Experimental Investigation of Two-Phase Flow Instabilities in Low-Mass-Flux Water Wall Tubes of Supercritical Circular Fluidized Bed Boilers

For the purpose of disclosing the hydrodynamic flow characteristics, under the low mass velocity conditions of the 600-MW supercritical circular fluidized beds boilers, experimental studies on instability of two-phase flow in parallel vertical internally ribbed tubes were conducted. Two kinds of oscillations, pressure-drop oscillation and density-wave oscillation, have been observed. In the range of test parameters the effects of pressure, mass flux, inlet subcooling, compressible volume, exit throttle, and asymmetric heat flux to the two-phase instability were explored and analyzed. Indications from experiment data are: To increase system pressure, mass flux and inlet subcooling will intensify the stability of water wall tubes. To increase exit throttle will intensify the instability of water wall tubes. The bounding pressure and bounding mass flux of density-wave oscillations and the bounding pressure of pressure-drop oscillation have been obtained. Based on the results of testing and using a homogeneous model, the threshold relational expressions of instability were obtained. The results may be used for the design and safe operation of parallel vertical rifled water wall tubes of supercritical circular fluidized beds boilers.     

Experimental analysis of the single phase pressure drop characteristics of smooth and microfin tubes

In this study, the single phase pressure drop characteristics of smooth and microfin tubes are investigated experimentally. The horizontal test section is a counter flow double tube heat exchanger with water flowing in the inner tube and cooling water flowing in the annulus. By means of experimental setup, required temperature and pressure measurements are recorded and friction factor coefficient and pressure drop of smooth and microfin tubes are determined. Experiments are conducted for mass flow rates in the range between 0.023 kg/s and 0.100 kg/s and effect of Reynolds number on pressure drop is investigated. By using experimental results, Blasius type friction factor equations are developed for both smooth and microfin tubes. Experimental results for both smooth and microfin tubes are compared with correlations given in the literature.     

Experimental study of the thermal hydraulic performance of sub-cooled refrigerants flowing in smooth,micro-fin and herringbone tubes

The present paper reports an investigation of the thermo-hydraulic performance of two standard microfin tubes with different number of fins and a herringbone microfin tube. As a reference for comparison purposes a smooth tube of equal external diameter and wall thickness has been tested for similar operational conditions. Copper tubes of 9.52 mm external diameter, electrically heated, have been used in the investigation. Most of the reported data has been gathered with refrigerant R134a though refrigerant R22 was also used in some of the tests with both the smooth and one of the standard microfin tubes. The test tube entrance state of the working fluid was kept constant at a temperature of the order of −2.5 °C and a pressure of 500 kPa, whereas its mass velocity varied in the range between 100 kg/s m² and 1350 kg/s m². It has been found that the thermal performance of the herringbone tube is superior to that of the two standard microfin tubes though the pressure drop obtained with the former is clearly higher. All the microfin tubes present significant heat transfer enhancement with respect to the smooth tube. According to the present results, the enhancement ratio of the microfin tubes attains an asymptotic value for Reynolds numbers of the order of 20,000, the herringbone tube presenting the higher asymptotic value. A figure of merit parameter has been introduced to qualify each of the microfin tubes tested, with the both standard microfin tubes being the best qualified with respect to the herringbone one, especially for applications that operate at the intermediate Reynolds numbers range.     

Plastic collapse of pipe bends under combined internal pressure and in-plane bending

Plastic collapse of pipe bends with attached straight pipes under combined internal pressure and in-plane closing moment is investigated by elastic–plastic finite element analysis. Three load histories are investigated, proportional loading, sequential pressure–moment loading and sequential moment–pressure loading. Three categories of ductile failure load are defined: limit load, plastic load (with associated criteria of collapse) and instability loads. The results show that theoretical limit analysis is not conservative for all the load combinations considered. The calculated plastic load is dependent on the plastic collapse criteria used. The plastic instability load gives an objective measure of failure and accounts for the effects of large deformations. The proportional and pressure–moment load cases exhibit significant geometric strengthening, whereas the moment–pressure load case exhibits significant geometric weakening.     

