非稳态横掠管束对流传热的数值模拟

为研究管束内的换热规律与管间距的关系,通过分析不同物理模型的流场、换热系数,验证了针对横掠管束充分发展段采用周期性边界条件和对称性边界条件的合理性。采用FLUENT软件,对一定雷诺数范围内、不同的管间距横掠顺排管束周期性充分发展段模型的流动换热进行数值模拟。结果表明:密集排列管束的换热效果明显要比稀疏排列的管束大,且对于纵向管间距S_n/d=1.25的管束,当横向管间距取S_p/d=2.0左右时,管束间的换热达到最强,可以为提高换热器换热能力提供工程实际参考。最后将数值模拟结果与已有的经验公式以及实验结果进行比较,验证数值模拟方法的正确性。     

球床反应堆流动与传热的CFD分析:燃料球尺度

采用大涡模拟方法研究了球床反应堆燃料球点接触体心立方球阵的流场与温度场,并与k-ε方法的结果进行了详尽的对比。结果表明k-ε方法对于流动分离的计算不够准确,而球床中大多都是球面,存在大量的流动分离区域,因此采用k-ε方法来研究球床的局部流动并不合适,得到的流场与温度场与大涡模拟方法的时均结果有较大差异;大涡模拟捕捉到的接触点附近的涡大大增强了该处的换热情况。结论对于设计人员来说具有一定的参考意义。     

水平位置对突扩通道内横掠圆管对流换热的影响

采用非稳态数学模型对突扩通道内横掠圆管流动与换热进行了数值模拟。分别对不同雷诺数Re以及圆柱的不同水平位置情况进行数值模拟。结果表明,随着Re的变化,数值解有稳态和周期振荡。同时随着雷诺数的增加,圆管附近的换热不断增强。对于圆管不同的位置,当10≤Re100和Re=300时,圆管的换热随着Xc的增加而变弱;当100≤Re≤200时,圆管换热并不单调减弱。     

水平单管外混合气体对流冷凝换热的理论研究

采用修正的膜模型与Nusselt凝结理论结合的方法,对含湿混合气体自上而下横掠水平管外时的对流冷凝换热机理进行研究,建立了液膜流动和传热模型,进行数值求解并分析了雷诺数、壁面温度及水蒸汽浓度等因素对混合气体冷凝换热的影响。计算结果表明:水管外壁液膜厚度分布很大程度上受气体边界层对液膜剪切力的影响。而局部努谢尔数不同于纯蒸气的的冷凝换热,它受气相热阻的影响很大,其分布状况类似于单相气体管外的对流换热。     

纵向涡强化换热的优化设计及机理分析

对带纵向涡发生器的椭圆管翅片换热器空气侧表面的换热和流动特性进行了三维数值模拟.深入分析了纵向涡对流场和温度场的影响,并通过场协同原理揭示了纵向涡强化换热的根本机理,即减小了速度和温度梯度之间的夹角,改善了速度场和温度场的协同性.在此基础上,对纵向涡发生器的布置位置(上游布置和下游布置)和纵向涡发生器的攻角α(15°,30°,45°,60°)进行了优化设计.结果表明:当纵向涡发生器布置于换热管下游时,具有更好的强化换热能力;在纵向涡发生器采用下游布置的前提下,当纵向涡发生器的攻角α=30°时,具有最佳的强化换热能力.     

采用热-流耦合方法对火焰筒壁温三维数值模拟

考虑了流场变化对换热的影响,使用热-流耦合方法,对某型燃烧室整个流场、温度场进行完全的数值模拟。该方法对流场和固壁内换热进行耦合计算,得出了三维燃烧室壁温分布。计算中,对完全发展的湍流燃气采用了标准“k-ε”湍流模型,运用DO模型计算了燃气的热辐射,燃烧模型使用涡-耗散模型来计算化学反应速度,固壁材料使用了变比热和变导热系数。数值模拟结果表明流场与固壁相互作用得更充分,能更精确地反映流场和温度场的整个形态,可以模拟出较为合理的流场和温度场分布以及相应的流动换热特性。     

加涡产生器的管片式换热器换热与阻力特性的数值研究

通过数值模拟的方法,研究了小翼式涡产生器对错排圆管管片式换热芯子换热与阻力特性的影响,比较了光板与加涡产生器强化板芯的速度场、横向平均Nu数以及平均对流换热系数、阻力系数的变化规律,为进一步提高其换热性能、改进翅片结构、设计新型换热器提供了理论依据。     

横掠螺旋管束流动传热的数值模拟

螺旋管是一种紧凑式换热器的芯体形式,在工程中有较多应用。本文对螺旋管管束的流动传热情况进行了数值模拟研究,无量纲螺距0.15～0.3,进口雷诺数1000～5000,获得了横掠复杂螺旋管束的流场和温度场。结果表明：管外平均努赛尔数随雷诺数的增大而增大,欧拉数随雷诺数的增大而减小;减小螺距使平均努赛尔数和欧拉数均增大,且对欧拉数的影响要明显大于努赛尔数。同时通过对数值结果进行拟合给出了管束换热的关联式,式中考虑了无量纲螺距的影响。     

