闭式喷雾冷却的瞬态传热过程研究

搭建了闭式喷雾冷却实验台,实验研究了喷雾冷却的瞬态传热过程,获得了准确描述其传热过程的实验曲线,分析了冷却初始温度、加热功率及工质类型对瞬态传热过程的影响。研究表明:对于喷雾冷却的瞬态传热过程,其表面温度变化趋势可分为急速下降、持续升高、二段下降3类。初始表面温度在经历启动初期增强效应后,若小于莱登弗罗斯特点(LFP)对应的温度Tf,则表面温度不断下降,在核态沸腾区实现热平衡;反之,表面温度升高,在膜态沸腾区实现热平衡;恒定加热功率的大小决定了表面温度变化速率,随着恒定加热功率的增大,表面温度下降或者上升的速率加快;同等条件下,对于不同类型介质,喷嘴入口压力及饱和温度越高,其Tf也越高。     

Discrete thermal element modelling of heat conduction in particle systems: Pipe-network model and transient analysis

In our recent work [Y.T. Feng, K. Han, C.F. Li, D.R.J. Owen. Discrete thermal element modelling of heat conduction in particle systems: basic formulations. Journal of Computational Physics. 227: 5072-5089, 2008], a novel numerical methodology, termed the discrete thermal element method (DTEM), is proposed for the modelling of heat conduction in systems involving a large number of circular particles in 2D cases. The method cannot be easily extended to transient analysis, which causes difficulties in combining the DTEM with the conventional discrete element method for modelling thermal/mechanical coupling problems in particle systems. This paper presents a simplified version of the DTEM, termed the pipe-network model, in which each particle is replaced by a simple thermal pipe-network connecting the particle centre with each contact zone associated with the particle. The model essentially neglects the direct heat transfer between the contact zones and thus significantly simplifies the solution procedure of the original DTEM. With this feature, transient heat conduction analysis can now be performed in a straightforward manner. In addition, the entire algorithmic structure of the pipe-network model is compatible with the discrete element method, leading to an effective scheme for simulating thermal-mechanical coupling problems. Numerical experiments are conducted to establish the solution accuracy of the proposed model.     

Modelling of transient mass transfer in liquid and liquid‐solid pulsating systems: Applications and extension to heat transfer

The dynamic surface renewal model of Maucci et al. (2001) is applied to transient mass transfer problems and extended to transient heat transfer measurements in pulsating, two‐phase flows. The model is also used to simulate mass transfer for square‐wave liquid velocity pulses in a liquid‐solid column. Experiments and simulation show that, when flow reversal occurs, the average mass transfer for a pulsating flow can be significantly higher than for steady state flow at the same bulk flow rate. This increase depends mainly on the relative pulse magnitude. The influence of pulse frequency and symmetry is second‐order. Apparent differences between various published studies are resolved.     

膜过程中热质耦合传递分析

研究了透过致密无孔膜的传热传质过程,考察了传热传质的相互作用,建立了膜过程中热质耦合传递的数学模型,并以湿空气透过薄膜分离过程为例,分析了温差及浓度差的变化对传热传质过程的影响,发现温差及浓度差的变化会引起热阻及湿阻的变化,从而进一步影响热流量和传质通量,所以对传热传质过程有加成作用。     

求解多维半透明介质内辐射传递的谱元法

直角坐标系下的辐射传递方程可以看作一类特殊的对流扩散方程,且具有强对流特性。采用数值方法求解该问题需要特别的稳定技术,否则计算结果往往会出现非物理振荡现象。本文发展了基于流向迎风彼得罗夫-伽辽金（SUPG）格式的谱元法来求解多维半透明介质内的辐射传递。采用3个算例对SUPG谱元法求解多维半透明介质内辐射传递的性能进行了检验。结果表明,与伽辽金谱元法相比,SUPG谱元法有效地消除了解的非物理振荡现象,同时与解析解及文献中的结果相比较,SUPG谱元法对于求解多维半透明介质内的辐射传递有着很好的精度。     

Pore-to-mesoscale network modeling of heat transfer and fluid flow in packed beds with application to process design

A dual-network model (DNM) representing the topological characteristics of both the pore space and solid fraction of a packed bed was developed to study coupled incompressible water flow and heat transport from the pore-scale to mesoscale (μm-cm) with the consideration of temperature-dependent fluid viscosity. The DNM was validated and used to study the temperature and velocity at the pore scale and their effects on fluid flow and heat transfer. Then the pore volume of the DNM was varied to illustrate the effect of bed porosity on transport processes, quantifying the trade-off between flow conditions and heat transfer. This work demonstrates the ability of the DNM to simulate pore-scale fluid flow and heat transfer simultaneously, which can then be averaged over the entire simulation domain to approximate meso/macroscopic parameters efficiently in relation to the pore geometry.     

