Evaporator Section Research Articles

Surface wettability modifications and applications in wickless heat pipes

Proper methods of surface fabrication and characterization are essential for various applications. For example, wickless heat pipes, such as flat plate pulsating heat pipes (FPPHP) and a compact flat thermosyphon (CFT), have massive potential for commercialization owing to simple construction, flexibility in using different working fluids, and better thermal contact with flat heat sources. In this work, various surface treatment methods such as chemical etching, electrochemical deposition, and laser texturing on a copper sample are investigated to prepare superhydrophilic or superhydrophobic surfaces, and the thermal durability of the as-prepared surfaces is tested through heating and boiling experiments for 15 days. The wettability modification methods are implemented on the surfaces of the FPPHP and CFT, with the different wettability combinations as either bare evaporator and condenser, completely superhydrophilic (that is, all surfaces including that of evaporator, condenser, and adiabatic section) or completely superhydrophobic or hydrophilic evaporator and superhydrophobic condenser. The thermal performance of the FPPHP and CFT subjected to different wettability combinations on the surface is investigated. Results of FPPHP are reported for water as the working fluid at 60 % filling ratio with power inputs given from 40 to 200 W. Similarly, for CFT, the results are reported for water filled at 50 % filling ratio with heat flux input from 2 to 20 W/cm2. On the FPPHP, the best thermal performance and lowest resistance were obtained when the entire surface was superhydrophilic with a maximum reduction of 60 % at 200 W. The CFT gives the best thermal performance with a bare evaporator and superhydrophobic condenser with a maximum decrease in thermal resistance of 42 % at 20 W/cm2. The best-performing wettability combination for FPPHP and CFT is contrasting and has to be chosen appropriately depending on the heat transfer device.

EXPLORING THE HEAT TRANSFER PERFORMANCE OF CYLINDRICAL HEAT PIPES WITH VARIED MOLAR Ag-DOPED GO HYBRID NANOFLUIDS - AN EMPIRICAL STUDY

In our modern era of technological advancements, the widespread use of high-powered electronic and electrical devices has resulted in the release of excessive heat, posing a threat to the environment. To address this issue, heat pipes have emerged as a promising solution for effective heat removal. As researchers strive to improve their performance, various approaches have been explored. This study took a unique approach by investigating the performance of miniature cylindrical copper sintered heat pipes using different molar concentrations (0.03 M, 0.06 M, and 0.09 M) of Ag-doped GO hybrid nanofluids, with a weight percentage of 0.05 wt.% as the working fluid. The study carefully assessed the heat transport capabilities of these nanofluids within the heat pipe, considering a range of heat inputs (10-130 W) in the evaporator section and different mass flow rates (8.33 g/s, 16.66 g/s, and 25 g/s) of the cooling fluid in the condenser section. The findings revealed that the 0.09-M Ag-GO nanofluid demonstrated superior performance, with a remarkable 50.48% reduction in thermal resistance and a significant 27.52% improvement in wall temperature distribution, particularly at a mass flow rate of 16.66 g/s and a heat input of 100 W. It reveals that altering the molarity of nanoparticles in hybrid nanofluids has the potential to enhance the heat transport abilities of heat pipes.

The DEPLOY! Project: Development of a Deployable Pulsating Heat Pipe experiment on a parabolic flight

