Many electronic devices run under relatively stable power loads, allowing traditional thermal management systems to operate in steady state. However, some electronic components display unpredictable and highly dynamic operational regimes, characterized by sudden increases in thermal load. These situations require a specialized thermal management solution with high, quickly-accessible thermal capacity, enabling short-term energy storage during operation and gradual heat dissipation over more extended periods. While energy storage alone is insufficient due to inherent capacity limits, a system that transfers heat to the environment and simultaneously provides extra energy buffering during thermal pulses is highly desirable. In this study, a novel thermal management concept, termed "Thermally Capacitive Heat Exchanger" (TCHX), is developed and explored. The TCHX employs solid-liquid phase change, integrated into a heat exchanger structure. The prototype includes three coaxial pipes, where the inner pipe and outer annulus function as flow passages for hot and cold heat transfer fluids (HTFs). In contrast, the middle annulus contains the PCM, which adds thermal capacity, and carefully designed fins that facilitate heat transfer across it. However, the system suggested, designed, built and explored in this work is not a usual "triple-pipe" heat exchanger or PCM storage unit, explored to some extent in the existing literature. The novel design allows for an actual heat transfer between the hot and cold fluids, whereas a built-in latent heat storage serves to compensate for the mismatch between the instantaneous thermal load to be removed and the available cooling capacity. A dedicated experimental setup was built around the unit to assess its performance under different thermal conditions. The experimental study included several typical operation cases, including charging, discharging, and hybrid scenarios involving simultaneous heat accumulation and transfer at various rates. Each hybrid experiment demonstrated the unit's dual-mode operation: steady state and transient. At steady state with no energy accumulation, the heat transfer rate ranged from 180 to 240 W, depending on the operational regime. During the transient stage, the TCHX acted as a thermal buffer between the HTFs, where the heat input from the hot stream exceeded the heat removed by the cold one, and the surplus energy was accumulated in the unit. In this stage, the peak heat transfer rate from the hot HTF reached 700 W. A dimensionless parameter called dominance was introduced in this study to serve as a possible "scenario classifier" for the TCHX operational regimes. A simplified numerical model was developed to predict the system's behavior trends under various scenarios. The model was validated for two distinct hybrid-operation cases, and then used to perform a numerical study of additional cases, going much beyond the specific configurations explored experimentally. In particular, it revealed the dominance effects on geometrically identical configurations, along with the effect of the configurations themselves on the device performance at the same dominance. The results of the current study demonstrate that the TCHX prototype has potential as a modular "building block" for future scalable thermal management systems.

Phase change materials (PCMs) are attractive for transient thermal management of electronic devices operating either over a limited period or intermittently with occasional thermal dissipation spikes. Since the low thermal conductivity of PCMs presents a significant challenge for heat removal, various ways have been suggested to overcome this issue, which can cause overheating of the electronic devices due to the associated thermal resistance. Commonly, various thermally conductive additives, like extended surfaces, porous structures or particles, have been suggested for this purpose. The present study is an advanced exploration of an alternative approach to mitigating the high thermal resistance of PCM-based systems, based on a concept termed “dynamic PCM”. Dynamic PCMs work by applying an external load to the solid PCM, causing it to move towards the heat source during melting. The melted PCM is squeezed away, and only a thin liquid layer of practically constant thickness separates the heated surface from the solid PCM phase. Following the recent studies where this concept was introduced and confirmed in principle, the objective of the present work is to demonstrate practical implementation and operation of a sealed, cyclic system, based on the same general idea but significantly modified to meet the real-world requirements. This device is based on the “hourglass” concept, introduced and devised in a previous study. Accordingly, in the present work, a fully metallic configuration is developed to demonstrate practical operation at room temperature. The system is simple, consisting of a cylindrical tube and two end caps. The external force, required for dynamic PCM, is created by a weight which, together with the PCM, is located within the system. The heat-generating component itself serves as the driving factor for detachment of solid PCM from the envelope. As a result, the system preserves all positive features of the earlier prototype but allows for stand-alone implementation with a heat-generating component. Following the system design, fabrication and proof-of-concept runs, controlled tests at three power levels, 30 W, 40 W, and 50 W, are conducted to characterize the thermal behavior, reproducibility of results, and effective melting dynamics of the system. The heating and dynamic melting stages are successfully characterized. Then, system recharge (solidification) is explored under the conditions of free and forced convection in air. Numerical simulations, validated using the experimental results, complement the experiments while revealing important details of the underlying processes. A robust and repeatable performance of the system is demonstrated.

