A phase change processor method for solving a one-dimensional phase change problem with convection boundary

A simple yet accurate iterative method for solving a one-dimensional phase change problem with convection boundary is described. The one-dimensional model takes into account the variation in the wall temperature along the direction of the flow as well as the sensible heat during preheating/pre-cooling of the phase change material (PCM). The mathematical derivation of convective boundary conditions has been integrated into a phase change processor (PCP) algorithm that solves the liquid fraction and temperature of the nodes. The algorithm is based on the heat balance at each node as it undergoes heating or cooling which inevitably involves phase change. The paper presents the model and its experimental validation.     

A level-set based adaptive-grid method for premixed combustion

A numerical method to simulate premixed combustion is analyzed. It consists of a Cartesian cut-cell flow solver for compressible viscous flows coupled with a level-set method which solves the G-equation to describe the kinematics of the premixed flame. The coupling of the two solvers is achieved via a dual hierarchical dynamic adaptive-mesh framework. Both solvers operate on different Cartesian hierarchical meshes sharing a common background grid level through which they are connected. For the flow solver, feature- and G-based adaptive mesh refinement is taken advantage of, while a uniform high-resolution grid is used for the level-set solver. The heat release due to combustion is described by a source-term formulation by which the reaction rate profile of the premixed flame can be attached to the flame front, the motion of which is governed by the G-equation. A flame–vortex interaction problem is discussed in detail to validate the proposed methodology and to demonstrate the benefits of solution-adaptive mesh refinement in the context of the level-set approach for premixed combustion. After a forced laminar Bunsen flame is considered as an example for attached flames, the coalescence of two spherical flame kernels is simulated to assess the performance of the method and the potential savings in terms of computational costs for three-dimensional problems. The results of the test problems show the artificial thickening of the flame and numerical errors in the level-set solution on coarser grids to possess a comparatively small impact on the overall accuracy. The best findings in the sense of efficiency and physical quality are achieved by the combined feature-/G-based adaptation method.     

The effects of liquid motion induced by phase change and thermocapillarity on the thermal equilibrium of a vapour bubble

A theoretical estimate is made of the liquid motion induced in the vicinity of a vapour bubble on a heated solid surface by evaporation and condensation at the bubble surface and by thermocapillarity effects. These results are used to examine the thermal equilibrium of the vapour bubble.     

Direct contact heat transfer with change of phase: Condensation of a bubble train

Solution of the coupled velocity and temperature fields associated with the condensation of a single or two-phase bubble train is used to obtain the bubbles' radii as a function of time (or height), frequency, temperature driving force and inerts concentration.The reliability of the solution procedure is demonstrated by its convergence at zero frequency to other solutions of single bubble condensation and by the good agreement of the calculated results with experimental data.     

Direct contact heat transfer with phase change: Theoretical expression for instantaneous velocity of a two-phase bubble

An analytical expression for instantaneous velocity of two-phase bubble evaporating through immiscible liquid, has been developed. This expression, predicts very well, the experimental data for n-pentane and furan drops evaporating through high viscosity aqueous glycerol.     

A non-parametric method for estimating enthalpy-temperature functions of shape-stabilized phase change materials

A method for thermodynamic characterization of shape-stabilized phase change materials (PCM) based on one-single sample and one-single experiment has been proposed. The simplicity of the experimental device is comparable to that of the T-History method. However, instead of simple energy balances as in T-History method, a numerical heat transfer model is used to retrieve the whole set of parameters/functions characterizing the PCM from temperature measurements at one-single point within the PCM. An efficient inversion technique has been proposed for that. Its most striking feature is that it allows non-parametric identification of the enthalpy-temperature function in an easy way. Such a function is retrieved by solving a problem of nonhomogeneous heat source term estimation by inversion of a linear heat conduction model.     

Collapse of one-component vapor bubble with translatory motion

Theoretical analysis of one-component vapor bubble collapse with translatory motion in uniformly subcooled liquid has been carried out. The bubble is spherical and flow in the region surrounding the bubble is potential. General solution is obtained in which the function R = R(τ) is defined implicitly by integral equation. General solution is reduced to the quasi steady state and quasi linear problem. Quasi steady state solution is used to obtain a set of simple and explicit expressions by which the bubble radius is determined in function of time. The results of theoretical analysis are compared with those given by other authors and available experimental data. The agreement between compared experimental data and theoretical results is very good.     

A modular phase change heat exchanger for a solar oven

A modular energy storing heat exchanger designed to use pentaerythritol for thermal storage (solid-solid phase change at 182°C) is tested in an oven by circulating heat transfer oil which is electrically heated in a manner to simulate a concentrating solar collector. Three efficiencies for heating the system under controlled and measured power input are determined—the heat exchanger efficiency, the efficiency of the heater with distribution lines, and the total system efficiency. Thermal energy retention times and cooking extraction times are determined, and along with the efficiencies, are compared with the results previously reported for a nonmodular heat exchanger. The modular configuration provides a highly improved extraction rate for cooking due to its wrap-around character and its increased surface-to-volume ratio. A full scale glass model of the copper tubing of the system is described and flow observations reported demonstrating how uniformly the parallel pumping branches perform and how trapped air pockets affect pumping power. A technique for measuring pumping power is described and its application to our system is quantified to show that less than 1 watt is required to circulate the heat transfer oil even when the system includes the solar collector and its longer connecting tubes.     

