Emitter discharge characteristics of vertical tube irrigation affected by various factors

To examine the working principle of vertical tube irrigation, variations in vertical tube emitter discharge and their causes were analyzed in the laboratory experiment. The effects of the pressure head, initial soil water content, and tube diameter on the emitter discharge of the vertical tube were studied. The results show that quantitative relationship between the time and cumulative infiltration and emitter discharge of the vertical tube is obtained, and R 2 is more than 0.98. Emitter discharge exhibits a positive and negative correlation with the pressure head and soil water content, respectively. Tube dia- meter has a nonsignificant effect on the emitter discharge. Changes of the soil water content around the emitter water outlet are the main causes of emitter discharge variations. In the experiments, the range of vertical tube emitter discharge is 0.056-1.102 L/h. The emitter of vertical tube irrigation automatically adjusts the soil water content and maintains the root zone soil water content within an appropriate range, which achieves continuous irrigation, and further achieves the effect of water-saving.

Passive heat transfer enhancement of laminar mixed convection flow in a vertical dimpled tube

Heat transfer enhancement in vertical tubes plays an important role on the thermal performance of many heat exchangers and thermal devices.In this work,laminar mixed convection of airflow in a vertical dimpled tube was numerically investigated.Three-dimensional elliptical governing equations were solved using the finite-volume technique.For a given dimpled pitch,the effects of three different dimple heights(h/D=0.013,0.027,0.037) have been studied at different Richardson numbers(0.1,1.0 and 1.5).The generated vortex in the vicinity of the dimple destructs the thermal boundary layer and enhances the heat transfer.Therefore,lower wall temperature is seen where the dimples are located.Fluid flow velocity at the near-wall region significantly increases because of buoyancy forces with the increase of Richardson numbers.Such an acceleration at the near-wall region makes the dimples more effective at higher Richardson number.Using a dimpled tube enhances the heat transfer coefficient.However,the pressure drop is not important.For instance,in the case of Ri=1.5 and h/D=0.037,20% gains in the heat transfer enhancement only costs2.5% in the pressure loss.In general,it is recommended using a dimpled tube where the effects of buoyancy forces are important.

Pressure Drop of Liquid–Solid Two-Phase Flow in the Vertical Tube Bundle of a Cold-Model Circulating Fluidized Bed Evaporator

A cold-model vertical multi-tube circulating fluidized bed evaporator was designed and built to conduct a visualization study on the pressure drop of a liquid–solid two-phase flow and the corresponding particle distribution.Water and polyformaldehyde particle(POM)were used as the liquid and solid phases,respectively.The effects of operating parameters such as the amount of added particles,circulating flow rate,and particle size were systematically investigated.The results showed that the addition of the particles increased the pressure drop in the vertical tube bundle.The maximum pressure drop ratios were 18.65%,21.15%,18.00%,and 21.15%within the experimental range of the amount of added particles for POM1,POM2,POM3,and POM4,respectively.The pressure drop ratio basically decreased with the increase in the circulating flow rate but fluctuated with the increase in the amount of added particles and particle size.The difference in pressure drop ratio decreased with the increase in the circulating flow rate.As the amount of added particles increased,the difference in pressure drop ratio fluctuated at low circulating flow rate but basically decreased at high circulating flow rate.The pressure drop in the vertical tube bundle accounted for about 70%of the overall pressure drop in the up-flow heating chamber and was the main component of the overall pressure within the experimental range.Three-dimensional phase diagrams were established to display the variation ranges of the pressure drop and pressure drop ratio in the vertical tube bundle corresponding to the operating parameters.The research results can provide some reference for the application of the fluidized bed heat transfer technology in the industry.

