Investigation on hydrodynamics and mass transfer characteristics of a gas-liquid ejector using three-dimensional CFD modeling

An investigation of the gas-liquid ejector has been carried out to study the influence of operating conditions and ejector geometries on the hydrodynamics and mass transfer characteristics of the ejector by using three-dimensional CFD modeling. The CFD results were validated with experimental data. Flow field analysis and prediction of ejector performance were also conducted. Variations of the operating conditions were made by changing the gas-liquid flow rates ratio in the range of 0.2 to 1.2. The length to diameter ratio of mixing tube (L _M/D _M) was also varied from 4 to 10. CFD studies show that at L _M/D _M=5.5, the volumetric mass transfer coefficient increases with respect to gas flow rate. Meanwhile, at L _M/D _M=4, the plot of volumetric mass transfer coefficient to gas-liquid flow rate ratio reaches the maximum at gas-liquid flow rate ratio of 0.6. This study also shows that volumetric mass transfer coefficient decreases with the increase of mixing tube length.     

Steady and non-steady flow through an i.c. engine inlet valve with heat transfer

Two methods are described for predicting flow conditions through an internal combustion engine inlet valve with heat transfer. Calculations using these methods show that the heat transfer effects may have a significant influence on the mass flow rate through the valve, the trapped mass in the cylinder and the trapped pressure and temperature.     

Application of small size cavitating venturi as flow controller and flow meter

The cavitating venturi is using to provide constant mass flow rate of liquid which is passing through a passage, independent of downstream pressure changes. The flow rate is a function of the upstream pressure, the throat area, the density and saturation pressure of the liquid. An experimental setup with capability of supplying water flow rate and constant upstream pressure was designed and manufactured. Three cavitating venturis with throat diameter of 5, 2.5, and 1 mm were designed and built to investigate the effect of venturi size on its mass flow rate. Three different sets of experiments were conducted to investigate the performance of the venturis. In the experiments, the mass flow rates were examined under different downstream and upstream pressure conditions and time varying downstream pressure. The results show for the ratio of downstream pressure to upstream pressure less than 0.8, the mass flow rate is constant and independent of the downstream pressure. Whenever the pressure ratio exceeds 0.8, the venturi acts like an orifice. This pressure ratio has been predicted analytically to highlight the affecting parameters, mainly the geometry of the venturi and viscous losses. It is found that the venturi size has no effect on its expecting function to keep mass flow rate constant. Also, it is shown that by applying a discharge coefficient and using only upstream pressure, the cavitating venturi can be used as a flowmeter with a high degree of accuracy in a wide range of mass flow rate.     

Convective heat transfer behaviour of water-ethylene glycol-mixture with silver nanoparticles under laminar flow conditions

In this work, we report the forced convective heat transfer performance and pressure drop of aqueous ethylene glycol seeded with silver nanoparticles for low temperature applications. Experiments were performed in a tube in tube counter-current heat exchanger using silver nanofluid as the hot fluid under laminar flow conditions. In this study, water-ethylene glycol mixture with 70:30 volume percent was used as the base medium. Silver nanofluid was allowed to flow through inner tube of the heat exchanger for varying nanofluid mass flow rates from 5 g/s to 30 g/s and three inlet temperatures of nanofluid viz. 2 °C, 5 °C and 10 °C. The increments in thermal diffusivity and viscosity are found to be ~37 % and ~69 % at 0.45 vol%, respectively. The enhancement in heat transfer coefficient at highest mass flow rate is found to be ~94 % for 0.45 vol%. The pressure drop in the silver nanofluid increases with respect to increase in volume percentage of nanoparticles due to increase in viscosity.

     

Electrochemical Behavior of Uric Acid at a Meso‐2,3‐Dimercaptosuccinic Acid Self‐Assembled Gold Electrode

Abstract

The fabrication and electrochemical characteristics of a meso‐2,3‐dimercaptosuccinic acid (DMSA) self‐assembled monolayer modified gold electrode were investigated. The DMSA self‐assembled electrode can enhance the electrochemical stability of uric acid (UA) and the electrochemical reaction of UA on the DMSA electrode has been studied by cyclic voltammetry and electrochemical quartz crystal microbalance. Some electrochemical parameters, such as diffusion coefficient, standard rate constant, electron transfer coefficient, and protons transfer number have been determined for the electrochemical behavior on the DMSA self‐assembled monolayer electrode. The electrode reaction of UA is an irreversible process which is controlled by the diffusion of UA with two electrons and two protons transfer at the DMSA/Au electrode. In phosphate buffer (pH 5.0), the peak current is proportional to the concentration of UA in the range of 8.0×10^?5?1.0×10^?2 mol L^?1 and 8.0×10^?5?8.0×10^?3 mol L^?1 for the cyclic voltammetry and differential pulse voltammetry methods with the detection limits of 1×10^?6 and 8×10^?7 mol L^?1, respectively. This method can be applied to the determination of the UA concentration.     

