Search

Heat dissipation and temperature uniformity are one of the key technologies for electronic equipment. With the rapid growth of computing demand and the wide application of multichip devices, heat sources are distributed discretely in space. Therefore, the flow boiling in the mini-channel with two discrete heat sources is investigated in this study. The effects of heat source distribution, inlet fluid temperature and channel diameter on flow pattern, heat transfer and pressure drop are discussed in detail. The results show that intermittent flow tends to form around discrete heat sources, especially near the heat source which is far from the channel inlet. Moreover, the distance between discrete heat sources shouldn't be too close to avoid heat concentration, nor too far to make full use of latent heat. When the distance between two heat sources is 70 mm, the temperature distribution is appropriate, and the heat dissipation is the best in this study. For the discrete heat source far from the inlet, the heat transfer coefficient increases significantly when the inlet fluid temperature increases due to the preheating of another heat source. [ABSTRACT FROM AUTHOR]

The metals are the most popular materials for the plate heat exchangers due to high thermal conductivity. The thermal performance of the metallic plate heat exchanger is degraded by longitudinal heat conduction in the plate. For the cabinet cooling system and ventilator, the metallic plate heat exchanger can be replaced by the plastic plate heat exchanger. In this article, the longitudinal heat conduction in the plastic heat exchanger is studied by using the heat transfer models with and without longitudinal heat conduction, which were proposed in our previous work. The effects of both transverse heat conduction and longitudinal heat conduction can be considered. The thermal performances of the plastic plate heat exchanger and metallic plate heat exchanger are compared. We find out that in the plastic plate, both the transverse heat conduction and longitudinal heat conduction are weakened because of the low thermal conductivity. The high-density polyethylene plastic plate heat exchanger has better thermal performance than that of the aluminum plate heat exchanger. [ABSTRACT FROM AUTHOR]

In this paper, film condensation of steam in inclined tubes in the presence of noncondensable gases is modeled using numerical method. A single-phase fluid model incorporating a wall-function boundary condition is proposed to simulate the heat and mass transfer interaction between the liquid condensate and the gas-vapor mixture region. A user defined function is implemented to impose the proposed boundary to the governing equations for steady state incompressible flow. The performance of applying this boundary condition and subsequent treatment of the heat transfer at the liquid-mixture interface is validated against published experimental results. The calculated profiles of local and average Nusselt number for various inlet conditions compares favorably to the previous published experimental results and correlations. For all simulated cases, increases in inlet and bulk concentrations of noncondensable gases resulted in a decrease in the thermal performance of the condensing tube. Additionally, it is found that the inclination angle of the condensing tube can impact the local distribution of the heat transfer coefficient and Nusselt number, while there is only small effect on the average along the length of the entire tube. [ABSTRACT FROM AUTHOR]

In this study, the effect of gradient and multi-layered porous media is assessed for the optimized properties and arrangement in the tested novel baffle design arrangement, which results in further enhancement in its thermo-hydraulic performance. The computational study is carried out using ANSYS FLUENT; wherein the permeability and porosity for each layer is varied. It's executed in two steps: case 1 – porous layers with variations implemented (constant, linear/stepwise, increasing/decreasing) and case 2 – porosity varied over the length. The findings show that the linear/stepwise increments in both cases give almost similar performance evaluation criteria (PEC) values. The increase in heat transfer rate and pressure drop can be compared using PEC, for better evaluation of the performance of different arrangements. With the simulation data genetic algorithm optimization model is used to find the optimal arrangement of the porous layers for both cases to maximize PEC further. The optimal arrangement for case 1 and case 2 taken individually, gives PEC of 2.425 and 2.401 respectively. And their simultaneous optimization gives highest PEC of 2.508. Additionally, to improve the heat transfer rate further, Al2O3 (5 w/v %) nanoparticles in water is tested with optimized conditions, which improves the PEC by almost double magnitude. [ABSTRACT FROM AUTHOR]

