Melting Time Research Articles

Limitations of the enthalpy-porosity method for numerical simulation of close-contact melting on inclined surfaces

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

Oscillatory instability of magneto-convection during the melting of non-Newtonian ferromagnetic nano-enhanced phase change materials at low Rayleigh number

This work complements mechanism of the oscillatory instability of magneto-convection in the process of melting heat transfer controlled by magnetic field at low Rayleigh number (Ra), aiming to provide a theoretical basis for improving the stability of energy storage system and melting manufacturing process. A latent heat storage cavity unit exposed in uniform magnetic field is designed to reveal the magnetic-controlled heat transfer in solid–liquid phase change process. The volume average method is used to describe porous media composite nano-enhanced phase change materials in latent heat unit. Special attentions are paid to the key parameters of Magnetic number (Mn) and Ra, and coupling flow heat transfer mechanism of magnetic force and natural convection effect is numerically studied. The evolution stages of different flow patterns are divided according to the Kelvin force, vorticity, velocity, flow field and isotherm distributions of characteristic points. Main results show that the magnetic field-modulated phase change process at low Ra can be divided into three stages, of which the second stage is characterized by periodic oscillations. Increasing Mn at fixed Ra accelerates the second stage of oscillatory convection heat transfer coming earlier and promotes melting. When Mn increase to 5 × 106, the full melting time is reduced to the greatest extent, resulting in 11.6 % reduction in heat storage but 17.4 % increase in heat storage efficiency. Three mechanisms of magnetic forced convection coupled with natural convection are found during the increase of Ra: synergistic (Ra < 2 × 103), antagonistic (2 × 103≤ Ra < 1 × 104), and natural convection-dominated (Ra = 1 × 104). Finally, the mode distribution map of Ra-Mn regulation is obtained, in which the unstable transformation mechanism of steady-state, periodic oscillation and chaotic oscillation heat transfer in the melting process under the control of magnetic field is described.

ANALYSIS OF AUGMENTATION IN PERFORMANCE OF PV MODULE INTEGRATED WITH FINNED PCM BY THREE-DIMENSIONAL TRANSIENT NUMERICAL SIMULATION

The overall performance of PV-PCM integrated with rectangular straight fins is analysed by three-dimensional transient numerical simulations. The influence of fin lengths, number of fins (n), and inclination (θ) of the system is investigated and compared with the PV-only system, and an optimal system configuration is then identified. Finite element analysis is used to conduct the simulations using COMSOL Multiphysics 6.0. The PV front surface is subjected to a constant flux of 1000 W/m2 for 180 min, and the PCM employed is RT25HC. The results indicate that the average PV temperature tends to drop with increasing inclination and fin length, thereby enhancing the PV efficiency, with maximum improvement attained for the full fin case for a given inclination and number of fins. Compared to the PV-only system, the highest PV temperature reduction and PV efficiency enhancement are 59.65 °C and 45.1%, respectively, for the horizontal system of full-length fins with a number of fins equal to 6. The full-fin PV-PCM system with 6 fins and 45° inclination gives the highest instantaneous power output of 14.16 W. The melting rate of PCM is strongly related to the heat transfer rate inside PCM, and the lowest melting time is obtained for the 8-finned PV-PCM system with θ = 45°. The peak velocity magnitude for all systems with different fin lengths is also examined to analyse the extent of convection levels within PCM.

Influence of fin shape design on thermal efficiency of phase change material-based thermal energy storage unit

The present numerical study treats the impact of fin shape design on the thermal efficiency of phase change material (PCM)-based thermal energy storage (TES) unit, focusing on the same surface area occupied by fins. Comparing two different finned TES units equipped with rectangular and triangular fin shapes, respectively, showed significant enhancements in PCM melting activity. Comparative analysis demonstrated that triangular fin shape lowers PCM melting time by 12.64% for equivalent fin numbers, and by 15.38% for equal fin lengths due to the enlargement of the heat transfer area provided by the triangular shape. Further examination of fins with triangular shape in terms of spacing and length, under fixed thickness and size parameters, revealed significant reduction in melting time with increasing fins length. Notably, 50.75% decrease in melting time was achieved by decreasing the number of fins to 20 while increasing fin length to 10 mm. Moreover, maintaining a heat transfer fluid (HTF) temperature 20 K higher than the melting PCM temperature maximizes TES thermal efficiency. These outcomes emphasize the importance of optimizing fin shape design for enhancing heat transfer without affecting the energy storage capacity of TES systems, with potential applications in building thermal management.

