Abstract In this study, both electro-dynamic balance (EDB) method and pendant droplet method were employed to study the evaporation and crystallization process of brine droplet. The EDB setup was used to levitate the charged micrometer sized droplets to study the evaporation process. The pendant droplet method could hang millimeter sized droplet to study the crystallization process. The evaporation of brine droplets with different mass concentrations was visualized by a high speed camera under different temperatures and relative humidity in the test chamber. The results showed that EDB method could get the accurate droplet evaporation results which obeyed the classic D2-law. It was found that the evaporation was increased with the decrease of relative humidity. Due to the attractive force provided by sodium and chloride ion on water molecules, evaporation rate of brine droplet was smaller than pure water, therefore the evaporation rate decreased with the increase of mass concentration by showing a linear relationship. In addition, a salt shell could be formed at the outside of droplet which still contained some amount of salt solution inside. Consequently, the water molecules need to overcome the pressure difference inside and outside the salt shell, and diffuse through the shell for further evaporation. For a higher relative humidity, a more round shell would be formed during the evaporation, and the growth of crystallization along the filament is weaker than that of smaller relative humidity. We hope this study can provide a different perspective to the heat transfer community about the evaporation of brine droplet.

Abstract The food-energy-water nexus considers critical resource challenges which must be resolved in order to meet the needs of a growing population. Agriculture is the largest global water user, accounting for two-thirds of global water withdrawals, including water for crop irrigation. Understanding and therefore reducing evaporation of water from soil is an approach to conserve water resources globally. This work studies evaporation of water from a simulated soil column and employs x-ray imaging to determine the location of water in the porous media. A 30-mL beaker was filled with approximately 1700 2-mm hydrophilic glass beads. Water (i.e., 5.5 mL) was added to the simulated soil, comprised of glass beads and a heat flux (i.e., 1500 W/m 2 ) was applied to the beaker using a solar simulator and the intensity was measured with a light meter. Real-time mass measurements were recorded during evaporation and X-ray imaging was utilized to capture liquid transport during evaporation. Images were post-processed using Matlab; the position of the liquid front was determined from this imaging. Across three replications, it took 47 hours on average to evaporate 5 mL of the total 5.5 mL of water. The transitions between evaporation Stage I, II, and III evaporation rates were determined using mass data and x-ray imaging; transition between Stages I and II occurred between approximately 4 and 9 hours, and the transition from Stage II to III evaporation occurred between approximately 18 and 24 hours. The result of this experiment will be useful to understand the liquid transport and formation of liquid bridges during evaporation from soil.

Abstract Recent studies have indicated that at slightly superheated surface temperatures, droplet evaporation on a nanoporous superhydrophilic surface exhibits onset of nucleation and nucleate boiling effects similar to pool boiling processes. This paper discusses water droplet evaporation experiments and pool boiling experiments conducted on nanostructured surfaces of a 45° downward facing pyramid copper and aluminum substrate. The nanostructured surfaces were used to conduct both droplet evaporation experiments and pool boiling experiments and thus allow direct comparison of the underlying heat transfer performance and mechanisms for these two different processes. The four surfaces tested were the following: bare copper surface, nanostructured surface on copper, bare aluminum surface, and nanostructured surface on aluminum. Mean heat flux values at varying superheats were obtained through temperature and time measurements. To better understand the heat performance of each surface, the wetting and wicking characteristics of each surface were also tested. Experimental results indicate that many of the mechanisms associated with pool boiling may also play a role in droplet vaporization, and their presence can produce levels of heat transfer performance comparable to, or even higher than, that observed in pool boiling at a comparable wall superheat. The results demonstrate that the nanostructured surface affects onset of nucleate boiling and maximum heat flux in both droplet vaporization and nucleate boiling on these surfaces. The implications of these results for strategies to enhance spray cooling and pool boiling are also discussed.

