Parallel Flow Channels Research Articles

Experimental investigation and analysis of two-phase flow instability of flow boiling in a mini-channel heat sink

High heat flux and good axial temperature uniformity of channel flow boiling demonstrate that it is ideally suited for next-generation cooling systems. Such systems, however, are hampered by undesirable flow fluctuations, making channel flow boiling ineffective due to the rapid bubble growth within the channels. Furthermore, especially in the mini-channel heat sink, the interactions between multiple parallel channels and inlet/outlet plenum induce complex hydraulic and thermal oscillations. Therefore, lots of consistent and detailed experimental data are still needed to design a reliable flow boiling system. However, only limited studies have examined the effects of channel interaction on flow boiling instability. Thus, to provide a better understanding of the flow boiling instabilities, a mini-channel heat sink was tested with near-saturated inlet conditions using FC-72 as the working fluid. The mini-channel heat sink consisted of a 150 mm long and 70.4 mm wide base area, having 22 identical rectangular channels measuring 1.6 × 4.8 mm2. The flow instabilities were analyzed by observing the vapor backflow and flow fluctuation in mini-channels with variations in the mass velocity, pressure, and heat fluxes. The analysis is based on the detailed flow visualization in the parallel channels and inlet plenum. It was found that the rapid bubble generation in individual channels promotes vapor backflow and hydraulic oscillations. Further observation of the high-speed camera images shows that the vapor crossing over between parallel channels through the inlet plenum led to the synchronization of vapor oscillations, which is responsible for the parallel channel flow instability. This visualization study revealed six dominant flow patterns showing multi-channel flow instability. Based on the classification of dominant flow patterns, a better understanding of the operating conditions for stable flow boiling was obtained.

Performance and mass transfer evaluation of PEM fuel cells with straight and wavy parallel flow channels of various wavelengths using CFD simulation

Polymer electrolyte membrane fuel cells (PEMFC) have the potential to replace conventional fossil fuels. The flow channel is a key part of the PEMFC that facilitates reactant mass transfer into the gas diffusion layer, catalyst layer, and membrane. Overall performance of the PEMFC mostly depends on the efficient mass transfer, pressure drop, and water management within this channel. In the straight channel mass transfer occurs by diffusion while in the wavy channel mass transfer occurs through convection. In this study, the performance and mass transfer evaluation of a PEMFC with a straight flow channel and wavy flow channels of different wavelengths (λ = 4, 4.8, and 6) were compared using a commercial solver (STAR-CCM+16.04). The simulation results were validated experimentally and the results were in good agreement. Comparison of the simulation results established that the mass transfer through diffusion and convection was more enhanced in the case of wavy channels.

Brief research report optimization of catalyst porosity arrangements for hydrogen production in microchannel reactors by methanol reforming

Optimization of catalyst porosity arrangements is carried out for hydrogen production through computational modeling of a thermally integrated microchannel reactor. The reactor has parallel flow channels for conducting simultaneous oxidation and reforming reactions. Numerical simulations are performed under a variety of velocity conditions to evaluate the effect of reforming catalyst porosity arrangement on the transport phenomena in the reactor system. The oxidation catalyst has a uniform porosity, and the porosity range of the reforming catalyst is from 30 to 70 percent. The porosity is uniform in each segmented region and the overall porosity is maintained 50 percent. The heat and mass transfer issues for the reactor system are highly complex. Performance comparisons are made in terms of methanol conversion, hydrogen yield, and heat of reaction between these porosity cases under different inlet velocity conditions. Dimensionless Nusselt and Sherwood number analyses are performed to understand the underlying cause for the performance difference. The dimensionless numbers in transport phenomena are principally analyzed to understand how important the transverse transport components are. The results indicate that optimization of catalyst porosity arrangements is required for thermal matching purposes. The optimum porosity arrangement depends upon the flow rates. The catalyst porosities must be configured to improve the kinetics in the upstream or downstream sections of the reactor so that the endothermic and exothermic processes are thermally matched. While advantages can be realized by using the two-segment design, the three-segment design yields no advantage. The processes of transverse transport are of great importance to the chemical reactions.

