Optimum dimensions of longitudinal rectangular fins and cylindrical pin fins with variable heat transfer coefficient

Considering a temperature dependent heat transfer coefficient, the optimum dimensions of longitudinal rectangular fins and cylindrical pin fins have been investigated analytically. Employing the parameters of fin thickness or fin diameter and tip temperatures, the optimum dimensions, heat dissipation, and efficiency of fins are obtained by Lagrange's multiplier method. It is shown that the fin efficiency of an optimum cylindrical pin fin is higher than that of an optimum rectangular fin. To facilitate the thermal design of the heat transfer components, very simple mathematical expressions for the optimum dimensions of fins are presented in this study.     

非热平衡多孔介质内反应与传热传质耦合过程

采用局部热不平衡假设,对发生强吸热化学反应的多孔介质体系建立了反应与传热、传质耦合问题的数学模型,采用Ergun-Forchheimer-Brinkman方程描述多孔介质中的流体流动.运用交替方向隐式(ADI)方法对模型离散求解,并采用文献中的实验数据对模型进行验证.计算了不同条件下颗粒物料层内气体和固体骨架的温度场、产物气体浓度场以及固体转化率分布,以得到多孔介质体系内固有化学反应时的传热、传质规律.结果表明,不能忽略固体骨架与流体间的温度差.入口渗流速度、入口气体温度以及固体颗粒尺寸是影响系统反应特性的重要参数.研究结果对具有强吸热反应的固定床反应器的设计和运行具有一定的参考作用.     

基于POD降阶模型的正弦波翅片扁管管翅式换热器流动与传热特性分析

为了提高正弦波翅片扁管管翅式换热器的数值设计效率,以FVM (finite volume method,有限体积法)的计算结果作为样本数据构建POD降阶模型,研究换热器空气侧的流动与传热特性。结果表明：单参数和双参数变化时POD降阶模型重构的温度场与FVM计算结果的偏差集中表现在正弦波翅片表面区域,且沿主流方向呈减小趋势,2种方法所得速度分布的差异主要集中在主流区域。随着变量参数个数的增多,POD降阶模型的计算精度及计算效率均有所降低。POD降阶模型重构的物理场与FVM结果间的平均相对偏差最大值为2.921%,平均计算速度比传统FVM计算速度最大提高了11752倍。     

NATURAL CONVECTION FROM A RADIATING FIN IN AIR—A CONJUGATE PROBLEM

The problem of natural convection from a thermally radiating fin to air is solved as a conjugate phenomenon, coupling the relevant consititutive equations. Integral method of analysis is employed to solve the non-linear thermal problem. From the results, it is observed that the process of heat dissipation from the lateral face of the fin is dependent on three dimensionless parameters viz., the temperature ratio term Ψ_T', the conduction-convection-radiation parameter Ψ_T' and the fin parameter At. Results are discussed to bring out the influence of these parameters. The efficiency characteristics presented in the analysis can be employed in the design of a fin taking into consideration the actual working conditions. Explicit expression is provided to calculate the average Nusselt number     

Research for Optimizing Porosity of Porous Thermal-insulating Materials

With the energy crisis and ecological environment deterioration, porous thermal-insulating materials become an advanced research hotspot,and the influence of pore distribution cannot be ignored. The mathematical model is established basing on the heat transfer theory,regarding the minimum heat flux density as the objective function,the constant total porosity as a constraint condition,using the BFGS method to optimize the pore distribution. The results show that when the heat flux is the minimum,in the case of the fixed total porosity,the high temperature zone has high porosity,the low temperature zone has low porosity; the maximal fluctuating amplitude of porosity between the adjacent discrete points has great impact on the thermal insulating performance,the greater the fluctuating amplitude,the better the thermal insulating ability. After calculating the temperature field of the corresponding physical model, it can be found that the temperature gradient is non-uniform,the temperature gradient of the high temperature zone is steep,and that of the low temperature zone is gentle.These results have guiding significance for preparation of porous thermal-insulating materials.     

