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MeOH and EtOH evaporating flow mechanisms in square and circular microchannels

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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 …

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 < q”s < 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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  • 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 Técnico, Technical University of Lisbon vania.silverio@dem.ist.utl.pt moreira@dem.ist.utl.pt
  • 2. APPLICATIONS Devices shrink in footprint and increase in functionality keep getting hotter Chevrolet Equinox Fuel Cell Electric Vehicle Computers Photovoltaics http://alternativefuels.about.com http://www.treehugger.com http://onyxgreenbuilding.wordpress.com 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 • Microchannels – etched directly into the component • dielectric fluids –  thermal resistances • 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 • Flow boiling –  heat removal rates –  pumping power –  €€ 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 𝑷𝒓𝒆𝒔𝒔𝒖𝒓𝒆 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 𝑻𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆 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
  • 7. PRESSURE DROP 𝑝 𝑖𝑛 = 𝑝 𝑚𝑒𝑎𝑠,𝑖𝑛𝑙𝑒𝑡 − ∆𝑝 𝑐𝑜𝑛 − ∆𝑝 𝑛𝐻𝑇,𝑖𝑛 ∆𝑝 𝐻𝑇 = 𝑝 𝑖𝑛 − 𝑝 𝑜𝑢𝑡 𝑝 𝑜𝑢𝑡 = 𝑝 𝑚𝑒𝑎𝑠,𝑜𝑢𝑡𝑙𝑒𝑡 + ∆𝑝 𝑒𝑥𝑝 + ∆𝑝 𝑛𝐻𝑇,𝑜𝑢𝑡 heated length 𝑝 𝑝 𝑖𝑛 𝑚𝑒𝑎𝑠,𝑖𝑛𝑙𝑒𝑡 ∆𝑝 inlet stagnation chamber ∆𝑝 𝑐𝑜𝑛 inlet contraction ∆𝑝 𝑛𝐻𝑇,𝑖𝑛 non-heated entrance length ∆𝑝 𝑛𝐻𝑇,𝑜𝑢𝑡 non-heated exit length single-phase ∆𝑝 𝑐𝑜𝑛 = 1 − 𝑝 𝑝 𝑜𝑢𝑡 𝑚𝑒𝑎𝑠,𝑜𝑢𝑡𝑙𝑒𝑡 ∆𝑝 𝑒𝑥𝑝 outlet expansion single-phase 𝐴 𝑐𝑠 𝐴 𝑖𝑠𝑐 2 + 𝐾 𝑐𝑜𝑛 1 2 𝐺 𝜗𝐿 2 ∆𝑝 𝑒𝑥𝑝,𝑠𝑓 = 𝐾 𝑐𝑜𝑛 = 0.0088𝛼 2 − 0.1785𝛼 + 1.6027 1 𝐾 𝐺 2 𝜗 𝐿,𝑜 2 𝑒𝑥𝑝 𝐾 𝑒𝑥𝑝 = - 2 x 1.33 two-phase ∆𝑝 𝑒𝑥𝑝,𝑡𝑓 = 𝐺 2 𝐴 𝑐𝑠 𝐴 𝑖𝑠𝑐 𝐴 𝑐𝑠 −1 𝐴 𝑖𝑠𝑐 𝜗 𝐿,𝑜 1 − 𝑥 𝑒𝑥𝑖𝑡 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 7 𝐴 𝑐𝑠 𝐴 𝑖𝑠𝑐 2 1− 1+ 𝐴 𝑐𝑠 𝐴 𝑖𝑠𝑐 5 1 + 2 𝑋 𝑉𝑉 𝑋 𝑉𝑉 ∆𝑝 outlet stagnation chamber
  • 8. TEMPERATURE 𝑻 𝒊𝒏𝒏𝒆𝒓 𝒘𝒂𝒍𝒍 one dimensional heat conduction 𝑇 𝑤,𝑖𝑛 = 𝑇 𝑤,𝑜𝑢𝑡 − 𝑞 "𝑠 𝐴 𝑐𝑠 𝑘 𝑠𝑢𝑟 𝑠 𝑓 𝑙𝑜𝑔 𝑠𝑓 = 1 𝐷𝑜 𝐷𝑖 2𝜋𝐿 𝐻𝑇 𝑠 𝑓 = 0.785 𝑻 𝒇𝒍𝒖𝒊𝒅 𝑇 𝑓 = 𝑇 𝑚,𝑖𝑛 + 𝑇 𝑠𝑎𝑡 = 1 − 𝐿 𝑠𝑎𝑡 = 𝑞 "𝑠 𝑃 𝑤 𝑧 𝑉 𝜌 𝐿 𝑐 𝑝,𝐿 𝑧 𝐿 𝐻𝑇 𝑉 𝜌 𝐿 𝑐 𝑝,𝐿 𝑇 𝑠𝑎𝑡,0 − 𝑇 𝑓,𝑖 𝑞 "𝑠 𝑃𝑤 (Single-phase region) 𝑇 𝑠𝑎𝑡 𝑃𝑖𝑛𝑙𝑒𝑡 + 𝐿 𝐻𝑇 𝑇 𝑓 = 𝑇 𝑠𝑎𝑡 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 𝑧 8 𝑇 𝑠𝑎𝑡 𝑃 𝑜𝑢𝑡𝑙𝑒𝑡 (Two-phase region)
