To mitigate the intermittency and instability of solar energy, phase change thermal storage is widely used for thermal regulation. However, phase change material (PCM) typically has low thermal conductivity, which restricts its practical application. Incorporating gradient metal foam enhances thermal conductivity while preserving natural convection, making it an effective strategy for improving solar thermal utilization. Based on this concept, this study numerically investigates phase change thermal storage enhanced by gradient-porosity metal foam. In a shell-and-tube thermal storage unit, ten gradient structures are designed by varying the number of filling layers (bilayer or trilayer) and their distribution direction (axial and radial). To decouple the effects of gradient distribution and volume fraction, gradient schemes are designed using an equal-volume segmentation method while maintaining a constant average porosity. The results indicate that both axial and radial positive gradient structures significantly accelerate the melting process. Among these configurations, the trilayer axial positive gradient structure exhibits the best overall performance. By strengthening the synergistic interaction between heat conduction in the lower region and convection in the upper region, the complete melting time decreases from 135.7 min for the homogeneous structure to 112.4 min, representing a reduction of 17.17%. Meanwhile, the time-averaged temperature uniformity index decreases by 7.1%, the average power increases by 15%, and the thermal responses at the measurement points become more uniform. Similarly, the trilayer radial positive gradient structure optimizes the coupled heat transfer between early-stage conduction and late-stage convection. As a result, the complete melting time decreases to 119.8 min, corresponding to a reduction of 11.72%, while temperature uniformity and average power are also improved. In summary, the trilayer axial positive gradient and trilayer radial positive gradient are identified as optimal configurations for gradientporosity structure optimization, significantly enhancing the performance of phase change thermal storage systems and the efficiency of solar thermal utilization.

Based on a hybrid-structured thermal energy storage (TES) system (high-temperature phase change material H-PCM, quartzite, low-temperature phase change material L-PCM), this study breaks through the limitation of the narrow operating temperature range of single-material systems through the synergistic effect of multi-level PCMs, achieving a higher thermal storage density. Specifically, the ternary carbonate Li2CO3-Na2CO3-K2CO3, quartzite, and NaNO3 serve as H-PCM, sensible heat material, and L-PCM, respectively. The heat transfer fluid (HTF) employed in the TES system is NaNO3-KNO3. Six types of hybrid structure packed beds are designed by adjusting the volume fractions of the PCMs. A continuous solid-phase model is adopted for numerical simulation, and the dynamic performance differences among the six hybrid-structured packed bed TES systems are comparatively analyzed under steady cyclic conditions. Furthermore, the effects of the operating parameter (cutoff temperature threshold Delta T) and structural parameter (phase change material capsule diameter d) of the hybrid-structured thermocline packed bed on the thermal storage performance of the TES system are investigated in detail. The results indicate that the hybrid-structured TES system exhibits optimal thermal storage performance when the total volume fraction of PCMs is 12 %. Additionally, within the scope of this study, the optimal thermal storage performance of the hybrid-structured TES system is achieved when the cutoff temperature threshold Delta T is 30 degrees C and the PCM capsule diameter d is 16.53 mm. This work provides a reference for the practical engineering application of hybrid-structured packed bed TES systems in concentrated solar power (CSP) plants.

Fouling is a prevalent issue in heat exchangers, significantly impairing their thermal performance. To address particle fouling in heat exchanger channels, this study selected a rectangular channel with dimensions: length 1000 mm, width 40 mm, and height 20 mm. The anti-fouling performance of various perforated vortex generator structures in pulsating channel at Re 6334 is optimized through experimental and simulation studies. We experimentally characterize particulate fouling and flow resistance in pulsating channel, a pulsating rectangular wing channel, and a pulsating perforated rectangular wing channel, and validate the numerical model. Additionally, numerical simulations analyze how different perforation sizes and perforation positions (longitudinal and transverse) affect flow resistance and particulate fouling. The results indicate that, compared with the pulsating channel and the pulsating non-perforated rectangular wing channel, the pulsating perforated wing channel achieves superior anti-fouling effect with lower flow loss. For perforation sizes variations, the optimal condition occurs at r/a is 0.6, where flow loss is minimized and the anti-fouling effect reaches 42.2%. At longitudinal position h/b is 0.2, flow loss remains low and the anti-fouling effect peaks at 44.1%. At lateral position d/a is 0.3, flow loss is low and the anti-fouling effect reaches 44.6%. Moreover, within the scope of this study, longitudinal perforation position exerts a greater influence on anti-fouling effect than lateral positioning.

