Enhancement Of Natural Convection Research Articles

Experimental study on natural convection heat transfer performance of microencapsulated phase change material slurry in a square cavity

Microencapsulated phase change slurry (MicroEPCMS), consisting of microencapsulated phase change material (MicroEPCM) and base liquid, represents a novel latent heat functional fluid that can absorb a large amount of heat within its phase change temperature range. It has found widespread use as a functional liquid cooling medium for heat dissipation in batteries, electronic devices, and other devices. In this work, MicroEPCMS with different MicroEPCM concentration were prepared. The physical properties of MicroEPCMS including density, thermal conductivity, and dynamic viscosity were tested within different temperature ranges. The results showed that the MicroEPCMS exhibits a superior thermal energy storage capacity compared to water. In the phase change range of MicroEPCM, the MicroEPCMS has a higher thermal conductivity compared to water. In addition, the natural convection heat transfer characteristics of MicroEPCMS in a square cavity were investigated under varying heating power. The results indicated that MicroEPCMS can enhance natural convection heat transfer by absorbing heat with a minimal temperature rise during the phase change process compared to water. Moreover, higher heating power enables natural convection enhancement of MicroEPCMS in a square cavity. These results underscore the significant potential of MicroEPCMS as a functional liquid cooling medium for applications in thermal energy storage and thermal management.

Natural convection heat transfer in a three-dimensional enclosure under the effect of heated pin-fins of various shapes

Natural convection enhancement for heat sinks with the help of pin-fins has been extensively researched, but further investigations are still required regarding the technological development. In order to understand the effect of different pin-fins shapes on the heat transfer and fluid flow pattern in a three-dimensional air-filled enclosure, a numerical investigation is conducted. The enclosure is heated by the left wall, on which pin-fins are attached, and cooled by the right wall, while the other walls are considered adiabatic. A validation has been made with both experimental and numerical results from the literature, and a good agreement has been observed. Five pin-fins shapes with the same base surface area and length are tested: circular, triangular, square, pentagonal, and hexagonal. The findings show that the circular pin-fins exhibit the highest thermal performance. Furthermore, for different Rayleigh numbers, increasing the number of pin-fins improves the thermal performance of heat sinks.

Numerical analysis of heat transfer enhancement in grooved vertical Multi-Cylinders at various groove geometries

In this study, the natural convection enhancement in grooved vertical multi-cylinders at various groove geometries is investigated. The effects of several grooves ranging from 3 to 7, groove thickness ranging from 0.25 to 1 mm, and cylinder surface temperature ranging from 350 to 500 K at different Rayleigh numbers are examined. The current study was simulated using the finite volume method using CFD with a laminar steady-state condition. The SIMPLE scheme is used for the pressure–velocity coupling discretization and the second-order upwind method is utilized to discretize the momentum and energy equations. The results obtained from the present research show that the presence of grooves on the cylinders will increase the heat transfer surface, create and intensify the secondary flow and mixing, and ultimately increase the heat transfer. Moreover, by increasing the number of grooves and its thickness, the amount of heat transfer increases dramatically. It’s also found that the groove thickness parameter's effectiveness on heat transfer is more than the groove number parameter. Ultimately, it’s demonstrated that using grooved cylinders leads to a 14 % augmentation in Nusselt number in comparison with employing plain cylinders.

Enhancement of natural convection of a nanofluid by stress-free patches in an L-shaped enclosure

Purpose This study aims at investigating the heat transfer characteristics of a nonsquare enclosure when hydrodynamic resistance is altered discontinuously along its inner surface. Particularly, it focuses on investigating how several essential factors collaboratively influence the natural convection, including the Rayleigh number (Ra), the aspect ratio (AR), the nanoparticle volume fraction (ϕ) and the locations of changing hydrodynamic resistance. Design/methodology/approach To achieve these objectives, an L-shaped enclosure of various AR is adopted, while zero local shear resistance is applied and modeled by stress-free (SF) patches of four distinct arrangements (corresponding to Cases 1–4). The nanofluid is modeled by Buongiorno’s two-phase model. The effects are explored using an in-house numerical framework based on a hybrid lattice Boltzmann-finite difference method with the total variation minimization scheme. Findings The results show that when Ra is sufficiently large, i.e. Ra = 105, SF patches can generally enhance the heat transfer performance regardless of other factors. However, the ways of achieving those enhancements are different, which mainly depend on the arrangement of the SF patches and AR but are nearly independent of ϕ. The maximum improvement of heat transfer can be achieved in Case 3 with AR = 0.6, Ra = 105 and ϕ = 0.04, where the averaged Nusselt number is enhanced by 8.89%. Originality/value This study presents a new scenario where the SF patches of various arrangements are applied to enhance the nanofluid natural convection of a nonsquared enclosure, and it reveals how the improvement is achieved and cooperatively affected by several important factors.

