Numerical Simulation in Atmospheric Water Generator By Ansys Fluent

Water crisis and providing water is one most essential problems that humans are facing. There are several ways for water extraction, depending on population and resources. In this simulation, water is extracted by a cooling tower from humid air. Humid air enters the buried pipes with fans. Then the air reaches the cooling tower. Because of the difference in temperature between air and pipes, air temperature decreases. After this process, there is an amount of residual water, extracted from humid air. The extracted water can be used as drinkable water or for agricultural purposes. In this project, we simulate airflow by Ansys Fluent. Then by using air condition relations, combined with the numerical solution, the amount of extracted water can be calculated. In addition, in the first half of the day, because the temperature difference between airflow and pipes’ wall is more than at the end of the day, the amount of extracted water is different throughout the day. We can obtain that in the second half of the day, the airflow temperature is more than the tower’s temperature, so the air is cooling the tower and preparing the system for the next day.


Introduction
Fresh water supply is always one of the most critical issues facing humans. Depending on the population and available water resources, different methods were provided for water supply. Because in less populated areas, water desalination is not economical, other methods should apply in these areas. One of these methods is water extraction from humid air, which can only use in hot and humid areas [1]. In this case, water is extracted by reaching the air to its dew point conditions. If the air temperature is lower than the dew point temperature, liquid water is released from the humid air through a constant pressure process [3]. For this process, a large pipe is placed in the middle of the ground and around it at a certain distance; several smaller pipes are connected to the main pipe are placed in the ground (Figure 1). The small tubes are for directing air into the main tube to perform the process. The airflow is directed into the pipes by placing a fan. If we consider the main pipe's length higher, the wind at the top of the pipe can suck the airflow and provide more flow speed. Then, due to the low temperature inside the main pipe, heat transfer starts between the humid air and the pipe. Also the air temperature decreases to the dew point temperature and liquid water can be extracted from humid air [2].

System Definition
According to Figure 2, air enters the buried pipes from the right through the inlet fans, after passing through the reducer (which is located in the middle of the pipe), enters the cooling tower at the left end of the pipe. In this simulation, there are 6 buried pipes that are connected to the cooling tower. Due to the temperature difference between the underground and the surface, there is heat transfer between the pipe and the airflow. Thus, the first part of water extraction is in this part. Buried pipes consist of a pipe with a diameter of 1 meter and a length of 30 meters, which is connected to another pipe with a diameter of 0.5 meters and a length of 30 meters by a reducer. Air enters these pipes through the fan and enters the cooling tower through these pipes.
In the cooling tower ( Figure 3), the inlet air enters the cooling tower from the buried pipe at a 3 degrees angle. For simplicity in simulation, the incoming air enters the tower from the bottom of the tower. Because of that, the vertical component is considered as the inlet velocity. The inlet air flows from the bottom to the top of the tower stages. In this simulation, there are three types of towers. In the 1 st type, which includes a coil in the first stage ( Figure 3 on the right) and a cone in other stages, inlet air cooling is available only on the first stage. In the other stages, heat transfer is only due to the temperature difference between the inlet air and the wall. In the 2 nd type, instead of having a coil in the first stage, glass plates are used for cooling and water extraction ( Figure 3 in the middle). These plates are similar in dimensions to the cones, and the cooling process is similar to the previous type. In the 3 rd type of tower, the cooling process is quite different from the other two types. In this one, there are cones with similar dimensions in all stages, and cooling is available at all stages. The cooling tower consists of a pipe with a diameter of 1.5 meters and a length of 20 meters. The inside of the cooling tower is divided into seven stages. In the first stage, three types of cooling process, glass plates, or cones are applied. In the remaining six stages in all three types, the cones are applied. The coil has a circular cross-section with a diameter of 0.2 m, which is about 2 m long and 1 m in diameter. Glass plates are also designed for more accessible in the simulation in the form of a cylinder with a diameter of 1 meter and a length of 2.45 meters. The cone inside the tower is 2.8 meters long and 1 meter in diameter.

Formulation
In buried pipe formulation, by using a 2-dimensional formulation for laminar flow, the approximate answers for pressure and velocity can be reached. In the reducer section, by having the velocity values from the previous section and by using convergent-divergent nozzle formulas, the pressure and velocity values after the reducer are calculated. In order to calculate the extraction rate, the air-condition formulation in references [4] and [2] is used.

Pressure and velocity in buried pipes
Assuming that the fan can rotate about 10,000 rpm, we can calculate the values of pressure and velocity for a 2dimensional laminar flow in a pipe.
= 3600 In this formula, rpm is the fan's rotation speed and A is the pipe's section area. [

Amount of extracted water
By using the air-condition formulation [4] we have:

Numerical Solution
The numerical solution is divided into four steps of 6 hours, in which according to time, the variables change. This solution includes underground pipes and the cooling tower. Each of the solutions is steady state, and the flow regime is laminar. In addition, the heat transfer process is included. After the solution is performed by Ansys Fluent, by applying a cross-section in each stage, the information about that stage is gathered for calculations. By extracting temperature and pressure data in all parts of the pipes and then averaging the data in each stage, the mean values are considered as the reference data, and calculations are performed with this data. For the next time step calculations, the temperature of the previous time step is used as a reference.
First, the flow is solved for the buried pipes. According to the results of this solution, by averaging the air temperature at the outlet section of the pipe, the inlet temperature of the tower is calculated for each step.

