汽-气液滴形管与圆管外凝结换热的研究

为了研究管型对汽-气凝结换热的影响和强化汽-气凝结换热过程,建立了汽-气在液滴形管与圆管外凝结换热所形成的气液膜的厚度及换热系数沿管壁分布的综合数学模型。通过有限差分的方法,以天然气燃烧产生的烟气为例对两种管型进行了计算比较。在有效换热面积相等的情况下,与圆管相比,液滴形管上半部分管径小,压力梯度大,有利于排液,下半部分表面曲率大,亦有利于排液;液滴形管表面形成的气膜薄,液膜亦薄,珠状凝结区域大,液珠尺寸小,凝结换热系数大;液滴形管对显热传递亦有一定的强化作用。通过实验对计算结果进行了比较验证,结果表明该模型亦适用于椭圆管和其它汽-气混合流体种类。     

内翅板蒸发式冷凝器降膜流动与传热特性

为了增强蒸发式冷凝器板外气液两相流动与传热,基于数值模拟方法对比分析了几种新型的板片结构,研究了板片构型、喷淋水喷淋密度、空气入口速度等因素对气液两相流流体流动及传热性能的影响。结果表明:半圆波纹板有较长的水膜稳定时间和较大的换热面积,换热性能较好;不同板片结构的蒸发式冷凝器具有不同的最佳喷淋水喷淋密度,最佳喷淋密度区间范围为0.48~0.93 kg/(m·s);空气入口速度一定时,半圆波纹板的壁面温度随喷淋密度增大而增大,气液界面温度随喷淋密度增大而减小;当空气入口速度小于2.5 m/s时,空气入口速度的适当增大能够有效减薄半圆波纹板板外水膜厚度,强化换热。     

船用冷凝器冷凝强化管换热性能研究

在船用制冷空调领域,制冷机组通常用海水作为冷却水冷却冷凝器中的制冷介质.为了提高冷凝换热管的抗海水腐蚀性能,船用冷凝器的管材常常采用Cu-Ni合金(如B10、B30等)或铝黄铜,这些材料往往导热性能较差[1,2].本文通过Cu-Ni合金冷凝管的凝结换热性能测试和试验分析,指出低导热系数材料对凝结换热效果的影响不仅仅表现为管壁热阻,还弱化了强化管管外的凝结换热,通过影响换热性能的机理分析,提出了采用复合管技术进一步提高低导热材料冷凝管换热效果的设想.     

空冷/水冷混合冷却竖管内LiBr溶液降膜吸收过程数值解

针对风冷和水冷联合冷却的竖管降膜吸收器,考虑汽液界面的阻力、变膜厚、横向对流和冷却水的冷却作用的影响,建立了降膜吸收过程中热质耦合数学模型和同心管环空内冷却水换热数学模型.计算了沿竖管内表面的液膜厚度、温度、浓度以及冷却水在混合冷却条件下的温度分布等参数.分析了冷却水进口温度、LiBr溶液Re数和PE数等参数对传热系数和吸收速率的影响.数学模型的计算结果与实验数据吻合较好.得出的结论对联合冷却吸收器的设计和优化具有指导意义.     

高效水冷冷凝器的研究

与传统的冷凝器进行比较,高效冷凝器壳体内设有挡液板,强制冷凝下来的液体沿壳体内壁流动,并通过挡液板把换热管纵向进行分割,从而使换热管在整个换热过程中均能有效维持在薄液膜冷凝的高效换热工况,降低了整个换热管的平均液膜厚度。     

重力热管蒸发段气液分布形式与换热能力分析

重力热管传热性能主要取决于蒸发段内液膜与液池的分布形式和换热过程.提出了许多液膜和液池换热机理与其影响因素之间的经验关联式.利用其中较完善的理论结果,总结了两者的换热机理分布图,通过计算传热系数的比值,发现在满足管内气液循环条件下,增加蒸发段内液膜段长度可以提高热管传热性能.     

波纹角对波纹竖板冷凝换热特性的影响

为了提高板式换热器的换热性能,探究换热板片的波纹角对波纹竖板冷凝侧换热特性的影响,优化其结构参数,采用建模计算的方法定量分析了波纹角对板片换热面积、液膜下落时间、流体努塞尔数、摩擦系数等参数的影响,并用可视化实验观察了不同波纹角下液膜流动路径的特点。分析结果表明,波纹角的变化对板片换热面积影响很小,可以忽略。波纹角越小,液膜下落时间越短,排液速度越快,液膜厚度越小,越有利于冷凝传热。波纹角越大,冷凝液的流动路径越接近波纹状流型,扰动越剧烈,增大了流体的雷诺数和努塞尔数,且雷诺数值越大,努塞尔数对波纹角的变化越敏感。流体与波纹板的摩擦系数也会随着波纹角的增大而增大,蒸气冷凝时产生的摩擦压降也越大。     