Mini-grooved flat heat pipe (FHP) is an efficient approach to cool the electronics within small space. As it was hard to measure the flow and thermal characteristics inside the FHP, a novel three-dimensional model was set up to couple velocity, pressure, temperature, and mass transport inside the FHP, anticipating understanding the working mechanisms of FHP filled with different working fluids under transient and steady states. Temperature-dependent physical properties of working fluid and simulated thermal conductivity of wick were employed. The numerical results were validated by experiment with the maximum deviations in thermal resistance and maximum temperature of 17.9% and 5.0%, respectively. It was found that working fluid types, properties and input heat affected the FHP performance. The deionized water filled FHP shows more excellent thermal characteristics. Under input heat of 10 W, the thermal resistance, maximum temperature, and temperature nonuniformity of deionized water filled FHP were numerically lower than those of anhydrous ethanol filled FHP by 48.1%, 11.5% and 56.4%, and lower than those of hexane filled FHP by 54.4%, 14.3% and 62.6%, respectively. However, the starting performance of deionized water filled FHP is comparatively poorer. FHP filled with anhydrous ethanol or hexane is more appealing for electronics cooling. [ABSTRACT FROM AUTHOR]

The 6th International Workshop on Heat/Mass Transfer Advances for Energy Conservation and Pollution Control (IWHT2021) was successfully held online from August 13 to 16, 2021, in Harbin, China. The workshop aimed to provide a global platform for the exchange of cutting-edge technological knowledge and research on heat and mass transfer issues related to energy conservation and pollution management. The event featured plenary lectures, keynote lectures, oral presentations, and posters covering various topics such as nuclear energy, advanced energy systems, heat exchangers, and heat/mass transfer enhancement techniques. Nine research papers from the workshop were selected for inclusion in a special issue of Heat Transfer Engineering. The 7th IWHT conference is scheduled to take place in Tokushima, Japan in 2023. [Extracted from the article]

This study analyzed the acoustic energy transfer process in expanded pipe of heat exchangers based on the field synergy principle. The synergy between the flow field and the acoustic field was deduced theoretically based on the energy and momentum equations pertaining to the acoustic field. The synergy angle of the flow field and the acoustic field was calculated numerically. The noise propagation process in the expanded pipe was measured experimentally. The results show that the sound pressure level and synergy angle both increase with the increase in flow velocity, whereas the acoustic energy transfer process decreased with the increase in synergy angle. An extended outlet of the expanded pipe segment can reduce the flow noise magnitude with a lower pressure drop. The flow noise can reach 123 dB and the pressure drop is 37.3％ and 24.7％ less than other cases. In conclusion, the theoretical analysis, experimental tests, and numerical simulation method are developed to study the acoustic energy transfer process in segments of expanded pipe based on synergy principle of flow and acoustic fields. [ABSTRACT FROM AUTHOR]

The enhancement with porous media involvement is estimated and compared for three varieties of shell and tube heat exchangers (STHXs), with helical, segmental and clamping anti-vibration baffles, with two variations, viz., porosity value and dimension of this media (radius). In this study, further improvement in the thermo-hydraulic performance of the anti-vibration baffles design of STHX was investigated using simulation, with porous media inclusion. The inclusion of porous medium in the shell side is in a way that the whole tube bundle is wrapped by it; inside the tubes, porous media are located in two ways: (1) at the center of tubes and (2) fixed to the inner lining of the tubes. The findings suggest that the porous media improve the heat transfer with the expense of pressure drop; it is also noted, of all the cases, the pressure drop is minimum in anti-vibrational baffles. The analysis showed that using porous media with specific porosity and dimension can increase heat transfer rate along with less pressure drop. For the clamping anti-vibration baffles STHX, enhanced and overall effective performance was found with porous media inclusion of 0.6 porous radius ratio and porosity of 0.6. [ABSTRACT FROM AUTHOR]

Printed circuit heat exchanger (PCHE) is a promising candidate of coolers used for SCO2 Brayton cycle. The thermal-hydraulic performance of SCO2 in a PCHE depends on the PCHE structure as well as the operating conditions of SCO2 due to its dramatic variation of thermal physical properties near the pseudo-critical point. This work experimentally investigated the thermal-hydraulic characteristics of zigzag-channel PCHEs between SCO2 and water at pressures that ranged from 8.0 to 11.0 MPa. Results showed that increasing the flow rate of SCO2 might affect the PCHE more pronouncedly in comparison with changing the flow rate of water, indicating that the major convective thermal resistance is on the SCO2 side. When increasing the operating pressure from 8.0 MPa to 11.0 MPa, the Nusselt numbers of PCHEs with zigzag angles of 0°, 15°, and 25° averagely decrease by 63.3%, 62.1%, and 54.0%, and the friction factors averagely decrease by 28.6%, 27.4%, and 21.0% respectively. Meanwhile, it is found that the buoyancy force is non-negligible during SCO2 cooling process in PCHEs. In this regard, Nusselt number and friction factor correlations considering the buoyancy effect are proposed with applicable ranges. [ABSTRACT FROM AUTHOR]

