A comparison of predictive models for the onset of significant void at low pressures in forced-convection subcooled boiling

The predictive models for the onset of significant void (OSV) in forced-convection subcooled boiling are reviewed and compared with extensive data. Three analytical models and seven empirical correlations are considered in this paper. These models and correlations are put onto a common basis and are compared, again on a common basis, with a variety of data. The evaluation of their range of validity and applicability under various operating conditions are discussed. The results show that the correlations of Saha-Zuber (1974) seems to be the best model to predict OSV in vertical subcooled boiling flow.     

Local characteristics of two-phase flow parameters in an annulus boiling channel

Local two-phase flow parameters were measured to investigate the internal flow structures of steam-water boiling flow in an annulus channel. Two kinds of measuring methods for the local two-phase flow parameters were investigated. A two-conductivity probe was used for local vapor parameters and a Pitot tube for local liquid parameters. Using these probes, the distributions of phasic velocities, interfacial area concentration (IAC) and void fraction are measured in a steam-water boiling flow. In this study, it is observed that the local void fraction is smoothly decayed out from the surface of a heating rod to the channel center in subcooled boiling without any wall void peaking, which were observed in air-water experiments. The distributions of the local IAC and bubble frequency coincide with those of the local void fraction for a given area-averaged void fraction.     

Feedback fluidic flowmeters with curved attachment walls

Feedback fluidic flowmeters with curved attachment walls instead of conventional straight attachment walls were designed and machined. We experimentally investigated the effects of the dimensions of the jet nozzle and feedback channel on the oscillatory frequency, using water as the working fluid. The results reveal that in most cases, feedback fluidic flowmeters with curved attachment walls have a greater oscillatory frequency than those with straight attachment walls. A performance characteristic curve related to only the oscillatory frequency f and Reynolds number Re was found to be insensitive to the jet nozzle width and feedback channel width, even for asymmetric feedback channels [f(Hz)=7.67·exp(–α/2.56)+31.2·exp(–α/0.554)+0.757, where α=ln(Re)/f]. The characteristic curve can be used to measure the flow rates of liquids through a feedback fluidic flowmeter with curved attachment walls without the need for frequent calibrations.     

A suggestion to enhance pool boiling heat transfer on a vertical tube surface

To find a way of improving pool boiling heat transfer on a vertical tube surface, a revised annulus has been investigated experimentally. The annulus with closed bottoms has a shorter outer tube than the inside heated tube. For the study, three tube diameters (16.5, 19.1, and 25.4mm) and water at atmospheric pressure were used. The annular gap covers from 3.2 to 19.3mm in size and is generated by several glass tubes, which are fabricated around the heated tube. To clarify effects of the revised annulus on heat transfer, experimental results of the annulus were compared to the data of unrestricted tubes. The heat transfer coefficients for the revised annulus increased remarkably in comparison to the unrestricted tube. This paper was recommended for publication in revised form by Associate Editor Jae Young Lee Myeong-Gie Kang received his B.S. degree in Precision Mechanical Engineering from Pusan National University, Korea, in 1986. He then received his M.S. and Ph.D. degrees from KAIST in 1988 and 1996, respectively. Dr. Kang is currently a Professor at the Department of Mechanical Engineering Education at Andong National University in Andong, Korea. He has served as an engineer and researcher in KEPCO for 7 years. His research interests include pool boiling heat transfer, flow induced vibration, and nuclear thermo-hydraulics.     

Imaging of gas–liquid annular flows for underbalanced drilling using electrical resistance tomography

The underbalanced drilling technique, which is also known as managed-pressure drilling, is playing an important role in oil and gas sector, as it reduces common conventional drilling problems such as minimal drilling rates and formation damage, differential sticking and lost circulation. Flow regime monitoring is one of the key topics in annular multiphase flow research, particularly for underbalanced drilling technique. Prediction of the prevailing flow regime in an annulus is of particular importance in the design and installation of underbalanced drilling facilities. Especially, for establishing a suitable pressure-drop model based on the characteristics of the active flow regime. The methods of flow regime prediction (or visualisation) in an annulus that are currently in use are very limited, this is evidently due to poor accuracy or they are simply not applicable to underbalanced drilling operation in practice. Therefore, this paper presents a monitoring method, in which Electrical Resistance Tomography (ERT) is used to rapidly image the prevailing flow regime in an annulus with a metallic inner pipe. Experiments were carried out using an air–water flow loop with a test section 50 mm diameter flow pipe. The two-phase air–water flow regimes are visualised in the upward vertical annulus with a radius ratio (r/R) 0.4. This paper highlights the visualisation results of only three flow regimes, namely bubble flow, transitional bubble-slug flow and slug flow. The flow regimes are visualised through axial images stacked from 50 mm diameter-pixels of 2D tomograms reconstructed with the Conjugate Gradient Method (SCG). Gas volume fraction profiles within the annular flow channel are also illustrated. The profiles are extracted using the Modified Sensitivity coefficient Back-Projection (MSBP) method with a sensitivity matrix generated from a realstic phantom in the finite element method software. The results are compared with visual observations (e.g. photographs) of the active flow regime at the time of ERT measurements.     