DEPLOY! Project aims at analysing the behaviour of Deployable Pulsating Heat Pipe (PHP) shaped as a helicoidal torsional spring in the adiabatic section on board a Parabolic Flight platform. A PHP is a passive thermal control device where the heat is efficiently transported by means of the self-sustained oscillatory fluid motion driven by the phase change phenomena. The microgravity environment allows to eliminate the buoyancy force contribution in the liquid phase momentum. Consequently, it is possible to isolate the contribution of the pressure drop caused by the 3D arrangement and infer on their effect on the PHP performance. As a result, a proper design based on the previous considerations would increase the flexibility of the PHP for use in space applications without significant reductions in efficiency. The goal of DEPLOY! is to demonstrate the functionality of a Deployable Pulsating Heat Pipe in various unfolding configurations by analysing its thermal-hydraulic response throughout a Parabolic Flight. The presented Deployable PHP is composed of an aluminium tube (inner/outer diameters 1.6mm/2.6 mm) and filled with HFE-7000. It is heated at the evaporator using a flat heater and cooled at the condenser with a water-cooled cold plate. T-type thermocouples are used to measure the wall temperature in several locations, while two pressure transducers (one located in the evaporator section and another in the condenser section of the same pipe) measure the local fluid pressure. Additionally, an IR Camera will be used to observe a section of the pipe for further analysis of the flow frequency. The device operation will be tested on ground and 0-g at different heat loads (24W, 34W), in multiple static positions corresponding to different opening angles (0°, 45°, 90°, 135°, 180°) and during its dynamic opening from 0° to 180°, thanks to a remotely controllable motion system.

Using Spatial Modeling to Evaluate LED Light Intensity and Uniformity in Commercial Broiler Houses during Brooding and Tunnel Conditions

Highlights During brooding, more than 97% of the floor area of new broiler houses was above target light intensity, however, in aged houses less than 0.2% of the floor area was above the target. During tunnel conditions, less than 6% of the floor area met the target light intensity range (0.18–0.22 lx) for both aged and new houses. All floor areas in the fan section exceeded the target light intensity in both aged and new houses in all the farms due to light intrusion from the tunnel fans. Bulbs need to be periodically replaced to meet minimum target levels during brooding and light dimmer adjustments need to be made to meet light intensity targets during tunnel ventilation. Abstract. Commercial broiler companies set specific light intensities and photoperiods during different stages of live production to manage bird behavior and growth and to support health and welfare. However, it is currently unknown how much of the floor area in commercial houses is within the target levels set by companies. Therefore, a field survey of twenty 18.3 × 152.4 m broiler houses on five farms was performed to evaluate in-situ light intensity and uniformity in two house sections during brooding [evaporative pad section (pad) and the middle of the house section (mid)] and three sections during tunnel ventilation [pad, mid, and tunnel exhaust fan section (fan)]. Houses ranged in age from new (never used) to aged (5–8 years old). Light intensity measurements were recorded at 70 locations in each of the house sections (140 per house during brooding; 210 per house during tunnel). For tunnel mode, target light intensity values were categorized into above target (>0.22), within target (0.18–0.22 lx), and below target (<0.18 lx). For brooding mode, target light intensity values were below target (<43 lx) and within target (=43 lx). Geostatistics was used to map and calculate the percentage of floor area above, below, or within target thresholds. During brooding and tunnel conditions, the percent of floor area below target (<43 lx and <0.18 lx, respectively) was higher for aged houses than for new houses in the pad and midsections. All the floor area in the fan section was above target due to light intrusion from operating fans. During tunnel ventilation in the new houses, 4.5%, 6.0%, and 0.0% of the floor area at the pad, center, and fan sections, respectively, were within the 0.18 to 0.22 lx target threshold. However, in the aged houses, only 0.6%, 0.7%, and 0.0% of the floor area at the pad, center, and fan sections, respectively, were within the target range. This research illustrates that the newer houses did a better job at meeting or exceeding target light intensities and that very little of the floor area in new and aged houses during tunnel ventilation was within a 10% threshold of target intensities. Lack of light dimmer adjustment to account for lumen depreciation and accumulation of dust on the bulbs in the aged house most likely led to the lower overall light intensities. Because brood bulbs have a 100% lumen capacity and are not dimmed, a deep cleaning of the bulbs and periodic replacement are recommended if they are not reaching lighting targets. Periodic checking of light intensity is recommended, especially in older houses. Keywords: Broilers, House age, Light intensity, Spatial variation, Target.

