Convective Heat Transfer Mechanism Research Articles

Влияние центробежного поля на процессы реакции-диффузии-конвекции в двухслойной системе несмешивающихся растворителей

The presence of a chemical reaction in a liquid medium can lead to changes in some physicochemical parameters of the system and, as a consequence, a number of hydrodynamic instabilities can develop. Studying the conditions of their occurrence in various force fields is an important task, given that the appearance of a convective mechanism of heat and mass transfer can increase the reaction rate by several orders of magnitude. This article presents the results of an experimental study of the effect that a centrifugal field has on the conditions of occurrence, intensity and structure of convective motion arising during a neutralization reaction occurring between reagents initially located in different layers of a two-layer system of immiscible solvents. The upper layer was a solution of hydrochloric acid in n-pentanol, the lower layer was an aqueous solution of sodium hydroxide. The use of inorganic reagents excludes the development of capillary types of instability, bringing to the fore gravity-dependent mechanisms of the convective motion formation. We studied the spatial-temporal distribution of the reagents and reaction product, the structure of the resulting flow by means of interferometric methods and by adding an acid-base indicator. The studies were conducted in parallel in the gravity field and in the centrifugal field. The influence of the centrifugal field leads to a radical change in the flow structure with a slight increase in the reaction rate due to convective motion, compared with experiments in the gravity field. Convective motion in a centrifugal field develops even under conditions when the system remains hydrodynamically stable in a constant gravity field. It is suggested that the reason for the change in the flow structure is the Coriolis force, the manifestations of which are traditionally neglected in theoretical studies due to, as it is believed, its smallness in comparison with the centrifugal force.

Mixed Convective Heat Transfer Characteristics and Mechanisms in Structured Packed Beds

Mixed Convective Heat Transfer Characteristics and Mechanisms in Structured Packed Beds

Heat transfer by liquid convection in particulate fluidized beds

In this work the theoretical model for heat transfer from a wall to a liquid-solid fluidized bed by liquid convective mechanism has been proposed and developed. The model is based on thickness of boundary layer and film theory. The key parameter in the model is the distance between two adjacent particles which collide with the wall. According to the proposed model, the liquid convective heat transfer in a fluidized bed is 4 to 5 times more intense than in a single-phase flow. Additionally, the wall-to-bed heat transfer coefficient has been measured experimentally in water?glass particles fluidized bed, for different particle sizes. Comparison of the model prediction with experimental data has shown that size of the particles strongly influences the mechanism of heat transfer. For fine particles of 0.8 mm in diameter, the liquid convective heat transfer model represents adequately the experimental data, indicating that particle convective mechanism is negligible. For coarse particles of 1.5?2 mm in diameter, the liquid convective heat transfer mechanism accounts for 60 % of the overall heat transfer coefficient.

Numerical Modelling of a Photovoltaic Thermal (PV/T) System Using Nanofluid With Parallel Flow Thermal Absorber

In the present study, a numerical model of photovoltaic thermal (PV/T) system using alumina (Al2O3) nanofluid, and pure water are used as working fluid. The proposed PV/T model consists of parallel riser tubes that are connected to two header tubes and it is attached to an absorber plate to simulate the conduction and convection heat transfer mechanism of a conventional PV/T system. The energy efficiency of the PV/T model is analyzed by varying the solar radiation (Heat Flux), inlet fluid velocity, and the volume percentage of the nanofluids. The numerical simulation is performed by using a conjugate heat transfer method with a computational fluid dynamics (CFD) software. According to the simulation data, the energy efficiency and the heat transfer coefficient of the PV/T system increased by increasing the inlet fluid velocity. In comparison with water, alumina nanofluid showed better thermal and electrical efficiency due to its high thermal conductivity. The thermal efficiency increased by 5.55% for alumina, compared to pure water and the electrical efficiency increased by 0.15% for alumina. Moreover, the effect of inlet fluid velocity ranging from 0.04m/s to 0.2m/s was also evaluated, and the results showed that the increase in thermal efficiency for pure water and alumina are 18.15% and 25.77%, respectively. Subsequently, the electrical efficiency increased by 0.52% and 0.56% for pure water and alumina using the new parallel flow thermal absorber, respectively.

