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Papers by Yoav Peles

Journal of Heat Transfer, 2009

ABSTRACT Flow boiling was experimentally studied using coolant HFE-7000 for two types of par-alle... more ABSTRACT Flow boiling was experimentally studied using coolant HFE-7000 for two types of par-allel microchannels: a plain-wall microchannel and a microchannel with structured re-entrant cavities on the side walls. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass fluxes ranging from G 164 kg/ m 2 s to G 3025 kg/ m 2 s and mass qualities (energy definition) rang-ing from x 0.25 to x 1. Comparisons of the performance of the enhanced and plain-wall microchannels were carried out. It was found that reentrant cavities were effective in reducing the superheat at the onset of nucleate boiling and increasing the heat transfer coefficient. However, they did not seem to increase the critical heat flux.

1st International Conference on Microchannels and Minichannels, 2003

ABSTRACT The design, fabrication, and evaluation of forced convection boiling two-phase flow, mic... more ABSTRACT The design, fabrication, and evaluation of forced convection boiling two-phase flow, micro scale heat exchanger are described. The micro heat exchanger consist of a heater, and 16 mm long multiple parallel triangular micro channels, with hydraulic diameter ranging from 50 μm to 200 μm. The system allowed simultaneously visualizing the flow regime, to measure the resistor temperature distribution, the pressure drop, and input power levels. Experiments were conducted using water with mass flow rate of 1–10 g /min and heat fluxes ranging from 10–60 W/cm2 in order to better understand the flow mechanism associated with micro scale forced convection boiling two-phase flow. The pressure drop, temperature, fluctuation and flow regimes map were obtained. The results are consistent with newly reported studies and show that flow fluctuation at that scale is exaggerated with respect to conventional size channels. A unique flow regime was observed and was named “rapid bubble growth”.

Volume 4: Fatigue and Fracture; Fluids Engineering; Heat Transfer; Mechatronics; Micro and Nano Technology; Optical Engineering; Robotics; Systems Engineering; Industrial Applications, 2008

ABSTRACT Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Associat... more ABSTRACT Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal

ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 1, 2007

ABSTRACT Flow boiling in parallel microchannels with structured reentrant cavities was experiment... more ABSTRACT Flow boiling in parallel microchannels with structured reentrant cavities was experimental studied. Flow patterns, boiling inceptions and heat transfer coefficients were obtained and studied for G = 83 kg/m2 -s to G = 303 kg/m2 -s and heat fluxes up to 643 W/cm2 . The heat transfer coefficient-mass velocity and quality relations had been analyzed to identify boiling mechanism. Comparisons of the performance of the enhanced and plain-wall microchannels had also been made. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation.

Encyclopedia of Microfluidics and Nanofluidics, 2013

ASME 2008 6th International Conference on Nanochannels, Microchannels, and Minichannels, 2008

ABSTRACT Flow boiling was experimentally studied in parallel microchannels using coolant HFE-7000... more ABSTRACT Flow boiling was experimentally studied in parallel microchannels using coolant HFE-7000. Subcooled nucleate boiling was achieved under various thermal-hydraulic conditions for mass velocities ranging from G = 164 kg/m2 ·s to G = 3025 kg/m2 ·s. Local surface temperatures were measured and flow visualizations were conducted to obtain flow morphologies, boiling curves, and heat transfer coefficients during boiling process. It was found that heat transfer was significantly enhanced during subcooled flow boiling by bubble agitation of the liquid.

Volume 8: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A and B, 2007

ABSTRACT Flow boiling instability in parallel microchannel and the ability of artificial reentran... more ABSTRACT Flow boiling instability in parallel microchannel and the ability of artificial reentrant cavities to suppress their occurrence was experimental studied. Experiments were conducted using two types of 226 μm hydraulic diameter parallel microchannel devices: microchannel with reentrant cavities and plain-wall microchannel. Onset of nucleate boiling (ONB), critical heat flux (CHF) condition, onset of flow oscillation (OFO), and local temperature were also obtained and compared. It was found that reentrant cavities on channel walls can assist mitigating the rapid bubble growth instability and postpone the parallel channel and compressible volume instability, which resulted in the delay of CHF.

ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels, Volume 1, 2011

ABSTRACT A numerical study of subcooled onset of nucleate boiling (ONB) in a micro-channel under ... more ABSTRACT A numerical study of subcooled onset of nucleate boiling (ONB) in a micro-channel under pulsed heating using the volume of fluids (VOF) model was conducted. The ONB time was determined when the void fraction at the microheater surface starts to exist. A smooth thin Pt heater was located between the water in the channel and the solid material. The theoretical superheat for homogeneous nucleation did not predict the transient ONB results of convective flow of water. Once heat load increases at the heater, transient flow boiling starts to occur. From a parametric study, it was found that the time constant increases with large substrate thermal diffusivity, low Reynolds number, and large channel diameter.

International Journal of Multiphase Flow, 2009

The effect of surface tension on adiabatic two-phase flow across a bank of 100 lm diameter stagge... more The effect of surface tension on adiabatic two-phase flow across a bank of 100 lm diameter staggered circular micro pillars, 100 lm long with pitch-to-diameter ratio of 1.5, for Reynolds number between 5 and 50, was investigated. Experiments with ethanol were performed and compared to results with water. Flow maps revealed similar flow patterns, but the transition lines were different for the two liquids. Void fraction measurements of the two fluids were also compared, and no significant deviations were observed. The two-phase pressure drop characteristics were significantly affected by the reduction in surface tension. Interfacial friction was attributed to this deviation, and a two-fluid model was developed to account for surface tension force. In addition, a modified form of Chisholm correlation was developed that accounts for surface tension.

ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 1, 2007

ABSTRACT Flow patterns, void fraction and pressure drop in adiabatic nitrogen-water two phase flo... more ABSTRACT Flow patterns, void fraction and pressure drop in adiabatic nitrogen-water two phase flows across a bank of micro pin fin were experimentally investigated for Reynolds number ranging from 5 to 50. Staggered cylindrical shaped micro pin fins with diameter and height of 100 μm were micro-fabricated into 1 cm long, 1.8 mm microchannel. Flow patterns were determined by flow visualization and classified as bubbly-slug flow, gas-slug flow, bridged flow and annular flow. The applicability of conventional scale models to predict two-phase frictional pressure drop was also assessed. The two-phase frictional multiplier was found to be a strong function of mass flux and flow patterns unlike the previous results observed in the microchannel studies. It was observed that models from conventional scale systems did not adequately predict the two-phase frictional multiplier at micro-scale and thus, a modified model accounting for mass flux and flow patterns have been developed in this work.

2013 American Control Conference, 2013

ABSTRACT This paper presents a dynamic model and feedback control strategies for vapor compressio... more ABSTRACT This paper presents a dynamic model and feedback control strategies for vapor compression cycles (VCC) in electronics cooling applications. A notable difference between traditional VCC and VCC for electronics cooling is that two-phase flow is required at the evaporator outlet in order to avoid burnout. Therefore, the control objective is to avoid critical heat flux during transient heating conditions. An emphasis is placed on the heated accumulator, which is a necessary component to guarantee superheated flow in the compressor suction-line. Addition of heat in the accumulator provides control actuation that may be used to avoid the critical heat flux via the effect on system pressure. In contrast to previous work, we present more detailed evaporator and accumulator models, implement the heated accumulator as a control actuator, and consider both single and multiple evaporator systems. For single evaporator VCC, we use frequency-domain techniques to design a dual-input, proportional-integral controller using accumulator heat and compressor speed. Both simulation and experiment show this design to be superior to strategies that do not actuate accumulator heat. We then use similar design strategies to develop a controller for the much more challenging two-evaporator VCC.