Abstract The dynamic behavior of impinging water droplets is studied in the context of varying surface wettability and wickability on smooth and nanostructured superhydrophilic surfaces. This study distinguishes the separate effects of wetting (contact angle), wickability, and inertia on the spreading and vaporization of water droplets deposited on nanoporous surfaces by considering experimental results in tandem with axisymmetric, volume of fluid (VOF) simulations of droplet spreading. High speed videos were obtained for water droplets spreading on nanoporous surfaces which exhibit very low (< 15°) contact angle and high wickability. In this study, the effect of wickability was assessed by comparing the experimental results, which include the low contact angle and high wickability effects, to predictions of the VOF model, which include only the ultralow contact angle. While a droplet touched to the nanostructured surface demonstrates spreading driven by wicking, droplets which hit the surface with a non-zero impact velocity demonstrate spreading characteristics similar to the smooth surface, which are driven by inertia and ultra-low contact angle. The presence of the nanoporous layer impacts the equilibrium position of the contact line and the final spread radius changes with impact velocity on the nanostructured surface. These results provide fundamental input for modeling of spray cooling systems with nanostructured surfaces.

Abstract Diatoms are a group of single-celled photosynthetic algae that use biochemical pathways to bio-mineralize and self-assemble three-dimensional photonic crystals with unique photonic and micro- & nano-fluidic properties. In recent years, diatom biosilica has been used in surface-enhanced Raman scattering (SERS) based optofluidic sensors for detection of a variety of chemical and biological molecules. In this paper, we present a study to develop a microfluidic pumping system using super-hydrophilic diatom thin films. The desire to develop such a system stems from the requirement to create a low-cost, self-powered microfluidic pumping system that can sustain a continuous flow over an extended period of time. The diatom biosilica acts not only as the driving force behind the flow, but also serves as ultra-sensitive SERS substrates that allows for trace detection of various molecules. Liquid is drawn from a reservoir to the tip of a 150μm inner diameter capillary tube positioned directly over the diatom film. A thin and long horizontal reservoir is used to prevent flooding on the diatom film when the liquid is initially drawn to the diatom film through a capillary tube from the reservoir. The connection of the meniscus from the capillary to the film was maintained from a horizontal reservoir for a recorded time of 20 hours and 32 minutes before the partially filled reservoir emptied. Flow rates of 0.38, 0.22 and 0.16μL/min were achieved for square biosilica thin films of 49mm 2 , 25mm 2 , and 9mm 2 at a temperature of 63°F and 45% relative humidity respectively. A temperature-controlled system was introduced for the 49mm 2 substrate and flow rates of 0.60, 0.82, 0.93, and 1.15μL/min were observed at 72, 77, 86, and 95°F at 21% relative humidity respectively. More testing and analysis will be performed to test the operation limits of the proposed self-powered microfluidic system.

Abstract Recent studies have indicated that droplet evaporation heat transfer can be substantially enhanced by fabricating a thin nanoporous superhydrophilic layer on a metal substrate. Earlier investigations have focused on how these surfaces affect low Weber number deposition of droplets and their subsequent evaporation on a horizontal, upward-facing heated surface. This investigation explores the effects of changing impact parameters — specifically how deposition, spreading, and vaporization on nanoporous superhydrophilic surfaces are affected by changing impact velocity and incident angle of the droplet motion relative to the surface. The results of droplet deposition and evaporation experiments are reported here for multiple droplet sizes (2–6 μL), and multiple incident angles (0–45°), and of 8 μL droplets from different drop heights (1.2, 40, and 80 mm). The results indicate that the strong capillary forces that enhance spreading on these surfaces remain dominant in the spreading and vaporization processes even when droplets strike the surface with significant velocity, and when the incident angle is oblique. The results indicate that increasing the Weber number has little effect on droplet evaporation and decreasing the incident deposition angle further enhances spreading and evaporation heat transfer. This paper also explores the implications of these results for spray cooling applications.

