Effect Of Forced Convection Research Articles

Effects of magnetic field, binary particle loading and rotational conic surface on phase change process in a PCM filled cylinder

In the present work, coupled effects of forced convection, rotational conic surface and magnetic field on the phase change dynamics are numerically explored by using finite element method for a phase change material (PCM) filled 3D cylindrical reactor. The PCM filled region has a conic shape and it is rotating. The study is conducted for various values of rotational Re number (between 0 and 2500), magnetic field strength (Hartmann number between 0 and 30) and conic surface aspect ratio (between 1 and 2). It is observed that the coupled interactions between the rotational surface and magnetic field significantly affect the phase change process dynamics and convective heat transfer between different phases. Optimum value of rotational Reynolds number for minimum complete phase transition time is achieved at Rew = 1000. The magnetic field has a positive impact on the phase change process while it is impact is profound without surface rotation. There are 98% and 65% reductions in the complete phase transition times when configurations at Rew = 1000 are compared with motionless conic surface case in the absence and presence of magnetic field. The effects of rotation are profound when different aspect ratios of the PCM filled region is considered. The transition time is increased up to 553% without rotation while this value is only 86% when cases with lowest and highest aspect ratio are compared. A modal analysis with 30 mode is used to capture the phase change dynamics and coupled interactions between the rotational surface and magnetic field on the variation of liquid fraction.

In-situ thermal transport measurement of flowing fluid using modulated photothermal radiometry

In situ thermal transport measurement of flowing fluid could be useful for the characterization and diagnosis of practical thermal systems such as fluid heat exchangers and thermal energy storage systems. Despite abundant reports on the ex-situ thermal conductivity measurement of stagnant fluids, a suitable technique for the thermal conductivity measurement of flowing fluid has been rarely reported. This paper presents the thermal conductivity measurement of flowing fluid within a pipe using a non-contact modulated photothermal radiometry (MPR) technique, where the surface of the pipe is heated by an intensity-modulated laser and the heat diffuses into the fluid with suitable modulation frequency. We design a tube section with small wall thickness suitable for the MPR measurements to maximize the sensitivity of the thermal response to the fluid properties while minimizing the lateral heat spreading effect. Intrinsic thermal conductivity of different fluids was obtained within a proper range of frequency and flow velocity where the forced convection effect is negligible. The forced convection effect became prominent at high flowing velocity and at low modulation frequency, leading to higher effective thermal conductivity of the fluid. It is found that the intrinsic thermal conductivity could be obtained when the flow velocity is less than 100 mm/sec and ReD1/2Pr1/3 < 100 for DI water and Xceltherm oil under the specified experimental conditions, where ReD is the Reynolds number and Pr is the Prandtl number.

Free dendritic growth model considering both interfacial nonisothermal nature and effect of convection for binary alloy

Considering both the effect of the nonisothermal nature of the interface as well as the effect of forced convection, an extended free dendritic growth model for binary alloys was proposed. Comparative analysis indicates that the effect of convection on solute diffusion is more remarkable compared with the ignorable effect of convection on thermal diffusion at low bath undercooling, due to the fact that solute diffusion coefficient is usually three orders of magnitude less than thermal diffusion coefficient. At high bath undercooling, the effect of convection on the dendritic growth is very slight. Furthermore, a satisfying agreement between the model predictions with the available experiment data for the Cu70Ni30 alloy was obtained, especially at low bath undercoolings, profiting from the higher values of interfacial migration velocity predicted by the present model with nonideal fluid case than that predicted by the one ignoring the effect of convection.

Unsteady magnetohydrodynamics nano fluid flows past a cut magnetic solid sphere when the mix convection included

