Role Of Natural Convection Research Articles

Thermal interaction between water and liquid paraffin wax inside an intertwined enclosure: The role of natural convection in design of latent heat storage units

This work numerically investigated free convective heat transfer between water in a tank and melted paraffin-wax in an enclosure mounted inside the tank. The mediums were separated by thermally conductive walls. Three aspect ratios (AR = 0.3, 0.5, and 0.7) and three Rayleigh numbers (Ra = 104, 105, and 106) were examined for side- and bottom-heating orientations. The computations were conducted separately for the convection and conduction cases to observe the effects of natural convective heat transfer between the enclosures of the storage tank. The results showed that the heating orientation had a remarkable impact on the heat transfer between two fluids inside the intertwined enclosures. In the side-heating orientation, the variation of the average Nusselt number (Nu) with respect to Ra did not exhibit a consistent trend. It increased by 23.1 % when Ra increased from Ra = 104 to 105, but it decreased by 52.8 % when Ra was further increased from Ra = 105 to 106, at AR = 0.5. The Nu along the bottom wall of the inner enclosure was up to 24-folds of that along its side walls for the side-heating orientation and was 2.45-folds in the bottom-heating orientation. Furthermore, accumulation of hot fluid around the inner cavity walls, which occurred in side-heating cases, reduced convective heat transfer because of the low temperature gradient in these regions. Considering the outcomes of the present work, the bottom-heating orientation with AR = 0.5 can be recommended for future applications involving phase change processes as it ensured an effective thermal interaction between the two fluids.

A hybrid study of a 4-stage compressed solar distiller based on experimental, computational and deep learning methods

In the present study, a hybrid technique is employed to demonstrate the thermal performance of a 4-stage compressed solar distiller with hydrophilic evaporators (CSDHE) regarding the role of natural convection in the air gap domain. Initially, the test is conducted under simulated solar radiation of 2.13 W·m−2 over 5400 s. A thermodynamic model incorporating diffusion is developed for the next step to analyze internal transient temperatures. In the third step, a 3D single-phase natural convection model of the CSDHE is simulated to achieve the research objectives. Based on experimental, thermal modeling, and computational results, a dual deep neural network is developed to predict the thermal behavior of the system and the variation of convection heat transfer coefficient for different air gap thicknesses over 10,000 s. The experimental results revealed a distilled water production rate of 3.806 kg·m−2h−1 with a gained output ratio (GOR) of 112 %. Moreover, the findings demonstrate that conduction accounts for 84.23 % of total heat transfer, while convection accounts for 6.36 % during 5400 s. We also found that, by employing a convolutional neural network (CNN) and directly harvesting data from pre-defined contours, the hybrid training dataset for deep forward neural network (DFNN) is trained 28.35 % faster than conventional methods.

Numerical Investigation of phase transition in different latent heat storage systems in the presence of natural convection and porous media

Securing a reliable supply of energy is critical given the ever-increasing demand for energy and the challenges posed by a growing population. Latent energy storage is being introduced as one of the most efficient solutions to harness renewable and waste energy and ensure a constant supply. The phase change material (PCM) in this type of energy storage suffers from low responsiveness, which slows down the industrialization of latent storage. This study succinctly identifies the influencing factors that affect the efficiency of PCM-based latent energy storage, including fluid temperature, the role of natural convection and the benefits of porous media. It also provides new insights through comprehensive configuration analysis, ultimately contributing to the understanding of the field and addressing sustainable energy demands. For the numerical computations, enthalpy-porosity methodology via ANSYS Fluent 18.2 is employed a rigid grid for precise dual-phase simulation. The working fluid temperature is of similar importance for three types of units, as an improvement of 22% has been obtained by increasing the inlet temperature by 5 degrees, while the pipe model has benefited from an improvement of 26% under the same condition. The Triplex Tube Heat Exchanger (TTHX) outperforms the other units in all scenarios, as the heat transfer surface of this type is greater than that of its counterparts. Natural convection phenomenon is most effective in the pipe model, as its absence slows the melting rate by 236%. The inclusion of porous media produced on average 93% faster systems due to the additional heat transfer surface, although it suppressed the gravitational movement.

