Wick Thickness Research Articles

Numerical and experimental investigation of floating wick solar still with a porous-media system

Wick-based solar desalination and solar-driven interfacial evaporation have attracted significant attention for their high efficiency in evaporation and salt removal. This study presents both experimental and numerical models for wick solar desalination. Cotton fabric, coated with black carbon for enhanced radiation absorption, was used as the evaporation medium in experiments due to its superior desalination, capillary, and salt rejection properties. Experimental results showed an evaporation rate of 1.57 kg/m2·h and efficiency of 81 % with freshwater. For saltwater (3.6 % salinity), the evaporation rate and efficiency were 1.24 kg/m2·h and 71 %, under 1000 W/m2 radiation. After validating the numerical model, the effects of porosity, capillary pressure, wick thickness, and thickness of the fabric’s upper (solar absorber) layer were examined. Results showed that reducing the solar absorber fabric thickness (from 9 mm to 0.6 mm), lowering porosity (from 0.97 to 0.3), and increasing wick thickness (from 1.5 mm to 10.5 mm) improved evaporation rates by 58.6 %, 27 %, and 28.4 %, respectively. Capillary pressure and fabric submerged length had minimal effects on evaporation. The model estimates that in real conditions at Jask coastal port (Iran), a configuration with greater wick thickness (6 mm vs. 3 mm), reduced solar absorber fabric thickness (1 mm vs. 1.5 mm), and lower porosity (0.75 vs. 0.93) could increase annual freshwater output by 26.6 % (from 796.6 to 1008.4 kg/m2). The cost per liter of produced water in the best-case scenario is estimated to be $0.0045.

Evaluation on capillary performance of multifunctional ultra-thin composite wick

Vapor chamber technology offers an effective solution for addressing challenges in battery thermal management due to its superior heat transfer efficiency per unit volume. However, achieving a lightweight and ultra-thin VC, which relies significantly on the wick structure, is crucial for its practical application. Nonetheless, reducing the thickness of the wick compromises its capillary performance, thereby impacting the thermal performance of the VC. To overcome this challenge, this paper proposes a Multifunctional Ultra-Thin Composite Wick (MUTCW) which is an integration of cover shell and wicks, aimed at enhancing capillary properties while simultaneously reducing wick thickness. A series of capillary rate-of-rise experiments were conducted to comprehensively evaluate the performance of the MUTCW. The results demonstrate that a MUTCW sintered with a 0.1 mm #200 mesh layer exhibits the most favorable capillary performance, with a wick coefficient of 19.299, an average capillary velocity of 3.5 mm/s, and an average mass flow rate of 3.32 mg/s.

Numerical investigation of the saturation effect on wick dry-out in a capillary driven evaporative cooling system

The influence of gravity in vertically oriented capillary-driven evaporative cooling systems utilizing porous structures in devices such as heat pipes, vapour chambers and direct wick-based cooling is nontrivial. This can impede the rise of liquid through the structure, diminishing wick saturation. Here we developed a model to quantify the variation of saturation in a vertically oriented porous structure operating at low heat fluxes, with a focus in their use in evaporative cooling of batteries. The model was validated within 15% of the experimental data, and a parametric study was conducted to assess the impact of wick parameters on saturation. Dry-out height and dry-out heat flux were also determined across a range of heat fluxes and wick thicknesses, considering various porosities. Findings indicate that saturation increases with increasing porosity and wick thickness, while decreasing with increasing particle diameter. Furthermore, as porosity and wick thickness increase, the thermal conductive resistance of the wick experiences a substantial increase. Wick porosity has the most significant influence on dry-out height, saturation, and thermal conductive resistance. These insights are valuable for designing capillary-driven evaporative cooling systems operating under gravity.

