Thermal energy storage is increasingly needed in a sustainable world because of its potential of capturing waste heat and being incorporated in solar power plants. For power generation, in particular, as turbine technology advances, a demand for higher temperature thermal energy storage materials also grows. For this purpose, latent thermal energy storage fits in well since it uses phase change materials (PCMs) which generally have a higher energy density compared to their sensible heat counterparts. In the present study, a eutectic Na₂CO₃(41.69%)-(33.1%)KCl-(25.21%)NaCl phase change material (PCM) with a melting temperature of 569 ° C was chosen as the storage material to experimentally assess the performance benefit of using a readily available stainless steel (ss304) wire mesh (as the periodic structure) to enhance heat transfer within the domain. In addition, for discharging, a numerical model was developed and compared with the experimental results. Furthermore, for discharging, a numerical investigation of the influence of the heat transfer fluid (HTF) flow-rate to the rate of heat transfer was performed. Overall, it was experimentally observed that the charging time for the composite case was shortened by about 35%, compared to the pure PCM case. For discharging, in the axial direction, the composite solidification time when compared to the pure PCM case was on average 10% shorter. Regarding the radial discharging performance of the composite, there was only about 5% improvement compared to the pure PCM case, which was expected due to the thermal contact resistance in the radial direction. Discharging experimental results were used to validate a discharging numerical model. Discharging results from the model showed that increasing the flow rate of the heat transfer fluid (HTF) reduced the time for solidification. It was observed that for the HTF flow rate of 5 L/min, 10 L/min, 20 L/min and 30 L/min, the discharge time was shortened by 23%, 30%, 33% and 35%, respectively.

The present study compares a modified variable height fin heat sink with the conventional constant height fin heat sink. The two heat sinks are filled with an equal volume of PCM (n-eicosane) and a fin volume fraction of 8 %. The experiments are performed for constant loads and also different power surge conditions. The pulsed heat loads are applied for two scenarios: 1. Constant load 4 W - power surge and constant load 4 W - power surge - 1800 s no-load condition, and 2. Power surge (50 s, 100 s, and 150 s) - no-load conditions of 1800 s. During experiments, the proposed variable height fin heat sinks possess better thermal performance for all load scenarios. Further, a 3D computational model is developed using ANSYS Fluent 19 to assess not only the effect of fin arrangement for different aspect ratios but also the impact of fin shape. The enclosure aspect ratio employed for the given study ranges from 0.3 to 0.8 for both the heat sinks. Regarding the fin structure in a heat sink, four types of fin shapes are adopted: square, circular, diamond, and triangular. The contour images of temperature and the liquid fraction are shown for the charging process. For the discharging process, the time required for the heat sinks to completely solidify the PCM is discussed. From the outcomes, variable height fin heat sinks provide enhanced melting/solidification for all the aspect ratios and fin shapes considered. As the aspect ratio increases, the time difference between the heat sink for the completion of the discharging cycle is reduced. Moreover, the triangular shaped fin shows a higher enhancement percentage of 2.29 % and 1.43 % during melting and 6.25 % and 12.5 % during solidification for both the heat sinks, respectively.

A passive method with phase change material (PCM) is an appropriate technique in electronic cooling. But, due to its poor thermal conductivity, many enhancers are employed to reduce the thermal resistance offered by the PCM. A partial filling strategy to reduce the cost and weight of foams with fins is used in this study. A hybrid heat sink with a combination of fins placed at the sidewalls of the enclosure and foams filled at certain heights such as 10, 20, and 30 mm is considered in this present work. A two-dimensional numerical model with n-eicosane as PCM is developed in ANSYS Fluent 19. A multi-objective optimization is carried out using a reliable multi criteria decision making approach. Different weightage is distributed to the objective functions in this method depending on the choice of the user. The pore size and density vary for various filling heights, and 60 cases are investigated for both charging and discharging cycles. The pore size of 0.8-0.95 and pore density of 5-25 pores per inch with a broad range is considered. From the discussions, guidelines for selecting a preferable pore size and pore density can be determined based on the filling height and applied weightage.

