetting phase saturation and the normalized interfacial area increase with decreasing contact angle. However, the wetting condition does not appear to influence the CO2 breakthrough time and saturation. We also show that the macroscopic capillary pressures based on the pressure difference between inlet and outlet can differ significantly from the phase averaging capillary pressures for all contact angles when the capillary number is high (log Ca > −5). This indicates that the inlet–outlet pressure difference may not be a good measure of the continuum-scale capillary pressure. In addition, the results show that the relative permeability of CO2 can be significantly lower in strongly water-wet conditions than in the intermediate-wet conditions.

Flow visualization of supersonic two-phase transcritical flow of CO2 in an ejector of a refrigeration system

Supersonic two-phase flow of CO2 in an ejector was investigated in flow visualization experiments. The ejector was installed in a transcritical CO2 ejector-expansion refrigeration system with a convergent primary nozzle and a rectangular mixing chamber. The flow fields in the suction chamber and the mixing chamber of the ejector were visualized by direct photography for various operating conditions. The results showed that the liquid in the primary flow after the primary nozzle exit increased with increasing primary flow and secondary flow pressures. The expansion angle of the primary flow at the nozzle exit decreased with increasing secondary flow pressures. The primary flow was blocked in some cases at the mixing chamber entrance due to the large expansion angle which reduced the entrainment performance. The entrainment ratio was inversely related to the expansion angle. The primary and the secondary flows had a short mixing region in the mixing chamber with the mixed flow quickly becoming uniform.

Study of the slippage of particle/supercritical CO2 two-phase flow

In this paper, the slippage velocity and displacement between particles and supercritical CO2 (SC-CO2) were studied to reveal the particle-SC-CO2 two-phase flow behavior. Visualization experiments were performed to directly measure the slippage velocity and displacement. Eight groups of experiments involving various pressures (7.89–10.96MPa), temperatures (38.6–47.5°C), particle diameters (0.3–0.85mm), particle densities (2630 and 3120kg/m3) and SC-CO2 flow rates (0.920–1.284m/s) were conducted. The measured particle slippage velocities in the flowing direction were approximately 10.3% of the SC-CO2 flow rate. The measured particle slippage displacements were all at the centimeter level, which indicated that SC-CO2 had a superior particle transporting capability that was similar to those of liquids even if it had a low viscosity that was similar to those of gases. A numerical model was built, and analytic slippage calculations were performed for SC-CO2 for additional analyses. The density of SC-CO2 was found to have a greater influence on the slippage than the viscosity. Moreover, a comparison of the slippage between SC-CO2 and water showed that the particle slippage in water was constant, while the particle slippage in SC-CO2 continually accumulated at an extremely slow rate.

A modified homogeneous relaxation model for CO2 two-phase flow in vapour ejector

In this study, the homogenous relaxation model (HRM) for CO2 flow in a two-phase ejector was modified in order to increase the accuracy of the numerical simulations The two- phase flow model was implemented on the effective computational tool called ejectorPL for fully automated and systematic computations of various ejector shapes and operating conditions. The modification of the HRM was performed by a change of the relaxation time and the constants included in the relaxation time equation based on the experimental result under the operating conditions typical for the supermarket refrigeration system. The modified HRM was compared to the HEM results, which were performed based on the comparison of motive nozzle and suction nozzle mass flow rates.

CO<sub>2</sub>冷凍サイクル用エジェクターの駆動ノズル部流れの速度計測

For the optimum design of two-phase flow nozzle of CO2 two-phase flow ejector, it is important to investigate fluid mechanics of high-speed gas-liquid two-phase flow. The purpose of this study is to investigate CO2 two-phase nozzle flow in view of both experimental and analytical aspects. In the experiment, CO2 in supercritical pressure condition was blown down into atmosphere through a divergent-convergent nozzle, where the mainstream profile of temperature was measured. Furthermore, velocity measurements of nozzle flow were conducted using by PIV system. Experimental data of both temperature and velocity show that the nozzle flow is considered to be supersonic flow. The experimental values of velocity are found to locate between the gas velocity and the liquid velocity of the 1-dimensional analysis by Kinoue et al.

Simulation of Supercritical CO2 Flow Through Circular and Annular Orifice

Supercritical CO2 (sCO2) is a promising working fluid for future high-efficiency power conversion cycles. In order to develop these cycles, it is necessary to understand supercritical and two-phase CO2 flow. This paper presents a methodology for the computational fluid dynamic (CFD) simulation of sCO2 flowing through a restriction under a wide range of flow conditions. Under an accidental situation, such as a pipe break, the inventory of sCO2 leaks out through a small restriction. In this research, we use circular and annular orifices to mimic the behavior of restrictions. As the atmospheric pressure is much smaller than the operating pressure in the pipe, a two-phase choked flow will happen. Such behavior is considered in the simulation. The homogeneous equilibrium model (HEM) is employed to model the two-phase state. To correctly simulate the behavior of the power cycle under this accidental scenario, the inventory leakage rate should be calculated precisely. Therefore, at the current state, this study only focuses on the prediction of mass flow rate through orifices.