Role of natural convection and battery arrangement for phase change material based battery thermal management unit

Phase change material (PCM) based battery thermal management (BTM) has been of increasing interest due to the passive concept with efficient cooling and temperature uniformity performance. Due to the inherent characteristics of solid-liquid phase change during operation, it is worth in-depth investigation of the influence of buoyancy driven natural convection (NC) on battery thermal behavior. Here, a typical PCM enclosed cylindrical battery unit is considered as the benchmark problem. The effects of battery arrangement and liquid PCM NC on melting process and thermal performance are investigated numerically based on rigorously validated model. It shows that excellent cooling performance could be achieved for battery with the function of latent heat storage of PCM, whether the unit is arranged vertically or horizontally. In addition, the horizontal arrangement (4.1 K) has a better temperature uniformity than the vertical arrangement (5.7 K), which can be ascribed to the varying melting rate of PCM. To further insight into the thermal behavior of BTM unit, the relationship between the melting rate and interface length associated with differential liquid fraction curve is proposed for the first time. When the liquid PCM is full of the upper half of the BTM unit, the length reaches its maximum value and after that the differential liquid fraction is significantly declined, indicating the poor heat transfer and thermal performance. The motivation is to not only explore the fundamental mechanism of passive phase change method, but also to serve as a basis for the design of more practical and efficient PCM based BTM in the future.

Study on dominant heat transfer mechanism in vertical smooth/finned-tube thermal energy storage during charging process

• Heat transfer mechanism of heat conduction is dominant in smooth-tube TES. • The heat transfer in finned-tube TES is dominated by secondary flow. • Heat conduction enhancement rate was defined to characterize the heat conduction intensity. • Heat conduction enhancement rate increases with the increase of the number of fin. • Aluminum finned TES obtains the best thermal storage efficiency. Thermal energy storage (TES) is an indispensable part of solar energy utilization system. This paper intends to explore the role of heat conduction, natural convection and secondary flow on the charging performance of phase change material in vertical smooth/finned-tube TES respectively. It was found that heat conduction is dominant in smooth-tube TES. The role of natural convection is to cause non-uniformity of melting rate of the lower and upper part. Meanwhile, secondary flow is dominant in finned-tube TES due to the generation of vortices between adjacent fins. Fin material has little effect on the melting characteristic of phase change material when fin pitch is larger than 40 mm in this study. Therefore, nonmetallic fin and cheap metallic fin could be used in finned-tube TES to increase thermal storage density and reduce cost of investment. Compared to conduction model, the total melting time was reduced by 9.7% and 34.2% in smooth-tube TES and aluminum finned-tube TES when convection model is considered in the computation. Besides, the heat conduction intensity and secondary flow intensity were explored in aluminum finned-tube TES. The heat conduction enhancement rate φ was defined to characterize the heat conduction intensity. With the fin pitch decreasing from 40 mm to 8 mm, the heat conduction enhancement rate increases from 15.2% to 39.6%, indicating that heat conduction plays a crucial role in the finned-tube TES. The aluminum finned TES exhibit the best thermal storage characteristic as a result of the best heat transfer enhancement and smaller density.