The use of metallic periodic structures was considered for melting rate enhancement of a phase change material (PCM) contained in a rectangular enclosure isothermally heated from the side. The critical (optimized) cell size, or pore size, of a periodic structure with fixed porosity, realising the shortest melting time by maximizing the convection and conduction heat transfer rate into the PCM, was studied. Furthermore, the effects of material properties (copper, aluminium, nickel, and stainless steel), enclosure length, wall-melting temperature difference and porosity were numerically investigated. It was observed that increasing porosity and/or reducing thermal conductivity enlarged the critical cell size (i.e. the optimal cell size that minimizes the melting time). The critical PPIs (pores per inch) of copper and aluminium periodic structures for all studied porosities were 10; for nickel, the critical values were 10 PPIs for porosity values of 0.75, 0.8 and 0.85 while it reduces to 5 PPI for the highest porosity considered here being 0.95. Interestingly, showing a different trend, the critical PPI of stainless-steel structures was 5 for the lowest porosity (0.75) and reduced to 3 for higher porosities. The results clearly demonstrated localised melting which was observed in all periodic structures except for the 10 PPI stainless-steel case. Scattered melting islands are observed as opposed to a moving interface when ϕ=(d_p/L)α_ligament/α_PCM>1. For such cases, localized melting occurs and the PCM is melted at the ligaments away from the heated wall before the melt front reaches those ligaments.

In this study, 3D printed open-cell foam were produced and reconstructed from open-cell metal foam structure using a tomography scanning method and two different additive manufacturing technologies. The materials used in the 3D printing were nylon powder and plastic acid. The porous morphology and surface finish of the 3D printed foams were investigated using a microscope. The results showed that the surface finish and structure strength depend on the printing process, used material and foam size. This study found that laser-sintering technology would have smoother pores with lesser residue than stereolithographic. However, the ligaments of the small-size 3D printed foam were fragile and could be easily broken.

Foodstuff has to be transported from where it is produced or packaged to the market. It is important to control the temperature and humidity of the delivered food to minimize the waste and ensure customer satisfaction. At the same time, unfortunately, the transport sector is among the polluting sectors. Therefore, innovative solutions have been proposed such as the adoption of Latent Thermal Energy Storage (LTES) systems based on Phase Change Materials (PCMs) to control the food temperature. Such systems minimize the fuel consumption and thereby reduce the emission. LTES systems rely on the phase change process to absorb and store heat in a more sustainable way while ensuring a nearly constant temperature for the foodstuff while being transported. Hence, we first present a general overview of the application of PCMs at low temperatures. Then, the literature on the adoption of PCMs in the refrigerated transport sector is critically surveyed. Different proposals offered in the literature are recovered, compared, and critically assessed based on their merits. Despite the differences between the proposed technologies, significant advantages in terms of temperature uniformity in the refrigerated space, product thermal stress reduction, product quality, energy consumption, and pollutant emissions reduction are reported in the literature as surveyed in this paper.

The rising energy demand for buildings has enhanced public awareness of sustainable energy sources and technologies. In particular, natural ventilation systems such as wind-catchers have attracted considerable new attention. A new wind-catcher design with single-stage direct-air evaporative cooling was proposed for indoor air conditioning. An Eulerian-Lagrangian approach employing the Realizable k-ε model was utilized to conduct the CFD simulations. Furthermore, the effects of inclining the bottom surface of the wind-catcher and installing a baffle across the flow path on the air temperature drop, water mass fraction, and air velocity distribution were studied. The inclined bottom surface led to more flow uniformity in the room compared to the conventional geometry. The baffled wind-catcher with β = 0, 30, 45, and 60° and unbaffled wind-catcher showed different flow patterns and thermal comforts. A methodology for evaluating the thermal comfort performance of evaporative cooling systems integrated into natural or passive cooling devices was also proposed based on the generated CFD results. The baffled wind-catcher with β = 60° combined with an evaporative cooling system significantly reduced the air temperature inside the building up to 17.4 °C and improved the occupants’ thermal comfort. The most suitable design for thermal comfort was also determined.

