Computational Fluid Dynamics Study of the Effect of Gravity on the Impact of a Drop onto Dry and Wet Surfaces

Transcription

1 Computational Fluid Dynamics Study of the Effect of Gravity on the Impact of a Drop onto Dry and Wet Surfaces Presenter : Murat Dinc MURAT DINC and DONALD D. GRAY, Dept. of Civil and Environmental Engineering, NICHOLAS HILLEN, J. STEPHEN TAYLOR, and JOHN KUHLMAN, Dept. of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV /20/2014 WVAS

2 Background The impact of liquid drops onto both dry surfaces and wet surfaces has been studied by many researchers for more than a century The interaction between a drop and a surface may involve inertial, surface tension, viscous, and gravitational forces Experimental Sprays include millions of drop impingement on a surface Spray cooling is the key part in thermal management of the design of next generation electronic, aircraft and spacebased systems Computational 3/20/2014 2

3 Goal of the Spray Cooling Project The Project involves both experimental and computational studies aiming to develop correlations for the previously developed computationally effective Monte Carlo Spray Simulation Model In this presentation; contact angle, gravity effects will be investigated quantitatively for: A single drop impact on a dry surface A single drop impact on a wet surface 3/20/2014 WVAS

4 Contact Angle/Wettability Lotus Leaf Surfaces with a contact angle < 90 are referred as hydrophilic, with an angle > 90 are referred as hydrophobic The higher the contact angle the higher the hydrophobicity of a surface, and results in less wettability Some plants show contact angles up to 160 and are called super-hydrophobic Plants like the lotus leaf can reach a contact angle of 170 Contact Angle Values Surface Glass Polymer Steel Teflon Liquid Water Water Water Water Contact angle, θ ( 0 ) /20/2014 WVAS

5 Gravity Gravity (g) is a natural phenomenon by which physical bodies attract with a force proportional to their masses. Newton s Theory of Gravity: F g = m g Gravity Values Earth Gravity (m/s 2 ) Lunar Gravity (m/s 2 ) Zero Gravity Negative Gravity (m/s 2 ) /20/2014 WVAS

6 Computational Approach Computational Model: CFD code: ANSYS 14 FLUENT The Navier-Stokes and continuity equations are solved in axisymmetric coordinates using the Finite Volume Method (FVM) for unsteady, incompressible and laminar flow Volume of Fluid (VOF) multiphase model is implemented with three phases (liquid drop, liquid layer and gas) to track the interface between the liquid phases and the surrounding gas 3/20/2014 WVAS

7 Initial and Boundary Conditions Pressure Outlet 0.05 x 0.05 [m 2 ] axisymmetric domain Pressure Outlet Boundary Conditions Axis Pressure Outlet Wall Initial Conditions Smallest cell size= Do/168 Drop Liquid Layer 3/20/2014 WVAS

8 Definition of Drop Impact Dry Surface Initial Stage Advancing Stage Receding Stage First Impact at t 0 Maximum Spread Drop receding D o V 0 V r V r Wet Surface D First Impact at t 0 Maximum crown height generation Crown collapse D o V 0 h H max V r V r 3/20/2014 WVAS

9 Validation of the Computational Model t=1 ms t=3 ms t=7.5 ms t=10 ms Comparison of results for D = 4.2 mm droplet impacting on a thin liquid film with h = 2.1 mm: We = 2009, and h * = 0.5 (Experimental results of Wang and Chen (2000) in the left column of images, numerical results of Asadi and Passandideh-Fard (2009) in the middle column, numerical results of the current study with uniform-fine mesh elements in the right column) 3/20/2014 WVAS

10 Non-Dimensional Parameters Dimensionless numbers in engineering and physics are used for data reduction of similar problems. Important parameters used in drops are: Reynolds Number: Froude Number: Re = U D o υ Fr = V2 gd Bond Number: Bo = gρd 2 σ Weber Number: Non-dimensional Film thickness: Non-Dimensional Numbers We = ρu2 D o σ h = h D o Surface Liquid We Bo Re Fr (Earth gravity) h/do Dry Water Dry Water Wet Water /20/2014 WVAS