As the global demand for energy and high-power computing grows, thermal management and storage remain key global challenges. While phase change materials (PCMs) have been explored as solutions to address both areas, they suffer from the fundamental physical challenge of melt front propagation, which reduces their power density. Dynamic Phase Change Materials (dynPCMs) have been investigated to eliminate the challenge of melt front propagation, enabling higher energy and power densities for thermal management and storage systems. DynPCMs have been explored in previous work, however many of their underlying assumptions, governing mechanisms, and implementation considerations have yet to be analyzed. Here, we introduce the governing principles of dynPCM and analyze the validity of commonly used assumptions to understand where they may break down. We use analytical and simulation approaches to understand the dynamics within the thin melt layer present in dynPCM, including the pressure and velocity distribution. We compare dynPCM to a variety of state-of-the-art technologies in both thermal management and storage and show that dynPCM can have a more than tenfold improvement in energy and power density compared to state-of-the-art thermal storage solutions. We show that dynPCM can have substantial improvement in effective heat transfer coefficient compared to state-of-the-art thermal management solutions, including a > 100 & times; increase over conventional PCMs. Finally, we explore the implementation of dynPCM into real systems, including a variety of balance-of-plant considerations and the design of a holistic approach for dynPCM selection and evaluation.

Phase change materials (PCMs) offer effective transient cooling due to their high latent heat of fusion and energy density. Unfortunately, PCMs generally have relatively low thermal conductivity, impeding effective heat dissipation from the heat source and limiting their power density. This work uses dynamic PCM (dynPCM) cooling for thermal management of high power electronics. DynPCM cooling uses pressure-enhanced closecontact melting of a PCM. The applied pressure causes liquid PCM to be pumped away from the heat transfer surface, maintaining a thin melt layer and high heat transfer. Through experimental investigations with a circuit board mounted 2 x 2 array of gallium nitride (GaN) power transistors integrated with heat spreaders of different thicknesses, we evaluate the cooling performance of dynPCM across various device heat dissipation levels (4.4 W/cm2 to 46.6 W/cm2) and under both homogeneous and heterogeneous heating conditions. Using paraffin as the PCM, we explore the effects of different pressures (0 Pa, 750 Pa, and 7.5 kPa) on dynPCM cooling effectiveness. DynPCM significantly enhances cooling for electronics operating at high power, achieving over a 50 % reduction in steady-state junction temperature when compared to both traditional air-cooled and hybrid PCMcooled systems at a 32.4 W/cm2 individual GaN device power loss. We developed a reduced-order thermal resistance model to assess heat transfer from the electronic devices through the heat spreader into the dynPCM. The model helps to illustrate the critical role of the heat spreader design and PCM geometry on cooling performance, offering design guidelines for dynPCM thermal management systems. This work highlights the potential of dynPCM as a thermal management strategy for high-power electronic devices, facilitating the advancement of more effective cooling methods for a variety of applications.

