Search

Influence of variable thermophysical properties is investigated in airflows established by natural convection inside vertical tubes. Systematic numerical results are obtained for covering the entire range laminar to turbulent (modified Rayleigh number ranging 1 0 − 3 – 1 0 9 ), for both isothermal and isoflux heating conditions. Particular attention is devoted to the effects of variable properties of air. Own functions are used to take into account the variation of thermophysical properties as a function of temperature. Results are compared to those computed assuming the Boussinesq approach. For isoflux heating, it is proven that the disruptive effect consisting of the emerging of a critical heat flux and the consequent thermal crisis phenomenon, appear in the regarded cylindrical geometry. An analytical estimation of the critical heat flux is developed for vertical tubes, valid for the laminar regime, and an overall correlation is proposed for the laminar-turbulent range. The impact of the height and the widening of a funnel-shaped adiabatic extension is explored for selected values of the modified Rayleigh number, and the conditions under which its inclusion is beneficial to postpone the appearance of the crisis phenomenon, are determined. • Behavior analysis of convective airflow in vertical tubes with variable properties. • Thermal crisis phenomenon for laminar/turbulent natural convection flows within tubes. • Analytical expression for critical heat flux applied to a vertical, circular, tube. • Widening of funnel-shaped adiabatic extension delays thermal crisis phenomenon. [ABSTRACT FROM AUTHOR]

Natural convection heat transfer in cavities commonly occurs in various engineering problems. Specifically, the problem of a differentially heated cavity with oscillatory boundary conditions arises in the design of energy-saving strategies or energy evaluations within the building sector. This work addressed numerically and experimentally the natural convection in a full-scale room subjected to diurnal heating and nocturnal cooling. The experiment was instrumented with temperature sensors and an air velocity sensor, inspired by works reported in the literature. The numerical model involved solving the equations governing natural convection, considering turbulent bi-dimensional and three-dimensional flow in a transient state. This research allowed the selection of an appropriate turbulence model and evaluated the performance of the two-dimensional and three-dimensional approaches in simulations. Results showed that the k-omega SST model performed best in predicting velocity, while the standard k-omega model performed best in predicting temperature for the two-dimensional simulation, and the RNG k-epsilon model performed best for the three-dimensional simulation. Additionally, the three-dimensional simulation more accurately reproduced the chaotic fluctuations observed in experimental velocity data, demonstrating up to 30 % better estimation during periods of high velocities. The study identified the reversal of convective cells in the cavity, with an approximate duration of 2 h and 40 min in a real case. This study contributes to improving the understanding of turbulent natural convection in a differentially heated cavity with oscillatory boundary conditions. These findings have significant implications for various environmental and engineering applications, including the design of thermally comfortable buildings. [ABSTRACT FROM AUTHOR]

We measured the natural convection heat transfer of a heated sphere with a point contact with an insulated sphere, varying the location of the contact point along the flow development. Two different sphere diameters of 15.8 mm and 40 mm were used, which correspond to Ra d 's of 3.35 × 108 and 5.43 × 109. Mass transfer experiments using a copper sulfate-sulfuric acid (CuSO 4 –H 2 SO 4) electroplating technique were performed based on the analogy, which visualized the local natural convection heat transfers by the plating patterns. For d = 15.8 mm, the natural convection heat transfers of the heating sphere were similar to that without the contact point regardless of the location of contact points. The copper plating pattern showed the enhancement of heat transfer near the contact point resulting from the redevelopment of the boundary layer compensating for the reduced heat transfer area. For d = 40 mm, the heat transfer of the heating sphere was weaker than that without the contact point only when the contact point was located at downstream from the separation point, which hindered the redevelopment of the boundary layer. This study provides the experimental evidence for the boundary layer redevelopment near the contact point by visualization, which compensates for the reduction in heat transfer area. • Natural convection heat transfer of a heated sphere with a point contact was measured. • Mass transfer experiments were conducted instead of heat transfer ones based on analogy. • Local heat transfer at the heated sphere were visualized by a copper plating pattern. • Heat transfer reduction by contact point is compensated by boundary layer redevelopment. • The redevelopment was not observed for contact points located above the separation point. [ABSTRACT FROM AUTHOR]

We present natural convection experiments at low-to-moderate porous media Rayleigh number R a in a fluid-saturated structured plastic foam with porosity of 0.83 heated from below. This is motivated by the lack of detailed experiments on thermal convection in these systems, although they are of great interest for many engineering applications. Our experiments were carried out using Magnetic Resonance Imaging (MRI), which allowed us to obtain quantitative measurements of velocity vectors together with temperature fields in the bulk of the fluid-saturated sample. The results obtained are compared with those for classical porous media (e.g. packed beds of spheres) and with theoretical predictions. For our system, we found the onset of convection at R a c = 41. 9 , with a critical wavenumber α c ≈ π. For R a < R a c , velocities always remain below the accuracy of the measurement and temperature fields remain homogeneous and linear in the vertical direction. For R a c ≤ R a < 308 , the flow has a stable convective patterns in forms of axisymmetric, straight or deformed rolls. At about R a = 308 , the flow enters in a transitional regime in which the convective rolls begin to deform. At the highest R a tested, the convective pattern takes the form of cross-rolls. The intensity of convection is estimated through the large-scale mean squared velocity U 2. For R a > 70 , we obtain U ∼ R a 1 2 . The heat transfer is investigated by evaluating the Nusselt number from temperature fields. We found that the local N u reflects the flow structure: higher values of local N u correspond to cold down-welling regions, and smaller N u to warm up-welling regions. Finally, we found that the averaged Nusselt number for the whole system increases linearly with R a with a slope equal to 2. • Coupling velocity and temperature fields during convection in solid foam using MRI. • Convective pattern evolves from rolls to cross-rolls with increasing Ra. • Large-scale mean velocity U scales as U ∼ Ra 0.5. • The Nusselt number increases linearly with Ra with a slope of 2. [ABSTRACT FROM AUTHOR]

The natural convection heat transfer mechanism of the body centered cubic (BCC) lattice structure needs to be clarified for its application in passive cooling thermal management. The BCC lattice heat sinks with various design parameters, such as strut diameter and unit cell length, were fabricated using the laser powder bed fusion process. Subsequently, the thermal performance was experimentally determined for variations in design parameters. The thermal performance tended to be maximized when the relative density of the lattice heat sink constituted approximately 10 %. The numerical analysis was performed to figure out the heat transfer mechanism and investigate the effects of inclination angles. According to numerical results, a lattice heat sink with a smaller strut diameter at the given unit cell length entrains more air than one with a larger strut diameter. However, decreasing strut diameter led to a negative impact on conduction and radiation heat transfer due to the reduction in solid region and surface area. Depending on the inclination angle, thermal performance of a lattice heat sink proportionally decreases. The main cause of performance degradation was the hindrance of plume motion by the base of the heat sink. [ABSTRACT FROM AUTHOR]

A 3D cellular automaton (CA)-lattice Boltzmann (LB)-immersed moving boundary (IMB) coupled model is established to study dendritic growth and particle pushing/engulfment during the solidification of particle-reinforced aluminum matrix composites (PAMCs). Following the validation of the models with several benchmarks, simulations of directional solidification of Al–3Cu/SiC p were carried out, reproducing several essential physical phenomena, including the particle movement, dendritic growth, and interaction between the particle and the liquid/solid interface, which were in good agreement with the experimental results. During the long-range interaction between the particle and the dendrite at the initial stage, the flow induced by the particle movement and the flow generated by the natural convection showed a competitive relationship, which influenced the solute distribution and the dendritic growth. Once the interaction became short range, the decrease in the concentration gradient between the particle and dendrite tip together with the increase in the horizonal concentration gradient below the particle caused the dendrite tip to engulf the particle. The results also showed that larger particles tended to be partially engulfed by dendrites, although sometimes they were at the grain boundary, and smaller particles were more easily pushed at the grain boundary by the dendrites on the side. • A 3D cellular automaton (CA)-lattice Boltzmann (LB)-immersed moving boundary (IMB) coupled model is established. • Simulations of directional solidification were carried out, reproducing several essential physical phenomena. • Effect of natural convection and particle motion on solute distribution and liquid phase flowing is taken into account. [ABSTRACT FROM AUTHOR]

