Winter water temperature variation and ice formation patterns in aqueducts under thermal-flow coupling

Water diversion aqueducts are highly susceptible to freezing and blockage at low temperatures in winter, particularly in the cold and arid regions of northern China. There are also significant declines in the water conveyance capacity and structural performance. A great threat has then been posed to the long-term safe operation of water diversion projects in these regions. Therefore, it is crucial to the patterns of winter water temperature and the mechanisms of icing evolution in aqueducts. However, it is still lacking in the winter operational characteristics of aqueducts. This study aims to investigate the spatiotemporal variations of water temperature and icing behavior under the influence of multiple coupled factors. A three-dimensional coupled numerical model was developed for the non-isothermal flow heat transfer in a closed aqueduct. Some parameters were then considered, including air temperature (T_a), flow velocity (u), inlet water temperature (T₀), solar radiation, and wind speed. The characteristic parameters were selected as the temperature drop value (ΔT) and temperature drop rate (Δf_u) of the water flow in the aqueduct. A systematic analysis was implemented to explore the temporal and spatial variation patterns of winter water temperature. Additionally, a predictive model was also established for the water temperature and freezing points. The average water temperature was then calculated at the outlet section of the aqueduct under various conditions. The icing locations of the water flow were predicted under specific temperature scenarios. A two-dimensional transient icing model was established to consider the effect of the ice-water phase transition on heat transfer in the water flow in an aqueduct. The icing evolution was also obtained in the aqueduct water flow. The correctness of the model was verified to compare the indoor test data with the simulation. The results show that there was a decrease in the water temperature of the aqueduct over time in winter, especially with the increasing water delivery length. Along the length of the aqueduct, the temperature drops of water flow exhibited an overall trend of rapid decline followed by a slower reduction. The ΔT value decreased with the increasing u under certain meteorological conditions, as T₀ rose. While there was an increase. The primary influencing factor on water temperature was the flow velocity u, with the largest temperature drop rate in the range of 0-1m/s. The temperature drops near the solid walls of the aqueduct were approximately 2 to 4 times greater than that at the center of the cross-section. Solar radiation caused a greater decrease in the water flow temperature near the aqueduct wall at night than during the day. In contrast, the water temperature at the center of the aqueduct cross-section was less affected by solar radiation. According to the water flow freezing in aqueducts, the release of latent heat during condensation shared a compensatory effect on the water temperature. The bank ice width was used as a quantitative indicator of icing. The amount of ice formation increased over time. There was a shorter critical length for the water flow near the shaded side wall of the aqueduct to reach the freezing point. The ice formation on the shady side was approximately three times that on the sunny side. This finding can also provide a strong reference for the safe operation of aqueducts at low temperatures in winter.