聚合反应工程——第5章 非牛顿流体的传递过程(四)

5.4 非牛顿流体在搅拌釜内的传热 5.4.1 一般传热关联式 在对搅拌聚合釜进行设计放大时,传热是一个重要问题。搅拌釜通常采用冷却夹套或其他内冷构件(如内冷管、导流筒、中空内冷搅拌桨叶等)移热。在湍流域操作时,常以内冷直管或盘管作内冷构件,直管还兼具挡板作用;在层流域和过渡流域操作时,多用螺轴-导流筒搅拌器,这时把导流筒设计成传热面,也有以中空内冷搅拌桨叶作为内冷构件的。     

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.     

非牛顿流体和牛顿流体在搅拌槽内的传热研究

本文根据边界层理论,通过搅拌槽壁面扭矩与搅拌桨旋转扭矩近似相等的关系,提出非牛顿流体在搅拌槽传热阻力层的实际表观粘度的计算方法.使佐野雄二的给热系数关联式也能适用于不同桨型的非牛顿流体的给热系数计算,从而获得了同时适用于牛顿流体和非牛顿流体在装有各种桨型的搅拌槽壁侧给热系数的统一关联式:     

搅拌槽中高粘度液体的传热研究

本文应用内外螺带-锚、双螺带-锚及螺带-螺旋轴-锚三种组合桨,研究了牛顿流体及假塑性流体,在层流域及过渡流域的传热关联式。分别获得了三种组合桨的传热膜系数关联式。为了得到三种组合桨的传热膜系数的通用关联式,本文采用了传热膜系数与单位质量功之间的关联。     

非牛顿流体在搅拌槽内传热的研究——直管冷却体系

用不同几何尺寸的 MIG、透平和三叶后掠式奖,研究了非牛顿流体在搅拌槽内的传热。搅拌槽内装有同时起冷却表面和挡板作用的垂直管。数据关联时使用了笔者提出的剪切速率模型。通过172—184次试验,得到了夹套侧和直管侧的给热系数关联式: 对MIG桨, Nu_j=0.452R~(*2/3)Pr~(*1/3)Vis~(1/6)(d/D)~(0.3)(∑bisinθ/H)~(0.337) Nu_c=0.0475R~(*2/3)Pr~(*1/3)Vis~(1/6)(d/D)~(0.477)(∑bisinθ/H)~(0.347)式中 Re~*为 200—18400,Pr~*为 80—2300。 对透平和和三叶后掠式桨, Nu_j=1.19Re~(*2/3)Pr~(*1/3)Vis~(1/6)(∑bisinθ/H)~(0.74) Nu_c=0.0767Re~(*2/3)Pr~(*1/3)Vis~(1/6)(∑bisinθ/H)~(0.523)式中 Re~* 为25—6310,Pr~* 为115—9000。 与以往研究者的模型相比,笔者的剪切速率模型不仅适用于不同类型的搅拌桨,不同的冷却体系,以及夹套侧和内冷管侧的给热系数关联,而且其关联精度较高。     

螺带桨搅拌槽内给热系数和所需功率的研究

本文详细研讨了螺带桨搅拌槽内给热系数及其与所需功率的关系.研究中使用了4个不同几何尺寸的螺带装以及8种不同稠度的假塑性非牛顿流体和高粘度牛顿流体.结果表明:在雷诺数小于0.1时,试验流体在槽内的给热系数与浆叶几何尺寸有关.通过回归分析法得到了4个对应于各螺带桨的给热系数关联式;雷诺数大于10时,给热系数和搅拌浆几何尺寸无关,从而得出一个适用于不同螺带浆的共同关联式.实验还研究了给热系数和所需搅拌功率的关系,并用多项式回归法得到了整个实验雷诺数范围的传热-搅拌功率关联式.     

非牛顿流体在过渡域的搅拌功率和混合效率

本研究测定了螺杆-导流筒、锚式、框式、三层三叶后掠式-D 形挡板和四层MIG-D 形挡板搅拌器的功率消耗 K_p、Metzner常数 Ks和混合时间 θ_M。得到了下列关联式:对牛顿流体,在层流域(Re<20),N_pRe=K_p;在过渡域(Re>20),1nN_p=A B 1n Re C(1n Re)~2。对非牛顿流体,只要用表观雷诺数Re~*代替上式中的雷诺数Re,螺杆-导流筒、三层三叶 后掠式-D 形挡板和四层MIG-D 形挡板搅拌器同样可用上述功率关联式;对锚式和柜式搅拌器,在层流域可用牛顿流体功率关系式,在过渡域,则会产生50％的偏差。作为工业设计,在层流域和过渡域,可选用螺杆-导流筒搅拌器;当Re>200时,也可选用三层三叶后掠式-D形挡板搅拌器。     