Abstract The effect of applied electric fields on the behavior of liquids and their interaction with solid surfaces has been a topic of active interest for many decades. This has important implications in phase change heat transfer processes such as evaporation, boiling, and condensation. Although the effect of low to moderate voltages has been studied, there is a need to explore the interaction of high electric fields with liquid drops and bubbles, and their effect on heat transfer and phase change. In this study, we employ a high speed optical camera to study the dynamics of a liquid drop impacting a hot substrate under the application of high electric fields. Experimental results indicate a significant change in the pre- and post-impact behavior of the drop. Prior to impact, the applied electric field elongates the drop in the direction of the electric field. Post-impact, the recoil phase of the drop is significantly affected by charging effects. Further, a significant amount of micro-droplet ejection is observed with an increase in the applied voltage.

The phenomenon of bubble growth on a heated surface is of fundamental importance in many scientific and engineering applications, including boiling heat transfer. Although the growth of a homogeneous bubble in a pool of hot liquid is well understood, bubbles growing on hot solid surfaces involve evaporation from a three-phase contact line and therefore exhibit several peculiar features. One of these is the effect of surface texture and wetting properties on the size and timing of bubbles that form and depart from a uniformly heated surface. Here, we present pool boiling experimental results elucidating this important phenomenon. Using high-speed optical imaging, we perform a comparative study of the process of growth and departure of bubbles on plain and rough surfaces and explore the different factors that dictate this behavior. Using scaling analysis, we analyze the primary forces acting on a growing bubble and show that the effect of surface roughness on bubble behavior can be explained in terms of the dependence of these forces on the rate of bubble growth and in-turn on the rate of thin-film evaporation from the three-phase contact line of the bubble.

An experimental study on local flow boiling heat transfer performance in two longitudinal dimple-grooved tubes and an equivalent smooth tube was performed. All three test cooper tubes have the same inner diameter of 11.5 mm; the working fluid is the near-azeotropic mixture, R410A; and all test runs are conducted in a 2 m long horizontal tube-in-tube heat exchanger. Constant evaporation temperature at 10 °C was maintained when heat flux ranging from 32 kW/m 2 to 37 kW/m 2 and refrigerant quality varied from inlet 0.1 to outlet 0.9 at mass flux 150 kg/(m 2 s). The local heat transfer coefficients were obtained for all test conditions using refrigerant R410A. The test results for evaporation were presented compared to the equivalent smooth tube. Wall temperature and local and average heat flux is measured; heat flux effect and surface superheat effect is discussed on the tube side evaporation. The enhanced heat transfer area of two longitudinal dimple-grooved tubes are 1.02 and 1.03, respectively.

An Experimental investigation was conducted to compare the evaporation characteristics of R410A inside three newly developed horizontal enhanced heat transfer (EHT) tubes with the same OD12.70mm and ID11.50mm, and the result of them are compared with that of a plain tube. The inner enhanced surface of 1EHT tube consists of dimples/protrusions and petal arrays, while that of 2EHT-1 tube and 2EHT-2 tube is composed by longitudinal grooves and dimples of different depths. The mass velocities are in the range of 70kg/m 2 s-200kg/m 2 s with a nominal saturation temperature fixed at 279K and the vapor quality in the test section varies from 0.2∼0.9. As the mass flux increases, both the heat transfer coefficient and pressure penalty increase accordingly. The heat transfer coefficient of EHT tubes can achieve 1.14–1.53 times higher than that of the smooth tube while the pressure gradients is 1.43–1.83 times larger than that of smooth tubes. Besides, the enhancement ratios of all the enhanced surface tubes are larger than their respective surface area ratio, and the enhancement ratio comparisons of heat transfer coefficient are made to obtain the enhancing mechanism. The results show that the EHT tubes appear higher performance at low mass fluxes. In all, the EHT1 tube has the best heat transfer performance at low mass velocity, which can be attributed to its special enhanced inner surface, resulting in the increase of nucleation sites, flow separation and turbulent fluctuations. The other two 2EHT tubes can enhance the evaporation greatly with small respective surface ratios as well as relatively little pressure drop penalty, and them shows outstanding performance especially at high mass velocity.