Widespread commercialization of PCM-based latent heat storage systems is limited by the low melting and solidification rates during the phase transition process. In this study, fins are used to enhance the phase change process. A parametric study is conducted to understand the effect of fins on the thermal performance of vertically- and horizontally-oriented rectangular storage tanks. Throughout simulations, the fin volume, and thereby the mass of PCM in the tank, were kept constant. It was observed that the horizontal enclosures can take advantage of the development of strong natural convection flow until near the end of the melting process, whereas with vertical counterparts the strength of the convection currents was diminished during the shrinkage stage. The results suggest that longer and thinner fins are more beneficial for enhancing the melting rate than shorter and thicker fins. It was concluded that for horizontal enclosures with fin lengths of 25 and 35 mm, increasing the number of fins does not necessarily shorten the melting time. The maximum melting time reduction compared to the 3-fin vertical enclosure with a fin length of 25 mm (chosen as our benchmark case) was 75.1% when nine 45-mm-long fins are used in a horizontal enclosure.

Insulation performance and buried depth of heating pipelines are the vital factors affecting the energy loss of directly buried heating pipelines. This study considers the thermo-economic assessment of insulation of directly buried heating pipelines for district heating. The optimal insulation thickness, energy-saving effect, and payback period of district heating pipelines with five nominal pipe diameters, four fuel types, four kinds of insulation materials, and four buried depths are calculated. A numerical code is developed based on Life Cycle Cost Analysis and is validated via comparing with results in the open literature. Three representative cities of Xi’an, Shenyang, and Harbin subjected to three different climatic zones in China are explored. The highest values of optimum insulation thickness are 176, 153, and 121 mm in Harbin, Shenyang, and Xi’an, respectively, which are reached using oil as fuel, rock wool as insulation material, nominal pipe diameter 500 mm, and buried depth 1 m. A sensitivity analysis is performed to indicate how much the optimum insulation thickness and payback period are sensitive to the changes of insulation, fuel, and buried depth. The results show that insulation and fuel have a greater influence on the optimum insulation and payback period than the buried depth.

In the present research, the synthesis of highly stable multiwalled carbon nanotubes in aqueous media is developed using a one-pot, covalent and green functionalization technique to improve the heat transfer and hydrodynamic behavior of a horizontal stainless-steel tube subjected to a uniform heat flow at its outer surface. Instead of using corrosive inorganic acids, the free radical grafting of gallic acids is used in this procedure. In this work, GA-functionalized multiwalled carbon nanotubes water-based colloidal suspensions (nanofluids) were prepared at three different weight concentrations. Various characterization techniques comprising Fourier transform infrared spectroscopy, Raman spectroscopy, High resolution transmission Electron Microscopy, and zeta-potential measurements were performed and confirmed the success of MWCNT functionalization. The thermophysical properties were evaluated experimentally and validated using empirical correlations available in the literature. To prove the colloidal suspension stability, ultraviolet-visible spectroscopy was used, and the results showed that nanofluid was stable for almost 60 days. When the GAMWCNT concentration was increased, there was a significant improvement in the thermal conductivity. Moreover, the nanofluids' dynamic viscosity experienced a slight increment up to 7.30% when the GAMWCNTs were loaded relative to distilled water. Following the confirmation of the experimental setup's analytical correlations, tests for the colloidal GAMWCNT suspension flowing through a heated horizontal tube were carried out in a fully developed turbulent state. A significant enhancement in the convective heat transfer coefficient was obtained, with only minor growth in the relative pumping power by 33.05 and 1.19%, respectively. More importantly, the reported positive performance index indicator for all the Reynolds numbers of ranges shows the possibility of using the synthesized GAMWCNT aqueous suspensions as an alternative working fluid in heat transfer systems.

Supercritical flow through a horizontal pipe leads to a non-uniform peripheral wall temperature distribution even when the wall heat flux is kept constant and uniform. This is attributed to lower heat transfer coefficient at the top section where the denser fluid tends to sink. Hence, to obtain a uniform wall temperature, a designed wall roughness is devised. Uniform sand-grain roughness is employed to only partly cover the top half of the pipe wall. Numerical simulations were conducted using the SST k−ω turbulence model. The simulation results indicate that our proposed design can lead to a more uniform heat transfer distribution over the wall periphery compared with the smooth pipe. An extreme case was also considered where the inner wall was completely covered with roughness elements. While heat transfer augmentation was observed for this case, the excess pressure drop was prohibitively higher compared with a pipe with designed wall roughness.

