Heat Transfer

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Heat Transfer Materials Development Research

• R.A. Wirtz
• Mechanical Engineering Department
• University of Nevada, Reno
• Rawirtz_at_unr.edu
• [phone]
• June 2, 2008

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Research Focus

• Temperature Control of Electronics Sensors
• Thermal Characterization Development of Heat
  Transfer Devices Systems
• Experimentation
• Mathematical (and some CFD) Modeling
• High Performance Coolers and Heat Exchangers
• Thermal characterization of Heat Sinks, Cold
  Plates
• Development of novel Heat Sinks
• Enhanced surfaces
• Engineered porous media (Reticulated Filament
  Media)
• Hybrid Thermal Energy Storage (TES) Coolers
• Novel Phase Change Materials (PCMs)
• Multifunctional Materials/Devices

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Principal Facility, HREL305

• Approximately 1000 ft2 laboratory
• Two open-loop (air) flow channels (Laminar Flow
  Elements), pool flow boiling test rigs.
• Temperature, heat flux and flow rate
  instrumentation
• Visualization
• Infrared imaging system
• Two interferometers (Mach-Zhender, Holographic)
• Medium Speed Video (to 1200 fps), 12 MPixel SLR,
  Borescope, Inspection Metallurgical
  Microscopes, Access to SEM
• Differential Scanning Calorimeter
• Thermal Conductivity Apparatus (polymer plates,
  powders)
• Materials Processing Equipment (fume hood, vacuum
  oven, controlled atmosphere furnaces to 1800C)
• Prototype Fabrication (access to machine shop,
  Plunger EDM, access to Wire EDM, copper diffusion
  bonding, copper and aluminum brazing)

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Research Sponsors (Current)

• DOE/NREL
• Gas Research Institute
• IBM Corp
• Intel Corporation
• MDA/AFOSR
• NASA
• National Science Foundation
• Nevada Applied Research Initiative
• Office of Naval Research

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Single Phase Heat Transfer
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Product Performance Evaluation
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MCM CoolersAir Cooling, 60 mm sq. footprint
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Porous Heat SinkExploit Large Sp. Surface Area,
ß of Porous Material
60mm x 60mm x 35mm
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Screen Laminate and 3-Filament Weave Heat
Exchange Matrices (Reticulated Filament Media)
Engineered Porous Media select filament size,
pitch, shape, orientation to achieve specific
objective (high ß, ke)
RFM Test Articles
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RFM Property Modeling Porosity, Specific Surface
Area (ß), Effective Thermal Conductivity (ke)
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DIFFUSION BONDING FACILITY

• Quartz tube furnace
• 5 CO mixed in Argon
• Rotometer
• Mechanical Vacuum Pump
• Graphite plates to hold sample (not shown)

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DIRECT DIFFUSION BONDING TEST ARTICLE TO HEAT
TRANSFER SURFACE

• Rolling mill to flatten wire mesh for increased
  bonding area
• 30 reduction in thickness on 80 mesh screen (Dh
  225 mm)

• 80 mesh diffusion bonded directly to copper block

Section View
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Heat Exchanger Implementation
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RFM Heat Exchange Heat Transfer Modeling
where
where
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Heat Transfer Experiments
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Screen-Fin Heat Sink
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Boiling Heat Transfer
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RFM-Enhanced/Un-enhanced Surfaces Pool Boiling
Water _at_ Psat 1 atm
RFM Enhanced, Dh 352 µm Time Compression 20
Un-Enhanced Surface Time Compression 20
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RFM Enhanced/Un-enhanced Surfaces
Boiling/Cooling Curves
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SOLID CORE RFM-ENHANCED EXTENDED SURFACE
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Math Modeling of Pool Boiling From Fin
Surface(un-enhanced surface)
4 Rectangular qb 20 W/cm2
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POOL BOILING TEST RIG
3.5 L Tank
To condensers and vacuum controller
Mirror
Aux. Heater
Test Article
Telecentric Lens
Camera
Ring light
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RFM-SURFACE ENHANCED BOILING
qb 105 W/cm2, DTsat 10C, Water 1 atm, 2
Layer 50 (2L50) mesh copper screen
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Boiling FC72 from an RFM Fin q gt 140 W/cm2
DTsat 3.3 C q 33.3 W/cm2
Edge View
DTsat 13.9 C q 61.4 W/cm2
DTsat 22.6 C q 82.7 W/cm2
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Thermal Energy Storage Systems(TES-Systems)
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TES Hybrid Cooler Concepts

• Cooler sized for nominal heat load
• Phase Change Material (PCM) stores/releases heat
  during high/low power operation
• Smaller, simpler, less power consuming cooler

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PCM CHARACTERISTICS

• Dry PCMs
• Solid state transition (no liquid phase)
• Encapsulated solid-liquid transition
• Liquid immobilized solid-liquid transition
• k-enhancement
• Conductive fillers
• Conductive foam
• Discrete elements
• hc-enhancement (contact conductance)
• Wetting agents

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Macro-Encapsulation of a Solid-State PCM in Foam
Al/Al-Sheet
118 j/cc heat storage capacity Potentially, a
40-fold increase in keff
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A Liquid-Immobilized Paraffin PCM Composite

• T lt 60C A waxy solid
• T gt 60C A jell (paraffin immobilized)

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TES-System Design/ModelingHybrid TES/Cold Plate
Finite Volume Heat Transfer Model Design
Optimization Algorithm
Optimized 10030 watt capacity unit
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Ultra-Mobile ApplicationPrototype Hand-Held Unit

• 4.5 x 2.5 x 0.625 115cc package (i-Phone)
• As(max) 202 cm2
• Phase Change Material (dry PCM) 35 lt Tt lt 65C
• Power Duty 10W on for up to 4 hrs, 10mW off
  mode
• Thermal Interface (electronics) temperature, TTI
  65C
• Units skin temperature, Ts 45C
• Ambient temperature, Te 35C
• Skin-to-ambient conductance 15 lt Us lt 50 W/m2K

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Thermal Energy Storage Pad
qo(t)
Thermal Interface to heat source _at_ Tti, Ac
Enhanced (dry) PCMC _at_ Ac, Tp
Skin _at_ Ts, As
Environment _at_ Te Conductance, UsAs
Thermal Interface and Skin are thin conductive
heat spreaders
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Math Model Benchmark (no k-enhancement)?
Solution in 10 sec (numerical 2.5 hrs)
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Multi-functional TES-Systems
Composites that can store heat (via latent heat
effect) while they serve a structural function.
Approach Encapsulate PCMs in sub-millimeter
scale metal or polymer matrix materials to form
structural elements having large PCM-to-matrix
contact area.
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Multi-functional TES-Systems Thermal Response
Modeling

• Write energy balances for
• Thermal Interface (TI)
• TI/Spreader interface
• Conductive spreader
• Liq/solid interface in TES-volume

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The End