Microencapsulated Phase Change Materials Research Articles

Experimental Investigation of the Viscosity and Density of Microencapsulated Phase Change Material Slurries for Enhanced Heat Capacity and Transfer

Working fluids that incorporate solid microencapsulated phase change materials (MPCMs) can benefit from properties such as density and viscosity, which are crucial for improving heat capacity and transfer. However, limited data are available on these parameters for specific slurry and mass ratios. In this study, we present a comparative analysis of the experimental results on the viscosity of three different MPCM aqueous dispersions, namely MPCM 31-S50, MPCM 25-S50, and Micronal 5428X. Varying MPCM mass ratios of distilled water were used to obtain different mass concentrations of the phase change material (PCM), and the resulting slurries were analysed at temperatures ranging from 15 to 40 °C. Our findings showed that all slurries exhibited non-Newtonian characteristics at low shear rates, with viscosity stabilising at higher shear rates, resulting in the characteristics of a Newtonian fluid. The viscosity results were highly dependent on the type of MPCM base dispersion, particularly at high mass ratios, with the slurries having viscosities higher than those of water. Furthermore, we conducted density experiments as a function of temperature, using a flow test setup and a Coriolis flowmeter (Endress+Hauser, Reinach, Switzerland) to determine the density of two MPCMs, namely MPCM 25-S50 and Micronal 5428X. The test samples were prepared at mass concentrations of 10%, 15%, and 20% of the phase change material. We found significant differences in density and viscosity for different MPCM slurries as a result of both the PCM concentration and the material studied. Our results also revealed an apparent PCM phase change process, in which the slurry density significantly decreased in the temperature range of the phase transition from solid to liquid.

Application of microencapsulated phase change materials in prefabricated building projects based on building information modeling technology

Faced with the rapidly increasing urban population, there is a surge in demand for building construction, accompanied by an incessant rise in building energy consumption. To promote sustainable development and mitigate building energy consumption, this study proposes the utilization of microencapsulated phase change materials in prefabricated buildings through the integration of building information modeling. We prepare polymethyl methacrylate/n-eicosane/wood cellulose nanocrystal microcapsules using Pickering emulsion through the solvent evaporation method. The results showed that the material exhibits optimal comprehensive performance at a core–shell ratio of 2:1, with a melting enthalpy of 152.10 J/g and a coating rate of 60.27%. However, when the core–shell ratio increases to 2.5:1, significant performance degradation occurs due to phase change material leakage. The materials show excellent cycling performance in cold-hot cycling tests, maintaining their melting enthalpy value above 97% even after 100 cycles. Moreover, materials with a core–shell ratio of 2:1 exhibit good thermal stability up to high temperatures (160 °C). In conclusion, this study has great significance for advancing the progress of prefabricated buildings and sustainable buildings.

Improving freeze resistance of cement-based materials with modified graphite-paraffin composite low-temperature microencapsulated phase change materials

This study examines the influence of microencapsulated phase change materials (MPCM) on enhancing the freeze-thaw resistance of cementitious structures, overcoming the limitations commonly associated with traditional phase change materials (PCM), such as insufficient thermal conductivity and non-optimal phase change temperatures. The research focused on developing MPCM with improved thermal conductivity and lower phase transition points, utilizing a binary mixture of tetradecane and hexadecane as the core material, supplemented by modified graphite. The phase change performance and physicochemical stability of the prepared MPCM were thoroughly assessed. Cement specimens, incorporating different contents of MPCM, were then subjected to a series of tests under both room temperature and simulated freeze-thaw conditions. These tests included mechanical testing, temperature response experiments, and monitoring of internal damage. The findings identified the optimal MPCM formulation as a 9:1 mixture of tetradecane and hexadecane, with an addition of 2 % hydrophobically modified graphite, which exhibited a phase change temperature near 0 °C and a range exceeding 6.7 °C, along with stable physicochemical properties. When 3 % MPCM was included in cement specimens cured for 28 days under freeze-thaw conditions, a significant reduction in internal damage caused by freeze-thaw cycles was observed, leading to an increase in compressive strength by 0.12 % and in flexural strength by 11.77 %. Additionally, temperature tests confirmed the efficacy of MPCM in stabilizing internal temperature fluctuations within the cement specimens. The results of this study present a viable approach to enhance the durability of cement-based infrastructure in cold climates.

