Convection Heat Transfer Modes Research Articles

Mathematical analysis of heat and mass transfer efficiency of bioconvective Casson nanofluid flow through conical gap among the rotating surfaces under the influences of thermal radiation and activation energy

In the current proceeding, the flow of incompressible non-Newtonian nanofluid called Casson nanofluid is considered. A conical gap occurred among the rotating disc and the cone filled with the fluid flow. Heat and mass transport through this nanofluid is done by the convection mode of heat transfer. The impacts of microorganisms, chemical processes, thermal radiation, minimal amount of energy, and magnetic field are also considered in the mathematical model of flow problem. The Casson nano-fluid governing equations are interpreted in cylindrical coordinates. By implementing proper similarity transformations, the modeling PDEs of energy, momentum, concentration, and microorganism density are transformed into non-linear ODEs. A set of non-linear ODEs that deals with the distributions of temperature, velocity, concentration, and motile microorganisms produced by this technique. MATLAB in-built ‘bvp4c‘ technique is utilized to solve these equations. Findings are displayed graphically and elaborated theoretically. The primary goal of this work is to examine the effects during the rotation of the disc and cone as well as the impacts of other variables on the rotation. The nano-fluid temperature and radial velocity are found to be negatively impacted by the rotation parameter whereas azimuthal velocity is positively impacted. The parametric values are taken as 0.1<Nb<0.7, 0.1<λ<0.4, 1.0<Pr<2.50.1<M<0.4, 1.0<Rb<4.0, 0.1<Nr<0.4, 0.1<Sc<3.0, 0.1<Nt<0.4 for the purpose of generating modified results. From published and current results noted that rotation parameter effects impacted the transfer rate of the nanofluid.

Inactivation of polyphenol oxidase and peroxidase in pineapple juice during continuous ohmic heating and modeling of inactivation kinetics during isothermal holding

AbstractContinuous ohmic heating (COH) is a novel thermal technology for fruit juice processing and preservation. The present work demonstrated the effect of COH parameters like electric field strength (EFS: 30–40 V/cm), isothermal holding (0–60 s), and temperature (70–90°C) on the inactivation of polyphenol oxidase (PPO) and peroxidase (POD) in standardized pineapple juice. Statistical parameters and Akaike information criteria were used to compare kinetic models like the first order, distinct isozymes, Weibull distribution, sigmoidal logistic, and fractional conversion for PPO and POD inactivation. The findings revealed that the COH parameters significantly affected the activities of both enzymes. A maximum inactivation of 68.2% of PPO and 82.2% of POD was observed at 90°C, 60 s, and 40 V/cm. The Weibull distribution model, compared to other models, was found to describe the inactivation kinetics better for both PPO (R2 &gt; 0.99; RMSE &lt; 0.0101) and POD (R2 &gt; 0.98; RMSE &lt; 0.042). The accuracy factor (Af) and bias factor (Bf) for both PPO (Af = 1.003–1.010; Bf = 1.000–1.004) and POD (Af = 1.007–1.072; Bf = 0.992–1.009) were closer to simulation line suggesting the accuracy of the Weibull model in predicting the enzyme inactivation. Also, Akaike increment (∆i &lt; 2) substantially supported the Weibull model. The shape factor (n &lt; 1) explained the tailing phenomenon of the enzymes. The PPO's scale factor (δ values) was higher than POD, suggesting higher thermal stability of PPO than POD. Thus, the Weibull model is a good tool for predicting the enzyme inactivation in COH‐treated standardized pineapple juice.Practical ApplicationsContinuous ohmic heating (COH) is a novel electro‐thermal technology that heats the fruit juice quickly, uniformly, and volumetrically. Generally, for inactivating spoilage enzymes like polyphenol oxidase and peroxidase during pineapple juice processing in an industrial set‐up, a conventional method of conduction and convection mode of heat transfer is used. This results in a high‐temperature gradient within the juice matrix, causing a problem of over‐ and under‐processing of juice. Ohmic heating is a potential and efficient way of inactivating these spoilage enzymes to a desired level in a shorter processing time. It can be used to preserve juice by extending its shelf life. COH can reduce the total heat damage to the juice by maintaining its flavor and nutrient compounds. Overall, ohmic heating is a versatile and effective technology for the continuous processing of fruit juice for improved product quality and production efficiency.