固液两相离心泵内部非定常流动特性研究

为研究固液两相流离心泵内部的非定常流动特性,基于滑移网格方法,采用RNGκ-ε湍流模型以及ASMM代数滑移混合物模型,对一台高比转速固液两相离心泵内部流场进行非定常流动的数值模拟,通过分析清水工况数值计算结果、外特性性能实验结果以及固液两相流非定常数值计算结果,获得了非定常条件下固液两相输送离心泵的瞬时外特性曲线和内部流动及磨损规律。研究结果表明:在一个转动周期内,离心泵的扬程、效率和轴功率均呈现正弦波动特征;动静干涉效应使得叶轮出口处的速度和静压分布均呈现周期性波动;模型泵叶轮前后盖板的磨损情况比蜗壳壁面的磨损严重。上述计算结果可为实现高比转速固液两相流离心泵的优化水力设计和减轻磨损提供一定的理论参考。     

Experimental study on bubble size distribution of subcooled flow boiling in cylinder head of internal combustion engines

Subcooled flow boiling is becoming an efficient and widely used heat transfer approach in internal combustion engine cooling systems. Bubble evolution behaviors are crucial for understanding the mechanism of subcooled flow boiling. In this study, a diesel engine test platform equipped with endoscopic high-speed photography system was built to investigate the characteristics of boiling bubble. Under various inlet liquid subcoolings and flow rates, the mean bubble diameters and bubble size distributions were measured and analyzed based on the image processing techniques. Most of the bubbles approximated the spherical and ellipsoidal shapes. The bubble size was distributed between 1 and 2.5 mm, and the bubble diameters increased with the decrease of the inlet subcooling and flow rate. The flow rate had a more significant effect on the formation of large boiling bubbles.

     

Cyclic flow characteristics in an idealized asymmetric abdominal aortic aneurysm model

The flow characteristics and the corresponding hydrodynamic stability in an idealized asymmetric abdominal aortic aneurysm (AAA) model have been investigated using a laser Doppler anemometer. A rectified sine waveform was used to simulate aortic flow conditions (Re(delta) = 806 and alpha = 12.2). The flow around the distal neck of the AAA model undergoes transition and becomes turbulent for a fraction of time shortly after the commencement of the deceleration phases at every flow cycle while the rest of the flow inside the model stayed laminar throughout the cycle. As a result of non-symmetric vortical structure development inside the model, the distribution of turbulent shear stresses was found to be highly uneven along the radial direction of the model; this is in contrast to results found by the present authors in the symmetrical AAA model. The maximum turbulent shear stress found at the straight side of the distal neck are four times more than the maximum turbulent shear stress measured at the most dilated side of the distal neck. One of the interesting biological implications of the results is that the outward dilation of the arterial wall may be a physiological response to avoid the high turbulent shear load from the momentarily turbulent blood flow.     

Numerical simulation of vapor volume fraction in a vertical channel under low-pressure conditions

This numerical study involved investigating void behavior under low-pressure subcooled flow boiling by using an Eulerian approach (two-fluid model). In the simulation, a vertical pipe with a length of 0.15 m and diameter of 0.01229 m was considered. Different levels of uniform wall heat flux, mass flux, and inlet subcooling temperature were applied although a constant pressure of 1.65 bar was used for all the simulations. A sensitivity study of the empirical coefficients used to access the predictive capacity of the existing mass transfer models was conducted. Thus, the k-epsilon model was used for the turbulence of the fluid. The axial vapor volume fraction profile, liquid temperature is compared at the operating pressure. Furthermore, the most sensitive flow characteristics of the channel were identified. The results indicated that the predictions of numerical phase evolution relative to the experimental observations were in good qualitative agreement with those obtained in extant studies. Additionally, the changes in drag coefficients were helpful in precisely predicting the void fraction. A commercial CFD solver was used for the implementation of the model.     

