土壤蓄冷与释冷过程的能量分析

在对土壤蓄冷系统冷量损失过程分析的基础上,提出了地下埋管换热器垫层冷量损失与温差传热冷量损失的概念,并建立了垫层冷量损失和温差传热冷量损失的计算模型;为衡量系统在运行期间地下埋管换热器管束系统的运行效率,建立了埋管换热器管束系统的平均释冷率模型。通过理论研究与模拟计算,分析了土壤初始温度、埋管间距及管束结构对土壤蓄冷系统冷量损失的影响,指出了土壤蓄冷系统冷量损失随土壤初始温度及埋管间距之间的变化规律,为土壤蓄冷系统地下埋管换热器的优化设计提供理论依据。     

DESIGN OF TWO-DIMENSIONAL STEFAN PROCESSES WITH DESIRED FREEZING FRONT MOTIONS

This paper presents a methodology for the design of two-dimensional solidification processes, It is aimed particularly at the calculation of the boundary flux or temperature for a body that solidifies with a desired motion of the freezing front. Such problems are of particular technological significance, considering that the freeing front motion is directly related to the quality of casting. Solution of these problems can be used to achieve a desired east structure, to optimize the time of casting, to prevent choking of the liquid flow, or to control liquid feeding to the contracting front so that cavities and partial solidification in complex castings are avoided. Spatial smoothing and a modification of Beck's future information method are used to solve this ill-posed design problem. A moving and deforming finite-element formulation is employed. The accuracy of the method is demonstrated through several two-dimensional numerical examples.     

EFFECTS OF RAREFACTION AND COMPRESSIBILITY OF GASEOUS FLOW IN MICROCHANNEL USING DSMC

The direct simulation Monte Carlo (DSMC) is performed for two-dimensional gaseous flow through a microchannel in both slip and transition regimes to understand the effects of compressibility and rarefaction. Results are presented in the form of axial pressure distribution, velocity profile, local friction coefficient, and local Mach number (Ma) and are compared with the available analytical and experimental results. The effect of compressibility is examined for the inlet to outlet pressure ratios ranging from 1.38 to 4.5. Low-pressure drop simulations with Knudsen numbers (Kn) ranging from 0.03 to 0.11 are performed to identify the effect of rarefaction. It was found that compressibility makes the axial pressure variation nonlinear and enhances the local friction coefficient. On the other hand, rarefaction does not affect pressure distribution but causes the flow to slip at the wall and reduces the local friction coefficient. In addition, it was found that the locally fully developed (LFD) assumption is valid for low Ma flows by comparison with DSMC results.     

EFFECTS OF LOBE GEOMETRY ON THE MIXING FLOW IN MULTILOBE MIXERS

This study is concerned with a numerical investigation of the mixing flow in multilobe mixers. Predictions are obtained using a finite-volume method which incorporates a second-order difference scheme. The irregular boundaries are fitted by curvilinear nonorthogonal grids and the grid points are arranged in the collocated manner. A method is described to generate suitable three-dimensional grids to cover both the lobe region and the mixing duct. Computations are undertaken to examine the effects of lobe geometry on the mixing performance. Results indicated that the convex lobe contour can lead to stronger vortex flow and, thus, better mixing because of its large slope. As for the geometry of the lobe trailing edge, a sinusoidal shape with wide peak region gives rise to well-organized vortices and the mixing performance is enhanced. Also revealed is the mechanism which, resulting in efficient mixing in the mixer, is clearly identified.     

青藏高原土壤冻融对地表辐射特征的影响分析

利用黄河源区气候与环境综合观测研究站.2006年10月1日～2007年4月30日的辐射和5cm土壤温度和湿度资料研究了土壤冻融对地表辐射特征的影响,结果表明:土壤冻结降低了土壤中的液态水含量,致使冻结阶段地表反照率大于冻结前的值,也大于土壤融化后的值;受总辐射和地表反照率共同作用,整个冻融过程中反射辐射可区分为两个比较明显的阶段;土壤冻结减缓了地表温度降低的速率,相应地减缓了地表长波辐射和大气逆辐射降低的速率;整个冻融过程中净辐射变化与总辐射的变化比较相近,在冻结阶段,地表反照率大于冻结前的值,也大于融化后的值,因此冻结阶段的净辐射值也更小一些。     