Thermo-hydraulic analysis of direct steam generation in a 500 m long row of parabolic trough solar collector using Eulerian two-fluid modeling approach

Direct steam generation (DSG) in the parabolic trough solar collectors is a potential alternative for economic and performance improvement of the solar thermal system. This process comprises the preheating, evaporation, and superheating of water/steam in the solar collectors. The modeling and simulation of the evaporation section are challenging because of the complex physics associated with the boiling phase change process. This work describes the Eulerian two-fluid modeling of the once-through mode of the DSG process in solar collectors. The interfacial interactions between phases have been modeled using the appropriate empirical correlations. The experimental data from the Spanish DISS test facility is used to validate the numerical model. Further, the 3-D thermal–hydraulic investigation of DSG in a 500 m long solar collectors' row has been performed for operating pressures of 60 bar and 100 bar; inlet mass flow rates of 0.4 kg/s and 0.6 kg/s; DNI of 750 W/m2 for the solar time at 12:00 h and 13:00 h. It has been observed that the maximum absorber temperature is reduced by 27 % at 60 bar and 20 % at 100 bar operating pressure when the inlet mass flow rate increases from 0.4 kg/s to 0.6 kg/s. Similarly, the maximum circumferential temperature difference reduces by 12 % and 14 %, respectively. Moreover, the fluid temperature, steam quality, absorber temperature distributions, fluid velocity, and pressure loss under the subjected boundary conditions have been summarized.

Experiment and modeling of an improvised atmospheric water harvester for arid and semi-arid conditions

Freshwater scarcity is a severe challenge in arid and semi-arid climatic conditions wherein the water harvesting from the atmosphere using active cooling condensation technique (ACC) is the only option. In this regard, we performed extensive experiments on a classical atmospheric water harvester (AWH) that relies on moisture condensation via a typical 1TR refrigeration cycle. We developed a MATLAB-SIMULINK based AWH model to note that the classical system has reasonable water harvesting rates in humid climates, but is inadequate for arid and semi-arid climatic conditions. We add an ultrasonic humidifier (160 W power input) before the evaporator section of the classical AWH system to address this issue. The modified system only needs a small amount of water (≈250 ml) for start-up, and further operates without perpetual water supply. We measure the water overflow from the humidifier tank (subtract the water added via the humidifier from the total water harvest) to show a net water harvesting in extremely hot and dry conditions (49 °C DBT and 12 % RH) resembling the weather conditions of Jaisalmer, India. A significant improvement of up to 70 % in water harvesting was also observed under warm and dry climates (35 °C DBT and 21 % RH) mimicking Jaipur, India. We believe that our system shows path for developing active AWH systems with substantial water harvesting in arid climates where active moisture condensation using the conventional strategy is otherwise not possible.

Three-dimensional CFD simulation of geyser boiling in high-temperature sodium heat pipe

A deep understanding of the characteristics and mechanism of geyser boiling and capillary pumping is necessary to optimize a high-temperature sodium heat pipe. In this work, the Volume of Fluid (VOF) two-phase model and the capillary force model in the mesh wick were used to model the complex phase change and fluid flow in the heat pipe. Computational Fluid Dynamics (CFD) simulations successfully predicted the process of bubble nucleation, growth, aggregation, and detachment from the wall in the liquid pool of the evaporation section of the heat pipe in horizontal and tilted states, as well as the reflux phenomenon of capillary suction within the wick. The accuracy and stability of the capillary force model within the wick were verified. In addition, the causes of geyser boiling in heat pipes were analyzed by extracting the oscillation distribution of heat pipe wall temperature. The results show that adding the capillary force model within the wick structure can reasonably simulate the liquid backflow phenomenon at the condensation; Under the horizontal and inclined operating conditions of the heat pipe, the phenomenon of local dry-out will occur, resulting in a sharp increase in local temperature. The speed of bubble detachment and the timely reflux of liquid sodium (condensate) replenishment in the wick play a vital role in the geyser temperature oscillation of the tube wall. The numerical simulation method and the results of this study are anticipated to provide a good reference for the investigation of geyser boiling in high-temperature heat pipes.