Charging and discharging enhancement of a vertical latent heat storage unit by fractal tree-shaped fins

The popular application of latent heat storage (LHS) units using phase change materials is limited in converting and utilizing renewable energy due to their poor thermal efficiency. To address this deficiency, a tree-shaped fin inspired by nature is employed for the thermal enhancement of vertical LHS units. A three-dimensional mathematical model using the enthalpy-porosity method is developed to comprehensively evaluate the heat charging/discharging process of the new and traditional LHS units, focusing on the role of heat transfer fluid (HTF) direction. The results indicate that tree-shaped fins improve the temperature uniformity and facilitate the melting/solidification rate, which exhibits a preferable advantage that the full melting/solidification duration shortens by 34.5% and 49.2%, and the time-averaged heat storage/release rate augments by 49.4% and 96.4%. Interestingly, natural convection significantly improves the heat charging performance, while the role of natural convection can be considered negligible during discharging processes. Moreover, the HTF temperature seriously affects the performance of LHS units, while the role of HTF flow rate is less prominent. It is found that the upward flow of HTF is beneficial to the heat charging improvement while a downward flow mode facilitates the discharging performance, providing useful advice for practical applications of vertical LHS units.

Thermal performance analysis of multi-slab phase change thermal energy storage unit with heat transfer enhancement approaches

A novel multi-slab thermal energy storage (TES) unit with NaNO3 as PCM is designed for concentrating solar thermal power plants due to its easy manufacture and high energy storage density. A numerical model, based on the k−ε turbulence model, is used in studying the thermal performance of the system. To enhance the efficiency, two heat transfer enhancement methods are compared with the original unit, which is adding plate fins or 10 wt%EG into NaNO3. The numerical model is validated to be reliable for simulating the heat transfer with natural convection by a lab-built rig. The numerical results show that adding fins shortens the phase change time by up to 18% with an HTF flow rate of 4.0 × 10−3 m3/s in each channel and adding 10 wt%EG is the most effective way to enhance the heat transfer. The role of natural convection in heat transfer is analyzed. In practical applications, the sensible heat during the charging/discharging process cannot be neglected, which occupies 45%–55% of the total heat stored/released. This paper is expected to provide a detailed investigation of the thermal performance of the unit with two different thermal enhancement methods to give some rational advice in optimizing the system design and operation.

Experimental study on the charging and discharging behaviour of capric–lauric acid/oleic acid mixture in a cold thermal energy storage system for cold storage applications

In this study a capric–lauric acid/oleic acid mixture was synthesized to form a low temperature PCM for cold storage applications. An experimental investigation was carried out in a double helical coil cold thermal energy storage unit (CTESU) to study the charging and discharging behaviour of capric–lauric acid/oleic acid mixture. The inlet temperature and flow rate of the heat transfer fluid (HTF) were selected as experimental parameters to analyse the cold thermal energy storage performance of PCM. The analysis includes charging and discharging behaviour, energy dissipation rate, energy storage rate, energy charging efficiency, energy discharging efficiency and influence of HTF inlet temperature and flow rate under varied experimental parameters. With the increased contact area, the developed double helical coil considerably improves the heat transfer performance of the cold thermal energy storage unit. The discharging rate was 18% faster than the charging process due to predominance of natural convection. The PCM solidification during charging process is controlled by conduction. With increasing HTF flow rate during the discharging process, the melting rate was 16% higher than lower flow rates. In contrast, as flow rates increased, the charging rate did not change significantly. The role of natural convection and stratification which leads to unnecessary mixing of PCM consequently lowering the cooling energy stored. Also results suggest that capric–lauric acid/oleic acid can be utilized as a potential PCM for low temperature storage applications.

Thermal Modeling of Transport Phenomena for a Pulsed Laser Melting Process

Single pulsed laser melting in a cylindrical titanium alloy work piece is studied numerically using an axisymmetric model. Finite volume method and Tri-Diagonal Matrix Algorithm (TDMA) are used for discretization of the energy equation and solving the resulting algebraic equation respectively in order to obtain temperature distribution inside the computational domain. Heat losses from the irradiated surface takes place through convection and radiation and other surfaces are kept insulated. A volumetric and Gaussian laser is irradiated on the work piece. Validation of the present model with the existing literature is done first and the results agree very well. Then, the detailed transport phenomena during the laser melting process is studied using the model. The enthalpy porosity technique is used track the melt pool shape and size. The role of natural convection and Marangoni convection in controlling the shape of melt pool is discussed. Maximum temperature results at domain centre and it then decreases exponentially along the axial and radial direction of the work piece because of Gaussian nature of the pulse. The numerical results obtained can provide the direction to develop models for all type of laser applications used in the industry.

