Resonant operation and cycle work from a MEMS-based micro-heat engine

The documentation of a new engine thermodynamic cycle on the micro-scale is presented. This new cycle is the result of resonant operation and cycle work production from a MEMS-based micro-heat engine. The engine is constructed of two thin membranes surrounding a cavity filled with working fluid. This new thermodynamic cycle is shown to include nearly constant volume pressure increase, expansion, heat rejection, and compression components. A thermal switch is integrated with the micro-engine to control heat rejection. The micro-engine is shown to produce up to 6.7 μW of cyclic mechanical power when operated on this cycle. Micro-engine natural frequency is shown to vary from 90 to 140 Hz. The Micro-engine is shown to operate across a low temperature gradient of 1.5°C.     

MEMS-based resonant heat engine: scaling analysis

In this article, a scaling model of a MEMS-based resonant heat engine is presented. The engine is an external combustion engine made of a cavity encapsulated between two thin membranes. The cavity is filled with a saturated liquid–vapor mixture working fluid. Both model and experiment are used to investigate issues related to scaling of the engine. The results of the scaling analysis suggest that the performance of the engine is determined by three major factors: geometry of the engine, speed of operation, and thermal-physical properties of engine components. Larger engine volumes, working fluids with higher latent evaporation property, slower engine speeds, and compliant expander structure are desirable. In experiments, the expander membrane material and thickness, and the thickness of engine cavity are varied. The experimental measurements show that the velocity amplitude of the expander membrane increases by approximately a factor of 20 when more compliant structure with less thermal inertia is used.     

基于热平衡和结构强度的发动机 分析平台开发及其应用

建立发动机缸体和缸盖的动力学、燃烧和流体有限元模型,进行发动机热平衡、冷却散热和结构强度研究。创建发动机正向设计和分析方法,革新产品开发流程,自主开发热平衡计算平台。建立发动机缸内和水套传热、流体、温度场、强度的计算模型库,设计水套优化方法和流场评价标准,正向计算发动机热平衡和水套散热。通过2个应用案例,证明该平台在发动机热平衡计算和结构分析与评估中的作用。该平台可为发动机热平衡、冷却散热和结构强度的正向设计提供基础。     

Optimal Configuration and Performance of Heat Engines with Heat Leak and Finite Heat Capacity

The optimal configuration of a class of two-heat-reservoir heat engine cycles in which the maximum work output can be obtained under a given cycle time is determined with the considerations of heat leak, finite heat capacity high-temperature source and infinite heat capacity low-temperature heat sink. The heat engine cycles considered in this paper include: (1) infinite low- and high-temperature reservoirs without heat leak, (2) infinite low- and high-temperature reservoirs with heat leak, (3) finite high-temperature source and infinite low-temperature sink without heat leak, and (4) finite high-temperature source and infinite low-temperature sink with heat leak. It is assumed that the heat transfer between the working fluid and the reservoirs obeys Newton's law. It is shown that the existence of heat leak doesn't affect the configuration of a cycle with an infinite high-temperature source. The finite heat capacity of a high temperature source without heat leak makes the cycle a generalized Carnot heat engine cycle. There exists a great difference of the cycle configurations for the finite high-temperature source with heat leak and the former three cases. Moreover, the relations between the optimal power output and the efficiency of the former three configurations are derived, and they show that the heat leak affects the power versus efficiency characteristics of the heat engine cycles.     

Development of a theoretical decoupled Stirling cycle engine

The Stirling cycle engine is gaining increasing attention in the current energy market as a clean, quiet and versatile prime mover for use in such situations as solar thermal generation, micro-cogeneration and other micro-distributed generation situations.A theoretical Stirling cycle engine model is developed. Using a theoretical decoupled engine configuration in which working space swept volume, volume variation, phase angle and dead space ratio are controlled via a black-box electronic controller, a model is developed that is to be used as a tool for analysis of the ideal Stirling cycle engine and the limits on its real-world realisation.The theoretical configuration approximates the five-space configuration common in Stirling cycle analysis. It comprises two working spaces and three heat exchangers: hot side, cold side and the regenerator between. The kinematic crank mechanism is replaced by electronically controlled motor/generator units, with one motor/generator controlling each of the working pistons. The use of stop valves permits the flow and non-flow processes inherent in the ideal cycle to be realised.The engine configuration considered here is not intended as a viable prime mover but rather a tool for study of the limitations of the cycle.     

