直接空冷凝汽器单元样机流动和传热性能研究

直接空冷凝汽器由一个个直接空冷单元组成.对直接空冷凝汽器单元的研究具有重要的意义.依托实际工程项目,对某135 MW直接空冷凝汽器单元样机进行流动和传热的性能研究.利用计算传热学(NHT)软件Fluent,对空冷单元样机的设计和实验工况进行数值模拟.分析了模拟结果与设计和试验数据存在一定偏差的原因.对直接空冷凝汽器外部空气的速度场、温度场的模拟、分析和研究,为直接空冷系统的优化设计提供帮助.     

结霜工况下变片距空气冷却器性能仿真与试验

以结霜工况下的变片距空气冷却器为研究对象,对空气冷却器建立分布参数计算模型,采用分子扩散理论建立霜层的生长模型。在计算过程中,采用空气冷却器迎风面的霜层厚度平均值作为变量迭代求解空气冷却器的风量。研究了变翅片间距结构对空气冷却器结霜工况下性能的影响,并进行了实验验证。实验结果表明:空气冷却器的风量、换热量变化趋势的实验值和计算值基本一致,偏差在5%左右。与定翅片间距空气冷却器相比,变翅片间距空气冷却器在结霜工况下具有较长的除霜周期和更好的传热性能。     

发动机废气再循环冷却器性能计算仿真

发动机废气再循环是有效降低Nox的重要方法,废气再循环冷却器的设计开发显得尤其重要。本文,结合现有EGR冷却器结构参数,利用DOE的设计方法,通过对三种不同结构形式的EGR冷却器进行传热和阻力性能测试。同时通过多元回归分析,建立EGR冷却器热侧传热和流动实验关联式,并进一步利用Visual Baisc开发工具,建立EGR冷却器性能计算仿真平台。该仿真平台的开发,有效的提高了EGR冷却器设计计算精度和产品开发速度。     

压缩机组空气冷却器传热失效分析

分析了某合成氨装置中空气冷却器的传热失效问题，对冷却器在设计、制造中存在的问题及其对传热的影响进行了探讨。     

SCAL型间接空冷塔内外空气流动和传热性能数值研究

利用计算流体力学软件CFD,对自然通风状态下,某电厂2×300 MW机组两机一塔的SCAL型间接空冷散热器空冷塔内外空气的流动和传热性能进行数值模拟、分析和研究。确定考核工况基准下,不同环境风速对空冷塔通风量和间接空冷散热器散热量以及机组背压的影响。模拟结果显示,随着环境风速的增加,空冷塔内外流场发生复杂变化,空冷塔通风量和散热器散热量降低,机组背压升高,将引起很大的经济损失。这为间接空冷系统空冷散热器的优化设计提供了参考。     

表面式间接空冷散热器传热特性分析北大核心

以表面式间接空冷散热器为例,建立了间接空冷散热器冷却单元的一维传热数学模型。通过该模型,计算并分析了冷却水流量、温度及空气进口温度(环境温度)、流速等对空冷散热器传热性能的影响。计算结果显示,随着冷却水流量、温度和空气流速、温度的增大,散热器总传热系数逐渐增大;散热器总传热系数更接近于空气侧表面传热系数,其中空气流速对总传热系数的影响尤为明显;积灰对散热器传热性能的负面影响较大,积灰越厚,散热器总传热系数越小。另外,利用本文模型,得到了一定条件下间接空冷系统冷却水最佳流量,为间接空冷机组的运行了提供一定的理论依据。     

大型循环流化床锅炉热力计算软件的开发及应用

针对大型循环流化床(CFB)锅炉的设计需要,研究提出了CFB锅炉热力计算方法,建立了炉膛传热计算模型及外置床传热计算模型,以此为基础开发了适用于M型及H型炉型的CFB锅炉热力计算软件。该热力计算软件可开展大型CFB锅炉的设计计算和校核计算,通过210 MW和330 MW CFB锅炉的应用表明,所开发的CFB锅炉热力计算软件计算结果准确可靠,为大容量CFB锅炉的设计提供了有力工具。     

IGCC系统空气分离集成对燃气轮机通流匹配约束影响研究

为了探究燃气轮机在应用于IGCC系统时,由于空气分离（以下简称空分）集成方式与燃料热值变化引起的复杂约束问题,以某F级燃气轮机为基础,构建IGCC系统燃气轮机及空分系统性能模型,研究不同运行约束条件下空分集成方式对燃气轮机通流匹配的约束边界,确定了保持进口导叶IGV全开时空分整体化率和氮气回注率的可行域与燃气轮机性能的变化规律,并针对限制因素给出压气机的流量设计需求及改进后的性能对比。结果表明：保持透平进气温度稳定在设计值的情况下,可实现的最小空分整体化率为0.15;受合成气热值及空分系统集成方式的限制,压气机与透平流量的匹配约束使低整体化率与高回注率的组合无法实现;保持透平排气温度稳定在设计值可行域的范围有所扩大;可行域内,降低空分整体化率、提高氮气回注率均有助于提升燃气轮机性能;对改型前后的燃气轮机性能进行比较,独立空分-氮气不回注组合下燃机效率提高到35.2%;在氮气不回注与氮气完全回注时,独立空分相较于完全整体化空分组合燃气轮机效率分别高19.63%和15.91%。     

