Two phase loop cooling system

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Two phase loop cooling system

  1. 1. Thermal management of high power density chips using a two-phase loop cooling system December 16, 2011 Jeehoon Choi
  2. 2. Background – Trends in Computing H/W Technologies High performance /Specialization Education Personal Gaming Design Computing Home Optimization Notebook, Netbook, Entertainment High performance, high heat flux Low-cost/lightweight market change PMP market Special marketImproved integration of technologyIncrease of processing speedAggressive miniaturization Move to the MID (Mobile Internet Device) market High performance High efficiency Power consumption Client / Server Computing Personalization Intelligence Reality
  3. 3. Background – H/W cooling in a data center Cost Reduction – (cost per kWh / DC : 0.0964 USD) Current Efficiency Level Efficiency Goal Energy Consumed Per Hour Current Efficiency Level Efficiency Goal Electricity used per Year 876,000kW 512,220kW Enter Total IT Load 50kW 50kW Annual Power Cost 84,446 USD 50,246 USD Total Facility Load 100kW 60kW Annual Cabon Footprint 528 Tons 314 Tons Annual Data Center Efficiency Savings Reduction In Kilowatt Hours Electricity Costs CO2 Emissions Equivalent To 1 Year 354,780 kW 34,201 USD 214 Tons 40 Fewer Cars 5 Years 1,773,900 kW 171,004 USD 1,070 Tons 202 Fewer Cars 10 Years 3,547,800 kW 342,008 USD 2,139 Tons 404 Fewer Cars
  4. 4. Background – How can we cool efficiently down servers and data rooms? Sever industries and markets have called for the development of enhanced cooling techniques that are able to meet these challenging needs under limitations imposed by small overall volume and weight, necessary in sever rack-mount or workstation systems. Besides the cooling technique is required to attend a large amount of heat transfer rates over considerable distances with minimal temperature drop. Out-of-control airflow in the data rooms
  5. 5. Background – History on thermal management of H/W CPUs’ Heat LoadsTrends/ Heat LoadsPC Technologies GPUs’ Heat Loads CPU Over-clocking : 200~250W The others (RAM & etc.) Xeon Series Increase in the heat generation Heat Load Trends 130W 150W operating problems at high temperatures 100W Requires high- performance 50W thermal systems Pentium RAM, M/B & etc. (15W) Increase heat flux PC popularization Next- generation cooing technology 1980 1990 2000 2005 2010 Passive & Active cooling Personal Water cooling computing cooling Heat pipe application coolingResearch/Products Server Active cooling computing cooling Water cooling
  6. 6. Background – Taking note of two-phase loop cooling systems The LHP is a two-phase heat transfer device with capillary pumping of working fluid. They are capable of transferring a large amount of heat Q  mc p T Q  mh fg for distances up to several meters. Sensible heat Latent heat Reservoir Loop Heat Pipe Vapor Line The advantages of LHP Evaporator Pump • High heat flux capability Condenser • Capability to transfer energy over long distances without restriction due to routing of liquid & vapor line Liquid LineWick • Ability to operate over a gravitational field gradient Conventional Heat pipe Liquid • No wick within the transport lines • Vapor and liquid flows separated; therefore, no entrainment Evaporator Condenser Vapor This system comprises an evaporator and a condenser, as in conventional heat pipes, but differs in having separate vapor and liquid line, rather like the layout of the single phase heat exchanger system.
