Phase Change Zone Research Articles

Simulation Analysis of Influencing Factors of High Pressure Phase Transformation in Single Crystal Germanium Ductile Cutting

The high-pressure phase Ge-II of germanium has similar metal cutting characteristics. In single-point diamond turning, in order to maximize the high phase transition zone of germanium, that is easily to make ductile cutting realized. The single factor test method is used to simulate the ultra-precision turning process under different machining process parameters and tool geometric parameters. Through the calculation of the area of the high-pressure phase change zone in the simulation results and the description of the cutting temperature change law, the influence law of different process parameters and tool geometry parameters on the evolution of the phase change zone is obtained, which provides a useful reference for the choice of turning test parameters.

Double-diffusive convection of a rotating circular cylinder in a porous cavity suspended by nano-encapsulated phase change materials

The main objective of this study is to simulate the suspension of a nano-encapsulated phase change material (NEPCM) during the double-diffusive convection flow inside a compound cavity. A novel cavity shape is formed from a central circular cylinder mounted with two rectangular shapes. An incompressible smoothed particle hydrodynamics (ISPH) method is used to solve the governing equations of the present physical problem. A heat capacity of the core-shell layers assisted the phase changes of the encapsulated nanoparticles. The main findings of the performed simulations showed that a fusion temperature controls both location and intensity of a phase change zone inside a cavity. Regardless of the impacts of a buoyancy ratio parameter on the flow speed, the melting-solidification zones are not changing under the variations in a buoyancy ratio parameter. An increase in a radius of a circular cylinder enhances the double diffusive convection and it reduces the intensity of a phase change zone. In addition, the increase in the radius of the circular cylinder powers the average Nusselt and Sherwood numbers. Increasing nanoparticles concentration enhances a phase change zone.

Impact of the dielectric duty factor on magnetic resonance in Ag-SiO2-Ag magnetic absorber

Magnetic absorber in optical frequency can be fulfilled through metamaterials designing. Therein, magnetic resonance in metal-dielectric-metal metasurfaces can be manipulated conveniently, and studying the parameters impacts is the primary for applications. In this work, through changing the grating width and the thickness of silica, the magnetic resonance modes have been studied, the conditions of the phase change zone from magnetic resonance (MR) to Fabry-Perot (FP) are given out in Ag−SiO2−Ag grating magnetic metasurfaces. The results indicate that the MR mode in metal-dielectric-metal configuration is mainly decided on the dielectric duty factor other than the sole behaviors of the thickness of dielectric and size of nanostructures. The physical mechanism is elucidated through simulated electromagnetic field distributions using finite difference time domain (FDTD) solution, and numerical analysis of effective refraction index of Ag−SiO2−Ag magnetic metasurfaces. This study may prompt development of metamaterials in basic research in condensed physics and in optical devices applications.

Calculation of working temperature of water wall of ultra-supercritical boiler

Ultra-supercritical coal-fired power generation technology is an advanced and efficient power generation technology. Due to the high parameters of the ultra-supercritical boiler, the heat transfer of the working substance in the water wall is complicated. Therefore, this paper takes an ultra-supercritical boiler as the research object and uses MATLAB to calculate the temperature of working substance inside the water cooling wall. The results show that when operating under low load there is a phase change zone in the working substance of the water cooling wall, the heat transfer is complex in this zone, and the temperature of the water cooling wall may rise to dangerous range. Therefore, it is necessary to pay attention to the fluctuation of temperature of working substance.

Microstructure and properties of laser-arc hybrid welding thick bainitic steel joints with different arc position

The realization of 12mm thick laser-arc hybrid welding technology is the prerequisite for efficient production of new high-speed train bogies. In this work, for the laser-leading hybrid welding, the problem of hump defects in the root of the weld is easy to occur. The arc-leading mode is chosen to obtain the welded joint of high-strength bainitic steel with no hump, no unfilled and no porosity defects. Then the microstructure distribution of the welds of the high-strength bainitic steel is analyzed, which the weld metal is mainly composed of acicular ferrite and lath bainite and the heat affected zone mainly concludes coarse-grained bainite and granular bainite. Then, under the optimal process, the hardness distribution, arc action area size and weld impact performance of the laser-leading and arc-leading welds were compared. It was found that the arc-leading weld hardness distribution was more uniform and stable, and the hardness peaks both appear in the HAZ (coarse lath bainite), and the valleys both appear in partial phase-change zones (Bulk ferrite). Meanwhile, the arc-leading weld impact performance is 21.49% higher than that of laser-leading.

