Fluid Convective Heat Transfer Research Articles

On the reticulate pattern and heat transfer performance of the topologically optimized microchannel heat sink

With the increasing integration of electronic devices, the rapid heat dissipation has become a key challenge for performance improvement. In this work, a 2D topology optimization for the channel pattern of a microchannel heat sink is conducted, and a novel reticulate pattern of optimized design of microchannel heat sink is found based on the theoretical and numerical analysis. This pattern has tertiary ranked reticulate channels resulting from the topology optimization, which are highly related to the convection heat transfer of fluid and the conduction in solid. The reticulate channels could significantly optimize the total number of channels in the heat sink, and accordingly regulate the flow distribution among the channels. The profiles of the temperature gradients and velocity exhibit high similarity in all reticulate channels, and lead to a synergistic enhancement of heat transfer. A full scale 3D simulation shows that the reticulate microchannel heat sink has better heat transfer performance than the other optimized heat sinks. With pressure drop of 1 kPa, the heat flux for the reticulate microchannel heat sink can reach up to 405 W/cm2, and its performance evaluation criteria (PEC) can reach up to 3.7.

Heat transfer analysis of impinging flames using field synergy principle

The principle of field synergy well reveals the physics of fluid convection heat transfer. This study was carried out to apply the field synergy principle in a special case by conducting a theoretical and numerical analysis of jet-flame-and-wall impingement heat transfer. Numerical simulations of both premixed-flame and diffusion-lame impingement heat transfer are performed. First of all, traditional evaluation indices successfully used for field synergistic analysis of fluid convection heat transfer are adopted in an attempt to synergistically examine flame impingement heat transfer. Then, a theoretical analysis is carried out, with the results pointing out that the role of internal heat source should be taken into account. Then, a new evaluation index St considering chemical reactions as internal heat source is proposed. The theoretic results of a derivation from St to forced convection heat transfer coupling with numerical treatment of heat release shows good consistence with flame heat transfer data. It hints that for putting forward a comprehensive evaluation index for flame convection heat transfer, great care should be given to coupling the effects of both convection and internal heat source.

Insight into hydrodynamics and heat transfer characteristics in a gas–solid spout fluidized bed from a multiple zone view

In this work, the hydrodynamics and heat transfer characteristics within a gas–solid spout fluidized bed were investigated from a multiple-zone view by employing the coupled computational fluid dynamic and discrete element method. Results show that both the axial and radial particle mixing rates are accelerated with the axial particle mixing rate much faster than the radial one when increasing the spouting gas velocity. Besides, the particle-scale and the multiple-zone heat transfer analyses show that the annulus zone makes the major contribution to inter-particle conduction with the particle–particle conduction and the particle–fluid-particle conduction heat fluxes taking up about 64 % and 73 %, followed by the fountain zone taking up about 28 % and 22 %, respectively. As for the particle–fluid convective heat transfer, the spouting and annulus zones make almost the comparative contributions being about 38 %, followed by the fountain zone being less than 24 %. Moreover, the full-bed time-averaged heat transfer analysis shows that the particle–fluid convective heat transfer plays a dominant role in the heat transfer process in the spout fluidized bed with the particle–particle conductive and the particle–fluid-particle conductive heat fluxes being about 0.1 % and 1 % of the particle–fluid convective heat flux. The results of this study can provide an efficient theoretical reference for the gas–solid flow-mixing-heat transfer processes optimization and intensification of spout fluidized beds.

