基于弱耦合方式的柴油机冷却风扇的数值模拟研究

基于流固耦合理论,采用VB语言对三维计算流体动力学软件SC/Tetra和有限元分析软件ANSYS进行了二次开发,编写了弱耦合分析的接口程序.利用此程序对某柴油机塑料冷却风扇在发动机标定工况下进行了数值模拟研究.结果表明:考虑风扇流固耦合作用后,计算得到的风扇流量更接近试验值;该方法为柴油机柔性风扇的设计与优化提供数值计算平台,具有普遍的指导意义.     

Numerical study of cell morphology effects on convective heat transfer in reticulated ceramics

Understanding the influence of foam morphology on the heat transport mechanism is an essential task for the design engineers. The assessment of foam thermal properties was performed using experimental techniques or simulation approaches such as Finite Elements analysis and/or computational fluid dynamics and was, up to now, mainly focused on describing the influence of some average parameters, such as cell size and porosity. Recent numerical analysis have instead demonstrated that local cell morphological structures can strongly influence thermal conduction in ceramic foams. Therefore, in the present work, the effect of morphological characteristics, namely ligament radius, cell inclination angle and ligament tapering, on the convective heat transfer of ceramic foams were studied. The approach used is Computational Fluid Dynamics (CFD) and foam geometries were schematically represented with tetrakaydecahedra geometries. The numerical simulations, performed with ANSYS/Fluent on different tetrakaydecahedra geometries, aimed at evaluating pressure drop and heat exchange through the foam. A heat exchanger efficiency parameter was defined and then evaluated for the different foam geometries at several air flow velocities. Results show the influence of the different morphological parameters and, in particular, that the heat exchanger efficiency of the foams decreases when increasing the air flow velocity.     

Effect of synchronized piezoelectric fans on microelectronic cooling performance

This study reports on the influence of dual vibrating fans on flow and thermal fields through numerical analyses and experimental measurements. Two piezoelectric fans were arranged face to face and were vertically oriented to the heat source. 3D simulation was performed with FLUENT and ABAQUS with the use of code coupling interface MpCCI to calculate the velocity and temperature distribution on the horizontal hot plate. The fans' motion was described as deformable parts by ABAQUS at their first mode vibration. The effects of vibration phase difference between the fans corresponding to in-phase (Φ = 0°) and out-of-phase (Φ = 180°) vibrations were explored in terms of transient temperature and flow fields. The purpose is to enhance heat dissipation from the microelectronic component. Comparison with the performance of a single fan is made to assess the significance of the additional fan on thermal performance. Good comparison results were achieved through accurate modeling of the most important features of the fans and through heat transfer. Computed results show that the single fan enhanced heat transfer performance within approximately 2.3 times for the heated surface. By contrast, the dual fans enhanced heat transfer performance within approximately 2.9 for out-of-phase vibration (Φ = 180°) and 3.1 for in-phase vibration (Φ = 0°).     

Numerical study of the hydrodynamics and heat transfer characteristics of liquid–liquid Taylor flow in microchannel

A numerical study of an oil–water Taylor flow is presented in this paper to explore its flow and heat transfer characteristics. Due to the large surface area to volume ratio in narrow channels, using slug flows, high heat and mass transfer rates could be achieved. Sound knowledge of the underlying physics of slug flow is required for the practical design of microfluidic devices. In this study, hydrodynamics and heat transfer characteristics of dispersed oil droplets flowing inside a vertically upward circular microchannel (D = 0.1 mm) with water being the carrier phase have been explored numerically. ANSYS Fluent was employed to capture the liquid–liquid interface using volume of fluid method. Two different boundary conditions were considered in the present study. First, an isothermal wall of 373 K and later a constant wall heat flux (420 kW/m²) were, respectively, prescribed over the wall of the microchannel. The numerical code was validated against the results available in the literature, and the significant results in the form of pressure drop and heat transfer rates have been discussed. A considerable increase in Nusselt number, up to 180% and 210%, was observed with the oil–water slug flow in contrast to the liquid‐only single‐phase flow inside the microchannel for isothermal and constant wall heat flux conditions, respectively.     

Analysis of three-dimensional heat and fluid flow induced by piezoelectric fan

In this study, the characteristics of the three-dimensional heat and fluid flow fields generated by the vibrating fan are examined by performing numerical simulations and experimental measurements. This paper considers two different arrangements of the heat source, namely vertical and horizontal. In performing the simulations, the fluid domain is meshed using a dynamic meshing scheme in order to take account of the time-varying shape and position of the vibrating blade. The results show that the interaction between the normal force exerted on the air surrounding the moving blade and the impingement jet flow produced at the blade tip prompts the formation of two counter-rotating screw-type flow circulations on either edge of the blade. An infrared thermal camera is used to measure the temperature distribution on the heated surface to examine the numerical results. It is indicated that the piezoelectric fan improves the heat transfer coefficient by 1.6–3.4 times when the heat source is vertically arranged and 1.8–3.6 times when the heat source is horizontally arranged.     