一种改进的回转式空气预热器热力计算方法

针对现有热力计算方法的不足,提出了一种改进的热力计算方法。该方法根据蓄热式换热器的工作特点,通过能量方程与传热方程的联立求解得到出口烟气和空气的温度,因而该方法具有计算精度高和计算工作量少的特点。该方法具有很好的通用性,既适用于两分仓,也适用于三分仓。给出了回转式空气预热器非稳定换热影响系数的数学模型,与目前仅根据空气预热器转速进行插值计算的方法相比,该模型具有更好的机理性和广泛适用性。表3参5     

Optimization of ASU Integration Rate of IGCC Power Plant

利用专用流程计算软件建立了400 MW级IGCC电站系统模型,在此基础上对不同整体化率设计下的系统性能进行了评估,得出最优的整体化率,并通过进一步分析,确定了影响整体化率选择的因素,定量分析了各因素取值变化对空分系统的影响.结果表明：IGCC电站的最优空分整体化率与燃气轮机性能、气化工艺、空分装置主空压机效率等有关,其中燃气轮机性能随抽气量的变化是影响空分系统整体化率取值的关键因素.     

基于Flowmaster的相变空冷器元件开发及应用

基于湿空气热力学理论,以Flowmaster换热元件为基础,通过引入相变计算开发了考虑相变潜热的空冷器元件。经计算验证,所开发的元件计算结果与理论计算值吻合性较好,相同边界条件下,总换热量偏差和含湿量偏差均小于1%。分析表明:在高温、高湿环境条件下,湿空气会在空冷器中形成冷凝水,产生大量相变潜热,而Flowmaster换热元件模型由于未考虑相变因素,气侧出口温度计算结果偏低。采用所开发的相变空冷器元件模型可有效提高气侧出口温度计算的准确性,减小因相变带来的仿真误差。     

Study on carbon dioxide gas cooler heat transfer process under supercritical pressures

In carbon dioxide transcritical air‐conditioning and heat pump systems, the high‐pressure‐side heat exchanger operating at supercritical pressures is usually called as gas cooler. The carbon dioxide gas cooler displays much difference from the traditional heat exchangers employing constant property fluids. The commonly used logarithmic mean temperature difference (LMTD) and effectiveness—heat transfer unit (ε‐NTU) fail for the gas cooler design calculation as the carbon dioxide properties change sharply near the critical or pseudo‐critical point in the heat transfer processes. The new effective heat transfer temperature difference expression for variable fluid property derived by the authors is verified by numeric simulation of the carbon dioxide gas cooler. Moreover, the available correlated models for the cooled carbon dioxide supercritical heat transfer are used to simulate the gas cooler. Detail analysis is made for the deviations among the different models, and for the distributions of local convective coefficient, heat flux, and local temperature of carbon dioxide along the flow path in the gas cooler. Copyright © 2002 John Wiley & Sons, Ltd.     

Numerical Modeling of Fin Heat Exchanger in Application to Cold Storage

The fin-and-tube heat exchanger are extensively used in refrigeration systems applied to cold storage. The performance of the heat exchanger affects the efficiency of the refrigeration system. Prediction of temperature, humidity, as well as velocity distribution in a cold storage chamber requires accurate prediction of operation of the finned air cooler. The operation of the air cooler unit is usually taken into account in modeling of operation of the cold storage chambers, but with very simplified geometry and physics. On the other hand numerical investigations of the heat exchangers are focused on thermal performance mostly in order to improve the overall heat transfer coefficient or to optimize its geometry. Results of numerical modeling using the computational fluid dynamics software ANSYS FLUENT of fin-and-tube air cooler applied on a cold storage chamber is presented in the paper. Two different approaches were used: the dual cell model, and porous media conditions. Numerical predictions of air temperature as well as air velocity at the air cooler outlet were also validated on the basis of the own experimental data.     

Experimental investigation on the performance of air cooler under frosting conditions

Evaluation of heat transfer performance of the air cooler under frosting conditions is of great importance for the refrigeration industry. In this paper, effects of frost growth on the performance of the air cooler have been studied with an experimental air cooler of industrial size with different fin spacings, i.e., 6, 8 and 10 mm. Results showed that factors affecting the heat transfer performance of the air cooler include the evaporation temperature, the frost height, the fin spacing and the air velocity. The overall heat transfer coefficients based on a logarithmic mean temperature difference (LMTD) and the energy transfer coefficients based on a logarithmic mean enthalpy difference (LMED) were calculated. As the frost accumulated on the air cooler, the overall heat transfer coefficient and energy transfer coefficient will drop gradually.     

