Dynamic crystallization characteristics of the working process of small type ice slurry maker

动态冰蓄冷是储能领域中广受制冷界重视的潜热储冷技术,基于平板刮削方式设计并搭建了小型动态制备冰浆实验装置,通过测试制冰溶液的动态结晶降温曲线、不同结晶时刻的含冰率和冰晶当量直径,分析了结晶时间、载冷剂温度、添加剂种类与浓度、刮削速度等因素对冰浆制备过程动态结晶特性的影响。结果表明：刮削速度低于200 r/min,溶液结晶过冷度随刮削速度的增大而增大,冰浆的含冰率以及冰晶粒径均随结晶时间的增加而增大,在冰晶粒径较大时刮削作用引起冰晶颗粒聚并、破碎,导致冰晶粒径增大减缓甚至减小;随添加剂浓度的增大,冰浆含冰率增大速率与冰晶粒径增大量均有所减小,且强极性无机化合物对冰晶生长的抑制效果好于乙二醇等醇类化合物。     

空间组合弯头气固两相流动磨损特性的数值模拟

采用计算流体力学(CFD)方法对空间组合弯头内气固两相流动及磨损特性进行了数值模拟,研究了气流速度、颗粒直径、颗粒浓度和弯头间连接长度对壁面磨损率的影响.结果 表明:空间组合弯头壁面磨损最严重位置在第1个弯头外侧,磨损严重区域自中心向周围过渡;气流发生空间转向,固体颗粒由于惯性会撞击在第2个弯头的侧壁面,导致第2个弯头...     

飞灰作用下的SCR系统催化剂磨损特性研究

采用实验与数值模拟结合的方法研究飞灰颗粒与催化剂的磨损问题,利用Lagrange法追踪飞灰颗粒在催化剂近壁区的运动轨迹,并结合催化剂磨损关联式计算催化剂的磨损率。结果表明:在研究工况范围内,催化剂前端磨损率为催化剂孔道磨损率的266～304倍,沿催化剂孔道出口方向,催化剂孔道磨损率在0～30 mm时下降较快, 30 mm之后时下降减缓;受催化剂钢网处飞灰的二次反弹和局部烟气速度高的影响,小飞灰粒径对催化剂前端产生两次撞击角度波峰,钢网边缘波峰处的磨损率较大,导致催化剂变薄,大飞灰粒径惯性大对催化剂前端中部只产生一次撞击角度波峰,引起磨损区域变大;催化剂前端磨损率随烟气速度呈指数增长,随飞灰粒径增大,飞灰粒径对催化剂前端磨损率的影响减弱。     

风沙环境下风力机叶片冲蚀磨损的数值研究

基于N-S方程,结合RNG k-ε湍流模型和DPM模型对风沙环境下风轮的冲蚀磨损行为进行数值计算。通过分析不同颗粒直径和浓度下风轮的冲蚀磨损特性,研究风沙对风力机叶片冲蚀磨损的主要部位,以及叶片磨损与颗粒粒径和沙尘浓度的关系。研究发现,叶片磨损最严重的区域位于叶片前缘部分,次严重区域为叶片压力面;颗粒粒径对叶片磨损区域和磨损速率的大小影响显著;颗粒浓度对磨损位置影响很小,叶片的磨损速率随着颗粒浓度的增加呈线性规律增加。     

波纹换热管管内强化传热实验研究

管内以水为介质,管外以饱和蒸汽为热媒,通过实验研究了φ5／φ19波纹管的传热特性。测试了 RP=1800~24000的换热系数,并通过拟合获得了该种规格波纹管的传热准则关系,并表明波纹管具有 良好的强化传热性能,传热系数比光滑直管大2.5~3倍;波纹管加工尺寸误差对传热性能的影响不明 显。     

小型WNS燃油锅炉改燃生物质后烟管磨损数值模拟分析

小型WNS燃油锅炉改燃生物质成型燃料后因烟气流速显著增大,并带有飞灰颗粒,可能磨损烟管。根据直径大小将飞灰颗粒分为D1、D2、D3和D4四类,通过数值模拟分析了四种颗粒在不同烟气流速下对烟管磨损位置和磨损量的影响。结果表明,D1磨损外排烟管进口边缘;D2磨损外排烟管壁面及边缘;D3和D4主要磨损内外排烟管壁面及边缘。最大磨损量随着流速上升而增大。低流速时,不同颗粒造成的最大磨损量及差别均较小,而高流速将显著加剧D1导致的磨损。     