Numerical analysis on thermal hydraulic characteristics of mesh-type ultra-thin vapor chambers

The mesh-type ultra-thin vapor chamber (UTVC) is a highly cost-effective device that spreads heat efficiently and maintains good temperature uniformity, meeting the present trend and demand for compact system heat dissipation. To analyze the thermal behavior of the mesh-type UTVC, a three-dimensional transient numerical model involving the non-isothermal flow and phase change processes of the working medium was developed to solve the hydraulic and heat transfer performance of the mesh-type UTVC. The model was validated by experimental results with a maximum relative error of 1.98 %. The effects of the mesh parameter including the support coarse mesh and the fine mesh wick on the heat transfer performance of the mesh-type UTVC are analyzed, and the thickness of the fine mesh wick is determined to be the influencing factor of the thermal response performance of the UTVC. With the design of limited space, the wick space ratio of the UTVC is reduced from 0.565 % to 0.085 %, and the thermal response time can be reduced by 22.7 %, while it is beneficial to improve the heat transfer performance of the mesh-type UTVC.

Heat transfer incipience of capillary-driven liquid film boiling

High-power electronic devices bring both unprecedented challenges and great opportunities to two-phase cooling technologies for efficient thermal management. Capillary-driven liquid film boiling with wicking structures combines the advantages of capillary evaporation and nucleate boiling, and thus could be one of the most promising cooling methods. However, a comprehensive understanding of liquid film boiling mechanism is lacking. For example, the current prediction of liquid film boiling incipience is based on the bubble nucleation model utilized in pool boiling, which assumes that bubbles nucleate inside the thermal boundary layer with linear temperature distribution due to heat conduction. In liquid film boiling, heat transfer processes are quite different, including both heat conduction inside the liquid film and evaporation at the liquid-vapor interface. In this work, based on the thermal resistance network considering evaporation at thin-film region near the tri-phase line, we develop a model to predict liquid film boiling incipience. Both visualization and heat transfer experiments are conducted using copper micromesh to validate this model. The model predictions of critical liquid film thickness, surface superheat, and heat flux at the onset of nucleate boiling (ONB) are compared with the experimental data on various wicking structures in the literature, including silicon micropillar, copper micropowder, and copper micromesh. The results show that the heat flux and superheat reduce as the wick thermal conductivity decreases, the wick thickness increases, and the wick porosity increases.

Effect of PTFE wick thickness on Loop heat pipe performance

Heat leakage has long been a problem in the development of Loop heat pipe (LHP). Among the strategies to suppress heat leakage, the use of low thermal conductivity polymer materials and adjustment of wick thickness are considered as one of the effective methods. However, although there are many recent reports on the effect of metal wick thickness on performance, there are few studies on the effect of polymer wick thickness such as PTFE wick. Therefore, this paper hopes to integrate the two factors and present the best results. This paper investigates the effect of PTFE wick thickness (1 to 2.75 mm) on LHP performance and proposes the thickness effects on both thermal resistance and flow drag. In addition, since PTFE is hydrophobic, water, with its high surface tension, cannot be used as working fluid. Therefore, in this study, a tiny amount of Butanol (6 wt%) is added to water to create self-rewetting fluid to be used as working fluid, overcoming the flow drag of PTFE wick and allowing the system to operate smoothly.Regarding the wick’s porosity and permeability, the best capillary effect and flow drag occurs at wick thickness of 2 mm. The LHP performance result shows, compared to using PTFE wick with water as working fluid where the system was unable to start up, the system using self-rewetting fluid can smoothly operate, overcome heat leakage and enhance performance. The best performance occurs at wick thickness of 2 mm, the maximum heat load reaches 600 W and heat flux 30 W/cm2; the results are all consistent with the performance predictions based on the wick parameters.

Effect of meniscus curvature on phase-change performance during capillary-enhanced filmwise condensation in porous media

Sustainably enhancing condensation heat transfer performance is a major challenge in thermal management and energy systems, since typical condensation enhancement methods (i.e., dropwise condensation with low surface energy coatings) have limited lifetime/durability, restricted compatibility with working fluids, and sustainability concerns due to the coating composition (e.g., fluorinated compounds). The robust and scalable capillary-enhanced filmwise condensation mode presented in this work demonstrates high heat transfer coefficients for water and low surface tension liquids condensing in a porous wick. Thin porous wicks offer the highest enhancements in heat transfer, however such thin porous wicks have thickness-dependent permeability, and the effective liquid thickness of the wick depends on the shape of the liquid-vapor interface. In this study, we leverage a spatially-discretized porous media model to characterize the effect of the wick thickness on condensation heat transfer performance. The model uses a spatially-varying permeability that depends on the local liquid-vapor interface shape/curvature and the resulting effective wick thickness. We apply this model to investigate the correlation between the heat transfer enhancement and various geometric factors, which enables the design of optimal porous structures for relevant phase-change application. We also predict favorable enhancement in condensation performance with a few common hydrocarbon and fluorocarbon fluid refrigerants. This study provides fundamental insight into the effects of the shape of the liquid-vapor interface on the phase-change performance in the capillary-enhanced filmwise condensation mode.

