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Abstract: We present pulsed evaporative transient thermometry, a metrology technique that utilizes the transient thermal response from pulsed heating on isolated microstructures to obtain temporally-resolved heat capacity and heat transfer conductance. We demonstrated the approach with two model systems, copper microwires and alumina nanoporous membranes. Temporal resolutions as high as 0.2s were achieved where peaks in heat transfer conductance were observed corresponding to the thin film evaporation stage. The metrology technique can also be extended to various other micro and nanostructures, which can provide increased understanding of thin film evaporation for the realization of advanced phase-change based thermal management solutions. [Copyright &y& Elsevier]

Abstract: This paper presents a new method to enhance thermal energy harvesting with pulsed heat transfer. By creating a phase shift between the hot and cold sides of an energy harvester, periodically pulsed heat flow can allow an available temperature gradient to be concentrated over a heat engine during each thermal pulse, rather than divided between the heat engine and a heat sink. This effect allows the energy harvester to work at maximum power and efficiency despite an otherwise unfavorable heat engine–heat sink thermal resistance ratio. In this paper, the analysis of a generalized energy harvester model and experiments with a mechanical thermal switch demonstrate how the pulse mode can improve the efficiency of a system with equal engine and heat sink thermal resistances by over 80%, although at reduced total power. At a 1:2 engine–sink resistance ratio, the improvement can simultaneously exceed 60% in power and 15% in efficiency. The thermal pulse strategy promises to enhance the efficiency and power density of a variety of systems that convert thermal energy, from waste heat harvesters to the radioisotope power systems on many spacecraft. [Copyright &y& Elsevier]

Abstract: Improvements in solar-to-thermal energy conversion will accelerate the development of efficient concentrated solar power systems. Nanofluid volumetric receivers, where nanoparticles in a liquid medium directly absorb solar radiation, promise increased performance over surface receivers by minimizing temperature differences between the absorber and the fluid, which consequently reduces emissive losses. We present a combined modeling and experimental study to optimize the efficiency of liquid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A one-dimensional transient heat transfer model was developed to investigate the effect of solar concentration, nanofluid height, and optical thickness on receiver performance. Simultaneously, we experimentally investigated a cylindrical nanofluid volumetric receiver, and showed good agreement with the model for varying optical thicknesses of the nanofluid. Based on the model, the efficiency of nanofluid volumetric receivers increases with increasing solar concentration and nanofluid height. Receiver-side efficiencies are predicted to exceed 35% when nanofluid volumetric receivers are coupled to a power cycle and optimized with respect to the optical thickness and solar exposure time. This work provides insights as to how nanofluids can be best utilized as volumetric receivers in solar applications, such as receivers with integrated storage for beam-down CSP and future high concentration solar thermal energy conversion systems. [Copyright &y& Elsevier]

Abstract: We present the modeling and optimization of a new hybrid solar thermoelectric (HSTE) system which uses a thermosyphon to passively transfer heat to a bottoming cycle for various applications. A parabolic trough mirror concentrates solar energy onto a selective surface coated thermoelectric to produce electrical power. Meanwhile, a thermosyphon adjacent to the back side of the thermoelectric maintains the temperature of the cold junction and carries the remaining thermal energy to a bottoming cycle. Bismuth telluride, lead telluride, and silicon germanium thermoelectrics were studied with copper–water, stainless steel–mercury, and nickel–liquid potassium thermosyphon-working fluid combinations. An energy-based model of the HSTE system with a thermal resistance network was developed to determine overall performance. In addition, the HSTE system efficiency was investigated for temperatures of 300–1200K, solar concentrations of 1–100 suns, and different thermosyphon and thermoelectric materials with a geometry resembling an evacuated tube solar collector. Optimizations of the HSTE show ideal system efficiencies as high as 52.6% can be achieved at solar concentrations of 100 suns and bottoming cycle temperatures of 776K. For solar concentrations less than 4 suns, systems with thermosyphon wall thermal conductivities as low as1.2W/mK have comparable efficiencies to that of high conductivity material thermosyphons, i.e. copper, which suggests that lower cost materials including glass can be used. This work provides guidelines for the design, as well as the optimization and selection of thermoelectric and thermosyphon components for future high performance HSTE systems. [Copyright &y& Elsevier]

Abstract: Microjet impingement cooling devices promise high heat removal rates for the development of advanced thermal management solutions. However, understanding of microjet hydrodynamics is needed to optimize cooling performance. In this paper, we combined experiments and modeling to obtain three-dimensional (3-D) microjet flows. We fabricated single-jet and multi-jet arrays with 50μm diameter orifices and used micron-resolution particle image velocimetry (μPIV) to capture two-dimensional (2-D) images of the flow field at different imaging planes. The data was subsequently used to obtain the out-of-plane (z-component) velocities, which play an important role in enhancing heat transfer at the impingement surface. The results from the reconstruction of the 3-D flow field offers new insights into the impact region of a single jet and optimized design of microjet cooling devices. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the boiling process and two-phase flow behavior in microchannels is the key to developing microchannel heat sinks for high-power microprocessors. We conducted experiments in micromachined silicon channels with a range of 27–171μm hydraulic diameters and varying surface roughnesses. Bubble nucleation, flow patterns, wall temperature, as well as transient pressure fluctuations were recorded and analyzed. We observed both typical nucleate boiling and eruption boiling with large amounts of wall superheat in these channels, and recorded up to 138kPa transient pressure fluctuations due to bubble nucleation. We found the boiling mechanism is strongly dependent on the wall surface roughness, and we explained the boiling mechanism in sub-150μm diameter channels with Hsu’s model. [Copyright &y& Elsevier]