埋地换热器在渗流中的三维温度场模拟

为确定地下水渗流对埋地换热器的影响,建立了考虑地下水渗流的三维有限长线热源模型,综合虚拟热源法、移动热源法和格林函数法得出了有渗流时半无限大介质中有限长线热源产生的非稳态温度响应的表达式。通过matlab编程模拟温度场,分析了单个钻孔周围土壤过余温度场。模拟结果发现:地下水渗流引起水平面上温度场的变形,当量渗流速度在10-6量级时,能有效缓解热堆积;沿钻孔不同深度处的土壤过余温度不同,在钻孔中部过余温度达到最大。该文中的三维模型可用来研究不同地区、不同土壤分层构造和分层地下水流速等复杂问题,从而更准确地预测分析实际工程运行后的地下温度场变化,具有重要意义,为进一步研究考虑土壤分层和地下水分层流动下地埋管周围温度场奠定了基础。     

地下水渗流条件下埋地换热器传热性能的实验研究

建立了根据地温分布确定地下水流动特性的简化数学模型。针对一个实际工程钻孔进行了地温分布测量,并测试了局部地下水渗流条件下埋地换热器的传热性能。结果表明,上述理论模型具有较好的预测精度。在该文实验条件下,地下水渗流对埋地换热器的传热性能具有明显的强化作用,埋地换热器的换热量平均提高11.35%。     

NUMERICAL SIMULATION OF THE THERMODYNAMIC,FLUID FLOW,AND HEAT TRANSFER PROCESSES IN A DIESEL ENGINE

A comprehensive model for the complete second-law analysis of a diesel engine has been developed. The model incorporates formulations for zero-dimensional estimation of the convective and radiative heat transfers in the engine cylinder. Models for the valve flow and manifold gas dynamics are also included in order to estimate the second-law losses. The results obtained help identify the engine processes where irreversible losses occur and the magnitudes of these losses relative to the work output. For example, at 2000 rpm and 0.7 overall fuel/air equivalence ratio, the indicated work for the single-cylinder engine simulated in this study is 49% of the fuel exergy. The loss due to irreversibilities is 27.6%, of which 14.8% is due to throttling loss in the exhaust valve, 9.3% is due to combustion irreversibilities, and the remainder is due to throttling toss in the intake valve.     

THERMAL EFFECTS ON FLOW PROPERTIES OF ALUMINUM ALLOYS

The flow properties of aluminum alloys under hot working conditions can be studied by hot torsion testing. However, the properties obtained may be severely affected by the thermal effects generated during testing. In this paper thermal analysis is made by a finite element analysis (FEA) to evaluate the temperature distribution over the test specimen, and the predictions of the flow stresses of an aluminum alloy AA5252 are modified by taking into account the temperature changes. It has been shown that the further flow softening at high strain rates, as a result of heat accumulation during testing can be predicted by the method presented.     

EFFECTS OF THE POLARITY OF WORKING FLUIDS ON VAPOR-LIQUID FLOW AND HEAT TRANSFER CHARACTERISTICS IN A CAPILLARY

For one polar and several apolar working fluids, the vapor-liquid flow and the heat transfer characteristics in a capillary tube are obtained and compared. The polarity of the working fluid has a close relation to the disjoining pressure, which affects the formation of the liquid film profile greatly. For a polar working fluid (water), the length of the evaporating interfacial region is much longer than those for apolar working fluids. And the thickness of its liquid film increases relatively more gently at the end of the evaporating interfacial region. For different apolar working fluids, the calculation results are similar and the above-mentioned lengths are shorter. The difference is due mainly to the fact that the stronger polar working fluid leads to much stronger disjoining pressure, and the evaporation is choked because the liquid molecules of polar working fluids on the vapor-liquid interface are strongly attracted by the solid wall. The effects of other factors on the length of the evaporating interfacial region and the profile of the vapor-liquid interface are more secondary.     