In order to study the deposition of fly ash particles on the heat exchanger tube wall, a model was developed by combining the direct simulation Monte Carlo (DSMC) method with the random function method (RFM). The model incorporates the collision (including rebound and coagulation), deposition, and erosion processes of fly ash particles. Dynamic mesh technology was employed to accurately track the dynamic evolution of particle deposition morphology on heat exchanger tube surface. The model findings are consistent with the experimental data, and the obtained deposition morphology is also similar to that observed in the experiments. In addition, the effects of different incident velocities and mass flow rates on the deposition of fly ash particles are investigated, both in the presence and absence of collisions. This study found that under identical working conditions, the deposition mass on the tube is greater when particle collisions are considered than when they are not considered.

Thermal energy storage (TES) systems are indispensable for the extensive utilization of renewable energy. To overcome the limits of fixed-diameter TES systems, this study designs a three-layer changed-diameter capsule structure with high-temperature phase change material (H-PCM), quartzite and low-temperature PCM (L-PCM). Fifteen cases of changed-diameter packed beds are set up. Under stable cycling conditions, the combined effects of changed-diameter structure and hybrid materials on thermo-economic performance are systematically investigated. In the thermodynamic analysis, the fifteen cases are classified into five groups by fixed H-PCM diameter, and the dynamic axial temperature evolution of HTF within each group is examined. Energy storage performance is comprehensively evaluated energy utilization efficiency, effective energy storage, exergy efficiency, and heat storage capacity. Cases showing favorable axial temperature evolution and energy storage efficiency are further assessed economically by comparing total system cost and heat storage cost. Results indicate that within each group, thermocline thickness increases with L-PCM diameter. The optimal cases identified are 1, 2, 5, and 7 (10-10-10, 15-10-10, 20-10-15, and 25-10-10). For H-PCM diameters of 25 mm and 30 mm, effective energy storage, and energy utilization efficiency decline as L-PCM diameter increases. Based on comprehensive evaluation, cases 1, 2, 5, and 7 demonstrate superior thermodynamic performance and are selected for economic assessment. Among them, case 5 (20-10-15) reduces total cost C by approximately 21.45% and heat storage cost Chs by 20% compared with the costliest case 1, achieving a Chs of 17.16 $/kWh. Within the study scope, case 5 (20-10-15) offers the optimal thermo-economic performance. These findings provide valuable references for the engineering application of changed-diameter hybrid packed-bed TES systems in concentrated solar power plants.

Enhancing the thermal storage performance of shell-and-tube solar latent heat thermal energy storage (LHTES) devices is crucial for achieving efficient utilization of thermal energy in solar heating systems. However, their performance is limited by the inherently low thermal conductivity of phase change materials (PCM). To address this issue, the present study focuses on composite fins integrated into a shell-and-tube LHTES device. By combining experimental and numerical methods, a comparative investigation is first carried out among annular fins, longitudinal fins, and composite fins with the same fin volume. Subsequently, two approaches, referred to as number variation and dimension variation, are employed to systematically examine how different volumetric ratios of annular and longitudinal fins within the composite configuration affect the melting performance of the device. The results demonstrate that, compared with configurations using only annular fins or longitudinal fins, composite fins provide a more uniform axial temperature distribution, accelerate the melting process of the PCM, and improve both the heat storage rate and the thermal response rate. For number variation of composite fins, the LHTES device exhibits better performance when the volumetric ratios of annular and longitudinal fins are both 50%. For dimension variation, favorable performance is observed when the volumetric ratios are 37.5% and 62.5%. Furthermore, comparison of the two approaches shows that when the volumetric ratio of annular fins is below 50%, dimension variation performs better, whereas when it exceeds 50%, number variation becomes more advantageous. Within the scope of conditions considered in this study, under dimension variation with a volumetric ratio of annular fins of 37.5%, the LHTES device achieves optimal performance. Compared with the better-performing volumetric ratio obtained under number variation, the complete melting time is shortened by 6.75%, while the heat storage rate and thermal response rate are increased by 7.60% and 7.14%, respectively.

