Plain Copper Research Articles

Experimental study on the effect of Tin-doped copper oxide capillary-porous surfaces on pool boiling heat transfer performance

The novel tin-doped copper oxide capillary-nano-porous surfaces were created using the easy and affordable sol-gel method. The pool's boiling incipience wall superheat on a capillary-nano-porous surface was lower than a non-coating surface. Bubble flow visualization investigation demonstrates that the coating's shape can significantly affect the processes involved in improving heat transmission. An investigation of the nano-porous surface's long-term stability was conducted using water. Repeated cycles of studies on created nano-porous surfaces revealed a little divergence in wall superheat and surface shape. The present study's tin-doped copper oxide coated surface (Sn-CuO-600) has a higher CHF (critical heat flux) and HTC (heat transfer coefficient). Heat transmission by boiling rises when a substantial quantity of bubbles are rapidly evacuated from the heating exterior. It is observed that a greater quantity of tiny, spherical bubbles are continually growing on the superhydrophilic Sn-CuO-600 surface. Compared to the plain copper surface, the Sn-CuO-600 surface experiences a significant reduction in bubble departure times. The rate of bubble formation is increased by superhydrophilic surfaces, which also reduce bubble dimensions and release times. Consequently, the Sn-CuO-600 surfaces exhibit superior heat transfer ability throughout boiling.

Experimental Investigation of Evaporation of Methanol and n‐Pentane on a Submerged Capillary Structure

AbstractThe heat transfer limit of heat pipes is investigated by pool boiling experiments in a standard apparatus with a capillary layer placed on a copper tube. The capillary layer is a hollow cylinder made of bronze with a wall thickness of 2 mm and the test fluids are methanol and n‐pentane. The setting is comparable to high‐flux tubes already investigated in the literature. In dependence on the heat flux, three to four sections with different characteristics are identified. Compared to plain copper tubes, the heat transfer is partially increased by a factor of 12 until the limit is reached at around 70 kW m−2.

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.

Speleothem-Inspired Copper/Nickel Interfaces for Enhanced Liquid-Vapor Transport by Marangoni and Soret Effects.

Geological formations have superior wickability and support the absorption of water and oils into narrow spaces of Earth's crust without external assistance. In this study, we present speleothem inspired heterogeneous porous and wicked copper (Cu)/nickel (Ni) interfaces for enhanced nucleate boiling of water/ethanol mixtures for energy-efficient separation processes. The incorporation of Ni strands within the copper particle matrix significantly enhanced heat transfer. Compared to plain copper, the Cu/Ni speleothem surfaces exhibited a 61% increase in the heat transfer coefficient for water/ethanol mixtures and a 332% increase for water, with a 58% faster onset of nucleate boiling. This enhancement was attributed to Marangoni and Soret effects at the Cu/Ni interfaces, driven by surface tension and concentration gradients. Furthermore, the synergistic wicking action of the Ni strands facilitated rewetting of the surface, replenishing liquid to the porous nucleation sites and preventing surface dry-out, thereby improving the overall heat transfer performance.

Saturated water flow boiling heat transfer and pressure drop in scalably microstructured plain and finned copper tubes

Well established understanding of flow boiling heat transfer and its hydrodynamic driving mechanisms have led to the inception of surface geometry augmentation techniques that can enhance thermal-hydraulic performance. Enhancing the flow boiling heat transfer at a reasonable pressure drop enables the deployment of more compact and efficient heat exchangers and thermal systems. To enable ease of manufacturing, simplicity, and reliability, passive augmentation techniques that do not require external stimuli are preferred. In this study, a scalable copper microstructuring technique was developed on both smooth and internally finned 0.25” tubes to investigate how it affects the thermofluidic behavior of relatively higher surface tension water during saturated flow boiling. The internally finned tubes used in this study comprised of 50 fins with a helical angle of 13°. To apply the conformal microstructures on the internal tube surfaces, chemical oxidation via an alkaline mixture was used, resulting in structures having characteristic length scales approaching 200 nm −1 µm. Our results show that microstructuring of plain copper tubes provides a water flow boiling heat transfer coefficient enhancement ranging from 10 % to 90 % compared to untreated specimens for vapor qualities ranging from 0 to 0.1 and mass flow rates ranging from 255 to 485 kg/(m2·s). Augmentation is attributed to the increased roughness of the plain copper tubes that creates additional nucleation sites for enhanced liquid entrapment and increases wettability. Associated with this heat transfer augmentation is an increased pressure drop ranging from 0.5 % to 38 % relative to untreated tubes due to the increased skin friction as well as add fluid flow disruption. Overall, the performance factor ranged between 0.86 to 1.81 demonstrating that the augmentation in heat transfer overshadowed the increase in pressure drop. Interestingly, applying the conformal microstructures on internally finned copper tubes decreased the heat transfer coefficient and pressure drop by as much as 75 % and 45 %, respectively, when compared to the un-structured finned copper tubes counterparts. The decrease in heat transfer performance and pressure drop was attributed to partial wetting of the fin roots due to the presence of a vapor layer near the inner tube walls. Our work develops a mechanistic understanding of the role that microstructures play during flow boiling of water on industrially applicable smooth and finned tube geometries and designs.

