Diabatic experiments of evaporating flows of pure methanol and ethanol are conducted to study heat transfer and pressure drop in both square and circular cross section tubes of hydraulic diameter 521um and 543um respectively, in a range of mass fluxes 60 < G < 700kg.m-2.s-1 and heat fluxes 50 < qโs < 140kW.m-2. The heat transfer coefficient is higher for low vapor qualities and shows little dependence of mass flux for G < 500kg.m-2.s-1. Nucleate boiling heat transfer decreases as the liquid film near the wall fully evaporates during long periods of vapor passage Instabilities are observed to occur under these conditions. The corners in the square cross section prevent complete evaporation and higher heat transfer coefficients are found. The experimental heat transfer data is compared to correlations developed for microchannel subcooled and nucleate flow boiling showing similar trends, but lower values than predicted. The results from visualization clearly show the transition between flow patterns with known and well defined frequency. They also present no difference between the top and bottom thin film thickness, meaning very low influence of gravity forces in the flow. Wavy plugs and fingerlike structures are observed with high-speed visualization and described. Flow pattern maps for methanol for both configurations are obtained. Results for circular cross section show discrepancies from previous studies as for square cross section the results are found to agree.
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MeOH and EtOH evaporating flow mechanisms in square and circular microchannels
1. Methanol and Ethanol Evaporating Flow
Mechanisms in Square and Circular
Microchannels
Laboratory of Thermofluids, Combustion and Energy Systems, LTCES
Center for Innovation, Technology and Policy Research, IN+
Instituto Superior Tรฉcnico, Technical University of Lisbon
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
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Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
2
3. MOTIVATION
โข
Microchannels
โ etched directly into the component
โข dielectric fluids
โ ๏ thermal resistances
โข integrate the microchannel structure into a layer that is closer to the heat producing device. This
removes layers of material in the thermal resistance path which can significantly improve the cooling of
the heat source
โข
Flow boiling
โ ๏ heat removal rates
โ ๏ pumping power
โ ๏ โฌโฌ
Macrochannel Flow Pattern Maps simply fail to apply
Instabilities are prominent
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
3
4. EXPERIMENTAL APPARATUS
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
4
5. EXPERIMENTAL CONDITIONS
Properties of the fluids (Tsat, 0.1MPa)
methanol
ethanol
800
CH OH CCS
3
CH OH SCS
3
.s - 1 ]
600
521
C H OH CCS
2 5
-2
G [kg.m
542
C H OH SCS
400
2 5
542
521
200
0
0
50
100
-2
q"s [kW.m ]
CCS_543
SCS_521
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
5
150
200
6. MEASUREMENTS
๐ท๐๐๐๐๐๐๐
40
10
8
Pressure Drop [kPa]
Pressure [kPa]
30
inlet measured pressure
20
outlet measured pressure
6
4
10
0
2
4
6
8
10
12
time [s]
14
16
18
20
0
0.5
1
1.5
Time [s]
2
2.5
3
๐ป๐๐๐๐๐๐๐๐๐๐
8
400
450
380
]
6
300
.K
-2
340
h [kW.m
350
-1
360
Temperature [K]
Temperature [K]
400
320
300
280
250
0
0
2
4
6
8
10
12
time [s]
14
16
18
20
20
40
60
80
2
0
100
0
Length [mm]
20
40
60
Length [mm]
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
4
6
80
100
12. HEAT TRANSFER COEFFICIENT
๐"๐ = 55kW.m-2; 130 < ๐ฎ < 700kg.m-2.s-1
square 521mm,C2H5OH
ethanol
Experimental
Kandlikar
Yu et al.
Saitoh et al.
