Natural Convection Heat Transfer Characteristics Research Articles

Natural Convection from a Corrugated Heated Surface at the Bottom of Vented Rectangular Enclosure

Natural convection heat transfer characteristics enclosures from a tilted rectangular enclosure heated at the corrugated bottom wall and vented by uniform slots opening at the top wall are experimentally investigated. The experiments were carried out to study the effects of the angle of opening of the corrugated surface, opening ratio, enclosure's tilt angle, and Rayleigh number on the passive cooling of the enclosure. The experiments were carried out at Rayleigh numbers ranging from 2 × 108 to 1.52 × 109 for enclosure tilt angles ranging from 0° to 90° and angle of opening of corrugated surface ranging from 0° to 25°. The top venting arrangement was studied at different opening ratios of 1, 0.75, 0.5, and 0.25. The results gave an optimum angle of opening of the corrugated surface at which Nusselt number is maximum.

三次元数値解析によるAl<sub>2</sub>O<sub>3</sub>－水ナノフルードの自然対流熱伝達特性の解明

三次元数値解析によるAl<sub>2</sub>O<sub>3</sub>－水ナノフルードの自然対流熱伝達特性の解明

Natural convection cooling characteristics in a plate type fuel assembly of Kyoto University Research Reactor during loss of coolant accident

The transient behavior of nuclear fuel during loss of coolant accident (LOCA) must be evaluated not only for power reactors but also for research reactors. Kyoto University Research Reactor (KUR) is a light-water moderated tank-type reactor operated at the rated thermal power of 5MW and has been widely utilized for a lot of scientific researches. The safety of the plate-type fuels in the KUR has been studied by a reactor coolant system analysis code. However, for the accurate prediction of the natural convection phenomena of air during LOCA, the empirical correlation for natural convection should be selected properly and an experimental validation should be performed. In this study, the natural convection heat transfer characteristics were experimentally investigated using a simulated fuel assembly which has the same dimension with the KUR fuels. Furthermore, a simplified unsteady heat conduction simulation was performed by taking the axial heat density distribution into account, and the cooling feature of the fuel plate during LOCA was discussed by estimating the time to the meltdown.

Natural convection heat transfer characteristics in vertical cavities with active and inactive top and bottom disks

Natural convection heat transfer was investigated in vertical cavities where either all surfaces were active, or only the vertical surface was active for four different geometries, which were varied by placing a disk at the top and/or bottom of the cavity. A cupric acid–copper sulfate electroplating system was employed for mass transfer experiments exploiting the analogy with heat transfer. The Rayleigh number was varied in the range 4.55×109⩽RaLw⩽3.79×1013. Preliminary tests for a vertical pipe, upward- and downward-facing horizontal disks showed good agreement with existing correlations. The measured Nusselt numbers in the vertical cavities with all surfaces active were always greater than those with only the vertical surface active, which is attributed to greater hydrodynamic interaction of the flows generated by different surfaces. When all surfaces were active, the bottom-closed cavity exhibited the largest heat transfer rates, followed by both-closed, top-closed, and both ends open cavities; this trend was observed with laminar and turbulent flows. With only the vertical surface active, similar trends were observed except that the heat transfer rates were almost identical for both-open and top-closed cavities, which is attributed to the weak influence of the top disk on the heat transfer characteristics. Using these results, empirical correlations were derived for both laminar and turbulent flow conditions.

Numerical simulation of natural convection of square enclosure filled with Cu/Al2O3-water mixed nanofluid based on lattice Boltzmann method

As an effective heat transfer medium, Nanofluid is used widely in heat transfer field. However, due to the contradiction between the heat conductivity coefficient of nanofluid and the cost of nanoparticles, a new mixed nanofluid is developed. In order to investigate the natural convection heat transfer characteristics and the interaction mechanism between nanoparticles, the lattice Boltzmann equations of nanofluid flow and temperature fields are deduced by multi-scale technique based on considering the interaction forces between nanoparticles, and the lattice Boltzmann model of Cu/Al2O3-water mixed nanofluid is established by coupling the evolution equations of flow with temperature fields. Nanoparticles distribution in enclosure and interaction forces between nanoparticles are investigated, it is found that Brownian motion force is far bigger than any other forces, and the effects of temperature difference driving force and Brownian motion force on nanoparticles distribution are biggest. In addition, the effects of nanoparticles fractions and Rayleigh number on natural convection are investigated, and the natural convection heat transfer characteristics of mixed nanofluid (Cu/Al2O3-water) are compared with those of single metal nanoparticle nanofluid (Al2O3-water). It is found that the mixed nanofluid has a higher heat transfer characteristic than other common nanofluid.