Numerical investigation on the heat and mass transfer in a direct evaporative cooler

A simplified mathematical model is developed to describe the heat and moisture transfer between water and air in a direct evaporative cooler. The mass of evaporated water is treated as a mass source of air flow, and the related latent heat of water evaporation is taken as a heat source in the energy equation. The momentum caused by water evaporation is taken into account in the momentum equations. The effective air viscosity and diffusion coefficient are decided experimentally. The models and methods are validated by comparing the numerical results with those of experiment for the same evaporative cooler. The influences of the inlet frontal air velocity, pad thickness, inlet air dry-bulb and wet-bulb temperatures on the cooling efficiency of the evaporative cooler are calculated and analyzed.The cooling effects of the direct evaporative cooler are predicted for use in four different regions in northwest China using the present numerical method and local weather data for air conditioning design. The predicted results show the direct evaporative cooler with high performance pad material may be well applied for air conditioning with reasonable choices for the inlet frontal velocity and pad thickness.     

基于CFD仿真的柴油机中冷器流动均匀性优化

基于某船用柴油机1D热力学模型提供的计算边界条件,采用3D CFD对该柴油机匹配的中冷器的瞬态流动及换热过程进行仿真模拟,计算分析中冷器入口截面的速度均匀性以及前进气箱压力损失。结果表明:中冷器入口截面的速度均匀性系数未完全达到设计目标。对中冷器前进气箱的结构进行优化设计,优化后的中冷器入口截面的速度均匀性系数有所提高,满足设计要求,同时中冷器前进气箱的压力损失处于较低水平。     

建筑墙体热、湿及空气耦合传递

为了研究建筑墙体的热质耦合传递规律，该文考虑了太阳辐射的影响，建立了在第三类边界条件下建筑墙体非稳态热、湿及空气渗透耦合传递的物理及数学模型，开发了相应的数值模拟计算软件。给出了墙体热、湿及空气耦合传递时各部位模拟计算温度随时间的变化曲线及不同使用年限湿容量沿墙体厚度的分布规律，并将墙体的模拟计算结果与现场实测热流及温度进行了对比分析。     

小型CO_2热泵系统用气体冷却器仿真研究

CO_2气体冷却器的结构和换热效果对CO_2跨临界循环影响较大.为设计出高效的气体冷却器,有必要对其性能进行模拟和优化.采用有限单元法建立了小型CO_2热泵热水器中气体冷却器稳态分布参数模型,分别对其CO_2侧和水侧的流动与换热进行了数值仿真,运用该模型分别针对CO_2侧进口压力对气体冷却器设计管长和CO_2换热性能的影响进行了分析.结果表明,CO_2侧进口压力在8~12 MPa时,从8 MPa开始每递增1 MPa,换热系数峰值比压力增加1 MPa前的依次递减约57.14%、33.33%、25.00%、9.83%,设计管长比压力增加1 MPa前的依次递减约55.60%、18.75%、11.33%、9.09%.综合考虑管道耗材与CO_2换热能力,针对小型CO_2热泵系统,气体冷却器CO_2侧进口压力取8.5~10 MPa较合理.研究可为气体冷却器设计提供理论指导.     

Quick Algebraic Estimate of the Thickness of Insulation for the Design of Process Pipelines with Allowable Heat Losses to Ambient Air

Adequate control of the total heat losses from in-tube fluid streams to neighboring ambient air is an important problem in heat transfer engineering. In this regard, the primary goal of this article is to demonstrate that an iterative solution of a nonlinear algebraic equation might allow thermal design engineers to estimate quickly the thickness of insulation of round tubes. The calculation methodology begins with the adoption of a powerful 1-D extended lumped energy model in favor of the customary 2-D differential energy model. The principal advantage of the former model is its ability to produce concise analytic expressions for the axial variation of the mean bulk temperature and also for the total heat transfer in the entire length of the tube. Particular attention was given to realistic situations in industry that account for laminar or turbulent velocities of single-phase viscous fluid flows in horizontal tubes rejecting heat by natural or forced cross flow of the ambient air. The total heat transfer to the air is the constraint design parameter in the set of design parameters. Once the magnitude of the total heat transfer was prespecified, the thickness of insulation of the round tube was easily computed numerically, exploiting the fixed-point iteration procedure for the solution of an adjoint nonlinear algebraic equation. Starting with a judicious guess of a root of the nonlinear algebraic equation, the correct root was surprisingly obtained in two or three iterations, thus furnishing immediately the required size of the thickness of the insulation annulus.