Thermal conductivity analysis of ceramic membranes for recovering water from flue gas

Condensation is the common mechanism of water intake from gases. Transport membrane condenser is a newly proposed technology to recover water from flue gas based on the mechanism of condensation. Most of the existing heat transfer research on transport membrane condenser focuses on water vapor phase change or fluid convection heat transfer, and there is a lack of research on the thermal conductivity of ceramic membranes. The significance of studying thermal conductivity is to optimize the overall heat transfer performance. The thermal conductivity of alumina ceramic solid materials is not good enough, which can affect the industrial application performance to a certain extent. There is currently a lack of experimental measurements or theoretical calculations to quantitatively characterize the thermal conductivity of ceramic membranes. This affects the optimization of overall heat transfer performance of transport membrane condenser. In this paper, coal fly ash is used as the raw materials to prepare macroporous ceramic membranes. Coal fly ash is a very cheap material. It can greatly reduce the manufacturing cost of transport membrane condenser using coal fly ash to prepare ceramic membranes. The classical theoretical model of thermal conductivity of porous materials is introduced, the calculation and experimental results are compared, and the validity and limitation of the classical models in analyzing the heat transfer problem of transport membrane condenser are discussed. Finally, based on the composite effective medium theory, combining with the experimental results, a modified model is proposed. The modified model reduces the average theoretical error to 7.20%, and enriches the research on the heat transfer theory of transport membrane condenser.

Particle convective heat transfer near the wall in a supercritical water fluidized bed by single particle model coupled with CFD-DEM

Supercritical water fluidized bed (SCWFB) is a promising reactor to gasify biomass or coal. Its optimization design is closely related to wall-to-bed heat transfer, where particle convective heat transfer plays an important role. This paper evaluates the particle convective heat transfer coefficient (hpc) at the wall in SCWFB using the single particle model. The critical parameters in the single particle model which is difficult to get experimentally are obtained by the computational fluid dynamics-discrete element method (CFD-DEM). The contact statistics related to particle-to-wall heat transfer, such as contact number and contact distance, are also presented. The results show that particle residence time (τ), as the key parameter to evaluate hpc, is found to decrease with rising velocity, while increase with larger thermal boundary layer thickness. τ follows a gamma function initially adopted in the gas–solid fluidized bed, making it possible to evaluate hpc in SCWFB by a simplified single particle model. The theoretical predicted hpc tends to increase with rising thermal gradient thickness at a lower velocity (1.5 Umf), while first decreases and then increases at higher velocity (1.75 and 2 Umf). hpc occupies 30%–57% of the overall wall-to-bed heat transfer coefficient for a particle diameter of 0.25 mm. The results are helpful to predict the overall wall-to-bed heat transfer coefficient in SCWFB combined with a reasonable fluid convective heat transfer model from a theoretical perspective.

Improved test method for convection heat transfer characteristics of carbonate fractures after acidizing etching

Understanding of convection heat transfer characteristics of fractures is of great significance to improve the heat extraction efficiency of carbonate reservoir. Previous studies on convection heat transfer of fluid flowing through rock fractures are on either granite or sandstone. Limited experimental research has been performed on carbonate fractures after acidizing etching. In this work, an improved test method is developed to analyze the convection heat transfer characteristics of carbonate fractures after acidizing etching under real-time high temperature and high confining pressure. In this method, the traditional test method of convection heat transfer coefficient is improved by monitoring the temperature of inner fracture surface and flowing water. Two thermocouples are especially arranged inside the sample to monitor the temperature of inner fracture surface along the flow direction, and two other thermocouples for the inlet and outlet water temperatures. The results show that the temperature differences between the fracture surface and the flowing water are significantly dependent on confining pressure, fracture roughness and flow rate, and the maximum temperature difference could be reached 2.2 ◦C, which leads to a significant difference in the convection heat transfer coefficient between the traditional and improved test methods. A larger number of pores, caves, and micro-fractures caused by acid etching are observed on the fracture surface by scanning electron microscopy. The special fracture morphology of carbonate is totally different from those of granite and sandstone in previous studies, and can increase the convection channel and increase the contact area with flowing fluid and results in the inapplicability of the hypothesis to carbonate fractures after acidizing etching. The present work could improve the knowledge of convection heat transfer characteristics of carbonate fractures. Cited as: Zhan, H., Dong, W., Chen, S., Hu, D., Zhou, H., Luo, J. Improved test method for convection heat transfer characteristics of carbonate fractures after acidizing etching. Advances in Geo-Energy Research, 2021, 5(4): 376-385, doi: 10.46690/ager.2021.04.04