Experimental study of heat transfer of ultra-supercritical pressure water in vertical upward internally ribbed tube

Under ultra-supercritical pressure, the heat transfer characteristics of water in vertical upward 4- head internally ribbed tubes with a diameter of 28.65mm and thickness of 8mm were experimentally studied. The experiments were performed at P = 25- 34MPa, G = 450- 1800kg/（m^2·s） and q = 200 600kW/m^2. The results show that the pressure has only a moderate effect on the heat transfer of uhra-supercritical water when the water temperature is below the pseudocritical point. Sharp rise of the wall temperature near the pesudocritical region occurs earlier at a higher pressure. Increasing the mass velocity improves the heat transfer with a much stronger effect below the pesudocritical point than that above the pesudocritical point. For given pressure and mass velocity, the inner wall heat flux also shows a significant effect on the inner wall temperature, with a higher inner wall heat flux leading to a higher inner wall temperature. Increasing of inner wall heat flux leads to an early occurrence of sharp rise of the wall temperature. Correlations of heat transfer coefficients are also presented for vertical upward internally ribbed tubes.

Wall-Liquid Mass Transfer for Taylor Bubbles Rising Through Liquid in a Vertical Tube

Wall-liquid mass transfer for Taylor bubbles rising through liquid column in vertical tubes is an important and fundamental topic in industrial processes.In this work,the characteristics of wall-liquid mass transfer for this special case of slug flow were studied experimentally by limiting diffusion current technique (LDCT). Based on the experimental results and the analysis of hydrodynamic mechanisms,it was proposed that four different zones exist,i.e.the laminar falling film zone,the turbulent falling film zone,the wake region and the remaining liquid slug zone.The corresponding correlations for all these zones were developed.

Two-phase slug flow in vertical and inclined tubes

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HEAT TRANSFER STUDIES OF HIGHLY VISCOUS NON-NEWTONIAN FLUIDS IN VERTICAL TUBES BY FINITE ELEMENT METHOD

A numerical method capable is developed for handling steady laminar flow and heat trans-fer of a highly viscous power-law fluid whose density,viscosity,specific heat and thermalconductivity,vary with temperature.The governing equations are found to be continuity,monmentumand energy expressions.Important effects such as varying viscosity,natural convection and viscousdissipation are incorporated in the theoretical model.These equations are being attracted by employing a decoupled finite element method.Galerkin’sprinciple is used to handle the momentum and continuity equations.Consistent(SU/PG)andnon-consistent(SU)streamline upwind methods are employed for the energy equation.Comparisonof calculated results and experimental data shows good agreement.Similar results are obtained withSU and SU/PG methods.Velocity and temperature profiles which provide insights into the processare also given.

A mechanistic model of heat transfer for gas–liquid flow in vertical wellbore annuli

The most prominent aspect of multiphase flow is the variation in the physical distribution of the phases in the flow conduit known as the flow pattern. Several different flow patterns can exist under different flow conditions which have significant effects on liquid holdup, pressure gradient and heat transfer. Gas-liquid two-phase flow in an annulus can be found in a variety of practical situations. In high rate oil and gas production, it may be beneficial to flow fluids vertically through the annulus configuration between well tubing and casing. The flow patterns in annuli are different from pipe flow. There are both casing and tubing liquid films in slug flow and annular flow in the annulus. Multiphase heat transfer depends on the hydrodynamic behavior of the flow. There are very limited research results that can be found in the open literature for multiphase heat transfer in wellbore annuli. A mechanistic model of multiphase heat transfer is developed for different flow patterns of upward gas-liquid flow in vertical annuli. The required local flow parameters are predicted by use of the hydraulic model of steady-state multiphase flow in wellbore annuli recently developed by Yin et al. The modified heat-transfer model for single gas or liquid flow is verified by comparison with Manabe＇s experimental results. For different flow patterns, it is compared with modified unified Zhang et al. model based on representative diameters.

Boiling Heat Transfer Enhancement in a Vertical Annulus by Introduction of Air in Liquid Flow

In this paper, boiling heat transfer in a vertical annulus with inner side heated with and without air introduction is experimentally studied. Results show that boiling heat transfer is significantly enhanced by the introduction of air. When air is introduced into the liquid with a temperature below boiling point, the enhancement of heat transfer is also detected. It is concluded from the study that the heat transfer enhanced by introduction of inert gas is due to the liquid vaporization at the gas-liquid interface near the wall, which removes a large amount of latent heat and lowers the interfacial temperature considerably. Thus the gas-liquid interface acts as a 'heat sink'and the heat transfer is augmented significantly.

