板翅式换热器物流分配特性及换热的研究/Fluid Flow Distribution and Heat Transfer

2018-08-25 06:44:09

The heat flow fluid 换热器



板翅式换热器由于具有效率高、体积小等突出优点,在航空、化工、HVAC等领域具有不可替代的地位。由于换热器在设计、制造、装配、安装、运行中的种种问题引起其内部流动的不均匀分配,从而造成换热效能的下降。如何改善换热器内部的物流分配,提高换热器整体效能的研究,一直得到了很多学者的共同关注。
本文首次数值地研究了目前工业中应用的传统型封头与导流片对板翅式换热器内部物流分配的影响。数值计算结果表明,换热器内部存在严重的物流分配不均匀性。在对物流分配不均匀性分析的基础上,对多种改进型封头和导流片结构进行了数值计算,计算结果表明,改进的封头和导流片结构可以有效地改善换热器内部的物流分配。 同时数值研究还发现,影响导流片导流性能的主要因素包括导流片进口流速不均匀度、导流角度、导流片开孔率等。
本文扩展完善了板翅式换热器物流分配不均匀性实验系统,在原先只能进行单相液体实验的基础上扩展成可以进行单相气体、气液两相物流分配实验以及换热实验。同时开发了一套基于可编程二次仪表的智能型﹑高精度计算机数据采集系统,对实验系统中的压力﹑流量﹑温度进行实时监测。同时对目前工业中广泛使用的板翅式换热器进行了结构模化设计,以便研究其内部物流分配的情况。
实验研究了不同的入口结构对换热器气态物流分配的影响,研究结果表明,目前空分系统中广泛使用的导流片与封头结构造成换热器内部严重的物流分配不均匀。研究还表明流体雷诺数对换热器内部物流分配不均匀的影响是非常严重的。同时得到了不同入口结构的物流分配不均性参数与流体雷诺数之间的关系式。
提出了孔板封头结构,并且实验研究了孔板上小孔的孔径、孔的分布、孔板长度以及封头尺寸大小对板翅式换热器内部物流分配不均匀性的影响。研究结果表明,孔板封头结构极大的改善了换热器内部物流分配,换热器内部的最大流速与最小流速之比由改进前的3~4倍降低到1.1~1.2倍,其中错排孔板封头具有最佳的改进效果。
实验研究了板翅式换热器两相流物流分配的问题,研究发现,两相流在板翅式换热器内部通道分布比单相流更加不均匀;进口流量和干度是影响板翅式换热器两相流物流分配的主要因素;对于两相流在换热器出口截面上的分配,其不均匀主要体现在液相上,其中又以横向不均匀为主。
在对板翅式换热器内部物流分配不均匀研究的基础上,进行了物流分配不均匀对换热器效能影响的实验研究。通过实验研究发现物流分配不均匀导致换热器效能的严重下降,同时通过实验得到了换热器物流分配不均匀参数与换热器效能恶化程度的关联式。
实验结果与数值计算结果对比表明二者吻合较好,说明了数学模型及算法建立的可靠性。本文的研究方法及结论对板翅式换热器的优化设计具有重要意义。


Plate-fin heat exchanger (PFHE) plays an important role in aerospace, chemical engineering and HVAC, because of its high heat transfer efficiency and compactness. As the result of all sorts of problem in design, manufacture, assembly, installation and operation of heat exchangers, the fluid flow maldistribution happens in PFHE, which leads to the performance deterioration of the heat exchanger. How to improve the fluid flow distribution so as to enhance the efficiency of heat exchanger attracts the attention of a lot of researchers.
The influences of the conventional distributor and header configurations used in industry at present on the fluid flow distribution in PFHE were numerically investigated for the first time. The numerical results showed that the fluid flow maldistribution is very serious in the heat exchanger. On the basis of the analysis of the fluid flow maldistribution, some improved header and distributor configurations were computed. The computational results indicated that the improved header and distributor configurations can effectively improve the performance of fluid flow distribution in the heat exchanger. It was also found that nonuniform inlet velocity, inlet angle and punched ratio are the main factors affecting the distribution performance of distributor.
The available experimental system only used to study liquid flow distribution has been expanded to investigate gas flow distribution, two-phase flow distribution and heat exchange. For the experimental study, an intellective data acquisition system with high precision has been developed based on the computer. It can accurately measure the parameters of the pressure, flow rate and temperature in the experimental system. The heat exchangers were simulated according to the configuration of PFHE used wildly in the industry for the experimental research.
The experimental study of the influence of different inlet configurations on the gas fluid flow maldistribution in PFHE has been completed. It is found that very serious is the fluid flow maldistribution caused by the header and distributor, which are used widely at present in the air separation plant. The influence of the fluid Reynolds number on the fluid flow maldistribution in PFHE is very heavy. The correlations of the flow nonuniformity parameter and the fluid Reynolds number under the different inlet configurations have been obtained
The header configuration with perforated plate was brought forward. The experimental investigations of the influence of the orifice diameter, the distribution of orifice, the perforated plate length and the dimension of header on the fluid flow maldistribution in PFHE have been performed. The experimental results showed that the header configuration with perforated plate greatly improves the fluid flow distribution in PFHE and the ratio of the maximum flow velocity to the minimum flow velocity of the heat exchanger drops from 3~4 to 1.1~1.2. The header configuration with the staggered arrangement perforated plate brings about the best improvement.
The two-phase fluid flow distribution in PFHE was experimentally studied. The experimental results indicated that the two-phase fluid flow maldistribution among the passages of PFHE is more obvious than the single-phase flow. Inlet flow rate and dryness are the main factors affecting the two-phase fluid flow distribution in PFHE. The liquid phase nonuniformity is more serious than that of the gas phase and the flow nonuniformity in the lateral direction is more serious than that in the gross flow direction.
On the basis of the study of fluid flow maldistribution in PFHE, the influence of fluid flow maldistribution on the performance of heat exchangers was studied experimentally. It is shown that the fluid flow maldistribution leads to a great loss of the heat exchanger effectiveness. The correlation of the flow nonuniformity parameter and the heat exchanger effectiveness loss has been obtained by the experimental study.
The numerical results are compared with the experimental results. They are basically consistent which indicates that the mathematical model and the calculating method are reliable. The method and conclusions of this paper are of great significance on the optimum design of PFHE.