Effects of freeze–thaw on bank soil mechanical properties and bank stability

Riverbank instability in the seasonally frozen zone is primarily caused by freeze–thaw erosion. Using the triaxial freeze–thaw test on the bank of Shisifenzi Bend in the Yellow River section of Inner Mongolia, we investigated the changes in the mechanical properties of the soil at different freezing temperatures and freeze–thaw times, and analyzed the bank’s stability before and after freezing based on the finite element strength reduction method. The results showed that the elastic modulus, cohesion, internal friction angle and shear strength of the soil tended to decrease with the increase in the number of freeze–thaw cycles and the decrease in freezing temperature. After 10 freezing cycles at − 5 ℃, − 10 ℃, − 15 ℃ and − 20 ℃, the modulus of elasticity of soil decreased by 40.84 ~ 68.70%, the cohesion decreased by 41.96 ~ 56.66%, the shear strength decreased by 41.92 ~ 57.32%, respectively. Moreover, the stability safety coefficient of bank slope decreased by 18.58% after freeze–thaw, indicating that the freeze–thaw effect will significantly reduce the stability of bank slope, and the bank slope is more likely to be destabilized and damaged after freeze–thaw.

Diffraction-based approach for real-time monitoring of nanosecond direct laser interference patterning structure formation on stainless steel

Direct Laser Interference Patterning (DLIP) stands out as a versatile and cost-effective method for functionalizing material surfaces at high throughputs. Monitoring the dynamics of the structure formation can lead to a deeper understanding of the interplay between the main factors governing the process and ultimately to optimize the final texture. Here, the formation of gratings on stainless steel by DLIP with ns-pulses is studied using a diffraction-based approach, which measures the time-resolved reflectivity (TRR) of the sample. Measurements are performed for single pulses across different laser fluences. The melting dynamics are analyzed and compared with numerical results. By correlating the recorded signals with the structure depths, growth rates of 11 nm/ns and 57 nm/ns were estimated for fluences of 1.9 J/cm2 and 5.3 J/cm2, respectively. Furthermore, two growth regimes are identified. In the fast growth phase, the melting time increased from 73 to 380 ns for fluences of 1.9 J/cm2 and 5.9 J/cm2, respectively, showing a good agreement with the performed thermal simulations.

Simulation on paraffin melting enhancement in shell-tube phase change thermal storage equipment induced by natural convection

To enhance the thermal performance of the shell-tube phase change thermal storage equipment, the transient two-dimensional mathematical models of the RT35 melting were established. The transient evolution of temperature and velocity distribution and the effect of the positions of the inner tube on the intensity of natural convection for the RT35 and the melting times were analyzed. The results showed that the evolution process of the liquid fraction of RT35 could be divided into two stages: the natural convection stage and the conduction stage. In the natural convection stage, due to the buoyancy effect, the RT35 with high-temperature flowed upward and convectively heat transferred with the solid RT35 on the upper side of the inner tube, resulting in the promotion of the melting of RT35 on the upper side of the inner tube. With the increase in the eccentricity ratio (ε), the gradual increase in the mass of RT35 on the upper side of the inner tube enhanced the effect of natural convection on the melting of RT35, leading to a significant reduction in melting time. When the heat exchange temperature differences (ΔT) ranged between 9 °C and 21 °C, increasing ε from 0 to 0.8 could shorten the melting time by 87.3 %–89.73 %.

Experimental investigation on the stability and heat transfer enhancement of phase change materials composited with nanoparticles and metal foams

Conventional phase change materials (PCMs) possess the challenge of low thermal conductivity, which hinders the energy exchange rate and storage efficiency. PCMs composited nanoparticles and metal foams have been a proven solution to enhance the thermal characteristics of PCMs. In this work, three enhanced PCM composites including nanoparticle/paraffin, metal-foam/paraffin and nanoparticle/metal-foam/paraffin were prepared using fusion blending and vacuum impregnation method. The effects of different nanoparticles with different mass fractions on the stability and heat storage and release characteristics of nanoparticle/paraffin were investigated. Compared with paraffin, the melting time and solidification time of nano-metal/paraffin are reduced by a maximum of 30.18 % and 43.15 %, respectively. In addition, the effect of pore density of metal foam on natural convective heat transfer process was evaluated. The results showed that the stability in descending order is as follows: nano‑aluminum, nano‑copper and nano‑silver. The optimal mass fraction of nanoparticles with good stability was 1.5 wt% (copper), 1.0 wt% (silver), 1.5 wt% (aluminum), respectively. The metal foam makes the internal temperature distribution of PCMs more uniform in the heat transfer process, while inhibiting the convective heat transfer process. With the increase of pore density, the effect on natural convection heat transfer process rises. When the PPI (pores per linear inch) of foamed metal is greater than 40, the natural convection process can be inhibited, obviously. The foam metal has a more significant effect on the heat transfer performance of nanoparticle/metal-foam/paraffin than that of the nanoparticles. The economic benefits of nanoparticle/paraffin, metal-foam/paraffin and nanoparticle and metal foam composite phase change materials are progressively decrease, with MFP being suitable for applications with high economic budgets.