Optimization of Blade Motion of Vertical Axis Turbine

In this paper,a method is proposed to improve the energy efficiency of the vertical axis turbine.First of all,a single disk multiple stream-tube model is used to calculate individual fitness.Genetic algorithm is adopted to optimize blade pitch motion of vertical axis turbine with the maximum energy efficiency being selected as the optimization objective.Then,a particular data processing method is proposed,fitting the result data into a cosine-like curve.After that,a general formula calculating the blade motion is developed.Finally,CFD simulation is used to validate the blade pitch motion formula.The results show that the turbine＇s energy efficiency becomes higher after the optimization of blade pitch motion;compared with the fixed pitch turbine,the efficiency of variable-pitch turbine is significantly improved by the active blade pitch control;the energy efficiency declines gradually with the growth of speed ratio;besides,compactness has lager effect on the blade motion while the number of blades has little effect on it.

核心筒偏置超高层混合结构的竖向变形差异分析

在核心筒偏置的超高层混合结构中,由于核心筒与外框的应力水平的差异,在重力荷载下便会产生较大的变形差异。竖向变形差异对结构构件和非结构构件均会产生不利影响,可能会导致幕墙、隔墙、机电管道和电梯等非结构构件受损,造成耐久性和建筑外观方面的问题,还可能会在伸臂桁架等构件中引起严重的附加内力。目前,国内外针对核心筒偏置的超高层混合结构的竖向变形差异问题研究较少。以某超高层结构作为案例,对核心筒完全偏置的超高层混合结构的竖向变形差异进行了详细分析研究,并针对性提出了应对措施与建议。

Flow and Heat Transfer Instability of Supercritical Carbon Dioxide in a Vertical Heated Tube

Supercritical carbon dioxide(S-CO_(2))is one of the most promising working fluids in energy conversion systems.However,the instability of the flow and heat transfer has caused great harm to the security of energy conversion systems.In this work,a transient model based on the Finite Volume Method is set up to investigate the flow and heat transfer instability of CO_(2) changing from a subcritical state to a supercritical state in a vertical heated circular tube.The instability occurs when the wall heat flux is higher than a critical value,which makes the density and mass flow rate variations large enough.A large variation of the density triggers self-sustained oscillations in the flow.The critical heat flux heightens with the higher inlet pressure and pressure drop,larger tube diameter,and lower inlet temperature,but it reduces with the lengthening of the tube.To reflect the density-variation degree for the corresponding heat flux,a dimensionless number N_(tpc)=qπDL/Mβ_(pc)(p)/C_(p,pc)(p)(trans-pseudocritical number)is introduced.The critical trans-pseudocritical number Ntpc,c first goes up and down with the increase of the inlet pressure and the reduction of the inlet temperature.The rise of the mass flow rate,the shortening of the tube length,and the enlargement of the tube diameter all induce the temperature difference along the radial direction to become large.These tendencies make the critical Ntpc,c small.Consequently,the stability boundary N_(tpc,c)=48.47(N_(psc))^(1.048)(Δp∗)^(0.359)(D/L)^(−0.026)(G∗)^(−0.335)(ρ∗)^(2.666) is obtained to distinguish the regions of the flow and heat transfer stability and instability.

西辽河通辽段河床沉积物渗透系数的现场测定

采用不同方法对西辽河通辽段河床沉积物渗透系数进行现场测定,获取各点位的垂向渗透系数。并分别在探坑、土样坑、渗透仪渗水试验、竖管试验、渗坑法渗水试验、双环渗水试验位置处及其附近,先从河底表面取沉积物样,再从岩性明显变化的不同深度取样,带回实验室,进行颗粒分析,探讨河床沉积物渗透系数与颗粒级配之间的关系。结果表明:随着河床沉积物中砂石含量的增加其渗透系数也增大;反之,河床沉积物中黏土含量增加其渗透系数减小。沉积物的颗粒级配是影响渗透系数的重要因素,颗粒级配对于沉积物的孔隙尺寸、孔隙的形状及大小都有很大的影响。研究结果可为计算地表水与地下水的交换量提供可靠的水文地质参数,进而提高地下水资源评价结果的精度。

