Numerical Investigation of the Influence of Microchannel Diffusion Welded Heat Exchanger Head Structure on Flow Characteristics
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摘要: 为认识和掌握封头几何结构对以超临界二氧化碳（SCO2）为工质的微通道扩散焊式换热器（简称MCD）流量分配能力的影响机理，并优化MCD封头结构设计，改善换热器流量分配均匀性，从而提高换热效能与安全性，本研究采用数值模拟方法，对不同结构MCD封头的流动和流量分配性能进行研究。为解决硬件条件对复杂换热器模型网格数量的限制，开发了一款可广泛用于MCD流体力学性能模拟的UDF程序，大量减少了重复性网格工作，降低了模拟计算的硬件门槛。运用Fluent软件，分析封头局部几何参数（不同封头壁面曲线参数、不同多孔挡板参数等）对压降、流量分配性能、流场的影响。研究结果表明，进口封头腔体内产生的涡旋以及出口封头的突缩结构会造成压力损耗，低高度的二次曲线壁面封头可以有效抑制涡旋产生，减少突缩结构造成的压力损耗，从而降低封头压降，提高流量分配性能。
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关键词:
- 微通道扩散焊式换热器（MCD） /
- 封头结构 /
- 流动特性
Abstract: In order to comprehend and grasp the influence mechanism of head geometry on the flow distribution capacity of microchannel diffusion welded heat exchanger (MCD) with supercritical carbon dioxide (SCO2) as working fluid, optimize the head structure design of MCD, improve the flow distribution uniformity of heat exchanger, and thus improve the heat exchange efficiency and safety, this study employed numerical simulation methods to investigate the flow and flow distribution performance of MCD heads with different structures. To address the limitations posed by hardware conditions on the grid resolution of complex heat exchanger models, a user-defined function (UDF) code which can be widely used for simulating fluid dynamics performance in MCDs was developed, which can greatly reduce the repetitive grid-related tasks and lower the hardware threshold of simulation calculation. The software Fluent was used to analyze the influence of local geometric parameters of the head (different head wall curve parameters, different porous baffle parameters, etc.) on pressure drop, flow distribution performance and flow field. The results show that the vortex generated in the cavity of the inlet head and the abrupt contraction-expansion structure of the outlet head can cause pressure loss. The head featuring lower-height secondary curved wall can effectively suppress vortex generation and reduce pressure loss attributed to the abrupt contraction-expansion structure, subsequently lowering header pressure drop and enhancing flow distribution performance.tributed to abrupt contraction-expansion structures, thus reducing the head pressure drop and improving the flow distribution performance. -
图 2 UDF功能
Pi—单个流道的进口的工质压力；Po—单个流道的出口的工质压力；Vi—单个流道的进口的工质流速； Vo—单个流道的出口的工质流速；$P_i^* $ —经UDF程序计算的进口工质压力；$V_{\rm{o}}^*$—经UDF计算的出口工质流速；ΔP—由流速计算出的流道内压降
Figure 2. UDF Function
图 7 不同网格方案下入口封头的出、入口压力大小
Figure 7. Inlet Pressure and Outlet Pressure of the Inlet Head at Different Mesh Schemes
图 16 各类型封头内部流体平均压降
Figure 16. Average Fluid Pressure Drop inside Various Types of Heads
表 1 封头模型边界条件
Table 1. Boundary Conditions of Head Model
入口边界参数 入口边界参数值 工质 CO2 温度/℃ 200 压强/MPa 10 流速/(m·s−1) 6 出口压力/Pa 0 
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表 2 网格划分方案
Table 2. Mesh Division Scheme
网格
方案最大网格
尺寸/mm最小网格
尺寸/mm微流道边
界层厚度/mm体网格
生长率网格总
数/万方案1 10 0.2 0.06 1.2 260.6 方案2 5 0.1 0.06 1.2 484.6 方案3 2.5 0.075 0.06 1.2 788.4 方案4 1.8 0.05 0.06 1.2 1707.2 方案5 1.3 0.03 0.06 1.2 3256.5
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表 3 各种类型封头的流量分配参数值
Table 3. Flow Distribution Parameter Values for Various Types of Heads
封头类型 整体不均匀度S 长轴不均匀度Sy 短轴不均匀度Sz 传统封头A 0.07814 0.05085 0.02578 传统封头B 0.08431 0.04756 0.03521 改进封头A 0.08629 0.04659 0.03651 改进封头B 0.08899 0.05703 0.02173 改进封头C 0.07758 0.03923 0.02593
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