An experimental investigation was performed to study the heat transfer and pressure drop characteristics of refrigerant R-134a boiling in a chevron-patterned brazed plate heat exchanger (BPHE) at low mass flux. The heat transfer coefficient and pressure drop characteristics are analyzed in relation to varying mass flux (30–50 kgm −2 s −1 ), saturation pressure (675 kPa and 833 kPa), heat flux (0.8 and 2.5 kWm −2 ), and vapor quality (0.1–0.9). The two-phase pressure drop shows a strong dependence on mass flux and significant saturation temperature drop at high mass flux. The two-phase heat transfer coefficient was both strongly dependent on heat flux (at vapor qualities below 0.4) and on mass flux (at vapor qualities above 0.4). There was also apparent dryout, as depicted by decreased heat transfer at high vapor qualities. These observations suggest that both nucleate and convective boiling mechanisms prevailed. Existing transition correlations however suggest that the experimental data is rather convection-dominant and not a mix of convection and nucleate boiling. The experimental data further strongly suggest the prevalence of both macrochannel and minichannel type flows. Several acknowledged semi-empirical transition criteria were employed to verify our observations. These criteria mostly support our observations that R-134a evaporating at low mass fluxes in a BPHE with a hydraulic diameter of 3.4 mm, has heat transfer and pressure drop characteristics typically indicative of macrochannel as well as minichannel flows. Disagreement however exists with accepted correlations regarding the prevalence of convective or nucleate boiling.

The development of hydrophilic surface coatings for enhanced wetting characteristics has led to improvement in heat transfer metrics like impinging droplet vaporization time and the heat transfer coefficient. Hydrothermal synthesis, a method of developing hydrophilic surfaces, has been previously shown to produce high performing heat transfer surfaces on copper substrates [1]. Our study applied this production method to aluminum substrates, which have the advantage of being cheaper, lighter, and a more widely used for heat sinks than copper. Previous experiments have shown that water droplets on ZnO nanostructure coated surfaces, at low superheats, evaporate via thin film evaporation rather than nucleate boiling. This leads to heat transfer coefficients as much as three times higher than nucleate boiling models for the same superheat. Our nanocoated aluminum surfaces exhibit superhydrophilicity with an average droplet liquid film thickness of 20–30 microns, which can produce heat transfer coefficients of over 25 kW/m 2 K. This study discusses characterization of ZnO nanostructured aluminum surfaces to better understand the related mechanisms which lead to such high heat transfer performance. All ZnO nanostructured aluminum surfaces produced for this study exhibited superhydrophilicity, with sessile droplet contact angles of less than 5 degrees. The challenge of achieving accuracy for such low contact angles led to the development of a new wetting metric related to the droplet’s wetted area on a surface rather than the contact angle. This new metric is predicated on the the fact that heat transfer performance is directly related to this wetted area, thickens, and shape of the expanding droplet footprint. Shape irregularity of droplets on these superhydrophilic surfaces is discussed in this study, where there appears to be advantages to irregular spreading compared with surfaces that produce symmetric radial spreading. One form of irregular spreading consists of liquid droplets spreading out both on top of the surface and within the microstructure of the surface coating. The liquid within the microstructure forms films less than 5 microns thick, making local heat transfer coefficients of greater than 100 kW/m 2 K possible. SEM microscope imaging provided additional insight to the underlying mechanisms which cause these surfaces to produce such exceptional spreading as well as irregular spreading, resulting in very good heat transfer performance. Experimental work was coupled with computational analysis to model the contact line of the droplet footprint. Image processing of experimental photos helps to analyze spreading characteristics, which can be directly related to heat transfer due to film thickness at various points during spreading. Approaches used to characterize these superhydrophilic surfaces advance understanding of the connections between nanoscale structural elements and macroscale performance characteristics in heat transfer. This understanding can reveal key insights for developing even better high performance surfaces for a broad range of applications.

