Liquid Fraction Distribution Research Articles

Experimental and numerical investigations on the intermittent heat transfer performance of phase change material (PCM)-based heat sink with triply periodic minimal surfaces (TPMS)

The excessive heat generated during the operation of electronic components leads to significant temperature increases, posing a significant risk to their service life. For the problem of efficient thermal management of electronic devices in confined spaces experiencing high heat flow, a scheme using triply periodic minimal surfaces and a phase change material-based active–passive cooling heat sink is proposed to control the temperature of electronic equipment. The apparent heat capacity method is employed to simulate the intermittent process of the phase change material-based heat sink. The optimization based on Box-Behnken Design and Nondominated Sorting Genetic Algorithm II is analyzed to obtain an improved triply periodic minimal surface structure. The effect of thermal performance (including base temperature, liquid fraction, and Grashof number) of an intermittent heat sink based on an improved structure is discussed. Visualization and temperature testing platforms are established. Through visualization experiments, the internal temperature and liquid fraction distribution of the heat sink are obtained, and the simulation results are in good agreement with the test results. The results show that the base temperature, liquid fraction, and Grashof number achieve a steady periodic variation during the intermittent process. Specifically, the base temperature remains stable in the range of 314 K to 340 K, the liquid fraction stabilizes between 0.8 and 1.0, and the Grashof number stabilizes between 100 and 1000. At the heating power of 30 W or below, the base temperature of the heat sink exhibits a steady periodic variation. At the heating power of 40 W, the maximum base temperature of the heat sink exceeds 370 K.

Numerical investigation of the discharging process in a latent cold storage unit

In this study, a latent cold storage unit using vertical cylindrical structure coupled with a concentric air duct is proposed. The cold discharging process of the unit is simulated by computational fluid dynamics (CFD) based on the enthalpy-porosity method. The temperature distribution and liquid fraction distribution of the latent cold storage unit are analyzed. In addition, thermal performance including the average temperature of phase change material, outlet air temperature, the liquid fraction and the cold discharging power are analyzed in detail.

Development and application of discontinuous Galerkin method for solidification problems in a semitransparent medium-filled cavity

Research on the phase change phenomenon has gained increasing attention due to its extensive engineering applications in the fields of energy storage and release. In the semitransparent medium, the phase change process is accompanied by the transfer of internal radiation energy, which can significantly affect the distributions of temperature and liquid fraction. Based on the enthalpy model, this work develops the discontinuous Galerkin method (DGM) to solve the enthalpy equation (EE) and radiative transfer equation (RTE) that govern the phase change process. By comparing with the analytic solution and other numerical results, the correctness of the proposed DGM framework for solving coupled radiation and phase change problems is validated. Results reveal that the DGM can accurately capture the heat and mass transfer process in solidification problems. The proposed DGM is further extended to analyze the solidification problem within a two-dimensional cavity considering the radiation effect. Effects of Planck number, medium scattering albedo, and refractive index on the temperature and liquid fraction distributions as well as the mushy zone location are investigated. It is found that these parameters have important impacts on the solidification rate. This work provides accurate numerical solutions on phase change problems in the presence of radiation.

Comparison of heat transfer enhancement between open and closed cell porous metal structures for solid–liquid phase change

PurposeThe purpose of this study is to analyze heat transfer for solid–liquid phase change in two inclined cavities assisted with open cell and closed cell porous structures for enhancement of heat transfer and compare them.Design/methodology/approachThe heat transfer analysis is done numerically. The set of conservation equations for mass, momentum and energy for phase change material (PCM) and conduction heat transfer equation for metal frame are solved. Furthermore, temperature and solid–liquid fraction distributions for a cavity filled only with PCM are also obtained for comparison. The porosity is 0.9 for both porous structures. Rayleigh number and inclination angle change from 1 to 108, and from −90° to 90°, respectively.FindingsThe present study reveals that the use of closed cell structures not only can make phase change faster than open cell structure (except for Ra = 108 and = 90°) but also provide more stable process. The use of a closed cell porous structure in a cavity with PCM can reduce melting period up to 55% more than a cavity with an open cell porous structure. The rate of this additional enhancement depends on Rayleigh number and inclination angle.Originality/valueTo the best of the authors’ knowledge, this is the first time that the comparison between closed cell and open cell porous structures for heat transfer enhancement in a solid/liquid phase change process is reported. Authors believe that the present study will lead more attentions on the use of closed cell porous structures.