Based on data-driven approaches, eight machine-learning algorithms have been proposed to characterize the heat transfer performance of turbulent jet impingement cooling. These algorithms are designed to cool high-heat-flux surfaces modified with single protrusions, multi-protrusions, or V-grooves. To train these algorithms, an experimental dataset was used, which included 184 experiments with Reynolds numbers ranging from 10,000 to 33,000, nozzle exit-to-plate distances ranging from 2 to 10 jet diameters, and two different heat inputs of 60 W and 90 W. Multiple linear regression algorithms in Python programming were used to model jet impingement cooling, showing that the proposed machine learning models provided more accurate predictions with a 20.4 % to 66.5 % relative residual improvement over the existing conventional regression model based on performance metrics like R2 and relative residual. This improvement was leveraged to quantify natural convection's impact on the effectiveness of heat transfer enhancement due to different types and sizes of roughness elements in high-temperature applications. The research found that as the protrusion surface area increased from 0.02 % to 0.188 % compared to the flat plate, the effectiveness of heat transfer enhancement decreased from 2.69 % to 8.37 % at similar operating conditions. These results revealed that natural convection, which opposes forced jet impingement, was strengthened with increased heat input values, roughness element surface area, and the local turbulence generated. The higher natural convection weakened the enhancement afforded by jet impingement on the modified plates relative to the flat plate. Additionally, the capabilities of machine learning algorithms were explored to unlock the full potential of the jet impingement technique and to overcome the associated complexity in jet impingement applications for next-generation electronics cooling. Finally, the Decision Tree Classifier algorithm was applied to classify the performance of the surface roughness element as either effective or ineffective. • Machine Learning models outperformed conventional regression models by 20 %–66 %. • ML models to predict roughness element performance for jet impingement cooling. • ML algorithms to unlock full potential of next-generation jet impingement technique. • Quantified the impact of natural convection on heat transfer by roughness elements. • Decision tree classifier to classify effective and ineffective roughness elements. [ABSTRACT FROM AUTHOR]

In the present study, thermal performance tests of horizontally oriented HS conical , HS cross-cut , and HS flat with a base plate dimensions of 80*80*5 mm3 were performed in natural convection. The effects of ambient temperature (30 and 40 °C), input power (16.5 and 33 W), and conical pin fin geometry on the thermal performance of the HSs were investigated. The thermal resistance of the heat sinks decreases while the convective heat transfer coefficient, average surface temperature, and average junction temperature increase with increasing ambient temperature and input power. However, the Nusselt number increases with increasing ambient temperature for all HSs at 16.5 W, while the Nusselt number decreases with increasing ambient temperature at 33 W. The highest convective heat transfer coefficients are 32.17, 30.68, and 19.61 W/m2K for HS conical , HS flat , and HS cross-cut at 33 W and 40 °C, respectively. The lowest thermal resistances are 2.75, 2.80, and 3.81 K/W for HS conical , HS cross-cut , and HS flat at 33 W and 40 °C, respectively. Thermal resistances of HS conical and HS cross-cut decreased by 27.78 % and 26.44 % at maximum and 27.06 % and 24.58 at minimum, respectively, compared to HS flat. The results show that the conical fins have better thermal performance in natural convection. • CPFHS exhibits the highest convective heat transfer coefficient in all tests conditions. • CPFHS shows a maximum reduction in thermal resistance of 27.78 % compared to FHS. • The overall fin efficiency of CPFHS is higher than CCPFHS in all test conditions. • CPFHS exhibits the highest thermal performance under natural convection in all tests. [ABSTRACT FROM AUTHOR]

The utilization of phase change materials (PCMs) holds tremendous potential of heat storage domain. The PCM's refractory at the latent heat thermal energy storage (LHTES) unit bottom hinders the heat storage efficiency, despite the significant improvement in thermal conductivity achieved through the addition of metal foam. This study employs numerical simulation to examine the impact of applying bottom cross-cut on PCM's spatial distribution in a horizontal LHTES unit. The manuscript analyzes parameters including melting fraction, complete melting time, Rayleigh number, natural convection heat transfer gain, melting phase interface, velocity and temperature distributions, and heat storage. The findings indicate that the proximity to the heating tube results in a reduction of solid volume at the LHTES unit bottom. A 0.6 bottom cross-cut ratio leads to an 18.84 % faster heat storage rate compared to a concentric-circle unit. Furthermore, a bottom cross-cut ratio of 0.5 enhances natural convection heat transfer gain by 3.28 times. [ABSTRACT FROM AUTHOR]

An experimental and numerical investigation has been performed on the quantitative determination of two-stage heat transfer characteristics of constrained melting process of phase change material (PCM) inside a horizontal cylinder. Melting experiments are conducted with paraffin in a cylinder subjected to different boundary temperatures. The solid-liquid interface and temperature field of PCM are captured using photography and infrared technique, respectively, which are then used to validate the numerical models established based on the enthalpy method. A scale analysis is performed to estimate the order of magnitude of the influence factors. With the validated models, the melting processes of PCM inside a horizontal cylinder covering a wide range of Rayleigh number (Ra), Stefan number (Ste) and Fourier number (Fo) are simulated and analyzed carefully. It is found that the transition point between the conduction and convection stages depends greatly on the Rayleigh number while has little relation with the Stefan number. Particularly, three important Rayleigh numbers are identified: when Ra < 5.0 × 103, the PCM melted in the conduction stage is more than 90%, indicating that the natural convection effect on the melting heat transfer is very weak; when Ra > 3.3 × 107, the PCM melted in the convection stage is more than 90%, so the natural convection dominates the melting process; when Ra = 5.3 × 104, the PCM melted in conduction and convection stages are both equal to 50%. For the conduction stage, the heat transfer characteristics depend only on the dimensionless melting time (FoSte), while for the convection stage, the heat transfer characteristics depend not only on the Fourier and Stefan numbers, but also on the Rayleigh number. Based on the numerical and the scale analysis results, the correlation for the dimensionless transition melting time (FoSte) tr (which can be used to distinguish between the conduction stage and convection stage) as well as the correlations of liquid fraction (α) and Nusselt number (Nu) for the conduction and convection stages, respectively, are proposed. These correlations, with a wide application range of Rayleigh and Stefan numbers, agree well with the experimental and theoretical results from both present study and open literature, and can well predict the two-stage heat transfer characteristics of constrained melting in a horizontal cylinder. • A quantitative study on two-stage melting heat transfer characteristics in a horizontal cylinder was performed. • Melting experiment was performed by combined use of photography and infrared technique. • Transition points between conduction and convection stages were examined. • Transition point correlation as a function of Rayleigh number was developed. • Liquid fraction and Nusselt number correlations for conduction and convection stages respectively were proposed. [ABSTRACT FROM AUTHOR]

