Experimental and Modeling Results for Flow Boilers and

Experimental and Modeling Results for Flow. Boilers (and Flow Condensers) that Operate in Annular Regimes and in High Heat-Flux Modes Narain, A. , Naik, R. , Kivisalu, M. , Gorgitrattanagul, P. , Bhasme, S. and Ranga Prasad, H. Michigan Tech Abstract Number HT 2016 -7464, Proceedings of 2016 ASME Summer Heat Transfer Conference, July 10 -14, Washington DC Acknowledgments Grants: NSF-CBET-1402702, NSF-CBET-1033591, NASA-NNX 10 AJ 59 G

Talk Outline Millimeter-scale Flow Boilers (and Condensers) Ø Broad Outline o Applications and Motivations o Proposed Innovative Operations Ø Experimental and Modeling /Computational Results for Innovative Flowboilers Ø Modeling/Computational Details Ø Conclusions 2

Examples of Contemporary Applications Needing Innovation for shear/pressure driven flow-boilers and flow-condensers Electronics/Data Center Cooling http: //www. pgal. com/portfolio/rice-university-data-center Space Based Thermal Management Systems and Power Generation Cycles http: //spaceflightsystems. grc. nasa. gov Ø Ground-based miniaturization - requires use of millimeter scale hydraulic diameters and flow rates that lead to shear driven flow Ø Space - requires such devices at all hydraulic diameters Ø Handling of high heat loads at high heat-fluxes is desired Ø Low weight and size requirements typically need to be met 3

Challenges with Traditional Shear/Pressure Driven Boiling Flows (mm-scale devices) Dh<5 mm and G(kg/m 2 s) is sufficiently large Traditional to shear/pressure driven operational challenges (channels heated from below) Ø Loss of buoyancy assisted nucleate boiling and associated heat transfer degradation Ø Increased lengths of ineffective non-annular regimes Ø Lack of repeatability due to extreme sensitivities 4

Proposed innovative mm-scale flow-boilers: How do they address the identified challenges? Key Ideas Ø Controlled recirculation of vapor (at negligible pumping power) ensures annular flow regimes. Ø Induced resonant pulsations in the vapor & liquid phases allow energy acoustics mediated formation of: standing waves of large amplitudes on micron-scale liquid film flows. This leads to very high heat flux operations. Ø Interactions with adsorbed layer on the wetting heat-exchange surface allows nucleate boiling type high heat-flux values through stable “sticking/dwelling” of the low-pressure wave-trough regions and induced convective motions. 5

Innovative Annular Flow Boiling Basics Vapor recirculation rate = ? How thin should this be ? Vapor Non-pulsatile Liquid Annular x = 0 Annular (suppressed nucleation) (with nucleation) How CHF/Dry-out instability can be avoided? 2 -D Simulations are made available (for the first time) for this x>0 region. 6

Flow Physics Hypotheses for Pulsatile Boiler and Condenser Operations Near interface schematic of instantaneous spatial film thickness profile Time-varying film thickness profile at location x = x* Ø The three intersecting circles represent: interaction zones of nm-scale, mm-scale, and macro-scale phenomena. Ø Between the liquid-vapor interface and the adsorbed layer, at the troughs, reduced pressure and shear allow enabling convective motion. The low-pressure is enabled both by the surface-tension and curvature effects as well as by adsorbed (yellow) layer through disjoining pressure effects. Ø Nucleation (of bubbles) is suppressed for thin annular boiling on wetting (or superhydrophilic) surfaces – yet the contact-line physics (Stephan et al. 2012; etc. ) advantages of nucleating bubbles are retained. 7

Motivation: Why is pool boiling effective when buoyancy is present? Ø Very high heat flux at the contact line (of an initially growing and subsequently shrinking and detaching vapor bubble) are experimentally observed (Stephan, et al. , 2012). Disjoining pressure assisted low-pressure in the micro-layer liquid films enable vigorous convective motion. Ø This physics is being used by the thin film flow-boiler to reproduce similar convection – during part of the wave’s time-period (as it approaches wetting-surface) but throughout the device - to obtain high heat flux with “through” flows. 8