气体横掠换热管束最佳质量流速的快速确定

本文提出一种快速确定气体横掠换热管束质量流速的计算方法，该质量流速将热交换器传热和阻力限制“耦合”在一起加以考虑，具有接近真值，迭代次数少，收敛快等优点，是该类热交换器设计气体流速的一种较佳确定方法。     

煤焦油燃烧特性数值模拟

利用数值模拟方法,选用标准k-ε湍流模型,化学反应采用涡-耗散模型,辐射模型应用P-1辐射模型,研究进油口尺寸、进气口尺寸一定,煤焦油蒸气速度和助燃气体速度不变,富氧条件下不同含氧助燃气体对燃烧室火焰空间温度场和气流场压力场的影响。结果表明,助燃气体含氧量越高,火焰温度越高,燃烧速率越快,尾气排放所带走热量越小,火焰空间温度场分布梯度变大。数值模拟结果对于煤焦油用于工业燃烧条件的设计和操作具有一定的理论指导和实践意义。     

Simulation of gas species and temperature separation in the counter-flow Ranque–Hilsch vortex tube using the large eddy simulation technique

A computational fluid dynamic model is used to predict the species and temperature separation within a counter flow Ranque–Hilsch vortex tube. The large eddy simulation (LES) technique was employed for predicting the gas flow and temperature fields and the species mass fractions (nitrogen and helium) in the vortex tube. A vortex tube with a circumferential inlet stream of nitrogen–helium mixture and an axial (cold) outlet stream and a circumferential (hot) outlet stream was considered. The temporal evolutions of the axial, radial and azimuthal components of the velocity along with the temperature, pressure and mass density and species concentration fields within the vortex tube are simulated. Even though a large temperature separation was observed, only a very minimal gas separation occurred due to diffusion effects. Correlations between the fluctuating components of velocity, temperature and species mass fraction were calculated to understand the separation mechanism. The inner core flow was found to have large values of eddy heat flux and Reynold’s stresses. Simulations were carried out for varying amounts of cold outlet mass flow rates. Performance curves (temperature separation/gas separation versus cold outlet mass fraction) were obtained for a specific vortex tube with a given inlet mass flow rate.     

Large eddy simulations of the flow field and temperature separation in the Ranque–Hilsch vortex tube

A computational fluid dynamic model is used to predict the flow fields and the associated temperature separation within a Ranque–Hilsch vortex tube. The large eddy simulation (LES) technique was employed for predicting the flow and temperature fields in the vortex tube. A vortex tube with a circumferential inlet stream and an axial (cold) outlet stream and a circumferential (hot) outlet stream was considered. The temporal evolutions of the axial, radial and azimuthal components of the velocity along with the temperature, pressure and density fields within the vortex tube are simulated. Performance curves (temperature separation versus cold outlet mass fraction) were obtained for a specific vortex tube with a given inlet mass flow rate. Simulations were carried out for varying amounts of cold outlet mass flow rates. Predictions from the present large eddy simulations compare favorably with available experimental measurements.     

非均匀热流下水平管内混合对流大涡模拟研究

针对以槽式太阳能集热器为背景的高密度、高度非均匀热流下水平管内的混合对流换热问题,采用大涡模拟方法,研究了热流密度非均匀性对水平管内混合对流瞬态涡结构、脉动强度、湍流热通量及局部平均壁温的影响;揭示了非均匀热流下自然对流对管内湍流特性的影响规律;提出了适用于不同热边界条件下管内混合对流换热的强化措施。结果表明:均匀热流时,自然对流会抑制管顶部的湍流脉动,使流动层流化,造成传热能力局部恶化;非均匀热流时,随着自然对流的增强,近壁面速度脉动强度先减小后增大,二次流逐渐增强,换热能力逐渐提高,故管内换热能力受湍流脉动与二次流协同影响;在自然对流影响下,均匀加热时管顶部可采用针对层流的强化换热措施,非均匀加热时需着重提高管底部高热流区域的湍流脉动与涡强度。     

Large eddy simulation of flow and heat transfer in a channel with a detached rib array

Ribbed channels are widely used to enhance heat transfer in various heat exchange equipment. However, the heat transfer is locally deteriorated immediately behind the rib due to the flow separation. To overcome this shortcoming, a detached rib array has been proposed recently. In the present study, large eddy simulations (LES) of turbulent flow and heat transfer in a channel with a detached rib array have been conducted. The no-slip and no-jump conditions on the rib surface are satisfied in the Cartesian coordinates by using an immersed boundary method. Experiments are conducted as well to validate the simulation. The velocity and temperature fields are obtained by a hot wire and a thermocouple, respectively. The surface heat transfer is measured using the thermochromic liquid crystal with a high spatial resolution. Compared with the attached rib, the detached one enhances the heat transfer underneath the rib, whereas the channel wall downstream of the rib shows lower heat transfer rate. By investigating the effect of the clearance between the rib and the wall, we have found an intermediate flow pattern, where the counter-rotating vortex pair and the separation bubble coexist. Including the new flow pattern, we discuss the flow characteristics such as the wake length and the locus of the saddle points, and the flow physics behind the local heat transfer distribution on the channel wall based on the instantaneous flow and thermal fields. The turbulence data near the solid surface are also presented, which have not been provided by previous experiments.     

