2nd International Seminar on ORC Power Systems, Rotterdam (Netherlands)
EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTCS AND
THERMAL STABILITY OF SILOXANES
Florian Heberle, Markus Preißinger, Theresa Weith and Dieter Brüggemann
Page 2
Introduction Siloxanes as working fluids in ORC Power Systems
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 2
• Siloxanes are potential working fluids for ORC power systems.
• Advantages: long-term experiences, low toxicity and GWP = 0.
• Mainly used as ORC working fluids for high-temperature heat sources
like biomass-fired power plants or waste heat recovery units.
Experimental investigation
Thermal stability Heat transfer coefficient
• Comparison to correlations
• Economic evaluation (pure
fluids and mixtures)
• Maximum process
temperatures
• Decomposition products
Page 3
Introduction Investigated working fluids
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 3
• Hexamethyldisiloxane (MM); n = 0
• Octamethyltrisiloxane (MDM); n = 1
• Decamethyltetrasiloxane (MD2M); n = 2
Fluid properties: T,s-diagram:
Structural
formula
Tcrit
(°C)
pcrit
(bar)
MM C6H18OSi2 245.6 19.4
MDM C8H24O2Si3 290.4 14.2
MD2M C10H30O3Si4 326.3 12.3 -1.0 -0.5 0.0 0.5 1.00
100
200
300
tem
pe
ratu
re (
°C)
entropy (kJ/(kgK))
MM
MDM
MD2M
Page 4
Heat transfer characteristics Experimental setup
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 4
• pmax = 25 bar
• Tmax = 260 °C
Test conditions:
• 𝑞 = 8 – 18 kW/m2
• 𝐺 = 50 – 400 kg/(m2s)
• Electrical heated
steel pipe (DC power)
• Length: 5 m
10 m
m
12 m
m
3 thermocouples
electrical
insulation
DC power supply
test section
inlet
test section
outlet
P/T
1.1
P/T
1.2
Page 5
Heat transfer characteristics Evaporation – Test section
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 5
Data reduction:
TTC
j = 0 j = 1 j = 2 TW,i
Tsat
ℎ𝑗 =𝑞
𝑇W,𝑖 − 𝑇sat(𝑝)
𝑇𝑊,𝑖 = 𝑇 𝑊,𝑜 +𝑞 i4𝜆
∙ (𝑟o2 − 𝑟i
2) +𝑞 i2𝜆
∙ 𝑙𝑛𝑟i𝑟𝑜
∙ 𝑟o2
𝑇 𝑊,𝑜 =𝑇𝑇𝐶,𝑡𝑜𝑝 + 2 ∙ 𝑇𝑇𝐶,𝑚𝑖𝑑𝑑𝑙𝑒 + 𝑇𝑇𝐶,𝑏𝑜𝑡𝑡𝑜𝑚
4
j = 10
Page 6
Results Variation of mass flux density – MM
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 6
• h increases with
increasing mass
flux density
• h decreases with
increasing
vapour quality
0.0 0.2 0.4 0.6 0.8 1.0
0
2
4
6
8
10
12
14
he
at
tra
nsfe
r co
eff
icie
nt
(kW
/m2K
)
vapour quality (-)
G (kg/(m2s))
50
100
200
300
400
MM
p = 9 bar;
q = 8 kW/m2
Page 7
Results Variation of heat flux density – MM
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 7
• No significant
influence of heat
flux density
• h decreases
with increasing
vapour quality
0.0 0.2 0.4 0.6 0.8 1.0
0
1
2
3
4
5
6
heat
transfe
r coeff
icie
nt
(kW
/m2K
)
vapour quality (-)
q ( kW/m2)
8
10
12
14
15.9
17
MM
G = 200 kg/(m2s);
Tsat
= 220 °C
Page 8
Results Variation of examined working fluid – statistical and systematic uncertainties
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 8
• Different behaviour
of MM and MDM
depending on
vapour quality
• Statistical
uncertainties
(5 repetitions)
• Systematic
uncertainties
(ΔA/A; ΔP/P,
ΔTW,o/TW,o,
Δpsat/psat)
0.0 0.2 0.4 0.6 0.8 1.0
0
1
2
3
4
5
6
he
at
tra
nsfe
r co
eff
icie
nt
(kW
/m2K
)
vapour quality (-)
MM
MDM
Tsat
= 198 °C; G = 200 kg/(m2s); q = 15.9 kW/m
2
Page 9
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 9
Comparison to correlations
0.0 0.2 0.4 0.6 0.8 1.0
0
1
2
3
4
5
6
heat
transfe
r coeff
icie
nt
(kW
/m2K
)
vapour quality (-)
Experimental Data
Kandlikar (1998)
Saitoh et al. (2007)
MM; G = 200 kg/(m2s); T
sat = 198 °C; q = 15.8 kW/m
2
Page 10
Results Comparison to correlations
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 10
0.0 0.2 0.4 0.6 0.8 1.0
0
1
2
3
4
5
6
heat
transfe
r coeff
icie
nt
(kW
/m2K
)
vapour quality (-)
Experimental Data
Kandlikar (1998)
Saitoh et al. (2007)
MM; G = 200 kg/(m2s); T
sat = 198 °C; q = 15.8 kW/m
2
Page 11
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 11
Comparison to correlations
0.0 0.2 0.4 0.6 0.8 1.0
0
1
2
3
4
5
6
heat
transfe
r coeff
icie
nt
(kW
/m2K
)
vapour quality (-)
Experimental Data
Kandlikar (1998)
Saitoh et al. (2007)
MDM; G = 200 kg/(m2s); T
sat = 198 °C; q = 15.8 kW/m
2
Page 12
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 12
Comparison to correlations
0.0 0.2 0.4 0.6 0.8 1.0
0
1
2
3
4
5
6
heat
transfe
r coeff
icie
nt
(kW
/m2K
)
vapour quality (-)
Experimental Data
Kandlikar (1998)
Saitoh et al. (2007)
MDM; G = 200 kg/(m2s); T
sat = 198 °C; q = 15.8 kW/m
2
Page 13
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 13
Comparison to correlations – working fluid: MM
• All measured
local h
• Mean relative
deviation (Kandlikar)
25.1 %
• Saitoh et al.
