多孔金属和方柱复合结构池沸腾数值模拟研究

针对一种多孔金属和方柱复合结构,采用双粒子分布函数格子Boltzmann方法对其池沸腾传热进行数值模拟研究。分别调整了下部方柱的柱宽/槽宽(W/D)、柱高H和上部多孔层的厚度,分析了不同结构的沸腾换热曲线和传热机理。流场和温度场分别用密度分布函数和温度分布函数来描述,底部固体采用定压边界条件。研究发现,方柱-多孔层复合结构可以有效的强化沸腾换热。结果表明,低热流密度下,具有较小的W/D的结构换热表现更好,高热流密度下,较大的W/D换热表现更好;换热效果随结构柱高H的增大而增加;多孔层厚度的变化对整体结构的强化效果影响不大。     

回热式多孔介质发动机的排放特性分析

研究了回热式多孔介质发动机的排放特性.使用Ferrenberg所提出的回热式发动机模型,其多孔介质面热器在气缸中作往复运动.用改进的KIVA-3V程序进行了二维模拟.探讨了在不同当量比和孔隙率条件下,气缸内燃烧和排放特性.结果表明,回热式多孔介质发动机在燃烧和排放方面较之常规都有明显的优势.采用低当量比能有效地降低污染物的排放,而孔隙率的影响则与当量比有关.分析证明,采用多孔介质回热器是发动机实现稀薄燃烧的一条可供选择的途径.     

高低热导率相间表面强化饱和池沸腾的格子玻尔兹曼模拟

采用格子玻尔兹曼方法模拟高低热导率相间表面的饱和池沸腾过程,研究不同表面高低热导率区域热导率比值、低热导率区域宽度和深度对沸腾换热性能的影响。对比均匀热导率表面与高低热导率相间表面的沸腾曲线发现：高低热导率相间表面的沸腾过程可被分为5个阶段,并且其临界热流密度最高可达均匀表面的12倍;高低热导率相间可促使表面维持一定的温度差异,从而保持明显的气液流动;随着低热导率区域宽度增大,气液分离更加明显,低热导率区域宽度存在一个最优值,其与毛细长度的量级接近;随着低热导率区域的深度增大,表面过热度的差异更加明显。     

电化学方法制备多孔微纳结构表面的池沸腾传热性能研究

电化学表面处理一直以来都是池沸腾传热强化领域的一种重要的表面改性方法。然而,在光滑表面通过不断沉积生长枝晶结构的同时,结构底层会形成不必要的堆积层,从而增加了底层的热阻,不利于表面沸腾传热系数（HTC）的提升。因此,本文通过结合电化学表面处理手段中的电化学腐蚀与电化学沉积两方面的工艺,制备了“电腐蚀+沉积”表面,来探究经预先腐蚀处理后的表面与直接沉积表面沉积形貌及沸腾传热性能的差异。SEM图像表明,相较于在未处理铜板上直接进行沉积的表面,“电腐蚀+沉积”表面具有更长的枝晶结构、更深的孔结构及较少的底层堆积,结构的整体粗糙度更大。稳态池沸腾实验结果表明,在未处理铜板上直接进行电沉积的表面,其临界热流密度值较未处理铜板提升141%,而“电腐蚀+沉积”表面的临界热流密度值相较于未处理铜板提升了193%,并且其整体的沸腾曲线相较于直接沉积表面左移。对表面沸腾过程中汽泡行为的观测及表面铺展速率测试结果表明,“电腐蚀+沉积”表面具有更强的回液能力,能更好地促进表面气泡的脱离过程。     

内表面烧结型多孔管的流动沸腾换热

采用流动沸腾传热试验平台,研究了2 m长铁基烧结型内表面多孔管竖直管内流动沸腾传热特性,利用流动沸腾传热学基本原理及公式计算了传热过程中的热通量、沸腾传热系数及相关参数,并考察了过热度和流速对多孔管流动沸腾传热性能的影响.结果表明:烧结型表面多孔管的流动沸腾传热能力优于同条件下的光滑管,内表面沸腾传热系数是同尺寸光滑管...     

表面亲疏水性改性多层铜网的池沸腾换热特性研究

通过化学处理金属铜网,制备了具有不同层数和湿润性的多层铜网样品。以去离子水作为液态工质,在相同的条件下,对制备的多组样品进行可视化池沸腾实验,分析了池沸腾换热特性和气泡动力学行为。研究结果表明：在低热流密度下,疏水结构换热性能略优;在大热流密度下,亲水结构换热性能更优;对于多层铜网结构,厚度可以增大换热面积和成核点位,但过厚会增大气泡逃逸阻力。实验中5层的亲水铜网换热性能最优,换热系数峰值可达7.4×10～4 W/（m²·K）,是相同热流密度下光滑铜表面沸腾换热系数的2.10倍。     