基于数值模拟的矩形腔体内对流传热研究进展

基于数值模拟的矩形腔体内对流传热研究进展

Buoyancy effect on the mixed convection flow and heat transfer of supercritical R134a in heated horizontal tubes

Thermal non-uniformity in the horizontal mixed convection heat transfer of fluids at the supercritical pressure is a major issue that must be addressed in a trans-critical organic Rankine cycle. However, the heat transfer mechanism is not fully understood. To further investigate the mechanisms of the buoyancy effect and property variations in a horizontal supercritical flow, mixed convection with supercritical pressure R134a is studied numerically herein using the AKN turbulence model. When the buoyancy effect is weak, the difference in the turbulent kinetic energy with ktop < kbottom is the dominating factor resulting in a non-uniform heat transfer. In a strongly buoyancy-affected mixed convection, the flow process is divided into three regions. In region I (Tw increasing section) and region III (gas-like section), ktop < kbottom because of the greater velocity gradient at the bottom; in region II, where Tw,top reaches a peak and subsequently decreases, ktop > kbottom is observed because the newly developed vortexes near the tube top intensifies the turbulence near the top. Heat transfer cases with various tube diameters and pressures are discussed. A stronger buoyancy effect is developed in larger tubes. No new vortex is formed in a 2-mm tube while multiple vortexes are developed in the upper region of 16-mm and 26-mm tubes, providing stronger turbulence for the heat transfer recovery. As the specific heat is sensitive to the pressure variation while the density variation with pressure is moderate, the pressure has less effect on heat transfer in a strong-buoyancy case than in a weak-buoyancy case.

Numerical simulation of mixed convection heat transfer of fluid in a cavity driven by an oscillating lid using lattice Boltzmann method

Lattice Boltzmann Method (LBM) has been applied for the simulation of a mixed convection heat transfer of two dimensional Newtonian fluid in a square cavity driven by a periodically oscillating lid. The top wall is maintained at higher variable temperature. The developed LBM code in Matlab, is applied for the oscillating lid to simulate the fluid flow and heat transfer in cavity, was validated using various Reynolds and Grashof numbers. Nusselt number was calculated for various pertinent dimensionless groups. Fluid flow and heat transfer characteristics were examined in the domain of Reynolds number, Grashof number, the dimensionless lid oscillation frequency and different values of temperature. Such that: 102⩽Re⩽103, 102⩽Gr⩽106, 0.1⩽ω⩽5, the Prandtl number was fixed as 0.71. The results show that the variation of the Reynolds and Grashof numbers has an effect on energy transport process and drag force behavior depending on the conduct of the velocity cycle. Moreover the variation of Rayleigh number and period of the heated portion has an effect on the transfer rate on convective structures.

Characteristics of Fluid Flow and Heat Transfer in the Shell Side of the Trapezoidal-like Tilted Baffles Heat Exchanger

Periodic whole cross-section computation models are established for segmental baffle heat exchanger, shutter baffle heat exchanger, and trapezoid-like tilted baffle heat exchanger. The reliability of models is verified by comparing the simulated results to the results obtained from the Bell-Delaware method. Due to the orthogonal assembly of the baffles, the shell side fluid shows the twisty flow of trapezoid-like tilted baffle heat exchanger. The essential mechanism on disturbing flow and heat transfer enhancement is revealed by defining the non-dimensional factor η of the shell side fluid flow direction of heat exchanger and the field synergy principle. The results show that at the same Reynolds number, the shell side fluid convection heat transfer coefficient of trapezoid-like tilted baffle heat exchanger is 12.43%-24.33% and 6.71%-11.51% higher than those of segmental baffle heat exchanger and shutter baffle heat exchanger, respectively. The shell side fluid flow velocity field and the pressure gradient field of trapezoid-like tilted baffle heat exchanger and shutter baffle heat exchanger decreases compared with that of segmental baffle heat exchanger, so the shell side fluid flow resistance and pressure drop is increased; the shell side comprehensive performance of trapezoid-like tilted baffle heat exchanger is 5.85%-9.06% higher than that of segmental baffle heat exchanger, and 15.27%-23.28% higher than that of shutter baffle heat exchanger. In this study, a baffle structure with higher efficiency of the energy utilization for the heat exchanger is provided.

