Higher Heat Transfer Coefficient Research Articles

Frictional pressure drop correlation of steam-water two-phase flow in helically coiled tubes

Helically coiled tubes are utilized as the heat transfer tubes for High-Temperature Gas-cooled Reactor (HTGR) steam generators due to their compactness, thermal expansion accommodation and high heat transfer coefficient. A clear understanding of the pressure drop characteristics is important for the design and operation of steam generators. In this study, experiments were conducted to investigate the frictional pressure drop characteristics in helically coiled tube with a very large curvature ratio of 0.109. The experiments were carried out in a steam quality range of 0.1–0.9, system pressure range of 3.5–7 MPa, mass flux range of 320–1100 kg/m2s. The results indicate that the frictional pressure drop increased with the increase of mass flux, decreased with the increase of system pressure. It first increased with the increase of steam quality, and reached a maximum value at a steam quality of approximately 0.8, then decreased until steam quality reached unity. Based on the present work and existing experimental database, 25 existing correlations are evaluated and none of them could satisfactorily fit all the data. Therefore, a new correlation based on the Martinelli and Nelson correlation was proposed. Compared with existing correlations, it has a broader range of applicability (operating pressure from 0.75 to 8 MPa, mass flux from 200 to 1100 kg/(m2·s), steam quality from 0.03 to 0.99, and curvature ratio from 0 to 0.109.), and higher accuracy (weighted mean MAPE of 8.92 % and arithmetic mean MAPE of 8.81 %). It also has better compatibility with straight tubes.

A Review of Thermochemical Energy Storage Systems for District Heating in the UK

Thermochemical energy storage (TCES) presents a promising method for energy storage due to its high storage density and capacity for long-term storage. A combination of TCES and district heating networks exhibits an appealing alternative to natural gas boilers, particularly through the utilisation of industrial waste heat to achieve the UK government’s target of Net Zero by 2050. The most pivotal aspects of TCES design are the selected materials, reactor configuration, and heat transfer efficiency. Among the array of potential reactors, the fluidised bed emerges as a novel solution due to its ability to bypass traditional design limitations; the fluidised nature of these reactors provides high heat transfer coefficients, improved mixing and uniformity, and greater fluid-particle contact. This research endeavours to assess the enhancement of thermochemical fluidised bed systems through material characterisation and development techniques, alongside the optimisation of heat transfer. The analysis underscores the appeal of calcium and magnesium hydroxides for TCES, particularly when providing a buffer between medium-grade waste heat supply and district heat demand. Enhancement techniques such as doping and nanomaterial/composite coating are also explored, which are found to improve agglomeration, flowability, and operating conditions of the hydroxide systems. Furthermore, the optimisation of heat transfer prompted an evaluation of heat exchanger configurations and heat transfer fluids. Helical coil heat exchangers are predominantly favoured over alternative configurations, while various heat transfer fluids are considered advantageous depending on TCES material selection. In particular, water and synthetic liquids are compared according to their thermal efficiencies and performances at elevated operating temperatures.

Study on bubble behaviors and heat transfer at shallow liquid boiling over open rectangular microchannels

The heat transfer performance of shallow liquid boiling differs from that of deep liquid boiling owing to distinct bubble venting mechanisms. Since the liquid level is of the same order of magnitude as the bubble departure diameter, bubble detachment is restricted by the surface tension of the free interface, which causes an untimely liquid reflux toward the nucleate site. Consequently, shallow liquid boiling has a high heat transfer coefficient (HTC) at a liquid height below the critical liquid level but at the expense of limited critical heat flux (CHF). This study aims to improve the CHF of shallow liquid boiling by utilizing open rectangular microchannels surface (ORMS), which can provide separated vapor–liquid pathways to facilitate the liquid return. The boiling heat transfer curves and bubble behavior of distilled water at shallow liquid levels are investigated using the ORMS. A critical liquid level of 5 mm was achieved with heat fluxes of approximately 15 W/cm2, 25 W/cm2, and 35 W/cm2 over ORMS. Compared to a plain copper surface (PCS), ORMS improves the upper limit of the heat flux for the critical liquid level. A simplified and efficient analytical model with two-sphere bubble is established to predict the critical liquid level for the ORMS by considering the dynamic and static force balance. The critical liquid level is approximately twice the bubble departure diameter, which may aid in designing the liquid filling rates for phase-change heat sinks for HTC improvement.