Enhancing heat transfer using fluid-structure interactions (FSI) has attracted significant interest in recent years. Previous effort on this topic was mainly devoted to forced convection. The present study adopts a flexible structure to enhance heat transfer by natural convection. A two-dimensional numerical simulation is carried out to study a flexible baffle horizontally attached to a vertical heated flat surface. An immersed boundary-lattice Boltzmann method (IB-LBM) is adopted to solve the coupled thermal FSI problem. The effects of the Rayleigh number (R a = 2.3 × 10 8 ∼ 3.5 × 10 9), the normalised bending rigidity of the baffle (E B = 1.75 × 10 5 ∼ 3.5 × 10 6), and the normalised baffle position (h = 0.2 ∼ 0.5) on baffle dynamics and consequent heat transfer performance are described. The dynamic response of the baffle and the interactions between the flexible baffle and the thermal boundary layer (TBL) are examined via spectral analysis. The present results reveal that the flexible baffle may experience either a stable mode or a periodic vibration mode within the parameter ranges considered. It is advantageous to replace a rigid baffle with a flexible one for enhancing heat transfer, and up to 12% enhancement of the total heat transfer may be achieved when the flexible baffle flaps in the periodic vibration mode at multiple frequencies. The enhancement is due to induced resonance of the TBL and an oscillatory wake behind the flexible baffle which carries hot fluid downstream. It is further revealed that for the highest Rayleigh number case, varying the bending rigidity within the present range leads to 8–10%, 9–12%, and 7–8% enhancement of the total heat transfer at h = 0.2, 0.3 and 0.5 respectively. The position of the flexible baffle has two opposing effects on enhancing the overall heat transfer through the heated surface, and thus an optimal position may be determined to achieve the best heat transfer enhancement. This work proves the concept of the fully passive strategy. The present findings may guide the design and optimisation of the fully passive and self-adaptive strategies for enhancing passive cooling of electronics. • Fully passive vibration of a flexible baffle attached to a vertical heated surface • Sustaining perturbations to thermal boundary layer by oscillatory wake behind baffle • Heat transfer enhancement due to induced resonance of thermal boundary layer • More heat transfer enhancement with a flexible baffle than a rigid one [ABSTRACT FROM AUTHOR]

The performance of a fractal geometry blade for cooling hot water by natural convection and evaporation was studied numerically and experimentally. The blade design includes a tree-like fractal pattern and the Sierpinski carpet. The fin is made up of a solid hollow structure whose core is embedded with a porous material through which liquid flows. For the experiments, temperature measurements were taken in the porous medium by holding the fin in a fixed vertical position while warmed water was fed through its flow inlet. Numerical simulations were performed using a two iteration scheme that first involves the calculation of the local convective heat transfer coefficients and then the estimation of the heat dissipated by evaporation. The behavior of the fractal blade was compared with that of a benchmark heat sink that does not include the Sierpinski pattern in its construction. The experimental and numerical results agree well with each other and show a better performance of the reference heat sink when cooling water. Although the windows in the Sierpinski pattern improve convective cooling, this effect is marginal compared to evaporative cooling, which is enhanced by the larger surface area of wet porous medium exposed to the environment in the benchmark fin. [ABSTRACT FROM AUTHOR]

The heat transfer potential of Phase Change Materials (PCMs) is augmented by the insertion of a metallic wool with high thermal conductivity. Apparent thermal heat conductivity has been determined experimentally in the solid and in the liquid phase for different wool contents. Heat transfer is enhanced by the addition of wool in solid phase, but is mitigated in liquid phase by the restriction of natural convection. The difference in heat transfer efficiency in the liquid and solid phase induces an asymmetric behavior between the melting and solidification configurations. The behavior of metallic wool-PCM composites is very similar to metallic foam-PCM hybrids studied in literature. As shown on the basis of rational material selection, the addition of a metallic wool constitutes in the practice a cost and process effective way to control the heat transfer potential. [ABSTRACT FROM AUTHOR]

Radiation induced natural convection has been extensively researched due to its rife presence in numerous natural flows. Recently, it has seen renewed and resurged interests in context of understanding the photo-thermal energy conversion and its redistribution in direct absorption solar collectors (DASCs). The present work serves to further this objective; in particular, convection induced by direct interaction of the fluid and radiation (irradiated from side) has been critically and comprehensively investigated. In addition to DASCs, the present study has significant applications in various other fields such as nuclear power plants, building design, and laser fusion. A detailed mechanistic theoretical modeling framework has been developed to quantify the impact of various fluid properties and incident flux parameters; viz., Rayleigh number (102 to 108 in the laminar regime), Prandtl number (0.01–1000), optical thickness (0.1–100), convective heat loss from the transmitting wall (controlled in the simulation by varying the modified Biot number, Bi* from 0 to 5), aspect ratio (0.1–10). Furthermore, in addition to magnitude of the incident flux, its distribution (i.e., whether uniform or linearly varying) has also been dealt with. Another distinguishing aspect of the study is the use of the "heatlines" to visualize the flow of energy as it occurs during convection caused due to volumetric heating. Detailed analysis reveals that, while a conduction dominated regime is apparent at Ra ≤ 102, a transition regime occurs at 103 ≤ Ra ≤ 104, and convection is the domineering mechanism when Ra > 104. At higher Rayleigh numbers, the pattern of heatlines begins to resemble that of the streamlines. Nature of the flux distribution at the side wall (along the depth direction) significantly impacts the temperature distribution. Furthermore, there exits an optimum value of optical thickness (for a given Rayleigh number), which ensures minimum heat loss and maximum useful energy gain. Overall, through the present work, we have been able to decipher key design and operational parameters (and quantify their impacts) which dictate the momentum and heat transport in side-irradiated volumetrically heated cavities. [ABSTRACT FROM AUTHOR]

Nanofluids have gained much interest in recent years in heat transfer applications due to their improved thermal properties compared to conventional pure fluids. In many applications, the nanofluid may be subjected to unsteady heating and to the effect of external magnetic field, which can have a considerable effect on the behavior of the nanofluid, especially if the nanoparticles are metallic. In the present study, the flow and heat transfer characteristics of Cu/water nanofluid in a semi-circular cavity are analyzed. The fluid is heated by a heat source located at the bottom wall and having a time-periodic temperature profile. The other walls are adiabatic. Two magnetic sources are located on the top wall. The equations governing the flow and thermal behavior of the nanofluid are presented and converted into the dimensionless form. The equations are then solved by means of the finite volume method. The results are presented in the form of flow and thermal contours, as well as in the variation of the sensible heat, thermal stratification, and Nusselt numbers as functions of time. The impacts of various parameters, namely the temperature frequency, the magnetic number, the porosity, Darcy and Rayleigh numbers, on the results are discussed. In particular, it has been found that the porosity possessed the most important effect on the flow and thermal behaviors of the nanofluid. A 60% increase in Nusselt number is notably observed when the porosity is changed from 0.9 to 0.1. [ABSTRACT FROM AUTHOR]

The effect of orientation on the laminar natural convection heat transfer from branching fins attached to a rectangular plate has been numerically investigated. The orientations are termed 'vBvF' (vertical Base vertical Fin) and 'hBvF' (horizontal Base vertical Fin) when the base plate is vertical and horizontal respectively and the branching fins are attached orthogonally on the base plate. The third orientation is 'vBhF' (vertical Base horizontal Fin) when the fins are arranged horizontally on a vertical base plate. Besides orientation effect, the investigation also involves the effect of fin dimensions on the heat dissipation performance. Optimum branching angle of a 'single' branching fin is determined in each orientation. Fin size influences the optimum angle only in hBvF orientation. By studying the characteristics of buoyant fluid flow over the heat sinks, we have observed 'vortex motion', 'geometry induced acceleration' for certain sizes and orientations of the fins. These flow features are specific to the branching fins in natural convection. The dependency of the average heat transfer coefficient on the fin dimensions is explained and correlations to predict the Nusselt number in terms of non-dimensional parameters are developed. Highest thermal performance is obtained in vBvF orientation followed by hBvF and vBhF orientations. A comparison test has indicated that the branching fins have more effectiveness and efficiency than rectangular fins of equal mass and equal base area in all orientations. [ABSTRACT FROM AUTHOR]