  • 9. HEAT TRANSFER COEFFICIENT -1 -1 -2 -1 -2 -1 -1 G=662kg.m .s G=483kg.m .s G=303kg.m .s -1 15 G=214kg.m .s -2 (Two-phase region) -2 -2 20 havg [kW.m .K ] 𝑇 𝑓 = 𝑇 𝑠𝑎𝑡 squareSCS_521, C2H5OH 521mm, ethanol -2 𝑞 "𝑠 ℎ= 𝑇 𝑤,𝑖𝑛 − 𝑇 𝑓 G=125kg.m .s 10 5 0 60 90 120 -2 q"s [kW.m ] 150 𝑞 "𝑠 = 𝐼2 𝑅 − ℎ 𝑙𝑜𝑠𝑠 𝑇 𝑤,𝑜𝑢𝑡 − 𝑇 𝑎𝑖𝑟 − 𝜀𝜎 𝑇 4 − 𝑇 4 𝑤,𝑜𝑢𝑡 𝑎𝑖𝑟 𝐴 𝐻𝑇 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 9
  • 10. HEAT TRANSFER COEFFICIENT 𝒒"𝒔 = 91kW.m-2, 𝑻 𝒔𝒂𝒕 =343K 𝒒"𝒔 = 99kW.m-2, 𝑻 𝒔𝒂𝒕 =357K square 521mm,CH3OH methanol -2 -1 -2 -1 -2 -1 -1 -2 -1 G=302kg.m .s G=214kg.m .s 4 0 0.0 0.2 Quality [-] Local Vapor Quality [-] -2 -1 -2 -1 -2 -1 -2 -1 -2 -1 G=662kg.m .s hlocal [kW.m-2 -1] havg [kW.m.K.K ] G=482kg.m .s -2 -1 h [kW.m-2 -1] hlocal [kW.m.K.K ] avg G=661kg.m .s 8 square 521mm,Cethanol 2H5OH 12 -2 12 0.4 G=483kg.m .s 8 G=304kg.m .s G=214kg.m .s G=125kg.m .s 4 0 0.0 0.2 0.4 Quality [-] Local Vapor Quality [-]  = 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 10 ℎ − ℎ 𝑠𝑙 ℎ 𝑓𝑔
  • 11. HEAT TRANSFER COEFFICIENT 66 < 𝑮 < 700kg.m-2.s-1, 𝑳 = 𝑳 𝑯𝑻 130 < 𝑮 < 700kg.m-2.s-1, 𝑳 = 𝑳 𝑯𝑻 circular 543mm, methanol CH3OH 12 circular 543mm, ethanol C2H5OH 12 " -2 " -2 " -2 q s=60kW.m " -2 q s=88kW.m " -2 q s=92kW.m 4 0 -0.2 0.0 0.2 Quality [-] Exit Vapor Quality [-] -1 8 " 0.4 4 0 -0.2 0.0 0.2 Quality [-] Exit Vapor Quality [-] 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 q s=124kW.m -2 8 h hlocal [kW.m-2.K-1] ] [kW.m .K avg q s=66kW.m -2 -1 h hlocal [kW.m-2.K-1] ] [kW.m .K avg q s=45kW.m 11 0.4
  • 12. HEAT TRANSFER COEFFICIENT 𝒒"𝒔 = 55kW.m-2; 130 < 𝑮 < 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, 𝐺= 0.11 – 1.84 kg m-2 s-1;  = 0.0 – 1.0; 𝑞"𝑠 = 11-170kW.m-2; 𝐷ℎ = 0.92,1.95mm subcooled and saturated flow boiling +3.0% R113, R134b, R123; 𝐺 = 50 – 570kg m-2 s-1;  =0.00 – 0.98; 𝑞"𝑠 = 5 – 91kW.m-2; 𝐷ℎ = 0.19 – 2.92mm strong presence of nucleate boiling +3.3% R134a, SUS304, 𝐺= 150-450kg m-2 s-1;  = 0.2 – 1.0; 𝑞"𝑠 = 5-40kW.m-2; 𝐷ℎ = 0.51, 1.12, 3.1mm convective and nucleate boiling contributions +10.2% Water, SS, 𝐺= 50 – 200kg m-2 s-1;  = 0.0 – 0.9; 𝑃= 200kPa; 𝐷ℎ = 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 𝐺 and  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 • Definitions adapted from Collier and Thome (1994) and Carey (2007) – Determined from simultaneous measurements of ∆𝑝, 𝑇 𝑤,𝑜𝑢𝑡 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