介质辐射换热有限元法的扩散综合加速算法

发展了一个基于有限元法的 “扩散综合加速算法”,用于改善传统“源项迭代”技术在求解散射占优型介质辐射问题时计算效率较低的缺陷。该算法通过P₁扩散近似方程修正散射源项,提高迭代计算的准确度,从而实现了求解效率的提升。通过几个算例的分析,结果表明：当前的加速算法是准确高效的,能够明显提高介质辐射有限元法的计算效率,缩短计算时间。随着介质光学厚度和反照率的增大,算法的加速效果越趋明显,计算时间大幅缩短。     

淬火过程流动与传递现象数值模拟

根据淬火过程中多相流动与传热的特点,提出了简化模型。基于两相流体动力学及流固耦合传热建立了流动与传热方程。推导出了计算模型并结合大型软件Fluent进行了用户子程序（UDF）设计,利用此方法对影响流动与沸腾传热的几个因素进行了数值研究,数值计算结果与理论分析吻合。最后对金属铝块的淬火过程进行了计算,计算得到的金属表面测点温度随时间变化历程与文献中实验数据误差在15%以下,表明计算方法可行并且把握了物理过程的本质。     

Mathematical and computational modeling of heat transfer and deformation in film blowing process

The blown‐film extrusion process was investigated both experimentally and theoretically. In experimental study, nonisothermal experiments were conducted using low‐density polyethylene. Rheological parameters were studied, considering the polymer melt as a power law fluid in nonisothermal conditions. Axial tension, bubble diameter, and film thickness at a variety of film extrusion conditions, that is, different flow rate, pressure difference across the film, and take‐up speeds were measured. In theoretical study, an analysis was employed to simulate the blown‐film extrusion process by setting up the force‐ and energy‐balance equations on the film bubble moving upward. Four nonlinear complex differential equations were integrated numerically, using an iterative backward shooting method and the fifth‐order Runge‐kutta technique. The program written, based on a mathematical model, predicts the bubble shape, temperature profile, and film thickness as a function of the distance along the machine axis. Furthermore, the model evaluates the elongational viscosity of LDPE in biaxial tension in terms of distance from die axis and take‐up speed. In this simulation, the total stress components in machine and the transverse directions were computed from the die exit up to the freeze line, the knowledge of which is necessary for evaluation of the elastic memory build up in heat‐shrinkable films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2115–2123, 2002     

Discrete element method modeling of non-spherical granular flow in rectangular hopper

Discrete element method (DEM) was developed to simulate the corn-shaped particles flow in the hopper. The corn-shaped particle was described by four overlapping spheres. Contact force and gravity force were considered when establishing the model. In addition, the velocity distribution and voidage variance of corn-shaped and spherical particles were investigated. The results show that the vertical velocity difference between centre and side wall and the horizontal velocity of corn-shaped particles are relatively larger than that of spherical particles. The mean voidage for corn-shaped particles is smaller than for spherical particles in any hopper. And the mean voidage values decrease with the increase of the ratio of width and length (D/L) and the ratio of height and width (H/D) for both corn-shaped and spherical particles. The local voidage profiles in hoppers with different D/L were also studied. It demonstrates that the wall effect on the voidage of spherical particles is more remarkable than that of the corn-shaped particles. The voidage fluctuations of corn-shaped and spherical particles decrease obviously with increasing D/L when the particles are far away from the wall. And when the particles are discharging, the wall effect on the spherical particles is more remarkable than the condition of packing naturally.     

溶液再生器传热传质过程的数值模拟

根据湿空气和溶液热质交换的基本理论,建立了叉流溶液再生器的传热传质数学模型,并将该模型简化,得出空气和溶液的质量、能量控制方程。根据数值求解的方法,对方程进行离散简化,利用Matlab语言编程模拟计算。将模拟结果和实验结果进行了比较,结果表明本模型可靠。     

CFD方法在固定床反应器传热研究中的应用

固定床反应器是一种常见的化学和生化反应器,由于内部结构十分复杂,固定床内的局部流动和传热过程研究一直是一个颇具挑战性的问题。有效参数法是长期以来固定床传热研究的常用方法,但是由于对固定床内温度场缺乏准确的了解,目前为止许多基于有效参数法的传热模型的普适性仍然存在问题。计算流体力学（CFD）的数值模拟方法是近年来应用于固定床流动和传热研究的一种新的研究方法,它通过数值方法求解流动和传递的微分方程组而获得流场和温度场。CFD数值模拟方法能够提供精确的局部流动和传递信息如速度分布、压力分布、温度分布、组分浓度分布等。本文综述和分析了CFD方法在固定床反应器的流动和传热研究领域的最新进展,讨论了CFD方法在该领域的应用前景。     

Continuous versus discrete modelling of heat transfer to agitated beds

Heating of free-flowing particles by contact with the wall of a rotary drum without inserts has been simulated in two dimensions by means of the thermal version of the Discrete Element Method (DEM). The results are in qualitative agreement with existing experimental data and with the classical penetration model (PM) for the following limiting cases: heat transfer controlled by a contact resistance at the wall of the drum; heat transfer to agitated beds with significant bed-side resistance; heat transfer to the stagnant bed. The latter can be used to establish an equivalence (calibration) between the discrete (DEM) and the continuous (PM) modelling approach. Thermal mixing times can be derived from asymptotic overall heat transfer coefficients obtained via thermal DEM for agitated beds. They are found to be significantly smaller than purely mechanical mixing times. For the investigated conditions, they are also much smaller than previous recommendations based on the PM. The ability of thermal DEM to provide information not accessible to the penetration model, like temperature distributions, is discussed. It is pointed out that a decrease of the high computational cost of the method is necessary in order to enhance its applicability.     

