混合板换热器板片结构对流道内触点分布及流动换热性能的影响

混合板式换热器作为余热利用系统中高效节能设备，其换热性能对系统效率及稳定性有重要影响。该研究基于RNG k-ε湍流模型，对M型和H型板式换热器流道内流动及传热过程展开数值模拟，研究板片结构对板间触点分布的影响，及雷诺数Re和板片组合对横向流道内流动及换热特性的影响规律。结果表明：M型板间触点呈“方格”， H型呈“菱形”，且H型流流体速度较大、温度场分布较均匀。随Re数增大，压降△P及平均努塞尔数$ overline {Nu} $均增大；Re数较低时，H型的$ overline {Nu} $明显提高，且△P增幅较小；而高Re数（Re=6 000）时，与M型相比，H型的$ overline {Nu} $增幅仅为△P的25%左右。随波纹间距s减小，触点数明显增多，△P和$ overline {Nu} $均增大；s=12～16mm较合适。研究结果为混合板式换热器的设计及优化提供理论依据。

Effect of plate structure of mixed-plate heat exchanger on contact distribution and flow heat transfer performance

Plate heat exchangers have been widely used as heat transfer equipment in solar heat utilization and waste heat recovery systems. There are also high heat exchange efficiency, compact structure, strong adaptability, low operating cost, easy disassembly and repair, as well as long service life. Among them, the mixed-plate heat exchanger can be adapted to fully meet the requirements of heat load and pressure drop under different conditions, particularly in many fields, such as heating ventilation air conditioning (HVAC), solar heat utilization, food processing and agricultural drying. The heat transfer performance of mixed-plate heat exchangers can also dominate the efficiency and stability of the system. However, there is a complicated flow path between the plates of the mixed-plate heat exchanger. It is necessary to investigate the flow and heat transfer mechanism in the flow channel, in order to improve the heat transfer efficiency. In this study, the three-dimensional models of M-type (30°-60°) and H-type (50°- 60°) mixed-plate heat exchangers were established using RNG k-ε turbulence model. A numerical simulation was performed on the flow and heat transfer process in the flow channel. Meanwhile, the velocity and temperature fields were first plotted to evaluate the pressure drop △P and the average Nusselt number $overline {Nu} $. Subsequently, a systematic investigation was carried out to explore the effect of plate structure on contact distribution between plates, and the effect of Reynolds number Re and plate pattern combination on flow and heat transfer. The results showed that the contacts between plates were distributed in "square" and "diamond" in M- and H-type mixed-plate heat exchangers, respectively. The fluid flew cross-over in the transverse channel, and there was the wake vortex area with the lower velocity at the tail of contacts. There was a stronger fluid disturbance in the H-type heat exchanger, a less wake vortex area, and a more uniform temperature distribution, compared with the M-type one. The pressure dropped △P, whereas, the average Nusselt number $overline{Nu} $ both increased, with the increase in Re. At the same time, $overline{Nu} $ in the H-type heat exchanger increased outstandingly, while △P increased little when Re was low (Re<4000). Once Re was high, the increment in △P was greater than that in $overline{Nu} $. Furthermore, the increment in $overline{Nu} $ of H-type was only about 25% of that in △P at Re=6 000, compared with M-type one. Therefore, the heat transfer performance was improved at the cost of a large pressure drop. The number of contacts between plates, and the fluid velocity increased outstandingly with the decrease of corrugation pitch s, while the temperature distribution was more uniform. Additionally, the pressure dropped △P, as the $overline{Nu} $ increased. The heat transfer performance was improved significantly when the corrugation pitch was too small. However, there was a large pressure drop, especially for the H-type plate heat exchanger. Consequently, it was appropriate to set s=12~16mm. The fluid velocity increased with the increase of corrugation height h, indicating a more uniform temperature field. The increase in h greatly contributed to the longitudinal distance between contacts increasing and the fluid channel between plates expanding, thus enhancing the fluid mixing for the high heat transfer between plates. In addition, the pressure drop decreased, while the heat transfer coefficient $overline{Nu} $ increased. The increment of $overline{Nu} $ decreased in the high Re, but △P decreased outstandingly. Therefore, the increasing corrugation height can be expected to achieve higher heat transfer performance at a smaller pressure drop, particularly for the H-type plate heat exchanger. These findings can provide theoretical guidance for the design and optimization of mixed-plate heat exchangers.