Number Of Flow Channels Research Articles

A pore-scale study on microstructure and permeability evolution of hydrate-bearing sediment during dissociation by depressurization

Research data and visual evidence to understand the interaction between hydrate dissociation and micro-seepage in sediment are currently lacking. In this study, X-ray computed tomography (CT) is used to reveal the dynamic interaction process between sediment pore-scale structural changes and permeability during hydrate dissociation. After Xe hydrate formation in spherical quartz and then decomposed the hydrate by depressurization to obtain the real-time permeability and 3D structure of hydrate-bearing sediments (HBSs) with CT scans. The results show that the heterogeneity of hydrate spatial distribution does not change the power function relationship between permeability and hydrate saturation. The ball-and-stick model shows that the number and size of flow channels dominate the flow resistance of the fluid at high hydrate saturation, while the size of flow channels dominates it at low hydrate saturation. The saturation cut-off point is between 16.35% and 25.37%. The CT scan images show that the hydrate morphology changes from patchy to load-bearing, and then to cementing. The transition process from load-bearing to cementing has the greatest impact on sediment permeability. Further PNM simulations show that the size and number of flow channels and capillary forces are the main factors affecting the ability of gas–water two-phase flow in the sediment. Most importantly, a relative permeability model related to hydrate saturation parameters is proposed based on image processing technology. The good agreement between the model and available laboratory/field measurements proves its practicability. The method of proposing this model provides a new idea to establish new relative permeability models.

Influence mechanism and optimization of the plugging capacity of temporary plugging zones with different particle sizes based on pore structure

Reducing the temporary plugging zone permeability is the key to the success of temporary plugging and diverting fracturing. The pore structure of the plugging zone determines the permeability of the temporary plugging zone. This paper introduces a new method based on micro-computed tomography (micro-CT) to quantitatively characterize the pore structure of temporary plugging zones with different particle size. The permeability of temporary plugging zone was measured, and seepage simulation was carried out based on pore network model (PNM). The particle compound method and formula were proposed to reduce the permeability of the temporary plugging zone. Finally, a permeability prediction model of temporary plugging zone based on pore structure was established. The results show that the porosity and tortuosity of the temporary plugging zone are independent of particle size under natural accumulation, and the pore radius decreases with the decrease in particle size. After compaction, the pore structure of the temporary plugging zone changes more obviously with smaller particle size. The seepage simulation shows that the number of flow channels in the temporary plugging zone formed by 2–3 mm particles is only 23.5% of the 0.6–1 mm flow channels. However, several obviously dominant channels are found in the 2–3 mm particles, resulting in a higher permeability. The 0.6–1 mm particles are used to fill the pores formed by 2–3 mm particles so that the dominant channel is divided into several small flow channels to significantly decrease the permeability of the 2–3 mm particles. When 2–3 mm and 0.6–1 mm particles are mixed according to the established particle size recombination formula, the pore radius and permeability of the temporary plugging zone decreased by 42.3% and 53.2% compared with 2–3 mm particles. The permeability prediction model can predict the permeability of temporary plugging zone well, and the average error is 9.145%. The sample preparation methods, pore structure testing and permeability prediction model were established in this work, providing a new method for the selection and design of temporary plugging agents.

Proton Exchange Membrane Fuel Cell as an Alternative to the Internal Combustion Engine for Emission Reduction: A Review on the Effect of Gas Flow Channel Structures

Proton exchange membrane fuel cells are a new energy technology with great potential due to advantages such as high efficiency and no pollution. The structure of the gas flow channels has a profound impact on the overall performance of the fuel cell. Different flow channel geometries have their own advantages and disadvantages, and a good understanding of the influence of these structures on performance can provide a reference for the design and improvement of flow channel geometries in various application contexts. Numerical models can be used as a reasonable and reliable tool to evaluate the influence of operating and structural parameters on cell performance and service time by simulating the transport processes of substances and heat as well as electrochemical reactions inside the fuel cell and can be used for the optimisation of cell design. This paper reviews the recent models of proton exchange membrane fuel cells, summarises and analyses the effect of gas flow channels on fuel cells, and organises and concludes efficient design of flow channel structures to enhance PEMFC performance in terms of the cross-section shape, length, width, number of flow channels, and baffle position.