Effects of Schmidt number on near-wall turbulent mass transfer in pipe flow

Large Eddy simulation (LES) of turbulent mass transfer in circular-pipe flow has been performed to investigate the characteristics of turbulent mass transfer in the near-wall region. We consider a fully-developed turbulent pipe flow with a constant wall concentration. The Reynolds number under consideration is Re _τ = 500 based on the friction velocity and the pipe radius, and the selected Schmidt numbers (Sc) are 0.71, 5, 10, 20 and 100. Dynamic subgrid-scale (SGS) models for the turbulent SGS stresses and turbulent mass fluxes were employed to close the governing equations. The current paper reports a comprehensive characterization of turbulent mass transfer in circular-pipe flow, focusing on its near-wall characteristics and Sc dependency. We start with mean fields by presenting mean velocity and concentration profiles, mean Sherwood numbers and mean mass transfer coefficients for the selected values of the parameters. After that, we present the characteristics of fluctuations including root-mean-square (rms) profiles of velocity, concentration, and mass transfer coefficient fluctuations. Turbulent mass fluxes and correlations between velocity and concentration fluctuations are also discussed. The near-wall behaviour of turbulent diffusivity and turbulent Schmidt number is shown, and other authors’ correlations on their limiting behaviour towards the pipe wall are evaluated based on our LES results. The intermittent characteristics of turbulent mass transfer in pipe flow are depicted by probability density functions (pdf) of velocity and concentration fluctuations; joint pdfs between them are also presented. Instantaneous snapshots of velocity and concentration fluctuations are shown to supplement our discussion on the turbulence statistics. Finally, we report the results of octant analysis and budget calculation of concentration variance to clarify Sc-dependency of the correlation between near-wall turbulence structures and concentration fluctuation in the vicinity of the pipe wall.

     

R410A在内螺纹强化管管内冷凝的传热性能试验研究

为了研究水平强化单管的管内冷凝性能,搭建了实验台。研究了在冷却水量不变的情况下,R410A在不同冷凝温度(35℃和40℃)和不同管径(5mm和9.52mm)下的换热情况。结果表明:总换热系数和压降随工质质量流量的增大而增大,质量流量对管内换热系数影响不是很大。冷凝温度40℃,5mm铜管的换热系数最高;冷凝温度40℃,9.52mm铜管的压降最小。     

Experimental study on the vortex flow in a concentric annulus with a rotating inner cylinder

This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0-600 rpm. Also, the visualization of vortex flows has been performed to observe the unstable waves. The results of present study reveal the relation of the bulk flow Reynolds number Re and Rossby number Ro with respect to the skin friction coefficients. In somehow, they show the existence of flow instability mechanism. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and, then, it gradually approach to zero for the turbulent flow regime. Consequently, the critical (bulk flow) Reynolds number Re_c decreases as the rotational speed increases. Thus, the rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.     

一种旋转振动圆柱获取能量的参数化研究

提出一种旋转振动圆柱从潮流能中主动获取能量的方法。基于Fluent软件的用户自定义函数求解旋转振动圆柱升力,采用动网格技术对一定速度下的旋转振动圆柱获取潮流能进行数值模拟研究。参照相关试验结果,进行数值模拟结果确认,得到的最大升力系数幅值与平均阻力系数幅值与试验结果基本吻合。分析相位差、振幅比及转动幅值对旋转振动圆柱获能效率的影响。结果表明,同振幅比、同转动幅值的旋转振动圆柱,能量利用率都是随着相位差先逐渐增大后减小;同相位差、同转动振幅的旋转振动圆柱,在有效获能范围,其振幅比越大则能量利用率越高;同相位差、同振幅比的旋转振动圆柱,随着转动幅值的增加,其能量利用率先逐渐增加后减小;当相位差为243°,转动幅值为3,振幅比为2.0时,能量利用率最高。     