  7. 7. Motivation and Objectives• One sever rack has the same heating power as a heating of a one-family house (~25kW).• Cooling, power supply and data center infrastructure need twice as much electricity than the computers itself.• Current techniques reached its physical limit. High Performance •Chip temperature - below 70 ℃ from a large amount of heat flux Space constraints • Stability/Reliability • Long distance transport lines • Smaller/ thinner evaporator • Light-weight system Design For Cost (DFC) •Design For Manufacturing (DFM) Hot w at er Cold w at er •Design For Assembly (DFA) The two-phase loop coolingsystem must be developed within Evaporator Severcertain constrains, associated, in Condenser Rack- m ount 4large part, with its applicationhigh density technologies. Sever Rack- m ount 3 CPU Sever RAM Rack- m ount 2 Power Supply HDD Sever Rack- m ount 1
  8. 8. Theory – Principle of operation When a heat load is applied to the evaporator fluid evaporates from the surface of the wick. The capillary pumping forces in the wick prevent the flow of the vapor from evaporator to LCC. As the pressure difference between evaporator and LCC increases, the liquid is displaced from the vapor transport line and the condenser and returned to the LCC. Liquid 4: vapor-liquid interface Heat out 3: condenser inlet Liquid flow through wick Vapor Evaporation Saturation line Pressure Liquid P1 1 transport 5 : condenser outlet 2 Condenser line Vapor P3 3 Liquid compensation transport Liquid 4 Chamber (LCC) line P5 5 Pc ,max 6: Liquid 7: wick-liquid 8: wick-vapor inlet interface Vapor removal channel P6 7 interface 6 Pw Vapor 8 P8 1: vapor 2 : vapor line inlet T6 T7 T4 T1 Wick Evaporator TemperatureEvaporating meniscus The wick structure provides capillary pressure force that transports the condensate liquid back to the evaporator and ensures working fluid is evenly distributed over evaporator surface.
  9. 9. Modeling – Temperature and Thermal resistance circuit The maximum heat transfer capacity of electronics cooling systems is determined based on the maximum permissible temperature at the Allowable junction temperature semiconductor chips which is normally less than 70°C. With a given constant value of ambient temperature, the operating temperature is iteratively solved for the given input value of applied heat load. Thermal grease Liquid transport line Heat source Tc Ths Q Te Condenser Q Tj Evaporator Ta Temperature profile Tvi Chip junction temp. Vapor transport line Evaporator temp. Vapor line inlet temp. Heat sink temp. Ambient temp. LocationTotal thermal resistance
  10. 10. Modeling – Heat sink design and simulation Max. heat dissipation capacity of condenser Cu AlOverall surface efficiencyof condenser Long Mean Temperature Difference (LMTD)Force convection coefficient Fin thickness and fin height at given heat transfer coefficients
  11. 11. Modeling – Pressure balance Capillary pressure limits – pressure balance analysis Ploss Pc ,max Ploss Pw Pv Pc Pl Pg Maximum capillary pressure Pc ,max Pv Pressure profile Wick Wick Pc Pw Vapor removal Pl channel Pw Tc Vapor transport Vapor transport line Condenser Liquid transport line line Location Pw Wick pressure drop, due to liquid flow through the wick thickness Pressure profile Vapor pressure drop, necessary to cause the vapor to flow Pv from the evaporator to the condenser. Pc Condenser pressure drop The hydrostatic head due to the unfavorable slopes of the system in the gravitational field, ΔPg ,which may be zero, positive or negative, depending on relative positions of the condenser and the evaporator. Pl Liquid pressure drop, required to return the liquid from the condenser to the evaporator. Pw Pv Pc Pl
  12. 12. Modeling – Driving force of system Wick structure for capillary pressure 2 Pc ,max Reff 2 P P2 P 1 1 1 R P P2 P Qlatent  mh fg 1 R1 R2 Pressure difference across Geometry of meniscus a curved liquid surface at liquid-vapor interface Vapor Tv removal CNTs space CNTs Revp Growth Twe Cu Wick ProvidedPrimary wick Rw Cu Substrate Alumina layer Twi Anodization Rb process Cu Wick The copper plate Te Provided Cu Substrate Q0
  13. 13. Modeling – Total pressure loss calculation (Component design) The pressure drop due to friction losses in L u2 Pressure loss liquid and vapor flow through the loop (for P f laminar or turbulent flows, circular or non- D 2 circular pipes, smooth or rough surfaces) is given by Darcy-Weisbach equation. Q u Qlatent  mh fg Ah fg Wick pressure drop Vapor line pressure drop Liquid line pressure drop  tw l m tw l Q 32 Lvi vuv2 32 Lvi vQ 32 Lli l ul2 32 Lli l Q Pw Pv P Aw l K Aw l h fg K Revl Dvl 4 Dvl v h fg l Rell Dll 4 Dll l h fg Hydrostatic head of liquid Condenser pressure drop Pg l gl sin 2 x2 (1 x) 2 x2 (1 x) Pcon G G v (1 ) l z z2 v (1 ) l z z1K Permeability 2G 2 z z2 z z2x Mass quality fv x2 2 v dz g sin [ v (1 )]x 2 dz v Dcon z z1 z z1 (vapor mass flux/total mass flux)G Mass flux (G  m / A) Void fraction 0.36 0.07 1 Correlations from the friction 0.64 1 x v l pressure drop in each phase 1 0.28 (Ref. the Lockhart-Martinelli method) x l v