Condensation of Steam on Pin-Fin Tubes: Effect of Circumferential Pin Thickness and Spacing

Experimental data are reported for condensation of steam at atmospheric pressure and low velocity on five three-dimensional pin-fin tubes. The main geometric parameters varied were the circumferential pin spacing and thickness, since these have been shown to have a strong effect on condensate retention, and the present study shows some evidence for an optimum circumferential fin spacing. Enhancements of the vapor-side heat-transfer coefficient of up to 4 were found, compared to a plain tube at the same vapor-side temperature difference. The measured enhancements are equal to, but do not exceed, those obtained from “optimized” two-dimensional integral-fin tubes reported in the literature—an observation that is also generally true for condensation of refrigerants. The evidence suggests, however, that three-dimensional fin profiles can produce worthwhile enhancement over those obtained from simple, two-dimensional, integral-fin tubes, but that more work is needed to understand the phenomena involved so that more efficient geometries can be developed.     

超临界压力锅炉水冷壁倾斜并联内螺纹管内两相流不稳定性的试验研究

针对国产首台600 MW超临界机组在亚临界压力条件下的水动力特性,进行了倾斜并联内螺纹管内的汽液两相流不稳定性试验研究,观察到压力降型和密度波型两种类型的脉动。探讨了影响不稳定性的主要参数。并给出了发生不稳定性的阈值。试验表明:在超临界锅炉滑压运行设计参数条件下,水冷壁管内不会发生两相流不稳定性。图11表1参7     

Experimental Investigation of the Effect of Tube-to-Tube Porous Medium Interconnectors on the Thermohydraulics of Confined Tube Banks

This experimental study investigates the effect of tube-to-tube copper porous interconnectors on the thermohydraulic performance of an in-line and staggered confined tube bank. The porous medium, having a transverse thickness equal to that of the diameter of the tube (9 mm), connects longitudinally six successive tubes kept as in-line and staggered arrangements with a square pitch of 2.0. The tubes are subjected to a constant and uniform heat flux and are cooled by forced convection under laminar-transition flow range (200 < Reynolds number < 1500) using air with a Prandtl number of 0.71 as cooling fluid. Experimental data presented here establish that by introducing tube-to-tube porous medium interconnectors for the maximum Reynolds number tested here, a reduction in the pressure drop by 18% is observed in the in-line configuration while the heat transfer is enhanced by 100% in the staggered configuration, when compared to their respective configurations without the porous medium. Defining an overall energy gain as the ratio of the heat transfer enhancement due to the presence of the porous inserts to the pressure drop incurred, it is seen that fixing the porous inserts in the in-line configuration is advantageous.     

Numerical investigation of heat transfer and pressure drop in enhanced tubes

This study investigates passive heat transfer enhancement techniques to determine the distribution of temperature and static pressure in test tubes, the friction factor, the heat flux, the temperature difference between the inlet and outlet fluid temperatures, the pressure drop penalty and the numerical convective heat transfer coefficient, and then compares the results to the experimental data of Zdaniuk et al. It predicts the single-phase friction factors for the smooth and enhanced tubes by means of the empirical correlations of Blasius and Zdaniuk et al. This study performed calculations on a smooth tube and two helically finned tubes with different geometric parameters also used in the analyses of Zdaniuk et al. It also performed calculations on two corrugated tubes in the simulation study. In Zdaniuk et al.'s experimental setup, the horizontal test section was a 2.74 m long countercurrent flow double tube heat exchanger with the fluid of water flowing in the inner copper tube (15.57–15.64 mm i.d.) and cooling water flowing in the annulus (31.75 mm i.d.). Their test runs were performed at a temperature around 20 °C for cold water flowing in the annulus while Reynolds numbers ranged from 12,000 to 57,000 for the water flowing in the inner tube. A single-phase numerical model having three-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the flow. The temperature contours are presented for inlet, outlet and fully developed regions of the tube. The variations of the fluid temperature and static pressure along tube length are shown in the paper. The results obtained from a numerical analysis for the helically tubes were validated by various friction factor correlations, such as those found by Blasius and Zdaniuk et al. Then, numerical results were obtained for the two corrugated tubes as a simulation study. The present study found that the average deviation is less than 5% for the friction factors obtained by the Fluent CFD program while Blasius's correlation has the average deviation of less than 10%. The corrugated tubes have a higher heat transfer coefficient than smooth tubes but a lower coefficient than helically finned tubes. The paper also investigates the pressure drop penalty for the heat transfer enhancement.     

