Suppression Of Natural Convection Research Articles

Energy storage, thermal-hydraulic, and thermodynamic characteristics of a latent thermal energy storage system with 180-degree bifurcated fractal fins

The low thermal conductivity of organic phase change materials limits the performance of latent thermal energy storage (TES) systems. Inspired by fractal theory, this study proposes an innovative 180° fractal fin for enhancing the thermal performance of latent TES systems. The effects of length ratios (l) and fractal levels (N) are numerically investigated employing the enthalpy-porosity method. Compared with traditional rectangular fins, the results indicate that the 180° fractal fins reduce the integral average value of the maximum velocity by 2.24%–48.51%, which indicates the suppression of natural convection by the latter. Increasing l and N result in a general increase in melting time. Compared to the TES systems without fins and with rectangular fins, the 180° fractal fins can respectively reduce melting time by up to 88.79% and 28.00%, increase integral average Nusselt number by up to 7.30 times and 34.21%, and enhance energy storage power by a maximum of 8.55 times and 38.71%. Moreover, flow viscous entropy generation can be neglected compared to thermal entropy generation. In contrast to rectangular fins, the employment of fractal fins leads to a maximum reduction of 90.06% and 99.10% in total frictional entropy generation and thermal entropy generation, respectively.

Heat transfer characteristics of ceramic foam/molten salt composite phase change material (CPCM) for medium-temperature thermal energy storage

Molten salts have been widely used as energy storage materials in medium- and high-temperature thermal energy storage. However, pure salt commonly suffers from low thermal conductivity and many conventional methods of heat transfer enhancement do not apply due to the serious corrosion and the extremely high temperature. In the current study, the open-cell SiC ceramic foam was integrated with solar salt (60wt% NaNO3 + 40wt% KNO3) to enhance the heat transfer of salt and avoid severe corrosion issues. A visualised experiment was for the first time conducted to investigate the melting phase change heat transfer in the ceramic foam/molten salt composite phase change material (CPCM). A representative elementary volume (REV)-scale simulation was simultaneously performed and the computational results were compared with experimental data. It is found that the conduction-friendly ceramic skeleton remarkably enhances the heat transfer in molten salt, especially in the region far away from the heat source. The spatial temperature difference across the composite is decreased in both horizontal and vertical directions and the local superheating is mitigated. The enhancement of heat conduction is greater than the suppression of natural convection; as a result, the melting rate of the CPCM is increased by 41.3%. This study provides crucial benchmark data of phase change heat transfer for medium-temperature thermal energy storage and paves the way for system design and optimization.

Numerical investigation on melting behaviour of phase change materials/metal foam composites under hypergravity conditions

The integration of electronic devices in aerospace vehicles leads to a significant increase in the heat dissipation, which makes thermal management a severe challenge. Due to the high thermal energy storage capacity and the excellent thermal diffusivity, phase change materials (PCM)/metal foam composite is a potential thermal management material. In order to deepen the understanding of melting behaviour of PCM/metal foam under hypergravity, the numerical model considering natural convection, flow resistance of metal foam, and non-equilibrium heat transfer is proposed, and the effects of inclination angle, hypergravity value, metal foam porosity, and pore density on the melting are analyzed. It shows that the inclination angle has a great influence on the morphology of the melting interface, while has limited influence on the variation of PCM liquid fraction. As the inclination angle changes from 0° to 180°, the heated wall temperature first decreases, and then increases after reaching the lowest value at 45°, which is the optimal inclination angle. With the rise of hypergravity value, natural convection is enhanced, the heated wall temperature is reduced, while the hypergravity value has limited effect on the PCM liquid fraction. Furthermore, with the increase of metal foam porosity, natural convection is enhanced, while the heated wall temperature and the melting time become larger. When the porosity is larger than 0.95, the effect of metal foam on natural convection suppression is greater than that of heat conductivity enhancement, leading to that the performance of PCM/metal foam is worse than that of the pure PCM. Besides, with the decrease of pore density, natural convection is enhanced, the heated wall temperature becomes lower. Therefore, metal foam with lower pore density is preferred. This paper provides guidance to the optimal design of PCM-based heat sink for aerospace applications.