1. 8 1. 6 3. 0 Hz Imposed Fluctuations, Innovative Flow Boiler 1. 4 1. 2 q ”W (x = 40 cm) data Expected Forthcoming experimental results 1 0. 8 0. 6 0. 4 0. 2 q ”W (x = 40 cm) trend Time - Averaged Heat Flux Measured at X = 40 cm (W/cm 2) 2 Enhancement Measured Local Heat-Flux (W/cm 2) Experimental Evidence of Significant Heat-flux Enhancements for Pulsatile Flows as Opposed to Non-pulsatile Flows 3. 5 3 2. 5 2 10. 6 Hz Imposed Fluctuations, Innovative Condenser 1. 5 1 0. 5 0 0 0 No Imposed Pulsations 28. 4 Hz Imposed Fluctuations, Innovative Condenser 0. 6 1. 2 1. 8 2. 4 Measured Pulsation Amplitude (k. Pa) 3 3. 6 No Imposed Pulsations 0 0. 2 0. 4 0. 6 0. 8 Pressure Amplitude at Heat Flux Meter location (k. Pa) Characteristics of Experimentally Observed Pulsatile Flows: Ø Much thinner mean film thickness achieved by pulsations (10 micro-meters). Ø Controlled amplitude and frequency (1 -15 Hz) for large amplitude standing waves. Ø Heat-flux enhancement over entire 0 ≤ x ≤ L : Ongoing Expt. 9 1

New Modeling Approach for Steady Innovative Operations First-principles based simulations (DNS & scientific) Simplified results with approximate correlations Empirically obtained/adjusted correlations Condensation Boiling h yp=0 q"w(x) x Δx q"w(x) is positive for boiling & negative for condensation 10 1 D design solver

New Modeling Approach for Steady Innovative Operations (Contd. ) --- Temp. Prescription Eq. (1) --- Heat flux Prescription Experiments and/or DNS based correlations ≡ --- Temp. Prescription --- Heat flux Prescription Boiler (+) Condenser(-) Combined solutions of Eq. (1) and Eq. (2) typically yield spatial variations in X, hx etc. 11 Eq. (2)

Experimental/Modeling/Computations are given in: Published scientific papers: Ø Kivisalu, M. T. , Gorgitrattanagul, P. , and Narain, A. , (2014), International Journal of Heat and Mass Transfer, 75, 381 -398. Ø Naik, R. , Narain, A. , Mitra, S. , (2016) Numerical Heat Transfer, part B: Fundamentals, 69(6), 473 -494. Ø Naik, R. , Narain, A. , (2016) Numerical Heat Transfer, part B: Fundamentals, 69(6), 495 -510. Papers on correlations-based design tools: Published: Ø Narain, A. , Naik, R. R. , Ravikumar, S. , Bhasme, S. S, (2015) Journal of Thermal Engineering, 1, pp. 307 -321. Submitted: Ø Ranga Prasad, H. , Narain, A. , Bhasme, S. S. , Naik, R. R, (2016) International Journal of Transport Phenomena. 12

DNS Modeling Requires Inlet Conditions Approach: Assume Adiabatic Flows Followed by Different Prior Heating Conditions Leading to Uniform Heating 13

Film Thickness, Δ (m) Steady Flow Boiling Results Based on DNS y=∆(x) Distance along the length of the channel, xp (m) Velocity, u. I (m/s) @ xp = 0. 02 m (I – 1 or 2) (b) Distance from heated surface, y (m) (a) Pressure, p. I (m/s) @ xp = 0. 02 m (I – 1 or 2) (c) (a) Plot of the steady film thickness profile Δ(x) for a transverse gravity case of gy=-g. (b) Cross-sectional profiles for x-component of velocity u. I, at xp = 0. 02 & (c) Cross-sectional profiles for Pressure p. I at xp = 0. 02 m. (Run parameters: Fluid – FC-72, U = 1 m/s, p 0 = 105. 1 k. Pa, ΔT = 10°C, channel height = 2 mm) 14