Experimental and numerical simulation study of sodium heat pipe with large aspect ratio

As a core cooling device of reactor, a high temperature sodium heat pipe is designed in this paper which has a large aspect ratio of 126. The effects of heating mode and evaporation section length on its start-up are studied experimentally. The results show that the axial temperature uniformity of the heat pipe is better under variable power heating mode. As the evaporation cross-section length increases, thermal resistance decreases by 50%. Compared with heat pipes with low aspect ratio, heat pipes with large aspect ratio are more difficult to start up. Furthermore, numerical simulation is conducted on the designed sodium heat pipe to acquire the velocity and pressure distribution during steady-state operation.

Numerical Study on Single Turn Pulsating Heat Pipe with Additional Branch Configuration

The simple wickless structure and strong heat transfer potential of pulsating heat pipes (PHP) have prompted widespread interest in transferring heat from electronic devices. This work proposes a novel single-turn PHP with convergent and divergent shaped additional branches (ABs) (C-PHP and D-PHP) in the evaporator section, which could increase latent heat transfer in the evaporator by minimizing the saturation temperature and improving the unidirectional flow and thermal performance of PHP. Numerical simulations were performed using computational fluid dynamics (CFD) techniques to understand the thermo-hydrodynamics and properties of the proposed novel PHPs. This study provides the outcomes of novel PHPs and a comparison with the performance of traditional PHP (T-PHP) without AB and straight AB PHP (S-PHP). The results demonstrated that the startup time of novel C-PHP and D-PHP were improved by 32.6% and 22.4%, respectively, compared to the T-PHP, “stopovers” and “local flow reversals” are minimized by the use of AB. The average flow velocity of novel C-PHP and D-PHP was improved by 16% when compared to T-PHP and 11.4% compared to S-PHP, the directional component in C-PHP is substantially stronger than S-PHP and D-PHP.

Experimentally validated CFD-tool for a freezing simulation in a small-scale freeze-dryer

This paper focused on the development of a 3-D coupled CFD model to predict and reduce the freezing time in small-scale freeze-dryers with an integrated freeze-drying chamber and ice trap. The proposed CFD model consists of three sub-models of different time and size scales to simultaneously consider thermal and flow processes inside the vacuum chamber, the food product, and the evaporator section. The model was validated against the experimental data collected for the commercially available unit. For this reason, the developed model is suitable for the forced airflow simulation in a closed chamber with stationary shelves and can easily be adapted as the CFD-design tool for other freeze-drying chambers. In consequence, the proposed model was used to improve the geometry of the freeze-dryer unit resulting in a 50% reduction in the freezing time compared to the reference design.

Optimization design and performance analysis of methanol reforming reactor for exhaust waste heat recovery of marine engine

In order to improve the heat utilization of exhaust from the small or medium-sized ships without exhaust boilers, four types of methanol reforming reactors were designed and optimized via the field synergy principle. The effect of the operating parameters on the performance of optimized reactor under typical loads was also investigated. The analysis based on the field synergy principle shows that the R(S,1) reactor has higher heat transfer synergy performance, better evaporation performance, and more uniform vapour distribution at the inlet of the reaction section, which is more beneficial to the reforming reaction. The effect of the operating parameters on reactor performance shows that the reactor performs better at 75% load with hydro-alcohol molar fraction ratios (S/C) of 1.2 or 1.3. The engine load affects the reactor performance by influencing the temperature of the evaporation and reaction sections. The S/C of the mixed solution at the inlet of the reactor were set at 1.2 and 1.3 to explore the effect of engine loads on the reforming performance under E3 test cycle. The results show that the reactor performs better at S/C = 1.3 under the loads of 100, 75, and 25%; at S/C = 1.2, the reactor performed better under 50% load.

