Zigzag Channels Research Articles

Thermo-Hydraulic performance of printed circuit heat exchanger as precooler in supercritical CO2 Brayton cycle

The Printed Circuit Heat Exchanger (PCHE) is one of the potential precooler candidates to be employed in the supercritical carbon dioxide (S-CO2) Brayton cycle with its high compactness, high efficiency and high endurance to extreme condition. Since the S-CO2 precooler operates in the condition close to the pseudo-critical region, it is of importance understanding the performance differences among different types of PCHE with the dramatical change of thermal-physical properties. Thus, in this study the effect of three channel configurations with zigzag, wavy and airfoil fin channel on heat transfer and flow resistance under precooler condition are investigated with numerical method. Based on the local analysis, it is found that the zigzag channel PCHE presents the better heat transfer performance compared to wavy and airfoil fin channel, and differences are increased as the temperature approaches the pseudo-critical region. Meanwhile, the airfoil fin channel shows the lower friction factor with only 6.12% and 41.31% of the zigzag and wavy channel, respectively. Furthermore, comprehensive performances are compared through the methods of entropy number, JF factor (ratio of Colburn j to friction factor) and Q/(ṁ∙ΔP) (ratio of heat exchange to pressure drop and mass flow). Based on the results, it is recommended for the airfoil channel to be adopted near the pseudo-critical region due to its larger surface area and smoother structural design.

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.

Controlled Fabrication of Wafer-Scale Zigzag Silicon Nanowire Arrays by Metal-Assisted Chemical Etching through Synergistic Effect of Viscosity and Temperature

Zigzag silicon nanowires (SiNWs) attract great interest due to their unique physical/chemical properties that make them appealing in various applications; yet, their fabrication has always been difficult and the formation mechanism for zigzag silicon nanostructures remains elusive. In this work, the influence of temperature on the morpholohies of SiNWs by metal-assisted chemical etching (MatEtch or MACE) in aqueous HF-H2O2-C3H8O3 solution is systematically analyzed. It is found that zigzag-shaped silicon nanowires can be fabricated through a slow etching process at low temperatures in the presence of glycerol (C3H8O3). The synergistic effect of glycerol and low etchant-temperatures varied from 15 °C to 35 °C facilitate the fabrication of zigzag-SiNWs. The arm length of the zigzag design increases with the increase of temperature. However, straight SiNWs are fabricated at higher temperatures, such as those greater than 35 °C. It is also shown that for dispersed AgNPs, straight channels can be obtained at higher temperatures, while low temperature favors zigzag channels with decreasing arm length. We anticipate that these findings provide scientific guidance for the controlled fabrication of large-scale zigzag SiNW arrays.

Numerical Study on Flow and Heat Transfer Characteristics of Supercritical CO2 in Zigzag Microchannels

The zigzag channel is the uppermost channel type of an industrial printed circuit heat exchanger (PCHE). The effect of geometric properties on the flow and heat transfer performance of the channel is significant to the PCHE design and optimization. Numerical investigations were conducted on the flow and heat transfer characteristics of supercritical CO2 (sCO2) in semicircular zigzag channels by computational fluid dynamics method. The shear stress transfer (SST) k–ω model was used as turbulence model and the National Institute of Standards and Technology (NIST) real gas model with REFPROP database was used to evaluate the thermophysical parameters of sCO2 in this numerical method. The effectiveness of the simulation method is verified by experimental data. Thermal hydraulic performance for zigzag channels with different pitch lengths, bending angles, and hydraulic diameters are studied comparatively based on this numerical method, with the boundary conditions which cover the pseudocritical point. The comparison results show that reducing the bending angle and pitch length will strengthen the effect of boundary layer separation on the leeward side of the wall and enhance the heat transfer performance, but the pressure drop of the channel will also increase, and the decrease of channel hydraulic diameter is beneficial to the heat transfer enhancement, but it is not as significant as that of the straight channel.