Double-diffusive natural convection of Al2O3-water nanofluid in an enclosure with partially active side walls using variable properties

Natural convection heat and mass transfer characteristics in a square enclosure using variable thermal conductivity and variable viscosity are numerically studied. The fluid in the enclosure is a water-based nanofluid containing Al2O3 nanoparticles. The top and bottom horizontal walls are insulated, while a source (Th, Ch) and a sink (Tc, Cc) are located at the vertical left and right walls as active parts, respectively, with Th>Tc and Ch>Cc. The governing equations in the two-dimensional space are discretized using the control volume method. A proper upwinding scheme is employed to obtain stabilized solutions. The study has been carried out for the Rayleigh numbers of 104 to 106, the buoyancy ratios of −5∼5, and different configurations of the source and sink. Results are presented in the form of the streamlines, isotherms and iso-concentraions as well as the average Nusselt and Sherwood numbers. It is observed that average Nusselt number is increased by adding the nanoparticles, while average Sherwood number is reduced. Moreover, both Nusselt and Sherwood number are increased as absolute value of the buoyancy ratio or Rayleigh number is increased.

Natural convection from a vertical wall embedded in a non-Darcy porous medium filled with nanofluids

Purpose – This work aims to focus on the study of natural convection heat transfer characteristics in a vertical wall embedded in a non-Darcy porous medium filled with nanofluids. Design/methodology/approach – The governed partial differential equations are transformed into ordinary differential equations which are obtained by similarity solution and then have been solved through homotopy analysis method. Findings – The results show that Cu-water as nanofluid enhances the local Nusselt number more than other nanofluids. Originality/value – Nanoparticles and nanofluid enhance the local Nusselt number.

Natural convection heat transfer on surfaces of copper micro-wires

The natural convection heat transfer characteristics and mechanism for copper micro-wires in water and air were investigated experimentally and numerically. The wires with diameters of 39.9, 65.8 and 119.1 μm were placed horizontally in water inside of a sealed tube and in air of a large room, respectively. Using Joule heating, the heat transfer coefficients and Nusselt numbers of natural convection for micro-wires in ultra pure water and air were obtained. A three dimensional incompressible numerical model was used to investigate the natural convection, and the prediction with this model was in reasonable accordance with the experimental results. With the decrease of micro-wire diameter, the heat transfer coefficient of natural convection on the surface of micro-wire becomes larger, while the Nu number of natural convection decreases in water and air. Besides, the change rate of Nu number in water decreases apparently with the increase of heat flux and the decrease of wire diameter, which is larger than that in air. The thickness of boundary layer on the wall of micro-wire becomes thinner with the decrease of diameter in both water and air, but the ratio of boundary layer thickness in water to the diameter increases. However, there is almost no change of this ratio for natural convection in air. As a result, the proportion of conduction in total heat transfer of natural convection in water increases, while the convective heat transfer decreases. The velocity distribution, temperature field and the boundary layer in the natural convection were compared with those of tube with conventional dimension. It was found that the boundary layer around the micro-wire is an oval-shaped film on the surface, which was different from that around the conventional tube. This apparently reduces the convection strength in the natural convection, thus the heat transfer presents a conduction characteristic.

Closed-Form Analytical Solutions for Laminar Natural Convection on Horizontal Plates

A boundary layer based integral analysis has been performed to investigate laminar natural convection heat transfer characteristics for fluids with arbitrary Prandtl number over a semi-infinite horizontal plate subjected either to a variable wall temperature or variable heat flux. The wall temperature is assumed to vary in the form T¯w(x¯)-T¯∞=ax¯n whereas the heat flux is assumed to vary according to qw(x¯)=bx¯m. Analytical closed-form solutions for local and average Nusselt number valid for arbitrary values of Prandtl number and nonuniform heating conditions are mathematically derived here. The effects of various values of Prandtl number and the index n or m on the heat transfer coefficients are presented. The results of the integral analysis compare well with that of previously published similarity theory, numerical computations and experiments. A study is presented on how the choice for velocity and temperature profiles affects the results of the integral theory. The theory has been generalized for arbitrary orders of the polynomials representing the velocity and temperature profiles. The subtle role of Prandtl number in determining the relative thicknesses of the velocity and temperature boundary layers for natural convection is elucidated and contrasted with that in forced convection. It is found that, in natural convection, the two boundary layers are of comparable thickness if Pr ≤ 1 or Pr ≈ 1. It is only when the Prandtl number is large (Pr > 1) that the velocity boundary layer is thicker than the thermal boundary layer.

