Natural Convection Phenomena Research Articles

Determination of Heat Transfer Coefficient from Housing Surface of a Totally Enclosed Fan-Cooled Machine during Passive Cooling

This paper presents the analytical calculation of the heat transfer coefficient of a complex housing shape of a Totally Enclosed Fan-Cooled (TEFC) industrial machine when it works below 20% of its nominal speed or close to stall. Therefore, passive cooling is dominant, and most of the heat is extracted by the combination of natural convection and radiation phenomena. Under these conditions, the area-based composite approach was used for the development of the analytical calculation method. A test rig using a TEFC Synchronous Reluctance Motor (SynRM) was constructed, and the collected experimental data was used to validate the proposed analytical method successfully.

Computational Performance of Disparate Lattice Boltzmann Scenarios under Unsteady Thermal Convection Flow and Heat Transfer Simulation

The present work highlights the capacity of disparate lattice Boltzmann strategies in simulating natural convection and heat transfer phenomena during the unsteady period of the flow. Within the framework of Bhatnagar-Gross-Krook collision operator, diverse lattice Boltzmann schemes emerged from two different embodiments of discrete Boltzmann expression and three distinct forcing models. Subsequently, computational performance of disparate lattice Boltzmann strategies was tested upon two different thermo-hydrodynamics configurations, namely the natural convection in a differentially-heated cavity and the Rayleigh-Bènard convection. For the purposes of exhibition and validation, the steady-state conditions of both physical systems were compared with the established numerical results from the classical computational techniques. Excellent agreements were observed for both thermo-hydrodynamics cases. Numerical results of both physical systems demonstrate the existence of considerable discrepancy in the computational characteristics of different lattice Boltzmann strategies during the unsteady period of the simulation. The corresponding disparity diminished gradually as the simulation proceeded towards a steady-state condition, where the computational profiles became almost equivalent. Variation in the discrete lattice Boltzmann expressions was identified as the primary factor that engenders the prevailed heterogeneity in the computational behaviour. Meanwhile, the contribution of distinct forcing models to the emergence of such diversity was found to be inconsequential. The findings of the present study contribute to the ventures to alleviate contemporary issues regarding proper selection of lattice Boltzmann schemes in modelling fluid flow and heat transfer phenomena.

Investigation of Microclimate Ventilation of Simulated Garment in Terms of Wind Speed and Air Gap Thickness

During the circulation of air in the garment’s microclimate, the body extra heat dissipates to the environment. This event is an important factor in thermal comfort and ventilation. In this study, a heating cylinder was designed and calibrated to assess the ventilation occurrences in garment as a simulator of the human body. Tubular samples with different sizes were prepared using double jersey weft knitted fabric in order to obtain five levels of air gap thickness, including −1.5, 0, 1, 1.5 and 2 cm. Experiments were carried out in two conditions including without wind and by wind blowing with eight different wind speeds include 0.5, 1, 1.5, 2, 2.5, 3, 5 and 7 m/s. In each condition, the air gap’s temperature was considered as the main parameter for examination of the microclimate ventilation. The obtained outcomes revealed that by increasing the air gap thickness to more than 1 centimetre, the ventilation rate increased significantly. In the absence of wind, the change in the air gap’s temperature was not considerable for all sizes due to the natural convection phenomena that were slow. An increase of wind speed to more than 1 m/s led to a rise in the forced convection heat loss and took less time for the system to reach the equilibrium temperature; hence, higher ventilation rate was achieved. Moreover, by the increment of wind speed, the reduction of microclimate temperature follows a power function and at higher wind speeds remained approximately constant. According to the statistical analysis of results, it was confirmed that the ventilation rate of garment was influenced by wind speed; however, the influence of air gap thickness only in the absence of wind is significant.

