Surface Engineering for Phase Change Heat Transfer: A Review

Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiencies at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930's to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoveries

Fluid Dynamics and Solidification of Molten Solder Droplets Impacting on a Substrate in Microgravity

This program investigates the fluid dynamics and simultaneous solidification of molten solder droplets impacting on a flat substrate. The problem of interest is directly relevant to the printing of microscopic solder droplets in surface mounting of microelectronic devices. The study consists of a theoretical and an experimental component. The theoretical work uses axisymmetric Navier-Stokes models based on finite element techniques. The experimental work is performed in microgravity to allow for the use of larger solder droplets that make feasible the performance of accurate measurements while maintaining similitude of the relevant fluid dynamics groups (Re, We) and keeping the effect of gravity negligible

Thermal Performance of a Liquid-cooling Assisted Thin Wickless Vapor Chamber

The ever-increasing need for power consumption in electronic devices, coupled with the requirement for thinner size, calls for the development of efficient heat spreading components. Vapor chambers (VCs), because of their ability to effectively spread heat over a large area by two-phase heat transfer, seem ideal for such applications. However, creating thin and efficient vapor chambers that work over a wide range of power inputs is a persisting challenge. VCs that use wicks for circulating the phase changing media, suffer from capillary restrictions, dry-out, clogging, increase in size and weight, and can often be costly. Recent developments in wick-free wettability patterned vapor chambers replace traditional wicks with laser-fabricated wickless components. An experimental setup allows for fast testing and experimental evaluation of water-charged VCs with liquid-assisted cooling. The sealed chamber can maintain vacuum for long durations, and can be used for testing of very thin wick-free VCs. This work extends our previous study by decreasing overall thickness of the wick-free VC down to 3 mm and evaluates its performance. Furthermore, the impact of wettability patterns on VC performance is investigated, by carrying out experiments both in non-patterned and patterned VCs. Experiments are first carried out on a wick-free VC with no wettability patterns and comprising of an entirely superhydrophilic evaporator coupled with a hydrophobic condenser. Thereafter, wettability patterns that aid the rapid return of water to the heated site on the evaporator and improve condensation on the condenser of the vapor chamber are implemented. The thermal characteristics show that the patterned VCs outperform the non-patterned VCs under all scenarios. The patterned VCs exhibit low thermal resistance independent of fluid charging ratio withstanding higher power inputs without thermal dry-outs.Comment: Presented at IEEE ITherm (Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems) 2023. Orlando, FL, US. Corresponding: [email protected]

Numerical simulation of droplet impact on wettability-patterned surfaces

© 2020 American Physical Society. Numerical simulations have unexplored potential in the study of droplet impact on nonuniform wettability surfaces. In this paper, we compare numerical and experimental results to investigate the application potential of a volume-of-fluid method utilized in OpenFOAM. The approach implements the Kistler model for the dynamic contact angle of impacting droplets. We begin with an investigation into the influence of the most important solver parameters to optimize the computational setup and reach the best compromise between computational cost and solution errors, as assessed in comparison to experimental results. Next, we verify the accuracy of the predictions for droplet impact on uniformly hydrophilic or superhydrophobic surfaces. Benchmarking the maximal spreading factor, contact, and spreading times, as well as contact-line behavior, we show strong agreement between the present numerical results and the models of Pasandideh-Fard, Phys. Fluids 8, 650 (1996)PHFLE61070-663110.1063/1.868850 and Clanét, J. Fluid Mech. 517, 199 (2004)JFLSA70022-112010.1017/S0022112004000904. Lastly, we demonstrate the capability of the model to accurately predict outcome behaviors of droplets striking distributed-wettability surfaces, which introduce 3D outcome characteristics, even in orthogonal impact. The model successfully predicts droplet splitting and vectoring, as reported in the experiments of Schutzius, Sci. Rep. 4, 7029 (2014)2045-232210.1038/srep07029. Finally, we demonstrate a configuration wherein a droplet centrally strikes a circular disk of different wettability than its surrounding domain. The main contribution of the present paper is a numerical model capable of accurately simulating droplet impact on spatially nonuniform wettability patterns of any foreseeable design

Exploring machine learning models to predict atmospheric water harvesting with an ion deposition membrane

This work investigates the performance of a novel membrane-based atmospheric water harvesting (AWH) unit under various operating conditions of ambient temperature, relative humidity (RH), and carrier fluid flow rate. Ion deposition membranes (IDMs) were selected for their ability to enhance water uptake by lowering the water vapor saturation pressure at the gas-membrane interface. This effect, achieved through metal ion implantation into PTFE-based membranes, improves water harvesting rates – especially under low RH conditions – by up to a factor of four compared to untreated membranes. The benchmark design was tested over all possible combinations of four distinct carrier fluid flow rates, three temperatures, and six RH values. The yield with a lab-scale prototype was as high as 354 ml/day of water, with an average of 155 ml/day, corresponding to water harvesting rates of 22.13 kg/m2/day and 9.69 kg/m2/day, respectively. The experimental dataset obtained was used to build three machine learning (ML) regression models to predict the amount of water harvested under specific operating conditions. The ML techniques are: Support Vector Regression, Gradient Boosting Regression, and Multilayer Perceptron. These methods achieved accuracy scores as high as 89 %, proving suitable for implementation in the regulation of AWH plants featuring this technology. The best-performing model (Multilayer Perceptron) was used to predict the water harvesting potential on a typical spring day in Jeddah, Saudi Arabia, a region facing severe water scarcity

