Microfluidics Lab 1. Engineering 1282H. Spring Team Y1. Mahnoor Naqvi. Spandan Shah. Stefan Heglas, Wednesday 3:00 PM

Transcription

1 Microfluidics Lab 1 Engineering 1282H Spring 2015 Team Y1 Mahnoor Naqvi Spandan Shah Stefan Heglas, Wednesday 3:00 PM Date of Submission: 03/09/15

2 Memorandum To: Stefan Heglas From: Team Y1 (Mahnoor Naqvi and Spandan Shah) Date: 03/07/2015 Re: Computational Fluid Dynamics I. Introduction Flow simulations are crucial in understanding the dynamics of fluid flow within various spaces. They allow for a controlled environment and flexibility in channel shape, structure, and fluid type. The importance of such simulations becomes crucial in micro and nano systems where it is difficult to gain an accurate understanding of fluid flow through physical experimentation due to the size of such systems. The purpose of this lab was to create a fluid simulation of water within a channel flow in order to gain an understanding characteristics such as velocity, shear stress and pressure across a specific flow channel. Modeling the flow enhances understanding of experimental results by showing where the how the pressure and velocity deviates across the chip channel. The SolidWorks simulation uses the same parameters as the Fluid Mechanics program to determine the flow rate and change in pressure in the channel. While in the program, only one point in the channel could be analyzed at a time, the SolidWorks simulation could show results across the entire channel depending on the size of the mesh cells.

3 II. Results and Description This analysis was conducted in two parts. The first part looked at the effect of mesh size on the accuracy of the fluid flow model. The second part looked at modeling flow in the entire channel of the standard chip utilizing the information gained in the first part of the analysis. The parameters and settings for both parts of the analysis are summarized in Table 1 and Table 2 in the Figures and Tables section in the attachments. III. Discussion In order to check the validity of the solution in the SolidWorks Flow Simulation several basic characteristics of fluid flow were compared to that of the simulation to check for a resemblance. The first check was whether the velocity was greatest at the center of the channel. This condition was met as evidenced by the 3D velocity contour in Figure 7 in the Figures and Tables, which shows that the velocity was the greatest at the center of the channel. The second check was whether the velocity decreased as one went away from the center to the edges. The 3D velocity contour can be referenced once again the confirm this check. The third characteristic was that the pressure decreases at the fluid flows across the channel. Figure 14 in the Figures and Tables, which denotes the Surface Contours of Pressure shows the pressure at the entrance being the highest and as the fluid moves across the length of the channel, the pressure linearly decreases.

4 The linear decrease in the pressure can be inferred from the Surface Contours of Pressure figure. There is a consistent decrease from the entrance where the pressure is Pa to the exit where the pressure is Pa. This represents as 1000 Pa pressure difference across the 25 cm channel. Another point of validity can be evaluated by looking at Figure 11 in the Figures and Tables, which is the shear stress plot. The velocity gradient value at the edges of this figure was relatively low compared to the velocity value at the center of the channel, but it was not exactly zero. Therefore, it is not entirely consistent with the no-slip condition based on the parameters of the simulation. The no-slip condition states that the velocity of a fluid at the edges is the same as the velocity of the edges. In both simulations, the edges were unmoving. This means that in order for the simulation to be valid, the velocity at the edges must be zero. There were slight discrepancies in the SolidWorks simulation when compared to the expectations of the flow. The primary discrepancy arose from the inconsistency of the simulation the the no-slip condition. The correct result would have shown a velocity of zero at the edges of the channel. The reason why the simulation isn t fully consistent is because the accuracy of the simulation depends on the mesh size. The smaller the mesh size, the more accurate the model. However, in order to get an absolutely accurate model, the mesh size would need to be infinitely small, which is extremely impractical.

5 Figure 5 in the Figures and Tables proves this by showing the velocity cut plot after the mesh has been changed to be less coarse. The mesh was changed by increasing the cells in every direction which caused the simulation to run longer. The best mesh was the one with the most accurate results and a reasonable run time. Figure 6 also shows the refined mesh velocity has higher velocity at the center of the plot and lower velocity around the edges is slower as compared to Figure 2 where the mesh is coarse. The parabolic 3D plot of the refined velocity contours Figure 7 in the Figures and Tables shows there is a fully developed flow at 0.010m from the end of the channel. At this point, the entrance length is at a distance far enough away to allow the water velocity in the center to reach the fastest velocity and the edge velocities to reach the slowest velocities. Figure 8 in the Figures and Tables shows the difference of velocities at the three distances shows the change in velocity of the water as the position in the channel changes. The area where the water velocity is fastest is shown to increase as the distance from the channel increases. This shows that as you get farther away from the entrance, the entrance effects change to fully developed flow effects. Applications of using the Flow Simulation could be to determine where the the flow is fully developed in the design channels and whether the test simulations run experimentally match the ones done on SolidWorks. IV. Conclusions The purpose of this lab was to create a SolidWorks simulation mesh that was small enough to provide an accurate representation of the velocity, shear stress, and pressure in the

6 channel. This was done testing a coarse mesh, and then adding cells to the mesh so it became more refined. By creating this model in SolidWorks, we can see where the water flow should be fastest due to shear stress and pressure. Applying this to the experimental results gives a better idea of how the flow in the channels work. Furthermore, testing the results from the simulation can help confirm the results from the experiment. Using the simulation also analyzes the flow rate and the change in pressure across the channel whereas the fluid mechanics program can only analyze the parameters at a point on the channel. Attachment: Figures and Tables

7 Figures and Tables Table 1: Channel Dimensions Length 25 mm Width 3 mm Height 200 μm Table 2: Flow Parameters Pressure Head (ΔP) 1000 Pa Dynamic Viscosity (μ) Pa-s Density kg/m 3

8 Figure 1: Goals Plot

9 Figure 2: Pressure Contours

10 Figure 3: Velocity Contours

11 Figure 4: Velocity Vectors

12 Figure 5: Flow Trajectories

13 Figure 6: Refined Mesh Velocity Contour

14 Figure 7: Refined Mesh 3D Velocity Contour

15 Figure 8: Velocity Contours at Channel Entrance

16 Figure 9: Shear Stress along the Bottom of the Channel

17 Figure 10: Delta Value of Goals Plot Figure 11: Lateral Cut Plot

18 Figure 12: Transverse Cut Plot Figure 13: Surface Contours of Pressure

19 Figure 14: Sheer Stress Surface Plot References [1] Microfluidics Lab 1 Procedure Part , March 9. [2] Microfluidics Lab 1 Procedure Part , March 9.