Role of microstructure and composition on natural convection during ternary alloy solidification

Solidifying ternary systems can exhibit complex natural convection phenomena, particularly due to the presence of two porous zones (cotectic and primary mush), and the rejection of two differently dense solutes. The primary objectives of this study are to investigate the following: (i) the natural convection patterns in various compositional regimes of a typical ternary system, and (ii) the role of the combined existence of the microstructure (facets and dendrites) in the porous zone on natural convection, with a motivation to enhance the current understanding of the microstructure–convection relationships. A ternary mixture is chosen such that different compositions of the three primary solidifying components lead to the formation of distinct ice, dendritic and faceted solid structures that cover the complete span of microstructure–convection relationships. The observations of flow in different compositional regimes show convection occurring in the form of plumes, random mixing and double-diffusive layering, as well as combinations of these, which are governed by the type of coexisting microstructures. The study reveals the occurrence of Rayleigh–Taylor instability with varying amounts of the heavier component. The bulk liquid composition showed a tendency to cross the cotectic line, and thus also change the nature of primary solidifying structure from faceted to dendritic in cases where facets and dendrites were present in cotectic mush, and facets in primary mush. These insights are believed to elucidate the complex mechanisms of ternary solidification, as well as provide important real-time data for direct numerical simulations.

Natural Convection Heat Transfer Characteristics of the Annular Fins on the Measuring and Monitoring Devices

The paper is based on the observations made in computational fluid dynamics (CFD) modelling of the natural convection phenomena in the annular fins on measuring or monitoring device along with its validation through the experimentation. Navier Stokes equations (N-S) and energy equations (in 3-D) have been fully numerically simulated for full scale monitoring device using multi grid solver of FLUENT 16.0. The experimentation was done to test the numerical simulations and correctness of the flow model. The CFD results were found to be matching with experimental results with some tolerance. Increasing the fin diameter and or thickness results in increasing surface area and volume of the fins which is equivalent to addition of thermal mass. The thermal mass is chunk of a material where some thermal energy gets accumulated. The variation of temperature and physics of the whole phenomenon is discussed using the variation of Nusselt number (Nu) and heat transfer coefficient (h) with respect to non-dimensional diameter(d*) and non-dimensional thickness(t*) along with role of thermal mass. Non-dimensional diameters in the range of 1.5–3.0 and non-dimensional thickness in the range of 0.06–0.16 were tested. Finally, it is concluded that the effect of addition of thermal mass is beneficial in case of increasing the fin diameter but the same is not beneficial in case of increasing fin thickness.

Lattice Boltzmann Modeling for Natural Convection in Power-Law Fluids Within a Partially Heated Square Enclosure

Abstract This paper reports the numerical investigations for steady-state natural convection in power-law fluids inside a square enclosure embedded with bottom discrete heaters. The lattice Boltzmann method (LBM) is employed to model the flow and heat transfer phenomenon at different combinations of power-law index, Rayleigh number, and heat source length for a constant Prandtl number. The buoyancy force is incorporated in the collision term of the LBM through Boussinesq approximation. Simulation outcomes are furnished using streamlines, temperature contours, velocity profiles, and variation of heat transfer on the nonadiabatic walls to probe natural convection phenomena. The results indicate that the temperature and the flow fields in the enclosure are symmetric about the vertical centerline. The detailed physical interpretations have been provided for the reported results. Further, the increase in the power-law index means a rise in viscosity and a decrease in thermal energy transport for other constant parameters. The outcomes also specify that the intensity of circulation and heat transfer enhances with the increase of Rayleigh number and size of the localized heater. Finally, though, a rise in the size of the confined heat source enhances the rate of total thermal transport, it does not change the trend of fluid flow and local heat transfer rate.

3D Modeling and optimization of SiC deposition from CH3SiCl3/H2 in a commercial hot wall reactor

In this paper, the 3D modeling and optimization of a commercial hot wall vertical reactor for SiC coating is presented to investigate the effect of various process parameters on the hydrodynamic stability of the CVD reactor. The correlation between experimental and simulated results was established by tuning the kinetic parameters for the surface reaction. Besides, the incorporation of various dimensional numbers such as Reynolds number (Re), Péclet number (Pe), and Grashof number (Gr) enabled the systematic investigation of the effect of the natural convection phenomena on film growth performance. It was found that the buoyancy-driven flow can occur inside the reactor at a high Reynolds number and Gr/Re2 ratio. The process optimization was performed using response surface methodology (RSM) to obtain the desired film quality. The CFD-RSM collaborative approach allowed a significant reduction in the number of experiments and simulations required to optimize the CVD process.

