(F-229) Computational Flow Simulations in a Vacuum-Sealed Flow Chamber

            Computational fluid dynamics (CFD) software (ANSYS Fluent) was used to  predict fluid flow behavior through an experimental, in vitro flow chamber. Specifically, to verify the flow profile and wall shear rate/stress values of an aqueous buffer through a vacuum-sealed, parallel plate chamber. The chamber was particularly designed to achieve steady-state, fully developed laminar flow within a minimal distance from the circular inlet, the use of modeling software was necessary to confirm these predictions. Confirmation of a sufficiently long fully developed flow regime enables the use of the device for experiments requiring a controlled wall shear rate/stress. Future simulations will include mass transport and surface reactions which will be comparable to experiments relevant to thrombosis.  

            The vacuum-sealed flow chamber was digitally replicated using ANSYS Workbench DesignModeler, where a three-dimensional model of the experimental flow chamber was produced. This model consists of a rectangular flow chamber with inlet and outlet tubes extruding from the top wall. The model was imported into ANSYS Fluent meshed, and the flow was simulated. Since the buffer characteristics are similar to water, the density and viscosity of water were applied to the fluid throughout this simulation. Boundary conditions applied including the no-slip condition at the chamber walls, a constant pressure outlet, and uniform constant velocity inlet. In order to achieve the desired shear rates of 100- 400 s^-1 (100-400 mPa shear stress) in the fully developed regime, the volumetric flow rates were computed according to:
Q = wh²γ / 6

Where Q is the volume flow rate, w is the flow chamber width, h is the chamber height, and γ is the desired shear rate. Subsequently, the inlet velocity at the circular inlet port was determined according to:
v = Q / A
Here, v is the inlet velocity, Q is the volume flow rate, and A is the cross-sectional area of the inlet.

The simulation results demonstrated that the expected wall shear rates were achieved with 98% accuracy. The flow fully developed within 17% of the circular inlet for wall shear rates of 100-400. Results achieved at the highest wall shear rate, at which the entrance length is the greatest, are presented as examples. Figure 1 displays the wall shear stress values along the center of the bottom plate. Away from the inlet and outlet tubes, the stress exerted on the wall is shown to stay precisely around the predicted 400 mPa. Using a point 0.23 cm past the inlet, the fluid velocity is shown as a function of the height of the flow chamber in Figure 2. This returns a symmetrical and parabolic velocity profile which can be observed to further support the fully developed flow assumption.

         Computational simulations supported the fully developed flow assumption in the vacuum-sealed flow chamber, thus this chamber can be used in biofluid experiments which require this behavior at this scale. They may also extend to additional wall shear rates/stresses and to include mass transfer and reactions as needed to compare to experimental findings.