Optimization of flow field structure for proton exchange membrane fuel cell stack by multi-physics coupling simulation

In this paper, the three demission multi-physics model for the proton exchange membrane fuel cell (PEMFC), which carefully considers the mass, momentum, energy, species and electric charges conserving and liquid water managing processes, has been completed. The grid independence verification and good agreements between the calculated results and the reported experimental results well illustrate the validation of the developed model. Then, it is further used to investigate the potential application of the novel compensation flow field by comparing the PEMFCs with the parallel flow channel and the parallel-serpentine-parallel flow channel. The research result show that although there is low pressure difference throughout the PEMFC, there is a big oxygen depleting zone over the electrolyte surface, while the parallel flow channel is adopted. Choosing the parallel-serpentine-parallel flow channel will have more uniform oxygen and electrochemical reacting distributions over the electrolyte surface and higher water management ability than that of the PEMFC with parallel flow channel.

Optimization and evaluation criteria of water-gas transport performance in wave flow channel for proton exchange membrane fuel cell

Flow channel optimization is an important method to improve the water-gas transport capacity and enhance the performance of proton exchange membrane fuel cells. In this study, the size ratio of the opposite sinusoidal wave flow channel (OSWFC) is optimized, and three-dimensional numerical models are developed to evaluate the comprehensive performance. The results show that OSWFCs obtain performance enhancement compared to the parallel flow channel. 1:1 wave channel has a maximum improvement of 19.53% in mass transfer capacity by the most frequent flow disturbance, while 1:3 has a maximum increasement of 37.5% in water removal performance by the virtue of gentle wave structure. Large pressure loss decreases the mass transfer efficiency, although it with the maximum mass transfer capacity, the mass transfer evaluation criterion of 1:1 wave channel is less than that of 1:3. Moreover, a significant correlation between water-gas transport capacity and electrochemical performance is demonstrated by Pearson correlation coefficient.

Researching and Predicting the Flow Distribution of Herschel-Bulkley Fluids in Compact Parallel Channels

There is growing interest in multi-nozzle array printing, as it has the potential to increase productivity and produce more intricate products. However, a key challenge is ensuring consistent flow across each outlet. In the heat exchangers, achieving uniform distribution of flow in parallel channels is a classic goal. To address this issue in multi-nozzle array direct printing technology, high-viscosity slurry fluids can be utilized in place of water, and the structure of compact parallel channels can be employed. This study experimentally and numerically investigated the flow distribution law of Herschel-Bulkley fluids (high-viscosity slurry fluids) entering each manifold of the compact parallel channels, which contained a single circular inlet and multiple outlets. The research identified two types of factors that impact the non-uniformity flow coefficient (Φ), which reflects the uniformity of flow distribution in each channel of the structure: entrance and exit conditions (V, P1, P2) that have a negligible effect on Φ, and structural dimensions (D, S, L, N, A, d) that are the primary influence factors. By analyzing the experimental results, a prediction model was derived that could accurately calculate Φ (error < 0.05) based on three structural dimensions: A, S, and L. Through proper design of these structural dimensions, a consistent flow rate of each channel of the parallel channels can be ensured.

One-dimensional modelling and analysis of flow excursion of cracking hydrocarbon fuel in SCRamjet parallel cooling channels

Endothermic hydrocarbon fuel is the key of regenerative cooling in SCRamjet. To investigate flow excursion of cracking hydrocarbon fuel in parallel cooling channels, a one-dimensional model with 2 steps/9 species pyrolysis mechanism of hydrocarbon fuel flow and heat transfer in parallel channels is developed and validated. Two types of flow excursions occur in the pyrolysis and trans-critical negative slope region of the fuel hydrodynamic curve. Multiple solutions (stable and unstable ones) of flow and temperature distributions are presented. Further simulations are developed at the pressure of 2.2–7.0 MPa, nominal heat flux of 0.5–3.0 MW/m2. Influencing factors such as back pressure, hot resistance, nominal heat flux, thermal deviation and parallel channels number are studied. Higher back pressure affects the fuel properties and negative slope of hydrodynamic curve disappears, which restrains the flow excursion. Hot resistance keeps promoting the pyrolysis flow excursion and presents a non-monotonic influence on trans-critical flow excursion. Three basic forms of hydrodynamic curve with negative slope regions are uncovered as nominal heat flux varies. Flow excursion states are simpler under High nominal heat flux condition (Ppeak1 < Pvalley2). As nominal heat flux decreases, two negative slope regions and corresponding flow excursions overlap. Combined flow excursion region takes shape and the flow excursion amplitude amplifies. When channel number increases, the feature of multiple solutions is much more complex. 23 flow excursion states and 25 flow excursion modes are uncovered.

