# 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.

### 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.