Spherical Capsules Research Articles

Experimental study on the performance of packed-bed latent thermal energy storage system employing spherical capsules with hollow channels

The applicability of packed bed latent thermal energy storage devices is restricted by the limited thermal conductivity of phase change materials (PCMs). As a cheap and simple heat transfer-enhanced construction, the hollow channel allows the heat transfer fluid to go through the capsule center directly where the melting rate of PCM would be enhanced. In this study, an experiment was conducted to explore the promotion effect of hollow channels. The influence of the hollow channels on the charging duration and outlet temperature characteristics at different inlet flow rates, inlet temperatures, and placement angles was investigated. The results showed that the charging rate was improved with a reduction in charging duration of about 16.6%. The improvement was consistent in scenarios with variable inlet temperatures and flow rates, with a difference of less than 1%. Furthermore, adding hollow channels resulted in a lower average outlet temperature. Finally, there is no significant influence of horizontal and vertical placements of the hollow channels on the performance of PBLTES. Therefore, random angle placement is advised to reduce costs in engineering. However, the addition of hollow channels drops the PCM filling rate by 3.7%, reducing latent thermal energy storage but increasing sensible thermal energy storage.

Modelling functionalized drug release for a spherical capsule

Advances in material design have led to the rapid development of novel materials with increasing complexity and functions in bioengineering. In particular, functionally graded materials (FGMs) offer important advantages in various fields of application. In this work, we consider a heterogeneous reaction-diffusion model for an FGM spherical drug release system that generalizes the multi-layer configuration to arbitrary spatially-variable coefficients. Our model proposes a possible form for the drug diffusivity and reaction rate functions exhibiting fixed average material properties and a drug release profile that can be continuously varied between the limiting cases of a homogeneous system (constant coefficients) and two-layer system (stepwise coefficients). A semi-analytical solution is then used to solve the model, which provides closed-form expressions for the drug concentration and drug release profiles in terms of generalized Fourier series. Our results show how the release rate of the proposed FGM drug release system can be controlled and continuously varied between a fast (homogeneous) and slow (two-layer) release while maintaining the same averaged values for the diffusivity and reaction rate.

An effective thermal conductivity correlation for three high-temperature chloride salts melting in a spherical thermal storage capsule

ABSTRACT Chloride salts are viable phase change materials (PCMs) for packed-bed thermal energy storage for third-generation concentrating solar power (CSP) systems. In the study, a two-dimensional, axisymmetric and transient model is developed to simulate the constrained melting process of three chloride salts (NaCl-CaCl2, LiCl and MgCl2) in a spherical capsule while considering the natural convection of the salt. The simulation results lead to the proposal of an effective thermal conductivity correlation for the liquid part of these three PCMs’ melting process in a sphere, represented as λ eff/λ l = 0.38Ra 0.26(δ/R i)0.25. Additionally, a simplified conduction-controlled numerical model has been developed for predicting the melting process of chloride salts in a spherical capsule with improved accuracy and efficiency. The proposed correlation improves the conduction-controlled model’s accuracy in predicting the melting process compared to other correlations previously reported in literature. The proposed correlation comprehensively reflects the conduction and natural convection of the chloride salts during the constrained melting process in a sphere.

Optimal design and evaluation for sphere capsules in the packed bed latent thermal energy storage

The high-performance PLTES basically relies on the heat transfer inside the sphere capsule. To achieve a high cost-performance enhancement, the sphere capsule with the copper foam partially filled is proposed, depending on the full utilization of the foam enhancement and natural convection. The melting performance of the partly filled sphere capsule and the optimization, including the geometric shape and foam material usage, is numerically investigated. The economic assessment of these initial cases and the optimized cases is also conducted to evaluate the economic feasibility. The numerical results indicate the partially filled strategy is effective and the optimal design is the spherical capsule with a copper foam geometric shape that resembles a high cone with a prominent center. This is because the buoyancy-driven natural convection is fully used to enhance the heat transfer in the upper section and the foam is used to enhance the heat transfer in the bottom section. Based on this analysis, a better effect can be obtained by maintaining consistent angle size while changing the height of the vertex of the high cone. Through this optimization, the usage of copper foam is reduced by 24.48 % at the same melting rate.