搅拌槽中垂直列管外壁的传热膜系数分布

在一内径为 12 0 0mm容积为 1.5m3 的搅拌槽中 ,使用CBY螺旋桨、 4 5°斜叶桨和六直叶涡轮桨 ,对牛顿流体采用量热法 ,研究了列管外壁的局部传热膜系数的分布情况。实验结果表明 ,对轴向流桨和径向流桨 ,列管外壁的局部传热膜系数有不同的分布 ;列管外壁各点的局部传热膜系数均与ε0 .197成正比。此外 ,文中还研究了桨叶离底距离、桨径等因素对列管外壁局部传热膜系数的影响。     

有无混合器对圆形直管内空气传热膜系数α影响的研究

研究了空气分别在有无混合器的圆形直管内湍流流动时的传热膜系数α,比较了两种情况下α及准数关联式的差异。结果表明:在相同条件下,空气在有混合器的强化传热管中其α远高于无混合器的圆形直管;且空气在强化管内湍流时的α其准数关联式不同于圆形直管。     

桨型和导流筒型式对气液两相体系气含率的影响

在高径比为1.5、双层组合搅拌桨的气液搅拌反应器中,研究了桨型组合及导流筒结构对气含率的影响.研究发现,下桨排出流向上的气含率比排出流向下时的高;反应器中双层导流筒结构时的气含率结果最优,上层导流筒的开孔率应为6.90%或16.4%,上层搅拌桨应为透平桨.并对最优组合情况的气含率进行了关联.     

Heat transfer to newtonian and non-newtonian liquids in a screw agitator and draft coil system

Heat transfer has been investigated for several Newtonian and non-Newtonian liquids mixed in a flat bottomed vessel equipped with a screw agitator and a coil acting simultaneously as draft tube and heat exchanger. Heat transfer rates from coil to liquid were determined for different coil designs, rheological properties and operating conditions in the heating and cooling modes. A circulation Reynolds number is defined, the characteristic length being the effective height of the heat exchanger (the coil) and the characteristic velocity being the circulation velocity of the fluid which was determined experimentally. Using this Reynolds number, it was possible to establish a single correlation for the Nusselt number as a function of Reynolds and Prandtl numbers. Geometric ratios do not appear in this correlation which could adequately represent more than 200 experimental data. A similar single correlation could not be obtained when using the conventional mixing Reynolds number. This novel system is shown to be very efficient for handling rheologically complex fluids.     

刮壁浆在槽式釜的传热研究

针对结刮壁搅拌槽壁面的传热提出渗透－对流模型。运用在搅拌浆上安装热电偶的测温方法，考察了刮壁搅拌槽中的温度分布情况，实验结果支持了渗透－对流模型。当Ｒｅ＞１００，Ｒｅ·Ｐｒ＞１０６时，刮壁残余层对壁面传热有明显影响，随着Ｒｅ的进一步提高，刮壁槽壁面的传热系数增加缓慢，关联得到Ｒｅ＜１００，８０００＜Ｐｒ＜１８０００时刮壁槽壁面传热系数的经验关联式。     

升膜蒸发管内流型可视化及传热性能

为了探究热流密度、真空度和流量对升膜蒸发器传热性能的影响,以及对升膜加热管内流体流型进行观测和分析,本文建立了升膜蒸发系统传热实验平台,对升膜蒸发器的传热特性和流体流型进行实验研究.实验所用升膜管管长2200mm,升膜管采用镀透明导电膜石英管,工作介质为水;升膜管蒸发侧采用电加热方式;研究了热流密度(6.71kW/m²≤ q≤ 26.79kW/m²)、流量(20L/h≤ M≤ 100L/h)和真空度(0≤ P≤ 15kPa)对升膜加热管流体流型和传热特性的影响.结果表明:通过电加热的方式可以实现石英管内溶液的升膜蒸发,并能观测到泡状流、块状流、弹状流、柱塞流、环状流和雾状流;热流密度低于6.71kW/m²时无法形成升膜蒸发,随着蒸发侧热流密度的增大,升膜管内环状流长度增大,管内传热系数增大;随着流量的增大,升膜管内液体湍流强度增大,管内传热系数增大;真空度对流体流型影响较大.     

