Characteristics In Channels Research Articles

Multi-dimensional characteristics of upward bubbly flows in a vertical large-size square channel

Gas-liquid two-phase flows frequently occur in various flow channels in engineering systems with heat and mass transport processes. The channel geometry affects thermo-fluid behavior generally. Extensive research is done on gas-liquid two-phase flows in circular and annular channels with a wealth of local two-phase flow measurement data. Square channels are used in various engineering disciplines, but there has only been limited experimental research on two-phase flows in square channels with local flow parameter measurements. For two-phase flow analyses in square channels, knowledge of the flow characteristics and model validation against some key parameters, such as the drift-flux parameters and the interfacial area concentration, is crucial. An experiment for upward bubbly flows was conducted in a large square channel with an inside cross-section of 0.136 × 0.136 m. A four-sensor optical probe and a hot-film anemometer were used to measure local two-phase flow parameters at 66 points in an octant symmetric triangular cross-section at three axial locations. The measured two-phase flow parameters were the void fraction, axial gas velocity, axial liquid velocity, interfacial area concentration, and bubble Sauter mean diameter. The flow characteristics through flow development were investigated based on the local measurement data. The local measurement data directly determined the drift-flux parameters through their definitions. The distribution parameters determined through experimentation could be reproduced by the existing correlation for large circular pipes indicating that the flow characteristics in large square channels may be similar to those in large circular pipes. Based on existing drift-flux correlations for large circular pipes, the void fractions in the current experiment could be fairly predicted. Furthermore, the interfacial area concentrations could be predicted by existing correlations with reasonable accuracy.

In-situ visualization of cathode flow channel in polymer electrolyte membrane fuel cell: Effect of GDL degradation

Polymer electrolyte membrane fuel cells (PEMFCs) are used in various fields, but their lifetime should be considered for commercial applications. The degradation of their gas diffusion layer (GDL), which enables fuel diffusion and water management, can reduce the performance and accelerate the degradation of other components. This study investigates the effect of GDL degradation on in-situ flow characteristics in cathode flow channels. A special cell is designed to visualize in-situ two-phase flows in the cathode flow channel with a high-speed camera. As the GDL degrades, the flow pattern changes from dropwise to slug and annular flows. Moreover, the amount of residual liquid after one water discharge cycle increases as the GDL deteriorates. These changes in the flow patterns and remaining liquid can disrupt the fuel supply to chemical reaction sites and degrade PEMFC performance.

Performance evaluation of a ventilated roof desiccant bed by direct use of solar energy for zero-energy buildings

Indoor dehumidification is usually achieved by mechanical cooling resulting in significant consumption of high-grade energy resources such as electricity. Conventional rotary dehumidification systems also require the use of high-grade energy to heat the desiccant and regenerate it. Utilization of renewable energy for indoor dehumidification is highly of concern. Ventilated roof desiccant bed system is a novel dehumidification method which can regenerate itself automatically with low-grade energy usage by taking advantage of solar energy and natural ventilation directly. Using renewable energy as an alternative, it can reduce the building energy consumption, which is beneficial to achieve nearly zero energy building (nZEB) target. This study establishes the heat and moisture transfer model of the ventilated roof desiccant bed while the moisture transfer model is validated by using the reference data. The flow-field characteristics in the airflow channels are numerically analyzed, and the regeneration process of the desiccant bed in a typical day is also simulated. Results show that both the temperature and air velocity in the channel are increasing from the inlet to the outlet. The air velocity increment can reach 0.5 m/s, and the temperature can rise about 6 °C under the same boundaries. On a typical day, the regeneration process of the ventilated roof desiccant bed lasts for approximately 3 h, and 80 g of water is desorped per unit. It means that about 18 units are sufficient to remove the moisture produced in 8 h by a man. The research results can provide data reference for the design of the system, and thus facilitates the realization of engineering application.