  • 14. FLOW PATTERN MAPS 𝒒"𝒔 = 81kW.m-2, 𝑻 𝒔𝒂𝒕 =342K circular 543mm, methanol CH3OH, CCS 600 400 200 0 0.0 𝐼 𝐵 0.2 0.4 0.6 Quality [-] Exit Vapor Quality [-] 𝐶𝐵 Revellin and Thome (2007) = 0.763 𝑅𝑒 𝑙𝑜 𝐵𝑜 𝑊𝑒 𝑙𝑜 0.8 Bubbly flow Confined flow Elongated flow IB/CB CB/A -1 -2 -2 -1 Bubbly flow Confined flow Elongated flow IB/CB CB/A square 521mm,OH, SCS CH3 methanol 800 Mass Flux, G [kg.m .s ] 800 Mass Flux, G [kg.m .s ] 𝒒"𝒔 = 73kW.m-2, 𝑻 𝒔𝒂𝒕 =360K 1.0 600 400 200 0 0.0 0.2 0.4 0.6 Quality [-] Exit Vapor Quality[-] 0.8 0.41 𝐶𝐵 𝐴 = 0.00014𝑅𝑒 1.47 𝑊𝑒 −1.23 𝑙𝑜 𝑙𝑜 Revellin and Thome (2007) 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 14 1.0
  • 15. CLOSURE ∆𝒑 • inlet contraction and outlet expansion as well as non–heated lengths were quantified and subtracted from the total two-phase flow pressure drops • determination of local 𝑻 𝒔𝒂𝒕 and 𝑻 𝒇 and of flow pattern regimes 𝑻 𝒘,𝒐𝒖𝒕 • 𝑇 𝑤,𝑜𝑢𝑡 varies non-linearly along the channel • determination of local 𝑻 𝒘,𝒊𝒏 , 𝑻 𝒇 , 𝒉 and of flow pattern regimes 𝒉 • 𝒉 𝒍𝒐𝒄𝒂𝒍 • • • is higher for low  and independent on 𝐺  incipience of boiling is lower for high  and independent on 𝐺  dry patches on the wall causing heat transfer decline 𝒉 𝒍𝒐𝒄𝒂𝒍,𝒐𝒖𝒕𝒍𝒆𝒕 • • • is higher for low 𝑞 "𝑠 and dependent on 𝐺 (𝐺 =662kg.m-2.s-1)  reduced space for convective flow to develop is lower for low 𝐺 and independent on 𝑞 "𝑠  dominance of nucleate boiling and annular evaporation; the effect of 𝑞 "𝑠 on ℎ overcomes the effect of 𝐺 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 𝑭𝒍𝒐𝒘 𝒑𝒂𝒕𝒕𝒆𝒓𝒏 𝒎𝒂𝒑𝒔 • flow patterns and flow pattern transitions for diabatic evaporation of ethanol and methanol obtained from 𝑻, 𝒑 and high speed imaging • flow patterns are qualitatively identical for both fluids and cross sections • similar trends with the model proposed by Revellin and Thome (2007) • deviations  Instabilities occurring inside the channel, due to pressure fluctuations, explosive boiling and long dryout periods that degrade the heat transfer • 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 “SURWET-COOLS”, 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