A novel structure for a shell-and-tube heat exchanger (STHX) with triple-layer flower baffles consisting of three blades is proposed in this article. This study also compares the performance of three heat exchangers with triple-layer flower baffles, double-layer flower baffles, and segmental baffles. In order to compare the hydrodynamics and heat transfer characteristics of the three heat exchangers, a water–water heat transfer experiment is designed. The experiment is carried out in a counter-current flow pattern with hot water in the shell and cold water in the tube. Based on the experimental data, at the same flow rate, the shell-side heat transfer coefficients of the STHXs with triple-layer flower baffles and double-layer flower baffles are 31.7% and 14.3% higher than those of traditional segmental baffles, respectively. With an increased number of blades, the STHX with triple-layer flower baffles achieves a comprehensive performance that is 23.7% improved over the STHX with double-layer flower baffles. Finally, an experimental correlative formula is fitted for the heat exchanger with flower baffles and an optimal design scheme for shell-side heat transfer enhancement is proposed. [ABSTRACT FROM AUTHOR]

Non-isothermal melting phenomenon widely exists in phase change process where mixtures are utilized as phase change materials. It is in the mushy zone that phase transition actually occurs during the melting process. Thus, an in-depth understanding of flow and heat transfer in the mushy zone can provide beneficial ways to improve melting technologies. This article focuses on the evolution of mushy zone for low-Prandtl material under Neumann boundary condition. A two-dimensional model composed of liquid zone, mushy zone, and solid zone is established based on the improved enthalpy-porosity method. The pressure and velocity are coupled with SIMPLER algorithm based on the finite volume method with a Fortran code. The results indicate that the temperature on the heating surface is uneven, which are different from the results under Dirac boundary condition. The existence of mushy zone can supply a buffer to the unsteady melting adjacent to the heating surface initially. And the mushy zone experiences expanding stage and shrinking stage. More time is needed for complete melting and the average temperature on the heating surface gets lower under greater melting temperature range. Increasing Ra can result in augmentation of heat transfer in liquid domain and more inclined interface, and the heat transfer is dominated by heat conduction when liquid phase fraction is less than 20%, and thereafter, it is dominated by natural convection. For the same material, increasing heat flux obviously shortens the total melting time. And the mushy zone is reduced for that greater temperature gradient is produced in the mushy area. [ABSTRACT FROM AUTHOR]

The flow and temperature fields in packed beds are rather complex but are of great importance since they will influence macroscopic parameters and the overall performance of the packed bed system. Regions with either too high or too low velocity are not preferred since they will decrease the heat and mass transfer characteristics of packed beds. The residence time distribution (RTD) is a way to assess the characteristics of the flow maldistribution, where a narrower distribution is closer to the ideal plug flow with less dispersion. In this article, the RTD in three typical structured packed beds, namely, the simple cubic (SC) packing, the body-centered cubic (BCC) packing and the face-centered cubic (FCC) packing, are studied by using ANSYS FLUENT. Results show that, firstly, the RTD curve can be used to diagnose the flow pattern in packed beds where the skewed peak indicates the channeling effect and a long tail relates to recirculation. It is revealed secondly that, FCC packing has the narrowest RTD curve in the studied structures, BCC packing comes second and SC packing has the flattest RTD curve, which demonstrates that the flow uniformity in FCC packing is the best. For the same packing form, the flow is less maldistributed at higher velocity. Finally, it is shown that the packing form with less dispersion, which behaves more like the ideal plug flow, has a more uniform distribution of the particle-to-fluid heat transfer coefficient near the particle surface. [ABSTRACT FROM AUTHOR]