Experimental and computational study of convective heat fluxes in swirling two‐phase flows

AbstractThe convective mode of heat transfer is mainly due to the bulk motion of the fluid. Its turbulent nature and enhanced heat transfer coefficients have always attracted the academic, scientific community, and industrialists for many decades. The current research is based on the experimental and theoretical investigations on the turbulent convective heat transfer in swirling (60–300 rpm) steam (1–3 bars) injection into cocurrently flowing water. An exponential increase in the convective heat transfers up till the most swollen part of the swirling steam‐water volume of fluids has been observed. However, the convective heat transfer of the remaining part of the steam's plume shows an almost stagnant decreasing trend. The range of Rayleigh number that supports the transition in trends of the convective heat fluxes is 2.84 × 1011–3.71 × 1011. This transition affects the magnitude of the convective heat fluxes and the extent of the effective momentum fluxes, which is evident in the dominant role of the flow instabilities acting across the length of the steam's plume. Computational Fluid Dynamic analysis also has supported the exhibition of the heat fluxes magnitudes under the influence of the interacting Kelvin–Helmholtz instabilities and inertial instabilities across and along with the cocurrently acting shear layer.

A methodology for measuring heat transfer coefficient and self-similarity of thermal regulation in microvascular material systems

Fluid transport through microvascular networks—a hallmark for homeostasis in living systems—has transcended to engineered materials, primarily made possible because of modern manufacturing advancements. Vascular-enabled multifunctionality, including thermal regulation and self-healing, holds great potential for extending the lifetime of structural materials and expanding the operational envelope. Prior studies on vascular-based active cooling use a “combined” heat transfer coefficient (HTC): a single parameter lumps convection and radiation effects. Although the resulting mathematical models are linear—an attractive feature for computational modeling, the combined coefficient approach may not be accurate or even applicable if the operating temperature is unknown, which is the case with many thermal regulation applications (e.g., space probes). In this paper, we illustrate the remarked limitations of the lumped approach and advocate the need to use a decoupled HTC by splitting convective and radiative heat transfer modes. We show the broad applicability of the proposed method by applying it to three material systems: glass and carbon fiber-reinforced polymer composites and an additive manufactured metal. We show, using numerical simulations, the differences in the predictions from the decoupled approach with that of the combined HTC; these differences are prominent at higher heat fluxes. Also, the decoupling has enabled us to establish a scaling law that allows transferring of solutions fields across material systems, strengthening further the validity and utility of our approach. This work's significance is two-fold. First, the research is fundamental, providing accurate measurement protocols for critical model parameters. Second, this work facilitates the development of mathematical models for vascular-based thermal regulation that are predictive even for hostile environments (which are often difficult to realize in laboratories), such as outer space.

Analysis of effects of moving wall direction in a one sided semi-circular porous cavity on mixed magneto-bioconvection in the presence of hybrid nanofluid with oxytactic bacteria

A computational analysis on the effects of direction of wall movement in a one sided semi-circular porous cavity on mixed magneto-bioconvection of hybrid nanofluid in the presence of oxytactic bacteria has been performed. The working fluid is the hybrid nanofluid. The enclosure is cooled from semi-circle side and heated from moving lid under the effect of uniform magnetic field in the horizontal direction. The numerical solution of governing equations is obtained by using the higher order finite element method (FEM). The reliability of designed solver is tested through the experimental and numerical data available in the literature. Two cases of the boundary conditions are considered related to the tendency of right vertical lid of the cavity. Other parameters are Richardson number, Darcy number, Hartmann number, Peclet number and porosity. It is found that moving lid direction is the most effective parameter for the flow field, isotherms, oxygen and microorganism distributions. Moreover, higher heat transfer rate is obtained in Case-I for different values of Darcy number. Around 10% increment is occurred on mass transfer with different Pe number in case of downward moving lid due to increment of convection mode of heat transfer inside the cavity.