An experimental study on flow and heat transfer characteristics of binary hydrate slurries in a horizontal tube

Fluid flow and heat transfer characteristics of two kinds of binary hydrate slurries CO2-TBAB and CO2-THF in a horizontal tube are investigated experimentally. It is found that CO2-TBAB hydrate slurry shows Herschel-Bulkley (H–B) fluid characteristics at low flow velocity (<0.35 m/s) and as a dilatants fluid at high flow velocity (>0.45 m/s). On the other hand, CO2-THF hydrate slurry exhibits dilatant fluid properties in the flow velocity range of 0.1–0.8 m/s. Moreover, heat transfer characteristics of CO2-TBAB slurry are studied at different TBAB initial concentrations, flow velocities and heating powers. The local convective heat transfer coefficient decreases in the slurry region and then increases in the aqueous solution region. The hydrate slurry produced with 15 wt% initial concentration of TBAB shows a longer slurry region and a shorter solution region. The local convective heat transfer coefficient of CO2-TBAB hydrate slurry with 5 wt% TBAB is in a range of 4413–5397 W/m2K and its phase change zone is at a temperature of 12–14 °C.

Parameter identification of a multi-stage, multi-load-demand experimental refrigeration plant

Parameter estimation of a multi-stage, multi-load-demand refrigeration plant is addressed. It is shown that dominating system dynamics are those of heat exchangers, and their heat-transfer-related parameters affect system statics but barely influence system dynamics. A novel identification procedure focused on the heat exchangers is presented. It is based on non-measurable refrigerant phase-change zones, considering an overall heat transfer coefficient at each zone. Consistent values of all parameters are obtained considering only steady-state experimental data and some orders of magnitude found in the literature. The identified parameters are validated considering the two-stage, two-load-demand plant configuration.

Synthesis and characterization of multifunctional energy composite: Solar absorber and latent heat storage material of high thermal conductivity

Phase change materials (PCM) used in thermal energy storage systems present high storage capacity and require low temperature variation to allow effective heat charge or discharge density. However, these materials involve too low thermal conductivities (<1Wm−1K−1) which can limit the heat transfer power between the heat transfer fluid and the phase change zone. In the first part of this study, high power level composites made of PCM (organic and also inorganic) and graphite have been elaborated in the thermal range from 50 to 350°C in order to increase the effective thermal conductivity. In a second part, the assessment of their use as solar absorbers has been realized. Results show that these composites present high absorptance with respect to the whole solar radiation spectra. This allows new potentiality of integration of such materials in solar thermal processes offering a direct management of heat variation at the very first step of the process.

On identifying steady-state parameters of an experimental mechanical-compression refrigeration plant

This paper describes the thermodynamic model of a vapour compression refrigeration system, focusing on the performance of the heat exchangers. A novel steady-state identification method is proposed, which is based on inaccessible refrigerant phase-change zones assuming that a constant overall heat transfer coefficient can be identified for each zone. The proposed method has been applied to a configurable two-stage, two-load-demand vapour compression refrigeration plant. Considering steady-state experimental data and taking into account orders of magnitude of the global heat transfer coefficients found in the literature, some relationships between them and the heat exchanger zone lengths are suggested. Then coherent values are obtained for all heat transfer coefficients, which confirms the validity of the hypothesis. Additionally the remaining elements of the refrigeration system have been identified using well-known models. Experiments have been carried out in order to validate this method, using a one-stage, two-load-demand configuration. Although all components are separately identified, a global validation method is implemented, of which results show good agreement with experimental data, with relative estimation errors around 10% in the key variables.

Research of Microstructure and Property of Laser Alloying on the Surface of P20 Steel Compared with Nitriding

The experiment of laser alloying on the surface of P20 steel was made. Tungsten carbide (WC) powder was used as alloying material. The microstructure and property of laser alloying layer and nitriding layer are studied. The research showed that laser alloying layer had better properties such as minute crystals, deeper layer, higher hardness and good metallurgical bonding with base metal. The average hardness of alloying zone was 600HV0.2. The average hardness of phase-change hard zone was 450HV0.2. P20 steel was widely used in the field of plastic mold manufacture, especially mold core and cavity. Abrasion, corrosion and pressure resulted in change of mold size and shape which could greatly affect molding precision. Using laser alloying, the good wear layer would be made on the surface of p20 steel and would greatly increase the mold useful life.