PERFORMANCE IMPROVEMENT EVALUATION OF A LATENT HEAT STORAGE UNIT ENHANCED BY VICSEK FRACTAL FINS

The mature and scale utilization of renewable energy is restricted by the deficient efficiency of latent heat storage (LHS) technology. To address this, an innovative Vicsek fractal fin is first proposed herein for the heat charging/discharging rate enhancement. The phase transition behavior and thermal characteristics of the innovative LHS unit are solved numerically and compared to the traditional one, employing the enthalpy–porosity algorithm and a two-dimensional transient model. The results indicate that Vicsek fractal fins remarkably promote the thermal efficiency of LHS units. Compared to the traditional LHS units, innovative LHS units with Vicsek fractal fins, respectively, reduce the melting/solidification duration by 16.1% and 28.7%. The melting process is divided into early thermal conduction, subsequent convective thermal transport, and later thermal conduction, while solidification processes are dominated by the initial natural convection and the subsequent continuous heat conduction. Moreover, the impact of free convection is higher on melting processes than on solidification processes. Importantly, the efficient heat transfer path of Vicsek fractal fins allows the inherent natural convection suppression to be much weaker than the improvement level of heat transfer efficiency, facilitating the interstitial thermal transport between the phase change material (PCM) and fins and improving the overall temperature uniformity of LHS units.

Enhanced thermal energy storage of a paraffin-based phase change material (PCM) using nano carbons

Thermal energy storage performance of a paraffin-based phase change material (PCM) enhanced by nano graphite and nano coconut shell charcoal was investigated. The nano carbon concentration was 0.02, 0.06, and 0.10 wt%, respectively. To understand the influence of nano carbons on the thermal behavior of PCMs, the melting process of nano-enhanced PCMs (NePCMs) was monitored using thermocouples and an infrared camera, in comparison with the melting process of pure paraffin. It was found that the melting process of NePCMs without dispersant was accelerated with nano carbons at concentration of 0.02 wt%, although the thermal conductivity of NePCMs with higher concentrations was higher. With increasing concentration, the melting process was decelerated because of the agglomeration of nano carbons and the suppression of natural convection. To eliminate the agglomeration, Span 80 and oleic acid were used as dispersants for the PCMs with 0.06 wt% and 0.10 wt% nano graphite. The melting process was accelerated further with the increasing concentration of the nano graphite. The shortest melting time was realized using 0.06 wt% nano graphite and 2.0 wt% oleic acid, which was 21% shorter than the pure paraffin. Moreover, it was noticed that the data from thermocouples were not able to represent the melting process accurately because of the limited measuring points, especially for the melting process with agglomeration and sedimentation.

Charging performance optimization of a latent heat storage unit with fractal tree-like fins

Abstract The significant deficiency of the latent heat storage (LHS) technology is the poor heat charging and discharging efficiency. Here invokes a fractal tree-like fin to strengthen the melting property of LHS units inspired by bionics in nature. A transient melting model of the shell-and-tube LHS unit with tree-like fins is built and solved numerically. The structure optimization of fractal tree-like fins is conducted for charging enhancement by response surface method (RSM). The results report that the length ratio and thickness index play considerable roles in the competition between natural convection suppression and heat conduction improvement during melting processes. Compared with the plate fin, the distribution of tree-like fins in a multi-level fashion provides point-to-area networks for high heat flows, which benefits to the melting rate acceleration and uniform temperature field owing to that the thermal conduction improvement is more remarkable than convection suppression. More significantly, the total melting duration of the LHS unit with optimal tree-like fins reduces by 26.7%, and the averaged heat storage rate increases by 45.4% relative to the plate fins case. Therefore, a fractal tree-like fin with α = 1.07 and Δ = 0.65 is recommended in practical applications for maximizing energy charging performance of LHS units.

