Surface structure enhanced microchannel flow boiling of low surface tension fluids
Microchannel flow boiling can meet the thermal management requirements of high power and high frequency integrated circuits, but the technology has been limited by the instabilities unique to their length scales. During microchannel flow boiling of water, many of these instabilities have been succe...
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| Format: | Thesis |
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Massachusetts Institute of Technology
2022
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| Online Access: | https://hdl.handle.net/1721.1/138987 |
| _version_ | 1826206547664961536 |
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| author | Sircar, Jay |
| author2 | Wang, Evelyn N. |
| author_facet | Wang, Evelyn N. Sircar, Jay |
| author_sort | Sircar, Jay |
| collection | MIT |
| description | Microchannel flow boiling can meet the thermal management requirements of high power and high frequency integrated circuits, but the technology has been limited by the instabilities unique to their length scales. During microchannel flow boiling of water, many of these instabilities have been successfully mitigated by the addition of surface microstructures. Thermohydraulic performance in terms of critical heat flux (CHF) and heat transfer coefficient (HTC) have also benefited from surface modification, without significantly adding to the overall flow pressure drop. For some microelectronic applications, water may not be the ideal fluid for microchannel flow boiling, such as in aerospace engineering where thermal management solutions must operate over a wide range of temperatures from subzero Celsius to optimal semiconductor temperatures. Alternatives to water as a working fluid include methanol, which has one of the largest ranges of operating temperatures suitable for electronics cooling, and hydrofluoroether (HFE) 7000, an environmentally friendly dielectric fluid. Though there are benefits to using these alternative working fluids, several of their thermophysical properties involved in generating capillary flows are significantly less than that of water—most notably their surface tensions. Prior studies have shown that the level of enhancement to the critical heat flux during the flow boiling of water, was positively correlated with the capillary-limited thin-film dryout heat flux. The same semi-analytical model suggested that thin-film dryout would occur at approximately one and two orders of magnitude smaller heat fluxes when switching from water to methanol and HFE 7000, respectively.
In this thesis, thermohydraulic changes from surface microstructures during intrachip flow boiling of lower surface tension working fluids, primarily methanol, was investigated. We fabricated microchannels on the heated bottom wall of silicon test samples, with/without micropillars of two different heights (25 and 75 µm) and two different cylindrical pillar solid fractions (~ 5% and 20%). For methanol, a maximum CHF of 494 W/cm^2 was achieved with a structured surface, a 61% enhancement compared to smooth surface. At higher heat fluxes, the maximum HTC increased by as much as 71%, to 271 kW/m^2 K, for the taller, sparser micropillar wicked channels compared to smooth microchannels. The presence of micropillars reduced the HTC or resulted in no significant change at lower exit quality conditions. The CHF enhancement among different geometries of wicks did not fully agree with the fluid wicking model, suggesting that capillarity may not be the dominant factor contributing to the enhanced performance. Annular films of methanol within smooth microchannels near CHF abruptly dewet from bulk of the heated wall, and resulted in inverted annular (film) flow boiling or transition boiling. High speed imaging coupled with hydraulic and thermal measurements, showed that the taller micropillars prevented this liquid film rupture. The importance of hydrodynamic effects resulting from the micropillar wick arrays was supported by force scaling analysis and finite element analysis. Observed experimental flow boiling behaviors near CHF for water, methanol, and HFE 7000, revealed that as surface tension decreased, the effectiveness of micropillar wicks in preventing the rupture and removal of annular films dwindled. Heat dissipation approaching ½ kW/cm^2 at a calculated wall superheat of less than 20 K during flow boiling of methanol in microstructured channels was achieved, suggesting that this can be a promising cooling strategy for high power-density electronic systems operating in challenging environments. Insights gained from this work will lead to the development of new design principles that will allow for even lower surface tension fluids, e.g. fluorinated dielectrics, to maximize their potential during flow boiling. |
| first_indexed | 2024-09-23T13:34:39Z |
| format | Thesis |
| id | mit-1721.1/138987 |
| institution | Massachusetts Institute of Technology |
| last_indexed | 2024-09-23T13:34:39Z |
| publishDate | 2022 |
| publisher | Massachusetts Institute of Technology |
| record_format | dspace |