A novel thermal-tailored strategy to mitigate thermal cracking of cement-based materials by carbon fibers and liquid-metal-based microencapsulated phase change materials

Massive concrete structures are susceptible to thermal cracking due to the large surface-to-internal temperature gradients induced by the exothermic cement hydration, threatening their durability and safety. This study explored a novel thermal-tailored strategy to mitigate thermal cracking of cement-based materials by incorporating carbon fibers (CF) and liquid-metal-based microencapsulated phase change materials (MEPCM). Specifically, spherical MEPCM composed of In-Bi-Sn alloy cores coated by polyvinyl alcohol shells was prepared through a high-speed liquid-phase dispersion method, with average diameters of 36.26 μm and a high volumetric latent heat of 214 MJ/m3. The synergistic effect of CF and MEPCM on the microstructure, mechanical and thermal properties of cement mortars was further investigated. Upon adding 0.3 wt% of CF and 3.2 vol.% of MEPCM, the flexural strength of cement mortars exhibited a significant increase of 30.3 %. Microanalysis revealed that the molten MEPCM could flow to fill the pores and microcracks within its surrounding matrix and form a network skeleton with well-dispersed CF, imparting flexibility to the typically rigid cement hydrates. By this way, the thermal conductivity of cement mortars was increased by 21.7 % and internal temperature rise was decreased by 12 ℃ by adding 4.8 vol.% of MEPCM, contributing to the mitigation of risks associated with thermal cracking. The research outcomes will provide valuable insights into the development of thermal cracking-resistant cement-based materials in a highly efficient way.

Development and application of low-melting-point microencapsulated phase change materials for enhanced thermal stability in cementing natural gas hydrate layers

In the exploration of deep-water oil and gas resources, encountering natural gas hydrate (NGH) layers poses significant challenges, particularly during cementing operations critical for wellbore stabilization. This study proposes a strategy employing cement slurries integrated with micro-encapsulated phase change materials (MPCMs) to address the destabilization of NGH layers, a key obstacle in deep-water cementing. MPCMs containing BaCO3 shell and low-melting binary PCMs in the core were synthesized by the self-assembly method. The structure and thermal properties of MPCMs were thoroughly characterized. MPCMs exhibited a distinct core-shell structure with a hydrophilic and well-sealed shell. Their phase change temperatures were in closely matched the temperatures found in NGH formations, at 3.5 °C and 13.9 °C. A comprehensive evaluation of the fundamental properties of cement slurries containing MPCMs (CSM) was conducted, including thermal stability, setting time, pore structure, and mechanical strength. Additionally, a laboratory device was employed to simulate cementing operations, allowing for intuitive analysis of the impact of hydration heat evolution on NGH stability—a notably underexplored aspect in related literature. The results demonstrated that MPCMs effectively reduced the hydration heat generated by cement slurry within the initial 36 h. Specifically, at a 9% MPCMs dosage, the hydration heat of CSM decreased by 19.0%, resulting in the sustained stability of NGH. In contrast, the control group's cementing process directly led to the complete decomposition of NGH. Even at lower MPCMs concentrations, the decomposed NGH was more likely to recrystallize and return to a stable state due to the slower hydration heat release rate of CSM. This study not only provides evidence of the effectiveness of MPCMs as heat inhibitors in reducing the disruption of hydrate stability during well cementing but also provides theoretical guidance for the design and application of cement slurries for NGH reservoirs. By enhancing the stability of NGH layers, this work supports the safe and efficient exploitation of undersea hydrocarbon resources, contributing to shaping future global energy strategies.