회전수 및 자기장강도 변화에 따른 이중원관내 자성유체의 자연대류 열전달 특성에 관한 실험적 연구

본 연구의 목적은 회전자기장에서 회전수 및 자기장강도 변화에 따른 이중원관내 자성유체의 자연대류 열전달 특성에 관하여 고찰하는 것이다. 회전자기장은 6극 3상 유도전동기를 사용하여 인가하였고, 인버터 드라이버와 전압변환기를 이용하여 회전수 및 자기장강도를 제어하였다. 실험에 사용된 경계조건으로 이중원관 내부원관은 <TEX>$30^{\circ}C$</TEX>로 유지하고 외부원관은 <TEX>$25^{\circ}C$</TEX>로 유지하였으며, 회전자기장은 반시계방향으로만 인가하여 실험을 수행하였다. 그 결과 이중원관내 자성유체의 자연대류 열전달 특성은 회전자기장의 회전수 및 자기장강도 증가에 따라 열확산이 증가하여 고온부과 저온부의 열교환이 활발히 이루어졌으며, 이중원관 내 자성유체의 열확산 방향은 인가자기장과 같은 방향인 반시계방향으로 진행되었다. The objective of this study is experimentally to investigate natural convective heat transfer characteristics of the ferrofluid for a concentric annuli under rotating magnetic field with variations of the revolution and the magnetic field strength. The rotating magnetic field was provided by induction motor with 6 poles and 3 phases and the revolution and the magnetic field strength were controlled by an inverter driver and a voltage meter, respectively. Temperatures of the inner pipe and the outer pipe in the tested concentric annuli were maintained at <TEX>$30^{\circ}C$</TEX> and <TEX>$25^{\circ}C$</TEX>, respectively, during the test and the direction of the rotating magnetic field was a counterclockwise. As a result, the natural convective heat transfer characteristics of the ferrofluid for a concentric annuli were increased with the rise of the revolution and magnetic field strength due to the increased heat dissipation between hot side and cold side of the concentric annuli.

Coupled thermal simulation of hydrogen storage tank-Dewar flask system

The thermal effects during hydrogen charge and discharge processes are critical for improving hydrogen storage performance. The heat transfer of the hydrogen storage tank couples with its surrounding Dewar flask in thermal simulation. An additional model for studying the natural convection characteristics in the Dewar flask of the hydrogen storage system is developed and integrated into the system model. The Navier–Stokes equation is used to describe the air flow in the Dewar flask. The thermal dispersion is considered to describe the effective thermal conductivity of the storage tank. In the hydrogen storage tank-Dewar flask system, eight monitoring points in hydrogen storage tank and eight points in Dewar flask are selected to monitor temperature variations. The simulated pressures and temperatures in the hydrogen storage tank-Dewar flask system-agree well with experiments. The simulation results show that the model can well describe the natural convective heat transfer characteristics in the Dewar flask and the hydrogen storage behavior in the porous activated carbon tank, and further prove the feasibility and correctness of the coupled thermal simulation of hydrogen storage tank-Dewar flask system. The heat flow rates from the hydrogen tank and the Dewar flask is obtained by integrations along their boundaries to analyze the heat transfer coefficients. The coupled model is useful for optimizing empirical heat transfer coefficients of hydrogen storage tank.

Free convection heat transfer in complex-wavy-wall enclosed cavity filled with nanofluid

A numerical investigation is performed into the natural convection heat transfer characteristics within an enclosed cavity filled with nanofluid. The left and right walls of the cavity have a complex-wavy geometry and are maintained at a low and high temperature, respectively. Meanwhile, the upper and lower walls of the cavity are both flat and insulated. The nanofluid is composed of Al2O3 nanoparticles suspended in pure water. In performing the analysis, the governing equations are formulated using the Smoothed Particle Hydrodynamics and the complex-wavy-surface is modeled as the superimposition of two sinusoidal functions. The simulations examine the effects of the volume fraction of nanoparticles, the Rayleigh number and the complex-wavy-surface geometry parameters on the flow streamlines, isotherm distribution and Nusselt number within the cavity. The results show that for all values of the Rayleigh number, the Nusselt number, increases as the volume fraction of nanoparticles increases. In addition, it is shown that the heat transfer performance can be optimized by tuning the wavy-surface geometry parameters in accordance with the Rayleigh number. Overall, the results presented in this study provide a useful insight into potential strategies for enhancing the convection heat transfer performance within enclosed cavities with complex-wavy-wall surfaces.