径向间隙及叶片型线对液环泵性能影响的分析

采用欧拉气液两相流动模型对液环真空泵内部三维非稳态气液两相流动进行数值模拟,通过数值模拟研究叶轮与壳体间的径向间隙及叶片型线对液环泵性能的影响,分析了液环泵内液相能量转换的规律,分析了前弯、后弯及直叶片不同叶片型线液环泵性能曲线,分析了叶轮径向间隙对液环泵性能的影响。数值模拟结果表明后弯叶片、径向直叶片和前弯叶片下液环泵的极限真空度和最大流量依次递增;随着叶轮径向间隙的减小,液环泵的极限真空度和最大流量逐渐增大。为液环泵的性能优化研究提供了理论依据。     

Neural network and CFD-based optimisation of square cavity and curved cavity static labyrinth seals

The pressure drop characteristics for leakage of water through circular grooved, square cavity and curved cavity static labyrinth seals are investigated. A semi-theoretical model employing two new terms named virtual cavity velocity and vortex loss coefficient, to determine the pressure drop across the seal is presented. Five different square cavity labyrinth seals (SCLS) were subjected to flow visualisation tests to observe the leakage flow patterns. Computational fluid dynamic (CFD) analysis was done using Fluent commercial code. The values of the vortex loss coefficient for the SCLS at turbulent flow conditions were obtained experimentally. Using the data pool, an artificial neural network (ANN) simulation model was employed to identify the optimal SCLS configuration. Based on the insights gained, two different curved cavity labyrinth seal (CCLS) geometries were developed and optimised using parametric CFD analysis. They were visualisation tested and experimentally found to have higher pressure drops and vortex loss coefficients as compared to the SCLS configurations. The studies show that the enhanced performance is due to the presence of multiple recirculation zones within their cavities, which dissipate higher amount of leakage flow momentum.     

A study on the combustion characteristies of coaxial jet furnaces with swirl flow

This study is an analysis of the turbulent diffusion flame with swirl flow and the calculated results are compared with experimental data in cases of various swirl numbers and air-fuel ratios. The mathematical model is restricted to single-phase, diffusion controlled combustion with swirl flow. Values of local flow properties are obtained by solving appropriate differential equation for continuity, momentum, stagnation enthalpy, concentration, turbulence energy, dissipation rate of turbulence energy, and the mean square of concentration fluctuation. The method is proposed for calculating the local probability of chemical reaction on the use of the probability density function for the mixture fraction.     

爪式流体机械中曲爪与直爪转子的性能对比

将爪式流体机械的曲爪型和直爪型转子在型线构成、工作过程、受力变形进行对比。分析两种转子在工作性能方面的不同,并对转子的流场和受力进行数值模拟。研究结果表明:在相同半径条件下,直爪型转子的相对余隙容积小比曲爪型转子,曲爪型转子的压力比和压缩效率高于直爪型转子。直爪转子的最大应力小于曲爪转子,爪背无应力集中现象,力学性能好,承载能力强。     

Numerical analysis of turbulent flow over a backward-facing step using reynolds stress closure model

Reynolds stress turbulence models are adopted and applied for calculating turbulent flow over a backward-facing step. For the diffusion term in the transport equations for the Reynolds stresses, two gradient-type models are employed and compared. In addition, investigations on the modified ∈ equations are carried out. The results of the computations are compared with the extant experimental data. As a consequence, it is concluded that the Reynolds stress models predict the flow field better than the standardk-∈ model in the recirculating region. However, after the reattachment the return to the ordinary turbulent boundary layer is shown to be too slow to predict the flow field irrespective of turbulence models.     

大变形曲梁多体系统动力学建模

为了提高大变形曲梁多体系统数值仿真的计算精度和计算效率,引入弧线坐标,采用曲梁单元,从非线性应变关系式出发,用绝对节点坐标法建立了大变形曲梁多体系统的动力学模型,该方法的特点是不需要引入单元转换矩阵,收敛性好。用曲梁单元模型对曲梁单摆进行数值仿真计算,将计算结果与将曲梁细化为若干个直梁单元的动力学模型的计算结果进行对比,验证了曲梁单元动力学模型的准确性和有效性。在此基础上用曲梁单元模型实现了四连杆曲梁多体系统的动力学仿真。     

Large eddy simulation of turbulent premixed combustion flows over backward facing step

Large eddy simulation (LES) of turbulent premixed combustion flows over backward facing step has been performed using a dynamic sub-grid G-equation flamelet model. A flamelet model for the premixed flame is combined with a dynamic sub-grid combustion model for the filtered propagation of flame speed. The objective of this study is to investigate the validity of the dynamic sub-grid G-equation model in a complex turbulent premixed combustion flow. For the purpose of validating the LES combustion model, the LES of isothermal and reacting shear layer formed at a backward facing step is carried out. The calculated results are compared with the experimental results, and a good agreement is obtained.     