Research on deicing performance and environmental impact of anti-freezing asphalt mixtures

Snow and ice on pavement surface can cause a significant safety concern for drivers. This research investigates the deicing performance of the asphalt mixtures containing a self-prepared salt-storage additive (SSA), as well as their environmental impacts. A pull-off test was developed to evaluate the adhesion between the ice layer and asphalt mixture surface. The self-prepared SSA was incorporated into the asphalt mixtures by replacing mineral filler at percentages of 0 %, 20 %, 50 %, and 100 % by volume. The influence of self-prepared SSA on the pavement performance of asphalt mixtures was firstly investigated in this study, including the high-temperature stability, low-temperature performance, and water stability. The deicing performance of the asphalt mixtures containing self-prepared SSA and commercial product was determined by using the freezing point tests, ice-melting tests, and self-designed pull-off tests. To evaluate the environmental impacts of those anti-freezing asphalt mixtures, corrosion tests and seed germination tests were also employed in this study. Testing results showed that the addition of self-prepared SSA compromised the pavement performance of asphalt mixtures, but all measured properties met the specified requirements. The self-prepared SSA can effectively reduce the freezing point of the asphalt mixtures and shorten the ice melting time by 30 %, which has a similar or even better deicing effect as compared to the commercial salt-storage additive. Notably, the pull-off tests showed that the self-prepared SSA can significantly reduce the adhesion force between ice layer and asphalt mixture surface. The environmental impacts of asphalt mixtures containing self-prepared SSA were found to be within a reasonable range.

Melting and Solidification Characteristics of PCM in Oscillated Bundled-Tube Thermal Energy Storage System

Phase change material (PCM) based thermal energy storage (TES) is an important solution to the waste of heat and intermittency of new energy sources. However, the thermal conductivity of most PCMs is low, which severely affects the thermal energy storage performance. Oscillation of the tube bundles in a TES unit can intensify the convection of liquid PCM and, therefore, enhance heat transfer. However, the energy storage performance of bundled-tube TES systems in response to oscillation at different amplitudes and frequencies has not been well understood yet, and the optimum time to apply the oscillation during phase transition remains unexplored. To address this issue, this present study was carried out. First, the melting behaviour of PCM with oscillation starting at different times was investigated. Then, the influences of oscillation frequency and amplitude on the melting performance were explored. Finally, the solidification behaviour of PCM with oscillation starting at different times was examined. Results show that the oscillation can accelerate the phase transition process by enhancing convective heat transfer. Compared to the case without oscillation, the complete melting and solidification times are reduced by 8.2 and 6.7% for the case with oscillation starting at 200 s, respectively. The effect of oscillation frequency on the melting enhancement is negligible, while the oscillation amplitude has an important effect on the melting enhancement.

Enhanced study of phase change thermal storage using active rotating tank wall coupled fins combined with eccentric design

To address the low thermal conductivity of phase change materials and e improve heat storage, a novel rotating T-LHTESU structure is proposed. Numerical simulations systematically analyze the effects of rotation speed, various eccentricities and wall temperature on the performance of phase change heat storage. The results show that the introduction of rotation in the intermediate sleeve shortens the melting time with increasing rotation speed, reaching a maximum reduction of 61.66 % at 2 rps. Furthermore, the melting rate at 1 rps and 2rps first decreases and then increases with increasing eccentricity. At an eccentricity of 10 mm and 2 rps, the melting time is reduced by 7.23 % compared to the non-eccentric model. In addition, a higher wall temperature accelerates the change in the liquid phase fraction and increases the energy input. At a wall temperature of 371.15 K, the melting time is reduced by 73.56 % compared to the stationary model, with an increase in total heat by 2.52 %. This study provides theoretical guidelines for subsequent research on phase change heat transfer.

