thermal systems design - university of...

Thermal Systems DesignENAE 697 -Space Human Factors and Life Support

U N I V E R S I T Y O FMARYLAND

Thermal Systems Design

• Fundamentals of heat transfer• Radiative equilibrium• Surface properties• Non-ideal effects

– Internal power generation– Environmental temperatures

• Conduction• Thermal system components

© 2009 David L. Akin - All rights reservedhttp://spacecraft.ssl.umd.edu

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Classical Methods of Heat Transfer

• Convection– Heat transferred to cooler surrounding gas, which creates

currents to remove hot gas and supply new cool gas

– Don’t (in general) have surrounding gas or gravity for convective currents

• Conduction– Direct heat transfer between touching components

– Primary heat flow mechanism internal to vehicle

• Radiation– Heat transferred by infrared radiation

– Only mechanism for dumping heat external to vehicle

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Thermodynamic Equilibrium

• First Law of Thermodynamics

heat in -heat out = work done internally• Heat in = incident energy absorbed• Heat out = radiated energy• Work done internally = internal power used

(negative work in this sense - adds to total heat in the system)

€

Q −W =dUdt

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Radiative Equilibrium Temperature

• Assume a spherical black body of radius r• Heat in due to intercepted solar flux

• Heat out due to radiation (from total surface area)

• For equilibrium, set equal

• 1 AU: Is=1394 W/m2; Teq=280°K

€

Qin = Isπ r2

€

Qout = 4π r 2σT 4

€

Isπ r2 = 4π r2σT 4 ⇒ Is = 4σT 4

€

Teq =Is4σ

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4



Effect of Distance on Equilibrium Temp

Mercury

PlutoNeptuneUranus

SaturnJupiter

Mars

Earth

Venus

Asteroids

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Shape and Radiative Equilibrium

• A shape absorbs energy only via illuminated faces

• A shape radiates energy via all surface area• Basic assumption made is that black bodies are

intrinsically isothermal (perfect and instantaneous conduction of heat internally to all faces)

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Effect of Shape on Black Body Temps

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Incident Radiation on Non-Ideal BodiesKirchkoff’s Law for total incident energy flux on solid

bodies:

where– α =absorptance (or absorptivity)– ρ =reflectance (or reflectivity)– τ =transmittance (or transmissivity)

€

QIncident =Qabsorbed+Qreflected +Qtransmitted

€

Qabsorbed

QIncident

+Qreflected

QIncident

+Qtransmitted

QIncident

=1

€

α ≡Qabsorbed

QIncident

; ρ ≡Qreflected

QIncident

; τ ≡Qtransmitted

QIncident

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Non-Ideal Radiative Equilibrium Temp

• Assume a spherical black body of radius r• Heat in due to intercepted solar flux

• Heat out due to radiation (from total surface area)

• For equilibrium, set equal

€

Qin = Isαπ r2

€

Qout = 4π r 2εσT 4

€

Isαπ r2 = 4π r 2εσT4 ⇒ Is = 4 ε

ασT 4

€

Teq =αεIs4σ

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(ε = “emissivity” - efficiency of surface at radiating heat)

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Effect of Surface Coating on

ε = emissivity

α =

abs

orpt

ivit

y

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Non-Ideal Radiative Heat Transfer• Full form of the Stefan-Boltzmann equation

where Tenv=environmental temperature (=4°K for space)

• Also take into account power used internally

€

Prad =εσA T 4 −Tenv4( )

€

Isα As + Pint =εσArad T4 − Tenv

4( )

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Example: AERCam/SPRINT

• 30 cm diameter sphere• α=0.2; ε=0.8• Pint=200W

• Tenv=280°K (cargo bay below; Earth above)

• Analysis cases:– Free space w/o sun– Free space w/sun– Earth orbit w/o sun– Earth orbit w/sun

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AERCam/SPRINT Analysis (Free Space)

• As=0.0707 m2; Arad=0.2827 m2

• Free space, no sun

€

Pint = εσAradT4 ⇒ T =

200W

0.8 5.67 ×10−8 Wm2°K 4

0.2827m2( )