口腔颌面锥形束CT在根折诊断中的应用

口腔颌面锥形束CT(cone beam computed tomography,CBCT)现已被广泛应用于口腔医学的各个领域。有关根折,尤其是应用CBCT诊断根纵裂一直是研究热点,其研究内容主要包括以下5个方面:①CBCT对根析的诊断效能研究;②扫描参数对CBCT诊断根折准确性的影响,如扫描视野、空间分辨率、管电流和管电压等;③图像后处理技术,主要是金属伪影消除技术的应用能否提高根管治疗后和根管内有桩核时根折诊断的准确性;④根纵裂临床诊断模式的建立与验证;⑤人工智能和对比剂在CBCT图像根折诊断中的应用。与根尖片相比,CBCT可以显著提高未经过根管治疗患牙的根折诊断准确性;但是对于根管治疗后的患牙,根纵裂的诊断需结合临床体征共同判定,其中患牙颊舌(腭)向的垂直性骨吸收是根纵裂的特征性指标。根纵裂的宽度是影响根纵裂诊断准确性的重要原因,而CBCT扫描时采用的体素大小并不是影响根纵裂诊断准确性的必要因素;除个别机型外,根纵裂的折裂方向不影响其诊断准确性;图像后处理技术,特别是金属伪影消除技术不能够提高根纵裂诊断的准确性,还可能降低诊断效能,临床中不建议应用。

超临界CFB锅炉深度调峰跨临界过程中水冷壁动态特性的试验研究

为了保证超临界循环流化床(CFB)锅炉具有良好的宽负荷运行特性及深度调峰的能力,对跨临界压力变化时工质与水冷壁间的动态特性进行试验研究。采用Φ25.0 mm×3.5 mm的垂直上升光管,在压力20.0~23.0 MPa,质量流速400~800 kg/(m^(2)·s)试验工况范围内开展了近临界稳态传热试验和跨临界压力阶跃动态特性试验。结果表明:近临界压力下,增大质量流速、减小内壁热负荷、降低压力都能使传热恶化发生时的干度减小,对应的流体焓值增大,使传热恶化推迟发生;跨临界压力阶跃变化时,受热管内流体可能发生传热恶化导致壁温飞升,但随着质量流速的增大温度又回落到正常值;壁温飞升点与内壁面的传热随时间依次经历传热恶化阶段,过冷沸腾传热强化阶段和单液相换热阶段;各参数对跨临界压力阶跃变化时传热恶化的影响与对近临界稳态试验的传热恶化的影响相同,质量流速减小和内壁热负荷增大会使传热恶化发生的位置提前,同时壁温飞升的数值更大。

超超临界锅炉垂直管圈水冷壁焊缝交错区域温度场建模及热应力模拟

以某660 MW超超临界锅炉为对象,构建垂直管圈水冷壁焊缝交错区域温度场模型,研究热应力分布对裂纹的影响。结果表明:焊缝交错区域管环焊缝向火侧顶点的温度高于直管段管壁向火侧顶点温度,发生超温时比直管段管壁向火侧更容易开裂;水冷壁管最大轴向拉应力位于焊缝交错区域管环焊缝焊趾向火侧顶点附近,在变负荷运行工况下易导致交变热应力,产生沿管环焊缝焊趾延伸的横向裂纹;焊缝交错区域应力集中处的等效应力大于直管段对应处,背火侧管环焊缝焊趾与鳍片角焊缝交接处等效应力最大,超过对应的高温屈服强度,材料在交变热应力作用下会诱发裂纹,裂纹沿管环焊缝焊趾向背火侧管壁延伸。