The bubble nucleation, growth, and departure cycle is a fundamental aspect of nucleate pool boiling. While much research on this subject has been performed in the previous century, new correlations and models have not been developed in light of important results in simulations and experiments in recent decades. In this work, we provide an updated understanding of nucleation, growth, and departure with analytical models that are validated by recent work. We found that nucleation at incipience is correlated to departure size, suggesting that the momentum-induced bubble snap-off at departure may be responsible for the size of trapped vapors in cavities. We also developed a bubble growth model that takes into account a thermal boundary layer whereas previous works typically considered a uniform superheating. In addition, we developed a bubble departure description, wherein the velocity boundary layer is responsible for closing the base of the bubble. Finally, we provide a methodology to calculate bubble growth and departure for contact angle changing with time, which is relevant in studying surfactant-enhanced boiling. [ABSTRACT FROM AUTHOR]

Highlights • Developed a thin film evaporation model accounting for local interfacial curvature. • Characterized the effects of geometry and curvature on heat transfer coefficient. • Quantified the tradeoff between dryout heat flux and heat transfer coefficient. • Guides design of wicks in terms of temperature constraints and the dryout limit. Abstract Porous wicks are of great interest in thermal management because they are capable of passively supplying liquid for thin film evaporation, a promising method to reliably dissipate heat in high performance electronics. While dryout heat flux has been well-characterized for many wick configurations, key design information is missing as many previous models cannot determine the distribution of evaporator surface temperature. Temperature gradients are inherent to the passive capillary pumping mechanism since the shape of the liquid/vapor interface is a function of the local liquid pressure, causing spatial variation of permeability and heat transfer coefficient (HTC). Here, we present a comprehensive modeling framework for thin film evaporation in micropillar wicks that can predict dryout heat flux and local temperature simultaneously. Our numerical approach captures the effect of varying interfacial curvature across the micropillar evaporator to determine the spatial distributions of temperature and heat flux. Heat transfer and capillary flow in the wick are coupled in a computationally efficient manner via incorporation of parametric studies to relate geometry and interface shape to local permeability and HTC. This model predicts notable variations of HTC (∼30%) across the micropillar wick, highlighting the significant effects of interfacial curvature. Further, we are able to quantify the tradeoff associated with enhancing either dryout heat flux or HTC by optimizing geometry. Our model provides all of the information needed to guide the design and optimization of micropillar wicks by resolving evaporator temperature distributions in addition to dryout heat flux. [ABSTRACT FROM AUTHOR]

Abstract: A numerical solution is introduced to investigate the effect of laminar flow with a suspension of phase change material nanoparticles (PCMs) in a microchannel. The nanoparticle suspension consisting of lauric acid nanoparticles in water is introduced into a microchannel of 50μm height and 35mm length, where a constant heat flux is applied to the bottom wall. Mass, momentum and energy equations are solved simultaneously using a fluid with effective thermo-physical properties. The effect of various parameters including mass flow rate (1×10−5–4×10−5 kg/s), heat flux (8000–20,000W/m2) and particle volume concentrations (0–10%) on the thermal performance is investigated using effectiveness ratio, performance index, and Merit number. The study is extended to include the optimum channel length for improved thermal performance. For a given particle concentration, an optimum heat flux to mass flow rate ratio exists that leads to the maximum effectiveness ratio of 2.75, performance index of 1.37 and Merit number of 0.64. Such a study facilitates understanding the parametric space to optimize heat transfer in microchannels for applications such as thermal management and energy conversion devices. [Copyright &y& Elsevier]

High-flux evaporators are important for various fundamental research and industrial applications. Understanding the heat loss mechanisms, especially the contribution of natural convection during evaporation is thus a ubiquitous process to predict and optimize the performance of evaporators. However, a comprehensive analysis on natural convection heat transfer, where the vertical Stefan flow due to evaporation couples with buoyancy driven convective flow has not been carefully considered. In this work, we developed a theoretical framework to elucidate the effect of Stefan flow on natural convection during evaporation. This theory incorporates the vertical Stefan flow into the conventional boundary layer theory. We found that a significant suppression of natural convection can be induced by a weak Stefan flow owing to the increase of boundary layer thickness. To understand this phenomenon, we discuss the governing mechanisms at different Stefan flow regimes. We provide a theoretical correlation to the overall heat transfer which includes both effects of the Stefan flow velocity and the buoyancy force. We finally predict the effect of natural convection on an evaporator at different operating temperatures. The heat loss from natural convection no longer monotonically increases with the superheat temperature due to the effect of Stefan flow suppression. As a result, there is an approximately 40% overestimation of the natural convection contribution at saturation temperature using conventional theory. This work improves the fundamental understanding of the natural convection during evaporation and can help guide future high-performance evaporator designs. [ABSTRACT FROM AUTHOR]