Evaluation of overall thermal performance for conjugate natural convection in a trapezoidal cavity with different surface corrugations

The effects of various surface corrugations and wall materials of the thick base wall of a trapezoidal cavity on conjugate natural convection have been examined in this study. Pinewood, plexiglas, dry concrete and glass fiber are considered as the materials of the solid bottom wall. In this cavity, both sidewalls are kept adiabatic, while the top wall is kept isothermally cold, and isothermal heating is applied at the bottom wall. Four types of regular surface corrugation, such as flat, rectangular, sinusoidal, and triangular-shaped corrugated surfaces are considered at the inner side of the base wall. The mathematical solutions of Navier-Stokes and thermal energy equations are obtained using Galerkin finite element method. Parametric simulation is carried out by varying Rayleigh number within laminar regime. The computed results are presented in terms of isotherm and streamline plots. The average Nusselt numbers along the solid-fluid interface, the average fluid temperature inside the cavity and the overall thermal performance ratio are also observed to assess the impact of wall materials and shapes of corrugation on convective heat transfer mechanism. From the viewpoint of overall heat transfer performance, the present results reveal that the surface material as well as the shape of the corrugation plays critically on the heat transfer enhancement of the trapezoidal cavity.

Experimental and theoretical investigation of heat transfer characteristics of cylindrical heat pipe using Al2O3\u2013SiO2/W-EG hybrid nanofluids by RSM modeling approach

Nanofluids are emerging two-phase thermal fluids that play a vital part in heat exchangers owing to its heat transfer features. Ceramic nanoparticles aluminium oxide (Al2O3) and silicon dioxide (SiO2) were produced by the sol-gel technique. Characterizations have been done through powder X-ray diffraction spectrum and scanning electron microscopy analysis. Subsequently, few volume concentrations (0.0125–0.1%) of hybrid Al2O3–SiO2 nanofluids were formulated via dispersing both ceramic nanoparticles considered at 50:50 ratio into base fluid combination of 60% distilled water (W) with 40% ethylene glycol (EG) using an ultrasonic-assisted two-step method. Thermal resistance besides heat transfer coefficient have been examined with cylindrical mesh heat pipe reveals that the rise of power input decreases the thermal resistance and inversely increases heat transfer coefficient about 5.54% and 43.16% respectively. Response surface methodology (RSM) has been employed for the investigation of heat pipe experimental data. The significant factors on the various convective heat transfer mechanisms have been identified using the analysis of variance (ANOVA) tool. Finally, the empirical models were developed to forecast the heat transfer mechanisms by regression analysis and validated with experimental data which exposed the models have the best agreement with experimental results.

Hydrothermal and energy analysis of flat plate solar collector using copper oxide nanomaterials with different morphologies: Economic performance

Flat plate solar collectors (FPSCs) have gotten a lot of attention in the last decade because of their ease of installation and design. The current study focused on numerical aspects of a three-dimensional (3D) inclined tube-on-absorbing sheet solar collector. The numerical model was considered to work under a conjugated laminar mixed convection heat transfer mechanism in the range of 100 ≤ Re ≤ 1300. CuO/H2O nanofluids were used as the working fluids. The impact of different parameters on the thermal efficiency of the current FPSC, including inlet temperature, nanoparticle size, volume concentration, and various copper oxide morphologies, on the heat transfer, thermodynamics, thermal, and economic performance, was studied. According to simulation results, the solar collector using distilled water (DW) under 293 K performed better in terms of heat transfer and energy efficiency than those using 303 K and 313 K. In comparison to other sizes and volume concentrations, nanosphere nanofluids with 20 nm and 4% showed higher hydrothermal performance characteristics. Nanoplatelets-shaped CuO nanofluids illustrated the higher values for pressure drop, heat transfer, energy gain, energy efficiency, and lower values for surface plate temperature and outlet temperature, followed by nanocylinders, nanoblades, nanobricks, and nanospheres, respectively. The economic performance indicators recommended that nanobricks CuO nanofluids with 1% volume fraction were the best fluid replacement for water in solar collector applications.