Performance studies of a membrane-based water and heat exchanger using serpentine flow channels for polymer electrolyte membrane fuel cell application

This study proposes a serpentine flow field to enhance the performance of a membrane-based water and heat exchanger (MWHE) to employ in polymer electrolyte membrane fuel cells. Two MWHEs (serpentine and parallel-flow channels) are numerically simulated and compared in terms of water vapor transmission rate (WVTR), water recovery ratio (WRR), temperature and dew point at the outlet of the dry side, and the dew point approach temperature (DPAT). For all mass flow rates at the dry and wet inlets, the outlet temperature of the dry side and the dew point at the dry side outlet of the MWHE with serpentine channels are higher compared to the one with parallel channels. Using serpentine channels, compared to simple parallel channels, the WRR is enhanced by 8.5 % to 20 % and the DPAT is diminished by 4 % to 13.6 % for a range of mass flow rates on the wet side. At higher wet-side flow rates, the use of serpentine arrangement has a more significant influence on WRR and DPAT. In both MWHEs, an increase in the dry side mass flow rate leads to a reduction in heat and water transfer rates and an increment in the wet side mass flow rate, resulting in a reduction in the DPAT and WRR. Enhancing the operating pressure has a negative impact on the performance of the MWHE. By changing the operating pressure from 100 to 150 kPa, WRR is reduced by 18.1 % and DPAT is enhanced by 10.2 %.

One-dimensional modelling and simulation of multiple solution characteristics of hydrocarbon fuel flow excursion in scramjet parallel cooling channels

To investigate flow excursion of hydrocarbon fuel in parallel cooling channels of the scramjet, one-dimensional steady-state model of hydrocarbon fuel flow and heat transfer in parallel channels is developed and validated. Flow excursion occurs in the negative slope region of the fuel hydrodynamic curve. Multiple solutions (stable and unstable ones) of flow and temperature distributions are presented. Further simulations are developed at the pressure of 2.2–7.0 MPa, heat flux of 0.5–3.0 MW/m2. Influencing factors such as back pressure, hot resistance, heat flux, thermal deviation and parallel channels number are studied. Higher back pressure affects the fuel properties and negative slope of hydrodynamic curve disappears, which restrains the flow excursion. Hot resistance presents a non-monotonic influence on flow excursion. Hot resistance that is too large or too small suppresses flow excursion. Higher heat flux magnifies the flow excursion amplitude and narrows the flow range of multiple solutions. When channel number increases, the feature of multiple solutions is much more complex and the flow excursion modes becomes tricky.

Dynamic response and stability performance of a proton exchange membrane fuel cell with orientational flow channels: An experimental investigation

The dynamic response of proton exchange membrane fuel cells with the load switching determines the operation stability and life of fuel cells. To investigate the dynamic response of proton exchange membrane fuel cells from the perspective of gas and water delivery, the dynamic response of the fuel cell with the parallel and orientational flow channels was tested under different loads and operating conditions. The experimental results show that the dynamic performance of the fuel cell with orientational flow channels is good after the current and voltage loads are switched. The orientational flow channels improve water management, make the water distribution of the flow channels uniform, increase the membrane hydration, and prevent flooding in the local electrode. The voltage variation coefficient of the cell with orientational flow channels is 1.57%, which is smaller than that of the cell with parallel flow channels 35.6%. Thus, the dynamic performance and stability are improved. The overshoot and undershoot of pressure drop are observed which is due to the changing of gas transferring to the electrode when the load is switched. The orientational flow channels improve the gas transfer capacity, which makes more gas enter the electrode to participate in reactions, thus the orientational flow channels improve dynamic performance and decrease dynamic time constant.

