In the flue gas of coal-fired power generation, particularly during low-load operation to meet the flexible peak-shaving requirements of the power grid and adapt to the fluctuations of renewable energy, it is impossible to prevent acid condensation on the low temperature heating surfaces. The precise prediction of the acid dew point (ADP) and acid condensation characteristics within the heat and mass transfer boundary layer on double H-type fin surfaces was numerically simulated. Furthermore, the impacts of six variables on the acid solution concentration (Csolution), acid condensation rate (Racid), and water condensation rate (Rwater) across the fin surfaces, along with the acid dew point (tADP), acid vapor content (yacid), and water vapor content (ywater) of flue gas within the computational domain, were investigated. The results demonstrate that the inlet velocity (vin), flue gas temperature (Tin), tube wall temperature (Twall), and tube wall temperature difference (∆Twall) modify the flow and heat transfer characteristics, consequently changing the fin surface temperatures and affecting the acid condensation behavior. When the flow and heat characteristics remain constant, elevated acid vapor content (yacid) and water vapor content (ywater) significantly enhance the probability of acid condensation on fin surfaces. The conclusions are crucial for the subsequent analysis and optimization of related heat exchange equipment.

The demand for metallic nickel has been growing steadily. Laterite nickel ore, as an oxide mineral, contains a higher nickel content than sulfide ores, enabling more economical smelting in the nickel extraction process. However, an extremely thick slag layer generated during side-blown smelting of laterite nickel ore alters the bath circulation characteristic of conventional smelting processes. To address this, this study employs a validated Multi-Fluid VOF model to analyze the flow field characteristics of the side-blown furnace bath under varying slag layer thicknesses. Within the slag thickness range of 1.5-2.5 m, the ascending air plume shifts horizontally along the furnace wall with increasing slag thickness, entraining matte into the slag and promoting its diffusion toward the settling zone. Matte diffusion is governed by bath circulation: backflow from the settling zone dominates at lower slag thickness, while vertical kinetic energy from splashed melt reflux dominates at higher thickness. This reflux-induced circulation is blocked by the dense matte layer, causing no disturbance to the bottom matte and thus favoring slag-matte separation. Increasing slag thickness raises the near-wall circulation zone and plume entry point, reducing gas holdup near the tuyere and weakening oxygen-matte contact.

This study systematically investigated flow boiling characteristics within a novel three-layer microchannel heat sink with 3/4 open-ring pin fin arrays, designed for high-heat-flux thermal management of low-carbon metallurgical reactors. Two-phase flow regimes, pressure drop, and wall temperature responses were analyzed. To evaluate the impact of functional surface material properties on thermo-hydraulic behavior, a hydrophilic nano-coating modification was applied to the inner copper channel walls for comparison. Increasing the flow rate triggered a transition from a vapor-dominated confined slug flow to a liquid-dominated dispersed bubble flow, which effectively improved the thermo-hydraulic stability. Hydrophilic surface modification resulted in an average pressure drop reduction of 33% and significantly diminished the sensitivity of flow resistance to velocity variations. Through hydrophilic treatment, the localized vapor film effect at high velocities was suppressed, and temperature field homogenization was promoted, yielding a maximum convective heat transfer coefficient of 7760 W/(m2·°C), i.e., 72.9% enhancement over the baseline heat sink. The underlying mechanism is attributed to the formation of a stable near-wall thin liquid film and the promotion of high-frequency nucleate boiling. These results will be of high relevance for developing efficient cooling solutions for power electronics, thereby supporting the advancement of low-carbon metallurgical reactors.

