ISNET Thermal Design

Altuğ Okan, MSc

22 Oct 2014, TÜBİTAK UZAY

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Thermal Analysis & Design

Copyright © 2014 by TUBITAK UZAY. All rights reserved.

OUTLINE

• Course Objectives• Introduction to Thermal Design and

Thermal Control Subsystem• Heat Transfer Basics• Hands-on exercise

22 Oct 2014ISNET/TUBITAK UZAY Workshop on

Small Satellite Engineering and Design

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COURSE OBJECTIVES

The participants will;• be aware of necessity of thermal control• learn heat transfer basics• perform basic thermal analysis

including trade-offs related to satellite configuration

• discuss analysis results with each other.



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• First artificial satellite Sputnik was launched by Russians in 4 October 1957.

• Hermetic satellite, air inside and had no clue about space environment. • Spacecraft environment

– Vacuum– Temperature Extremes (From solar max to radiation to almost absolute Zero temperature) – Radiation– ...

• Most effective space environment effect is the orbital heat fluxes under vacuum conditions.

• Temperature of each component in the spacecraft must be within defined, tolerable limits which could be handled by Thermal Control Subsystem

INTRODUCTION



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• Environmental Heat Loads– Solar Flux (1326 – 1417 W/m2 depending on seasons)– Albedo (Reflected portion of the incoming Solar Flux, typically

%30 of Solar)– Earth Infrared Emission (IR Emission of the Earth at 255K =

~240W/m2)• Heat Dissipation

– Batteries– Other Electronic Equipments

• Radiation to Space– Infrared emission from exterior surfaces of spacecraft

SPACECRAFT THERMAL ENVIRONMENT



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Aim;QSun + QAlb + QEarth + Qinternal – QSpace = 0

where• QSpace is radiation of spacecraft exterior surfaces

to deep space with 4th power of each external surfaces.

• Deep space temperature is about 4 K (~269°C) and behaves as heat sink and black body (perfect absorption/emission)

Finally; • The temperatures of any spacecraft equipment

shall stay within allowable limits after heat in and out is balanced.

• The system enabling this requirement is the Thermal Control Subsystem of the spacecraft.

ENERGY BALANCE



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Thermal Design

and Analysis

Mathematical

Model

Thermal Control Subsystem

Thermal vacuum/bala

nce tests

INTRODUCTION – Thermal Design & Thermal Control



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• Heat transfer mechanisms– Conduction– Convection– Radiation

• Conduction is the way of heat transfer within spacecraft subsystems/equipment.

• Convection is negligible in space, but used in launch vehicle – spacecraft coupling during launch.

• Radiation is the major heat transfer mechanism for balancing heat in space. It is also good way of heat transfer within spacecraft subsystems/equipment for high temperature differences.

HEAT TRANSFER BASICS



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One dimensional heat flow (x-> ) [Fourier, 1822]

Qx= -k*A*(dT/dx)

Qx : Heat flux in x-axis (W) ,k : conduction coefficient (W/(m*K)) ,A : conductance area (m2) ,T : temperature (K) ,x : Distance in x direction.i j

Qi,j= Qx

dxdTAkQx

HEAT CONDUCTION



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i jQi,j

Ki,j

ij,

c,,

j,i TTL

AkQ

ji

jiji

Conduction Heat Transfer Equation

• All materials greater than 0 K transfers heat via thermal radiation• In electromagnetic spectrum, thermal radiation covers 0.1 100 m

of wavelength

• Black body is the ideal body that emits and absorbs all the energy in all wavelengths and defined by Boltzmann rule:

E = T4

: Stefan-Boltzmann constant( = 5.6696*10-8 W/(m2*K4) ),T : Surface temperature (K).

In reality, there is no ideal surface (black body) emitting/absorbing 100% of its energy.

THERMAL RADIATION - Basics



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Real Surfaces, (Emissivity) = Ereal surf / Eblack

surfEreal surf= T4

Total Energy transfer

THERMAL RADIATION – Real Surfaces

Reflected, (*E)

Incoming flux, E

Absorbed, (*E)

Transmitted, (*E)

+ + = 1



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View Factor (Radiation Exchange Factor for ideal surfaces; ε=1 )

Radiation Exchange Factor (for real surfaces where ε<1)

THERMAL RADIATION – View and Radiation Exch. Factors



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j,irij,i A

ji j

Qi,j

i,j

i

44,, ijji

riji TTAQ

Qi,j

wherei,j : Radiation Exchange Factor between surfaces i and j

THERMAL RADIATION – Radiation Exchange

Radiative Heat Transfer Equation



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Energy Balance

Time Dependent Energy Equation

tTCp q

zTk

zyTk

yxTk

x

HEAT TRANSFER EQUATION (1/2)

QSun + QAlb + QEarth + Qinternal – QSpace = 0

Conduction and Radiation Terms

In Orbit Temperatures of all the satellite



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Discretized Energy Equation

HEAT TRANSFER EQUATION (2/2)