气化炉内气体穿越液池过程中气泡特性的数值模拟

采用数值模拟方法对含尘气体通过下降管穿越液池过程中的气泡特性进行了研究,展现了该过程中池内气泡生成、破碎和液滴飞溅等一系列行为过程,分析了下降管出口气流速度、静态液位和突扩比等实际操作参数对气泡分布和气液界面面积等气泡特性的影响．结果表明：轴向含气率分布随气流速度的变化存在一个转折点;液面以上空间的含气率随着突扩比的减小而降低,而在液面以下区域,含气率随着突扩比的增大而降低;随着气流速度的增大、静态液位的升高及突扩比的减小,气液界面面积增大．     

多孔介质回热微燃烧器预混燃烧特性研究

设计了多孔介质回热徽燃烧器,对微燃烧器内H2/Ak的预混燃烧特性进行了实验研究和数值模拟,实验结果表明,当过量空气系数1.0＜α＜3.0时,微燃烧器具有较高的燃烧效率,出口烟气温度和较低的燃烧热损失率,且燃烧热功率P越高,α越大,热损失率越小.当P=100 W时,其出口烟气温度最高可达到1 232 K,当α=3.0时,燃烧效率仍达到96.85％,而热损失率仅为14.87％.数值模拟结果表明,由于采用了回热夹层和多孔介质回热结构,有效地回收了热量损失,使得微燃烧器具有良好的热性能.证明设计的多孔介质回热微燃烧器是一种燃烧效率高、热损失率低的微燃烧器.     

表面强化管外池沸腾传热特性研究

为实现节能降耗,开发了多种强化沸腾传热的高效换热管。以水为工质,在0.1MPa下对垂直光管、烧结多孔管和T槽管进行了池沸腾传热实验研究,并分析了沿管子轴向的温度分布。实验结果表明,烧结多孔管与T槽管能显著降低起始沸腾过热度、强化沸腾传热:烧结多孔管和T槽管的起始沸腾过热度比光管的低1.5K左右;烧结多孔管和T槽管的核态沸腾传热系数分别为光管的2.4～3.2倍和1.6～2.0倍。此外,烧结多孔管和T槽管能降低相同热流密度下的壁面温度,且有利于降低管子轴向的温差。     

多孔介质对太阳池传热传质影响的研究

通过实验与数值模拟相结合的方法研究在太阳池底部增设多孔介质水层对太阳池热盐双扩散及储热量、稳定性等的影响.在实验室研究了多孔介质对盐扩散的抑制作用.在海边沙滩建造了两个小型太阳池,测量了有无多孔介质太阳池的温度分布和盐度分布并进行数值模拟,模拟值与实验值吻合较好.计算了有无多孔介质太阳池盐梯度的分布、池子稳定性情况以及多孔介质对太阳池的储热量的影响.结果表明,在太阳池底部加设多孔介质水层可以提高太阳池LCZ温度.多孔介质水层有利于提高太阳池的储热量,有利于抑制盐分向上扩散,可以节省太阳池的盐资源消耗,并有利于提高非对流层的盐梯度和稳定性.     

Experiment Analysis of Pool Boiling Heat Transfer in Bead Packed Porous Layers

The heat transfer of pool boiling in bead packed porous layers was experimentally investigated to analyze the effects of the bead material, bead diameter and the layer number of the porous bed on the transport of flux and the heat transfer coefficients. The glass and copper bead, the bead sizes of 4 mm and 6 mm as well as the bead packed porous structures ranging from one to three layers were chosen in the experiments. The pool boiling heat transfer in the bead packed porous structures and that on the plain surface were compared to analyze the enhancement of pool boiling heat transfer while the bead packed porous layers were employed. The maximum relative error between the collected experimental data of the pure water on a plain surface and the theoretical prediction of pool boiling using the Rohsenow correlation was less than 12%. Besides, the boiling bubble generation, integration and departure have a great effect on the pool boiling and were recorded with a camera in the bead stacked porous structures of the different layers and materials at different heat flux. All these results should be taken into account for the promotion and application of bead packed porous structures in pool boiling to enhance the heat transfer.     

Dynamics of discrete bubble in nucleate pool boiling on thin wires in microgravity

A space experiment on bubble behavior and heat transfer in subcooled pool boiling phenomenon has been performed utilizing the temperature-controlled pool boiling (TCPB) device both in normal gravity in the laboratory and in microgravity aboard the 22^nd Chinese recoverable satellite. The fluid is degassed R113 at 0.1 MPa and subcooled by 26°C nominally. A thin platinum wire of 60 μm in diameter and 30 mm in length is simultaneously used as heater and thermometer. Only the dynamics of the vapor bubbles, particularly the lateral motion and the departure of discrete vapor bubbles in nucleate pool boiling are reported and analyzed in the present paper. It’s found that these distinct behaviors can be explained by the Marangoni convection in the liquid surrounding vapor bubbles. The origin of the Marangoni effect is also discussed.     

