Thermal Control

Robert ManningAAE450Spring 2007

Outline Fundamentals Thermal Control Devices Heat Shield (TPS) Resources & Considerations

Fundamentals: Steady-state thermal modeling is

simply an energy balance. Q is heat flux or transfer (Watts) q is heat flux per unit area (W/m2) Area is ALWAYS normal to transfer. Three method of heat transfer:

radiation, conduction, & convection.

Fundamentals: Conduction Simple one dimensional condition:

K = Thermal conductivity (W/m/K) dt/dx = Temperature gradient (K/m) Derivative can be approximated

using two temperature (T1 and T2)

x

TTKq

dx

dTKq

21

21

Fundamentals: Convection Newton’s Law of cooling:

h = Transfer Coefficient (W/m2-K) Empirical equation. Use Nusselt number

correlations to determine h. Laminar/Turbulent?

Free convection/external/internal?Boiling/Condensation?

)( 2121 TThq

Fundamentals: Radiation

Heat emitted is governed by Stefan-Boltzmann Law. is emissivity. is 5.67x10-8 J/(K4-m2-s)

Heat absorbed is governed by the absorbitivity coefficient .

Use view factor relationship (Incropera Chapter 13)

incidentabsorbout qqTq 4

Fundamentals: Tricks Area is projected area of radiation. If no heat is generated in body,

temperature can be controlled by examining /.

We can treat thermal conductance as an electrical resistor:

R

TQ

KA

xR

Thermal Control Devices Passive Thermal Control:

System without any moving parts or electrical input

Active Thermal Control:Anything that has moving parts and/or electrical input

Multi-layer Insulation

MLI is typically part of micrometeorite protection.

Use Effective Emmittance(~0.005):

Chapter 13.2.5 from Incropera

Outer Cover

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Reflector

Cover & Structure

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QTTA CH )(* 44

Pumped-Loop Systems Active Control Transfers heat from one location to

another using a pumped liquid. Typically use water for human habitat. Ammonia or Freon used for external or

non-habitat portions. Use counter-flow heat exchangers! Chapter 11 of Incropera

Radiators Active Control Used in conjunction with pumped-

loops to radiate heat into space. Two types:

body-mounted or deployable Use Flash Evaporators when not

deployed

Thermal Protection System Difficult. Ask Prof. Schneider! Establish characteristics of entry:

Velocity-altitude profilebluff or streamlined bodyKnudsen numberablative vs. no ablation

Consider using existing data or codes!

TPS: Flow characteristics Chemical reaction at high temperatures

Oxygen: T > 2000 K, Nitrogen: T > 4000 K Possible ionization Turbulent, separated, shock interactions Convection vs. Radiation

Knudsen: kn > 0.1 => no continuum

length ccharacteri

path freemean

kn

Resources: Books

1) Excellent Thermal Design Book:David G. Gilmore. Spacecraft Thermal Control Handbook.

2) Incropera, DeWitt, et al. Fundamentals of Heat and Mass Transfer.

3) Anderson, John. Modern Compressible Flow or Hypersonic and High Temperature Gas Dynamics.

Resources: Web Code for aero-thermal modeling:

http://roger.ecn.purdue.edu/~aae450s/methods.pdf

TPSX:http://tpsx.arc.nasa.gov/

Resources @ Purdue SODDIT:

Sandia One-Dimensional Direct and Inverse Thermal Code

Newton’s Method:

Predicts Cd and Cl for high mach numbers Prof. Schneider