Thermal transport analysis in stagnation-point flow of Casson nanofluid over a shrinking surface with viscous dissipation

In arteries blood flow is the usual example for a Casson fluid flow among the other vital utilizations of this fluid model. Thus, it would be helpful to study the Brownian motion diffusion and thermophoresis diffusion in Casson fluid for biomedical applications. In this analysis, the electrically conducting Casson nanofluid, with suction and convective boundary condition, has been addressed in a shrinking surface. The mechanism of convective heat transfer has been elaborated with Ohmic heating and viscous dissipation effects. The numerical solutions to the governing equations have been obtained by applying the similarity transformation to the nonlinear partial differential equations. The present model is employed to examine the viscoplastic characteristics in the porous regime. This dimensionless ruling problem, along with physical boundary conditions, is handled numerically by using a Runge–Kutta Fehlberg scheme. The outcomes of the present study show that the rate of heat and mass transfer at the surface of the shrinking sheet enhances with the growth in Casson fluid parameter and magnetic parameter. Furthermore, it is observed that the shear stress at the wall rises with the increment in magnetic parameter. Moreover, unsteadiness parameter is the decreasing function of heat and mass transfer rates.

Nanofluid jet impingement heating of a cooled surface with a constant heat flux in the presence of porous layer

PurposeThe purpose of this study is to numerically investigate the confined single-walled carbon nanotube-water nanofluid jet impingement heating of a cooled surface with a uniform heat flux in the presence of a porous layer. The analysis of the convective heat transfer mechanism is introduced considering the buoyancy force effect under local thermal non-equilibrium conditions.Design/methodology/approachThe governing equations for the nanofluid and solid phase are discretized by the finite volume method and the SIMPLE algorithm is used to solve these equations.FindingsIt is observed that there is an increase in a local variation of temperature along the upper wall with increasing Reynolds, Darcy and Grashof numbers. For given parameters, the optimum values of thermal conductivity ratio and porous layer thickness leading to better heating on the upper wall are found as Kr = 1.0 and S = 0.5, respectively. The maximum and minimum values of temperature on the upper wall are obtained in the case of higher nanoparticle volume fraction at Re = 100, however, the temperature values get higher along the upper wall with increasing nanoparticle volume fraction at Re = 300.Originality/valueThe effects of various parameters, such as Reynolds number, Darcy number and Grashof number, on thermal behavior and nanofluid flow are examined to determine the desirable heating conditions for the upper wall. This paper provides a solution to problems such as icing on the surface with a suitable thermal design and optimum geometric configuration.

Derivation of unifying formulae for convective heat transfer in compressible flow fields

Although many theoretical and experimental studies on convective heat transfer exist, the consistent analytical expression of advection heat flux vector in convection as well as its reference temperature in the thermal driving force remains unclear. Here we show theoretically and experimentally the unifying formulae for three-dimensional (3D) heat flux vector of forced and natural convections for compressible laminar flows based on the first law of thermodynamics. It is indicated for a single-phase compressible fluid that advection is no other than heat transfer owing to mass flow in the forms of enthalpy and mechanical energy by gross fluid movement, driven by the temperature difference between the fluid temperature and the potential temperature associated with the relevant adiabatic work done. A simple formula for the total convective heat flux vector of natural convection is also suggested and reformulated in terms of logarithmic density difference as the thermal driving force. The theoretical calculations agree well with the laminar flow experiment results. Our discovery of advection heat transfer for compressible flows caused by the temperature differential in which the potential temperature is regarded as the unifying reference temperature represents a previously unknown thermal driving mechanism. This work would bring fundamental insights into the physical mechanism of convective heat transfer, and opens up new avenue for the design, calculation and thermal management of the 3D convection heat flux problems using the novel thermal driving force for compressible laminar and turbulent flows.