Experimental investigation on flow boiling characteristics in offset strip fin channels under different sloshing conditions

In order to reveal the effects of sloshing movement on the performance of the plate-fin heat exchangers applied in floating liquefied natural gas platforms and environment control system of aircrafts, the flow boiling characteristics in offset strip fin channels of plate-fin heat exchangers under different sloshing conditions were investigated experimentally, covering rolling, pitching and swaying movements. The results show that, the rotational type movements enhance the transient heat transfer coefficient by a maximum of 21.1% at low vapor quality condition and weaken the transient heat transfer coefficient by a maximum of 27.7% at high vapor quality condition, while the transitional type movements enhance the heat transfer at high vapor quality. Fast Fourier transform method was used in the spectrum analysis of flow instabilities, and there are two peak frequencies caused by the inlet fluctuations, the distribution of the liquid phase and the enhanced turbulence. The time-average factors were proposed to evaluate the average heat transfer performance during the sloshing period. The ranges of sloshing time-average factors for rolling, pitching, and swaying conditions are 0.861 ∼ 1.079, 0.923 ∼ 1.061, 0.834 ∼ 1.064 and 0.994 ∼ 1.090, respectively. As the mass flux and sloshing period increase, the time-average factors trend to 1.0, and increase at the transitional sloshing movement. With the increasing amplitude, the time-average factors increase from 1 to a maximal value of 1.093. The correlations to predict the time-average two-phase heat transfer coefficients under rolling, pitching, swaying, and heaving conditions for offset strip fins have been developed, and the deviations are within ± 10 %.

Dean instability and thermal characteristic in sinusoidal structure

Sinusoidal structures have been widely used in various heat exchangers and micro heat sinks. In this paper, the flow and heat transfer characteristics in sinusoidal channels are numerically investigated. In particular, the unsteady fluctuation of Dean vortices has been studied in detail. It is found that a critical Reynolds number exists in sinusoidal channel. When the Reynolds number exceeds the critical Reynolds number, the flow is in an unsteady state, which is manifested by the irregular fluctuation of the Dean vortices. In addition, the POD (Proper Orthogonal Decomposition) method is used to perform mode analysis of the Dean vortices motion, and the Swirling number is used to quantify the degree of Dean instability for the first time. It is found that the fluctuations of the Dean vortices have similarity in the decomposition of modes when reaching the critical state, and the locations of the main fluctuations at 90° and 180° cross sections change. It is also found that the Dean instability has a significant effect on improving the thermal homogenization of the channel wall. In detail, 82.83% and 20.20% enhancements of thermal homogenization at the upper and lower walls at the 90° cross section are obtained, respectively. 14.06% and 14.33% enhancements of thermal homogenization are obtained at the upper and down walls at the 180° cross section, respectively.

PIV and CFD Study on Crossflow Characteristics in a 7-Pin Wire-Wrapped Bundle Channel

China initiative Accelerator Driven System (CiADS) combines a linac, spallation target and a Lead-cooled Fast Reactor (LFR) together, which is designed to transmute nuclear waste and accelerate the progress of China’s energy technology research towards the goal of carbon neutrality. A LFR uses helical wire-wrap spacers as positioning components to enhance crossflow mixing in the reactor core. To study the velocity distribution and crossflow characteristics in wire-wrapped rod bundle channels, a 2 : 1 magnified scale 7-pin bundle fuel assembly model was fabricated using polymathic methacrylate. Particle image velocimetry (PIV) and computational fluid dynamics (CFD) simulations were used to investigate the velocity distribution in the 7-pin bundle flow channels at Reynolds number of 1250~5000 in the x z plane and Reynolds number of 1500 and 2500 in the x y plane. The deviation between CFD simulation results and PIV experimental data was small, and the Reynolds Average Navier-Stokes model could accurately simulate the flow characteristics of the wire-wrapped fuel rod bundle channels. The maximum crossflow velocity caused by helical wires was about 40% of the axial bulk velocity. The normalized crossflow velocity at the subchannel interface varied approximately sinusoidally with the axial height. As the Reynolds number increased, the velocity distribution trend and the loss rate of axial velocity in flow channels remained essentially constant while the peak value of crossflow velocity increased. The contour images of velocities with different axial heights were obtained from the x y plane, and their velocity distribution had a certain periodicity. The axial velocity loss rate in each subchannel caused by wire-wrap spacer resistance was between 7.35% and 38.51%, and the axial velocity loss rates in inner subchannels were usually higher than those in edge subchannels.