Analytic solutions of heat and mass transfer in flat heat pipes with porous wicks

Flat heat pipes have been used in cooling computer chips. However, previous models impose heat rate through the pipe as an input parameter, preventing optimization of heat transfer. We consider a horizontal flat heat pipe with an idealized porous wick on either one or both walls. A constant temperature difference is imposed across the two ends of the pipe, and we seek a steady-state solution. The pores in the wick are straight circular capillaries running along and across the wick, and are filled with a partially-wetting liquid. Its vapor fills the center region of the pipe and is connected to the liquid through circular pore openings on the wick surface. At the hot end, the liquid evaporates from the pore openings into vapor, which moves to the cold end where it condenses and the condensate flows in the wick to the hot end to complete a cycle. The mass evaporation rate at each pore opening on the wick surface is found from evaporation kinetics. This pore-level result is incorporated into pipe-level mass, momentum, and energy balances. The mass evaporation rate is found to depend only on the pipe temperature and is essentially independent of the vapor pressure. Consequently, the pipe behaves like a fin with evaporative cooling instead of convective cooling. Thus, the analytic fin solution holds for the pipe temperature and contains only one dimensionless number S (S2 is the ratio of evaporative to conductive heat rate through the pipe). It is observed that in heat pipes, S≫1 and thermal energy is chiefly transferred by vapor flow. Analytic solutions are subsequently found for the evaporation rate, the heat rate through the pipe, and the liquid and vapor flows along the pipe. Our idealized-wick parameters are converted to the porosity and permeability of a porous medium, so that we can compare directly with two published flat heat-pipe experiments. We analyze the effects of pipe length and wick thickness on the heat rate through the pipe, and provide guides for future design of flat heat pipes.

Improving the thermal performance of thin vapor chamber by optimizing screen mesh wick structure

• Effect of mesh layer and mesh number on the thermal performance of TVCs are investigated. • Thermal performance of the singular mesh wicked TVCs is dependent on effective capillary radius and wick thickness. • Larger thickness and smaller effective capillary radius of mesh wick can enhance the thermal performance of TVCs with wick thickness below 0.12 mm. • The multilayer composite mesh wick can improve the thermal performance of TVCs with a 0.4 mm thick mesh wick. Due to the booming of portable electronic devices, thin vapor chambers (TVCs) have been used to effectively transfer high heat flux in narrow space. In this study, various-layer singular meshes and multilayer composite mesh whose thicknesses are below 0.4 mm are developed by electrical resistance welding. For improving the thermal performance of TVCs, the various-layer singular mesh wicked TVCs are investigated to reveal the effects of mesh layer (mesh thickness) and mesh number (effective capillary radius of pores) on the thermal performance of TVCs, and the multilayer composite mesh wicked TVC is also studied as a comparison. The results show that the thermal performance of various-layer singular mesh wicked TVCs is mainly dependent on effective capillary radius along height direction and mesh thickness. Among the mesh wicks whose thicknesses are below 0.12 mm, the mesh wick with a larger thickness and smaller effective capillary radius can enhance the thermal performance of TVCs. The multilayer composite mesh wick with larger pores on top of smaller pores can improve the thermal performance of TVCs with 0.4 mm-thick mesh wick. In addition, three-layer 350 in -1 singular mesh wicked TVC shows the best performance, the maximum heat transfer capacity is 240 W and the lowest thermal resistance is 0.115 K/W.