High latent heat of phase change materials (PCMs) can be utilized for thermal management of electronic devices operating in transient conditions, either over a limited period of time or in an intermittent fashion. However, because of the low thermal conductivity of PCMs and associated thermal resistance, the device can exceed its allowed operation temperature. A common approach to deal with this problem is by using extended surfaces and their analogs, like porous structures or conductive inclusions. An alternative approach to mitigating thermal resistance in PCM-based systems, explored in the present study, is based on a novel concept termed "dynamic PCM", in which a load applied to the solid PCM causes the latter to move towards the heat source and melt in such a manner that the melted PCM is squeezed away and the melted region retains its rather small, and practically constant, thickness. Being an extension of close-contact melting (CCM), caused only by the PCM's own gravity, in "dynamic PCM" the resulting heat transfer is both enhanced and may be controlled via the regulated applied load. A novel device, termed an "hourglass" system because of its shape and its way of operation, is introduced and explored experimentally. Its dimensions and features, specifically the transparent envelope, are chosen to obtain a clear picture of the process via visualization. The force exerted on the PCM is created by completely passive means, namely, a weight which moves freely through the liquid PCM but pushes the solid PCM. The system is sealed and can be used for cyclic operation, without the need to be opened or refilled. It contains up to 220 g of eicosane, whereas the stainless steel weight is 250 or 500 g. The explored heat inputs are up to 60 W, corresponding to the heat fluxes of up to 3.8 W/cm(2). The results clearly demonstrate the advantages of the dynamic PCM: at the heat input of 30 W, the excess temperatures of the heated body over the PCM melting point are about 11 and 8 centigrade for the weight of 250 g and 500 g, respectively, while in pure CCM without a weight this value is about 24 degrees C. Robust and repeatable performance of the system is demonstrated for various heat inputs, PCM amounts and added weights. A complete periodicity of the operation is achieved, with turning the system around for recharge being the one and only mechanical action required because of the cyclic character of its operation. To provide further insights into the physical phenomena involved, an in-house numerical model is formulated, allowing for the inclusion of the added weight in the force balance. Then, a physically-meaningful dimensional analysis is performed, based on the appropriately defined Stefan, Fourier and Archimedes numbers and involving the mass ratio of the added weight and the PCM itself. This analysis successfully generalizes the predicted and experimentally achieved melt fractions and Nusselt numbers. It indicates that for the systems of this type, it is possible to define system parameters needed to cope with expected heat loads in a prospective application. The ways of practical implementation of the explored type of systems are outlined and discussed.

The present study deals with melting in a geometry suitable for Latent-Heat Thermal Energy Storage (LHTES) systems, which are of importance for future industrial installations utilizing solar energy or waste heat. The intrinsically high thermal resistance of phase-change materials (PCM) can be remedied by taking advantage of close contact melting (CCM), where the solid phase is separated from a hot surface by only a thin liquid layer. The basic CCM takes place on a horizontal flat surface, but during the last decade its application in various finned LHTES units has been demonstrated experimentally and analyzed numerically.Specifically, the configuration studied in the present work is relevant to a horizontal double-pipe concentric storage unit with a longitudinally finned inner tube. While this rather simple geometry has been extensively studied in the past, the present work is completely novel in its demonstration and analysis of simultaneous close-contact melting on two generally asymmetric surfaces created by the longitudinal fins. A unique experimental apparatus is introduced, based on the so-called 'Mercedes' configuration of the fins. It is designed transparent, allowing for real-time observation of melting sequences. Close-contact melting is achieved by supplying heat to the outer shell of the unit: the solid phase is detached from the shell and moves, by translation and rotation, in the liquid phase. The results from these experiments demonstrate the feasibility of CCM on two asymmetric walls within this system, hinting at the potential for optimizing the melting rate by utilizing a larger portion of the extended surface for CCM.A key component of this study is the development of a reliable numerical approach, which goes beyond the limitations of widely-applied enthalpy-porosity method and combines the general enthalpy formulation, convective heat transfer and general rigid body motion that includes rotation. Therefore, a new numerical model has been devised, using an in-house code realized in MATLAB. A full set of the governing conservation equations is solved using a finite-difference framework, integrated with advanced numerical methods for the fluid-solid interaction and an enthalpy formulation for the phase change process. The model is validated carefully, and then numerical studies are conducted to elucidate the melting process.The present paper presents a further proof of the special role that close-contact melting (CCM) can play in properly designed thermal energy storage units and finned systems in general. It is demonstrated that the fins, when properly designed and oriented, can induce CCM in their vicinity, contributing to a very significant increase in the melting rate, which reflects charging of the unit.