Experimental investigation of heat transfer characteristics of inclined aluminium two phase closed thermosyphon

Abstract A reduction in the size of electronic equipment increases the heat generation rate. Failure of electronic equipment occurs if the heat is not dissipated properly. This paper examines the performance of aluminium two-phase closed thermosyphon for cooling electronic equipment. Acetone charged aluminium two-phase closed thermosyphon was fabricated with an inside diameter of 17.05 mm and 1 mm thickness. A series of experimentations were performed for inclination angles of 10°–90° at selected filling ratios of 30, 60 and 100 %, along with heat inputs of 100, 200 and 300 W. The condenser section flow rate of water was maintained constant. Minimum thermal resistance was obtained at a 30° inclination angle for all filling ratios and heat inputs. The evaporator and condenser sections have a maximum heat transfer coefficient at a 30° inclination angle. Thermosyphon, with a 30 % or 60 % filling ratio, performed better than a 100 % filling ratio for all inclination angles and heat inputs. As the heat input was increased, the heat transfer coefficients of the evaporator and condenser section were increased, whereas total thermal resistance decreased. For 300 W heat input and 30 % filling ratio, the minimum thermal resistance at a 30° inclination angle was 0.158 °C/W. It is found that, the same heat input and filling ratio, the maximum heat transfer coefficient value for the evaporator and condenser section at a 30° inclination angle was 1602 W/m2 °C and 5652 W/m2 °C, respectively.

Performance tests on triple composite mesh-groove-powder wicks in a visualizable flat-plate heat pipe

Two types of novel triple composite mesh-groove-powder wicks are proposed and tested in a visualizable flat-plate heat pipe. The powders are only added in the evaporator in the triple composite wicks to increase the capillarity therein. Outside the evaporator is the mesh-groove wick, with a layer of 200-mesh copper screen sintered over parallel grooves, which has been shown to exhibit high permeability and anti-gravity ability. The first type (Triple A) is to add powders in the mesh-groove wick at the evaporator section, while the second type (Triple B) contains an evaporator with only copper powders. The parallel grooves have a semi-elliptic cross-section with a width of 0.18 mm and a depth of 0.075 mm. According to visualization, the powders in the mesh-groove evaporator appear to increase the flow resistance and prompt partial dry-out in some grooves at high heat loads. The experiment results for the Triple B wick with a powder-only evaporator demonstrate a maximum heat load (Qmax) 20 % higher than that for the double mesh-groove wick at the horizontal orientation, with evaporator resistances also slightly higher. However, the Triple A wick, with powders added in the mesh-groove evaporator, displays a slightly lower Qmax than the original mesh-groove evaporator due to the excessive flow resistance associated with powder-filled meshed grooves.

Innovative experimental approach for the dynamic Multi-Variable investigation of Pulsating heat Pipes

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices characterized by a simple structure and high heat transfer capabilities. Despite this, their large-scale application is still hindered by the actual unpredictability of their dynamic behavior during the startup and the thermal crisis. An innovative experimental apparatus is designed to systematically investigate the above-mentioned phenomena. It consists in a square loop made of four borosilicate transparent glass tubes joined at corners by means of brass connectors. The external tube surface is coated with several transparent Indium Tin Oxide heaters. The device is used to topologically reproduce 5, 7, and 11 evaporator/condenser sections PHPs with a 2 mm inner diameter tube, filled with pure ethanol and tested in horizontal position. First a steady-state characterization is performed. The condenser temperature is varied from 10 °C to 40 °C; the input power goes from 10 W to 40 W. Results show that both the increase of number of evaporator sections and condenser temperature enhance the PHP performance in terms of equivalent thermal resistance. Then, the effect of the condenser temperature on the initial liquid phase distribution is analyzed. It is observed that at low condenser temperatures (10 °C, 20 °C), the liquid phase tends to gather in the condenser sections rather than the evaporator sections and it is arranged in short liquid plugs; conversely, for high condenser temperatures (30 °C, 40 °C), the amount of liquid phase in evaporators is higher and it is arranged in long liquid plugs. Finally, the transient behavior is studied in terms of startup time. It is observed that the parameter influencing most the startup time is the initial fluid distribution; the startup time increases with the increase of evaporator sections volume occupied by liquid phase and with the length of liquid plugs; on the other hand, the startup time decreases with the increase of number of menisci in the evaporator sections.