Neutron radiography of liquid foam structure near a vertical wall.

At a solid boundary, the structural formation of bubbles is different from that in the bulk of a liquid foam. The presence of a solid boundary imposes additional constraints, resulting in a crystalline arrangement of the bubbles. For dry and monodisperse foam, the Kelvin and Fejes-Tóth structure is expected in the vicinity of the wall, while a random ordering should occur in the bulk. In this study, we investigate the transition from a crystalline to a random structure near a vertical wall located in the middle of a flat foam cell. The corresponding layering of the liquid was quantified by measuring the distribution of liquid fraction within the cell using neutron radiography. The amplitude of the liquid fraction distribution and its decay with distance from the solid boundary were correlated with the foam bubble size and polydispersity. Furthermore, by applying forced drainage, we measured the corresponding permeability and wetting front velocity near the vertical wall. We found that the crystalline sorting reduces the permeability and wetting front velocity compared to a randomly packed foam.

Effect of fins and nanoparticles in the discharge performance of PCM thermal storage system with a multi pass finned tube heat exchange

This work studies the heat exchange process of a latent heat thermal energy storage (LHTES) system equipped with a compact finned tubes heat exchanger (HE) as this is one of the most important aspects of the storage system, the capacity for effectively delivering its stored energy. This work fills in a literature gap for 3D, transient heat transport fluid (HTF) flow models concerning storage systems with phase change materials (PCMs) with fins and nanoparticles allowing for an evaluation on the quality of heat delivered by the system. Numerical simulations, for full turbulent conditions of the HTF flow, were developed to access the influence of the fin pitch and the PCM thermal properties in the performance of the energy discharge process. Samples of commercial paraffin-wax A53 doped with graphene based nanoplatelets were tested and characterised. Different types of nanoplatelets were employed in the range of 0.5% to 6% weight. Measured data of the thermal conductivity, specific heat and fusion latent heat are presented. The simulations were developed for three fin pitch values 5 , 10 and 20 mm and for 1%wt and 6%wt nanoparticles loads. The effect of fins and combination of fins and nanoparticles in the outlet temperature and liquid fraction distribution inside the LHTES unit during the discharge process in a 3D full scale model was analysed. The system performance was evaluated based of off the outlet temperature of HTF to ascertain both the quantity and quality of the heat provided. The results show that the PCM thermal conductivity is significantly enhanced by the addition of graphene nanoparticles with a high aspect ratio. The addition of only 1%wt doubled the solid phase PCM thermal conductivity and for a 6%wt load the thermal conductivity increased by a factor of 3.5. Meanwhile, specific and latent heat values of the samples are relatively unaffected. The numerical results further show that applying thin fins is an effective approach to enhance LHTES systems discharge performance. Increasing the fin number significantly enhances the heat transfer rate and the HTF discharge temperature during solidification and has a positive impact in the useful discharge heat capacity, providing better quality heat. Combining fins and nanoparticles improves the discharge process, nevertheless the role of nanoparticles becomes secondary as the fins number increases. The results demonstrate that standardised compact finned heat exchangers ubiquitously used in the HVAC industry can successfully overcome the low thermal conductivity of common PCMs without compromising the useful heat discharge capacity or resorting to nanoparticles decreasing the discharge time between 60 and 77% with adequate fin number. • Samples of paraffin-wax with graphene nanoplatelets employed and characterised. • The paraffin thermal conductivity improved via nanoparticles with a high aspect ratio. • The paraffin specific and latent heat marginally affected by the nanoparticles. • 3D simulations, for full turbulent conditions of the heat transport fluid (HTF) flow. • Results showed employing HE fins with proper fin pitch replaces need of particles.

