Experimental study on the effect of surface temperature on the frost characteristics of an inverted cold plate under natural convection

Abstract

• Frosting experiments on inverted cold plate at micro scale are carried out • Four frosting stages are illustrated with three key dividing moments defined • Uneven frost layer could be quantitatively reflected by its surface roughness • Big converged droplets and falling of branches influence the frosting process • Reverse melting phenomenon and its dynamic variation law are analyzed Frosting is frequently seen in nature and engineering fields. To avoid the drawbacks and take advantage of frost, analyzing the frost characteristics is meaningful. Frosting experiments on an inverted cold plate at surface temperatures of −30 ∼ -10 °C under natural convection are carried out. Frost characteristics, including frosting stages, frost layer thickness and surface roughness, dynamic frosting rate and reverse melting, are analyzed and discussed. Results show that stages of droplet condensation and coalescence, solidified liquid tip-growth and frost layer growth are within 220 s with their frost thickness less than 450 × 10 -6 m. Formation of large condensate droplets under influence of gravity causes a longer solidification time to 160.2 s and decreases the surface roughness. As surface temperature decreases, frost thickness increases and surface roughness fluctuates more intensely, with its amplitude maximum at 102.29 × 10 -6 m. Dynamic frosting rates at different surface temperatures increase linearly and then declines in a curve, and their trend varies when first reverse melting occurs. At the surface temperature of −25 °C, amount and time of reverse melting are 9.42 × 10 -8 m 2 and 1.25 s, respectively, leading to the largest downward of dynamic frosting rate. Contributions of this study are expected to provide a more in-depth understanding of frosting and thus provide a reference for optimal defrosting control in engineering applications.