Morphological and molecular characterization of a new freshwater Ceratomyxa species (Cnidaria: Myxozoa) from the yellow catfish, Trachysurus fulvidraco in China.

Ceratomyxa Thélohan, 1892 is one of the largest genera of the sub-phylum Myxozoa, and has a worldwide geographical distribution. Ceratomyxa species mainly infect the gallbladder of marine fish and rarely infect histozoically or coelozoically freshwater fish. In the present study, yellow catfish, Trachysurus fulvidraco (Siluriformes, Actinopteri) collected from the low reach of Yellow River was first found to be infected with an unknown Ceratomyxa species in their gallbladder which was identified to be new to science and nominated as Ceratomyxa huangheensis n. sp. by an integrative taxonomic approach for myxosporeans. Spores are typical of the genus Ceratomyxa, and matures spores are arcuate, with posterior angle of 139.2±11.6 (137.0-156.0)° and rounded ends, and measures 4.7±0.6 (3.3-5.5) μm in length, and 10.7±1.3 (8.5-13.3) μm in thickness. Two spore valves are slightly unequal and present elongated ovoid in the lateral view. Two equal spherical polar capsules, measuring 2.2±0.4 (1.4-2.8) μm×2.0±0.4 (1.0-2.5) μm in size locates adjacent to the straight suture line. The obtained partial small subunit ribosomal DNA (SSU rDNA) sequence was unique among all myxozoans in GenBank, and the highest similarity is 85.3% with Ceratomyxa sparusaurati, a marine fish-infecting congener. Phylogenetic analysis further indicated that this novel species did not cluster with other freshwater fish-infecting congeners from South America to form an independent clade, but was phylogenetically positioned within the marine fish-infecting clade. The possible presence of different radiation trajectories between Ceratomyxa huangheensis n. sp. and Amazonian freshwater Ceratomyxa lineage was discussed. This is the first report of Ceratomyxa species in the Yellow River basin and the second freshwater Ceraotomyxa species in China, even in Asia.

Modeling and analysis of PCM‐encapsulated solar thermal energy storage system for space heating

AbstractIn this study, a phase change material (PCM)‐encapsulated packed‐bed thermal energy storage (PB‐TES) system is intended for Day‐round space heating in the winter. Solar concentric collectors act for day‐time heating and TES charging, while the stored energy can be used at night and intermediate time for space heating. A region‐wise techno‐economic analysis of PCM candidates is made to compare the costs and sizes of solar collectors and PB‐TES. A spherical capsule filled with sodium nitrite (NaNO2) and sodium nitrate (NaNO3) is numerically investigated during the PCM charging at constant thermal input. After complete liquefication, the charging time for NaNO2 and NaNO3 capsules is 3280 and 4300 s, respectively, holding the maximum stored energy of 733.4 KJ/kg at 285°C and 687.27 kJ/kg at 310.12°C, respectively. In addition, the stored exergy is obtained 238.78 and 234.74 kJ/kg for NaNO2 and NaNO3, respectively; however, the average exergetic effectiveness for NaNO2 and NaNO3 are 0.54 and 0.46. Despite the lower energy storage capacity (kJ/kg) of NaNO3, the PB‐TES volume is less with NaNO3 than with NaNO2 due to higher density. However, the average cost ($/tonne) of NaNO3 PCM is 50% higher than NaNO2. Subsequently, the thermal efficiency of the proposed system is estimated at 27.68% and 26.94% during day and night‐time operations.