Pore scale analysis for flow and thermal characteristics in metallic woven mesh

Metallic woven mesh screens are representative porous media with periodic structures, which have great application prospects in emerging technology and industrial equipment. However, the effect of their pore structure on macroscopic properties is not clear, which limits their large-scale engineering application. In this paper, to investigate the influence mechanism of stacking patterns and wire contact conditions on flow and heat conduction from the pore scale, a representative elementary volume model of the square-shaped plain copper micromesh was reconstructed according to the geometric parameters obtained by the optical microscope and porosity test experiment. Furthermore, based on the understanding of microscopic mechanism, the theoretical calculation model for macroscopic characteristics was proposed. The results show that the flow characteristics in meshes are mainly affected by stacked patterns. When ReK is 0.08 and 0.28, the flow in single layer and staggered stacked woven meshes transitions from Darcy flow to Forchheimer flow. In Forchheimer flow regime, neglecting the inertia force causes a maximum 50 % error for permeability calculation. The permeability empirical correlations for Darcy and Forchheimer flow are proposed according to Kozeny-Carman and Kozeny-like model. The z-direction effective thermal conductivity of copper woven meshes is significantly affected by wire contact conditions, the maximum value is from 2.5 to 13.5 W/(m·K). The z-direction effective thermal conductivity of tangent and gap contact conditions can be described by the ZBS model, while the random model is suitable for that of overlapped contact conditions. The x-direction thermal conductivity is little affected by contact conditions, the maximum value is 50.3 W/(m·K), but the tortuosity effect of copper wires should be considered for calculation. The calculation model of anisotropic thermal conductivity for single-layer woven meshes is established according to Kirchhoff's law, which is also extended to the staggered stacked woven meshes by introducing a compactness factor.

Molecular dynamics simulation of argon pool boiling: A comparative study of employing nanoparticles and creating tree-root type nanostructures

This paper is aimed to investigate the effectiveness of innovative nanostructured surfaces in terms of heat transfer and evaporation rate enhancement during the pool boiling process of argon atoms by employing molecular dynamics simulation. LAMMPS software and Lennard-Jones potential are used for the simulation and determination of the force field. The fundamental thought of creating these novel geometries (tree-root type nanostructures) is to use the branches as a barrier against liquid cluster separation, which leads to fluid temperature enhancement. First, the framework of simulation is validated, and then the results of creating two types of copper tree-root nanostructures are presented and compared with three cases which include: adding platinum, aluminum, and copper nanoparticles to the argon fluid on the plain copper substrate. The results revealed that creating tree-root type nanostructures postponed the separation of the liquid cluster in comparison with adding nanoparticles. In addition, the tree-root type nanostructures can enhance the evaporation rate and the argon temperature up to 60.05% and 17.13% more than adding nanoparticles, respectively. On the other hand, these nanostructures improve the maximum heat flux and corresponding convective heat transfer coefficient by 15.46% and 26.86% compared to the cases of adding nanoparticles, respectively.

Heat Transfer Characteristics of Pool Boiling with Scalable Plasma-Sprayed Aluminum Coatings