Enhancement of Natural Convection for Cooling Active Antenna Unit Device in 5G Base Station

Enhancement of Natural Convection for Cooling Active Antenna Unit Device in 5G Base Station

Entropy-based analysis and economic scrutiny of magneto thermal natural convection enhancement in a nanofluid-filled porous trapezium-shaped cavity having localized baffles

This article presents a modeling approach to predict the thermal natural convection inside a porous trapezium-shaped zone with baffles installed on the adiabatic side-walls. The cavity is filled with nanofluid and exposed under the effect of a magnetic field. The equations which govern the phenomenon have been solved numerically. For various configurations of the installed baffles, temperature field, the flow structure, heat exchange rate, and entropy production have been characterized for sundry figures of parameters i.e. Rayleigh (Ra), Hartmann (Ha), Darcy (Da), material’s porosity (ϵ), and nanoparticles’ concentration (ϕ). In addition, a cost–benefit analysis of the convection enhancement process within the system based on nanomaterials has been proposed. It has been ascertained that rising Ra, Da, and ϵ while lowering Ha improved the convective nanofluid flow. The influence of porous medium properties (material's porosity and permeability) on the convective flow intensity is more effective at moderate Ra. The average cooling rate increases with rising Ra, Da, and ϕ and declining Ha and ϵ. Moreover, obtained results characterize a positive economic feasibility of utilizing nanomaterials to improve thermal natural convection in the presence or absence of a magnetic field.

Thermal performance improvement of energy storage materials for low operating temperature solar water heating systems

Recent studies have shown the attractive energy storage capabilities of phase change materials (PCMs); however, PCMs may not be fully effective due to their poor heat transfer characteristics. Moreover, the melting point of PCMs restricts direct integration into a solar collector for low operating solar water heating (SWH) applications. This study investigates the heat transfer enhancement of PCMs as well as aiming to lower their melting point onset to expedite the latent heat storage initiation. The selected types of PCMs are paraffin waxes with melting point temperatures of 28–72 °C. In Phase I, silicone oil is selected as a heat transfer medium to be used with the PCMs to establish natural convection enhancement, which led to maximum melting point depression of approximately 3 °C in the PCM72. In Phase II, the conductive heat transfer enhancement was studied by adding nanoparticles with varying mass concentrations. From this analysis, it was determined that PCM72 with 1% multi-walled carbon nanotubes (MWCNT) has shown an increase of approximately 3.81% in thermal conductivity. The obtained results from this study can lead to overall efficiency improvement of SWH systems by employing the PCM mixtures with low melting point, while keeping the high latent heat capacity, and high thermal conductivity.

Analysis of the effects of porous media parameters and inclination angle on the thermal storage and efficiency improvement of a photovoltaic-phase change material system

The heat generation in the photovoltaic (PV) panels leads to the rise in temperature, which subsequently deteriorates the electrical efficiency of PV panels. The application of phase change materials (PCM) can reduce the operating temperature and perform as a cooling mechanism. However, the cooling performance and energy storage capacity of PCM are drastically affected by its poor thermal conductivity. Therefore, in this paper, the impact of the insertion of metal foam on the heat transfer through PCM and the electrical efficiency of PV-PCM systems has been numerically investigate. A comparative study on the impact of the porosity of metal foam ranging from 0.7 to 0.9 has been carried out. Furthermore, different inclination angles, 0° to 90°, have been considered to evaluate the impact of inclination angle on the natural convection enhancement through liquid PCM. Results indicated that increasing the inclination angle results in better natural convection of molten PCM and better heat transfer through PCM. Moreover, results represented that compared with the PV-PCM system, 6.8% and 9.8% enhancement in the average temperature and electrical efficiency, in the case of insertion of metal foam with porosity of 0.9, is achieved.

