梭式窑对流换热的模拟研究

在梭式窑空气动力模型上，采用不同规模的烽嘴，研究了烧嘴的流量与喷速对窑内对流换热的影响。提出了换热不均匀度的定义式，推导了入窑气体温度与换热不均匀度的关系，得到了梭式窑对流换热关系式。     

可变化烧制空间体积梭式窑内温度场均匀性研究

结合现代自吸式烧嘴与动力式烧嘴梭式窑各自的优势,设计两种烧嘴结合的新型可变化烧制空间体积梭式窑。利用三维物理模型和稳态传热模型,采取控制烧嘴位置、烧嘴烟气流速等条件,基于ANSYS15.0软件,对窑炉内部温度场进行数值模拟分析。结果表明,窑内坯体表面温度差在8℃之内,窑内温度均匀性良好,是一种先进窑炉。     

梭式窑温度场影响因素的数值研究

为深入了解梭式窑内温度场分布情况,利用ANSYS软件,采用湍流标准动能一动能耗散率(k-ε)方程,对影响梭式窑温度均匀性的因素用正交试验法进行了有限元数值模拟研究,得出了温度场的分布特征。分析了影响温度不均匀分布的成因。结果表明:烧嘴和排烟口布置方式对梭式窑温度均匀性有重要影响,底烧和窑顶排烟相结合温差小,底烧和顶烧相结合有利于温度场的均匀。在气流减速或倒流的边角、死角区适当设置排烟口,可以引导气流流向,促进气流的循环,使气流在窑内的分布趋于均匀。而烟气喷速在高速情况下,提高速度对温度场的影响不明显,最佳喷速在100m/s左右。数值模拟结果与有关物理模型实验数据符合较好,并互为补充。     

梭式窑温度场影响因素的数值模拟

为深入了解梭式窑内温度场分布情况,利用ANSYS软件,采用湍流标准动能———动能耗散率(K-ε)方程,对影响梭式窑温度均匀性的因素用正交试验法进行了有限元数值模拟研究。得出了温度场的分布特征。分析了影响温度不均匀分布的成因。     

2m~3梭式窑高温段数值模拟对比性研究

通过建立相同尺寸为2 m~3的梭式窑模型,对传统梭式窑与全自动梭式窑进行数值模拟对比研究。结论是全自动梭式窑气体扰动明显比传统的梭式窑,大大提高高温烟气换热的效率,全自动温度梭式窑炉温度均匀性比传统梭式窑温度场的温度均匀性更好,更有利于陶瓷烧制的产品,为未来智能梭式窑生产提供了更好的理论基础。     

全氧燃烧玻璃熔窑火焰空间的数值模拟

采用基于k-e湍流模型、非预混化学反应模型、DO辐射传热模型的数值模拟方法,研究全氧燃烧玻璃熔窑火焰空间的温度场与流场分布。研究表明:火焰长度约为燃烧空间长度的2/3,其最高温度为2602℃,燃烧空间非火焰区平均温度为1290℃,辐射传热效率高。燃烧空间主要存在3个回流,其中流经火焰上下部空间的高温气体回流有效地与玻璃液面进行对流换热。在辐射与对流换热的综合作用下,玻璃液面上均匀分布5个高温区,玻璃液面平均温度约为1255℃,综合平均温差最大值约为5℃,温度分布非常均匀。烧嘴对面及烟道口附近的墙体温度约为1320℃,热冲侵蚀严重,其它燃烧空间的墙体温度约为1265℃。     

隧道窑烧成带对流传热模型研究

烧成带的气体流动和传热在隧道窑三带中最为复杂。本文认为:在一定的窑型和料垛码法时影响烧成带对流传热的主要因素是:来自两侧烧嘴的横向射流和由他们汇集而成流往预热带方向的主流.本文根据相似论原理,利用空气动力模型,采取局部模拟方法,研究了隧道窑烧成带的对流传热.研究结果指出:1.相对安排烧嘴时,描述隧道窑烧成带对流传热的准数方程式是 Nu=4.160Re~(0.110)Reo~(0.254),烧成带对流传热受主流影响较弱,受烧嘴射流影响较强.烧成带对流传热方程式完全不同于预热带和冷却带的情况.2.一般情况下,交错安排烧嘴可提高烧成带内平均对流传热,高速烧嘴的这种作用更明显。一般隧道窑烧成带使用高速烧嘴不宜相对安排,应交错安排。     

