Effect of ΔT- and Spatially Varying Heat Transfer Coefficient on Thermal Stress Resistance of Brittle Ceramics Measured by the Quenching Method

A study was conducted of the effect of a spatial variation in heat transfer coefficient during forced convection and the effect of temperature dependence of the heat transfer coefficient during free convection on the magnitude of thermal stress encountered during the thermal shock testing of brittle ceramics by the quenching method. For specimens with circular geometry, the results obtained by the finite element method indicate that, depending on the value of the Biot number, a factor of three spatial variation in heat transfer coefficient changes the tensile thermal stresses on cooling by a maximum of about 17% over the value for spatially uniform heat transfer. On heating, the corresponding tensile thermal stresses are lower by a maximum of about 6%. In free convection for a heat transfer coefficient, h , proportional to Δ Ti^1/4 , where Δ T_i is the instantaneous temperature difference between the specimen and quenching medium, the stresses are appreciably less than those for Δ T_i -independent heat transfer for the same initial value of h at the onset of the thermal quench. The validity of the dependence of h on Δ T_i was established experimentally. The relevance of this result to the interpretation of thermal quenching studies is discussed.     

动力电池热管冷却效果实验

基于对动力电池在大负荷运行模式下的产热行为分析,组装了一种采用热管式冷却的电池热管理系统。在空气自然对流、强制对流和热管冷却3种模式下,研究了电池模块中各电池在放电过程的温度变化趋势。结果表明,镍氢MH-Ni动力电池采用热管式冷却方式具有良好的冷却效果,可确保电池在最佳的工作温度范围内运行,并且具有拉平电池温度的能力。与自然冷却及强制对流冷却相比,在3728 mA电流放电情况下,热管冷却方式使电池温升最多可以降低10 ℃左右;持续放电8 min后电池温度也不超过43℃。     

Optimization of Fischer‐Tropsch Process in a Fixed‐Bed Reactor Using Non‐uniform Catalysts

Optimization of Fischer‐Tropsch (FT) process in a fixed‐bed reactor is carried out using non‐uniform catalysts. The C₅⁺ yield of the reactions is maximized utilizing a combination of non‐uniform catalysts across the bed. A 1D heterogeneous model is developed to simulate the bed containing uniform and non‐uniform catalysts. It is found that the egg‐shell and surface‐layered catalysts result in higher C₅⁺ yield. Moreover, effects of cooling temperature are studied. Genetic Algorithm (GA) and Successive Quadratic Programming (SQP) methods are applied. Feed and cooling temperature are selected as decision variables together with distribution of non‐uniform catalysts along the bed. The optimization result shows 14.47 % increase in the C₅⁺ yield with respect to the base condition.     

Modeling mass transfer in a Taylor flow regime through microchannels using a three-layer model

The process of convection and diffusion in a Taylor regime of gas–liquid flow through microchannels is modeled based on a three-layer mass transfer model in the axisymmetric formulation. The circulation circuit of Taylor vortices is described in the form of outer and inner layers surrounded by a thin film that does not participate in circulation. Due to the assumption about the predominance of convection over radial diffusion in the inner layer (the Peclet number order of magnitude is nearly 10⁵) and a uniform concentration distribution over the cross sections of each layer, the problem is confined to a quasi-one-dimensional problem with boundary conditions of special type. The obtained numerical solutions allow us to determine the kinetics of mass transfer from a liquid to a channel wall in detail, calculate the average mass-transfer coefficient, and reveal optima in the dependences of the mass-transfer coefficient on the two-phase flow velocity and the capillary diameter; furthermore, there is also an optimum for the length of a liquid slug. The obtained results allow us to understand the pattern of Taylor convection and diffusion fluxes and to reveal the reasons for a decrease in mass fluxes under nonoptimal conditions. This enables the correction of selected geometric and process parameters during the design of microreactor equipment.     

内设/不设中心管两股催化剂混合预提升结构内的颗粒浓度分布

采用光纤探针分别考察了内设与不设中心管两种预提升结构内FCC催化剂颗粒浓度的分布,并引入径向不均匀指数RNI(ri)对颗粒浓度径向分布的不均匀性进行定量分析. 结果表明,在扩径区(中心管区),内设中心管的预提升结构内颗粒浓度梯度较大;而在提升管入口区,内设中心管的预提升结构内颗粒浓度分布更均匀;相比传统结构,无中心管的提升管入口区内RNI(ri)约减小65%,内设中心管时约减小75%;给出了内设中心管的预提升结构内平均固含率轴向分布的经验模型,计算值与实验值吻合较好,可供工程设计参考.     

