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A Simple Representation of the Impact of a Loop Heat Pipe on a Space - - PowerPoint PPT Presentation

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A Simple Representation of the Impact of a Loop Heat Pipe on a Space System Perry Ramsey Andrew Hensley ITT Industries Space Systems Division Fort Wayne, IN TFAWS '04 31 August 2004 Abstract Abstract As two phase thermal control

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A Simple Representation of the Impact

  • f a Loop Heat Pipe on a Space System

Perry Ramsey Andrew Hensley ITT Industries Space Systems Division Fort Wayne, IN

TFAWS '04 31 August 2004

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Abstract Abstract

  • As two phase thermal control devices like loop heat pipes become more common, it is

necessary to develop a method to simply represent their effects in a system-level model. Thermo-hydraulically simple models are available, but they still are too slow to include in system models that will be used routinely during concept development. This discusses an attempt to create a model that will represent the gross impact of an LHP on an orbiting space vehicle.

  • The approach taken is to model the LHP as a node connected to the radiator through an array
  • f variable conductors. The value of each conductor is dependent on the source temperature.

By varying the shape of the conductance function, a broad range of potential responses is possible.

  • The thermo-hydraulic model of the loop heat pipe was run at various input powers over its

range of viability. The variable conductor model was correlated against the results using the Sinda solver. The variable conductor model was then embedded in a spacecraft system model.

  • Though the variable conductance model is applicable only over a fairly limited range of
  • perating conditions, it has proven very useful the development of the system and has

provided excellent insight into the behavior of the operating space vehicle.

31 August 2004 TFAWS '04

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An Geostationary Instrument Deals With Widely Varying Heat Loads An Geostationary Instrument Deals With Widely Varying Heat Loads

We are developing

concepts for a geostationary instrument

GOES experience shows

that varying heat loads must be managed

  • Diurnal
  • Annual

31 August 2004 TFAWS '04

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One Way to Deal With the Load is a Loop Heat Pipe One Way to Deal With the Load is a Loop Heat Pipe

LHP is a viable design option

  • Can transport internal loads to external radiator
  • Can control evaporation temperature

Need some way to model it at system level Thermo-Hydraulic Model Simulates Two-Phase

Physics (Reference 1)

Converted from SinAps to Thermal Desktop/FloCad

  • Changed variables to reflect our design, and added heated

compensation chamber

31 August 2004 TFAWS '04

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Top Level LHP Behavior Inspires Reduced Representation Top Level LHP Behavior Inspires Reduced Representation

31 August 2004 TFAWS '04

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Liquid evaporates Condensation in

radiator until all input heat load is rejected

Beyond, radiator

decoupled from load

At increasing

load, full condensation takes longer

Load removed High conductance

from evaporator to radiator

Low or zero

conductance from evaporator to radiator

More radiator is

connected LHP Behavior Reduced

Outlet Inlet

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LHP Integrated with Radiator in Orbit Produces Credible Results LHP Integrated with Radiator in Orbit Produces Credible Results

Controlled compensation

chamber temperature

Ramp up of evaporator

temperature

  • Drop ~ 40 W/K

Radiator opens as load

increases

  • Eventually becomes fully
  • pen

Fluid Model used as “True”

behavior in future calculations

230 240 250 260 270 280 290 300 310 100 200 300 400 500 Input Load (W) T em p eratu re (K )

Evaporator Outlet Inlet

31 August 2004 TFAWS '04

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Model is ‘Expensive’ to Run Model is ‘Expensive’ to Run

A thermohydraulic model, even simplified, is difficult to

incorporate

  • Takes a lot of calculations per time step
  • Needs small time step to resolve fluid physics

– Fluid variations are not particularly important to external thermal

performance with slowly varying boundary conditions

– Spending a lot of time on things that don’t affect the results much

  • Transient instabilities can stop the entire analysis run

We need a representation that is

  • Fast enough for parametric analysis of the system
  • Robust
  • Accurate enough that we don’t make big mistakes in

architecture

31 August 2004 TFAWS '04

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Try to Capture the Essence of Behavior Without Modeling All the Physics Try to Capture the Essence of Behavior Without Modeling All the Physics

In simplest terms, a controlled LHP looks like a

variable resistor

  • Resistance is dependent on source temperature

We’re most interested in the temperature drop from

the evaporator to the radiator

Secondary goal is temperature variation across the

radiator

31 August 2004 TFAWS '04

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Model LHP as Array of Variable Conductors Model LHP as Array of Variable Conductors