Abstract Heat pipes and vapor chambers act as efficient heat spreaders since they rely on phase change of the coolant. The evaporator design is critical and typically high performance is characterized by the high heat dissipation capability with low thermal resistance. In the past, there have been numerous experimental and modeling studies focused on the design of evaporator wicks of different geometries, but systematic studies to simultaneously optimize both the heat flux and thermal resistance have been limited. In this work, we developed a comprehensive model that considers both aspects to provide design guidelines for evaporator micropillar wicks. We show that capillary limited heat dissipation is best captured with a recently developed numerical model as compared to previous analytical models. We also developed a numerical model to obtain the effective wick thermal conductivity, which is a function of pillar diameter, pitch, and height. Smaller diameters with smaller pitches of the pillars had more thin film area and had larger effective wick thermal conductivities. Our parametric investigations show that trade-offs between lowest thermal resistance and maximum heat carrying load exists, and the actual wick geometry will be dictated by application specific requirements. Finally, we highlight the importance of accurately obtaining the accommodation coefficients to predict the effective wick thermal conductivity. The present work would enable in optimal design of micropillar wicks (with low thermal resistance and high dry-out heat flux) and the same methodology can be extended to other types of wick structures as well. [ABSTRACT FROM AUTHOR]

Climate control applications in the form of heating and cooling account for a significant portion of energy consumption in buildings and transportation. Consequently, improved efficiency of climate control systems can significantly reduce the energy consumption and greenhouse gas emissions. In particular, by leveraging intermittent or continuous sources of waste heat and solar energy, thermally-driven energy storage systems for climate control can play a crucial role. We demonstrate the concept of a thermophysical battery, which operates by storing thermal energy and subsequently releasing it to provide heating and cooling on demand. Taking advantage of the adsorption-desorption and evaporation-condensation mechanisms, the thermophysical battery can be a high-power density and rechargeable energy storage system. We investigated the thermophysical battery in detail to identify critical parameters governing its overall performance. A detailed computational analysis was used to predict its cyclic performance when exposed to different operating conditions and thermodynamic cycles. In addition, an experimental test bed was constructed using a contemporary adsorptive material, NaX-zeolite, to demonstrate this concept and deliver average heating and cooling powers of 900 W and 650 W, respectively. The maximum power densities and specific powers observed were 103 W/l and 65 W/kg for heating, and 78 W/l and 49 W/kg for cooling, respectively, making the thermophysical battery competitive with the state-of-the-art climate control systems that provide relatively lower power densities. Additionally, with further opportunities for development and innovation, especially in synthesizing novel adsorptive materials, the thermophysical battery can achieve significantly higher power densities. With its ability to function using thermal energy input while being compact and lightweight, the thermophysical battery offers an option to address the energy challenges associated with the rising demand for climate control. [ABSTRACT FROM AUTHOR]

The generation of concentrated heat loads in advanced microprocessors, GaN electronics, and solar cells present significant thermal management challenges in defense, space and commercial applications. Liquid to vapor phase-change strategies are promising due to the high latent heat of vaporization of the working fluid. In particular, thin-film evaporation has received increased interest owing to advances in micro/nanofabrication and the potential to dissipate high heat fluxes by increasing the evaporative meniscus area. Yet, predictive tools to design various wicking structures are limited due to the complexity of the thermal–fluidic transport. In this work, we performed systematic experiments to characterize capillary-limited thin-film evaporation from silicon micropillar wicks in the absence of nucleate boiling. The insights gained from experiments were used to model the capillary pressure, permeability, and thermal resistance. Accordingly, we developed a semi-analytical model to determine the capillary-limited dryout heat flux and wall temperature with ±20% accuracy, compared to our experiments. The model provides a versatile platform to design and optimize micropillar wicks for next generation thermal management devices. [ABSTRACT FROM AUTHOR]

LTA zeolite layer was successfully grown on a superhydrophilic mesoporous titania layer coated onto porous α-alumina substrate. Mesoporous titania layer was formed as an intermediate bridge in the pore size variation between the macroporous α-alumina support and micro-porous LTA zeolite layer. In-situ aging microwave heating synthesis method was utilized to deposit the LTA zeolite layer. Mesoporous titania layer was pre-treated with UV photons and this was observed to have played a major role in improving the surface hydrophilicity of the substrate leading to formation of increased number of Ti–OH groups on the surface. This increase in Ti–OH groups enhanced the interaction between the synthesis gel and the substrate leading to strong attachment of the amorphous gel on the substrate, thus enhancing coverage of the LTA zeolite layer to almost the entire surface of the 1-inch (25.4 mm) diameter membrane. LTA zeolite layer was developed via in-situ aged under microwave irradiation to study the effect of synthesis parameters such as in-situ aging time and synthesis time on the formation of the LTA zeolite layer. Optimized process parameters resulted in the formation of crack-free porous zeolite layer yielding a zeolite–titania–alumina multi-layer membrane with a gradient in porosity. [ABSTRACT FROM AUTHOR]