基于ANSYS的机械密封副温度场模拟

机械密封中,密封副端面的温度对整套密封装置的安全性、稳定性起着关键作用,为了模拟实际工况下密封副端面温度场的变化,建立了机械密封副的传热模型,在合理的假设条件下,通过相关计算,确定了动环、静环各自传递的热流率密度、与密封流体的对流传热系数以及冲洗流体的影响等关键参数,最后利用有限元分析软件ANSYS10.0进行了数值计算,得到了密封副温度场的变化规律、最大值及其位置,认为冲洗是降低密封端面温度的一种有效措施,为机械密封方案的设计提供了理论依据。     

A naphthalene sublimation study on heat/mass transfer for flow over a flat plate

It is important to completely understand heat/mass transfer from a flat plate because it is a basic element of heat/mass transfer. In the present study, local heat/mass transfer coefficient is obtained for two flow conditions to investigate the effect of boundary layer using the naphthalene sublimation technique. Obtained local heat/mass transfer coefficient is converted to dimensionless parameters such as Sherwood number, Stanton number and Colburnj-factor. These also are compared with correlations of laminar and turbulent heat/mass transfer from a flat plate. According to experimental results, local Sherwood number and local Stanton number are in much better agreement with the correlation of turbulent region rather than laminar region, which means analogy between heat/mass transfer and momentum transfer is more suitable for turbulent boundary layer. But average Sherwood number and average Colburnj-factor representing analogy between heat/mass transfer and momentum transfer are consistent with the correlation of laminar boundary layer as well as turbulent boundary layer.     

微通道内流体流动及换热特性的数值分析

采用Navier—Stokes方程与滑移边界条件联立的理论分析模型，对等壁温、等热流及无温度梯度工况下，气体在微通道中的流速分布、阻力系数变化趋势（Cf·Re）和传热特性（努塞尔数）进行了数值研究。结果表明：气体稀薄效应可显著减小管内的摩擦阻力和努塞尔数，增大气体流速；壁面的速度滑移和温度跳跃对微圆管内换热特性的影响相反，温度跳跃的影响更大；等热流加热与等壁温加热两种情况下，努塞尔数随克努森数的变化趋势明显不同。     

A non-intrusive method for evaluation ammonia mass flow rate in the condenser for diffusion-absorption refrigerators

Studies of diffusion-absorption (DAR) refrigerators have intensified, as well as the market's interest in increasing energy efficiency through cogeneration systems. Several studies have been published with the objective of improving the coefficient of performance, through the most different procedures. This article evaluates a non-intrusive method for the calculation of the ammonia mass flow rate at the inlet of the expansion device. The required parameters are geometric data and condenser temperature. The air-side convection heat transfer coefficient was calculated considering six empirical correlations. Then, throughout an energy balance in the condenser, the mass flow rate was computed. Four correlations predicted similar heat transfer coefficient. Thus, it is possible to estimate the ammonia mass flow using only the geometric data and the condenser temperatures. The strategy adopted to achieve this goal it was to use only the geometric data and the temperatures of the condenser and to calculate the energy balance in the superheated and saturation phases, since the absence of the subcooling in the condenser is a characteristic of the DAR cycle.     

An analytic study on laminar film condensation along the interior surface of a cave-shaped cavity of a flat plate heat pipe

An analytic approach has been employed to study condensate film thickness distribution inside cave-shaped cavity of a flat plate heat pipe. The results indicate that the condensate film thickness largely depends on mass flow rate and local velocity of condensate. The increasing rate of condensate film for circular region reveals about 50% higher value than that of vertical region. The physical properties of working fluid affect significantly the condensate film thickness, such as the condensate film thickness for the case of FC-40 are 5 times larger than that of water. In comparison with condensation on a vertical wall, the average heat transfer coefficient in the cave-shaped cavity presented 10-15% lower values due to the fact that the average film thickness formed inside the cave-shaped cavity was larger than that of the vertical wall with an equivalent flow length. A correlation formula which is based on the condensate film analysis for the cave-shaped cavity to predict average heat transfer coefficient is presented. Also, the critical minimum fill charge ratio of working fluid based on condensate film analysis has been predicted, and the minimum fill charge ratios for FC-40 and water are about Ψ_crit= 3-7%, Ψ_crit=0.5-1.3%, respectively, in the range of heat fluxq” = 5-90kW/²     

Forced convective boiling in vertical tube for binary refrigerant mixtures of R11 and R113

An experimental study was carried out on convective boiling heat transfer for mixtures of R11 and R113 flowing in a uniformly heated vertical tube by measuring the wall and bulk temperatures, and the results were compared with an existing correlation. A reduction of the average heat transfer coefficient for mixtures was verified for flow boiling. It was observed that two kinds of boiling behavior existed depending on mass flux. It was also found that the Chen's correlation was particularly successful for the case of high mass rate flow in which convective boiling prevailed. However in the case of low mass rate flow where nucleate boiling was dominant, the Chen's correlation was found to be inappropriate. Mass transfer resistance in the liquid film played a vital role for determining the heat transfer coefficient of refrigerant mixtures. It has been also found that the equilibrium assumption was hardly applicable to the convective boiling phenomena.     