Parametric Study on Ammonia-Based Loop Heat Pipe

The loop heat pipe (LHP) is a two-phase heat transfer device used for thermal management in applications like space, mobile, laptop, electronic cooling. In the present study, modeling and parametric analysis of LHP was done by applying energy balance approach at different sections using Dev-C++ software. The effects of various parameters like sink temperature (5 °C, 10 °C, 22 °C), condenser effectiveness (0.4–0.9), vapor line diameter (0.0043 m, 0.0053 m, 0.0063 m), wick thickness (0.0073 m, 0.0048 m) and wick porosity (40%, 60%, 80%) were studied on the performance of LHP. The results obtained were validated with the results published in the literature. With increase in condenser effectiveness from 0.40 to 0.90, steady state operating temperature got decreased by 4.5 °C at a heat load of 600 W. The studies on wick porosity revealed that it has significant effect on the heat leak from evaporator to compensation chamber and the lower porosity leads to higher heat leak. From the analysis, an optimum performance of LHP was observed with 60% wick porosity, 0.0073 m wick thickness, 5 °C sink temperature, vapor line diameter of 0.0053 m and condenser effectiveness as 0.9 among the various parameters under investigations.

A multiscale composite silicon carbide wick with excellent capillary performance

Wick is the key component of the heat pipe. Composite wick with multiscale porous structure has gained increasing interest for the high permeability and large capillary pressure. In this study, a grooved silicon carbide wick (GSCW) with a multiscale porous structure was fabricated via sintering and laser ablation. The resultant wick is composed of main grooves, the surrounding porous space formed by the stacking silicon carbide (SiC) particles, the microscale netted pores, and nanoscale pores on the groove walls. The effects of laser energy density on the surface morphology, wettability, capillary rise velocity, and overall capillary performance parameter of the GSCW are investigated. The presented wick is hydrophilic. The capillary rise velocity in the initial stage of GSCW was 7 times that of sintered SiC wick. The capillary performance parameter increased with the increasing laser energy density. The laser-ablated grooves improve the capillary performance parameter over 50 times. The rising performances of water, acetone, methanol, and ethanol were tested. The wick thickness has a significant effect on the capillary performance. The comparison of GSCW with other composite wicks in the literature reveals that it has very good capillary performance, making it an excellent wicking structure for heat pipe.

Experimental Study of an Inclined Wick Solar Still Operating in Drop by Drop System Under The Climatic Conditions of Hodna's Region, Algeria

Solar distillation is a very economical process for the fresh water production, especially in arid and semi-arid areas where solar energy is abundant. Several types of solar stills have been designed and manufactured for a purpose to increase their performances. This work aims to study experimentally under outdoor conditions of Hodna’s region, two types of solar still, the first is an inclined solar still with different wick thicknesses (thick, medium and thin) operating in drop by drop system of feed water and the other is conventional. Four clear days of the April months have been chosen to experiment the solar stills performances. The results showed that the solar stills performances are notably influenced by design and operating conditions, as revealed by the daily production and efficiency values of the inclined solar still with thin wick which achieved 4.14 liter/m2.day and 46.66%, with an improvement of about 23.21% and 12.56 % respectively, compared to those of the conventional solar still, which reached 3.36 l/m2 and 34.1% respectively. In addition, the economic analysis illustrates that the low cost of one liter of distilled water and the quicker payback period at the same time are for the inclined solar still with a thin wick of about 0.011$/l and 77 days respectively.

Digital holographic interferometry for temperature measurement of oil lamp flames with different wick thicknesses

Under the existing threat of depleting reserves of fossil fuels and environmental pollution, the present work focuses on the use of different renewable fuel alternatives in oil lamps under optimized conditions. In the present work, we measure temperature profiles of flames of four types of oil lamps (with different wick thicknesses), each of them fuelled successively by mustard, sesame, castor and butter oil, using digital holographic interferometry for the first time to the best of our knowledge. In addition, a comparative study of temperature variation with change in wick thickness of oil lamps for all the fuels has also been performed. It has been observed that peak temperature of oil lamp flame fuelled by mustard and castor oil decreases with an increase in wick thickness whereas peak temperature of sesame and butter oil lamp flame enhances with an increase in wick thickness. Remarkably, we also find that maximum peak temperature (1052 K) among sixteen flames is achieved from the oil lamp using butter oil as a fuel with maximum wick thickness (1.60 mm). Temperature profiles of the flames determined by this technique are compared with temperatures measured through thermocouple and are found to be in good agreement.