Fractional-order simulations for heat and mass transfer analysis confined by elliptic inclined plate with slip effects: A comparative fractional analysis

In this investigation, the novel fractional order simulations are performed in order to inspect the characteristics of mass and heat transfer over porous inclined surface subject to the magnetic force impact. The radiative phenomenon and slip wall constraints are utilized to make the model more comprehensive. The convective thermal mechanism of heat transfer is predicted with assumptions of ramped temperature. The recent definitions of fractional derivatives are followed. The famous technique of fractional approach namely Atangana-Baleanu (AB) and Caputo-time fractional derivative are employed for the solution procedure. Various relations expressing the mathematical functions for the fractional derivatives are encountered. The major outcomes are listed for parameters govern for the model. This obtained result reflects many engineering and industrial applications like thermal processes, processing industries, nuclear reactions, solar systems, energy production etc.

Three-Dimensional Simulation of Vapor Bubble Growth in Superheated Water Due to the Convective Action by an Interface Tracking Method

Abstract In this paper, the growth of a rising vapor bubble in superheated water was numerically studied using an advanced interface tracking method, called the intersection marker (ISM) method. The ISM method is a hybrid Lagrangian–Eulerian front-tracking algorithm that can model an arbitrary three-dimensional (3D) surface within an array of cubic control volumes (CCV). The ISM method has cell-by-cell remeshing capability that is volume conservative, maintains surface continuity, and is suited for tracking interface deformation in multiphase flow simulations. This method was previously used in adiabatic bubble rise simulation with no heat and mass transfers to or from the bubble were considered. This work will extend the ISM method's application to simulate vapor bubble growth in superheated water with the inclusion of additional physics, such as the convective heat transfer mechanism and the phase-change. Coupled with an in-house variable-density and variable-viscosity single-fluid flow solver, the method was used to simulate vapor bubble growth due to the convective action. The forces such as the surface tension and the buoyancy were included in the momentum equation. The source terms for the mass transfer were also modeled in the computational fluid dynamics governing equations to simulate the growth. Bubble properties such as size, shape, velocity, drag coefficient, and convective heat transfer coefficient were predicted. Effects of surface tension and temperature on the bubble characteristic were also discussed. Obtained numerical results were compared against the analytical and past works and found to be in good agreement.

The influence of ultrasonic vibration amplitude and magnetic field intensity on microstructural characteristics in laser drilling of Ti6Al4V

Nowadays, laser drilling has found extensive medical applications such as drilling of titanium implants. However, laser drilling of such implants has encountered several restrictions such as low penetration depth, high thickness of recast layer and heat affected zone (HAZ). Therefore, various approaches such as magnetic field and/or ultrasonic vibration aided laser drilling have been proposed to overcome these limitations; among them, few studies have been conducted considering the simultaneous effect of magnetic field and ultrasonic vibrations on microstructural characteristics. Therefore, in the present paper, the effects of magnetic field intensity and ultrasonic vibration amplitude (with a frequency of 28 kHz) have been investigated on the formed phases, thickness of recast layer and thickness of HAZ in laser drilling of Ti6Al4V. According to the obtained results, adding ultrasonic vibrations to the laser drilling process will lead to an average decrease of 29.40% and 28% respectively for the thickness of HAZ and recast layer. However, with the addition of a magnetic field (0.1 Tesla), the thicknesses of HAZ and recast layer were increased by 7% and 11%, respectively. Furthermore, increasing the ultrasonic vibration amplitude was associated with the increase in the acicular alpha phase (α′) as well as more dense, and fine-grained and uniform structure. This can be attributed to the strengthening of convective heat transfer mechanism and higher cooling rate. Additionally, by increasing the intensity of the magnetic field, the structure of the acicular alpha (α′) became finer and the density of lateral branches decreased.