The system of engine in a car is one of the magnetohydrodynamic (MHD) implementations. This system applies the basic concept of MHD, which is fluid motion that can conduct electric current because of a magnetic field. Several researchers therefore commenced to develop these ideas by observing the characteristics of the numerical simulation solutions. Because of the significance of this research, we are interested in assessing the effect of forced convection on MHD fluid flow, especially nano-fluids, when passing over a magnetic object. We therefore consider the MHD analysis of nano-fluids through sliced magnetic spheres that are induced by mixed convection problem using a mathematical model. This mathematical model is developed from the continuity equation, the momentum equation and the energy equation. The model is then elucidated numerically by means of the finite difference method with the Keller-Box scheme and MATLAB software. Furthermore, within numerical simulations investigated the effects of magnetic parameters (M), slice angle parameters (θs), and volume fraction of nano-fluid (X) towards fluid velocity and temperature. The numerical solutions express that when the magnetic parameter (M) rises, the velocity of the nano-fluid drops and the temperature of the nanofluid growths. When the cut angle parameter (θs) enlarges, the velocity of the nano-fluid enhances and the temperature of the nano-fluid lessens. When the volume fraction of nanofluid (X) expansions, the velocity of the nano-fluid falls and the temperature of the nanofluid rises.

Microwave‐assisted forced convection drying effect on bioactive compounds of the Canadian blueberry leaves (Vaccinium corymbosum)

Canadian blueberry leaves (Vaccinium corymbosum) are a potential source of polyphenols (catechins) and antioxidants (FRAP and PT). In this study, the technique of drying by microwave-assisted forced convection was carried out to find the optimal conditions which would allow obtaining the highest polyphenol content under control parameters such as microwave power (100, 300, and 400 W), hot air velocity (0.5 and 2.0 m/s), leaf size (whole and fractional), and temperatures (50, 60, and 70°C). It was found that 30 W/g microwave powers and a 60°C temperature for the process in whole leaves were the optimal conditions that allowed obtaining the highest content of polyphenols (>10 mg/gdb (+) catechin) and antioxidant capacities (100–150 mg GAE/gdb), more than in fractional leaves. It was observed that different hot air velocities did not affect the concentration of the polyphenol’s contents; this was verified using the factor mixture analysis technique. Practical applications The MWFC process allows the preservation of antioxidants in Canadian blueberry leaves (Vaccinium corymbosum). These leaves have high antioxidant capacity for nutraceutical and pharmaceutical uses. This leaf, with high polyphenol content, can be used to obtain extracts in infusions and human food through the mixture of this leaf with multiple powdered products traditionally consumed or food supplements. This research presents the optimal conditions for the preservation of antioxidant compounds in dried Canadian blueberry leaves.

Evaluation of Active Heat Sinks Design under Forced Convection-Effect of Geometric and Boundary Parameters.

This study shows the performance of heat sinks (HS) with different designs under forced convection, varying geometric and boundary parameters, via computational fluid dynamics simulations. Initially, a complete and detailed analysis of the thermal performance of various conventional HS designs was taken. Afterwards, HS designs were modified following some additive manufacturing approaches. The HS performance was compared by measuring their temperatures and pressure drop after 15 s. Smaller diameters/thicknesses and larger fins/pins spacing provided better results. For fins HS, the use of radial fins, with an inverted trapezoidal shape and with larger holes was advantageous. Regarding pins HS, the best option contemplated circular pins in combination with frontal holes in their structure. Additionally, lattice HS, only possible to be produced by additive manufacturing, was also studied. Lower temperatures were obtained with a hexagon unit cell. Lastly, a comparison between the best HS in each category showed a lower thermal resistance for lattice HS. Despite the increase of at least 38% in pressure drop, a consequence of its frontal area, the temperature was 26% and 56% lower when compared to conventional pins and fins HS, respectively, and 9% and 28% lower when compared to the best pins and best fins of this study.

Heat and mass transfer analysis of combined convection in a horizontal rectangle

AbstractIn this study, the heat and mass transfer of combined free and forced convection in the horizontal rectangle is explored. The governing equations together with the boundary conditions are solved numerically by using the finite volume method. The innovative idea in this study is to appropriately modify the Semi‐Implicit Method for Pressure‐Linked Equations algorithm and thereby, the numerical solutions of the flow variables such as the temperature and the concentration in addition to the components of velocity and the pressure are computed. The Richardson numbers (Ri) for distinct gases and liquids are calculated for different Rayleigh numbers at low (Re = 50) and high (Re = 5000) Reynolds numbers. The dimensionless parameters, such as the Reynolds number (Re), the Prandtl number (Pr), and the Schmidt number (Sc) are appropriately chosen to calculate the Richardson numbers. Consequently, combined free and forced convection effects are analyzed. Furthermore, the heat and mass transfer aspect for distinct gases and liquids is critically examined using empirical correlations. The accuracy and the validation of these results are ensured owing to the solutions obtained from correlations being advised in this study and those are existing in the literature.