• Developed model capable of capturing sorption and desorption of hygroscopic hydrogels. • Validated model against experimental data and demonstrated good agreement between theory and experiments. • Model shows the relevance of simultaneous vapor, water, and heat transport. • Identified key differences in transport behavior depending on how the hydrogel is being heated during desorption. • Performed parametric analysis of the hydrogel thickness, the effective thermal conductivity, the heat transfer coefficient to the ambient, and the hydrogel shear modulus to guide the practical design of hydrogels for sorption applications. Sorption and desorption with hygroscopic hydrogels hold significant promise for thermal management, passive cooling, thermal energy storage, and atmospheric water harvesting. However, a comprehensive understanding of the energy and mass transport mechanisms in hygroscopic hydrogels remains missing, impeding accurate modeling and optimization. In this work, we develop a model for the simultaneous vapor, water, and heat transfer in hygroscopic hydrogels during sorption and desorption processes. We show that by considering vapor diffusion in the hydrogel micropores, water diffusion in the polymer mesh, and heat transfer in the porous hydrogel, we can accurately capture experimentally observed thermally-driven desorption rates in these hydrogels. Furthermore, we consider three typical operating configurations of hydrogels and elucidate the differences in the transport mechanisms depending on the configuration. Finally, for each of these configurations, we identify key design parameters, including hydrogel thickness, hydrogel shear modulus, heat transfer coefficient, and thermal conductivity, and we parametrically show that by varying these parameters, a hygroscopic hydrogel can desorb up to 128.5%, 14.9%, 69.7%, and 9.6% more water, respectively, relative to the initial water content. This work provides a generic framework to model sorption and desorption processes in hygroscopic hydrogels which can guide the design and optimization in applications of thermal management, passive cooling, thermal energy storage, and atmospheric water harvesting with hydrogels. [Display omitted] [ABSTRACT FROM AUTHOR]

• Multiscale porous ceramic heat exchanger for high temperature applications. • Hierarchical models to predict and optimize heat exchanger core. • 2.8 × enhancement of the surface area to volume ratio compared to existing designs. The efficiency of a heat engine can be significantly improved by operating in a high-temperature and high-pressure environment, which is crucial for a wide range of applications such as hybrid and electric aviation as well as power generation. However, such extreme operating conditions pose severe challenges to the heat exchanger design. Although recently developed superalloys and ceramics can survive high-temperature and high-pressure loads, using these materials in a traditional heat exchanger design requires high cost and yields low power density. In this work, we propose an ultrahigh power density ceramic heat exchanger for high-temperature applications enabled by a multiscale porous design. By optimizing the design of centimeter-scale macrochannels and microchannels, significant improvement to both heat transfer and structural strength is predicted, with a negligible pressure drop penalty (< 1%). Based on finite element simulations, an optimized heat exchanger core design is expected to achieve power densities of 717 MW/m3 and 300 kW/kg, which indicates more than 2.5× enhancement in thermal performance compared to printed-circuit heat exchanger design. Furthermore, the heat exchanger design features low material costs and scalable fabrication, enabling highly customizable applications in aerospace and terrestrial power generation. [ABSTRACT FROM AUTHOR]

Wepresent a systematic study of carbon nanomaterials with different geometries and thermal properties, including few-layer graphene (FLG), graphene oxide (GO), and functionalized carbon nanotubes (fCNT) as additives to enhance the thermal conductivity of microporous adsorbentmaterials. The dimensionality and intrinsic thermal conductivity of the additives were found to be critical for both maximizing the thermal conductivity enhancement, and minimizing the reduction in the adsorption capacity of the active materials. We demonstrated that two-dimensional (2D) FLG was the most effective thermal additive for zeolite (ZT) adsorbents due to its high thermal conductivity and preferential 2D geometry. Meanwhile, negligible enhancement was observed from one-dimensional (1D) additives such as fCNTs, which is consistent with the predictions froma modified effective medium analysis (EMA). Our work provides insights for the development of additives to enhance the thermal performance of porous materials in applications such as adsorption heat pumps, gas storage, and separation processes. [ABSTRACT FROM AUTHOR]

Current technologies that provide climate control in the transportation sector are quite inefficient. In gasoline-powered vehicles, the use of air-conditioning is known to result in higher emissions of greenhouse gases and pollutants apart from decreasing the gas-mileage. On the other hand, for electric vehicles (EVs), a drain in the onboard electric battery due to the operation of heating and cooling system results in a substantial decrease in the driving range. As an alternative to the conventional climate control system, we are developing an adsorption-based thermal battery (ATB), which is capable of storing thermal energy, and delivering both heating and cooling on demand, while requiring minimal electric power supply. Analogous to an electrical battery, the ATB can be charged for reuse. Furthermore, it promises to be compact, lightweight, and deliver high performance, which is desirable for mobile applications. In this study, we describe the design and operation of the ATB-based climate control system. We present a general theoretical framework to determine the maximum achievable heating and cooling performance using the ATB. The framework is then applied to study the feasibility of ATB integration in EVs, wherein we analyze the use of NaX zeolite–water as the adsorbent–refrigerant pair. In order to deliver the necessary heating and cooling performance, exceeding 2.5 kW h thermal capacity for EVs, the analysis determines the optimal design and operating conditions. While the use of the ATB in EVs can potentially enhance its driving range, it can also be used for climate control in conventional gasoline vehicles, as well as residential and commercial buildings as a more efficient and environmentally-friendly alternative. [ABSTRACT FROM AUTHOR]

