Mixing Length and Pressure Optimization in Ethanol-Gasoline Pipelines

Description

Project Description:

In this project, we aim to design a T-shape pipeline. The gasoline flows in the main pipe, and the ethanol is injected from a side pipe. The ultimate goal of the project is to consider the shorter mixing length while maintaining the specific constraints listed below.

• Constraints:
• Pressure before the T-shape (upstream): 600 kPa
• The outlet volume fraction of ethanol: 8%
• Gasohol flow rate: 36 m³/h
• Pressure after the mixing: above 80kpa

Limitations:

In industrial applications, the flow velocity varies between 1 to 5 m/s on average. Additionally, the standard pipelines have diameters ranging from 6 to 48 inches. Therefore, we should keep in mind the standard dimensions.

Geometry and Mesh:

For all cases, the geometry is created using ANSYS Design Modeler software. Then, the mesh grid is generated in ANSYS Meshing software. To model the mixture behavior near the wall regions, a boundary layer is applied.

Methodology:

The mixture multiphase model is employed in order to predict the behavior of fluids. Because there isn`t any tangible difference in densities, note that the density of ethanol and gasoline is 789 and 750kg/m^3, respectively.

Result of Designs:

• Design #1:

 In the first case scenario, we select a 10-inch diameter pipe intending to prevent any exceeding of velocity. However, the average velocity could be calculated by a simple calculation; the maximum value is unpredictable considering the T-joint.

The other challenge we encounter during the designing process is the upstream pressure. Based on the literature, the pressure drop in a pipeline depends on diameter, length, fluid viscosity, fittings, valves, etc. Regarding the low viscosity of fluids, we know that there wouldn`t be any strict pressure drop along the pipe. Still, we need an exact value of the pressure drop. So, in this case, we put finding the pressure drop as a priority.

The boundary conditions are shown in the table below:

The results prove that the pressure drop inside a 10-in pipeline is about 1170pa. Therefore, if 600kpa pressure is applied before the T-shape joint at the outlet, the pressure would be 598828pa. By having this parameter, we can get to the next design and consider the correct outlet pressure to satisfy our constraints.

• Design #2:

In this design, we are aware of the exact pressure drop in the 10-in pipeline. Thus, we alter the previous boundary conditions:

Now we are assured of the pressure behind the t-shape joint. Obviously, it is 600kpa. At the outlet, the volume fraction of ethanol is 7.68%, close to the expectations. The velocity contour indicates that the velocity is below 2m/s at its maximum. So, we can decrease the pipe diameter to increase the velocity and turbulence of the better mixture. Also, we have created a line in the center of the main pipe along the z-direction and plotted the ethanol volume fraction on it (figure 3b). It can be seen that the fluids are fully mixed after 300 inches. In other words, 280 inches from the t-shape joint.

• Design #3:

In this new design, we decreased the pipe diameter to 6 inches. As a result, we should repeat the pressure-drop study just like what we have done in Design 1. As mentioned earlier, the pipe diameter could have a direct impact on the pressure drop. After that, in a new simulation, we applied the boundary condition:

It is worth mentioning that the pressure drops increase by using a thinner pipe. It reaches 11225pa, about 9 times higher than the previous case.

Figure 4b proves that the mixture length decreases to below 220 inches or 200 inches from the t-shape joint. Also, the maximum velocity in the fluid domain is 2.95m/s, which is still in the standard range (figure 4c). Due to standards, we cannot decrease the pipe diameter more, so this would be the best-case scenario which satisfied all constraints as well.