Computational Fluid Dynamics model is developed to simulate pool boiling for liquid nitrogen. Instead of using the user defined function (UDF), the evaporation-condensation model is adopted to take care of the energy term when phase change happens. Test cases with different excess temperature are simulated to study the important heat transfer features in different boiling regimes — free convection boiling, nucleate boiling, transition boiling and film boiling. The heat flux for each case is calculated. Simulation results are compared with the experimental and numerical data in the literature, good agreements are observed. By using the evaporation-condensation model, the pool boiling procedure is successfully simulated, and the complex physical process such as the pool vaporization can be clearly analyzed. At last, some comments and improvements are discussed to increase the accuracy of the simulation.

The power density of power electronics in electric vehicles (EVs) is expected to be significantly enhanced, which would result in higher heat flux generation. Therefore, it is critical to find an efficient heat spreading and removal technology to reduce the junction temperature for high power density power electronics application. Vapor chamber using evaporation and condensation of the inside working fluid (i.e., water) as a heat spreader can remarkably improve the uniform heat spreading and heat removal for the power electronics compared to the traditional copper heat spreader. Furthermore, high thermal conductivity graphite foam (GF) would be embedded in the vapor core of the vapor chamber to further enhance the heat transfer performance. Numerical heat transfer simulations were performed for copper heat spreader, vapor chambers (with and without the presence of graphite foam) by using ANSYS. Various parametric effects on vapor chamber thermal performance were investigated, including the effects of heat flux from power electronics, heat transfer coefficient at heat sink, vapor chamber thickness, and graphite foam. Through the simulation studies, it was found that thinner vapor chamber (1.35 mm thickness) had better heat transfer performance than thicker vapor chamber (5 mm thickness) because of the extreme high effective thermal conductivities of ultra-thin vapor chamber. Furthermore, the effect of graphite foam on thermal performance improvement was very minor for ultra-thin vapor chamber, but significant for thick vapor chamber. The GF could help reduce the junction temperature by 15–30% in the 5-mm thick vapor chamber. Use of GF embedded vapor chamber could achieve 250–400 W/cm 2 local heat removal for power electronics.

Phase change driven heat transfer has been the topic of interest for a significantly long time. However, in recent years on demand sweating boosted evaporation which requires substantially less amount of the liquid medium has drawn attention as a possible way of increasing/supplementing heat transfer under convective conditions where the convective heat transfer coefficient has already reached its maximum value as well as where dry cooling is a desired objective. In this study, a numerical study is conducted to obtain insight into the ‘hybrid’ system where evaporation and convection both contribute to the heat transfer effect. The system modeled consists of evaporation of thin liquid (water) film under a laminar flow condition. The mathematical model employed consists of coupled conservation equations of mass, species, momentum and energy for the convection-evaporation domain (gaseous), with only mass and energy conservation being resolved in the liquid film domain. The evaporative mass flux is obtained from a modified Hertz-Knudsen relation which is a function of liquid-vapor interface temperature and pressure. A two-dimensional rectangular domain with a pre-prescribed thin liquid water film representative of an experiment is simulated with the developed model. The thin rectangular liquid film is heated by uniform heat flux and is placed in the convection-evaporation domain with an unheated starting length. A moving boundary mesh is applied via the“Arbitrary Lagrangian-Eulerian” technique to resolve the receding liquid interface resulting from evaporation. The prescribed relative displacement of the moving interface is calculated from the net mass flux due to evaporation and is governed by the principle of mass conservation. Simulations were conducted over a range of Reynolds number, heat flux conditions and liquid film thickness. The numerical predictions indicate that under convective-evaporative conditions the overall heat transfer coefficient increases significantly (∼factor of a five) in comparison to the purely forced convection scenario. An increase in the heat transfer coefficient is observed with Reynolds number and vice versa for film thickness. A critical Reynolds number is identified beyond which the heat transfer coefficient does not continue to increase significantly rather tends to plateau out.