Understanding ecological response to physical characteristics in side channels of a large floodplain-river ecosystem

Side channels in large floodplain rivers serve a variety of important ecological roles, particularly in reaches where habitat conditions have been degraded or diminished. We developed hypotheses regarding side channel ecological structure whereby we expected species richness of young-of-year fishes to generally be higher in shallower, more physically heterogeneous side channels with lower velocities, with differences based on reproductive guild. We also hypothesized species richness of adult fishes to be higher in side channels with greater heterogeneity that could support diverse foraging resources and provide refugia during extreme flow conditions. To test these hypotheses, we used a 28-year fish community dataset from the Upper Mississippi and Illinois Rivers. Across six study reaches, we assessed metrics of side channel physical size, heterogeneity, and connectivity that were hypothesized to explain variance of fish community response, while accounting for site-level factors across 52 side channels using multilevel models. We then used these side channel-level characteristics in a K-means cluster analysis to classify 1126 side channels across 32 reaches of the river system. Our results indicated that the relative explanatory contributions of physical metrics varied by response variable, providing varying evidence in support of our hypotheses, and indicating that different forms of heterogeneity matter in different ways. Side channel-level factors were more explanatory of fish community responses in side channels of upstream reaches compared to downstream reaches and percent wet forest was the most explanatory side channel-level factor of fish community responses across all models. Our classification of side channels indicated strong spatial contrasts in the abundance and diversity of side channels across reaches. Scaling up to understand how the diversity and abundance of different types of side channels contributes to landscape-scale ecological functions and processes would be useful for establishing targets for reach-scale physical heterogeneity.

Laboratory investigation on flow structure and turbulent characteristics in low sinuous compound channels with vegetated floodplains

The flow-vegetation interactions and management of floodplain vegetation can be helpful for morphological stability and ecological maintenance of the rivers. Experiments were conducted in a fluvial hydro-ecological laboratory to study the hydrodynamic behavior of the compound meandering channel of low sinuosity with different layouts of flexible vegetation on floodplain. The presence of high density of floodplain vegetation resulted in increase of streamwise velocity up to 23% in main channel as compared to other vegetation layouts. The floodplain vegetation enhanced the streamwise velocities along the curvature, but the magnitude of the increase was dependent on the flow depth. The shear layer at the interface of the vegetated floodplain is expanding laterally towards the main channel with varying vegetation density. At the channel-floodplain interface, the velocity gradient in the lateral direction increases with increasing vegetation density, generating a strong shear layer at that position. The turbulent structures near the channel-floodplain interface are observed to increase with increasing vegetation density. This resulted in an increase in turbulence parameters such as turbulence intensity, Reynold’s shear stresses and turbulent kinetic energy showing its peak near the inner bank. The variability of flow angle at apex, bend and crossover region is observed to be less dominant in emergent conditions than vegetation configurations in submerged conditions. These findings provide insights of the shear layer dynamics and other turbulent structures in a low sinuous channel with the effect of floodplain vegetation.

АНАЛИТИЧЕСКОЕ ИССЛЕДОВАНИЕ ВЛИЯНИЯ ИОНОСФЕРЫ НА ЭНЕРГЕТИЧЕСКИЕ СООТНОШЕНИЯ В СПУТНИКОВЫХ СИСТЕМАХ СВЯЗИ МЕТОДАМИ ФИЗИЧЕСКОЙ КИНЕТИКИ ПЛАЗМЫ

In civil aviation, an urgent and important task is to provide air traffic participants with uninterrupted radio communication. Satellite communication is a global mobile network covering the entire surface of the Earth, including polar regions, which provides a two-way data and voice communication exchange. An analysis of the principles of functioning of satellite communication systems (SCS) showed that the determination of energy characteristics is of great importance. Energy characteristics in satellite communication channels and the level of interference determine SCS operational capabilities and quality indicators. One of the critical factors that determine the effectiveness of the SCS functioning is the conditions for the propagation and absorption of radio waves, including ionospheric phenomena, which are not yet fully understood, and this determines the research relevance. This article is devoted to theoretical studies of the ionospheric plasma by the methods of physical kinetics to increase the energy potential of the radio links of a satellite communication system.

