Experimental studies of power consumption for agitated vessels equipped with non-standard baffles and high-speed agitator

The influence of the geometrical parameters of non-standard baffles on power consumption has been investigated for agitated vessels equipped with a high speed agitator (Rushton disc turbine, pitched-blade turbine or propeller) The following geometrical parameters of the baffles have been tested: number J, width B, length L and distance p between the lower edge of the baffle and the bottom of the vessel. Measurements were carried out according to a second-order rotatable design of experiment, using the strain gauge technique. The results of the experiments have been approximated in the form of equations.     

充气条件下多层桨悬浮颗粒的临界转速

在直径为386 mm 的通气多层桨搅拌釜中,实验考察了下层搅拌桨型、挡板和气体分布器等对颗粒悬浮临界搅拌转速的影响,这对工业过程的设计和放大具有一定的指导意义.     

曲面弓形折流板换热器壳程流体流动与传热

提出一种新型折流板--曲面弓形折流板,并构造曲面弓形折流板换热器,采用数值模拟和实验相结合的方法研究其壳程传热和流动阻力性能。在实验方面,设计了实验用曲面弓形折流板和普通弓形折流板换热器试样,其中换热器管束采用可拆连接形式,以考察不同折流板结构和板间距的影响。通过改变管程及壳程流量和管程流体进口温度,获得了大量对应于不同折流板结构的壳程压力降和传热系数实验数据。在模拟方面,利用Fluent软件建立了曲面弓形折流板换热器和普通弓形折流板换热器流体数值分析模型,得到了壳程流体流场分布及壳程压力降和传热系数。结果发现,在相同结构参数和流动条件下,曲面弓形折流板换热器壳程压力降比普通弓形折流板换热器降低9%～24%,而壳程传热系数比普通弓形折流板换热器提高3%～11%。     

Critical Rotational Speed for a Floating Particle Suspension in an Aerated Vessel

The critical suspension speeds of floating particles in a gas‐liquid‐floating particle three‐phase system were measured in a multiple‐impeller agitated vessel. Three types of impellers, i.e., simple axial‐flow impeller upflow (SPU) and downflow (SPD), disk turbine (DT) and wing turbine (WT), twelve types of baffles and three kinds of gas spargers were used. The influences of impeller types, baffle configurations, gas spargers, gas superfacial velocity and particle loading on the critical suspension speeds of floating particles were systematically investigated. The optimum regressions of critical suspension speeds were respectively obtained for some better combinations of impellers, bafffles and spargers, such as (a) the 45SPU+45SPD+DT triple impellers, two high‐level baffles and two low‐level baffles (symmetric allocation), gas spargers, (b) the 45SPU+45SPD+DT three‐impeller, standard baffle and small gas sparger. Their errors were smaller than 11 %.     

Experimental studies of momentum transfer in a gas-liquid system in the vicinity of an agitated vessel wall

The results of an experimental investigation of transport processes at the wall of an agitated vessel in a gas-liquid system are presented. Measurements were carried out by means of the electrochemical method. The experiments were performed in a standard agitated vessel of diameter 0.3 m, equipped with a Rushton disc turbine and baffles. The distributions of the diffusion current along the vessel wall were obtained for different values of agitator speed and superficial gas velocity. On the basis of these data, distributions of the shear rate, shear stress, dynamic velocity, friction coefficient and energy dissipated were evaluated. The mean values of the variables were approximated analytically. This paper was presented at the International Workshop on Electrodiffusion Diagnostics of Flows held in Dourdan, France, May 1993.     

非牛顿流体在搅拌槽内的传热

基于搅拌桨旋转扭矩和搅拌槽内所有壁面的平均扭矩近似相等的关系,引入流况系数,提出了计算非牛顿流体表观粘度的剪切速率模型.并对内蛇管、外蛇管和直管以及不同几何尺寸的MIG、透平、三叶后掠式、板式、锚和半椭圆片桨组成的多种搅拌体系,用该模型关联夹套侧和冷却管侧的给热系数,分别得到了统一不同几何尺寸搅拌桨的给热系数关联式.该模型适用于不同冷却构件.不同类型、不同几何尺寸的搅拌桨、过渡域和湍流域以及夹套侧和冷却管侧的非牛顿流体给热系数关联.另外,对多种剪切速率模型进行了比较和讨论.     

折流板间距对换热器性能影响的数值研究

采用CFD数值模拟方法,研究了简化模型下弓形折流板和螺旋折流板换热器,对应于不同间距/螺距时,流动参量的变化对换热器整体流动与传热性能的影响。结果表明,两种结构对应的壳程压力损失和换热系数均随壳程流量的增加而增大,而螺旋折流板结构单位压降下换热系数大于弓形折流板,并且其性能受折流板螺距变化的影响较小,体现了螺旋折流板结构的优越性。计算结果与现有实验结论吻合较好。     