竖直管内导流叶片形成的气液旋流场的数值模拟研究

利用CFD FLUENT对竖直管内导流叶片形成的气液旋流场进行数值模拟研究分析。发现气液混合流体经过导流叶片后会形成环状流,且气液分离效果沿着管线逐步增加,存在气液分离最佳位置。在管道中心和管壁处的切向速度最小,随着流动发展,切向速度变小。轴向速度沿着管线先变大后变小,管道中心处出现最大值。管内压力场呈现中心低外围高,且沿着流动方向,外围和中心压差逐渐减小,压力曲线趋于平缓。     

开架式气化器换热管预冷过程传热数值模拟

换热管作为开架式气化器的基本换热单元,工作时存在从环境温度降至低温的预冷过程,采用数值模拟的方法,对开架式气化器换热管预冷过程进行数值模拟,得到管内流体温度场和速度场变化情况。分析发现:预冷过程中,海水膜厚度越厚,换热管内流体平均温度和平均速度越高;进入稳定换热后,海水膜厚度不再影响换热管内流体温度场和速度场的分布。结果有助于指导实际的预冷操作,对缩短预冷时间,提高预冷效率,减小海水用量,降低运行成本有意义。     

圆锥槽结构强化低压水蒸气的膜状凝结实验研究

通过在冷凝面上构造圆锥槽结构,使冷凝液膜内部形成结构梯度,利用圆锥槽内冷凝液膜产生的轴向拉普拉斯压差使冷凝液自发向圆锥槽半径减小的方向运动,实现冷凝液快速排出,从而强化低压蒸气膜状凝结。实验结果表明,冷凝流量较小时,圆锥槽内凝结液膜能快速排出,膜状凝结换热得到增强,而光滑表面由于接触角滞后造成冷凝液块钉扎在冷凝面,造成传热热阻增大,凝结换热恶化;而在较大冷凝流量下,圆锥槽表面冷凝液膜变厚,液膜内部轴向压力梯度减小,轴向输运能力减弱,光滑表面冷凝液块脱离频率增加,同时合并路径上的其他液块与液珠,加快表面更新频率,造成蒸气在光滑表面的冷凝速率略大于圆锥槽表面。     

R404A在水平强化管外的冷凝实验及数据处理方法

针对一种双侧强化换热管,实验测试和分析了制冷工质R404A在管外凝结与水在管内对流的传热规律,采用"Wilson图解法"和"Gnielinski法"两种不同的方法对实验数据进行了处理。经理论分析和实验研究表明,Wilson图解法对于双侧强化换热管管内、管外表面传热系数实验容易产生较大误差,"Gnielinski法"是更合适的方法。实验得出了管内对流传热和管外凝结传热的计算关联式及传热的强化倍率。对于制冷剂R404A,在强化管外凝结的表面传热系数随着壁面过冷度的增加而增大,呈现出与纯工质光滑管外冷凝时不同的变化趋势。     

微肋表面强化液氮温区冷凝传热特性的数值模拟研究

开展了微肋表面强化氮蒸气凝结传热的机理研究,基于CFD数值方法建立了竖直方向平板和微肋板上氮蒸气冷凝的三维数值模型,对比分析了平板表面和微肋表面的冷凝换热效果,通过表面冷凝的液膜分布和速度分布等内在参数揭示平板和微肋表面的冷凝换热机理。结果表明,氮的冷凝传热性能在微肋表面上得到显著提升,在1.5 K的冷凝传热温差下,其平均传热系数提高1.6倍,初步揭示了微肋表面(肋高H=0.6 mm、节距p=1 mm)对强化氮冷凝的有效性。分析显示,微肋的弯曲结构改变了液膜的空间分布,通过减薄局部液膜厚度实现了冷凝的强化。     

Condensation of steam on a vertical tube in a granulated material

The heat transfer enhancement was studied during condensation of steam on a chilled vertical surface of a tube packed into a granulated material with different contact angles of wetting. The dimensionless values of heat transfer at condensation on a surface in filling, obtained for a vertical tube in the range of Reynolds numbers from 70 up to 400, exceed Nu* values for a smooth tube by the factor of 2–3. The intensification of heat transfer on a vertical tube, housed in a granulated layer, is conditioned by the several interdependent phenomena: 1 — capillary ascent of some liquid near the meniscuses, and as a consequence, reduction of the mean film thickness; 2 — burble of a film at the points of sphere contact with a surface of condensation at Re>10; 3 — removal of some film liquid by a granulated layer; accompanied by simultaneous film burble at the points of sphere contact with a cooling surface at Re>83. Results of the current research can be used for the development of heat exchanging devices under the conditions of microgravitation.     