Numerical investigation of block structure parameters on liquid water removal in novel block flow channels of PEMFC

ABSTRACT In proton exchange membrane fuel cell (PEMFC), liquid water transport and removal is very important to the performance and durability. In this study, based on the volume of fluid (VOF) method, liquid water transport and removal characteristics in novel block channels of PEMFC are numerically investigated considering the dynamic wetting effect. Effects of geometric parameters of the block, namely the height, width, and length of the block, are analyzed on the water removal ability and pressure drop of block channels. The results show that the height, width, and length of the block are all decisive factors on whether droplets can be directly dragged away from the GDL surface, achieving faster water removal, and they are all positively correlated with the pressure drop. Based on earlier water removal time and lower pressure drop, the height, width, and length of the block are investigated and optimized sequentially, and the range of optimum values are suggested.

Effects of rolling motion on flow and heat transfer characteristics in a tube bundle channel

The usage of nuclear energy in the ocean is growing, and the most significant fundamental motion of ocean conditions is rolling motion. Due to the significant quantity of tube bundle channels in marine nuclear power equipment, it is crucial to investigate the single-phase and boiling heat transfer characteristics in tube bundle channels under rolling motion. The paper compares the single-phase convection, sub-cooled boiling, and saturated boiling heat transfer characteristics under rolling and vertical conditions. The result demonstrates that the two-phase time-averaged heat transfer is lower under rolling motion compared with that under vertical conditions, and the rolling motion has the strongest impact on the time-averaged sub-cooled boiling. Time-averaged heat transfer coefficient decreases with the increase of the rolling amplitude and period. Due to the dearth and the necessity of research on transient sub-cooled boiling under rolling motion, the instantaneous sub-cooled boiling heat transfer fluctuation characteristics are investigated. The fluctuation amplitude of the heat transfer coefficient increases with increasing rolling period and frequency. The change of bubble behavior caused by rolling motion is the main reason for the change of transient heat transfer characteristics compared with that under vertical conditions. At last, a series of correlations based on the Gungor-Winterton correlation is improved and proposed to predict the instantaneous sub-cooled boiling heat transfer coefficient. Thereinto, the average relative error is 7.36% for forecasting the time-averaged heat transfer coefficient, and the average relative error for predicting the fluctuation amplitude of the heat transfer coefficient is 11.58%.

Experimental study of flow boiling characteristics in minigap channels over a wide heat flux range

The flow boiling characteristics in minigap channels were investigated by observing the two-phase flow patterns, measuring the two-phase pressure drops, and calculating the two-phase heat transfer coefficients and critical heat fluxes in minigap channels with heights of 0.5–2 mm. The working fluid was deionized water with mass fluxes of 200–400 kg m−2 s−1. The results show that bubbly flow, sweeping flow, churn flow, and churn-annular flow occur in sequence with increasing heat fluxes. An unstable two-phase flow regime was observed during the sweeping flow. The pressure drop varied in three regimes that were related to the flow patterns. The maximum pressure drops are 12.2, 3.8 and 1.6 kPa, respectively in 0.5 mm, 1 mm and 2mm channels at 400 kg m−2 s−1. The effect of gap height on the heat transfer coefficient first increases and then decreases with increasing heat flux. The heat transfer coefficients of all channels are at a high level when the heat flux is high. The maximum heat transfer coefficients are 7.3, 6.9 and 5.8 W cm−2 K−1 in the 0.5 mm, 1mm and 2mm channels at 400 kg m−2 s−1, which occurred before critical condition. The critical heat flux increased with increasing mass flux and gap height. In addition, a pressure drop correlation was developed to more accurately predict the pressure drop in microgap channels with an accuracy of 10.8%. The Liu-Winterton correlation and the Shah correlation give the best predictions for the subcooling boiling heat transfer coefficient. The Sun-Mishima correlation and the Kandlikar correlation provide the most accurate predictions for the saturated boiling heat transfer coefficients.