Comparison of One-Dimensional Analysis with Experiment for CO<sub>2</sub> Two-Phase Nozzle Flow

The aim of this study is to investigate CO2 two-phase nozzle flow in terms of both experimental and analytical aspects for the optimum design of two-phase flow nozzle of CO2 two-phase flow ejector. In the experiment, it is measured that the temperature profile in the stream-wise direction of a divergent-convergent nozzle through which CO2 in the supercritical pressure condition is blown down into the atmosphere. In the analysis, a one-dimensional model which assumes steady, adiabatic, frictionless, and equilibrium is proposed. In the convergent part of the nozzle the flow is treated as single-phase flow of liquid, whereas in the divergent part the flow is treated as separated two-phase flow with saturated condition. The analytical results indicate that the temperature and the pressure decrease rapidly in the divergent part, and the void fraction increases immediately near the throat. Although this analysis is quite simple, the analytical results can follow the experimental results well within this study.

Numerical Modeling and Validation of Supersonic Two-Phase Flow of CO2 in Converging-Diverging Nozzles

Carbon dioxide is an attractive alternative to conventional refrigerants due to its low direct global warming effects. Unfortunately, CO2 and many alternative refrigerants have lower thermodynamic performance resulting in larger indirect emissions. The effective use of ejectors to recover part of the lost expansion work, which occurs in throttling devices, can close this performance gap and enable the use of CO2. In an ejector, the pressure of the motive fluid is converted into momentum through a choked converging-diverging nozzle, which then entrains and raises the energy of a lower-momentum suction flow. In a two-phase ejector, the motive nozzle flow is complicated by the nonequilibrium phase change affecting local sonic velocity and leading to various types of shockwaves, pseudo shocks, and expansion waves inside or outside the exit of the nozzle. Since the characteristics of the jet leaving the motive nozzle greatly affect the performance of the ejector, this paper focuses on the details of flow development and shockwave interaction within and just outside the nozzle. The analysis is based on a high-fidelity model that incorporates real-fluid properties of CO2, local mass and energy transfer between phases, and a two-phase sonic velocity model in the presence of finite-rate phase change. The model has been validated against the literature data for two-phase supersonic nozzles and overall ejector performance data. The results show that due to nonequilibrium effects and delayed phase change, the flow can choke well downstream of the minimum-area throat. In addition, Mach number profiles show that, although phase change is at a maximum near the boundaries, the flow first becomes supersonic in the interior of the flow where sound speed is lowest. Shock waves occurring within the nozzle can interact with the boundary layer flow and result in a ‘shock train’ and a sequence of subsonic and supersonic flow previously observed in single-phase nozzles. In cases with lower nozzle back pressure, the flow continues to accelerate through the nozzle and the exit pressure adjusts in a series of supersonic expansion waves.

Hydrodynamics of liquid CO2 with hydrate formation in packed bed

The purpose of the present study is to clarify the influence of CO2 hydrate formation on liquid CO2 flow injected into a packed bed which simulate seabed. In order to reveal the influence of CO2 hydrate on liquid CO2 flow, differential pressure and temperature are measured under the conditions with CO2 hydrate formation and without CO2 hydrate formation. As the result, for liquid CO2 flow with the hydrate formation, differential pressure at the upstream part of the packed bed becomes large compared with other sections. And, the amount of CO2 hydrate estimated from temperature rise decreases as Reynolds number increases. Friction factor is also estimated. As the result, the difference of the friction factor between both conditions becomes small with increase of Reynolds number in the upstream part of the packed bed. These results suggest that the hydrate formation is not so significant for liquid CO2 injection into the packed bed under the present high Reynolds number conditions. Furthermore, friction factor and water saturation are compared with the relative permeability model. As the result, it is suggested that pressure drop for water–liquid CO2 two-phase flow with the hydrate formation in the packed bed can be estimated by using the relative permeability model within the present experimental conditions.

Two-Phase Flow of CO2 with Phase Transfer

A model for two-phase pipeline flow of CO2 with phase transfer is presented. Two different relaxation models for phase transfer are developed. The system of equations is solved by splitting it into a hyperbolic conservation law and a relaxation ODE, solved by a multi-stage (MUSTA) finite volume scheme and the backward Euler method, respectively. The stiffened-gas equation of state is used for calculating thermodynamic properties. Simulation results from a depressurisation case of a CO2 pipe, causing phase change and cooling, are presented, showing that statistical rate theory predicts solutions close to those of an equilibrium model with instant phase transfer.