Impact of using single heated obstacle on natural convection inside porous cavity under non-Darcy flow and thermal non-equilibrium model: A comparison between horizontal and vertical heated obstacle arrangements

The current study implements a numerical investigation on natural convection inside non-Darican porous duct to analyse the actions of using a single heated obstacle attached horizontally/vertically at the heated surface under Local thermal non-equilibrium (LTNE). Importance has been given on how the heated obstacle length and its position manipulate the enhancement of natural convection. Solutions confirm that Nu is extremely depends on ratio of thermal conductivity (Kr), obstacle length (Z), obstacle position (X), modified Rayleigh number (Ra*) and inertia coefficient (Fs/Pr*) as compared to heat transfer coefficient (H). Results display that the rising in Z leads to recede free convection activity as compared to the smallest one, where Z and X have extremely impact on Nu. Results displayed that Nus, Nuf and NuT obtained for the horizontal case are higher than vertical case, which reveals a better thermal performance. Optimum enhancement in Nu has been observed when using small Z and it horizontally attached at higher place inside the duct. The density of LTNE regime is extremely reduced with rising in H, Kr and Ra* and reducing in Z and Fs/Pr*. Finally, the horizontal heated obstacle can be advised to be used in heat exchanger for techniques implementing porous media.

Natural Convection Enhancement in the Annuli Between Two Homocentric Cylinders by Using Ethylene Glycol / Water Based Titania Nanofluid

The natural convection heat tranfer in annulate region confined between two homocentric cylinders were numerically studied in the present work. The annulus was filled by ethylene glycol / water based Titania nanofluid. Both internal and external cylinders are presereved at an isothermal hot and cold temperatures respectively, whereas, the upper and lower walls are adiabatic. The numerical solution is obtained by applying the finite volume method along with the SIMPLER, and TDMA algorithms. In the current study, the solid volumetric fraction is varied as (0 % ? ? ? 3 %), the volume ratios of EG to water are varied as (0:100 %, 40 : 60 %, 100:0 %), while both the Rayleigh number and radii ratio are considered fixed at (Ra = 104 and ? = 2). The obtained results indicated that, the average Nusselt number increases as the solid volumetric fraction and the volume ratio of ethylene glycol in the base fluid increase. Moreover, the velocity profiles reach their maximum value in the half region adjacent the internal hot wall when TiO2 – water nanofluid is used. Also, the temperature profiles decrease along the radial distance for all considered values of volume ratios of EG to water.

Numerical investigation of natural convection enhancement in latent heat energy storage units with punched-fin and slit-fin

We study the melting process of phase change materials (PCM) inside a shell-and-tube heat exchanger with punched-fin and slit-fin numerically based on the enthalpy-porosity model in the present paper. The phase change material used is paraffin wax. A comparison of the liquid fraction, the total melting time, and the average temperature was performed to assess the improvement of the melting processes for six fin designs. The numerical results reveal that the usage of punched-fins and slit-fins can improve the melting rate because of the decrease of the blockage effect. For slit-fin cases, the reduction percentage in the total melting time has nearly the same value at different Stephan numbers, and an almost 14.0% improvement is observed compared to the straight fin case, while for punched-fin cases, the reduction percentage varies for different Stephan numbers, and the maximum value of the reduction percentage is 10.1%.

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.

Finding a low cost energy multi-DBD plasma actuator for natural heat transfer enhancement in a vertical duct

Current study is focused on numerically finding an optimal active control system to augment the natural heat transfer through a vertical duct. This control system consists of a multiple set of wall embedded Dielectric-Barrier-Discharge (DBD) plasma actuators, which inherently consume the electric power to push the momentum near their embedded electrodes. On the other hand, in current study which the vertical duct is exposed to a constant heat flux on one side, the enhancement of natural convection by wall momentum pushes cause the upward flow to recirculate through the channel and consequently decelerates the mass flow rate to give higher outflow temperatures. The numerical simulation deals with the effects of variable parameters such as Rayleigh number, number of DBD actuators, and the vertical DBD arrangements. A descriptive enhancement ratio: The ratio of the average Nusselt number affected by plasma actuation to the average Nusselt number without DBD control, is used to evaluate capability of the heat transfer augmentation. The numerical results showed that the enhancement ratio in the presence of the DBD actuator reaches to about 2.75. However, the results briefly show the sensitivity of the heat transfer enhancement to the Number of the actuators and their arrangement. As the main goal and the final step, the overall thermal efficiency (OTE) as a parameter expressed to further clarify the performance of plasma actuator system from the perspective of energy consumption is defined as the rate of the augmented heat transfer by the DBD actuator to the actuation electric power consumption. Also, it can be realized that by increasing the number of DBD actuators from 1 to 4, the La increases, and over 4 it is constant. The results show that the case N = 4 can be a candidate for the optimum design.

