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.     

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.     

非牛顿流体在搅拌槽中的传热(下)

三、非牛顿流体在搅拌槽中的传热 1.两种常用的关联方法 牛顿流体和非牛顿流体在搅拌槽中的传热机理基本上是相同的。对非牛顿流体来说,关键在于如何处理式(1)中的Re、Pr和η_b/η_w三者中的粘度值,以使牛顿流体和非牛顿流体可用统一的关联式来关联。由于搅拌功率一股用准数方程式N_p=K_p/Re~x来关联,对于关联非牛顿流体的搅拌功率,研究     

非牛顿流体在搅拌槽内传热的研究——蛇管冷却体系

基于搅拌槽内所有壁面的平均扭矩与搅拌桨旋转扭矩近似相等的关系,并引入流况系数f的概念,提出了比以在研究者所用模型适用面更广,所得传热关联式精度更高的新的剪切速率模型: r=0.4~(1/n)N~[2-f(2-n)]/n f=exp(-0.00705d~2N~(2-n)ρ/K)用不同直径的平板桨和半椭圆片桨,经过320次实验,得到了搅拌槽壁侧和蛇管侧的给热系数关联式: Nu_j=0.42Re~(*2/3)Pr~(*1/3)ViS~(1/6)(d/D)~(0.414) Nu_c=0.039Re~(*2/3)Pr~(*1/3)ViS~(1/6)(d/Dc)~(0.028)式中,广义雷诺数Re~*为 30—1.86×10~4,广义普朗特数 Pr~*为 40—6500。两关联式的平均偏差分别为9.4％和10.5％。可供设计工业搅拌槽时使用。     

双轴表面更新型搅拌釜中高粘牛顿流体与非牛顿流体的功耗

实验测定了双轴表面更新型卧式釜中采用三种不同形式的搅拌桨、多种桨间距、广泛的粘度范围内牛顿流体和非牛顿流体的功率特性;统一关联了牛顿流体与非牛顿流体的搅拌功耗,得到:N_PRe=C(l/D_f)~(-1)(V_L/V~*)~D式中的C、D是搅拌桨的结构参数.实验结果还表明,该装置的Metzner常数是转速的函数.     

非牛顿流体在搅拌槽中的传热(上)

早在1944年Chilton就发表了牛顿流体在搅拌槽中的传热关联式,但到1966年才有非牛顿流体在搅拌槽中的传热的报道。以牛顿流体在搅拌槽中的传热,非牛顿流体在搅拌槽中的流动及功率消耗,以及有关平板上和圆管中传热的边界层理论等三方面的研究为前导,二十年来非牛顿流体在搅拌槽中的传热研究获得了很大的进     

组合桨搅拌槽内非牛顿流体的气液分散特性

采用Brookfield黏度仪测量了质量分数为0.5%,1.0%,1.5%的羧甲基纤维素钠(CMC)水溶液在298.15 K的流变特性。在直径为0.48 m的平底搅拌槽中,对搅拌槽内不同的3层组合桨的气液二相的分散特性进行了实验研究,考察了非牛顿流体浓度、桨型组合、通气量和搅拌转速对通气功率、相对功率和气含率的影响规律。结果表明:CMC水溶液为假塑性流体,幂律方程能很好地关联其流变特性。在相同操作条件下,随着浓度的增加气含率减小,功率消耗增加;在相同的功率输入时,下压桨型组合的气含率更高。相对功率(PG/P)、气含率关联式等研究结果对非牛顿流体的气液搅拌器设计和操作具有一定参考价值。     

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。     

表面更新型搅拌釜中非牛顿流体脱挥特性的研究

探讨了高粘流体脱挥过程的容积传质系数,提出用氧探头放在气相测试的方法,建立并求解了测定模型,利用该法测定了双轴表面更新釜中采用三种不同型式的桨搅拌非牛顿流体时的脱挥性能,对结果作了关联,并对各种桨型的脱挥性能进行了评价。     

顺丁聚合釜给热系数的测定

参照国内顺丁橡胶的生产过程,以 CMC水溶液为模拟液,用非稳态法测定了非牛顿流体在搅拌釜内壁的给热系数,分别得到了偏框搅拌器和螺带搅拌器的给热系数关联式。     

Non-Isothermal mixing of rheologically complex fluids with close-clearance impellers: Effect of natural convection

Transient heat transfer in a mechanically agitated vessel is studied in the case of an anchor and an helical ribbon impeller using Newtonian and shear thinning fluids. Temperature stratification is found more pronounced with the anchor, making this impeller clearly inadequate for heat transfer. The impact of natural convection is evaluated first using the classical Gr/Re² ratio. It is shown that the use of this criterion in viscous mixing is somewhat misleading. A new Grashof number is then proposed to assess the significance of the viscous and buoyancy effects in non-isothermal, non-Newtonian mixing applications. It is shown that the interpretation of this new number is strongly related to the concept of process viscosity.     

