Model Based Development of The Enhanced

Multi-Mission Radioisotope Thermoelectric

Generator and Effect of Thermoelectric

Element Length on eMMRTG

Swapnil Magdum

APRIL 2019

Western Michigan University

Mechanical and Aerospace Engineering

1

Outline

Introduction

Literature Review

Project Scope

Dimensional Investigation

3-D Modelling

Analytical Model

Numerical Modelling and Simulation

Results

Conclusion

Future Scope

2

Introduction - MMRTG3

Images taken from http://www.space.com/12004-nasa-mars-

rover-curiosity-photos-mars-science-laboratory.html

Figure 1- Conceptual image of the Curiosity Mars Rover Figure 2- The Curiosity Mars Rover at JPL during the final testing

Launched -11/2011, Landed - 07/2012

Introduction - eMMRTG

Figure 3- Cutaway of eMMRTG Figure 4- CAD model of eMMRTG and its component

Image taken from Woerner D. (2016). Image taken from Holgate et al. (2016).

4

General Purpose Heat Source (GPHS)5

Figure 5 - GPHS used in eMMRTG Figure 6- Pu 238 as a fuel for GPHS

Image taken from Hammel et al. (2016)

Thermoelectric Couple

Same cross-sectional area for

both legs.

Both use Skutterudite.

This reduces overall

complexity and analysis.

This leads to an overall 25%

power increase from the

MMRTG.

Images taken from Woerner D.(2016).

Figure 7 - MMRTG and eMMRTG thermoelectric couples

6

Introduction - MMRTG vs eMMRTG7

Holgate T., et al (2015).

Note: The only difference between the MMRTG and eMMRTG is the TE material used. Otherwise the two designs are identical.

Project Scope

Future Mars Rover

Modification in the design of eMMRTG to obtain more output power

Assumptions

Constant Heat Generation

Simplified Model and Estimated dimensions

Material properties are independent of temperature

8

ANSYS - Dimensional Investigation9

Images taken from Woerner D.(2016)Figure 7 - Cutaway of eMMRTG

10

11

Exploded view of the reproduced eMMRTG

12

Fin shell and fin

Module bar

Mica

Heat distribution

block

TEG module

Liner

Aerogel insulation

General Purpose

Heat Source

Figure 8 - Exploded view of the reproduced eMMRTG

14

Analytical Modelling

15

A Simple Analytical Model16

Lee, H.(2017).

Figure 9 - A system with only heat sink

Numerical Modelling and Simulation

17

18

Heat flux Input

Symmetry

Setting up the model with

different input parameters

Figure 10 – Setting up the model

19

Figure 11- Static temperature contour of all domains Figure 12- Static temperature contour of P-type thermoelement

20

Temperature distribution in the

thermoelectric element

Figure 13- Static temperature contour of thermoelectric couple

Temperature (K) ANSYS Result JPL Result

Hot Side 818.1 873

Cold Side 408.3 373-473

Table 2 - comparison of ANSYS result and JPL result

300

400

500

600

700

800

900

0 2 4 6 8 10 12 14

Tem

pra

ture

(K

)

Leg Length (mm)

21

Figure 14- Heat transfer from fin to atmosphere (XZ plane) Figure 15- Heat transfer from fin shell to atmosphere (YZ plane)

22

Figure 16 - Velocity streamlines (YZ plane) Figure 17 - Temperature profile along the fin

23Thermal Boundary Layer Thickness

Image taken from Bardy, E. (2008).

Figure 18 - Theoretical velocity and

thermal profile in natural convection

along a vertical wall Figure 19 - Thermal boundary layer thickness

290

300

310

320

330

340

350

0 5 10 15 20 25 30 35

Tem

pe

ratu

re (

K)

Length of the fin (mm)

on the Mars on the Earth

24

Image taken from Bardy, E. (2008).

