38th Dayton-Cincinnati Aerospace Sciences Symposium Computational Study of Sprays for the Development of a Monte Carlo Model Presenter: Murat Dinc West Virginia University Donald D. Gray West Virginia University DCASS 2013
Outline Introduction Basic Definitions Spray Impact and Cooling Background Computational Model Computational Approach Initial and Boundary Conditions Results Droplet Velocity Distribution Droplet Diameter Distribution Liquid Film and Impact Efficiency Conclusions 3/5/2013 DCASS 2013 2
Introduction Introduction Sprays Sprays are utilized in many engineering areas (e.g. spray cooling etc.) Sprays may include millions of drop impingements per second on a surface After spray impact onto a dry surface, liquid film is formed (thickness ranges from a few microns to hundreds of microns Nozzle Spray Spray Droplets Liquid Film Impact Surface Kreitzer, P. J., Kuhlman, J. M., Spray Cooling Droplet Impingement Model, Paper AIAA-2010-4500, AIAA 10th AIAA/ASME Joint Thermophysics and Heat Transfer Conference, Chicago, IL, June 28-July 1, 2010. 3
Basic Definitions Spray Impact and Cooling Spray Cooling Spray cooling is the key part in thermal management of electronic, aircraft and space-based systems The aim is to get the highest rate of heat transfer by (one of the major mechanisms in spray cooling): preventing the loss of droplet liquid contact with the surface by minimizing drop splashing and crater retraction time Spray Impact and Cooling Silk, E. A., Golliher, E. L., R. Paneer Selvam, R. P., "Spray Cooling Heat Transfer: Technology Overview and Assessment Of Future Challenges for Micro- Gravity Application", Energy Conversion and Management, Vol. 49, 2008, pp. 453 468 Spraying System Nozzle pressure Spray flow rate Spray half angle Nozzle-to-surface distance Nozzle geometry Spray Liquid (Coolant) Density Surface tension Viscosity Temperature Thermal properties Impact Surface and Ambient Conditions Surface temperature Surface roughness, contact angle Ambient gas temperature Ambient gas type, pressure Gravity The basic parameters that affect the spray impact and cooling efficiency 4
Background Background A first principles simulation of a spray of millions of drops is impractical using present computers (High computational time and storage) The overall goal of this project is to derive correlations to include in previously developed Monte-Carlo Spray Cooling Model (Kreitzer and Kuhlman, 2010) Computationally (time and storage) efficient Embody sufficient physics to yield reliable predictions This presentation shows some initial isothermal simulations of sprays impacting surfaces: Full 3D spray simulations are performed for two water sprays having same mass flow rate and nozzle pressure but different: Nozzle-to-surface distance (h) Spray half angle (θ) Kreitzer, P. J., Kuhlman, J. M., Spray Cooling Droplet Impingement Model, Paper AIAA-2010-4500, AIAA 10th AIAA/ASME Joint Thermophysics and Heat Transfer Conference, Chicago, IL, June 28-July 1, 2010. 5
Computational Model Computational Approach CFD code: ANSYS 14.5 FLUENT The Navier-Stokes and continuity equations are solved in full 3D coordinates using the Finite Volume Method for: unsteady, incompressible, isothermal and turbulent flow (the k-ε model) The Discrete Phase Model (DPM) is used to calculate: the atomization of spray o The pressure swirl atomizer model, The KH-RT secondary droplet breakup model the trajectories of spray droplets droplets interactions with each other and airflow o Collision and coalescence effects, two-way coupling spray-wall interactions o Liquid film accumulation, liquid film height, liquid film velocity, etc. 3/5/2013 DCASS 2013 6
Computational Model Initial and Boundary Conditions Simulations are performed using a full 3D cylindrical domain The surrounding gas is air at room temperature (25 C), the spray liquid is water at room temperature Nozzle tip Liquid film formation Sheet break-up D Spray half angle, θ Case Gravity (m/s 2 ) Spray Liquid Parameters of Spray Simulations Mass Flow Rate (kg/s) Nozzle Pressure (Bar) Nozzletosurface, h (mm) Spray half angle, θ ( ) Impact Diameter, D (mm) 1-9.81 Water 0.003 10 40 10 14.1 2-9.81 Water 0.003 10 35 18 22.7
Results Case 1 Droplet Velocity Distribution Velocity Magnitude (Red shows 26.7-29.7 m/s, blue shows 0-3 m/s velocity range) Case 2 0.5 ms 3.5 ms 0.5 ms 3.5 ms t = 0.5 ms 1.5 ms t = 3.5 ms 4.5 ms 8ms t = 0.5 ms 1.5 ms t = 3.5 ms 4.5 ms 25ms t = 1.5 ms 2.5 ms t = 4.5 ms 6 ms t = 1.5 ms 45ms 2.5 ms t = 4.5 ms 6 ms t = 2.5 ms t = 6 ms t = 2.5 ms The velocity distribution is similar and the velocity decreases dramatically for both cases as spray approaches the impact surface t = 6 ms 8
Results Droplet Velocity Distribution Velocity Components (x, y, z) (Red shows the positive direction, blue shows the negative direction based on the coordinate system) Case 1 (at 5 ms) Case 2 (at 5 ms) X-Velocity + - Case 1: X-Velocity Distribution of Spray 8ms Case 2: X-Velocity Distribution of Spray Y-Velocity + Case 1: Y-Velocity Distribution of Spray 25ms 45ms Case 2: Y-Velocity Distribution of Spray Z-Velocity + - Case 1: Z-Velocity Distribution of Spray Case 2: Z-Velocity Distribution of Spray 9
