توصيفگر ها :
نرخ تبخير , جريانهاي آبي , جريان باد , تابش نور , توازن انرژي
چكيده انگليسي :
Evaporation from surface water resources, especially rivers, is an essential parameter in hydrological and ecological studies. Evaporation can severely affect the environmental conditions of rivers and streams by impacting water energy balance and thermal regimes. Although many studies have been conducted on evaporative loss from lakes and water storages, evaporation from rivers and water channels has not been studied comprehensively. In the present study, the effects of different physical, meteorological, and hydraulic factors on evaporation rate from water flows were investigated by well-controlled laboratory measurements and analytical modeling. All the tests were conducted in a laboratory flume with 0.082 m width, 0.145 m height, and 4.77 m length for three different water flow velocities of 1.5, 4.5, and 7.5 cm/s. Different experimental conditions were established to estimate the evaporation rate in the test section. In the first condition, experiments were conducted without radiation and wind flow. The second and third types of the tests were performed only in the presence of wind flow and radiation, respectively. The effect of wind flow and radiation were simultaneously applied for the last sort of the tests. Water temperature, air temperature, and relative humidity were measured at regular intervals every 30 min since the onset of measurements. Energy balance methods were used to analytically model evaporation rate under different boundary conditions and water velocities. Investigations showed the significant role of meteorological parameters and water flow velocity on evaporative losses from flowing water bodies. In the absence of wind and radiation, the evaporation rate was lower than the other cases, and the evaporation rate increased with increasing water velocity. Increasing the water flow velocity increased the relative velocity between the water surface and overlying air and thus decreased the thickness of the viscous sublayer over the surface. The exclusive use of radiation resulted in higher energy transfer and water temperature, which increased the evaporation rate. The obtained results showed that the evaporation rate doubled with increasing radiation from 0 to 200 W/m2, and from 200 W/m2 to 400 W/m2, it increased by about 60%. The wind flow was applied in three directions: headwind, crosswind, and tailwind relative to the water flow direction. It was also found that the evaporation rate was lowest when the tailwind was applied. When the headwind was applied, the evaporation rate was the highest. It was found that in the case of the crosswind, by increasing the wind flow velocity from 0 to 2 m/s, the increment of evaporation rate is 150%. In addition, comparing the effect of crosswind flow and headwind flow, it was found that the evaporation rate increased by about 10% when the headwind was applied. Also, when the tailwind flow was applied, the evaporation rate was reduced by about 10% compared to the presence of the crosswind flow. The highest evaporation rate was observed when wind flow and radiation were applied simultaneously. Four dimensionless parameters are obtained by the Pi-Buckingham method of dimensional analysis and the effective parameters available in research. We call these parameters Ee, Rd, Ve, and Re. The parameter Ee is the change in evaporation rate, Rd is defined as the change in radiation, Ve is the ratio of wind flow velocity to water flow velocity, and Re is the Reynolds number. We note that with each increase in the independent parameters Ve, Rd, and Re, the dependent Ee increases. From the effects of the dimensionless and dimensional parameters on the evaporation rate, we can conclude that the evaporation rate increases as the water flow velocity, wind flow velocity, and radiation increase. Finally, a general relationship for estimating evaporation rate under different conditions was introduced using SPSS software.