Ground heat exchangers (GHEs) are key technology for efficient shallow geothermal utilization, while the strong bidirectional coupling between winter heat extraction and soil freeze-thaw cycles in cold regions remains a critical challenge. This study establishes a fully coupled thermal-hydraulic-mechanical (THM) model integrating unsaturated soil phase transition and GHEs heat extraction processes, systematically evaluates system responses under varying reservoir properties and technical parameters, and identifies the key performance-controlling parameters. The results reveal significantly hierarchical influences of reservoir and technical parameters on frozen zone evolution, outlet temperature and soil deformation. Multiple correlation analyses quantify the influence priority: initial water content > porosity > hydraulic conductivity > specific heat capacity (reservoir properties), and injection temperature > heat exchange section dimension > injection mass flow rate > burial depth (technical parameters). All parameters show pronounced phased dependence, with complete correlation polarity reversal between freezing and thawing stages (e.g., hydraulic conductivity: 0.290 to-0.342; initial water content:-0.731 to 0.403). This non-monotonic performance trend is dominated by dynamic THM coupling, rooted in the bidirectional feedback between heat extraction and freeze-thaw behavior. This study provides critical theoretical guidance for site selection and parameter optimization of shallow geothermal projects, with important implications for freeze-thaw geological disaster prevention.

Viscous ash layers and low-temperature corrosion pose major challenges to the high-efficiency operation of heat exchange equipment in the flue gas of coal-biomass co-firing power stations. The field experiments on low-temperature fouling layers were carried out in a 135 MW coal-fired power station and a 35 MW biomass-fired power station. The micro characteristics of ash samples were tested to explore the forming mechanisms of viscous ash layers. The results show that the reactions of ash particles and ammonium salt crystallization (NH4HSO4, (NH4)2SO4, NH4Cl), acid condensation (H2SO4, HCl and HF), deliquescence of hygroscopic salts (CaCl2, MgCl2, NH4Cl, etc.), and water condensation determine the viscous ash layer formation covering heating surfaces in coal-biomass co-firing power stations.

Flow resistance and thermal resistance, reflected by velocity and temperature gradient layers near the heat exchange surface, impede the heat transfer process in an air heat exchanger. To reduce the resistance, the audible sound waves are introduced. With reference to the boundary layer definition, the evolution of maximum velocity and temperature gradient layers in an air-to-air heat exchanger is examined. Results show that the sound waves induce oscillating flow, causing periodic fluctuation in velocity, while the temperature distribution remains stable. The maximum velocity gradient layer exhibits distinct evolution characteristics across different surfaces, whereas the maximum temperature gradient layer remains stable. The heat transfer characteristics are influenced by the combined effect of both gradient layers. Specifically, fluctuations in heat flux show an inverse trend relative to those of the velocity gradient layer and lag behind its evolution, and an increase in heat flux corresponds to a decrease in the thickness of the temperature gradient layer. Furthermore, the time averaged thicknesses of the velocity and temperature gradient layers in regions farther from the inlet decrease by more than 30% and 40%, respectively, as the sound pressure level increases from 0 to 140 dB. Concurrently, the heat flux in these regions increases by over 30%, while that near the inlet remains largely unchanged. This indicates that the incidence of sound waves has minimal impact on heat transfer performance near the inlet but gradually enhances it along the flow direction, the flow resistance and thermal resistance decrease, and the oscillating flow accelerates the heat transfer process.

In this study, flow and heat transfer characteristics in novel non-closed 3/4 ring-shaped micro-pin-fin arrays with in-line and staggered layouts were investigated numerically. The flow distribution, wake structure, vorticity field and pressure drop were examined in detail, and convective heat transfer features were explored. Results show that vortex pairs appeared earlier in the ring-shaped micro-pin-fin array compared with the traditional circular devices. Pressure drop across the microchannel varied with layout of the fins, while little difference in pressure drop was observed between ring-shaped and circular fins of the same layouts, with the maximum difference being 1.43%. The staggered ring-shaped array was found to outperform the in-line array and the circular arrays in convective heat transfer. A maximum increase of 21.34% in heat transfer coefficient was observed in the ring-shaped micro-pin-fin array in comparison with the circular micro-pin-fin array. The overall thermal-hydraulic performance of the microstructure was evaluated, and the staggered ring-shaped array with a fin height of 0.5 mm exhibited the best performance among the configurations studied.