Numerical investigation and optimization of the melting performance of latent heat thermal energy storage unit strengthened by graded metal foam and mechanical rotation

In this paper, a combined passive graded metal foam and active mechanical rotation strategy is proposed to simultaneously solve the problem of slow melting rate and non-uniform phase change problem of the latent heat thermal energy storage (LHTES) technology. The enthalpy-porosity method and fixed grid structure are employed to numerically investigate the effects of porosity range, the number of graded layers, and the rotational angular velocity. Moreover, the response surface method (RSM) is further selected to optimize the structural parameters and obtain the function of melting performance and various factors. The result shows that there is an optimal porosity range and the total melting time decreases first and then increases with the increase of porosity range. When the porosity range is 12 % and the graded layer is three, it can shorten the melting time by 35.54 % and increase the thermal energy storage rate (TESR) by 51.57 %. In addition, the melting performance is gradually improved as the number of graded layers increases. The strengthening effect of graded metal foam with just four layers is close to that of the maximum layers considered in this paper. The trend of the influence of rotation speed on the melting performance of the LHTES unit is consistent with the porosity range, indicating the existence of the optimal rotational speed. The RSM optimization structure, with a 14.969 % porosity range, 5-layer graded number, and 2.267 rpm rotational speed, shortens the melting time by 42.42 % and increases the TESR by 62.75 % compared with the stationary case with uniform metal foam. This work verifies the effectiveness of active rotation coupling with passive graded porous structures, which could provide a new strengthening approach to enhance LHTES.

The Effects of Four Different Thawing Methods on Quality Indicators of Amphioctopus neglectus.

Amphioctopus neglectus is a species of octopus that is favored by consumers due to its rich nutrient profile. To investigate the influence of different thawing methods on the quality of octopus meat, we employed four distinct thawing methods: air thawing (AT), hydrostatic thawing (HT), flowing water thawing (FWT), and microwave thawing (MT). We then explored the differences in texture, color, water retention, pH, total volatile basic nitrogen (TVB-N), total sulfhydryl content, Ca2+-ATPase activity, and myofibrillar protein, among other quality indicators in response to these methods, and used a low-field nuclear magnetic resonance analyzer to assess the water migration that occurred during the thawing process. The results revealed that AT had the longest thawing time, leading to oxidation-induced protein denaturation, myofibrillar protein damage, and a significant decrease in water retention. Additionally, when this method was utilized, the content of TVB-N was significantly higher than in the other three groups. HT, to a certain extent, isolated the oxygen in the meat and thus alleviated protein oxidation, allowing higher levels of Ca2+-ATPase activity, sulfhydryl content, and springiness to be maintained. However, HT had a longer duration: 2.95 times that of FWT, resulting in a 9.84% higher cooking loss and a 28.21% higher TVB-N content compared to FWT. MT had the shortest thawing time, yielding the lowest content of TVB-N. However, uneven heating and in some cases overcooking occurred, severely damaging the protein structure, with a concurrent increase in thawing loss, W value, hardness, and shear force. Meanwhile, FWT improved the L*, W* and b* values of octopus meat, enhancing its color and water retention. The myofibrillar protein (MP) concentration was also the highest after FWT, with clearer subunit bands in SDS-PAGE electrophoresis, indicating that less degradation occurred and allowing greater springiness, increased Ca2+-ATPase activity, and a higher sulfhydryl content to be maintained. This suggests that FWT has an inhibitory effect on oxidation, alleviating protein oxidation degradation and preserving the quality of the meat. In conclusion, FWT outperformed the other three thawing methods, effectively minimizing adverse changes during thawing and successfully maintaining the quality of octopus meat.