Performance indicators for solar pipes with phase change storage

Performance indicators for a solar pipe system in which solar radiation is stored as latent heat of a phase changing material are proposed. These performance indicators are aimed at serving as a yardstick by which such multivariables systems are evaluated. The indicators are the melt and solidification times obtained for standardized systems and conditions. These indicator enable the comparison between the suitability of systems with different materials and configurations to store and release thermal energy. The indicators are obtained from numerical solutions of the nonlinear heat conduction problem of the axisymmetric liquid-solid interface motion within the solar pipe. Longitudinal dimension required for the determination of the solidification time is added by an axial superposition of axisymmetric sections. This simplified approach enables a simple numerical solution for an otherwise complicated problem. Estimates of performance characteristics that are based on a simplified model and realistic materials point to the practical potential of solar pipe utilization.     

Quasi-steady-state model of a counter-flow air-to-air heat-exchanger with phase change

Using mechanical ventilation with highly efficient heat-recovery in northern European or arctic climates is a very efficient way of reducing the energy use for heating in buildings. However, it also presents a series of problems concerning condensation and frost formation in the heat-exchanger. Developing highly efficient heat-exchangers and strategies to avoid/remove frost formation implies the use of detailed models to predict and evaluate different heat-exchanger designs and strategies. This paper presents a quasi-steady-state model of a counter-flow air-to-air heat-exchanger that takes into account the effects of condensation and frost formation. The model is developed as an Excel spreadsheet, and specific results are compared with laboratory measurements. As an example, the model is used to determine the most energy-efficient control strategy for a specific heat-exchanger under northern European and arctic climate conditions.     

Fire retardants for phase change materials

Fire-retarded form-stable phase change material (PCM) products consisting of paraffin (RT21) (or propyl ester), high density polyethylene (HDPE) and fire retardants were prepared using the Brabender Plastograph. The properties of the form-stable PCM, containing different types of fire retardants such as magnesium hydroxide, aluminium hydroxide, expanded graphite (EG), ammonium polyphosphate (APP), pentaerythritol (PER) and treated montmorillonite (MMT) were classified using vertical burning test, thermogravimetry analysis (TGA) and differential scanning calorimeter (DSC). The results from the vertical burning test have shown that the form-stable PCM which contained APP + PER + MMT and APP + EG showed the best improvement in fire retardancy since it can self-extinguish by forming a large residue. The TGA graphs showed that addition of fire retardants has increased thermal stability of material by increasing the amount or residues formed, which was also supported by the Con Calorimeter testing, while DSC results showed that adding fire retardants to PCM did not change its thermal properties significantly.     

Optimization of a phase change material wallboard for building use

In construction, the use of phase change materials (PCM) allows the storage/release of energy from the solar radiation and/or internal loads. The application of such materials for lightweight construction (e.g., a wood house) makes it possible to improve thermal comfort and reduce energy consumption. A wallboard composed of a new PCM material is investigated in this paper to enhance the thermal behavior of a lightweight internal partition wall. The paper focuses on the optimization of phase change material thickness. The in-house software CODYMUR is used to optimize the PCM wallboard by the means of numerical simulations. The results show that an optimal PCM thickness exists. The optimal PCM thickness value is then calculated for use in construction.     

Analysis of a flat plate collector with fluid undergoing phase change

This paper presents a theoretical analysis of the performance of a flat plate solar collector with the heat removal fluid undergoing a phase change. The resultant efficiency expression is a modified Hottel-Whillier-Bliss equation. Numerical computations are made to investigate the effect of vaporisation and operational parameters on the collector's performance. The collector's efficiency increases with the increase in liquid length until a point is reached when the region of superheating the vapour disappears. The efficiency is higher when a heat removal fluid of high latent heat of vaporisation is used in the collector. An increase in the saturation temperature of the working fluid (with increase of pressure) in the collector reduces its efficiency.     

Cooling of helmet with phase change material

This paper presents the design of a helmet cooling system using phase change material (PCM) to absorb and to store the heat produced by the wearer head so as to achieve comfort cooling for the wearer. The PCM is packed into a pouch and placed between the helmet and the wearer head. The heat from the wearer head is transferred to the PCM by conduction through a heat collector which is spread over the wearer head. No electrical power supply is needed for the cooling system. The temperature on the wearer head is maintained just above the PCM temperature, thus the wearer would not suffer from an uncomfortable and dangerous hot environment on the head which will affect the wearer alertness. The cooling unit is able to provide comfort cooling up to 2 h when the PCM is completely melted. The stored heat from the PCM pouch would then have to be discharged by immersing in water for about 15 min to solidify the PCM before re-use. The PCM helmet cooling system is simple and has potential to be implemented as a practical solution to provide comfort cooling to the motorcycle riders.     

纳米复合相变材料固液相变储能过程研究进展

对纳米复合相变材料固液相变储能过程的若干最新研究进行了回顾,从相变储能系统的动态性能和典型的凝固、熔化传热过程两方面总结了相关研究的进展,并重点评述了数值模拟研究中纳米复合相变材料有效热物性预测方法的适用性及其与实验结果之间的偏差,最后对纳米复合相变材料固液相变储能过程的未来发展和重点研究方向进行了展望。