When the microchannel heat exchanger operates as an evaporator, frosting inevitably forms on cold surfaces at various inclination angles. In the long-term operation process, the frost layer will melt and then solidify. To investigate these phenomena in depth, this paper proposes a frost prediction model for inclined cold surfaces in microchannel heat exchangers that incorporates melting and re-solidification processes. By comparing the average thickness of the frost layer, the local thickness, and the volume fraction of the ice phase in the frost model with/without melting and re-solidification, the accuracy of the model was verified by experiments. Furthermore, based on the melting-re-solidification frosting model, we analyzed how inclination angle affects frost surface morphology evolution, ice phase volume fraction, and the position and area of melting-re-solidification regions. Results indicate that the frost model incorporating melting and re-solidification predicts average frost layer thickness, local thickness fluctuations, and ice phase volume fraction distribution more accurately and with broader applicability than the model without these processes. Further analysis reveals that both average and local frost-layer thickness increase with inclination angle. Under the condition of prolonging the frosting time, the local thickness will increase rapidly with the increase of the inclination angle, and then appear the evolution characteristics of slow decline after reaching the peak. Higher inclination angles also induce significant fluctuations in frost-surface morphology. Analysis of ice-phase volume fraction distribution shows that on horizontal cold surfaces, ice concentrates in narrow regions, whereas on inclined surfaces, gravity causes broader diffusion. As inclination angle increases, melting-re-solidification regions expand and shift position accordingly.

Efficient cooling systems are essential for concentrating photovoltaic (CPV) systems to enhance their role in the energy transition and reduce carbon emissions in the power industry. Filling part of the phase change radiator with metal foam is an effective way to improve thermal management performance. In the phase change radiator, 75 % of the volume was filled with metal foam. The thermal management performance of 12 layout configurations was systematically evaluated by dividing the pure PCM region into single, double, and triple zones, and placing them at the top, upper-middle, middle, lower-middle, and bottom positions. Average temperature, electrical efficiency, and temperature uniformity were used as evaluation indicators to identify the optimal layout. The results show that different layout configurations have little impact on the average temperature and electrical efficiency of photovoltaic cells, but significantly affect temperature uniformity. Among these, the double-zone layout greatly improves overall temperature uniformity. In contrast, placing the PCM region in the middle intensifies the hotspot at the cell center, thereby reducing temperature uniformity. When the pure PCM region is arranged at the upper-middle and bottom positions (Case 9), it exhibits the most effective vertical thermal diffusion capability, reducing the time-averaged temperature non-uniformity index by 31.25 %. This configuration greatly improves the temperature distribution of photovoltaic cells and effectively reduces hotspots on both sides, thereby achieving optimal thermal management performance. Therefore, in a phase change radiator partially filled with metal foam, dividing the pure PCM region into two zones and positioning them at the upper-middle and bottom locations maximizes the thermal management performance of photovoltaic cells.

This paper proposed a particle deposition model that took into account inter-particle collisions. The deposition characteristics of particles in a two-dimensional flow channel were studied by numerical simulation. To account for the influence of particle collisions on their movement and deposition, the direct simulation Monte Carlo method (DSMC) was introduced to calculate inter-particle collisions. The random function method was used to model the deposition of particles. The proposed model was used to calculate the dimensionless deposition velocities of particles with five different particle diameters (1 mu m, 3 mu m, 5 mu m, 9 mu m, and 16 mu m) at inlet velocities of 2.2 m/s, 5.3 m/s, and 9 m/s, and compared to the experimental data. The simulation results were mostly consistent with the experimental results, demonstrating that the proposed model is accurate and effective. At the same time, compared to the results of particle deposition without considering collision, the results considering particle collision were closer to the experimental data. This indicated that collision must be considered in the calculation of particle deposition.