In the present paper, the forced convective heat transfer was experimentally studied in packed beds with uniform and non-uniform spheres. The pore structures of different packings were reconstructed with discrete element method, and the lateral porosity distributions were analyzed. Furthermore, the pressure drop, heat transfer and overall heat transfer performances of different packings were investigated and compared with each other. Firstly, it is found that, using laterally layered composite packing (LLM), smaller pores would be formed close to the tube wall and big flow channels would be formed in the inner-tube region, which would reduce the tube-wall effect in the near tube wall region of packed bed. When compared with mono-sized packing with lower tube to particle diameter ratio, both the heat transfer and overall heat transfer capacities would be improved with LLM packing method. Secondly, it is found that, using randomly composite packing (RCM) or axially layered composite packing (ALM), both the lateral porosity magnitude and amplitude of packed bed are lower than those of mono-sized packing with higher tube to particle diameter ratio in the near tube-wall region. The tube-wall effect would be further reduced, and both the heat transfer and overall heat transfer capacities would also be improved with RCM and ALM packing methods. [ABSTRACT FROM AUTHOR]

This article is the second part of a two-part study of the modeling and optimizing of anode-supported solid oxide fuel cells (SOFC) with gradient anode. Based on the 3D numerical model established and validated in part I of this two-part article, two kinds of continuous gradient anodes at the micro-scale level (continuous gradient porosity and continuous gradient particle size) were introduced and described in this article. Two widely used neural networks (NNs), i.e. back propagation neural network (BPNN) and radial basis neural network (RBNN), were tested to find the proper approximation between various microstructure parameters distribution and cell electrical performance. For the optimization of both continuous gradient anodes, BPNN shows a better approximation of the objective function in comparison with RBNN. Based on the BPNN, the genetic algorithm (GA) was used to find the optimal microstructure parameters distribution for the highest electrical performance of SOFC. The optimal porosity distribution was found to be expressed as ε(z) = (0.57–0.70)z29.73/73429.73 + 0.7 (0 µm ≤ z ≤ 734 µm). The optimal particle diameter distribution was expressed as dp(z) = (0.10–0.60)z65.12/73465.12 + 0.6 (0 µm ≤ z ≤ 734 µm). Compared with the experimental maximum output power density (Pmax) of 0.31 W·cm−2 at 800 °C and 125 mL·min−1 in Part I, Pmax of the optimal porosity and particle diameter distribution is increased by 17.10% and 37.42%, respectively. Compared with the optimal porosity distribution, the optimal particle size distribution is more advantageous to improve the cell electrical performance. These results can provide a guidance of microstructure parameters distribution during the experimental fabrication for cell performance enhancement. [ABSTRACT FROM AUTHOR]

Grading the porous anode microstructure, i.e., gradient anode is an effective way to enhance the electrical performance of anode-supported solid oxide fuel cells (SOFCs). This article is the first part of a two-part article which presents a 3D numerical model of a gradient anode SOFC and conducts a model validation in comparison with the experiments for the gradient anode SOFC modeling and optimizing. An experimental test setup was constructed first to evaluate the electrical performance of a SOFC button cell with a gradient anode. Then the microstructure characterization of each cell component was obtained by combining the mercury porosimetry with image analysis. Parameters relating to the microstructure mainly included the layer thickness, porosity, pore size, and particle size. Based on the microstructure parameters and measured electrical performances, the numerical model of the gradient anode SOFC button cell was established, which related the cell microstructure to the electrochemical characteristics. The results show that the microstructure parameters of each cell component can be quantificationally obtained by the backscatter electron image analysis. The modeled i-V and i-P curves at three flow rates of fuel gas (75/100/125 mL·min−1) agree well with the experimental data. This validation process indicates that the numerical model is able to accurately describe the electrical performance of the gradient anode SOFC and it can be further extended to optimize the anode microstructure distribution. [ABSTRACT FROM AUTHOR]

Thermoelectric power generators (TEGs) have received considerable attention in the vehicular waste heat recovery. Large temperature difference between hot-side end and cold-side end of TE modules should be maintained to keep large power generation. However, the scaling-up problem of inevitably temperature drop of exhaust gas along the flow direction of vehicles is introduced when multiple TE modules are assembled. In this work, a three-dimensional fluid–thermal–electric multiphysics model with equivalent properties of TE modules is established. The influencing mechanism of longitudinal vortex generator (LVG) pair number and distribution in the hot-side heat exchanger on the local power generation performance of the TEGs is examined. It is found that the extent of the local power generation enhancement gets smaller or even deteriorates at the downstream TE modules by uniformly increasing the LVG pair number along the flow direction. Hence, to make the heat flux efficiently utilized, a nonuniform segmented enhancement method is used for the hot-side heat exchanger. The results show that compared with the TEG with a uniform heat exchanger or a downstream-denser heat exchanger, the total Seebeck voltage generated by the TEG with an upstream-denser heat exchanger is 28% higher, and its output power is 64.4% higher at the matched load resistance. The TEG with the upstream-denser heat exchanger significantly improves the overall power generation performance without additional pumping power. [ABSTRACT FROM AUTHOR]