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where• m = mass [ kg ]• Cp = Specific Heat [ J/(kg*K) ]• = num. coefficient [=1 fully-implicit, =0 fully-

explicit] • Qi = heat flux coming to note i• N = number of nodes in the satellite• n = time at t; n+1 is the time at t+t

1ni

N

j

41ni

41njj,i

N

j

1ni

1njj,i

ni

1ni

i QTTRTTKtTTCm

ip

N

j

ni

N

j

4ni

4njj,i

ni

njj,i QTTRTTK)1(

Temperature Requirement of Typical Spacecraft Equipment

THERMAL DESIGN REQUIREMENTS - Temperatures



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Equipment NO min (C) Op. min (C) Op. max (C) NO max (C)

Optical Imager 0 13 23 40Battery 0 0 30 30OB Computer -30 0 50 60Other Electronics -30 -20 50 60

Solar Panels -100 -100 100 100

Antennas -50 -50 100 100NO: Non-OperatingOp: Operating

Critical Equipment1. The batteries must operate at preferably between 15°C and 25°C to

increase lifetime.2. Optical imager optics must operate in very narrow temperature

bandwidth (±5°C or less) for less thermal distortion.

Rule of Thumb 1Keep everything simple: the more you increase the complexity, the harder you analyze and solve problems

Set up your model with isothermal nodes for each equipment instead of Finite Element modeling with many meshes

Use worst hot and worst cold cases to stay within temperature limits

Always prefer commercial off the shelf coatings and materials to keep the energy balance at moderate temperatures.

TRADE OFF



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Rule of Thumb 2Radiation from exterior surfaces is the key for thermal design.

Choose appropriate coatings/tapes to keep satellite in moderate temperatures

TRADE OFF



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Equipment αs εIR Remarks

Optical Solar Reflector 0.08 0.80 Solar flux rejector, Preferred for cooling

First surface mirror 0.14 0.05 Preferred for keeping warmMLI* 0.31- 0.6 0.85-0.96 Radiatively decouples from

environmentBlack Paint 0.95 0.90 Preferred for heat rejection inside

s/cWhite Paint 0.20 0.85 Preferred for coolingSolar Cells 0.70 0.70 Used in solar panel modeling*: effective emissivity <

0.04

Rule of Thumb 3Use appropriate thermal control hardware for specific thermal problems

TRADE OFF



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Thermal Hardware FunctionMLI Decouples radiative heat exchangeOptical Solar Reflector Rejects heat

Radiator Used for emitting heat to space

Heat Pipe Acts as very high conductive material and allows to carry heat from heat source to radiators

Paints / Coatings Cooling or keeping the energy depending on α/εHeaters Helps to increase tepmerature of specific equipment or regionThermostat Help to control heaters Sensors Measures temperatures

Hands-on Exercise



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Typical Thermal Design and Analysis requires • specific modelers and pre/post processors• many details & iterations = time and

effort

Hand calculation is an option but takes too much time

Therefore, we set up affordable simple satellite model for Hands-on Exercise

• 2- node satellite model (Platform and Payload)

• Averaged input and output (steady-state solution)

• Thermal data, orbital fluxes and all calculations in Excel spreadsheet

HANDS-ON EXERCISE CHALLANGES

THERMAL MATHEMATICAL MODEL – Node Definition



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Node Item1 Platform

2 Payload

3 Space

Given in the Nodes tab of the Excel sheet

THERMAL MATHEMATICAL MODEL – Heat Loads



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Environmental Heat Loads

Given in the Heat Loads tab of the Excel sheet

Internal Heat Generation

Assumptions• Steady-state orbital fluxes (averaged) on

each surface for worst hot and worst cold conditions

• Only environmental loads on Node 1 (since Node 2 is the payload and should be thermally stable and independent of orbital heat fluctuations for thermal stability ; α =ε << 1 )

• 3 axis-stabilized• +X axis is the velocity vector• +Z axis indicated the Earth

THERMAL MATHEMATICAL MODEL – Links



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Given in the Conduction and Radiation Links tab of the Excel sheet

Links Formula

K1,2 k x Ac / l

1,3 σ x Ar x ε x F1,2

Discretized heat equation given in Slide 15:

is simplified to Steady-State 1D heat transfer problem

Q1,2 = Q2 = K1,2*(T2-T1) and Q1,3 = (Q1+Q2) = R1,3*(T14-T3

4)

which are calculated in the Results tab of the Excel sheet

THERMAL MATHEMATICAL MODEL – Calculations



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Q1,2

Q1,3

1ni

N

j

41ni

41njj,i

N

j

1ni

1njj,i

ni

1ni

i QTTRTTKtTTCm

ip

N

j

ni

N

j

4ni

4njj,i

ni

njj,i QTTRTTK)1(

Documents

ISNET Thermal Design