Experimental Study of the Effect of Intermittent Electromagnetic Stirring on the Columnar-Equiaxed Transition in Sn–10 wt % Pb Alloy Solidification

Abstract Experimental analysis of stratification, electromagnetic stirring (EMS), and solidification were carried out for two solidification experiments for a binary Sn–10 wt % Pb alloy. The objective of this study is to examine the effect of forced convection driven by an intermittent traveling magnetic field on the solidification process. Several aspects were investigated, namely, thermal field, macrostructure, and finally segregation behavior, as well as morphology. The effect of the both thermal and solutal stratification on the intensity of the flow is discussed yet showed that stratification has a stabilizing effect for the flow, which can also slow the convective hydrodynamic movements generated by the buoyancy forces. The consequence of this stratification on macrosegregations and channel segregation, which develop during the solidification period, is experimentally analyzed. EMS by intermittent traveling magnetic promotes the development of the columnar-equiaxed transition mechanism (CET), more particularly the refinement of the grain size. The results illustrate, also, that EMS effectively diminish macrosegregations significantly, while remaining inactive for reducing channels segregation development.

Effect of forced convection on the hygrothermal performance of a wood frame wall with wood fibre insulation

Wood-based thermal insulation materials have interesting hygroscopic characteristics because of their high moisture capacity. The present paper investigates the moisture conditions in wood-frame walls with wood fibre thermal insulation. The effect of forced convection of moist air into the construction is studied in particular. Laboratory measurements were carried out on a full-scale wall model divided in five sections with various configurations. The wall sections had different combinations of exterior air barrier, thermal insulation and interior vapour retarder. Each section was prepared with an air leakage to simulate supply of moist air to the wall construction through forced convection. The measurement results provided insight in the moisture conditions in the wall and internal distribution of moisture in the wall sections. The results show that walls with wood fibre insulation may have the same risk of high moisture levels as walls with mineral wool insulation. However, the results indicate that wood fibre insulation absorbs condensation and melting water, while the mineral wool does not. Hence, the results imply that the wood fibre insulation has the benefit of distributing the moisture over a larger volume than the mineral wool insulation. Furthermore, the investigations indicate that using an exterior air barrier with high thermal resistance results in a generally lower moisture level in the wall construction, which can be considered favourable regarding risk of mould growth.

Unsteady development of Marangoni convection in a sidewall moving open cavity

In this study, simultaneous effect of free, forced and Marangoni convection on fluid flow and heat transfer in a sidewall moving open cavity is investigated. The cavity is filled with low Prandtl number fluid (Pr = 0.054); such a situation can be find in the application of liquid metal and semiconductor melts. Free convection is induced due to the heating of fluid from the bottom of the cavity. Forced convection is originated due to the translation of the sidewalls either in upward or downward direction. Whereas, the Marangoni convection is generated in the upper portion of the fluid due to imposed temperature gradient along the top free surface. The transient phenomena of the combined effects are analyzed numerically in dimensionless forms utilizing an in-house CFD code. The analyses are conducted under different pertinent parameters such as Marangoni number, Biot number, Reynolds number and Richardson number. The study is conducted to explore the flow-field structures and heat transfer characteristics in the presence of three convection modes with their relative strengths.

Sustainable Electrochemical Extraction of Lithium from Natural Brine: Part II. Flow Reactor

An electrochemical flow reactor for the extraction of lithium chloride from natural brine has been designed and experimentally tested. The reactor comprises two three-dimen-sional porous packed bed electrodes and a porous separator immersed in electrolyte. The packed bed electrodes were filled with conducting petroleum coke particles covered respectively with LiMn2O4 selective to lithium ion and polypyrrole selective to anions. The reactor operates in two steps: In the first step the porous electrodes and separator were filled with natural brine to extract lithium and chloride by intercalation and adsorption respectively. After rinsing with water, in the second step the reactor was filled with a dilute LiCl recovery solution and by reversing the electrical current LiCl is recovered in the electrolyte. A two dimensional mathematical model which describes the diffusion and migration of different ions in the electrolyte with the Nernst-Planck equation, the convective flow of electrolyte and the lithium ion intercalation in LiMn2O4 has been developed using a finite element method under the COMSOL environment. The model captures the effect of forced convection on the efficiency of lithium extraction due to diffusion gradients in the porous LiMn2O4 cathode and predicts the best operation parameters.