滴形管换热性能的实验研究

采用特殊形状和表面的管子是最为常用、有效的强化换热手段。本文基于滴形管换热器回收天然气锅炉排烟余热,提出了烟气侧的换热系数实验关联式。通过改变换热管间的排列间距,在不同烟气流量下,对圆管和滴形管的换热性能及影响因素进行了分析。与实验数据比较,验证了实验关联式可正确反映凝结换热的特性。结果表明:不同烟气量通过滴形换热管的压损小于圆管,约为圆管的0.33~0.38倍;烟气温度降大于圆管;冷却水通过滴形管的温升高于圆管;换热系数滴形管比圆管的提高约7%,表明滴形管的换热性能优于圆管。因此对于有凝结换热过程发生时,滴形换热管具有强化换热的作用。     

Flow regimes during condensation in superheated zone

Heat transfer experiments with flow visualization were performed with R134a in a smooth horizontal tube in order to verify that condensation occurs outside the two-phase region. The visualization performed for this work is unusual in that a diabatic visualization section was used, more closely approximating the behavior of the fluid in an actual condenser than typical adiabatic visualization experiments. In addition to the flow visualization, liquid film thickness measurements were taken and used to determine the cumulative latent heat transferred from the refrigerant. The measured film thicknesses and recorded videos confirm the presence of liquid in the bulk superheated region. Condensate appeared first as droplets and rivulets on the tube wall and then formed a thin film. The flow later passed through misty annular and wavy flow regimes as the refrigerant transitioned to the two-phase region.     

Analysis of Condensation Heat Transfer Performance in Curved Triangle Microchannels Based on the Volume of Fluid Method

Numerical simulations of condensation heat transfer of R134a in curved triangle microchannels with various curvatures are proposed. The model is established on the volume of fluid (VOF) approach and user-defined routines which including mass transfer at the vapor-liquid interface and latent heat. Microgravity operating condition is assumed in order to highlight the surface tension. The predictive accuracy of the model is assessed by comparing the simulated results with available correlations in the literature. Both an increased mass flux and the decreased hydraulic diameter could bring better heat transfer performance. No obvious effect of the wall heat flux is observed in condensation heat transfer coefficient. Changes in geometry and surface tension lead to a reduction of the condensate film thickness at the sides of the channel and accumulation of the condensate film at the corners of the channel. Better heat transfer performance is obtained in the curved triangle microchannels over the straight ones, and the performance could be further improved in curved triangle microchannels with larger curvatures. The minimum film thickness where most of the heat transfer process takes place exists near the corners and moves toward the corners in curved triangle microchannels with larger curvatures.     

R404A和R407C在水平强化管外的凝结换热实验研究

实验研究了近共沸制冷工质R404A与非共沸制冷工质R407C在水平强化换热管管外的凝结换热性能。采用"Wilson图解法"对实验数据进行处理。结果表明:对于R404A和R407C,强化管外的凝结换热系数随着壁面过冷度的增加而增大,呈现出与纯工质冷凝时不同的变化趋势,这主要是近共沸或非共沸工质凝结过程中,某些组分的凝结会遇到其它组分的凝结气膜热阻所造成的;随着过冷度增加,易挥发组分开始凝结,气膜变薄,冷凝传热系数增大。R407C在强化换热管管外的凝结换热系数比R404A要小70%左右,这是由于R407C的温度滑移较R404A要大,管外形成的凝结扩散气膜造成的影响更大。R407C在高热流密度工况下的换热效果提升明显,故应尽量工作在高热流密度区域。     

Experimental correlation of falling film condensation on enhanced tubes with HFC134a; low-fin and Turbo-C tubes

The objectives of this paper are to develop experimental correlations of heat transfer for enhanced tubes used in a falling film condenser, and to provide a guideline for optimum design of the falling film condenser with a high condensing temperature of 59.8 °C. Tests are performed for four different enhanced tubes; a low-fin and three Turbo-C tubes. The working fluid is HFC134a, and the system pressure is 16.0 bar. The results show that the heat transfer enhancement of low-fin tube, Turbo-C (1), Turbo-C (2) and Turbo-C (3) ranges 2.8–3.4 times, 3.5–3.8 times, 3.8–4.0 times and 3.6–3.9 times, respectively, compared with the theoretical Nusselt correlation. It was found that the condensation heat transfer coefficient decreased with increasing the falling film Reynolds number and the wall subcooling temperature. It was also found that the enhanced tubes became more effective in the high wall subcooling temperature region than in the low wall subcooling temperature region. This study developed an experimental correlation of the falling film condensation with an error band of ±5%.