间接蒸发冷却板式换热器换热性能的数值模拟

应用计算流体力学(CFD)方法建立了间接蒸发冷却板型换热器内三维层流流动与传热的数学物理模型;对影响蒸发冷却换热器换热性能的主要因素进行了数值计算和预测;通过计算表明,换热器通道间距、空气的迎面风速,以及一次风的干球温度的变化对换热器效率有很大的影响;分析了不同参数时通道内流场、能量场以及换热器效率的变化.     

间接蒸发冷却器不可逆(火用)损失研究

通过CFD方法模拟了逆流、顺流和叉流三种流动形式的间接蒸发冷却过程,分析了换热器内的温度和湿空气的值变化情况,讨论了有用能的转化关系和不可逆损失。为深入理解间接蒸发冷却器内的传热、传质过程和能量的传递与转换过程提供了参考。     

有冷凝和没有冷凝的板式间接蒸发冷却器性能分析比较

在不同的参数条件下，对一次空气通道有冷凝和没有冷凝的板式间接蒸发冷却过程进行了三维数值模拟。根据间接蒸发冷却过程的热力学特性，定义了间接蒸发冷却器的[火用]效率计算公式。通过对不同的入口速度、浓度以及通道宽度条件下的模拟和计算结果进行分析比较，得到了影响换热效率的因素，并从热力学第二定律的角度对换热器进行了[火用]分析。     

间接蒸发冷却器不可逆[火用]损失研究

通过CFD方法模拟了逆流、顺流和叉流三种流动形式的间接蒸发冷却过程，分析了换热器内的温度和湿空气的[火用]值变化情况，讨论了有用能的转化关系和不可逆[火用]损失。为深入理解间接蒸发冷却器内的传热、传质过程和能量的传递与转换过程提供了参考。     

基于空间扫描思想的复杂蒸发冷却式换热器 通用模拟方法

针对现有蒸发冷却式换热器数值计算模型中存在的不足,提出了基于空间扫描思想的复杂蒸发冷却式换热器的通用模拟方法。该方法将三维复杂蒸发冷换热器按空间划分为若干个节点,每个节点视为独立的微型换热器,利用流体在节点之间的流动建立节点联系,根据稳定流动和稳态换热时节点之间的能量和质量守恒关系,构建迭代更新算法,利用扫描迭代的方法对模型进行求解。将无填料型和有填料两种闭式冷却塔的模拟结果与文献实验数据进行对比,结果表明：所建模型具有良好的精度,可用于复杂结构、复杂流动、多模态蒸发冷却式换热器的仿真。     

一次空气有冷凝的间接蒸发冷却过程的三维数值模拟

应用计算流体力学(CFD)方法,建立了板式间接蒸发冷却器的三维教学物理模型;探讨了内部边界耦合的传热、传质过程,对一次空气通道有冷凝和没有冷凝的间接蒸发冷却过程进行了三维数值模拟;在模拟的数据基础上,对有冷凝和无冷凝的间接蒸发冷却过程进行了比较, 分析了影响冷凝的因素。     

包覆吸水性材料椭圆管式间接蒸发冷却器的理论与实验研究

从吸水性材料、换热器结构及布水方式入手，设计了采用功能性纤维套、铝箔椭圆管和间歇性供水方式等强化换热措施的间接蒸发冷却器结构。功能性纤维套采用异形涤纶和Lanseal纤维混纺而成，大大加强了水膜导热和水膜表面的蒸发能力；间歇性供水方式保证了管外纤维套凹坑及纤维凹槽不会被水膜堵塞，提供了足够的换热面积；使用铝箔椭圆管并对排列的几何参数进行优化设计，使间接蒸发冷却器的换热性能达到最优化。实验结果表明这种新型管式间接蒸发冷却器和板式间接蒸发冷却器有相近的换热效率E，但阻力明显小于板式。     

椭圆管式间接蒸发冷却器液膜蒸发的数值模拟

通过对椭圆管式间接蒸发冷却器建立三维传热、传质数学模型,并利用CFD技术得出了二次空气入口参数对水膜蒸发量的影响,并预测了椭圆管式间接蒸发冷却器内部的含湿量场,对间接蒸发冷却技术的应用和推广可起到一定的促进作用。     

管式间接蒸发空气冷却器传热传质模型的建立及验证

回顾和分析现有间接蒸发冷却器的热工性能和数学模型,并在分析管式间接蒸发空气冷却器传热、传质过程及特点的基础上,建立针对管式间接蒸发空气冷却器热工计算模型。基于模型中管外二次空气侧空气与水膜之间的传热、传质系数是影响模型精度的重要因素,对管外二次空气侧空气与水膜之间的传热、传质系数进行深入分析,将模型用于水平单管外蒸发传热、传质系数的计算,并将计算结果与文献中的实验数据进行对比,证明所选模型的正确性,为下一步对管式间接蒸发空气冷却器整体热工性能的数值模拟奠定坚实的基础。     

