Gas-liquid Transport Research Articles

A comparative numerical investigation of effect of mechanical compression on the transport properties of gas diffusion layer fabricated by wet and dry laid methods

The gas diffusion layer (GDL), a core component of the proton exchange membrane fuel cell (PEMFC), is integral to thermoelectric and gas-liquid transport. Recognizing the impact of mechanical compression on transport properties and cell performance, it is crucial to comprehensively study the GDL's effect on effective transport performance. In this study, two types of GDLs, Toray GDL fabricated by wet-laid and Freudenberg GDL fabricated by dry-laid, are first reconstructed. Then, their stress-strain distributions under various mechanical compression ratios from 0 to 40 % are simulated using the finite element method, along with the effects of mechanical pressures on the microstructural parameters. Finally, a pore scale model is utilized to obtain the effective transport properties of the two types of GDLs under ten different compression ratios. The results show that Freudenberg GDL exhibits a more rapid stress transfer with a more uniformly distributed stress, while Toray GDL displays a slower stress transfer with a more differentially distributed stress. Notably, the pore size decreases significantly by approximately 60 % in both GDL types when the compression ratio increases from 0 % to 40 %. As the mechanical compression ratios increases from 0 to 40 %, the tortuosity of Toray GDL and Freudenberg GDL in the in-plane/through-plane direction increases by 78 %/50 % and 81 %/86 %, respectively. The conductivity of Toray GDL and Freudenberg GDL increases by 150 %/650 % and 130 %/140 % in the in-plane/through-plane direction, respectively, when the compression ratio is increased to 40 %. Additionally, a 20 % compression ratio is identified as an optimal point for both GDLs to balance gas diffusion and thermoelectric conduction.

Analysis and identification of gas-liquid two-phase flow pattern based on multivariate multi-scale dispersion entropy and interconnected dispersion pattern complex network

Gas-liquid two-phase flow is polymorphic and unstable. The investigation of the flow behavior of two-phase flow is vital for the application of gas-liquid transport in ocean engineering areas. We propose an analytical framework that combines multivariate multi-scale dispersion entropy (mvMDE) and interconnected dispersion pattern complex networks (IDPCN) for flow patterns. The resistance sensor array is used to capture the two-phase flow fluctuation signals in the riser, and the dimensionally reduced time series are nonlinearly mapped into a multivariate symbolic sequence. The mvMDE of the multivariate symbolic sequence is computed to measure the complexity of the flow pattern signals at the level of amplitude variation. In addition, a new multis-cale complex network based on the symbolic sequence is proposed to study the pseudo spatial coupling behavior of flow patterns. The evolution of the flow pattern from bubble to churn flow is quantitatively characterized using the mean value of the complex network indices in the optimal scale range. Finally, we construct the joint plane of mvMDE and two network indices to complete flow pattern identification. The research results show that the framework can effectively fuse multi-channel flow pattern information at different time scales to reveal the evolution mechanism of gas-liquid two-phase flow.

Artificial intelligence vision technology application in sustainability evaluation of solar-driven distillation device

The process of transforming brackish water into freshwater utilizing solar thermal energy is referred to as solar-driven distillation device, or solar still. Such device without fossil fuel provides significant environmental and health benefits by reducing air pollutants and remedying brackish water. The yield of purified water from conventional solar stills remains insufficient, prompting the necessity for research to enhance it by understanding the coupling effect between steam flow, heat transfer, and mass transfer. As such, computer-aided brackish water treatment comparison of the hemispherical solar still and other multi-slope solar stills is conducted in this paper, and an automatic steam&heat flow detection algorithm is developed to solve the problem of difficult data acquisition for gas-liquid transport processes. Firstly, double slope, four slope, and hemispherical solar stills are exposed to the sun in outdoor experiments, therefore the solar thermal performance of each still is analyzed through pairwise comparisons. Secondly, a large amount of experimental image data is input into neural network for training, and by continuously adjusting network parameters, the network can accurately recognize different types of images. Finally, the image to be recognized is input into the trained neural network, which outputs the category labels of the image to achieve automatic image recognition. Based on the data above, the best structure of solar-driven device is hemispherical structure, due to that the hemispherical solar still possesses the best performance in terms of distilled water yield, energy efficiency, exergy efficiency and energy payback.