With the rapid advancement of modern industry, heat exchangers often operate with fluids containing fine particulate impurities that easily deposit within heat exchange channels, forming particulate fouling. To address this issue, this study combines pulsating flow with wing vortex generators. First, particulate fouling and pulsating flow models were developed, and their accuracy was validated through experimental testing. Numerical simulations were then conducted to compare the anti-fouling performance of three types of wing vortex generators under pulsating flow conditions. Finally, a detailed analysis was carried out on the effects of the relative height and length of rectangular wing vortex generators on particulate deposition. The results indicate that, compared to smooth channels, the use of pulsating flow offers a certain degree of anti-fouling performance, and combining pulsating flow with wing vortex generators further enhances this effect. Among the three types of wing vortex generators, the rectangular wing vortex generator exhibited the most effective anti-fouling performance under pulsating flow conditions. As the relative length and height of the rectangular wing vortex generator increased, fouling resistance first decreased and then increased. The minimum fouling resistance occurred when the relative length and height were 0.1 and 0.45, respectively, resulting in an anti-fouling performance improvement of up to 42.1 %.

At present, heat exchangers are widely used in industrial production processes, but fouling will lead to an increase in their energy consumption, and the application of ultrasonic can effectively reduce fouling. This study presents a crystallization fouling model under the action of ultrasonic, aimed at investigating the inhibitory effect of dual-frequency ultrasonic on CaCO3 fouling in heat exchanger tubes. Based on the constructed model, this study primarily compares the effects of single-frequency and dual-frequency ultrasonic, with a detailed analysis of the impact of pressure amplitude and frequency of the dual-frequency ultrasonic. The results indicate that dual-frequency ultrasonic at 20 + 40 kHz exhibits the fouling inhibition rate 19.9 % and 36.2 % higher than single-frequency ultrasonic at 20 kHz and 40 kHz, respectively. The fouling layer thickness under ultrasonic action increases progressively with tube length, while the wall shear force varies periodically over time. The average of wall shear force under dual-frequency ultrasonic action is higher than that under single-frequency. In the research range of dual-frequency ultrasonic, increasing pressure amplitude enhances its fouling inhibition effect. As pressure amplitude increases from 100 kPa to 250 kPa, the fouling inhibition rate rises by 21 %. An increase in frequency weakens the fouling inhibition effect. When the co-frequency increases from 30 + 30 kHz to 60 + 60 kHz, the fouling inhibition rate decreases by 19.9 %. When the different-frequency increased from 20 + 40 kHz to 50 + 70 kHz, the fouling inhibition rate decreased by 24.1 %. Additionally, the fouling inhibition rate at different-frequency ultrasonic is consistently higher than that at the co-frequency, indicating that different-frequency ultrasonic provides better inhibition. Furthermore, the fouling layer thickness decreases with increasing pressure amplitude and increases with frequency.

Concentrating photovoltaic (PV) systems have significant potential for promoting low-carbon emissions. However, high temperatures can compromise system safety, highlighting the need for effective cooling devices. Metal foam (MF) enhances the cooling capacity of phase change radiators, but it is costly when fully filled. This study investigates the effects of different metal foam filling conditions in MF-PCM radiator (seven filling ratios: 100 %, 90 %, 80 %, 75 %, 70 %, 60 %, and 50 %; five filling positions of pure PCM regions: top, upper-middle, middle, lower-middle, and bottom regions) on thermal management performance (average temperature and electrical efficiency, as well as temperature uniformity of photovoltaic cells). The goal is to optimize the metal foam filling to maintain performance while reducing production costs. The results indicate that a filling rate of 75 % effectively combines PCM's natural convection with the high thermal conductivity of metal foam, significantly reducing metal foam usage without compromising radiator performance. Furthermore, positioning the pure PCM region in the middle-upper region achieves optimal thermal management performance. Compared to the top region, the average photovoltaic cell temperature shows no significant change, while temperature uniformity improves by 27.08 %, ensuring a more uniform temperature distribution. Therefore, when the metal foam filling rate of 75 % combined with the middle-upper pure PCM region placement substantially reduces production costs without significantly impacting thermal management performance.

