The antireflective potential of dropwise condensation

Abstract

The transmissivity of fogged glass to visible light incident on the dry side is studied with ray tracing to show that condensation can act as an optically thick antireflective coating. A new simulation method is described that uses symmetry relations and analytical expressions for the intersection of rays and surfaces to include all drop-drop and drop-surface interactions between an infinite number of drops. Angle of incidence, droplet contact angle, and surface coverage are varied. The simulation reveals that in the optimal contact angle range, dropwise condensation can decrease the reflectance of glass to below even that of glass coated with a water film.

Transmittance of transparent windows with non-absorbing cap-shaped droplets condensed on their backside

“…4(b)]. Similar simulations were performed by Tow [15]. Excellent agreement was found between the two approaches confirming the validity of the algorithms and proper consideration of the optical phenomena and boundary conditions (see Fig.…”

“…Moreover, in some cases (e.g., for θ c ¼ 30°and f A 4 85%), the normal-hemispherical transmittance exceeded that of transparent glass window covered with a non-absorbing water film of arbitrary thickness denoted by T nh,f and expressed as [32] where ρ aw , ρ wf , and ρ fa are the specular reflectivities under normal incidence at the air/window, window/film, and film/air interfaces respectively given by ρ ij ¼ (n i -n j ) 2 /(n i þ n j ) 2 . This could be attributed to total internal reflection occurring at the top surface of the glass window, so that incidence radiation was reflected back into the droplets and eventually transmitted, as suggested by Tow [15].…”

“…Tow [15] investigated the directional-hemispherical reflectance by the ray-tracing method at wavelength 500 nm for non-absorbing monodisperse cap-shaped water droplets (n d ¼ 1.33, k d ¼ 0) in a hexagonal arrangement on the backside of a non-absorbing glass window (n w ¼ 1.5, k w ¼ 0) as a function of (i) contact angle ranging from 0°to 90°, (ii) incident angle between 0°and 80°, and (iii) droplet surface area coverage up to 90%. The glass window was 3 mm thick and the monodisperse droplets were 2.66 mm in diameter.…”

“…The effects of water droplets condensate on the transmittance of plane parallel slabs or films of various semi-transparent materials (e.g., glass or plastic) have been investigated both experimentally and theoretically [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. There exist two types of condensation namely (i) filmwise condensation referring to the formation of a liquid film covering the window and (ii) dropwise condensation corresponding to discrete droplets forming, growing, and potentially coalescing on the window [1].…”

Contactless steam generation and superheating under one sun illumination

“…However, an additional advantage of the CSES design is the built-in vapour collection, since steam is delivered to a single outlet tube. Previous works generated vapour distributed over a wide area, requiring collection by a semi-transparent condensing cover 21 , 40 , 41 . When losses due to the condensing cover are taken into account, the CSES performance is comparable with the lower-temperature evaporation systems that we have investigated 28 , which achieve efficiencies in the range 21–24% with collection.…”