Thermo-mechanical analysis of microencapsulated phase change material incorporated in concrete pavement

This study investigates the potential of enhancing thermal performance in pavement concrete by integrating microencapsulated phase change materials (PCM@SiO2) using a Sol-gel process for encapsulation. With a concentration range from 0% to 20% in 5% increments, the effects of PCM@SiO2 on both the mechanical strength and thermal performance of concrete were evaluated. Although the addition of PCM@SiO2 resulted in a compromise in mechanical strength, significant improvements in thermal performance were observed, demonstrating a promising approach to mitigating temperature stresses and the urban heat island effect in pavement construction. This balance between mechanical and thermal properties underscores the potential of PCM incorporation as a strategic material choice for innovative urban development.

Utilization of waste foam concrete with MPCM as a substitution material for cement in mortars

Given the rising popularity of foam concrete (FC) for both structural and insulating purposes, evaluating the feasibility of recycling after its lifespan is crucial in the context of the growing emphasis on sustainable building practices. One approach to recycling FC incorporating microencapsulated phase change material (MPCM) involves utilizing recycled foam concrete powder (RFCP) as an additive in cement composites. This article aims to investigate the impact of RFCP without and with MPCM when employed as a partial replacement for cement in mortars. Furthermore, the study verifies various processing methods such as crushing, grinding, and heat treatment for RFCP. The results reveal that introducing RFCP, regardless of the MPCM presence and processing method, significantly affects the properties of both cement and mortar. The presence of MPCM in RFCP negatively influences the flowability of fresh mortars, delays the setting time, and reduces the hydration heat within the first 48 h. However, the presence of MPCM does not significantly affect mortars' strength and water absorption but simultaneously it increases shrinkage and decreases thermal conductivity. Grinding RFCP mitigates the adverse effects of MPCM, while thermal processing removes MPCM from RFCP, albeit with an associated increase in water demand. A noteworthy finding is that mortars having 20 % RFCP, with or without MPCM, exhibit compressive strengths exceeding 16 MPa and 42.5 MPa after 2 and 28 days, respectively. These results meet the requirements outlined in EN-196-1 for cement of class 42.5, highlighting the potential to produce CEM II/A-F 42.5 using RFCP with MPCM.

Heat transfer enhancement in a loop thermosyphon with microencapsulated phase change material suspension

The microencapsulated phase change material suspension (MPCMS) is a novel functional thermal fluid. The application of MPCMS to loop thermosyphon is an innovative attempt, which can effectively improve the heat transfer efficiency of loop thermosyphon. In this paper, a three-phase loop thermosyphon using water-based MPCMS as the medium is developed. A comprehensive experimental investigation was conducted to study the heat transfer performance by varying the filling ratio (50 %, 60 %, and 70 %), heat flux (25 − 125 W/cm2), and mass concentration (0 %, 0.1 wt%, and 1 wt%). The findings of this study indicate that the loop thermosyphon incorporating MPCMS exhibits improved heat transfer performance compared to a two-phase loop thermosyphon utilizing pure water. The addition of MPCMS has been observed to reduce the wall temperature by up to 6.6 °C and decrease the loop thermal resistance by 13.5 %. Furthermore, the loop thermosyphon with MPCMS demonstrates superior start-up characteristics and thermal homogeneity when compared to water-based systems.

Study on the Interaction of Polymeric Chemical Additives with Phase Change Materials in Air Lime Renders.

The interaction of microencapsulated phase change materials (PCMs) with polymeric chemical additives in an air lime binding matrix was studied. These polymer-based additives included an adhesion booster (derived from starch) and a superplasticizer (polycarboxylate ether). Two different PCMs with melting points of 18 °C and 24 °C were assayed. The microcapsules were composed of melamine, with paraffin-based PCM cores. Measurements of zeta potential, particle size distribution, adsorption isotherms, and viscosity analyses were performed to comprehend the behavior of the polymer-based additives within the air lime matrix and their compatibility with PCMs. Zeta potential experiments pointed to the absence of a strong interaction between the lime particles and the microcapsules of PCMs. At the alkaline pH of the lime mortar, the negative charge resulting from the deprotonation of the melamine shell of the microcapsules was shielded by cations, yielding high positive zeta potential values and stable dispersions of lime with PCMs. The polycarboxylate ether demonstrated the ability to counteract the increase in mixing water demand caused by the PCM addition in the lime matrix. The dispersing action of the superplasticizer on the lime particles was seen to exert a collateral dispersion of the PCMs. Conversely, despite the positive values of zeta potential, the addition of the starch-based additive resulted in the formation of large PCM-lime clumps. Air lime renders incorporating 5, 10, and 20% PCMs by weight with various dosages of these chemical additives were experimented with until the optimal formulation for the specific application of the mortars as renderings was achieved. This fine-tuned formulation effectively tackled issues commonly associated with the addition of PCMs to mortars, such as poor adhesion, crack formation, and reduced fluidity.

