Numerical simulation of glass doors fogging and defogging in refrigerated display cabinets P. D’Agaro * , G. Croce, G. Cortella DiEM, Dipartimento di Energetica e Macchine, Universita ` di Udine, Via delle Scienze 208, 33100 Udine, Italy Received 22 September 2005; accepted 21 January 2006 Available online 3 March 2006 Abstract A numerical procedure for the simulation of fogging and defogging phenomena is presented and is applied to the prediction of the demisting process on the glass door of a refrigerated closed display cabinet. The physical problem depends on the flow field along the glass, both inside and outside the refrigerated cabinet, on the unsteady con- duction through the multi-glazed door and on the heat and mass transfer phenomena within the thin water layer on the fogged surface. Thus a suite of routines for the unsteady simulation of the water layer evolution is coupled with FVM solvers for the Navier Stokes and energy equations inside the cabinet and for the conduction problem in the glass. The convective heat transfer coefficient on the exterior side of the glass door is evaluated through empirical correlations. The codes are iterated simultaneously to reproduce the physical transient, before, during and after the door opening. Simulations are carried out for a frozen food display cabinet with an air curtain flowing along the internal side of the door. Both activated and deactivated configurations of the electric heaters inside the glass are considered. The results show a good agreement with the experimental ones and highlight the importance of taking into account the shape and the distribution of the dew droplets. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Fogging; Defogging; Display cabinet; Droplet; Evaporation; Condensation 1. Introduction Cold glass fogging and defogging phenomena are of great interest in several application fields mainly because of the related visibility problems. Glass fogging occurs when the condensation of atmospheric humid air takes place on a cold surface. The condensed water layer is made up of a collection of droplets that, due to the scattering of the light, produces a clouded appearance. This is a major concern, as an example, for automotive or aeronautic industries where the certification processes require a quick defogging of the inside of vehicle windshields, in order to ensure adequate visibility and preserve passenger safety. In commercial refrigeration a typical fogging problem concerns the mist deposition on the glass surfaces in dis- play cabinets, particularly on the door of frozen food cab- inets where the internal temperature reaches 28 °C. When the door is open, condensation of atmospheric humidity occurs on the cold surface: as soon as the door is closed a quick defogging must be achieved to re-establish proper visibility of the product through the glass. Defogging is usually obtained via the use of an air curtain flowing along the glass surface and it is frequently accelerated by electric heaters embedded in the multi-glazed door. Unfortunately, the heaters yield undesired energy consumption, due both to their heat release and to the refrigeration system power required to remove such heat. Thus, these systems should be carefully designed and the prediction of condensation– evaporation is an essential step of such a design process. Recently anti-fog coatings, common in the optical applica- tion, are being employed also in the refrigeration industry, thus requiring the understanding and the prediction of the effect of the surface wettability on coating performance. 1359-4311/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2006.01.014 * Corresponding author. Tel.: +39 432 558017; fax: +39 432 558027. E-mail address: dagaro.paola@uniud.it (P. D’Agaro). www.elsevier.com/locate/apthermeng Applied Thermal Engineering 26 (2006) 1927–1934