Transient response of plate heat exchangers considering effect of flow maldistribution

Plate heat exchangers have been playing important role in the power and process industries in the recent past. Hence, it is important to develop simulation strategies for plate heat exchangers accurately. This analysis represents the dynamic behaviour of the single pass plate heat exchangers, considering flow maldistribution from port to channel. In addition to maldistribution the fluid axial dispersion is used to characterise the back mixing and other deviations from plug flow. Due to unequal distribution of the fluid, the velocity of the fluid varies from channel to channel and hence the heat transfer coefficient variation is also taken into consideration. Solutions to the governing equations have been obtained using the method of Laplace transform followed by numerical inversion from frequency domain. The results are presented on the effects of flow maldistribution and conventional heat exchanger parameters on the temperature transients of both U-type and Z-type configurations. It is found that the effect of flow maldistribution is significant and it deteriorates the thermal performance as well as the characteristic features of the dynamic response of the heat exchanger. In contrast to the previous studies, here the axial dispersion describes the inchannel back mixing alone, not maldistribution, which is physically more appropriate. Present method is an efficient and consistent way of describing maldistribution and back mixing effects on the transient response of plate heat exchangers using an analytical method without performing intensive computation by complete numerical simulation.     

Numerical analysis of heat transfer under a radiant warmer

The main objective of this paper is to present numerical modeling and assessment of heat transfer in neonatology. In the present study, numerical simulation is performed over a simplified infant model with specific boundary conditions in a closed chamber. The proposed approach is based on three‐dimensional (3D) computational fluid dynamics (CFD) simulation to capture the combined effect of air flow and heat transfer phenomena: natural convection and radiation heat transfer taking place around an infant and radiant lamp. A 3D model is numerically investigated using the commercial CFD package StarCCM+. The results presented are compared and found to be in qualitative agreement with the results available in the literature and published measurement data.     

Numerical investigation of nonuniform transverse magnetic field effects on the flow and heat transfer of magnetic nanofluid in a sintered porous channel

In this paper, fluid flow and convective heat transfer of a ferrofluid (water and 4 vol% Fe₃O₄) in sintered Aluminum porous channel, which is subjected to a nonuniform transverse magnetic field have been studied. The numerical simulations supposed an ordinary cubic and staggered arrangement organized by uniformly sized particles with a small contact area for the porous media and constant heat flux at the surface of the microchannel. A wire, in which the electric current passes creates a nonuniform magnetic field, which is perpendicular to the flow direction. To do this simulation, the control volume technique and the two‐phase mixture model have been employed. The results show that the obtained local heat transfer coefficient on the channel surface increased with increasing mass flow rate and decreased slightly along the axial direction. Moreover, exerting the above‐mentioned magnetic field increases the Nusselt number that enhances the heat transfer rate while it has no effect on the pressure drop along the channel.     

CFD simulation of a Gifford–McMahon type pulse tube refrigerator

Pulse tube refrigerator has the advantages of long life and low vibration over the conventional cryocoolers, such as Gifford–McMahon (GM) and Stirling coolers because of the absence of moving parts in low temperature. This paper performs a two-dimensional computational fluid dynamic (CFD) simulation of a Gifford–McMahon type double inlet pulse tube refrigerator (DIPTR), operating under a variety of thermal boundary conditions. A commercial Computational Fluid Dynamics (CFD) software package Fluent 6.1 is used to model the oscillating flow inside a pulse tube refrigerator. Helium is used as working fluid for the entire simulation. The simulated DIPTR consists of a transfer line, an after cooler, a regenerator, a pulse tube, a pair of heat exchangers for cold and hot end, an orifice valve with connecting pipe, a double inlet valve with connecting pipe and a reservoir. The simulation represents fully coupled systems operating in steady-periodic mode. The externally imposed boundary condition is sinusoidal pressure inlet by user defined function at one end of the tube and constant temperature or heat flux boundaries at the external walls of the hot end and cold-end heat exchangers. The general results, such as the cool down behaviors of the system, phase relation between mass flow rate and pressure at pulse tube section and the temperature profile along the wall of the cooler are presented.The simulation shows the minimum decrease in temperature at cold-end heat exchanger for a particular combination of cryocooler assembly. The CFD simulation results are compared with available experimental data. Comparisons show that there is a reasonable agreement between CFD simulation and experimental results.     