Heat transfer augmentation in a heating surface covered by variable-shape or double-layer porous media subjected to water jet impingement

In this study, a cooling scheme for high-power and highly-integrated electronic devices is proposed in which the heating wall is covered with a variable-shape or double-layer porous medium. The porous media impacted by water jet impinging is covered over the heating surface, wherein heat from bottom wall to porous matrix is transmitted by thermal conduction and dissipated by convection generated by water jet impinging in porous layer. A combination of Brinkman–Darcy–Forchheimer model with energy equations in local thermal equilibrium is employed to describe the hydrothermal transport details in porous matrix, and the k-ε model is used to interpret water jet impinging. The impacts of porous shapes, porosity distributions, and thickness ratios among lower and upper layers with various porosities in gradient variation configuration on heat dissipation capacity are investigated numerically. The numerical calculations are well verified by published experiments. Results show that rectangular porous media covered heating surface is more sensitive to changes in jet velocity, while the mode with triangular porous media results in lower surface temperature and higher heat transfer coefficient at stagnation point. Besides, the rise ratio of heat transfer coefficient of porous configuration with gradient increase in porosity from 0.6 to 0.8 is 11.84% higher than that with gradient decrease in porosity from 0.9 to 0.6. Finally, the optimized shape, porosity arrangement and thickness ratio, jointly affect the heat transport. The results obtained can be used to generalize and apply the coupling of jet impingement with porous media above the heating surface to augment heat transfer.

Study on bubble behaviors and heat transfer at shallow liquid boiling over open rectangular microchannels

The heat transfer performance of shallow liquid boiling differs from that of deep liquid boiling owing to distinct bubble venting mechanisms. Since the liquid level is of the same order of magnitude as the bubble departure diameter, bubble detachment is restricted by the surface tension of the free interface, which causes an untimely liquid reflux toward the nucleate site. Consequently, shallow liquid boiling has a high heat transfer coefficient (HTC) at a liquid height below the critical liquid level but at the expense of limited critical heat flux (CHF). This study aims to improve the CHF of shallow liquid boiling by utilizing open rectangular microchannels surface (ORMS), which can provide separated vapor–liquid pathways to facilitate the liquid return. The boiling heat transfer curves and bubble behavior of distilled water at shallow liquid levels are investigated using the ORMS. A critical liquid level of 5 mm was achieved with heat fluxes of approximately 15 W/cm2, 25 W/cm2, and 35 W/cm2 over ORMS. Compared to a plain copper surface (PCS), ORMS improves the upper limit of the heat flux for the critical liquid level. A simplified and efficient analytical model with two-sphere bubble is established to predict the critical liquid level for the ORMS by considering the dynamic and static force balance. The critical liquid level is approximately twice the bubble departure diameter, which may aid in designing the liquid filling rates for phase-change heat sinks for HTC improvement.

Effects of adding micro/nano-scale surface roughness on upward flow boiling of R-134a through annular tubes

The enhancement of heat transfer efficiency is crucial for development of high-performance thermal systems in various industrial applications. This study investigates heat transfer and pressure drop during upward saturated flow boiling of R-134a in annular tubes featuring smooth and micro/nano-scale roughened copper surfaces achieved through sandblasting and chemical etching. A high-pressure chamber with a glass side is designed to observe surface wettability and measure contact angles of R-134a on both surfaces. The annular tubes, with inner and outer diameters of 28.6 and 42 mm respectively, and a total heated length of 1000 mm, undergo varying mass flux, vapor quality, and saturation pressure within ranges of 6.73–20.19 kg m−2 s−1, 0.14–0.95, and 530–1000 kPa, alongside heat flux variations from 608 to 2432 W m−2. Results indicate that the application of micro/nano-scale roughness to copper surfaces minimally reduces the contact angle of R-134a which slightly improves its wettability. However, this modification significantly increases the heat transfer coefficient by 256 % compared to the smooth tube, albeit with an associated increase in pressure drop. Additionally, increasing mass flux and vapor quality increases both the heat transfer coefficient and pressure drop. Furthermore, higher heat flux results in a higher heat transfer coefficient with a small increase in pressure drop. Also, the experimental results are compared with some of the available correlations in the literature. The novelty and significance of this work advances our understanding of effect of surface roughness on flow boiling characteristics. Understanding these phenomena is crucial for optimizing the design and performance of heat exchangers and other thermal systems used in various industries, including refrigeration, air conditioning, and power generation.