Planar water disinfection reactor with parallel-channel configurations

A novel planar water disinfection reactor is proposed to treat potable water with UV-C LED. Parallel-channel configurations are employed to increase the flow uniformity, microbial exposure time and UV fluence for microorganisms. The effects of flow rate, baffle width and number of flow channels (n) are examined to elevate the efficiency of sterilizing Escherichia coli (E. coli). Experimental results indicate that the four-channel reactor exhibited the highest inactivation efficiency owing to high flow uniformity, sufficient exposure time and circulation elimination in the flow channel. The reactor achieved 4.0 log at a flow rate of 29 mL/min and a total radiative power of 80 mW. The disinfection flow rate was 45% higher than that of the reactor without channel configuration. Increasing the number of channels helps eliminate circulations and achieve uniform flow distribution. However, microbial exposure time and UV fluence decrease with increased number of channels. Additionally, when the baffle width was less than 6 mm, the log inactivation increased with baffle width due to the increase in the degree of flow uniformity. The flow reactor achieves optimal disinfection efficiency at baffle width 6 mm due to an optimal combination of flow variation and water velocity.

Study on the Curved Channel Model in the Initial Core Loading of Pebble Bed High Temperature Reactors

Continuous on-line fuel cycling is the essential feature of the pebble bed high temperature reactor (PB-HTR). The flow speed of the fuel pebbles in a PB-HTR presents a radial distribution in the reactor core, mainly due to the friction between the pebbles and the wall and the conical structure at the core bottom. In the VSOP fuel shuffling model, the simulation of unequal pebble flow speed is achieved by dividing the reactor core into some vertical flow channels with different numbers of the equal-volume regions in each channel. However, the fuel shuffling with equal-volume batches bring complexity when dealing with the change of fuel composition, such as the fuel fraction of fuel-graphite pebble mixture, during the initial core loading and early running-in phase. In this work, a curved channel model with unequal flow speed and the bottom cone is established based on the DEM simulation of pebble flow in the HTR-PM. The batch-wise fuel shuffling strategy is adapted to fit the complex situation during mixing and re-assigning the discharged fuels by employing a rounding strategy for the actual volume of fuels with similar irradiation history. The key of the adapted strategy is to divide the total number of the mixed batches with similar irradiation history by the number of flow channels, and round the quotient as the number of reloaded batches in each top region. The fuel loading process to build up the initial core, accompanied by the low-power reactor running to compensate the reactivity provided by the fresh fuels, is simulated by using the fuel shuffling model mentioned above. On the other hand, the simulation on the same process with an effective cylindrical core mesh and straight flow channels is carried out, in which dividing and rounding the batch numbers are unneeded. The results of both models are compared, indicating that the curved channel model presents less core reactivity and shorter fuel loading period than those of the cylindrical model. From the point of view of fidelity, the former is more suitable for the simulation of initial core loading process. The results in this work are important for enhancing the economy of fuel cycling of PB-HTRs.

Numerical study on serpentine design flow channel configurations for vanadium redox flow batteries

Abstract A flow channel is a significant factor determining the performance of vanadium redox flow batteries (VRFBs). In this study, we use a three-dimensional numerical model to investigate the complexities of the fluid dynamics and electrochemical reactions of VRFBs considering a number of serpentine design flow channels. The results show that cell voltage increases with the reduction in the number of flow channels and increase in the electrolyte flow rate during the discharging process. The higher electrolyte flow rate significantly improves the uniformity of vanadium concentration at both electrodes. We observe that the pressure drop in the VRFB cell decreases with increasing number of flow channels. Moreover, the electrolyte disturbance and flow resistance reduces when the number of flow channels increases, which lead to a smaller pressure drop. As a result, the pumping power derived from the pressure drop decreases with the increase in the number of flow channels. The maximum power-based efficiency calculated for the quadruple serpentine design flow channel at an electrolyte flow rate of 60 ml/min is 97.18%. This serpentine design shows the lowest pumping power and achieves the optimum power-based efficiency. The proposed numerical approach provides comprehensive insights into serpentine design flow channel configurations for VRFBs.