Convective heat transfer mechanisms of molten phase change material in a vertical slender rectangular cavity: A numerical case study

In this paper, the convective heat transfer pattern of molten phase change material (mPCM) in a slender rectangular cavity with constant heat flux boundary condition at one side and flow boundary condition at opposite side is numerically investigated. The influences of the Rayleigh number (constant heat flux boundary condition, Ra = 104, 105 and 106), the Reynolds number (flow boundary condition, Re = 1×104, 2×104, 3×104, 4×104 and 5×104) and aspect ratio (AR = 5, 7.5 and 10) on the convective heat transfer mechanisms of the mPCM are analyzed. Several dimensionless parameters are used as quantitative indexes to describe and evaluate the convection and heat transfer process in the cavity. The results show that the convective heat transfer efficiency can be improved by increasing the Ra or Re. With longer length of the cavity in the direction of buoyancy, the internal natural convection heat transfer of mPCM is more intense. Especially, due to different boundary conditions, the heat transfer intensity at two vertical walls of the cavity is different. In addition, the mixing degree of cold and hot fluid in the cavity will also affect the local heat transfer effect at the wall.

Turbulent convection in a cube with mixed thermal boundary conditions: low Rayleigh number regime

Turbulent thermal convection was numerically investigated in a cubic cavity under homogeneous and inhomogeneous heating for Rayleigh number Ra=107 and Prandtl number Pr=6.46. Inhomogeneous heating was created only at the lower boundary using mixed boundary conditions. Three configurations of the heated regions distribution of the same heating area were considered. The total heat flux through the lower boundary substantially depends on the distribution of the heated regions and increases with decreasing of the heaters size, which is in a good agreement with Ripesi et al., 2014 and Bakhuis et al., 2018. It was found that in turbulent thermal convection under inhomogeneous boundary conditions large-scale circulation (LSC) is formed. Dynamics and structure of LSC strongly depends on the distribution of conducting regions. In case of Rayleigh-Bénard convection, the main flow is more complex with higher-order modes prevailing. Time-averaged velocity field revealed the existence of two vortex rings rotating in opposite directions and located near horizontal walls. Decomposing the velocity into free-slip modes allowed us to find that these vortex rings are stable and provide the most of system energy. The absence of mean LSC results in a specific convective heat transfer mechanism when mean and turbulent heat fluxes are spatially separated.

A review of flow and heat transfer in cavities and their applications

The study of fluid flows in a cavity and their effect on thermal performance in heat transporting and entropy generation are found in many heating and cooling engineering applications such as energy storage geothermal reservoirs, boilers, solar collectors, underground water flow, lakes, nuclear reactors. This paper presents a comprehensive review of the recent numerical and experimental studies on both heat transfer and entropy generation and their applications in cavities. The effect of the fluid thermal properties, the cavity configuration, the boundary, and initial conditions, the magnetic field, the thermal source discretion, and other geometrical and physical parameters on heat transfer and entropy generation are discussed. This study also presents the effect of several important dimensionless parameters such as the Reynolds, Richardson, Grashof, Rayleigh, Darcy, Hartmann, and Prandtl numbers on both natural and combined convection heat transfer mechanisms in cavities.

