When subjected to high heat flux, the drastic variations in thermophysical properties of supercritical CO2 could lead to heat transfer deterioration. This study conducted numerical simulations to investigate the mitigation of heat transfer deterioration using teardrop dimples in vertically upward heated channels. The results indicate that teardrop dimples channels reduce the peak wall temperature by up to 10 % compared to smooth channels, and enhance the minimum heat transfer coefficient by as much as 86 %. The effects of various structural parameters of teardrop dimples on supercritical CO2 flow characteristics and thermodynamic features in the channel induced by different vortex structures were comprehensively analyzed. Vortices induced by the teardrop structures disrupt the boundary layer, enhance the turbulent mixing, improve energy transport efficiency, strengthen the flow acceleration effect and alleviate the buoyancy effect. Increasing the teardrop radius and number, as well as decreasing dimples spacing, contributes significantly to heat transfer enhancement, while causing only a slight increase in pressure drop. Rational parameter adjustment achieves optimal performance evaluation criteria, ensuring efficient heat transfer while minimizing pressure drop, optimizing fluid flow paths, and reducing operational costs.

The characteristics of various factors in a fixed-bed dehumidification system can provide the guidance of the design, such as a practical dehumidification system. In the present study, a fixed-bed dehumidification system, encapsulated with calcium alginate hydrogel/silica gel/expanded graphite composite adsorbent, was numerically studied with COMSOL Multiphysics and verified with experimental data. A multi-field coupling model of the fixed-bed was established by coupling the energy conservation, mass transfer rate and adsorption isotherm equations, including the transfer of Darcy's Law, heat transfer of porous media and partial differential equation with coefficient form. The adsorption heat characteristics were dynamically analyzed, and multi-parameter effects were systematically investigated. The effects of air moisture content, air velocity, air temperature at inlet, and structural dimensions of the fixed-bed on the dehumidification performances were revealed extensively. The results indicate that the numerical results of the three dimensional model match well with the experimental data, within the maximum deviation of about 10.56 %. When the air moisture content at inlet increases, the average dehumidification rate of the system increases due to the large potential difference between the water vapor and solid adsorbent, and the adsorption heat increases consequently. When the air velocity and temperature at inlet increase, the average dehumidification rate of the system decreases on the contrary. Different geometries of the fixed-bed affect the dehumification performance, e.g.,when the height to diameter ratio increases from 1:1 to 4:1, the flow time of air in the fixed-bed increases, inducing the increase of the average dehumidification rate of the system, from 0.858 g/(kg s) to 2.075 g/(kg s). The variation law of adsorption heat together with pressure drop is important for the design of a dehumidification system.

This study numerically investigated the suppression mechanism of heat transfer deterioration of supercritical carbon dioxide (CO2) in heated vertical channels with an internal protruded teardrop dimple (IPTD). The effects of IPTD structure parameters and management on flow field, vortex structure, buoyancy effect, thermal acceleration, heat transfer, pressure drop, and performance evaluation criterion (PEC) were analyzed. The results revealed that the complicated vortex reformation induced by IPTD could effectively suppress the heat transfer deterioration witnessed by peak wall temperature reduction and well-distributed as well as heat transfer coefficient improvement. It was indicative that the internal protrusion in teardrop dimple without additional heat surface could mitigate the formation of large primary-recirculation flow in the dimple cavity and generate the small secondary-recirculation flow surrounding the protrusion, shifting the flow separation and reattachment, weakening the flow impingement, while reducing the form drag and friction drag, which contributed to heat transfer enhancement and less pressure drop penalty, consequently PEC improvement. In addition, dimple depth increased, neighbor dimple space decreased, circumferential dimple number increased, and disordered arrangement could further boost the heat transfer performance by promoting vortical motion, flow pattern transition, and intensified flow mixing.