Temperature control for the synthesis of n-butylmagnesium bromide Grignard reagent based on n-octadecane/polystyrene microPCMs

Thermal runaway accounts for a high proportion of chemical safety accidents and poses a serious threat to the operational safety of personnel. Phase change materials (PCMs) have a high enthalpy of melting, if the latent heat energy of their phase change can be stored and used to absorb the heat released during thermal runaway, this phenomenon can provide a new approach for the control of reaction thermal runaway. In this work, microencapsulated phase change materials (microPCMs) were prepared by the suspension polymerization method using n-octadecane as the core material and polystyrene (PS) as the shell material. The prepared microPCMs had regular spherical morphologies with smooth surfaces, and the average particle size was 7.54 μm, the thermal conductivity was 0.05964 W/m∙K. The melting enthalpy of the microPCMs was 142.2 J/g, and the encapsulation percentage of the PCMs was 62.15%. The synthesis of the n-butylmagnesium bromide Grignard reagent occurred via a strong exothermic reaction, which was very likely to lead to overpressure venting once control was lost. The prepared microPCMs were applied as inhibitors in this reaction to investigate the temperature control effect under different operating conditions. The results showed that the amount of microPCMs added, and the timing of the addition affected the control effect. The addition of microPCMs could effectively reduce the reaction temperature and slow the rate of temperature increase. Even in the event of an emergency situation, such as a control failure, time could be gained for other emergency measures, effectively reducing the probability of thermal runaway accidents.

Heat-transfer characteristics of the two-phase-change process between a microencapsulated phase-change-material-slurry droplet and air: A numerical study

A concept is proposed for spray-heat exchangers with the aim of addressing the mismatch of heat supply-demand within and outside a water droplet. The concept involves using a microencapsulated phase-change-material (MPCM) slurry as the working medium, enabling two-phase-change heat transfer with air. The temperature of the water droplet rapidly decreases owing to continuous evaporation, leading to a rapid decrease in its heat-transfer capacity. The phase change of the MPCM particles not only provides sufficient heat for evaporation but also maintains a high surface temperature, thus sustaining effective evaporation. A numerical model was developed to theoretically investigate the effect of the MPCM phase change on external convection. The heat-mass transfer of a droplet with a radius of 0.25 mm composed of water and 20 vol% MPCM was simulated. The results indicate that the MPCM phase change effectively reduces the temperature decrease rate. However, when the overall surface temperature of the MPCM droplet exceeds that of water, the total heat-transfer coefficient is not necessarily higher. Further local analysis reveals that maintaining both an overall high surface temperature and a reasonable local temperature distribution is crucial to achieve a higher total heat-transfer coefficient. The MPCM phase change significantly influences the local surface-temperature distribution.

Enhancing the hygrothermal performance of corn cob residue-based eco-friendly building materials through biochar and microencapsulated phase change material incorporation

Amid mounting environmental concerns and the relentless advance of climate change, this study delves into the potential of biochar derived from corn cob waste to enhance the hygrothermal performance of bio-based building materials. Focused on eco-conscious construction, the research explores the integration of biochar and phase change materials (PCMs) to reduce carbon emissions and improve energy efficiency. Results show that adding biochar at 5 % and 10 % ratios increases surface area and porosity, enhancing PCM loading and energy storage. Biochar also improves thermal stability by approximately 13.3 %, delaying PCM oxidation and boosting resilience. The addition of 10 % corncob biochar yields optimal heat capacity and energy storage. Furthermore, biochar increases the water vapor resistance coefficient by 84.57 % compared to materials without biochar. Although there is a slight decline in thermal conductivity across different humidity levels, attributed to the enhanced water vapor resistance coefficient, the overall insulation performance remains robust. In summary, this study demonstrates the enhanced hygrothermal potential of biochar derived from bio-waste and exhibits high compatibility with phase change materials, contributing to sustainable building.