Effect of Ptfe Wick Thickness on Loop Heat Pipe Performance

Effect of Ptfe Wick Thickness on Loop Heat Pipe Performance

Model prediction of the critical heat flux and thermal resistance of a rectangular thin-strip evaporator with distributed liquid supply capillary wicks for high heat flux cooling

• A rectangular thin-strip evaporator with distributed liquid supply capillary wicks (RTEDCW) is designed and the performance predicted using a thermal-hydraulic model. • CHF is attained when the liquid pressure drop in the wick equals the wick's capillary pumping capability. • The overall thermal resistance is the ratio of the wick superheat and the corresponding input heat flux. • CHF increases from 275 W/cm2 to 655 W/cm2 as the particle size increases from 30 mm to 100 mm. • The overall thermal resistance increases from 0.045 K/(W/cm2) to 0.175 K/(W/cm2) as the particle size increases from 30 mm to 100 mm. The heat transfer performance characteristics, mainly the upper limit of heat flux and the overall thermal resistance, of two-phase liquid-vapor-based heat spreaders primarily developed for concentrated-heat dissipation in microelectronics are dependent, predominantly, on the thickness and particle size of the evaporation wick as well as on the distribution of the high-permeable liquid supply channels, also known as arteries. In this study we develop a heat and mass transfer (thermal-hydraulic) model to predict the critical heat flux and the overall thermal resistance of a novel wick with rectangular thin-strip evaporator (with a large length-to-thickness ratio, L / t ≥ 3.5), with distributed liquid supply capillary wicks in a downward-facing orientation, each made of sintered copper particles, for microgravity applications. The liquid supply capillary wicks are made of 350-μm-diameter particle sizes. With the thermal-hydraulic model, we predict the CHF and overall thermal resistance for three evaporation wick particle sizes, i.e., 30, 60, and 100 μm, monolayer each, and show that both performance characteristics are primarily controlled by the hydraulic properties of the evaporation wick. The predicted CHF and minimum thermal resistance are 275 (12.3 °C superheat) and 0.045 K/(W/cm 2 ), 599 (56.7 °C superheat) and 0.095 K/(W/cm 2 ), and 655 W/cm 2 (114.4 °C superheat) and 0.175 K/(W/cm 2 ), for the 30, 60, and 100 μm particles, respectively. Finally, we compare the results of the present work with some of the existing numerical and experimental data for different thin wick designs found in the literature.

Digital Holographic Interferometry for Temperature Measurement of Flame without Phase Unwrapping

Fast and precise measuring technique is required for vigilant monitoring of risk assessment of fire. In the present work, digital holographic interferometry has been applied for temperature profile measurement of single and coalesced candle flames without undergoing the tedious phase unwrapping procedure. Hence, it has also reduced time and effort. The impact of wick thickness on the temperature profile of single and coalesced candle flames has also been investigated. It is inferred that the peak temperature of the single and coalesced candle flame diminishes with an increase in the wick thickness of the candle. It has also been deduced that the coalescence of flame enhances the peak temperature of the flame. Maximum enhancement of peak temperature (103 K) on the coalescence of flame has been observed in the merged flames of the thinnest wick. The experimentally calculated value of temperature is in proximity to the temperature measured through the thermocouple.

Effect of pipe flattening on pressure drop in vapor core and thermal characteristic of miniature round and flat-shape heat pipe with sintered fiber wick