Simulation of phase change drywalls in a passive solar building

Integration of phase change materials (PCMs) into building fabrics is considered to be one of the potential and effective ways of minimizing energy consumption and CO 2 emissions in the building sector. In order to assess the thermal effectiveness of this concept, composite PCM drywall samples (i.e. randomly-mixed and laminated PCM drywalls) have been evaluated in a model passive solar building. For a broader assessment, effects of three phase change zones (narrow, intermediate and wide) of the PCM sample were considered. The results showed that the laminated PCM sample with a narrow phase change zone was capable of increasing the minimum room temperature by about 17% more than the randomly-mixed type. Even though there was some display of non-isothermal phase change process, the laminated system proved to be thermally more effective in terms of evolution and utilization of latent heat. Further heat transfer enhancement process is however required towards the development of the laminated system.

Phase-change drywalls in a passive-solar building

Integration of phase-change materials (PCMs) into building fabrics is considered to be one of the potential and effective ways of minimising energy-consumption and CO 2-emissions in the building sector. In order to assess the thermal effectiveness of this concept, composite PCM drywall samples (i.e., randomly mixed and laminated PCM drywalls) have been evaluated in a model passive-solar building. For a broader assessment, the effects of three phase-change zones (narrow, intermediate and wide) of the PCM sample were considered. The results showed that the laminated PCM sample with a narrow phase-change zone was capable of increasing the minimum room temperature by about 17% more than the randomly mixed type. Even though there was some display of a non-isothermal phase-change process, the laminated system proved to be thermally more effective in terms of evolution and utilisation of latent heat. A further heat-transfer enhancement process is, however, required for the development of the laminated system.

98/02387 TIM-PCM external wall system for solar space heating and daylighting : Manz, H. et al. Solar Energy, 1997, 61, (6), 369–379

Integration of phase-change materials (PCMs) into building fabrics is considered to be one of the potential and effective ways of minimising energy-consumption and CO2-emissions in the building sector. In order to assess the thermal effectiveness of this concept, composite PCM drywall samples (i.e., randomly mixed and laminated PCM drywalls) have been evaluated in a model passive-solar building. For a broader assessment, the effects of three phase-change zones (narrow, intermediate and wide) of the PCM sample were considered. The results showed that the laminated PCM sample with a narrow phase-change zone was capable of increasing the minimum room temperature by about 17% more than the randomly mixed type. Even though there was some display of a non-isothermal phase-change process, the laminated system proved to be thermally more effective in terms of evolution and utilisation of latent heat. A further heat-transfer enhancement process is, however, required for the development of the laminated system.

FINITE-ELEMENT ANALYSIS OF PHASE-CHANGE PROBLEMS USING MULTILEVEL TECHNIQUES

Abstract A simple and effective finite-element procedure is presented for the analysis of transient heat transfer with phase changes. An averaged specific heat is employed to simulate the elements including the phase-change zone in this finite-element analysis with fixed mesh. Due to the use of a Gauss-Seidel method and a multigrid algorithm, the computational complexity can be reduced from O(n3) to 0(n In n) with high storage efficiency. The computational results are also in very good agreement with the exact solutions. Results show that the magnitudes of the lime step and freezing-temperature interval have little effects on the maximal and average errors. As a result, a larger time step can be used to save computing time and achieve the same order of accuracy. This algorithm is available for pure metals and alloys that exhibit discontinuous specific heat.

Effect of Turbulent Heat Transfer on Continuous Ingot Solidification

For many continuous ingot casting processes, turbulent heat transfer in the molten pool plays a critical role which, along with buoyancy and surface tension, is responsible for the quality of the end products. Based on a modified low Reynolds number k-ε two-equation closure, accounting for the phase change and mushy zone formation, the effect of turbulent heat transfer on the solidification characteristics during titanium alloy ingot casting in an electron beam melting process is investigated. The overall heat transfer rate is enhanced by turbulent transport via two sources, one through the correlated velocity and temperature fluctuations present for both single- and multi-phase flows, and the other through the correlated velocity and release of latent heat fluctuations which are unique to the flows with phase change. The roles played by both mechanisms are identified and assessed. The present turbulence model predicts that although the mushy zone defined by the mean temperature field is generally of substantial thickness as a result of the convection effect, the actual instantaneous zone thickness varies substantially due to turbulence effect. This finding is in contrast to the traditionally held viewpoint, based on the conduction analysis, of a generally thin mushy zone. The impact of turbulent heat transfer on local dendrite formation and remelting is illustrated and the issues involved in model development highlighted.