To ensure adequate reliability in two-phase cooling systems involving boiling, it is essential to enhance the heat transfer coefficient and maximize the critical heat flux (CHF) limit. A key technique to avoid surface burnout and increase the CHF limit in pool boiling is the frequent coolant supply to the probable dry-out locations. In the present work, we have explored the plasma-spray coating as a surface modification technique for enhancing heat transfer coefficient and CHF value in pool boiling applications. Three plasma-coated aluminum surfaces (C-15, C-20, and C-25) are fabricated on a copper substrate at three different plasma powers of 15, 20, and 25 kW, respectively. Detailed surface morphologies of the plasma-sprayed coatings are presented, and their roles in pool boiling heat transfer mechanisms are analyzed. Plasma-coated surfaces exhibit wickability characteristics and enhanced wettability compared to the plain copper surface. Saturated pool boiling experiments are performed with DI (deionized) water at atmospheric pressure. Plasma spray-coated surfaces show favorable boiling incipience with less wall superheat and more active nucleation sites than the plain copper surface. Compared to the plain copper surface, enhancement values of nearly 68, 60.7, and 55.5% in the heat transfer coefficient are observed for C-15, C-20, and C-25 plasma-coated surfaces, respectively. Experiments could not be performed beyond the heat flux of 197 W/cm2 due to repeated failure of the cartridge heaters. Based on the experimental measurement of wickabilities, the CHF values of plasma-coated surfaces have been theoretically calculated. Compared to the plain copper surface, a maximum 2.39 times higher CHF value is observed for C-15 plasma-coated surface. Improved wettability and wickability are responsible for CHF enhancement in the case of plasma-coated surfaces. At higher heat flux, capillary wicking and frequent rewetting of the dryout locations delay the burnout phenomenon, enhancing CHF in plasma-coated surfaces. The plasma-spray coating is a robust and scalable process, which can be a potential candidate for high heat flux dissipation in various industrial applications.

Experimental investigation and modeling of falling film heat transfer on partial dry-out condition

Falling film heat transfer has been applied for decay heat removal in more and more nuclear reactors, in which the coolant feed rate is set small to prolong the effective cooling time during accident conditions. Previous works have shown that the cooling surface is under partial dry-out condition for most of the time during the accident, while the existing falling film experiments are mainly focused on fully-wetted condition. In this paper, falling film experiments were conducted on a horizontal plain copper tube within 0.019–0.099 kg/m·s coolant flow rate, 0–164.1 kW/m2 heat flux, and dry-out condition is emphasized. In the experiments, it is found that the overall liquid coverage rate measured by infrared equipment was between 45.2 % and 100 %, and the dry-out process is recorded and analyzed as well. Moreover, HTC (Heat Transfer Coefficient) at wetting area is defined to characterize the heat transfer on the surface covered by liquid and is obtained based on a three-dimensional heat conduction model of test tube. Boiling HTCs on falling film are found 10 % approximately higher than that in pool boiling condition. The difference is attribute to the different mechanism of bubble departure and bubble growth environment, with which the falling film condition is found to result in a 1–3 times bubble departure diameter and a 0.85 times bubble growth rate compared with pool boiling condition. Finally, a general falling film heat transfer model is developed based on the conservation equations with consideration of liquid coverage, gas–liquid interface evaporation and liquid sub-cooling. The model shows effective for falling film heat transfer prediction on both fully-wetted and dry-out, convection and boiling conditions with different working fluids through the validation with a number of experimental data. The findings in this work could be helpful for the design and analysis of nuclear cooling system, as well as the falling film evaporation system.

Bubble growth on a smooth metallic surface at atmospheric and sub-atmospheric pressure

Bubble growth rate is one of the most important parameters required for the development of accurate mechanistic nucleate boiling heat transfer models. It is also very important for understanding the hydrodynamic forces and the mechanism of bubble departure. This paper presents an experimental study on bubble growth measurements in saturated pool boiling of deionized water on a plain copper surface at atmospheric and sub-atmospheric pressure. The measurements were conducted using a high-speed, high-resolution camera with a microscopic lens. The mechanisms of bubble growth are discussed, while the microlayer evaporation mechanism has been evaluated and discussed using the measured bubble growth curve. The estimated contribution of microlayer evaporation to a single bubble growth is about 70 %, while the contribution of latent heat transfer (evaporation) to the total heat transfer rate from the surface is about 30 %. The remaining 70 % is due to other mechanisms, i.e. conduction and convection. These values were obtained based on the analysis of the bubble growth curve only and agreed with some researchers who conducted local heat transfer measurements using integrated sensors or infrared thermography. These detailed measurement techniques cannot be used with the thick copper block tested in the current study, which was also tested by many researchers in literature and is representative of industrially used surfaces. It was also found that the bubble departure mechanism at atmospheric pressure is due to a static balance between surface tension and buoyancy forces while at sub-atmospheric pressure, it was between buoyancy and liquid inertia forces. The pressure did not have a significant effect on the characteristics of the dynamic contact angle, which was also measured from the instantaneous images of the bubble. It was concluded also that the force balance required for the accurate prediction of departure diameter should be conducted when the two forces are equal, which occurred at time less than the departure time and dynamic contact angle of about 45. In most bubble departure models, researchers recommended the balance to be conducted at the moment of departure when the bubble forms a neck with contact angle of 900 (underestimation to the surface tension force). The analysis of one of the commonly used homogeneous growth models indicated that for homogeneous bubble growth models to be applicable in nucleate boiling, an allowance must be made for the fact that the degree of superheat varies with time during a bubble growth period.