ANALYSIS OF CONVECTIVE HEAT TRANSFER DURING IMMERSION FRYING

ABSTRACT

Immersion frying was studied as a high temperature drying process and analogies drawn between periods found in drying and similar periods in immersion frying. Analysis of external heat transfer during immersion frying showed a highly complex system of free and forced convection augmented by boiling conditions. Oil flow was found to be driven downward by buoyancy forces due to cooling at the sample surface then upward due to entrainment in vapor bubbles during boiling conditions. Experimental work showed the convective heat transfer coefficient to be a dynamic property ranging from 300 to 1100 W/m²°C and to be strongly coupled with bulk movement of the oil. Heat flux measurements found a peak flux of nearly 30,000 W/m³. Based on analysis of bubble dynamics it is hypothesized that heat flux increases with increased oil degradation through a reduction in vapor bubble size and increase in bubble frequency due to changes in interfacial properties of the oil.     

Residence time distribution in a packed bed bioreactor containing porous glass particles: Influence of the presence of immobilized cells

An experimental investigation of the liquid phase residence time distribution (RTD) in a packed bed bioreactor containing porous glass particles is presented. For Re < 1, intraparticle forced convection is negligible and only diffusion, characterized by an effective diffusion coefficient, must be considered to describe the mass transfer process between the extraparticle and the intraparticle fluid phase. For Re > 1, the mass transfer rate becomes dependent on the liquid flow rate, indicating the existence of intraparticle convection. A model including axially dispersed flow for the external fluid phase and an ‘apparent’ effective diffusivity that combines diffusion and convection, predicts experimental RTD data satisfactorily. Yeast cells immobilized inside the porous glass beads did not affect the mass transfer rate at low biomass loading. At high biomass loading (0·02 g yeast cells g^?1 carrier), the mass transfer rate between the extraparticle and intraparticle fluid phase was significantly decreased. Comparison of the RTD data from experimets performed in the presence and absence of cells in the external fluid phase revealed that the mass transfer rate is influenced by the cells immobilized inside the porous particles and not by the cells present in the external fluid phase.     

CHARACTERISTICS OF HEAT TRANSFER FROM A SPHERICAL SOLID BODY UNDER PULSATING FORCED CONVECTION

Heat transfer from a solid sphere having a Biot number Bi ? 0.1 to a flow medium in a cooling process under pulsating forced convection is experimentally examined. In the experiment, two kinds of pulse modes, which are the type of pulsating forced convection with continuous feed and the feed type mixing forced and natural convection, are considered as a pulse feed technique. Initially, the evaluation method of the Nusselt number Nu is derived in the cooling process and the validity of that method is verified by comparison with the experimental results. With regard to the effect of pulsating feed, it is shown that the pulsating feed conditions have a great influence on the characteristics of the heat transfer. The enhancement of heat transfer under the condition of the forced convection pulsatively fed with continuous feed is recognized, and the characteristics of heat transfer for the pulsating feed including natural convective heat transfer region are aggravated. Besides, it is clear that the decrease in the heat transfer characteristics is dependent on the duration of the natural convective heat transfer. Furthermore, Ranz-Marshall correlations for each pulsating feed condition are presented.     

内设中心管的两股催化剂混合预提升管内颗粒速度的分布

采用PV-6D光纤速度仪考察了内设/不设中心管两种催化裂化提升管预提升段内FCC催化剂颗粒速度沿轴、径向的分布,引入径向不均匀指数RNI对颗粒速度的径向不均匀性进行定量分析,并与传统结构内的RNI进行了对比. 结果表明,设中心管的提升段内颗粒速度沿轴向分布更均匀,两种结构内颗粒速度沿径向分布均匀程度扩径区(中心管区)设中心管结构<无中心管结构,底部区及提升管入口区设中心管结构>无中心管结构;相比传统结构,无中心管提升段内RNI减小40%,设中心管减小60%. 给出了设中心管提升段内截面平均颗粒速度轴向分布的经验模型,计算值与实验值吻合较好,可供工程设计参考.     