Phase change materials (PCMs) present opportunities for efficient thermal management due to their high latent heat of melting. However, a fundamental challenge for PCM cooling is the presence of a growing liquid layer of relatively low thermal conductivity melted PCM that limits heat transfer. Dynamic phase change material (dynPCM) uses an applied pressure to pump away the melt layer and achieve a thin liquid layer, ensuring high heat transfer for extended periods. This paper investigates heat transfer during dynPCM cooling when the heated surface has extended features made from high thermal conductivity copper (Cu). Using experiments and finite element simulations, we investigate the heat transfer performance of dynPCM paraffin wax on finned Cu surfaces. A total of 102 transient temperature measurements characterize the performance of dynPCM with extended surfaces and compare the performance with other cooling methods including hybrid PCM and air cooling. The study examines the effects of fin geometry, applied power (20–65 W), and pressure (0.97–12.5 kPa). For dynPCM on a finned surface and a heating power of 65 W, the thermal conductance is 0.45 W/cm2-K, compared to 0.22 W/cm2-K for dynPCM on a flat surface and 0.10 W/cm2-K for hybrid PCM. The heat transfer is highest at the fin tips where the melt layer is thinnest, providing valuable design guidelines for future high performance dynPCM cooling technologies.

To accommodate societal electrification and decarbonization, renewable energy resources continue to expand their share of the global energy market. The intermittency of renewable energy technologies as well as the high power density of modern electrified platforms necessitates the need for both efficient thermal management and high-density thermal storage. Phase change materials are a promising passive thermal energy storage solution. However, difficulties with efficient system implementation stemming from the inherent melt pool formation hinder their potential. We develop an innovative strategy, termed dynamic phase change material “dynPCM,” to address this thermal transport issue by ensuring a constant, thin, melt layer. We analyze the fundamental limits of dynPCMs, characterize the peak achievable heat flux and energy/power densities, estimate the power consumption of dynPCM systems, and investigate the fundamental physics which govern dynPCM behavior. We show that dynPCM can eliminate the classical trade-off seen between energy density and power density and achieve ultrahigh heat fluxes, ∼105 W/cm2, with heat flux-to-required power ratios as high as ∼107. We also demonstrate achievable power densities as high as ∼100 W/cm3 at energy densities as high as ∼10 kJ/cm3. Throughout this work, we develop a methodology to evaluate the operating limits, enabling adaptation of the dynPCM system concept to a variety of applications and industries.

Abstract One of the main challenges for latent thermal energy storage (LTES) systems is low heat transfer rates due to the low thermal conductivity of most phase change materials (PCM). Close-contact melting (CCM) can accelerate melting times in LTES systems but the current numerical techniques for solid liquid phase change have difficulties with accurately predicting this process. In this study, close-contact melting of PCM on an inclined surface is simulated using the enthalpy-porosity method in ANSYS Fluent. All PCM properties, including density, are temperature-dependent. In this way, phenomena such as natural convection, volume change and buoyancy between the solid and liquid are taken into account. The volume change is compensated by a gaseous expansion volume. Both 2D and 3D simulations are used to show discrepancy between state-of-the-art enthalpy porosity modelling and experimentally observed phenomena in the case of CCM. The mushy zone constant, which is set to 105 to allow motion of the solid bulk, causes the solid phase to deform as a highly viscous fluid instead of moving as a rigid body. The velocity differences inside the solid are more than 50 % of its sinking velocity. As a result, the movement of the solid resembles creep behaviour and the obtained CCM patterns are not physically accurate. Furthermore, the density difference between the solid and liquid phases causes an avalanching effect in the mushy zone, which artificially strengthens convection. In conclusion, the enthalpy porosity method exhibits significant limitations in accurately capturing close-contact melting phenomena.