Lattice Boltzmann simulation of melting heat transfer in a composite phase change material

Thermal performance enhancement of the phase change material (PCM) is of particular significance to the energy storage applications. The composite PCM with metal particles is introduced here for maximizing the thermal performance. Based on the description of a composite PCM by the QSGS method, a lattice Boltzmann model of the melting composite PCM was developed and numerically solved. The melting front evolution and transient temperature response in the composite PCM are analyzed and compared to the pure PCM. Furthermore, the effects of the dispersity, volume fraction, and diffusion direction of the metal particles on the melting performance of composite PCMs are comprehensively analyzed. The results indicate that the presence of metal particles benefits the thermal performance enhancement of PCMs. The melting evolution and the heat transfer in the composite PCM are much quicker due to the enhanced thermal conduction. Besides, the melting front morphology maintains an approximate line during the whole melting process arising from the natural convection suppression. For maximizing the melting heat transport, the metal particles in the composite PCM are recommended to present a larger dispersity, and their diffusion direction is suggested to be in the same direction as the heat flow. Interestingly, even though the volume fraction of the metal particles is identical, the melting performance of composite PCM is also relevant to the dispersity and diffusion direction of the metal particles.

Experimental Study on the Thermal Performance of a Finned Metal Foam Heat Sink with Phase Change Material

The present research conducts an experimental study on the thermal performance of the finned metal foam (FMF) heat sinks with phase change material. The dynamic temperature response of an FMF heat sink is analyzed and compared with the corresponding finned heat sink. The effects of porosity and pore density on the thermal performance of FMF heat sinks are analyzed. Furthermore, the enhancement ratio and heat exchange capability are evaluated for the optimization of FMF heat sinks. The results indicate that the thermal conduction enhancement caused by the addition of metal foam exceeds the heat transfer loss arising from the suppression of natural convection. A decrease in porosity leads to an increase in heat exchange capability and contributes to a higher enhancement ratio. Considering the tradeoff between the low operating temperature and longer duration of reliability, the porosity of 0.9 is the best choice for FMF heat sinks. Though the porosity is identical, the thermal performance of an FMF heat sink is better with the increase of pore density. Considering the maximum energy charging capacities are identical, the FMF heat sink with larger pore density is recommended for practical engineering applications.

Performance optimization of a finned shell-and-tube ice storage unit

The solidification enhancement of the finned shell-and-tube ice storage (STIS) unit with water is numerically analysed with consideration of natural convection. An unsteady model of the discharging process is presented to investigate the role of fin geometry in ice storage performance, which is validated by a discharging experiment in a finned STIS unit. The temperature response and ice front evolution in a finned STIS unit are analysed and compared with a corresponding unit without fins. The effects of structural parameters on the solidification performance are comprehensively discussed, including fin height, fin thickness, and fin number. Furthermore, a new evaluation criterion of the dimensionless cooling power mass penalty is proposed to optimize fin geometry. The results indicate that the natural convection is not conducive to the solidification performance. The addition of fins effectively enhances the energy discharging performance of STIS units due to the coupling effect of natural convection suppression and thermal conduction enhancement. It is highly efficient to increase fin height for better energy discharging performance since fin height is the most important parameter. Fin thickness and fin number also affect ice storage performance. Considering the trade-off between the weight and ice storage performance of STIS units, the optimal fin thickness and fin number in this paper are respectively recommended to be 3 mm and 8 to achieve the largest performance enhancement with the least mass penalty.

