基于热力转换机理仿真圆柱体零件形体边界约束的热变形

提出了热变形精确分析理论与一种仿真方法以保证热误差影响下机械零件的最佳配合。从微观分子力学理论出发,在充分考虑材料属性、温度以及形体边界约束对热变形影响的基础上,提出了热力转换热变形分析理论,将零件热变形问题等效为力变形问题。针对一组铝制空心圆柱体零件进行了热变形测量实验,给出了基于ANSYS Workbench仿真环境的热力转换热变形仿真方法,并对比分析了实验结果与仿真结果。结果显示:铝制空心圆柱体零件受高温变形时,不同内外径尺寸的空心圆柱体其热膨胀系数是不相同的;外圆径向热膨胀系数大约是内圆径向热膨胀系数的2倍;基于热力转换理论的仿真结果与实验结果吻合较好,吻合度达到98%以上,验证了热力转换热变形分析理论的合理性和有效性。提出的方法弥补了ANSYS Workbench传统热分析完全忽略形体边界约束影响零件热变形的缺陷,为后期热鲁棒性结构设计提供了重要参考。

线接触弹流润滑下固体表面热应力的计算

线接触热弹流润滑下,与油膜接触的两固体表面会产生温升变化,固体表面必定会产生热应力,由于固体表面受到了非均匀油膜压力的约束,数值求解固体表面热应力的难度较大。将热力转换原理和热弹性力学理论引入弹流润滑计算中,给出了数值计算方法,实现了固体表面线胀系数的数值求解,解决了热应力和热弹性变形的数值计算,通过算例得到了满足收敛精度的润滑特性和热应力的数值解,并讨论分析了线载荷变化对固体表面热应力的影响。研究内容可为热弹流润滑分析和约束条件下固体热应力分析提供理论基础和数值方法。

汽车变速箱形体热变形研究

针对传统热变形公式存在一定近似性、无法准确反映物体实际热变形的问题,根据机械形体热变形非相似性理论,引入形体边界约束参数对传统热变形公式进行修正,提出并论证了非相似性热变形公式。同时为便于应用有限元仿真软件进行快速仿真,提出了热变形热力转换分析方法。对某型号变速箱关键部位的热变形进行有限元仿真和实验,结果表明:相较于传统热变形仿真,非相似性热变形仿真结果和实验结果吻合程度大幅提升,验证了热力转换分析方法和非相似性热变形公式的合理性和有效性,为变速箱后期热鲁棒性结构设计提供了理论依据和参考。

线接触热弹流润滑中材料热膨胀系数的计算

运用线接触热弹性流体动压润滑理论,结合热弹性力学理论和热力转换原理,提出了一种求解线接触热弹流润滑下接触固体表面材料热膨胀系数的方法,能解决油膜压力约束下的非均匀温度场引起的固体表面热弹性变形的求解问题。通过算例求解得到了满足收敛条件的数值解,进一步分析了载荷、卷吸速度和固体材料变化对材料热膨胀系数的影响。研究结果表明:同种金属材料时,半径大的固体表面在计算域内各点的材料热膨胀系数相对较大;载荷增大时,固体表面各点材料热膨胀系数变大;速度增大时,两固体表面材料热膨胀系数会减小;固体密度增大时,固体表面的材料热膨胀系数减小。

Thermodynamic Modeling of Wax Precipitation in Crude Oils

Most of the crude oils contain waxes which precipitate when temperature drops, resulting in deposition in pipelines and production equipment. It is necessary to set up a model which can predict the wax appearance tem-perature and the amount of solid precipitated in the different conditions. A modified thermodynamic solid-liquid equilibrium model to calculate wax precipitation in crude oil systems has been developed recently. The assumption that precipitated waxes consist of several solid phases is adopted in this research, and the solid-solid transition is also considered in the modified model. The properties of the pseudo-components are determined by using empirical correlations. New correlations for properties of solid-solid and solid-liquid transitions are also established in this work on the basis of the data from the literature. The results predicted by the proposed model for three crude oil systems are compared with the experimental data and the calculated results from the literature, and good agreement is observed.

Flow resistance characteristics of water in narrow annulus during heat exchange

Considering the special resistance characteristics of fluids flowing through ducts with small gaps, experiments are performed to investigate the resistance characteristics of single-phase water, which is forced to flow through ver tical annuli. The gap sizes are 0.9, 1.4 and 2.4mm, respectively. The experiments are conducted under condition of 1atm. The water in the annuli is heated by high temperature water reversely flowing through the inner tube and the outer annulus. The results show that the flow pattern begin to change from laminar to turbulent before Reynolds number approaches 2000, the flow resistance in annulus has little relations with the temperature difference and ways of being heated, but mainly depends on the ratio of mass flux to the width of annulus.

Novel Optimization Approach to Mixing Process Intensification

An approach was presented to intensify the mixing process. Firstly, a novel concept, the dissipation of mass transfer ability(DMA) associated with convective mass transfer, was defined via an analogy to the heat-work conversion. Accordingly, the focus on mass transfer enhancement can be shifted to seek the extremum of the DMA of the system. To this end, an optimization principle was proposed. A mathematical model was then developed to formulate the optimization into a variational problem. Subsequently, the intensification of the mixing process for a gas mixture in a micro-tube was provided to demonstrate the proposed principle. In the demonstration example, an optimized velocity field was obtained in which the mixing ability was improved, i.e., the mixing process should be intensified by adjusting the velocity field in related equipment. Therefore, a specific procedure was provided to produce a mixer with geometric irregularities associated with an ideal velocity.