NUMERICAL SIMULATION OF THE EFFECTS OF BUOYANCY FORCES AND FLOW CONTROL DEVICES ON FLUID FLOW AND HEAT TRANSFER PHENOMENA OF LIQUID STEEL IN A TUNDISH

Fluid flow and heat transfer of liquid steel in a tundish of a continuous slab caster was numerically simulated. The importance of natural flow convection was established through a dimensionless number given by the ratio Gr/Re². Buoyancy forces proved to be as important as inertial ones, especially in the extremes of the tundish far from the entering liquid jet. The usual flow control devices like weirs and dams were not as effective as turbulence inhibitors, a kind of impact pad with lips, interior squared angles, and a square cross section. The simulations indicated that this device helps to decrease fluid turbulence in the zone of the entering liquid jet and has damping effects of step inputs of cold or hot steel, allowing for better control of the casting temperature.     

草原土壤微生物能量流动的数学模拟

对草原土壤微生物的能量流动过程进行了分析,建立了草原土壤微生物能量流动的分室模型。运用微分方程理论构建了对草原土壤微生物的能量流动进行动态模拟物数学模型,并根据该模型对草原土壤微生物能量流稳定性进行了分析,得出了许多重要的结论。最后给出了应用实例。     

环境因子对土壤水分蒸散的影响

在有作物生长的条件下，土壤散失到大气中的水分主要通过土壤表面蒸发和作物叶面蒸腾两种方式进行，二者之和称为土壤水分蒸散。由于作物冠层的遮荫作用，作物覆盖下土壤水分的蒸发强度与裸土情况下显然不同。本文建立了土壤水分蒸散率的计算公式，并对几个环境因子的影响进行了讨论。计算结果表明，冠层净辐射强度和空气饱和差对蒸散率影响较大，而气温和风速的影响相对较小。当冠层净辐射强度和空气饱和差增大时，蒸散率和日蒸散总量的增加均较为明显。另外，作物的叶面积指数对蒸散也有较大的影响，同时它对蒸发和蒸腾在蒸散中所占的比重影响很大，叶面积指数增大时，叶面蒸腾所占的份额增大，而蒸发所占的份额相应地减小。     

采集凝固热热泵技术凝固及换热性能的理论分析

介绍了一种新型的采集凝固热热泵技术,建立了凝固热采集装置中层流流动水在常壁温条件下的管内凝固问题的数学模型。采用准稳态近似方法进行了凝固特性的分析,使用当量平均表面换热系数和潜热显热比两个参数讨论了冰层对换热性能的影响。计算结果与所得结论对凝固热采集装置的设计和热泵系统性能的改善有一定指导作用。     

EFFECTS OF ASPECT RATIO ON VORTEX FLOW PATTERNS IN MIXED CONVECTION OF AIR THROUGH A BOTTOM-HEATED HORIZONTAL RECTANGULAR DUCT

Abstract

Buoyancy-induced vortex flow structures and the associated heat transfer were numerically investigated in a mixed convective airflow in a bottom-heated horizontal rectangular duct of different aspect ratios. The unsteady three-dimensional Navier-Stokes and energy equations were directly solved by a higher order upwind finite difference scheme. Results were presented in particular for Reynolds numbers ranging from 5 to 15, Rayleigh numbers up to 9000, and aspect ratios from 4 to 12. The predicted results clearly show significant differences in vortex structures induced in ducts with small and large aspect ratios. For an aspect ratio less than 6 the transverse vortex rolls are periodically generated in the duct entry and gradually transform into longitudinal rolls when moving downstream. The resulting vortex flow eventually evolves to a time periodic state with the upstream and downstream portions of the duct dominated by the transverse rolls and longitudinal rolls, respectively. For a large aspect ratio (A > 9) the transverse rolls prevail in the duct core, with two to three longitudinal rolls existing near each sidewall. The flow oscillation in the region dominated by the transverse rolls is much higher than that dominated by the longitudinal rolls. At high Ra the flow becomes chaotic in time, and the duct is filled with unstable irregular vortex rolls.     