Haynes and Fletcher
60
-2
-1
hlocal [kW.m-2 .K
havg [kW.m .K-1] ]
80
40
20
0
0.0
0.4
Comments
Maximum deviation
R11 and R123; Copper, ๐บ= 0.11 โ 1.84 kg m-2 s-1; ๏ฃ = 0.0 โ 1.0;
๐"๐ = 11-170kW.m-2; ๐ทโ = 0.92,1.95mm
subcooled and saturated flow
boiling
+3.0%
R113, R134b, R123; ๐บ = 50 โ 570kg m-2 s-1; ๏ฃ =0.00 โ 0.98;
๐"๐ = 5 โ 91kW.m-2; ๐ทโ = 0.19 โ 2.92mm
strong presence of nucleate
boiling
+3.3%
R134a, SUS304, ๐บ= 150-450kg m-2 s-1; ๏ฃ = 0.2 โ 1.0;
๐"๐ = 5-40kW.m-2; ๐ทโ = 0.51, 1.12, 3.1mm
convective and nucleate boiling
contributions
+10.2%
Water, SS, ๐บ= 50 โ 200kg m-2 s-1; ๏ฃ = 0.0 โ 0.9;
๐= 200kPa; ๐ทโ = 2.98mm
Haynes and Fletcher(2003)
Kandlikar and Balasubramanian (2004)
Yu et al (2002)
0.2
0.3
Quality [-]
Exit Vapor Quality [-]
Application range
nucleate boiling dominates
over a large ๐บ and ๏ฃ range
+21.5%
Correlation
Saitoh et al. (2007)
0.1
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
12
13. FLOW PATTERNS
โข
Definitions adapted from Collier and Thome (1994) and Carey (2007)
โ
Determined from simultaneous measurements of โ๐, ๐ ๐ค,๐๐ข๐ก and high speed imaging
Bubbly flow
Confined flow
Elongated flow
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
13
15. CLOSURE
โ๐
โข
inlet contraction and outlet expansion as well as nonโheated lengths were quantified and subtracted from the total two-phase
flow pressure drops
โข
determination of local ๐ป ๐๐๐ and ๐ป ๐ and of flow pattern regimes
๐ป ๐,๐๐๐
โข
๐ ๐ค,๐๐ข๐ก varies non-linearly along the channel
โข
determination of local ๐ป ๐,๐๐ , ๐ป ๐ , ๐ and of flow pattern regimes
๐
โข
๐ ๐๐๐๐๐
โข
โข
โข
is higher for low ๏ฃ and independent on ๐บ ๏ incipience of boiling
is lower for high ๏ฃ and independent on ๐บ ๏ dry patches on the wall causing heat transfer decline
๐ ๐๐๐๐๐,๐๐๐๐๐๐
โข
โข
โข
is higher for low ๐ "๐ and dependent on ๐บ (๐บ =662kg.m-2.s-1) ๏ reduced space for convective flow to develop
is lower for low ๐บ and independent on ๐ "๐ ๏ dominance of nucleate boiling and annular evaporation; the effect of ๐ "๐ on
โ overcomes the effect of ๐บ
comparison of the experimental results with correlations for subcooled boiling and flow boiling show similar trends, but the
experimental values are below prediction
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
15
16. CLOSURE
๐ญ๐๐๐ ๐๐๐๐๐๐๐ ๐๐๐๐
โข
flow patterns and flow pattern transitions for diabatic evaporation of ethanol and methanol obtained from ๐ป, ๐ and high speed
imaging
โข
flow patterns are qualitatively identical for both fluids and cross sections
โข
similar trends with the model proposed by Revellin and Thome (2007)
โข
deviations ๏ Instabilities occurring inside the channel, due to pressure fluctuations, explosive boiling and long dryout periods that
degrade the heat transfer
โข
further experimental research is needed to generate more data at higher vapor qualities and different heat fluxes and mass
fluxes, for the developing of more accurate flow pattern maps
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
16
17. QUESTIONS
Simultaneous measurements of Temperature, pressure and high-speed imaging
in well defined homogeneous transparent channel walls with constant wall heat flux
is a major asset to assist in the comprehension of fluid flow behavior in microscale flows
Acknowledgements
Professor Nunes de Carvalho and his team for thin film deposition.
Financial support:
Project โSURWET-COOLSโ, PTDC/EME-MFE/109933/2009
Portuguese Science and Technology Foundation, grant SFRH-BD-76596-2011
Laboratory of Thermofluids, Combustion and Energy Systems
Methanol and Ethanol Evaporating Flow Mechanisms in Square and Circular Microchannels
vania.silverio@dem.ist.utl.pt
moreira@dem.ist.utl.pt
17