The growing interest in phase-change materials (PCM) is related to their possible role in thermal energy storage and thermal management. The choice of materials depends strongly on the required temperature range, whereas the latent heat of solid–liquid phase transition has to be as high as possible. Among other organic PCM, sugar alcohols have gained some attention due to their availability and certain advantageous properties. However, the thermal processes in these materials still require investigation. In the present work, we focused on the materials with solid–liquid phase change within 80 °C–100 °C. A comprehensive literature survey was conducted to elucidate the available sugar alcohols relevant to this range. It was found that the use of pure materials of this type is not very practical, because of their scarcity in the required range and their specific features, like difficulties with crystallization and solidification. On the other hand, based on the literature, we have discerned three eutectic mixtures of erythritol with other organic materials, namely, erythritol–xylitol, erythritol–urea and erythritol– trimethylolethane (TME). In all those cases, it is remarkable that while the components commonly have rather high melting temperatures, the eutectic mixtures had the phase transitions in the required range. Still, each of these mixtures has its own peculiar features, especially at cooling and solidification. An extensive experimental study was performed to provide detailed visualization of these major processes. The results revealed the melting temperature and latent heat of the mixtures to be: 84 °C and 190 J g −1 for erythritol–xylitol, 82 °C and 227 J g −1 for erythritol–urea. Erythritol–TME has two phase transitions at 82 °C and 97 °C, with total latent heat of 198 J g −1 . Based on the present findings, the erythritol–urea mixture is the best PCM candidate for the melting range within 80 °C–100 °C.

The use of phase change materials (PCMs) has gained much attention for applications of transient ther-mal management of electronic systems due to their high latent heat and ability to absorb heat near -isothermally. Because of their low thermal conductivity, PCMs are usually integrated with heat sinks to be used more efficiently. In this study a PCM-based heat sink with a generic structure of plate fins was investigated experimentally and numerically. The intentionally simple fin topology allowed focusing on the effects of various material properties rather than the commonly investigated geometry effects. Hence, two types of heat sink material, copper and aluminum, were examined, and two PCMs with similar melt-ing temperatures but distinctly different thermal properties - a metallic alloy (Field's metal) and an or-ganic paraffin (n-Octacosane), were used. Experimental findings allowed for validation of the numerical approach, used for a comprehensive parametric numerical study. The latter facilitated a more detailed investigation of the transient heat transfer processes, such as melting patterns and heat accumulation analyses, where the superior thermal properties of the metallic PCM manifested in more efficient latent heat accumulation, resulting in reduced system peak temperatures. It was found that systems with Field's metal were able to accommodate up to 80% of the energy in the form of latent heat, which is 10 percent-age points higher than achieved using the organic paraffin. A dimensional analysis accounting for power inputs and material properties was conducted, and a generalized behavior was achieved for a normalized time in terms of Fourier and Stefan numbers, and thermal diffusivities ratio. (c) 2023 Elsevier Ltd. All rights reserved.

Thermal energy storage (TES) is considered vital for the advancement of renewable energy solutions. Latent heat thermal energy storage (LHTES) captures the thermal energy via a solid-liquid phase transition that occurs in phase-change materials (PCM). The PCM is usually encapsulated in some way. In this study, we consider PCM melting in a vertical cylindrical enclosure, that is a prototype of a capsule used in a future storage system. Moreover, we achieve the highly desirable close-contact melting (CCM), which speeds up the charging process significantly.First, different types of experiments are conducted to elucidate the melting behavior in an original experi-mental device that allows different types of melting in such configuration, demonstrating the prominent role of close-contact melting. The device main features are transparency of the melting chamber, separate heating methods for the periphery and the bottom of the chamber, and axisymmetric melting. To further investigate the heat transfer modes in the system, an in-house numerical model for combined convective and close-contact melting in an axisymmetric cylindrical geometry is validated with the experimental results. Then, this original model is used to simulate a vertical cylindrical shell in a practical configuration, namely, when an enclosure is exposed to fluid flow normal to its bottom. The results clearly illustrate the importance of close-contact melting for the overall melting process under practical conditions. The conclusions from this work would aid the design of LHTES installations which involve macro-encapsulated PCM.

