Introduction to Loop Heat Pipes p p Jentung Ku g NASA/ Goddard - PowerPoint PPT Presentation

https://ntrs.nasa.gov/search.jsp?R=20150018090 2018-05-15T17:56:28+00:00Z Introduction to Loop Heat Pipes p p Jentung Ku g NASA/ Goddard Space Flight Center GSF C· 2015 Introduction to LHP - Ku 2015 TFAWS

Outline • From Heat Pipe to Loop Heat Pipe (and Capillary Pumped Loop) • LHP Operating Principles LHP Operating Principles • LHP Components Sizing and Fluid Inventory • LHP Operating Temperature Control • LHP Start-up • LHP Shutdown • • LHP Analytical Modeling LHP Analytical Modeling • Recent LHP Technology Developments 2 Introduction to LHP - Ku 2015 TFAWS

From Heat Pipe to Loop Heat Pipe From Heat Pipe to Loop Heat Pipe and Capillary Pumped Loop 3 Introduction to LHP - Ku 2015 TFAWS

Heat Pipes - Heat Transport Limit • For proper heat pipe operation, the total pressure drop must not exceed L L L e c a Heat Source Heat Sink its capillary pressure head . wall  P tot ≤  P cap,max Liquid Flow  P tot =  P vap +  P liq +  P g  P cap,max =  cos  /R p Vapor Flow • Heat Transport Limit Liquid Flow – (QL) max = Q max L eff wall Adiabatic – L eff = 0.5 L e + L a + 0.5 L c Evaporator Condenser eff e a c Section Section S ti Section – (QL) max measured in watt-inches or watt-meters • Capillary pressure head: Vapor Vapor Vapor  P cap  1/ R p P 1/ R Pressure Drop • Liquid pressure drop: Capillary Pressure Liquid Liquid Pressure  P liq  1/ R p 2 Pressure Drop • An optimal pore radius exists for A ti l di i t f Liquid Li id No Gravity Force maximum heat transport. Adverse Gravity Force • Limited heat transport capability L e L a L c • Limited pumping head against gravity p p g g g y Distance b) Vapor and liquid pressure distributions 4 Introduction to LHP - Ku 2015 TFAWS

Constant Conductance Capillary Pumped Loop Heat In Wick Evaporator Liquid In Vapor Out Heat Out Heat Out S Subcooling Condenser Liquid Leg Duct High Velocity Vapor Plus Liquid Wall Vapor Vapor Film Fil Bubble Liquid “Slug” Flow Forces Predominate Surface Tension Forces Predominate • Wicks are present only in the evaporator, and wick pores can be made small. • Smooth tubes are used for rest of the loop, and can be separately sized to reduce pressure drops. • Vapor and liquid flow in the same direction instead of countercurrent flows. • Operating temperature varies with heat load and/or sink temperature. 5 Introduction to LHP - Ku 2015 TFAWS

Variable Conductance Capillary Pumped Loop Reservoir Evaporator Liquid Line Vapor Line Condenser • The reservoir stores excess liquid and controls the loop operating The reservoir stores excess liquid and controls the loop operating temperature. • The operating temperature can be tightly controlled with small heater power. • The loop can be easily modified or expanded with reservoir re-sizing. • Pre-conditioning is required for start-up. • Evaporator cannot tolerate vapor presence, may be prone to deprime during start-up. • Polyethylene wick with pore sizes ~ 20 µm 6 Introduction to LHP - Ku 2015 TFAWS

Capillary Two-Phase Thermal Devices Heat In Heat In Reservoir Reservoir Evaporator Evaporator ne Vapor Lin Heat Liquid Line In/Out Liquid Line Condenser/Subcooler Vapor Line Line Condenser Heat Out Q IN Q OUT Container Wall Wick Structure Evaporization Condensation Vapor Flow Liquid Flow q 7 Introduction to LHP - Ku 2015 TFAWS

CPL and LHP Flight Applications – NASA Spacecraft TERRA 6 CP TERRA, 6 CPLs HST/SM - 3B; 1 CPL AURA, 5 LHPs Launched Dec Launched July 2004 Launched Feb 2002 1999 ICESat, 2 LHPs SWIFT, 2 LHPs GOES N-Q, 5 LHPs each Launched Jan 2003 Launched 2006 Launched Nov 2004 8 Introduction to LHP - Ku 2015 TFAWS

CPL and LHP Flight Applications – NASA Spacecraft SWOT, 4 LHPs GOES R-U, 4 LHPs each ICESat-2, 1 LHP To be launched To be launched To be launched • LHPs are also used on many DOD spacecraft and commercial satellites. 9 Introduction to LHP - Ku 2015 TFAWS