Turbulent Convection Heat Transfer Analysis of Supercritical Pressure CO2 Flow in a Vertical Tube Based on the Field Synergy Principle

ABSTRACTTurbulent convection heat transfer of fluids at supercritical pressures can experience abnormal heat transfer phenomena as the fluid temperature approaches the pseudo-critical temperature. A better understanding of these abnormal heat transfer characteristics is essential to develop ways to assure the security and reliability of engineering designs. In this study, we analyze the turbulent convection heat transfer characteristics of supercritical pressure fluid flow in a vertical heated tube using the field synergy principle. A turbulent Prandtl number model that is a function of the flow conditions as well as the physical properties is used in a low Reynolds number turbulence model with validations against experimental data. The validity of the field synergy principle is extended here to analyze convection heat transfer of supercritical pressure fluids where the heat transfer rate is a function of the synergy between the velocity and temperature gradients. Numerical simulation results show that when heat transfer deterioration occurs, the synergy angle between the velocity and temperature gradient vectors increase. The field synergy number could be used as an indicator to quantitatively identify the degree of heat transfer deterioration. The results presented here provide a basis for further research on enhancing turbulent convection of supercritical pressure fluid.

Borehole heat exchanger with nanofluids as heat carrier

This paper presents numerical study on the use of nanofluids to replace conventional ethylene glycol/water mixture as heat carrier in a BoreHole Heat Exchanger. Nanofluids contain suspended metallic nanoparticles: increasing their concentration, in comparison to the base fluid, the thermal conductivity increases and the volumetric heat capacity decreases. The first effect is positive for the reduction of borehole thermal resistance, since it causes the grow of fluid convective heat transfer coefficient, while the second one is detrimental, due that it decreases the heat transfer between fluid and borehole wall.A numerical model based on energy and momentum balances is used to evaluate which is the best nanofluid with low nanoparticles volumetric concentration (0.1%–1%) that ensures the highest decreases of borehole thermal resistance and minimum increases of pressure drop among silver, copper, aluminium, alumina, copper oxide, graphite and silicon oxide. Moreover, a simple economic analysis was done.Results show that copper is characterized by highest borehole thermal resistance reduction, that reaches the value of about 3.8%, in comparison to that of base fluid, when nanoparticles volumetric concentration is 1%, but also the second one highest pressure drop. In this case, the cost of copper-based nanofluid is about 10€m−1, i.e. about 12% of total cost of BoreHole Heat Exchange – Ground Source Heat Pump system.

Experimental study on convection heat-transfer characteristics of BCG-CO2 fracturing fluid

A BCG-CO2 fracturing fluid system is especially suitable for shale gas stimulation because it has many advantages, including minimizing water consumption, reducing damage to the formation, and recovering more rapidly and efficiently. A major objective of this experimental study is to ascertain the convection heat-transfer characteristics of a BCG-CO2 fracturing fluid system. A high-parameter foam fracturing fluid test system was adopted in this study. The measurement of convection heat transfer is based on the double-wall heat-transfer theory. Through experimental research, the correlations for BCG-CO2 fracturing fluid convection heat transfer have been obtained under different foam qualities. The average error of the fitting formula for all experimental data is 10.7%. The results indicate that the convection heat-transfer coefficient decreases with increasing temperature, with an average decrease of about 10%. The convection heat-transfer coefficient shows a decreasing trend with increasing foam quality. There is a linear relationship between the convection heat-transfer coefficient and shear rate, and the shear rate has the most pronounced effect on convection heat-transfer characteristics. The results of this study can be used to calculate heat-transfer parameters to determine the temperature field of BCG-CO2 fracturing fluid flow in fracturing design, and they are significant for the engineering application of BCG-CO2 fracturing fluid.