Melting of multiple PCMs with different arrangements inside a heat exchanger for energy storage

Latent heat storage using Phase Change Materials (PCMs) is an important promising solution for addressing the renewable energy fluctuation problem. The low thermal conductivity of PCMs that substantially contributes to very slow storage and recovery of energy is a major obstacle. This study investigated the performance of multiple PCMs for different arrangements in a horizontally positioned heat exchanger during melting of the PCMs. The results identified the effects of conduction and natural convection heat transfer for different PCM arrangements in the heat exchanger. Numerical simulations were conducted for the defined geometries. The model developed for the study was validated against a published related study. Results in the form of liquid fraction contour and Nusselt number ratio plots are presented and discussed. It was found that the application of multiple arrangement of the PCMs has different effects on the heat transfer mechanisms. Natural convection has a significant positive effect on the heat transfer characteristics of these systems in all the arrangements studied. The parallel arrangement enhances conduction, leading to 40% reduced melting time, when only conduction heat transfer is considered, but it suppresses natural convection, while the series arrangement enhances natural convection heat transfer significantly. Compared to all the arrangements studied, the series arrangement was found to be superior for energy storage. With it, 15.5% reduction in complete melting time was achieved compared to the single PCM arrangement. Natural convection enhancement in this arrangement led to 50% complete melting time reduction, compared to the case of only conduction heat transfer. It was also found that the application of multiple PCMs increases the storage capacity by about 25%.

Ultrasounds Used as Promoters of Heat-Transfer Enhancement of Natural Convection in Dielectric Fluids for the Thermal Control of Electronic Equipment

We present an experimental investigation of the effect of ultrasound application to increase the heat-transfer coefficient for natural convection of a dielectric fluid. An experimental analysis is carried out to estimate the increase of the convective heat-transfer coefficient between an electronic board and a refrigerant fluid, the Fluorinert Electronic Fluid FC-72. For this purpose, an experimental apparatus composed of an electronic board, its electronic control circuit, and data acquisition systems have been designed and implemented. The data collected appear to confirm in some situations of practical interest the enhancement effect of the convective heat-transfer coefficient in connection with the use of ultrasound. The most favorable condition was observed with the fluid in quite low subcooled conditions.

Enhancement of natural convection for improvement of Trombe wall performance. An experimental study

This experimental work shows that the natural convective heat transfer in the active enclosure of a conventional Trombe wall assembly is enhanced by the interposition of transparent and vertical partitions. Quantification of the average Nusselt number corresponding to both versions with and without partitions was carried out by means of a 1/5 scale assembly. The glass cover is maintained isothermal at cold temperature while the active wall generates a variable heat flux simulating the incident solar radiation. The distance between the two active and parallel walls is variable. Four ratios between this distance and the height of the wall are considered, associated with a wide range of Rayleigh number reaching 4.1 × 109. An error calculation is carried out for all the processed configurations, taking into account the experimental uncertainties of the measured physical parameters. The maximum error found on the average Nusselt number is low, of about 5%. Measurements made by means of an interstitial medium without partitions are consistent with the results of other work carried out by experimental and numerical approaches in specific ranges of Rayleigh number. This study reveals the partitions effectiveness, since the natural convective heat transfer's increase lies between 10.0% and 14.4% according to the considered configuration. Correlations are proposed in the present work in order to calculate the average natural convective Nusselt number for the conventional Trombe wall without partitions and for its improved version, for any aspect ratio and in the whole range of the considered Rayleigh number. They contribute to the optimization of the thermal design of this interesting assembly.