In this study, natural convection in solar air heaters with different fins is numerically investigated. Effect of fin shape (rectangular, elliptical and triangular), the inclination angle (45–90°) on mass flow rate, temperature and performance are studied by considering the effect of shadow created by fins. Three different cases, solar air heater with fins considering shadow effect (Shadow Effect Considered: SEC), ones with fins neglecting shadow effect (Shadow Effect Neglected: SEN), and ones without fins (WF) are compared. It can be concluded that shadow created by fins, which eliminates some part of absorber from playing a direct role in heat transfer, has a significant impact on finding the best performance and dimensions of fins. For example, the performance of rectangular and triangular fins in case SEC are about 10% and 20% less than case SEN. Due to fins shadow, mass flow rates in solar air heaters without fins (WF) is more than solar air heaters with fins in higher inclination angles. While the highest mass flow rate is achieved for depth between 0.005 and 0.010 m, but the best depth from performance point of view is about 0.025 m and improvement in performance is negligible for larger depths. • Effect of fins' shadow on solar air heaters are numerically studied. • For β > 80°, using Triangular and elliptical fins is not beneficial. • The highest mass flow rate is obtained by rectangular fins. • For β > 60°, using fins lead lower mass flow rates. • Considering the performance of solar heater, a fin depth of 25 mm is recommended. [ABSTRACT FROM AUTHOR]

It is complicated and costly to directly simulate the natural convection influenced melting processes involving multiple PCM units in packed bed phase change system. An effective method to simulate such melting process is using an equivalent conduction model, in which the effective thermal conductivity (ETC) of PCM considering the natural convection effects is an important thermal parameter needing to be determined. In this paper, the ETC of PCM is investigated and determined by comparing numerical results based on models considering natural convection and pure conduction, respectively. Parameters of interest include melting fraction (α), Rayleigh number (Ra , 1.5 × 104 ≤ Ra ≤ 6.3 × 107), Prandtl number (Pr , 17 ≤ Pr ≤ 550) and Stefan number (Ste , 0.011 ≤ Ste ≤ 0.276.). Results show that the ETC of PCM has a strong relation to α and Ra , while it has little relation to Pr and Ste. Hence, an ETC correlation is developed as a function of Ra and α. Validation and application of the ETC correlation are further carried out, and it is found that the melting fraction predicted based on the present ETC correlation can better reflect the effect of liquid zone geometry on the melting heat transfer with natural convection than that predicted based on the previously reported correlations using a liquid gap parameter. • The ETC is determined by comparing results based on natural convection and pure conduction models. • The correlation is proposed as a function of melting fraction and Rayleigh number. • The proposed correlation is applicable to a wide range of Rayleigh numbers. • The proposed correlation shows better predictions than the reported correlations. [ABSTRACT FROM AUTHOR]

A numerical simulation has been performed to study the natural convection cooling of an IRS device by varying Rayleigh number in the range of 1010 to 1012. Computations are carried out by varying the diameter ratio (DR) in the range of 1.02–1.3. Navier-Stokes equations and energy equation have been numerically simulated for a full-scale IRS device using the algebraic multi-grid solver of FLUENT 16 with a two-equation eddy viscosity based turbulence model. It was found that with the increase in diameter ratio the heat transfer rate increases but the average Nusselt number initially increases and then decreases following a peak value. The effect of diameter ratio, funnel overlap and Ra on Nusselt number, induced mass flow rate and cooling time has been discussed along with the correlations for average Nusselt number and induced mass flow rate which can be very beneficial to the shipping industry. [ABSTRACT FROM AUTHOR]

Abstract The influence of the suspender on the evaporation of droplets under quiescent atmospheric conditions was studied experimentally using pendant droplets of ethanol (volatile) and water (non-volatile). Conducting and non-conducting suspenders of different materials and configurations (a steel needle, glass needle, steel rod, and glass rod) were evaluated. For both liquids, it is observed that the rate of evaporation is higher with steel suspenders. More importantly, for ethanol droplets, there is a difference in the evaporation rates between the needle and rod configurations of the same material. Thus, it is shown for the ethanol droplet that the configuration (needle versus solid rod) also plays a role in the evaporation process, apart from the material. The water droplet does not show this dependence. Visualization of internal flow and heat transfer calculations explain this observation. The results will be useful in choosing an appropriate suspender while using the pendant droplet method for studying complex interactions in an evaporating droplet. [ABSTRACT FROM AUTHOR]

Abstract In the present work, natural convection heat transfer from a thick hollow horizontal cylinder placed on ground is studied numerically. 3 D numerical simulations have been performed by varying the Rayleigh number (Ra) from 104 to 108 (preferably within laminar regime). All the simulations have been carried out by varying length to outer diameter ratio (L/D) and thickness ratio (d/D) in the range of 0.2–20 and 0.4–0.9 respectively. Nusselt number, heat transfer rate and contribution to total heat transfer rate from different surfaces have been calculated and their variation with the above parameter is represented graphically. Similarly, thermal plume and velocity vector are shown pictorially at different sections of the cylinder and physical phenomenon of fluid flow and heat transfer could be described from such pictures. Later on effect of above parameters on cooling time is also discussed. It has been observed that with increase in thickness, Nu decreases at low L/D (L/D ≤ 2), whereas at higher L/D a reverse trend is observed. Heat transfer rate from the inner surface increases initially with increase in L/D and after a particular L/D, it attains constant value whereas for outer surface, it increases continuously. It has been also observed that at a particular L/D and d/D, contribution of heat transfer rate from inner surface increases with Ra, whereas reverse trend is observed for outer and side flat surface. A correlation for Nusselt number as a function of Rayleigh number, L/D and d/D has been developed which may be useful for industrial purposes. [ABSTRACT FROM AUTHOR]

Abstract This study numerically investigates the thermal performance of horizontal tube with circular fins under natural convection. The natural convection from horizontal finned-tube heat sink has been understood in an average sense since most previous studies were experiment-based. The present study reveals the flow and heat transfer characteristics in details with numerical analysis. The parameters include fin diameter (D), fin spacing (b), fin material, and tube diameter (d), under a fixed fin thickness of 0.5 mm and temperature difference of 50 K between the tube and the ambience. The distributions of fin temperature and heat transfer coefficients are calculated for stainless-steel (SS) and aluminum (Al) fins under various conditions. For Al fins with small fin temperature variations, decreasing tube diameter leads to higher heat transfer rates due to the larger fin surface areas; but for SS fins with large fin temperature variations, increasing tube diameter enlarges heat transfer rates. On large SS fins, negative local heat transfer coefficients may exist above the tube as the hot plume generated by the horizontal tube transfers heat back to the fin. A larger d leads to a slightly larger optimum fin spacing b opt. In addition, various existing empirical b opt formulae are examined for their reliability with respect to different D / d ratios and Al and SS fins, respectively with small and large fin temperature variations. Highlights • Aluminum and stainless-steel fins are examined to present different fin temperature variations. • Under small fin temperature variations, decreasing tube diameter increases heat transfer rate. • Under large fin temperature variations, increasing tube diameter increases heat transfer rate. • On large SS fins, negative local heat transfer coefficients may exist above the tube. • The optimum fin spacing is studied versus tube diameter and fin temperature variation. [ABSTRACT FROM AUTHOR]

Abstract A detailed numerical investigation is carried out in this work to understand the effect of atmospheric wind on the heat transfer in natural convection based solar air heaters. A 3D inclined rectangular channel has been considered for the numerical simulations. The top wall of the channel is maintained at constant heat flux condition, and other walls of the channel are assumed to be adiabatic in all simulations. The effect of surface radiation between the channel walls and channel walls to the atmosphere is also considered. All the inner surface of the walls of the channel are assumed to be black and the emissivity (ε) of the plates are taken to be 0.95. Numerical simulations are carried out by varying the values of heat flux, q" = 250, 500 and 750 W/m2 , velocity of the atmospheric wind V wind = 0.0, 0.2, 0.6, and 1.0 m/s, and the inclination angle of the channel θ = 15°, 30°, 45°, and 60°. In the parametric study, various directions of free stream flow are also considered such as cross flow over the channel, in the main flow direction of the channel and the diagonal to the channel. In this paper, it is found that external wind imparts significant effects on the heat transfer and flow pattern of the air inside the solar air heater channel. Highlights • Atmospheric wind effect on natural convection based solar air heater is studied. • Atmospheric wind effect is significant at high angle of inclination of the channel. • Effect of surface radiation is significant between the walls of solar air heaters. [ABSTRACT FROM AUTHOR]

