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[en] A hybrid numerical method is developed to solve one-dimensional phase change problems with the mushy zone. This hybrid numerical method involves the control volume formulation for the space domain and the Laplace transform technique for the time domain. In the present study, nonlinear terms are linearized by using the Taylor series approximation. The growth of the mushy zone is unknown a priori and is predicted by using the least squares concept. To show the efficiency of the present numerical method, various comparative examples are illustrated. It can be seen that excellent agreement is observed between present numerical results and those of early works
[en] Recently, electronic and electrical products have problems how to reduce heat in trend reducing size and increasing speed. heat pipes worked by latent heats can solve problems for effective and quiet electronic applications. Heat Pipes have to be suitably designed for the external conditions due to showing optimum performance. it has influence on efficiency of heat pipes to the exterior structure changed by length, bending angle, diameter. Designing heat pipes has depended on experience from trial and error. this method wasted too many resources, but can't guarantee efficiency. to prevent those wastes, this study aims at making the thermal transfer coefficient predicting efficiency. In this study, the thermal transfer coefficient has been made from experimental results that used variables - lengths between heat source and radiation, bending angles, diameters of heat pipes. variables become non-dimensional in modeling process for making the coefficient
[en] Highlights: • Comprehensive review of the main types of electrospun ultrafine PCFs since 2006. • Review relationship between fiber morphology, composition and thermal properties. • Discuss future challenges and opportunities of electrospun ultrafine PCFs. - Abstract: Over the last 30 years, phase change fibers (PCFs) have been extensively investigated and applied as high-performance nonwoven fabrics and coatings. As a prospective renewable and clean material, PCFs with micro-scale have been successfully prepared by melt/wet spinning for applications in thermal energy storage (TES) and temperature regulation. With the development of fiber manufacturing techniques, e.g. electrospinning, ultrafine PCFs have been exploited and investigated in the last decade. This paper considers the state of investigations and developments in ultrafine (submicro-scale) PCFs by electrospinning technique since 2006. Electrospun ultrafine PCFs individually using long-chain aliphatic hydrocarbons (and paraffin waxes), polyethylene glycol, fatty acids (and their eutectics), and other solid-liquid phase change materials (PCMs) as latent heat storage (LHS) component are reviewed. The relationship between morphology, composition, and thermal properties are discussed for providing guidance for fabricating appropriate ultrafine PCFs with desired thermophysical properties for various applications. The further challenges and opportunities of electrospun ultrafine PCFs for TES and other applications are also discussed.
[en] Highlights: • This paper analyzes the performance of a building-integrated thermal storage system. • A wall opposing a glazed surface serves as phase change materials thermal storage. • The study is based on both experimental and simulation studies. • Heat is stored and released up to 6–8 h after solar irradiation. • Yearly heating requirements are reduced by 17% in a cold climate. - Abstract: As energy availability and demand often do not match, thermal energy storage plays a crucial role to take advantage of solar radiation in buildings: in particular, latent heat storage via phase-change material is particularly attractive due to its ability to provide high energy storage density. This paper analyzes the performance of a building-integrated thermal storage system to increase the energy performances of solaria in a cold climate. A wall opposing a highly glazed façade (south oriented) is used as thermal storage with phase change materials embedded in the wall. The study is based on both experimental and simulation studies. The concept considered is particularly suited to retrofits in a solarium since the PCM can be added as layers facing the large window on the vertical wall directly opposite. Results indicate that this PCM thermal storage system is effective during the whole year in a cold climate. The thermal storage allows solar radiation to be stored and released up to 6–8 h after solar irradiation: this has effects on both the reduction of daily temperature swings (up to 10 °C) and heating requirements (more than 17% on a yearly base). Coupling of the thermal storage system with natural ventilation is important during mid-seasons and summer to improve the PCM charge-discharge cycles and to reduce overheating. Results also show that cooling is less important than heating, reaching up to 20% of the overall annual energy requirements for the city of Montreal, Canada. Moreover, the phase change temperature range of the material used (18–24 °C) is below typical summer temperature levels in solaria, but the increase in thermal capacity of the room alone can reduce annual cooling requirements by up to 50%.