11 Results/Comparisons Results: Gravity and Contact Angle effect on dry surfaces 3/20/2014 WVAS

12 Normal Gravity (m/s2) Lunar Gravity (m/s2) Positive Earth Gravity 0 ms 5 ms Dry Surface-Gravity Effect Zero Gravity Negative Gravity (m/s2) Negative Earth Gravity 0 ms 5 ms Contours of Volume Fraction, 90 Contact Angle (Blue shows liquid drop while red shows air) Liquid We Re Fr h/do Do (mm) Vo (m/s) Water ms 15 ms 10 ms 15 ms 20 ms 25 ms 20 ms 25 ms 30 ms 30 ms 3/20/2014 WVAS

13 Dry Surface-Gravity Effect Contours of Volume Fraction (Blue shows liquid drop while red shows air) Zero Gravity Lunar Gravity 0 ms 5 ms 0 ms 5 ms 10 ms 15 ms 10 ms 15 ms 20 ms 25 ms 20 ms 25 ms 30 ms 30 ms 3/20/2014 WVAS

14 Dry Surface-Contact Angle Effect Contours of Volume Fraction Earth Gravity 77 0 ms 5 ms Liquid We Re Fr h/do Do (mm) ms 5 ms 0 ms 5 ms Vo (m/s) Water ms 15 ms 10 ms 15 ms 10 ms 15 ms 20 ms 25 ms 20 ms 25 ms 20 ms 25 ms 30 ms 35 ms 30 ms 35 ms 30 ms 35 ms 40 ms 40 ms 40 ms 3/20/2014 WVAS

15 Results/Comparisons Results: Gravity and Contact Angle effect on wet surfaces 3/20/2014 WVAS

16 Normal Earth Gravity (0 Contact Angle) Wet Surface-Contact Angle Effect Contours of Volume Fraction (Red shows liquid drop, green shows liquid layer while blue shows air) Liquid We Re Fr h/do Do (mm) Vo (m/s) Water Normal Earth Gravity (180 Contact Angle) 0 ms 5 ms 0 ms 5 ms 8 ms 16 ms 8 ms 16 ms 25 ms 35 ms 25 ms 35 ms 45 ms 50 ms 45 ms 60 ms 3/20/2014 WVAS

17 Wet Surface-Gravity Effect Contours of Volume Fraction (Red shows liquid drop, green shows liquid layer while blue shows air) Negative Earth Gravity 0 ms 5 ms Zero Gravity 0 ms 5 ms 8ms 16ms 8ms 16ms 25ms 35ms 25ms 35ms 45ms 60ms 45ms 60ms 82ms 80ms 3/20/2014 WVAS

18 Conclusions-1 At advancing stage flow behavior is obtained very similar since Fr >> 1 (intertial forces are dominant over gravity) However, small differences start to appear at receding stage Generally, differences are not so much since Bond (Bo) number ~ 1 that makes gravitational forces comparable to surface tension forces Chandra and Qiao (1996) also showed same phenomenon where they experimentally tested a single water drop impact on a dry steel surface in normal and low gravity 3/20/2014 WVAS

19 Conclusions-2 Results show that higher surface contact angles (hydrophobic), and negative gravity environment cannot be effective for spray cooling due to none or less drop liquid remaining on the surface However, analysis with heated surface is needed in order to study the heat transfer effectiveness of drops on dry and wet surfaces on low and normal earth gravity Computational approaches can be useful tool to study multiphase flow simulations such as drops and sprays, and can be correlated into the previously developed Monte Carlo Simulation Model 3/20/2014 WVAS

20 Acknowledgments We gratefully acknowledge the financial support of this project under NASA Cooperative Agreement NNX10AN0YA. 3/20/2014 WVAS

21 QUESTIONS/COMMENTS 3/20/2014 WVAS

22 THANK YOU 3/20/2014 WVAS