Investigation of thermal-flow performance in close circuit cooling towers

Abstract

The main aim of this study is to investigate computational fluid dynamics (CFD) models to better understand the counter-current air–water interaction and thermal behavior in a packaged-type closed-circuit cooling tower (CCCT). The objectives of the study include examining key parameters such as flow behavior, specific humidity, water and air temperature, air velocity, heat transfer coefficient, pressure drop, and evaporation rate. To this end, both the Mixture multiphase method and the Discrete Phase Model (DPM) were implemented in two dimensions, while a three-dimensional DPM model was also developed to capture more realistic film distribution. The CFD results were validated by comparing the heat transfer coefficient between the water film and the outer tube surface with experimental data, aiming to provide a deeper understanding of heat and mass transfer mechanisms in CCCTs. Initially, a two-dimensional CFD model using the Mixture multiphase method combined with the Shear Stress Transport (SST) k–ω turbulence model was tested to represent the air–water interface. However, this approach tended to overpredict evaporation and the spray-side heat transfer coefficient. Therefore, a more advanced two-dimensional DPM model was developed, which incorporated wall film boundary conditions, evaporation–condensation mass transfer, and species transport for water vapor diffusion. Simulations were performed in ANSYS Fluent using a structured mesh of approximately 160,000 elements, and the spray water was directed toward the coil surfaces to capture realistic film formation and evaporation dynamics. Finally, the analysis was extended to a three-dimensional DPM model to overcome the uniform distribution assumption inherent in 2D simulations. The 3D approach was able to capture more detailed flow structures which strongly affect local heat and mass transfer. This model provided more physical fidelity in terms of film temperature, evaporation rate, and heat flux compared to the 2D model. To support the numerical approach, an experimental study was conducted using a CCCT equipped with 10 rows of staggered serpentine tubes under real atmospheric conditions. During testing, inlet and outlet air temperatures were recorded to evaluate the cooling performance of the system. The heat transfer coefficient between the falling water film and the outer tube surface was determined using empirical correlations and resistance-based calculations. These experimentally obtained values were then used to validate the CFD model by comparing the numerical results with measured data, ensuring that the simulation accurately captured the air-water interaction and coupled heat-mass transfer processes within the cooling tower. In terms of experimental data, the dry and wet bulb temperatures of inlet air were recorded as 29°C and 19.7°C, respectively. The process water entered the tower at 34.9°C and was cooled to 25.9°C at the outlet. Data were averaged over one hour under steady-state conditions to ensure accuracy. The range and approach values were calculated as 6.97°C and 6.17°C, respectively. Based on appropriate thermal resistance calculations, the spray heat transfer coefficient was determined to be 1163 W/m²·K. The total evaporation loss observed in experiment was 0.54 m³/h. However, spray heat transfer coefficient was found to be around 1526 W/m²·K based on the simulation results. Simulation results showed slight agreement with experimental findings, with the heat transfer coefficient exhibiting an error margin of approximately 30%, which indicates that the heat transfer between the water film and the outer tube surface is adequately captured in the 3D model.