Optical, electrical, and thermal performance of low-concentrating photovoltaic/thermal system using microencapsulated phase change material suspension as a coolant

Concentrated photovoltaic/thermal (C-PV/T) systems exhibit superior output compared to common PV/T collectors. To address potential issues caused by excessive concentration of conventional parabolic concentrators and enhance the electro-thermal co-generation capability, this work proposes a low-concentrating PV/T (LC-PV/T) system that integrates two improved concentrator designs and a coolant of microencapsulated phase change material suspension (MPCMS). A numerical model is established to investigate the effects of working conditions and meteorological conditions on the optical, electrical and thermal performances. The results indicate that the improved concentrator designs effectively prevent over-concentration of sunlight while maintaining a comparable concentration ratio (CR). The electrical output increases with an increase in mass fraction, inlet mass flow rate and solar irradiance, but decreases with an increase in ambient temperature and inlet temperature. Conversely, thermal output increases with an increase in all of these parameters except for the inlet temperature. This system can achieve 17 % electrical efficiency and 72 % thermal efficiency due to the high-efficiency solar cells, optimized concentrator design, and unique MPCMS coolant. The system can produce a maximum electrical power of 4.76 kW/m2 and a maximum thermal power of 19.53 kW/m2. Throughout the day, total electrical and thermal energies are approximately 4.1 kWh and 58.8 MJ.

Experimental investigation on the physical and mechanical properties of silty clay enhanced by microencapsulated phase change materials

Microencapsulated phase change materials (mPCM) can absorb or release heat by transforming their core phase. This study investigated the effect of mPCM on the thermal and mechanical properties of silty clay in seasonally frozen strata. It analyzed and researched the thermal and mechanical properties of silty clay blended with different contents (2%, 4%, and 6%) of mPCM, as well as its changing rules, using the DSC thermal cycling test, the specific heat capacity test, the freeze-thaw (F-T) cycling test, the no-limit compressive strength test, and the microscope observation test. In addition, this study utilized hyperspectral equipment to assess the applicability of improved silty clay soils in practical engineering on seasonally frozen ground. The results show that mPCM has no supercooling phenomenon and has stable and reversible transformation characteristics, which improves the thermal stability of silt. The specific heat capacity of silt increased with the increase of mPCM dosage. The unconfined compressive strength of silty clay increased to 162.5 kPa at 2% dosage, which was 16.8% higher than that of silty clay, while the unconfined compressive strength of silty clay at 6% dosage decreased to 119.1 kPa, which was 14.4% lower than that of silty clay after freeze-thaw cycles. Adding mPCM reduces the microscopic damage to the pore structure of silty clay soils caused by the freeze-thaw process and mitigates the macroscopic attenuation of their mechanical strength. The mPCM can effectively reduce the solar radiation reflectivity of silty clay, thus providing long-term utility for winter projects.

Large supercooling of Al[sbnd]Si alloy in microcapsule composed of Al2O3 shell

Al–Si-based microencapsulated phase change materials (MEPCMs) with a eutectic temperature of 577 °C are promising thermal energy storage (TES) materials owing to their large latent heat. Various preparation methods for Al–Si-based MEPCMs with Al2O3 shells have been reported; interestingly, Al–Si-based MEPCMs have large supercooling that does not depend on the preparation method. Supercooling is an important property of MEPCMs in the engineering aspects of designing a TES system. However, limited studies have reported the mechanism of supercooling in MEPCM. Moreover, the large supercooling of AlSi alloys in microcapsules compared with bulk materials is an interesting phenomenon in the field of physical chemistry. This study investigates the supercooling phenomenon in eutectic, hypoeutectic, and hypereutectic AlSi alloy MEPCMs, that is, AlSi alloys in a microcapsule comprising an Al2O3 shell. For the MEPCM with a hypoeutectic composition, supercooling is not observed because the primary Al acts as a nucleation site. For MEPCMs of eutectic and hypereutectic compositions, supercooling is observed at 25–37 °C. When the MEPCMs are heated to 1000 °C and then cooled, the supercooling degree increases significantly. In MEPCMs with a hypoeutectic composition, the precipitation of primary Al crystals does not occur under these conditions. Such findings contribute to the optimization of the design and operation of TES systems.