Heat transfer augmentation in a heating surface covered by variable-shape or double-layer porous media subjected to water jet impingement

In this study, a cooling scheme for high-power and highly-integrated electronic devices is proposed in which the heating wall is covered with a variable-shape or double-layer porous medium. The porous media impacted by water jet impinging is covered over the heating surface, wherein heat from bottom wall to porous matrix is transmitted by thermal conduction and dissipated by convection generated by water jet impinging in porous layer. A combination of Brinkman–Darcy–Forchheimer model with energy equations in local thermal equilibrium is employed to describe the hydrothermal transport details in porous matrix, and the k-ε model is used to interpret water jet impinging. The impacts of porous shapes, porosity distributions, and thickness ratios among lower and upper layers with various porosities in gradient variation configuration on heat dissipation capacity are investigated numerically. The numerical calculations are well verified by published experiments. Results show that rectangular porous media covered heating surface is more sensitive to changes in jet velocity, while the mode with triangular porous media results in lower surface temperature and higher heat transfer coefficient at stagnation point. Besides, the rise ratio of heat transfer coefficient of porous configuration with gradient increase in porosity from 0.6 to 0.8 is 11.84% higher than that with gradient decrease in porosity from 0.9 to 0.6. Finally, the optimized shape, porosity arrangement and thickness ratio, jointly affect the heat transport. The results obtained can be used to generalize and apply the coupling of jet impingement with porous media above the heating surface to augment heat transfer.

Numerical investigation of the sensible heat storage in novel electrostatic precipitators with liquid-filled collection electrodes

Heat recovery from flue gas holds significant potential for energy conservation. A strategy is proposed for recovering and storing heat in electrostatic precipitators by utilizing liquid-filled collection electrodes. In the prototype, the hollow collection electrode is connected to the heat storage tank. Convective heat transfer occurs between the gas stream and the liquid contained in the collection electrode. The heat storage mechanism is revealed by numerical simulation of corona discharge, flow and heat transfer processes in a simplified electrostatic precipitator. The results indicate that both the gas-side and liquid-side heat transfer coefficients reach steady values within approximately 100 s. The buoyancy-driven liquid flow results in a heat transfer coefficient that is 28 times higher than that for gas-side forced convection. Increasing the gas inlet temperature leads to a greater buoyancy force and, consequently, a higher heat transfer coefficient on the liquid side. Both ionic wind and inlet velocity affect the gas-side heat transfer. The ionic wind disturbs the thermal boundary layer, leading to a 27.9 % increase in the gas-side heat transfer coefficient at an inlet velocity of 0.5 m/s. Furthermore, this enhancement becomes more pronounced as the inlet velocity decreases. The flow pattern of liquid in the collection electrode and heat storage tank is crucial to the heat storage process. Additionally, installing the heat storage tank above the electrostatic precipitator provides added benefits. During the process of heat storage, the largest heat resistance is found on the gas side.