Modular and compact adsorption heat pumps (AHPs) promise an energy-efficient alternative to conventional vapor compression based heating, ventilation and air conditioning systems. A key element in the advancement of AHPs is the development of adsorbents with high uptake capacity, fast intracrystalline diffusivity and durable hydrothermal stability. Herein, the ion exchange of NaY zeolites with ingoing Mg 2+ ions is systematically studied to maximize the ion exchange degree (IED) for improved sorption performance. It is found that beyond an ion exchange threshold of 64.1%, deeper ion exchange does not benefit water uptake capacity or characteristic adsorption energy, but does enhance the vapor diffusivity. In addition to using water as an adsorbate, the uptake properties of Mg, Na-Y zeolites were investigated using 20 wt.% MeOH aqueous solution as a novel anti-freeze adsorbate, revealing that the MeOH additive has an insignificant influence on the overall sorption performance. We also demonstrated that the lab-scale synthetic scalability is robust, and that the tailored zeolites scarcely suffer from hydrothermal stability even after successive 108-fold adsorption/desorption cycles. The samples were analyzed using N 2 sorption, 27 Al/ 29 Si MAS NMR spectroscopy, ICP-AES, dynamic vapor sorption, SEM, Fick’s 2nd law and D–R equation regressions. Among these, close examination of sorption isotherms for H 2 O and N 2 adsorbates allows us to decouple and extract some insightful information underlying the complex water uptake phenomena. This work shows the promising performance of our modified zeolites that can be integrated into various AHP designs for buildings, electronics, and transportation applications. [ABSTRACT FROM AUTHOR]

Adsorption based heat-pumps have received significant interest owing to their promise of higher efficiencies and energy savings when coupled with waste heat and solar energy compared to conventional heating and cooling systems. While adsorption systems have been widely studied through computational analysis and experiments, general design guidelines to enhance their overall performance have not been proposed. In this work, we identified conditions suitable for the maximum utilization of the adsorbent to enhance the performance of both intermittent as well as continuously operating adsorption systems. A detailed computational model was developed based on a general framework governing adsorption dynamics in a single adsorption layer and pellet. We then validated the computational analysis using experiments with a model system of zeolite 13X-water for different operating conditions. A dimensional analysis was subsequently carried out to optimize adsorption performance for any desired operating condition, which is determined by the choice of adsorbent-vapor pair, adsorption duration, operational pressure, intercrystalline porosity, adsorbent crystal size, and intracrystalline vapor diffusivity. The scaling analysis identifies the critical dimensionless parameters and provides a simple guideline to determine the most suitable geometry for the adsorbent particles. Based on this selection criterion, the computational model was used to demonstrate maximum utilization of the adsorbent for any given operational condition. By considering a wide range of parametric variations for performance optimization, these results offer important insights for designing adsorption beds for heating and cooling systems. [ABSTRACT FROM AUTHOR]

Highlights: [•] Framework water capacity was determined as 35±2N/UC for MFI zeolites. [•] Infiltration pressure was estimated to be 95–100MPa for MFI zeolites. [•] Textural porosity and pre-adsorbed water contribute to discrepancies in capacity. [•] Lower framework capacity of nano-sized zeolites due to uncrystallized primary units. [ABSTRACT FROM AUTHOR]

Abstract: Solar thermophotovoltaic (STPV) systems convert solar energy into electricity via thermally radiated photons at tailored wavelengths to increase energy conversion efficiency. In this work, we report the design and analysis of a STPV system with 2D photonic crystals (PhCs) using a high-fidelity thermal-electrical hybrid model that includes the thermal coupling between the absorber/emitter/PV cell and accounts for non-idealities such as temperature gradients and parasitic thermal losses. The desired radiative spectra of the absorber and emitter were achieved by utilizing an optimized two-dimensional periodic square array of cylindrical cavities on a tantalum (Ta) substrate. Various energy loss mechanisms including re-emission at the absorber, low energy emission at the emitter, and a decrease in the emittance due to the angular dependence of PhCs were investigated with varying irradiation flux onto the absorber and resulting operating temperature. The modeling results suggest that the absorber-to-electrical efficiency of a realistic planar STPV consisting of a 2D Ta PhC absorber/emitter and current state of the art InGaAsSb PV cell (whose efficiency is only ~50% of the thermodynamic limit) with a tandem filter can be as high as ~10% at an irradiation flux of ~130kW/m2 and emitter temperature ~1400K. The absorber-to-electrical STPV efficiency can be improved up to ~16% by eliminating optical and electrical non-idealities in the PV cell. The high spectral performance of the optimized 2D Ta PhCs allows a compact system design and operation of STPVs at a significantly lower optical concentration level compared with previous STPVs using macro-scale metallic cavity receivers. This work demonstrates the importance of photon engineering for the development of high efficiency STPVs and offers a framework to improve the performance of both PhC absorbers/emitters and overall STPV systems. [Copyright &y& Elsevier]