超临界机组流动加速腐蚀边界层流动模拟与分析

流动加速腐蚀(FAC)是电厂高压漏水、管道断裂等主要事故的原因,采用Fluent软件模拟管道的流动加速腐蚀规律,将微观与宏观研究联系起来,分析了温度、流速、管型与边界层流动及流动加速腐蚀之间的规律。结果表明:与其他温度相比,150℃时管道边界层处的Fe2+质量浓度增大,边界层的厚度更薄,边界层的腐蚀较严重,对流动加速腐蚀的影响也较大,为最易腐蚀温度;腐蚀速率随流速的增大而增大;T型三通管相比直管易发生腐蚀,且在流向改变处腐蚀最严重。     

轴流压气机叶片磨损位置模拟方法研究

采用气固两相流计算方法,对某型燃气轮机压气机实际工作环境中沙尘颗粒对压气机叶片磨损特性进行了全三维数值模拟研究,分析了叶片表面磨损位置的分布情况。研究表明：同级与不同级压气机叶片均呈现出明显的非均匀磨损特性,同级叶片的最大磨损浓度可以达到最小磨损浓度的2.9倍,不同级叶片的平均磨损浓度最大相差17.8倍;同一叶片上,颗粒对叶片前缘的磨损程度高于尾缘;随着转速的增大叶片的磨损率最大增加120%且非均匀性进一步增强。     

尾气颗粒对可变混流涡轮增压器喷嘴环冲蚀磨损特性的数值模拟

可变涡轮增压器在运行过程中其涡轮喷嘴环会受到尾气颗粒的冲蚀磨损,造成喷嘴环叶片失效和涡轮运行效果下降。借助CFD软件对可变混流涡轮内部的气固两相流进行数值模拟计算,分析喷嘴环开度的变化和颗粒粒径对喷嘴环磨损规律的影响。结果表明：喷嘴环处于不同开度下时,磨损率和磨损区域均有所不同,磨损区域主要集中在喷嘴环压力面的中后段区域,随着开度的增大,磨损率和磨损区域均减小,压力面的磨损程度明显大于吸力面;尾气中的小颗粒因惯性较小对气流的跟随性较好,主要撞击喷嘴环前缘,且开度对喷嘴环前缘的磨损影响较小;尾气中大颗粒的运动轨迹趋于直线,主要撞击喷嘴环压力面的中后段区域;由于涡轮进气涡壳结构的周向不对称性,使得涡轮内部流场沿周向分布不均匀,导致不同周向位置的喷嘴环磨损率和磨损区域有所不同,且随着开度的增大,各喷嘴环之间的磨损差异也增大。     

颗粒直径对向心透平冲蚀磨损的影响

为了探讨不同直径颗粒对无导叶向心透平输送气固两相流过程中的冲蚀磨损问题,对颗粒与各过流部件的冲击速度与冲击角度进行数值模拟。结果表明:小直径颗粒在蜗壳和叶轮流道中分布相对均匀,几乎不与过流部件表面发生碰撞,对过流部件的冲蚀磨损率相对较小。大直径颗粒运动中更倾向于从蜗壳尾部进入叶轮,在叶轮流道中,颗粒轨迹易向叶片压力面靠拢,且易与叶片压力面发生碰撞。随着颗粒直径的增加,颗粒将以更大的角度和速率撞击叶片压力面,且存在多次撞击过程,造成压力面中部靠近前盘处和吸力面靠近出口区域严重磨损。     

Heat Transfer and Pressure Drop Characteristics of Nanofluid Flows Inside Corrugated Tubes