Effect of end plates on heat transfer of plate heat exchanger

Steady-state heat transfer data for single-phase (water) in both frame-and-plate (FPHE) and brazed plate heat exchangers (BPHE) are presented with various number of chevron plates in U-type flow arrangement. Analysis of the experimental results indicates that the end plates, instead of being adiabatic, function as fins due to the contact between adjacent plates. The experimental data is used to validate a thermal conduction model in ANSYS, which indicates that the end plates fin efficiency is a function of fluid convective heat transfer coefficient and conductive thermal resistance. In the FPHE, the pressing force of the frame may affect the contact thermal resistance, thus change the fin efficiency. In BPHE, the fin efficiency is much higher due to the larger contact area and higher conductivity of the brazing material. Although the effect of end plates is quickly diluted by the increased number of plates in real applications, it could be significant when plate number is small, as is often the case in laboratory settings for the development of heat transfer correlations.

Influence of T-semi attached rib on turbulent flow and heat transfer parameters of a silver-water nanofluid with different volume fractions in a three-dimensional trapezoidal microchannel

This study aimed at exploring influence of T-semi attached rib on the turbulent flow and heat transfer parameters of a silver-water nanofluid with different volume fractions in a three-dimensional trapezoidal microchannel. For this purpose, convection heat transfer of the silver-water nanofluid in a ribbed microchannel was numerically studied under a constant heat flux on upper and lower walls as well as isolated side walls. Calculations were done for a range of Reynolds numbers between 10,000 and 16,000, and in four different sorts of serrations with proportion of rib width to hole of serration width (R/W). The results of this research are presented as the coefficient of friction, Nusselt number, heat transfer coefficient and thermal efficiency, four different R/W microchannels. The results of numerical modeling showed that the fluid's convection heat transfer coefficient is increased as the Reynolds number and volume fraction of solid nanoparticle are increased. For R/W=0.5, it was also maximum for all the volume fractions of nanoparticle and different Reynolds numbers in comparison to other similar R/W situations. That's while friction coefficient, pressure drop and pumping power is maximum for serration with R/W=0 compared to other serration ratios which lead to decreased fluid-heat transfer performance.

Numerical modeling of particle to fluid heat transfer during ultrasound assisted immersion cooling

Convective heat transfer between a sphere and the surrounding medium and conductive heat transfer within the sphere were numerically modeled at the presence or absence of ultrasound. Different Nusselt (Nu) number correlations were evaluated and ultrasound assisted cooling was simulated for different intensities (190–2800Wm−2), cooling medium temperatures (−5–−20°C) and materials (copper and potato). The simulations were compared with experimental data. Both simulations and experimental data revealed that the particle to fluid convective heat transfer rate could be enhanced significantly by the aid of ultrasound irradiation. The results revealed that the selection of an appropriate Nu correlation was critical. With the chosen Nu correlation, the numerical model simulated the experimental data accurately. Ultrasound intensity was also incorporated into the Nu correlation while considering the ultrasonic heat generation in the heat transfer equation, and the simulation profiles matched to experimental profiles very well. The prominent advantage of the model was its ability to estimate the convective heat transfer coefficient at different temperatures. The results of this study are useful in different food and chemical processes involving heat transfer such freezing, chilling, crystallization and heat preservation.