Abstract Natural convection heat transfer in closed enclosure exists in a wide range of engineering applications. Although huge researches were conducted for natural convection in non-circular enclosures, researches for annular enclosures are very limited, especially finned enclosures. In the present results natural convection air flow and heat transfer characteristics in un-finned and finned annulus enclosed between two concentric horizontal cylinders are numerically investigated. The effects of the Rayleigh numbers, annulus width, fins geometrical dimensions, fins numbers, fins shapes and fins arrangements on the heat transfer, fluid flow and effective thermal conductivity of the annulus were investigated. The results show the increase of the heat transfer and the effective thermal conductivity of the annulus with the increase of the annulus thickness, Rayleigh number, number of fins and the fins width. For the same fins areas, the longitudinal fins showed more heat transfer rate and effective thermal conductivities enhancements compared to the annular fins. Highlights • The increase of the effective thermal conductivity of the annulus with the annulus thickness and Rayleigh number. • The presence of the annular fins or rectangular fins on the annulus surface enhances the heat transfer. • The effective thermal conductivity increases with the number of fins and the fins width. • The longitudinal fins showed more effective thermal conductivities enhancements than the annular fins. [ABSTRACT FROM AUTHOR]

Abstract Convection of a magnetic fluid within a perspex container was investigated experimentally and complemented by a computational Finite-Element model built according to the same physical specification. The enclosure was heated at two opposite side walls and exposed to a magnetic field provided by a Neodymium-Iron-Boron permanent magnet placed either above or below the container. The spatial temperature distribution on the front side wall of the container was recorded via infrared thermography (IR) and compared to computational results that reproduced the spatial temperature fields. The results show a significant effect on heat transfer by the location of the permanent magnet and gave evidence that the Kelvin body force can be much stronger than buoyancy. As both body forces are temperature sensitive an increase in temperature difference increased both, buoyancy and Kelvin body force, albeit with a different intensity that was explained via Curie's Law and expressed as a temperature dependent magnetisation through the pyromagnetic coefficient, K. The heat transfer was characterised by the Nusselt number and a suitable modified Rayleigh number that took the orientation of both buoyancy and Kelvin body force in account. The degree of heat transfer enhancement reported varied between a 23% reduction to a 20% enhancement. [ABSTRACT FROM AUTHOR]

Abstract In this paper, magnetohydrodynamic buoyant flow of liquid metal in a cubical enclosure is studied for low to high Hartmann numbers at various Rayleigh numbers with, and without, the use of wall-function treatment. The numerical calculations have been carried using a 3-D MHD numerical code developed in-house by our research group. We also consider the conjugate thermal and electromagnetic problem for flow-wall coupling and compute the conduction heat transfer and electric potential field in the walls. The numerical calculations are performed for Hartmann numbers of 100 − 5000 , Rayleigh numbers of 10 4 − 10 6 and wall conductance ratios of 0 − ∞ at P r = 0.01. It is observed that an increase in the applied magnetic field decreases the flow velocity and Nusselt number. At R a = 10 4 , Hartmann numbers beyond 200 cause the complete suppression of the core velocity. For higher R a , complete suppression is found only at increased values of H a (H a = 500 and H a = 1000 for R a = 10 5 and R a = 10 6 respectively). At higher H a , the heat transfer is mostly purely due to conduction and the Nusselt number is close to unity. For a fixed H a and R a , it is seen that an increase in wall conductivity decreases convective motion and the average Nusselt number, as the walls become more conducting. The effect of the magnetic field applied parallel to the temperature gradient is more pronounced over the other orthogonal directions because it directly acts against the buoyancy force. Highlights • A robust 3-D MHD code developed by our research group is used for simulations. • Wall-function treatment is used for modeling the Hartmann layers. • The convective motion is completely suppressed at large magnetic field strengths. [ABSTRACT FROM AUTHOR]

Abstract In this article, we present application of the Cascaded Thermal Lattice Boltzmann Method (CTLBM) in simulations of natural convection in differentially heated square cavity with adiabatic top and bottom walls. This classical benchmark problem is solved for wide range of Rayleigh numbers ( 10 6 – 10 10 ) and compared with data from the literature. For high Rayleigh numbers we present comparison of Nusselt number and wall shear stress distributions along hot wall with experimental and direct numerical simulation (DNS) data. Results for Rayleigh numbers up to 10 6 are also compared with previous results obtained by MRT-LBM simulations of Wang et al. The results are in good agreement with the existing ones obtained numerically and experimentally. Highlights • The cascaded thermal lattice Boltzmann method (CTLBM) is applied to natural convection heat transfer. • A wide range of Rayleigh numbers 103–1010 is considered. • In-depth and quantitative analysis are presented and compared with various LBM, experimental and DNS results. • Nusselt number and wall shear stress distributions have been computed for very high Rayleigh numbers. • The results are in strong agreement with the existing experimental and numerical results. [ABSTRACT FROM AUTHOR]

Abstract Three dimensional numerical simulations have been carried out by solving the continuity, momentum and the energy equations to predict the flow and the temperature field around an internally finned horizontal cylinder in natural convection. The fin height, fin spacing or the fin number and the length of the cylinder were varied at different Ra to predict the heat loss from the finned cylinder and interesting findings were obtained. The heat loss from a short cylinder (L/D < 1) can be maximum for certain height of the fin and certain fin number after which the heat loss can reduce if the fin height increases or the fin spacing decreases in the laminar range of Ra to within 107. When the length of the cylinder increases (L/D > 1) the maximum point of heat loss vanishes for all heights of the fins and fin spacing. A table has been generated to show the optimum configuration of fin height, fin number where maximum heat loss can take place. The average Nu of the finned cylinder was seen to be decreasing with H/D, L/D and increasing with Ra. From the enormous numerical simulations that has been done in the present study a general correlation for the Nu as a function of L/D, H/D, fin number, N and Ra has been developed to within an accuracy of ± 6 % which can be beneficial to the industry and practical designers. Highlights • The present work captures the phenomenon of natural convection heat transfer from an internally finned horizontal cylinder. • The heat loss from a short cylinder (L/D < 1) can be maximum for certain height of the fin and certain fin number. • The average surface Nu of the finned cylinder was seen to be decreasing with H/D, L/D and increasing with Ra. • A table has been generated to show the optimum fin configurations where maximum heat loss can take place. [ABSTRACT FROM AUTHOR]