某柴油机缸盖强度计算

以某4缸轿车用柴油机铝合金缸盖为研究对象,用某CFD软件计算得到水套表面的温度和对流换热系数,用Abaqus计算缸盖稳态温度场、装配载荷下冷机的应力分布、装配状态下的热机应力分布和各缸爆发时刻缸盖的应力分布,结果表明缸盖强度低于材料屈服极限.基于应力结果计算缸盖的高周疲劳安全因数和低周循环次数,结果表明缸盖疲劳安全因数满足使用要求.     

Generalized Thermodynamics of Maximum Work in Finite Time

We consider thermodynamic behaviour of thermal machines founded on kinetic rather than static origins. Their models, which are formulated for finite time transitions, simplify to models of classical thermodynamics in the limiting case of an infinite duration. An extended exergy is derived as a finite-time extension of the classical thermodynamic work delivered from a system of a body and its environment. With this quantity enhanced bounds can be determined for active continuous and cascade processes, in which there is an indirect energy exchange between two sybsystems through the working fluid of an engine, a refrigerator or a heat pump. These bounds refer to systems with finite exchange area or with a finite contact time. An economic framework of this theory is outlined.For both continuous and discrete processes, nonlinear thermodynamic models are derived from a combination of the energy balance and transfer equations. These models serve as constraints in the problem of work optimization. Variational and optimal control approaches are developed which are analogous to those found in analytical mechanics. Variational calculus is used along with some aspect of the canonical transformation theory to maximize work and discuss the role of a finite process intensity and of a finite duration.The optimality of a definite irreversible process for a finite-time transition of a controlled fluid is pointed out as well as a connection between the process duration, optimal dissipation and the optimal process intensity measured in terms of a hamiltonian, a dissipative quantity. It is shown that limits of the classical availability theory should be replaced by stronger limits which are obtained for finite time processes, and which are closer to reality. A hysteretic property of the generalized exergy describes a decrease of the maximum work received from an engine system and an increase of work added to a heat pump system, the features which are particularly important in high-rate regions of thermodynamic processes. For an infinite sequence of infinitesimal thermal machines, an optimal temperature strategy is obtained in the form similar to that known in the theory of simulated annealing.     

Design and fabrication of a cross flow micro heat exchanger

A cross flow micro heat exchanger was designed to maximize heat transfer from a liquid (water-glycol) to a gas (air) for a given frontal area while holding pressure drop across the heat exchanger of each fluid to values characteristic of conventional scale heat exchangers. The predicted performance for these plastic, ceramic, and aluminum micro heat exchangers are compared with each other and to current innovative car radiators. The cross flow micro heat exchanger can transfer more heat/volume or mass than existing heat exchangers within the context of the design constraints specified. This can be important in a wide range of applications (automotive, home heating, and aerospace). The heat exchanger was fabricated by aligning and then bonding together two identical plastic parts that had been molded using the LIGA process. After the heat exchanger was assembled, liquid was pumped through the heat exchanger, and minimal leakage was observed     

Molecular dynamics–continuum hybrid simulation for condensation of gas flow in a microchannel

A molecular dynamics-continuum coupling method combining fluid flow and heat transfer is developed to study the condensation process of gas flow in a microchannel. The computational domain is decomposed into particle (P), continuum (C) and overlap (O) regions with solving approaches of molecular dynamics simulation, finite volume method and the developed coupling method, respectively. Continuities of momentum and energy in O region are ensured by constraint dynamics and the Langevin method. The validity of the developed method is confirmed by a good agreement between hybrid results and analytical solutions from two cases including the unsteady dynamical and thermal problems. For the condensation process of gas flow, the hybrid transient velocity and temperature fields indicate that the process does not progress smoothly but wavily with noticeable fluctuation, leading to oscillation in temperature field and recirculation flow in velocity field. Analysis based on heat and mass transfer is carried out in P region, and the Kapitza resistance and the thermal conductivity in liquid are obtained with the satisfying agreement with experimental data, which shows the availability of the developed model for the investigation on the thermal boundary resistance. The good performance had demonstrated that the developed coupling method and computational model are available to provide a multiscale overview in dynamical and thermal problems including phase-transition from nanoscale to microscale, which will show significantly potential in micro fluidics and thermal engineering.     