CFD modeling of a triple‐walled direct absorption evacuated tube solar collector based on hybrid nanofluid/microencapsulated PCM

AbstractNowadays, direct absorption solar collectors are a new concept of solar collectors which have attracted special attention. The type of heat transfer fluid (HTF) has a significant influence on the efficiency of this kind of collector. Therefore, in this study, a wide range of working fluids including mono nanofluid ( and CuO), binary nanofluid, and especially the incorporation of a new hybrid combination of nanofluid and microencapsulated phase change material (MPCM), are used as the working fluids of a triple‐walled direct absorption evacuated tube solar collector. The computational fluid dynamics (CFD) method is used for simulating the collector and investigating the effect of different parameters including volume fraction, base fluid, mass flow rate, and type of absorber tube structure (double‐ and triple‐walled) on collector thermal performance. Results show that binary nanofluids of 0.06% /0.002% CuO/water and 0.06% /0.002% CuO/ethylene glycol (EG) have the largest working fluid temperature differentials equal to 48.31 and 66.5 K, respectively. It was inferred that, at the mass flow rate of 2.7 , the efficiency is obtained to be 60.29% employing binary/EG nanofluid, which is 8.68% and 15.31% higher than CuO/EG and /EG nanofluids, respectively. Inserting the hybrid CuO nanofluid/MPCM leads to an improvement of 1.14% and 1.22% in the efficiency of triple‐ and double‐walled collectors, respectively, with respect to the individual usage of the nanofluid. The thermal efficiency of the double‐walled structure is higher than triple‐walled considering all HTFs.

Phase-Transition-Promoted Thermoelectric Textiles Based on Twin Surface-Modified CNT Fibers.

With the fast development of new science and technology, wearable devices are in great demand in modern human daily life. However, the energy problem is a long-lasting issue to achieve real smart, wearable, and portable devices. Flexible thermoelectric generators (TEGs) based on thermoelectric conversion systems can convert body waste heat into electricity with excellent flexibility and wearability, which shows a new direction to solving this issue. Here in this work, polyethylenimine (PEI) and gold nanoparticles (Au NPs) twin surface-modified carbon nanotube fibers (CNTFs) were designed and prepared to fabricate thermoelectric textiles (TET) with high performance, good air stability, and high-efficiency power generation. To better utilize the heat emitted by the human body, microencapsulated phase change materials (MPCM) were coated on the hot end of the TET to achieve the phase-transition-promoted TET. MPCM-coated TET device could generate 25.7% more energy than the untreated control device, which indicates the great potential of the phase-transition-promoted TET.

Physico-mechanical properties and thermal monitoring performance of thermal enhanced cement slurry-coated LWAs containing microencapsulated phase change material

Over the past decade, phase change materials (PCMs) have emerged as promising solutions for thermal energy storage (TES) systems, aimed at minimizing heating and cooling energy requirements in buildings. Nevertheless, despite their potential, there are some significant challenges in effectively integrating PCMs into building components. As part of this study, lightweight aggregates (LWA) were coated with a cement slurry containing microencapsulated phase change material (MPCM) to produce lightweight concrete (LWC) with the aim of investigating its mechanical and thermal properties. The LWAs were coated with MPCM at proportions of 2.5%, 5%, and 7.5% of their weight, and an LWC-MPCM was produced using these coated aggregates. The LWC-MPCM exhibited a decrease in dry unit weight up to 1248 kg/m3 and a reduction in thermal conductivity up to 0.60 W/mK with negligible loss of strength. SEM examinations revealed that the cement slurry coating provided strong adhesion to the aggregates, resulting in a robust concrete-aggregate interface. The room with the LWC-MPCM experienced a decrease of approximately 0.23 °C in center temperature compared to the reference room during the daytime. Additionally, after sunset, the LWC-MPCM showed an increase of approximately 1 °C in room center temperature. These advantageous physico-mechanical and thermal properties establish LWC-MPCMs as promising and energy-efficient components for producing thermo-regulative building materials.