Natural convective boiling in vertical annular tubes filled with porous medium

Experimental and theoretical studies were carried out on the natural convective boiling heat transfer and critical heat flux (CHF) in uniformly heated vertical annular tubes filled with a porous medium and submerged in saturated water and R11 liquid. The heat transfer experimental results were compared with the case without a porous medium. It was shown that heat transfer is greatly enhanced by the porous medium in the region of low heat flux. By adopting a simple mixing flow model, a generalized approximate relationship was derived for predicting the CHF. The prediction agrees relatively well with the CHF experimental data. © 2000 Scripta Technica, Heat Trans Asian Res, 29(6): 447–458, 2000     

黏性液体中锐孔处气泡的形成

考察一定流量气体,通过锐孔在静止黏性液体中连续溢出气泡的过程。应用动力学平衡半经验关系式,综合考虑气泡受力,分析气泡形成过程,给出合理假设,预测气泡直径。分析表面张力、气体流速、锐孔直径及液相物性对气泡脱离尺寸的影响,找到影响气泡脱离尺寸的主要因素。计算预报值与实验结果符合良好。     

多孔太阳墙结构热特性及应用分析

结合多孔太阳墙建筑,运用数值模拟方法分析了不同太阳墙结构对室内送风参数的影响,比较了太阳能房间与普通房间在工作区范围内的热舒适性,并对太阳能房间的窗墙面积比进行了优化改进。研究表明:对太阳墙的设计优化,增大了太阳墙面积,提高了室内热舒适性;改进后房间窗墙比为0.303,窗地比为0.2,符合节能建筑要求。     

同轴套管式换热器周围介质层的传热特性

建立了一套利用同轴套管式换热器模拟周围饱和型多孔介质层传热特性的实验装置,得到了一些实验数据,建立了理论模型并采用实验与理论相结合的方法,为在更广的范围内预测或确定地下土壤层换热速率提供依据。     

Cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film

The cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film were experimentally investigated in order to estimate the cooling performance of a rotating roll in a hot mill system. The following four conclusions were reached in the study. (1) In the case of a single spray jet, the local heat transfer coefficient at the center position depends on the forced convective heat transfer by the impinging jet. However, the average heat transfer coefficient is proportional to the flow rate density of the cooling water, and it does not depend on the distance between the nozzle and heated surface. (2) In the case of a double spray jet, liquid film interference occurs. The local heat transfer coefficient at the center position is greater, and the cooling performance increases with the increasing flow rate density of the cooling water. (3) The cooling performance of a multispray jet is proportional to the flow rate density of the cooling water. It does not depend on the nozzle construction, distance, or specifications. Also, there is no relation to the liquid film interference. (4) When the optimum specifications of the spray nozzle are used, thermal analysis of a rotating roll shows that the temperature at a depth of 1.3 mm from the surface is below 130 °C. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 280–299, 2000     

Analysis of flow and heat transfer characteristics of porous heat-storage wall in greenhouse

The flow and heat transfer characteristics of porous heat-storage wall in greenhouse are studied by using the one-dimensional steady energy two-equation model for saturated porous medium. The results show that the heat exchange between the air and the solid matrix of the porous heat-storage wall depends upon the inlet air velocity, the porosity and the permeability of porous medium, and the thermal conductivity of the solid matrix. Because the incidence of solar radiation on the porous heat-storage wall is not uniform, the new composite porous solar wall with different porosity is proposed to reduce the disadvantageous effect. __________ Translated from Journal of Engineering Thermophysics, 2008, 29(2): 284–286 [译自: 工程热物理学报]     

The use of phase-change coatings for estimating the heat transfer characteristics in a rotating disc system

Phase-change coatings have been applied to the axial-clearance rotor-stator cavity for the estimation of the transient heat transfer characteristics of the surface of the rotating disc. The tests were conducted for an air mass flow coefficient C_w = 1220, a gap ratio G = 0.1, an axial-clearance ratio G_ca = 0.05 and for rotational Reynolds numbers of Re? = 1 × 10⁵ and 2 × 10⁵. The phase-change coating used had a melting point of 38°C. From a video recording of the transient movement of the melt-line on the rotor (coated with the phase-change material) blown with heated air, it was possible to compute the heat transfer coefficients. The data reduction was made using the ‘semi-infinite slab’ approximation to the governing one-dimensional transient heat conduction equation.     

Natural convection and its fractal for liquid freezing in a vertical cavity filled with porous medium

The numerical simulation for a freezing liquid-saturated porous media in a vertical cylindrical cavity under the third kind of thermal boundary condition is reported in this paper. It shows that the effect of natural convection in the liquid phase decreases the freezing layer thickness and the freezing front has a wave shape instead of a stable plane, with one or more pair of eddy cells. This indicates a fractal existence. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 165–171, 1999