Bubble growth models in saturated pool boiling of water on a smooth metallic surface: Assessment and a new recommendation

Prediction of bubble growth rate is very important for the development of accurate models for bubble departure diameter and thus the heat transfer rates in nucleate boiling. This paper presents an evaluation study to the existing homogeneous and heterogeneous bubble growth models using our experimental data for bubble growth in saturated pool boiling of deionized water on a plain copper surface. The experiments were conducted at pressures 1, 0.5 and 0.15 bar and superheat in the range 5.1 – 19.5 K. To start with, the paper presents a critical review on bubble growth models in homogeneous and heterogeneous boiling. It was found that homogeneous growth models achieved some partial agreement with the experimental data at some conditions and thus they should be used carefully in heterogeneous boiling. There was a good agreement between some of the models that were suggested based on the assumption that bubble growth occurs due to evaporation from the superheated boundary layer around the bubble. The models based on microlayer evaporation only could not explain the experimental data, i.e. partial agreement at some conditions. The model that predicted the data very well at all conditions was the “relaxation boundary layer” model by Van Stralen [25]. This model was generalized in the current study by suggesting two new empirical models for the departure diameter and departure time.

Experimental study of heat transfer in a microchannel with pin fins and sintered coatings

ABSTRACT Increasing processing capacity without modifying the size of electronic devices has made thermal management important in the electronic industry. Commercialized thermal management, such as in a conventional air cooling system, is insufficient for electronic devices with high heat flux dissipation. Pool boiling is a better method for heat transfer because it can dissipate a substantial amount of heat at low wall superheats. This study focused on heat transfer enhancement using passive approaches, including nanostructures and microporous sintered surfaces over open microchannel surfaces and microchannels with pin-fins. In the present work, seven structures were studied in pool boiling, wherein experiments elucidated the effects of microchannels, sintered, and pin fins (micropillar) on boiling heat transfer from a copper chip in a pool of degassed water. Boiling performance is ascertained via critical heat flux (CHF) and heat transfer coefficient (HTC). The best heat transfer performance showed a heat flux of 243.75W/cm2 at 15.46°C on the pin-fins chip, which was 1.9 times the heat flux of the plain chip. The highest HTC was 181.03 kW/(m2 oC) at a heat flux of 172.61 W/cm2 for the microchannel with single pin-fins. The HTC enhancement was 2.8 times greater than the plain surface. It was found experimentally that HTC and CHF improved on all modified surfaces compared to the plain copper chip baseline.

Saturated pool boiling heat transfer enhancement of R245fa based on the surface covered by sintered copper powder with and without nanostructure

The pool boiling characteristics on the enhanced surfaces with sintered copper powder coatings with and without nano-treatment were experimentally investigated. Three kinds of enchanted surfaces formed by sintered porous layers with copper powder particles and their further nano-treated surfaces were adopted as the test surfaces. The thermal oxidation process was used to form nanostructures with super hydrophilic characteristic on the surface of sintered copper particles. Refrigerant R245fa was selected as the test fluid due to the potential the application in electronic cooling field. The saturation temperature of boiling experiments was maintained about 20 °C. The result showed that the boiling heat transfer performance was effectively improved by coating sintered copper powder porous layer on the plain copper surface, and the maximum heat transfer coefficient and CHF value enhancement could reach 151% and 160% of the plain one, respectively. The enhanced surfaces no matter with or without nano-treatment could trigger the nucleation process at the wall superheats lower than 12 °C rather than the value as high as 30 °C for plain copper surface. After nano-treatment, the heat transfer enhancement for sintered porous surfaces was further improved at medium and low heat flux, where the bubble nucleation or detachment remained robust. The CHF peaked as 140% of untreated surface, while the heat transfer coefficient increased to 108% at most.