Experimental comparison of specific heat capacity of three different metal oxides with MWCNT/ water-based hybrid nanofluids: proposing a new correlation

The main objective of this study is to investigate three distinct non-commonly used metal oxide-based nanoparticles, i.e., CuO + MWCNT, MgO + MWCNT, and SnO2 + MWCNT with a weight mixture ratio of 80:20 each (by weight ratio) and DI water as a base fluid along with the addition of the CTAB surfactant at 3:2. The study incorporates the effect of varying size, dp (20–50 nm), different temperature range (25–50 °C), and volume concentration range of 0.25–1.5%. After examining the stability of prepared HNF over the 30th day from the preparation day and some of the meticulous experimentation on isobaric heat capacity, a new correlation is developed to predict the specific heat of hybrid nanofluid. Considering various parameters like various volume concentration (φ = 0.25–1.50%) and temperature range (T = 25–50 ℃), specific heat of nanofluid, specific heat of base fluid, density of nanofluid, density of base fluid, and average diameter of used nanoparticles. The developed correlation was with 2.93% and 0.903% maximum and average absolute deviation, respectively. Experimental results show the maximum decrement in specific heat value is about 15.09% compared to base fluid DI water at 25 ℃ and 1.50% volume concentration with an average particle diameter of 20 nm for the MgO + MWCNT (80:20)/water HNF. However, it is found that the rate of decrement decreases with the increase of dp from 20 to 50 nm. From the current experimental results, it is concluded that the usage of HNF on relatively higher temperature significantly influences the overall thermophysical properties such as enhancement in thermal conductivity and decrement in viscosity of the working fluid, by paying the marginal loss in specific heat capacity. Therefore, it recommended that the usage of HNF on higher temperature is more beneficial for critical heat transfer application such as HVAC, effective thermal management of the system, natural and forced convection phenomenon.

Experimental and theoretical study of a vertical tube in shell storage unit with biodegradable PCM for low temperature thermal energy storage applications

This article presents the experimental investigations of the coconut oil-based TES module for HVAC applications in the ambient and-sub ambient temperature range. To properly study this problem modular experimental module and test loop were developed. Special attention has been paid to study the physical mechanism of the melting/solidification process for natural substance (coconut oil) which has perspectives to be used in thermal energy storage systems. It has been a highlight that the melting/solidification process is much more complex for non-eutectic mixtures and there are limited analytical models witch could be used to design TES working with such substances. Due to the limit of literature data for thermophysical properties of coconut oil own experimental data has been conducted.The article presents both visual and thermal characteristics of the melting and solidification process. It has been confirmed the strong influence of natural convection phenomena at the melting process. What is more, it has been observed that in the case of coconut oil it is better to talk about appearance melting temperature than melting temperature. The experimental analysis confirmed that appearance melting temperature is a function of radial distance from the heat source. In the case of solidification, the influence of natural convection is reduced quickly due to the increasing domination of the conduction heat transfer mechanism. For the solidification temperature profiles are characteristic of the occurrence of subcooling temperature. The level of temperature drop in sub cooling is a function of radial distance from the heat source. It has been also confirmed that in case of melting as well as solidification process the shape of the phase-change profile seems to form a truncated cone (in case of melting inverted).To validate the engineering approach to vast discrepancies in the literature data own experiment has been evaluated with a simple mathematical model to verify the applicability of the literature data in system behavior prediction. Based on the best-fit approach the parameters thermal resistance of the module R = 1.3 W/K was found. Additionally the latent heat of the coconut oil of 178 kJ/kg with Gaussian distribution (σ=1.1) for melting in temperature of 25 °C and solidification at 21 °C. The proposed correlation can be easily used to optimize latent thermal energy storage cooperated with the HVAC system.