Pore-level numerical simulation of methane-air combustion in a simplified two-layer porous burner

A simplified two-dimensional model of two-layer porous burner based on pore level is developed.The heat transfer of solid phase in porous burner is seen as the synergistic effects of conduction through con-necting bridges and surface radiation between the solid particles in the model.A numerical simulation study on the characteristics of flow,combustion and heat transfer in the two-layer porous burner is car-ried out using the pore level model,and the effects of the control parameters such as the inlet velocity and solid thermal conductivity on thermal non-equilibrium are investigated.The results show that the flame structure is highly two-dimensional based on pore level.Obvious thermal non-equilibrium in the burner for the two phases and solid phase are observed,the largest temperature difference between the gas and solid phases is observed in combustion zone,while the temperature difference inside the solid particles is largest near the flame front.The results also reveal that thermal non-equilibrium of por-ous burner is much affected by the inlet velocity and solid thermal conductivity.     

Optimum designs of longitudinal fins

Employing fin parameter and tip temperature, a new concept with Lagrange's multiplier method is proposed to obtain the optimum width, height, heat dissipation, and efficiency of longitudinal fins for boiling heat transfer. A power-law type heat transfer coefficient is assumed in this analysis. Longitudinal fins of rectangular, convex parabolic, triangular, and concave parabolic profiles are first investigated. It is found that the optimum dimensions of a fin depends upon fin profile area, heat transfer coefficient at the fin base, and the thermal conductivity of the fin. Then, the least material fin profile is derived for each single heat transfer mode. To facilitate the thermal design of heat transfer components, simple mathematical expressions for the optimum dimensions of fins are presented in this study.     

Modelling and simulation of coal gasification process in fluidised bed

A one-dimensional steady state mathematical model and a numerical algorithm have been developed to simulate the coal gasification process in fluidised bed. The model incorporates two phases, the solid and the gas. The gaseous phase participates in the emulsion (with the solid phase) and forms the bubble. The solid phase is composed of carbonaceous material, limestone and/or inert bed material. The model can predict temperature, converted fraction, and particle size distribution for the solid phase. For the gaseous phase, in both emulsion and bubble, it can predict profiles of temperature, gas composition, velocities, and other fluid-dynamic parameters. In the feed zone, a Gaussian distribution for the solid particle size is considered. This distribution changes due to attrition, elutriation, consumption and drag inside the reactor. A system of 29 differential and 10 non-linear equations, derived from the mass, energy and momentum balances for each phase, at any point along the bed height, are solved by the Gear and Adams Method. Experimental data from the Universidad de Antioquia and Universidad Nacional-Medellin have been used to validate the model. Finally, the model can be used to optimise the gasification process by varying several parameters, such as excess of air, particle size distribution, coal type, and geometry of the reactor.     

非线性偏最小二乘法及可视化火焰检测系统的燃煤锅炉NOx排放特性的研究

为了研究燃煤电站锅炉NOx排放特性,利用可视化火焰检测系统对燃煤锅炉燃烧火焰和温度场进行了测试研究。通过测量,由双色测温法计算得到了炉内温度场,采用数字图像处理技术提取了火焰图像特征参数,进而借助非线性偏最小二乘法建立了由炉内燃烧温度场及火焰图像特征参数来预测NOx排放量的模型。结果表明,所建模型预测值与实测值具有一致性,从而为燃煤锅炉通过燃烧调整,以降低NOx排放和提高锅炉效率提供了有效手段。     

高温金属熔液在旋转多孔介质内的渗流传热过程

针对转动坐标系中铝熔液在SiC多孔介质内的流动传热现象 ,考虑离心力对渗流传热过程的影响 ,采用局部非热平衡假设建立多孔介质渗流传热数理模型 ,研究不同工况下流体的流速、压力损失及铝熔液和多孔介质的温度变化 .计算结果表明 :在渗透区域 ,液固两相存在温差 ,且液固温差随渗透界面的移动而减小 ;在非渗透区域 ,固体的温度曲线基本不变 .离心转速或孔隙率的增加都使渗透前沿区域液固两相温差增大 .孔隙率对流场和压力损失有较大影响.     