Steady Flow Boiling Results Based on DNS (Contd. ) Velocity magnitude (m/s) Distance from the heated surface, y (mm) h= Distance along the length of the channel, xp (m) Correlations are being developed for annular boiling. (On-going activity/Sample Reported) 15

Steady Flow Boiling Results Based on DNS (Contd. ) (a) , non-dimensional pressure gradients, etc. are being correlated in forms discussed earlier. 16 (b)

Signature of Instability in Steady Base Flow: Energy Transfer Mechanisms and Flow Variables x A|0 g x. A|1 g XA ≡ ”NA-A” transition location x. A|0 g Plot of Characteristic Wave Speed and Viscous Plot of significant Energy Transfer Mechanism Terms Dissipation rates in the presence of transverse gravity Analyzing the steady base flows’ features for a number of cases: Ø In the absence of transverse gravity, x. A|0 g corresponds to the extremum in the net mechanical work terms per unit width, transferred primarily as the sum of net pressure work per unit width (Δx) terms (PWL-conv & PWL-int) & interfacial viscous work per unit width (Δx) term VWL-int. These are dissipated by equal and opposite outgoing net internal viscous dissipation per unit width term VDL. These are the predominant terms in the energy transfer mechanisms. Ø In the presence of transverse gravity, x. A|1 g approximately corresponds to the maximum associated with peaking and dropping nature of the characteristic wave speed. Its estimate provides an upper bound. 17

Steady/Unsteady Flow Boiling Results Based on DNS (Contd. ) Vapor Recirculation Inlet Non-annular Vapor Xin < Xcr|NA-A Annular Xcr|NA-A Xin Vapor Recirculation Inlet Ṁv-recirc Vapor ∆0 Annular Liquid Xcr|NA-A Xin 18 Xin > Xcr|NA-A

Unsteady Simulations: Instability Identification Capability Not available for Flow-boiling. Expected to be analogous to the ones in flow-condensation. Velocity magnitude (m/s) Unsteady flow streamline patterns Run parameters: Fluid – FC 72, Inlet Speed U = 1 m/s, Temperature Difference ΔT = 20 °C, Channel height h = 2 mm, gy = -9. 81 m/s 2 19

Fundamental Modeling/Simulation Supported Correlation: Flow. Condensation 20

Fundamental Modeling/Simulation Supported Correlation: Flow-Boiling 21

Modeling / DNS CFD Approach Outline (for Annular Flow Boiling) Channel gap, h – 2 to 4 mm Channel Length, L – 10 to 100 cm Inlet Velocity, U – 0. 1 to 3 m/s Inlet Pressure, pin – 1 to 2 bar Vapor – Refrigerant FC 72, R 113, etc. 22

Simulation Tool Development 2 D/Engineering 1 D For 1 -D (mass and momentum balance is solved for the chosen vapor velocity profile) Heat transferred Vapor Liquid Δx Δm P Δm Wall for channel geometry or axis of symmetry for cylindrical tube. P Thin film approximation (analytical solution of momentum and energy balance are used) Governing Equations : Mass, momentum and energy equations in the fluid Interface Conditions : Flow physics restrictions on mass-momentum-energy transfer, definition of normal component of the surface velocity & continuity of tangential velocity, and thermodynamic restrictions 23

Scientific CFD Simulation Tool Development • Single phase domain approach solves CFD for each phase on COMSOL using FEM – all governing equations • Solutions of the two domains “talk” to each other through interface conditions – arrange on MATLAB/COMSOL program. The “talk” results from embedded interface conditions in the CFD formulation for each domain • One of the interface conditions is the well known Interface Tracking Equation which is solved on MATLAB on a separate x-grid (or x-y grid for 2 -D Level-set or x-y-z grid for 3 D Level-set). For unsteady simulation, the grid is “fixed” for a set of time-instants, but changes with marker time-instant (current time instant). 24

Inlet Condition-Extended Prior to Inlet & Assume Different Prior Heating Condition 25

Conclusions Ø Innovations proposed and demonstrated for mm-scale boilers and condensers. Ø Experimental and modeling support structure has been developed. 26

Thank you! Questions? 27

Governing Equations Back 28

Interface Conditions Back 29