Novel effective thermal conductivity numerical model for distinct shaped pure paraffins (C14–C33)

This paper presents a novel numerical model for “effective thermal conductivity” designed to address the limitations of previous models, which required different constants for each distinct shape. The constants C and m incorporates the natural convection effect through the Rayleigh number (CRam), obviating the need to solve velocity equations such as those for continuity and momentum. Notably, this model is applicable to a wide range of phase change materials (PCMs) using the same constants. Prior investigations predominantly focused on spherical capsule shapes and specific PCMs with varying C and m constants. The present study extends the limitations from spherical to cylindrical, square and rectangular shapes. The PCMs evaluated includes pure paraffins ranging from C14 to C33, all assessed using constant values for the C and m. Initially, the proposed model for effective thermal conductivity, along with the constrained model accounting for natural convection, was verified against experimental data for C18 alkane, achieving a root mean square error of 0.0177 with experimental and 0.0077 with constrained model. Subsequently, the present model's validity was affirmed against a reference constrained model across a wide range of alkanes. Simulations were carried out for both cylindrical and square shapes. For rectangular configurations, the aspect ratio was incorporated into the Rayleigh number to account the effects of both vertical and horizontal orientations.

Performance enhancement of ice storage capsules by biomimetic fins

Ice storage enhanced by biomimetic fins in spherical capsules is studied numerically based on enthalpy-porosity method. The evolution of water/ice interface, the temperature variation, and the cooling energy storage capacities are examined quantitatively to gain further insight into the solidification process in capsules. Especially, the geometrical configurations of fins, including the maximum bifurcate level and fractal dimension of length, are investigated. In addition, four evaluation criteria are employed to determine the optimum design for the biomimetic fins. The results show that the biomimetic fins are significantly effective in enhancing the heat transfer in the stage of cooling energy storage through sensible heat in ice. The performance enhancement is facilitated as the level of fins is increased that average 24% reduction in solidification time is achieved by the fin-Ⅲ series with only 1.24% loss of cooling energy storage capacity averagely. An optimum design for biomimetic fins is the fin-Ⅲ with a fractal dimension of 3, which results in a 30.01% increase in dimensionless cooling energy storage capacity per unit time and weight compared to the case without a fin. Aluminum is a favorable fin material considering both the enhancing performance and material cost of the capsule. A prediction formula of ice volume under different cooling conditions is generalized.

An improved effectiveness-NTU method for streamlining the design and optimization of packed bed latent thermal energy storage

The packed bed thermal energy storage system (PBLTES) is one of the promising and effective ways to solve the contradiction between the supply and demand of renewable energy. As a simplified mathematical representation, the effectiveness-number of transfer units (NTU) method can characterize the PBLTES faster than numerical simulation, which is convenient for PBLTES design, analysis, and optimization. The p factor, which is a key parameter for calculating the equivalent thermal resistance in the effectiveness-NTU method, is used to define the path of charging and discharging of the PBLTES, that is, the synchronization of phase change processes of phase change materials in different layers, which determines the accuracy of the model. In this paper, the calculation method of average effectiveness is further refined based on existing studies and mathematical derivations. The phase change duration characteristic of the PBLTES is incorporated into the method. Then, a numerical simulation model of PBLTES with spherical capsules was established to investigate the influence of relevant parameters on the p-factor and the impact mechanism. Finally, the p-factor was fitted based on numerical simulation. The results show that the effectiveness solved by the fitting p-factor equation has a maximum error of less than 10 % and an average error of 5.11 % compared to the experimental results. The establishment of the p-factor calculation equation allows the effectiveness-NTU method to be used more accurately and quickly for the design and optimization of the structural and operating parameters of the PBLTES, which can advance the engineering applications of PBLTES.

Experimental and numerical investigation of the melting process of solar salt in a spherical capsule

Solar salt is commonly employed as phase change material in various industrial applications, particularly in latent heat-based thermal storage systems such as packed beds in solar thermal power technologies. Here, a series of experimental and numerical studies are conducted of the melting process of a binary nitrate salt inside a spherical capsule under a wide range of large Stefan numbers, specifically ranging from 0.56 to 1.12. For a deep understanding of the melting behavior inside the sphere and its heat transfer characteristics under different constant wall temperatures and sphere sizes, the visualized high-temperature phase change experiment along with a 2D axisymmetric numerical model are presented. To confirm modelling reliability, the numerical model well-realized the sinking phenomena visualized in the experiment with a suitable mushy-zone parameter value of 6*108 kg/(m3·s). Subsequently, the temperature field, flow field, melting front, total transfer rate and melting time are explored. Further, a detailed parametric investigation is applied, including the effects of dimensionless parameters such as Gr, Ste on the melting process. And finally, the dimensionless correlation is succeeded to be proposed for unconstrained melting process with 0.56 ≤ Ste ≤ 1.12 and 1.80E+04 ≤ Gr ≤ 2.02E+05.