Break radiator into segments Connect each to evaporator through a variable

conductor

When cold, all are off As the heat input increases, conductors turn on in

sequence based on rising evaporator temperature

31 August 2004 TFAWS '04

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Conductor Behavior Tailored to Mimic Reality Conductor Behavior Tailored to Mimic Reality

Model it in Sinda Derive values of conductors

31 August 2004 TFAWS '04

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Outlet Inlet

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In Sinda, this is a SIVA In Sinda, this is a SIVA

A SIVA is similar to more familiar SIV conductor

  • Interpolation is based only on the first node temperature

Data in the array drives the network behavior

Scaling factor Evaporator node Radiator node Value array

SIVA 101,HUB.10, RAD.101, hp.A101,fac . . . SIVA 125, HUB.10, RAD.125, hp.A125,fac

31 August 2004 TFAWS '04

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Rising Ramp Allows Smooth Turn On Rising Ramp Allows Smooth Turn On

Each conductor

ramps from 0 to 100%

Some tweaks

needed for stability

0% 20% 40% 60% 80% 100% 120% 274.5 275.0 275.5 276.0 276.5 Temperature [K] F ra c tio n o f F u ll C o n d u c ta n c e Conductor 2 Conductor 3 Conductor 4 T incr G Base G Near On T LHP On

31 August 2004 TFAWS '04

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Use Optimizer to Match Simplified to Detailed Model Use Optimizer to Match Simplified to Detailed Model

Run the thermo-hydraulic model for range of loads

  • 50 to 350 W input in 50 W steps
  • Varying beta angles
  • NOT trying to handle anything but normal operation

Fit simplified to detailed model using the Sinda

  • ptimizer

31 August 2004 TFAWS '04

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Two Systems Have Comparable Performance Two Systems Have Comparable Performance

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Evaporator and radiator segment temperatures Some differences are apparent, but evaporator

response is similar

LHP Fluid Model LHP Simplified Representation

220 230 240 250 260 270 280 290 50 100 150 200 250 300 350 400 LHPload [W] Temp [K] 220 230 240 250 260 270 280 290 50 100 150 200 250 300 350 400 LHPload [W] Temp [K]

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Installed in System Model, Performance Met Program Needs Installed in System Model, Performance Met Program Needs

Able to run system model with fast turnaround Not driven by simulated LHP performance User has to be careful to keep the simulation within

applicable bounds

  • Exceeding shutdown limits
  • Overrunning maximum heat input

31 August 2004 TFAWS '04

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Variation in LHP Heatload Variation in LHP Heatload

Load input to LHP – up to 250 W diurnal variation

50 100 150 200 250 300 350 400 12 PM 3 PM 6 PM 9 PM 12 AM 3 AM 6 AM 9 AM 12 PM Time of Day LHP Input (W) Beta 0 Beta 8.7 Beta 23 31 August 2004 TFAWS '04

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Stable Temperature Over Large Variation in Heat Load Stable Temperature Over Large Variation in Heat Load

Temperatures at LHP baseplate vary ~ 10°C

5 10 15 20 25 12 PM 3 PM 6 PM 9 PM 12 AM 3 AM 6 AM 9 AM 12 PM Local Time of Day Temperature (°C) beta 0 beta 8.7 beta 23 31 August 2004 TFAWS '04

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Opportunities for Improvement Opportunities for Improvement

We’re correlating a model to a model

  • Test data would be helpful

Impact of external load variations require examination

  • Varying loads due to, beta angle, property variation

Gradient in radiator too high Are we optimizing on the right thing Simulation of return line environment

31 August 2004 TFAWS '04

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Simplified Representation Functional, Useful for Conceptual Trades Simplified Representation Functional, Useful for Conceptual Trades

A representation of LHP behavior has been created

  • Suitable for concept development

Within the limits of its capability, the representation

has been useful

  • Fast enough for parametric analysis of the system
  • Robust
  • Provides an accurate enough result to guide architecture

development

31 August 2004 TFAWS '04

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References References

1) Cullimore, B. and J. Baumann, “Steady State and

Transient Loop Heat Pipe Modeling”, SAE Paper Number 2000-ICES-105

31 August 2004 TFAWS '04

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Acknowledgements Acknowledgements

This work performed under contract NAS5-01119, ABI

Formulation Phase

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