Experimental and heat transfer studies of MHP-PV/T by Reynolds number and heat transfer coefficient enhancement

The photovoltaic/thermal (PV/T) system, as an energy conversion system to generate electricity and heat, has great application potential in northwest zone of ample solar energy resource in China. The working media inside the micro heat pipe (MHP) of previous studies was acetone. Compared to acetone, R141b has better stability and lower solubility. For working fluid as R141b in the MHP, higher Reynolds Number (Re) theoretically means better heat transfer. During the typical winter season, when the inclination of the PV panel was 45°, the average power conversion efficiency (PCE) and thermal conversion efficiency (TCE) can reach 12.8 and 26.4 %. Furthermore, in order to reduce the simulation time and facilitate the research, the study establishes the fitting equation of MHP-PV/T surface temperature based on solar radiation intensity and environmental temperature with an average error of 7.6 %. Furthermore, a three-dimensional mathematical model of MHP-PV/T system was developed and validated with experimental results, investigating the Re of R141b in the MHPs and calculating the related heat transfer coefficient (h) based on Re. The simulation showed that the Re and h at the condensation section of the MHP were bigger than those at the evaporation section. The Re and h increased with the water temperature decrease of airfoil heat exchanger and solar radiation intensity rise. Lastly, when water temperature of airfoil heat exchanger was unchanged, the impact of solar radiation intensity on h was greater than Re. When the solar radiation intensity remained unchanged and the water temperature decreased, Re was the main reason for affecting the change of h. The research results will give a scientific foundation and technical application for the MHP-PV/T, as well as more efficient solar energy applications in the future.

Thermal and viscous irreversibilities in the heat exchanger of individually finned heat pipes using freon R404A as the working fluid

This work aims to apply a theoretical procedure to determine the performance of the heat exchanger of individually finned heat pipes used in an air conditioning system. The relevant physical quantities are defined and specified locally in the evaporator and condenser sections. The results obtained in the sections are associated with the theoretical determination of the global performance of the heat exchanger. Global theoretical results are compared with global experimental results. Thermal effectiveness, heat transfer rate, pressure drop, thermal and viscous irreversibilities, and thermodynamic Bejan number are determined at the evaporator, condenser, and heat exchanger. The relevant variables used to determine the results are the number of fins per heat pipe and rows of heat pipes. The theoretical-experimental comparison demonstrates that the localized model applied in the analysis is consistent and can be used as a design and comprehensive analysis tool for finessed heat exchangers. The performance of the heat exchanger demonstrated exceptionalness when comparing irreversibilities through the Bejan number, indicating a favorable cost-benefit ratio for the fins less than 30 and the number of heat pipes equal to 49. Bejan’s thermodynamic number, which uses results related to thermal and viscous irreversibilities, demonstrated that one should look for the relationship between thermal irreversibility versus total irreversibility and that fin numbers between 10 and 20 for heat pipes equal to 49 provide a better cost-benefit ratio. The absolute percentage errors obtained between theoretical and experimental values, for an experimental number of fins equal to 30, for the overall heat transfer rate and overall thermal effectiveness range from 2.0% to 42.1%.

Theoretical thermal performance of cross-flow finned heat pipe heat exchanger used for air conditioning in surgery rooms

The thermal efficiency of heat exchangers was applied for theoretical analysis of the thermal performance of a finned heat pipe heat exchanger (FHPHE) used as an auxiliary device to control the temperature and quality of the air conditioning in operating rooms. The theoretical analysis performed is punctual and distributed. It is divided into three aspects: analysis of the evaporator section, analysis of the condenser section, and analysis of the heat exchanger in terms of overall performance. The distributed procedure contrasts with the theoretical-experimental study, which uses the concept of thermal effectiveness (ε-NTU) for global heat exchanger analysis. The developed approach considers the number of heat pipes, the number of fins, and the flow rate variation at the inlet of the heat exchanger as fundamental parameters for determining the thermophysical quantities of interest. Theoretical values were determined for the average velocities, Nusselt numbers, thermal effectiveness, heat transfer rates, and exit temperatures. The localized theoretical-experimental comparisons are consistent, and the absolute relative error for the global heat transfer rate ranges from 0.5% to 35%.