Influence of TiO2 thin film on critical heat flux and performance enhancement of pool boiling at atmospheric pressure

An increase in the rate of heat generation due to the compactness and performance enhancement of electronics equipment has been observed in the recent years. Though the efficient heat recovery to improve safety and to increase the life span of the equipment is a challenging task, Pool boiling is considered as a technique to recover an abundant quantity of heat at minimum wall superheat. The performance and safety factors are explored using boiling heat transfer coefficient and critical heat flux. The present investigation is carried out on a plain copper surface and copper surface coated with TiO2 thin film with three different thicknesses, namely 250, 500 and 750 nm. This work also explores the influence of wettability and roughness of the TiO2 thin film coated surfaces. The morphological and structural studies are conducted by scanning electron microscopy and x-ray diffraction scanning electron microscopic patterns. The critical heat flux of TiO2 thin film coated surface at 750 nm thicknesses was better augmented by 69% than the plain copper surface. The heat transfer coefficient was better augmented by 117% than the uncoated copper surface. The excellence in hydrophillic nature of the TiO2 thin film creates numerous nucleation sites which in turn enhance the capillary wicking. The continuous wetting and rewetting characteristics of the TiO2 thin film enhance the critical heat flux and boiling heat transfer coefficient.

Pool boiling heat transfer characteristics of a stepped microchannel structured heating surface

Structured heating surfaces modify the bubble ebullition cycle in pool boiling, thereby enhancing heat transfer. The present work explores an innovative structured heating surface for heat transfer enhancement in pool boiling. Two types of microstructured, i.e., stepped microchannels (SMC) and stepped-segmented microchannels (SSMC) surfaces, have been fabricated on a heating surface, and their heat transfer characteristics are experimentally investigated. Saturated pool boiling experiments are performed with deionized water at atmospheric pressure, and the performance of both the structured surfaces is compared with a plain copper heating surface. All three heating surfaces are fabricated using a wire-EDM process with the same footprint area of 10 × 10 mm2. The SSMC structured surface exhibits the maximum heat transfer coefficient (HTC) and results in 296% heat transfer enhancement compared to the plain copper surface. The surface also makes a 224% enhancement in the critical heat flux (CHF) compared to the plain copper surface. Compared to the plain copper surface, the SMC structured surface shows an enhancement of 200% in HTC and 162% in CHF. For the same operating conditions, the SSMC structured surface shows better pool boiling heat transfer characteristics than the similar structured surfaces reported in the literature. More nucleation sites, better bubble evolution along the expanding cross-section stepped channel, and efficient rewetting of the heating surface contribute to the excellent thermal performance of the SSMC structured surface.

UV illumination control and enhancement of heat transfer during pool boiling process

An increasing number of studies have focused on controlling the boiling flow field using external effects. This study is the first to propose a transition of bubble nucleation by triggering titanium dioxide (TiO2) surfaces with ultraviolet (UV) illumination during the pool boiling process. In the presence of UV light, the wettability of the TiO2 surface changed from hydrophobic to hydrophilic owing to photocatalysis. An opportune moment to activate the surface wettability transition using UV illumination occurred during the boiling process of isolated bubbles. High-speed camera images showed that the transition of the surface wettability with UV illumination changed the liquid vapor dynamics in the pool and affected the boiling heat transfer performance. The TiO2-coated surfaces of the test samples, upon UV illumination, showed an enhancement of 72% in the critical heat flux (CHF) and 49% in the heat transfer coefficient (HTC) compared with plain copper surfaces.

Influence of bubble departure control on nucleate pool boiling heat transfer of electrodeposited copper foam: Experiments and correlation

Pool boiling is highly suitable for cooling in various industries due to its high heat transfer capability. Among pool boiling enhancement methods, embedding open-cell metal foams to the heater surface are particularly beneficial as they increase the effective heat transfer area. In this study, by analyzing 30 samples, the effect of pore density, thickness, and channels with different diameters on copper metal foams were investigated. In addition, the electrodeposition method was used to deposit a porous layer on the edges of copper metal foam to increase boiling efficiency in atmospheric conditions with deionized water as a working fluid. As an innovation, holes with different diameters were created inside the specimens using machining to construct a suitable exit path for the bubbles. According to the results, creating a channel with a diameter of 3 mm inside foams with thicknesses of 8 mm and 10 mm, both in the untreated and electrodeposited modes, improved the heat transfer coefficient (HTC) compared to foam without the channel. On the other hand, a 5 mm diameter channel decreased boiling efficiency due to a substantial reduction in effective surface area compared to a 3 mm channel. At low heat fluxes, the electrodeposited sample with a pore density of 20 ppi, a thickness of 10 mm, and a channel diameter of 5 mm achieved an HTC of 13.56 W/cm2. K that was approximately 3.5 times higher than the plain copper sample. A high-speed camera was utilized to observe the bubble departure from the channels during changing the heat flux. Finally, by considering the dimensionless parameters of the problem, an experimental correlation was derived to predict the HTC values.