Phase-change materials for storing heat extracted from heat pump condensers

This study aims to investigate the storage of heat from a heat pump's condenser in a well-insulated cylindrical tank filled with spherical capsules containing a phase-change material (PCM). Using a detailed physical model and the enthalpic method, the system's operation and phase change phenomena are simulated. The impact of various operational parameters, such as condenser power, water flow rate, storage volume, and PCM type, is examined. The results reveal that increasing condenser power significantly reduces the PCM loading time, achieving a 66 % reduction within the 5 kW to 15 kW power range. Additionally, decreasing the water flow rate enhances storage efficiency by 15 % when the flow rate drops from 2.3 m3/h to 1 m3/h. The use of a PCM with a melting temperature of 44 °C is deemed more suitable for the system compared to other options.

Influence of carbon graphene flakes enriched PCM on partial charging conditions for cool thermal energy storage system applications

The present research work is aimed at studying the influence of carbon graphene flakes nanoparticle concentrations in DI water at various heat transfer fluid temperatures of −6, −9, and −12 °C. The carbon graphene flakes (CGF) with higher concentrations of 0.4, 0.8, and 1.2% with the base PCM is investigated experimentally in a stainless steel spherical capsule of 100 mm diameter, and the thermophysical properties of the CGF PCMs were interpreted with the charging behaviours of the base PCM. The CGF PCMs help to reduce the solidification time by 47 to 56% for the higher bath temperature of −6 °C in partial charging conditions. For the complete solidification, the maximum reduction in solidification time was attained between 14 and 21% for the CGF PCM with a concentration of 1.2%. The partial charging conditions favour the increase in heat flux two-fold when the bath temperature is operated at −6 °C, this reduces the demand for energy during the peak hours and shifts the peak load conditions. It is concluded that the CGF PCMs with a concentration of 1.2% help in enhancing the heat transfer rate and significantly compress the solidification duration compared to the bath temperatures of −9 °C and −12 °C.

Encapsulating potential and functional properties of exopolysaccharide from Limosilactobacillus reuteri KCTC 14626BP isolated from human breast milk

Exopolysaccharides (EPS) are natural, nontoxic, biocompatible and biodegradable macromolecules produced by microorganisms, including the Lactic acid bacteria, to enhance protection against environmental stress conditions. The current study focused on the encapsulation and functional efficiency of EPS produced by probiotic strains isolated from human milk. Among 27 isolates, the potential high EPS-producing strain Limosilactobacillus reuteri KCTC 14626BP was selected based on biofilm production. The structural Characterization of EPS was performed based on FTIR, NMR and functional properties were determined; further, the encapsulation efficiency of EPS was determined with caffeic acid. The results indicate that L. reuteri produced EPS major component consisting of glucose, galactose and arabinose with the ratio of (0.78:0.16: 0.05). The antioxidant efficiency of EPS-LR was determined on DPPH (60.3 %) and ABTS (48.9 %); EPS showed enhanced functional activities. The absence of toxicity was confirmed based on Caenorhabditis elegans. The EPS-loaded Caffeic acid (CA) EPS-LR indicated spherical capsules with rough surfaces, with sizes ranging from 1.39 to 6.75 μm. These findings indicate that EPS-LR can be applied as a bioactive compound and encapsulating material in food, cosmetics, and pharmaceutical industries.

