Analytical solution for transient temperature and thermal stresses within convective multilayer disks with time-dependent internal heat generation,Part II: Applications

This work investigates the analytical solution for transient temperature and thermal stresses within three circular geometries. First, the transient temperature and thermal stresses within a composite disk are addressed. Then, two examples regarding transient temperature and thermal stresses throughout circular heaters are analyzed. Pulsed and sinusoidal internal heat generations are incorporated into the second and third examples, respectively. For the composite hollow-disk example, merely the separation of variables method (SVM) is used to overcome the energy partial differential equation. For the other two examples, the combination of the SVM and Duhamel's theorem are adopted to solve the partial differential equations. Accordingly, assuming plane stress formulation, the transient thermal stresses within structures are obtained.     

Modified theoretical models of film condensation in horizontal microfin tubes

The previously proposed theoretical models of film condensation in horizontal microfin tubes have been modified to describe the characteristics of condensing two-phase flow more accurately. The stratified flow regime and the annular flow regime were considered. For the stratified flow regime, the previously proposed theoretical model was modified to take account of the curvature of stratified condensate due to the surface tension force. For the annular flow regime, a more accurate expression for the interfacial shear stress was incorporated. Generally, the modified theoretical models predicted a lower circumferential average heat transfer coefficient than the previously proposed ones. Comparison of the theoretical predictions with available experimental data for six tubes and five refrigerants revealed that a good agreement (r.m.s error of less than 21.1%) was obtained for all cases when the higher of the two theoretical predictions were adopted as the calculated value.     

Ambient ammonia in terrestrial ecosystems: a comparative study in the Tennessee Valley, USA

Atmospheric ammonia has been shown to degrade regional air quality and affect environmental health. In-situ measurements of ammonia are needed to determine how ambient concentrations vary in different ecosystems and the extent to which emission sources contribute to those levels. The objective of this study was to measure and compare ammonia concentrations in two Tennessee Valley (USA) ecosystems: a forested rural area and a metropolitan site adjacent to a main transportation route. Integrated samples of atmospheric ammonia were collected with annular denuder systems for ~ 4 weeks during the summer of 2009 in both ecosystems. Ancillary measurements of meteorological variables, such as wind direction and precipitation, were also conducted to determine any relationships with ammonia concentration. Measurements in the two ecosystems revealed ammonia concentrations that were mostly representative of background levels. Arithmetic means were 1.57 ± 0.68 μg m^− 3 at the metropolitan site and 1.60 ± 0.77 μg m^− 3 in the forest. The geometric mean concentrations for both sites were ~ 1.46 μg m^− 3. Wind direction, and to a lesser extent air temperature and precipitation, did influence measured concentrations. At the metropolitan site, ammonia concentrations were slightly higher in winds emanating from the direction of the interstate highway. Meteorological variables, such as wind direction, and physical factors, such as topography, can affect measurement of ambient ammonia concentrations, especially in ecosystems distant from strong emission sources. The 12-h integrated sampling method used in this study was unable to measure frequent changes in ambient ammonia concentrations and illustrates the need for measurements with higher temporal resolution, at least ~ 1-2 h, in a variety of diverse ecosystems to determine the behavior of atmospheric ammonia and its environmental effects.     

旋转气体介质对环膜液体射流破碎不稳定性影响的研究

利用线性不稳定性理论研究了旋转气体介质对黏性环膜液体射流破碎的影响。研究结果表明，无论是轴对称模式还是非轴对称模式，由液体环膜内部气体介质旋转所产生的离心力是液体射流的失稳因素，有助于液体射流的破碎。另外，由液体环膜外部气体介质旋转所产生的离心力是液体射流的促稳因素，不利于液体射流的破碎。当相同强度的旋转同时存在于内部和外部气体介质中时，对于轴对称模式，内部气体介质的影响显著，而对于非轴对称模式，则外部气体介质的影响更为明显。通常情况下，非轴对称模式的扰动增长率强于轴对称模式的扰动增长率，因此会在环膜液体射流的破碎中占据主导地位。     

Characterization of two-phase flow in a single rock joint

An experimental study using a triaxial apparatus was used to analyze the two-phase flow patterns in jointed rock specimens. Rock specimens having a single natural fracture were tested for two-phase flow of water and air. Triaxial tests were conducted to characterize the two-phase flow through fractured granite specimens at low confining pressures. It was found that for a relatively smooth joint (JRC<6), bubble flow pattern occurred within the rock joint when the gas velocity is below 15 m/s. The average velocity of water usually varied between 0.1 and 0.5 m/s for bubble flow patterns. In this velocity range, air bubbles were able to form along the joint walls or to be randomly displaced within the water phase. When the gas velocity inside the rock joint exceeded 22 m/s, the flow patterns took annular form for non-zero capillary pressures (i.e., injected gas pressure is not equal to injected water pressure). At elevated (>0.25 MPa) gas injection pressures, the gas occupied the main part of the fracture and the liquid was able to flow as an unstable film forming an annular flow along the joint. When the annular flow developed, the mixture flow pattern was independent of the air flow velocity. This was due to the fact that once the injected air velocity reached a critical value (i.e., 20 m/s), water velocity inside the joint was negligible for a given confining pressure and injected water pressure. Further increase in inlet air pressures developed a single-phase air flow with no water flow.     