ASME 3rd International Conference on Microchannels and Minichannels, Parts A and B, 2005

ABSTRACT Incompressible flow through constriction elements (orifices, nozzles and venturis) are c... more ABSTRACT Incompressible flow through constriction elements (orifices, nozzles and venturis) are commonly encountered in several macro and micro scale engineering applications.. The current research endeavor experimentally investigates single-phase incompressible flows of de-ionized water through rudimentary micro-constriction configurations such as rectangular slot micro-orifices entrenched inside microchannels. Additionally, the effects of micro-orifice and microchannel size on the discharge in single-phase flows have been evaluated, and experimental data suggests that the flow rate is dictated by the constriction element opening rather than the microchannel area. The discharge coefficients associated with incompressible flows through multifarious micro-orifices have been estimated, and the effects of Reynolds number, micro-orifice size and microchannel area on the discharge coefficients have been explored. The discharge coefficients are calculated based on standardized D-D/2 (One channel diameter upstream and half channel diameter downstream) pressure tap specifications, and can be directly employed in the design of micro-valves and other micro-constriction devices. Furthermore, experimental results indicate that the discharge coefficient rises and peaks at a critical Reynolds number (200 ≤ ReCrit ≤ 500), which indicates the emergence of turbulence immediately downstream of the micro-orifice and re-laminarization further downstream. The discharge coefficient stabilizes and reaches a steady value after the critical Reynolds number has been transgressed. Finally, a correlation for the discharge coefficient, which includes the Reynolds number and the ratio between the hydraulic diameter of the micro-orifice and the microchannel, is presented to aid designers of MEMS devices involving micro-constriction components.

Volume 2: Fora, 2005

ABSTRACT Hydrodynamic cavitation in micro systems is a fundamental engineering problem that is po... more ABSTRACT Hydrodynamic cavitation in micro systems is a fundamental engineering problem that is poorly understood. The lack of knowledge on cavitation relevant to MEMS devices is impeding the practical realization of novel high-velocity microfluidic machines. This paper divulges differences between cavitation occurring inside micro and conventional systems, and also indicates the limited applicability of conventional knowledge to predict and understand cavitating flows in micro-domains. A detailed discussion delineating the possible reasons of such a divergence is presented in this article. Additionally, selected results obtained from preliminary experiments on cavitation in micro-domains are presented.

Volume 2: Fora, 2005

ABSTRACT Hydrodynamic cavitation, the explosive growth and catastrophic collapse of vapor bubbles... more ABSTRACT Hydrodynamic cavitation, the explosive growth and catastrophic collapse of vapor bubbles, has immense impact on the design and performance of hydraulic machinery in the macro world. However, cavitation in high-speed microfluidic systems has received scarce attention and hardly been reported. This article reports the presence of hydrodynamic cavitation in the flow of de-ionized water through 11.5–40micron wide rectangular slot micro-orifices entrenched inside 100–200micron wide microchannels. Pioneering experimental investigations on hydrodynamic cavitation in rudimentary microfluidic configurations such as slot micro-orifices has been presented and unique cavitating flow patterns have been identified. Assorted cavitating (two-phase) flow patterns including incipient, choking and supercavitation have been detected. Designers of high-velocity microfluidic systems, especially Power-MEMS devices, need to be aware of the deleterious effects of cavitation as it can significantly affect device performance. The effects of micro-orifice and microchannel size on cavitation have been discussed and results indicate the existence of strong scale effects. Incipient and choking cavitation numbers are observed to increase with increasing micro-orifice size, while the orifice discharge coefficient plummets once cavitation activity erupts. In addition, inlet pressure effects on several cavitation parameters have been discussed and compared with established macro-scale results. The cavitating flow patterns encountered are significantly influenced by the micro-orifice and microchannel size. Flow rate choking occurs irrespective of the inlet pressures and is a direct consequence of cavitation inside the micro-orifice. Cavitation hysteresis is observed but its effects are more marked for the smallest micro-orifice.