Suppression Of Natural Convection And Radiation Heat Losses In Solar Cavity Receivers: A Novel Approach

The concentrating solar power technology has great potential to be used for energy production and it is a promising alternative to conventional fossil fuel-based energy technologies, such as coal power plants, due to the abundance of solar energy as an energy resource, as well as its minimal impact on the environment. The parabolic dish receiver assembly is one such promising concentrating solar power technology. It usually consists of a reflector in the form of a dish with a downward-facing receiver at the focus of the dish. A cavity receiver is used to maximise the absorption of the concentrated flux. However, the receiver is subjected to environmental variations, as well as changes in receiver inclination angle, which lead to heat losses that affect the overall receiver’s performance. The need for the commercialisation of economically viable parabolic dish systems necessitates further in-depth investigation into cavity receiver designs. As the cavity receiver plays a critical role in transferring solar heat to the engine, any heat loss from the cavity receiver can significantly reduce the efficiency and, consequently, the system’s cost effectiveness. It is therefore essential to assess and effectively minimise heat loss in the cavity receiver to improve the thermal performance of the system, which can contribute to the commercialisation of this type of technology. This novel approach of suppressing natural convection heat loss in a cavity receiver was investigated. The proposed model has not been observed in literature. A cavity receiver with plate fins attached to the inner aperture surface was investigated as a possible low-cost means of suppressing natural convection heat loss in a cavity receiver. Employing air as the working fluid, laminar natural convection heat transfer from the cavity receiver with plate fins attached to the inner aperture surface was investigated for a range of Rayleigh numbers, inclination angles, and fin heights and thicknesses

Stefan flow induced natural convection suppression on high-flux evaporators

High-flux evaporators are important for various fundamental research and industrial applications. Understanding the heat loss mechanisms, especially the contribution of natural convection during evaporation is thus a ubiquitous process to predict and optimize the performance of evaporators. However, a comprehensive analysis on natural convection heat transfer, where the vertical Stefan flow due to evaporation couples with buoyancy driven convective flow has not been carefully considered. In this work, we developed a theoretical framework to elucidate the effect of Stefan flow on natural convection during evaporation. This theory incorporates the vertical Stefan flow into the conventional boundary layer theory. We found that a significant suppression of natural convection can be induced by a weak Stefan flow owing to the increase of boundary layer thickness. To understand this phenomenon, we discuss the governing mechanisms at different Stefan flow regimes. We provide a theoretical correlation to the overall heat transfer which includes both effects of the Stefan flow velocity and the buoyancy force. We finally predict the effect of natural convection on an evaporator at different operating temperatures. The heat loss from natural convection no longer monotonically increases with the superheat temperature due to the effect of Stefan flow suppression. As a result, there is an approximately 40% overestimation of the natural convection contribution at saturation temperature using conventional theory. This work improves the fundamental understanding of the natural convection during evaporation and can help guide future high-performance evaporator designs.

Self-diffusion Measurements of Liquid Sn Using the Shear Cell Technique and Stable Density Layering

By utilizing the shear cell technique and achieving stable density layering with the addition of an alloying element Bi, the self-diffusion coefficients of liquid Sn were measured on the ground at 573 K, 773 K, and 973 K (300 °C, 500 °C, and 700 °C). Moreover, the impurity diffusion coefficients of Bi in the liquid Sn were simultaneously measured, to confirm the suppression of natural convection. From the experimental results, natural convection was confirmed to be suppressed, given that the impurity diffusion coefficients of Bi were in good agreement with the microgravity reference data. Upon changing the amounts of added Bi within 5 at. pct Bi, the self-diffusion coefficients of liquid Sn did not vary significantly. Moreover, the SnBi system within 5 at. pct Bi can be regarded as a dilute solution by calculating the activity of Sn in the SnBi system beforehand. The self-diffusion coefficients of the liquid Sn were in good agreement with the power law of temperature dependence from the microgravity reference data. After confirming the suppression of natural convection and that the SnBi system is a dilute solution, the self-diffusion coefficient of liquid Sn was determined as 5.00 ± 0.16 × 10−9 m2 s−1 at 773 K (500 °C). The effectiveness of the experimental method used in this study for the measurement of the self-diffusion coefficients was confirmed, under the condition that the liquid system could be regarded as a dilute solution.