Investigation of deformation mechanics and forming limit of thin-walled metallic bipolar plates

The present study is conducted on forming of the metallic bipolar plates made of 316 stainless steel sheet with a parallel serpentine flow field. The plastic deformation of straight and curved microchannels, forming limit criteria, and deformation mechanics during the process are investigated partially to present a reliable model for estimating fracture initiation. For this purpose, experimental stamping tests are employed to fabricate metallic bipolar plates and the process is simulated by finite element software. The validity of simulation results is examined by comparing thickness distribution and force-displacement curves reflecting 4.76% and 3.85% error rates, respectively. According to experimental observations, fracture starts at a channel depth of 0.610 mm. Hence, for determining the forming limit and predicting the fracture during the process, the deformation mechanic is studied at different points of the microchannels. Results of stress states analysis indicate that the stress state of plane-strain tension up to biaxial tension governs this process. Despite the presence of different loading paths during the process, the critical element in each channel is deformed under plane-strain tension. Therefore, a fracture model is developed based on thinning percentage and equivalent strain to predict the instability of metallic bipolar plates. According to the results, both the equivalent strain and thinning percentage criteria with critical limits of 0.56 and 33.45%, respectively, are considered as an allowable range of plastic deformation during the conventional stamping process of bipolar plates. Results indicate that maximum thinning in all directions is lower than 33.45% by using the modified stamping process.     

低气压下平直翅片管换热器空气侧刘易斯因子的实验研究

在高空低气压环境模拟舱内对平直翅片管换热器的传热传质特性进行了实验,研究了不同环境压力下刘易斯因子的变化规律和影响因素,并引入了压力修正因子用来预测不同环境压力下的刘易斯因子。实验结果表明:当环境压力从101.3 kPa下降到40 kPa,刘易斯因子变化显著,最大降幅为22.2%,最大增幅为53.7%;当换热器翅片表面为部分湿工况时,入口空气含湿量对刘易斯因子影响较大,全湿工况下入口空气含湿量影响不明显;随环境压力的下降,翅片表面发生部分湿工况到全湿工况的转变,转变前刘易斯因子随环境压力下降而减小,转变后刘易斯因子随环境压力的下降而增大;引入压力修正因子后关联式能较好地预测不同环境压力下刘易斯因子,关联式和99%实验数据点的相对误差在±20%以内。     

Interfacial friction factor in vertical upward gas-liquid annular two-phase follow

This paper presents new data on the gas-liquid interfacial friction factor in annular two-phase upward co-current flow in a vertical circular pipe. Different from most previous work, the present studies have been performed at relatively high film thickness, taking into consideration the effect of the entrained droplets which occur from the breakup of the disturbance waves. The test section has an inner diameter of 29 mm and the length of 3 m. The porous wall injector is used to introduce the liquid into the test section. The two phase pressure drop is measured by two static pressure tubes connected with a manometer. The film thickness is measured by calibrated stainless ring electrodes mounted flush in the tube wall. The electrode operates on the principle of the variation of electrical resistance with changes in the liquid film thickness between two parallel eletrode rings. The entrained liquid flow rate is measured by using a sampling probe connected with a cyclone separator. The entrainment flow rate in the gas core is calculated from an assumption that the sampling is carried out in an isokinetic manner. The results from the experiments are compared with those calculated from correlations reported in the literature. A new empirical correlation for predicting the interfacial friction factors for practical applications is proposed.     