Abstract The study describes natural convection through a vertical channel, open on four sides and bound by two isothermal walls. Both balancing and infrared experimental investigations were performed in air ( Pr = 0.71) to estimate the impact of channel width s and wall-to-ambient temperature difference ( t w - t ∞) on natural convective heat transfer, the formation of air flow patterns and the rate of flow pattern formation. The study was conducted on two parallel vertical plates of height H = 0.5 m and width B = 0.25 m, with the heated surfaces facing each other, thus creating peripherally open channels of different widths s = 0.045, 0.08, 0.180 and ∞ m. The surface temperature t w , identical for both heating plates, was changed every 10 K and set at t w = 40, 50, 60, 70 and 80 °C, while the ambient temperature was maintained within the 18–25 °C range. In the balance method, heat fluxes were determined based on measurements of voltage and electric current supplying the heaters placed inside the walls. In the gradient method, the heat fluxes were calculated from the temperature distribution in air, within a plane perpendicular to the heating plate surfaces. Temperature fields were visualized using a plastic detecting mesh and a thermal imaging camera. The distribution of temperature t x,y , and its gradient at the walls d t /d x | x =0, y were obtained at different heights y along the channels. The gradient values obtained and the results, presented as dependencies of the Nusselt number on the Rayleigh number, indicate that the channel width has a significant impact on heat transfer. Compared to the vertical plate s = ∞, the following levels of convective heat transfer enhancement were observed: 29.5% ( s = 0.045 m), 38.8% ( s = 0.085 m) and 61.6% ( s = 0.180 m). Highlights • Analysis of optimum channel width for natural convective heat transfer has been conducted. • Gradient method with the use of a thermal imaging camera has been utilized. • This study shows that ∂t/∂x│ x=0,y distributions along the channel and Nu ( Ra ) can be obtained. • The channel width was found to be correlated to the intensity of convective heat transfer. • This method may prove useful in practical applications, e.g. multi-plate convectors. [ABSTRACT FROM AUTHOR]

An analytical and numerical study of laminar convection in a horizontal cylindrical enclosure heated from below is performed. It is found analytically that uniform outer magnetic field can cause instability of the equilibrium state of magnetic nanofluid or influence gravity convection. Particular attention is paid to the steady convection, structure transients and the onset of unsteadiness. The Prandtl number from 0.7 to 700, different Rayleigh numbers (up to 10 6 ) and magnetic Rayleigh number are considered. For a nonmagnetic fluid, we studied the influence of the Prandtl number on the Rayleigh number values for which the equilibrium state becomes unstable, convective flow becomes bi-cellular, or becomes nonstationary. The effect of the magnetic Rayleigh number and the thermal conductivity of the solid block surrounding the enclosure on the onset of the convection is investigated. It is found that the magnetic permeability of the magnetic fluid filling the enclosure is an important factor. Four modes of fluid flow are studied analytically and then two of them are demonstrated during numerical simulation. It is shown that the outer uniform magnetic field turns to nonuniform in the magnetic fluid heated from below with not only the vertical, but also with the horizontal components of the gradient. So, there is a weak no-threshold convective flow even for very small magnetic Rayleigh numbers. The influence of the convective flow on magnetic field structure in the volume of a non-conducting magnetic fluid is found. Critical temperature difference for the experiment is predicted. [ABSTRACT FROM AUTHOR]

We report direct numerical simulations of the fully developed turbulent natural convection flow of a Boussinesq fluid (Pr = 0.71) in a vertical channel. One wall is heated with a constant heat flux and the other is assumed to be perfectly insulated. Simulations were performed at three different Rayleigh numbers (R a = 5 × 105, 106 and 5 × 106). Predictions of the time averaged velocities, temperatures and intensities of the fluctuations agree with experiments reported in the literature. Under the fully developed flow conditions considered in this study the shear stress is greater on thermally active wall than that on the adiabatic wall, but the turbulent intensities and the turbulent shear stress are greater near the insulated wall. Conversely, the intensities of the temperature fluctuations and turbulent heat fluxes are higher near the thermally active wall than those near the adiabatic wall. Contrary to the vertical channel with constant but different wall temperatures, where buoyancy and turbulent shear dominate the generation of turbulent kinetic energy, in the asymmetrically heated channel the turbulent shear stress is the principal mechanism of turbulent kinetic energy production as in a forced convection channel flow. [ABSTRACT FROM AUTHOR]

In the present study, the migration and deposition of multi-walled carbon nanotube (MWCNT)-Fe 3 O 4 nanocomposite particles in a square enclosure for natural convection hybrid nanofluid flow are analysed numerically using the Eulerian–Lagrangian model with one-way coupling. The governing equations are solved by the finite volume method and Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm. The location and velocity of 3000 selected composite particles in the host nanofluid are obtained by employing the Lagrangian approach. The influences of Rayleigh number and nanoparticle volume fraction on the thermal and flow behaviours of the system are presented. It is observed that the increasing volume fraction leads to a rise in the average Nusselt number at low Rayleigh numbers, while the case of ϕ =0.1% is the optimum volume fraction of nanoparticle in terms of heat transfer enhancement and strength of flow structure at R a ≥ 1 0 4 . The simulations of Lagrangian particle tracking demonstrate that the minimum deposition rate of composite particles with d p =0.5 μ m is obtained for a given Rayleigh number. The number of deposited large composite particles for the case of ϕ =0.1% is slightly more than for ϕ =0.3%, while the deposition rate of ultra-fine composite particles gets a higher value in the case of ϕ =0.3%. Moreover, the thermophoresis force plays a more significant role in small particle deposition than the Brownian random force. [ABSTRACT FROM AUTHOR]

This study experimentally investigates the supercooling and natural convection behavior in three PCM, namely pure octadecane (organic), a eutectic PCM, and sodium acetate trihydrate (inorganic). The aim is to explore the effect of different factors on the degree of supercooling and on the natural convection, and to provide data for numerical simulation models. Results show that the type of PCM, the temperature of the cooling fluid, and mechanical shocks have a direct effect on the degree of supercooling. The supercooled PCM shows a high sensitivity to any changes in the applied conditions. The thermal insulation of the unheated PCM boundaries has a major effect on the solidification and melting dynamics and topology, particularly regarding the solid-liquid interface. The cross-sectional temperature profiles, the inclination of the solid-liquid interface, and the melting rate show that regardless of the PCM, natural convection plays a significant role during heating but not during cooling. [ABSTRACT FROM AUTHOR]

Molten salt is a promising heat transfer fluid with its high heat capacity, high boiling point, and chemical stability. In liquid fueled molten salt reactors and spent fuel reprocessing unit, molten salt carries heat generating fuel itself in a liquid state. To investigate the natural convection heat transfer performance of molten salt, a vertical heater vertical cooler rectangular loop filled with chloride salt containing liquid fuel, NaCl-KCl-(Pu, 238 U)Cl 3 , is simulated using an open source CFD package, OpenFOAM under various heat input conditions; external heat flux, internal heat generation (IHG), and a combined heat input type condition. Different aspects of natural convection heat transfer according to heat input condition are found for same amount of total heat. The external heat flux condition generates a high temperature gradient near the heater wall with very thin thermal boundary layer due to the low thermal conductivity of molten salt. On the other hand, the IHG condition has a relatively flat temperature profile from its uniform volumetric heating. Due to the different behaviors of temperature evolution, local upward flow is formed only at the wall region under the heat flux condition, but global natural circulation flow is promoted under IHG condition. The combined type condition is largely dependent on how much heat exists in heat flux type, which strongly influences local heat and fluid phenomena in natural convection heat transfer. Molten salt has a high dependence on the heat input condition from its thermo-physical properties. Therefore, the distinct characteristics of molten salt and heat input conditions should be considered, when designing and analyzing the molten salt systems with both external and internal heating conditions. [ABSTRACT FROM AUTHOR]

