We conducted a series of frost buildup simulations under turbulent flow using DNS on flat plates and fin-and-tube heat exchangers. The air and frost layers are coupled using the immersed boundary method. A number of challenges need to be overcome to make these simulations feasible. One very important physical challenge is the vast disparity in the time scales for the turbulent air flow and the frost buildup. Frost builds up at a rate of few millimeters per hour, whereas the turbulent flow evolves at a much faster rate, which makes the simulations prohibitively expensive. In this work we use the slow-time acceleration technique to accelerate frost buildup by a predetermined factor.When the first frost layer forms on the cold plate, its temperature matches the plate. As the frost thickens, its surface temperature rises above that of the plate, leading to changes in frost properties like density and thermal conductivity. To handle this, we use a densification scheme that removes vertical variations in frost properties, allowing only lateral differences.We varied the plate temperature from −10°C to −5°C, and the free-stream temperature and humidity ratio from 0°C to 20°C and 3.77 × 10⁻³ to 1.47 × 10⁻², respectively. At a shear Reynolds number of 180, heat and mass transfer rates are found to be three times higher than in laminar flow. Additionally, we find the Nusselt and Sherwood numbers, when scaled by the frost surface to free-stream temperature and humidity ratio difference, respectively, to become independent of free-stream temperature and humidity ratio as well as plate temperature. The scaling can be used in conjunction with the frost layer conservation equations of mass and energy to provide an estimate for the temporal evolution of the frost layer under turbulent flow conditions for a range of plate temperatures as well as free-stream temperatures and humidity ratios.In the case of finned tubes in heat exchangers, frost deposition and growth are investigated for a bulk Reynolds number of 240. The results show significant spatial variation, with frost deposition being much greater on the fins than on the tubes. Nusselt and Sherwood numbers follow the same trend, with higher values on the fins.
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