Reaction pathway and kinetics of CdO nanoparticles prepared from CdCO_3 precursor using thermal decomposition method

The non-isothermal kinetics of CdO nanoparticles prepared from CdCO3 precursor using thermal decomposition method was investigated. A model-fitting Malek approach and a model-free advanced isoconversional method of Vyazovkin were applied to the analysis of the DSC and TGA data. The results showed that CdO nanoparticles prepared from CdCO3 followed an autocatalytic reaction. Sestak–Berggren model could favorably describe the studied reaction process. Moreover, the apparent activation energy of CdCO3 decomposition was calculated to be (119.19±9.97) kJ/mol and the explicit rate equation form of CdCO3 decomposition was established.

Testing Thermal Properties of Cooling Device with Heat Pipes to Improve His Heat Transfer Ability

This paper deals about testing thermal properties of the cooling device with heat pipes at inclination position, in consequence of using the natural convection to improve heat transfer properties. Head point testing of cooling device is monitoring temperature on the aluminium block of energy converter, heat pipes and ribs under temperature condition 30 ℃ in thermostatic chamber. Testing of the device was performed at tilt angles positions 0, 10 and 20° from the vertical level. The heat flux loaded to energy converter was 450 W. The next goal of the paper is to research on influence working position of the wick heat pipe on their thermal performance. In this research heat pipes were made with capillary structure sintered from copper powder granularity 100, 63 and 50 μm filled with water and ethanol. Next heat pipe thermal performance was performed by measuring heat source and working positions. Knowledge of these two research goals can bring potential improvements in purpose of cooling device for effective heat sink from high power electronic components.

Numerical Study of Thermoelectric Generation within a Reciprocal Flow Porous Media Burner

A numerical study based on direct thermal to electric energy conversion was performed in a reciprocal flow porous media burner embedded with two layers of thermoelements. The burner lean combustibility limit was sought in order to maximize global efficiency of thermal to electrical energy conversion by minimizing fuel consumption. Once the pairs of operational variables, composition and filtrational velocity of gas inlet mixture were found, the optimal length and placement of thermoelectric elements within the reactor high thermal gradients were sought to maximize the electric current, thermoelements and system overall efficiency. A two temperature-resistance model for finite time thermodynamics was developed for the thermoelectric elements energy fluxes. Results indicate a distribution of current and efficiencies that presents a maximum at different themoelements length. Maximum values for current and system efficiency obtained were 44.3 m A and 2.5%, respectively.

Study on Combination of Engine/Steam Turbine Cycle for Improving the Energy Conversion

This work discusses the combination of two thermodynamic cycles seeking to improve the overall chemical energy conversion rate into mechanical energy. Here one engine operates according a Rankine cycle in order to use part of the thermal energy released to the boundary, i.e., the neighboring atmosphere. The analysis of this combined cycle shows that it might, under proper condition, represent a gain of 1.2% in the overall delivered engine power.

Research on Marine Boiler's Pressurized Combustion and Heat Transfer

The effect of pressure on combustion and heat transfer is analyzed. The research is based on the basic combustion and heat transfer theorem. A correction for the heat calculation method for pressurized furnace is made on the basis of the normal pressure case. The correction takes the effect of pressurizing into account. The results show that the correction is reasonable and the method is applicable to combustion and heat transfer of the marine supercharged boiler.

Analyses of thermodynamic performance for the endoreversible Otto cycle with the concepts of entropy generation and entransy

In this paper, the endoreversible Otto cycle is analyzed with the entropy generation minimization and the entransy theory. The output power and the heat-work conversion efficiency are taken as the optimization objectives, and the relationships of the output power, the heat-work conversion efficiency, the entropy generation rate, the entropy generation numbers, the entransy loss rate, the entransy loss coefficient, the entransy dissipation rate and the entransy variation rate associated with work are discussed. The applicability of the entropy generation minimization and the entransy theory to the analyses is also analyzed. It is found that smaller entropy generation rate does not always lead to larger output power, while smaller entropy generation numbers do not always lead to larger heat-work conversion efficiency, either. In our calculations, both larger entransy loss rate and larger entransy variation rate associated with work correspond to larger output power, while larger entransy loss coefficient results in larger heat-work conversion efficiency. It is also found that the concept of entransy dissipation is not always suitable for the analyses because it was developed for heat transfer.

A MULTISTEP FINITE VOLUME METHOD WITH PENALTY FOR THERMAL CONVECTION PROBLEMS IN TWO OR THREE DIMENSIONS

The author considers a thermal convection problem with infinite Prandtl number in two or three dimensions. The mathematical model of such problem is described as an initial boundary value problem made up of three partial differential equations. One equation of the convection-dominated diffusion type for the temperature, and another two of the Stokes type for the normalized velocity and pressure. The approximate solution is obtained by a penalty finite volume method for the Stokes equation and a multistep upwind finite volume method for the convection-diffusion equation. Under suitable smoothness of the exact solution, error estimates in some discrete norms are derived.