基于元体能量平衡法的垂直U型埋管换热特性的研究

基于元体能量平衡法建立了垂直U型埋管的传热模型,模型考虑了流体温度的沿程变化,并通过引入热干扰角与等效传热间距反映两管脚间的热干扰问题,使之更符合实际的传热情况。通过所建模型对U型埋管的换热特性进行了数值模拟,结果表明:增加土壤与回填物导热系数、管脚间距、管内流体流量及减小管脚热干扰角与进口流体温度(供热工况下)均可以增强埋管的换热效果,其中土壤导热性影响最为显著。但回填物导热系数不可无限制增大,其大小还要考虑对增强管脚间热干扰的影响及其与管脚间距的相互关联性。同时,流量的增加要考虑对流动阻力增加的限制,可以采用变流量设计来进行调节与优化。此外,为了充分发挥地源热泵的能效,实际设计应该考虑埋管、热泵及负荷三者间的相互匹配性。     

换热管内转子强化换热的数值模拟

本文概述了转子组合式强化传热装置的强化传热和自清洁原理。分别通过建立光管及内置多个转子换热管的三维流动模型,对换热管内流场、温度场、压力场以及换热过程进行了模拟,得到了管内流体的流动特性和传热特性。对比模拟结果表明,内置转子换热管管内的三维流动比较复杂,转子与管壁之间缝隙内的流体有着明显的环绕流动,切向速度和径向速度也增大到一定范围,转子旋转半径内的流体整体呈螺旋流动。总的来说,内置转子的换热管内较光管有较强的湍流度,尤其是近壁区域,因此强化了管内的对流换热,传热系数显著提高,从而验证了转子具有强化传热和自清洁的双重功能。     

Fluid Flow and Heat Transfer in Plate-Fin and Tube Heat Exchangers in a Transitional Flow Regime

The unsteady behaviors of fluid flow and heat transfer in plain plate-fin and tube heat exchangers with a wide range of fin spacings from 2.06 mm to 16.48 mm and tube diameter 8.28 mm are studied by a large eddy simulation technique (LES). Velocity fluctuations and vortex sheddings induced by the tubes in the channel are modeled. The results found that the flow in passages of large spacings is quite different from that of small spacings. The flow is co-determined by two effects: the duct effect and the tube bank effect. The tube bank effect is more dominant with increasing fin spacings.     

螺旋槽管换热过程的三维速度场与温度场耦合数值模拟

根据螺旋槽管换热器结构特点及传热特性,建立了以水为工质的换热器流动与传热的三维几何模型。运用有限元分析软件ANSYS模拟出换热器在换热过程中速度场与温度场的状况,分别得到了螺旋槽管内壁与外壁的对流换热系数。结果表明:槽深越大,随着Re增大,换热性能越好;当Re较小时,螺距越大,换热效果降低。其与该类光管换热器相比,得出螺旋槽管的换热系数是光管的2.5倍左右,强化了传热,为此产品的进一步理论研究和推广应用提供了依据。     

Large eddy simulation of turbulent flow and heat transfer in outward transverse and helically corrugated tubes

Turbulent flow and heat transfer in outward transverse and helically corrugated tubes are performed with large eddy simulation by the ANSYS Fluent software. The prediction accuracy is validated by comparison with experimental data and empirical correlations for a wavy surface wall and smooth tube, respectively. The turbulent flow patterns, local heat transfer, and friction factor are discussed. The results show that the secondary and turbulent eddies are inhibited by the spiral flow. Otherwise, the flow impact of the wall is the key factor for heat transfer enhancement, and the spiral flow has of small effect on heat transfer performance, however it can decrease the flow resistance significantly. The overall heat transfer performance for the helical corrugated tube is 1.23, which is superior to the value of 1.18 for the transverse corrugated tube.     

垂直环管内旋流换热与流动的数值模拟研究

基于垂直环管内旋流对流动边界层的扰动机理,采用数值模拟的方法研究了叶片角度、雷诺数以及进口水温对管内换热以及流动特性的影响,揭示了重力对环管内旋流流动的内在影响机制。结果表明：与水平环管相比,垂直环管的综合换热性能变化平缓,主要受到重力对压降的影响;与雷诺数相比,叶片角度对流场以及温度场的影响最显著;在雷诺数小于15 000,叶片角度为30°时管内的换热性能最佳。