Preparation and crystallization behavior of sensitive thermochromic microencapsulated phase change materials

A novel sensitive thermo-chromic microcapsules with inorganic/organic nanoparticles-embedded composite shell was designed and synthesized, and the thermal properties, thermo-chromic performance, crystallization behavior of ethyl myristate (EM) in confined spaces were studied in this paper. As the reaction proceeded, the aminated silica (NH2-SiO2) nanoparticles modified outer shell in the aqueous phase and isophorone diisocyanate (IPDI) in the oil phase gradually self-assembled at the inner interface, leading to the formation of an inorganic/organic polymers composite shell. The effect of NH2-SiO2 nanoparticles content on the crystallization behavior, microstructure, morphology, and phase-change properties as well as the thermal stability of MPCMs were investigated in detail. Interestingly, the covalent attachment of NH2-SiO2 nanoparticles on the capsules surface offered long-term chemical stability compared to that of conventional physical adsorption. The NH2-SiO2 nanoparticles were found distributed uniformly on the surface of MPCMs, and the onset crystallization temperature of MPCMs increased by 7.3 °C with NH2-SiO2 nanoparticles addition of 15% content, which counteracts the super-cooling phenomenon. Microcapsule temperature change optical microscopy and non-isothermal crystallization kinetics revealed that the crystallization of RS-MPCMs were homogeneous as well. RS-MPCMs have significant potential in green energy applications due to their latent heat storage, thermal stability, thermal conductivity, leakage prevention, and mechanical properties.

Field synergy analysis on thermal-hydraulic behavior of phase change slurry in porous microchannel heat sink with graded porosity configuration

In this study, a novel design of porous microchannel heat sink (MCHS) with graded porosity configuration is proposed in the hope of attaining lower flow and thermal resistances than constant porosity (CP) configuration. The efficacy of new scheme is validated by utilizing a three-dimensional solid-fluid conjugate model based on the finite volume method, which considers the flow and heat transfer of microencapsulated phase change material slurry (MPCMS) inside the porous fins. It is reported that the thermal performance of porous MCHS with constant and graded porosity configurations is significantly better than that of non-porous MCHS. The feasibility of new design is verified by comparing the hydrothermal behavior of multiple graded porosity MCHSs and CP configuration for different fin designs (involving the number of graded equidistant fins N, graded length ratio R, and fin-to-pitch width ratio β) and operating conditions (including Reynolds number Re, mass fraction cm, and inlet velocity uin). The comprehensive performance improvement mechanism of MPCMS in graded porosity microchannels is discussed through the analysis of flow and thermal details, field synergy angle (θ) and field synergy number (Fc), and the overall performance of new design is evaluated by performance evaluation factor (PEF). Results indicate that the six types of porous MCHS with stepwise varying porosity exhibit lower thermal resistance than CP mode, and better thermal behavior and larger PEF can be obtained at N = 2 and β = 0.7. Smaller and larger R should be chosen for stepwise increasing porosity (SIP) and stepwise decreasing porosity (SDP) modes, respectively, to achieve higher heat transfer enhancement while avoiding greater friction losses. The Fc value of porous MCHS with various porosity configurations is about two orders of magnitude less than 1 and shows a monotonically reducing trend with Re, and the synergistic effect deteriorates at high Re. Compared to the CP mode, a low-θ “drift” toward the low porosity region of porous fin occurs within the graded porosity MCHS, which decreases and increases in the downstream and upstream directions of the “drift”, respectively. Among the novel MCHS designs, the z-directional SIP mode acquires the best overall performance due to the excellent synergy between the velocity and temperature fields, and its PEF consistently exceeds 1.13 and reaches up to 1.33. Generally speaking, the fin porosity of graded porosity MCHS near the channel cavity, channel top, and channel inlet should be greater for superior comprehensive performance.