The distribution of friction heat between a stationary pin and a rotating cylinder

The knowledge of the distribution of friction heat between two bodies in sliding contact is necessary for the calculation of the elevated temperatures at the contact area. An analytical model has been devised for the calculation of the coefficient for the friction heat distribution between a rotating cylinder and a stationary pin. This model covers a wide field of practical applications. The results of this analysis are in very good agreement with the experimental results existing in the bibliography. Graphs are included to facilitate the numerical calculations. The proposed model takes into consideration the specific geometric configurations and the surface heat transfer conditions of the bodies coming in sliding contact.     

基于Fluent汽油机缸盖冷却性能仿真分析

针对某型号汽油机缸盖的冷却性能进行了流场仿真分析。基于现代反求技术获得汽缸盖精确的3D模型,通过布尔运算获得内部真实流道模型。使用ICEM对其进行网格划分并设置恰当的边界条件,通过Fluent运算最终得到流道的速度流线场、换热系数场和压力分布场。对其进行分析可知该缸盖整体冷却性能正常,但4缸之间冷却效果存在显著差异。进行优化分析后可知总体冷却效果提升显著,但对改善流道速度分布均匀性作用不大,并指出了局部流动死区。     

射流条件下采用纳米流冷却液对柴油机缸盖冷却的研究

为提高柴油机缸盖的散热性能,提出一种将纳米流冷却液作为冷却工质,利用射流技术提高缸盖进排气门鼻梁处传热系数的方法。试验研究发现,相同射流参数下,不同体积分数的纳米流冷却液较传统冷却液都可有效提升传热系数,但体积分数增加的作用有限。同体积分数的纳米流冷却液会因为不同射流参数而表现出不同的传热性能,同时也会因为其本身黏性的增加而耗费更大的驱动功率。采用纳米流冷却液可以有效提高缸盖鼻梁处的传热系数。     

Computational/experimental study of a variable critical nozzle flow

Recently, critical nozzles have been extensively utilized to measure the mass flow rate in a variety of industrial applications. For the measurement of the mass flow rates in a wide range of operation conditions, the critical nozzle is required to be designed with different diameters. The objective of the present study is to investigate the effectiveness of a variable critical nozzle. A rod with a small diameter is inserted into the critical nozzle to change the effective cross-sectional area of the critical nozzle. Experimental work is performed to measure the mass flow rate of the critical nozzle with rod. Computational work is carried out using the two-dimensional, axisymmetric, compressible Navier–Stokes equations which are discretized using a fully implicit finite volume method. The diameter of the rod is varied to obtain different mass flow rates through the variable critical nozzle. Computational results predict well the measured mass flow rates. The boundary layer displacement and momentum thickness at the throat of the critical nozzle are given as a function of Reynolds number. The discharge coefficient of the critical nozzle is given as an empirical equation.     

Analytic approach to analyzing the performance of membrane dehumidification by pervaporation

As life quality has been greatly improved recently, the importance of humidity control has increased. Various kinds of humidifiers and dehumidifiers have been developed and utilized widely in our daily lives and industrial processes. In this work, we focused on dehumidification facilitated by a pervaporation system that employs a nonporous membrane. The system mainly consists of two parallel chambers separated by the membrane and a vacuum pump that constantly drives moisture removal. The membrane-pump couple sets out a vapor concentration gradient across the membrane to permeate water vapors from the feed chamber of ambient wet air to the low-pressure permeate. The experiments were performed in a constant temperature and humidity chamber to supply the constant concentration of vapor to the feed chamber. We measured the outlet humidity and the water vapor transport rate, the mass of vapor permeated through the membrane, under various experimental conditions of different feed flow rates and feed chamber heights. As the flow rate increases, the outlet humidity is decreased, whereas the water vapor transport rate is increased. However, they are independent of the height in our experiment range. Combining the mass transport theory of the membrane and volume conservation of an infinitesimal control volume, we have established theories of the outlet humidity and the water vapor transport rate. The experimental data points are entirely consistent with our theory curves. Comparing the experimental results to the theories, we also have derived the membrane coefficient.