Heat Transfer Analysis of Supercritical Methane in Microchannels With Different Geometric Configurations on High Power Electromechanical Actuator

Abstract The issue of regenerative cooling is one of the most important key technologies of flight vehicles, which is applied into both the engine and high-power electrical equipment. One pattern of regenerative cooling channels is the microchannel heat sinks, which are thought as a prospective means of improving heat removal capacities on electrical equipment of smaller sizes. In this paper, three numerical models with different geometric configurations, namely, straight, zigzag, and sinusoid, respectively, are built to probe into the thermal hydraulic performance while heat transfer mechanism of supercritical methane in microchannel heat sinks for the heat removal of high-power electromechanical actuator is also explored. In addition, some crucial influence factors on heat transfer such as inlet Reynolds number, operating pressure, and heating power are investigated. The calculation results imply the positive effect of wavy configurations on heat transfer and confirm the important effect of buoyancy force of supercritical methane in channels. The heat sinks with wavy channel show obvious advantages on comprehensive thermal performance including overall thermal performance parameter η and thermal resistance R compared with that of the straight one. The highest Nu and average heat transfer coefficient αm appear in the heat sink with zigzag channels, but the pumping power of the heat sink with sinusoidal channels is lower due to the smaller flow loss.

Paper-Based Microfluidics Perform Mixing Effects by Utilizing Planar Constricted-Expanded Structures to Enhance Chaotic Advection.

This study aimed to design and fabricate planar constricted–expanded structures that are integrated into paper-based channels in order to enhance their chaotic advection and improve their mixing performance. Chromatography papers were used to print paper-based microfluidics using a solid-wax printer. Three different constricted–expanded structures—i.e., zigzag, crossed, and curved channels—were designed in order to evaluate their mixing performance in comparison with that of straight channels. A numerical simulation was performed in order to investigate the mixing mechanism, and to examine the ways in which the planar constricted–expanded structures affected the flow patterns. The experimental and numerical results indicated that the proposed structures can successfully mix confluents. The experimental results revealed that the mixing indices (σ) rose from the initial 20.1% (unmixed) to 34.5%, 84.3%, 87.3%, and 92.4% for the straight, zigzag, curved, and cross-shaped channels, respectively. In addition, the numerical calculations showed a reasonable agreement with the experimental results, with a variation in the range of 1.0–11.0%. In future, we hope that the proposed passive paper-based mixers will be a crucial component in the application of paper-based microfluidic devices.

Dissecting the Chain Length Effect on Separation of Alkane-in-Water Emulsions with Superwetting Microchannels.

Oil/water separation is an essential process in the petrochemical industry, environmental remediation, and water treatment. Alkanes are the major components of crude oil and are difficult to separate once they form emulsions in water. Much less attention has been focused on the feature of liquid alkanes that could, in turn, influence the separation process. The role of chain length is systematically studied herein by separating the alkane-in-water emulsions with superwetting titanium microchannels of 14-55 μm. The chain length covers the entire liquid alkane spectrum with carbon numbers ranging from 6 to 16. The separation efficiency decreases while the TOC content increases with the chain length of liquid alkanes for a given channel. This is attributed to the small Ostwald ripening rate with the long chains, which stabilize the oil droplets of small sizes that could pass through the zigzag channels. Accordingly, a high separation efficiency of >99.97% and a low TOC content of <5 ppm are achieved with superhydrophilic channels of 14 μm for alkanes with less than 12 carbons. The metallic microchannels surpass the conventional organic membranes and inorganic frameworks over the entire liquid n-alkane spectrum, paving the way for the future development of oil/water separation using porous metals.