Results Droplet Diameter Distribution (Red refers bigger droplets, blue refers to smaller droplets) Case 1 (at 5 ms) Case 2 (at 5 ms) 8ms 25ms 45ms Red refers to 188-208 μm, yellow refers to 126-146 μm, green refers to 84-105 μm, and dark blue refers to less than 22 μm diameter Red refers to 301-334 μm, yellow refers to 201-234 μm, green refers to 134-168 μm, and dark blue refers to less than 35 μm diameter Maximum droplet diameter is bigger for Case 2 (334 μm) compared to Case 1 (208 μm) at 5 ms. Splashing droplets are smaller 10
d 32 (μm) d 10 (μm) Results Droplet Diameter Distribution Total Average Droplet Diameters (d 32 and d 10 ) d 32 (μm) d 10 (μm) 100 90 80 70 60 Case 1 Case 2 28 26 24 22 20 18 8ms Note: Case 2 initial impact time is at around 1.5 ms and Case 1 initial impact time is little after 1.5 ms This is why Case 1 has bigger droplets at t = 1.5 ms Case 1 Case 2 50 16 14 25ms 40 12 30 0 1 2 3 4 5 6 7 8 t (ms) 10 45ms 0 1 2 3 4 5 6 7 8 t (ms) The average droplet diameters become smaller with time due to splashing droplets. The Sauter mean drop diameter is comparable to the splashing droplets (e.g. the average d splash ~ d 32 ) Splashing droplets 3/5/2013 DCASS 2013 11
V s (m/s) m s (mg) Spray impaction efficiency, η (%) Results 6 5 Liquid Film and Impact Efficiency Liquid Film Mass, m s (mg) 30 Spray Impact Efficiency, η (%) 4 25 3 Case 1 Case 2 20 2 1 0 0 1 2 3 4 5 6 7 8 t (ms) 30 25 Liquid Film Velocity, V s (m/s) 15 10 5 0 8ms 25ms 0 1 2 3 4 5 6 7 8 t (ms) Case 1 Case 2 20 15 10 5 0 0 1 2 3 4 5 6 7 8 t (ms) Case 1 Case 2 η (%) = M s / M (M s : Mass flux of liquid film, M : Mass flux of Spray) (Note that t = 0 ms is the time when spray is injected, and t ~ 1.5 ms is the time when spray droplets start to impinge and accumulate on the surface) Spray impact efficiency is higher for Case 2 until t ~ 4.2 ms, then the efficiency of Case 1 becomes larger than Case 2 12
Results Case 1 (at 5 ms) Liquid Film Contours of Liquid Film Height (Red refers the height liquid film region, blue refers to dry surface) Impact surface Case 2 (at 5 ms) 8ms 25ms Red refers to 2.04-2.27 μm, yellow refers to 1.36-1.59 μm, green refers to 0.91-1.13 μm, and dark blue refers to less than 0.27 μm 45ms Red refers to 1.46-1.62 μm, yellow refers to 0.97-1.13 μm, green refers to 0.65-0.81 μm, and dark blue refers to less than 0.16 μm In Case 1, the maximum thickness of 2.27 μm is at the center of the spray, and the film thickness declines monotonically with increasing radius. Case 2 is more complex with a dimple near the centerline and an off center maximum of 1.62 μm 13
Major: Conclusions The average droplet diameters are similar and do not depend on the spray half angle (θ) and nozzle-to-surface (h) distance The velocity distributions are similar. The velocity decreases for both cases as the spray approaches the impact surface The decrease in spray impact efficiency (η) for both cases is largely due to increase of splashing droplets since η (%) = M s / M in which M s : Mass flux of liquid film that has accumulated on the impact surface and M : Total mass flux of spray Since the aim is to obtain more effective spray cooling mechanism, reducing splashing can increase the spray impact efficiency which may also increase the spray cooling efficiency BECAUSE incoming spray droplets are also included among the secondary splashing droplets. THUS, if we can reduce splashing, more incoming spray droplets can contact with the heated surface after spray impact onto the liquid film. This can increase transient heat conduction which is one of the major heat transfer mechanisms in spray cooling 3/5/2013 DCASS 2013 14
Conclusions Improvements: More cases need to be simulated to better justify these conclusions mentioned on the previous page. Heat transfer will be also studied More time range (later spray impact time) should be included in all analysis It was concluded based on parametric spray simulations that 2D axisymmetric model could be used instead of full 3D (or even ¼ 3D model) based on total computational time (2D Axi ~ 3-4 days, 3D ~ several months) The model will also be validated with the experimental measurements performed by our research group and other computational studies in literature 3/5/2013 DCASS 2013 15
Acknowledgments The financial support of this work under NASA Cooperative agreement NNX10ANY0YA Helpful discussions with Dr. Eric Silk, NASA GSFC, Greenbelt, MD Helpful discussions with Dr. Kirk Yerkes, AFRL, Wright-Patterson AFB, OH Helpful discussions with Dr. John M. Kuhlman, Nicholas L. Hillen, and J. Steven Taylor in our research project meetings at West Virginia University, WV 3/5/2013 DCASS 2013 16
QUESTIONS??SNOITSEUQ Morgantown, WV 3/5/2013 DCASS 2013 17