二酸化炭素高速気液二相ノズル流れの研究（実験と１次元解析の比較）

For the optimum design of two-phase flow nozzle of CO2 two-phase flow ejector, it is important to investigate fluid mechanics of high-speed gas-liquid two-phase flow. The purpose of this study is to investigate CO2 two-phase nozzle flow in view of both experimental and analytical aspects. In the experiment, CO2 in supercritical pressure condition was blow down into atmosphere through a divergent-convergent nozzle, where the mainstream profile of temperature and the jet force at nozzle exit were measured. In the analysis, one-dimensional model which assumes steady, adiabatic, frictionless, and equilibrium was proposed. The flow in the nozzle of convergent part was treated as single-phase flow, whereas the flow in the nozzle of divergent part was treated as separated two-phase flow with saturation. Although this analysis is quite simple, the analytical results can follow the experimental results well within the cases in this study.

863 超音速炭酸ガス二相流に発生する斜め衝撃波に関する実験的研究(OS4-1 気液二相流の伝熱・流動,OS4 伝熱の基礎と応用技術)

863 超音速炭酸ガス二相流に発生する斜め衝撃波に関する実験的研究(OS4-1 気液二相流の伝熱・流動,OS4 伝熱の基礎と応用技術)

PURE NH3 AND CO2 HEAT TRANSFER AND PRESSURE DROP IN HORIZONTAL SMOOTH TUBES FOR NH3 TWO-STAGE AND NH3/CO2 CASCADE REFRIGERATION SYSTEMS: A REVIEW

This paper presents a review of differences and similarities of in-tube heat transfer and pressure drop between ammonia (NH3) and carbon dioxide (CO2) from the perspective of the design of heat exchangers for NH3 two-stage and CO2/NH3 cascade refrigeration systems. The focus is on differences in thermophysical properties and thus different characteristics of heat transfer and pressure drop. A brief summary of published literatures about CO2/NH3 cascade refrigeration systems is provided and literature review of available correlations and developed correlations are presented for flow boiling and condensation heat transfer and pressure drop. Because of large deviation of calculated values with exiting correlations from measured results, a new correlation to predict flow condensation heat transfer coefficients was developed based on experimental results for CO2 at -15°C. From comparison of measured and predicted values, it is shown that some correlations, previously published in open literature, can be used to calculate flow boiling heat transfer coefficients for NH3 at -20°C, if a flow pattern can be appropriately determined for a flow condition. Also, it is presented that existing correlations can predict well the heat transfer coefficients for CO2 flow boiling at -15 and -30°C. It is shown that some correlations can predict pressure drop relatively well for NH3 and CO2 two-phase flow. The NH3 and CO2 flow evaporation heat transfer and pressure drop characteristics at -40°C are compared with predicted values.

Shock Waves in Supersonic Two-Phase Flow of CO2 in Converging-Diverging Nozzles

CO2 is a promising alternative to hazardous, ozone-depleting and global-warming refrigerants. It is more suitable to the ejector refrigeration cycle than to the vapor compression cycle. However, shock waves significantly reduce the efficiency of the ejector nozzle and, therefore, they must be investigated to improve the efficiencies of the nozzle and the ejector and the coefficient of performance of the ejector refrigeration cycle. This paper elucidates the types of shock waves in two-phase flow of CO2 in converging-diverging nozzles and their relationship to inlet conditions and two-phase thermodynamic states. Shock waves in supersonic liquid-vapor flows with low and medium quality through the diverging sections of the nozzles were investigated. Strong-and thin-equilibrium shock waves were calculated and considered as the ideal limiting case of actual shock waves. Pseudo-shock waves and dispersed shock waves were obtained from the experiment. Both were weaker than the equilibrium shock waves and indicated relaxation phenomena. Based on a theoretical model, the pseudo-shock waves had long relaxation times. The large liquid droplets formed could not be easily decelerated by the vapor. Conversely, the dispersed shock waves had short relaxation times. The small liquid droplets formed could be easily decelerated by the vapor, but the deceleration was less rapid than that in the equilibrium shock waves.

Modeling of Supercritical CO2 Flow Through Short Tube Orifices

The transcritical cycle of carbon dioxide (CO2) is a promising alternative approach to heat pumps and automobile air conditioners. As an expansion device, the short tube orifice in a transcritical CO2 system usually receives supercritical fluid at the entrance and discharges a two-phase mixture at the exit. In this work, a two-fluid model (TFM) is developed for modeling the flow characteristics of supercritical CO2 through the short tube orifice. The deviations between the TFM predictions and the measured mass flow rates are within ±20%. Meanwhile, the TFM predicts reasonable pressure, temperature, and velocity distributions along the tube length. The small values of interphase temperature difference and velocity slip indicate that the nonequilibrium characteristics of the two-phase flow of CO2 in the short tube orifice are not significant. Consequently, the homogeneous equilibrium model reduced from the TFM gives a good prediction of the mass flow rate as well.