Experimental analysis of heat transfer with phase change in porous media crossed by a fluid flow

Experimental analysis of the heat transfer in an unconsolidated porous medium was conducted. A fluid flowed through the medium in an upward direction. The fluid was heated by one or both of the vertical sides. Several flow and thermal configurations were examined, single-phase and phase-change. Experimental analysis was carried out on the set of temperature values measured throughout the enclosure. All the temperatures in the medium were evaluated by using polynomial fitting from local readings. The validity of the technique is confirmed by the good correspondence, for single-phase flow, between the experimental and calculated temperatures obtained with a numerical model. In phase-change conditions, the experimental observations revealed the formation of zones. In the case of forced convection for steady-state conditions, three zones were observed and localized: a liquid-saturated zone at the bottom, a vapor zone at the top, and a phase-change zone between them. For a liquid initially at rest, these three zones were also observed during a finite time.

Application of infinite elements to phase change situations on deforming meshes

AbstractIn recent years progress has been made in applying moving and deforming mesh systems to phase change problems. This allows the numerical attention where it is needed, near the migrating phase change zone. In spatially unbounded problems one hopes that numerically finite outer boundaries either escape significant activity or are automatically pushed further away as activity nears. Not infrequently this approach fails. Temperature activity often spreads more rapidly than phase change, thereby reaching far boundaries; stretching of the mesh by movement of far boundaries can challenge mesh control and cause ill‐conditioning. In this paper the advantages of time dependent mesh adaption are enhanced by the joining of a new formulation for infinite elements to far boundaries. This is accomplished through a co‐ordinate transformation within the framework of conventional 2‐D quadratic, biquadratic, and linear–quadratic elements. Standard 2 by 2 Gauss–Legendre quadrature suffices throughout and normal Galerkin finite element features are undisturbed, including strict conservation of energy. The formulation is independent of global co‐ordinates, entails no restrictions on the unknown function and should be applicable to other problem types. All test cases on quadrilateral and triangular grids show very significant improvements with infinite elements relative to comparable solution systems using strictly finite grids.

A finite element analysis of fire-resisting cabinets using an adaptive remeshing technique

Fire-resisting cabinets are specially designed for the protection of valuable documents such as magnetic tapes or floppy discs against fire hazard. Special phase change materials are employed to absorb the intensive heat generated by the combustion process. The phase change region for these materials usually occurs within a narrow range of temperature. When the problem is spatially discretized via the finite element method using a fixed mesh, the range of phase change also depends on the fineness of the mesh in the area undergoing the change. Therefore the range of phase change is widened when a coarse mesh is employed. To accurately simulate the phenomena, an adaptive remeshing technique is introduced that tracks the moving front of the phase change zone. At certain time steps a scan is made to find out the points at which phase change is occuring. This procedure is carried out at intervals that are at the discretion of the user and will obviously be influenced by the accuracy required in the solution and the computational facilities available. Thus remeshing may be done to produce a refined mesh at the salient points only and continue the computing process. The results obtained illustrate that the method is both efficient and accurate.

Solidification in castings by finite element method

AbstractA self-contained CAD (computer aided design) system capable of analyzing foundry casting processes in sand and gravity dies is being developed at the University College of Swansea. The work involves preprocessing, postprocessing, and a finite element code with some novel numerical techniques. The solidification of castings is a heat transfer problem involving phase change, which may occur in a narrow range of temperatures. To simulate the phenomena accurately, very fine meshes must be used and the solution of such a system becomes very expensive. In the Swansea system, an adaptive remeshing technique is introduced, which tracks the moving front of the phase change zone. At every time step, a scan is made to determine the points at which phase change is occurring, so that the remeshing may be done to produce a refined mesh at such points. The computing process is then continued. Examples have illustrated that the method is efficient and accurate. In addition, an interfacial heat transfer model is i...

Solidification in castings by finite element method

A self-contained CAD (computer aided design) system capable of analyzing foundry casting processes in sand and gravity dies is being developed at the University College of Swansea. The work involves preprocessing, postprocessing, and a finite element code with some novel numerical techniques. The solidification of castings is a heat transfer problem involving phase change, which may occur in a narrow range of temperatures. To simulate the phenomena accurately, very fine meshes must be used and the solution of such a system becomes very expensive. In the Swansea system, an adaptive remeshing technique is introduced, which tracks the moving front of the phase change zone. At every time step, a scan is made to determine the points at which phase change is occurring, so that the remeshing may be done to produce a refined mesh at such points. The computing process is then continued. Examples have illustrated that the method is efficient and accurate. In addition, an interfacial heat transfer model is introduced to improve the simulation of the casting process. Advective heat transfer in the liquid is also modelled.MST/1041