Heat Transfer Characteristics Of Ice Slurry In Finned Tube Heat Exchangers

Ice slurry is a kind of particular solid-liquid two-phase flows which phase change will occur during flowing and heat exchanging processes. Ice slurry has high energy capacity owning to the huge latent heat from the ice crystals melting. Meanwhile, it has good liquidity. Thus, it has been widely applied into fishery industry, medical treatments, food preservation, building cooling and so on. In district cooling systems, using ice slurry instead of conventional chilled water as cooling medium has many potential benefits, for instance, reducing flow rate, downsizing pipe networks and decreasing pump power. Furthermore, using ice slurry as cooling medium and then combining with the ice thermal storage technology has a positive effect on keeping the electric power network load stability. Because it can store cooling capacity during the low electric power load at the night, and then use it during the daytime to satisfy the peak load. In addition, the operation cost of the system can be cut down in the region where off-peak electricity tariffs are available. Hence, using ice slurry as cooling medium is a kind of feasible and desirable ways to improve the energy efficiency and decrease the operating consumptions of the buildings.Unlike traditional solid-liquid two-phase flows which composition concentration of the flows is constant, the concentration of ice crystals in ice slurry is variable owing to ice crystals melting during the flowing and heat exchanging processes. According to the published literature, the present commercial heat exchanger designed for the single phase flow cannot take full advantage of the potential of ice slurry. Besides, there is little theoretical research on the heat transfer characteristics of ice slurry in the finned tube heat exchanger which is a kind of common terminal heat exchangers in HVAC systems. It is necessary to makes in-depth analysis on the major factors that mainly influence the heat transfer performance of the finned tube heat exchanger. Thus, a numerical model has been established in the present study by introducing the VOF (Volume of Fluid) method and considering the influence of ice crystals melting. In addition, a PDEs (partial differential equations) model based on the principle of energy balance is also developed and its analytical solution is obtained by solving the model. Calculated results from both numerical simulation and the analytical solution are compared and validated by the experimental data from the existing literature. The variation and its impact of ice concentration and heat transfer characteristics of ice slurry in the finned tube heat exchanger has been analyzed by both the validated numerical model and analytical solution. Moreover, an experiment has been conducted on a custom-built finned tube heat exchanger with ice slurry as cooling medium in the laboratory. The experimental results are used to further verify and modify the numerical model. Finally, the heat transfer performance of the ice slurry finned tube heat exchanger is analyzed by the modified numerical model.Research results show that the heat transfer performance of the finned tube heat exchanger is significantly affected by the ice concentration of ice slurry. Heat transfer rate of the heat exchanger increases with the inlet ice mass fraction growing. Nevertheless, the growth rate of the heat transfer rate decreases at the same time. The heat transfer rate no longer rises when the inlet ice mass fraction increases to a critical value which is usually 30-35%. At this condition, the effect of ice mass fraction on the heat transfer performance of the heat exchanger will be optimal. As to heat transfer coefficient of ice slurry, it is affected by velocity, flow regime and ice mass fraction of ice slurry. Heat transfer coefficient of ice slurry grows with the velocity and ice mass fraction increasing. In addition, when ice slurry flow is in laminar region, the growth rate of ice slurry heat transfer coefficient along with the ice mass fraction is larger than that in turbulent region. Besides, the numerical results show that, extending the width of the fins at the downstream direction of the airflow is favorable to making full use of the constant low temperature of ice slurry and improving the performance of the heat exchanger.