Introduction to Loop Heat Pipes 10 Introduction to LHP - Ku 2015 TFAWS

Schematic of an LHP Primary wick Secondary wick Compensation Arteries Evaporator Chamber A Bayonet Primary wick Secondary wick A Liquid line Vapor Grooves Vapor grooves Vapor line Bayonet y Section A-A Condenser • Main design features − The reservoir (compensation chamber or CC) forms an integral part of the evaporator assembly integral part of the evaporator assembly. − A primary wick with fine pore sizes provides the pumping force. − A secondary wick connects the CC and evaporator, A secondar ick connects the CC and e aporator providing liquid supply. 11 Introduction to LHP - Ku 2015 TFAWS

Main Characteristics of LHP Heat In Heat In Reservoir Evaporator Vapor Line uid Line Heat In/Out Liqu V Condenser/Subcooler Heat Out Heat Out • High pumping capability – Metal wicks with ~ 1 micron pores – 35 kPa pressure head with ammonia (~ 4 meters in one-G) p ( ) • Robust operation – Vapor tolerant: secondary wick provides liquid from CC to evaporator • Reservoir is plumbed in line with the flow circulation. – Operating temperature depends on heat load, sink temperature, and surrounding temperature. – External power is required for temperature control. – Limited growth potential Limited growth potential • Single evaporator most common 12 Introduction to LHP - Ku 2015 TFAWS

LHP Operating Principles – Pressure Balance • The total pressure drop in the loop is the sum of viscous pressure drops in LHP The total pressure drop in the loop is the sum of viscous pressure drops in LHP components, plus any pressure drop due to body forces:  P tot =  P groove +  P vap +  P cond +  P liq +  P wick +  P g (1) • The capillary pressure rise across the wick meniscus:  P cap = 2  cos  /R (2) • • The maximum capillary pressure rise that the wick can sustain: The maximum capillary pressure rise that the wick can sustain:  P cap, max = 2  cos  /r p (3) r p = radius of the largest pore in the wick g p p • The meniscus will adjust it radius of curvature so that the capillary pressure rise matches the total pressure drop which is a function of the operating condition:  P  P cap =  P tot  P (4) (4) • The following relation must be satisfied at all times for proper LHP operation:  P tot   P cap max  P tot   P cap, max (5) (5) 13 Introduction to LHP - Ku 2015 TFAWS

Pressure Profile in Gravity-Neutral LHP Operation Capillary Force Driven Bayonet Secondary 6 Primary Wick Vapor Wick Channel  P 1 2 1 P 2  P 3 P 4   sure Evaporator Reservoir P 5 P 6 1 7 Press Liquid Vapor Line Line Condenser P P 7 5 3 4 Location 7 (Liquid) (Liquid) • Evaporator core is considered part of reservoir.  • P 6 is the reservoir saturation pressure.  • All other pressures are governed by P 6 p g y 6 • All pressure drops are viscous pressure drops. 1 (Vapor) (Vapor) wick wick 14 Introduction to LHP - Ku 2015 TFAWS

Thermodynamic Constraints in LHP Operation (1) • • For the working fluid in a saturation state there is a one-to-one For the working fluid in a saturation state, there is a one-to-one correspondence between the saturation temperature and the saturation pressure. • There are three LHP elements where the working fluid exists in a two-phase state, i.e. evaporator, condenser and reservoir. • There is a thermodynamic constraint between any two of the above-mentioned three elements, i.e. the pressure drop and the temperature drop between any two elements are temperature drop between any two elements are thermodynamically linked. E.G. P E – P cc = (dP/dT) (T E – T cc ) • The derivative dP/dT can be related to physical properties of the working fluid by the Clausius-Clapeyron equation: dP/dT =  / (T cc  v) 15 Introduction to LHP - Ku 2015 TFAWS

Thermodynamic Constraints in LHP Operation (2) Bayonet Secondary Primary 6 Vapor Wick Wick Saturation Channel Curve  2 1  P 1   4  P 4  ressure Evaporator Reservoir 6 P 6  1 7 Li Liquid id Pr Liquid Vapor Line Vapor Line Condenser T 4 T 1 T 6 5 5 3 3 Temperature 4 P E – P cond = (dP/dT) (T E – T cond ) P cond – P cc = (dP/dT) (T cond – T cc ) P E – P cc = (dP/dT) (T E – T cc ) • Gravity affects the pressure drop, and hence the temperature difference. 16 Introduction to LHP - Ku 2015 TFAWS

LHP Operating Principles – Energy Balance Reservoir Reservoir Q sub Q IN m Evaporator Q leak Q Vapor Line e Liquid Line Q RA Q LA Condenser/Subcooler m Q C,1 Φ Q C,2 Φ   Q Q Q  IN E L T f ( m , L , T )    c , out c , 2 c , wall   Q G T T     L E , CC E CC   T T f f T T , m m , L L , D D , T T IN c , out LL LL amb Q E   m     Q m C T T      Q m 2 D L h ( T T ) sub P CC IN    c , 2 c c , 2 c , 2 CC c , wall    Q Q Q 0 leak sub RA 17 Introduction to LHP - Ku 2015 TFAWS