Experimental and numerical investigation on the performance of carbon-based nanoenhanced phase change materials for thermal management applications

Phase change material (PCM)-based thermal management is one of the most promising approaches for the thermal management of energy conversion devices such as electric vehicle batteries and photovoltaic cells. However, most of the PCM present inherently low thermal conductivities, which result in deficient temperature control capacity in these applications. Several types of carbon-based nanoparticle have been explored to enhance the thermal conductivity of pure PCM. Despite the significant improvements in their thermal conductivity, the heat transfer in the liquid phase nano-enhanced PCM is expected to decrease due to the presence of the nanoparticles matrices. In this study, three types of carbon-based nanostructures are embedded in a paraffin to investigate their heat transfer performance for thermal management applications. The experimental results indicate that the addition of nanoadditives can improve the heat conduction of solid phase paraffin with slight latent heat degradation. But it also drastically increases the dynamic viscosity of composites which suppresses the natural convection heat transfer in the melted PCM. The thermal behavior of nanocomposites during the melting process is experimentally and numerically examined. Results reveal that adding carbon nanofibers and graphene platelets deteriorate the thermal performance of the pure paraffin. However, an enhanced thermal response observed when graphite-based nanocomposites at 7.5 and 10wt% are used due to their 620% and 1100% solid phase thermal conductivity enhancement, respectively. The results of the current work suggest that there is a trade-off between the thermal conductivity enhancement and natural convection suppression of nanocomposites that can be used in the optimal design of PCM-based thermal management systems.

Suppression of Natural Convection in a Thermoacoustic Pulse Tube Refrigerator

The effects of gravity on the efficiency of thermoacoustic engines are investigated theoretically and experimentally, especially for thermoacoustic pulse tube refrigerators. The significant effects of gravity are found to be due to the presence of natural convection in the thermoacoustic pulse tube when the hot side of the tube is lower than the cold side. This kind of natural convection influences and reduces the efficiency of the thermoacoustic working system. Thus, how to suppress this natural convection becomes important for increasing the efficiency of thermoacoustic engines. Unlike the method of inserting a silk screen in a pulse tube, the present study uses a numerical simulation method to research the natural convection in pulse tubes, and we try to change the shape of the pulse tube to suppress this convection.

Integrated study of lead iodide crystal growth and doping processes

Due to its high atomic number, high density and wide band-gap energy, lead iodide is a promising nuclear-radiation detector material. The radiative transition in PbI2 is mainly due to donor–acceptor pair (D–A) recombination associated with impurities, which suggests improving scintillating performance of lead iodide by doping certain impurities. Before the doping study, high-purity PbI2 with the least undetermined impurities is obtained by purifying the commercial PbI2 with zone-refining technique. After well mixed with certain dopants (AgI and LaI3), the purified PbI2 is used as starting material for single crystal growth by vertical Bridgman method. Numerical studies are conducted to study temperature fields in both zone-melting and Bridgman furnaces. The solid/liquid interface is determined according to the heat balance on the growth front. The results are found to agree well with experimental measurements. Distribution of impurity concentration in purified ingot is studied experimentally, while that in grown crystal is studied analytically. By comparing analytical solutions with experiments, the effective redistribution coefficients of dopants in lead iodide crystal are determined. We further conclude that diffusion-dominated mechanism of dopant redistribution accounts for the uniform distribution of dopants in crystal, and that the suppression of natural convection in the melt is critical. By integrated study of heat and mass transfer phenomena during purification and growth processes of lead iodide, important information is achieved for the understanding and improvement of hot-zone design in the furnaces as well as for the parametric optimization of both processes.

Digital Selective Growth of ZnO Nanowire Arrays from Inkjet-Printed Nanoparticle Seeds on a Flexible Substrate

In this article, we introduce fully digital selective ZnO nanowire array growth on inkjet-printed seed patterning. Through proper natural convection suppression during hydrothermal growth, successful ZnO nanowire local growth can be achieved. Without any need for photolithographic processing or stamp preparation, the nanowire growth location can be easily modified when the inkjet printing process is integrated with a CAD (computer-aided design) system to allow a high degree of freedom when the design needs to be changed. The current proposed process is very fast, low-cost, environmentally benign, and low-temperature. Therefore, it can be applied to a flexible plastic substrate and scaled up for larger substrates for mass production or roll-to-roll processing.

