Visualization study of frosting characteristics on a horizontal cold plate under natural convection condition

Frosting has always been a vital problem in many applications and could induce efficiency degradation or even accidents. Despite its critical role in heat and mass transport, the influence of transient airflow patterns on frosting characteristics has not been fully investigated. In this paper, the Schlieren method was combined with conventional apparatus to visually study the frosting on a horizontal cold plate under natural convection conditions. The relationship between cold surface velocity and frosting characteristics was quantitatively analyzed for variable humidities and surface temperatures. A clear reduction in horizontal velocity by more than 90 % was observed when the frost layer accumulated on the surface. Humidity and cold surface temperature showed obvious effects on airflow near the surface. The average characteristic velocities increased from 6.4 × 10⁻³ m/s to 8.9 × 10⁻³ m/s in the early frosting stage when humidity increased from 50 % to 80 % at a cold surface temperature of -9 °C. A maximum reduction in characteristic velocity of 52.6 % was observed when the cold surface temperature increased from -9 °C to -5 °C with different humidities. The natural convection influenced the heat transfer rate and the growth rate of the frost layer, particularly at the edge region of the cold plate. A criterion was proposed to identify the initial and the subsequent stages by the dimensionless number Re. When Re > 2, the frosting was in the initial stage and the heat transfer coefficient increased with humidity; when Re < 2, the frosting became stable and the heat transfer coefficient scattered around 6.58 W/m²⋅K in the present experiments.