Inflection of flow transition curves by magnetic effect in annulus and consequential bifurcation map in wide ranges of Rayleigh and Hartmann numbers

In this study, the bifurcation phenomenon of natural convection at a Prandtl number of 0.3 is analyzed for a circular magnetic field applied to an annulus of a diameter ratio of 2. Flows in the annulus have been categorized as upward and downward flows according to the flow patterns observed near the upper stagnation region of the annulus. In previous studies on the bifurcation of natural convection without magnetic effects, no bifurcation was observed at very low (<7000) or high (>45,000) Rayleigh conditions. However, the results presented here indicate the bifurcation can occur even at high Rayleigh conditions (>45,000) under specific magnetic conditions. A bifurcation map was thus created over a range of Hartmann numbers from 0 to 20 and Rayleigh numbers from 3000 to 80,000 and the inflection of the transition lines was studied. As a result, the transition Rayleigh and Hartmann numbers and the critical Rayleigh and Hartmann numbers were redefined.

Effect of natural convection and thermal storage system on the electrical and thermal performance of a hybrid PV-T/PCM systems

Waste heat management is one of the prominent method of increasing the electrical performance of photo-voltaic (PV) panels by decreasing its operating temperature. In this paper, investigation on integrating natural convection phenomena with phase change materials (PCM) beneath the PV panel for heat extraction from the panel has been reported. The PV temperature is regulated by PCM and increase its efficiency. A numerical model was developed for this PV-T/PCM system and validated with the experimental data. A comparative numerical study has been performed to analyze the electrical and thermal characteristics of PV, PV/PCM and air integrated PV-T/PCM system with an incident solar radiation of 800 W/m2. A drop of 25% and 35% has been observed in PV cell temperature for PV/PCM system and air PV-T/PCM respectively with reference to base PV system. As a result of this temperature drop the electrical efficiency of PV cell has been augmented by 14.12% and 19.75% for PV/PCM system and air PV-T/PCM respectively.

Numerical Analysis of the Thermal Performance in Traditional and Developed Shapes of Thermosyphon

In recent years, the research in renewable energy is remarkably increased to reduce the reliance on fossil fuels. Solar energy took the lead among the other sources of renewable energy due to its availability everywhere and the feasibility of its use for various circumstances. Thermosyphon is one of the most successful systems for solar energy conversion. In the present study, a CFD code is employed to analyze the natural convection phenomenon which is inherent in the performance of a thermosyphon system. A steal pipe of 25 mm internal diameter and 1250 mm length is considered as the condenser of the thermosyphon. Water jacket is used to cool the condenser with an internal diameter that is five times that of the condenser. Two types of pipes are considered in the present study, namely; straight and helical pipes. The case of helical pipe assumes five different numbers to turns, which are; 10, 20, 30, 40 and 45 turns. A solar heat flux of 500 W/m2 and above is used in the analysis. The results obtained from the numerical analysis showed that the helical pipe is more efficient than the straight pipe in heating the water inside the condenser jacket for the same operating conditions. The ratios of increase in jacket water temperature of the five cases of the helical pipe to that of the straight pipe were 23.8%, 36.5%, 42.1%, 45.3% and 46.2%, respectively.

A centrifugal buoyancy formulation for Boussinesq‐type natural convection flows applied to the annulus cavity problem

SummaryTraditionally, the Boussinesq approximation is adopted for numerical simulation of natural convection phenomena where density variations are supposed negligible except through the gravity term of the momentum equation. In this study, a recently developed formulation based on a Boussinesq approximation is presented in which the density variations are also considered in the advection terms. Extending density‐variations to the advection terms captures centrifugal effects arising from both bulk enclosure rotation and within individual vortices, and thus more accurate results are expected. In this respect, the results of the proposed formulation are compared against the conventional Boussinesq simulations and weakly compressible approximation in the concentric horizontal annulus cavity. A new relation is established which maps the magnitude of the non‐Boussinesq parameter of incompressible flow to the corresponding relative temperature difference of a compressible flow simulation which is in agreement with the maximum allowed Gay‐Lussac number to avoid unphysical density values. For comparison purposes, variations of different thermo‐fluid parameters including average and local Nusselt number, entropy generation, and skin friction up to Ra = 105 are computed. Results obtained under the proposed approximation agree with the classical Boussinesq approximation up to Ra = 103 for large non‐Boussinesq parameter corresponding to the large relative temperature difference, but at Ra = 105, computed thermo‐fluid parameters via the two approaches are not identical which justifies the inclusion of large Gay‐Lussac number for convection dominated regime in natural convection problems.