Experimental study of the flow and heat transfer performance of a PCHE with rhombic fin channels

The printed heat exchanger (PCHE) has the advantages of high efficiency, compact size and low thermal inertia and is considered to be suitable for the supercritical CO2 (SCO2) Brayton cycle. In this work, the thermal-hydraulic performance of a PCHE with rhombic fin channels was experimentally investigated. The combined effects of turbulent intensity and physical property of SCO2 on the PCHE performance were analyzed. Based on the experimental data, an iterative method combined with genetic algorithm (GA) was put forward to derive the heat transfer correlation. The results indicate that the overall heat transfer coefficient of the PCHE is roughly linearly dependent on the Geometric Mean of Peclet numbers of the cold and hot fluids. The iterative method combined with GA is an effective way to obtain the heat transfer correlation of SCO2 in PCHEs with tremendous property variation and limited boundary parameters. The friction coefficient of the SCO2 in PCHE is closely pertinent to the channel structure and the friction coefficient correlations of the cold and hot fluid are often different considering their different channel structure at the inlet and outlet regions. The rhombic fin channel can realize the heat transfer rate per unit volume equivalent to the zigzag channel at the expense of relatively low pressure drops.

Experimental Study on Flow and Heat Transfer of Supercritical Carbon Dioxide in Zigzag Channels with Bending Angle 30 Degrees

Experimental Study on Flow and Heat Transfer of Supercritical Carbon Dioxide in Zigzag Channels with Bending Angle 30 Degrees

Experimental Study on Flow and Heat Transfer of Supercritical Carbon Dioxide in Zigzag Channels with Bending Angle 30 Degrees for Advanced Nuclear Systems

Experimental Study on Flow and Heat Transfer of Supercritical Carbon Dioxide in Zigzag Channels with Bending Angle 30 Degrees for Advanced Nuclear Systems

Structural Analysis of the Zigzag Channel Pche as an Ihx in a Htgr Power Plant

Structural Analysis of the Zigzag Channel Pche as an Ihx in a Htgr Power Plant

INVESTIGATION OF THERMO-HYDRAULIC PERFORMANCE OF NANOFLUIDS IN A ZIGZAG CHANNEL WITH BAFFLES

In this study, the thermo-hydraulic performance of Al2O3-water nanofluid in a zigzag channel with baffles were numerically investigated. The mass, momentum and energy equations are discretized with finite volume approach and iterations are solved with SIMPLE algorithm. Reynolds number (200 ≤ Re ≤ 1600), and nanoparticle volume fraction (0.01 ≤ φ ≤ 0.03) were changed, and other parameters kept constant. The lower and upper zigzag surfaces of the channel were kept at constant temperature, and the Nusselt number and friction factor along the channel were calculated. The velocity and temperature contours in the channel were obtained in order to observe the effects on flow and heat transfer of the paramerers. The results shown that the increasing particle volume fractions and Reynolds numbers improved the heat transfer, while the friction factor increased slightly. The best thermo-hydraulic performance was obtained at Re = 1400 and φ = 3% as approximately 1.15.

Multi-Objective Optimization of Different Channel Shapes in Heat Exchangers

The effect of geometric parameters of the zigzag, rectangular, and serpentine channels on convective heat transfer coefficient and pressure drop was investigated using computational fluid dynamics (CFD). In all channels, the same boundary conditions were considered, and the number of steps was equal to 10. The simulations were performed for turbulent flows (liquid water as the operating fluid), and Reynolds number (Re) range between 20000 and 60000 was selected. The zigzag channel showed a best thermal performance and the serpentine channel showed the best hydraulic performance. The thermal-hydraulic performance (THP) factor was employed for comparing the channels. As the complexity of the channels surfaces increased, the two parameters of convective heat transfer coefficient (positive factor) and pressure drop (negative factor) increased simultaneously. Therefore, predictive correlations for friction factor and Nusselt number were presented using genetic algorithm (GA), and the multi-objective optimization was performed to obtain the most appropriate Nusselt number and minimum friction factor as the two basic objective functions. The resulting Pareto set, which includes the optimum geometric dimensions of the heat exchangers, allows a designer to choice the geometries based on higher heat transfer or lower pumping power.

