A compact setup to study homogeneous nucleation and condensation

An experiment is presented to study homogeneous nucleation and the subsequent droplet growth at high temperatures and high pressures in a compact setup that does not use moving parts. Nucleation and condensation are induced in an adiabatic, stationary expansion of the vapor and an inert carrier gas through a Laval nozzle. The adiabatic expansion is driven against atmospheric pressure by pressurized inert gas its mass flow carefully controlled. This allows us to avoid large pumps or vacuum storage tanks. Because we eventually want to study the homogeneous nucleation and condensation of zinc, the use of carefully chosen materials is required that can withstand pressures of up to 10(6) Pa resulting from mass flow rates of up to 600 l(N) min(-1) and temperatures up to 1200 K in the presence of highly corrosive zinc vapor. To observe the formation of droplets a laser beam propagates along the axis of the nozzle and the light scattered by the droplets is detected perpendicularly to the nozzle axis. An ICCD camera allows to record the scattered light through fused silica windows in the diverging part of the nozzle spatially resolved and to detect nucleation and condensation coherently in a single exposure. For the data analysis, a model is needed to describe the isentropic core part of the flow along the nozzle axis. The model must incorporate the laws of fluid dynamics, the nucleation and condensation process, and has to predict the size distribution of the particles created (PSD) at every position along the nozzle axis. Assuming Rayleigh scattering, the intensity of the scattered light can then be calculated from the second moment of the PSD.     

Study of divergence angle influence for sonic nozzle in non-equilibrium condensation

The condensation happens generally in a nozzle during expansion of compressed steam from convergent to the divergent part of the nozzle. The divergence angle is the angle measured from the throat of the nozzle to the outlet. In this paper, the outlet is kept constant and the throat diameter is varied. In turn, the divergence angle of the sonic nozzle is altered. The effect of divergence angle on condensation phenomena is investigated with wet steam in a sonic nozzle. For analyzing the wet steam properties, the non-equilibrium condensation model is used. This model is the classical nucleation theory coupled with the droplet growth rate equation. The base nozzle is designed with the throat diameter of 4.5 mm and other dimensions are calculated according to ASME nozzle formulas. Furthermore, the chosen divergence angles are 3°, 4.2°, and 6° for which the throat diameters are 4.5 mm, 3 mm, and 1.5 mm, respectively. As the divergence angle is gradually increased, the position of maximum Mach number of the flow moves upstream, the static temperature of the flow near the throat reaches the lower value, and the droplet nucleation rate is increased. The condensation shock gets gradually stronger with decreasing the divergence angle.

     

Passive prandtl-meyer expansion flow with homogeneous condensation

Prandtl-Meyer expansion flow with homogeneous condensation is investigated experimentally and by numerical computations. The steady and unsteady periodic behaviors of the diabatic shock wave due to the latent heat released by condensation are considered with a view of technical application to the condensing flow through steam turbine blade passages. A passive control method using a porous wall and cavity underneath is applied to control the diabatic shock wave. Two-dimensional, compressible Navier-Stokes with the nucleation rate equation are numerically solved using a third-order TVD (Total Variation Diminishing) finite difference scheme. The computational results reproduce the measured static pressure distributions in passive and no passive Prandtl-Meyer expansion flows with condensation. From both the experimental and computational results, it is found that the magnitude of steady diabatic shock wave can be considerably reduced by the present passive control method. For no passive control, it is found that the diabatic shock wave due to the heat released by condensation oscillates periodically with a frequency of 2.40 kHz. This unsteady periodic motion of the diabatic shock wave can be completely suppressed using the present passive control method.     

Non-equilibrium condensation process of water vapor in moist air expansion through a sonic nozzle

Non-equilibrium vapor condensation phenomenon through a sonic nozzle is very complicated and closely related to the flow measurement of the sonic nozzle. The gas–liquid two-phase flow Eulerian models for homogeneous and heterogeneous nucleation of moist gas in transonic nozzle flow were built to investigate the effect of vapor condensation on the mass flow rate of the sonic nozzle. Grid independence was achieved by using a solution-adaptive refinement. The CFD models were carefully validated by published experimental data and analytical results. It was shown that the flow rate of the sonic nozzle is affected by both homogeneous and heterogeneous nucleation. In comparison with the experimental data, the effects of vapor condensation on the mass flow rate of the sonic nozzle were obtained. Besides, experiments on periodic, unsteady condensation flow in the sonic nozzle were also reported. This unsteady flow will also affect the flow rate of the sonic nozzle. The results of experiments accorded well with simulations and semi-empirical formula. All the results can be used to further analyze the effect of unsteady flow induced by vapor condensation on the flow rate of the sonic nozzle.     