Simulation of the water controlling ability of an adaptive inflow control device

Traditional adaptive inflow control device (AICD) has low water control efficiency. Based on adaptive inflow control device, numerical simulation method is used to analyze the internal flow of the device. Influence of relevant design parameters on its water control capacity is studied. Internal structure of AICD is optimized to change internal flow and the flow path of oil flow in the restrictor. Purpose of controlling the flow of oil by using different viscosities of two flow states is achieved. Results show that setting the baffle to distinguish the annular passage and control chamber can improve pressure drop of water flow. The direction and number of flow channels have significant influence on AICD water control ability. Optimizing the shape of flow channel can change entrance speed and entrance angle, improving the water control capacity of inflow controlling device. The AICD achieves the optimal control capacity when it adopts the one transitional flow channel with 45°A angle.

An experimental investigation on the heat transfer and pressure drop characteristics of nanofluid flowing in microchannel heat sink with multiple zigzag flow channel structures

This research reports the thermal performance and flow characteristics of nanofluid flows in two different types of microchannel heat sink (MCHS) with multiple zigzag flow channel structures experimentally with regard to the continuous zigzag flow channel (CZ-HS) and the single cross-cutting zigzag flow channel (CCZ-HS). SiO2 nanoparticles with particle loadings of 0.3, 0.6, and 0.8vol.% and dispersed in deionized water (DI water) are used as the working medium. Both CZ-HS and CCZ-HS are made from copper material. Their dimensions are approximately 28×33mm. Hydraulic diameter and number of flow channels are equally designed as seven 1-mm flow channels, respectively. The heat transfer area of CZ-HS is approximately 1176 mm2 and that of CCZ-HS is 1238 mm2. The effects of single cross-cutting of the flow channel, Reynolds number, and particle concentration on the Nusselt number and pressure drop characteristics are investigated. The experimental data indicate that the nanofluid-cooled heat sink provided larger thermal performance than the heat sink cooled by water of approximately 3–15%. Similarly, the results indicated that the thermal performances of the CCZ-HS are larger than those of the CZ-HS by an average of 2–6%. For the pressure drop, the measured data showed that particle concentration and cross-cutting of the flow channel have a small effect on the pressure-drop data.

Thermal and Hydraulic Performances of Nanofluids Flow in Microchannel Heat Sink with Multiple Zigzag Flow Channels

This article presents an experimental investigation on the heat transfer performance and pressure drop characteristic of two types of nanofluids flowing through microchannel heat sink with multiple zigzag flow channel structures (MZMCHS). SiO 2 nanoparticles dispersed in DI water with concentrations of 0.3 and 0.6 vol.% were used as working fluid. MZMCHS made from copper material with dimension of 28 × 33 mm. Hydraulic diameter of MZMCHs is designed at 1 mm, 7 number of flow channels and heat transfer area is about 1,238 mm 2 . Effects of particle concentration and flow rate on the thermal and hydraulic performances are determined and then compare with the common base fluid. The results indicated that the heat transfer coefficient of nanofluids was higher than that of the water and increased with increasing particle concentration as well as Reynolds number. For pressure drop, the particle concentrations have no significant effect on the pressure drop across the test section.