非牛顿流体管板式换热器的传热特性

对管板式换热器在Re<2、270条件下中、高粘非牛顿流体传热进行了研究,并给出相应的关联式,传热数据处理采用修正Wilson算法。为了减少流动不均一分布对实验结果的影响,本文用测温法对换热器内流体流动分布进行了研究。     

LAMINAR BOUNDARY LAYER HEAT TRANSFER TO POWER-LAW FLUIDS

The heat transfer for the steady two-dimensional incompressible laminar boundary layer of power-law non-Newtonian fluids past a semi-infinite static or moving flat plate is studied by local similarity method. Four cases are considered: static plate with prescribed wall temperature and heat flux; and moving plate with prescribed wall temperature and heat flux. The effescts of the flow index, streamwise distance and modified Prandtl number on the temperature distribution and heat transfer rate are discussed in detail.     

LAMINAR BOUNDARY LAYER HEAT TRANSFER TO POWER-LAW FLUIDS

The heat transfer for the steady two-dimensional incompressible laminar boundary layer of power-law non-Newtonian fluids past a semi-infinite static or moving flat plate is studied by local similarity method. Four cases are considered: static plate with prescribed wall temperature and heat flux; and moving plate with prescribed wall temperature and heat flux. The effescts of the flow index, streamwise distance and modified Prandtl number on the temperature distribution and heat transfer rate are discussed in detail.     

Experimental determination of the optimal geometry of baffles for heat transfer in an agitated vessel

Experimental determination of the optimum values of geometrical parameters of baffles for heat transfer in a jacketed agitated vessel is the purpose of this research. The influence of the geometrical baffle parameters (number, width, length and distance between the lower edge of the baffle and the bottom of the vessel) upon both power consumption and heat transfer was examined for an agitated vessel with an inner diameter of 0.45 m. The experiments were performed with a disc turbine, a pitched-blade turbine and a propeller. An experimental design strategy was used in this research. The results of the experiments were approximated analytically. Based on the optimization criterion, an agitated vessel equipped with a disc turbine and optimally designed baffles is proposed as the most advantageous for heat transfer processes.     

Heat transfer similarities between impinging jets and axial-flow impellers

Similarities in the flow and heat transfer characteristics between simulation results of a round impinging jet and experimental data obtained on an agitated vessel with an axial flow impeller are presented in this paper. The electrodiffusion method was used in measuring the local heat transfer coefficients on a flat bottom of an agitated vessel. A small axial impeller has been built to provide a clearly defined flow imitating a submerged confined jet impinging the vessel bottom. Our simulation and experimental results show that the flow pattern in the impinging jet region below the axial flow impeller can be compared to the stagnation region of a round impinging jet with a corresponding tangential velocity component. CFD simulations of an impinging jet showed also the importance of different boundary conditions on small electrodes used with the electrodiffusion method and give an approximation of necessary correction factor.     

Friction and heat and mass transfer for turbulent pseudoplastic non-newtonian fluid flows in rough pipes

Friction factor and heat and mass transfer coefficient correlations are developed for turbulent pseudoplastic (purely viscous or inelastic) non-Newtonian fluids flowing through rough pipes. A correlation for friction factor which is applicable in the region of transition from the smooth to the fully-rough regime is proposed. It is obtained by adding the two correlations for the limiting situations, i.e. smooth pipes and fully-rough pipes. The predictions of the proposed correlation are compared with published experimental data and other previous correlations. A model for heat and mass transfer is developed on the basis of the Levich three-zone model. It is found that the proposed model is capable of predicting satisfactorily heat and mass transfer coefficients for Newtonian and pseudoplastic non-Newtonian fluid flow in rough pipes.     

COMBINED FORCED AND FREE CONVECTION IN BOUNDARY LAYER FLOW OF NON-NEWTONIAN FLUID ON A HORIZONTAL PLATE

An analysis is presented to study the effects of buoyancy-induced streamwise pressure gradients on laminar forced convection heat transfer to non-Newtonian fluids from a horizontal plate. Numerical solution of the transformed boundary layer equations has been carried out for different values of the flow behavior index and the buoyancy parameter by means of the local nonsimilarity method. A discussion is provided for the effect of the buoyancy force on the friction factor, the dimensionless heat transfer rate and the details of the velocity as well as temperature fields.