Full Melting Time Research Articles

Effect of porous media on the heat transfer enhancement for a thermal energy storage unit

Thermal energy storage (TES) can effectively recover thermal energy from low-temperature waste heat and it has now been received increasing attentions in practical engineering applications. Nevertheless, the relatively low thermal conductivity of engineering available phase change materials (PCMs) greatly limits the energy efficiency of TES applications. To enhance the phase change process, open-cell metal foam with a porosity of 0.94 and pore density of 15 PPI (pore per inch) was employed to be inserted either in heat transfer fluid (HTF) or in phase change material (PCM). A two-dimensional axis-symmetric problem was numerically solved and was validated through comparing temperature history at selected points. Results demonstrated that the involvement of open-cell metal foam can effectively enhance the phase change heat transfer, greatly reducing the full melting time. By comparing the four cases (without metal foam, inserting metal foam into HTF, PCM and both domains), the case that both HTF and PCM domains were embedded with porous media can provide the best heat transfer enhancement, from which practical applications with thermal engineering may benefit.

Experimental investigation on the melting behavior of phase change materials in open-cell metal foams in an inclined rectangular enclosure

To explore the heat and mass transfer characteristics during phase change, a well-designed test rig with melting front visualization was designed and fabricated. The results show that the different inclination angles have a great influence on the melting process of pure paraffin. Compared with the 60-degree inclination, the full melting time of pure paraffin at 30-degree obliquity decreased by 25.30%. The visualization demonstrated an inclined melting front, which indicated the contribution of local natural convection in the fluid phase. However, there seems to be little influence of inclination angles for the melting process of paraffin embedded in metal foam. Local thermal non-equilibrium between metallic ligaments and the saturating PCM (paraffin) was experimentally observed. This revealed the feasibility of two-equation model for modelling the phase change process in open-cell metal foam.

Melting process in porous media around two hot cylinders: Numerical study using the lattice Boltzmann method

Instantaneous melting of ice is accelerated inside a horizontal rectangular cavity with two vertically arranged cylinders using metallic porous matrix made of Ni-Steel alloys. The enthalpy-based lattice Boltzmann method (LBM) with a double distribution function (DDF) is employed at the representative elementary volume (REV) scale. Single-phase model is used and there is a local thermal equilibrium between porous structure and ice. Inserting metallic porous material into the base PCM results in the enhancement of the heat conduction and weakening of the natural convection flow. Concentric pattern of the solid–liquid interface persists in porous samples comparing to the pure PCM melting. Reducing porosity causes decrease of the full melting time and thermal storage capacity of the system. Thermal conductivity ratio has to be enlarged in porous samples with higher porosity from energy saving viewpoint.

Constrained ice melting around one cylinder in horizontal cavity accelerated using three heat transfer enhancement techniques

Constrained melting around one hot cylinder inside a horizontal square cavity is inefficient from an energy-saving viewpoint because of the elevated thermal stratification and sluggish advancement of the melting front at the bottom. The situation is exacerbated by the low thermal conductivity of the phase change materials (PCM). To overcome it, four hot cylinders instead of one cylinder are selected in the same cross-section area. Besides, the base PCM is mixed with the small volumetric concentration of the nanometer-sized Cu particles to enhance the melting rate. It has to be clarified in which array the NEPCM is the most effective way. However, increasing the dynamic viscosity of the base PCM using nanoparticles may have a negative effect on the speed of the natural convection flow. Finally, the metallic porous matrix with the thermal characteristics of the Nickel-Steel alloys is inserted in the base PCM to enhance the thermal conductivity and melting rate and to abate the full melting time. The weakening of the natural convection effect within the porous structure could be worthy of attention. The enthalpy-based lattice Boltzmann method (LBM) with a D2Q9-double distribution function (DDF) model at the REV scale is used to model the ice melting in the absence of the subcooling.

Thermal performance of a shell-and-tube latent heat thermal energy storage unit: Role of annular fins

This study conducts numerical investigations on melting process in a shell-and-tube latent heat thermal energy storage (LHTES) unit with annular fins. Commercial grade paraffin is employed as the phase change material (PCM) and water serves as the heat transfer fluid (HTF). Finite-volume-method (FVM) based numerical simulations are performed to investigate the effects of fin number, height and thickness on the phase change process. Particular attention is made to justify the contribution of local natural convection to the overall phase change process. Results demonstrate that the full melting time could be maximally reduced by 65% by inserting annular fins into PCM. For maximizing thermal performance, an optimal group fin parameter (fin number N=31, thickness t/l=0.0248 and interval l/L=0.0313) is recommended for the present study.