Droplet orthogonal impact on nonuniform wettability surfaces

The vast majority of prior studies on droplet impact have focused on collisions of liquid droplets with spatially homogeneous (i.e., uniform-wettability) surfaces. But in recent years, there has been growing interest on droplet impact on nonuniform wettability surfaces, which are more relevant in practice. This paper presents first an experimental study of axisymmetric droplet impact on wettability-patterned surfaces. The experiments feature millimeter-sized water droplets impacting centrally with on a flat surface that has a circular region of wettability (Area 1) surrounded by a region of wettability (Area 2), where (i.e., outer domain is less wettable than the inner one). Depending upon the droplet momentum at impact, the experiments reveal the existence of three possible regimes of axisymmetric spreading, namely (I) interior (only within Area 1) spreading, (II) contact-line entrapment at the periphery of Area 1, and (III) exterior (extending into Area 2) spreading. We present an analysis based on energetic principles for , and further extend it for cases where (i.e., the outer domain is more wettable than the inner one). The experimental observations are consistent with the scaling and predictions of the analytical model, thus outlining a strategy for predicting droplet impact behavior for more complex wettability patterns

Evaluation of Thermal Performance of a Wick-free Vapor Chamber in Power Electronics Cooling

Efficient thermal management in high-power electronics cooling can be achieved using phase-change heat transfer devices, such as vapor chambers. Traditional vapor chambers use wicks to transport condensate for efficient thermal exchange and to prevent "dry-out" of the evaporator. However, wicks in vapor chambers present significant design challenges arising out of large pressure drops across the wicking material, which slows down condensate transport rates and increases the chances for dry-out. Thicker wicks add to overall thermal resistance, while deterring the development of thinner devices by limiting the total thickness of the vapor chamber. Wickless vapor chambers eliminate the use of metal wicks entirely, by incorporating complementary wettability-patterned flat plates on both the evaporator and the condenser side. Such surface modifications enhance fluid transport on the evaporator side, while allowing the chambers to be virtually as thin as imaginable, thereby permitting design of thermally efficient thin electronic cooling devices. While wick-free vapor chambers have been studied and efficient design strategies have been suggested, we delve into real-life applications of wick-free vapor chambers in forced air cooling of high-power electronics. An experimental setup is developed wherein two Si-based MOSFETs of TO-247-3 packaging having high conduction resistance, are connected in parallel and switched at 100 kHz, to emulate high frequency power electronics operations. A rectangular copper wick-free vapor chamber spreads heat laterally over a surface 13 times larger than the heating area. This chamber is cooled externally by a fan that circulates air at room temperature. The present experimental setup extends our previous work on wick-free vapor chambers, while demonstrating the effectiveness of low-cost air cooling in vapor-chamber enhanced high-power electronics applications.Comment: Presented at IEEE ITherm (Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems) 2023, Orlando FL. Corresponding author: [email protected]

Surface engineering for phase change heat transfer: A review

Owing to advances in micro- and nanofabrication methods over the last two decades, the degree of sophistication with which solid surfaces can be engineered today has caused a resurgence of interest in the topic of engineering surfaces for phase change heat transfer. This review aims at bridging the gap between the material sciences and heat transfer communities. It makes the argument that optimum surfaces need to address the specificities of phase change heat transfer in the way that a key matches its lock. This calls for the design and fabrication of adaptive surfaces with multiscale textures and non-uniform wettability. Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiency at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930s to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoverie

INK JET PROCESSING OF METALLIC NANOPARTICLE SUSPENSIONS FOR ELECTRONIC CIRCUITRY FABRICATION

ABSTRACT A novel approach in creating circuit electrodes with features as fine as 100 µm is demonstrated using a single 38 µm diameter orifice, piezoelectrically driven print head to deposit metallic nanoparticle suspensions. The suspensions consist of gold particles of ~20 nm diameter suspended in toluene solvent. The amount of gold nanoparticles present in the suspension is 30% wt. Inductor and capacitor electrode patterns are deposited onto a glass substrate and thermally processed at 300°C for 15 minutes to drive off the solvent and allow the nanoparticles to sinter, thereby yielding a conductive path with a resistivity of O(10 -7 ) Ω m

Carbon Nanotubes as Thermally Induced Water Pumps

Thermal Brownian motors (TBMs) are nanoscale machines that exploit thermal fluctuations to provide useful work. We introduce a TBM-based nanopump which enables continuous water flow through a carbon nanotube (CNT) by imposing an axial thermal gradient along its surface. We impose spatial asymmetry along the CNT by immobilizing certain points on its surface. We study the performance of this molecular motor using molecular dynamics (MD) simulations. From the MD trajectories, we compute the net water flow and the induced velocity profiles for various imposed thermal gradients. We find that spatial asymmetry modifies the vibrational modes of the CNT induced by the thermal gradient, resulting in a net water flow against the thermal gradient. Moreover, the kinetic energy associated with the thermal oscillations rectifies the Brownian motion of the water molecules, driving the flow in a preferred direction. For imposed thermal gradients of 0.5-3.3 K/nm, we observe continuous net flow with average velocities up to 5 m/s inside CNTs with diameters of 0.94, 1.4, and 2.0 nm. The results indicate that the CNT-based asymmetric thermal motor can provide a controllable and robust system for delivery of continuous water flow with potential applications in integrated nanofluidic devices