Effect of Uniform Magnetic Field on Melting at Various Rayleigh Numbers

Melting phenomena occurs in various industrial applications, such as metal castings of turbine blades, environmental engineering, PCM-based thermal storage devices, etc. During the design of these devices, they are designed for efficient heat transfer rate. To improve the heat transfer rate, understanding of the important flow processes during the melting (and solidification) is necessary. An objective of the present work is to study the effect of natural convection and magnetic field on interface morphology and thereby on melting rate. In this work, therefore, an effect of uniform transverse magnetic field on the melting inside a cavity, filled initially with solid gallium, at various Rayleigh numbers (Ra=3×105, 6×105, and 9×105) is presented. A 2D unsteady numerical simulation, with the enthalpy-porosity formulation, is performed using ANSYS-Fluent. The magnetic field is characterized by the Hartmann number (Ha) and the results are shown for the Ha = 0, 30 and 50. The horizontal walls of the cavity are considered insulated and vertical walls are respectively considered hot and cold. It is observed that the role of natural convection during the melting is significant on the temperature distribution and solid-liquid interface. The increased magnetic field (Ha = 30 and 50) found to have a suppressing effect on the dominance of natural convection at all Rayleigh numbers (Ra=3×105, 6×105, and 9×105).

Experimental investigation of the effect of natural convection on the evaporation characteristics of small fuel droplets at moderately elevated temperature and pressure

The role of natural convection in droplet evaporation is experimentally investigated for a range of micro-sized alkane fuel droplets at moderately elevated pressure and temperature. This paper assesses the impact of buoyant mass transfer from two perspectives; the resulting variation in steady-state evaporation rate, and the potential relation to non-linear temporal d2 profiles. The steady-state evaporation rate is found to increase linearly with initial droplet diameter at all tested conditions, where the relative improvement in evaporation rate depends upon the fuel type, pressure, and temperature. In addition to modifying the steady-state evaporation rate, the various environmental conditions also impact the occurrence and timing of d2 deviation, which manifests as a strong reduction in evaporation rate near the end of a droplet’s lifetime. Pressure tends to encourage the appearance of d2 deviation for both heptane and decane by increasing the proportion of evaporation for which natural convection is responsible. The effect of temperature depends upon the droplet volatility, and the results differ between the two test fuels. Overall, the relative importance of natural convection, as compared to pure molecular diffusion, appears to govern both the expected improvement in, and potential deviation from, steady-state evaporation. Proposed Grashof correlations justify the above hypothesis.

Numerical method and model for calculating thermal storage time for an annular tube with phase change material

For calculating the thermal storage time for an annular tube with phase change material (PCM), a novel method is proposed. The method is suitable for either low-temperature PCM or high-temperature PCM whose initial temperature is near the melting point. The deviation fit is smaller than 8% when the time is below 2×104 s. Comparison between the predictions and the reported experimental data of thermal storage time at same conditions is investigated and good agreements have been got. Based on this method, the performance of the thermal storage unit and the role of natural convection are also investigated. Results show a linear relation between the maximum amount of stored heat and thermal storage time, and their ratio increases with the height of the thermal storage unit. As the thickness of the cavity increases, natural convection plays an increasingly important role in promoting the melting behavior of paraffin. When the thickness of the cavity is small, natural convection restrains the melting behavior of paraffin.

Melting inside a horizontal cylindrical capsule

The role of natural convection on solid-liquid interface motion during constrained melting within a horizontal cylindrical capsule was investigated. A numerical code is developed using an unstructured finite-volume method and an enthalpy porosity technique to solve for natural convection coupled to solid-liquid phase change. Flow patterns for different Rayleigh numbers are presented. The resulting melt shapes and the temperature in the PCM provide conclusive evidence of the importance of natural convection on heat transfer in the melt region.