Numerical analysis of convective heat transfer characteristics of supercritical hydrocarbon fuel in cooling panel with local flow blockage structure

The convection heat transfer of hydrocarbon fuel at supercritical pressure has a great influence on the regenerative cooling technology of a scramjet engine. A three-dimensional numerical simulation was conducted for the convection transfer of hydrocarbon fuel in the cooling panel of a combustion chamber wall. And the flow field around the local flow blockage structure and the outlet flow rate distribution characteristics of fuel in the cooling channels were analyzed in detail. The results of analyses indicate that with the optimized local flow blockage structure, the outlet flow rate distribution of fuel among the cooling channels become more uniform, as the area of local flow dead zone decreases. However, as the fuel temperature increases, the dramatic variation of thermodynamic physical properties of fuel has a strong influence on the flow field around the local flow blockage structure. Especially, a local flow dead zone can be easily formed in the supercritical temperature region. Meanwhile, transverse pressure gradient around the throat region of blockage structure and additional loss, which is caused by turbulence fluctuation and energy exchange of fluid in the downstream area, affect the outlet flow rate distribution of fuel among the coolant passages seriously. It can therefore be concluded that the local flow blockage structure is more suitably designed in the subcritical temperature region by taking above-mentioned factors into consideration.     

Hydrodynamic studies in natural and forced convection electrowinning cells

Horizontal and vertical mapping of the cathode mass transfer coefficient was used to compare the hydrodynamics of full height model electrowinning cells employing either natural convection or two methods of forced convection. The most sgnificant improvement in performance was achieved with uniform agitation resulting from sparging gas over the cathode surface.     

Prandtl number effect on combined free and forced laminar convection in the thermal entrance region of a horizontal tube

This note presents the Prandtl number effect on combined free and forced laminar convection in the thermal entrance region of a horizontal tube by a numerical vorticity-velocity method. The tube wall is heated with a uniform wall heat flux. Vorticity-velocity governing equations in cylindrical coordinates and an evaluation of boundary vorticity on the tube wall are presented along with a numerical method of solution. Local Nusselt number variations are shown for Pr = 100 and 10 and compared with the existing data for Pr → ∞. It is observed that the large Prandtl number assumption is valid for Pr = 10 within a difference of 4% at Ra = 1 × 10⁵. The numerical results are compared against the available experimental data with good agreement.     

Simultaneous heat and mass transfer through laminar boundary layers in combined forced and free convection

Heat and mass transfer rates from a vertical plate are reported for combined free and forced convection. Heat transfer coefficients were obtained from temperature gradients at the wall and mass transfer rates from thickness decrease measurements of naphthalene coatings. The free convection data cover a Grashof number range from 100 to 2 × 10⁸ and the combined flow parameters Gr/Re² and Gr'/Re² vary from 0.2 to 100 and 2 × 10^?8 to 3.25, respectively. Predictions from boundary layer solutions are in good agreement with the data. Absorption of ultraviolet light by naphthalene vapor was used to measure free convection concentration profiles. Results prove the versatility of this method.     

Modelling of crystallization process and optimization of the cooling strategy

To obtain a uniform and large crystal in seeded batch cooling crystallization, the cooling strategy is very important. In this study, an optimal cooling strategy is obtained through simulation and compared to linear and natural cooling strategies. A model for a crystallization process in a batch reactor is constructed by using population balance equation and material balance for solution concentration, and a prediction model for meta-stable limit is formulated by the dynamic meta-stable limit approach. Based on this model, an optimal cooling strategy is obtained using genetic algorithm with the objective function of minimizing the unwanted nucleation and maximizing the crystal growth rate. From the simulation results, the product from the optimal cooling strategy showed uniform and large crystal size distribution while products from the other two strategies contained significant amount of fine particles.     

Investigation of airflow and particle behavior around a subway train running in the underground tunnel

Airflow around an eight-passenger-car subway train running in the underground tunnel at a cruise speed of 70 km/h was numerically simulated, and the trajectories of the particles that were assumed to be re-suspended from the ground or generated at the contact points between the wheels and rails were predicted. In addition, field experiments were conducted to measure airflow velocity and PM₁₀ mass concentration under a T-car (trailer car without a driving cab) during the running of a subway train in straight sections of the underground tunnel of the Seoul Subway Line 5. The numerically predicted airflow velocities agreed well with the experimental data with the error of less than 30%, and the predicted particle distribution showed a similar tendency to the experimental results. The airflow under the T-car was predicted to be relatively uniform compared to the airflow under other passenger cars. Both numerical results and experimental data signified that a lot of particles could drift under the T-car by showing a higher particle concentration in the central area of the space under the T-car than in the edge area. As a result, the space underneath the T-car is anticipated to be a good place for installing a dust-removal system.