Numerical study of convection heat transfer in ordered three-dimensional porous media

Saturated fluid convective heat transfer in ordered three-dimensional porous media has been investigated numerically using a CFD software. Spherical particles with diameters dp equal to respectively 14, 9.4, 7 mm have orderly arrangements constituting the porous media skeleton. There is a copper plate of constant heat flux density above the porous skeleton. The distribution of temperature and local convective heat transfer coefficient in the channel, as well as factors affecting the convective heat transfer are investigated by the method of fluid-solid coupling. Results show that the thickness of the thermal penetration and the thermal boundary layer increase in the flow direction, and decrease with the flow rate. When the thermal conductivity of the skeleton is relatively high, the convective heat transfer increases slightly with decreasing particle diameter. The convective heat transfer coefficient decreases with the increase in mass concentration of polyacrylamide solution, because the viscous dissipation weakens convection heat transfer.

Thermal DEM–CFD modeling and simulation of heat transfer through packed bed

Abstract Heat transfer between particles takes place in a variety of industrial applications, but because of the stress and contact heterogeneities inherent to these applications, it is poorly understood, even in simple cases. Although particle–particle heat transfer is relatively weak compared with particle–fluid convection heat transfer, it may become very important in applications such as packed beds, where the fluid flow can be neglected or where fluid/particle stagnation areas can be observed. During our previous studies, CFD–DEM models and numerical simulations were developed to account for momentum and heat transfer between the fluid and the solid particles. The predictions of the simulations were validated with experimental data. In the present study, particle–particle and particle–wall heat transfer models were developed and integrated into our in-house unsteady three-dimensional discrete element method (DEM) software. Heat transfer simulations for predicting the effective thermal conductivity (ETC) of particulate beds under compression were conducted to validate these models with published experimental data. The results showed good agreement with the experimental data, and it was found that heat conduction through the bed's pores cannot be neglected even when particle thermal conductivity is much larger than that of the air. The influence of particle roughness on heat transfer through packed bed was examined. It was found that heat conduction through the packed bed pores cannot be neglected for rough particles.

Conjugate Wall Conduction-Fluid Natural Convection in a Three-Dimensional Inclined Enclosure

Laminar conjugate conduction-natural convection heat transfer in a 3-D inclined cubic enclosure comprised of finite thickness conductive walls and central cavity is numerically investigated. The dimensionless governing equations describing the convective flow and wall heat conduction are solved by the high accuracy multidomain pseudospectral method. Computations are performed for different Rayleigh numbers (103 ≤ Ra* ≤ 106), thermal conductivity ratios (1 ≤ k ≤ 100), dimensionless wall thickness (0 ≤ s ≤ 0.25), and enclosure inclinations (−30° ≤ α 1 ≤ 30°, 0° ≤ α 2 ≤ 45°). The effects of the above controlling parameters on the heat transfer performances of the enclosure system are investigated in detail, with emphases on the variations of wall conduction and fluid convection heat transfer, and the interactive heat transfer conditions between solid walls and fluid in the central cavity. Numerical results reveal that the existence of enclosure walls reduces the temperature gradient across the cavity and alters the temperature distribution within the solid walls; thus, the fluid convection is complexly determined by the combined effects of k and s, and is greatly affected by enclosure inclinations at high Rayleigh numbers. Moreover, the temperature distributions and solid-fluid interactive heat transfer conditions are provided for further interpretation and demonstration of the effects of the solid walls.

Transient natural convection heat transfer of fluids with variable viscosity between concentric and vertically eccentric spheres

The transient natural convection has been analyzed for the temperature dependent viscosity of fluids in spherical annulus and between two vertically eccentric spheres. Using the modified Sorenson’s method to generate the grid line can get the grid system with orthogonality along all boundaries. The grid system goes along with weighting function scheme to discretize the general governing equation. Numerical solutions were obtained for Rayleigh numbers (5.0 × 10 3–6.5 × 10 4) at a radius ratio of 2.0 with the dimensionless vertical eccentricity of the outer sphere (0– ± 0.65) for variable viscosity fluids at different Prandtl numbers (158, 405 and 720). The results of this analysis show that heat and flow patterns vary with the Rayleigh number and the eccentricity; besides, the effect of variable viscosity is investigated. The present calculations applied to constant viscosity are compared with the results of other papers, and these comparison results are agreeable.