Encyclopedia of Microfluidics and Nanofluidics, 2015

Encyclopedia of Microfluidics and Nanofluidics, 2008

Fourteenth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2014

Small, 2008

Phase change through boiling is used in a variety of heat-transfer and chemical reaction applicat... more Phase change through boiling is used in a variety of heat-transfer and chemical reaction applications. The state of the art in nucleate boiling has focused on increasing the density of bubble nucleation using porous structures and microchannels with characteristic sizes of tens of micrometers. Traditionally, it is thought that nanoscale surfaces will not improve boiling heat transfer, since the bubble nucleation process is not expected to be enhanced by such small cavities. In the experiments reported here, we observed unexpected enhancements in boiling performance for a nanostructured copper (Cu) surface formed by the deposition of Cu nanorods on a Cu substrate. Moreover, we observed striking differences in the dynamics of bubble nucleation and release from the Cu nanorods, including smaller bubble diameters, higher bubble release frequencies, and an approximately 30-fold increase in the density of active bubble nucleation sites. It appears that the ability of the Cu surface with nanorods to generate stable nucleation of bubbles at low superheated temperatures results from a synergistic coupling effect between the nanoscale gas cavities (or nanobubbles ) formed within the nanorod interstices and micrometer-scale defects (voids) that form on the film surface during nanorod deposition. For such a coupled system, the interconnected nanoscale gas cavities stabilize (or feed) bubble nucleation at the microscale defect sites. This is distinct from conventional-scale boiling surfaces, since for the nanostructured surface the bubble nucleation stability is provided by features with orders-of-magnitude smaller scales than the cavity-mouth openings.

Scripta Materialia, 2008

ABSTRACT The prediction of the strength of complicated structures based on material test data is ... more ABSTRACT The prediction of the strength of complicated structures based on material test data is important for high-power density microelectromechanical systems. Probalistic analysis is used to predict the strength of pressure-tested microfabricated silicon structures from simpler test specimens. The predictions are found to be non-conservative. This is probably due to interactions between the etching process and the structural geometry, which changes the flaw distribution between the two statistical populations. In both cases secondary smoothing etches recover the strength.

Physics of Fluids, 2006

The existence of hydrodynamic cavitation in the flow of de-ionized water through micro-Venturis h... more The existence of hydrodynamic cavitation in the flow of de-ionized water through micro-Venturis has been witnessed in the form of traveling bubble cavitation and fully developed streamer bubble/ supercavitation, and their mechanisms have been discussed. High-speed photography and flow visualization disclose inchoate cavitation bubbles emerging downstream from the micro-Venturi throat and the presence of a single streamer bubble/supercavity, which is equidistant from the micro device walls. The supercavity initiates inside the diffuser section and extends until the microchannel exit and proceeds to bifurcate the incoming flow. This article strives to provide numerical data and experimental details of hydrodynamic cavitation taking place within micro-Venturis.

Journal of Heat Transfer, 2009

ABSTRACT Flow boiling was experimentally studied using coolant HFE-7000 for two types of par-alle... more ABSTRACT Flow boiling was experimentally studied using coolant HFE-7000 for two types of par-allel microchannels: a plain-wall microchannel and a microchannel with structured re-entrant cavities on the side walls. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass fluxes ranging from G 164 kg/ m 2 s to G 3025 kg/ m 2 s and mass qualities (energy definition) rang-ing from x 0.25 to x 1. Comparisons of the performance of the enhanced and plain-wall microchannels were carried out. It was found that reentrant cavities were effective in reducing the superheat at the onset of nucleate boiling and increasing the heat transfer coefficient. However, they did not seem to increase the critical heat flux.