The aim of this work is to obtain experimental information on the thermal and fluid dynamics of a distribution transformer. The transformer of interest is of the oil natural – air natural type and our interest is centered in the natural circulation that is produced in the oil volume. An experimental device was built to simulate a representative slice of the transformer and a purpose built PIV setup was used to obtain the flow velocity field. The device overall dimensions and the refrigerating oil are the same as those of commercial transformers of 1000 KVA (Tubos Trans Electric S.A. 1000 KVA Standard Distribution Transformer) Natural circulation is produced by a heating element simulating the heat produced by copper losses in the transformer. A double glass window allows visual access for flow visualization and PIV capture. The slice comprises two fins and the air side is bounded by two isolating boards, forming three air channels. Two reference powers were tested for the experiment: The power corresponding to design operating conditions and a lower power to observe the sensibility to power variations. The result reported in this work shows features of the flow pattern that give important information on the sort of turbulent mixing to be expected. The transient evolution was measured and the thermal characteristic time of the system was estimated. It was found that the convection during the start-up transient is significantly stronger than during steady state operation. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical analysis of transient conjugate heat transfer in a double space gas filled enclosure with conducting solid walls has been carried out. This configuration can be found in cooling process of some electronic equipment in which the generated heat must be removed to avoid failure. Natural convection equations in the enclosures are solved in terms of the dimensionless stream function and vorticity using finite-difference technique. The discrete ordinates method (DOM) is used to solve the radiative transfer equation (RTE) in the filling gases which are considered to be absorbing, emitting and scattering. The effects of Rayleigh number and optical thickness of two radiating media in transient cooling process from the hot surface toward the cold one are investigated. The transient results show that when the optical thickness of the gas layer closed to the heat source is lower than the second layer, more heat penetrates into the enclosure which consequently leads to higher cooling performance and lower rates of thermal failure. [ABSTRACT FROM AUTHOR]

Fog is omnipresent in the natural environment, namely valleys and mountains. A strong comprehension of the fluid flow dynamics pertaining to fog formation is of crucial importance. In the present study, the two-dimensional numerical method is used to investigate the transient natural convection in a valley-shaped triangular cavity initially filled with stratified water. A wide range of Rayleigh numbers (2.26 × 10 5 –2.26 × 10 9 ) and aspect ratios (0.1–1.0) are considered. The numerical results are verified against experimental results. The development of natural convection flows in the cavity from the start up to the steady state is classified into two stages: an early stage and a transitional stage. Transient natural convection flows in the cavity are described. Spectral analysis is performed for different governing parameters. A simple scaling analysis is performed for the thermal boundary layer and the time scale of the stratification breakup describing the disappearance of fog in the valley is obtained and validated by numerical results. Additionally, mass and heat transfer in the cavity is measured and the scaling relation between the Nusselt number and the Rayleigh number for different aspect ratios are presented. [ABSTRACT FROM AUTHOR]

In this work, a scale analysis is applied over the two-dimensional steady flow of stable-stratified fluid in a lid-driven square cavity. Scales for the viscous and thermal boundary layers are pursuit, and adjusted with numerical results. The flow parameters investigated covers the range 10 3 ≤ G r < 10 6 , 100 ≤ R e ≤ 2500 , and 10 − 1 ≤ P r ≤ 10 2 . The outcome of the scale analysis is an expression for the surface-averaged Nusselt number which is capable of predicting the heat transfer within a 10% error range. [ABSTRACT FROM AUTHOR]

This study aims at investigating numerically the Rayleigh-Bénard Convection (RBC) in viscoplastic fluids. A Casson fluid is considered in a bidimensional square cavity heated from below. The effects of the dimensionless yield stress, the Bingham number B n , on the heat transfer and motion is investigated in the range 5.10 3 < R a < 10 5 for the Rayleigh number and P r = 10 , 100 , 1000 for the Prandtl number. One shows that the yield stress has a stabilizing effect, reducing the convection intensity. Above a certain value of B n m a x , the convection does not occur and the heat transfer is only due to conduction. For moderate B n values, truly unyielded regions are located in the center of the cavity; their areas grow with increasing Bingham number and invade the whole cavity at the threshold value B n m a x . [ABSTRACT FROM AUTHOR]

The three-dimensional characteristics of natural convection from isothermal vertical plate arrays with finite lengths are numerically analyzed. Studied are single- and multi-channel plate arrays with different plate length/height ( L / H ) ratios. To account for the extra plume buoyancy and the entrance flow resistance, extended computation domains encompassing the whole plate arrays are arranged. Highly non-uniform distributions of heat transfer coefficient exhibit along the plate, with the highest value at the plate end due to the effect of side flow. Inward, the heat transfer coefficient decreases drastically with boundary layer development but re-rises over the inner region as a result of higher plume buoyancy therein. As L / H increases, the stronger plume buoyancy promotes the overall heat transfer coefficient to approach the 2-D value with a difference of less than 4% when L / H  ≥ 2. The extra plume buoyancy varies in different channels, weakest in the edge channel, leading to complex flow above the outermost channels. The plume-buoyancy enhancement on the overall heat transfer coefficient is less pronounced for arrays with finite L / H ratios than for a 2-D array due to the plume shrinkage associated with the side flow. [ABSTRACT FROM AUTHOR]

The gaseous low-pressure flow of a steady-state two-dimensional laminar natural convection heat transfer in a cavity with an attached two solid fins to the hot wall is numerically investigated. Such flows can be found in many engineering applications such as the “evacuated” solar collectors and in the receivers of the solar Parabolic Trough Collectors (PTCs), nuclear reactor cooling and electronic equipment cooling. Buoyancy effect is modeled using Boussinesq approximation. Effects of Knudsen and Rayleigh numbers, location and length of the fins, conductivity ratio and fins porosity, and the tilt angle on the flow and heat transfer characteristics are investigated. Physical parameters ranges in this study are as follows; 0 ≤ K n ≤ 0.1 , 10 3 ≤ R a ≤ 10 6 , L F takes the values of 0.25, 0.5 and 0.75 m, H F takes the values of 0.25_0.5, 0.25_0.75 and 0.5_0.75, 1 ≤ K r ≤ 8000 , 0 ≤ ε ≤ 1 and 0 ∘ ≤ θ ≤ 90 ∘ . It is shown that Nusselt number depends directly on Rayleigh number and inversely on Knudsen number. In addition, it is found that attaching two solid fins to the hot wall will enhance the heat transfer for such flows. Moreover, it is found that the porous fins are superior to the solid fins as far as the heat transfer is concerned. In addition, it is shown that for the geometry where the lower fin height to cavity length ratio is 0.25 and the upper fin height to cavity length ratio is 0.75, the best heat transfer is achieved. In addition, for the fin lengths considered in the study, it is concluded that by increasing the length of the fin, a better heat transfer is achieved. Also, it is found that by increasing the tilt angle of the cavity, a better heat transfer is achieved up to a certain value which was found to be (30°) where beyond this value, Nusselt number will decrease. In addition, it is found that by increasing the conductivity ratio up to a certain value (10 3 ), better heat transfer is gained. Finally, a correlation among Nusselt number and the parameters investigated in this research proposed as N u = 0.02197 ε 0.00932 sin ( θ ) 0.00373 K r 0.0273 ( h 2 / L F ) 2.814 R a 0.13 ( ( h 2 − h 1 ) / L F ) 1.373 ( h 1 / L F ) 1.615 K n 0.37 . [ABSTRACT FROM AUTHOR]

This article contains numerical simulations of the influence of external magnetic field on free convection heat transfer in ferrofluid flow through a square cavity filled with water based ferrofluid when a heated square blockage with different aspect ratios (0.25 ≤ A/H ≤ 0.5 ) is placed at the centre of enclosure. Vertical boundaries of enclosure are assumed insulated, top horizontal wall is taken as cold while bottom horizontal wall is heated uniformly. The square obstacle is present at the centre of cavity which also serves as heat source in fluid. The mathematical model is presented in the form of nonlinear PDE's, which are solved using Galerkin finite element method. Results are shown for wide ranges of physical parameters like Rayleigh, Prandtl and Hartmann numbers etc. The heat and flow structures are noticed to be significantly dependent on strength of magnetic field, Rayleigh number and concentration of ferroparticles present in the base fluid. [ABSTRACT FROM AUTHOR]

A warming room air in a cold weather is an urgent need for people to keep performing normal activities in a comfortable environment. In the current study, a heater attached to one of the room's walls induces the natural convection flow. The flow is simulated by a turbulent unsteady compressible Navier-Stokes solver at a Rayleigh number of 3.06 × 10 11 . Two cases are investigated and their boundary conditions are identical except the right side where is adiabatic or isothermal. Although the adiabatic case had a significant importance in the domestic heating, it wasn't investigated in the literature. The transient results show that the hotter streams occupy the top zone where the velocity is relatively high and then move downward. As a whole, the average temperature of the adiabatic case is higher than that of the isothermal case, but the isothermal case has a higher temperature gradient. [ABSTRACT FROM AUTHOR]

