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-ɛ 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 \overlineNu both increased, with the increase in Re. At the same time, \overlineNu 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 \overlineNu . Furthermore, the increment in \overlineNu 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 \overlineNu 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=12-16 mm. 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 \overlineNu increased. The increment of \overlineNu 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.