This study reveals the mechanism and optimization strategies for enhancing heat transfer in segmented helical insert tubes for supercritical CO2 through numerical simulation. The results indicate that when contrasted against smooth tubes, segmented helical inserts can significantly suppress the peak wall temperature by 70 K and improve the overall heat transfer performance by 1.65. These inserts enhance the intensity of secondary flow and helicity by inducing three-dimensional swirling flows with spatial continuity, which continuously disrupts the thermal boundary layer and promotes vigorous mixing of the fluid in both the core and near-wall regions. By optimizing parameters (increasing the cross-sectional length of the insert, reducing the helix height and spacing), the spatial continuous distribution of the swirling flow enhancement effect can be achieved. This optimization enhances flow mixing intensity, significantly increases turbulent kinetic energy, and effectively mitigates the influence of buoyancy effect. The obtained insights offer engineering solutions for maintaining operational stability, enhancing thermal efficiency, and ensuring system safety in supercritical CO2 Brayton cycle power systems.

This study conducted computational simulations to comparatively investigate the effects of various typical clawshaped lattice structure arrays (LSAs) on the flow and heat transfer performance of supercritical CO2 in a vertically upward heated tube. The results indicate that the LSAs could effectively mitigate heat transfer deterioration (HTD) of supercritical CO2, by regulating the coupling mechanism between buoyancy and flow acceleration effects, reducing wall temperature, and enhancing heat transfer coefficient. Furthermore, the influence of buoyancy and flow acceleration on heat transfer characteristics are systematically analyzed, revealing the advantages of claw-shaped lattice structures in suppressing HTD. The findings demonstrate that the four-claw LSA exhibits the most significant heat transfer enhancement, as its complex lattice geometry could induce stronger and more intricate vortex system, thereby facilitating momentum exchange and energy transport between the near-wall and core flow regions. Additionally, the four-claw LSA exhibits excellent robustness in heat transfer performance across varying structural parameters. Moreover, a generalized heat transfer prediction model applicable to both claw-shaped LSA tubes and smooth tubes is established. The proposed model accurately captures over 99 % of the simulation data, with a prediction error within +/- 5 %, aiming at providing crucial theoretical support and engineering guidance for optimizing supercritical CO2 heating systems.

The present study investigates the flow boiling heat transfer and flow pattern of R245fa in a smooth, horizontal microchannel with an inner diameter of 1 mm. The experiments cover a range of mass flux from 300 to 700 kg/m2s and heat fluxes varying between 10 and 94 kW/m2, with corresponding saturation temperature of 23–54 °C. Heat transfer experiment results indicate that the local heat transfer coefficient (HTC) presents two distinct variation trends under different heat and mass flux conditions. In addition, the heat flux is positively correlated with the local HTC at each measure point. Visualization experiment results show that annular flow forms at low vapor quality and significantly influences heat transfer as the primary flow pattern. The flow boiling heat transfer of R245fa in the 1 mm microchannel can be characterized by three regimes: bubble/slug regime, liquid film evaporation regime, and intermittent dryout regime. Thin liquid film evaporation and intermittent dryout during annular flow predominately contribute to heat transfer. Finally, the experimental HTC is compared with five different predictive methods, revealing that the three-zone model proposed by Thome et al. [46] holds immense potential for estimating the local HTC in the microchannel.

This study numerically explores the fluid flow and heat transfer features of supercritical carbon dioxide (CO2) in lattice structure array channel. The mitigation mechanism of the miniature cylindrical lattice structure array on heat transfer deterioration (HTD) of supercritical CO2 in vertically upward heated channel is studied. This paper evaluates the effect of several key influential parameters (length, diameter, pitch and number) of cylindrical lattice structure array on fluid flow field involving vortex structures, heat transfer coefficient, buoyancy effect and accelerated effect. The results indicate that the lattice structure can effectively suppress the HTD by generating the vortex structure which promoting the turbulent mixing effect and enhancing the turbulent kinetic energy (TKE) of the fluid. Compared with the smooth channel, the peak wall temperature of the lattice structure channel is reduced by about 50 °C, with a corresponding increase in the heat transfer coefficient of about 140 %. The average heat transfer coefficient of the channel is increased by more than 1/3. Furthermore, by increasing lattice length can reduce flow layer stratification and by increasing lattice diameter can creating a composite vortex, which enhance flow mixing strength. The larger the lattice pitch, the worse the overall suppression effect. A denser lattice arrangement produces more temperature valleys, thereby increasing the heat transfer coefficient. The conclusions in this study could provide the main theory support for security and stability of supercritical CO2 heat exchangers in power system.

