Natural convection and wall condensation or evaporation in humid air-filled cavities subjected to wall temperature variations

Abstract

Heat and mass transfer by natural convection coupled to wall surface condensation or evaporation in a two-dimensional cavity subjected to uniform, but time-dependent wall temperatures is investigated numerically. At initial state, the cavity is filled with quiescent humid air at uniform temperature and density. By decreasing the wall temperature, condensation occurs at the four wall surfaces until an equilibrium thermodynamic state is reached. The walls are then Ra m solutal Rayleigh number,Sh local Sherwood number (Eq. 36)Greeks β m solutal coefficient of volumetric expansion

Effect of surface radiation on natural convective flows and onset of flow reversal in asymmetrically heated vertical channels

“…For dry air, c p (T ), k(T ) and µ(T ) are calculated according to the polynomial relationships given by Lide and Kehiaian [46], valid in the range [100 K, 600 K]. These properties variations were used in [47] where the specific relationships employed herein can be found.…”

Transient natural convection in enclosures filled with humid air, including wall evaporation and condensation

“…Some unsteady studies can be encountered concerning the specific case of enclosures filled with humid air, where some phase change can occur, as in [5][6][7][8], and in the references therein. Even if the majority of the reported studies refer to numerical works, some experimental work is also reported in the literature [1,9].…”

“…Present work is mainly motivated by the very recent one in [8], where the walls of a rectangular enclosure filled with humid air are subjected to some specific temperature variations, such that condensation or evaporation can occur at the walls. However, the circular cross section systems are also very interesting both in natural and engineered systems, and a similar study needs to be conducted concerning this geometry.…”

“…This corresponds, for example, to circular cross section ducts filled with humid air when off operation, or to horizontal cylindrical containers. Additionally, considering the work in [8] as a reference, some additional features can be incorporated into the physical model, namely: (i) Consideration of the total (absolute) pressure as varying over the enclosure, and depending on the gravity and also on the fluid motion; (ii) Consideration of the viscous dissipation together with the work of pressure forces into the thermal energy conservation equation [10]; (iii) The way how the saturation condition is numerically imposed at the walls, when saturation conditions exist there; and (iv) The relevance of the chosen driving force for mass diffusion when saturation conditions are present. Even if many of these issues are of minor relevance for enclosures of small dimensions, and especially for enclosures with small height, the complete and consistent physical model must consider all them.…”

“…Results are obtained for the rectangular enclosure as studied in [8], taken as a reference situation, and the emphasis is given to the enclosure of circular cross section. Three different situations are considered, namely: (i) The condensation situation, starting with uniform temperature and vapor concentration distributions the temperature of the walls first linearly decreases with time, with condensation of some humidity over the walls, and after the temperature of the walls is maintained at a constant value, such that a final steady-state situation for the saturation conditions is reached; (ii) The evaporation situation, starting with uniform temperature and concentration distributions corresponding to the saturation conditions, with some condensed liquid water over the walls of the enclosure, the temperature of the walls first linearly increases with time, with evaporation of some humidity from the walls, and after the temperature of the walls is maintained at a constant value, such that a final steady-state situation far from the saturation conditions is reached; and (iii) The periodic situation, starting with given temperature and concentration distributions, temperature of the walls varies following a time periodic distribution, and a periodic solution is reached after some few periods of time, evaporation and condensation occurring at different phases in each period.…”