Despite the fact that engineered surface enabling remarkable phase change heat transfer have elicited increasing attention due to their ubiquitous applications in thermal management, the underlying mechanisms of intrinsic rough structures as well as the surface wettability on bubble dynamics remain to be explored. Therefore, a modified molecular dynamics simulation of nanoscale boiling was conducted in the present work to investigate bubble nucleation on rough nanostructured substrates with different liquid–solid interactions. Specifically, the initial stage of nucleate boiling was mainly investigated and the bubble dynamic behaviors were quantitively studied under different energy coefficients. Results shows that as the contact angle decreases, the nucleation rate increases, because liquid obtains more thermal energy there compared with that on less wetting surfaces. The rough profiles of the substrate can provide nanogrooves, which can enhance initial nucleate embryos, thereby improving thermal energy transfer efficiency. Moreover, atomic energies are calculated and adopted to explain how bubble nuclei are formed on various wetting substrates. The simulation results are expected to provide guidance towards surface design in state–of–the art thermal management systems, such as the surface wettability and the nanoscale surface patterns.

A gas cooler is one of the important parts of a carbon dioxide (CO2) heat pump water heater, and it must meet the needs of not only pressurization but also heat transfer. It is important to study gas coolers. In this paper, a heat exchanger with a spiral channel is studied. ANSYS CFX software was used to analyze the flow and heat transfer characteristics of the heat exchanger (single-plate model). The influences of the cooling pressure of CO2, the mass flux of CO2, the mass flux of water and the channel radius of CO2 are discussed. In this paper, the results show that the cooling pressure of CO2, the mass flux of CO2 and the channel radius of CO2 all have a large influence on the local heat transfer coefficient: with an increase in the cooling pressure of CO2, the peak value of the heat transfer coefficient of CO2 decreases and the average heat transfer coefficient decreases; with an increase in the mass flux of CO2, the peak value of the heat transfer coefficient of CO2 increases and the average heat transfer coefficient increases; and with a decrease in the channel radius of CO2, the peak value of the heat transfer coefficient of CO2 increases. The water mass flux has only a slight effect on heat transfer, and the lower cooling pressure of CO2 corresponds to a higher peak heat transfer coefficient, which can reach 27.5 kW∙m−2∙K−1 at 9 MPa.

A novel measurement system for mixing property of binary mixtures in three-dimensional fluidized beds is developed based on capacitance probe method. The mixing processes at multi-positions of the bed are acquired simultaneously. A new dispersion coefficient is proposed to characterize the local dispersion of particles and a new mixing index is proposed to evaluate the local mixing quality in three-dimensional fluidized beds. The effect of convection and diffusion mechanism on particle mixing is discussed separately. Results show that the governing mechanism of particle mixing at the center and top of the beds is convection; meanwhile the governing mechanism for particle mixing at the bottom and near the wall is diffusion. The radial dispersion coefficient at the half-radius of the bed is mainly between 0.0038 and 0.026 m2/s, which is about 1.5 times that near the wall. The vertical dispersion coefficient is about 2.5 times that the radial dispersion coefficient.

A novel measurement system for radial particle mixing in annular fluidized beds is designed on the basis of the capacitance probe method. Mixing parameters at different radial positions are acquired. The effects of the convection/diffusion mechanism on radial mixing are analyzed individually. It is found that the governing mechanism of mixing at the axial line is convection; meanwhile, diffusion is the governing mechanism of mixing near the wall. The effect of convection on radial mixing at the upper part is more important than that at the lower part. The radial dispersion coefficient ranges from 0.006 to 0.072 m2/s. At the upper part, the radial dispersion coefficient at half-radius is between 0.016 and 0.072 m2/s and that near the wall is 0.006 and 0.028 m2/s, which is four times and twice that at the same radial position and at the lower part, respectively. The radial dispersion coefficient is about 1.5 times that in two-dimensional fluidized beds.