Melting widely exists in various industrial applications and melting control through an extra magnetic field is a promising subject. Recent studies mainly focus on the suppression effect of the magnetic induced Lorentz force. However, the influence of magnetic force is less investigated. The present study concentrates on investigating the impact of magnetic force, along with gravity, on the melting process. The magnetic quadrupole field is used to generate magnetic force and the temperature distribution, velocity field and melting front are obtained through lattice Boltzmann method. Results indicate that the magnetic force can enhance the flow and heat transfer during the melting process and it has the potential to be used to control the process. Besides, the gravity accelerates the melting process when the dimensionless magnetic force parameter is less than 20, whereas its effect can be ignored when the parameter is larger than 20. [ABSTRACT FROM AUTHOR]

Magnesium alloy is one of the most promising biomaterials in vascular clogging and bone injury. But it still has some defects to overcome and the key task is to control the degradation velocity. In this study, the reaction between NaCl solution and MgO is simplified as the first stage of the degradation of magnesium alloy stent, and the adsorption properties of NaCl solution on the MgO surface are investigated by MD simulation. The distribution of each component of the solution perpendicular to the MgO surface is analysed and the diffusion coefficient is calculated. Besides, a parameterised analysis is carried out. The results show that there is a solution layer formed at the surface of the MgO, and the existence of metal oxide restricts the diffusion of the solution. The adsorption capacity and the diffuse rate have an opposite variation tendency with the change of temperature, concentration and velocity. The self-diffusion coefficient of the solution increases with the increase in temperature as well as velocity, inversely adsorption capacity decreases with the increase in velocity. Besides, the influence of temperature on the adsorption capacity is small. What is more, the diffusion coefficient decreases while the adsorption capacity increases with the increase in concentration. [ABSTRACT FROM AUTHOR]

In this paper, the flow and heat transfer performances inside small pores of structured packed beds of dimple particles are numerically investigated for the first time and some interesting transport phenomena are obtained. Three-dimensional Navier-Stokes equations and SST k-ω turbulence model are adopted for the simulations. The effect of dimple depth is studied in detail, and the flow and heat transfer performances in the packed beds with dimple particles and smooth particles are also compared with each other. It is found that, with the same inlet velocity, the pressure drop and heat transfer in the packed bed with dimple particles would be lower than those in the packed bed with smooth particles, while the overall heat transfer efficiency of packed bed with dimple particles is higher. Furthermore, for the packed bed of dimple particles, the effect of dimple depth is remarkable. With the same inlet velocity, both the pressure drop and heat transfer rate of the packed bed decrease as dimple depth increases, while the overall heat transfer rate is similar for the packed bed with different dimple depths. [ABSTRACT FROM AUTHOR]

As a new type of heat exchanger, the printed circuit heat exchanger (PCHE) with high compactness can work under high pressure and high temperature, and can be applied to the very high temperature gas nuclear reactors. The chemical etching technology is one of main processes during PCHE fabrication, which determines whether PCHE flow channels can be properly produced according to design. However, many factors can affect the etching quality and the poor etching process can cause a great deal of uncertainty on the precision of stainless steel flow channels. In this paper, a two-dimensional (2-D) cellular automata model (CA) is used to simulate the etching process on stainless steel to examine the change of channel features varying with time under certain etching rates. Moreover, the simulation results are compared with our previous experimental etching data. On the one hand, from the comparison of the simulation results with channels data by spray etching, 2-D CA model can realize the simulation of etching process and display channel features under different etching rates and lateral erosion rates in every time period. On the other hand, from the comparison of the simulation results with channels data by immersion etching under different FeCl3 concentration and etching liquid temperature, 2-D CA model vividly presents channel features under different factors, which can easily and clearly explain the influence of factors on the etching quality, puddling effect and so on. Therefore, the simulation result strengthens the understanding of the etching mechanism and is helpful to realize an efficient and precise PCHE fabrication. [ABSTRACT FROM AUTHOR]

