Study of the jet impingement of the hot air on the windshield for defrosting problem.

1. Study of the jet impingement of the hot air on the windshield for defrosting problem.

2. MODEL • Windshield • Plane of symmetry • Velocity Inlet • Pressure Outlet • Cabin • Walls • Ice-Layer

3. Mesh Description • Meshing is done by using T-Grid. • Volume mesh - Tetrahedral cells(134558) • Surface mesh - Triangular elements • Wedge elements are used for the laminates of ice and windshield.

4. Solution Considerations • The flow inside the passenger compartment is assumed to reach “steady - state”. • Only the de-icing problem is considered transient.

5. Steps involved in solving the problem • Problem has been solved by using Finite Volume code, FLUENT 5.4 • Read the volume mesh. • Set up physical models defining a de-icing problem. • Set up material properties and boundary condition. • Solve for the steady state solution.(pressure and velocity field) Enable flow and turbulence equations and disable energy equation. • Take the steady state solution as the initial condition and solve the problem for transient flow. • Disable all equations except energy.

6. Phase Change Model • Why ? - During the deicing, some part of the ice-layer melts and its phase changes to the liquid. • Melt-Freez Model • New material type, liquid-solid, is added to the material panel. • Three new properties are added to material(fluid) panel - solidus temperature, liquidus temperature and melting heat, which is required to be defined for each material.

7. Phase • Phase : A phase is defined as being a finite volume in the physical system within which the properties are uniformly constant, i.e. don’t experience any abrupt change in passing from one point to another point in the volume. • Liquidus Line : It is the line which separates liquid and liquid-solid phase and the temperature associated with it is called liquidus temperature. If we lower the temperature of the substance below the liquidus temperature, its phase starts changing from liquid to solid and both phase are in equilibrium until it reaches to the solidus temperature. • Solidus Line : This line is defined as the loci of thepoints at which solidification completes upon equilibrium cooling or at which melting starts upon heating. • Tripple Point : At some specific pressure and temperature, all three phases (solid, liquid and vapor) of the substance are in equilibrium, which is called Tripple point.

8. Phase Diagram For Water • ONE PHASE EQUILIBRIUM : • AOC - solid phase • AOB - liquid phase • BOC - vapor phase • TWO PHASE EQUILIGRIUM : • AO - solid-liquid equilibrium - melting curve • CO - solid-vapor equilibrium - sublimation curve • BO - liquid-vapor equilibrium - evaporation curve • TRIPPLE POINT : • O - Tripple point • Homogenous phase - System contains one equilibrium phase. • Heterogeneous phase - System contains two or more equilibrium phase.

9. Ice-Water (Ice cells over the windshield) Air ( Cabin volume) Glass (Windshield) Solidus temperature = 271k Liquidus temperature = 273k Melting Heat = 334960 J/Kg By enabling the melt-freez model, we also need to enter above three properties for air,but in actual problem it is not required because phase change is not taking place inside the cabin and so we enter the above properties in such a way that it will show liquid fraction = 1 for the entire volume of air. New material type is defined for the windshield. Material Types

10. Inlet Boundary Condition • Velocity Inlet • Velocity = 3.2 m/s (constant) • Hydraulic Diameter = 7.87 inches • A user defined function is used to calculate the temperature of the air coming out of the blower for different time-steps. Air temperature is a function of the time in this problem. • Tmax = 344k, t = [0,300,600,900,1200] • T = [255.2, 290.2, 311.8, 329.1, 334.1] • If T > Tmax, T = Tmax condition is applied. • If t > 1200, T = (0.01667*t + 314.1) K condition is applied. • In the case when, t < 1200

11. Outlet Boundary Condition • Pressure Outlet • Gauge pressure = 0 Pascal (atmospheric pressure) • Temperature = 273 K • Hydraulic Diameter = 39.37 inches • Cabin-Walls : • Cabin walls are taken as the adiabatic boundary. No heat is coming in or going out through the cabin walls.

12. Solution • Solving the problem for steady-state by enabling flow and turbulence equation.(Disable energy equation) • Standard K-epsilon turbulence model is used to solve the problem • 0.0001 is the convergence criteria for continuity, x,y,z-velocity momentum , k and epsilon equation. • This steady state solution is used as the initial condition to solve unsteady state deicing problem. • Flow and turbulence equations are disabled and energy equation is enabled to solve unsteady problem. • Time step = 5 seconds, No. of time steps = 200, Total time=1000 sec • Save the case and data file.

13. (2) Temperature contours on the ice layer after t = 1000 seconds(unsteady state) • Cooling effect is maximum where hot air jet impinges directly on the windshield, so local heat transfer coefficient in this part is high. • Cooling effect is less near the periphery of the windshield, so local heat transfer coefficient is small in this part. • T = Air temperature coming out of blower(t = 1000 seconds) = 331 K • Cooling effect also can be observed at different time by giving different number of time steps.

14. (3) Contours of Liquid-Fraction on the ice layer after t = 1000 seconds • At t = 0, liquid-fraction = 0 on the entire ice layer, that means its in pure ice form. • In this figure(t = 1000), it can be seen that most of the ice has been melt and in some parts liquid-solid phase is in equilibrium while some blue part shows the ice in pure solid phase.

15. Velocity Contours at y= 20 plane • Velocity contours are same for the steady and unsteady solution. • After impingement, a major portion of the incoming air attaches the windshield upwards and creates a big recirculation with its eye in the middle of the plane. • Velocity of air particles which creates recirculation is less than that of the particles which attaches the windshield.

16. Velocity vectors at y=20 plane and colored by the temperature • The air particles which attaches the windshield after the impingement and exits through the pressure outlet are at constant high temperature. • The temperature of the air particles which creates the recirculation inside the cabin has been dropped.

17. Tasks Ahead • Apply the same condition in our windshield model and solve the problem for defrosting and defogging. • Study of Navier-Stokes equations used for this model. • Study of K-epsilon turbulence model.