超临界CFB锅炉灵活运行方式下的流动传热及动态特性试验研究

为了给超临界350 MW循环流化床(CFB)锅炉灵活运行提供指导,对亚临界压力下垂直上升光管内低质量流速汽水工质的流动传热及动态特性进行了试验研究。流动传热试验的压力为12.0~15.0 MPa,质量流速为350~440 kg/(m^(2)·s),内壁热负荷为135~220 kW/m^(2)。在26 MPa稳态试验基础上,通过压力阶跃减小和热负荷阶跃增加研究了垂直上升光管的动态特性。流动传热试验的结果表明:改变压力会影响饱和沸腾区的传热性能和饱和沸腾区的焓值区间;质量流速对传热性能影响仅体现在过冷区,质量流速增大过冷区的传热性能增强;增加热负荷会使饱和沸腾区的传热性能下降。根据流动传热试验数据,拟合得到了亚临界条件下垂直上升光管内汽水工质的传热和摩擦阻力关联式。动态特性试验的结果表明:压力阶跃变化量越大,试验段内的质量流速增量越大,出口比焓和出口壁温变化程度越大;热负荷阶跃仅会影响出口比焓,热负荷阶跃变化量越大,出口比焓和出口壁温变化程度越大。本研究可以为超临界CFB锅炉低负荷、灵活运行过程提供指导。

含不凝气体水蒸气竖直管内冷凝传质传热研究

针对竖直管内含不凝气体水蒸气冷凝传质传热过程开展研究.基于ANSYS Fluent软件平台建立了多相多组分三维数值计算模型,通过将管内中心轴线和壁面上多点温度分布与Kuhn试验结果进行对比,验证了模型的可靠性.在此基础上,考察了混合气体进口温度、速度、压力以及不凝气体体积分数对管内冷凝传热过程的影响规律,并基于不凝气体体积分数修正,提出了竖直管内含不凝气体水蒸气冷凝传热关联式,关联式误差范围为-20%~18%;同时,探究了不凝气体对水蒸气冷凝传质过程的影响规律,发现冷凝水率和平均传质速率随不凝气体体积分数的增大呈现先增大后减小的趋势,少量不凝气体具有促进水蒸气冷凝的作用,不凝气体体积分数为23.2%左右时,冷凝水率和平均传质速率最高.研究结果可为含不凝气体水蒸气的传热关联式和传质模拟研究提供参考.

不同管径管外含空气/CO_(2)蒸汽冷凝传热特性分析

当反应堆发生严重事故时,堆芯熔融物可能与安全壳中的混凝土发生相互作用,产生大量CO_(2)并释放到安全壳的气空间中。为了研究CO_(2)对安全壳内蒸汽冷凝的影响,本文采用数值模拟的方法对不同管径条件下竖直圆管外含空气/CO_(2)的蒸汽冷凝传热特性进行了数值模拟分析,考察了CO_(2)浓度的变化在不同条件下对含不凝性气体的蒸汽冷凝的影响。研究结果表明:在不同管径条件下,CO_(2)的存在都会增大冷凝传热系数,提升幅度可达30%以上。此外,传热管管径与CO_(2)的引入都不会改变局部冷凝传热系数在传热管轴向上的分布趋势,即由传热管顶部至底部,局部冷凝传热系数先迅速降低后缓慢增大。

立管式间接蒸发冷却器的结构与性能试验研究

为探索立管式间接蒸发冷却器可行性、结构与性能,设计了4组不同结构尺寸的冷却器并测试其降温性能,并对试验数据进行对比分析。结果表明,各试验冷却器温降均在6℃以上、湿球效率均在56%以上;增加换热面积可优化降温性能,冷却器单位面积处理风量宜控制在100~200 m^(3)/h之间;增大迎风面积、采用叉排布置增大流程可延长冷却时间,提高温降;增大管径可减小二次空气侧阻力,换热管宜采用30~50 mm管径圆管;采用管内直接布水与溢水相结合,并采用“上粗下细”的异径圆管,可保证换热管内壁形成稳定的贴附水膜,降温效果稳定。所得结果对立管式间接蒸发冷却器推广应用具有参考价值。