1st International Conference on Microchannels and Minichannels, 2003

ABSTRACT The design, fabrication, and evaluation of forced convection boiling two-phase flow, mic... more ABSTRACT The design, fabrication, and evaluation of forced convection boiling two-phase flow, micro scale heat exchanger are described. The micro heat exchanger consist of a heater, and 16 mm long multiple parallel triangular micro channels, with hydraulic diameter ranging from 50 μm to 200 μm. The system allowed simultaneously visualizing the flow regime, to measure the resistor temperature distribution, the pressure drop, and input power levels. Experiments were conducted using water with mass flow rate of 1–10 g /min and heat fluxes ranging from 10–60 W/cm2 in order to better understand the flow mechanism associated with micro scale forced convection boiling two-phase flow. The pressure drop, temperature, fluctuation and flow regimes map were obtained. The results are consistent with newly reported studies and show that flow fluctuation at that scale is exaggerated with respect to conventional size channels. A unique flow regime was observed and was named “rapid bubble growth”.

Volume 4: Fatigue and Fracture; Fluids Engineering; Heat Transfer; Mechatronics; Micro and Nano Technology; Optical Engineering; Robotics; Systems Engineering; Industrial Applications, 2008

ABSTRACT Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Associat... more ABSTRACT Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal

ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 1, 2007

ABSTRACT Flow boiling in parallel microchannels with structured reentrant cavities was experiment... more ABSTRACT Flow boiling in parallel microchannels with structured reentrant cavities was experimental studied. Flow patterns, boiling inceptions and heat transfer coefficients were obtained and studied for G = 83 kg/m2 -s to G = 303 kg/m2 -s and heat fluxes up to 643 W/cm2 . The heat transfer coefficient-mass velocity and quality relations had been analyzed to identify boiling mechanism. Comparisons of the performance of the enhanced and plain-wall microchannels had also been made. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation.

Encyclopedia of Microfluidics and Nanofluidics, 2013

ASME 2008 6th International Conference on Nanochannels, Microchannels, and Minichannels, 2008

ABSTRACT Flow boiling was experimentally studied in parallel microchannels using coolant HFE-7000... more ABSTRACT Flow boiling was experimentally studied in parallel microchannels using coolant HFE-7000. Subcooled nucleate boiling was achieved under various thermal-hydraulic conditions for mass velocities ranging from G = 164 kg/m2 ·s to G = 3025 kg/m2 ·s. Local surface temperatures were measured and flow visualizations were conducted to obtain flow morphologies, boiling curves, and heat transfer coefficients during boiling process. It was found that heat transfer was significantly enhanced during subcooled flow boiling by bubble agitation of the liquid.

Volume 8: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A and B, 2007

ABSTRACT Flow boiling instability in parallel microchannel and the ability of artificial reentran... more ABSTRACT Flow boiling instability in parallel microchannel and the ability of artificial reentrant cavities to suppress their occurrence was experimental studied. Experiments were conducted using two types of 226 μm hydraulic diameter parallel microchannel devices: microchannel with reentrant cavities and plain-wall microchannel. Onset of nucleate boiling (ONB), critical heat flux (CHF) condition, onset of flow oscillation (OFO), and local temperature were also obtained and compared. It was found that reentrant cavities on channel walls can assist mitigating the rapid bubble growth instability and postpone the parallel channel and compressible volume instability, which resulted in the delay of CHF.

ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels, Volume 1, 2011

ABSTRACT A numerical study of subcooled onset of nucleate boiling (ONB) in a micro-channel under ... more ABSTRACT A numerical study of subcooled onset of nucleate boiling (ONB) in a micro-channel under pulsed heating using the volume of fluids (VOF) model was conducted. The ONB time was determined when the void fraction at the microheater surface starts to exist. A smooth thin Pt heater was located between the water in the channel and the solid material. The theoretical superheat for homogeneous nucleation did not predict the transient ONB results of convective flow of water. Once heat load increases at the heater, transient flow boiling starts to occur. From a parametric study, it was found that the time constant increases with large substrate thermal diffusivity, low Reynolds number, and large channel diameter.