机械搅拌反应器中挡板的结构设计

研究了内径为0．786m的搅拌釜中挡板尺寸及结构对圆盘透平桨RT和翼型桨k5及其组合在气液两相中的气体分散与混合特性的影响。对不同形式的挡板的搅拌功率、气含率及气液混合特性进行了对比分析。研究结果表明：挡板尺寸结构应根据搅拌特性需要进行优化设计;挡板系数为0．12时,组合浆的功率输入已与同一转速下的全挡板系数时的功率输入相近,它同时可改善微观混合、提高混合效率：采用开槽挡板可提高复杂快反应的选择性,混合效率提高20％～25％。     

Solid–liquid mass transfer characteristics of an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller

To develop an enhanced form of solid‐liquid apparatus, an unbaffled agitated vessel has been constructed, fitted with an agitation system using an impeller whose rotation alternates unsteadily in direction, i.e. a forward‐reverse rotating impeller. In this vessel, solid‐liquid mass transfer was studied using a disc turbine impeller with six flat blades. The effect of impeller rotation rate as an operating variable on the mass transfer coefficient was evaluated experimentally using various geometrical conditions of the apparatus, such as impeller diameter and height, in relation to the impeller power consumption. Mixing of gas above the free surface into the bulk liquid, i.e. surface aeration, which accompanied the solid‐liquid agitation, was also investigated. Comparison of the mass transfer characteristics between this type of vessel and a baffled vessel with a unidirectional rotating impeller underscored the sufficient solid‐liquid contact for prevention of gas mixing in the forward‐reverse rotation mode of the impeller. Copyright © 2008 Society of Chemical Industry     

Effects of Geometrical Factors on Liquid Mixing in a Gas-Induced Agitated Tank

An agitated tank having two 45° pitched blade downward turbines in series and four thin baffles which were narrower than the conventional ones was studied. At higher impeller speeds, a gas and liquid vortex induces the gas above the liquid surface vertically downward into the turbines, which in turn cuts the gas stream into bubbles which then circulate in the liquid. Consequently gas-induction increases the gas utilization ratio as well as simplifying the ancillary equipment otherwise required to recycle the escaped gas. The relationship between tank geometrical factors and the liquid mixing time was also investigated. The experimental variables investigated were the impeller speed, the baffle width, the turbine diameter, the space between two impellers, the liquid level, and the clearance between the lower turbine and the bottom of the tank. The results showed that all but the clearance between the lower turbine and the bottom of the tank had significant effects on the mixing time under normal gas-induction, revealing results in contrast to the conventional agitated tank. © 1997 SCI.     

Mechanism of mixing enhancement with baffles in impeller-agitated vessel, part I: A case study based on cross-sections of streak sheet

Enhancement mechanism of mixing with baffle in agitated vessel using rotated two-bladed paddle impeller was investigated under a laminar condition. In mixing pattern, the toroidal isolated mixing region in the baffled vessel becomes distortive and much smaller than that of unbaffled vessel. From the visualization of streak cross-sections in the baffled vessel, interestingly, the renewed streak folds (streak lobes) are generated at the vicinity of baffles in both the vertical and horizontal cross-sections. These behaviors of streak are unlike the unbaffled case that the streak stretches straightforwardly. The streak lobe is known as the mixing template that its number and size are key factor for laminar mixing in agitated vessel. The results suggest that baffles can effectively transform the circumferential flow to vertical and/or radial flows. Consequently, in the baffled vessels, not only the vicinity of vessel wall but also the tip of baffles can become the origination of streak lobe formation, and folds of streak in the vertical and circumferential directions are further enhanced with baffles.     

扭转流换热器结构参数对流场和温度场的影响

建立扭转流换热器和弓形折流板换热器两种周期性全截面模型,采用计算流体力学方法对两种换热器的传热系数、阻力、综合性能进行了数值研究。分析了扭转流换热器管束支撑物类梯形导流板结构参数对换热器传热和流阻性能的影响。结果表明：导流板倾角和相邻两组导流板间距对扭转流换热器传热性能的影响显著,导流板宽度的影响次之,每组导流板的数量的影响较小。扭转流换热器优化结构,压降较弓形折流板换热器降低42.5%~46.9%,综合性能提高7.2%~14.1%。研究结果为管壳式换热器结构优化和强化传热提供了新方案。     

基于颗粒动力学理论的搅拌器中固液流动的数值模拟

在欧拉双流体模型基础上引入颗粒动力学理论(KTGF),对带挡板圆盘涡桨式搅拌器内的固液两相流动进行数值模拟。结果表明,搅拌器底部颗粒温度分布与固相浓度分布趋势吻合,转速低于600 r/min时,槽底会形成明显的颗粒沉积,转速从600 r/min增至1500 r/min,堆积区向轴中心收缩,基于颗粒动力学理论可以合理解释挡板及叶轮转速对固相浓度分布的影响。随叶轮转速增大,搅拌器内固液两相湍流运动加剧,颗粒温度、湍动能及轴向速度增加,颗粒分布更均匀,但达到完全悬浮状态后颗粒温度趋于稳定。搅拌器底部和挡板处颗粒堆积导致了局部颗粒浓度增加及颗粒平均自由行程减少,颗粒温度反而降低;同时挡板布置使搅拌器内形成了双循环回路,加强了流体的湍流程度,增强了湍动能,但导致颗粒在挡板处积聚,不利于固相在挡板处均匀分布。     

