Parametric and internal study of the vortex tube using a CFD model

A computational fluid dynamics (CFD) model is used to investigate the energy separation mechanism and flow phenomena within a counter-flow vortex tube. A two-dimensional axi-symmetric CFD model has been developed that exhibits the general behavior expected from a vortex tube. The model results are compared to experimental data obtained from a laboratory vortex tube operated with room temperature compressed air. The CFD model is subsequently used to investigate the internal thermal-fluid processes that are responsible for the vortex tube's temperature separation behavior. The model shows that the vortex tube flow field can be divided into three regions that correspond to: flow that will eventually leave through the hot exit (hot flow region), flow that will eventually leave through the cold exit (cold flow region), and flow that is entrained within the device (re-circulating region). The underlying physical processes are studied by calculating the heat and work transfers through control surfaces defined by the streamlines that separate these regions. It was found that the energy separation exhibited by the vortex tube can be primarily explained by a work transfer caused by a torque produced by viscous shear acting on a rotating control surface that separates the cold flow region and the hot flow region. This work transfer is from the cold region to the hot region whereas the net heat transfer flows in the opposite direction and therefore tends to reduce the temperature separation effect. A parametric study of the effect of varying the diameter and length of the vortex tube is also presented.     

Energy separation mechanism in unconfined laminar compressible vortex

A theoretical investigation from the view point of gas-dynamics and thermodynamics was carried out, in order to clarify the energy separation mechanism in an unconfined laminar compressible vortex, as a primary flow element of a vortex tube. The mathematical solutions of density and temperature in a viscous compressible vortical flow, with tangential velocity, were examined using an evaluation equation of total temperature. It is found from the results that a hotter gas in the peripheral region of the vortex is mainly generated by heat caused by viscous dissipation. A colder gas in the vortex center is mainly generated by viscous shear work done by the fluid element onto the surface of the surrounding gas. In addition, it is also found that the larger the representative Mach number of a vortex is, the lower the total temperature at the center of the vortex is, and at the same time, the higher the maximum total temperature in the peripheral region is. The increase in specific heat ratio of the working gas has the same effect, as increasing the representative Mach number of the vortex, on the total temperature in the vortex.     

Heat Transfer in a Micropolar Fluid along a Non-linear Stretching Sheet with a Temperature-Dependent Viscosity and Variable Surface Temperature

In this paper, heat transfer characteristics of a two-dimensional steady hydromagnetic natural convection flow of a micropolar fluid passed a non-linear stretching sheet taking into account the effects of a temperature-dependent viscosity and variable wall temperature are studied numerically for local similarity solutions by applying the Nachtsheim-Swigert iteration method. The results corresponding to the dimensionless temperature profiles and the local rate of heat transfer are displayed graphically for important material parameters. The results show that in modeling the thermal boundary layer flow with a temperature-dependent viscosity, consideration of the Prandtl number as a constant within the boundary layer produces unrealistic results and therefore it must be treated as a variable rather than a constant within the boundary layer. The results also show that the local rate of heat transfer strongly depends on the non-linear stretching index and temperature index.     

Temperature distribution in tools used for cutting iron,titanium and nickel

Temperature gradients in high speed steel tools used to turn iron, titanium and nickel have been measured using a method based on the structural changes in high speed steel heated to temperatures over 560°C. The relationship between temperature gradient, cutting speed and feed rate have been demonstrated through the heat affected region close to the cutting edge. The most emphatic feature of the results is that each of the three metals imposed its own and greatly different pattern of temperature distribution within the tool. It was shown that this controls some of the basic wear processes which limit the rates of metal removal. When cutting iron there was a relatively cool zone close to the cutting edge in the most highly stressed region, while the hottest part was well back from the edge; the temperature gradients were very Bteep. When cutting titanium the higher temperatures were much closer to the cutting edge, and the total heated region was smaller, while the temperature gradients were again very steep. When cutting nickel the tool edge was at least as hot as elsewhere in the heat affected area, and temperature gradients.near the edge were much less steep. The results have Televance to the understanding of tool wear, the design of cutting tools and the application of cutting lubricants.     

Dissimilar titanium/aluminum friction stir welding lap joints by experiments and numerical simulation

Dissimilar lap joints were produced by friction stir welding (FSW) out of Ti6Al4V titanium alloy and AA2024 aluminum alloy sheets. The joints, welded with varying tool rotation and feed rate, were studied by analyzing the maximum shear strength, Vickers microhardness and optical observations. A dedicated numerical model, able to take into account the presence of the two different alloys, was used to highlight the effects of the process parameters on temperature distribution, strain distribution, and material flow. The combined analysis of experimental measurements and numerical predictions allowed explaining the effects of tool rotation and feed rate on the material flow. It was found that tool rotation had a larger impact on the joint effectiveness with respect to feed rate. A competition between material mixing and heat input occurs with increasing tool rotation, resulting in higher joint strength when lower values of tool rotation are used.     

