Thermal Modeling of In-Depth Thermocouple Response in ...Thermal Modeling of In-Depth Thermocouple...

Thermal Modeling of In-Depth

Thermocouple Response in Ablative

TPS Materials

Thermal & Fluids Analysis Workshop (TFAWS 2008)

San Jose State University, San Jose, CA

August 18, 2008

Jose A. Santos Robin A. Beck Timothy K. Risch

Sierra Lobo, Inc. NASA Ames Research

Center

NASA Dryden Flight

Research Center

2

Outline

• Background and Scope

– MSL, Entry, Descent, Landing Instrumentation (MEDLI)

– MEDLI Instrumented Sensor Plug (MISP)

• Two dimensional thermal model

– Boundary Conditions

– Geometry

– Case Studies

• Conclusions

• Future Work

• Acknowledgments

Background

3

• MEDLI is an instrumentation suite installed in the heat shield of

the Mars Science Laboratory (MSL) Entry Vehicle that will

gather data on the atmosphere and on aerothermal, Thermal

Protection System (TPS) and aerodynamic characteristics of the

MSL Entry Vehicle during entry and descent providing

engineering data for future Mars missions.

• MEDLI consists of 7 pressure ports and 7 integrated sensor

plugs (containing four thermocouples and an isotherm sensor)

all installed in the forebody heat shield of the MSL entry vehicle.

Cruis e S tage

Backs he ll

Des cent S tage

Rover

Heats hie ld

Cruis e S tage

Backs he ll

Des cent S tage

Rover

Heats hie ld

MSL (For Reference)

MEDLI Instrumented Sensor Plug (MISP)

• Sensor plug contains four in-depth Type K TCs 2.54 mm,

5.08 mm, 11.43 mm, and 17.78 mm from the surface

• One isotherm-following sensor to measure char depth

4

Figure courtesy K. Edquist (NASA LaRC)

c/o R. Beck (NASA ARC)

1. Apex (reference heat flux)

2. Stagnation point

3. Windward heating

augmentation

4. Leeward shoulder heating

5. Leeward shoulder heating

6. Transition onset cusp

7. Leeward acreage recession

MISP Locations:

• Sensors located so as to

provide enough information to

recreate heat flux distribution on

the centerline

Problem Description and Solution Approach

• How does one model the response of the thermocouples

embedded in the fully decomposing thermal protection system

material?

– Existing material response codes do not account for multi-

dimensional pyrolysis gas flow (even the available multi-

dimensional codes still assume one-dimensional pyrolysis gas flow

through the material)

• Conservative approach is taken:

– Compute surface recession and wall temperature boundary

conditions using a 1D material response program and input into a

2D thermal model of the instrumented sensor plug

5

CFD Trajectory

Output:

ρeUeCH(t)

Pw(t)

Hr(t)

Hw(t)

1D Material Response

Code Output:

ds(t)/dt

Tw(t)

Thermal FEM output:

TTC

TTPS

Focus of this presentation

Surface Boundary Conditions to CMA04 Material

Response Program

• Time-dependent input heating and surface pressure profiles

(trajectory considered early in the design of the MSL heat

shield)

6

0

50

100

150

200

250

0 50 100 150 200

Heat Flux (W/cm

2)

Time (s)

Peak Heating

Stagnation Heating

0

5

10

15

20

25

30

35

0 50 100 150 200Surface Pressure (kPa)

Time (s)

Peak Heating

Stagnation Heating

• Peak heating profile: qmax = 217 W/cm2, Pmax = 23 kPa

• Stagnation point heating profile: qmax = 48 W/cm2, Pmax = 30 kPa

2D Heat Conduction Model Boundary Conditions

• CMA04 material input file

– SLA-561V High Fidelity Response Model (HFRM), peer reviewed at NASA ARC,

and Mars “B prime tables” (dimensionless mass surface flux) are used

• Computed wall temperature and recession rate time histories:

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Peak Heating Profile

0

500

1000

1500

2000

2500

3000

0.000

0.005

0.010

0.015

0.020

0.025

0 50 100 150 200 250

Surface Temperature (K)

Recession rate (mm/s)

Time (s)

Recession rate

Surface Temperature

Stagnation Point Heating Profile

0

500

1000

1500

2000

2500

3000

0.000

0.005

0.010

0.015

0.020

0.025

0 50 100 150 200 250

Surface Temperature (K)

Recession rate (mm/s)

Time (s)

Recession rate

Surface Temperature

• Predicted total recession in peak heating case is only 1.22 mm;

consequently, no thermocouples burn through

• No recession is predicted for the stagnation point

Geometry for 2D Thermal Model

• Thermocouple is modeled as a two-dimensional

object based on its cross-section

8

“Land length”