Three-dimensional nonequilibrium molecular dynamics simulations are performed to investigate the tribological characteristics of the high-speed sliding system. A sliding simulation model with two blocks is built. The friction force, the evolution of the structure of interface and the temperature profiles of the sliding system are obtained. The relationship among tribological characteristic, friction heat, and interface structure is studied. The influence of the sliding velocity is investigated. The velocity field of the interface is also considered. The theoretical analysis of the mixing layer is also built to study the flow and heat transfer characteristics in the interface. The results show that structure of the interface can greatly affect the friction force and temperature rising. The structure factors and the velocity field of the interface evidently suggest that atomic flow during sliding is similar to fluid flow, which is beneficial to reduce the friction force and heat dissipation. With the increase of sliding velocity, the thickness of the mixing layer increases, the steady state friction force decreases. The atom distribution and the radial distribution function of the interface indicate that the degree of short-range disorder of the mixing layer increases. [ABSTRACT FROM AUTHOR]

The melt pool is the general process of the laser additive manufacturing process. An in-depth understanding of the evolution of the melt pool will facilitate obtaining an optimum mechanical performance. In this paper, a novel 2D transient model is presented, which considers most physical phenomena taking place in the laser melt process and incorporates a coupled multiphase capture method. This model is validated by comparing it to Han’s research. Then an investigation on the transient thermal dynamic in the melt pool including quenching time is carried out, and the effects of evaporation and alternative material properties on the evolution of the melt pool are discussed. The results show that the deformation of the free surface and the temperature in the workpiece both increase with energy input. After quenching, the temperature in the workpiece decreases sharply and the melt pool also shrinks. The evaporation effect is vigorous in the center of the melt pool and negligible in the periphery. The material properties have significant influence on the evolution process because the convection and diffusion in the melt pool are directly dominated by them. [ABSTRACT FROM PUBLISHER]

Based on the volume of fluid (VOF) method, a steady three-dimensional numerical simulation of laminar film condensation of water vapor in a horizontal minitube, with and without non-condensable gas, has been conducted. A user-defined function defining the phase change is interpreted and the interface temperature is correspondingly assumed to be the saturation temperature. An annular flow pattern is to be expected according to a generally accepted flow regime map. The heat-transfer coefficient increases with higher saturation temperature and a smaller temperature difference between the saturation and wall temperatures, but varies little with different mass flux and degree of superheat. The existence of a non-condensable gas will lead to the generation of a gas layer between vapor and liquid, resulting in a lower mass-transfer rate near the interface and higher vapor quality at the outlet. In consequence, the heat-transfer coefficient of condensation with a non-condensable gas drops sharply compared with that of pure vapor condensation. Meanwhile, the non-condensable gas with a smaller thermal conductivity would cause a stronger negative effect on heat flux as a result of a higher thermal resistance of heat conduction in the non-condensable gas layer. [ABSTRACT FROM AUTHOR]

The spiral-wound heat exchangers are widely used in chemical industrial applications, but the mechanism of flow and heat transfer in shell side has not been clarified yet. A three-dimensional model is developed based on the FLUENT software in this study, with emphasis on quantifying the effects of the main geometry parameters, such as the number of tube bundles, the number of tube layers, the thickness of space bars, the number of tubes per circle in the first layer, the tube external diameter, the central cylinder diameter, and the tube pitch in the first layer, on the flow and heat transfer. Based on the numerical simulation results, the Taguchi method is used to estimate the effect of these geometrical factors on flow and heat transfer and the contribution rate of every factor is obtained. Meanwhile, the multivariate correlation is developed by MATLAB software to calculate the Nusselt number and friction factor in the shell side. [ABSTRACT FROM PUBLISHER]

As a newly developed industry, refuse incinerating power plants with air-cooled heat exchanger (ACHE) systems benefit both environment and energy savings. Since the exhaust steam of power plant is cooled in the atmosphere, the pressure of the exhaust steam fluctuates with variation of air temperature. As a result, the generated electric power of a power plant with an ACHE system is considerably influenced by meteorological conditions in the region where this type of power plant is built. The present work studies three different designs of a refuse-incinerating power plant with ACHE for a real project. Under the local meteorological conditions, the overall flow and temperature fields of the air in the whole power plant, especially around the air-cooled heat exchanger, are numerically investigated with the commercial computational fluid dynamics (CFD) code FLUENT and the feasibility of the project is then evaluated. In addition, a comparison of these three designs is also provided, including the performance of the exchangers and the fans, the collocation of the exhaust gas pipe, and the costs of the ACHE system. [ABSTRACT FROM AUTHOR]

