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Windshield De-fogging Analysis using EWF Model

Windshield De-fogging Analysis using EWF Model Using ANSYS Fluent

Introduction

Fogging in cold windshield i.e., condensation of water-vapor in the form of small droplets and the related reduction of visibility through the glass, is a major safety concern for several automotive or aeronautic companies. Defogging is the process of demisting the windshield panes by blowing hot air and it is very important for passenger safety. In this tutorial, Eulerian Wall Film (EWF) Model is used to model the defogging phenomenon on windshield.
The EWF model can be used to predict the formation and flow of thin liquid films on wall surfaces. Here, the film is not resolved using mesh normal to the surface. Instead, this model uses a virtual film on the surface, which flows parallel to the wall and does not affect the core flow momentum field, but affects the temperature and species concentration of the core flow. EWF model can only be used with 3D solver and it can be coupled with :

  1. Discrete Phase Model (particles are collected on the wall to form the film),
  2. Eulerian Multiphase Model (secondary phase mass is collected on the wall to form the film), and
  3. Species Transport Model (vapor species of core flow condensate to form liquid film or film evaporates to vapor species).

Defogging of the fogged liquid film in the windshield will be modeled as thin film via EWF model. Liquid film evaporates to vapor species and increases the vapor mass fraction of the core flow; and it will be modeled by Species Transport.

Problem Description

This tutorial considers defogging analysis on the windshield of an automobile passenger compartment. Windshields are modeled as solid volume (glass) and volume of passenger compartment (filled with moist air mixture) as fluid. The computational domain contains two air inlets and one outlet. A film of finite thickness at the windshield surface is initialized via EWF model and from inlet hot air flows to evaporate this liquid film. Objective is to study the transient of evaporation of this liquid film.

Preparation

  1. Copy the mesh file (defog-tut.msh.gz) and the UDF source file (defog.c) to the working folder.
  2. Use FLUENT Launcher to start the 3D version of ANSYS FLUENT.
  3. Enable Double-Precision in the Options list.
  4. Click the Environment tab and ensure that the Setup Compilation Environment for UDF is enabled.

The path to the .bat file which is required to compile the UDF will be displayed as soon as you enable Setup Compilation Environment for UDF. If the Environment tab does not appear in the FLUENT Launcher dialog box by default, click the Show More Options button to view the additional settings.

Result

     

Summary

In this tutorial, you modeled the defogging of wind shield using EWF model coupled with species transport. You have also learned how to post process the film variables with EWF model.

Appendix

UDF is used for:

  1. Define boundary condition for species mass-fraction of water-vapor at the inlets, and
  2. define mass diffusivity of moist-air mixture.

Relative humidity of the incoming hot air at the inlet is taken as constant and corresponding species mass-fraction is calculated through UDF. User can change this value of relative humidity in the UDF by changing the value of inlet phi which is taken as 0.5 (i.e. relative humidity = 50%) in the present case.
Initial species mass fraction for water-vapor is set as 0.01976907, which corresponds to 90% of relative humidity under normal condition.

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