Enhancing Heat Removal and H2O Retention Capability of Passive Air-Cooled Polymer Electrolyte Membrane Fuel Cells by Tailoring Cathode Flow-Field Design

This paper reports novel cathode flow-field designs for passive typed air-cooled polymer electrolyte membrane fuel cells (PEMFCs) to help alleviate electrolyte dehydration and performance degradation issues under excess dry air supply conditions. The proposed flow-field designs include 7 three-dimensional (3D) patterned designs in addition to a parallel channel configuration equipped with rectangular baffles to control the airflow for more efficient heat removal. The designs were evaluated numerically using 3D, two-phase PEMFC simulations. Compared to a typical parallel flow channel configuration, the proposed flow-field designs show better heat removal and water retention capability. The improvement in single cell voltage was around 13–75 mV at an operating current density of 0.5 A cm−2, whereas the larger pressure drops around = 6.9–317.6 Pa cm−1 were required because of the more complex flow-field configurations compared to the simple straight parallel channel geometry ( = 7.4 Pa cm−1). This work presents a comprehensive understanding of air-cooled PEMFC operating characteristics under excessive dry air supply conditions and a new design strategy for cathode flow-fields.

Optimization of blocked flow field performance of proton exchange membrane fuel cell with auxiliary channels

The flow field optimization design is one of the important methods to improve the performance of proton exchange membrane fuel cell (PEMFC). In this study, a new structure with staggered blocks on the parallel flow channels of PEMFC and auxiliary flow channels under the ribs is proposed. Through numerical calculation method, the effect of blocks auxiliary flow field (BAFF) on pressure drop, reactant distribution and liquid water removal in the fuel cells are investigated. The results show that when the operating voltage is 0.5 V, the current density of BAFF is 21.74% higher than that of the straight parallel flow field (SPFF), and the power density reaches 0.65 W cm−2. BAFF improves performance by equalizing the pressure drop across sub-channels, promoting the uniform distribution of reactant, and enhancing transport across the ribs. In addition, through parameter analysis, it is found that BAFF can discharge liquid water in time at the conditions of high humidification, high current density and low temperature, which ensures the output performance of the fuel cell and improves the durability of the fuel cell. This paper provides new ideas for the improvement of PEMFC flow field design, which is beneficial to the development of PEMFC with high current density.

Operando Neutron Radiography Measurements of a Zero-Gap Alkaline Electrolysis Cell

Low temperature alkaline water electrolysis is a well-known technology for hydrogen production and advantageous because less noble metals are needed compared to Proton Exchange Membrane (PEM) electrolysis. However, the two-phase flow behavior and especially the gas bubble distribution and flow during the operation in a zero-gap alkaline electrolysis cell have not been investigated in sufficient detail. Previous studies used e.g. transparent model electrolysis cells or focused on single flow phenomena e.g. gas bubble nucleation, growth or detachment.Recent publications show evidence that the ohmic resistance of zero-gap alkaline electrolysis cells is greater than the ohmic resistance of the used diaphragm, which has been believed the only term contributing to the ohmic resistance in zero-gap cells [1]. This suggests that the evolving gas bubbles might have a relation to the total ohmic resistance of the cell. As possible reasons nano bubbles in the pores of the diaphragm and a possible finite gap between electrodes and diaphragm is mentioned [2]. Haverkort et al. assume that the frontal area of the electrodes might be inactive [3].To prove the hypothesis and to deepen the general understanding of the two-phase flow inside an alkaline electrolysis cell we conducted operando cell measurements and visualized the electrolyte-gas bubble flow in an alkaline electrolysis cell by using neutron radiography. Thanks to the powerful neutron source at Institute Laue-Langevin in Grenoble, it was possible to observe the whole flow field with all flow channels at once with a sufficient temporal resolution to observe the movement of single gas bubbles, from nucleation to the moment of exiting the channel. The temporal resolution was 50 fps, which makes these measurement, to the best authors’ knowledge, the first to show the transient effects inside of a zero gap alkaline electrolysis cell. For the measurements, the NeXT (Neutron and X-Ray Tomograph) instrument was used which is the imaging station with one of the highest neutron fluxes of the world (1.4 E10 n/cm²/s at the end of the guide) [4]. The neutron beam was used to carry out through-plane measurements of the electrolysis cell.The measurements were conducted at a variety of operation conditions, varying the current density, the electrolyte volume flow and the operation temperature. At different operation conditions Electrical Impedance Spectroscopy (EIS) measurements have been conducted to measure the ohmic resistance of the cell. The used cell is an alkaline single cell out of a Nickel alloy with vertical parallel flow channel and a pocket milled to the backside of the cell in order to increase the transmission through the cell material.From the results of the measurements we observe a relation between current density (forming gas amount) and the ohmic resistance and thus establish a relation to the gas bubble behavior inside the electrolysis cell. Furthermore, we analyzed the velocity of the gas bubbles inside the channel depending on the different operation conditions, as well as the agglomeration behavior of the bubbles and their flow regime.[1] Phillips et al.,Minimising the ohmic resistance of an alkaline electrolysis cell through effective cell design, 2017[2] de Groot et al., Ohmic resistance in zero gap alkaline electrolysis with a Zirfon diaphragm,2021[3] Haverkort et al., Voltage losses in zero-gap alkaline water electrolysis,2021[4] Tengattini et al., NeXT-Grenoble, the Neutron and X-ray tomograph in Grenoble, 2020 Figure 1