CARBON POLLUTION CONCENTRATIONS PROGNOSTICATION: CASE STUDY OF ROAD POLLUTION IN CONCEALED TUNNELS

Air pollution carries significant risks for human health and the environment. Vehicles powered by fossil fuels are major contributors to air pollution. In order to reduce carbon related ailments among road users, the polluted air in our roads need to be decarbonized. The aim of this paper is to present a two dimensional grid based numerical carbon pollution model prognosticating carbon pollutant concentrations in roads, generated by car exhausts with a case study of concealed tunnels. The results of the findings on concentration of carbon pollutants in concealed road tunnels with ventilations and ventilation fans on road sections of the tunnel have been presented. Forced convection has been brought in by introducing suction pumps and accelerating the air into the tunnel to achieve the desired results. Godunov finite volume technique is used to help in prognosticating carbon pollutant concentrations at different levels from the source. With the technique, it has been possible to predict the carbonaceous deposits in the atmosphere and the effect of forced convection on concentration of carbon pollutants from the source.

Computational investigation on heat transfer augmentation of a circular tube with novel hybrid ribs

Present study reports a computational investigation on heat transfer and pressure drop characteristics for flow through a heat exchanger tube fitted with novel hybrid ribs by using magnetic nanofluid (Fe3O4). Effects of different rib geometry on heat transfer and pressure drop characteristics have been investigated for Reynolds number ranging from 3 000 to 22 000. Until now, there is little information available in the literature on the method of quantifying the effect of forced convection on the heat transfer and pressure drop of hybrid rib (HR) inserts by using magnetic nanofluid (MNF). The transition SST models along with governing equations (continuity, momentum, and energy equations) are numerically solved with ANSYS Fluent 19.2. The simulation results are validated with established correlations and excellent agreement was found. Heat transfer coefficient is more in combined arrangement (HR and MNF) compared to acting alone arrangement (only MNF).

Thermal performance enhancement of phase change material using aluminum-mesh grid foil for lithium-capacitor modules

Nowadays, the most used thermal management system (TMS) is air-cooling, but due to weight and volume limitations, more promising cooling methods such as PCMs with high latent heat seems to be attractive. However, their disadvantages, like low thermal conductivity, should be managed to have an efficient TMS. Hence, aluminum-foil (PCM-Al) is added to the PCM in this work to enhance the PCM conductivity, and accurate experiments are accomplished to verify the simulation results. In this paper, a phase change material (PCM) with aluminum mesh grid foil is proposed to enhance cooling and temperature uniformity of a high-power dual-cell lithium capacitor (LiC) module. The optimization objective is keeping the temperature of the system below the safe range while maximizing the uniformity and minimizing the cost of the system. The effect of forced-convection (zero-PCM strategy), pure PCM, PCM-Al, PCM thickness, and influence of dual PCMs by different phase change temperatures on the thermal behavior of the module are studied numerically. The results indicate that the PCM-Al technique shows better thermal performance while reducing the maximum module temperature by 20%, and 13% in comparison with forced-convection and pure PCM, respectively. Moreover, 7 mm thickness is quite optimal, considering the cost, weight, and volume.