Influences of temperature and grain size on the material deformability in microforming process

This paper investigated the influences of temperature and grain size on the deformability of pure copper in micro compression process. Based on the dislocation theory, a constitutive model was proposed taking into account the influences of forming temperature, Hall-Petch relationship and surface layer model. Vacuum heat treatment was employed to obtain various grain sizes of cylindrical workpieces, and then laser heating method was applied to heat workpieces during microforming process. Finite element (FE) simulation was also performed, with simulated values agreed well with the experimental results in terms of metal flow stress. Both the FE simulated and experimental results indicate that forming temperature and grain size have a significant influence on the accuracy of the produced product shape and metal flow behaviour in microforming due to the inhomogeneity within the deformed material. The mechanical behaviour of the material is found to be more sensitive to forming temperature when the workpieces are constituted of fine grains.     

CPCM/液冷复合电池热管理方式优化设计

目的为了解决锂电池组在放电倍率为2.5 C,环境温度为308.15 K下工作时,其最高温度、最大温差可能超过适宜温度的情况。方法建立基于复合相变材料(CPCM)/液冷复合的电池组散热模型,首先通过实验测得锂电池单体相关性能参数,然后利用数值模拟方法讨论CPCM厚度对电池组散热性能的影响。分析得出当CPCM厚度在一定范围内变化时,单一的相变材料冷却方式不能将电池组最高温度控制在适宜的温度范围内,因此提出CPCM/液冷复合散热方式,以复合相变材料厚度、液冷通道间距、液体流速为设计变量,电池组最高温度和最大温差为优化目标进行多目标优化设计。结果结果表明,优化后的电池组最高温度和最大温差分别为316.88K和0.30K,满足设计要求,但相变材料在相变过程中存在泄露的风险。结论相较于单一的相变材料冷却方式,优化后的复合冷却模型能够大幅度降低电池组的最高温度,同时将最大温差控制在安全范围内;在保证散热模型最外层包装结构具有较高导热性的同时也要加强其结构设计,防止相变材料泄露。     

Joint formation mechanism of high depth-to-width ratio friction stir welding

High depth-to-width ratio friction stir welding is an attractive method for the joining demands of aluminum profiles, which is sparked with its extremely low heat input and high mechanical performance. In this study, the joint formation mechanism was studied by a numerical model of plastic flow combined with experimental approaches. A fluid-solid-interaction algorithm was proposed to establish the coupling model, and the material to be welded was treated as non-Newtonian fluid. The thread structure and the milling facets on tool pin promoted drastic turbulence of material. The thread structure converged the plasticized material by its inclined plane, and then drove the attached material to refill the welds. The milling facets brought about the periodic dynamic material flow. The thread structure and the milling facets increased the strain rate greatly under the extremely low heat input, which avoided the welding defects. The condition of the peak temperature of 648 K and the strain rate of 151 s⁻¹ attributed to the lowest coarsening degree of precipitate. The tensile strength of the joint reached 265 MPa, equivalent to 86% of base material. The amelioration via the material flow model inhibits the welding defects and optimizes the parameter intervals, providing references to extracting process-structure-property linkages for friction stir welding.     

航空电子模块储热器工作时间预测

航空电子模块储热器是一类广泛应用于航空电子设备的热管理装置.为了给航空电子模块储热器的工程设计提供理论支撑,对其工作时间与功耗、设定温度之间的关系进行研究.首先,综合考虑自然对流和热辐射的影响,运用传热学理论构建了储热器在实际工作环境下的热模型.然后,运用焓-多孔介质法对储热器的瞬态储热过程进行了数值模拟,并通过求解相...     

Thermomechanical finite element simulations of ultrasonically assisted turning

Ultrasonically assisted turning (UAT) is studied with finite element (FE) simulations and compared with conventional turning (CT) using both computational results and infrared thermography experiments. The two-dimensional thermomechanically coupled FE model of both UAT and CT utilizes MSC MARC general FE code and incorporates temperature dependent material properties, strain rate effects, heat generated due to plastic flow, contact interaction and friction at the cutter/workpiece interface. Material separation in front of a cutting edge and automatic remeshing of distorted elements are implemented in the developed computational scheme. Influence of friction on resultant temperatures and chip shapes in turning for both UAT and CT is discussed. Temperature fields in the cutting region and in the cutting tool for CT and UAT are studied and compared with the experimental data. A role of various heat transfer parameters on thermal processes in UAT and CT is investigated.     