• Variation in reported CHF values on flat metal surfaces cannot be explained by hydrocarbon contamination alone. • We studied CHF on flat Cu and Ni surfaces, accounting for surface oxidation during boiling. • Formation of rough Cu 2 O nanostructures was accompanied by an increase in CHF. • Flat NiO formation resulted in a relatively stable morphology and CHF over time. • Formation of hydroxide (Ni(OH) 2) led to a notably higher CHF without an increased roughness. The critical heat flux during pool boiling has been investigated for a range of applications including electrical power generation and thermal management. Reported experimental CHF values during pool boiling of water on flat metallic surfaces, however, show a large discrepancy across studies. Here, we address this discrepancy in CHF values by accounting for oxidation of metallic surfaces during boiling. We studied the effect of in situ oxidation on flat Cu and Ni surfaces by changing the duration that samples were held in saturated water before conducting boiling experiments. The morphology and chemical composition of surfaces after the boiling experiments were analyzed by atomic force microscopy and X-ray photoelectron spectroscopy, respectively. Cu surfaces showed gradually increasing CHF values as the duration in saturated water increased, which could be attributed to the increase in roughness due to the formation of Cu 2 O nanostructures. Conversely, Ni surfaces showed relatively stable CHF and morphology as a nearly flat layer of NiO formed, with one exception: formation of a highly wetting hydroxide, Ni(OH) 2 , on a Ni coupon held in saturated water for 24 h resulted in a uniquely high CHF value, signifying the importance of surface chemistry in addition to morphology. The fundamental mechanisms resulting in the wide spread of CHF values on metallic surfaces elucidated in this work will lead to more accurate estimation of CHF as well as a deeper mechanistic understanding of CHF values on engineered surfaces. [ABSTRACT FROM AUTHOR]

Abstract: Phase change material (PCM) suspensions have received wide spread attention for increased thermal storage in various thermal systems such as heat sinks for electronics and solar thermal applications. To achieve further heat transfer enhancement, this paper investigates the effect of focusing micron-sized phase-change particles (PCMs) to a layer near the heated wall of a parallel plate channel. A numerical model for fully-developed laminar flow with a constant heat flux applied to one wall is developed. Melting of the focused PCMs is incorporated using a temperature-dependent effective heat capacity. The effect of channel height, height of the focused PCM stream, heat flux, and fluid properties on the peak local Nusselt number (Nu∗ ) and the averaged Nusselt number over the melting length (Numelt ) are investigated. Compared to the thermally-developed Nusselt number for this geometry (Nuo =5.385), Numelt and Nu∗ enhancements of 8% and 19% were determined, respectively. The local heat transfer performance is optimized when the PCMs are confined to within 30% of the channel height. The present work provides an extended understanding of local heat transfer characteristics during melting of flowing PCM suspensions, and offers a new method for enhancing heat transfer performance in various thermal-fluidic systems. [Copyright &y& Elsevier]

Abstract: The development of efficient solar thermal receivers has received significant interest for solar to electrical power conversion and heating applications. Volumetric flow receivers, where the incoming solar radiation is absorbed in the volume of a heat transfer fluid (HTF), promise reduced heat loss at the surface compared to surface absorbers. In order to efficiently store the thermal energy in the volume, nanoparticles can be suspended in the HTF to absorb the incoming radiation. In such systems, compact models are needed to design and optimize the performance. This paper presents an analytical model that investigates the effect of heat loss, particle loading, solar concentration and channel height on receiver efficiency. The analytical model was formulated by modeling the absorption of solar radiation by the suspended nanoparticles as a volumetric heat release inside the flowing HTF. The energy equation was solved with the surface heat losses modeled using a combined radiative and convective heat loss coefficient. The analytical solution provides a convenient tool for predicting the effect of different parameters, in terms of dimensionless numbers (Pe, NuE , , and θamb ), on two-dimensional temperature profiles and system performance. By combining the receiver efficiency with a power generation efficiency, idealized by the Carnot efficiency, an optimum receiver length where the total efficiency is maximized is determined. However, in practice, the maximum efficiency depends on the maximum allowable temperature of the working HTF. As a case study, predictions were made for Therminol® VP-1 with suspended graphite nanoparticles in a 1cm deep channel with a solar concentration of 10. The model predicts an optimum total system efficiency of 0.35 for a dimensionless receiver length of 0.86. Finally, the analytical model was used to estimate the optimum efficiency and the corresponding optimum receiver length for different design configurations with varying NuE and . The results from this paper will help guide experimental design of volumetric flow receivers for solar thermal based power systems. [Copyright &y& Elsevier]

Abstract: The enhancement of convective heat transfer through a finned heat sink using interdigitated impeller blades is presented. The experimentally investigated heat sink is a subcomponent of an unconventional heat exchanger with an integrated fan, designed to meet the challenges of thermal management in compact electronic systems. The close integration of impeller blades with heat transfer surfaces results in a decreased thermal resistance per unit pumping power. The performance of the parallel plate air-cooled heat sink was experimentally characterized and empirically modeled in terms of nondimensional parameters. Dimensionless heat fluxes as high as 48 were measured, which was shown to be about twice the heat transfer rate of a traditional heat sink design using pressure-driven air flow at the same mass flow rate. [Copyright &y& Elsevier]