Velocity Boundary Layer Thickness

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 5 10 15 20 25 30 35

Ve

loc

ity

(m

/s)

Length of the fin (mm)

on the Mars on the EarthFigure 20 - Theoretical velocity and

thermal profile in natural convection

along a vertical wall Figure 21 - Velocity boundary layer thickness

25

Average Convention Heat Transfer Coefficient (h) for

Vertical Natural Convection

𝑅𝑎 =𝑔. β. 𝑇𝑠 − 𝑇𝑖𝑛𝑓 . 𝐿3

α. υ

𝑁𝑢 = 0.6 +0.387. 𝑅𝑎

16

1 +0.559𝑃𝑟

916

827

2

ℎ =𝑁𝑢. 𝑘𝑎𝐿

𝑔 = 9.807𝑚

𝑠2𝑘𝑎 = 27.1𝑒 − 3

𝑊

𝑚2𝐾α = 26.3𝑒−6

𝑚2

𝑠υ = 18𝑒−6

𝑚2

𝑠β =

1

𝑇𝑓Pr = 0.72

𝐿 = 0.465 𝑚

26

Local Heat transfer coefficient along the vertical wall of

the fin shell

ℎ𝐴𝑖𝑟 = 2.64 W/m2K

ℎ𝐶𝑂2 = 1.12 W/m2K

Analytical Average

heat transfer

coefficient on the

Earth and on the

Mars

27

Figure 22 – Thermal-Electric Analysis

• Power calculations-

I = 1.62A𝑅𝑙𝑒𝑔𝑠 = 0.03605 Ω

Total 𝑅𝑙𝑒𝑔𝑠 = 6.9234 Ω

𝑅𝑙𝑜𝑎𝑑 = 6.9234 Ω

𝑃𝑢𝑛𝑖𝑡 = 𝐼2𝑅𝑙𝑜𝑎𝑑𝑃𝑢𝑛𝑖𝑡 = 18.17 W𝑃𝑡𝑜𝑡𝑎𝑙 = 145.36 W

28

Table 3 - Comparison of the numerical and the analytical results with the literature

Numerical and Analytical results comparison with the literature

ParametersLiterature Results

[JPL Results]Numerical Results Analytical Results

The hot junction

temperature (K)873 818.1 922.38

The cold junction

temperature (K)373-473 408.3 325.13

Current induced in the

circuit (A)- 1.62 1.64

The output power of the

1/8th unit (W)- 18.17 18.62

The total output power of

the unit (W)145-170 145.36 148.92

Effect of ceramic material on the power

output

29

30

Table 4 – Effect of the ceramic material on the power output

Effect of the ceramic material on the power output

MaterialThermal Conductivity

(W/mK)

Hot Side

(K)

Cold side

(K)

Current

(A)

Power of the 1/8th unit

(W)

Total power output

(W)

Alumina 22 818.1 408.3 1.62 18.17 145.36

Aluminum nitride

140 862.2 442.72 1.71 20.23 161.87

Effect of thermoelectric element leg length on

the power output

31

32

Figure 24 - Effect of the thermoelectric leg length on the power output

Effect of the thermoelectric element leg length on the power output

Figure 23 - Thermoelectric couple

Image taken from Hammel et al. (2016)

155

165

175

185

195

205

215

225

235

0 2 4 6 8 10 12 14

Ou

tpu

t p

ow

er

(W)

Thermoelectric element leg length (mm)

Power

33

Table 5 – Thermoelectric element leg length effect on the power output

Effect of the thermoelectric element leg length on the power output

Leg

length

(mm)

Load resistance

(Ω)

Current

(A)

Power of the 1/8th unit

(W)

Total power output

(W)

Rise in the total power output

(%)

1 0.55 7.03 27.18 217.45 49.59 %

3 1.64 3.80 23.68 189.44 30.32 %

6 3.27 2.59 21.93 175.44 20.69 %

9 4.91 2.04 20.45 163.60 12.55 %

12.7 6.92 1.71 20.23 161.87 11.35 %

Conclusion

Effect of ceramic material – 11.35% power improvement

Effect of Thermoelectric element leg length – 49.59% power improvement

Up to 6 mm- a little bit improvement in the power output

At 3 mm and 1 mm – drastic improvement

Enable to reduce weight (for the Spacecraft)

Future Scope

ZT value improvement

Investigation of the optimum load resistance to internal resistance ratio

34

35Questions