A novel PCM/expanded graphite composite sphere with high thermal conductivity and excellent shape stability used for a packed-bed thermal energy system

In this research, a novel phase change material (NPCM) sphere was developed and its application in a packed-bed thermal energy storage (PBTES) system during thermal energy storage was studied. Paraffin wax (PW) with a melting point of 40 °C was selected as phase change material (PCM), expanded graphite (EG) as thermal conductivity enhancement material, and epoxy resin (ER) as a surface coating. The results show that the three-dimensional heat conduction network constructed by EG foam improves the thermal conductivity of PW/EG. The thermal conductivity of NPCM is anisotropic. The thermal conductivities perpendicular and parallel to the EG compression direction are 9.03 (W/m·K) and 5.58 W/(m·K). NPCM has excellent shape stability with a leakage rate of only 2.95 %. Then, a PBTE system was built to record the temperature evolution of NPCM spheres at various inlet temperatures and volume flow rates during thermal energy storage, and thermodynamic analysis was performed. The results are as follows. (1) The temperature of the NPCM sphere near the inlet rises more rapidly and the melting time of PCM is shorter. (2) Increasing the inlet temperature improves the heat transfer and shortens the melting time of PCM while increasing the temperature inhomogeneity of the NPCM spheres. Increasing the inlet temperature increases the energy storage capacity. However, higher inlet temperature leads to lower energy efficiency and exergy efficiency. The energy efficiency and exergy efficiency are 49.66 % and 67.64 % for the inlet temperature of 60 °C, respectively. When the inlet temperature increases from 60 °C to 80 °C, the energy efficiency and exergy efficiency decrease by 16.13 % and 16.22 %, respectively. (3) Higher inlet volume flow rate also promotes heat transfer and reduces the melting time of the PCM, but has little effect on the temperature inhomogeneity of the NPCM spheres. Increasing the inlet volume flow rate does not increase the energy storage capacity. Similarly, increasing the inlet volume flow rate also results in a decrease in energy efficiency and exergy efficiency. At the inlet volume flow rate of 19.28 m3/h, the energy efficiency and exergy efficiency are 41.65 % and 56.67 %, respectively. When the inlet volume flow rate increases from 19.28 m3/h to 25.16 m3/h, the energy efficiency reduces by 7.80 % while the exergy efficiency reduces by 7.41 %. This research provides a foundation for the application of the NPCM sphere.

Experimental and numerical investigation of a real-scale air to multiple PCM heat exchanger

Energy consumption by residential sectors has proliferated due to urbanization and lifestyle changes. Passive cooling and heating systems can reduce energy consumption and CO2 emissions in residential and commercial sectors. Phase change materials provide an efficient solution for passive energy storage, addressing a building's needs for free cooling and heating. These materials absorb and release heat energy, enhancing the overall efficiency of the energy management system. By employing energy storage devices, it becomes feasible to reduce and shift peak loads to off-peak hours. This study optimized the size and configuration of air-to-multiple PCM heat exchanger through the utilization of a 2D ANSYS (Fluent 19.2) model. This study employs multiple phase change materials (PCMs) with varying melting temperatures as a heat transfer technique to reduce the melting and solidifying times of the PCMs. Furthermore, it was observed that the arrangement of PCMs in series affected melting and solidification times, so two scenarios have been examined. The air-to-multiple PCM system has been investigated through numerical simulations and experimental analyses, focusing on the liquid fractions within the PCM and the outlet air temperatures across the air channels. The total time for entirely melting PCMs RT25HC and RT21HC is less than 4 h, but the solidification time for PCM-RT21HC needs more than 12 h. In both scenarios, PCM-RT21HC melted first, while PCM-RT25HC solidified first.

Formula optimization, physicochemical characterization, and sensory evaluation of beetroot-based blended frozen dessert

The purpose of this study was to optimize the formulation of beetroot-based blended frozen dessert in terms of sensory and physicochemical characteristics. The results showed that the concentration of beetroot juice, pineapple juice, honey, and lemon juice affected ∆E, melting time, pH, and panelists' preference of the blended frozen dessert. Optimization was conducted using the d-optimal method in the Design Expert 11.0 program and resulted in an optimal blended frozen dessert formula consisting of 60.146 % beetroot juice, 37.354 % pineapple juice, and 2 % honey. Based on the Indonesian food regulations, the blended frozen dessert using the optimal formula can be claimed as a low-calorie, low-sodium food, and a source of vitamin C. Optimization of the formula with the d-optimal method can significantly increase panelists' preference compared to unformulated beetroot blended frozen dessert.