Measuring the particle mixing parameters at multi positions in three-dimensional fluidized beds continuously remains a challenging task. A novel measurement method for the mixing and segregation of particles inside three-dimensional fluidized beds is developed based on capacitance probe. The measurement error is generally below 7%. The particle mixing parameters and dispersion coefficients at multi-positions of the three-dimensional fluidized bed are acquired. The effect of convection and diffusion mechanism on particle mixing is discussed. Results show that the governing mechanism of particle mixing at center and top of the bed is convection; meanwhile the governing mechanism for particle mixing at bottom and near the wall is diffusion. The radial dispersion coefficient at half-radius of the bed is mainly between 0.0038 and 0.026 m2/s, which is about 1.5 times that near the wall. The axial dispersion coefficient is mainly between 0.004-0.056 m2/s, which is about 2.3 times that in radial direction.

Detailed hydrodynamics in microchannels with in-line and staggered circular micro pin fin structures over Reynolds number ranging from 50 to 700 are experimentally studied with micro-particle image velocimetry. The velocity field, vortex structure and vorticity distribution is obtained and analyzed, and differences in flow behavior in the two arrangements are examined. The result indicates a delay in the onset of vortex shedding in micro pin fin arrays as compared to their macro scale counterpart due to the confinement effect of channel walls. Flow pattern in the in-line arrangement is featured by the microchannel-like main stream between the pin rows and the double vortex wake flow behind the pins, while flow in the staggered arrangement is characterized by the wavy main stream and a pair of accelerated flow streams on two sides of the pins. Vorticity bands along the sides of the wake region intensify with the Reynolds number, diffuse further downstream, but narrow in width. Visualization and data extraction results reveal that flow behavior in the staggered arrangement differs from that in the in-line arrangement in terms of a smaller area occupied by the low-velocity wake region and a more sufficient mixing of fluid.

采用微观粒子图像测速法(Micro-PIV),实验研究了 Reynolds数(Re)=50～800范围内去离子水在微肋直径D)=0.4 mm微肋阵内的绕流流场特性,获得了不同Re下错排和顺排微肋阵内的流线分布与速度场,分析了Re与微肋排布方式对旋涡结构、流速分布等流场特性的影响规律.研究结果表明,在Re=50～700范围内,错排和顺排微肋阵内均出现涡结构,当Re=800时错排微肋阵内开始发生旋涡脱落;错排微肋阵内旋涡长度随着Re的增大而增加,而对于顺排微肋阵,在低Re时旋涡长度随着Re的增大而增加,当Re≥300后,旋涡长度保持微肋间距不再增加;顺排微肋阵内主流区顺流速度较错排微肋阵大,而错排微肋阵内横向速度大于顺排微肋阵且最大值比顺排微肋阵高约25％,微肋错排布置增强了流体的掺混.

Micro pin fin heat sink is a very attractive cooling technique for high power density microelectronics. Optimization of its cooling performance requires insightful understanding on fundamental physics of flow inside it, especially around a single pin fin. In the present study, an experimental investigation on flow past an isolated low aspect ratio (height-to-diameter ratio) pin fin embedded in a microchannel is conducted using Micro-PIV. The flow field and vorticity distribution at different channel heights under various Reynolds numbers are obtained. Endwall effect is found to play an important role in flow past the pin fin in the microchannel, and the critical Reynolds numbers are larger than that at the conventional scale. Vorticity concentrations are formed on both sides of the pin fin along the shear layer and intensify with the increase in Reynolds number. Flow fields and vorticity distributions at different heights exhibit different characteristics, especially at higher Reynolds numbers, indicating three-dimensionalities of the flow. Viscous resistance of the endwalls leads to lower overall velocity, smaller extent of the recirculation zone and weaker vorticity in flow layer closer to the top and bottom channel walls. Pressure drop and flow resistance characteristics in the microdevice is analyzed. The effect of aspect ratio of the pin fin on the wake flow is also studied, and the results show that three-dimensionalities increase but critical Reynolds numbers decrease with larger aspect ratios. A comparison with flow across micro pin fin arrays is conducted and differences are observed in velocity field and wake flow features.

基于显微粒子成像测速(Micro-PIV)技术,对微通道内单柱绕流特性展开实验研究,分析了10＜ Re＜ 350范围内不同高度流层的速度场、涡量场及旋涡特性.结果 表明:微尺度绕流现象相比宏观尺度存在滞后,首次出现旋涡的第一临界Re约为10.随着Re的增大,尾流区长度不断增加,旋涡尺度逐渐增大,旋涡中心位置向下游延伸.涡量强度随Re的增加而提高,涡量向下游扩散能力增强,高涡量区变窄.不同高度流层的速度场与涡量场存在差别,体现出三维效应.