300MW汽轮机组启停胀差变化和控制

汽轮机在启停过程中,由于蒸汽温度、压力的变化、转子与汽缸的热交换条件不同,造成它们在轴向的膨胀也不一致,即出现相对膨胀。相对膨胀通常也称为胀差。胀差的大小表明了汽轮机轴向动静间隙的变化情况。监视胀差是机组启停过程中的一项重要任务。为避免轴向间隙变化影响机组的安全运行,不仅应对胀差进行严格的监视,而且应充分认识机组在启、停过程中的胀差变化规律。本文将简要介绍机组启、停时胀差的变化和控制。     

Time-dependent characteristics of the nonequilibrium condensation in subsonic flows

High-speed moist air or steam flow has long been of important subject in engineering and industrial applications. Of many complicated gas dynamics problems involved in moist air flows, the most challenging task is to understand the nonequilibrium condensation phenomenon when the moist air rapidly expands through a flow device. Many theoretical and experimental studies using supersonic wind tunnels have devoted to the understanding of the nonequilibrium condensation flow physics so far. However, the nonequilibrium condensation can be also generated in the subsonic flows induced by the unsteady expansion waves in shock tube. The major flow physics of the nonequilibrium condensation in this application may be different from those obtained in the supersonic wind tunnels. In the current study, the nonequilibrium condensation phenomenon caused by the unsteady expansion waves in a shock tube is analyzed by using the two-dimensional, unsteady, Navier-Stokes equations, which are fully coupled with a droplet growth equation. The third-order TVD MUSCL scheme is applied to solve the governing equation systems. The computational results are compared with the previous experimental data. The time-dependent behavior of nonequilibrium condensation of moist air in shock tube is investigated in details. The results show that the major characteristics of the nonequilibrium condensation phenomenon in shock tube are very different from those in the supersonic wind tunnels.     

节流阀内天然气组分凝结规律研究

节流制冷低温分离工艺是天然气集输过程中常见的脱水脱烃技术,节流凝结过程是天然气露点温度能否达标、工艺系统能否优化运行的关键。基于天然气在节流阀内的凝结过程,建立均质凝结和异质凝结统一的数值模型,研究天然气中易发生凝结的四种烷烃(正戊烷、正己烷、正庚烷、正辛烷)组分的凝结规律,分析背压比和总温等因素对凝结性质的影响。结果表明:节流阀内部流场非常复杂,呈现强湍流特性,节流孔内达到极低的温度使烃类组分的凝结在节流孔出口前基本完成。背压比和总温增加会减弱节流温降,使各组分的凝结量和液滴尺寸同时减小。节流过程中各组分的凝结性质存在较大差异。分子量较小的轻烃类组分,节流孔出口凝结量大,且凝结量和液滴尺寸受背压比和总温的影响大。     

温压补偿在MED装置蒸汽流量测量中的应用

为解决低温多效蒸馏（MED）淡化过程中蒸汽温度、压力波动引起的蒸汽流量测量不准确的问题,依托30t/d MED淡化中试装置,本文分析了进料蒸汽的温度、压力变化特点,研究选择适用于MED工艺的补偿公式和数学模型,并通过PLC控制实现补偿。结果表明：通过温度、压力补偿提高了MED蒸汽流量测量的准确度,增强了MED淡化系统的运行稳定性和经济性。     

水蒸气喷射泵设计中有关引射系数计算的若干问题探讨

用工作蒸气膨胀与混合气体压缩的热力平衡关系计算各级喷射器的引射系数,计算时应依据喷射器出口处水蒸气的实际过热状态,并以其内发生的气体流动过程处于绝热工况。由此设计的水蒸气喷射泵其实际抽气特性与设计相符程度高,在相同工作真空度下运行时,单位质量的工作蒸气所抽吸的被抽气体质量流量增加,能源消耗降低。     

Effects of heat flux on dropwise condensation on a superhydrophobic surface

The condensation heat transfer efficiencies of superhydrophobic surfaces that have ∼160° contact angle under atmospheric conditions were investigated experimentally. The departing diameter and the contact angle hysteresis of droplets were measured by capturing front and tilted side views of condensation phenomena with a high speed camera and an endoscope, respectively. Condensation behaviors on the surface were observed at the micro-scale using an Environmental scanning electron microscope (ESEM). Apparently-spherical droplets formed at very low heat flux q″ ∼20 kW/m² but hemispherical droplets formed at high q″ ∼440 kW/m². At high q″, heat transfer coefficients were lower on the superhydrophobic surface than on a hydrophobic surface although the superhydrophobic surface is water repellent so droplets roll off. The results of contact angle hysteresis and ESEM image revealed that the reduced heat transfer of the surface can be attributed to the large size of departing droplets caused by adhesive condensed droplets at nucleation sites. The results suggest that the effect of q″ or degree of sub-cooling of a condensation wall determine the droplet shape, which is closely related to removal rates of condensates and finally to the heat transfer coefficient.