A novel effective suppression of natural convection in pulse tube coolers

Due to natural convection, the cooling performance of the pulse tube cooler (PTC) greatly degrades if the pulse tube cold-head is placed upward. In our previous study, it was found that the natural convection in the pulse tube can be well suppressed by inserting a certain number of circular screens. In this paper, a further detailed investigation into this suppression method are made, including the effect of number and material of screens, input acoustic power, and some other factors on suppressing natural convection at different inclination angles θ in the range of 0–180°. In our experiment of a single stage inline type PTC driven by a linear compressor, the orientation-dependence is considerably weakened with 6-layer copper screens in the pulse tube, in the cold temperature range of 40–80 K. Moreover, the cooling performance deterioration at θ = 0° induced by the inserted screens can be kept in an acceptable range. This novel suppression of natural convection greatly improves adaptability of pulse tube cryocoolers’ application.

Numerical solution of optimal magnetic suppression of natural convection in magneto-hydrodynamic flows by empirical reduction of modes

The control of natural convection in magneto-hydrodynamic (MHD) flow is investigated by means of the Karhunen–Loève Galerkin procedure [Int J Numer Meth Engng 1998;41:1133–51]. The Karhunen–Loève Galerkin procedure, which is a type of Galerkin methods that employs the empirical eigenfunctions of the Karhunen–Loève decomposition as basis functions, can reduce non-linear partial differential equations to sets of minimal number of ordinary differential equations by limiting the solution space to the smallest linear subspace that is sufficient to describe the observed phenomena. In the present investigation, it is demonstrated that the Karhunen–Loève Galerkin procedure is well suited for the problems of control or optimization, where one has to solve the governing equations repeatedly but one can also estimate the approximate solution space from the range of control variable. The performance of the Karhunen–Loève Galerkin procedure for solving the optimal control problem of natural convection is assessed in comparison with the traditional technique employing the Boussinesq equation, and is found to be very accurate as well as efficient.

Natural convection suppression and crystal growth during unidirectional solidification of a binary system

We studied experimentally the interaction between natural convection and dendritic growth in the mushy layer during unidirectional solidification of aqueous ammonium chloride solutions cooled from below. Small amounts of hydroxyethylcellulose were added to the solution to increase its viscosity, leading to the suppression of convection. Natural convection consists of salt fingers in the liquid phase and plumes in the mushy layer for a low-viscosity solution, but the onset of plumes is suppressed for a high-viscosity solution. The mushy layer becomes sparsely packed, and the primary and secondary arms of the dendrites grow to noticeable sizes with increasing its viscosity, which yields a low solid fraction, such as 1% on average in the mushy layer for a viscosity ratio of 25.5. This demonstrates that natural convection strongly affects the morphology of dendrites. © 2000 Scripta Technica, Heat Trans Asian Res, 29(2): 120–131, 2000

Natural convection suppression and crystal growth during unidirectional solidification of a binary system

We studied experimentally the interaction between natural convection and dendritic growth in the mushy layer during unidirectional solidification of aqueous ammonium chloride solutions cooled from below. Small amounts of hydroxyethylcellulose were added to the solution to increase its viscosity, leading to the suppression of convection. Natural convection consists of salt fingers in the liquid phase and plumes in the mushy layer for a low-viscosity solution, but the onset of plumes is suppressed for a high-viscosity solution. The mushy layer becomes sparsely packed, and the primary and secondary arms of the dendrites grow to noticeable sizes with increasing its viscosity, which yields a low solid fraction, such as 1% on average in the mushy layer for a viscosity ratio of 25.5. This demonstrates that natural convection strongly affects the morphology of dendrites.