New thermal-hydraulic correlations for printed circuit heat exchangers (PCHEs) with zigzag channels under high Reynolds numbers

PurposeThe purpose of this paper is to investigate the impact of semi-circular zigzag-channel printed circuit heat exchanger (PCHE) design parameters on heat transfer and pressure drop of flows under high Reynolds numbers and provide new thermal-hydraulic correlations relevant to conditions encountered in natural gas processing plants.Design/methodology/approachThe correlations were developed using three-dimensional steady-state computational fluid dynamics simulations with varying semicircular channel diameter (from 1 to 5 mm), zigzag angle (from 15° to 45°) and Reynolds number (from 40,000 to 100,000). The simulation results were validated by comparison with experimental results and existing correlations.FindingsThe results revealed that the thermal-hydraulic performance was mostly affected by the zigzag angle, followed by the ratio of the zigzag channel length to the hydraulic diameter. Overall, smaller zigzag angles favored heat transfer intensification while keeping reasonably low pressure drops.Originality/valueThis study is, to date, the only one providing thermal-hydraulic correlations for PCHEs with zigzag channels under high Reynolds numbers. Besides, the broad range of parameters considered makes the proposed correlations valuable PCHE design tools.

Study of novel flow channels influence on the performance of direct methanol fuel cell

The existing flow channels like parallel and gird channels have been modified for better fuel distribution in order to boost the performance of direct methanol fuel cell. The main objective of the work is to achieve minimized pressure drop in the flow channel, uniform distribution of methanol, reduced water accumulation, and better oxygen supply. A 3D mathematical model with serpentine channel is simulated for the cell temperature of 80 °C, 0.5 M methanol concentration. The study resulted in 40 mW/cm2 of power density and 190 mA/cm2 of current density at the operating voltage of 0.25 V. Further, the numerical study is carried out for modified flow channels to discuss their merits and demerits on anode and cathode side. The anode serpentine channel is unmatched by the modified zigzag and pin channels by ensuring the better methanol distribution under the ribs and increased the fuel consumption. But the cathode serpentine channel is lacking in water management. The modified channels at anode offered reduced pressure drop, still uniform reactant distribution is found impossible. The modified channels at cathode outperform the serpentine channel by reducing the effect of water accumulation, and uniform oxygen supply. So the serpentine channel is retained for methanol supply, and modified channel is chosen for cathode reactant supply. In comparison to cell with only serpentine channel, the serpentine anode channel combined with cathode zigzag and pin channel enhanced power density by 17.8% and 10.2% respectively. The results revealed that the zigzag and pin channel are very effective in mitigating water accumulation and ensuring better oxygen supply at the cathode.

Numerical investigation on thermal hydraulic performance of supercritical LNG in PCHEs with straight, zigzag, and sinusoidal channels

Numerical investigation on thermal hydraulic performance of supercritical LNG in PCHEs with straight, zigzag, and sinusoidal channels

Numerical investigation on the thermal-hydraulic performance of helical twine printed circuit heat exchanger

The printed circuit heat exchanger (PCHE) is a high-efficiency heat exchanger with high compactness, pressure resistance, and stability. It can also withstand high temperatures and ultra-low temperatures. Therefore, it has attracted a large number of scholarly interests in recent years. In order to improve the thermal and hydraulic performance of traditional printed circuit heat exchangers, a three-dimensional helical twine printed circuit heat exchanger (HTPCHE) is designed and the laminar flow is numerically investigated using water as the working fluid. Taking Colburn factor j, Fanning friction factor f and comprehensive index j/f as parameters, the thermal-hydraulic performance of HTPCHE and zigzag-type PCHE with different inclined angles are compared. It is found that when the inclination angle of HTPCHE is 10–25°, its overall performance is better than that of traditional PCHE. When the inclined angle is 10° and Rein = 828, the overall performance is 37.6% better than that of 15° zigzag channel. In addition, the impact of the pitch of HTPCHE and the thickness of the transition plate on its heat transfer and flow characteristics is explored. It is discovered that when the given fixed inclined angle, the overall performance can be improved by increasing the pitch while the thickness of the transition plate does not have any significant impact.