Encapsulation of Amyloglucosidase in Chitosan-SDS Coacervates as a Means to Control Starch Hydrolysis in Plant-Based Beverages

Starch enzymatic hydrolysis is a key technology for preparing plant-based dairy alternative beverages, preferred for its low energy consumption, high product yield, and quality. However, the high-temperature requirement to terminate reactions and challenges in continuous operations hinder enzyme applicability. Therefore, encapsulation techniques have been explored to address these limitations, enhancing the enzyme’s stability and facilitating process control. This study developed a novel amyloglucosidase encapsulation system based on chitosan-SDS hydrogel capsules produced by simple coacervation. The enzymatic activity of free and immobilised amyloglucosidase was assessed using corn starch and maltose as substrates. The encapsulation process successfully yielded self-supporting structured and spherical capsules, achieving an immobilisation yield of 71% and an enzyme efficiency of 83%. The residual enzymatic activity after 28 days of storage remained at 76%. A feedback inhibition investigation was performed by varying the concentration of corn starch, and favourable applicability of encapsulated amyloglucosidase for continuous processes was found. The encapsulation process offered several advantages, such as a straightforward separation step to halt enzymatic reactions and no need for high temperatures that may affect food product attributes. Lastly, the amyloglucosidase-chitosan-SDS coacervates show promise in improving enzyme stability, facilitating hydrolysis control, and expanding enzyme utilisation in dairy alternative plant beverage manufacture.

Numerical analysis of oxygen uptake processes by red blood cells in stopped-flow measurements: Effects of cell shape, membrane permeability and unstirred layer

The transport process of oxygen and other gas species across red blood cell (RBC) membrane is of great importance for better understanding the critical biological functions of RBCs, and the stopped-flow experiments have often been employed for such investigations. In previous stopped-flow analyses, the RBC had usually been represented by a spherical capsule based on the RBC volume, and an assumed unstirred layer (USL) thickness had been used to determine the membrane permeability. In this research, unlike these previous studies, we simulate the oxygen uptake process with different RBC shapes (shperical, ellipsoidal and biconcave) and examine the effects of USL thickness and membrane permeability over broad ranges based on literature values. Our results show that the excess membrane area can greatly improve the oxygen transport efficiency, and a same uptake half-time can be obtained using different combinations of USL thickness and membrane permeability. These findings raise concerns on the reliability and uncertainty for the results and conclusions in previous studies, and also call for more complete numerical models, for example, with the fluid flow and cell deformation considered, and more in-depth investigations on the oxygen transport processes.

Microencapsulation of moringa oil in bio-polymer by simple solvent evaporation technique

Moringa oil (MO) contains various bioactive components and pharmacology. It is attractive to use as a raw ingredient in various products. However, there are limitations on its direct utilization, especially MO's instability and hastening the active ingredient's degradation from external environmental factors, including temperature, humidity, oxidation, light, and heat. To solve these problems, in this work, microencapsulation of MO using different biopolymers as cellulose acetate butyrate (CAB), ethyl cellulose (EC), and poly(L-lactic acid) (PLLA) were carried out by a simple solvent evaporation technique. The prepared polymer microcapsule suspensions were highly colloidal stable for all types of biopolymers and ratios. The spherical biopolymer capsules were formed to a micrometer size after solvent evaporation under all conditions. However, when the microcapsules were dried, aggregation was found with the polymer microcapsules at a ratio of PLLA to MO of 50:50 for all three types of polymers, possibly due to the low amount of polymer to completely encapsulate all of MO. When polymer contents increased to 70%, the dried spherical polymer microcapsules were smoothly produced. Using 70% polymers, the PLLA microcapsule surface was smoother than the polymer microcapsules prepared by CAB and EC which exhibited the dent or hole on the outer surface. Micrometer size, spherical polymer capsules with a core-shell morphology were fabricated. Due to the higher hydrophilicity of the polymer than the MO, the polymer moves outward, forming a strong shell around the MO. Then, all three biopolymers can be used for the microencapsulation of MO at a suitable polymer to MO ratio. However, using PLLA at a ratio of PLLA to MO of 70:30 presented the highest encapsulation efficiency (74.08%), which may be due to its high molecular weight. Because of the non-toxicity and biodegradability of biopolymers, the fabricated microcapsules would be well applied in cosmetic products.