翅片管式气-液换热器变工况下传热特性研究

采用FLUENT软件对高温空气-混合硝酸盐在翅片管式换热器中的换热进行了三维数值模拟,研究其换热与流动特性。模拟主要考察对于不同压力工况下及不同Re数的高温空气,换热器的换热及阻力特性。计算结果表明:随着空气侧流速及空气压力的增加,空气侧表面换热系数都有显著增加,同时流动阻力也有所增加。低压力工况时的换热及阻力特性曲线几乎随空气流速呈线性相关,高压力工况流动和换热呈非线性趋势。将数值模拟结果与实验结果进行了对比,对数值模拟结果的准确性进行了验证,并得出了流体物性对换热器性能的影响,给出了翅片管换热器在不同条件下的换热准则方程式。     

地热对井系统裂隙岩体三维渗流传热耦合的等效模拟方法

研究地热对井系统中的裂隙岩体渗流传热问题对于开采深层地热能和发展可再生清洁能源利用技术具有重要价值。基于渗流传热耦合理论和离散裂隙网络模型,提出了裂隙岩体三维热流耦合的等效模拟方法:考虑由岩块基质及复杂离散裂隙网络组成的双重介质,采用无厚度单元模拟裂隙、线单元模拟对井,通过裂隙、对井和岩块三者之间的流量和热量交换实现渗流和传热过程耦合分析。通过与解析方法和精细模拟方法相比较,验证了等效模拟方法的有效性;并将其应用于含大规模裂隙岩体地热对井系统热采过程的数值模拟,获取了储层内温度场的分布规律,评价了裂隙开度对储层平均温度和整体开采率的影响。结果表明:该文方法能够对裂隙及井筒中的渗流传热行为进行细致模拟,在保证精度的前提下,可大幅减小计算量和计算时长;裂隙网络的非均匀及各向异性分布导致岩体温度场分布呈现高度不均匀性,反映了热流耦合的早期热突破和长尾效应等特点;裂隙内水的对流传热作用明显,冷锋面沿储层内的主要贯通裂隙网络移动,裂隙开度是影响岩体温度场分布的重要因素。     

等离子+缆式焊丝脉冲GMAW复合焊熔池流体行为研究

目的 研究等离子+缆式焊丝脉冲GMAW复合焊过程的熔池流体行为。方法 综合考虑传热学以及流体动力学,建立Fluent数值分析模型。使用双椭球–锥体热源模型代表等离子弧焊传热模型,用双椭球热源表征GMAW电弧传热并考虑熔滴传热,同时考虑熔池受到的电磁力、浮力、表面张力、等离子流力等作用力。基于Fluent软件,对复合焊过程中熔池的温度场和流场进行研究,模拟脉冲电流为150 A时复合焊熔池的流体行为,并进行实验验证对比。结果 在固定等离子焊接电流为100 A、焊接速度为30 cm/min、GMAW脉冲平均电流为150 A时,模拟显示熔池的熔宽为10.57 mm、熔深为2.58 mm、余高为3.66 mm,模拟结果与实验结果吻合良好。结论 建立的GMAW复合焊数值模型能够有效指导该焊接过程。     

Influence of developing flow on the heat transfer in laminar oscillating pipe flow

A theoretical analysis was performed to evaluate the influence of developing flow on the heat transfer associated with laminar oscillating pipe flow. Simplified analytical approaches are briefly discussed before an investigation based on the numerical solution of the conservation equations for mass, momentum and energy is presented, assuming constant wall temperature and an incompressible viscous fluid. Focusing on operating conditions as found in heat exchangers of regenerative thermal machines, like Stirling engines or Vuilleumier heat pumps, numerous calculations of the flow field and the heat transfer have been executed covering wide ranges of the characteristic dimensionless groups. The results are presented in tems of correlations of the mean Nusselt number averaged on the pipe length and a distribution function describing the local heat transfer. Furthermore, it is shown that the derived correlations are also applicable to compressible fluid flow, provided that the relative pressure amplitude is within the limits typical of regenerative thermal machines.     

Experimental Analysis and Simultaneous Heat and Moisture Transfer with Coupled CFD Model for Convective Drying of Moist Object

Convective drying of rectangular-shaped moist object has been analyzed both experimentally and numerically. Transient mass of the potato sample is measured experimentally. Moisture content, diffusivity, and density of the object are calculated at different drying air temperatures from 40°C to 70°C with an air velocity of 2 m/s. A three-dimensional (3D) finite volume method (FVM) based numerical model is developed to predict the temperature and moisture distribution. A computational fluid dynamics (CFD) code is used for predicting heat and mass transfer coefficients required in the boundary conditions of the heat and mass transfer model. The experimental and numerical data are compared and good agreement is observed.     