折流式旋转床静圈对其性能的影响

折流式旋转床是一种新型的超重力旋转床,其核心部件是动、静结合的转子,转子由安装了动圈的动盘和安装了静圈的静盘上下相互嵌套而成。本实验中使用了一个常规的折流式转子（转子Ⅰ）和一个具有较短静圈的折流式转子（转子Ⅱ）,在常压下分别以乙醇-水体系和空气-水体系对两个折流式转子进行了传质性能与流体力学实验,考察了静圈对折流式旋转床的传质、压降和功耗的影响。结果表明,静圈能明显强化传质过程,与转子Ⅰ相比,转子Ⅱ理论塔板数大约降低了的50%;转子Ⅰ的传质效率随转速的增大而增大,当转子的转速从400 r/min增加到1200 r/min,转子Ⅰ的传质效率增大了约40%,而转子Ⅱ的理论塔板数变化不明显;静圈使折流式转子具有较大的压降和功耗,转子Ⅱ的压降为转子Ⅰ的20%～50%,轴功率为转子Ⅰ的60%～80%。     

螺旋折流板换热器三角区漏流阻塞的实验研究

目前普遍使用的螺旋折流板换热器壳程的一个螺距主要采用4块平面折流板结构,但在相邻两块折流板的直边搭接处存在顶角相对的两个三角漏流区,使壳程流体偏离理想的螺旋流,严重影响了换热器的性能。采用折面折流板结构,可以封闭原始折流板之间的外侧三角漏流区,使壳程流体更接近连续的螺旋流动。实验结果表明,采用折面折流板代替原始的平面折流板后,换热器总传热系数增加6.7%～19.1%,平均增大16.9%,表明此折面折流板能有效提高螺旋折流板换热器的换热性能。虽然壳程压降也随之增大,但带来的泵耗功率增量非常有限。换热器的热性能因子均大于1.0,平均值为1.071,表明结构改进后的换热器,其综合性能平均提高7.1%。基于实验数据,拟合了结构改进前后的换热器壳程传热系数和阻力系数的相关实验关联式。研究结果对于换热器的节能优化具有重要的指导意义。     

折流杆换热器

介绍了折流杆换热器的结构形式和使用特点,以同1台冷凝器为例,对比了采用折流杆或采用折流板换热器的使用效果,结果表明,在相同阻力降条件下,折流杆换热器传热系数大,传热面积小,壳体也小。     

当量螺旋角对三分螺旋折流板换热器性能的影响

提出了非连续螺旋折流板换热器的当量螺旋角定义,并建议以组合数ho/Δp1o/3来反映换热器的综合性能。对可拆卸芯体管束折流板的倾斜角为10°扇形、15°扇形、15°椭圆、20°扇形、20°扇形搭接的三分螺旋折流板换热器和弓形折流板换热器进行了传热和压降性能测试实验。实验结果表明:其中当量螺旋角最大的20°倾斜角扇形折流板方案的壳侧换热系数最高且压降较低,其综合性能指标ho/Δp1o/3比弓形折流板换热器的数值高26%左右;20°倾斜角扇形搭接方案与当量螺旋角相仿的15°倾斜角扇形方案的性能大致相当;15°扇形方案比当量螺旋角较小的15°椭圆方案的性能更高。可见对换热器的性能起决定作用的是当量螺旋角而不是倾斜角;实验预示在当量螺旋角更大的范围存在最佳值。     

三叶孔板换热器壳程传热与压降

三叶孔板换热器是一种新型纵向流换热器,由于其具有传热效率高、压降低、抗振结构性能优越等诸多优点而广泛应用于核电行业。搭建三叶孔板换热器壳程传热与压降测试平台,对传热和压降的测量结果进行不确定度分析。对4台三叶孔板换热器模型进行实验研究,结果表明随着Reynolds数的增大,壳程对流传热系数和压降在对数坐标内线性增大;在Reynolds数相同的情况下,随着支撑板间距的增大,三叶孔板换热器壳程Nusselt数逐渐减小,压降逐渐降低,同时压力梯度逐渐减小。为了进一步分析说明三叶孔板换热器壳程传热与阻力性能,基于Bell-Delaware法设计了具有相同结构参数的折流板换热器。与折流板换热器的对比结果表明：三叶孔板换热器壳程Nusselt数平均为折流板换热器的1.25倍,壳程整体压力平均为折流板换热器的0.77,综合性能平均为折流板换热器的1.62倍。     

Gas‐Liquid Floating Particle Mixing in an Agitated Vessel

The influence of impellers and baffles on the mixing of gas‐liquid floating particles in agitated vessels was investigated. Fifty‐two kinds of impeller combinations and twelve types of baffle arrangements were used. The associated power, gas holdup and solids concentration at the vessel bottom were measured. It is concluded that the mixing characteristics of three‐stage impellers were superior to those of two‐stage impellers for aspect ratios larger than 1.6. The optimal combination of impellers and baffles was proposed. The correlations of the relative power and the gas holdup for the optimal combination of impellers under all types of baffles were obtained.