49.0 %
• Mean relative
deviation (MDM –
Kandlikar)
40.9 %
0 1 2 3 4 5 6 70
1
2
3
4
5
6
7
- 40%
experim
enta
l heat
transfe
r coeff
icie
nt
(kW
/m2K
)
calculated heat transfer coefficient (kW/m2K)
+ 40%
Kandlikar
Page 14
Heat transfer measurements Main results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 14
• Heat transfer coefficients are measured for process temperatures up to 250 °C.
• Empirical model of Kandlikar shows a good agreement to the experimental data.
Experimental investigation
Thermal stability Heat transfer coefficient
• Comparison to correlations
• Economic evaluation (pure
fluids and mixtures)
• Maximum process
temperatures
• Decomposition products
Page 15
Thermal stability Experimental setup
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 15
• pmax = 30 bar
• Tmax = 500 °C
Test conditions:
• 𝑡 = 72 h
• 𝑇 = 240 – 420 °C
• Electrical heated by
heating wire
• Analysed by
gas chromatography/
mass spectroscopy
Thermocouple Pressure
device
Heating
wire
Page 16
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 16
Liquid phase, 360 °C, 144 h
• Fluid: Wacker® AK 0.65
• Purity: > 97 mass-%
• Formation of higher
chained siloxanes in
accordance to
Dvornic, Gelest, Inc.
time (min)
counte
r (-
) counte
r (-
) before
after
D5
MM
MDM
MD2M
MD4M
MD3M
MM
Page 17
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 17
Gas phase, 72 h
• Averaged molar
concentration before
tests: 99.4 mol-%
• Formation of
methane and ethane
in accordance to
Manders and Bellama,
Journal of Polymer
Science, 1985
Page 18
Conclusions and Future work
• Heat transfer and thermal stability measurements were carried out
for selected siloxanes.
• The correlation of Kandlikar shows the best agreement to experimental data.
• No significant amount of decomposition products for heat transfer test conditions.
• Heat transfer characteristics of the mixture MM/MDM and MM/MDM/MD2M.
• Investigation of enhanced tubes and alternative working fluids.
• Long-term and dynamic tests concerning thermal stability.
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 18
Page 19
Acknowledgements
The authors gratefully acknowledge financial support from
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 19
Free provision of Wacker® AK 0.65
“Fluid mixtures for efficiency increase of
Organic Rankine Cycles in selected
applications” (Grant no. 1713/12-1 and -2)
Partial financing of the
thermal stability test rig
www.zet.uni-bayreuth.de
Thank you
Florian Heberle, Markus Preißinger, Theresa Weith and Dieter Brüggemann
Page 21
Heat transfer characteristics Evaporation – Test section
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 21
10
mm
12
mm
3 thermocouples TC
electrical
insulation
DC power supply
test section
inlet
test section
outlet
P/T
1.1
P/T
1.2
i = 1 – 10 '
'' '
ii
h hx
h h
1 1i
i i i
TF
Ph h h h
m 0 ( 0.5 )sath h T K
Data reduction:
TTC
subcooled
i = 0 i = 1 i = 2 TW,i
Ts
Page 22
Results Variation of saturation pressure- MM
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 22
0.0 0.2 0.4 0.6 0.8 1.0
0
1000
2000
3000
4000
5000
6000
7000
hea
t tr
ansf
er c
oef
fici
ent
(W/m
2K
)
vapour quality (-)
psat
(bar)
5.8
7.75
9
13
MM
G = 200 kg/(sm2);
q = 15.8 kW/m2
Page 23
Results Variation of examined working fluid
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 23
0.0 0.2 0.4 0.6 0.8 1.0
0
1
2
3
4
5
6hea
t tr
ansf
er c
oef
fici
ent
(kW
/m2K
)
vapour quality (-)
MM
MDM
MD2M
Tsat
= 220 °C;
G = 200 kg/(m2s);
q = 15.9 kW/m2
Page 24
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 24
Comparison to correlations – Model of Kandlikar
ℎ𝑡𝑐tp = ℎ𝑡𝑐tp,nbd ℎ𝑡𝑐tp,cbd
ℎ𝑡𝑐tp,nbd = 0.6683 ∙ 𝐶𝑜−0.2 ∙ 1 − 𝑥 0.8 ∙ ℎ𝑡𝑐LO + 1058 ∙ 𝐵𝑜0.7 ∙ 1 − 𝑥 0.8 ∙ 𝐹fl∙ ℎ𝑡𝑐LO
ℎ𝑡𝑐tp,cbd = 1.136 ∙ 𝐶𝑜−0.9 ∙ 1 − 𝑥 0.8 ∙ ℎ𝑡𝑐LO + 667.2 ∙ 𝐵𝑜0.7 ∙ 1 − 𝑥 0.8 ∙ 𝐹fl ∙ ℎ𝑡𝑐LO
𝐵𝑜 =𝑞
𝐺 ∙ ∆ℎ=
𝐴cs ∙ 𝑞
𝑚 ∙ ℎ′′ − ℎ′ 𝐶𝑜 =
𝜌g
𝜌l
0,51 − 𝑥
𝑥
0,8
ℎ𝑡𝑐LO =
𝜁2 𝑅𝑒LO − 1000 ∙ 𝑃𝑟𝑙
1,0 + 12,7 ∙𝜁2 𝑃𝑟l
23 − 1
∙𝜆l𝑑i
Page 25
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 25
Comparison to correlations – Model of Saitoh et al.