The collapse of heat exchanger tubes with ovality and simulated defects

Heat exchanger tubes of a pressurised water reactor steam generator form part of the boundary between the primary and secondary circuits of the reactor. It is important that no gross breach of this boundary occurs at anytime. Normally the tubes are subjected to internal pressure. However, should a loss of coolant occur in the primary circuit, these tubes are subjected to external pressure.This investigation compares theoretical and experimental results for such tubes, firstly with varying amounts of ovality and secondly with defects artificially induced to simulate: 1, Longitudinal tube cracking; 2, Fretting at tube supports; 3, Erosion.The collapse is compared with the strength of an original perfect tube. Collapse was found to be of a ductile nature in every instance, and in no case did the defect propagate through the wall of the tube.The findings are also of general interest for any externally pressurised tubing.     

CFD-driven surrogate-based multi-objective shape optimization of an elbow type draft tube

Draft tube is the part of Francis turbines which is used to both discharge water and recover kinetic energy at the exit of the runner. A design optimization study of an elbow type draft tube based on the combined use of Computational Fluid Dynamics (CFD), design of experiments, surrogate models and multi-objective optimization is presented in this study. The geometric variables that specify the shape of the draft tube are chosen as input variables for surrogate models and the pressure recovery factor and the head loss are selected as output responses. It is determined that, pressure recovery factor, which is the main performance parameter, can be increased by 4.3%, and head loss can be reduced by %20 compared to the initial CFD aided design. Pressure recovery factor, is represented with a second order polynomial regression model in terms of the geometrical parameters based on the optimization results. The verification of the model is also provided by comparison with CFD results for different draft tubes other than that are used in the development of the model. The model is verified using 30 different design points and it can predict the pressure recovery factor with an error of less than 8%. This model allows the fast and correct design and optimization of elbow type draft tubes, without the need for further CFD simulations.     

Use of the cylindrical instability stress for blunt metal loss defects in linepipe

Assessment of metal loss defects in gas pipelines can be analysed by a number of methods. In analyses with finite element methods a failure criterion is required. A material property is introduced, the cylindrical instability stress, which determines the plastic collapse of cylindrical pressure containing vessels. The use of this is extended to cover blunt metal loss defects. Some published finite element studies of defected vessels are re-analysed using this failure criterion.

The cylindrical instability stress is a more accurate failure criterion for plastic collapse in pipelines and pressure vessels than commonly used measures such as flow stress, Specified Minimum Yield Stress plus 10 ksi or multiplied by 1.1. It can be used in determining burst pressure of defected and un-defected pressure vessels and piping.     

带无叶扩压器离心压气机的稳定性分析

结合实验数据,采用数值模拟方法对某带无叶扩压器离心压气机的性能与稳定性进行了详细分析,比较了设计点与近失速点下无叶扩压器内部的流动特征．结果表明：在发生喘振瞬间,扩压器进口最先出现了压力崩溃;扩压器进口流动特别是轮盖侧流动是导致压气机失稳的重要因素,对扩压器进口逆叶轮旋转方向的轮盖侧啧蒸汽能够提高扩压器的稳定性．     

无功支撑对改善电压崩溃作用的研究

以单负荷无穷大母线系统推到得出电压不稳与功角不稳之间的关系,为了论证电压崩溃与电压不稳以及电压崩溃与功角不稳之间的关系,又以电力系统单机无穷大模型为基础,在此系统中只存在功角失稳问题,不存在电压崩溃问题,之所以这样是因为无穷大系统中有足够的无功支撑。倘若此处的无功补偿有限,当达到无功补偿的上限的时候,一个负荷扰动,便会使电压崩溃问题出现。电压不稳并不意味着电压崩溃,足够的无功支撑虽不能改变运行点处于电压不稳的现状,但是可以改善此运行点不发生电压崩溃的效果。电压崩溃与功角不稳发生的先后顺序取决于事故点的无功支撑能力的大小,并用仿真算例验证了所提的结论。     