Suspensions of viscoelastic capsules: effect of membrane viscosity on transient dynamics

Membrane viscosity is known to play a central role in the transient dynamics of isolated viscoelastic capsules by decreasing their deformation, inducing shape oscillations and reducing the loading time, that is, the time required to reach the steady-state deformation. However, for dense suspensions of capsules, our understanding of the influence of the membrane viscosity is minimal. In this work, we perform a systematic numerical investigation based on coupled immersed boundary–lattice Boltzmann (IB-LB) simulations of viscoelastic spherical capsule suspensions in the non-inertial regime. We show the effect of the membrane viscosity on the transient dynamics as a function of volume fraction and capillary number. Our results indicate that the influence of membrane viscosity on both deformation and loading time strongly depends on the volume fraction in a non-trivial manner: dense suspensions with large surface viscosity are more resistant to deformation but attain loading times that are characteristic of capsules with no surface viscosity, thus opening the possibility to obtain richer combinations of mechanical features.

Topology optimization design of inner fins in spherical phase change capsules

With the development of thermal storage stacking beds, the research application of spherical phase change capsules has become a hot topic. However, the low thermal conductivity of the phase change material makes the thermal response time of spherical phase change capsules slow and the thermal storage/exhaust efficiency is poor. In order to solve this problem, this paper proposes a method for designing the inner fins of spherical phase change capsules by applying topology optimization techniques. In a two-dimensional hemispherical model, the density method is used to calculate the topology optimization results for different objective functions and to construct models to simulate the melting and solidification of a spherical phase change capsule. The results show that the fin models have a large Nusselt number, which effectively speeds up the heat transfer inside the capsule and reduces the melting and solidification time. On balance, the topology optimization fin model with minimized dissipation of thermal conductivity as the objective function has the best performance, being able to reduce the melting time by 34.22 % compared to the No Fins model and shorten the solidification time by 24.34 % compared to the straight fin model. In addition, this paper finds that the fins can influence the generation of natural convection, splitting the large vortices and accelerating the melting rate on both sides inside the capsule. This paper can provide a new idea for the fins optimal design inside the spherical phase change capsule.

Numerical investigation of the melting characteristics of spherical-encapsulated phase change materials with composite metal fins

A spherical phase change material (PCM) capsule with embedded composite fins for latent thermal energy storage is proposed and studied in this paper. A three-dimensional numerical model is established and validated by comparing with experimental data in the literature. The influences of the embedded fins with different structures, capsule size, and heating temperature on the melting performance are analyzed. Results show that the insertion of composite metal fins promotes the melting process owing to the extension of thermal surface. The fin with perforations exhibits higher energy storage performance as compared to the non-perforated one, which can be attributed to the reduction of the blockage effect as well as the increment in the filling volume of PCM. The punched fin with three perforations in each sheet is preferable to adopting in spherical PCM capsule in view of the less time required for completely melting and the higher energy storage capacity. However, as the size of capsule increases, the heat transfer enhancement contributed by the inclusion of the embedded fins reduces. In addition, the melting process can be accelerated significantly by raising the temperature of external shell surface from 340 to 360 K, while such improvement tends to be less pronounced for the further increase in heating temperature. This study can provide useful guidance for the optimal design of thermal energy storage system with spherical PCM capsules.

Effects of fluid rheology on dynamics of a capsule through a microchannel constriction

This paper numerically investigates the impact of fluid rheology on the behaviors of a spherical capsule through a microchannel constriction. Different flow scenarios are considered: a Newtonian capsule in a viscoelastic matrix, a Newtonian capsule in a Newtonian matrix, and a viscoelastic capsule in a Newtonian matrix. The results demonstrate that the capsule's lengths undergo oscillations during the passage through the constriction, with three stages of evolution. When approaching the constriction, the capsule respectively experiences increase and decrease in its length and height. While within or exiting the constriction, the length of the capsule continuously decreases, and the height generally increases. As the capsule moves away from the constriction, the capsule relaxes to different profiles in different flows. Detailed analysis on the effects of the fluid viscoelasticity on the capsule's lengths in different stages is provided. In addition, the behaviors of a red blood cell passing through a microchannel constriction are also examined. This study sheds light on the complex behaviors of a spherical capsule and red blood cell in microchannel constriction, emphasizing the significant influence of fluid rheology on their deformation and shape changes.