© 2016 American Association for Aerosol Research     

SLK分级机两种进风口的数值模拟与实验

在Fluent软件中选用标准的k-ε模型对分级机送料筒内气流分布进行模拟,得到两种不同进风口时的气流分布情况。分级机采用对侧进风口时,送料筒内气流运动属于对冲、挤压、压扁与转向的过程,气流分布不均。分级机采用环面进风口时,送料筒内气流运动是一个汇集、分散、转向的过程,气流分布相对均匀,有利于提高分级精度。实验研究中,保持喂料速度、系统风量、分级机转速一定的条件下,通过改变进风口面积,分别测试了两种风口在不同进口风速时的切割粒径、分级精度的变化。实验结果表明,随着风口面积的减小,进口风速增大,两种风口的分级机的分级粒径基本不变,分级精度先增大后减小。通过分级精度的对比,说明环面进风对于颗粒的分散性能较对侧面进风更为优越。     

热风循环隧道烘箱的流场模拟及结构优化

采用CFD数值模拟方法研究热风循环隧道烘箱的内部温度及流场分布,提出了6种结构优化方案,考察了不同方案下的气流分布特性。结果表明,烘箱出口面局部温差在1 K以内,温度分布较均匀;气流速度分布是影响烘干品质的主要因素,原始结构下出口气流分布极不均匀,出口面角落位置出现局部高速区,最大速度达3.48 m/s,平均速度为0.94 m/s,相对均方根值为0.73;相比原始结构,6种改进方案中,弯头三隔板结构出口面的相对均方根值最低,最大速度降低51.4%,最佳风速区域占比提升至75.17%,整流效果最明显;弯头内设置三块隔板后,风罩内再设置隔板对气流均匀性的改进作用不大。     

Intergranular Air Movement and Grain Drying in Silos with Fully,Partially and Slanted Perforated Floor

ABSTRACT

Intergranular air movement and its relation with moving drying fmnt in a grain bulk were investigated experimentally. An improved computer simulation model was developed to analyze the drying or cooling process in a bed of grain with non-parallel airflow from a partially or slanted perforated floor. The model first simulates the airflow, then calc lates the heat and mass transfer between air and grain along the airflow streamlines. The results were compared with the simulated and experimental results from a fully perforated floor configuration. The degree of agreement between the computed and the experimental moisture contents depended upon the floor configuration. The model was not capable of predicting the temperatures to the accuracy of the predicted moisture contents, especially when airflow stream lines were not parallel to each other. For further improvement of the drying simulation model, an improved airflow model and computer program is required. Comparison of the simulated results with experimental data also     

聚丙烯气体辅助注射成型冷却过程中温度分布的研究

通过对聚合物气体辅助注射成型冷却过程进行合理的假设与简化,对聚丙烯（PP）气体辅助注射成型冷却过程进行了实验与数值模拟研究。结果表明,注射氮气后,PP的冷却速度显著加快,并在气-熔界面处出现小范围的结晶平台。采用MATLAB软件对氮气辅助注射成型PP冷却过程中的温度分布进行了数值模拟,将计算所得模拟值与温度采集的实验值进行比较,发现在熔体降温阶段温度分布的模拟值与实验值吻合程度很高;在固相冷却阶段由于聚合物本身复杂特性以及气体的渗透效应,PP的模拟值略高于实验值,而氮气的模拟值低于实验值。     

THE GROWTH OF ICE DENDRITES UNDER MIXED CONVECTION CONDITIONS

Dendritic solidification of ice from a supercooled melt is studied under conditions which include thermal convection, forced convection, and the effect of the moving solid-liquid interface, which manifests itself as apparent convection. A theory based on Oseen rectilinear flow suggests that heat transfer from the tip of a growing dendrite can be described by a linear superposition of Peclet numbers due to the individual mechanisms of convection by forced bulk motion of the fluid and by the moving boundary which creates the apparent convection effect

This simple result was tested by plotting the experimental data over the full range available of rates of dendritic growth, in terms of the Nusselt number, as a function of the Peclet number, Pe^*, which is based on the tip radius and the sum of the forced velocity of the fluid and the velocity of the tip of the dendrite. This procedure clearly reveals three regions in which different mechanisms control the process: 1) Dendritic growth is dominated by apparent and natural convection when the Peclet number, Pe^*, is below about 2 10^-3 2) When Pe^* is equal to or greater than 10^-1, dendritic growth is dominated by forced convection. 3) Between these limits, the combined effects of apparent and natural convection compete for dominance with forced convection. The equation which correlates the data over the full range of operating conditions, including the mixed convection region is obtained by non linear superposition of the Nusselt number correlations for Pe^* less than 2 10^-3 and greater than 10^minus;1. The resulting Nusselt number represents the experimental data well over the full range studied.