Predicting local surface heat transfer coefficients by different turbulent k-ε models to simulate heat and moisture transfer during air-blast chilling

A numerical investigation using a computational fluid dynamics (CFD) code is carried out to predict the turbulent flow field, and heat and moisture transfer in a three-dimensional air-blast chiller with cooked meats of cylindrical and elliptical shapes. Three turbulent models [standard, low Reynolds number (LRN) and RNG k-ε model] have been used in these simulations. Based on local heat transfer coefficients on the surface of the meat calculated by CFD code, the unsteady heat and mass transfer were simulated which took into account of the effects of conduction within the meat, forced and natural convection, radiation and moisture evaporation on the surface of the cooked meat joint. The model allowed the simultaneous CFD prediction of both temperature distribution and weight loss of the meat throughout the chilling process. Good agreement with experimental results was obtained. The effect of using different models on the accuracy of the simulation of local heat transfer coefficient is presented.     

Numerical modelling of fluids mixing,heat transfer and non‐equilibrium redox chemical reactions in fluid‐saturated porous rocks

Non‐equilibrium redox chemical reactions of high orders are ubiquitous in fluid‐saturated porous rocks within the crust of the Earth. The numerical modelling of such high‐order chemical reactions becomes a challenging problem because these chemical reactions are not only produced strong non‐linear source/sink terms for reactive transport equations, but also often coupled with the fluids mixing, heat transfer and reactive mass transport processes. In order to solve this problem effectively and efficiently, it is desirable to reduce the total number of reactive transport equations with strong non‐linear source/sink terms to a minimum in a computational model. For this purpose, the concept of the chemical reaction rate invariant is used to develop a numerical procedure for dealing with fluids mixing, heat transfer and non‐equilibrium redox chemical reactions in fluid‐saturated porous rocks. Using the proposed concept and numerical procedure, only one reactive transport equation, which is used to describe the distribution of the chemical product and has a strong non‐linear source/sink term, needs to be solved for each of the non‐equilibrium redox chemical reactions. The original reactive transport equations of the chemical reactants with strong non‐linear source/sink terms are turned into the conventional mass transport equations of the chemical reaction rate invariants without any non‐linear source/sink terms. A testing example, for some aspects of which the analytical solutions are available, is used to validate the proposed numerical procedure. The related numerical solutions have demonstrated that (1) the proposed numerical procedure is useful and applicable for dealing with the coupled problem between fluids mixing, heat transfer and non‐equilibrium redox chemical reactions of high orders in fluid‐saturated porous rocks; (2) the interaction between the solute diffusion, solute advection and chemical kinetics is an important mechanism to control distribution patterns of chemical products in an ore‐forming process; and (3) if the pore‐fluid pressure gradient is lithostatic, it is difficult for the chemical equilibrium to be attained within permeable cracks and geological faults within the crust of the Earth. Copyright © 2005 John Wiley & Sons, Ltd.     

Application of IR thermography as a measuring method to study heat transfer on rotating surface

This paper presents an experimental and CFD numerical study of convective heat transfer and flow field characteristics in a rotating environment. Surface temperature distribution on a rotating blade was measured by means of infrared thermography. The experimental facility, IR thermography method, and the CFD numerical model that was made according to the actual test rig geometry and operating conditions, are described in detail. For the present study, tests were carried out at relatively low fluid temperatures in several operating points, defined by rotational, Reynolds and hot-to-cool air mass flow ratio. Experimental and numerical results for the observed blade side are compared in terms of surface temperature distribution (2D) and 1D charts along the blade midspan. Temperature distributions are statistically evaluated and show very good agreement.     

Numerical study of transient 2-D compressible flow with heat and mass transfer using the finite volume method

In this paper, we used a pressure-based finite volume method to investigate the problem of transient 2-D compressible flow with heat and mass transfer in a rectangular domain. We have used this method to solve the governing equations with given initial and wall slip boundary conditions. We implemented the SIMPLE-TS algorithm in order to compute the numerical solutions for the flow variables, viz., velocity, pressure, temperature, concentration, density. The variation of density of the fluid along the horizontal and vertical line through geometric center of the domain has been studied. The transient solutions of temperature and concentration indicate that, the transient flow though dominates initially, it finally settles down to steady states solutions after elapse of some time. Nusselt number and Sherwood numbers were used to predict the behavior of heat transfer and mass transfer, respectively, at the center line of the rectangular domain.     

Mass transfer from a finite strip near an oscillating stagnation point – implications for atherogenesis

The mass transfer from a finite-length strip near a two-dimensional, oscillating stagnation-point flow in an incompressible, Newtonian fluid is considered. The problem is investigated using a combination of asymptotic and numerical methods. The aim of the study is to determine the effect of the location of the strip, relative to the time-averaged position of the stagnation point, on the mass transfer into the fluid. The study is motivated by the problem of mass transfer from an injured region of the arterial wall into the blood, a process that may be of considerable importance in atherogenesis. For physiologically realistic parameter values, it is found that the fluid flow is quasi-steady, but the mass transfer exhibits genuine time-dependence and a high-frequency asymptotic solution provides an accurate prediction of the time-average mass transfer. In this regime, there is a significant reduction in mass transfer when the centre of the strip is located at the point of zero time-averaged wall shear rate, or equivalently wall shear stress, which may serve to explain, at least partially, the correlation between arterial disease and regions of low wall shear stress.