Analysis of HTFs, PCMs and fins effects on the thermal performance of shell–tube thermal energy storage units

In solar thermal power plant, the thermal energy storage (TES) system is a key component which includes thermal energy storage unit, thermal energy storage material and heat transfer fluid (HTF). We numerically simulate the dynamical thermal performances of shell–tube TES units with diverse geometric configurations, phase change materials (PCMs) including pure NaNO3 and a composite of NaNO3/expanded graphite (EG), and HTFs such as synthetic oil and molten salts. In the charging/discharging process, the conjugated heat transfer behavior between the thermal storage unit and thermal carriage channel is specially concerned. Effects of the extended fin, PCMs and HTFs on improving heat performance are examined and quantified with total heat flux and effective Nusselt numbers. Together with the presented diagrams of PCM phase evolution, the roles of heat conduction and natural convection in influencing the thermal behavior of the TES units are clearly revealed. The nonuniform phase distribution of PCM due to natural convection results in a considerable time spent in the end of the charging/discharging processes. The results show that both the full melting time and solidification time can be shortened at least 14%, benefited from using the extended fins and the composite. This study is expected to provide an apprehensive interpretation of thermal transport mechanism and rational advices in optimizing the TES system operation.

A comparison study on melting inside the rectangular and curved unit with a vertical heating wall

Efficient and rational use of thermal energy requires the design and development of suitable and cost-effective latent heat storage systems. In this study, a novel curved thermal storage unit is put forward and its thermal behavior has been numerically compared with the traditional rectangular PCM-based thermal storage unit. The solid–liquid interface evolution, liquid flow pattern and temperature distribution are presented. Transient Nu at the heating wall and full melting time are also calculated. The numerical results show the high impact of the enclosure geometry on melting and nature convection. A reduction of 30.6 % in thermal storage time has been achieved by changing the unit from rectangular to curve. Moreover, parametric studies are conducted to assess how thermal performance of the units is affected by the heating wall temperature and PCM thermal conductivity. Correlations encompassing a wide range of parameters are developed in terms of full melting time. Results indicate that proportion of full melting time reduced for the curved unit is almost a constant value and does not vary significantly with these factors. All these may be very helpful for the design of more efficient thermal storage units.

Effect of the length ratio on thermal energy storage in wedge-shaped enclosures

In this paper, a fundamental practical unit, namely the wedge-shaped enclosure, is proposed as a novel and efficient latent heat storage unit for thermal energy storage. The enthalpy–porosity method that treats the solid and liquid zones as a single domain is employed. Effect of the mushy zone constant C on melting is analyzed and a suitable value is obtained by comparing the numerical results with experimental data in the literature. A series of simulations are conducted to analyze the transient melting coupled with natural convection as well as the heat transfer process. Fourteen units those have different length ratios between top and bottom of the enclosures are investigated and compared by the analysis of transient temperature fields, vertical velocity distributions, and evolution of the melting fronts. It is found that the length ratio n dramatically affects the full melting time and heat transfer intensity. An enclosure of n = 5.5, which has the shortest completion time and the highest heat transfer intensity, is determined as the optimal unit. Compared with the base geometry (n = 1), charging time of the optimal unit (n = 5.5) decreased by 32.8 %, while the heat transfer intensity increased by 45.7 %. This is a significant improvement in the field of latent heat storage.

RETRACTED: On the expedited melting of phase change material (PCM) through dispersion of nanoparticles in the thermal storage unit

Heat transfer enhancement of phase change material (PCM) during the charging (melting) process is crucial in thermal energy storage (TES) systems. This study presents the enthalpy-based lattice Boltzmann method (LBM) simulation of ice melting enhanced through dispersion of Cu-nanoparticles in annuli. A double distribution function (DDF) model is used to solve the convection-diffusion equation including nonlinear convection and isotropic diffusion terms. The simulations are carried out under the following conditions: Pr=6.2,Ste=1,Ra=2x10^5. The effects of nanoparticles volume fraction and the position of the hot cylinder on the temperature contour, streamline and full melting time are illustrated.

Simulation of natural convection melting in a cavity with fin using lattice Boltzmann method

SUMMARYIn this study, a numerical investigation of melting phenomenon with natural convection in a cavity with fin has been performed using enthalpy‐based lattice Boltzmann method. The lattice D2Q9 model was applied to determine the density and velocity fields, and the D2Q5 model for the temperature field. The effect of vertical position and length of the fin on the melting rate was studied. The simulations were carried out for Stefan number of 10, Rayleigh number of 105and relative thermal conductivity (kfin∕kfluid) ranging from 5 to 30. The obtained results show that the rate of melting increases when the relative thermal conductivity and the length of the fin become greater. We also found that the variation of vertical position of the fin from bottom to middle has an insignificant effect on melting while it causes the increase of full melting time when the fin is mounted on the top of the cavity. Copyright © 2011 John Wiley & Sons, Ltd.