Numerical study of PCM thermal behavior of a novel PCM-heat pipe combined system for Li-ion battery thermal management

A three-dimensional numerical model is developed and validated against experimental data for a cylindrical lithium-ion battery module with a compact hybrid cooling system of phase change material (PCM) and heat pipes. This cooling system is particularly well-suited for small or medium-sized battery module or pack. The PCM process and its effect on battery thermal behavior are numerically investigated for cases based on different C-rate discharge processes (0.5C, 1C and 2C), different PCM properties (different mass fractions of expandable graphite) and different charge–discharge cycles of the battery module. The results indicate that the PCM liquid fraction distribution is not uniform during the discharge process, the PCM starts to melt from its outermost neighboring the batteries and its upper part gets more liquid due to natural advection of liquid PCM. The different melting rates within the PCM lead to slightly enlarged temperature difference between batteries. Increasing discharge C-rate further accentuates this temperature difference. A tested composite PCM of 12 wt% EG (expanded graphite) shows better heat dissipation performance than that of pure PCM, and consequently it is better suited for battery thermal management systems. The simulations for different charge–discharge cycles indicate that lowering C-rate and increasing recovery time can improve thermal performance of the battery module and thermal control is thus more effective.

Simulation of solid-liquid phase change at pore scale using lattice Boltzmann method with central moments in thermal energy storage

The solid-liquid phase change process is of significant importance to the thermal energy storage and electronics cooling using phase change material. In this paper, the central moments multiple-relaxation time (CM-MRT) collision model has been introduced into the solid-liquid phase change lattice Boltzmann model, in order to solve the solid-liquid phase change problem in porous medium at pore scale. To evaluate the influences of different collision functions, the problems of one-region phase change and natural convection with phase change at pore scale have been investigated. The results have showed that the CM-MRT model possessed higher accuracy in revealing temperature and liquid fraction distribution. Compared with mesh system of 256 × 256, the CM-MRT model with 64 × 64 possess the minimum error, 0.014%. Besides, in the case of high Rayleigh number (5 × 107) and low Prandtl number (0.005), the CM-MRT model presented extraordinary stability and the clear phase interface when investigating the heat transfer of porous medium composite phase change materials at the pore scale.

Ultrasound imaging of liquid fraction in foam

Flotation is an important process in liquid-solid and solid-solid separation, whereby desired solids in suspensions are recovered by their attachment to gas bubbles. Monitoring the process is essential for an increased grade and reduced water consumption. However, in situ measurements of the froth’s phases (liquid, air, particles) and / or volume flow that would be easily integrable in industrial processes are not available. The constant alternation of phases remains challenging for measurement instrumentations. In this paper, we report on pilot tests of novel instrumentation that allows for measurement of low liquid fraction based on electrodes and ultrasound transducers measurements. It offers high milliscale spatial and temporal resolution in the low liquid fraction range of foam (≤ 0.83∙10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ). Improving the accuracy of prospective volume flow estimations, a high penetration depth of 9.2 cm was achieved. The measurement system was calibrated using homogeneous, steady foam and an integral conductivity measurement. A backscatter model was applied to reduce the effect of ultrasound shadowing and enhance quantitative liquid fraction estimates. Additionally, six cases of inhomogeneous and dynamic liquid fraction distributions were investigated qualitatively. The quantitative and qualitative validation was conducted with a reference neutron radiography measurement, showing good correspondence. This investigation distinguished between two of the three froth phases within the analyzed range. Whereby a first step towards in situ monitoring of foam parameters in industrial applications was achieved, which is especially relevant to widely used froth flotation processes.

Numerical Predication of Solidification Phenomena of Phase Change Material in Concentric Annulus Pipe

Two-dimensional numerical simulation is performed aiming to understand the role of buoyancy force convection during restricted solidification of phase change materials (PCMs) inside a shell and tube heat exchanger according to annulus cross section. Where the transient history of PCM solidification evolution was studied. The governing equations of mass, momentum and energy are solved to study the solidification behavior inside the annulus geometry. The fluid flow in the mushy zone was accounted for using the Darcy drag source term in momentum, and the liquid percentage in each cell was updated using the enthalpy-porosity method. Thermal conditions of the outer cylinder insulated (adiabatic) and the inner cylinder at constant temperature (isothermal). The results are presents as a temperature contour and liquid fraction distribution in the domain. The predicted result shows the capturing phenomenon: primary heat conduction in all regions, then heat convection and conduction become dominant in the top and bottom regions, respectively. The max. and min. temperature changes near the outer pipe surface during 16 hrs. are 56.25% and 42.5%, respectively.