  14. 14. Outcome – Simulation program on the basis of system pressure balance The results of the predicted design parametershelp not only to understand the systemperformance of at various conditions, but also toprovide details of how the system operates. Wehave developed the simulation program for thesystem design optimization with the effect ofdesign parameters using the objective function,design variables and design boundaries. Design variable and boundaries Visualization program for optimization design and simulation Parametric studies of the design variables and boundaries
  15. 15. Future work – Improvement factors Max. Heat transfer capacity System operating temperatureThe capillary pumping performance of -Vapor temperaturewick structure has important effect on system. ;less than 40°C level at given heat loadsIt is necessary to develop the high performanceevaporator mounted with superior wick structures. n dP Pi T i 1 dT Straight pores Liquid flow through wick Evaporation Saturation line Pressure P1 1 2 P3 3 Liquid 4 P5 5 Pc ,max P6 7 6 Vapor 8 P8 T6 T7 T4 T1 Temperature
  16. 16. Future work – Material properties 1Working fluid and Material selection As part of a design procedure for a targeted Operating temperature rangeapplication, the selection of the working fluid is the of various working fluidsfirst step. The working fluid determines the range ofthe operating temperature and must satisfy anumber of criteria. In the second step, it isnecessary to decide the compatibility between theworking fluid and the material to avoid chemicalreactions. The chemical reactions would cause thedegradation of the mechanical strength or thetightness of the LHP, or the existence of non-condensable gases which could alter the LHPoperation. Compatibility metals with working fluids (C: compatible, NC: incompatible)
  17. 17. Future work – Material properties 2Merit number h fg The effect of working fluid on heat transferThe ranking of working fluid c pl capability and/or system operating temperature l h fg l h fg l l The friction factor associated M c l pl Maximum capillary pressure c pl with pressure loss of system l l l l
  18. 18. Future work – Material properties 3 Constant Temperature Tcritical pressure curves Sub-cooled liquid region B A C D Superheated Liquid vapor mix/ vapor region Saturated two-phase region Saturated liquid line vapor line Entropy Vapor temperature vs. Vapor pressure Change hfg by controlling the evaporator pressure
  19. 19. Summary – Research roadmap Design objective Design requirements MaterialsSever rack-mount cooling Heat load, Chip temperature, (with working fluid) Ambient temperature, Weight, Space constraints, Noise level, Transport line length, Cost, etc. Component Models Experiment System Model Investigation Condenser, Transport line, Evaporator, Airflow, etc. System Simulation Wick fabrication Design parameters (key correlations) Prototype Theoretical Design and Modeling Pressure loss, Void fraction Models Heat transfer coefficient, etc. Prototype test
  20. 20. Summary – Works Project schedule and plan Detailed design prototypes Detail design and concept generation Heat source Total heat dissipation condition on one or multi chips (Specially, CPUs)-A prototype model with compact condenser with a fan can ; more than 300W be mounted inside a sever rack-mount. (One CPU is more than 14 W/cm2) Allowable Below 60 ℃-Analyzing parameters, operating conditions, constraints, junction * Steady state condition etc. to build the systems. temp. of chips-Thermodynamic cycle and operating limits to understand Evaporator 40mm (width) x 40mm(length) the physical concepts and operating principle of the loop dimension x less than 5mm (height) scheme. Transport line Between 0.5 m and 1.0 m Length (Taking note of flexible tubes)-Selection of working fluid/material/wick structure. Evaporator sample fabrication and investigation Evaporator Condenser-Theoretical and experimental investigation (Obtaining wick characters – Nano/Micro application) Updated system algorism and design refinement CPU based on previous findings RAM -To apply new parameters to the optimization program Power Supply HDD - Comparison of experimental reviews with simulation results
  21. 21. Thank you very much !!!

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