碳化硅晶体生长炉辐射换热特性的数值模拟

采用C语言搭建了碳化硅(SiC)晶体生长炉三维温度场数值模拟平台,基于柱坐标系构建生长炉物理模型,采用有限体积法离散数学模型,利用S2S(Surface to Surface)辐射模型考察了生长室内的辐射换热特性,提出判断辐射面可视性的最短距离法. 模拟了电流强度1250 A、电流频率16 kHz条件下的生长炉温度场,定量揭示了生长室内的辐射换热强度;采用标准偏差法研究了线圈结构对晶体内部温度及温度梯度均匀性的影响. 结果表明,螺旋电磁加热线圈容易导致生长炉内部温度场呈非轴对称特性分布;辐射热流较导热热流大102~103倍,辐射换热促使生长室内温度分布均匀;螺旋线圈的布置方式使晶体截面温度呈非轴对称分布,造成温度梯度的均匀性变差,晶体生长过程中容易产生热应力,影响晶体质量.     

300Mw机组容量脱硫喷淋塔内部流场模拟研究

采用Fluent软件对300 Mw机组容量的脱硫喷淋塔的内部流场进行了数值模拟,使用Gambit软件对该模型采用非结构化四面体网格进行网格划分。研究了烟气的不同的入口形式,不同的入口角度对流场的影响。结果表明:120°和180°双入口烟道,入口角度为15°斜向下时,具有较好的流场分布。     

永磁梯度磁场布置方式对空气自然对流换热的影响

梯度磁场可用于控制非导电弱磁性介质的自然对流换热过程。利用钕-铁-硼永磁系统的不同空间布置，构建了具有不同磁场强度分布的梯度磁场，通过数值模拟得到了不同永磁梯度磁场的磁场强度和磁加速度。对不同梯度磁场作用下的二维封闭腔内的空气自然对流换热过程进行了数值研究，获得了空气自然对流的流场和温度场，以及壁面局部Nusselt数和平均Nusselt数并进行了比较。结果表明：空气自然对流换热可以通过施加具有不同磁加速度的梯度磁场得到强化或控制。     

Convective heat transfer in a rotary kiln

An experimental study of convective heat transfer from hot air to the solid charge and walls in a non-fired rotary kiln is reported. Ottawa sand was heated by passing it counter-current to a flow of preheated air in a 2.5 m × 0.19 m I.D. rotary kiln. Axial temperature profiles of gas, wall and solids were measured. Local and average convective heat transfer coefficients from gas to solids and from gas to wall were determined assuming plug flow of gas and solids. Solid feed rates to 1750 kg/m² h and air rates to 3300 kg/m² h were investigated at rotational speeds to 6 r/min, holdup ratios to 17% and gas temperatures from 350–590 K. The gas/solids convective coefficient was found to depend on the gas through-put and to a lesser extent on solids holdup and rotational speed. Over the range tested, the angle of kiln inclination, solids throughput and particle size showed no significant effect on heat transfer. Gas/wall coefficients were about a factor of ten below gas/solid coefficients. Heat transfer results are compared to the limited data available in the literature, and to commonly used equations. Correlations of the experimental data on gas/solids, and gas/wall coefficients are presented; data from the literature on the wall/solids heat transfer coefficient are summarized.     

水平管油气两相段塞流及其传热特性

海底油气管道的冷却传热过程是结蜡、水合物等海洋石油工业流动保障问题的关键控制因素。采用电容探针与热电偶、热电阻等流动及温度测量手段对不同冷却条件下空气-油段塞流的流动参数和传热参数进行实验测量,分析了空气-油段塞流流动参数对传热特性的影响,并与空气-水对流换热进行对比。结果表明,空气-油段塞流对流传热系数主要受液相折算速度的影响,且冷却液温度越低,管底热流体黏度越大,导致热边界层越厚,传热系数降低;受黏性力及边界层影响,对流传热系数远小于空气-水;沿管壁周向,从管顶到管底的对流传热系数不断增大。提出了适用于冷却条件下的油气段塞流传热关联式和传热模型。     

粗糙肋面上湍流热量传递中场协同关系的数值分析

用数值模拟的方法将湍流流动的传热分为传热层流底层和湍流层,来考察传热层流底层中温度梯度矢量与速度夹角对湍流流动换热的影响,用场协同理论分析了湍流条件下粗糙肋面的传热强化问题.     