Study on the performance of a new type of microchannel cold plate for shrinkage/expansion parallel flow channel design

This paper presents a new design scheme of microchannel cold plate for the heat dissipation of airborne high heat flux electronic equipment. In this scheme, the flow path of the cold plate is composed of a contraction channel and an expansion channel alternately arranged. The performance of cold plate is simulated by computational fluid dynamics method. The results show that the convective heat transfer performance in the contraction channel is better than that in the expansion channel, but the flow resistance is significantly higher than that in the latter channel. During the actual operation, the cold plate will distribute the flow according to the characteristics of the flow resistance, and the larger share of the flow will be obtained in the expansion channel. This flow distribution mechanism keeps the total heat transfer of the two channels at a close level, and effectively stops the large increase of flow resistance.

Experimental study on the frosting and heat transfer characteristics of cooling surfaces in confined parallel and counter flow channels

Experimental study on the frosting and heat transfer characteristics of cooling surfaces in confined parallel and counter flow channels

Performance of a fuel cell‐based proton battery with varied designs of micro‐flow channel for enhanced hydrogen storage

AbstractWe present an experimental investigation on a proton battery, that could run both as electrolyser and fuel cell, using six different flow channel orientations viz. pin‐type, parallel, interdigitated, spiral, serpentine, and parallel serpentine. The battery stores energy in the form of hydrogen ions and its performance is determined in terms of hydrogen wt. % stored in an integrated carbon‐based electrode. In this paper, the fabrication of electrode, development of an experimental proton battery and its testing are disclosed. The characterization of the employed activated carbon electrode is done using scanning electron microscopy, X‐ray diffraction analysis, energy dispersive spectroscopy, and fourier transform infrared spectroscopy. The fabricated proton battery is tested by charging (E‐mode operation) and discharging (FC‐mode operation) subsequently at normal temperature and pressure conditions. In the E‐mode operation, electrochemical hydrogen storage of the battery with six different flow channels is found to be in the range of 1.36–1.97 wt. %. The corresponding desorption of hydrogen in the FC‐mode operation is found to be in the range of 1.047–1.479 wt. %, respectively, for six different flow channels. The comparative result analysis revealed that parallel serpentine flow channel outperformed, with energy storage capacity of 1.97 wt. %; and pin type channel design showed comparatively low performance with energy storage capacity of 1.36 wt. %. The presented research work is an effort towards meeting the set targets of hydrogen storage set by the U.S. Department of Energy to make the hydrogen technology commercially viable.Statement of Industrial RelevanceThe presented research work has relevance with the power industry as it reveals the optimum design of flow channels for enhanced hydrogen adsorption. It is a way forward towards developing a fuel‐cell based proton battery that does not emit harmful fumes like lithium batteries and therefore, is environmentally friendly.Novelty or SignificanceThe presented research work is a maiden attempt of testing different designs of micro‐flow channels in an experimental proton battery that is a single unit capable of performing dual tasks i.e., electrolyser and fuel cell to store energy and give out power when required.