The use of phase change materials (PCM) for latent heat thermal energy storage (LHTES) is receiving considerable amount of attention in recent years. Experimental findings have demonstrated that the so-called 'close contact melting' (CCM) enhances heat transfer during the melting process remarkably. Yet, the commercially-available numerical schemes are not suitable for the modeling of CCM. Therefore, in this study a new numerical model for combined convective and close-contact melting in an axisymmet-ric cylindrical geometry is devised. A full set of the governing conservation equations is solved using finite differencing framework, integrated with an advanced immersed boundary method for the fluid-solid in-teraction and enthalpy formulation for the phase change process into an original in-house code. First, the model is validated carefully for each physical phenomenon with known benchmarks. Then, a numerical study is conducted to elucidate the melting process in a vertical cylindrical enclosure heated isother-mally from the bottom and side wall. Twelve study cases are considered in order to reveal the transient phase-change sequence dependence on the aspect ratio and the excess temperature. Detailed data on the flow and temperature fields are obtained. The overall results are generalized using a dimensional analysis, which includes the Fourier, Stefan and Archimedes numbers and the enclosure aspect ratio. A correlation for the melt fraction, suitable for all cases studied, is suggested. (c) 2021 Elsevier Ltd. All rights reserved.

This study investigates time-dependent heat transfer with periodic excitation in micro-scale systems. Specifically, this study sheds light on time and length scales relevant to periodic heat transfer in micro systems. First, a system's substrate is modeled as a slab of finite thickness, in which the heat conduction equation is solved analytically for a periodic temperature boundary condition over the entire range of transient-periodic process. Using the analytical solutions, the system reaction in time is characterized for time scales and material properties typical for micro-systems. A "penetration depth" is defined as a parameter which indicates the maximum distance from the periodically-heated boundary/surface at which the periodic-thermal excitations are still noticeable. Then, as a case study, an experimental device is examined that uses a round, impinging water jet to cool a surface heated by pulsing laser. Finally, a three-dimensional numerical simulation, validated versus experiments, is used to elucidate the system's expected thermal behavior, including spatial and temporal temperature field variation, relevant time scales for measurements, and the spatial distribution of the heat transfer coefficient. It is demonstrated that the analytical findings can serve to characterize the real behavior rather accurately. The findings can assist in the design of systems with unsteady heating, and in future studies aiming at understanding more complex physically-driven transient phenomena, like flow boiling in micro-systems.

An experimental and numerical study was conducted to elucidate local heat transfer processes under multiple microscale jet impingements using a dielectric coolant, HFE-7000. The micro device used in this experiment was made of a 400-mu m thick silicon wafer, a 210-mu m thick vinyl sticker, and a 1-mm thick Pyrex substrate. Fourteen jet orifices were etched using deep reactive ion etching (DRIE) on the silicon wafer, and four 100-nm thick resistance temperature detectors (RTDs) and a heater were fabricated from titanium on the Pyrex substrate. The double-sided vinyl sticker was used to bond the layers and to form a micro fluidic channel having dimensions of 1.9 mm x 14.8 mm x 210 mu m. Jet Reynolds numbers in the experimental study ranged from 162 to 4057 and nominal heat fluxes ranged from 10 W/cm(2) to 80 W/cm(2). A three-dimensional numerical model was developed to predict the jets hydrodynamics and the convection heat transfer coefficients using a turbulent flow model for the turbulent range of the flow. Good agreement was found between the numerical predictions and experiments. The numerical results also provided valuable insight into the flow patterns formed due to multiple jet interactions. Average Nusselt number and pressure drop coefficient values found in the present study correspond well with the existing correlations pertinent to jet array impingement.