Buoyant Marangoni convection of nanofluids in right-angled trapezoidal cavity

The phenomena of buoyant Marangoni natural convection in a right-angled trapezoidal enclosure filled with water-based nanofluids have been studied numerically. The left vertical wall of the trapezoidal cavity is heated uniformly, the right inclined wall is cooled, and the top and bottom walls are insulated (adiabatic). The finite element method (FEM) is implemented to solve the non-dimensional governing equations. The investigation has been done for different values of parameters: the aspect ratio of the enclosure, Ar the nanoparticles volume fraction, the Marangoni numbers, Ma bf and the Rayleigh numbers, Ra bf

Experimental study of turbulent air natural convection in open-ended vertical parallel plates under asymmetric heating conditions

In this study, we investigated the turbulent air natural convection phenomena between open-ended vertical parallel plates under asymmetric heating conditions. The rectangular air channel consisted of two insulating walls and two heated walls: one of the vertical walls is heated to maintain uniform wall temperature conditions at various temperatures from 50 to 500 °C, and the other wall is indirectly heated by thermal radiation from the directly heated wall. We measured both wall temperature distributions in the vertical direction and the air temperature profiles with the horizontal direction at certain heights.The results showed that when both walls are heated asymmetrically, reverse flow was absent at the outlet of the channel, as confirmed by a simple visualization method. In addition, we confirmed the practical result of a deviation between the mean air temperature and the measured air temperature at the center of the channel. From the results of the wall temperature distributions and the air temperature profiles, we suggest a reasonable flow behavior, which is the generation of two different recirculation flow patterns that are not affected by changes in the thermal boundary. In addition, we compared the ratio between the convective and radiative heat rates on the directly heated wall. Based on the experimental results, we analyzed the natural convection heat transfer between vertical parallel plates and we proposed two correlations. The heat transfer correlation agreed well with the correlation for turbulent natural convection on a single infinite vertical plate. The correlating equation related to the induced flow rate was presented as a modified Rayleigh number to estimate the flow rate.

Simulasi numerik kombinasi perpindahan panas konveksi alami dan radiasi pada square cavity

Natural convection is very important in some cases such as glass windows, solar collectors, electronic measuring devices (cooling devices for electronic instruments) and building materials that use insulation. In this research, will be seen the phenomenon of natural convection that occurs in square cavity. The method used for simulation is computational fluid dynamics or CFD. The model used is the 2D cavity model. In this simulation the value of Rayleigh Number (Ra) was varied i.e. 103,104,105 and 106. From this research, it is found that the higher the value of Ra in cavity, the higher the value of the average Nusselt Number (𝑁̅𝑢 ) on the wall which has a high temperature. Ra = 103 so that 𝑁̅𝑢 = 1.17. Ra =104, so that 𝑁̅𝑢 = 2.2. Ra = 105, so that 𝑁̅𝑢 = 4.47. Ra = 106, and then so that 𝑁̅𝑢 = 8.95. Based on the value and contour of the flow the smaller the Ra value in the square cavity, the isothermal line phenomenon that appears will be more vertical. The greater the Ra value in the cavity, the contour of the stream function will change to an ellipse and finally two vortices will appear.Keywords: Numerical simulation, konveksi, radiasi, square cavity.