In the present work, the performance of bi-textured micro pillar arrays has been modeled as a wicked evaporator to provide steam flow via the thin film evaporation mechanism. Bi-textured micro pillar evaporator consists of an array with rough hydrophilic pillar bases and smooth hydrophobic tips. Water wicks between the rough hydrophilic sections of the micro pillar array to cover the surface, and vaporizes from the thin films that are formed in the vicinity of the pillar walls. The stability of the phase change mechanism is increased due to the change in direction of the capillary forces at the rough-smooth interface of micro pillars. The experimental results show that the pure evaporation mechanism occurs for a surface temperature above saturation on the bi-textured micro pillar array. The numerical analysis shows that there are optimal micro pillar dimensions for each surface temperature. The evaporation mass flow rate at the optimum dimensions is higher than the pool boiling mass flow rate on a bare surface at the same surface temperature. However, the wicked evaporator performance decreases for larger evaporator sizes.

The present study is motivated by interest in understanding of physical mechanisms that govern the effect of material and micro-structural characteristics of heat surface on boiling heat transfer and burnout at high heat fluxes. The effect was reported and investigated experimentally and analytically over several past decades. Only recently, with the advent of nanotechnology including microscale manufacturing, it becomes possible to perform high heat-flux boiling experiments with control of surface conditions. Of particular importance for practice is the potential for significant enhancement of boiling heat transfer (BHT) and critical heat flux (CHF) in pool and flow boiling on heaters with specially manufactured and controlled micro-structured surfaces. This enhancement is very important to a very wide range of engineering applications, like heat exchanger and cooling system, where maximum flux is needed. Currently, there are many controlled experiments that investigate such effect and they lend themselves a subject for detailed computational analysis. The focus of this study is micro-hydrodynamics of the evaporating thin liquid film at the receding triple contact line, corresponding to formation of dry spot in the footprint of a growing bubble. Parametric investigations are performed to assess the hypotheses that micro-structured surfaces enhance resilience to burnout due to residual liquid in the dry patch after contact line receding. Towards the study objective, a particle-based (mesh-less) method of computational fluid dynamics called Smoothed Particle Hydrodynamics (SPH) is adopted. The SPH method is selected for its capability to handle fluid dynamics in complex geometries and free surface problems without mass loss (characteristic of alternative interface capturing schemes used in mesh-based methods). Both surface tension and surface adhesion (hydrophilicity) are implemented and tested. The solid (heater) surface and manufactured micro-structures are represented by solid-type particles. Heat transfer, phase change (evaporation) and vapor dynamics are not included in the present simulation. The bouncing drop case measures the contact time of water droplet with solid surface. This case is used for “mesh” sensitivity (particle size) study and calibration of boundary conditions and surface tension coefficient. Subsequently, case studies are formulated and performed for contact line dynamics on heater surfaces with the fabricated Micro Pillar Arrays surfaces (MPA) and smooth surface. Variable characteristics include surface tension and pillar density on structured surface (modified by changing distance between pillars). First of all, residual fluid are found in all simulations with structured surface, while fluid are drained for smooth cases. For structured surface, it’s found that after the contact line recedes, fluid with higher surface tension resides in the dry patch more than fluid with lower coefficient, and the relation tends to be non-linear. While for smooth surface, all fluid will be drained after certain time and the relations are non-monotonic; it’s also found that the amount of residual fluid increase as the distance between pillars decreases until a limit. The fluid then starts to decrease with pillars being set further apart. The increase starts from 30 μm and the limit is around 10 μm .