The air inlet flow direction is not orthogonal to the heat exchanger surface in many cases. To study the performance of the heat transfer and pressure drop of a heat exchanger with different air inlet angles, this paper shows the experimental system about a finned oval-tube heat exchanger inclined toward the air incoming flow direction. The heat transfer and pressure drop characteristics of four air inlet angles (90°, 60°, 45°, and 30°) are studied separately for the Reynolds number ranging from 1300 to 13000 in this study. The experimental correlations of Nusselt number and resistance coefficient of the air side are acquired. The results show that the overall heat transfer coefficients become smaller and smaller with the decrease of the air inlet angles, while the pressure drops have significant changes. The heat transfer performances of the heat exchanger under the three inclined air inlet angles are worse than that at 90°. Among the three inclined angles, the performance at 45° is the best under identical mass flow rate criterion and at low Reynolds number under identical pressure drop criterion; that at 60° is the best at large Reynolds under identical pressure drop criterion. Finally, some conclusions are attained about the effects of the air inlet angles on the heat transfer and pressure drop performance of the finned oval-tube heat exchanger. [ABSTRACT FROM AUTHOR]

The growth of global energy demand during the 21st century, combined with the necessity to master greenhouse gas emissions, has led to the introduction of a new and universal energy carrier: hydrogen. The Department of Energy (DOE) proposed using a bayonet-type heat exchanger as a silicon carbide integrated decomposer (SID) to produce the sulfuric acid decomposition product sulfur dioxide, which can be used for hydrogen production within a sulfur–iodine thermochemical cycle. A two-dimensional computational model of SID having a boiler, superheater and decomposer was developed using GAMBIT and fluid. The thermal and chemical reaction analyses were carried out in FLUENT. The main purpose of this study is to obtain the decomposition percentage of sulfur trioxide for the integrated unit. Sulfuric acid (H2SO4), sulfur trioxide (SO3), sulfur dioxide (SO2), oxygen (O2), and water vapor (H2O) are the working fluids used in the model. Concentrated sulfuric acid liquid of 40 mol% was pumped into the inlet of the boiler and the mass fraction of concentrated sulfuric acid vapor obtained was then fed into the superheater to obtain sulfur trioxide. The decomposer region, which houses the pellets, placed on the top of the bayonet heat exchanger acts as the porous medium. As the decomposition takes place, the mass fraction of SO3is reduced and mass fractions of SO2and O2are increased. The percentage of SO3obtained from the integrated decomposer was compared with the experimental results obtained from Sandia National Laboratories (SNL). Further, effects of various pressures, flow rates, and acid concentrations on the decomposition percentage of sulfur trioxide were studied. [ABSTRACT FROM PUBLISHER]

The heat transfer enhancement of the longitudinal vortex (LV) is a kind of technology with high efficiency and low thermal resistance. An LV is produced by longitudinal vortex generators (LVGs). Due to their relative long influence distance and simple structure, the LVGs may be used in narrow channels with a flat surface. In this paper, the dimension of a narrow rectangular channel is 600 mm (length) × 40 mm (width) × 3 mm (height), and one LVG is 14 mm (length) ×2.2 mm (width) × 1.8 mm (height). The rectangular blocked LVGs are periodically laid out in the heated plate, and the attack angle of LVGs is 44°, the longitudinal pitch between LVGs is 100 mm, and the transverse pitch between LVGs is 4 mm. The test section is visual with three surfaces and heated with one surface by direct current. The working fluid is water. The experimental results show that the boiling heat transfer coefficient on the heated surface is increased by 25.8%, while the pressure drop along the test section is increased by 50.6%. At the same time, the visual experimental data shows that the bubbles' behavior has been intensively affected by LVs, the growth and gathering of bubbles have been depressed, and the thermal boundary layer in the test section has been greatly damaged and reduced; as results, the momentum and energy exchange in the test section have been strengthened. Thus, the heat transfer is obviously enhanced by LVs. [ABSTRACT FROM AUTHOR]