Effect of Traverse/Rotational Speed on Material Deformations and Temperature Distributions in Friction Stir Welding

A fully coupled thermo-mechanical model was developed to study the temperature fields and the plastic deformations of alloy AL6061-T6 under different process parameters during the friction stir welding （FSW） process. Three-dimensional results under different process parameters were presented. Results indicate that the maximum temperature is lower than the melting point of the welding material. The higher temperature gradient occurs in the leading side of the workpiece. The calculated temperature field can be fitted well with the one from the experimental test. A lower plastic strain region can be found near the welding tool in the trailing side on the bottom surface, which is formed by the specific material flow patterns in FSW. The maximum temperature can be increased with increasing the welding speed and the angular velocity in the current numerical modelling.     

Analysis on Simulation of Quasi-Steady Molecular Statics Nanocutting Model and Calculation of Temperature Rise During Orthogonal Cutting of Single-Crystal Copper

This paper uses quasi-steady molecular statics method to carry out simulation of nanoscale orthogonal cutting of single-crystal copper workpiece by the diamond tools with different edge shapes. Based on the simulation results, this paper analyzes the cutting force, equivalent stress and strain, and temperature field. For the three-dimensional quasi-steady molecular statics nanocutting model used by this paper, when the cutting tool moves on a workpiece, displacement of atoms is caused due to the effects of potential on each other. After a small distance that each atom moves is directly solved by the calculated trajectory of each atom, the concept of force balance is used. And Hooke-Jeeves direct search method is also used to solve the force balance equation, and obtain the new movement position. When chip formation and the size of cutting force during cutting are calculated, further analysis is made. After the position of an atom's deformation displacement is acquired, the shape function concept of finite element is employed to obtain the atomic-level equivalent strain. With the stress-strain curve obtained from experiment of the numerical tensile value of nanoscale copper film taken as the foundation, regression treatment is made, and then the flow stress-strain relational equation is acquired. The flow stress-strain curve is used to calculate the equivalent stress produced under equivalent strain of element. This paper further supposes that workpiece temperature is mainly produced from two heat sources: plastic deformation heat and friction heat. Thus, this paper uses the acquired equivalent stress and strain to calculate plastic deformation heat. Besides, this paper additionally develops a method to calculate the numerical value of friction heat produced by the workpiece atoms on the tool face and the numerical value of temperature rise of workpiece atoms on tool face. Finally, the temperature rise produced from the two heat sources is added up for calculation of temperature field of the cut single-crystal copper workpiece during nanoscale orthogonal cutting, and for making analysis.     

制冷剂-空气热交换器集成式数字化设计软件

为了满足采用同一仿真工具对制冷系统中常用的不同类型的换热器进行仿真,建立了一种通用的制冷剂-空气换热器三维分布参数模型,并据此开发了具有3D交互式图形界面的换热器集成式数字化仿真设计软件HXSIM。HXSIM采用统一的仿真框架对换热器进行模拟,并将翅片管、微通道管翅、丝管和板管划分为四类控制单元。且每个单元均包括制冷剂,空气和管翅(板)三个对象。并根据三个对象的结构特点和传热特性建立了相应的控制方程。为了便于用户使用,在HXSIM中采用OpenGL技术开发了界面友好的3D交互式图形界面。HXSIM对不同类型的换热器都有较高的仿真精度,实验验证表明其换热量的最大预测误差小于10%,空气侧压降的最大预测误差小于1.16 Pa。     

Numerical analysis of drying characteristics of frozen material immersed in fluidized bed at low temperature under reduced pressure

A spherical frozen material was dried in a fluidized bed of inert particles at a low temperature (lower than the melting point of water) under reduced pressure. To evaluate the drying characteristics of the frozen material, the heat and mass transfers in the material during the drying process were calculated using one-dimensional differential equations. The fitting parameters (accommodation coefficient of internal sublimation and heat transfer coefficient at the surface of the material) were determined to fit the calculation results and the experimental data. Drying characteristics, such as the distributions of moisture content and temperature in the material during drying, were calculated. Operational conditions, such as bed temperature, humidity, and heat transfer coefficient (gas velocity) at the surface of the material were varied in the calculation, and the effects on the end time of drying were estimated. Sublimation in the interior of the material governs the drying characteristics. The dry region in the material became resistant to heat transfer. The calculation results are reasonable for expressing the drying characteristics of freeze-drying, that is, our calculation method can be used to estimate the drying characteristics of frozen material in a fluidized bed.     