Better understanding of phase change phenomena can be obtained through local measurements of the heat transfer process, which can’t be attained by traditional thermocouple point measurements. Infrared (IR) technology, which has been used by many researchers in the past, cannot be used under certain circumstances due to spectral transparency issues present in some materials. In the current study, Quantum Dots (QDs) are proposed as a novel tool for heat transfer measurements. QDs are nano-sized semiconductor materials which fluoresce upon excitation by blue or UV light. The light intensity emitted by QDs drops with temperature, which can be utilized to obtain the surface temperature distribution at a camera pixel resolution. If QDs are distributed on a surface of interest and optical access to that surface is available, the heat transfer processes can be examined using inexpensive equipment such as CCD/CMOS cameras and LED excitation sources. In this paper, a description of a QD based technique is given, where it is applied to visualize the heat transfer associated with ethanol droplet evaporation.

The use of a tourniquet leads to nerve damage, even if applied for short periods of time. This damage can be minimized if the limb is cooled. Because of the low conductivities of human tissue, core limb cooling is slow unless the surface temperature is very cool. Subzero surface temperatures can lead to skin injury (i.e., frostbite). Ideally one would adjust the limb surface temperatures as a function of time to maximize the cooling rate while avoiding permanent tissue damage. One possible approach is to use a thermoelectric cooler (TEC) in conjunction with a programmable power supply. TEC performance varies strongly with heat absorption rate, a function of limb thermal properties, and hot side temperatures that are strongly affected by the surface conditions on the hot side, i.e., overall heat transfer coefficients and ambient conditions. The paper describes the use of finite element simulation to predict the usefulness of using thermoelectric coolers applied to the surface of a limb when compared to the standard approach of using ice packs. Since the TEC performance is strongly influenced by its warm side thermal conditions, experimental results are presented for different ambient temperatures, free and forced convection, and evaporation of water from a wickable covering.

In power plants, the condenser section plays an important role in total thermal efficiency. Extensive considerations by researchers are put toward increasing the heat dissipation efficiency while maintaining low cost and water consumption. One way to archive such situation is to consider replacing the water cooled condenser (WCC) with an air cooled condenser (ACC), but resulting in a significant increase in the condenser size due to the relatively low heat transfer coefficient (HTC = 20–50 W/m 2 K) of the air compared to that of the water. Inspired by the phase change heat transfer of water during the perspiration of mammals, a sweating boosted air cooling approach is proposed herein to dramatically increasing the HTC and minimizing the water consumption. In this experimental study, a wind tunnel and distilled water dripping system were used to examine the thermal performance of copper samples with a tested surface area of 2 in by 2 in. Two surface enhancement approaches were adapted herein. In the first approach, the testing samples were integrated with microchannels, copper woven meshes (200 meshes/in) that were sintered on top of the surface, and finally nanostructures that was synthesized by a hot alkaline oxidation process. In the second approach, the tested samples were coated with TiO2 via an atomic layer deposition (ALD) process. This method allows for rapidly spreading of the dripping water droplets over the whole tested surface. This rapid spreading behavior is due to two main reasons, first the low resistance flow of microchannel which delivered the water globally; second the high capillary pressure generated by the micro-/nano-structures which delivered the water locally. Three different flat-surface samples were developed in this study, as flat surface with sintered copper meshes (design A), grooved surface with sintered copper meshes (design B); and grooved surface with sintered copper meshes coated with ALD TiO2 film (design C). The performance of the surfaces of this approach was quantitatively characterized with the wick testing. The heat transfer performances for all samples were also examined. The experimental results showed that the convection heat transfer plays a limited role in the heat dissipation. In addition, HTC was enhanced by increasing the dripping water rate consumption until the surface reached the flooding condition. The results showed that the evaporation rate of water was augmented with the increase of Reynolds number. The maximum HTC was 182.45 W/m 2 K with a water dripping rate of 12 ml/h, resulting in an enhancement approximately 214.07% compared to the case without water dripping. Further research on a higher HTC requires an optimized combination of Reynolds number and water consumption.