Effect of a shield on the hydraulic and thermal performance of a plate-fin heat sink

The hydraulic and thermal performance of a plate-fin heat sink undergoing cross flow forced convection with the introduction of a shield was investigated. With a CFD numerical method, the influence of fin width, fin height, number of fins and Reynolds number were assessed without and with a shield. A shield that tends to decrease the bypass flow effect has a great influence on the variation of the thermal fluid field and the performance of the heat sink. The results of attaching a shield show that more coolant fluid is forced to flow into the fin-to-fin channel to enhance the heat transfer, increasing the pressure drop; this trend is significant at low Reynolds numbers. The decrease of thermal resistance due to the shield diminishes with increasing fin height, but increasing the width of the fins has a more radical effect. For a shield at a particular Reynolds number, the fin geometry should be selected carefully to fit the demands of enhanced effectiveness of heat transfer and decreased power consumption.     

Numerical and experimental investigation for heat transfer in triplex concentric tube with phase change material for thermal energy storage

This paper addresses a numerical and experimental investigation of a thermal energy storage unit involving phase change process dominated by heat conduction. The thermal energy storage unit involves a triplex concentric tube with phase change material (PCM) filling in the middle channel, with hot heat transfer fluid (HHTF) flowing outer channel during charging process and cold heat transfer fluid (CHTF) flowing inner channel during discharging process. A simple numerical method according to conversation of energy, called temperature & thermal resistance iteration method has been developed for the analysis of PCM solidification and melting in the triplex concentric tube. To test the physical validity of the numerical results, an experimental apparatus has been designed and built by which the effect of the inlet temperature and the flow rate of heat transfer fluid (HTF, including HHTF and CHTF) on the thermal energy storage has been studied. Comparison between the numerical predictions and the experimental data shows good agreement. Graphical results including fluid temperature and interface of solid and liquid phase of PCM versus time and axial position, time-wise variation of energy stored/released by the system were presented and discussed.     

Numerical Investigation of Heat Transfer Performance of Rectangularly Coiled Pipes

A numerical simulation was conducted to investigate convective heat transfer from small and compact coiled pipes heat exchangers using computational fluid dynamics (CFD) software Fluent V6. One fluid (air) moves over the coiled pipe while a second fluid (refrigerant R141B) at different temperature flows through the pipe. The studied heat exchanger is composed with bends and straight tubes. Calculations were done for two cases with different outside flow arrangements. The simulation results showed remarkable differences in the flow characteristics and heat transfer rate of different single tubes of the entire heat exchangers. The temperature distribution and heat transfer are mainly influenced by temperature gradient, backflow conditions, exterior flow velocity, and surface area. The results also show the effect of the bends on the flow in straight tubes and vice-versa.     

刚性壁面法向振动对流动换热的影响

采用理论推导与数值模拟结合的方法,通过求解流动方程及能量方程得到壁面法向振动下流体流动特性变化规律,并分析振动对换热的影响。结果表明:层流状态下,低强度振动使壁面附近流体质点产生法向速度,但影响范围很小,对换热影响有限;随着振动强度的增大,流场逐渐转变为湍流,导致换热系数提升。通过数值模拟计算壁面平均努塞尔数随振动强度及来流速度的变化规律。结果表明:对于低速流体,当振动达到某一临界值后能增强换热效果,努塞尔数随振动强度增大而增加,最佳换热相位角在200°左右,低雷诺数下振动强化换热效果较为明显,对流换热系数最大可提升300%。     

On the design of the RTO unit using CFD

The objective of this work is to study alternative designs for a flow mixing tank to be installed ahead of a regenerative thermal oxidation (RTO) unit in an asphalt plant. A study of the steady-state flow field in the unit was conducted applying computational fluid dynamics (CFD). The governing equations (mass, momentum, energy and turbulence) were solved using the commercial code Fluent. The simulation results provided insight into the effectiveness of the numerical predictions of conditions in the mixer and led to practical responses in terms of a better design of the industrial RTO unit.     

Validation of a CFD model for the simulation of heat transfer in a tubes-in-tank PCM storage unit

A computational fluid dynamics (CFD) model was developed for the simulation of a phase change thermal energy storage process in a 100 l cylindrical tank, horizontally placed. The model is validated with experimental data obtained for the same configuration. The cold storage unit was charged using water as the heat transfer medium, flowing inside a horizontal tube bundle, and the selected phase change material (PCM) was microencapsulated slurry in 45% w/w concentration. The mathematical model is based on the three-dimensional transient Navier–Stokes equations with nonlinear temperature dependent thermo-physical properties of the PCM during the phase change range. These properties were experimentally determined using analytical methods. The governing equations were solved using the ANSYS/FLUENT commercial software package. The mathematical model is validated with experimental data for three different flow rates of the heat transfer fluid during the charging process. Bulk temperature, heat transfer rate and amount of energy stored were used as performance indicators. It was found that the PCM bulk temperatures were predicted within 5% of the experimental data. The results have also shown that the total accumulated energy was within 10% of the observed value, and thus it can be concluded that the model predicts the heat transfer inside the storage system with good accuracy.     