Bead weld

MISPTC Depth

U-shape Cross Section

y

z

x

z

“L-shape” design:

L-shape Cross Section

y

z

Horizontal

z

x MISP

Coated TC

wire

TC wire

x

y

∞

TPS

z

Construction of Thermal Model

• 2-D Boundary conditions for side and back walls

– Insulated side walls are assumed

– Radiation from back wall to an ambient temperature of 203 K (-70 °C)

• Initial conditions

– All materials are set to 203 K at time zero

• Material properties are input as functions of temperature

– Virgin properties of SLA-561V from the High Fidelity Response Model

developed at NASA ARC are used

• Material Stack-up:

9

RTV-560

(0.254 mm thick)

SLA-561V

(22.86 mm thick)

2024 Aluminum

(6.35 mm thick)

TC Cross-section

∑

∑=

i

ii

i

ipii

eqpA

CA

Cρ

ρ ,

,

∑

∑=

i

i

i

ii

eqA

kA

k

∑

∑=

i

i

i

ii

eqA

A ρ

ρ

Thermal Model Details (cont’d)

• Computational tool for 2D model:

COMSOL Multiphysics

– Commercial finite-element

package

• User may use pre-defined

physics modules

(transient conduction)

and/or input differential

equations in coefficient or

general form

– Arbitrary Lagrangian-Eulerian

(ALE) moving boundary is

used to provide recession rate

and surface temperature

boundary conditions as a

function of time

– Multiphysics capability of

COMSOL couples moving

mesh with heat conduction in

a solid material

10

Temperatures are in Kelvin

Screenshot showing surface recession of L-shape TC

configuration at time t = 70 sec of the peak heating profile

Thermal Model Details (cont’d)

• Mesh construction

– Approx. 4000 triangular elements total• ~ 75 elements for the alumina coating

• ~ 140 elements for the thermocouple wire

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Examination of 2D Assumptions

• Assumption of no internal decomposition examined with 1D

dimensional CMA04 analysis

• CMA04 code is run for two cases with different boundary conditions:

• No decomposition – time-dependent surface temperature and recession rate

• Fully decomposing – ρeUeCH(t), Pw(t), Hr(t), Hw(t) taken from CFD

12

0

500

1000

1500

2000

2500

3000

0 50 100 150 200 250

Time (s)

Temp (K)

Lines: Fully decomposing analysis with surface energy balance

Symbols: Non-decomposing analysis with Tsurf and sdot defined

Tsurf

0.1"

0.2"

0.3" 0.4"

0.5"

0.9"

Peak heating

point

• Fully decomposing trace

lags behind the non-

decomposing result

• Expect 2D non-

decomposing thermal

model to be conservative:

actual thermocouple

response time should be

faster than what the

computations predict

Model Traceability

• Comparison of COMSOL predicted in-depth temperatures with

CMA04 non-decomposing analysis:

13

0

500

1000

1500

2000

2500

0 50 100 150 200 250

Temp (K)

Time (s)

COMSOL TC1

COMSOL TC2

CMA Non-decomposing, TC1

CMA Decomposing, TC1


CMA Decomposing, TC2TC1 (2.54 mm)

TC2 (5.08 mm)

Peak heating

Peak Heating Profile

0

500

1000

1500

2000

2500

0 50 100 150 200 250

Temp (K)

Time (s)

COMSOL TC1

COMSOL TC2


CMA Decomposing, TC1


CMA Decomposing, TC2TC1 (2.54 mm)

TC2 (5.08 mm)

Stagnation point heating

Stagnation Point Heating Profile

• Analysis Matrix (valid for both peak heating and stagnation point heating profiles):

Test Cases

• Thermocouple error is relative to the temperature the TPS would achieve in the absence of instrumentation and subjected to the same boundary conditions

• Analysis focuses on locations of TC1 and TC2: 2.54 mm and 5.08 mm, respectively, from the top surface of the sensor plug

– Since negligible error (< 3 K) is seen at the TC3 location (11.43 mm depth), analysis of TC4 (17.78 mm depth) was not considered

• Aluminum back plate, RTV-560 bonding agent remained isothermal

14

100/)(% ×−= TPSTCTPS TTTError

0.165 mm 0.305 mm 0.396 mm No TC

U-shape × × × ×

L-shape × ×

Thermal Model Simulation

15

• Temperature-time histories for peak heating profile:

0

200

400

600

800

1000

1200

1400

1600

1800

0 20 40 60 80

Temp (K)

Time (s)

