Ice thermal energy storage (ITES) is one of the key solutions for peak-load shifting in commercial and industrial cooling systems. Natural convection during phase change is crucial to efficient energy management. However, the performance of the heat transfer can depend strongly on the complex process. It is still lacking in the contrasting role in solidification and melting. This study aims to numerically simulate the asymmetric regulation of the natural convection throughout a full charging–discharging cycle in a vertical ice-on-coil unit. A systematic investigation was also made to quantify the influence of the natural convection during solidification and melting. A theoretical framework was then provided for the dual promotion–inhibition behavior. A three-dimensional model of a concentric tube unit was developed using the enthalpy–porosity technique in the ANSYS Fluent platform. Furthermore, the nonlinear density–temperature relationship of the water (including the maximum at 4°C) was precisely incorporated to capture the buoyancy-driven flows. A series of tests was carried out to validate using published experimental data on the stearic acid solidification. The results showed that the better performance was achieved in the high accuracy (average relative error less than 0.2%), indicating a reliable framework for the coupled conduction–convection–phase change. Natural convection also exhibited a dual promotion–inhibition behavior during charging (solidification). Initially, the density-driven dual vortices enhanced the heat transfer. Among them, the peak storage rate increased by 15.6%, and the phase onset was advanced by 15.1%. This flow induced the nonuniform ice growth. Thus, an inverted-cone interface was formed with the top layer, nearly twice as thick as the bottom. The resulting uneven thermal resistance extended the total solidification time by 3.6%. The convection influence factor (θ) declined to about 0.95 in the later stage, indicating an overall inhibitory effect. Conversely, the natural convection was provided a sustained enhancement during discharging (melting). Expanding liquid regions promoted the stable multi-scale vortices that continuously transported the heat to the melting front. Complete melting time was shortened by 6.1%, whereas the 80% energy release was accelerated (φ = 0.2) by 13.1%. There was the expression of θ>1 and peaks at 1.08 during melting, indicating the positive and dominant role. The asymmetric behavior of the natural convection was governed by the phase-change direction. Specifically, the solidification was used to constrict the liquid domain, thereby forming the self-limiting feedback that suppressed the flow, whereas the melting was expanded to generate a self-amplifying loop that sustained the convection. The suppression of the natural convection during charging (e.g., via fins) was enhanced during discharging, in order to maximize the overall cycle efficiency. These findings can offer theoretical insights for the ITES system optimization.

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 $\overline{Nu}$ 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=12-16 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 $\overline{Nu}$ 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.