三相下喷式环流反应器的传质性能

在三相非牛顿型流体体系中，对下喷式环流反应器传质特性进行了实验研究。讨论了表观气速、能量耗散速率、导流筒直径与反应器直径比、喷嘴直径、导流筒下端距反应器底部的距离、固体装填量、羧甲基纤维素钠（ＣＭＣ）溶液浓度及其流变特性对它的影响。实验结果表明，容积传质系数随表观气速和能量耗散速率的增加有所增加，在实验条件下，发现最优的导流筒直径与反应器直径比在０．４～０．４５这一范围、固体装填量大约为３％（体积百分比）、导流筒下端距反应器底部的距离为０．０８ｍ左右。同时提出了容积传质系数的经验关联式。     

Bestimmung von Wärmeübergangskoeffizienten mit Hilfe des Wilson‐Plot‐Verfahrens

The heat transfer coefficient on the service side of a double‐pipe heat exchanger is determined with the Wilson‐plot method. Condensing hexanol vapor is used as heating medium and water in single phase counter current flow on the product side. In addition to the Wilson‐plot, two alternative methods are used for calculating the service side film heat transfer coefficient: the Nusselt theory for film condensation and the extraction from an overall heat transfer coefficient based on an energy balance and employing the Gnielinski equation for the tube side heat transfer. As the result, heat transfer coefficient can be calculated as a function of the transferred heat flux employing the Wilson‐plot.     

三叶孔板换热器热力性能及其影响因素分析

建立了三叶孔板换热器壳程周期性全截面模型,利用商用软件Fluent14.0及RNG k-ε湍流模型对8种不同结构参数的换热器壳程流体流动及传热性能进行数值模拟。分析了支撑板间距、三叶孔孔高、导流筒结构形式等结构参数对三叶孔板换热器传热及阻力性能的影响,并对比不同结构换热器的综合换热性能。结果表明：壳程传热系数与压力梯度都分别随着支撑板间距和开孔高度的增加而减小,且支撑板间距和三叶孔孔高对三叶孔板换热器壳程压降的影响大于其对传热的影响;六边形结构的导流筒换热器换热性能优于圆形导流筒换热器;8种换热器模型中,支撑板间距400mm、三叶孔高3.3mm（模型4-2）的换热器综合性能最好,支撑板间距400mm、三叶孔高1.8mm（模型2-2）的换热器综合性能最差。     

复合中空热管两相流的观测及充液比对热管等温性的影响

为了解决工业中大量低品质烟气余热的回收利用和烟气酸露点腐蚀导致设备容易失效的工程问题,提出了一种复合中空热管传热元件,介绍了其结构及工作原理。采用可视化实验对管内气液两相流的传热传质机理进行了分析,同时,对不同充液比情况下管壁的等温性进行了实验研究。研究结果表明:在内管采用酒精炉加热、外管采用空气自然对流冷却的条件下,复合中空热管管内液池为核态沸腾换热,外管壁内侧是饱和蒸气层流膜状凝结;复合中空热管在外管壁温度30℃时,加热时间2 min之内就能正常快速启动工作;充液比33%时,复合中空热管的等温性好。实验研究结果为复合中空热管换热器的工程应用提供了基础。     

Non-isothermal laminar pipe flow — II. Experimental

In a previous article [1] theoretical predictions of velocity distributions, pressure drops and mean Nusselt-numbers are given for non-isothermal pipe flow of power-law fluids having a temperature dependent consistency-index. In the present investigation these solutions are checked for Newtonian fluids with extensive experimental data.A flow visualization technique was used for measuring the non-isothermal, laminar velocity profiles of glycerol in a round tube. Furthermore, accurate pressure drop measurements were carried out for non-isothermal tube flow of a viscous. Newtonian liquid. Finally logarithmic mean heat transfer coefficients were measured in laminar flow heat transfer.The experimental velocity profiles, pressure drops and heat transfer coefficients show a good agreement with those predicted theoretically.     

Search for optimal geometry of a conical turbine agitator for heat transfer in stirred vessels

Simultaneous measurements of power consumption P and heat transfer coefficient α for a jacketed, baffled, agitated vessel of diameter D = 0.45 m, equipped with a conical turbine, were carried out. The following geometric parameters of the agitator were tested: diameter d, blade width b and number Z, and distance from the bottom of the vessel to the lower edge of the blade h. The results of these investigations were generalized mathematically. On the basis of optimization, the best geometric parameters with respect to heat transfer are proposed for the conical turbine agitator.     

INFLUENCE OF NON-NEWTONIAN FLOW BEHAVIOUR ON MASS TRANSFER INBUBBLE COLUMNS WITH AND WITHOUT DRAFT TUBES†

Gas hold-up and mass transfer were examined in a column with and without a draft tube. It was found that the introduction of a draft tube increases the gas hold-up but decreases the volumetric mass transfer coefficient in Newtonian fluid systems. For non-Newtonian fluid systems, both parameters were increased by the presence of the draft tube. Empirical correlations are proposed for the gas hold-up and the volumetric mass transfer coefficient in the bubble column with Newtonian and non-Newtonian fluid systems. The correlations are in general agreement with the data in this work and in the literature. They should be useful for design and scale-up purposes. It was also found that introduction of an ancillary impeller improves the mass transfer in non-Newtonian fluids due to the break-up of large bubbles.