International Journal of Multiphase Flow, 2009

The effect of surface tension on adiabatic two-phase flow across a bank of 100 lm diameter stagge... more The effect of surface tension on adiabatic two-phase flow across a bank of 100 lm diameter staggered circular micro pillars, 100 lm long with pitch-to-diameter ratio of 1.5, for Reynolds number between 5 and 50, was investigated. Experiments with ethanol were performed and compared to results with water. Flow maps revealed similar flow patterns, but the transition lines were different for the two liquids. Void fraction measurements of the two fluids were also compared, and no significant deviations were observed. The two-phase pressure drop characteristics were significantly affected by the reduction in surface tension. Interfacial friction was attributed to this deviation, and a two-fluid model was developed to account for surface tension force. In addition, a modified form of Chisholm correlation was developed that accounts for surface tension.

ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 1, 2007

ABSTRACT Flow patterns, void fraction and pressure drop in adiabatic nitrogen-water two phase flo... more ABSTRACT Flow patterns, void fraction and pressure drop in adiabatic nitrogen-water two phase flows across a bank of micro pin fin were experimentally investigated for Reynolds number ranging from 5 to 50. Staggered cylindrical shaped micro pin fins with diameter and height of 100 μm were micro-fabricated into 1 cm long, 1.8 mm microchannel. Flow patterns were determined by flow visualization and classified as bubbly-slug flow, gas-slug flow, bridged flow and annular flow. The applicability of conventional scale models to predict two-phase frictional pressure drop was also assessed. The two-phase frictional multiplier was found to be a strong function of mass flux and flow patterns unlike the previous results observed in the microchannel studies. It was observed that models from conventional scale systems did not adequately predict the two-phase frictional multiplier at micro-scale and thus, a modified model accounting for mass flux and flow patterns have been developed in this work.

2013 American Control Conference, 2013

ABSTRACT This paper presents a dynamic model and feedback control strategies for vapor compressio... more ABSTRACT This paper presents a dynamic model and feedback control strategies for vapor compression cycles (VCC) in electronics cooling applications. A notable difference between traditional VCC and VCC for electronics cooling is that two-phase flow is required at the evaporator outlet in order to avoid burnout. Therefore, the control objective is to avoid critical heat flux during transient heating conditions. An emphasis is placed on the heated accumulator, which is a necessary component to guarantee superheated flow in the compressor suction-line. Addition of heat in the accumulator provides control actuation that may be used to avoid the critical heat flux via the effect on system pressure. In contrast to previous work, we present more detailed evaporator and accumulator models, implement the heated accumulator as a control actuator, and consider both single and multiple evaporator systems. For single evaporator VCC, we use frequency-domain techniques to design a dual-input, proportional-integral controller using accumulator heat and compressor speed. Both simulation and experiment show this design to be superior to strategies that do not actuate accumulator heat. We then use similar design strategies to develop a controller for the much more challenging two-evaporator VCC.

ASME 3rd International Conference on Microchannels and Minichannels, Parts A and B, 2005

ABSTRACT Incompressible flow through constriction elements (orifices, nozzles and venturis) are c... more ABSTRACT Incompressible flow through constriction elements (orifices, nozzles and venturis) are commonly encountered in several macro and micro scale engineering applications.. The current research endeavor experimentally investigates single-phase incompressible flows of de-ionized water through rudimentary micro-constriction configurations such as rectangular slot micro-orifices entrenched inside microchannels. Additionally, the effects of micro-orifice and microchannel size on the discharge in single-phase flows have been evaluated, and experimental data suggests that the flow rate is dictated by the constriction element opening rather than the microchannel area. The discharge coefficients associated with incompressible flows through multifarious micro-orifices have been estimated, and the effects of Reynolds number, micro-orifice size and microchannel area on the discharge coefficients have been explored. The discharge coefficients are calculated based on standardized D-D/2 (One channel diameter upstream and half channel diameter downstream) pressure tap specifications, and can be directly employed in the design of micro-valves and other micro-constriction devices. Furthermore, experimental results indicate that the discharge coefficient rises and peaks at a critical Reynolds number (200 ≤ ReCrit ≤ 500), which indicates the emergence of turbulence immediately downstream of the micro-orifice and re-laminarization further downstream. The discharge coefficient stabilizes and reaches a steady value after the critical Reynolds number has been transgressed. Finally, a correlation for the discharge coefficient, which includes the Reynolds number and the ratio between the hydraulic diameter of the micro-orifice and the microchannel, is presented to aid designers of MEMS devices involving micro-constriction components.

