ABSTRACT
This paper aims to report a numerical study of the assessment of heat and mass transfers by evaporation of a large impoundment under Burkina Faso climate conditions. This impoundment is considered as a parallelepiped which upper face, in contact with the ambient environment and subject to solar radiation, is the seat of a natural convection-based evaporation. The intensity of this evaporation is modeled by a correlation in the literature. Transfers into water are made by natural convection. They are caused by temperature differences due to solar radiation and ambient conditions (wind, hygrometry of the air,) on water. These transfers are described by the Navier-Stokes equations and energy and the initial and boundary conditions associated with them. The finite volume method and the SIMPLE algorithm were used for speed-pressure coupling. The systems of algebraic equations deduced from the discretization of transfer equations and boundary conditions associated with them are solved with Thomas’ algorithm, the SIMPLE algorithm and an iterative procedure because evaporated water quantity depends on the temperature and concentration of water vapor at the surface of the impoundment which are the unknowns of the problem. The numerical model developed is validated in relation to previous work and experimental data from Burkina Faso meteorology. The results obtained concern the evolution of the evaporated water flux under dense solar flows, a relative humidity of the air proportional to the wind speed and also the evolution of the evaporated water flux against the solar flux density for high relative moisture content. Also the evolution of the evaporated water flow against the depth of the impoundment for a solar flux density, relative humidity and the temperature of the surface of the body of water is given. The determination of evaporated water flux for typical years was calculated on a 10-year period. The results obtained show that the flux of evaporated water increases with a high solar flux rate and decreases for a high relative humidity level.
ABSTRACT
This paper is an assessment of aerosols impact on solar potential available in Burkina Faso in 2017. Three measurement stations were selected from the North to the South according to the climatic zones, with sites at Dori (14.035°N, 0.034°W) in the North, Ouagadougou (12.20°N, 1.40°W) in the Center and Gaoua (10.29°N, 3.25°W) in the Southwest, respectively. This study is based on in-situ measurements, satellite observations and a tropospheric standard model of the Streamer radiative transfer code of atmospheric particles. The results show a high availability of solar irradiation with average monthly values ranging between 4.46 kWh/m²/d and 6.82 kWh/m²/d. The most favorable periods with maximum radiation are observed in Spring in March and in Fall in October. Yet, the qualitative comparison between the evolution of aerosols and that of solar potential clearly shows aerosols capacity to influence the radiation at the crossing of the atmosphere. Thus, the aerosols maxima correspond to the solar potential minima. Moreover, a comparison between the day cycles of solar radiation and those of the simulation model shows a good accuracy of the Streamer code to estimate the solar flows at the surface in a standard atmosphere without clouds in Burkina Faso.However, a quantification of the aerosol impact by the Streamer code reveals a reduction in the normal direct flow compared to clear days defined by aerosol optical depth (AOD) less than 0.2 (AOD<0.2) of nearly 75.04% at the Dori site in the North, 57.33% at the Ouagadougou site in the Center and 40.89 % at the Gaoua site in the Southwest during polluted days corresponding to AOD higher than 0.8.This corresponds to an increase in the diffuse flow of 279.69 W/m², 246.05 W/m² and 226.09 W/m², respectively calculated on the same sites. In case of a mixed day (0.2 <AOD <0.8), this decrease in direct solar flow is estimated at 41.25%, 22.11% and 37.13% with an increasein the diffuse solar flux of 115.04 W/m², 150.43 W/m² and 79.58 W/m² at the sites of Dori, Ouagadougou and Gaoua, respectively.