Energy storage, thermal-hydraulic, and thermodynamic characteristics of a latent thermal energy storage system with 180-degree bifurcated fractal fins

The low thermal conductivity of organic phase change materials limits the performance of latent thermal energy storage (TES) systems. Inspired by fractal theory, this study proposes an innovative 180° fractal fin for enhancing the thermal performance of latent TES systems. The effects of length ratios (l) and fractal levels (N) are numerically investigated employing the enthalpy-porosity method. Compared with traditional rectangular fins, the results indicate that the 180° fractal fins reduce the integral average value of the maximum velocity by 2.24%–48.51%, which indicates the suppression of natural convection by the latter. Increasing l and N result in a general increase in melting time. Compared to the TES systems without fins and with rectangular fins, the 180° fractal fins can respectively reduce melting time by up to 88.79% and 28.00%, increase integral average Nusselt number by up to 7.30 times and 34.21%, and enhance energy storage power by a maximum of 8.55 times and 38.71%. Moreover, flow viscous entropy generation can be neglected compared to thermal entropy generation. In contrast to rectangular fins, the employment of fractal fins leads to a maximum reduction of 90.06% and 99.10% in total frictional entropy generation and thermal entropy generation, respectively.

Effect of novel infrared–ohmic thawing method on the thawing rate and minced turkey breast quality

SummaryThe study evaluated the effect of the ohmic thawing method with IR pre‐treatment (unilateral and bilateral) on ground turkey breast that was frozen using various freezing methods, including snap freezing, freezing at −70 °C and −20 °C. The results revealed that the thawing time in ohmic thawing with unilateral and bilateral IR pre‐treatment (in the snap freezing method) was 69.23% and 50.14% shorter compared to the ohmic thawing method without pre‐treatment, respectively. Additionally, the snap freezing method exhibited a significantly higher thawing rate compared to the other freezing methods. Implementing IR pre‐treatment, particularly bilaterally, along with snap freezing, resulted in a reduction of all losses ranging from 20% to 70%. The results of Raman spectroscopy showed that the ohmic thawing method with bilateral IR pre‐treatment resulted in better preservation of protein secondary structure, protein solubility and reduced oxidation of frozen turkey meat. Although the pH changes were not affected by the thawing method, the pH value decreased as the freezing speed reduced. The preservation of meat colour quality was enhanced by increasing the freezing speed and implementing IR pre‐treatment. The results indicate that using IR pre‐treatment can enhance the positive effects of ohmic thawing. Also, the IR type applied showed better results obtained in bilateral mode (P &gt; 0.05).

Performance of a rotating latent heat thermal energy storage unit with heat transfer from different surfaces

Due to the rapid growth in the demand for fast and efficient latent heat thermal energy storage system, multiple heat transfer enhancement techniques have been proposed and widely investigated. Actively or passively, rotation of the energy storage unit affects the internal natural convection and the heat transfer performance. Hence, this study is conducted to evaluate the potential of the heat transfer enhancement by rotation. A two-dimensional model with the phase change material filled in the annular space of a double tube energy storage unit is developed. The numerical simulations for the heat transfer through the internal or external surfaces of the annular space are respectively conducted. Both solidification and melting of the phase change material are taken into account by the enthalpy-porosity method. According to different heat transfer surfaces and the occurrence of solidification or melting, four arrangements are formed. It is found that the rotation of the thermal energy storage unit can reduce the solidification time as high as 46 % compared to its still counterpart, but variation of the rotation speed only produces a minor influence. The effect of rotation on the melting depends on the heat transfer direction (or surface). When the heat is transferred into the solid phase change material through the internal wall, the melting time can be gradually reduced as the rotation speed increases with a maximum of 69 %. However, when the heat is transferred through the external wall, influence of the rotation results in adverse effect. This study can guide the design of innovative high performance latent heat thermal energy storage system containing rotation unit.

Melting and removing wax deposition by thermal washing in oil well

Wax precipitation has proven to be a serious problem when extracting the crude oil from current oilfields. Thermal washing is one of the common methods to remove wax deposition in oil wells. Mastering the melting process and the melting time of the wax layer in thermal washing is essential to optimize operating parameters and further improve the efficiency of thermal washing. In this paper, the scouring model and the melting model are carried out to simulate wax melting behaviour during the thermal washing process in a vertical oil well. Furthermore, with the melting model, the effects of five factors on the melting time have been studied. The results reveal that the melting time is mainly influenced by the thickness of the wax layer and the water flow rate, while the initial temperature of the wax layer, water temperature and inner wall temperature of tubing these three temperatures have a relatively slight influence on the melting time. Finally, with the numerical simulation results, a correlation is developed to predict wax melting time and verified with thermal washing time of the Cha well. It indicates that the melting model is applicable for most of the waxing section, while the scouring model is only applicable when the water initially reaches the bottom of the waxing section, so the melting model is more practical for thermal washing in the oil well.