Role of natural convection in the dissolution of sessile droplets

The dissolution process of small (initial (equivalent) radius $R_{0}<1$ mm) long-chain alcohol (of various types) sessile droplets in water is studied, disentangling diffusive and convective contributions. The latter can arise for high solubilities of the alcohol, as the density of the alcohol–water mixture is then considerably less than that of pure water, giving rise to buoyancy-driven convection. The convective flow around the droplets is measured, using micro-particle image velocimetry (${\rm\mu}$PIV) and the schlieren technique. When non-dimensionalizing the system, we find a universal $Sh\sim Ra^{1/4}$ scaling relation for all alcohols (of different solubilities) and all droplets in the convective regime. Here $Sh$ is the Sherwood number (dimensionless mass flux) and $Ra$ is the Rayleigh number (dimensionless density difference between clean and alcohol-saturated water). This scaling implies the scaling relation ${\it\tau}_{c}\propto R_{0}^{5/4}$ of the convective dissolution time ${\it\tau}_{c}$, which is found to agree with experimental data. We show that in the convective regime the plume Reynolds number (the dimensionless velocity) of the detaching alcohol-saturated plume follows $Re_{p}\sim Sc^{-1}Ra^{5/8}$, which is confirmed by the ${\rm\mu}$PIV data. Here, $Sc$ is the Schmidt number. The convective regime exists when $Ra>Ra_{t}$, where $Ra_{t}=12$ is the transition $Ra$ number as extracted from the data. For $Ra\leqslant Ra_{t}$ and smaller, convective transport is progressively overtaken by diffusion and the above scaling relations break down.

Conceptual analysis and design of a partitioned multifunctional smart insulation

Building insulation performance in walls and roofs is typically assessed in terms of its R-value, a metric related to its ability to resist heat flow under steady state conditions. Past and present efforts by numerous researchers have resulted in a continued increase in achievable R-values. However, for most climates, there are times during a typical day and throughout a calendar year where it would be advantageous to enable switching between highly insulated and conductive states. A large energy savings potential exists for such an adaptive insulation by decreasing the load imposed on the heating or cooling system; however, practical realizations of adaptive insulation have not been fully developed. A new multifunctional insulation is presented in this paper where thin polymer membranes are positioned within a wall to create layers of air such that the role of natural convection becomes negligible. The heat passing through the wall must therefore travel through alternating layers of stagnant air and polymer membrane. To achieve the low R-value condition, the air is removed and the layers are compressed, essentially leaving only conduction through the polymer membranes. The focus of this paper is the analysis of such a multilayered wall in both the insulated and conductive states. Design strategies are presented for selecting suitable materials and wall geometry. The conceptual analysis presented here provides the framework for future studies focused on fabrication and experimental design of such a multifunctional smart insulation.

Thermal conductivity improvement of phase change materials/graphite foam composites

Effective thermal conductivity of composites of graphite foam infiltrated with phase change materials (PCM) was investigated numerically and experimentally. Graphite foam, as a highly-conductive, highly-porous structure, is an excellent candidate for infiltrating PCM into its pores and forming thermal energy storage composites with improved effective thermal conductivity. For numerical simulation, the graphite structure was modeled as a three-dimensional body-centered cube arrangement of uniform spherical pores, saturated with PCM thus forming a cubic representative elementary volume (REV). Thermal analysis of the developed REV was conducted for unidirectional heat transfer and the total heat flux was determined, which leads to the effective thermal conductivity evaluation. For experimental verification, a sample of graphite foam was infiltrated with PCM. The effective thermal conductivity was evaluated using the direct method of measuring temperature within the sample under fixed heat flux in unidirectional heat transfer. The results indicate a noticeable improvement in the effective thermal conductivity of composites compared to the PCM. Our numerical and experimental results are in agreement and are also consistent with reported experimental results on graphite foam. Moreover, the role of natural convection within the pores is found to be negligible.