Design of multi-tubular heat exchangers for optimum efficiency of heat dissipation

The optimum design of compact heat exchangers made of a linear metal cellular material is presented. A novel representation of the cylindrical multi-tubular configuration is used. The aim is to maximize the heat dissipation rate while minimizing the prescribed flow pressure by optimizing the multi-tube configuration. The optimum distribution of cellular material for square-cell morphology (cell density and size over given cylindrical cross-section) is found using a structural topology optimization approach. The optimized thermal performance is compared using numerical analysis including both axial temperature fields and variations within the cross-sectional area. The results for the effects of different cross-section shapes, thermal boundary conditions and flow rates are discussed and compared. Interestingly, the present formulation leads to a non-uniform distribution of cellular structures which mimic natural biomaterials. Based on these results, design guidelines for a compact multi-tubular heat exchanger are presented.     

非金属绝热材料低温热导率测试装置

建立了一套非金属绝热材料低温热导率测试装置,测试采用稳态轴向热流法,高真空绝热,通过可控气体热开关提高了样品的降温速度,运用该装置对聚氨酯隔热材料液氮到室温温区热导率进行了测量,并对装置的重复性以及漏热和误差进行了分析。     

Turbulent flow through a circular pipe: Resistance and heat exchange at the constant boundary temperature

In this work, problems of the velocity profile, hydraulic resistance and heat exchange at constant equal temperature on the walls, steady-state turbulent flow, and established heat exchange in a straight channel limited by coaxial circular cylinders (a circular pipe) are solved. A moving incompressible fluid is considered as the medium, its viscous and heat-conducting properties being defined not only by its physical properties, but also by stable vortex structures that are formed upon the turbulent flow and generate local anisotropy of the medium. A vector called the director is a characteristic parameter of anisotropy. Director dynamics within the flow is assigned by a separate equation. The flow region consists of two near-wall subregions, which are adjacent to solid flow boundaries. The boundary between the subregions is determined during solving the problem. A closed set of equations is formulated for the desired values (velocity, temperature), and boundary conditions are laid. The velocity profile and temperature field in the flow were obtained in form of solutions to the corresponding boundary problems. The results of solution are compared with the experimental data and empirical formulas.     

Numerical simulation of non-adiabatic capillary tubes considering metastable region. Part II: Experimental validation

A detailed one-dimensional steady and transient numerical simulation of the thermal and fluid-dynamic behavior of capillary tube–suction line heat exchangers considering metastable region and separated flow has been developed in Part I of this paper. The developed numerical model allows analysis of aspects such as geometry, type of fluid, critical or non-critical flow conditions and metastable region. The accuracy of the detailed simulation model is demonstrated in this part (Part II) of the paper by comparing simulation results with a wide range of steady state experimental data from the technical literature, which include the refrigerant mass flow rate, outlet suction line temperature, and temperature profile along concentric and lateral capillary tube–suction line heat exchangers. Of the 196 data points evaluated for mass flow rate 96.4% are within an error of ±15%, 81.1% are within ±10% with a mean deviation of ±6.3%. Of the 143 data points evaluated for outlet suction line temperature 89.5% are within an error of ±2 °C, with a mean deviation of ±0.98 °C.The numerical results obtained are used to understand the refrigerant flow behavior inside non-adiabatic capillary tubes. Some divergence problems in the numerical solution process is found to be the discontinuity in non-adiabatic capillary tube flow characteristics caused by re-condensation of the refrigerant within the heat exchanger zone; this aspect needs special attention while modeling the non-adiabatic capillary tube flow. Other important parameter to be evaluated experimentally with special care is the capillary tube internal diameter due to its strong influence on the refrigerant flow results (results of any study based on the nominal diameter are to be used with caution).     

Influence of machine parameters on material flow behavior during channeling in modified friction stir channeling

Material flows during friction stir processes are very complex and not fully understood. Although the most literature reviews are presenting for material flow path, but Modified Friction Stir Channeling (MFSC) is a novel process based on Friction Stir Processing (FSP), which is being utilized to produce internal channel in Monolithic plates. A new flow pattern, in this study, is proposed to investigate material flow path and channel formation mechanism. The validity of this model is demonstrated by observing the cross-section and created keyhole using stop-action technique. The main difference between Friction Stir Channeling (FSC) and MFSC is the tilt angle. The effect of tilt angle and process parameters such as rotational speed and traverse speed on material flow path is studied. The results indicate that the tilt angle is an effective factor on forging of material behind of pin in the Advancing Side (AS) and also in the upper portion of channel roof imperfections. The rotational speed is an effective parameter on extruded material around the tool pin body and the extracted material in front of tool pin, because of the changing in the slip–stick condition and generated heat by tool. Traverse speed was an effective parameter on forging action of material and to keep material nearby tool pin in the behind of pin.