Optimization of Brake Disc Profile Based on the Thermal Performance for Electric Vehicles

The brake disc market has reached the value of USD 20.9 billion in 2021 at a CAGR of 4.9% and it is expected to grow to USD 26.5 billion by 2026. However, the difficulties associated with the brake discs are significant rise in the temperature which results in, early wear, thermal cracks and brake fade leading to uneven braking and premature replacement. In this context many researchers have been studying the parameters like brake disc material and type of ventilation. The objective of the work was to study of the effect of ventilation types on thermal responses and optimize the brake disc profile for better thermal performances. Center of gravity and dynamic load transfer were calculated for two wheeler electric vehicle experimentally which was followed by computing the force required for braking process and heat flux on the brake disc analytically. Ventilation of type cross drilled holes (CD) was adopted in brake disc modelling, and was carried out in SOLIDWORKS 2022 and finite element model for the brake disc was created in ANSYS 17.2 with mesh size of 2 mm for lamellar graphite iron (LGI) with temperature dependent material properties. The transient thermal analysis for three braking cycles were carried out with the considerations such as: initial velocity of the vehicle of 96 km/h, ambient temperature of 22°C, conduction and convection modes of heat transfer and 5 s of deceleration and acceleration time. Transient thermal analysis was carried out by applying the heat flux on the frictional surface of the brake disc. For the solid disc without ventilation, it was found that the maximum temperature at the end of first braking cycle (0 to 5 s) was 157.72°C, at end of second braking cycle (10 to 15 s) was 256.95°C and at the end of third braking cycle (20 to 25s) was 349.73°C. As the wear rate for lamellar graphite iron grows exponentially beyond 300°C, ventilations were provided on the brake disc to reduce the temperature on the disc. For the optimized brake disc with ventilation type VT-12, it was found that maximum temperature at the end of first braking cycle (0 to 5 s) was 139.43°C, at end of second braking cycle (10 to 15 s) was 222.68°C and at the end of third braking cycle (20 to 25s) was 299°C. The overall character of these shows a quick rise in temperature at the start of the process, followed by the achievement of the maximum value and, eventually there is a drop in temperature. In this research the future work can be carried out by implementing the noise, vibration and harness studies which would benefit the industry with better braking system.

Free convective trickling over a porous medium of fractional nanofluid with MHD and heat source/sink

Nanofluids are considered as smart fluids that can improve heat and mass transfer and have numerous applications in industry and engineering fields such as electronics, manufacturing, and biomedicine. For this reason, blood-based nanofluids with carbon nanotubes (CNTs) as nanoparticles in the presence of a magnetic field are discussed. The nanofluid traverses the porous medium. The nanofluids move on a vertical plate that can be moved. The free convection heat transfer mode is considered when the heat source and heat fluxes are constant. Convective flows are often used in engineering processes, especially in heat removal, such as geothermal and petroleum extraction, building construction, and so on. Heat transfer is used in chemical processing, power generation, automobile manufacturing, air conditioning, refrigeration, and computer technology, among others. Heat transfer fluids such as water, methanol, air and glycerine are used as heat exchange media because these fluids have low thermal conductivity compared to other metals. We have studied the effects of MHD on the heat and velocity of nanofluids keeping efficiency in mind. Laplace transform is used to solve the mathematical model. The velocity and temperature profiles of MHD flow with free convection of nanofluids were described using Nusselt number and skin friction coefficient. An accurate solution is obtained for both the velocity and temperature profiles. The graph shows the effects of the different parameters on the velocity and temperature profiles. The temperature profile improved with increasing estimates of the fraction parameter and the volume friction parameter. The velocity of the nanofluid is also a de-escalating function with the increasing values of the magnetic parameter and the porosity parameter. The thickness of the thermal boundary layer decreases with increasing values of the fractional parameter.