| spelling | mit-1721.1/1389872022-01-15T03:26:22Z Surface structure enhanced microchannel flow boiling of low surface tension fluids Sircar, Jay Wang, Evelyn N. Massachusetts Institute of Technology. Department of Mechanical Engineering Microchannel flow boiling can meet the thermal management requirements of high power and high frequency integrated circuits, but the technology has been limited by the instabilities unique to their length scales. During microchannel flow boiling of water, many of these instabilities have been successfully mitigated by the addition of surface microstructures. Thermohydraulic performance in terms of critical heat flux (CHF) and heat transfer coefficient (HTC) have also benefited from surface modification, without significantly adding to the overall flow pressure drop. For some microelectronic applications, water may not be the ideal fluid for microchannel flow boiling, such as in aerospace engineering where thermal management solutions must operate over a wide range of temperatures from subzero Celsius to optimal semiconductor temperatures. Alternatives to water as a working fluid include methanol, which has one of the largest ranges of operating temperatures suitable for electronics cooling, and hydrofluoroether (HFE) 7000, an environmentally friendly dielectric fluid. Though there are benefits to using these alternative working fluids, several of their thermophysical properties involved in generating capillary flows are significantly less than that of water—most notably their surface tensions. Prior studies have shown that the level of enhancement to the critical heat flux during the flow boiling of water, was positively correlated with the capillary-limited thin-film dryout heat flux. The same semi-analytical model suggested that thin-film dryout would occur at approximately one and two orders of magnitude smaller heat fluxes when switching from water to methanol and HFE 7000, respectively. In this thesis, thermohydraulic changes from surface microstructures during intrachip flow boiling of lower surface tension working fluids, primarily methanol, was investigated. We fabricated microchannels on the heated bottom wall of silicon test samples, with/without micropillars of two different heights (25 and 75 µm) and two different cylindrical pillar solid fractions (~ 5% and 20%). For methanol, a maximum CHF of 494 W/cm^2 was achieved with a structured surface, a 61% enhancement compared to smooth surface. At higher heat fluxes, the maximum HTC increased by as much as 71%, to 271 kW/m^2 K, for the taller, sparser micropillar wicked channels compared to smooth microchannels. The presence of micropillars reduced the HTC or resulted in no significant change at lower exit quality conditions. The CHF enhancement among different geometries of wicks did not fully agree with the fluid wicking model, suggesting that capillarity may not be the dominant factor contributing to the enhanced performance. Annular films of methanol within smooth microchannels near CHF abruptly dewet from bulk of the heated wall, and resulted in inverted annular (film) flow boiling or transition boiling. High speed imaging coupled with hydraulic and thermal measurements, showed that the taller micropillars prevented this liquid film rupture. The importance of hydrodynamic effects resulting from the micropillar wick arrays was supported by force scaling analysis and finite element analysis. Observed experimental flow boiling behaviors near CHF for water, methanol, and HFE 7000, revealed that as surface tension decreased, the effectiveness of micropillar wicks in preventing the rupture and removal of annular films dwindled. Heat dissipation approaching ½ kW/cm^2 at a calculated wall superheat of less than 20 K during flow boiling of methanol in microstructured channels was achieved, suggesting that this can be a promising cooling strategy for high power-density electronic systems operating in challenging environments. Insights gained from this work will lead to the development of new design principles that will allow for even lower surface tension fluids, e.g. fluorinated dielectrics, to maximize their potential during flow boiling. Ph.D. 2022-01-14T14:42:55Z 2022-01-14T14:42:55Z 2021-06 2021-06-29T20:14:44.667Z Thesis https://hdl.handle.net/1721.1/138987 In Copyright - Educational Use Permitted Copyright MIT http://rightsstatements.org/page/InC-EDU/1.0/ application/pdf Massachusetts Institute of Technology |
| spellingShingle | Sircar, Jay Surface structure enhanced microchannel flow boiling of low surface tension fluids |
| title | Surface structure enhanced microchannel flow boiling of low surface tension fluids |
| title_full | Surface structure enhanced microchannel flow boiling of low surface tension fluids |
| title_fullStr | Surface structure enhanced microchannel flow boiling of low surface tension fluids |
| title_full_unstemmed | Surface structure enhanced microchannel flow boiling of low surface tension fluids |
| title_short | Surface structure enhanced microchannel flow boiling of low surface tension fluids |
| title_sort | surface structure enhanced microchannel flow boiling of low surface tension fluids |
| url | https://hdl.handle.net/1721.1/138987 |
| work_keys_str_mv | AT sircarjay surfacestructureenhancedmicrochannelflowboilingoflowsurfacetensionfluids |