Study on winter water temperature variation and ice formation patterns in aqueducts under thermal-flow coupling

Water diversion aqueducts in the cold and arid regions of northern China were highly susceptible to issues such as freezing and blockage during low-temperature winter conditions. These problems led to significant declines in water conveyance capacity and structural performance, posing a threat to the long-term safe operation of water diversion projects in these regions. Therefore, understanding the patterns of winter water temperature changes and the mechanisms of icing evolution in aqueducts was crucial. However, current research on the winter operational characteristics of aqueducts lacked an in-depth investigation into the spatiotemporal variations of water temperature and icing behavior under the influence of multiple coupled factors. Based on this, a three-dimensional coupled numerical model of non-isothermal flow heat transfer in a closed aqueduct was developed in this study. The effects of air temperature (T_a), flow velocity (u), inlet water temperature (T₀), solar radiation, and wind speed were considered. The temperature drop value (ΔT) and temperature drop rate (Δf_u) of the water flow in the aqueduct were used as characterization parameters to explore the temporal and spatial variation patterns of winter water temperature. Additionally, a predictive model for water temperature changes and freezing points was constructed to calculate the average water temperature at the aqueduct's outlet section under various conditions and to predict the icing locations of the water flow under specific temperature scenarios. Considering the effect of the ice-water phase transition on heat transfer in the water flow, a two-dimensional transient model for icing in an aqueduct is established. The characteristics of icing evolution in the aqueduct water flow were investigated, and the model's correctness was verified by comparing indoor model test data with simulation results. The results show that in winter, the water temperature in the aqueduct decreases over time and with increasing water delivery length. Along the length of the aqueduct, the temperature drop of the water flow exhibits an overall trend of rapid decline followed by a slower reduction. Under certain meteorological conditions, ΔT decreases with increasing u and increases as T₀ rises. The primary factor affecting the change in water temperature is the flow velocity u, with the largest temperature drop rate occurring in the range of 0-1m/s. Under the same conditions, the temperature drop near the solid walls of the aqueduct is approximately 2 to 4 times greater than at the center of the cross-section. Solar radiation causes a greater decrease in 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 is less affected by solar radiation. According to research on water flow freezing in aqueducts, the release of latent heat during condensation has a compensatory effect on the water temperature. The bank ice width (ΔB_i) is used as a quantitative indicator of icing, the amount of ice formation increases over time, and the critical length for the water flow near the shaded side wall of the aqueduct to reach the freezing point is shorter than in other areas, the ice formation on the shady side is approximately three times that on the sunny side. The research results of this study can provide a reference for the safe operation of aqueducts in low-temperature winter environments.