A CFD-based experimental analysis on the effect of free stream cooling on the performance of micro processor heat sinks

Heat sinks are widely used to remove the heat from the microelectronic devices including Pentium and AMD processors. In the present study, four different types of heat sink have been used i.e. Pentiums III and IV, AMD Athlon and Duron heat sinks; in order to analyze their performance. The paper presents the comparison of heat sinks of Pentium and AMD families. The simulation and experimental investigations have been made at different Reynolds numbers. The Fluent 6.2 software which is a computational fluid dynamics (CFD) code has been used in the simulation to predict the temperature and the flow fields. The experiments have been carried out by using an air chamber with nozzle at different Reynolds numbers. The Nusselt number and temperature distributions have been plotted against Reynolds number for all heat sinks. The simulation results obtained are found in satisfactory agreement with the experimental results.     

Effect of aspect ratio on saturated boiling flow in microchannels with nonuniform heat flux

Microchannel two‐phase flow is an effective cooling method used in microelectronics, in which the heat flux density is unevenly distributed usually. The paper is focused on numerical study the effect of aspect ratio on the flow boiling of microchannels with nonuniform heat flux. The heat source is a three‐dimensional (3D) integrated circuit. 3D microchannel model and volume of fluid method are coupled in numerical simulation. The results show that the aspect ratio has no relationship with the two‐phase pressure drop of the microchannel. It has a certain influence on the distribution of bubble shape. In terms of the heat transfer coefficient, the aspect ratio has a certain influence on a section of the inlet. Due to the nucleate boiling, the convective heat transfer in the remaining areas is the dominant factor and the average heat transfer coefficient is mainly determined by the heat flux at the bottom of the channel.     

Flexibly using results of CFD and simplified heat transfer model for pulverized coal‐fired boilers

The efficient use of pulverized coal is crucial to the utility industries. The use of computational fluid dynamics (CFD)‐based numerical models has an important role in the design of new boiler furnaces or in retrofitting situations. The results of CFD simulations can be used to better understand the complex processes occurring within the boiler furnace. The use of these results to support boiler operation and training of operators requires that the CFD models can be easily accessed and the results are easily analysed. This paper discusses two ways to simulate the heat transfer process in boiler furnaces. The method directly applying CFD results is employed, in which the grid for solving the energy equation is the same as the flow grid in the CFD simulation while radiation heat transfer is solved in another relatively coarse grid. Comparison of the prediction results between CFD and Heat Transfer code (Simple model) is performed under boiler full load (100%) with one side wall fouling, as well as for different boiler loads (100, 98 and 95 per cent boiler full load, respectively). Finally, the flexible use of the results of CFD and the simple model for pulverized coal‐fired boilers is presented. To facilitate the use of the system, a user‐friendly interface was developed which enables the user to manipulate new calculations and to view results, namely performing ‘what–if’ analysis. Copyright © 2000 John Wiley & Sons, Ltd.     

VOF modelling of gas–liquid flow in PEM water electrolysis cell micro-channels

In this study, the gas–liquid flow through an interdigitated anode flow field of a PEM water electrolysis cell (PEMEC) is analysed using a three-dimensional, transient, computational fluid dynamics (CFD) model. To account for two-phase flow, the volume of fluid (VOF) method in ANSYS Fluent 17.2 is used. The modelled geometry consists of the anode channels and the anode transport layer (ATL). To reduce the complexity of the phenomena governing PEMEC operation, the dependence upon electro-chemistry is disregarded. Instead, a fixed source of the gas is applied at the interface between the ATL and the catalyst layer. An important phenomenon that the model is able to capture is the gas–liquid contact angle on both the channel wall and ATL-channel interface. Particularly, the latter interface is crucial in capturing bubble entrainment into the channel. To validate the numerical simulation, photos taken of the gas–liquid flow in a transparent micro-channel, are qualitative compared against the simulation results. The experimental observations confirm the models prediction of long Taylor bubbles with small bubbles in between. From the simulation results, further intriguing details of the flow are revealed. From the bottom to the top of the outgoing channel, the film thickness gradually increases from zero to 200 μm. This increase in the film thickness is due to the particular superficial velocity field that develops in an interdigitated flow. Here both the superficial velocities change along the length of the channel. The model is capable of revealing effect of different bubble shapes/lengths in the outgoing channel. Shape and the sequence of the bubbles affect the water flow distribution in the ATL. The model presented in this work is the first step in the development of a comprehensive CFD model that comprises multiphase flow in porous media and micro-channel, electro-chemistry in catalyst layers, ion transport in membrane, hydrogen evolution, etc. The model can aid in the study of gas–liquid flow and its impact on the performance of a PEMEC.