Capillary-Driven Separate Gas-Liquid Transport: Alleviating Mass Transport Losses for Efficient Hydrogen Evolution.

Developing earth-abundant transition metal electrodes with high activity and durability is crucial for efficient and cost-effective hydrogen production. However, numerous studies in the hydrogen evolution reaction (HER) primarily focus on improving the inherent activity of catalysts, and the critical influence of gas-liquid countercurrent transport behavior is often overlooked. In this study, we introduce the concept of separate-path gas-liquid transport to alleviate mass transport losses for the HER by developing a novel hierarchical porous Ni-doped cobalt phosphide electrode (CoNix-P@Ni). The CoNix-P@Ni electrodes with abundant microvalleys and crack structures facilitate the gas-liquid cotransport by separating the bubble release and water supply paths. Visualization and numerical simulation results demonstrate that cracks primarily serve as water supply paths, with capillary pressure facilitating the transport of water from the cracks to the microvalleys. This process ensures the continuous wetting of electrolytes in the electrode, reduces hydrogen supersaturation near the active site, and increases hydrogen transport flux to the microvalleys for accelerating bubble growth. Additionally, the microvalleys act as preferential sites for bubble evolution, preventing bubble coverage on other active sites. By regulating the amount of nickel, the CoNi1-P@Ni electrode exhibited the smallest and densest microvalleys and cracks, achieving superior HER performance with an overpotential of 51 mV at 10 mA cm-2. The results offer a promising direction for constructing high-performance HER electrodes.

Exploring the effect of the inlet gas volume fraction on the energy-conversion features of a multiphase pump using energy-transport theory

This work sought to reveal the role of the inlet gas volume fraction (GVF) on the energy-conversion features of a multiphase pump. To this end, a self-developed single-stage multiphase pump was used as the research object, and a gas–liquid transport medium was examined based on the energy-transport theory. The role of the GVF in the pressure-gradient work, Lamb-vector divergence, and vortex pseudo-energy dissipation in the pressurization unit of the multiphase pump were analyzed, and the impact of the GVF on the energy-conversion features of the multiphase pump was also investigated. The results indicate that under various GVFs, increasing the tip clearance prevents the pressure gradient from exerting its intended function. Furthermore, as the GVF is increased, the scale of the tip-leakage vortex increases, the flow field in the pressurization unit is disturbed, and the vortex pseudo-energy dissipation in the impeller increases. As a result, the energy loss increases, the pressurization effect of the pump is weakened, and its work performance decreases. These results offer a reference for enhancing the efficiency of multiphase pumps.

Effect of perforated cracks on liquid water in microporous layers by lattice Boltzmann method

Microporous layer (MPL), as a critical constituent of the proton exchange membrane fuel cell (PEMFC), plays a key role in mass, heat, electron, and species transport. The cracks formed during the deposition process on the surface of MPL change the overall transport capacity and effect the output performance of PEMFC. In this study, a 2D Lattice Boltzmann Method (LBM) pseudopotential multicomponent model is used to investigate the effects of perforated cracks in MPL on distribution, pressure drop and flow state of liquid water. The simulation results show that Capillary number (Ca) of liquid water in perforated cracks is around two orders of magnitude higher than Ca in twisty pores. Perforated cracks in MPL can not only reduce capillary resistance but also convert liquid water from capillary fingering to viscous fingering essentially, thus enhancing the permeability of liquid water in the through-plane direction while reducing the invasion in the in-plane direction. In consideration of gas-liquid transportation and other structural characteristics of GDL, the proper interval between perforated cracks with width of 10 μm is 3–6 crack diameters. This study provides reference for the design of functional pore structure MPL and conducts guidance for further research on PEMFC.

