S型小管径管束表面氨水降膜流动与吸收的特性研究

The circular absorption amount of an ammonia water absorption refrigeration system will decrease due to the weak heat and mass transfer performance of the absorber which results in the decrease of system performance and the increase of absorber bulk. The falling film absorbers employed in middle and small scale ammonia water absorption refrigeration systems have drawbacks of small specific area for heat and mass transfer and weak heat and mass transfer abilities due to the constraints of flow pattern and the feature size of heat and mass transfer. They results in a bottleneck that the heat and mass transfer rate in unit volume is difficult to be broken through. In view of this, this project is planned to construct an S pattern tube bundle with small diameter tubes (inner diameter less than 2 mm) to make high specific area, high heat and mass transfer coefficient, enough heat and mass transfer period, good wettability and surface renewal induced by mixed solution available thus the heat and mass transfer amount in unit volume could be improved. The visualization method is employed to make the flow characteristics of the film on the surface clear and the critical sprinkle density ranges of flow pattern conversion determined. The effects of key parameters on ammonia water falling film absorption characteristics of the S type small diameter tube bundle are revealed by experimental method. The correlations of heat transfer number Nu and mass transfer number Sh are established according to the experiment results. Based on the flow characteristics of the film, an ammonia water falling film absorption model of the S type small diameter tube bundle is established and a numerical analysis is conducted to illuminate the internal absorption mechanism and features. The research of the project is to provide theory basis of the insight of the flow and absorption characteristics and the thermal design of the novel falling film absorber.

吸收器热质传输性能差会引起氨水吸收式制冷系统循环吸收量减小，使得系统性能下降、吸收器体积增大。由于传热传质特征尺寸和液膜流型的约束，当前中小型氨水吸收式制冷系统所采用的降膜吸收器存在比表面积小、传热传质差的缺点，导致单位体积吸收率难以突破。本项目拟构建S型小管径（内径小于2 mm）管束实现充分传热传质时间、良好润湿性、溶液混合诱导紊流促进表面更新、高比表面积以及高传热传质系数，从而强化吸收传热传质、提高单位体积吸收率。拟采用可视化方法研究S型管表面氨水溶液流动特性，获得溶液流型转换临界喷淋密度范围。以实验方法揭示关键参数下S型小管径管束表面氨水降膜吸收特性，建立Nu和Sh的函数关系式。基于S型管表面溶液流动特点，建立氨水降膜吸收数值模型，通过数值研究阐明S型小管径管束表面氨水降膜吸收内部传热传质机理与特性。本项目的研究为理解S型小管径管束氨水降膜吸收的机理与特性及进行热设计提供完整的理论基础。

吸收传热传质性能对吸收式制冷系统的体积和性能具有显著的影响，当前中小型氨水吸收式制冷系统所使用的降膜吸收器存在比表面积小、传热传质差的缺点，导致吸收器体积热流密度较低。基于提高氨水降膜吸收的体积热流密度，本项目构建了对称的S型小管径管束，通过理论和实验研究，获得了S型小管径管束氨水降膜吸收的耦合传热传质机理和性能，验证了其高体积热流密度的特点。主要研究内容、重要结果和关键数据如下。.(1) 建立了努谢尔特平整层流膜假设下S型管表面降膜流动和传热模型，以体积热流密度及S型管内沿程流动阻力为优化目标，以管径、弯曲中心距以及管内流动雷诺数为优化参数，构建了管外径为2.5mm的S型管束降膜换热器。.(2) 实验研究了不同喷淋密度下S型管表面溶液流型，包括干壁态、液膜态、液滴态和液帘态，获得了流型转换的临界喷淋密度范围，明确了液膜态运行的喷淋密度范围。发现了液膜为波动膜、弯管段接触点有溶液流量两侧再分配的特点。.(3) 实验研究了S型小管径管束氨水降膜吸收过程传热传质性能，通过实验数据建立了传热传质实验关联式，并进行了溶液入口过冷度的修正，传热传质实验关联式与实验数据误差在±15%以内。相比于文献中外径为9.5mm的横管降膜吸收的实验结果，S型小管径管束降膜吸收器的比表面积提高了108%，单位传热温差体积热流密度提高了133%，膜侧传热系数提高了12.6%。.(4) 基于双膜理论，使用获得的传热传质实验关联式建立了S型小管径管束氨水降膜吸收耦合传热传质模型，获得了气相、液相和气液界面的温度分布和浓度分布，揭示了吸收过程耦合传热传质机理。在不同的实验条件下，模型预测与实验测量的溶液出口浓度最大误差为24.8%、最小误差为2.9%；溶液出口温度最大误差为26.5%，最小误差为4.1%。.本项目提出了一种高体积热流密度的S型小管径管束氨水降膜吸收器，通过实验研究和理论建模，建立了传热传质实验关联式，阐明了其吸收过程传热传质机理，提供了S型小管径管束氨水降膜吸收器的热设计基础。