ℎ𝑡𝑐tp = 𝐹ℎ𝑡𝑐𝑙 + 𝑆ℎ𝑡𝑐𝑝𝑜𝑜𝑙
ℎ𝑡𝑐𝑙 = 0.223𝜆𝑙𝐷
∙𝐺 1 − 𝑥 𝐷
𝜂𝑙
0.8
∙𝑐𝑝,𝑙𝜂𝑙𝜆𝑙
0.8
𝐹 = 1 +1
𝑥
1.05
+ 1 +𝑊𝑒𝑔−0.4
S = 1/1 + 𝑅𝑒𝑡𝑝 ∙ 10−41.405
ℎ𝑡𝑐𝑝𝑜𝑜𝑙 = 207𝜆𝑙𝑑𝑏
∙𝑞 𝑑𝑏𝜆𝑙𝑇𝑙
0.745
∙𝜌𝑔
𝜌𝑙
0.581
∙ 𝑃𝑟𝑙0.533
Page 26
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 26
Comparison to correlations – working fluid: MM
• Measured
local htc
• Mean relative
deviation (Kandlikar)
25,1 %
• Saitoh et al.
49,0 %
0 1 2 3 4 5 6 70
1
2
3
4
5
6
7
- 40%
exp
erim
enta
l
hea
t tr
ansf
er c
oef
fici
ent
(k
W/m
2K
)
calculated experimental heat transfer coefficient (kW/m2K)
+ 40%
Saitoh et al.
Page 27
Results Homogenous temperature profile
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 27
0 1 2 3 4 5
0
20
40
60
80
100
120
140
inner
wal
l te
mper
ature
(°C
)
length of test section (m)
t1
t2
t3
t4
Page 28
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 28
0 1 2 3 4 5
195
200
205
210
215
220
225
230
inn
er w
all
tem
per
atu
re (
°C)
length of test section (m)
MM top MM bottom
Page 29
Results
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 29
0 1 2 3 4 5
195
200
205
210
215
220
225
230
inn
er w
all
tem
per
atu
re (
°C)
length of test section (m)
MDM top MDM bottom
Page 30
Results Flow regimes - MM
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 30
0
50
100
150
200
250
300
350
400
450
500
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
G [
kg/m
²s]
x
x_m_IA
m_wavy
m_mist
m_strat
m_bubbly
Annular
Intermittent
Mist flow
Stratified wavy
Stratified
Page 31
Results Flow regimes – MDM
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 31
0
50
100
150
200
250
300
350
400
450
500
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
G [
kg/m
²s]
x
x_m_IA
m_wavy
m_mist
m_strat
m_bubbly
Page 32
Results Fluid properties
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 32
psat
(bar) pred
ρv
(kg/m3)
ρl / ρv
σ
(N/m)
MM 9.03 0.47 53.54 10.05 0.0154
MDM 2.90 0.21 20.82 29.44 0.0166
• Higher vapour density for MM lower vapour velocity at same mass flux.
• Nucleate dominates at low vapour qualities, caused by low surface tension and
liquid-to-vapour density ratio.
• Lower surface tension increase the probability of liquid entrainment in the vapour
core.
• Suppression of nucleate boiling is delayed by higher vapour density (lower velocity)
Page 33
Thermal stability Temperature distribution
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al. Page 33
temperature (°C)
heig
ht
of
reacto
r(-)
Page 34
Outline
08.10.2013 EXPERIMENTAL INVESTIGATIONS OF HEAT
TRANSFER CHARACTERISTICS AND THERMAL
STABILITY OF SILOXANES - F. Heberle et al.
Test procedure