Visualizing and disrupting liquid films for filmwise flow condensation in horizontal minichannels

This paper investigates the effects of hemispherical mounds on filmwise condensation heat transfer in micro-channels. Also investigated were the impacts that spatial orientation of the three-sided condensation surface (i.e., gravitational effects) on steam condensation, where the cooled surfaces were either the lower surface (i.e., gravity pulls liquid towards the condensing surfaces) or upper surface (i.e., gravity pulls liquid away from the condensing surfaces). Two test coupons were used with 1.9-mm hydraulic diameters and either a plain copper surface or a copper surface modified with 2-mm diameter hemispherical mounds. Heat transfer coefficients, film visualization, and pressure drop measurements were recorded for both coupons in both orientations at mass fluxes of 50 kg/m2s and 125 kg/m2s. For all test conditions, the mounds were found to increase condensation heat transfer coefficients by at minimum 13% and at maximum 79%. When the test section was inverted (i.e., condensing surface on the top of flowing steam), minimal differences were found in mound performance, while the plain coupon reduces heat transfer coefficients by as much as 14%. Flow visualization suggests that the mounds enhanced heat transfer due to the disruption of the film as well as by reducing the thermal resistance of the film. Pressure drops followed parabolic behavior with quality, being higher in the mound coupon than the plain coupon. No significant pressure drop differences in the inverted orientation were observed.

Experimental study on saturation pool boiling heat transfer characteristics of R245fa on the surface covered by sintered copper powder

In this paper, the mechanism of pool boiling heat transfer enhancement on the surface covered with sintered copper powder porous layer has been investigated. The refrigerant R245fa with great application prospect in electronic cooling field was selected as the test fluid. According to the results, the boiling heat transfer performance of R245fa could be effectively enhanced by attaching the sintered copper powder porous layer to the plain copper surface. Even when the wall superheat was as low as 2.2 °C, the sintered copper powder surface with the particle size of 110 μm and thickness of 0.6 mm could still be efficiently triggered to boiling. The heat transfer coefficient and CHF were significantly improved up to 247.71% and 255.15% compared with that of plain copper surface, respectively. However, with the thickness of porous layer increased from 0.6 mm to 0.9 mm, the maximum heat transfer coefficient for the enhanced surface with particle size of 30 μm went up to 136.01%, while the CHF value decreased by 92.52%. For 65 μm and 110 μm surfaces, the change of porous layer thickness had little effect on the heat transfer coefficient in the initial boiling stage, but presented the contrary influence trend at high heat flux conditions.

Physical mechanisms for delaying condensation freezing on grooved and sintered wicking surfaces

Heat pipes are passive heat transfer devices crucial for systems on spacecraft; however, they can freeze when exposed to extreme cold temperatures. The research on freezing mechanisms on wicked surfaces, such as those found in heat pipes, is limited. Surface characteristics, including surface topography, have been found to impact freezing. This work investigates freezing mechanisms on wicks during condensation freezing. Experiments were conducted in an environmental chamber at 22 °C and 60% relative humidity on three types of surfaces (i.e., plain copper, sintered heat pipe wicks, and grooved heat pipe wicks). The plain copper surface tended to freeze via ice bridging—consistent with other literature—before the grooved and sintered wicks at an average freezing time of 4.6 min with an average droplet diameter of 141.9 ± 58.1 μm at freezing. The grooved surface also froze via ice bridging but required, on average, almost double the length of time the plain copper surface took to freeze, 8.3 min with an average droplet diameter of 60.5 ± 27.9 μm at freezing. Bridges could not form between grooves, so initial freezing for each groove was stochastic. The sintered wick's surface could not propagate solely by ice bridging due to its topography, but also employed stochastic freezing and cascade freezing, which prompted more varied freezing times and an average of 10.9 min with an average droplet diameter of 97.4 ± 32.9 μm at freezing. The topography of the wicked surfaces influenced the location of droplet nucleation and, therefore, the ability for the droplet-to-droplet interaction during the freezing process.