Effect of sediment transport on flow characteristics in non-prismatic compound channels

Sediment transport in open channels and rivers is considered one of the most important and complex processes in hydraulic engineering. In this study, we investigated the effect of sediment transport in non-prismatic compound channels on flow characteristics using the Computational Fluid Dynamics (CFD) software ANSYS-Fluent. Contrary to many previous numerical studies, the momentum exchange between both primary (water) and secondary (sediment) phases was considered by activating the Discrete Phase Model (DPM). The aim was thus, to investigate the behavior of particles from a Lagrangian viewpoint and a discrete perspective. This enabled to monitor the momentum lost or gained by transported sediment particles that were injected in the continuous phase. Moreover, the influence of discrete phase trajectories on the continuum continuous phase was considered. Four sediment particle sizes 1, 5, 50, and 500 µm and three sediment mass discharges 5.19, 10.38, and 15.56 kg/s were considered during simulations of the non-prismatic compound channel. Results showed that the stream-wise velocity of flow deceased along the length of the main channel and floodplain for sediments with small diameter. No considerable change was noted for larger particles. Sediment mass discharge had a considerable effect on flow velocity. As sediment mass discharge increased, flow velocity decreased. The flow velocity at the channel outlet was 0.52 and 0.50 m/s corresponding to sediment mass discharge of 5.19 kg/s and 15.56 kg/s, respectively. Results revealed that flow turbulence was attenuated in the presence of sediments and the degree of attenuation increased with increasing sediment mass discharge in flow, especially near the free channel surface. Moreover, suspended sediment concentrations were higher in the presence of gradual enlarged floodplain as compared to the straight one. Maximum concentrations were 13 and 190 kg/m 3 in the gradual enlargement reach for sediment diameter 1 and 5 µm, respectively, and 4.5 and 170 kg/m 3 in the same reach for the prismatic compound channel, respectively. Generally, and based on the results, it can be concluded that the effect of the secondary sediment phase on the primary water phase was more pronounced in case of small diameters and high mass discharge of sediments. In addition, the joint effect of floodplain width enlargement and sediment particle size on suspended sediment concentration was more pronounced for fine sediments than that for coarser sediment along the compound channel.

Numerical study of flow characteristics in compound meandering channels with vegetated floodplains

Large eddy simulations were conducted to simulate the flow in compound meandering channels whose main channel sinuosity was 1.381. Then, the floodplain vegetation was generalized using the momentum equation coupled with the drag force formula. The mean flow pattern, secondary flow, coherent structure, turbulence characteristics, and lateral mass and momentum transport with and without floodplain vegetation with relative depths (Dr) of 0.3–0.5 were studied. Results showed that the floodplain vegetation enabled the flow of the main channel to be more concentrated. The maximum average velocity in the cross section of the main channel increased by 100% and 30% when the relative depth was 0.3 and 0.5. Under the influence of floodplain vegetation, the secondary flow cell transformed greatly with the change in relative depth. When Dr < 0.3, the vegetation caused the vortex center of the secondary flow to move closer to the concave bank side, and the secondary flow distribution presents a flow pattern not flooding the floodplain. When Dr > 0.3, the spatial change in the secondary flow was not obvious. In addition, the floodplain vegetation did not change the large-scale vortex that was separated from the boundary layer of the convex bank side. Meanwhile, the floodplain vegetation increased the overall turbulence intensity, turbulent kinetic energy, and Reynolds stress of the main channel, and it increased the range of lateral mass exchange of the inbank flow and the mean and turbulent transport flux of each cross section.