Numerical study on flow and thermal characteristics of a micro-channel separated heat pipe under various surface wettability

This paper aims to investigate the influences of surface wettability under different contact angles on the thermal and flow characteristics of the refrigerant inside the evaporator of a micro-channel separate heat pipe (MCSHP). The outside louvered fins were simplified by an empirical model, and the inside refrigerant flow and heat transfer behavior was simulated by CFD. The simulation results were validated by previous experimental data. In the hydrophilic channels, there was lubricating liquid film between bubbles and wall surface, while the bubbles in the hydrophobic channels were more likely to contact with wall directly. The heat transfer rate, fluid velocity and initial nucleate spot decreased with the increase of contact angle. And the temperature and liquid fraction distribution along the X and Y axis also varied with contact angle. It could be concluded that hydrophilic surface could enhance the heat transfer rate, while hydrophobic surface could accelerate bubble nucleation. These results may have benefits on the optimization of the design and material selection of MCSHP.

Linear, spatio-temporally resolved ultrasound measurement of the liquid fraction distribution in froth

Abstract Froth flotation is an important process for separating metal particles from gangue. A single flotation circuit for copper uses approx. 44 billion litres water a year. In situ process monitoring of the foam’s parameters and closed-loop control can reduce the resource use. However, no measurement technique is broadly employed that yields the liquid fraction distribution in the froth. Optical measurements are prevented by the bulk foam’s opacity. Though, ultrasound in the low frequency range is able to penetrate froth. In this paper we investigate the application of ultrasound to measure the local liquid content of aqueous foam in the axis of the ultrasound beam. Assuming a dependency of the backscattering coefficient b on the foam’s liquid fraction ϕ, we developed a model to calculate b from the echo signal. Local backscattering coefficients b n {b_{n}} can be determined for time gated windows and show a trend in the dependency on the foam’s liquid fraction ϕ (for ϕ &lt; 0.8 \phi &lt;0.8 %). The absolute uncertainty of the liquid fraction determined by means of an electrical reference measurement is σ ϕ = 0.079 {\sigma _{\phi }}=0.079 %. We demonstrated the capability of spatio-temporally resolved measurements with a frame rate of 0.33 Hz and an axial resolution of 7.9 mm in an experiment with a time-varying, inhomogeneous liquid fraction. This research work is contributing to a determination of in situ information of the foam’s parameter in a flotation process.

Evaluation and optimization of melting performance in a horizontal thermal energy storage unit with non-uniform fins

The shell and tube latent heat thermal storage systems are widely used in the storage and utilization of fluctuating thermal energy resources due to high thermal storage density, constant discharge temperature, simple processing and low cost. Adding fins to phase change materials is an effective method to expand the heat transfer area and improve the heat transfer rate. However, the obvious thermal stratification phenomenon and slow melting rate problem are exited in the horizontal latent heat thermal energy storage system. In order to improve the melting rate and uniformity, different non-uniform fin arrangements were designed and investigated in this study to improve the thermal performance. A numerical model of the melting characteristics of phase change materials was established and validated by the experimental results in the literature. The effects of fin arrangement, angle and length on the thermal performance of the melting process were quantitatively evaluated by analyzing the temperature field, velocity field and liquid fraction distribution before and after melting. Results indicated that when the fin angle α=25° and the fin length L=40 mm, the melting time is decreased by 83.9% and thermal storage density is improved by 466%, which meant that the lower fin arrangement realized the synergistic enhancement of natural convection and heat conduction. On this basis, the effect of the heat transfer fluid temperature on the thermal storage performance of the optimal scheme was further studied. The non-uniform fins design proposed in this study can provide theoretical support for the design method of thermal storage system, and contribute to the efficient utilization of intermittent thermal energy resources such as solar energy and industrial waste heat.