With the advancement of modern industry, water is commonly used as a working fluid for heat energy applications. However, this frequently leads to the accumulation of deposits and suspended particulates at specific locations on heat exchange surfaces, forming particulate fouling. This study introduces a local particulate fouling model for the liquid side within a channel, utilizing dynamic mesh technology to investigate the characteristics of localized particulate fouling. The validity of the model was established by comparing simulation results with experimental data. Additionally, a comparative study was performed to assess local particulate fouling characteristics with and without the application of dynamic mesh technology. Detailed analysis was conducted on the effects of various operating parameters on local fouling characteristics using the dynamic mesh approach. Results indicate that the local fouling model with dynamic mesh technology aligns more closely with experimental observations. Evaluation of local fouling resistance along the channel showed that resistance values obtained with dynamic mesh technology are higher than those without it. Measurement of fouling layer thickness within the dynamic mesh channel revealed a rapid increase in thickness, which stabilized after approximately 15 h. The increasing fluctuation range of the thickness curve suggests a rise in inhomogeneity over time. Furthermore, the morphology of the final fouling layer on the deposition wall closely matched experimental findings, both exhibiting an uneven distribution. Comparison across different operating conditions showed that average local fouling resistance decreases with increasing flow velocity, while higher wall temperatures and particulate concentrations result in increased fouling resistance. Specifically, in the numerical range of this paper, when the flow rate is 0.2 m/s, the wall temperature is 333 K, and the particle concentration is 400 mg/L, the thickness and unevenness of the fouling layer reach the maximum.

Frosting is a complex physical process involving multiphase phase changes and porous media. The porous frost layer formed during frosting significantly increases the heat transfer resistance and alters the flow characteristics of microchannel heat exchangers, thereby reducing equipment efficiency. As the fins of microchannel heat exchanger are limited by some conditions in actual operation, they may be arranged in an inclined manner (0 degrees to 90 degrees). However, most existing frosting models are based on horizontal cold surfaces and are not directly applicable to predicting the frosting process on inclined cold surfaces. To address this limitation, this study uses the ice volume fraction threshold to determine the shear stress of the frost layer and subsequently constructs a frost prediction model for inclined cold surfaces in microchannel heat exchangers based on the frost shear function. Then, the model is verified by the experimental platform. Results indicate that the average error in frost layer thickness between model predictions and experimental data is 9 % for inclined cold surfaces (0 degrees to 90 degrees), with most error values within +/- 10 %, thereby demonstrating the model's accuracy and versatility. Furthermore, using the constructed model, the effect of various inclination angles on frost layer thickness is investigated. It is found that both the average thickness of frost layer and the local thickness of each region of frost layer increase with the increase of the inclination angle of cold surface, and the local thickness of frost layer increases rapidly to the peak value along the flow direction of wet air, then gradually decreases to a stable value, and increases slightly to the end. Finally, the effects of various operating parameters on the frosting process of inclined cold surfaces are analyzed. The analysis reveals that air humidity exerts the greatest influence on frost thickness across various inclination angles, whereas air velocity has the least impact.

Annular fins are widely employed in latent heat thermal energy storage (LHTES) systems to improve heat storage and release performance. However, their presence can obstruct the flow of phase change material (PCM) within the storage unit, thereby suppressing natural convection effects. To mitigate this limitation, perforations can be introduced into traditional annular fins. In the present study, the thermal performance of three annular fin configurations, namely uniform annular fins, non-uniform annular fins, and non-uniform annular perforated fins, is systematically evaluated through comparative analysis. Building upon this analysis, the structural parameters of the non-uniform annular perforated fins, including relative hole pitch, relative aperture, and hole number, are optimized using response surface methodology (RSM). The results indicate that both non-uniform configurations exhibit a significantly higher liquid fraction during the melting and solidification processes compared to the uniform design. Among them, the non-uniform annular perforated fins achieve the shortest total duration for the complete heat storage and release cycle, highlighting their superior thermal performance. Further parametric optimization involving 48 different perforation configurations reveals that the minimum total time of 45.27 h is obtained when the relative hole pitch is 0.33, the relative aperture is 0.44, and the hole number is 6. In addition, a fitting formula for the hole structure parameters of non-uniform annular perforated fins was established using RSM, which is based on the total melting and solidification time of the latent heat energy storage unit as the response objective. By substituting the global optimal parameter combination into the fitting equation, it was found that the deviation between the predicted total time and the actual value was less than 3.9 %, confirming the reliability of the optimal combination and the feasibility of the fitting formula.