• Experimentally demonstrated that pool boiling CHF enhancement on hemi-wicking surfaces cannot be explained by roughness or wickability alone. • Experimental results of systematically designed micropillar surfaces showed that CHF depends on both roughness and wickability. • Performed a scaling analysis to derive a relationship for CHF with a unified descriptor associated with thin film density and volumetric wicking rate. • Thin film density and volumetric wicking rate enhance CHF by accelerating bubble departure frequency and delaying the dry-out at bubble base. • CHF values from our experiments and literature data showed a positive linear correlation with the unified descriptor. Boiling heat transfer is dictated by interfacial phenomena at the three-phase contact line where vapor bubbles form on the surface. Structured surfaces have shown significant enhancement in critical heat flux (CHF) during pool boiling by tailoring interfacial phenomena. This CHF enhancement has been primarily explained by two structural effects: roughness, which extends the contact line length at the bubble base, and wickability, the ability to imbibe liquid through surface structures by capillary pumping. In this work, we show that CHF enhancement on structured surfaces cannot be described by roughness or wickability alone. This result was confirmed using systematically designed micropillar surfaces with controlled roughness and wickability. Further, we performed a scaling analysis and derived a unified descriptor, which represents the combined effects of thin film density and volumetric wicking rate. This unified descriptor shows a reasonable correlation with CHF values with our experiments and literature data. This work provides important insights in understanding the role of surface structures on CHF enhancement, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer. [ABSTRACT FROM AUTHOR]

• A mechanistic and predictive theory for the boiling crisis, combining the thermo-fluidic interaction between bubbles and the stochastic interaction of nucleation sites, is proposed. • A dimensionless boiling crisis constant during the saturated pool boiling crisis is identified. • Quantitative and simultaneous predictions of the critical heat flux and the corresponding wall superheat are achieved. The boiling crisis determines the maximum heat flux for the safe operation of boiling equipment, which is widely used in various applications including power generation, thermal management of electronics and water desalination. Here we present a mechanistic and predictive theory for the boiling crisis, combining the thermo-fluidic interaction between bubbles and the stochastic interaction of nucleation sites. Using Rayleigh and Poisson distributions, we demonstrate that the boiling crisis occurs when the population of isolated nucleation sites reaches the maximum. We identified a dimensionless boiling crisis constant 1/ πe , which universally relates the bubble base diameter to the isolated nucleate site density during the saturated pool boiling crisis. This finding is supported by our direct numerical simulation as well as by previous numerical and experimental results. Combining the thermo-fluidic and stochastic interaction, quantitative and simultaneous predictions of the critical heat flux (CHF) and the corresponding wall superheat at the CHF were achieved, which agrees with existing experimental data. Our theory thus offers a new avenue for understanding the boiling crisis, and therefore can serve as a guideline for the future boiling enhancement design. [ABSTRACT FROM AUTHOR]

• A unified relationship between bubble departure frequency and diameter during saturated nucleate pool boiling. • Two dominant heat transfer processes for the recovery of bubble base temperature due to different regimes of bubble sizes. • Two characteristic timescales associated with two dominant heat transfer processes. • Prediction of bubble departure behaviors for various combinations of heating substrates and working fluids. The relationship between bubble departure frequency and diameter is fundamental to the boiling process and needs to be fully understood for prediction of overall boiling heat transfer performance. Hydrodynamic models for bubble departure were developed in previous studies. However, these models could not explain the dependence of bubble frequency on properties of the heating substrate and surrounding liquid, which was observed in many experiments. In this work, we develop a unified bubble departure theory for saturated nucleate pool boiling. The heat transfer in the bubble base region after bubble departure is taken into consideration. Two characteristic timescales, representing two dominant heat transfer processes for different regimes of bubble sizes, are extracted. These timescales, which depend on substrate and liquid properties, are used to determine bubble departure frequency. The results from our theory show reasonably good agreement with existing experimental data. The proposed model provides a unified relationship between bubble departure frequency and diameter for various combinations of heating substrates and working fluids in the saturated nucleate pool boiling regime. [ABSTRACT FROM AUTHOR]

• The difference between the triggering mechanisms of critical heat flux (CHF) and film boiling is clarified. • It is demonstrated that the heat flux of the wet region on the heater surface increases while the wet area fraction decreases with superheat, leading to the CHF. • It is shown that a vapor recoil force plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF. Boiling is a ubiquitous process in many applications including power generation, desalination, and high-heat flux electronic cooling. At the same time, boiling is a complicated physical process involving hydrodynamics and interfacial heat and mass transfer on multiple scales. One of the key limiting factors of boiling is the critical heat flux (CHF), beyond which a vapor blanket forms on the heating surface and catastrophic device burnout occurs. Yet, detailed understanding of the mechanism that triggers CHF remains elusive. In this paper, we elucidate the CHF mechanism by studying the evolution of wet/dry region on the heater surface using lattice Boltzmann simulations. We incorporate the equation of state for real gases in the liquid-vapor phase change model for direct numerical simulations of the CHF phenomenon. The results of this framework clarify the difference between the triggering mechanism of CHF and film boiling by analyzing the pool boiling curve. We demonstrate that the heat flux of the wet region on the heater surface increases while the wet area fraction decreases with superheat, leading to the CHF. We show that a vapor recoil force due to the interfacial heat and mass transfer plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF. Compared with previous CHF models which treat CHF as an isolated point on the boiling curve, this work elucidates the triggering mechanism of CHF from a perspective of the dynamic evolution of the wet/dry region with increasing superheat, which could potentially serve as a guideline for future CHF enhancement designs. [ABSTRACT FROM AUTHOR]

