Laboratory investigation of the effect of wind speed on evaporation from dam reservoirs

Abstract
Background and Objectives: Nowadays, freshwater management is very important due to the reduction of atmospheric precipitation. With the increase in global temperature and climate change, water evaporation occurs rapidly. One of the causes of evaporation can be called wind. Winds have a 20 to 40 percent effect on the speed of water evaporation. By controlling the wind speed, it can help reduce water evaporation in reservoirs and dams. This study aimed to investigate the effect of wind speed at different air temperatures. Another aim of this study was to use the CFD method to determine the areas and percentage of wind impact with the reservoir surface so that the wind kinetic energy and wind power applied on the water surface can be calculated. By investigating the effect of different wind speeds on the water surface at the two specified air temperatures, it was observed that water evaporation has a direct relationship with the volume of wind input to the water surface. Using the information obtained in this study, most water evaporation occurs at the beginning of the water reservoir in the wind direction.
Novelty: This study is to investigate the amount of wind force applied to the surface of the water by the CFD method and the amount of water evaporation by measuring its weight at two temperatures of 40 and 30 degrees Celsius and several different wind speeds.
Method and Material: In this study, CFD and laboratory methods were used to investigate the effect of wind energy on water evaporation. In the CFD method, the effect of wind speed and its effect on different areas of a reservoir was applied to the surface of the reservoir using Ansys Fluent software and the amount of wind power to observe which area of the reservoir can have the most evaporation. For further investigation of the CFD method, first, the water tank was drawn in 3D using Solidworks design software to be analyzed in the workspace of Ansys Fluent software. In the tank design, a 40 * 31.5 cm plate with 24 cm diameter air inlet ducts was used to control and measure the amount of air entering the tank. The diameter of this plate allows the volume of incoming air to be easily calculated. In this study, the method of water mass loss rate over time through evaporation (water weight changes) was used, thus, by applying wind speed at a specific temperature and at a specified time, the weight of water lost was calculated, and the amount and effect of wind speed and temperature on the rate of water evaporation were calculated. Investigating how wind interacts with the water surface using the CFD method: Experimental, numerical, or wake flow models can usually be used to study airflow. In a laboratory study, a reservoir with a length of 63 cm, width of 31.5 cm, and height of 13 cm was constructed and used to determine the effect of wind on the rate of evaporation. A wind generator was used to create the wind flow at the desired speed, a heater was placed behind the wind generator to increase the temperature, a temperature control module was used to adjust the temperature of the air entering the test environment, a wind meter was used to measure the wind speed and adjust the wind generator. To investigate the changes in water weight, a digital scale was used. The weight of the water in the tank was 16534 grams. The wind speed in this experiment is 3, 4, 5, and 6 m/s, respectively. The air temperature was set at 30 and 40 °C, respectively. To calculate the volume of air entering the surface of the water, a wall with a height of 40 cm and a width of 31.5 cm and a 24 cm diameter vent in the center of this wall has been used for air intake.
Results: According to the output of CFD software, it can be understood that most of the air contact with the water surface takes place in the first 20% of the water level of the reservoir. The lowest wind speed is at the bottom of the reservoir, which includes about 25% of the total reservoir. The velocity changes along the path of the reservoir begin to decrease, resulting in a decrease in the amount of evaporation at the bottom of the reservoir compared to the beginning of the reservoir. Using the water evaporation data, it can be concluded that by creating a suitable windbreak, the wind speed can be further reduced to reduce water evaporation. The purpose of calculating the kinetic energy of the wind and the power generated by the wind when it hits the surface is to show that these two factors cause the water droplets to separate from its surface, which causes evaporation. The power exerted by the wind on the surface of the water creates waves and can increase the rate of evaporation when water hits the air. According to the results of the laboratory, the maximum wind power exerted on the surface of the water is 9 W/m2, which corresponds to the wind speed of 6 m/s. The maximum kinetic energy is 4.76 m2/s. In this study, it was found that a wind speed of 6 (m/s) with an air temperature of 40 degrees Celsius caused water evaporation equivalent to 156 grams, which increased evaporation by 50% compared to the result of a temperature of 30 degrees Celsius. At wind speeds of 3, 4, and 5 m/s, the difference in water evaporation at temperatures of 40 and 30 °C is 36 gr.
Conclusion: With the studies conducted on water evaporation using laboratory methods and Ansys Fluent software, it can be concluded that the impact of the wind flow on the water surface is greater at the beginning of the tanks. Wind speed and temperature are two very important factors in increasing the speed of water evaporation. The speed of water evaporation in a wind with a speed of 6(m/s) at an air temperature of 40 ℃ can be 50 percent higher than the speed of water evaporation at the same wind speed at an air temperature of 30 ℃. At wind speeds of 3, 5, and 4 (m/s), the evaporation of wind at a temperature of 40 ℃ is at least 30 percent higher than at the same wind speeds specified at a temperature of 30 ℃. Wind temperature, velocity, and surface area of water are among the factors affecting water evaporation by increasing kinetic energy and creating a moisture difference between air and water, which causes water evaporation. At night, when the effect of solar radiation decreases, it can be said that the effect of wind on water evaporation is very high.
Keywords: CFD, Evaporation, Energy, Experimental, Wind.