A comparison between a linear- and a non-linear three-dimensional thermal analysis of a radial gas turbine wheel

Various rotating components of a gas turbine engine are subjected to high temperatures as well as high centrifugal forces. Abrupt temperature variations often introduce thermal stresses. These must of course be considered when the designer is faced with the problem of predicting the low cycle fatigue life of the relevant component.Particularly at higher temperatures the material behaviour is non-linear and may influence the accuracy of the computed results. Both the thermal conductivity- and the heat capacity properties become more and more temperature-dependent in higher temperature regions. In addition, convective heat transfer at heat exposed surfaces are temperature-dependent.This paper describes a linear- and two non-linear three-dimensional temperature field analyses of a radial gas turbine wheel. In one of the non-linear analyses the material properties were made temperature-dependent, whereas the second non-linear analysis had constant material properties but temperature-dependent heat transfer coefficients.The results obtained show that for the analyzed gas turbine wheel, a linear analysis will produce results which deviate insignificantly from the results obtained in a much costlier non-linear analysis. This is true for material properties and convective heat transfer coefficients that are non-linear only to a moderate extent. It is also fair to assume that this may be concluded for heat conduction problems of the same category, e.g. analysis of other machinery components, nuclear reactor design problems, etc.     

MEMS壁剪应力传感器热效应的数值模拟

借助计算流体动力学(CFD)商业软件FLUENT,采用数值模拟的方法,对基于MEMS的壁剪应力传感器热交换效应进行了分析。计算结果表明:在壁剪应力传感器的热膜下方加入真空腔或者空气腔是十分必要的。针对水流中测量的计算结果显示,真空腔和空气腔在整个计算区域的温度场分布以及对流体的传热效率的差别不大,而空腔可以明显地减小底层的热损失,这对提高剪应力传感器的灵敏度是十分有利的。此外,MEMS壁剪应力传感器的尺寸效应对传热效率也存在影响。     

微/纳米流体控制用冰阀器件执行过程的数值模拟

冰阀是一种利用微/纳米流道内流体自身的冻结/融化相变过程来实现关断或开启作用的新型流体元件.由于无运动部件、无泄漏、不会引起流体污染且易于实现电控等优点,该器件在微/纳米流体系统的控制中具有重要的应用前景.针对冰阀的工作原理,基于相应的流体力学及传热学数学模型,从数值计算角度对微流道冰阀器件的执行过程进行了模拟,并开展了相应的参数化研究,在此基础上可对冰阀的工作状态及控制过程更好地加以理解和剖析.     

一种新型MEMS器件中的近场辐射传热现象研究

以近场辐射传热方式利用光电器件中的热能制作热光电器件从而提高整个器件转换效率的思想已经被提出。本文基于该思想设计一种新型的具有双悬空薄膜的器件,两个薄膜面对面相互平行,间距为1 ?m。每个悬空薄膜中制作白金薄膜电阻。这个器件利用MEMS工艺中的牺牲层技术制作。在存在近场辐射传热和不存在近场辐射传热两种情况下,通过测量将下方结构加热到相同温度的输入功率差,测量出两个薄膜间的辐射热功率。实验数据表明该器件中薄膜间的传热已经大于黑体辐射传热;并且,当上方薄膜温度为317.2 K时,通过近场辐射传热可以使下方薄膜的温度从293 K升高294.2 K,该温度变化为热电转换提供了条件。     

Thermal interactions in nanoscale fluid flow: molecular dynamics simulations with solid–liquid interfaces

Molecular dynamics (MD) simulations of nano-scale flows typically utilize fixed lattice crystal interactions between the fluid and stationary wall molecules. This approach cannot properly model interactions and thermal exchange at the wall–fluid interface. We present a new interactive thermal wall model that can properly simulate the flow and heat transfer in nano-scale channels. The new model utilizes fluid molecules freely interacting with the thermally oscillating wall molecules, which are connected to the lattice positions with “bonds”. Thermostats are applied separately to each layer of the walls to keep the wall temperature constant, while temperature of the fluid is sustained without the application of a thermostat. Two-dimensional MD simulation results for shear driven nano-channel flow shows parabolic temperature distribution within the domain, induced by viscous heating due to a constant shear rate. As a result of the Kapitza resistance, temperature profiles exhibit jumps at the fluid–wall interface. Time dependent simulation results for freezing of liquid argon in a nano-channel are also presented.     

Equations of properties as a function of temperature for seven fluids

Equations are developed for property data of seven common fluids. These equations are useful in computer applications where it is desirable to determine properties without using tables or requiring the user to input data during the execution of programs.An equation is found that accurately approximates the variation of the fluid property data with temperature and is in a simple and convenient form.Equations are presented for the properties of the following fluids: air, liquid water, water vapor, carbon dioxide, Freon-12, engine oil, and mercury. The properties presented in equation form are: density, dynamic viscosity, constant pressure specific heat, thermal conductivity, and Prandtl number. Properties are presented for wide ranges of temperature and at atmospheric pressure.     