Effect of Radial Porosity Oscillation on the Thermal Performance of Packed Bed Latent Heat Storage

Owing to its high heat storage capacity and fast heat transfer rate, packed bed latent heat storage (LHS) is considered as a promising method to store thermal energy. In a packed bed, the wall effect can impact the packing arrangement of phase change material (PCM) capsules, inducing radial porosity oscillation. In this study, an actual-arrangement-based three-dimensional packed bed LHS model was built to consider the radial porosity oscillation. Its fluid flow and heat transfer were analyzed. With different cylindrical sub-surfaces intercepted along the radial direction in the packed bed, the corresponding relationships between the arrangement of capsules and porosity oscillation were identified. The oscillating distribution of radial porosity led to a non-uniform distribution of heat transfer fluid (HTF) velocity. As a result, radial temperature distributions and liquid fraction distributions of PCMs were further affected. The effects of different dimensionless parameters (e.g., tube-to-capsule diameter ratio, Reynolds number, and Stefan number) on the radial characteristics of HTF and PCMs were discussed. The results showed that different diameter ratios correspond to different radial porosity distributions. Further, with an increase in diameter ratio, HTF velocity varies significantly in the near wall region while the non-uniformity of HTF velocity in the center region will decrease. The Reynolds and Stefan numbers slightly impact the relative velocity distribution of the HTF—while higher Reynolds numbers can lead to a proportional improvement of velocity, an increase in Stefan number can promote heat storage of the packed bed LHS system.

Numerical Simulation of the Effect of Process Parameters on Cooling Rate and Secondary Dendrite Arm Spacing in High-Speed Twin Roll Strip Casting of Al–15 wt % Cu Alloy

Abstract Twin roll casting is a process used to produce thin strips of metals by continuously pouring melt on to rotating rolls. In order to make the process more productive and economical, high roll speed is recommended. The numerical simulation of high-speed twin roll casting is performed by analyzing fluid flow, heat transfer, and solidification behavior of Al–Cu hypo-eutectic alloy. The flow field, temperature, liquid fraction distribution, and cooling rate are analyzed by solving governing transport equations of continuity, momentum, energy, and turbulence. The low-Re Turbulence model is used to capture turbulence effects in the process and enthalpy-porosity technique used to account for the rise in viscosity due to phase change. The effect of melt pool height and roll velocity on average cooling rate along the strip surface is investigated. It is found that the increase in melt pool height and roll speed increases the average cooling rate along strip surface due to rise in heat transfer up to certain roll velocity but beyond that process fails due to breakout. The average cooling rate of process affects the microstructure and properties of strips. It is found that higher cooling rates result in a decrement of secondary dendrite arm spacing (SDAS) of 1 mm thin strip along strip surface results in the fine and homogeneous microstructure.

Non-equilibrium spontaneous condensation flow in cryogenic turbo-expander based on mean streamline off-design method

In a modern cryogenic liquefaction system, the turbo-expander is used to operate in the two-phase region so that a high cooling capacity can be provided. Compared with the expansion process in a conventional over-heated turbo-expander, non-equilibrium condensation occurs in the duct when the turbo-expander operates in the two-phase region, which varies the expansion process. In our previous works, the simulation by CFX can predict the nucleation process controlled by gaseous sub-cooling well, and the outlet liquid fraction has a good agreement with our experimental data. As the non-equilibrium condensation simulated by using CFX consumes much time and massive computing resource, it is not feasible to simulate the two-phase expansion in the turbo-expander in a wide off-design range. In this study, we investigate the non-equilibrium spontaneous condensation along the mean streamline of a cryogenic turbo-expander passage by using an off-design computational code compiled by Matlab. The two-phase off-design computational code is developed from our previous overheated off-design code. The calculation function of thermodynamic parameters on the mean streamline and non-equilibrium condensation module are integrated with this code. The variations of thermodynamic parameters on the mean streamline in a non-equilibrium condensation process are simulated. Nucleation onset, droplet growth/vaporization process, liquid fraction distribution are revealed. Droplet developing region is divided according to the droplet critical and mean radius. Simulation results of five two-phase cases are compared and analyzed, and different non-equilibrium condensation features and liquid fraction distributions are found. At last, isentropic efficiency and outlet liquid fraction are used to evaluate two-phase expansion performance, and it is found that the performance results from the simulation in five cases agree with experimental data well.