[en] Spin casting is being used widely in the prototyping industry as a secondary process to convert a master model into a functional metal or plastic part. The main problem of the spin-casting process is the poor thermal conductivity of silicone rubber as mould material--which leads to a long cooling time between casting processes, a short lifespan of the mould and therefore quality problems with respect to the final product. In order to address these problems different cooling methods, such as the λ-ρC method and latent heat storage method, have been developed, investigated experimentally and described in this paper. Experimental results show that some metals--such as plain carbon steel--can be used to control the thermal process in spin casting effectively
[en] Electric energy storage is considered to become a key element of the future electricity infrastructure. PTES (Pumped thermal electricity storage) represents an emerging thermo mechanical storage technology based on the transformation of low temperature heat by surplus electricity. After transformation, the high enthalpy heat is stored. During the discharge process, this heat is used to drive a thermodynamic cycle generating electricity. This concept allows storage of energy in the multi-MW range for several hours without any specific geographical requirements. Various combinations of thermodynamic cycles and storage types have been suggested for implementation using either low temperature storage (<200 °C) or high temperature storage (>500 °C). In contrast to these PTES concepts, the Compressed Heat Energy STorage (CHEST) concept presented in this paper is based on a medium temperature conventional Rankine cycle combined with a latent heat storage unit according to the current state of the art. This concept attains an efficiency of 70% while the maximum temperature is below 400 °C. The integration of heat provided by low temperature sources during the charging process represents an additional option of the CHEST concept; losses can be compensated, the electric work delivered during the discharge process might even outweigh the work needed during the charging process. - Highlights: • New concept for storage of electrical energy in the multi-MWh range is presented. • State of the art medium temperature storage technology is applied. • Maximum temperature is below 400 °C. • Roundtrip efficiency in the range of 70% is calculated. • Integration of low temperature heat sources allows compensation of losses
[en] The present study is an experimental investigation of nucleate boiling heat transfer mechanism in pool boiling from wire heaters immersed in saturated FC-72 coolant and water. The vapor volume flow rate departing from a wire during nucleate boiling was determined by measuring the volume of bubbles, varying 25 μm, 75 μm, and 390 μm, from a wire utilizing the consecutive-photo method. The effects of the wire size on heat transfer mechanism during a nucleate boiling were investigated by measuring vapor volume flow rate and the frequency of bubbles departing from a wire immersed in saturated FC-72. One wire diameter of 390 μm was selected and tested in saturated water to investigate the fluid effect on the nucleate boiling heat transfer mechanism. Results of the study showed that an increase in nucleate boiling heat transfer coefficients with reductions in wire diameter was related to the decreased latent heat contribution. The latent heat contribution of boiling heat transfer for the water test was found to be higher than that of FC-72. The frequency of departing bubbles was correlated as a function of bubble diameters
[en] The study focuses on the heat transfer performance of two-phase closed thermosyphons with plain copper tube and tubes having 50, 60, 70, 80, 90 internal grooves. Three different working fluids(distilled water, methanol, ethanol) are used with various volumetric liquid fill charge ratio from 10 to 40%. Additional experimental parameters such as operating temperature and inclination angle of zero to 90 degrees are used for the comparison of heat transfer performance of the thermosyphon. Condensation and boiling heat transfer coefficients, heat flux are obtained using experimental data for each case of specific parameter. The experimental results are assessed and compared with existing correlations. The results show that working fluids, liquid fill charge ratio, number of grooves and inclination angle are very important factors for the operation of thermosyphons. The relatively high rate of heat transfer is achieved when the thermosyphon with internal grooves is used compared to that with plain tube. The optimum liquid fill charge ratio for the best heat transfer performance lies between 25% and 30%. The range of the optimum inclination angle for this study is 20 .deg . ∼30 deg. from the horizontal position
[en] Highlights: •Testing of a latent heat storage for solar cooling applications is proposed. •The storage is made of a finned heat exchanger and a paraffinic blend. •The effect of operating parameter and incomplete phase change have been evaluated. •Thermal conduction is the dominant heat transfer process in the storage system. -- Abstract: The paper presents the realization and experimental characterization of a lab-scale latent heat storage, specifically developed for solar cooling applications. The latent heat storage is based on a compact fin-and-tube stainless steel heat exchanger (HEX) and a commercial paraffin blend, having a nominal melting temperature of 82 °C, suitable for solar cooling plants employing non-concentrating solar collectors technology. The realised heat storage has been experimentally characterised in lab, by means of a test rig able to simulate the working boundary conditions of a solar cooling plant. Charging and discharging tests have been performed both simulating a completed charge phase followed by a complete discharge phase, to analyse system efficiency and achievable energy storage density. Furthermore, dynamic tests, simulating short consecutive charge/discharge phases (with incomplete phase change), have been accomplished, to analyse the heat transfer efficiency inside the reactor. Main results confirmed that the heat storage density increases of about 50%, compared to sensible water storages. Satisfactory discharge efficiency, ranging between 45% and 60% has been obtained under analysed working conditions. Average discharging power between 0.7 and 1.2 kW has been measured, which confirms the necessity to further optimize the HEX efficiency as well as the thermal conductivity of the employed PCM.
[en] In this study, to obtain basic information for the design of a latent heat storage system, (1) the cylindrical type and the rectangular type of latent heat storage elements were designed, (2) the finite element method was adopted for the prediction of temperature profile of phase change material in heating and cooling process, and (3) experiments were performed to verify the numerical solutions, and then (4) the optimum size of latent heat storage units was predicted by the computer simulation. The results could be summarized as follows : (1) In cooling process, the predicted temperatures of latent heat storage units by computer simulation were in good agreement with measured. (2) The effective diameter of cylindrical element was observed to be 28 mm and the effective thickness of rectangular element was observed to be 21 mm