Spatiotemporal simulation of gas-liquid transport in the production process of continuous undulating pipelines

Large sections of gas accumulation easily form in continuously undulating liquid pipelines to impede the commissioning process affecting the safe operation of the pipeline. To solve the gas resistance and overpressure during the commissioning of continuously undulating pipeline in complex terrain, a whole process commissioning simulation model is proposed to quantitatively calculate the location and volume of gas accumulation and to monitor the pressure drop across the line in the real time. Based on a single V-shaped pipe compression process model, the proposed model considers the influence of back pressure, gas-liquid migration and integrates different process such as high point exhausting, pigging process and pump-pipe coordination. Multiple operating conditions of two real pipelines are simulated, and the results show that the proposed model has an accuracy of within ±15% for predicting the pressure along the pipeline and tracking the pigging device, indicating that the gas transport model of the continuous undulating pipeline production process has acceptable accuracy. In addition, the applicability of the proposed model under different operating condition is also verified. Specifically, the gas accumulation ratio and safety pressure of the entire pipeline are calculated by considering multiple sets of different exhaust Schemes for different flow rates, high points, and the use of pigging devices on two different real pipelines, demonstrating that the model has good generality and feasibility. Secondly, the model can calculate the pressure drop at different positions along the entire pipeline, monitor the position of the pigging device in real time, and simulate comprehensive production exhaust Schemes, demonstrating that the model has comprehensive functionality and some generality. In summary, the proposed model can provide validation and guidance for the subsequent production of related continuous undulating liquid pipelines.

Flow Performance Analysis of Double-Chamber Gas–Liquid Transferring Device

Gas–liquid transportation is an efficient and economical transportation technology developed in recent years. It mainly uses some transportation equipment to simultaneously transport crude oil, associated natural gas, produced water, and so on for the purpose of reducing costs and energy consumption. In this paper, a double-chamber gas–liquid transferring device was selected as the research object. The VOF multiphase flow model of FLUENT software was used to simulate the gas–liquid two-phase flow performance under the design condition. The relationships between the two-phase flows, pressure, temperature, liquid level in the tank, outlet flow rate of the device, and time were calculated and analyzed. The results show that the operating cycle of the device can be divided into three processes, namely ‘suction-compression-discharge’, in which the cycle period formula is proposed as well. The pressures of gas and liquid in the device are basically the same, but the temperatures of the two phases are quite different in the compression process. In the compression process, the outlet valve is closed so that the outlet flow rate of the device is zero, and the gas compression process can be approximately treated as isothermal compression in engineering design. The comparison between the measured performance data in the oil field and the calculation results indicates that the calculation method in this paper is feasible for the performance prediction and optimization of the double-chamber gas–liquid transferring device.

Study on the Influence of Gas Volume Fraction on the Flow Field and Deformation Characteristics of Gas Liquid Transportation Single Screw Pump

Study on the Influence of Gas Volume Fraction on the Flow Field and Deformation Characteristics of Gas Liquid Transportation Single Screw Pump

Effects of Cathode GDL Gradient Porosity Distribution along the Flow Channel Direction on Gas–Liquid Transport and Performance of PEMFC

The gas diffusion layer (GDL) is an important component of proton exchange membrane fuel cells (PEMFCs), and its porosity distribution has considerable effects on the transport properties and durability of PEMFCs. A 3-D two-phase flow computation fluid dynamics model was developed in this study, to numerically investigate the effects of three different porosity distributions in a cathode GDL: gradient-increasing (Case 1), gradient-decreasing (Case 3), and uniform constant (Case 2), on the gas-liquid transport and performance of PEMFCs; the novelty lies in the porosity gradient being along the channel direction, and the physical properties of the GDL related to porosity were modified accordingly. The results showed that at a high current density (2400 mA·cm-2), the GDL of Case 1 had a gas velocity of up to 0.5 cm·s-1 along the channel direction. The liquid water in the membrane electrode assembly could be easily removed because of the larger gas velocity and capillary pressure, resulting in a higher oxygen concentration in the GDL and the catalyst layer. Therefore, the cell performance increased. The voltage in Case 1 increased by 8% and 71% compared to Cases 2 and 3, respectively. In addition, this could ameliorate the distribution uniformity of the dissolved water and the current density in the membrane along the channel direction, which was beneficial for the durability of the PEMFC. The distribution of the GDL porosity at lower current densities had a less significant effect on the cell performance. The findings of this study may provide significant guidance for the design and optimization of the GDL in PEMFCs.