A novel experimental method for the lateral mixing of binary solids in bubbling fluidized beds was developed based on the capacitance probe technique. The evolutions of local mixing ratios in a fluidized bed which can be assumed as one mixing cell were analyzed in detail. The solids mixing within one mixing cell was resolved and the effect of convection and diffusion mechanism on lateral mixing was evaluated individually. The results show that at lower part of the fluidized bed, convection plays a more important role in the mixing process near the wall; meanwhile, diffusion is very important for the mixing around the center line. This is opposite with that at the higher part. A lateral micro dispersion coefficient was proposed to characterize the lateral mixing within the mixing cell and the value is generally between 0.005 and 0.025 m/s. A new mixing index was proposed to evaluate the lateral mixing quality of binary solids. It was found that at the lower part of the fluidized bed, the best mixing is acquired at the half radius, whereas mixing at the center line is the worst. At the higher part, solid mixing is better when increasing the distance from the wall. The influences of gas velocity and static bed on the lateral mixing were also discussed from a microscopic perspective.

采用微观粒子成像系统(Micro-PIV)实验研究了6<Re<300范围内微通道内D=0.4mm圆柱的绕流特性,获得并分析了不同Re下不同高度流层的速度场、涡量场、湍流强度场及回流区漩涡结构.研究结果表明,微圆柱绕流出现漩涡的第一临界Re在10左右,随着Re的增大,尾流区涡长度和宽度增加,尾流区域增大,漩涡中心后移;由于黏性阻滞,越靠近微通道壁面,主流速度越低且分布越均匀;不同高度下回流区长度相同,远离壁面的平面尾流区漩涡中心沿流动方向后移;高涡量区与高湍流强度区分布在微圆柱两侧,说明该位置流体混合较为剧烈,随着Re的增大,涡量增加,高涡量区变窄、变长,湍流强度及高湍流强度区域增大,当Re>200,不同高度流层的湍流强度差别较小.

The mechanisms of pore-scale foam transport in porous media help further understanding how foam flows in porous media with different permeabilities. In this work, the pore-scale behaviors of N2 foam flows in porous media with different permeabilities have been investigated using transparent polydimethylsiloxane (PDMS) micromodels with foam qualities in the range of 50 %–90 %. The foam transport in homogeneous and heterogeneous micromodels, and the effects of permeability on foam texture and gas trapping, were evaluated through direct visual observations. Bubble generation based on the pre-existing foam bubbles occurred for trapped and flowing bubbles, which could change the distribution of flowing and trapped gases in the micromodel. Two foam generation mechanisms, namely snap-off and lamella division, were observed in this study. The mean size of bubbles in the low-permeability micromodel was smaller than that in the high-permeability micromodel especially at high foam qualities. The trapped gas saturation in the low-permeability micromodel was larger than that in the high-permeability micromodel. For foam flow in the heterogeneous micromodel, the process of foam diversion from the high-permeability region to the low-permeability region was directly displayed at the pore scale. Regeneration of large trapped bubbles in the low-permeability region under the pressure difference across the micromodel played an important role in this process.

采用Micro-PIV实验系统和压差测试系统,研究了含有单个微圆柱的通道内去离子水在10<Re<430范围内的流动特性,得到了通道不同高度流层的流线、无因次速度场、湍流强度分布以及压降.结果表明,微圆柱绕流的第一、第二临界Re分别在10和430左右;尾流区漩涡内流体存在垂直流动方向的速度,漩涡三维特征明显;当Re较低时,压降及漩涡长度随Re增加较为缓慢,黏性阻力所占的比重较大,随着Re的增加漩涡发展,形状阻力占据的比重增加,压降随Re增加幅度上升,当Re达到400左右压降随Re变化曲线斜率增大,此时尾流区流动向着漩涡脱落过渡.