Effects of surface roughness in multiple microchannels on mixed convective heat transfer

• Effect of process-induced surface roughness on mixed convection was investigated. • Height, width and average roughness parameters were studied on multiple microchannels. • Surface roughness is an important geometric parameter and affects heat transfer. • Mixed convection increases with decreasing cross-section and increasing roughness. The most widely used type of heat transfer in applications is convection heat transfer, and both natural and forced convection modes have an effect in total heat transfer to a certain extent, whether any of these modes is neglected or not. In this study, the contribution of natural and forced convection effects to total heat transfer in rectangular microchannels was investigated experimentally for determining the effect of manufacturing process-induced surface roughness in multiple microchannels fabricated using stainless steel material. The experiments were conducted with microchannel heat sinks having three different surface roughness values of 1.1 µm, 1.8 µm, and 3.0 µm, three microchannel widths of 300 µm, 500 µm and 700 µm and two heights of 300 µm and 450 µm. During the manufacturing process, surface roughness values were controlled by changing electro-erosion fabrication parameters, and detailed surface maps were generated for the microchannel heat sinks. In the experiments using pure water as the working fluid, constant heat flux was applied to the bottom surface of the microchannel heat sink. The experiments were conducted in the range of 10–80 Reynolds numbers to ensure mixed convection conditions. In conclusion, it was determined that surface roughness had a significant effect on mixed convective heat transfer. An increase in surface roughness from 1.1 µm to 1.8 µm in microchannel heat sinks with cross-sections of 300 µm × 300 µm, 700 µm × 300 µm, 300 µm × 450 µm, and 700 µm × 450 µm s led to an increase of about 24 %, 19 %, 17 %, and 13 %, respectively, in the Nusselt number. Likewise, the increase in surface roughness from 1.8 µm to 3.0 µm resulted in an increase of 17 %, 15 %, 14 %, and 10 %, respectively, in the Nusselt number in the same channel cross-sections. In general, an increase in hydraulic diameter led to a reduction in the effect of surface roughness on overall heat transfer. For a constant microchannel width and height, the effect of surface roughness on mixed convection could be observed only with the increase in the Grashof number. Among the eighteen microchannel heat sinks with different geometric parameters, the best heat transfer results were obtained for the smallest microchannel cross-section of 300 µm × 300 µm and the highest roughness value of 3.0 µm.

Thermal efficiency in hybrid (Al2O3-CuO/H2O) and ternary hybrid nanofluids (Al2O3-CuO-Cu/H2O) by considering the novel effects of imposed magnetic field and convective heat condition

Comparative heat transfer analysis in conventional (Al2O3-CuO/H2O)hnf and modified hybrid nanofluid known as ternary hybrid nanofluid (Al2O3-CuO-Cu/H2O)mhnf is one of the potential topics of interest and is the demand of modern technological world to cope with the heat transfer problems faced by the industrialists and engineers. Therefore, the study is organized for hybrid and tri-hybrid nanofluids for their thermal storage. To improve thermal energy storage in the above-mentioned fluids, novel effects of a uniform magnetic field, convective heat transfer mode, and newly developed empirical correlations are used to synthesize tri-hybrid nanofluids. The comparative analysis under various physical constraints reveals that tri-hybrid nanoliquids have ultra-high thermal efficiency than conventional nanoliquids. Therefore, these fluids are very beneficial for industrial uses more specifically for purification purposes or where ultra-high heat transfer is required to accomplish the process.

Comparative Study on Thermal Analysis of Extended Surface Using ANSYS Simulation

Extended surface are heat exchangers generally made on the external circumference of an object that are mainly used to enhance the heat transfer rates through a combination of conduction and convection mode of heat transfer. In general, extended surface are made up of the materials whose thermal conductivity is maximum to facilitate more amount of heat transfer to or from the environment and also available in different profiles with respect to the applications of the components. Among the different types, the most commonly used are longitudinal type of fins, i.e. when the direction of flow of the fluid is parallel to the axis. In this research work, an attempt has been made to compare three different profiles like rectangular, trapezoidal, and concave parabolic fins using ANSYS simulation software. Thermal analysis using ANSYS simulation is one of the important techniques to analysis the efficiency of the fin with different profiles. The model of the fin profiles is made by fusion 360 and the comparative study on thermal analysis is carried by employing ANSYS simulation. From this research output, the trapezoidal of the extended surface has better results than the conventional rectangular as well as profile concave parabolic in various thermal properties, and hence it is great replacement of in near future.