A Review of Heat Transfer and Pressure Drop Correlations for Laminar Flow in Curved Circular Ducts

Heat transfer and pressure drop correlations for fully developed laminar Newtonian fluid flow in curved and coiled circular tubes are reviewed. Curved geometry is one of the passive heat transfer enhancement methods that fits several heat transfer applications, such as power production, chemical and food industries, electronics, environment engineering, and so on. Centrifugal force generates a pair or two pairs of cross-sectional secondary flow (based on the Dean number), which are known as the Dean vortices, and improves the overall heat transfer performance with an amplified peripheral Nusselt number variation. The main purpose of this review paper is to provide researchers with a comprehensive list of correlations and concepts that they may need during their research. The paper begins with an introduction to the governing equations and important dimensionless numbers for the flow in curved tubes. The correlations for developing flow in curved and coiled circular tubes are also presented. The main contribution of this study is reviewing the numerical and experimental correlations to calculate friction factor and Nusselt number in curved circular tubes. Nusselt number correlations are categorized based on the thermal boundary condition, as well as on the method. A Dean number range of 1 to 20,000 for the pressure drop correlations and 1 to 7000 for the heat transfer correlations and a Prandtl number range of 0.1 to 7,000 are covered with the reviewed correlations.     

Allowable sizes of axial flaws in pressurized pipes made of moderate toughness materials

Failure stresses for axially part-through flawed pipes made of moderately tough materials are predicted by several fracture mechanics. However, allowable flaw sizes using these fracture mechanics cannot be simply described because there are many effective parameters such as pipe diameter, wall thickness, material properties, etc. To establish codes and standards to evaluate flaws for piping of light water reactors, we determine unified allowable sizes for axial flaws in pipes subjected to internal pressure from J-integral based fracture mechanics. The allowable sizes are simply tabulated using a single parameter which consists of pipe geometry and material properties.     

An experimental investigation in forced convective heat transfer and friction factor of air flow over aligned round and flattened tube banks

Heat transfer coefficients and pressure drop are studied experimentally for airflow over aligned round and flattened tube configurations. The Reynolds number is based on the outer diameter of the round tube or the outside transverse diameter of the flattened tube, which is used for various flows, ranging from 133 to 800 with a constant input heat flux. In the present work, a total of 30 samples of round and flattened tubes heat exchangers with three transverse pitches, 2.0, 3.0, and 4.5, were studied to investigate their thermal performance. The results indicate that the relative gain in the overall Nusselt number is about 32.5 to 60.6% in flattened tubes, while the reduction range in the friction factor is about 11 to 30.6%. Correlations are proposed for the overall Nusselt number, friction factor, and Colburn j–factor for both round and flattened tube banks. A higher value means that a deviation error of 9.9% in the round tube banks and 10.1% in the flattened tube banks are expected. In addition, the best value for thermal performance for the flattened tube bundle was found to be coincident with a smaller Reynolds number.     

Allowable flux density on a solar central receiver

The allowable flux density on a solar central receiver is a significant receiver parameter and is related to the receiver life span and economics. The allowable flux density has gradually increased as receiver technologies have developed and is related to various factors, such as the material characteristics, tube sizes, and internal tube flow conditions. A mathematical model was developed to calculate the allowable flux density for the Solar Two receiver which agrees well with published data. The model was then used to show that a higher allowable flux density can be obtained by increasing the allowable strain of the tube material and the fluid velocity and decreasing the tube thermal resistance, the convective thermal resistance, and the tube diameter and wall thickness. A sensitive analysis shows that the most important influence is the wall thickness, followed by the tube diameter and fluid velocity. Finally, a molten salt receiver gives a much higher allowable flux density than water/steam receivers and is even better than a supercritical steam receiver.