energies Review Evaporator Frosting in Refrigerating Appliances: Fundamentals and Applications Christian J. L. Hermes 1, *, Joel Boeng 1 , Diogo L. da Silva 2 , Fernando T. Knabben 1 and Andrew D. Sommers 3   Citation: Hermes, C.J.L.; Boeng, J.; da Silva, D.L.; Knabben, F.T.; Sommers, A.D. Evaporator Frosting in Refrigerating Appliances: Fundamentals and Applications. Energies 2021, 14, 5991. https:// doi.org/10.3390/en14185991 Academic Editor: Andrej Kitanovski Received: 19 July 2021 Accepted: 17 August 2021 Published: 21 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 POLO Laboratories, Department of Mechanical Engineering, Federal University of Santa Catarina, Florianópolis 88040-900, Brazil; joel@polo.ufsc.br (J.B.); fernandok@polo.ufsc.br (F.T.K.) 2 Laboratory of Vehicular Refrigeration, Department of Mobility Engineering, Federal University of Santa Catarina, Joinville 89218-035, Brazil; diogo.londero@polo.ufsc.br 3 Department of Mechanical and Manufacturing Engineering, Miami University, 56 Garland Hall, 650 East High Street, Oxford, OH 45056, USA; sommerad@miamioh.edu * Correspondence: hermes@polo.ufsc.br Abstract: Modern refrigerators are equipped with fan-supplied evaporators often tailor-made to mitigate the impacts of frost accretion, not only in terms of frost blocking, which depletes the cooling capacity and therefore the refrigerator coefficient of performance (COP), but also to allow optimal defrosting, thereby avoiding the undesired consequences of condensate retention and additional thermal loads. Evaporator design for frosting conditions can be done either empirically through trial-and-error approaches or using simulation models suitable to predict the distribution of the frost mass along the finned coil. Albeit the former is mandatory for robustness verification prior to product approval, it has been advocated that the latter speeds up the design process and reduces the costs of the engineering undertaking. Therefore, this article is aimed at summarizing the required foundations for the design of efficient evaporators and defrosting systems with minimized performance impacts due to frosting. The thermodynamics, and the heat and mass transfer principles involved in the frost nucleation, growth, and densification phenomena are presented. The thermophysical properties of frost, such as density and thermal conductivity, are discussed, and their relationship with refrigeration operating conditions are established. A first-principles model is presented to predict the growth of the frost layer on the evaporator surface as a function of geometric and operating conditions. The relation between the microscopic properties of frost and their macroscopic effects on the evaporator thermo- hydraulic performance is established and confirmed with experimental evidence. Furthermore, different defrost strategies are compared, and the concept of optimal defrost is formulated. Finally, the results are used to analyze the efficiency of the defrost operation based on the net cooling capacity of the refrigeration system for different duty cycles and evaporator geometries. Keywords: frost; defrost; heat exchanger; refrigeration; appliance 1. Introduction Frost is likely to build up whenever moist air flows over a chilled surface whose temperature is sub-zero and below the local dewpoint of the air stream. Such a combination of psychrometric conditions and a cold substrate give rise to frosted media that can be observed in different engineering applications, spanning from agriculture to aerospace devices. A niche of particular interest lies in the evaporators of household and commercial refrigerating appliances, where the performance is dramatically affected by frost accretion, which not only imposes an additional thermal resistance to the heat flux but also diminishes the air flow passage, thus decreasing the air flow rate for a fixed pumping power. Either way, the evaporator heat duty (also known as the cooling capacity) is depleted over time, thereby demanding longer compressor cycles (and therefore more energy input) to accomplish the same refrigerating effect [1]. Energies 2021, 14, 5991. https://doi.org/10.3390/en14185991 https://www.mdpi.com/journal/energies