Heat transfer measurement near injection hole of supersonic nozzle with a sonic jet injection

Heat transfer measurements were conducted to investigate surface heat transfer characteristics resulting from the interaction between a sonic jet and supersonic nozzle crossflow. A sonic jet was injected into the nozzle where the average Mach number is 2.88. Experiments were carried out for three different momentum flux ratios (J=0.5, 1.0, 1.5). A half-nozzle was introduced to measure heat transfer on the inner walls of the nozzle using Infrared thermography. Pressure measurements using PSP and oil flow visualization were also conducted to understand the heat transfer. From the oil flow visualization and heat transfer results, it was found that the area around the hole influenced by recirculation vortexes exhibited high heat transfer coefficients, showing up to 5 times higher values compared to the freestream. Furthermore, as the momentum flux of the jet increased, both the heat transfer augmentation area and the heat transfer coefficient also increased. To analyze the heat transfer characteristics between the nozzle and flat surface, comparisons were made using dimensionless heat transfer coefficients. The nozzle exhibited higher heat transfer in a smaller interaction area compared to that of a flat surface, but the average heat transfer coefficient near the hole was lower. Accordingly, distinct thermal treatment strategies should be employed for nozzles and flat surfaces, especially near injection holes. These insights could be valuable for the thermal design of rockets and other supersonic/hypersonic mobility systems.

Flow and Heat Transfer Experimental Study for 3D-Printed Solar Receiving Tubes With Helical Fins at Internal Surface

Abstract 3D-printing technology was applied to fabricate novel solar thermal collection tubes that have internal heat transfer enhancement fins and external surfaces with high solar absorptivity and low emissivity due to the ability to use different materials in one tube. Helical fins were selected to introduce circumferential flow and thus minimize the circumferential temperature difference of the tube that receives sunlight on one side. The structures of the helical fins were previously optimized from computational fluid dynamics (CFD) analysis with the objective of low entropy production rate by looking for high heat transfer coefficient and relatively lower pressure loss. High-temperature alloy, Inconel-718, was used to 3D print the tubes, which can resist corrosion for the potential application of molten chloride salts as heat transfer fluid. Experimental tests were carried out using water as the heat transfer fluid with the high heat flux provided by a tubular furnace heater. The tested Reynolds number ranges from 3.9 × 103 to 6.1 × 104. Heat transfer coefficients of up to 2.8 times that of the smooth tube could be obtained with the expense of increased pressure loss compared to that of the smooth tube. The total system entropy generation can be significantly reduced due to the benefit of heat transfer enhancement that is greater than the expenses of the increased pressure loss. The experimental results of the 3D-printed heat transfer tubes confirmed the CFD-based results of fin optimization. The novel heat transfer tube is recommended for application in concentrating solar power systems.

An approach for topology optimization of heat sinks for two-phase flow boiling: Part 1 – Model formulation and numerical implementation

Two-phase flow boiling in heat sinks and cold plates is attractive for various thermal management applications due to high effective heat capacity rates and high heat transfer coefficients compared to single-phase operation. In addition, multi-physics topology optimization is sought for generation of high performance thermal management components. Yet, the design of heat sinks using topology optimization has been restricted to single-phase flow physics, due to incompatibility between available two-phase flow models and traditional single-phase topology optimization approaches. Interface-capturing two-phase flow models are too computationally expensive for optimization; mixture and two-fluid models that treat the liquid and vapor phases as an interpenetrating continuum with interfacial transport phenomena captured through empirical correlations are computationally tractable for heat-sink-scale optimization. However, the penalization approaches commonly used to define the structures formed during topology optimization results in arbitrary fin and channel geometries for which a universal two-phase flow correlation is not available. To enable coupling with two-phase mixture models, this work leverages a homogenization approach to topology optimization wherein heat sink designs are represented using spatially varying microstructures with optimized dimensions. A predefined microstructure geometry is chosen for which two-phase flow correlations exist and therefore topology optimization can be performed. This work is the first to develop a topology optimization algorithm using a mixture model for simulating two-phase flow boiling to design heat sinks and cold plates. Part 1 of this study develops and implements the algorithm to investigate topology optimized heat sinks at various heat inputs, topology optimization grid sizes, and maximum vapor quality constraints. Topology optimized heat sinks designed for single-phase versus two-phase flow are compared. There are significant differences in hydraulic and thermal responses of the single-phase and two-phase designs due to high effective heat capacity rates and high heat transfer coefficients of flow boiling. A companion paper (Part 2) focuses on manufacturing, model calibration, and experimental validation of the topology optimized heat sinks under two-phase flow boiling. The algorithm demonstrated in this work extends the capabilities of topology optimization to two-phase flow physics, and thereby enables the design of various two-phase flow components such as evaporators, condensers, heat sinks, and cold plates.