06/01521 Study on paraffin/expanded graphite composite phase change thermal energy storage material: Zhang, Z. and Fang, X. Energy Conversion and Management, 2006, 47, (3), 303–310

Effective thermal conductivity of composites of graphite foam infiltrated with phase change materials (PCM) was investigated numerically and experimentally. Graphite foam, as a highly-conductive, highly-porous structure, is an excellent candidate for infiltrating PCM into its pores and forming thermal energy storage composites with improved effective thermal conductivity. For numerical simulation, the graphite structure was modeled as a three-dimensional body-centered cube arrangement of uniform spherical pores, saturated with PCM thus forming a cubic representative elementary volume (REV). Thermal analysis of the developed REV was conducted for unidirectional heat transfer and the total heat flux was determined, which leads to the effective thermal conductivity evaluation. For experimental verification, a sample of graphite foam was infiltrated with PCM. The effective thermal conductivity was evaluated using the direct method of measuring temperature within the sample under fixed heat flux in unidirectional heat transfer. The results indicate a noticeable improvement in the effective thermal conductivity of composites compared to the PCM. Our numerical and experimental results are in agreement and are also consistent with reported experimental results on graphite foam. Moreover, the role of natural convection within the pores is found to be negligible.

An experimental study for the role of natural convection in the dissolution of GaSb into InSb melt, and the growth of In<SUB align=right>xGa<SUB align=right>1-xSb crystals

The article presents an experimental study for the role of convection occurring during the dissolution of GaSb into InSb melt, and the growth of InxGa1-xSb crystals. Experiments were carried out in a GaSb(seed)/InSb/GaSb(feed) sandwich system under an imposed temperature gradient. In the experiments, the GaSb feed crystal dissolved into the InSb melt to supply the required GaSb component for the growth of InxGa1-xSb crystal. Experiments were carried out for two different reference temperatures and two sample diameters. It was observed that the shape of the GaSb(seed)/InGaSb interface became flatter when the reference temperature was lower and the sample diameter was smaller. The growth length of uniform composition section was larger in the case of higher reference temperature. Experiments show that by varying the reference temperature and the sample diameter, the growth interface and the composition of the grown crystal can be controlled by controlling the natural convection in the melt.

A Numerical Study for the Role of Natural Convection in the Melting of a GaSb/InSb/GaSb Sandwich System

The role of natural convection in the melting of a GaSb/InSb/GaSb sandwich system was numerically investigated. The melting process was assumed unsteady, and the governing equations of the model were solved by the finite difference method using a boundary fixing technique. The solutal natural convection (caused by concentration gradients) in the melt enhances melting in the lower part of the sample, and the thermal natural convection (caused by temperature gradients) is responsible for the development of a melt-solid interface that is slightly concave toward the crystals. These results are in qualitative agreement with the experimental observations made previously by the authors.

Role of Natural Convection on the Emission of Volatiles from a Bed of Contaminated Particles

A theoretical model describing the emissions from a bed of porous particles imbued with volatile organic compounds is presented. The new and most important feature of the model is that, besides diffusion, it accounts for the natural convection, induced by concentration gradients, which establishes itself when both the top and the bottom of the bed are vented to the atmosphere. Experiments are carried out arranging particles of a porous rock tuff imbued with one or more volatiles, in a packed bed geometry. The volatiles are benzene, chlorobenzene, and 1,3-dichlorobenzene. The investigation shows that for volatiles having a vapor pressure like that of the compounds used in the experiments, natural convection predominates the overall release process. The release by fickian diffusion from the top of the bed is negligible. The theoretical model represents the experimental data with good accuracy and predicts closely the time required for the release of the volatiles. From a more practical point of view, the mo...