Analysis of Supercritical CO2 Cycle Using Zigzag Channel Pre-Cooler: A Design Optimization Study Based on Deep Neural Network

The role of a pre-cooler is critical to the sCO2-BC as it not only acts as a sink but also controls the conditions at the main compressor’s inlet that are vital to the cycle’s overall performance. Despite their prime importance, studies on the pre-cooler’s design are hard to find in the literature. This is partly due to the unavailability of data around the complex thermohydraulic characteristics linked with their operation close to the critical point. Henceforth, the current work deals with designing and optimizing pre-cooler by utilizing machine learning (ML), an in-house recuperator and pre-cooler design, an analysis code (RPDAC), and a cycle design point code (CDPC). Initially, data computed using 3D Reynolds averaged Navier-Stokes (RANS) equation is used to train the machine learning (ML) model based on the deep neural network (DNN) to predict Nusselt number (Nu) and friction factor (f). The trained ML model is then used in the pre-cooler design and optimization code (RPDAC) to generate various designs of the pre-cooler. Later, RPDAC was linked with the cycle design point code (CDPC) to understand the impact of various designs of the pre-cooler on the cycle’s performance. Finally, a multi-objective genetic algorithm was used to optimize the pre-cooler geometry in the environment of the power cycle. Results suggest that the trained ML model can approximate 99% of the data with 90% certainty in the pre-cooler’s operating regime. Cycle simulation results suggest that the cycle’s performance calculation can be misleading without considering the pre-cooler’s pumping power. Moreover, the optimization study indicates that the compressor’s inlet temperature ranging from 307.5 to 308.5 and pre-cooler channel’s Reynolds number ranging from 28,000 to 30,000 would be a good compromise between the cycle’s efficiency and the pre-cooler’s size.

Heat transfer performance comparison of printed circuit heat exchangers with straight, zigzag and serpentine flow channels for waste heat recovery

In the present study, a printed circuit heat exchanger (PCHE) is investigated numerically by solving the steady-state Navier–Stokes equations and energy equation. The PCHE is equipped with straight, wavy, and serpentine flow channels, allowing us to investigate the PCHE performance on a systematic basis of effectiveness and pressure drop between the inlet and outlet of the flow channels. Liquid water is utilized as a working fluid at temperatures of 30°C to 90°C, with geothermal heating rarely targeted for using water from hot springs. The numerical results are validated with experimental data, confirming that the shear stress transport (SST) turbulence model is suited for providing adequate accuracy. This study is intended to delve into several possible designs of PCHE's flow channels to reduce the pressure drop while increasing the effectiveness. Given the PCHE with 13 zigzag flow channels of the bending angle of 150°, the results show that the PCHE with the zigzag flow channels at a mass flow rate of 0.113 kg/s possesses significant heat transfer enhancement. When compared to the other two tested PCHEs (straight flow and serpentine flow passages), a 65% increase in effectiveness and a 39% rise in pressure drop are exhibited. The PCHE can be regarded as a recuperator of the Organic Rankine Cycle (ORC) and is generally thought to be an important part of maximizing the efficiency of the cycle. The favorable features of the zigzag PCHE make it possible to seamlessly integrate the zigzag PCHE into ORC as part of a waste heat recovery system in a feasible way to improve the ORC efficiency.

Measurement and correlation for two-phase frictional pressure drop characteristics of flow boiling in printed circuit heat exchangers

The two-phase frictional pressure drop characteristics of flow boiling in zigzag channels of the printed circuit heat exchangers (PCHEs) under different conditions were investigated experimentally. The research results show that, the friction factor f of zigzag channel is increased by 9%~17% than that of the straight channel; as the vapor quality increases, the frictional pressure drop initially increases and then decreases, reprensting a maximum value at vapor quality of 0.9; the frictional pressure drop always decreases with the increasing saturated pressure when the void fraction is higher than 0.90, and it increases with the increasing heat flux due to the larger average vapor quality and the smaller average pressure. A new correlation for the frictional pressure drop of flow boiling in PCHEs was developed within a deviation of ±25%.