Solar energy, as a kind of renewable energy, offers a large reserve to be harvested at a reasonably low cost for engineering applications. To decouple the temporal and spatial relevance of the continuous energy supply of solar energy, latent heat thermal energy storage can deal with this problem at different temperatures. Aiming to improve energy efficiency, a novel hybrid metal foam-pin fin structure is designed and assessed. Upon conducting measurements on a well-designed experimental bench, the phase change processes of paraffin that is filled in fins, metal foam, and a combination of both (hybrid structure) are evaluated. During the experiments, the transient melting interface is snapshotted and temperature development is documented under five different heat source temperatures of 61 °C, 63 °C, 65 °C, 68 °C, and 70 °C. In the foreground of the novel hybrid structure, each segment of the hybrid is also justified and discussed. Results indicate that the hybrid structure augments marked heat transfer. Compared to pure PCM, complete melting time decreases by 63.4% and simultaneously the temperature response rate increases by 143.9% as implementing the hybrid. Attempts to design hybrid structure find a solution to assess and operate thermal storage applications for solar engineering.

Theory of dynamical cavitation threshold for vapor and non-condensable gas bubble nuclei is proposed based on a model equation constructed from Rayleigh-Plesset equation for glycerol, the liquid with viscosity higher than that of water by 1500 times, under a finite duration of strong tension. The model equation is ascertained to be valid in cases of strong tension under which cavitation occurs. Our model enables the study of dynamics of nuclei on the phase plane of the nucleus radius and the growth velocity, by which the full details of the threshold are revealed. We propose a dimensionless parameter to be used to classify the threshold of cavitation. Our model offers a simple mathematical expression to calculate the maximum radii attained, while under tension, for each of these three recognized patterns. For each observed pattern, we present unique predictive correlations for the radius of the nucleus growing for the tension duration. Moreover, we elucidate that the dynamics of the nuclei, grown up to certain sizes, is fully controlled by tension independent of the viscosity. The discrepancy between the dynamical threshold and the conventional Blake’s threshold is discussed. Finally, the utility of the theory presented here is demonstrated through numerical examples.

Phase change materials (PCM) RT-28HC, RT-35HC, and RT-44HC with three different melting temperatures, 29 °C, 36 °C, and 44 °C, with similar thermal properties, are considered. The PCM is oriented from the left to right side of the heat sink in its increasing order. The fins are attached to the heat sink longitudinally, and its orientation effects are studied low (100–500 W/m²) and high (1000–5000 W/m²) heat fluxes applied on the horizontal bottom surface of the heat sink. A 2D model is developed using ANSYS Fluent 19, and the fin orientation effects are investigated numerically. The orientation of fins at different angles such as 0°, +15°, +30°, +45°, +60°,-15°,-30°,-45°, −60° are considered. The effect of fins on the charging cycle is assessed by comparing a single and double PCM heat sink. Three initial conditions are investigated by altering the initial temperature 300 K, 303 K, and 310 K. At increasing heat input, the negative angled fins possess a higher melting rate. For different initial conditions, −60° provides higher enhancement, and +60° possesses prolonged melting for almost all cases. The performance of a triple PCM design is compared with single and double PCM counterparts under similar conditions.

High-pressure drops characteristic of microchannel heat sinks (MCHS) is an issue that needs to be addressed to reduce the size of heat-removing devices in compact electronic devices. Supercritical carbon dioxide (s-CO₂) is a suitable candidate being proposed as an alternative coolant to enhance the cooling of the microchannel heat sink (MCHS), with high heat flux, due to its favorable thermophysical properties near its critical point. In this study, numerical simulations are conducted to evaluate the thermal and hydraulic performance of a channel for a designed heat sink with s-CO₂ (at constant (Formula presented.)) and compare it with conventional liquid coolant (water). The effect of coolants mass flow rate ((Formula presented.)), channel aspect ratio (AR), and inlet temperatures on the thermal and hydraulic performance of one channel is studied by varying (Formula presented.) from 0.004 to 0.03 kg/s and AR from 0.33 to 10. The results show that, for the same aspect ratio, same geometry, and constant heat flux, s-CO₂ offers a higher overall heat transfer coefficient (32%) with a lower friction factor (pumping power) compared with the water at the same inlet temperature (T = 32°C). The results of pumping power comparison between two coolants reveal that for CO₂ in supercritical conditions ((Formula presented.), T = 32°C), the consumed power varies by change of the aspect ratio, which is 1.85 times lower than water for AR = 0.33 and is 3.6 times higher for AR = 10. However, in the subcooled state, the reverse effect of the aspect ratio is seen.