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= 354°K

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AERCam/SPRINT Analysis (Free Space)

• As=0.0707 m2; Arad=0.2827 m2

• Free space with sun

€

Isα As + Pint = εσAradT4 ⇒ T =

Isα As + PintεσArad

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= 362°K

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AERCam/SPRINT Analysis (LEO Cargo Bay)

• Tenv=280°K

• LEO cargo bay, no sun

• LEO cargo bay with sun

€

Pint =εσArad T4 − Tenv

4( )⇒ T =200W

0.8 5.67 ×10−8 Wm 2°K 4

0.2827m2( )

+ (280°K)4

14

= 384°K

€

Isα As + Pint =εσArad T4 − Tenv

4( )⇒ T =Isα As + PintεσArad

+ Tenv4

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= 391°K

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EVA Thermal Equilibrium

• Human metabolic workload = 100 W• Suit electrical systems = 40 W• Total heat load = 140 W

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Q = !"AT 4 ! A =Q

!"T 4

A =140

(0.8)(5.67! 10!8)(295)4= 0.41 m2



EVA Thermal Equilibrium with Sun

• Human metabolic workload = 100 W• Suit electrical systems = 40 W• Total internal heat load = 140 W

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Q + !AIs = "#AT 4 ! A =Q

"#T 4 " !Is

A =140

(0.8)(5.67! 10!8)(295)4 " 0.2(1394)= 2.2 m2



Sublimation

• Water heat of sublimation = 46.7 kJ/mole• @ 18 gm/mole = 2594 W-sec/gm• Mass flow for 140 W = 0.54 gm/sec• per hour = 194 gm/hr• 8 hr total EVA time = 1.55 kg of water• @ 2 EVA/day and 180 days = 559 kg of water

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Sitting on a Planetary Surface

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IsTb

Th

Th



Radiative Insulation

• Thin sheet (mylar/kapton with surface coatings) used to isolate panel from solar flux

• Panel reaches equilibrium with radiation from sheet and from itself reflected from sheet

• Sheet reaches equilibrium with radiation from sun and panel, and from itself reflected off panel

Is

Tinsulation Twall

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Multi-Layer Insulation (MLI)• Multiple insulation

layers to cut down on radiative transfer

• Gets computationally intensive quickly

• Highly effective means of insulation

• Biggest problem is existence of conductive leak paths (physical connections to insulated components)

Is

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Emissivity Variation with MLI Layers

Ref: D. G. Gilmore, ed., Spacecraft Thermal Control Handbook AIAA, 2002

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MLI Thermal Conductivity


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Effect of Ambient Pressure on MLI


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1D Conduction

• Basic law of one-dimensional heat conduction (Fourier 1822)

whereK=thermal conductivity (W/m°K)A=areadT/dx=thermal gradient

€

Q = −KAdTdx

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3D Conduction

General differential equation for heat flow in a solid

whereg(r,t)=internally generated heatρ=density (kg/m3)

c=specific heat (J/kg°K)K/ρc=thermal diffusivity

€

∇ 2Tr r ,t( ) +

g(r r ,t)K

=ρcK∂T

r r ,t( )∂t

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Simple Analytical Conduction Model

• Heat flowing from (i-1) into (i)

• Heat flowing from (i) into (i+1)

• Heat remaining in cell

TiTi-1 Ti+1

€

Qin = −KATi − Ti−1Δx

€

Qout = −KATi+1 −TiΔx

€

Qout −Qin =ρcK

Ti( j +1) − Ti( j)Δt

… …

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• Time-marching solution

where

• For solution stability,

Finite Difference Formulation

! =k

"Cv

= thermal di!usivityd =!!t

!x2

Tn+1i

= Tn

i + d(Tn

i+1 ! 2Tn

i + Tn

i!1)

!t <!x2

2!

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Heat Pipe Schematic

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Shuttle Thermal Control Components

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Shuttle Thermal Control System

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ISS Radiator Assembly

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thermal systems design - university of...

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