OPTICAL MEASUREMENT OF MICROSCALE TRANSPORT PROCESSES IN DROPWISE CONDENSATION

The optimum use of interfacial free-energy gradients to control fluid flow in small regions naturally leads to simpler passive heat transfer systems. In this context, "passive" refers to the natural pressure field for fluid flow due to changes in the intermolecular force field resulting from an imposed nonisothermal temperature field. Although the particular constrained vapor bubble (CVB) discussed can be viewed as a large version of a wickless heat pipe, it is a much more general heat transfer concept. Herein, it is an ideal system for the optical study of microscale transport processes in droplet condensation due to interfacial phenomena. This article concerns the movement of a single condensed ethanol sessile drop into a concave liquid film. The intermolecular force is found to be much larger than the gravitational force and dominates condensate removal. A dimensionless force balance for viscous shear stress demonstrates the effect of changes in the contact angle and curvature. A dimensionless difference in free energy is identified as the cause of spontaneous condensate removal.     

TEMPORAL BEHAVIOR OF FLOW IN ROTATING CAVITIES

Temporal flow behaviors in a heated rotating cavity with an axial throughflow and sealed annulus are explored. Generally, for both systems, the ratio of the flow angular velocities to the system boundaries are found, even close to boundaries, not equal to unity. Both systems exhibit similar temporal velocity gradient component distributions. Most unsteadiness corresponds to regions of high tangential velocity gradients. As a result of the flow drift, these spatial gradients are converted to temporal gradients. For the present flows, computations suggest judicious coordinate system angular velocity choices may reduce demands on both convective term treatments and computer resources.     

径流描述方法对水库优化调度的影响

本文采用确定性、独立随机性、简单马氏性过程及混合性过程对径流进行了描述,分别建立了数学模型,并对有时段径流预报的随机动态规划法的递推方程求解过程做了改进,使得占用计算机内存减少、计算速度提高。最后结合实例在IBM-PC微机上进行求解,得出优化调度成果,比较并分析了不同径流过程描述方法对水库优化调度成果的影响。     

MODELING OF BUOYANCY-DRIVEN FLOW AND HEAT TRANSFER FOR AIR IN A HORIZONTAL ANNULUS: EFFECTS OF VERTICAL ECCENTRICITY AND TEMPERATURE-DEPENDENT PROPERTIES

In this article fluid dynamic and thermal fields are presented for numerical simulations of laminar, steady, two-dimensional buoyancy-driven flows in an annulus between two vertically eccentric pipes using the penalty finite element method. The simulations were accomplished for Rayleigh numbers between 10 3 and 10 5 and radius ratio of 2.6, with various eccentricities. For most of the work, the usual Boussinesq approximation was made. However, in order to demonstrate the effects of temperature-dependent physical properties for natural convection in a concentric annulus, a modified Boussinesq approximation with temperature-dependent viscosity and thermal conductivity has been used. The formulation was based on primitive variables. Numerical results are presented in terms of isotherms, streamlines, and Nusselt numbers. The results are compared with recent publications and excellent agreement has been found. Stable solutions were obtained when the model was modified to incorporate temperature-dependent viscosity and thermal conductivity and applied to the case of a concentric annulus. The varying viscosity had the most effect on the fluid velocity, while the effects of varying thermal conductivity were most noticeable in the temperature profiles and local Nusselt numbers.     

EFFECTS OF THERMAL BOUNDARY CONDITION ON BUOYANCY DRIVEN TRANSITIONAL AIR FLOW IN A VERTICAL CYLINDER HEATED FROM BELOW

A three-dimensional unsteady numerical computation was carried out here to investigate the effects of the thermal boundary condition on the convection flow in a vertical, bottom heated cylinder containing air. The thermal condition at the sidewall of the cylinder is assumed to be thermally well insulated or perfectly conducting. Results were obtained for air in a cylinder of finite aspect ratio (Gamma = 2) for various thermal Rayleigh numbers. The predicted results indicate that the flow in the sidewall insulated cylinder is highly asymmetric even at steady state and contains multicellular vortices. The flow formation processes leading to the above structures are relatively complicated. In the early transient two axisymmetric circular vortex rolls, one on top of another, appear. Then the rolls merge asymmetrically. In the late stage the flow deflection by the cylinder top and bottom results in a very complex flow. In the cylinder with a perfectly conducting sidewall the transition from a steady to a time dependent flow is subcritical. However, in the cylinder with an insulated sidewall the flow transition is supercritical.