孔隙分布对多孔介质内流动和传热的影响

采用Sierpinski地毯分形技术建立多孔介质内流动和传热模型,通过改变固体基质位置研究了孔隙分布结构对多孔介质内流动特性和热效率的影响,3种孔隙分布为每分形一次固体基质分布在中心位置(A)、分布在中上方(B)和分布在右上方(C),当流体稳定流过多孔介质时,不同的孔隙分布表现出不同流动和传热特性. 结果表明,孔隙分布是影响多孔介质传输特性和传热效率的重要因素,无量纲渗透率k*C>k*B>k*A,多孔介质孔隙率大于0.8时更明显;流体流过不同孔隙分布的多孔介质时,相同孔隙率时与流体接触的固体基质面积A>B>C,传热效果A最佳、C最差. 孔隙分布影响了无量纲局部熵产率,在3种孔隙分布下用Be表示的热传导引起的熵产率占主导.     

超声波辅助提取红阳猕猴桃花黄酮的工艺研究

为了优化猕猴桃花黄酮的提取工艺,采用超声波辅助提取猕猴桃花中的黄酮,在液料比、乙醇浓度、时间、温度和提取次数单因素实验基础上,进行了正交实验优化工艺参数.研究结果表明：最佳提取工艺条件为∶液料比25∶1,乙醇浓度60％,温度60℃,超声时间为40 min,在最优条件下红阳猕猴桃花黄酮提取率为2.10％.采用常规的回流加热提取法黄酮提取率仅为1.82％.     

一种扁管管翅式换热器的高效数值设计方法

传统的数值模拟方法计算量大,计算时间长,很难满足现代工业发展的需求。以扁管管翅式换热器为例,采用适体坐标与最佳正交分解（POD）相结合的方法构建了低阶模型,在等热流边界条件下对扁管管翅式换热器中流动与传热过程进行计算,并将POD计算结果与有限体积法（FVM）计算结果进行了对比。结果表明：POD方法能准确地捕捉到不同数量参数变化情况下的温度场及速度场信息。对于3变量工况重构速度场及温度场的相对偏差平均值的最大值分别为1.90%、0.308%。采用POD方法在保证计算精度的前提下将FVM计算速度最大能提高3093.4倍。研究对于提高扁管管翅式换热器数值设计效率、拓展POD方法的工程应用领域有一定的理论参考价值。     

水冷壁气化炉温度分布及影响因素分析

对气流床水冷壁气化炉炉壁的温度分布及影响因素进行了计算和分析。建立了气流床水冷壁气化炉的三维传热数学模型,运用有限元方法,计算出水冷壁的温度分布,炉壁导热主要通过渣钉完成;探讨了水冷管内工质传热系数、炉内温度、渣钉间距、鳍片厚度、鳍片宽度及熔渣厚度对渣钉、水冷管、鳍片端部和根部等关键部位的最高温度点温度的影响;为了验证热分析的正确性,在实验室的小型气化炉上进行试验,鳍端背火侧温度计算值与实验测量值吻合良好,误差在5%之内;其计算模型和分析结果可为水冷壁气化炉的设计提供相应依据。     

Temperature of post‐flashover compartment fires—Calculations and validation

This paper describes and validates by comparisons with tests a one‐zone model for computing temperature of fully developed compartment fires. Like other similar models, the model is based on an analysis of the energy and mass balance assuming combustion being limited by the availability of oxygen, ie, a ventilation‐controlled compartment fire. However, the mathematical solution techniques in this model have been altered. To this end, a maximum fire temperature has been defined depending on combustion efficiency and opening heights only. This temperature together with well‐defined fire compartment parameters was then used as a fictitious thermal boundary condition of the surrounding structure. The temperature of that structure could then be calculated with various numerical and analytical methods as a matter of choice, and the fire temperature could be identified as a weighted average between the maximum fire temperature and the calculated surface temperature of the surrounding structure as a function of time. It is demonstrated that the model can be used to predict fire temperatures in compartments with boundaries of semi‐infinitely thick structures as well as with boundaries of insulated and noninsulated steel sheets where the entire heat capacity of the surrounding structure is assumed to be concentrated to the steel core. With these assumptions, fire temperatures could be calculated with spreadsheet calculation methods. For more advanced problems, a general finite element solid temperature calculation code was used to calculate the temperature in the boundary structure. With this code, it is possible to analyze surrounding structures of various kinds, for example, structures comprising several materials with properties varying with temperature as well as voids. The validation experiments were accurately defined and surveyed. In all the tests, a propane diffusion burner was used as the only fire source. Temperatures were measured with thermocouples and plate thermometers at several positions.     