For the frosting issue of microchannel heat exchangers, this paper develops a long-term frosting model on the cold surface of microchannel heat exchangers and verifies its accuracy through experiments. The study primarily investigates the long-term growth process of the frost layer on the surface (including temperature, air velocity, and ice phase volume fraction) as well as the thermal resistance of the frost layer. The results indicate that as the frost layer grows layer by layer, the temperature boundary layer moves upwards and gradually thins, the air flow velocity in the wet air zone gradually increases, and locations with a higher ice phase volume fraction experience more frost accumulation. Furthermore, the thermal resistance of the frost layer initially increases and then decreases, with a lower thermal resistance observed when considering the melting-reconsolidation process. During the growth of the frost layer, melting occurs earlier at the front end, and the melted water, influenced by gravity, gradually infiltrates through the pores of the frost layer and moves vertically downwards along the plate surface into the internal regions of the frost layer, increasing its density.

The corrosion of the heat exchanger in the actual operation process will make the heat exchange wall rough, and the rough heat exchange wall will cause local fouling deposition in the channel. Based on the constructed local crystallization fouling model, this study compares the local fouling deposition of smooth channel and triangular roughness element channel, and analyzes the relative spacing of triangular roughness elements and the effect of different working condition parameters in detail. Results indicate that compared to smooth channels, triangular roughness elements significantly influence fouling deposition, appearing an extreme value in local fouling resistance. As the relative spacing decreases, the average value of the local fouling resistance initially decreases, reaching its minimum at a spacing of 0.125, before slightly increasing and stabilizing. For the average value of fouling resistance, the increase of inlet velocity or the decrease of calcium carbonate concentration will reduce it; an increase in wall temperature will increase it. Additionally, the thickness of the fouling layer decreases with increasing inlet velocity, decreases with lower concentration of calcium carbonate, and increases with rising wall temperature, both in the smooth zone outside the roughness element and in the roughness element affected zone.

In this paper, an experimental study is presented to investigate the start-up and quasi-steady operation characteristics of a loop pulsating heat pipe (LPHP). The effects of the localized start-up of the LPHP, along with the communication pipe, on the overall start-up process are analyzed, considering the temperature oscillations. In this regard, the temperature oscillations of each channel during the quasi-steady operation are analyzed to characterize and highlight the effects of different LPHP structural parts on the process. The results showed that there is a localized start-up process when employing the side-heating arrangement. Under this arrangement, the start-up time and average temperature at start-up are significantly reduced compared to the three other arrangements. Overall, using a side-heating arrangement, along with the periodic temperature oscillations in the quasi-steady state stage and the multiple quasi-steady state phenomena makes the LPHP withstand higher heat input loads.

This study investigates how factors like inlet wet air temperature, relative humidity, cold surface temperature, and wet air velocity affect frosting on cold surfaces of various angles under forced convection. Using frost layer thickness and quality as the primary evaluation indicators, a single- variable method is employed to study various operating conditions on a horizontal cold surface (0 degrees). For cold surfaces with different inclination angles (0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees), an orthogonal experimental approach is further adopted to delve into the frost layer growth process on these surfaces in detail. The results indicate that, under certain operating conditions, frost thickness and quality on a horizontal cold surface increase with higher relative humidity, higher inlet wet air temperature, lower cold surface temperature, and faster wet air velocity. As the inclination angle of the cold surface increases, both frost thickness and quality first increase and then decrease, reaching their maximum values at an inclination angle of 90 degrees and their minimum values at an inclination angle of 180 degrees. Additionally, as the inclination angle continues to increase, the frost layer transforms from a smooth surface to one with more needle-like frost branches, which is particularly evident at an inclination angle of 180 degrees.