GaN-on-Si Calorimetric Thermal Conductivity Gas Sensor

Introduction Thermal conductivity gas sensors are used in a multitude of fields where identifying concentrations of certain gasses within mixtures is important, such as in exhaust emission monitoring, chemical process control to name some examples. There is also a growing demand to be able to place sensors in more extreme environments closer to the source of the reaction/emissions. Traditionally, silicon-on-insulator (SOI) is the material of choice for MEMS for use in harsh environments, however SOI has a limit of around 300°C before it can no longer reliably sense [1].In this work, we present a Gallium Nitride-on-Silicon (GaN-on-Si) thermal conductivity calorimetric gas sensor. Depending on the specific process, devices fabricated from GaN can survive temperatures up to 1000°C [2] [3]. As a wide band-gap semiconductor, GaN devices do not suffer the adverse effects of excessive carrier generation that silicon-based devices do [4]. Using GaN, it is possible to fabricate heterogenous devices by growing an alloy layer on top of the GaN layer. In the case of this work, the alloy is Aluminium Gallium Nitride (AlGaN). Just below the interface of the two materials, piezoelectric polarisation induced by the lattice mismatch of the two materials causes a thin layer (angstroms) of electrons to form a 2-dimensional electron gas (2DEG) [5]. Currently several factors limit GaN’s mass market appeal, such as yield and cost, although with more development, GaN is a candidate for the harsh environment sensing domain.This work presents for the first time a thermal conductivity calorimetric gas sensor fabricated in a GaN-on-Silicon process, with 2DEG sensing elements on a membrane structure. Thermal Conductivity Sensing Thermal conductivity gas sensors use heat transfer principles to detect specific gasses or gas concentrations. A heating element used in conjunction with one or more temperature sensing elements can be used to measure the transfer of heat from heater to the gas mixture. Measuring this heat transfer is the basis for this method of gas detection. The Sensor The sensor die is square at 1.6 mm x 1.6 mm. The device consists of a heating element placed in the centre of the membrane with thermopile temperature sensor upstream and downstream of the heater [6][7]. The thermopiles are made up of 5 thermocouples where the junction materials are gold and the AlGaN/GaN 2DEG. The combination of upstream and downstream temperature monitoring along with the heating from the hot wire allow the temperature profile of the gas to be detected and thus the concentration. Figure 1 shows an optical image of the sensor die with the specific devices highlighted. Experiment The experiment was conducted using a custom gas rig consisting of 3 mass flow controllers (MFCs) interfaced with a Raspberry pi. The different MFCs enable the gas concentration to be controlled so that measurements can be conducted to measure the sensitivity of the sensor to various gas concentrations. In this work, zero air is used as a carrier gas with Hydrogen being the gas of interest. Hydrogen was chosen principally due to its high thermal conductivity (185 mW/mK) compared to zero air (28 mW/mK). Flow rate was kept constant at 200 SCCM/Min so that the effect of forced convection due to flow velocity on the temperature profile can be discounted.Initial Results will be presented at the conference. Acknowledgments This work was supported by the Innovate UK project “GaN Sense” (file ref. 103446).

Flow and heat transfer simulation in a wall-driven porous cavity with internal heat source by multiple-relaxation time lattice Boltzmann method (MRT-LBM)

Laminar mixed convection characteristics in a wall driven porous cavity with an isothermally heated square blockage inside have been investigated numerically by the Non-orthogonal multiple-relaxation time lattice Boltzmann method (MRT-LBM). Various directions of wall driven and placement of the blockage have been considered. In the current study, the geometrical and flow parameters being investigated are the directions of wall driven, blockage position (ex, ey), the Richardson number (Ri). From the analysis of the mixed convection process with different directions of wall driven, the results show that under the configuration of central placement of the blockage, the most preferable heat transfer is obtained in the right wall driven at any values of the Richardson number. Because the movement of the wall can lead to an assisting effect on the buoyancy flow in the right wall driven. Since the right wall driven has the best heat transfer effects, the influences of blockage location and Richardson number is investigated in this condition. When the value of the Richardson number is 0.1, the most preferable heat transfer is obtained when the blockage is placed at the top right and bottom right. When the value of the Richardson number is 1.0 or 10.0, both the middle left and middle right blockage placement have the best heat transfer rate. That is mainly due to the influence of the size, strength and position of the vortex, the combined effect of forced convection and natural convection is stronger when the blockage is placed at those locations.

Thick exchange layer evaporation model with natural convection effect and evaporation experimental study for multicomponent droplet