A review of drag reduction and heat transfer enhancement by riblet surfaces in closed and open channel flow

Turbulence and heat transfer are two critical factors in the chain of world energy consumption. Hence, development of more efficient energy transfer techniques in engineering applications through control and optimization of the thermal turbulent boundary layer and turbulence continues to be a subject of intense interest. This is why reviewing a passive flow control technique with a focus on unique characteristics of heat transfer in devices called riblets is beneficial as technology pushes to revolutionize design and manufacturing of heat exchanger equipment. Riblet surfaces are considered beneficial to many commercial applications as they reduce forces between surface and fluid, and in some cases can enhance heat transfer in the turbulent flow regime. As there is no literature study that focuses on heat transfer and riblets, the current study reports experimental and numerical investigations reviewing both drag reduction and heat transfer using riblets in closed and open channels. Drag reduction studies are limited to water and oil flow studies, and heat transfer studies are expanded to air flow studies as there are none on water or oil flow. In addition, physical mechanisms of fluid drag and convective heat transfer are discussed. This paper outlines the challenges of fabricating riblets and provides a design summary for optimal drag reduction and heat transfer. This review also looks at applied numerical methods and their accuracy in evaluating drag reduction and heat transfer over riblet surfaces. The important findings and a possible future optimization approach on riblet surfaces are highlighted as well.

Understanding the dynamic behavior and the effect of feeding policies of a direct contact membrane distillation for water desalination

In this study, a dynamic mathematical model for direct contact membrane distillation (MD) is developed and validated. Two model formulations were considered each of which can be cast in lumped or spatial structure. All types of models were verified against experimental transient data for the permeate and brine temperatures. The proposed models provided a perfect prediction of the measured data except at the lowest flow rates. Judicious model modification helped to enhance the model performance. The tuned model is used to analyze the process dynamic in one-dimensional space. It is found that the process possesses an enhanced convective heat transfer mechanism substantiated by the high values for the temperature polarization and Biot number. At very low flow rates, the membrane conduction resistance prevails limiting the heat transfer at the membrane interface and hence the process performance. Preliminary analysis of the special transient operation of the MD process to address challenging operations such as scheduled production strategy and/or intermittent energy sources is outlined.

Enhancement mechanism of convective heat transfer via nanofluid: An analysis by means of synchrotron radiation imaging

The enhancement mechanism on the heat transfer coefficient of SiO2 nanofluid compared to the ethylene glycol/water solvent was investigated by means of synchrotron x–ray radiographic imaging at 300.15 K. The experimental results showed a low concentration of local particles near the wall in the channel cross–section. This anomalous particle distribution alters the flow velocity distribution. Based on the law of conservation of momentum, the thickness of the thermal boundary layer becomes thin in the nanofluid, consequently enhancing the convective heat transfer. The thickness of the thermal boundary layer in the nanofluid based on the particle distribution visualized from x–ray imaging depends on the particle diameter, and its dependency is consistent with the magnitude relation of the heat transfer coefficient.

A Novel Method for Thermal Modelling of Photovoltaic Modules/Cells under Varying Environmental Conditions

Temperature has a significant effect on the photovoltaic module output power and mechanical properties. Measuring the temperature for such a stacked layers structure is impractical to be carried out, especially when we talk about a high number of modules in power plants. This paper introduces a novel thermal model to estimate the temperature of the embedded electronic junction in modules/cells as well as their front and back surface temperatures. The novelty of this paper can be realized through different aspects. First, the model includes a novel coefficient, which we define as the forced convection adjustment coefficient to imitate the module tilt angle effect on the forced convection heat transfer mechanism. Second, the new combination of effective sub-models found in literature producing a unique and reliable method for estimating the temperature of the PV modules/cells by incorporating the new coefficient. In addition, the paper presents a comprehensive review of the existing PV thermal sub-models and the determination expressions of the related parameters, which all have been tested to find the best combination. The heat balance equation has been employed to construct the thermal model. The validation phase shows that the estimation of the module temperature has significantly improved by introducing the novel forced convection adjustment coefficient. Measurements of polycrystalline and amorphous modules have been used to verify the proposed model. Multiple error indication parameters have been used to validate the model and verify it by comparing the obtained results to those reported in recent and most accurate literature.