U-shape 0.165 mm TC1



L-shape 0.305 mm coated TC1

U-shape no TC1

Peak heating

0

200

400

600

800

1000

1200

1400

1600

1800

0 50 100 150 200 250 300Temp (K)

Time (s)




L-shape 0.305 mm coated TC2

U-shape no TC2

Peak heatingPeak heating

TC 1 Depth of 2.54 mm from Surface TC 2 Depth of 5.08 mm from Surface

Results – Peak Heating Profile

• Results of parametric studies run to determine effect of wire

diameter and thermocouple installation method

16

• Only the 0.165 mm diameter wire maintains an error below 5% for

the entire duration of the trajectory

• In the case of the thermocouple located 5.08 mm from the surface,

the error peaks at the time of peak heating

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140

Temperature Error (%

)

Time (sec)

U-shape, 0.165-mm wire dia.



L-shape, 0.305-mm coated

5%error

Peak Heating, TC 2

qmax = 217 W/cm2

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140


)

Time (sec)





5% error

Peak Heating, TC 1

Curves truncated after 1370 °C

Type K temperature limit

qmax = 217 W/cm2

Results – Stagnation Point Heating Profile

• Lower overall heating at the stagnation point significantly reduces

error for thermocouple configurations

• Only the 0.165 mm and 0.305-in diameter bare wires remain below

an error of 5%

17

0

2

4

6

8

10

0 20 40 60 80 100 120 140


)

Time (sec)





5% error

Stagnation Point Heating, TC 2

qmax = 48 W/cm2

0

2

4

6

8

10

0 20 40 60 80 100 120 140


)

Time (sec)





5% error

Stagnation Point Heating, TC 1

qmax = 48 W/cm2

18

Conclusions

• 2D finite element model constructed in COMSOL

Multiphysics has been developed to estimate the error

associated with thermocouple lag and allows for rapid

turnaround of trade studies

• Moving boundary achieves excellent agreement with

CMA04 for a non-decomposing analysis

• Peak heating location produces larger errors than the

stagnation point (low heating) profile

• Bare wire (uncoated) thermocouple diameters of 0.165

mm and 0.305 mm consistently achieved the best results

among the configurations considered

• No significant thermal lag is predicted with this model for

in-depth thermocouples located at depths 11.43 mm and

below

19

Future Work

• Determine the effect of one thermocouple on another as

a function of separation distance

• Run the model for other trajectories and TPS materials

• Relax the perfect thermal contact assumption and model

the gap between the thermocouple and TPS material

– Radiation between wire and TPS

– Gas flow

• Develop model to account for internal decomposition

with multi-dimensional pyrolysis gas flow

Thermal Model

(Videos)

20

Acknowledgments

• MSL, Entry, Descent, Landing Instrumentation

(MEDLI) project for funding this work

• Dr. Michael J. Wright (NASA-ARC) for providing the

CFD trajectory data input as boundary conditions into

the CMA04 calculations

21

References

• Oishi, T., E. Martinez, and J. Santos. Development and Application of a TPS Recession

Sensor for Flight. AIAA 2008-1219. January 2008.

• Anonymous, User’s Manual Aerotherm Charring Material Thermal Response and Ablation

Program CMA04. ITT Industries, Advanced Engineering & Sciences Division. 2004. ITT

Document No. 1909-04-001.

• Chen, Y.-K. and F. S. Milos. Ablation and Thermal Response Program for Spacecraft

Heatshield Analysis. AIAA 98-0273. 1997.

• Chen, Y.-K. and F. S. Milos. Two-Dimensional Implicit Thermal Response and Ablation

Program for Charring Materials on Hypersonic Space Vehicles. AIAA 2000-0206. 2000.

• Chen, Y.-K. and Frank S. Milos. Three-Dimensional Ablation and Thermal Response

Simulation System. AIAA 2005-5064. 2000.

• “Physical Properties of Chromel-Alumel Alloys.” Cleveland Electric Laboratories. Email

Communication. January 2007.

• Anonymous, COMSOL Multiphysics User’s Guide. Version 3.2. September 2005.

• Standard Practice for Internal Temperature Measurements in Low-Conductivity Materials.

ASTM E377-96. Reapproved 2002.

• Beck, Robin and B. Laub. “Characterization and Modeling of Low Density TPS Materials for

Recovery Vehicles.” SAE Paper 941368, presented at the 24th International Conference on

Environmental Systems and 5th European Symposium on Space Environmental Control

Systems, Friedrichshafen, Germany, June 20-23, 1994.

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Thermal Modeling of In-Depth Thermocouple Response in ...Thermal Modeling of In-Depth Thermocouple...

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