     

Calibration of flowmeters in superheated and wet steam

The vapour mass fraction (dryness fraction) of wet-steam flows such as those encountered in the steam main of a power or process plant will rarely be less than 90%. In flowmetering of high-quality saturated steam, the moisture content in the steam is allowed for by correcting the mass flow indicated by the steam flowmeter. Although there are a number of different ways of correcting the mass flow for water content, there is still doubt as to the methods used. In the work reported in this paper, three types of flowmeter were calibrated: first in superheated steam and then in an equilibrium wet-steam mixture to determine their wet-steam correction factors. The correction factors were found to be dependent on the types of flowmeter. For consistency with recommended practice, the method recommended by the ‘Shell Flowmetering Engineering Handbook’ (Ed. G. W. A. Danen, McGraw-Hill, 1985) can be used universally with reasonable accuracy provided a high-efficiency separator is placed upstream of the steam meter to increase the dryness fraction to above 95%.     

CO2跨临界循环中膨胀过程的亚稳态特性研究

介绍了采用膨胀机的CO2跨临界循环的系统流程及典型工况，详细分析了膨胀中经历的亚稳态过程的成因和机理，指出了气化滞后对系统的危害，并对相变膨胀过程中的成核现象进行了热力学分析，在此基础上，结合CO2超临界膨胀过程的特点，阐明了气液相变介质中声速的确定方法，认为CO2超临界膨胀过程可近似按照经典热力学的准静态理论进行分析，并指出结论的理论依据。     

使用照相法测量汽轮机湿蒸汽中水滴尺寸的研究

为了蒸汽轮机的运行安全，准确可靠地测量蒸汽轮机中湿蒸汽水滴的粒径分布，利用图像测量的方法对湿蒸汽特征进行分析，采用Canny边缘检测算子准确检测水滴的边缘，然后根据水滴图片的放大倍数计算出水滴的实际尺寸，最后根据水滴的尺寸分布估计湿蒸汽的湿度及湿蒸汽的危害程度并发出报警信息。     

A new method of two-phase flow measurement by orifice plate differential pressure noise

The mechanism of differential pressure noise of orifices in two-phase flow has been investigated and a theoretical model has been developed for measurement of the double parameters, i.e. mass flow rate as well as phase fraction (steam quality). The model has been proved in a set of orifice experiments in a two-phase flow system at a pressure range of 5.8–12.1 MPa and steam quality of 0.05–0.95, and a practical model has been fitted. The r.m.s. errors of mass flow rate and steam quality estimated by the model are 9.0 and 6.5%, respectively. The results of the studies create a method to measure double parameters of two-phase flow at once using only a single orifice.     

Measurement of high-water-content oil-water two-phase flow by electromagnetic flowmeter and differential pressure based on phase-isolation

When the oil field has been exploited by long-term water-flooding, it will be in high water-content stage of production. However, it is a great challenge for high-water-content measurement due to oil droplets extremely dispersed in the water. In this paper, we developed a phase-isolation based method for high-water-content oil-water two-phase flow measurement. Phase-isolation was realized by axial-flow swirler to concentrate scattered and random oil droplets into the pipe center and change the inlet flow pattern into a particular annular flow before measuring. Owing to the axisymmetric velocity and phase distribution, the electromagnetic flow meter avoided the effect of random distribution of insulating phase, and then had a good measurement performance for total volume flow rate. Furthermore, we respectively studied using axial pressure drop, radial pressure drop and the ratio of the two pressure drops to measure water content. The results showed that the ratio of the two pressure drops not only improves the resolution of oil and water, but also effectively reduces the impact of error transfer. In the dual-parameter measurement experiment, the relative errors of total volume flow rate and water content were almost within ±5%.     