Gas–Liquid Transport Behaviors and Mass Transfer Mechanism During Oxygen Dissolution and Evolution Processes in a Micropump

Abstract On-orbit refueling and space circulation technologies involve the use of a space micropump to transport gas–liquid mixed fluids, which affects the gas–liquid mass transfer and dynamic behaviors. To predict dynamic mass transfer processes, our proposed dissolved and released models were applied to space micropump calculation after the verification of dissolved oxygen concentration and micropump energy characteristics. The mass transfer characteristics and gas–liquid states were investigated by combining the correlation analyses. The results show that the dissolved concentration and the volume fraction are considered to be strongly related to the mass transfer rate, and the effect of turbulence kinetic energy cannot be ignored particularly in the impeller and volute. Based on this, the gas–liquid state parameters are focused on unidirectional dissolved and bidirectional released-dissolved conditions. The released gas occupied the head of the suction surface of the long blades and developed downstream, and its presence causes a significant gas increase downstream. According to the mass-transfer characteristics comparisons, the oxygen increment decreases as the inlet dissolved oxygen concentration increases, exhibiting the similarity of the two-film theory. In addition, the evolution increases the fluctuation in the gas volume fraction and the total hydraulic loss. The current study guides the fueling gas–liquid mixed delivery status, and the dissolved gas concentration must be controlled strictly to avoid the evolution of gas to ensure safety and decrease the flow loss.

Perfluorocarbon nanoemulsions as hydrogen carriers to promote the biological conversion of hydrogen and carbon dioxide to methane

The formation of methane (CH4) via a biomethanation process using hydrogen (H2) and carbon dioxide (CO2) as feedstock is impeded by H2 gas-liquid mass transfer limitations. To address these limitations and improve the CH4 production rate in H2/CO2 biomethanation, perfluorocarbon (PFC) nanoemulsions exhibiting a high solubility for gases were proposed as H2 transfer carriers to promote the H2 utilization rate and methanogenesis kinetics. The presence of PFC nanoemulsions in a volume fraction of 1.5% increased the peak rate of CH4 production by 30.6% and decreased the lag time by 84.5%. These nanoemulsions, composed of PFC and surfactants, efficiently transported H2 molecules to alleviate gas-liquid transport limitations in biological solutions. Furthermore, lipophilic PFC, which is nonspecifically bound to cell membranes of Methanosarcina barkeri (M. barkeri) cells, also provided the transport of H2 molecules from PFC nanoemulsions to the cells. The PFC nanoemulsions separated from these cell membranes primarily through physical oscillations. However, physical separation was difficult when using 2 vol% PFC nanoemulsions due to their strong combination with M. barkeri cells, which hindered H2 transmission and utilization.

Effects of pin shapes on gas-liquid transport behaviors in PEMFC cathode

In this work, three pin-type flow field (FF) designs for a proton exchange membrane fuel cell (PEMFC) cathode with the circle, diamond, and square pins are proposed. The gas-liquid transport phenomena inside the PEMFC cathode are investigated to evaluate the water management performance of the designs. The volume of fluid model is used for the observance of the liquid water behaviors. A validated dynamic contact angle model is applied to improve the accuracy of the numerical simulation. From the results, it can be concluded that the pin shape affects greatly the emergence of liquid water from the catalyst layer, gas diffusion layer (GDL) to the FF. The study also offers insights into the airflow and liquid behaviors inside the pin-type FFs. Among the three FFs, the diamond pins remove water fastest with the lowest pressure drop. The square-pin FF also has low water mass in the computational domains but suffers from the highest pressure drop and uneven velocity, water, and pressure distributions. The circle pins have the most water content but can provide low pressure drop with the most uniform distributions. Thus, considerations for water removal ability should be taken for the pin-type FF designs regarding PEMFC performance and reliability.