A parametric study of the novel flow divider-type turbulator on heat transfer enhancement in a circular tube

The outcomes of numerical findings on a parametric study of flow divider-type turbulators in forced convection heat transfer mode were presented in this research study. A numerical study has been conducted using air as the working fluid within the Reynolds number range 7000–21000 under uniform wall heat flux boundary conditions. The turbulators were used with a pitch to tube diameter ratio of 0.54, with an alternate angle of twist and the cyclic angle of twist equal to 90°, 45° and, 30°. The CFD simulation outcomes show that the Nusselt number enhances by 2.34–2.40 times, for an alternate angle of twist of 30°. The maximum thermal enhancement efficiency equal to 1.09–1.15 is obtained for turbulators with an alternate angle of twist of 30°. The Nusselt number enhances by 1.67–2.13 times for tubes fitted with a turbulator with the cyclic angle of twist equal to 45°.

A simple modelling on transmission torque of eddy‐current axial magnetic couplings considering thermal effect

This paper proposes modelling on the transmission torque of an eddy-current axial magnetic coupling (EAMC) considering the thermal effect. A new and simple equivalent reluctance network is established to obtain the magnetic flux density based on the two-dimensional (2D) finite-element analysis. Furthermore, the expressions of the transmission torque and the thermal loss are derived by establishing the computational coordinate system for the conductor sheet (CS) eddy current. Then, a simple and effective thermal network considering different convective heat transfer modes is presented to obtain the temperature rise, and the influence of temperature on some key parameters is given to correct the transmission torque. Furthermore, the proposed modelling is in good agreement with the three-dimensional (3D) finite-element method (FEM) and experiments under normal working conditions. Moreover, some suggestions for the design and optimization of EAMCs are provided.

The evolution of microstructure and composition homogeneity induced by borders in laser powder bed fused Inconel 718 parts

Laser powder bed fusion (LPBF) is a robust metal additive manufacturing technology which can produce precise parts with high complexity and freedom. In a recent work by the authors, a significant number of microstructural defects was observed on the surface of the LPBF-fabricated parts, which could account for the poor tensile strength of the parts. It was revealed that the regions near the surface of the part are more affected by the convection heat transfer mode compared to the areas located away from the surface. In this study, for the first time, a novel technique is proposed to address the issue of microstructure inconsistency in the LPBF-fabricated Inconel 718 parts. This approach is based on the design and fabrication of a border surrounding the main part during the LPBF fabrication. It is hypothesized that such a border, if designed appropriately, would mitigate the undesirable convention heat transfer mode in the areas near the surface of the LPBF-fabricated parts and may result in microstructure homogeneity. To this aim, cubic samples surrounded by borders at two different gap distances of 0.5 mm and 2 mm were fabricated to control the rate of heat dissipation and thus control the microstructure along the cross section and reduce surface defects such as microcracks and porosities. Reference cubic samples were fabricated without borders to compare them with the samples fabricated with borders. Scanning electron microscope (SEM) and optical microscope were used to study the changes in microstructure, X-ray diffraction (XRD) was used to perform compositional analysis, and Vickers hardness test was conducted to study the hardness properties. For the sample fabricated with a smaller gap, the smaller melt pools overlapped to form deeper melt pools and led to the formation of more columnar grain structures elongated parallel to the build direction. The fabrication of borders around the sample also helped in controlling the heat transfer of the sample and thus reducing the surface porosities and defects. A higher level of secondary strengthening phases (γ′, γ″) was seen to precipitate into the matrix for the samples fabricated with borders. Vickers hardness values were higher for the samples fabricated with borders than that of the reference sample.