Fragility curves for road embankments exposed to adjacent debris flow

Studies of recent decades have shown thousands of kilometers of transportation networks that have presented damage or failure from different types of flows, causing important traffic disruptions. Debris flows running adjacent (or in parallel) to river channels often explain the structural damage to road embankments caused by slope erosion. The probability of expected structural damage caused by a natural hazard may be modeled using fragility curves, which have been developed for transportation infrastructures like bridges and roads exposed to debris flows and are used in risk assessment. There are even fragility curves available to estimate the fragility of road embankments exposed to perpendicular debris flows. However, currently no model is available to estimate the road damage probability of embankments exposed to adjacent debris flows despite their important effects on traffic. This paper aimed to develop fragility curves for road embankments exposed to adjacent debris flows, considering mechanisms of exposed slopes subject to erosion and subsequent embankment instability. Models were calibrated considering flow characteristics in straight and bend channels including three damage states in terms of road traffic capacity loss. Monte Carlo simulations were performed to model potential damage, obtaining fragility curves for two types of roads. Curves were fit to log-normal distributions with a 99.5% confidence level. The analysis demonstrated that the geometric characteristics of road embankments explain their fragility, wherein lower heights and platform widths result in more probable expected damage. The analytical model developed confirmed that the erosive process intensifies in bend zones of channels, resulting in higher damage probability.

Thermal and flow characteristics in a flat plate pulsating heat pipe with ethanol-water mixtures: From slug-plug to droplet oscillations

The thermal performance and the internal flow regimes of a closed-loop flat plate pulsating heat pipe (FPPHP) are experimentally investigated. Reports on the FPPHP using the ethanol-water mixtures as working fluids are scarce in the literature. The binary mixtures with different boiling point components are suitable for a wide range of heat fluxes. Therefore, the results are reported for the ethanol-water mixtures of ratios 3:1, 1:1, and 1:3, and the corresponding pure liquids filled in the range of [40–80] % with power inputs given from 40 to 200 W. The effect of different condenser cooling modes, such as forced convective water cooling, forced convective air cooling, and natural convective air cooling on the thermal performance of the FPPHP, is also reported. With the increase in the power input, the observed flow characteristics in the FPPHP channels are: no oscillations, slug-plug oscillations, droplet oscillations, and the evaporator dry out. The binary mixtures with increased ethanol content give better slug-plug flow oscillations with smaller thermal resistances and fewer evaporator drying out instances than the pure working fluids. For power inputs of less than 120 W, the ethanol:water mixture ratio of 3:1 at all filling ratios gives a larger slug departure frequency in the evaporator. The smallest thermal resistance measured is 0.1 K/W, a decrease of 27% over pure ethanol. For power inputs greater than 120 W, the mixture ratio of 1:1 at all filling ratios performs better with continuous droplet oscillations. The smallest thermal resistance measured at the 80% filling ratio is 0.12 K/W, a decrease of 22% over pure ethanol. When the condenser cooling mode is changed to air cooling, the evaporator temperatures reach around 100 °C for power inputs greater than 40 W and 80 W for natural and forced air convection. Thus, the FPPHP filled with ethanol-water mixtures with the water-cooled condenser gives a stable flow regime and better thermal performance for a long-range of power inputs.

Three-dimensional shape optimization of fins in a printed circuit recuperator using S-CO2 as the heat-transfer fluid

Printed circuit heat exchangers (PCHEs) are among the most promising candidates for recuperators in the S-CO2 Brayton cycle owing to their high-pressure resistance and high compactness, where the fins are usually installed in channels to improve the thermal-hydraulic performance. The fin shape optimizations have been performed in several previous studies for enhancing the heat transfer and reducing the pressure drop. However, most of them are limited to two-dimensional optimization where the shape variation along the height direction was ignored. In the present study, three-dimensional fin shape optimizations are performed for the PCHE recuperators using S-CO2 as heat transfer fluid to further improve the thermal-hydraulic performance. The heat transfer and pressure drop characteristics in channels with optimized fins are comprehensively evaluated by comparing against conventional cylindrical fins and airfoil fins. The results indicate that the optimized three-dimensional fins feature variable cross-sections along the height direction. Such three-dimensional fins can significantly diminish the impact area of fluid, which is conducive to reducing the local flow resistance caused by high-velocity gradient and consequently improving the comprehensive heat transfer performance. Under the typical working condition studied in this paper, the heat transfer capacity in hot channels of recuperators with the optimized three-dimensional fins is 21.2% greater than the cylindrical fins, and 17.3% greater than the airfoil fins when the identical pressure drop is fixed; while the heat transfer capacity in cold channels with the optimized three-dimensional fins is 24.3% greater than the cylindrical fins, and 29.3% greater than the airfoil fins. In addition, the robustness analysis indicates that the optimized three-dimensional fins can yield more excellent thermal-hydraulic performance than conventional cylindrical fins and airfoil fins over a wide range of working conditions.