Signal selection for identification of multiphase flow patterns in offshore pipeline-riser system

Multiphase flow pattern identification is an urgently desirable approach for flow assurance of gas-liquid transportation in deep-sea pipelines. Experiments are performed on a system with 1657 m horizontal pipe and 16.7 m S-shaped riser. A new method of sample preparation using only data from liquid accumulation stage is proposed for identification of early-stage severe slugging. A set of signal evaluation and selection criteria covering the recognition effect and practicability of signals are proposed for identification of severe slugging, oscillating flow and stable flow. For oilfield applications including above-water and underwater scenarios, pressure signals at different locations of the pipeline-riser system are selected to meet the actual demand with the shortest sampling duration. Using the combination of above-water signals at the top of the riser, the recognition rate of 90.0% is achieved at a sampling duration of 37.2 s. Feature dimension is reduced by principal component analysis, and good recognition results are obtained using the optimal signal combinations in different classifiers.

Improving cell performance for anion exchange membrane fuel cells with FeNC cathode by optimizing ionomer content

To improve the performance of anion exchange membrane fuel cells (AEMFCs) with platinum-group-metal (PGM)-free cathode, significant efforts are still needed. Herein, we prepare high oxygen reduction reaction activity FeNC catalyst and integrate such catalyst into AEMFCs with different ionomer/catalyst (I/C) ratios from 0.1 to 1.0. We show that suitable quaternary ammonia poly (methyl-piperidine-co-p-terphenyl) (QAPPT) ionomer content can provide better catalyst layers (CLs) microstructure, in which the transfer efficiency of electron and charge can be improved so as to decrease the active polarization. High ohmic resistance is caused by either low or excess ionomer which leads to inconsecutive ionic network of CLs or high coverage of non-electronic conductor. In addition, mass-transfer polarization is also brought out by excess QAPPT ionomer which fills up the gas–liquid transport pores inside FeNC/QAPPT aggregates. With the I/C ratio of 0.7, AEMFC with FeNC cathode exhibits the best cell performance achieving a peak power density of 660 mW cm−2 at 1500 mA cm−2 under H2/air (CO2-free). To verify the feasibility of FeNC cathode in realistic applications, an AEMFC stack with 29 cells of 270 cm2 MEAs was assembled with a power output of 508 W under H2/air.

Stainless Steel-Supported Amorphous Nickel Phosphide/Nickel as an Electrocatalyst for Hydrogen Evolution Reaction.

Recently, nickel phosphides (Ni-P) in an amorphous state have emerged as potential catalysts with high intrinsic activity and excellent electrochemical stability for hydrogen evolution reactions (HER). However, it still lacks a good strategy to prepare amorphous Ni-P with rich surface defects or structural boundaries, and it is also hard to construct a porous Ni-P layer with favorable electron transport and gas–liquid transport. Herein, an integrated porous electrode consisting of amorphous Ni-P and a Ni interlayer was successfully constructed on a 316L stainless steel felt (denoted as Ni-P/Ni-316L). The results demonstrated that the pH of the plating solution significantly affected the grain size, pore size and distribution, and roughness of the cell-like particle surface of the amorphous Ni-P layer. The Ni-P/Ni-316L prepared at pH = 3 presented the richest surface defects or structural boundaries as well as porous structure. As expected, the as-developed Ni-P/Ni-316L demonstrated the best kinetics, with of 73 mV and a Tafel slope of ca. 52 mV dec-1 for the HER among all the electrocatalysts prepared at various pH values. Furthermore, the Ni-P/Ni-316L exhibited comparable electrocatalytic HER performance and better durability than the commercial Pt (20 wt%)/C in a real water electrolysis cell, indicating that the non-precious metal-based Ni-P/Ni-316L is promising for large-scale processing and practical use.