This study numerically investigates the suppression mechanism of heat transfer deterioration (HTD) of supercritical carbon dioxide (SCO2) in heated vertical channels with lattice structure arrays (LSA). The influences of lattice structure parameters on the velocity field, temperature field, heat transfer coefficient, buoyancy effect and flow acceleration effect are analyzed. The results demonstrate that compared to smooth channels, the tri-claw LSA can significantly reduce the maximum peak wall temperature by up to 57 K. The heat transfer coefficient substantially increases, and the fluid turbulent kinetic energy (TKE) quintuples. The vortex structures generated by lattice structures can enhance turbulence intensity and heat transfer, which effectively mitigates buoyancy effect and ultimately suppresses HTD. Increasing the lattice claws number can create more complex vortex structures, lengthening the lattice disrupts the laminar flow within the boundary layer, increasing the lattice diameter can produce a composite vortex that enhances the strength of flow mixing. An increase lattice number across the cross-section strengthens the suppression effect, but an excessive number may lead to instability. A dense lattice arrangement can continually suppress HTD, achieving an overall improvement in heat transfer within a certain range. This paper could support the design of heat exchangers in SCO2 Brayton cycles.

Two-phase flow boiling visualization and heat transfer characteristics in the 1 mm microchannel are experimentally investigated. The experiments are carried out at mass fluxes of 300 to 700 kg/m2s and heat fluxes of 10 to 94 kW/m2. Five flow patterns are observed, of which the annular flow is the dominant flow pattern, which is already present in the low vapor quality region and has a significant influence on the heat transfer. In addition, dynamic flow processes such as bubble coalescence in bubbly and slug flow, the rupture of the elongated bubble and the chasing of droplet mist and liquid film wave in churn flow are also recorded. For the heat transfer coefficient profile, the trend is determined by three main regimes: bubble/slug regime, liquid film evaporation regime and intermittent dryout regime. Moreover, the thin liquid film evaporation along with the intermittent dryout is found to dominate the boiling heat transfer of the microchannel in the present study. Finally, the experimental heat transfer coefficients are compared with five predictive correlations, and the prediction accuracies are analyzed. Some modifications are made to improve the accuracy, and the modified three-zone model [41] presents a satisfactory accuracy with MAE=26.29% and θ=88.02%.

This study explores experimentally turbulent convection heat transfer characteristics of supercritical carbon dioxide (CO2) in cooled miniature tubes (Dh = 1 mm) with various inclined angles (theta = 0, 30, 60 and 90 degrees). The influence of tube inclined angles, CO2 mass flow rates and wall heat fluxes on wall temperature and heat transfer coefficient were analyzed. Results related to buoyancy effect and thereby affected heat transfer were discussed. It was found that the wall temperature and the heat transfer coefficient markedly increase with the increased inclined angle, indicating the gradual weakness of mixed convection flow with the increased inclined angle. This effect was strengthened as mass flow rate decreased. In addition, the liquid-like region witnesses more significant influence of buoyancy effect on the heat transfer coefficient. Increasing mass flow rate could promote the heat transfer intensification, accompanied by the flow shifting from mixed convention flow to forced convection flow. The heat transfer coefficient is independent of the wall heat flux in liquid-like region, and slightly increased with the augment of wall heat flux in gas-like region. Given that the previously existing heat transfer correlations showed large deviations in predicting the present experimental data, a set of new correlations were proposed for cooling heat transfer prediction of supercritical CO2 in inclined miniature tubes and yield excellent predictive capability, evidenced by overall mean obsolete error of 11.13%, 7.72%, 12.30% and 14.24% for the inclined angle theta = 0 degrees, 30 degrees, 60 degrees and 90 degrees respectively.