Large eddy simulation of heated vertical annular pipe flow in fully developed turbulent mixed convection

The goal of the present study was to perform a large eddy simulation of vertical turbulent annular pipe flow under conditions in which fluid properties vary significantly, and to investigate the effects of buoyancy on the turbulent structures and transport. Isoflux wall boundary conditions with low and high heating are imposed. The compressible filtered Navier-Stokes equations are solved using a second order accurate finite volume method. Low Mach number preconditioning is used to enable the compressible code to work efficiently at low Mach numbers. A dynamic subgrid-scale stress model accounts for the subgrid-scale turbulence. Comparisons were made with available experimental data. The results showed that the strong heating and buoyant force caused distortions of the flow structure resulting in reduction of turbulent intensities, shear stress, and turbulent heat flux, particularly near the wall.     

USE OF THE INTERCHANGE MODEL TO PREDICT ENTRAINMENT IN VERTICAL ANNULAR FLOW

Entrapment, E, in vertical gas-liquid annular flows may be pictured as a balance between the rate of atomization, RA, of the liquid layer and the rate of deposition of drops, R D. Laboratory measurements of RA, RD and E are reviewed. Theoretical analyses are discussed which picture RA as related to the growth of wavelets through a Kelvin-Helmholtz instability and RB as being directly proportional to the root-mean-square of the turbulent velocity fluctuations of the drops. An equation for E can be developed, which assumes that the deposition constant is independent of drop concentration and that the rate of atomization varies linearly with the flow rale of the liquid in the film. Limitations of this approach, suggested by measurements of RA and RD at large liquid flows, are discussed.     

LDV measurements of particle velocity distribution in an annular stripper

Particle descent velocities in an annular stripper were measured by a laser Doppler velocimetry (LDV) system. In the radial direction, particle descent velocity was relatively constant in the mid-region of the stripper and increased towards the walls on both sides, exhibiting an anti-U-shaped distribution. Particle descent velocity in the radial mid-region increased with the increase of superficial gas velocity, and the maximum in the outer wall region increased significantly with the increase of solid mass flux. Superficial stripping gas velocity had stronger effect on particle velocity distributions near the stripper gas distributor, and such effect weakened with the increase of the distance from the distributor. Local particle velocity and its radial profiles could be adjusted by changing the superficial stripping gas velocity. Empirical formulas were established to describe the relationships between the local particle velocity and cross-sectional averaged velocity based on the effects of operating conditions and measuring positions. The result showed that the predicted data was in good agreement with the experimental value.     

气体钻井预防钻具失效分析

气体钻井具有自身特点,气体钻井钻具受高速流体冲蚀作用,钻具磨损严重,空气锤反馈的冲击波使钻具接头螺纹容易断裂,气体钻井钻具频繁失效进而影响钻进速度,增加钻井成本。基于岩石力学和气固两项流理论,对气体钻井钻具受流体冲蚀进行分析 ,建立了环空混合物返速模型,实例计算回压对环空返速的影响。为避免钻具受高速流体冲蚀,应使环空混合物返速介于最小携岩速度和临界冲蚀速度之间。基于力学理论,提出增大井口回压的方法,推导出钻具接头螺纹在预紧力作用下的变形协调方程,应用有限元对钻具螺纹进行应力分析,找出钻具螺纹接头各螺纹牙载荷的分布规律,得出螺纹应力集中区,给出预防钻具失效提高螺纹强度的措施。     

Modified three-phase-lag Green–Naghdi models for thermomechanical waves in an axisymmetric annular disk

Abstract

A novel modification for Green and Naghdi thermoelasticity model of type III is proposed in this article. This model is joined to other ones to treat a thermoelastic wave propagation problem in an annular disk. The closed-form solution of thermoelastic analysis of the annular disks has been presented to deduce temperature, radial displacement, dilatation and stresses. Results are illustrated and reported to compare the simple Green–Naghdi II and III and their modified single-, dual- and three-phase-lag models. Concluding remarks are added.