The effective use of focused shadowgraphy for single bubble nucleate pool boiling investigations

Nucleate pool boiling is known for its high heat transfer coefficients. Despite being widely implemented, the prediction of nucleate pool boiling mechanisms remains complex and single bubble heat transfer analysis is helpful to simplify the problem. Owing to the interesting bubble behaviour during nucleate pool boiling, publications tend to focus on the bubble dynamics while the experimental setup is only briefly discussed. Small but critical information on the experimental setup is often omitted, which makes it challenging or even impossible to reproduce or compare experimental data. Therefore, the purpose of this study is to provide a systematic approach to using focused shadowgraphy for the investigation of single bubble dynamics using R245fa. A pool boiling experimental setup has been built and equipped with temperature, pressure and heat flux sensors, as well as a high-speed camera and light source. The nucleate pool boiling takes place at a single cavity on a copper block. The influences of the heat transfer surface area, light source, and diffuser films were investigated to provide a systematic approach to the use of focused shadowgraphy for the investigation of single bubble dynamics.

Effect of Marangoni flow during the solidification of a Fe-0.82wt.%C steel alloy

A two-phase Mixture model is proposed to simulate the liquid-solid phase transition of a Fe-0.82wt%C steel alloy under the effect of Marangoni flow. This model simplifies computations by solving a single momentum and enthalpy equation for the mixture phase using a three-dimensional finite volume method. The simulation involves solidifying a rectangular ingot (100 × 10 × 100 mm3) from the cold bottom surface towards the hot-free surface at the top. To facilitate heat exchange with the surrounding environment, a high heat transfer coefficient of h = 600 W/m2/K was applied on the bottom surface to establish an upward solidification direction. However, a lower heat transfer coefficient of 20 W/m2/K was applied on the top free surface, which was considered flat. This study aims to examine the effect of Marangoni flow generated by surface tension on flow and segregation patterns. The results show that the Marangoni flow emerges at the free surface and penetrates into the liquid depth, leading to the formation of hexagonal patterns along the liquid thickness. Upon full solidification, macro-segregation also exhibits hexagonal structures, mirroring the stationary hexagonal shapes generated by Marangoni flow.

Multi-scale modelling of boiling heat transfer: Exploring the applicability of an enhanced volume of fluid method in sub-micron scales

The advancement of technology has led to a significant increase in thermal loads, thus presenting new challenges in heat dissipation. Traditional single-phase cooling systems are often inadequate to meet these demands. As a result, phase-change technologies utilizing boiling and condensation, which can achieve high heat transfer coefficients, have garnered considerable attention. To delve into the complex physics of boiling heat transfer, researchers are increasingly turning to numerical simulation methods such as the Volume of Fluid (VOF) and the Diffuse Interface (DI) approaches. The VOF method, widely employed for macro-scale simulations ranging from micrometers to millimeters, effectively tracks bubble growth and detachment. Conversely, the DI method represents the interface as a continuous phase field and is primarily used for mesoscale simulations spanning from nanometers to micrometers. While the DI method excels in resolving mesoscale interfacial phenomena, it is computationally expensive for larger domains. Considering the strengths and weaknesses of both the VOF and DI methods, there is a growing interest in developing a multi-scale modeling approach that amalgamates their benefits. To pursue this objective, initial efforts are being made to evaluate the scaling capability of VOF towards lower spatial and temporal limits. Hence, an enhanced and customized VOF methodology has been developed within the OpenFOAM toolbox. This methodology is employed to investigate various bubble growth scenarios, progressively exploring its applicability at lower temporal and spatial scales to identify the lower limits of its application. By taking this first step towards combining the strengths of both the VOF and DI methods through a multi-scale modeling approach, the presented paper paves the way for enhancing the accuracy and efficiency of modelling approaches for boiling heat transfer while tackling a challenge associated with varying spatial and temporal scales. This endeavor not only pushes the boundaries of computational fluid dynamics but also holds promise for addressing real-world thermal management issues in diverse technological applications.