This study experimentally investigated the cooling heat transfer characteristics of supercritical carbon dioxide (CO2) in circular horizontal and vertical (downward and upward) micro-channels (D-h = 1 mm). Measurements were performed for operating pressure ranging from 7.5 to 8.2 MPa, mass flow rate ranging from 1.323 to 2.567 kg/h, wall heat flux ranging from 14.05-39.34 kW/m(2). The effects of mass flow rate, wall heat flux, operating pressure and flow direction on heat transfer coefficient and buoyancy effect were explored. The experimental results revealed that heat transfer intensification benefited from increased mass flow rate and decreased operating pressure. Wall heat flux exhibited relatively mild influence on heat transfer coefficient. A significant heat transfer enhancement in downward flow was discerned compared with horizontal flow. The buoyancy effect significantly influences the heat transfer characteristics during supercritical CO2 cooling process as thermal properties dramatically changes with the bulk temperature. The forced convection aligned region where the buoyancy effect was less dominated witnessed distinctly higher heat transfer coefficient in contrast with mixed convection flow. Furthermore, a comparative study was performed by comparing several existing correlations with experimental results of Nusselt number, and large deviations were reasonably observed due to limited applicability. (c) 2021 Elsevier Ltd. All rights reserved.

This study examines the advantages and disadvantages of micro- and macro-channel flow boiling for high-heat-flux cooling applications using both computational and theoretical/empirical methods. The computational simulations are conducted in ANSYS FLUENT using the Volume of Fluid (VOF) method along with the Lee phase change model, and accounting for both shear lift force and conjugate heat transfer along the channel walls. Computational results for both channel sizes are compared with theoretical/empirical results obtained using the Homogeneous Equilibrium Model (HEM) and Separated Flow Model (SFM), and both HEM and the Homogenous Frozen Model (HFM) are used to assess the potential for two-phase choking. The computational results show bubbles in micro-channels are highly confined and tend to grow longer in the flow direction. The two methods show good agreement in predicting wall temperatures. Overall, micro-channel heat sinks are shown to fare much better than macro-channels in terms of heat transfer performance, evidenced by both significantly higher heat transfer coefficients and lower wall temperatures, but this comes at the cost of significantly higher pressure drop and pumping power requirements. It is also shown micro-channels are prone to choking due to high two-phase Mach number.

A numerical study on cooling heat transfer and pressure drop of supercritical CO2 in horizontal wavy microchannels with consistent crests and troughs (WMCCT) and wavy microchannels with opposite crests and troughs (WMOCT) was conducted. The aim is to provide reference for optimal design in trans-critical CO2 gas cooler equipment. The influence of the channel structure on heat transfer coefficient, pressure drop, buoyancy effect, flow fields and overall thermal performance was analyzed. WMOCT and WMCCT show great advantages in heat transfer performance, while get slightly larger pressure loss punishment when compared with straight microchannel (SM). It is worthy to note that both heat transfer coefficient and pressure drop increase with increased amplitude or decreased wavelength in both WMOCT and WMCCT, which is particularly evident in WMOCT. Furthermore, the analyses carried out identified an optimal channel structure that could improve the overall thermal performance of microchannel gas coolers.

A numerical research on flow regime of R134a in horizontal minichannels was conducted. The model was on the basis of the volume of fluid approach, and inserted user-defined routines which cover condensation mass and heat transfer. The observed annular flow, injection flow and intermittent flow of pure steam condensation, and annular (annular-wavy) flow, injection flow, annular-intermittent flow and intermittent flow of gas mixture condensation were qualitatively compared against experimental data. Flow patterns of lower non-condensable gas mole concentration show similar with that of pure steam, while the fluctuation of gas-liquid interface in annular flow and the noticeable annular-intermittent flow of higher non-condensable gas mole concentration were noted. Besides, injection flow characters with larger head volume and longer neck, and intermittent flow characters with larger slug sizes in existence of non-condensable gas were found. Furthermore, the influence of mole concentration of non-condensable gas and operating conditions on flow pattern transition lines were analyzed after validating the flow pattern transition. The results showed that the injection flow transition point has a relative fixed point at pure vapor condensation, while various in presence of NCG. Besides, lower non-condensable gas mole concentration, smaller inlet gas mass flux and larger wall heat flux could lead to earlier flow pattern regime.