Results
According to the calculations, the results are sorted for each part of the system.

Buried Pipe
Using the air-condition formulation obtained in the relations extraction section and according to the obtained temperature values, the amount of water extracted in the buried pipe is calculated. As shown in Figure 1, by reaching the middle of the day, the water extraction rate reaches its maximum and then decreases. The cause of this phenomenon is a decrease in heat transfer due to a decrease in temperature. Over time, the temperature difference between the inlet air and the average temperature of the outlet air in the buried pipe decreases, and in the last time step, the temperature difference between the inlet air to the pipe and the outlet air is minimized. For this reason, the efficiency of the system in the first half of the day is higher than in the second half of the day.

1 st Type tower
The 1 st type of tower includes a cooling a coil in the first stage and cones on other stages. In addition, in this tower, cooling takes place only on the first stage and in the cooling coil, while on the other floors, the heat transfer is only due to the difference in air temperature in two different time steps.
The cooling process is only fully available in the first stage and in other stages, the heat transfer in the first half of the day is favorable. By using the extracted formulas, the rate of water extraction is 1337 kg per day.

Second type tower
On the first floor of the 2 nd type tower, unlike the 1 st , glass plates are used for cooling. The use of glass plates increases the amount of heat transfer and the rate of water production in the system. The rest of the tower floors are the same as the 1 st type of tower, and its conditions are similar to the previous. According to the calculations of the amount of water extracted in the glass plates in one day is more than the cooling coil, it can be concluded that the efficiency of the glass plates is more than that of the cooling coil.

Third type tower
In the 3 rd type of tower, cones are used instead of coils and glass plates, and heat transfer is included in all stages. According to this assumption, the efficiency of this cycle all day and at all stages is favorable. Due to the difference between the values of water production rate in different stages in this tower, the results of each stage are examined separately. In this case, due to the heat transfer in all stages, the difference between the inlet and outlet air temperature compared to the other two types is significant. With these results, 3 rd type tower has the highest rate of water extraction among the other two types.
In this cycle (with buried pipe), there is a total of 18054kg of water extraction in 24 hours, which is the highest value among these three types.

Conclusions
In this project, a numerical simulation of water extraction rate from the atmospheric water system has been done. This heat transfer simulation was performed for three different geometries for a full-day simulation. According to the results, the highest rate of extraction is in buried pipes. The rate of water extraction in a buried pipe depends on factors such as the number of pipes, the length of the pipe, the volume of the pipe, the dimensions, the reducer and the depth that the pipes are placed. Among these factors, the most important are the number of pipes, the location of the pipe and the length and volume of the pipe. The location of the pipe and the length of the pipe due to the temperature difference between the fluid and the wall affect the increase and decrease of heat transfer rate. Also, the volume of the pipe and the number of pipes affect the amount of fluid entering the pipe. In addition, there is heat transfer in the cooling tower due to the temperature difference, but this temperature difference is not as significant as the buried pipe. It should be noted that in the cooling tower, in stages where there is no cooling process, in the second half of the day, the temperature of the fluid is higher than the temperature of the tower wall. Therefore, water extraction is possible only in the first half of the day and in the next half, the cycle is being prepared and returning to the initial conditions for the next day. According to the results, the highest rate of water extraction in this system is related to the buried pipes. The reason is that the buried pipes have more temperature differences and contact surfaces than other parts of the system. For maximum utilization of the water production cycle from humid air, cooling can be used in the whole tower so that water can be produced throughout the day (3 rd type). If cooling is not possible in the whole tower, using glass plates is more efficient than the cooling coil due to the higher contact surface.
Placing pipes buried deeper in the soil, increasing the cooling rate in the tower, increasing the size of the tower to increase the contact surface, increasing the air inlet pipes, increasing the fan speed and changing the size of the cone to reduce pressure in the tower are all solutions to increase the efficiency of this system.

Abstract
Water crisis and providing water is one of the most important problems that humans are facing. There are several ways for water extraction, depending on population and resources. In this simulation, water is extracted by a cooling tower from humid air. Humid air enters the buried pipes with fans. Then the air reaches the cooling tower. Because of the difference in temperature between air and pipes, air temperature decreases. After this process, there is an amount of water extracted from humid air. The extracted water can be used as drinkable water or for agricultural purposes. In this project, we simulate air flow by Ansys Fluent. Then, by using air condition formulas, combined with the numerical solution, the amount of extracted water can be calculated. In addition, in the first hours of the day, because the temperature difference is more than at the end of the day, the amount of extracted water is different throughout a day. We can find out that in the second half of the day, the air is cooler than the tower, so the air is cooling the tower and preparing the system for the next day.