Pool boiling characteristics on the micro-pillar structured surface with nanostructure and heterogeneous wettability

The nucleate boiling phenomenon is significantly affected by the wettability of the surface because it is greatly influenced by the motion of the solid–liquid–gas triplet formed by the behavior of bubbles on the heated surface. Hydrophilic surfaces have a high critical heat flux, and hydrophobic surfaces have low onset of nucleate boiling points and high heat transfer coefficients. To design an optimal boiling surface with the advantages of both hydrophilic and hydrophobic surfaces, in this study, parametric experiments were performed according to the spacing of the micropillar, the presence or absence of nanostructures, and the presence or absence of hydrophobic treatment on the surface where the nanostructure was hierarchically modified to the micropillar array structure in which the upper surface of the pillar was hydrophobic. Using the micro-pillar fabrication process and the heterogeneous hydrophobic surface fabrication process, a micro-pillar structure with hydrophobic treatment on top of the pillar was fabricated. Pool boiling experiments were conducted on uniform hydrophilic surfaces, uniform hydrophobic surfaces, hydrophobic patterns on hydrophilic surfaces, micro-pillar surfaces, micro-pillar with nanostructure surfaces, micro-pillar surfaces with heterogeneous wettability, and micro-pillar with nanostructure surfaces with heterogeneous wettability. Based on the experimental data, we analyzed the change in boiling heat transfer coefficients and critical heat flux according to the surface characteristics. Based on experimental results, it was confirmed that the optimal boiling surface should be a surface with heterogeneous wettability, microstructure, and nanostructure.

Overview of the application of open cell foam heat exchangers

PURPOSE. Review modern highly porous cellular heat exchangers. METHODS. We conducted a broad literature review on highly porous cellular structures used as heat exchangers. We studied both domestic and foreign literature. RESULTS. We analyzed highly porous heat exchangers of various structures: stochastic (foam with open and closed cells) and ordered (honeycombs and lattices). Methods for producing open/closed cell foams and additive technologies for producing honeycomb and lattice structures have been studied. The basic properties of highly porous structures are described. The factors influencing heat transfer and hydrodynamics in highly porous cellular heat exchangers are analyzed. A review of theapplication areas of highly porous metal heat exchangers is carried out. CONCLUSION. Heat transfer and hydrodynamics in highly porous materials depend on structural parameters, such as porosity, cell size and geometry, diameter, and geometry of the strands. Increasing porosity and cell size leads to a decrease in heat transfer coefficient and pressure drop. Changing the cell geometry affects the specific surface area of the heat exchanger and the pressure drop. Cells with complex geometries, such as octet, have a large surface area and provide a high heat transfer coefficient but high resistance to coolant flow. Cells with simple geometries, such as a cube, on the other hand, provide low flow resistance and low heat transfer coefficient. In general, any structural parameter change affects heat transfer and hydrodynamics.

Experimental study on transpiration cooling through additively manufactured porous structures

Film cooling and thermal barrier coating are the most common external cooling techniques used for gas turbine components. In film cooling, coolant air is brought to the external surface via rows of discrete holes. This study investigates an alternative geometry, referred to as transpiration cooling. Here, the coolant air is brought to the external surface through an additively manufactured porous structure. An experimental study was performed to characterize the downstream cooling performance by obtaining film cooling effectiveness and heat transfer coefficient using thermochromic liquid crystals. The experimental setup was first validated using a cylindrical film cooling sample and the results were compared to results obtained from previously published film cooling studies. After validation, transpiration cooling samples were investigated. A total of three porous structure samples with different thicknesses were subjected to testing. The thickness of the porous structure had insignificant effect on both the heat transfer coefficient and the film cooling effectiveness. The samples were tested at blowing ratios ranging from 0.5 to 2.0. Results showed an increase in laterally averaged heat transfer coefficient with increasing blowing ratio and compared to cylindrical film cooling, transpiration cooling yielded significantly higher distributions. The spanwise averaged film cooling effectiveness displayed a similar trend with higher values for transpiration cooling, mainly due to a more even mixing in the lateral direction. Values of the heat flux ratio showed that the high heat transfer coefficient penalizes the porous cooling method and only for low blowing ratios may transpiration cooling be the beneficial choice. From visualizations with water and carbon dioxide, it could be concluded that the inclination angle is close to 90° which yields a chaotic flow field and, in turn, high values of the heat transfer coefficient. By changing the additive manufacturing's build direction, a more favorable inclination angle should be achievable and, thereby, an opportunity to utilize the porous structure's excellent mixing in the spanwise direction.

