Learning Forum

[Leila Esfahanizadeh]

We are involved in simulating the discharge and dispersion of supercritical CO2 through the nozzles of a cutting tool. Our primary objective is to enhance the cooling efficiency of the cutting-edge zone in machining applications. 

Problem Overview:

The geometry comprises a nozzle with an approximate discharge diameter of 0.2 mm, an inlet boundary with supercritical CO2 at 90 bar and 311K, and an outlet at atmospheric conditions, where CO2 disperses into the air. Our critical focus lies in the downstream conditions upon expansion, with a specific emphasis on solid CO2 volume fraction, CO2 mass fraction, and temperature distribution.

with the assumption of the outlet to be above the triple point (10 bar), our simulation has thus far achieved convergence for equilibrium phase change modeling of CO2 discharge from the nozzle using ANSYS CFX 2021 R2. We have utilized RGP tables for liquid CO2, vapor CO2, and saturation properties in their homogeneous binary mixtures. Furthermore, we've enabled Beta features in our simulations to account for subcooled liquid properties.

In the next phase, we aim to model the equilibrium phase change of CO2 and its dispersion in air (ideal gas at 25°C) upon expansion. Given that we've prescribed thermophysical and transport properties using RGP tables for liquid and vapor phases, we seek guidance on how to incorporate air alongside CO2 homogeneous binary mixtures in the equilibrium phase change model for CO2.

We have already explored CFX tutorials for equilibrium and non-equilibrium phase change models and have examined related forums and CFX model templates. However, the "evaporating_drops.ccl" template simulates the evaporation of water droplets in air, assuming constant density for liquid droplets. In our case, liquid properties are sourced from RGP tables, and there appear to be limitations with the "liquid evaporation model" in CFX. Can you provide insights on simulating the evaporation of liquid CO2 drops upon expansion into air? Which model, the homogeneous binary mixture with air or the liquid evaporation model, would be more suitable? Are there expert parameters to consider liquid phase properties sourced from RGP tables during the phase change process for CO2 discharge in air?

Additionally, we would appreciate information on whether applying RGP tables for liquid and vapor properties estimation requires any specific setup or adjustment of parameters in RGP tables or within the model to accurately account for the phase change during the process. It is essential that we ultimately simulate the non-equilibrium phase change of CO2 while considering metastable state properties sourced from RGP tables to faithfully represent the sudden expansion of CO2.

[rfblumen]

As you determined, the Liquid Evaporation Model (based on the Lagrangian Particle Tracking model) assumes constant particle density so it wouldn't be applicable if you wish to used RGP properties for the liquid.  Also, defining the fluid as a homogeneous binary mixture assumes an Equilibrium Phase Change approach, whereas you're wanting to model the phase change process as non-equilibrium.

Since this problem involves both air and CO2, a multi-component multiphase approach similar to what's done in the Steam Jet tutorial may be appropriate for phase change occurring in the subcritical region.  Mass source/sink terms using CEL expressions are used to specify phase change.

You may also want to explore Fluent as it has a wider range of multiphase models.

[Leila Esfahanizadeh]

Thank you for your prompt reply.

I have attempted to conduct my simulation following the procedure outlined in the steam jet tutorial, utilizing mass-source and mass-sink definitions for phase change specification. However, I have encountered a challenge as the CFX tutorial documents do not explicitly address the Sherwood number definition in the context of interphase mass transfer rate. Consequently, I am uncertain about how to appropriately define it for CO2, particularly given that the existing tutorial is based on the interphase mass transfer rate of water.

Could you please provide more details and proper references on how to estimate the expressions for mass source and mass sink to accurately specify the interphase mass transfer rate? 

[rfblumen]

Unfortunately, this goes beyond the scope of the assistance we can offer in this forum.  Please submit an Ansys service request and we can look into this in more detail.