• Highest HTC with jumping droplets, superhydrophobic surface in confined flow. • Reduced heat transfer at high subcooling due to superhydrophobic surface flooding. • Vapor velocity dependent heat transfer in dropwise mode (hydrophobic surface). • Lowest vapor ΔP with jumping droplets on superhydrophobic surfaces in confined flow. • Pressure drop decreased with condensation rate due to deceleration of vapor flow. Condensation is an important process in the Rankine cycle that significantly affects overall efficiency. Condensate typically forms a liquid film due to the high surface energy of industrial condenser materials; by engineering the condenser surface with a superhydrophobic layer, however, we can increase condensation heat transfer by an order of magnitude with the jumping droplet mode of condensation. While the basic phenomenon of jumping droplet condensation has been explored in depth, its effects on heat transfer and pressure drop in confined vapor flow inside a condenser tube, as in power plant condensers, have not been considered. Here, we report an experimental study of internal forced convective condensation with hydrophilic, hydrophobic, and superhydrophobic surfaces to study condensation in the filmwise, dropwise, and jumping droplet modes, respectively. The condenser tube samples were tested in a closed system internal flow condensation setup, and the heat transfer and pressure drop behavior were characterized over various operating conditions. In the jumping droplet mode, the heat transfer coefficient was highest at lower condensation heat flux and condenser surface subcooling, but a transition to the flooded mode at higher subcooling resulted in a heat transfer coefficient comparable to filmwise condensation. For dropwise condensation in the hydrophobic tube, the condensation heat transfer coefficient increased with the vapor velocity, similar to observations in past work. In addition to a large heat transfer coefficient, the pressure drop with the superhydrophobic tube samples was the lowest. These experimental results demonstrate the viability of harnessing the jumping droplet mode of condensation to enhance heat transfer and reduce pressure drop for internal forced convective flow condensation in industrial condensers. [ABSTRACT FROM AUTHOR]

• Developed a new thermodynamic model to evaluate and predict efficiencies of adsorption-based AWH systems. • Identified the thermodynamic limits, efficiencies, and optimal operating conditions based on adsorbent material properties. • Metal-organic framework (MOF)−801, MOF-303, and Ni 2 Cl 2 BTDD are studied at various operating conditions. Adsorption-based atmospheric water harvesting (AWH) technologies can enable decentralized and distributed water supplies in arid and water scarce regions with limited infrastructure. Recent advances in novel adsorbents, such as metal-organic frameworks (MOFs) and advanced zeolites, with high sorption capacity at low humidity and facile regeneration, promise the development of efficient AWH technologies. However, a comprehensive thermodynamic analysis based on fundamental material properties to predict optimal operating parameters and system-level efficiency has not been pursued. In this work, we present a generalized theoretical framework to optimize the energetic performance of thermally-driven adsorption-based AWH systems using fundamental material properties, such as adsorption isotherms. Using example characteristics of recently reported MOFs (MOF-801, MOF-303, and Ni 2 Cl 2 BTDD) with step-wise adsorption isotherms, we present AWH system-level theoretical efficiencies of each MOF based on the First and Second Law of Thermodynamics. We show the impact of heat source temperature from realistically achievable low-grade heat sources (up to 100 °C) on the overall efficiency. We also present the concept of a cascaded system which operates two adsorbent beds in series, and by capturing the condensation heat of the first bed, an increase in the overall efficiency can be achieved. At ambient conditions with relative humidities (RHs) below 40%, which is typical of arid climates, we show theoretical thermal (thermal energy to water conversion) and Second Law efficiencies of 0.33 and 0.18 with MOF-801 and MOF-303, and 0.56 and 0.19 with Ni 2 Cl 2 BTDD, respectively. For the cascaded system, a thermal efficiency of 0.7 and Second Law efficiency of 0.23 can be achieved with Ni 2 Cl 2 BTDD, over an order of magnitude greater than state-of-the-art refrigeration systems. Our framework presented can identify optimal operating parameters, and enable system-level predictions using materials properties for AWH and other related applications, including thermal energy storage, dehumidification, and desalination. [ABSTRACT FROM AUTHOR]

• Modeling framework of thermally-localized multistage solar stills (TMSS) is developed. • Heat and mass transfer in the TMSS are analyzed in detail. • Optimization strategies for the TMSS are presented. • Ultrahigh solar-thermal cumulative efficiency over 700% is predicted. Seawater desalination is a promising solution to global water shortage. Commercially available desalination technologies typically require large installations which can be impractical for developing regions without well-developed infrastructure. Passive solar desalination promises a viable solution, but can suffer from low efficiencies. Recent advances in the thermal design of small-scale solar desalination systems have demonstrated the potential for high-efficiency solar desalination in portable systems. In particular, the concept of a thermally-localized multistage solar still (TMSS) – which combines localized heating of a capillary flow with condensation heat recycling – has been experimentally demonstrated very recently and achieved over 100% solar-thermal cumulative efficiency. However, a fundamental understanding of the heat and mass transfer, efficiency limits and optimization strategies are missing in the literature. This work presents a modeling framework that evaluates the thermal and vapor transport in a model TMSS system with varying device configuration and predicts its solar desalination efficiency. We demonstrate that an ultrahigh solar-thermal cumulative efficiency, many times higher than that of conventional solar stills, can be achieved by optimizing the number of stages and device geometry. Specifically, our modeling shows that the efficiency of the capillary fed TMSS is limited by the dissipation of thermal energy to the environment during condensation and significant gains in efficiency can be achieved by minimizing this loss. This work provides insights into physical processes critical for thermally-localized portable solar distillation which could lead to high-performance desalination or water purification technologies. [ABSTRACT FROM AUTHOR]