High viscous liquids as a source in micro-screw heat exchanger: fabrication, simulation and experiments

Viscous liquids can be encountered in many applications of micro devices. In this paper, an experimental and numerical simulation of a micro screw concentric tube heat exchanger is presented to determine the overall heat transfer coefficient and the amount of heat created by friction. The screw surface area and the increase in the viscosity of the liquid can raise the amount of heat created by friction and subsequently increase the amount of heat transferred in the micro concentric tube heat exchanger. Increasing the viscosity of the source can increase the overall heat transfer coefficient. The micro heat exchanger consists of a tube inside the screw placed inside a micro-channel. When the screw rotates, a net force is transferred to the fluid due to differential pressure on the depth of the thread and pressure gradient along the screw axis, thus causing the fluid to displace outside and inside the screw. A micro-screw heat exchanger was designed to maximize heat transfer from a liquid (viscous) in the cylinder to water in a tube inside a screw. Three-dimensional simulation of micro-screw heat exchanger was performed. The results obtained from CFD simulation have been compared with the experiments.     

Effect of Heat Transfer Law on the Ecological Optimization of a Generalized Irreversible Carnot Engine

The optimal ecological performance of a irreversible Carnot engine with the losses of heat-resistance, heat leak and internal irreversibility, in which the transfer between the working fluid and the heat reservoirs obeys a generalized heat transfer law Q ∝ δ(Tⁿ), is derived by taking an ecological optimization criterion as the objective, which consists of maximizing a function representing the best compromise between the power and entropy production rate of the heat engine. Some special examples are discusses. A numerical example is given to show the effects of heat transfer law, heat leakage and internal irreversibility on the optimal performance of the generalized irreversible heat engine. The results can provide some theoretical guidance for the designs of practical engine.     

Design and simulation of a thermo transfer type MEMS based micro flow sensor for arterial blood flow measurement

Thermo transfer type MEMS (Micro Electro Mechanical System) based micro flow sensing device have promising potential to solve the limitation of implantable arterial blood flow rate monitoring. The present paper emphasizes on modeling and simulation of MEMS based micro flow sensing device, which will be capable of implantable arterial blood flow rate measurement. It describes the basic design and model architecture of thermal type micro flow sensor. A pair of thin film micro heaters is designed through MEMS micro machining process and simulated using CoventorWare; a finite element based numerical code. A rectangular cross section micro channel has been modeled where in micro heater and thermal sensors are embedded using the same CoventorWare tools. Some promising and interesting results of thermal dissipation depending upon very small amount of flow rate through the micro channel are investigated. It is observed that measuring the variation of temperature difference between downstream and upstream, the variation of fluid flow rate in the micro channel can be measured. The numerical simulation results also shows that the temperature distribution profile of the heated surface depends upon microfluidic flow rate i.e. convective heat transfer is directly proportional to the microfluidic flow rate on the surface of the insulating membrane. The simplified analytical model of the thermo transfer type flow sensor is presented and verified by simulation results, which are very promising for application in arterial blood flow rate measuring in implantable micro devices for continuous monitoring of cardiac output.     

微/纳米冰镊操作器执行过程的数值模拟

冰镊是一种借助于针尖与作用对象之间形成的极微小冰晶来实现对物体灵巧操纵的微/纳米操作技术,应用该器件不仅可以实现如拾取、摆放等简单动作,更可以方便地实施如拉伸、旋转等复杂操作,且不受对象的形状、带电与否、重量、材料以及质地等限制,并可与其它机构结合,组成微观意义上的自动化设备.针对冰镊的工作原理,基于相应的流体力学及传热学数学模型,从数值计算角度对微纳米尺度下冰镊的执行过程进行了模拟,在此基础上可望更好地理解冰镊的工作状态及控制过程,从而有助于设计优化新的微/纳米冰镊器件.     

Thermal convection of viscoelastic fluid with Biot boundary conduction

Two-dimensional unsteady natural convection of a non-linear fluid represented by Criminale–Erickson–Filbey (CEF) fluid model in a square cavity is studied in the fluid for Rayleigh–Benard convection case. The governing vorticity and energy transport equations are solved numerically either simple explicit and ADI methods, respectively. The two-dimensional convective motion is generated by buoyancy forces on the fluid in a square cavity, when the vertical walls are either perfectly insulated or conducted with Biot boundary conduction condition. The contributions of the elastic and shear dependent characteristics of the liquid to the non-Newtonian behaviour are investigated on the temperature distribution and heat transfer. The effect of the Weissenberg (which is a measure of the elasticity of the fluid), Rayleigh and Biot numbers on the temperature and streamline profiles are delineated and this has been documented first time for the viscoelastic fluid.