PFR Flow Simulation

Engineering Summary

This modeling study evaluates how porous reactor internals influence velocity distribution, pressure loss, and residence-time uniformity in a plug flow reactor (PFR). The model was developed as a digital extension of a previously constructed physical PFR, allowing theoretical flow behavior to be tested and visualized before further design refinement.

Modeling Approach

A three-dimensional CFD model was developed in ANSYS Fluent using the porous-media formulation. Flow enters through a contracted inlet, passes through a centrally located porous cylindrical zone, and exits through a contracted outlet. The porous region was modeled using the Darcy–Forchheimer framework, which captures both viscous and inertial resistance effects in packed beds.

Key assumptions included:

• Steady-state, incompressible flow

• Laminar regime within the porous medium

• Homogenized porous structure

  
  

These assumptions were chosen to isolate the hydraulic impact of the packed bed rather than detailed pore-scale effects..

Tools & Software

• ANSYS Workbench 2025 R2

• ANSYS DesignModeler for geometry design

• ANSYS Meshing for volume discretization

• ANSYS Fluent (3D) for CFD simulation and visualization

Geometry Design

The model was built to reflect the dimensions of my previously constructed PFR:

• Inlet pipe → contraction → porous cylindrical reactor body → contraction → outlet pipe

• Length: ~0.3 m

• Porous zone located centrally

• Smooth transitions to avoid artificial separation

The geometry was created based on the profile shown in DesignModeler

Model Setup (Fluent)

Flow Regime: Laminar

Inlet: Velocity inlet, 25 m/s

Outlet: Pressure outlet, 0 Pa gauge

Porous Zone Properties:

• Porosity = 0.4

• Permeability: 3.8 x 10^7

• Viscous and inertial resistances defined according to the Darcy–Forchheimer model

Solver Settings:

• Pressure-velocity coupling (SIMPLE)

• Second-order spatial discretization

• Residual convergence monitored to <10⁻³ for all components

Results

Convergence Behavior

Residuals decreased several orders of magnitude over ~600 iterations with stable late-stage behavior. Minor mid-simulation oscillations correspond to model stabilization at the porous interface, normal for Darcy-Forchheimer models.

Velocity Contour & Vectors

The flow accelerates at the inlet contraction, rapidly decelerates upon entering the porous media, and re-accelerates at the outlet. Velocity vectors show jet entry, energy dissipation inside the porous zone, and smooth flow recovery toward the outlet.

Reactor Performance Implications

The porous media dampens velocity gradients, suppresses localized high-speed zones, and promotes controlled outlet flow. These effects are critical in environmental reactors used for filtration, adsorption, and biologically mediated treatment processes.

Engineering Interpretation

1. Pressure Drop & Energy Loss

The sharp deceleration at the porous entrance confirms significant viscous resistance. This corresponds to the steep pressure gradient predicted by Darcy’s Law. Inside the packed bed, energy losses stabilize and the flow becomes more uniform.

2. Flow Uniformity

Velocity contours show the hallmark of plug-like flow:

• High-speed jet → dissipated rapidly

• Near-uniform velocities within the porous region

• Controlled outlet acceleration

This is desirable in PFR systems that rely on consistent residence time.

3. Influence of Porous Media

Porous structures dampen turbulence and reduce velocity gradients. The simulation illustrates how reactor internals modify mixing patterns and flow distribution, which are key for treatment efficiency in filtration, adsorption, or biological reactors.

4. Model Validity

The results match expected porous media behavior, aligning with theory and my prior experimental PFR work. This confirms the correctness of the setup and supports future optimization of media selection and geometry.

Skills Demonstrated

• 3D reactor geometry modeling

• Mesh generation & quality control

• Porous media modeling in Fluent

• Laminar/Darcy–Forchheimer flow simulation

• Interpretation of CFD results (pressure drop, streamlines, velocity distribution)

• Environmental reactor design principles

• Integration of CFD with physical design