A numerical study of three-dimensional backward-facing step flow

The laminar and turbulent flow over a backward-facing step placed in a square duct was investigated numerically. The aspect ratio of the step (step width/step height) was 3 and the area expansion ratio was 2∶3. Three-dimensional effects were significant due to the small aspect ratio. To simulate turbulent flows, both a standardk-? model and a non-lineark-? model were employed and the results were compared. The non-linear model was found to yield better results. From the numerical results, the existence of the corner vortex and the flow field associated with it were clarified. The reattachment length of the three-dimensional flow was found to be considerably shorter than the corresponding two-dimensional flow. The evolution of longitudinal vortices was visualized. Surface flow patterns which clearly demonstrate three-dimensional aspects of the flow were presented. Based on various data available, topological flow pattern was also sketched. To support the findings explored in the present work, expeirmental data were compared with the numerical data where applicable.     

Experimental determination of time lag due to phase shift on a flexible pipe conveying fluid

A finite element model of a flexible tube conveying fluid is developed in MATLAB^© based on the principle of virtual work, using a three-node isoparametric beam element. Finite element equations are formulated by applying Galerkin technique on the coupled equations of pipe conveying fluid. The present element developed is capable to model three-dimensional flexible tubes by including curved geometry, effects of damping, velocity and gyroscopic effects. The external excitation applied at the middle of the tube in the lateral direction produced a time lag between the lateral responses, which were measured at two equidistant points from the excitation point. This is due to the Coriolis effect, and the same is simulated using the developed code. An experiment, supported with a robust error analysis, is also conducted on a straight polyurethane tube conveying water, subjected to a sinusoidal excitation at the center between the clamped supports. The measured time responses are compared with the numerical values predicted by the code. The time lags for both cases are obtained from the temporal shift along the time axis, between the zero crossing points of the time–response curves. The results obtained agreed well. The code can be used to predict the time lag, which is correlated to the mass flow rate. The proposed method will help to design Coriolis mass flow meters for existing pipelines, without altering the system.     

Effect of surface roughness on pool boiling heat transfer in subcooled water-CuO nanofluid

We investigated the effect of surface roughness on pool boiling heat transfer in subcooled water-CuO nanofluid. Experiment was performed using 0.1% volumetric water-CuO nanofluid and pure water for comparison. The following results were obtained. The heat flux tended to increase as the liquid subcooling increased in the region of low wall superheat. However, the effect of liquid subcooling gradually decreased as the wall superheat increased. The heat flux of pure water and nanofluid was almost similar in the region of low wall superheat. As the wall superheat increased, however, the heat flux of nanofluid decreased compared to that of pure water. This was attributable to the fact that the nanoparticles mixed with pure water reduced the heat flux by deteriorating boiling on the heat transfer surface. The heat flux increased as the surface roughness increased in the pure water, but the effect of surface roughness on heat flux was unclear in the nanofluid. This was attributable to the decreased difference of surface roughness, which was caused by the coating or deposition of nanoparticles on the heat transfer surface during the experiment.     

Efficiency analysis of straight fin with variable heat transfer coefficient and thermal conductivity

In this study, one type of applicable analytical method, differential transformation method (DTM), is used to evaluate the efficiency and behavior of a straight fin with variable thermal conductivity and heat transfer coefficient. Fins are widely used to enhance heat transfer between primary surface and the environment in many industrial applications. The performance of such a surface is significantly affected by variable thermal conductivity and heat transfer coefficient, particularly for large temperature differences. General heat transfer equation related to the fin is derived and dimensionalized. The concept of differential transformation is briefly introduced, and then this method is employed to derive solutions of nonlinear equations. Results are evaluated for several cases such as: laminar film boiling or condensation, forced convection, laminar natural convection, turbulent natural convection, nucleate boiling, and radiation. The obtained results from DTM are compared with the numerical solution to verify the accuracy of the proposed method. The effects of design parameters on temperature and efficiency are evaluated by some figures. The major aim of the present study, which is exclusive for this article, is to find the effect of the modes of heat transfer on fin efficiency. It has been shown that for radiation heat transfer, thermal efficiency reaches its maximum value.