Helical baffles are employed increasingly in shell-and-tube heat exchangers (helixchangers) for their significant advantages in reducing pressure drop, vibration, and fouling while maintaining a higher heat transfer performance. In order to make good use of helical baffles, serial improvements have been made by many researchers. In this paper, a general review is provided of developments and improvements on helixchangers, which includes the discontinuous helical baffles, continuous or combined helical baffles, and the combined multiple shell-pass helixchangers. Extensive results from experiments and numerical simulations indicate that these helixchangers have better flow and heat transfer performance than the conventional segmental baffled heat exchangers. Based on these new improvements, the conventional heat exchangers with segmental baffles might be replaced by helixchangers in industrial applications to save energy, reduce cost, and prolong the service life and operation time. [ABSTRACT FROM AUTHOR]

High voltage air-core reactors are extensively used as components in electrical power distribution systems for many purposes. The noise of an air-core reactor can be reduced significantly using noise reducing cover. In the present article, the natural convection cooling performance of an air-core reactor with and without noise reducing cover is investigated to prevent the overheating problem. Six parameters, which all have five levels (values) of noise reducing cover, are considered. A Taguchi method is used as a systematic approach to plan the computational fluid dynamics (CFD) studies on the natural convection cooling performance. The optimum levels of six controllable parameters of noise reducing cover are determined. The relative contribution ratios of each parameter to the natural convection cooling performance are obtained. The contribution ratio of X1, the radius of the bottom opening of noise reducing cover, is the most significant one. This allows design efforts to be concentrated on the most sensitive factors. The present CFD-Taguchi combined method can be applied and conducted quickly and inexpensively to plan a more reliable and robust optimization. [ABSTRACT FROM AUTHOR]

Instead of the traditional trial-and-error process, a genetic algorithm (GA) is successfully applied to thermal design of fin-and-tube heat exchangers (FTHEs). The design method uses a GA to search and optimize structure sizes of FTHEs. The minimum total weight or total annual cost of FTHEs is taken as the objective function in the GA, respectively. Seven design parameters are varied for the optimization objectives. The implementation of the design method consists of a GA routine and a thermal design routine. In the GA routine, binary coding for tournament selection, uniform crossover, and one-point mutation is adopted. In the thermal design routine, thermal design of the FTHE is carried out according to the conditions of the structure sizes that the genetic algorithm generated, and the log-mean temperature difference method is used to determine the heat transfer area under the combined structure sizes for a given heat duty. Optimization shows that it is possible to achieve a great reduction in cost or weight, whenever such objectives have been chosen for minimization. The method is universal and may be used for thermal design and optimization of FTHEs under different specified duties. [ABSTRACT FROM AUTHOR]

The behavior of carbon particles (mostly 1 µm in diameter) in a natural-convection flow field of air and a magnetic field was studied numerically. One thousand particles were released randomly in a vertical cylinder whose height is equal to its radius. The bottom of the enclosure was heated and the side wall was cooled. A coil with electric current was set coaxially with the cylindrical enclosure. For the nano- and micro- order particles, Brownian motion was taken into consideration and the Langevin equation was solved for the particles. The results show that the behavior of 1-µm-sized particles depends mainly on the flow field. The magnetic field changes the flow mode of natural convection of air due to the paramagnetic susceptibility of oxygen and affects the behavior of the particles indirectly. The coil level, the aspect ratio of the cylinder, and the geometric arrangement of the hot and cold plates produce different flow fields, which makes the behavior of the particles different. The particles larger than 1 µm circulate along the streak lines or cluster, but those smaller than 1µm are strongly driven by the Brownian motion. [ABSTRACT FROM AUTHOR]

Abstract Two-phase flow is widely used in the cooling system of electronic equipment, where the heat source is localized. However, two-phase flow may lead to flow instabilities, which have adverse effects on the system such as heat transfer deterioration and vibration. In this study, the effects of related variables on Ledinegg instability in single and parallel channels under localized heat source are investigated. Both models of single and parallel channels have been verified, and the maximum relative error is within ±5%. Based on these two models, the results indicate that the high heating power and low inlet subcooling promote Ledinegg instability, whereas Ledinegg instability is independent of the channel inclination. The average negative slope of continuous heat source, −0.09557, is smaller than that of the heat source close to inlet, −0.07408, which shows the improvement of localized heat source on Ledinegg instability. For the parallel system, when the flow excursion occurs, the total mass flow rate, which is 0.028 kg/s, is equal to two times of the mass flow rate of maximum value in branch pressure curve at which pressure is equal to 0.778 MPa, if two branches are identical. The current work provides qualitative conclusions on how Ledinegg instability will be affected with the change of relevant variables. [ABSTRACT FROM AUTHOR]