Long-range underwater acoustic communication of M-eyry spread spectrum with cyclic prefix

The Spread Spectrum is an effective transmission method for complex channels in underwater acoustic (UWA) communication because of the high correlation of pseudo-random noise (PN) sequence. In particular, the spread spectrum method is effective in an environment with a low signal-to-noise ratio. The bandwidth of UWA communication channel is limited due to the fact that sound wave attenuation in water is positively associated with frequency. The limited bandwidth directly leads to the low data rate of conventional direct-sequence spread spectrum UWA communication. M-ary spread spectrum (MSS) schemes using multiple orthogonal PN sequences were proposed as a solution to this problem. However, even in the MSS method, the performance may be limited by the delay spread characteristics in the multipath propagation channels. We propose a more robust communication method in the UWA channel by including Cyclic Prefix (CP) to achieve a more robust performance in the multipath channel. In the proposed method, a CP is inserted for every symbol. In the 160 km long-range UWA communication experiment conducted in the East Sea, the proposed method showed more reliable results than the conventional MSS method. [Work supported by the Agency for Defense Development, South Korea, Grant No. UD200010DD.]

Numerical simulation on flow boiling characteristics in sinusoidal channels of airborne micro heat exchanger

With the miniaturization of airborne system, heat dissipation has become a critical issue. Since sinusoidal channel can enhance flow disturbance, it has a stronger heat exchange capacity than that of straight channel. However, flow boiling characteristics in sinusoidal channel are more complex. Here, numerical simulations of flow boiling in sinusoidal and straight microchannels are conducted based on the VOF model with the inner diameter of 1 mm, mass flux of 500 kg/m2 s and heat flux of 50 kW/m2. The effects of channel structure and hypergravity are obtained. The simulation results indicate that bubble evolution in sinusoidal channel is more complex than that in straight channel, and bubbles are more likely to coalesce, deform and break. Vortices appear in sinusoidal channel and disrupt the near-wall boundary layer. Bubble evolution combined with vortices affect the flow boiling performances. For straight channel, dryout is less likely to occur, while for sinusoidal channel, it is more likely due to more drastic flow changes. Furthermore, the transition of flow patterns is faster under hypergravity. For sinusoidal channel, flow patterns transform earlier into slug and circular flows.

Numerical Modeling of Compound Channels for Determining Kinetic Energy and Momentum Correction Coefficients Using the OpenFOAM Software

Abstract The non-uniformity of the flow velocity distribution in each section of compound channels and in the main channel-floodplain interface area causes errors in estimating water surface profile, flood routing, pollution transfer, and so on. To reduce the impacts of non-uniformity on the exact calculation of kinetic energy and momentum, α and β correction coefficients are used, respectively. However, the determination method of these coefficients is a challenging issue in river engineering. This study used the OpenFOAM Software to determine these coefficients numerically for two laboratory models of compound open channels of which the data are available, using the single-phase pimpleFoam solver to do modeling in the mentioned software and the k-ωSST turbulence model to calculate the flow characteristics. Based on the results, the highest difference (13%) between the results estimated by the software and those obtained from the lab experiments was seen in the low flow depth where the flow left the main channel and entered the floodplain of a very shallow depth, possibly due to the grid generation of this area. This difference decreased as the flow depth increased, and its average was 6.65% for α coefficient and 2.32% for β coefficient in all cases, which means the results of numerical modeling and the experimental data conformed well, and the OpenFOAM software can be successfully used in flow modeling and analyzing flow characteristics in compound channels.