Study of the effect of tilting lateral surface angle and operating parameters on the performance of a vertical shell-and-tube latent heat energy storage system

In this paper, a comprehensive mathematical model is developed to study the thermal energy storage performance in vertical shell-and-tube latent heat thermal energy storage (LHTES) systems with different tilting lateral surface angles. This model integrates heat transfer in heat transfer fluid (HTF), HTF tube and phase change materials (PCM). It is validated using experimental data and then applied to investigate effects of tilting lateral surface angles on the enhancement of natural convection in the LHTES system. The results show that increasing the tilting lateral surface angle largely enhances heat transfer in the melting process whilst it has negative effects on the solidification process. The total melting time reduces by 45% as the tilting angle increases from 0° to 7°. An optimal tilting angle of 4° is identified based on the trade-off between melting and solidification processes. Furthermore, the effect of HTF operating parameters on the performance of the LHTES system is evaluated. It shows that the melting and solidification time of the LHTES system with a tilting angle 4° reduces by 22% and 8.5%, respectively, as the HTF flow Reynolds number increases in laminar and transition flow regions. However, increasing the HTF flow Reynolds number does not show significants effect under turbulent flow regions. The variation of the HTF outlet temperature is also investigated. The temperature difference between the HTF inlet and outlet is found small at all studied HTF flow rates, which indicates that assumption of the constant HTF surface temperature boundary in simulations in the literature is reasonable.

Flow and heat transfer in a rotating heat pipe with a conical condenser

The flow and heat transfer phenomena in a rotating heat pipe (RHP) with a conical condenser were numerically studied by using a comprehensive numerical model based on the full discretized Navier-Stokes equations. The impact of parameters such as the heat transfer rate, rotational speed, fluid fill charge ratio and working fluid property on the performance of the RHP were examined. The operation characteristics of the RHP was captured in detail by using the present model. It is shown that as the heat transfer rate increased, the temperature gradient along the RHP experienced an increase. The overall thermal resistance of the RHP was reduced due to the enhancement of natural convection in the liquid at the evaporator section with larger heat transfer rates. The RHP has better performance at higher rotational speed in virtue of the enhancement of natural convection induced by larger centrifugal acceleration. The charge of fluid has impact on the performance of the RHP since the thermal resistance of the evaporator experienced an increase with the increase in the fluid fill charge ratio. As the working fluid, water has a better performance than ethanol, due to a larger thermal conductivity, a larger latent heat, and a bigger thermal expansion coefficient.

Electrohydrodynamic-Enhanced Natural Convection in an Enclosure by a Nonsymmetric Electric Field

The present study investigates heat transfer enhancement of natural convection in an air-filled enclosure by a nonsymmetric electric field. The geometry considered is a two-dimensional cavity with an aspect ratio of five. The electrical field is generated from a wire electrode charged with a high voltage of direct current with a positive polarity. Ionized air produced by corona discharge is issued from the wire, which impinges on the differentially heated walls and leads to substantial heat transfer enhancement. Numerical calculations have covered a wide range of parameters (that is, , 15, and 18 kV; and ). In the presence of an electric field, the flow and temperature fields may become steady, steady periodic, or nonperiodic. At low Rayleigh numbers, it is observed that the flow and temperature fields are basically oscillatory in nature. When the Rayleigh number is sufficiently increased, a steady state may be reached. Owing to the presence of oscillatory flows, there is a significant increase in heat transfer. It is found that heat transfer enhancement increases with the applied voltage but decreases with the Rayleigh number. In addition, it is found that heat transfer enhancement can be maximized by placing the electrode closer to and leaning toward the leading edge of the heat transfer surface: that is, to perturb the thermal boundary layer as early as it begins to develop.

A method to evaluate natural convection heat transfer in microencapsulated phase change material (MPCM) slurry: An experimental study

Immersion of microencapsulated phase change material (MPCM) into carrying fluid to form MPCM slurry is an effective method to guarantee both heat transfer efficiency and latent heat absorption. This article reports an experimental method to evaluate the natural convective heat transfer of MPCM slurry by means of setting the test rig in three angles to represent specific heating positions, i.e., θ = 0o (heated from bottom), θ = 90o (heated from side) and θ = 180o (heated from top). The thermal performances of MPCM slurry are analyzed by comparing with another two control groups: pristine PCM and MPCM particles. Results show that MPCM slurry gives the best thermal control amongst the three materials in the temperature range considered due to the thermal enhancement of natural convection, followed by pristine PCM, then MPCM particles. The surface temperature reduction of 16.3 °C is devoted by natural convection of carrying fluid in MPCM slurry, while this contribution is more pronounced (40 °C) in pristine PCM. However, the natural convection of core PCM is fully suppressed in MPCM particle. MPCM slurry with lower concentration consumes less time to reach a lower melt onset temperature at the cost of having less thermal management time.