Novel multi-tubular fixed-bed reactors’ shell structural analysis based on numerical simulation method

In the present contribution, a numerical study of fluid flow and heat transfer performance in a pilot-scale multi-tubular fixed-bed reactor with a novel configuration for propylene-to-acrolein oxidation reaction is presented using a three-dimensional computational fluid dynamics method (CFD) to ensure the uniformity condition using molten salt as a heat carrier medium on shell side. The effects of multiscale structural parameters including the number of baffles, baffles cut, central nontube region and the number of flow channels on pressure drop and heat transfer are considered. The simulations suggest that heat transfer coefficient per pressure drop is reduced with increasing number of baffles. By the single factor sensitivity analysis it was shown that the central region is the key factor in the structural design of a multi-tubular fixed-bed reactor.

Analytical model of flow maldistribution in polymer electrolyte fuel cell channels

Gas–liquid, two-phase flow through channels of a polymer electrolyte fuel cell (PEFC) is of great interest as reactant oxygen is supplied and liquid product water is removed via these PEFC channels. Gas diffusion layer (GDL) intrusion in the channels, which is inherent to the process of PEFC cell and stack assembling, increases the local flow resistance in the intruded channels and consequently lowers their flowrates. This flow maldistribution renders the intruded channels more susceptible to liquid water accumulation or flooding. A one-dimensional analytical model is developed in this work to elucidate the two-phase flow maldistribution in PEFC channels resulting from GDL intrusion. Relative humidity (RH) and the stoichiometric flow ratio of inlet gases are found to be the two key parameters controlling the flow maldistribution in PEFC channels. Interestingly, our analysis shows that decreasing the inlet RH worsens flow maldistribution. As GDL intrusion in channels is inevitable, a good flow-field design must be inherently tolerable to flow maldistribution. Using the analytical model presented herein, the number of flow channels and their U-turns are optimized to minimize the detrimental effect of GDL intrusion.

Combined Experimental and Simulation (CFD) Analysis on Thermal Performance of Plate Heat Exchanger Using Kerosene as Working Fluid

The plate heat exchanger exhibits excellent heat transfer characteristic, which allows a very compact design with ease of maintenance and modification of heat transfer area by adding (or) removing plates. Constructional parameters such as flow path, trough angle and corrugation can affect the performance of plate heat exchangers by altering effectiveness (?) and number of transfer unit (NTU). Especially plate heat exchangers play a vital role in petroleum industries for wide range of temperature application. Hence, it was proposed to choose kerosene as cold fluid and hot water as hot fluid in this present investigation. A vertical type of plate heat exchanger, in which flow pattern is maintained as co-current, has been used to conduct the experimental runs. The numbers of plates in the plate heat exchanger used in the present study are 10. The number of flow channels (space maintained between two consecutive channels) allocated for both fluids are 9. Experimental runs have been conducted for different combinations of hot fluid flow rates and cold fluid flow rates for single phase flow, in which hot water is considered as hot streams and kerosene as cold fluid. The thermal performance of plate heat exchanger has been analyzed based on calculated parameters using experimental data set. A similar corrugated plate heat exchanger model having the same dimensions as that of the experimental one was developed with aid of CFD tool. The model was simulated at different operating conditions and compared with experimental results. The simulated results are in good agreement with experimental data. The percentage deviation between experimental results and simulation results for over all heat transfer coefficient is less than ±6%.

The effect of flow arrangement on the pressure drop of plate heat exchangers

For the optimal design of plate heat exchangers (PHEs), an accurate thermal–hydraulic model that takes into account the effect of the flow arrangement on the heat load and pressure drop is necessary. In the present study, the effect of the flow arrangement on the pressure drop of a PHE is investigated. Thirty two different arrangements were experimentally tested using a laboratory scale PHE with flat plates. The experimental data was used for (a) determination of an empirical correlation for the effect of the number of passes and number of flow channels per pass on the pressure drop; (b) validation of a friction factor model through parameter estimation; and (c) comparison with the simulation results obtained with a CFD (computational fluid dynamics) model of the PHE. All three approaches resulted in a good agreement between experimental and predicted values of pressure drop. Moreover, the CFD model is used for evaluating the flow maldistribution in a PHE with two channels per pass.