The effect of pipe flattening on pressure drop in vapor core and also heat transfer characteristic of the sintered fiber heat pipes has been investigated. The original diameter of heat pipes were 2 mm and 3 mm, respectively, with the effective length of 100 mm. Thermal performance of the flat-shaped heat pipe with the sintered fiber bundle wick was experimentally investigated under a horizontal orientation in this study. The proper fiber bundle wicks were fabricated at porosity of 0.54, and firmly filled within internal heat pipe wall by the sintering process. The heat pipes were pressed into two final thickness by the flattened process which results in vapor core minimizing. The numerical method was established in modified forms to represent the related wick-heat pipe design parameters, for example, wick thickness, wick porosity, wall thickness, flattened width and height, corresponding to thermal performance of fiber wick heat pipes. The numerical analysis for pressure drop was only focused on vapor core region. The vapor flow was assumed to be laminar and incompressible. It is found that when the heat pipe was flattened, the pressure drop increased. The flattening of heat pipe caused the smaller space of vapor core, therefore the working fluid in vapor phase could not easily transfer heat. In this case, the critical final thickness of 0.45 mm (for original diameter of 2 mm) which the vapor core area nearly approaches to zero, has an effect on the high value of thermal resistance considered in vapor core (z5). In addition, the overall thermal resistance estimated from ESDU79012 agrees well with that obtained from the experiment with the standard deviation (STD) of ±15 %, and it shows that the thermal resistance model can provides a reasonable prediction for the thermal resistance of flat-shaped heat pipes with sintered fiber wicks. Moreover, the new normalized parameter h′v/hv is proposed to represent the appearance of vapor core after pipe flattening which can be used to suggest a desirable normal region of flattened heat pipe with the sintered fiber wick. The critical remaining vapor core height after pipe flattening has been further analyzed to be 0.06 (about 74% from original diameter).

Analysis of heat transfer performance and vapor–liquid meniscus shape of ultra-thin vapor chamber with supporting columns

Abstract Establishing the theoretical analysis model of vapor chamber is helpful to predict its heat transfer performance. It provides a valuable reference for heat transfer enhancement and structural design of ultra-thin vapor chamber (UTVC). In this paper, a theoretical analysis model of ultra-thin vapor chamber with supporting columns is established. This model considers the influence of supporting columns on vapor flow and the Marangoni effect on liquid flow and gives the analytical solution of temperature distribution and heat transfer limit of the UTVC. The accuracy of the model is verified by experiments. Compared with the experimental results, the maximum error of theoretical heat transfer limit is less than 10%. The meniscus shape under different heating power is displayed. The effects of key parameters such as the vapor space thickness, wick thickness, and the supporting columns arrangement on the heat transfer performance of the UTVC are analyzed. According to the theoretical analysis model, the critical thickness of vapor space is 0.3 mm. Thinning the wick thickness improves the uniformity of temperature. In addition, reducing the number of supporting columns on the flow section can effectively improve the heat transfer limit of UTVC.

Numerical study on thermohydraulic behavior in evaporator section of wicked copper-water heat pipe at low superheat

Previous numerical studies have focused on the temperature profiles, flow pattern, heat transfer characteristics and transient behavior of wickless heat pipes. With the increase of heat dissipated power, the heat transfer capacity of the wickless heat pipe is no longer satisfied. Therefore, the research on flow and heat transfer of wicked heat pipe is more meaningful. In this paper, the numerical model of the heat pipe evaporator with a simplified rectangular wick structure was simulated. The present work to be aimed at the numerical model investigating on possible heat transfer performance. The effects of wick thickness, wick porosity, contact angle, single and double layer on heat transfer and flow characteristics were studied. For the wick thickness of 0.25, 0.5 and 1 mm, the 0.5 mm wick exhibits good versatility in heat transfer and flow. The porosity of 0.3 exhibits better heat transfer than that of 0.5, 0.7, 0.9. The value of the Nusselt number of the porosity of 0.3 reaches 142. For the contact Angle of 0, 30, 60, 90 degrees, 30 degrees has better heat transfer performance. Compared with single-layer wick, the double-layer wick can better promote thermal-hydraulic behavior.

Analysis on heat transfer and start-up performance of mercury heat pipe

Mercury heat pipe has the advantages of good thermal stability and low saturated vapor pressure, which is the best choice for the transition from water heat pipe to liquid metal heat pipe. The effects of heating power and heat pipe structure on start-up time and steady-state heat transfer performance of mercury heat pipe were studied by using transient thermal network model. The results showed that: 1) Increasing the length of condenser is beneficial to reducing the start-up time and thermal resistance; 2) Increasing the heating power or wall thickness will reduce the thermal resistance, but increase the start-up time, and increasing the porosity of wick is just the opposite; 3) Increasing the thickness of wick can increase both the start-up time and the thermal resistance.