卧式螺旋管内紊流传热

在宽广的参数范围内对卧式螺旋管内紊流换热特性进行了详细的试验研究, 试验结果表明,螺旋管内强化换热是紊流流动与二次流动共同作用的结果,Reynolds数越大,二次流对强化换热的贡献越小,平均换热系数也越接近于等条件下的直管换热;换热系数在圆截面上的分布很不均匀,最外侧换热系数可达最内侧的3～4倍,最内侧是平均换热系数的0.5倍,并得到了截面上局部换热系数的计算式;换热系数在流动方向的分布具有周期性。     

HEAT TRANSFER IN A SUBCOOLED WATER FILM FALLING ACROSS A HORIZONTAL HEATED TUBE

A numerical simulation and an experimental study were carried out for sensible heat transfer for a subcooled water film falling across a horizontal heated tube. A laminar model and a turbulent model were adopted to calculate the heat transfer coefficient. The whole wetting zone on the tube surface was divided into two zones: the top stagnation zone and the lateral free film flow zone. The initial boundary conditions for the free film flow zone were determined by calculating the fluid and the temperature fields in the stagnation zone. A modified wall function method was used for the turbulent model. The comparisons between the experimental data and the numerical solutions show that the experimental data agree reasonably well with the laminar model solutions. Finally, two simple correlations were proposed for predicting the convective heat transfer of a falling film for engineering applications.     

低质量流率蒸汽真空水平管内凝结传热特性的实验研究

对换热长度为3.4 m、内径为38 mm的真空水平管内的蒸汽凝结流动换热特性进行了实验研究。分析了蒸汽质量流率小于9 kg/(m²·s),蒸汽饱和温度为50、60和70℃,换热温差为3~7℃时对凝结过程的影响。通过对分层流动冷凝换热机理分析,建立了热分区角计算模型。实验结果表明,热分区角随着质量流率的增加而增加,随着传热温差的增大而增大;饱和温度对管内凝结的局部传热系数和热分区角影响较小。通过以热分区角为分区界限,建立了局部传热系数经验关联式,在预测实验工况下,对于管顶部膜状冷凝区,预测精度在±25%以内;对于管底部冷凝液对流换热区,预测精度在+25%~-35%。     

回转窑传热模型与数值模拟

对回转窑内传热过程进行了分析 ,在已有研究基础上归纳了各项换热系数的关联式 ,尤其结合热渗透模型完善了回转壁面与料床之间传热机制的研究 ,并提出通用的计算关联式。进而提出了内热式炉型的一维轴向传热模型 ,并根据已发表的试验数据验证了该模型和各换热系数的适用性。计算表明在窑内低温段 ,物料的受热主要来自其覆盖的回转壁面对其加热 ;而在高温段 ,气体的辐射热量成为加热料床的主要热源。此外 ,由物料进口端沿轴向窑壁散热增大 ,在窑内高温段窑壁的散热甚至高于物料吸热量 ,因此在回转反应器的设计中应充分考虑窑壁散热造成的热效率降低并采取相应措施。     

辊道窑不同辊下高度气体流场与温度场的数值模拟

利用CFD软件Fluent对不同辊下高度的辊道窑烧成带的气体流场和温度场进行了模拟研究,得出了不同辊下高度的窑内气体流场和温度场的分布,分析了流场对窑内气体温度场和对流换热的影响。     

The Role of Nonuniformity in Convective Heat and Mass Transfer through Porous Media,Part 1

Through-air drying is commonly used in the drying of high-quality tissue and towel products. A representative elementary volume method was used to model the fluid flow and heat and mass transfer during through drying in heterogeneous porous biobased materials such as tissue and towel products. Results of flow both upstream and downstream of a modeled porous sheet allowed visualization of the effects of mixing at the top and bottom of the porous medium. The effect of initial nonuniformity on fluid flow and convective heat and mass transfer in heterogeneous porous media was studied. The effect of material nonhomogeneity and associated transport properties on moisture content of the porous material as a function of drying time was studied. Modeling results indicate that for the first time it is possible to simulate the effect of nonuniformity on fluid flow and convective heat and mass transfer in porous media during through-air drying of paper. Moisture and structural nonuniformity contributing to nonuniformity in air flow might contribute significantly to drying nonuniformity. Depending on the moisture regimes and degree of saturation of the convective medium, heat and mass transfer coefficients may have varying effects on the overall drying.     

Modelling the Rotary Lime Kiln

A three‐dimensional steady‐state model to predict the flow and heat transfer in a rotary lime kiln is presented. All important phenomena are considered for the pre‐heat and calcination zones including turbulent gas flow, buoyancy, all modes of heat transfer, evolution and combustion of species and granular bed motion with calcination reaction. The model is based on a global solution of three sub‐models for the hot flow, the bed and the rotating wall/refractories. Information exchange between the models results in a fully coupled 3‐D solution of a rotating lime kiln. The overall model is validated using UBC's pilot kiln trials (5.5 m laboratory kiln). Results for this case are presented and potential implications are discussed.