Design and Performance Analysis of Air Pre heater for Water Tube Boiler to improve its Efficiency

Air preheater is a heat transfer surface in which air temperature is raised by transferring heat from other media such as flue gas which is coming from boiler exhaust. The paper presents the design of regenerative –air preheater to designed to meet specific performance requirements, using the software application CFD (Computational Fluid Dynamics) / CADD. An analytical study was planned to find out the various heat transfer performance parameters like outlet and inlet air temperature of the air preheater and the boiler, Pressure drop inside the Air preheater and the boiler heat transfer coefficients, heat transfer rate, overall heat transfer coefficient and Velocity of the air and flue gas also Conduction and convection modes of heat transfer were found. These heat transfer parameters are critical in designing and functioning of the air Preheater and to calculate the efficiency of the boiler.

Mixed convection heat transfer in multi-Lid- driven trapezoidal annulus filled with hybrid nanofluid

Numerical study of mixed convection heat transfer in multi-Lid driven concentric trapezoidal annulus filled with H2O-Cu-Al2O3 hybrid nanofluid has been investigated. Three cases for multi-Lid driven have been studied: single lid-driven, double lid-driven move in the same direction, double lid-driven move in the opposite direction. The lid-driven walls move with a constant speed with constant cold temperature TC and the other inclined walls are insulated while the inner trapezoidal cylinder heated at constant temperature Th. Finite volume method used to solve the continuity, momentum, and energy equations by SIMPLE algorithm. The results validated by comparing with previous study with a good agreement of accuracy. The working fluids was: water with hybrid nanoparticles (volume fraction ϕ = 0 to 10%). The Richardson numbers changed from 0.01 to 10, to cover all convection heat transfer modes, and aspect ratios were 0.5 and 1. The results show that, the opposing flow produced highest maximum stream function. Moreover, in aiding flow (case 2) produced a heat transfer coefficient on the top and bottom walls of outer cylinder higher than that produced by the opposing flow (case 3). Generally, the skin friction increases with increase in the volume fraction of nanoparticles due to increasing the viscosity of fluid causes increase in shear stress and leads to increasing the pressure drop. Additionally, the aiding flow produced fiction factor higher than the opposing flow.

Effect of evaporator numbers on water production of a free convection air-water harvester

An experimental study was performed to determine the effect of the number of evaporators on the water production and coefficient of performance of an air-water harvester. Water was obtained on the evaporator walls in free convection and condensation heat transfer modes. The evaporator dimension was 480 mm × 285 mm × 6.25 mm, and it was made of copper pipes. The evaporators were installed in a parallel arrangement in an open box (top and bottom open) with a size of 500 mm × 500 mm × 500 mm. The refrigerant R134a was used as the working fluid. The experiments were performed at a low pressure of 40 psi and a high pressure of 180 psi. In the outside evaporator walls, air flowed naturally and part of the water vapour in the air condensed. Three evaporator variations were tested: Case A (one evaporator), Case B (two evaporators), and Case C (three evaporators). Each evaporator contained 25 copper pipes and the length of each pipe was 285 mm. The maximum water production and the evaporator efficiency attained were 0.51 L a day and 13 % respectively. The COP of the machine was ranging from 5.2 to 13.3. Increasing the evaporator number elevated the water production.

Numerical Study Using Microstructure Based Finite Element Modeling of the Onset of Convective Heat Transfer in Closed-Cell Polymeric Foam.

The thermal performance of closed-cell foams as an insulation device depends on the thermal conductivity. In these systems, the heat transfer mode associated with the convective contribution is generally ignored, and studies are based on the thermo-physical properties that emerge from the conductive contribution, while others include a term for radiative transport. The criterion found in the literature for disregarding convective heat flux is the cell diameter; however, the cell size for which convection is effectively suppressed has not been clearly disclosed, and it is variously quoted in the range 3–10 mm. In practice, changes in thermal conductivity are also attributed to the convection heat transfer mode; hence, natural convection in porous materials is worthy of research. This work extends the field of study of conjugate heat transfer (convection and conduction) in cellular materials using microstructure-based finite element analysis. For air-based insulating materials, the criteria to consider natural convection () is met by cavities with sizes of 9.06 mm; however, convection is developed into several cavities despite their sizes being lower than 9.06 mm, hence, the average pore size that can effectively suppress the convective heat transfer is 6.0 mm. The amount of heat transported by convection is about 20% of the heat transported by conduction within the foam in a , which, in turn, produces an increasing average of the conductivity of about 4.5%, with respect to a constant value.