Measurement of steam flow rates using a clamp-on ultrasonic flowmeter with various wetness fractions

Most of the heat in industrial plants is supplied by steam. To minimize energy waste, measuring the steam flow rates in existing pipes is important. Clamp-on ultrasonic flowmeters are used for this purpose, for which the sensors are attached to the pipe wall. However, flow conditions that can be used are limited because the signal-to-noise ratio of the ultrasonic signal in a steam flow is low. Furthermore, the steam wetness increases with heat losses, which may affect measurement results. Therefore, flow rate measurements in wet steam flows using clamp-on ultrasonic flowmeters have not been fully established. In this study, steam flow rates with various wetness fractions and system pressures were measured using a laboratory-made clamp-on ultrasonic flowmeter. The results show that flow rates in wet steam could be determined within a 10% error under general conditions in a steam piping system, although the conversion factor from line-average to area-average velocities was calibrated in superheated conditions, and the speed of sound in saturated conditions at each pressure was used. However, the error of the flow rates tended to increase with the wetness fraction and was biased toward positive values. The speed of sound and liquid volume fraction were evaluated at different wetness fractions. The flow rate error due to the change in sound speed was less than 1%, and 1.2% of the flow rates were overestimated owing to the liquid volume fraction. The velocity distribution in wet steam was considered different from that in the superheated steam owing to the existence of the liquid phase, and the change in velocity profile may lead to an overestimation of the steam flow rates in the wet steam condition.     

换热器中流体及管壁温度变化关系式及其修正

借鉴推导无相变顺、逆流换热器对数平均温差的方法,导出了单流程管式无相变换热器中流体及换热管壁温度变化关系式。这些关系式将换热器设计参数、流体物性参数及过程参数集中在一个表达式中,反应了它们的内在联系。因在推导时作了几点假设,文中对这些关系式进行了分析,探讨了实际应用时的补充和修正方法。     

压水堆核电站湿蒸汽透平的研究与制造

本文就核电站湿蒸汽透平和常规电站透平的特点作了比较。指出湿蒸汽透平的蒸汽膨胀过程对设计及制造带来了诸如叶片受侵蚀、功率损失及叶片庞大等问题。特别在高压缸中,高密度湿蒸汽引起的侵蚀问题的尚待解决。在引进技术和设备的同时,也必须通过自己的科研工作才能掌握技术。为此,应积极开展湿蒸汽与水滴测量技术的基本研究,必须进行湿蒸汽膨胀的模拟试验以及湿蒸汽透平模型试验,以便在此基础上建立核电站湿蒸汽透平的正确设计计算方法。     

滚滑轴承滑块的油-气两相流润滑分析

为改善滚滑轴承的润滑,运用两相流理论对其滑块进行油气润滑设计,建立滑块的油-气两相流CFD模型,分析不同入口角度、进气速度、进油速度和润滑油黏度对流场油相分布的影响。结果表明:油-气混合润滑方式能在内外滚道接触区形成有效的润滑油膜;油气管道夹角影响油滴分布,角度过大时大量油滴会在滑块侧面上附着,角度过小时油滴会在外滚道入口处堆积,造成供油连续性不好,油膜稳定性下降;进气速度过大会降低油滴附着率,无法形成有效油膜,而进油速度过大会造成润滑油累积,出现搅油现象,因此选择合适的进气和进油速度,才能控制油滴的大小和保持润滑过程的连续性;润滑油黏度会影响油滴在滑块上的附着效果,合理地选择润滑油黏度,才能保证流场油相分布均匀。     

Flow behavior control in immersion lithography

Over the past decade, immersion lithography has been the primary technology for exposure process in semiconductor manufacturing. Compared with traditional dry lithography method, this technology improves the exposure resolution greatly by inserting a high index liquid into the gap between lens and wafer surface. Keeping the immersion liquid pure and uniform as well as avoiding residual droplets during high speed scanning motion are two challenges faced by the development of immersion lithography. Contaminations, particles, bubbles, heating and stress in the liquid will destroy the continuity of the refractive index. High speed motion of the wafer during scanning may break the meniscus stability on the interface between the liquid and the gas surrounding it, and then generate residual droplets on the wafer. All above phenomena will affect the exposure performance of immersion lithography and corresponding flow behavior control methods are required to solve the problems. This paper gives a review of the studies on flow behavior control in immersion lithography, which contains two parts: studies for liquid purity and uniformity and studies for meniscus stability. In each part, the mechanism and character of the flow behaviors as well as their effects to exposure performance are presented firstly. Then, control approaches adopted by now, including material and surface modifications as well as immersion head utilization, are introduced. In addition, the challenges faced in future studies are also pointed out. The purpose of this review paper is to help the researchers to understand the flow control problems in immersion lithography and to provide better control approaches for exposure performance improvement.