The optimization of parallel flow channel structure of plate heat exchangers

The optimization of parallel flow channel structure of plate heat exchangers

New direction and perspectives in elastic instability and turbulence in various viscoelastic flow geometries without inertia

We shortly describe the main results on elastically driven instabilities and elastic turbulence in viscoelastic inertialess flows with curved streamlines. Then we describe a theory of elastic turbulence and prediction of elastic waves Re ≪ 1 and Wi ≫ 1, which speed depends on the elastic stress similar to the Alfvén waves in magneto-hydrodynamics and in a contrast to all other, which speed depends on medium elasticity. Since the established and testified mechanism of elastic instability of viscoelastic flows with curvilinear streamlines becomes ineffective at zero curvature, so parallel shear flows are proved linearly stable, similar to Newtonian parallel shear flows. However, the linear stability of parallel shear flows does not imply their global stability. Here we switch to the main subject, namely a recent development in inertialess parallel shear channel flow of polymer solutions. In such flow, we discover an elastically driven instability, elastic turbulence, elastic waves, and drag reduction down to relaminarization that contradict the linear stability prediction. In this regard, we discuss briefly normal versus non-normal bifurcations in such flows, flow resistance, velocity and pressure fluctuations, and coherent structures and spectral properties of a velocity field as a function of Wi at high elasticity number.

Numerical Investigation on the Performance of Proton Exchange Membrane Fuel Cell With Zigzag Flow Channels

Reasonable flow channel designs play a significant role in improving the performance of proton exchange membrane fuel cells (PEMFC). The effect of the zigzag flow channels with three different numbers of turns on the performance of PEMFC was investigated in this paper. The polarization curves, molar concentration of oxygen and water, and power density were analyzed, and the numerical results showed that the overall performance of the zigzag flow channels was significantly better than that of the conventional parallel flow channel. With the increase of the number of turns from 3 to 9, the performance of PEMFC was gradually improved, the diffusion capacity of oxygen to the interface of the electrochemical reaction was also promoted, and the low oxygen concentration regions were gradually reduced. When the number of turns was 9, the current density of PEMFC was 8.85% higher than that of the conventional parallel channel at the operating voltage of 0.4 V, and the oxygen non-uniformity at the between gas diffusion layer (GDL) and catalyst layer (CL) interface was the minimum with a value of 0.51. In addition, the molar concentration of water in the channel also decreased. Due to the relatively large resistance of the zigzag flow channels, the maximum pressure drop of the zigzag flow channel was 263.5 Pa, which was also conducive to the improvement of the drainage effect of the conventional parallel flow channel. With the increase of the number of turns in the zigzag channel, the pressure drop and parasitic power density gradually increased. The 9-zigzag flow channel obtained the maximum pressure drop and net power density, which were 263.5 Pa and 2995.6 W/m2, respectively.

Design and numerical investigation of multi-channel cooling plate for proton exchange membrane fuel cell

Proton exchange membrane fuel cells need cooling plates with strong heat exchange capacity to maintain temperature balance. In order to obtain better cooling performance, four new types of flow field distribution of cooling plate are designed, including local serpentine channel, local parallel channel and the flow field in which the four channels are symmetrically distributed along the diagonal and horizontal centerline of the cooling plate. The maximum temperature, temperature difference, temperature uniformity index and pressure drop of the multi-channel cooling plate under different working conditions are analyzed by numerical simulation. The results show that the multi-channel design can effectively enhance the heat transfer effect of the cooling plate and reduce the pressure drop. The local serpentine channel design can increase the fluidity of the coolant, avoid local reflux, make the temperature distribution more uniform and avoid local overheating. The maximum temperature of local serpentine channel flow field is 1.2 K lower than that of local parallel channel flow field. The pressure drop of internal parallel flow field design is 2.58 kPa, which is 62% lower than that the design of internal serpentine flow field. Increasing the inlet coolant flow can strengthen the heat transfer capacity of the cooling plate, when the flow rate increases from 2e-6 m3/s to 4e-6 m3/s, the maximum decrease is 8.53% of Model6. But it will increase the pressure drop of the channel with long cooling channel, the maximum pressure drop increase is 924 kPa when the inlet flow increases from 6e-6 m3/s to 8e-6 m3/s.