Flow uniformity in and its effect on the performance of polymer electrolyte membrane fuel cell stacks

In this work the effect of flow uniformity on a PEM fuel cell stack performance has been investigated to optimize the stack design. Based on the hydraulic resistance network method, a correlation for the ratio of pressure drop in flow channel, to that in stack manifold, has been derived analytically as a measure of flow uniformity among the cells in the stack. It has been shown that the amount of flow variation can be predicted via the pressure drop ratio, and is found that they are inversely proportional to each other. The results indicate that the output voltage degrades rapidly as the amount of flow variance is increased. Sufficient flow uniformity is crucial to minimize the cell‐to‐cell voltage variation. However, the space available for the manifold is limited on bipolar plate and excessive flow uniformity may result in net performance degradation either due to a reduction in the active cell area or excessive pumping power. Optimization has been carried out based on net output power which is obtained by subtracting the pumping powers for the anode and cathode streams from the stack output power. The effect of minor loss on cell‐to‐cell voltage variation as well as stack output voltage has been investigated and it may become considerable when the number of flow channels per bipolar plate is small.

Performance analysis and optimization of PEM fuel cell stacks using flow network approach

The performance of polymer electrolyte membrane (PEM) fuel cell stacks can be improved significantly by optimizing the design and operating conditions. In this study, performance modeling and optimization of a PEM fuel cell stack have been conducted. The pressure and molar flow rate distributions for the fuel and oxidant streams in the stack are determined with a flow network model incorporating the minor losses. The distributions are then used in the single cell model developed previously to evaluate the performance of PEM fuel cell stack. Analysis has been carried out for different fuel and oxidant flow configurations and bipolar plate designs. It was found that the minor losses increase the stack operating pressure and the power requirement for oxidant supply and alter the cell-to-cell voltage variations in the stack. A symmetric double inlet–single outlet topology provides optimal stack performance with reasonably low compressor power requirement for the reactant flow and minimum cell-to-cell voltage variations. The stack performance is considerably affected by the size and the number of flow channels on bipolar plate. Optimal stack performance requires the matching of the stack manifold designs, flow channels on the bipolar plates and the stack operating conditions.

Porous media: A model system for the physics of disorder

We discuss several examples of applications of soft matter and statistical physics to various problems related to porous and fractured media. The structure of porous media often displays multiple scale features which can be analysed by neutron diffraction or electron microscopy and can be approached by fractal models. Such characteristics are also observed on the surface of natural fractures.The relation between various transport parameters such as permeability or conductivity introduce characteristic microscopic length scales which can sometimes be independently determined. In some cases, if the porous medium get clogged or if the number of flow channels or fractures is low enough so that threshold effects appear which can be analysed in terms of percolation models. Disorder physics approaches are particularly useful to analyse non miscible diphasic flows in some cases in which multiscale heterogeneities of the fluid mixture composition appear. This is for instance the case for very slow non wetting invasions and of the fast injection of a low viscosity fluid : these processes can be described respectively by the “invasion percolation” and the “diffusion limited aggregation” models. Finally tracer dispersion provides an application of random walk models to disordered systems : examples of the response of this measurement in partly saturated and double porosity media are presented.

Effect of fresh air purging on the efficiency of energy recovery from exhaust air in rotary regenerators

To prevent the carry-over of contaminated air to the fresh air in rotary heat regenerators, a small part of the incoming fresh air is used to purge the heat transfer matrix during its transfer from the hot to the cold zone. This purging causes a loss in the effectiveness of heat transfer in the regenerator. A simple numerical scheme employing heat transfer theory for elemental heat exchangers has been developed. This scheme was obtained by dividing the gas stream and the ‘stream’ of matrix material into a number of flow channels, and the effect of fresh air purging has been calculated over a wide range of parameters. The results are presented numerically, graphically and in the form of approximate algebraic expressions.