Experimental investigation on the effect of transverse grooves in a channel on the thermal performance of a flat-plate pulsating heat pipe

This study experimentally investigates the effect of transverse grooves in a channel on the thermal performance of a flat-plate pulsating heat pipe (PHP). Rectangular grooves are engraved on the sidewalls of the channel in the evaporator section in a direction transverse to the oscillating flow, and these grooves extend from the top surface to the bottom surface of the sidewalls. Two PHPs are fabricated for experimental evaluation using MEMS techniques: one with a grooved channel and another with a smooth channel. Both PHPs have overall dimensions of 29 × 29 × 1.5 mm3. A rectangular meandering channel is etched on a silicon wafer, and this channel possesses dual hydraulic diameters with alternating widths of 1 mm and 0.4 mm and a height of 0.5 mm. In the PHP with the grooved channel, the dimensions of the grooves are analytically determined to maximize the film evaporation heat transfer rate: the depth, the width, and the spacing are determined to be 50 μm, 20 μm, and 20 μm, respectively. R-236fa is used as the working fluid, and the filling ratio is approximately 55 % by volume. Thermal performances of the PHP with the grooved channel and the PHP with the smooth channel are evaluated and compared through thermometry. Experimental results reveal that engraving transverse grooves on sidewalls of the channel effectively enhances the thermal performance of the PHP: the thermal resistance is decreased by an average of 34.6 % across the entire range of input heat fluxes, from the heat flux required for startup to the maximum allowable heat flux. High-speed photography is used, and it is observed that the liquid film in the grooved channel exhibits an 69.2 % thinner average film thickness than that in the smooth channel. This decrease in the average film thickness contributes to the increased evaporation heat transfer rate.

Experimental study on the thermal performance of ultra-thin flat heat pipes with novel multiscale striped composite wick structures

The rapid development of power-intensive and flexible electronic devices requires thinner heat-dissipation devices with better thermal performance. Ultra-thin flat heat pipe (UTFHP) with striped wick structure is a promising candidate for this application, but its wick structure and thermal performance have not yet been thoroughly studied and optimized for the small concentrated heat source, which is commonly encountered in electronics. In this study, several concentrated striped composite wick (CSCW) structures for 0.6 mm thick UTFHPs are proposed and experimentally investigated. The CSCW consists of copper foam with striped passages converging in the heating zone and double layers of copper screen mesh. The thermal performance of UTFHPs with various composite wick structures is experimentally evaluated. UTFHPs with the proposed structures are also compared with a UTFHP with a more conventional parallel passage composite wick structure. Experimental results show that the CSCW with the hollow structure at the evaporation section is preferred, due to the directed liquid working medium reflux and a large vapor-liquid evaporation interface. Besides, the passage width of the copper foam significantly affects the thermal performance. With the best-performing wick structure, the UTFHP gives the lowest thermal resistance of 0.79 °C/W at a heat load of 23.34 W. Its effective thermal conductivity is approximately 7 times that of copper. The proposed striped wick structure for UTFHPs provides an alternative to handle the hot-spot challenge of electronic devices.

Characteristics of droplet transportation on feather-shaped superhydrophilic-superhydrophobic patterns

As a new structure, the Wick-free vapor chamber (WF-VC) breaks through ultra-thin and ultra-light technical difficulties. The convergence and dripping of droplet in the condensation section and the rapid diffusion of the working fluid at the evaporation section have become hot spots. In this research, a feather-shaped superhydrophobic-superhydrophilic pattern (SSP) was prepared for the droplet's directional wetting and rapid diffusion. The main path is responsiblefor transporting droplets over long distances, and the branch is designed to diffuse the droplet rapidly toward the end of the branch. The highest transportation velocity reaches 174 mm/s. After 29 orthogonal experiments and performing a response surface analysis of the four structural parameters, the optimal structural parameters were determined through multi-objective optimization of transportation time and mass point.