Performance Assessment of a Triangular Integrated Collector Using Neural Networks

A numerical study is achieved on a new shape of temperature saver solar collector using an artificial neural network. The storage collector is a triangle face and a right triangle pyramid for the volumetric shape. It is obtained by cutting a cube from one upper corner at 45°, down to the opposite hypotenuse of the base of the cube. The numerical study was carried out using the computational fluid dynamics code (ANSYS-Fluent) software with natural convection phenomenon in the pyramid enclosure. Elman backpropagation network is used for his ability to find the nearest solution with the smallest error rate. The network consists of three layers, each of different corresponding weights. The results show that the temperature and velocity distributions throughout the operating period were obtained. The influence of introducing an internal partition inside the triangular storage collector was investigated. Also the optimum geometry and location for this partition were obtained. The enhancement was best at y = 0.25 m, whereas the height of triangular collector was 0.5 m. The hourly system performance was evaluated for all test conditions. The performance of the NN to train a model for this work was 0.000207, while the error of the calculation was 1×10-2 as average.

Effects of temperature and fluid velocity on beer pasteurization in open and closed loop heating systems: numerical modeling and simulation

Abstract A two-dimensional symmetric heat transfer model and a fluid rotation model were established to study beer pasteurization process through the COMSOL Multiphysics software. Two heating modes, including closed-loop heating (CLH) and open-loop heating (OLH), were considered. There was a significant natural convection phenomenon in both heating systems. However, the natural convection became weaker with a gradual increase in the heating temperature of the beer. The maximum fluid velocity (FV) in CLH and OLH modes was 69.34 and 43.74 mm/s, respectively. After heating at 333.13 K for 20 min, the minimum and maximum pasteurization unit (PU) values in CLH were 55 and 59, respectively, while the corresponding values for OLH were 30 and 55, respectively. The pasteurization effect under the CLH mode was better than the OLH one. The heat transfer was also affected by fluid flow (laminar and turbulence) patterns. The PU value was nonlinearly related to the FV. The optimal FV can be obtained at ∼50 mm/s.

Natural convection in a partially heated porous cavity to Casson fluid

Abstract In the present study, the phenomenon of natural convection in a square porous cavity containing Casson fluid is investigated, numerically. The non-Newtonian model of Casson fluid is utilized to obtain the governing equations of a flow problem. The thermal control inside the cavity is managed by partially heating the bottom wall and symmetric cooling of the side walls while the top wall is taken to be adiabatic. The penalty finite element method is used to solve the non-linear coupled equations of the flow problem. The effect of different lengths of heated zone e, Rayleigh number Ra and Darcy number Da has been examined on flow velocity and temperature distribution inside the cavity. The impact of effective viscosity of the fluid by varying Casson fluid parameter γ on the flow of fluid and heat transport has also been investigated. The results are demonstrated by plotting streamlines, isotherms and average Nusselt number over wide ranges of governing parameters. The obtained results show that with an increase in Casson fluid parameter γ and Darcy number Da, heat transfer and flow circulation increases. It is also observed that the conduction dominant heat transfer takes place for low Darcy number over a wide range of the Rayleigh number (102 ≤ Ra ≤ 106).

Numerical investigation on the effect of magnetic field on natural convection heat transfer from a pair of embedded cylinders within a porous enclosure

The present work examines the influence of magnetohydrodynamic field on natural convection phenomena inside a porous square enclosure with a pair of embedded hot circular cylinders. Numerical investigations are performed to understand the effects of interspacing distance between the embedded cylinders, Hartmann number, Rayleigh number and Darcy number on the thermal transport process and the total irreversibility generation. It is observed that the isotherm distribution is strongly affected by the presence of magnetic field although the distribution of streamlines remains independent of the strength of magnetic field. This underlines the fact that magnetic field strongly influences the heat transfer process and entropy generation characteristics. It reveals that the natural convection is suppressed in the presence of a higher magnetic field as evident from the reduction in Nusselt number. It is observed that an increase in the spacing between the cylinders increases the heat transfer rate, and moreover, the effect of the magnetic field on heat transfer is more pronounced at higher interspacing distance between the embedded cylinders. The heat transfer rate increases significantly with the increase in the permeability of the medium. The entropy generation rate is independent of the strength of applied magnetic field. Further, the contribution of the entropy generation owing to friction is found to be negligible in total irreversibility obtained at lower values of Rayleigh number irrespective of Darcy number. However, the contribution of irreversibility owing to heat transfer is found to be minimal at higher values of Rayleigh number.