An experimental investigation is carried out to study the heat transfer and pressure drop characteristics of multiwalled carbon nanotubes (MWCNTs)/heat transfer oil nanofluid flows inside horizontal corrugated tubes under uniform wall temperature condition. To provide the applied nanafluids, MWCNTs are dispersed in heat transfer oil with mass concentrations of 0.05, 0.1, and 0.2 wt%. The Reynolds number varies between 100 and 4,000. Three tubes with hydraulic diameters of 11.9, 13.2, and 15.5 mm are applied as the test section in the experimental setup. Tubes are corrugated four times on the cross section; that is, there are four different helices around the tube. Depths of the corrugations are chosen as 0.9, 1.1, and 1.3 mm, and pitch of corrugation is 14 mm. The acquired data confirm the increase of heat transfer rate as a result of utilizing nanofluids in comparison with the base fluid flow. However, corrugating the tubes decreases the heat transfer rate at low Reynolds numbers. The highest increase in heat transfer rate is observed for the Reynolds numbers for which the smooth tube is in the transition regime and the corrugated tube reaches the turbulent flow, that is, Reynolds number in the range of 1,000 to 3,000. Rough correlations are proposed to predict the Nusselt number and friction factor.     

Study on the heat transfer and flow characteristics in a spiral-coil tube

In the present study, numerical and experimental results of the heat transfer and flow characteristics of the horizontal spiral-coil tube are investigated. The spiral-coil tube is fabricated by bending a 8.00 mm diameter straight copper tube into a spiral-coil of five turns. The innermost and outermost diameters of the spiral-coil are 270.00 and 406.00 mm, respectively. Hot and cold water are used as working fluids. The k-ε standard two-equation turbulence model is applied to simulate the turbulent flow and heat transfer characteristics. The main governing equations are solved by a finite volume method with an unstructured nonuniform grid system. Experiments are performed to obtain the heat transfer and flow characteristics for verifying the numerical results. Reasonable agreement is obtained from the comparison between the results from the experiment and those obtained from the model. In addition, the Nusselt number and pressure drop per unit length obtained from the spiral-coil tube are 1.49 and 1.50 times higher than those from the straight tube, respectively.     

Analysis of coiled-tube heat exchangers to improve heat transfer rate with spirally corrugated wall

Steady heat transfer enhancement has been studied in helically coiled-tube heat exchangers. The outer side of the wall of the heat exchanger contains a helical corrugation which makes a helical rib on the inner side of the tube wall to induce additional swirling motion of fluid particles. Numerical calculations have been carried out to examine different geometrical parameters and the impact of flow and thermal boundary conditions for the heat transfer rate in laminar and transitional flow regimes. Calculated results have been compared to existing empirical formulas and experimental tests to investigate the validity of the numerical results in case of common helical tube heat exchanger and additionally results of the numerical computation of corrugated straight tubes for laminar and transition flow have been validated with experimental tests available in the literature. Comparison of the flow and temperature fields in case of common helical tube and the coil with spirally corrugated wall configuration are discussed. Heat exchanger coils with helically corrugated wall configuration show 80–100% increase for the inner side heat transfer rate due to the additionally developed swirling motion while the relative pressure drop is 10–600% larger compared to the common helically coiled heat exchangers. New empirical correlation has been proposed for the fully developed inner side heat transfer prediction in case of helically corrugated wall configuration.     

Condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes

This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000 mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7 mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2 mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46 mm. Similarly, the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The test conditions are performed at saturation temperatures of 40–50 °C, heat fluxes of 5–10 kW/m², mass fluxes of 200–700 kg/m² s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes.     

Research on low-temperature heat exchange performance of hydrogen preheating system for PEMFC engine

The durability of membrane electrodes, which are the core components of the Proton Exchange Membrane fuel cell (PEMFC), seriously affects the service life of the stack. Under the action of long-term low temperature, the gas diffusion layer at the entrance will be aged and its micro-porous layer structure will be destroyed, which will hinder the removal of liquid water and gas transport, so it is necessary to preheat the anode hydrogen. In the present study, the influence of different pitch ratio and diameter ratio on heat transfer of corrugated tube heat exchanger is simulated by means of thermal-fluid coupled numerical simulation and periodic unit model, the effects of coolant flow rate and temperature on the overall heat transfer performance were also studied. The validity of the simulation results is verified by experiments, and the effect of hydrogen preheating on the stack performance is also tested. The simulation results show that the corrugated joint will disturb the flow of hydrogen, which increases the temperature gradient along the radial direction of the main flow and enhances the heat transfer. When Re is lower than 4000, the friction factor decreases quickly and then gradually flattens out. Compared with 0.5 for bellows with a pitch ratio of 1, the friction factor increases by 17%. With the increase of Re, the j values of different pitch ratios differ greatly and decrease linearly. For every 5 cm increase in the length of the corrugated tube, the total heat exchange capacity is increased by about 20%，and the total heat transfer increases about 100 W with the increase of the coolant flow 0.04 kg/s.     