In order to investigate the high-temperature evaporation characteristics of multicomponent liquid fuel, three kinds of blended fuel: n-heptane/n-decane/RP-3 aviation kerosene-ethanol were experimentally studied with and without forced convection. Further, based on zero-diffusion and infinite diffusion concept, this study expanded Thick Exchange Layer evaporation model with Natural Convection effect (NC-TEL) to multicomponent liquid fuels. The experimental results show that the droplet evaporation rate increases significantly with the increase of ambient temperature. Higher temperature leads to more significant relationships between the composition ratio and the evaporation rate. The effect of forced convection is not obviously under the circumstance in this paper. Then, the evaporation models were validated by experimental data. In general, the new NC-TEL model behaves better than the Ranz-Marshall (R-M) model, and the prediction accuracy at high temperature is improved by 8% to 35%. In lower temperature conditions, the prediction of zero-diffusion NC-TEL model is better than the infinite diffusion NC-TEL model. In high-temperature conditions, for n-heptane-ethanol droplet, the predictions of NC-TEL model are accurate, but for n-decane/RP-3 aviation kerosene-ethanol, the predictions are lower than experimental results. This may be caused by the micro-explosion phenomenon and the Marangoni phenomenon.

Enhanced resolution of generator-collector studies of enzymatic structures by means of hydrodynamic scanning electrochemical microscopy

In this report, the effects of forced convection on scanning electrochemical microscopy (SECM) studies of enzymes in the context of the generator-collector mode (G/C mode) were investigated. Forced convection was generated via an electrical high precision stirrer integrated into the electrochemical cell. Circular spots of glucose oxidase were immobilized on a gold support serving as model substrate. The diffusion layer of enzymatically generated H2O2 was characterized recording probe scan curves (PSCs) in z-direction. Furthermore, the enzyme-modified surfaces were investigated via constant-height SECM imaging in feedback mode and in G/C mode. For methodical comparison all sets of experiments were performed in quiescent solution (conventional approach) and with forced convection, respectively. In contrast to a growing diffusion layer without forced convection by applying forced convection, a constant diffusion layer of produced H2O2 was observed. Hence, via hydrodynamic SECM time-independent images within a reasonable time scale of SECM measurements in G/C mode were enabled and their resolution was enhanced.

Simulation of eutectic growth using phase field method and LBM based on OpenCL

During the solidification of alloys, numerical simulation of forced convection eutectic growth is essential for accurate prediction and control of solidified microstructure. The phase field lattice Boltzmann model (PF-LBM) is established by coupling the phase field model of KKSM model with the LBM model of computational fluid. In order to solve the problems of PF-LBM model, which has a large amount of computation, the simulation region is small and can not be solved in a reasonable time. The parallel computing architecture based on OpenCL can give full play to the powerful computing power of NVIDIA GPU and accelerate the implementation of the algorithm to improve the efficiency of numerical simulation. The effect of forced convection on the eutectic growth of CBr4-C2Cl6 eutectic alloy is systematically studied. The results showed that liquid flow altered the eutectic evolution by affecting the distributions of the solute ahead of the eutectic solid/liquid interface. Compared with serial CPU code, a single GPU can achieve a maximum acceleration ratio of 136.X. Solved the problem of 3-D numerical simulation of forced convection eutectic growth due to computational limitations.

Microstructural Changes and Quality Improvement of Al7Si0.2Mg (356) Alloy by Die Vibration

The influence of amplitude and frequency of die vibration during solidification on microstructural evolution of Al–7Si–0.2Mg (356, LM-25) alloy was studied. The amplitude of die vibration was varied from 0.0 to 1.05 mm at 50 Hz frequency, and the frequency was changed from 30 to 50 Hz at a amplitude of 0.75 mm. Structural examination and quality of the casting were evaluated in terms of porosity at various processing conditions. Vibration modified and refined structure during gravity die casting of the alloy. Macrostructure of casting prepared in vibrating die consisted of fine equiaxed grains. In contrast, macrostructure of casting produced in stationary die typically consisted of columnar grains at the periphery and equiaxed grains at the center. Die vibration resulted in microstructure of mixed type comprising of globular and dendritic primary α-Al with interdendritic eutectic Si particles. On the contrary, microstructure of casting produced in stationary die consisted of dendritic α-Al structure and eutectic Si particles. In addition, die vibration reduced secondary dendritic arms spacing (SDAS) to 18.98 μm from 34.38 μm obtained without vibration. Since SDAS is a measure of cooling rate, its reduction due to die vibration implies an increase in cooling rate of casting. This is attributed to the forced convection effect generated by die vibration. Consequently, the higher cooling rate owing to the die vibration reduced microsegregation of Si and Mg in the casting. Further, structural modification and refinement due to die vibration improved the quality of casting significantly in terms of porosity.