Equation of state of polytetrafluoroethylene for calculating shock compression parameters at megabar pressures

Semi-empirical equations of state (thermal and caloric) are obtained to calculate not only the kinematic parameters (shock wave velocity, particle velocity, and reverberation of waves) but also the thermodynamic parameters (temperature, pressure, and compression) of monolithic and porous polytetrafluoroethylene at high shock pressures. The equations of state are used to model wave interaction in shock-wave experiments using the developed hydrocode. The equations are verified by comparison simulation results with published results of experiments and the data of our shock compression tests of solid and porous samples of PTFE in the range of 10–170 GPa.     

LTNE条件下界面对流传热系数对部分填充多孔介质通道传热特性的影响

在多孔介质区考虑局部非热平衡,采用Brinkman-extended Darcy模型结合应力跳跃条件对部分填充多孔介质通道内流体传热特性进行分析。获得了各区域温度分布及Nusselt数解析解,并分析了各参数对温度及Nusselt数的影响。结果表明：界面对流传热系数H_s较小时,界面应力跳跃系数β和Darcy数Da的增加会减小流固两相间温差。而在高H_s下,Da减小也会减小两相温差。在Da、H_s和固流两相热导率之比K较大且空心率S（自由流体区高度与通道高度之比）和Biot数Bi较小时,流固两相间会在接近多孔介质区中部出现最大温差,而该最大温差会随着S增加和Da及H_s的减小向界面区移动。对于不同K与Bi,Nusselt数Nu与S的关系曲线存在不同的类型,与模型A（界面处多孔介质固相和流相根据各自温度梯度和热导率划分总热流）不同的是,采用模型C（界面处固相热流分配与自由流体区流相的热交换相关）所获得的Nu曲线类型与H_s有关。在K较小时,β对Nu的影响大于H_s对Nu的影响;而在K较大时,H_s对Nu的影响要远大于β对Nu的影响,且H_s增加会明显提高通道内的Nu。     

Modelling and simulation of a continuous fluidized-bed dryer

A fairly rigorous mechanistic model of a continuous fluidized-bed dryer has been developed. It depicts the dynamic interactions between gaseous and solid phases in detail. The performance of the dryer has been simulated numerically based on the model. The effects of the operating parameters on the performance characteristics of the dryer have been investigated. These parameters include the superficial gas velocity, the inlet temperature of the drying gas, the mean residence time of solids and the dryer-wall temperature. The results of simulation based on the present model are compared with those based on an existing model. This comparison shows that the former is a substantial improvement over the latter.     

Condensation on vertical porous metal fins

The problem of condensation of vapors on a porous plate fin is formulated and the dimensionless heat transfer coefficients are found to be dependent on two system variables, viz., the fin and the porosity parameters. Numerical simulations indicated that the condensation heat transfer coefficients for a porous fin are larger than those for a non-porous one of similar cross-section. For design purposes, a dimensionless equation for predicting the mean condensation heat transfer coefficient is proposed. The significant increase in the condensation heat transfer coefficient suggests that the condenser section of a flat plate heat pipe can be made more efficient in operation with the use of porous plate fins instead of impermeable fins of similar cross-section.     

Modelling of non-catalytic gas—solid reactions—I. Transient analysis of the particle—pellet model

A transient analysis of the problem of non-catalytic gas—solid reaction based on the particle—pellet model, which considers the particulate nature of the pellet and includes the external transport resistances, is presented. The method of solution of the resulting non-linear partial differential equations is based on the orthogonal collocation technique. The transient model has been compared with the pseudo-steady analysis of the problem. The effect of various parameters on the temperature profiles in the pellet and the conversion of the solid reactant has been discussed.