Thermal performance analysis and optimization of a spherical PCM capsule with pin-fins for cold storage

A spherical phase change material (PCM) capsule with circular pin-fins for latent cold thermal energy storage is proposed and studied in this paper. Experiments were conducted to monitor the PCM temperature variation in each capsule during solidification in order to validate the simulated results calculated based on the enthalpy method. Good agreement was found between the simulated values and experimental data and the deviations were lesser than 6.9%. Thermal performances, including the temperature variation, liquid fraction distribution as well as cold charging rate, of the capsules with various pin-fin configurations were numerically compared. Results demonstrate that the cold charging rate is improved dramatically due to the insertion of fins, and the cold charging time is decreased by more than 50% in the capsule with six fins (approximately 3600 s) compared to that without fins (approximately 7700 s). In addition, the increasing extent is positively related to the pin-fin number, despite the slight reduction of PCM filling ratio. Furthermore, it is indicated that the thermal performance can also be improved by means of optimizing geometric parameters of pin-fins, such as their length L and diameter D. As for the spherical capsule with an inner radius of R = 20 mm, the optimal parameter is regarded to be L/R = 75% and D = 2 mm.

Simulation of cesium desorption behavior of porous nickel

To achieve the application of artificial plasma technology in high technology, the desorption behavior of cesium metals with low ionization characteristics in porous nickel is studied in this paper. The simulated results, which are consistent with the experimental desorption results, demonstrated that the liquid-phase rate of cesium gradually decreased along the axial direction and the distribution of liquid fraction exhibited a similar trend. The results that the removal efficiency reached above 95% provide a reference for studying the controllability of release of cesium metal and the adsorption and desorption of other alkaline-earth metal experiments with porous nickel.

CFD simulation on the heat transfer and flow characteristics of a microchannel separate heat pipe under different filling ratios

This paper established a CFD model of the evaporator of a microchannel separate heat pipe (MCSHP) used for special indoor environment cooling (e.g., telecommunication station) to study its heat transfer characteristics and flow mechanisms under different filling ratios. The louvered fin model outside the pipe was simplified to shorten the computation time without ignoring the thermal enhancement. The CFD simulation results were validated by experimental data. The optimal refrigerant filling ratio was from 68% to 100%. The bubble flow existed inside the whole pipe during the early stage of evaporation, and converted to slug flow under continuously heating. It was found that the cooling capacity increased with the filling ratio, and was high to 4087 W at the filling ratio 78%. Due to the formation and movement of refrigerant bubbles, the liquid fraction distribution inside the evaporator varied with the filling ratio and dimensionless time. The distribution of wall temperature and the liquid fraction both indicated that the effective heat transfer area of two-phase region was a key parameter affecting the cooling capacity. This study can be used to simplify experiment procedure and to optimize the system design and operation of MCSHP.

Numerical investigation of passive stabilization of the surface temperature using PCM and metal foam

Stabilization of the temperature of PVs is of great importance, due to strong relation between operating temperature and its efficiency. Due to low thermal conductivity of PCM, intensification of heat transfer to the operating material is needed. The four fully passive PV’s temperature stabilization systems, based on the phase change material (PCM) are numerically investigated in this paper. Apart from the natural convection promotion, intensification of heat transfer is done with aluminium fins and aluminium foam. The simplified computational model based on the equilibrium formulation of energy transport equation, taking into account viscous and inertial fluid flow resistances in the porous material, is formulated and solved with the general purpose software - ANSYS Fluent. Proposed model is succesfully verified by comparing the results with available in literature numerical solutions.Simulations outcomes are presented, the temperature and liquid fraction distributions in proposed geometry configurations, temperature variations determined in selected points and averaged on the heated surface temperature plots. Presented results help to assign the best configuration.