This experimental work investigates buoyant flow in a differentially heated vertical channel located inside a water cavity. The flow is found to be highly unsteady, and the key aspect of this study is to consider this unsteady behavior as a succession of states that turn out to be driven by the flow outside the channel. A conditional mean operator with respect to the average wall temperature is used to disentangle the different states through which the flow passes. Most of these states are characterized by a transition from laminar heat transfer in the bottom part of the channel to turbulent heat transfer, with a transition point that moves toward the exit as the average wall temperature increases. For the highest values of the average wall temperature, no transition is observed, and the heat exchange is found to be similar to that along a single vertical plate. For an intermediate range of wall temperature, a transition zone with turbulent heat transfer is observed in the upper part of the channel, and the heat transfer is found to follow the same laws as found for a symmetrically heated channel. For the lowest values of the wall temperature, the beginning of the turbulent zone is observed near the entry. The analysis is extended to several channel widths. The origin of the unsteady behavior is attributed to the flow in the whole cavity, and the conditional mean operator allows characterization of the flow inside the channel independently from the flow in the surroundings. [ABSTRACT FROM AUTHOR]

Numerical investigation of a natural convection flow with a localized thermal disturbance is presented here. The configuration of a differentially heated cavity is considered with air as working fluid. In this study, the initial flow regime is steady and close to the transition to unsteadiness. Different cases of local wall temperature disturbances are tested in a thin area at the onset of the hot vertical boundary layer. These cases are distinguished by the time-averaged temperature of this area (higher or lower than the hot wall temperature) and by their temporal evolution (steady or periodic). In cases with a periodic disturbance, the imposed frequency corresponds to the first frequency emerging in the unsteady regime. The results show the spread of temperature fluctuations across the whole cavity with the periodic disturbance, in particular near the upper corner of the cavity. The hot steady disturbance trigs the onset of time-dependent flow. Moreover, the local modification of the hot wall temperature has an impact on global heat transfer: about 16% downstream the disturbance area and about 2% on the opposite cold wall. [ABSTRACT FROM AUTHOR]

This paper details a numerical study of solid-liquid phase change heat transfer in a rectangular enclosure with the aim of optimizing the number, dimension, and positioning of fins in the enclosure. The enclosure is exposed on one side to a constant heat flux of 1000 W/m 2 for 3 h and both the total fin mass and the PCM mass are kept constant in all simulated cases. The heat diffusion equation in the PCM uses the equivalent heat capacity method while natural convection driven PCM motion in the enclosure is modeled through the buoyancy force, using a modified viscosity and an additional volume force term in the Navier-Stokes momentum conservation equation. Three efficiency assessment parameters are used: (i) the height-averaged temperature of the front hot plate which should remain as low as possible for the longer possible time, (ii) the energy stored inside the PCM as a function of time, and (iii) the heat transfer rate and heat flux between the front plate and the PCM. For each simulated case, complementary data such as the standard deviation of temperature along the heated plate and the ratio of sensible heat to latent accumulation are also calculated. Results show that increasing the number of fins diminishes both the stabilization temperature and the stabilization time of the front plate during phase change, and accelerates the sensible and latent storage of energy in the PCM. Using thinner but longer fins provides the same impact except that the stabilization time also increases as the fin length is increased. Besides, at constant fin mass, varying fin spacing have marginal impact on the latent heat storage performance and on the hot plate temperature stabilization. The study also shows that the surface area plays the largest role in the increase of the heat transfer rate between the front plate and the PCM, while configurations which can promote stronger natural convection lead to higher heat flux. Finally, it was observed that systems allowing easier heat transfer to the back plate during melting provide a higher ratio of sensible heat to latent accumulation, while the standard deviation of the front plate temperature decreases as the number of fins or the spacing between fins increases. [ABSTRACT FROM AUTHOR]

Natural convection and radiation in rectangular cavities with aspect ratios of 1, 2.09, 3, 4, 5 and 6 has been studied. All six cavities have a height of 340 mm and a depth of 210 mm. The cavities have different lengths. The cavity is closed, filled with air, and has one active vertical wall. The opposing vertical wall is inactive. The other four walls are adiabatic. Experiments are performed to study the heat transfer in the cavity. Thermocouples are used to measure the temperature. In each cavity case, the temperature distribution between the vertical walls of the cavity is obtained at 35 positions in the length, seven positions in the height and one position in the depth directions. Temperature field is presented at the mid-depth of the cavity. In the central region of the cavity, the temperature is nearly constant, and the fluid is almost stagnant. Heat transfer correlation is obtained for the Rayleigh number range of 1.60 × 10 5 to 4.67 × 10 7 , and the aspect ratio range of 1 to 6. The variations of the temperature profile and the local Nusselt number along the cavity height are presented. [ABSTRACT FROM AUTHOR]

This paper presents the results of numerical modeling of the evolution of thermal plumes in a fluid layer heated locally from below. Special attention is paid to the development of a boundary layer that precedes thermal plume generation. Equations for natural convection are solved using the ANSYS Fluent software package. The calculated temperature fields are compared with the results of a full-scale experiment. The variation of parameters of the numerical model gives two mechanisms of heat transfer from a local heat source – conductive and convective. In the first case, uniform heating of fluid above the heater surface leads to the formation of a single plume. In the second case, the boundary layer near the heater edges experiences deformation that causes a convective jet to bifurcate. The purpose of the present work is to study the effect of heat source shapes (triangular, square, pentagonal and circular) on the boundary layer structure. For each heater shape, the critical value of the Rayleigh number determined. It is shown that bifurcation of a thermal plume is initiated when the Rayleigh number exceeds a critical value. [ABSTRACT FROM AUTHOR]

Numerical model for heat transfer through air gap in thermal protective clothing was coupled with bio-heat transfer model for multi-layer human skin. Coupled CFD-radiation heat transfer model was used for heat transfer through air gap and Pennes' bio-heat transfer model was used for heat transfer through human skin. First and second degree skin burn times were calculated using Henriques' burn integral. Effect of air gap width and air gap orientations were analyzed on heat transfer through air gap and skin burn injuries. Significant impact of air gap width and air gap orientations were observed in first and second degree burn times. First and second degree burn times and hence the thermal protection were found to be higher in case of vertical air gap orientations as compared to horizontal air gap orientations. Air gap orientation influences the thermal protection more at higher air gap widths. Analysis were also performed for determining outcome of heterogeneous air gaps on heat transfer through air gap and protective performance. Heterogeneous air gap cases results in more heat transfer and lower thermal protection as compared to homogeneous air gap cases for both the air gap orientations. Further, effect of fold aspect ratio was also analyzed on first and second degree burn times in case of the heterogeneous air gap. [ABSTRACT FROM AUTHOR]

The instability of natural convection in a vertical porous layer is analysed. The plane parallel boundaries of the vertical layer are modelled as open and subject to Robin-type temperature conditions. The latter conditions describe heat transfer to the external environment, with a finite conductance. The basic state is given by a stationary fully-developed flow with linear velocity and temperature profiles. Instability arises when the Darcy-Rayleigh number exceeds its critical value. This value depends on the Biot number associated with the temperature boundary conditions. The most unstable normal modes turn out to be transverse. By solving numerically the stability eigenvalue problem, it is shown that the critical Darcy-Rayleigh number is a decreasing function of the Biot number when the Biot number is sufficiently small. For larger Biot numbers, a minimum is attained, and then the critical Darcy-Rayleigh number becomes an increasing function of the Biot number. [ABSTRACT FROM AUTHOR]