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Trial Lecture:Pulsating Heat Pipes: Principles, Applications and ChallengesM. Sc. Nicolas La Forgia

Norwegian University of Science and TechnologySummaryBasics concept of Heat transportation,Introduction to heat pipes, Principles of operations, Advantages, Types of heat pipes, Introduction to Pulsating Heat Pipes (PHP),Principle of operation,Characteristics parameters, and limitations,Difference with others heat pipes,Applications,Challenges,Summary and conclusions.22

Heat transport in solid conductorsHeat Conduction in copper cylinder.

HeatThermal resistanceTemperature differencelA3Introduction to Heat Pipes4What are Heat Pipes?HP are highly effective passive devices for transmitting heat at: high rates, over considerable distances, with extremely small temperature differences.

Thermal resistance considerable better than solid conductor.

5Principle of operationThree main parts:Evaporator section,Adiabatic section,Condenser section.Vapor phase created in the evaporator.Liquid phase created in the condenser.Pressure difference between evaporator and condenser.

66Capillarity - Heat PipesLiquid travelling through a wick structure.Orientation independent,Liquid move by capillarity forces.

EvaporatorCondenserAdiabaticWick

77Wick?

8

Compared with solid conductorL=150mm, d=6mm,

HP, -> thickness =0.2mm, Compress to 1mm,9AdvantagesTransportation of large quantities of heat.Considerable distance, The length of the heat pipe varies from 10mm to 15m long.Small cross- sectional area,No additional power input to the system.No pumpsVery small temperature drops, (Thermal resistance of the heat pipe is several times smaller than that of solid conductor).Simple design and manufacture,Ability to control and transport high heat rates at various temperature levels,The weight of heat pipe are smaller compared to solid conductor. Convenient for aerospace applications and personal computing.

1010Types of heat pipesTwo phase Thermosyphon,Rotating Heat pipes,Capillary driven Heat Pipe,Annular Heat PipeVapor Chamber Heat PipeGas loaded Heat Pipes,Inverted Meniscus Heat Pipe,Mono-groove Heat Pipe,Micro and miniature Heat Pipes,Loop Heat Pipes,Capillary pumped loop Heat Pipe,-> Pulsating Heat Pipes

11Introduction to Pulsating Heat Pipes12Pulsating Heat Pipes

Construction: made from a long capillary tube bent into many turns, Evaporator and condenser sections located at the turns. Two main types of PHPs:Looped,UnloopedNo Wick structure

1313Liquid-vapor characteristicConfined/elongated bubbles, slug/plug flow=> Mini- and Micro-channels,

Optimal liquid-vapor ratio between 40% and 60%. 1414Fluid motion and heat transferThe heat is transfer from evaporator to condenser by the means of pulsating action of vapor slug and liquid slug.

Flow induced by evaporation and condensation of bubbles (thermally induced pressure pulsations). Flow pulsation appears as a non-equilibrium chaotic process, Continuous operation requires non-equilibrium conditions inside the parallel channels.

1515Parameters affecting performance of PHPFluid properties:The type of fluid and the operating pressure inside the pulsating heat pipe depend on the operating temperature of the heat pipe.Diameter: Slug/plug flow pattern, pressure drop. Performance increase with diameter,Minimum heat flux for continue fluid motion.Latent Heat, heat flux, pressure drop (fluid viscosity, pipe roughness, pipe inner diameter)Maximum heat flux for dryout,Latent Heat, heat flux, Number of turns vs gravity, Diameter, working fluid, heat flux.Performance is higher with increasing the number of turnsTotal length, Filling ratio,Optimal value around 40-60%,1616Comparison with HPNo wick material, Meaning less weight and simpler construction.Heat transfer depends mostly on sensible heat and fluid motion,Fluid motion depends on heat flux and latent heat.Most types of HP are affected by liquid entrainment due to counterflow current. Loop and capillarity driven loop heat pipe are the exception.PHP presents a minimum heat flux required for fluid motion. HP perform slightly better than PHP.1717Aplications18ApplicationsThe applications can be divided into three main categories:Separation of heat source and sink, High thermal conductivity, efficiently transport heat from a source to a remote sink. Temperature equalization, Thermal conductivity provide an accurate method of temperature equalization. For example, minimizing thermal stresses. Temperature control,Source with varying flux can be kept at a constant temperature (within the operating range of the heat pipe).1919ApplicationsCooling of micro-electronic devices,Thermal management of high dense electronics systems, temperature control of lasers and thermal imaging devices, etc. Heat transportation to heat exchanger -> modern laptops/notebooks use heat pipes in CPU cooling systems.Energy production: Solar power collection and transportation, thermal control of nuclear power reactors, thermal cycles, etc.Thermal control of furnaces, Isothermal applications for chemical process and heat treatments.Metal production: Cooling of electrodes in production of metals. Special applications:Heat rejection: space craft. Heat removal: from leading edge of hypersonic aircraft.Microgravity environment applications.Cooling of large scale data centers.

2020Application: PHP vs HPApplications where weight is importantWick structure avoided,Specially suited for aerospace applications, Low cost applications,Lack of materials (wick structure) needed for fabrication.

21Challenges22Application related challengesComputer market requirements: Thermal resistance from chip to heat sink < 1 K/W, High heat transport, up to 300 W for new intel computer chip, High heat flux, 100 W/cm2 for new intel computer chip, Miniaturization , Long term reliability, Low cost (Why not?).

Main Challenges -> Performance estimation.2323Modelling ChallengesComplex thermo-hydrodynamic coupling that governs the thermal performance.Pulsating fluid motion, Temperature oscillations, pulsating heat transfer.Non-equilibrium due to the coupling of hydrodynamic phenomena from the different turns in the PHP,Void fraction distribution in the pipes,Flow pattern, Mathematical modeling and theoretical analysis of PHPs has been carried out using very simplified approaches that do not represent the complete thermo-hydrodynamics phenomena. Most existing efforts on modeling have focused on slug flow.2424Main parameters characterizing the performance of PHPWorking fluid: Surface tension, latent heat, sensible heat, viscosity, etc.Diameter, Slug/plug flow, pressure drop,Total tube length and critical number of loops, Length of evaporator and condenser sections, Optimal filling ratio, Effect of Inclination angle,

2525Summary and Conclusions26SummaryHeat transportation,HP:Better performance than solid conductors,Advantages,Different designs and implementations,PHP:Principle of operation,Comparison with others heat pipes,Parameters affecting performance,Applications,Challenges, (future application requirements and predicting models)

27ConclusionsPHP presents and interesting approach for heat transportation,Performance of PHP is slightly lower than capillarity driven heat pipes. (It is still better than solid conductors).

Recommendations for further research:Development of mathematical models for studying thermal performance.Effect of sensible and latent heat on heat transfer.Effects of fluid properties on the coupling of thermo-hydrodynamical phenomena and flow pattern.Geometric design, (Number of turns, total length and evaporation/condensation length)Measurements oriented in the development and assessment of models for estimating performance.

28Questions ?29