Natural convection heat transfer and entropy generation in wavy-wall enclosure containing water-based nanofluid

A numerical investigation is performed into the natural convection heat transfer characteristics and entropy generation of water-based nanofluids in an enclosure bounded by wavy vertical walls and flat upper and lower surfaces. In performing the analysis, it is assumed that the left wall is heated by a constant heat flux while the right wall is maintained at a constant low temperature. In addition, the upper and lower walls are both assumed to be insulated. The analysis considers three different nanofluids, namely Cu–water, Al2O3–water and TiO2–water. The governing equations are modeled using the Boussinesq approximation and are solved using the finite-volume numerical method. The analysis examines the effects of the nanoparticle volume fraction, the type of nanofluid, the Rayleigh number and the wavy surface geometry parameters on the mean Nusselt number and total entropy generation. The results show that for all values of the Rayleigh number considered in the present study (Ra=104∼106), the mean Nusselt number increases and the total entropy generation reduces as the volume fraction of nanoparticles increases. In addition, it is shown that the Cu–water nanofluid yields the best heat transfer performance and the lowest total entropy generation of the three nanofluids. Finally, it is shown that for a given nanofluid, the mean Nusselt number can be maximized and the total entropy generation minimized via an appropriate tuning of the wavy surface geometry parameters. Overall, the results presented in this study provide a useful source of reference for enhancing the natural convection heat transfer performance in wavy-wall enclosures while simultaneously reducing the entropy generation.

Natural Convection Heat Transfer of Annular Metal Porous Medium Heat Sink of LED

This study used metal porous medium heat sink to replace traditional cooling fins to form a high performance LED cooling system. The metal foamed material has high permeability and cooling area as large as several times of that of traditional fins. With a proper configuration design, it can improve the heat transfer capability of natural convection effectively. This study experimentally investigated the natural convection heat transfer characteristics of the annular metal foamed material, and determined the optimal configuration. The experimental results showed that 1) the heat transfer coefficient (h) increased with ΔT; 2) the (h) decreased as PPI (pores per inch) increased when the thickness (t) of the annular metal foams equaled 5 mm, but the (h) increased as PPI increased when t=11 and 14.5 mm; 3) the (h) increased and then decreased as (t) increased, and there was better heat transfer effect when t=11 mm as shown in the experimental data.

Numerical analysis of natural convection in sodium plenum below the grid plate of a fast reactor during a severe accident

Heat transfer characteristics of liquid sodium are quite different from that of normal liquids because of its very low Prandtl number. Empirical correlations for convective heat transfer in sodium are rather limited because of inherent difficulties in performing sodium experiments. In this paper, transient and steady natural convection developing inside a cylindrical enclosure heated from the top and cooled along the vertical side wall is studied by a numerical analysis. This problem is of relevance to the safety analysis of sodium cooled pool type fast reactor. The focus of the present investigation is to determine the effect of plate thickness on the natural convection heat transfer characteristics. For the case of a thick top plate, in the transient regime, heat transfer is conduction dominated and Nusselt number is found to be inversely proportional to square root of time as in the case of a thin plate. But the steady state heat transfer coefficient between the lower surface of thick hot plate and liquid sodium is less by about 25% than for a thin plate at the same temperature as the lower surface. The reason for this behaviour is traced to the weaker convective current developed in the case of a thick plate. Based on detailed parametric studies, a correlation for Nusselt number is derived with a correlation coefficient of 0.99, for use in the reactor safety analysis. The Nusselt number is found to be a function of Boussinesq number, thermal conductivity ratio between the wall material and liquid sodium and non-dimensional thickness of the plate. The validity of the correlation is further checked by applying it to different data sets not used for the derivation of the correlation.

Natural convection in a square porous annulus

The current study is focused to investigate the natural convective heat transfer characteristics in a porous square annulus. Finite element method is used as a tool to simplify the partial differential equations that govern the heat and fluid flow characteristics inside the porous medium. A simple three noded triangular element is used to divide the porous domain into smaller segments known as elements. The algebraic set of equations resulting from the finite element equation are assembled into a global matrix and then solved iteratively to get the solution variables. Thermal equilibrium as well as non equilibrium in porous domain is considered. The effect of various geometric and physical parameters are investigated. The boundary conditions are such that the inner walls of the annulus are heated isothermally to temperature Th, and the outer surfaces are exposed to cool temperature Tc. The width ratio defined as the ratio of hollow portion to the length of the cavity is varied from 0.125 to 0.875. Results are discussed with respect to width ratio, Rayleigh number, radiation parameter and viscous dissipation parameter.