Effects of rolling motion on transient flow behaviors of gas-liquid two-phase flow in horizontal pipes

The investigation of transient flow behaviors of gas-liquid two-phase flow under ocean conditions is vital to the application of gas-liquid transport in ocean engineering area. In this paper, the gas-liquid two-phase flow numerical model of rolling horizontal pipe is established based on CFD code combined with user-defined functions (UDF). The three-dimensional rolling flow fields are simulated using the volume of fluid (VOF) model, RNG k−ε turbulence model and additional inertia force method. First, the simulation results are compared with the experimental data, which revealed a reasonable agreement. Then, the variation in transient flow behaviors by horizontal pipe (L = 2.0 m, D = 0.035 m) with varying rolling conditions (θm = 2°-6°, T = 0.5–4.0 s) and flow velocities (jG = 0.1–10.0 m/s, jL = 0.1–0.5 m/s) are investigated. The results showed that the rolling motion caused agglomeration of dispersed bubbles and gas slugs into a continuous gas column, increased the cross-sectional void fraction significantly. The variation of additional inertial force and pressure drop increased with the increase of rolling angle and the decrease of the rolling period. Compared with the rolling angle, the transient flow behaviors are more sensitive to the rolling period. In addition, as the initial gas volume fraction increased, the effects of rolling motion on the transient flow behaviors decreased gradually. These qualitative conclusions may serve as a reference for gas-liquid transport simulations under rolling conditions.

Visualized-experimental investigation on a mini-diameter loop thermosyphon with a wide range of filling ratios

The mini-diameter loop thermosyphon, which has an excellent heat transfer performance with simple, compact structure and pump-free advantages, can effectively solve the problem of heat transfer under high heat load in a confined space. The mini-diameter loop thermosyphon exhibits different gas-liquid transport and heat transfer patterns than the conventional-diameter one due to various gas-liquid phase transformation properties and a strong friction force in the mini-diameter channel. Furthermore, data on the visual analysis of gas-liquid transport and heat transfer properties of the mini-diameter loop thermosyphon under various filling ratios is still lacking. In this paper, a visible experimental research on a mini-diameter loop thermosyphon (3 mm) in a wide filling ratio range(44% -97%) is undertaken. The maximal heat transfer capacity increases initially and subsequently falls with the filling ratio, according to the testing data. At a low filling ratio, the flow pattern changes as slug flow - churn flow - annular flow with the heat load increase, leading to a low loop thermal resistance, and dry out occurs at 185 W heat load under 50% filling ratio. The gas-liquid flow shifts to slug flow-geyser boiling-bubble flow at high filling ratios (73% and 82%), and the bubble departure diameter steadily decreases with increasing heat load. Consequently, the heat transfer performance (thermal resistance) has been enhanced, and the maximum heat transfer capacity has been increased to 280 W. Single-phase flow is plainly seen in the loop when the filling ratio reaches 91% or even 97%, thus reduced heat transfer capacity. Therefore, the high-filling-ratio mini-diameter loop thermosyphon is better for high-load heat dissipation. However, a low thermal resistance shows at the low filling ratio, but the heat load is also low. As a result, a loop thermosyphon with a low filling ratio should be selected for the low heat load situation.

Study of bubble behavior in high-viscosity liquid in a pseudo-2D column using high-speed imaging

Bubbles, as a critical microscale phenomenon, play an important role in determining the performance of gas–liquid transport and reactions. Reactors should be designed and scaled up suitably to investigate their bubble dynamics, which is generally influenced by the liquid viscosity. Compared to low-viscosity systems, high-viscosity systems have rarely been investigated for their bubble behavior (especially their bubble swarm behavior, which is often involved in relevant industrial processes). This paper reports a case study of nitrogen–glycerin viscous systems, conducted via high-speed imaging. Bubbly flows with naturally generated interactions and collisions are investigated quantificationally, and the special conjunct bubble is observed qualitatively. The correlation of equivalent bubble diameters is obtained for the relevant viscous system by fitting the experimental results. The aspect ratio that characterizes the bubble shape is correlated, and a novel EoRe–E equation is derived.

Effects of Pin Shapes on Gas-Liquid Transport Behaviors in Pemfc Cathode

Effects of Pin Shapes on Gas-Liquid Transport Behaviors in Pemfc Cathode