新型热法磷酸燃磷塔传热分析与计算

在介绍新型燃磷塔结构的基础上,提出了计算其理论燃烧温度和烟气排放温度的工程方法。同时分析了燃磷量、过剩空气系数及燃磷塔结构型式等因素对其传热特性的影响。结果表明,在空气过剩系数或空气量不变时,最高燃烧温度和烟气排放温度随燃磷量增加呈单调递增趋势;而在燃磷量一定时,过剩空气系数越大,最高燃烧温度越低,烟气排放温度越高。     

基于离散单元法的丝状颗粒传热数学模型

丝状颗粒作为一类长径比较大的非球形颗粒,其传热特性及相关技术广泛应用于工农业生产的诸多领域。但目前颗粒在运动过程中传热问题的研究还很不充分,特别是对于丝状颗粒,更是缺乏有效的数学模型进行描述。从颗粒传热机理出发,提出了一种基于离散单元法的丝状颗粒传热模型,模型中综合考虑了颗粒碰撞（接触）传热、颗粒的内部导热以及颗粒与气体间的对流换热。利用该模型,对固定床中堆积丝状颗粒的热量迁移过程进行了数值模拟,着重比较了各种传热方式对传热过程的影响。研究表明,对流换热对整体传热量的贡献较大。此外,还获得了不同工况下颗粒温度随时间的变化规律。     

超声波场中蒸汽气泡凝结过程及传热特性

利用高速摄像仪记录有、无超声波时注入过冷水中蒸汽气泡的凝结过程,以分析超声波对蒸汽气泡凝结过程及传热特性的影响。结果表明:在超声波场中,蒸汽气泡表面会形成晶格状毛细波,有效增加气泡表面积,并加强气泡周围流体热边界层扰动,从而导致凝结换热的强化及气泡凝结速度加快。基于15～60 K过冷度下,有、无超声波时较大蒸汽气泡凝结的实验数据,拟合得出有、无超声波时的气泡凝结换热经验关联式,预测误差在±30%以内。     

Effects of crystal growth rate and heat and mass transfer on solute distribution

The effects of crystal growth rate and heat and mass transfer on solute distribution during solidification of binary melt have been theoretically investigated on the basis of a new theory of solute distribution proposed by the present authors. The solute distribution factor f at the solid-liquid (SL) interface is in inverse proportion to the one-half power of the dimensionless growth rate U. The growth rate U is in proportion to the second power of the normalized concentration difference between the SL interface and bulk melt. A new transport factor K, which describes heat and mass transfer in melt, gives an important contribution to the crystal growth and the solute distribution at the SL interface. The transport factor is used successfully to control the solidification of melt. The flow structure in melt exerts essential influence on the solid purity.     

Experimental and numerical heat transfer in a plate heat exchanger

In the present study a virtual prototype of a four-channel plate heat exchanger with flat plates was developed using computational fluid dynamics (CFD). Parallel and series flow arrangements were tested and experimental results were compared to numerical predictions for heat load obtained from the 3D CFD model and also from a 1D plug-flow model. The CFD model represents channels, plates and conduits of the exchanger and takes into account the unequal flow distribution among channels and the flow maldistribution inside the channel. CFD results are in good agreement with experimental data, especially for the series arrangement.     

超临界反溶剂过程传质数学模型

建立了有机溶剂(甲苯)液滴与超临界反溶剂(超临界CO2)之间的传质模型,用于模拟超临界反溶剂制备微纳米粉体材料的传质过程。该模型考虑了双向传质过程,既有反溶剂向溶液的扩散过程,又有溶液中的溶剂向反溶剂的“汽化”过程。液滴的传质行为是影响颗粒形态和尺寸分布的关键因素。假定传质是在一个孤立的微小液滴与包围着它的反溶剂连续相间进行的,利用描述液滴内和液滴外某一点行为的连续方程、扩散方程、能量方程和动量方程,及界面上的守恒条件进行耦合,从而建立传质过程的数学模型,并给出求解方程和求解的边界条件和初始条件,进行数值求解。     

The imaginary planes method for the calculation of radiative heat transfer in industrial furnaces

Radiation is usually the dominant mode of heat transfer in furnace enclosures. The imaginary planes method is one of the new methods for calculating the radiative heat transfer in such systems. In this study, a general procedure was developed for its application in furnace modelling. Various case studies were carried out for a number of furnaces, and the results were compared with those of the zone method. It was found that the new method combines good accuracy with low computation time and appears to be a promising method for the determination of heat flux and temperature distributions within furnace enclosures.