Numerical investigation of free convection through a horizontal open-ended axisymmetric cavity

Objectives: The purposes of this work are to investigate the free convective heat transfer in an axis-symmetric open-ended cavity heated from below and to propose useful correlations of Nusselt number. Methods: The governing equations that model the fluid flow and the temperature field are solved using a control volume-based finite differences method. Under steady state condition, the natural convective flow is considered to be laminar, incompressible and axisymmetric. The Boussinesq approximation with constant thermophysical properties is adopted. Numerical experimentations are performed to deduce the optimum sizes of the calculation domain and the mesh grid. Findings: the obtained results indicate that when Rayleigh number (Ra) and aspect ratio (A) are low the heat transfer is weak and mainly conductive. The increase of Ra and A enhances the convective heat transfer mode thereby the heat transfer is ameliorated. Unlike the Rayleigh Bénard convection, the transition from conduction to convection produces at critical value of Rayleigh number (Rac) that is variable dependent on A. Novelty: To the best of authors knowledge, the formula of (Rac) elaborated in this work for the studied cavity is the first attempt. As well, correlation of Nusselt numbers (Nu) for the cold upper plate in terms of Ra and A is performed. Comparisons between Nu at the lower plate given in previous work and Nusselt number at the upper plate is conducted. Keywords: free convection; circular plates; Nusselt number correlations; open ended cavity; critical Rayleigh number

Multi-fluid Eulerian simulation of binary particles mixing and gas–solids contacting in high solids-flux downer reactor equipped with a lateral particle feeding nozzle

The performance of binary particles mixing and gas–solids contacting, which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds, is carefully investigated by means of a numerical approach in the newly developed high solids-flux downer lignite pyrolyzer (φ0.1 m × 6.5 m). Since binary particles are used in this system, a reasonably validated 3D, transient, multi-fluid model, in which three heat transfer modes relating to the convection, conduction and radiation are considered, is adopted to simulate the flow behavior, temperature profiles as well as volatile contents. The simulation results showed that the solids stream impinges the left wall surface initially and turns towards the right wall in the further downward direction and then shrinks during this process resulting in that the solids concentrate a little more at the central region. In the further downward section of the downer, the particle flow disperses near the right wall and develops uniformly. Meanwhile, the coal phase is slowly heated in the downer and it is found that most of the heat absorbed by the coal is from the convection heat transfer mode. To explore the heat transfer mechanism more quantitatively, two indexes (mixing index and contacting index) are proposed, and it is found that the mixing index initially increased fast and later remained at a relatively flat state. For the contact index, it shows a trend with a first rising and then falling, finally rising continuously. Also, it is found that the convection heat transfer is closely correlated to the contacting status of gas–coal which indicates that the improving of the gas–coal contacting efficiency should be an effective way to strengthen the coal particle heating process.

Performance enhancement of high heat generating IC chips using paraffin wax based mini-channels - A combined experimental and numerical approach

The present study aims at the experimental and numerical investigations of the Paraffin wax-based mini-channels for the performance enhancement of seven high heat-generating IC chips under the natural convection heat transfer mode. Two mini-channels (left and right) are fabricated at the periphery of the substrate board and are filled with the Paraffin wax. The IC chips are placed adjacent to these channels and supplied with non-uniform heat inputs leading to the constant volumetric heat generation in the ranges of 0.4 × 107–1 × 107 W/m3. The numerical analysis is carried out to support the experiments and it is seen that both the results match well with each other within the IC chip temperature variation of 4–5 °C. The temperature of the IC chips is reduced by 37.34–45.79% using the Paraffin wax and thus enhances their performance by 2.33–4.63 times. This confirms the significance of Paraffin wax for the cooling of the IC chips and ultimately enhances the system performance and reliability.