Elucidating the effects of surface wettability on boiling heat transfer using hydrophilic and hydrophobic surfaces with laser-etched microchannels

This study explores the nuanced interplay between surface wettability and morphology in nucleate boiling heat transfer. Surfaces with identical microstructures but varying wettability are created on copper substrates using laser texturing and a hydrophobic agent. The spacing between laser-etched microchannels is adjusted to achieve different contact angles and percentages of laser-textured areas. Boiling performance is assessed with variations in channel spacing and surface coverage, revealing significant differences between hydrophilic and hydrophobic surfaces. The critical heat flux (CHF) enhancement mechanisms are identified, emphasizing either enhanced liquid replenishment or separate boiling and liquid-supply regions. A critical laser line spacing of 100 μm is determined, where hydrophobic and hydrophilic variants exhibit similar CHF. Interestingly, hydrophilic surfaces show a weak correlation between the heat transfer coefficient (HTC) and contact angle, while hydrophobic surfaces demonstrate a strong correlation. This separation of surface morphology and wettability confirms that lower wettability promotes early nucleate boiling and high HTC, whereas hydrophilic variants contribute to enhanced CHF with lower HTC. These distinctions are most prominent at high coverage with laser-textured areas and diminish as coverage decreases. The findings provide valuable insights into optimizing surface properties for efficient boiling heat transfer applications.

Wise Choice of Showerhead Patterns: How to Save Energy during Showering While Maintaining Thermal Comfort

Heat transfer coefficients between shower water and human skin could significantly impact occupants’ thermal sensation and energy consumption during showering. A recent study found that heat transfer coefficients varied considerably among showerhead patterns. However, the specific effects of the showering heat transfer process on the showerhead patterns have yet to be determined. Two experiments were conducted to quantify the spray patterns during showering, and the impacts of the patterns’ parameters on the heat transfer coefficient were examined using different statistical methods. Five showerheads with 18 spray patterns were tested in this study. The resistance factor, water supply pressure, and nozzle area ratio of these patterns were measured to qualify their shower performance. The results indicated that all the tested parameters significantly impacted the heat transfer coefficient in general, and using resistance factor and nozzle area ratio could accurately predict the heat transfer performance of the showerhead pattern. Additionally, this study demonstrated that changing to a showerhead with a higher heat transfer coefficient could save considerable energy while maintaining the same thermal sensation during showering. The influence of water spray patterns on the heat transfer coefficient could provide residents with scientific references when selecting showerheads in their bathrooms.

Numerical investigation and data-driven prediction of flow boiling heat transfer in manifold microchannels with slopes

The manifold microchannels (MMC) with high heat transfer coefficient and low pressure drop has received much attention. Flow boiling heat transfer inside MMC is promising for dissipating heat of high-power devices. In this study, flow boiling heat transfer inside MMC is further numerically studied with slopes added in the upstream and downstream corners of MMC. The results show that downstream slope leads to lower local temperature and thermal resistance by removing the upstream vapor more easily, and upstream slope leads to higher heat flux corresponding to the turning point in the flowing boiling curve by restraining the local high temperature at upstream corner. The combination of the upstream and downstream slopes can reduce the thermal resistance by 5.45% and increase the heat flux related to the turning point by 14.28% compared to the MMC without slopes. A new flow boiling heat transfer correlation considering the slopes is developed with prediction accuracy of mean absolute deviation (MAD) of about 0.05%. An artificial neural network (ANN) is further built to predict the vapor saturation inside the MMC based on the outside temperature at the MMC bottom surface, which presents high accuracy of 97.03%.