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Research Goals
Research Goals
Design and prototype an "Wickless" Oscillating Heat Pipe for future commercial applications
Understanding and characterizing
Heat transfer
Fluid flow regimes
Capturing flow dynamics correlating them with thermal performance
Visualize with high speed imaging
Use this as feedback to improve performance
Channel design, substrate materials & surface treatment and working fluid selection
Develop expertise in two-phase flow visualization
Extendable to other systems (pump-driven, gravity-driven)
Why thermal management? Maintains reliability and prevents device failure
Why thermal management? Maintains reliability and prevents device failure
Higher Cooling rate → Greater Density & Speed → Increased Functionality
Modern electronics thermal management faces considerable challenges in the wake of component miniaturization, which has led to higher demands on net heat flux dissipation. With the electronics industry closely following Moore’s Law, the future demands are becoming increasingly more challenging.
A variety of cooling technologies exists, but few offer next-generation heat removal solutions.
A variety of cooling technologies exists, but few offer next-generation heat removal solutions.
Air Cooling
Single Phase Liquid Cooling
Heat Pipes (keff>~20000 W/m/K vs. Copper ~400 W/m/K)
Wicked Heat Pipes utilize Capillary Forces
Oscillating Heat Pipe rely on Liquid-Vapour Entrainment
Standard thermal management technologies (eg. air cooling) are tried techniques but have already been exhausted in attempting to reduce heat loads, they are no longer practice for future next generation cooling technologies.
Single Phase Liquid cooling offers interesting prospects, but raises concerns with leaks and fundamental heat transport limitations
Alternatively, two-phase liquid cooling technologies are proven present day solutions and exhibit vastly superior effective conductivities (keff) and heat removal by taking advantage of phase change. Evaporation takes with it considerable energy, substantially reducing surface temperatures!
Conventional heat pipe technology has been successfully applied for the thermal management of a variety of applications, but is quickly running into its own limitations, leading to evolution of novel concepts addressing present industry demands: the "Wickless Heat Pipe" or Pulsating/Oscillating Heat Pipes (P/OHP).
Oscillating Heat Pipe Technology for Passive Two-Phase Thermal Management
Oscillating Heat Pipe Technology for Passive Two-Phase Thermal Management
Equilibrium PSat between liquid and vapour phases at ambient.
Operating conditions:
Evaporator: +∂q/∂t → Phigh; vapour volume expands
Condenser: -q ̇→ Plow; vapour volume contracts
Vapour flows from Thigh→ Tlow (Phigh→ Plow)
Non-equilibrium Heat Transfer Device!
Pulsing pressure field drives fluid motion
Generally, OHP functions via thermally excited oscillations induced by the cyclic phase change of an encapsulated working fluid. It consists of a train of liquid slugs and vapor bubbles which exist in serpentine capillary tubes/channels.
Successful operation depends primarily on sustaining non-equilibrium conditions within the system. The liquid & vapor slug transport comes about because of the pressure pulses caused in the system arising from the continual condensation (in the condenser) and evaporation (in the evaporator) of the working fluid.
This pulsating vapor pressure field drives fluid motion between adjacent channels. Since these pressure pulses are fully thermally driven, there is no external mechanical power source required for the fluid transport: the device is passive.
Heat & Fluid Transport Regimes
Heat & Fluid Transport Regimes
A mechanical analogue for the flow regimes can be imagined by considering a multiple spring mass-damper system in a loop:
Initially there are pulses when non-equilibrium stress is experienced, these ripple throughout the system and continue to grow as that source of stress (in this case the temperature gradient) is not removed.
If the flow is given time to develop it forms an oscillating pattern which cycles around the closed path. The frequency of this cycling continues until the forwards velocity of vapour exceeds a critical value.
Annular flow is the maximum efficiency of the system; at this point the spring-mass-dampening explanation no longer exists, the system is entirely governed by the vapour flow.
Comparison with conventional heat pipe technology
Comparison with conventional heat pipe technology
Similarities
Thin liquid films give high heat transfer coefficients:
In OHP – occurs between vapour and channel wall;
In wicked heat pipe – occurs within pores.
Evaporative cooling benefits from water’s high heat of vaporization.
Differences and Improvements
Conventionally, wicks with small pores generate large capillary forces (~2γ/rpore) which can passively transport liquid over large distances.
Various limits: capillary limit, sonic limit, critical heat flux limit, etc.
Very sensitive to gravitational orientation
In OHP – oscillating pressure field drives transport.
OHP has a maximum heat transport capability (annular flow)
Less sensitive to gravity
OHP becomes appealing as a next-generation thermal management solution because
Can be made thinner and lighter in weight, which offers huge weight advantages when using materials such as aluminum.
Less gravity sensitive than conventional wicked heat pipes
Recent experimental work indicates that – no. turns ↑ & channel r ↓ → even lower gravity dependence is observed (Lin et al. 2009; Smoot 2013)
Heat transfer efficiency improves with greater input power (until maximal heat transport capacity)
Relies on simpler fabrication
Performs better with distributed heat loads, which offers advantages for cooling circuit packs with arrays of modules
Design & Prototyping
Design & Prototyping
Geometric Variables:
Overall length of the heat pipe, diameter/size and shape of the tube
Critical Radius based on Bond Number: r≤rcrit=√(σ/(g∗(ρliq - ρvap))), where g = accel. due to gravity [m/s^2]; σ is surface tension [N/m]; ρ density [kg/m^3]
Number of turns of evaporator/condenser/adiabatic section
Physical Variables:
Quantity of the working fluid (filling ratio, ≤~50%)
Physical properties of the working fluid
Window and channel/tube material selection
Operational Variables:
Open loop or closed loop operation
Heating and cooling methodology
Orientation of the heat pipe during operation
Use of check valves
Designed and fabricated prototype single & serpentine channel OHP systems
Investigated different materials (borosilicate glass, ceramic coated plastics, polymer films) to ensure adequate to vacuum sealing requirements and long-term fluid confinement.
Measured temperature changes for different configurations (working fluids and volume fraction).
Examining background loss to ambient (no fluid flow) and conduction contribution.
Assessed heat dissipation with under oscillation, circulation, and annular flow (estimating cooling in terms of thermal resistance [C/W]).
Liquid Vapour Entraiment
Liquid Vapour Entraiment
After appropriate calibration, assessment of working fluid parameters, and maintenance of partial vacuum, liquid-vapour entrainment was observed on both single-loop and serpentine OHP prototypes!
Note: published information and released material is generally limited due to its propriety and confidential nature. The scientific and technical information disclosed has been approved for release by Bell Laboratories, Nokia. Images courtesy of Bell Laboratories, Nokia. Image of conventional heat pipe courtesy of WikiCommons, Wikipedia, Wikimedia Foundation. Image of comparative open and closed loop OHP courtesy of "Effect of Working Fluids and Fill Ratio on Design Parameters of Closed Loop Pulsating Heat Pipe : A Review", SS Bhakre, et al. 2015.
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