This study explores experimentally and computationally fluid flow and heat transfer characteristics of FC-72 condensation along a cooling module containing multiple 1 mm x 1 mm square channels. The module is cooled along its underside by a counterflow of water. The computational portion of the study adopts the VOF method and Lee interfacial phase change model, and is executed using ANSYS FLUENT. Computed are dominant flow patterns as well as spatial variations of both bottom wall temperature and fluid temperature for FC-72 mass velocities ranging from 68 to 367 kg/m(2)s. The computed flow patterns show good agreement with those captured experimentally using high-speed video. Captured correctly are dominant smooth-annular, wavy-annular, transition, slug, bubbly, and pure liquid flow patterns. And predicted variations of wall temperature show good agreement between computational results, with average deviation ranging from 1.46% to 6.81%. The computational method is capable of predicting fluid temperature, which cannot be measured experimentally in a small channel. Detailed spatial variations of fluid temperature are provided both perpendicular to the bottom wall and along the channel. These variations show close correspondence with axial spans of the dominant flow patterns. (C) 2019 Elsevier Ltd. All rights reserved.

A numerical study of condensation heat transfer in curved square and triangle microchannels with various curvatures is conducted. The model is based on the volume of fluid approach and user-defined routines. The predictive accuracy of the numerical results is assessed by comparing the heat transfer coefficient with available correlation. After the validation, the effects of the mass flux and heat flux on heat transfer coefficient are analyzed in detail. Increasing mass flux could strengthen heat transfer performance, while wall heat flux has little effect on condensation heat transfer coefficient. The curved microchannels show an advantage in heat transfer enhancement comparing with the straight ones, and the enhancement performs better in curved microchannels with larger curvatures. Besides, the heat transfer performance could be enhanced in triangle microchannels in comparison with square microchannels.

Numerical simulations of condensation heat transfer in triangle microchannels are presented.The model is established on the Volume of Fluid (VOF) approach and the user-defined routines which includes heat transfer at the vapor-liquid interface and latent heat.The predictive accuracy of the numerical model is assessed by comparing the heat transfer coefficient with the available empirical correlations in the literature.The influence of gravity and surface tension on the liquid-vapor interface distribution and heat transfer performance are analyzed.No obvious effect of the gravity is observed in the liquid-vapor interface distribution and the average cross sectional heat transfer coefficient.Surface tension,which plays a dominate role during the condensation in non-circular microchannels,leading to reduction of the condensate film thickness at the sides of the channel and accumulation of the condensate at the corners of the channel,giving rise to smaller thermal resistance and better heat transfer performance.

Numerical simulations of condensation heat transfer of R134a in curved triangle microchannels with various curvatures are proposed. The model is established on the volume of fluid (VOF) approach and user-defined routines which including mass transfer at the vapor-liquid interface and latent heat. Microgravity operating condition is assumed in order to highlight the surface tension. The predictive accuracy of the model is assessed by comparing the simulated results with available correlations in the literature. Both an increased mass flux and the decreased hydraulic diameter could bring better heat transfer performance. No obvious effect of the wall heat flux is observed in condensation heat transfer coefficient. Changes in geometry and surface tension lead to a reduction of the condensate film thickness at the sides of the channel and accumulation of the condensate film at the corners of the channel. Better heat transfer performance is obtained in the curved triangle microchannels over the straight ones, and the performance could be further improved in curved triangle microchannels with larger curvatures. The minimum film thickness where most of the heat transfer process takes place exists near the corners and moves toward the corners in curved triangle microchannels with larger curvatures.

ABSTRACT A numerical research on flow regime transition in wavy microchannels was conducted. The model was based on the volume of fluid approach and user-defined routines including interfacial mass transfer and latent heat. The observed droplet flow, annular–wavy flow, injection flow, and slug–bubbly flow were qualitatively compared against experimental data and transition lines were established. The effects of inlet vapor velocity, wall heat flux, and microchannel geometry characteristics on the annular length, occurrence frequency of injection flow, initial slug volume, and bubble detachment frequency were investigated.