Natural convection heat transfer performance in complex-wavy-wall enclosed cavity filled with nanofluid

A numerical investigation is performed into the natural convection heat transfer characteristics within an enclosed cavity filled with nanofluid. The left and right walls of the cavity have a complex-wavy geometry and are maintained at a high and low temperature, respectively. Meanwhile, the upper and lower walls of the cavity are both flat and insulated. The nanofluid comprises Al2O3 nanoparticles suspended in pure water. In performing the analysis, the governing equations are formulated using the Boussinesq approximation and the complex-wavy-surface is modeled as the superimposition of two sinusoidal functions. The simulations examine the effects of the volume fraction of nanoparticles, the Rayleigh number and the complex-wavy-surface geometry parameters on the flow streamlines, isotherm distribution and Nusselt number within the cavity. The results show that for all values of the Rayleigh number, the Nusselt number increases as the volume fraction of nanoparticles increases. In addition, it is shown that the heat transfer performance can be optimized by tuning the wavy-surface geometry parameters in accordance with the Rayleigh number. Overall, the results presented in this study provide a useful insight into potential strategies for enhancing the convection heat transfer performance within enclosed cavities with complex-wavy-wall surfaces.

Experimental evaluation on natural convection heat transfer of microencapsulated phase change materials slurry in a rectangular heat storage tank

The main purpose of this experiment is to evaluate natural convection heat transfer characteristics of microencapsulated PCM (phase change material) slurry (MPCMS) during phase change process in a rectangular heat storage tank heated from the bottom and cooled at the top. The microencapsulated PCM is several material compositions of n-paraffin waxes (mainly nonadecane) as the core materials, outside a layer of a melamine resin wrapped. In the present study, its slurry is used mixing with water. And the specific heat capacity with latent heat shows a peak value at the temperature of about T=31°C. We investigate the influences of the phase change process of the MPCMS on natural convection heat transfer. The experimental results indicate that phase change process of the MPCMS promote natural convection heat transfer. The local maximum heat transfer enhancement occurs at approximately TH=34°C corresponding to the heated plate temperature. With high mass concentration Cm, the onset of natural convection gets easier for the MPCMS. The temperature gradient is larger near top plate and bottom plate of a rectangular heat storage tank. Heat transfer coefficient increases with the phase change of the PCM. And it summarizes that the phase change process of the PCM promote the occurrence of natural convection.

Characteristics and Enhancement of Heat Transportation From a Block Heat Source Module in Three-Dimensional Cabinets to an Ambient Natural Convective Air Stream

This study aims to numerically investigate the natural convective heat transfer characteristics and cooling performance enhancement for a module comprised of a board and arrays of discrete heat generating blocks in three-dimensional cabinets. The objective of this study has three aspects. First, efforts are performed to investigate the influence of thermal interaction between the air streams inside and outside the cabinet on the natural convective heat transfer characteristics for the heat source module. Second, attention is given to investigate the cooling performance enhancement of the module by installing plate fins onto the back surface of board. Finally, the cooling performance enhancement of the module by constructing air vents in the cabinet wall is conducted. The computation domain covers the cabinet and surrounding area, so that the temperature and velocity fields of the cabinet and surrounding are solved simultaneously. The results show that the maximum difference in hot spot temperatures of the heat source module for the situations with and without consideration of thermal interaction can be up to 22.7 as Pr = 0.7, 104 ≤ Ra ≤ 106, K bf = K pf = K wf = 100, K Ff = 8000, and the ranges of other parameters listed in the text. When plate fins are installed onto the back surface of the board, the hot spot temperature can be reduced by 25.5%. Furthermore, the reduction in hot spot temperature is about 47.8% when air vents are constructed in the cabinet wall.

Numerical investigation into natural convection heat transfer enhancement of copper-water nanofluid in a wavy wall enclosure

Numerical investigations are performed into the natural convection heat transfer characteristics within a wavy-wall enclosure filled with Cu-water nanofluid. In the paper, the bottom wall of the enclosure has a wavy geometry and is maintained at a constant high temperature, while the top wall is straight and is maintained at a constant low temperature. The left and right walls of the enclosure are both straight and insulated. In performing the simulation, the Boussinesq approximation is used to model the governing equations. The study examines the effect of the nanoparticle volume fraction, the Rayleigh number, the wave amplitude, and the wavelength on the heat transfer characteristics. It is shown that the heat transfer performance can be enhanced as the volume fraction of nanoparticles increases. It is also shown that for a given Rayleigh number, the heat transfer effect can be optimized via an appropriate changing of the geometry conditions.