A comparison of flow characteristics of refrigerants flowing through adiabatic straight and helical capillary tubes

This paper presents a numerical investigation of the flow characteristics of helical capillary tubes compared with straight capillary tubes. The homogenous two-phase flow model developed is based on the conservation of mass, energy, and momentum of the fluids in the capillary tube. This model is validated by comparing it with the experimental data of both straight and helical capillary tubes. Comparisons of the predicted results between the straight and helical capillary tubes are presented, together with the experimental results for straight capillary tubes obtained by previous researchers. The results show that the refrigerant flowing through the straight capillary tube provides a slightly lower pressure drop than that in the helical capillary tube, which resulted in a total tube length that was longer by about 20%. In addition, for the same tube length, the mass flow rate in the helical capillary tube with a coil diameter of 40 mm is 9% less than that in the straight tube. Finally, the results obtained from the present model show reasonable agreement with the experimental data of helical capillary tubes and can also be applied to predict the flow characteristics of straight capillary tubes by changing to straight tube friction factors, for which Churchill's equation was used in the present study.     

多工况下高水头水泵水轮机压力脉动特性分析

为研究多工况下高水头水泵水轮机内部的压力脉动特性,以某抽水蓄能电站水泵水轮机模型为例,采用SST湍流模型对非设计工况点下的水泵水轮机进行三维全流道非定常数值模拟,同时监测了固定导叶与活动导叶间、无叶区及尾水管处的压力脉动。结果表明,对于固定导叶与活动导叶之间的区域,水轮机工况下的压力脉动主频为叶片通过频率,而水泵工况下的最高扬程和最低扬程工况的主频分别为转频和叶片数通过的频率;对于无叶区,由于受到强烈的动静干涉效应,水轮机、水泵工况下的主频均为转轮叶片数通过频率,且脉动幅值较大;对于尾水管区域,直锥段处的频率分布规律与流量有关,水轮机小流量工况下,尾水管内主要为0.3倍转频的低频压力脉动,而水轮机大流量工况下,脉动频率主要以2.6倍转频为主。     

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.     

在第二类吸收式热泵中对螺旋槽管的换热性能研究

为了提高在以溴化锂为工质的第二类吸收式热泵吸收器的性能,在第二类吸收式热泵吸收器内对不锈钢螺旋槽管,即不锈钢光滑管的传热传质性能进行了实验研究.发现螺旋槽管的传热传质性能约为光滑管3倍,螺旋槽管内热媒工质-水的流体阻力系数是光滑管的17～20倍;应用于第二类吸收式热泵中间大大降低换热面积,促进热泵的高效紧凑化.     

Correlations for evaporation heat transfer coefficient and two-phase friction factor for R-134a flowing through horizontal corrugated tubes

Correlations for the evaporation heat transfer coefficient and two-phase friction factor of R-134a flowing through horizontal corrugated tubes are proposed. In the present study, the test section is a horizontal counter-flow concentric tube-in-tube heat exchanger with R-134a flowing in the inner tube and hot water flowing in the annulus. Smooth tube and corrugated tubes with inner diameters of 8.7 mm and lengths of 2000 mm are used as the inner tube. The corrugation pitches are 5.08, 6.35, and 8.46 mm and the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The outer tube is made from smooth copper tube with an inner diameter of 21.2 mm. The correlations presented are formed by using approximately 200 data points for five different corrugated tube geometries and are then proposed in terms of Nusselt number, equivalent Reynolds number, Prandtl number, corrugation pitch and depth, and inside diameter.