Volume 2: Fora, 2005

ABSTRACT Hydrodynamic cavitation in micro systems is a fundamental engineering problem that is po... more ABSTRACT Hydrodynamic cavitation in micro systems is a fundamental engineering problem that is poorly understood. The lack of knowledge on cavitation relevant to MEMS devices is impeding the practical realization of novel high-velocity microfluidic machines. This paper divulges differences between cavitation occurring inside micro and conventional systems, and also indicates the limited applicability of conventional knowledge to predict and understand cavitating flows in micro-domains. A detailed discussion delineating the possible reasons of such a divergence is presented in this article. Additionally, selected results obtained from preliminary experiments on cavitation in micro-domains are presented.

Volume 2: Fora, 2005

ABSTRACT Hydrodynamic cavitation, the explosive growth and catastrophic collapse of vapor bubbles... more ABSTRACT Hydrodynamic cavitation, the explosive growth and catastrophic collapse of vapor bubbles, has immense impact on the design and performance of hydraulic machinery in the macro world. However, cavitation in high-speed microfluidic systems has received scarce attention and hardly been reported. This article reports the presence of hydrodynamic cavitation in the flow of de-ionized water through 11.5–40micron wide rectangular slot micro-orifices entrenched inside 100–200micron wide microchannels. Pioneering experimental investigations on hydrodynamic cavitation in rudimentary microfluidic configurations such as slot micro-orifices has been presented and unique cavitating flow patterns have been identified. Assorted cavitating (two-phase) flow patterns including incipient, choking and supercavitation have been detected. Designers of high-velocity microfluidic systems, especially Power-MEMS devices, need to be aware of the deleterious effects of cavitation as it can significantly affect device performance. The effects of micro-orifice and microchannel size on cavitation have been discussed and results indicate the existence of strong scale effects. Incipient and choking cavitation numbers are observed to increase with increasing micro-orifice size, while the orifice discharge coefficient plummets once cavitation activity erupts. In addition, inlet pressure effects on several cavitation parameters have been discussed and compared with established macro-scale results. The cavitating flow patterns encountered are significantly influenced by the micro-orifice and microchannel size. Flow rate choking occurs irrespective of the inlet pressures and is a direct consequence of cavitation inside the micro-orifice. Cavitation hysteresis is observed but its effects are more marked for the smallest micro-orifice.

Encyclopedia of Microfluidics and Nanofluidics, 2015

Encyclopedia of Microfluidics and Nanofluidics, 2008

Fourteenth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2014

Small, 2008

Phase change through boiling is used in a variety of heat-transfer and chemical reaction applicat... more Phase change through boiling is used in a variety of heat-transfer and chemical reaction applications. The state of the art in nucleate boiling has focused on increasing the density of bubble nucleation using porous structures and microchannels with characteristic sizes of tens of micrometers. Traditionally, it is thought that nanoscale surfaces will not improve boiling heat transfer, since the bubble nucleation process is not expected to be enhanced by such small cavities. In the experiments reported here, we observed unexpected enhancements in boiling performance for a nanostructured copper (Cu) surface formed by the deposition of Cu nanorods on a Cu substrate. Moreover, we observed striking differences in the dynamics of bubble nucleation and release from the Cu nanorods, including smaller bubble diameters, higher bubble release frequencies, and an approximately 30-fold increase in the density of active bubble nucleation sites. It appears that the ability of the Cu surface with nanorods to generate stable nucleation of bubbles at low superheated temperatures results from a synergistic coupling effect between the nanoscale gas cavities (or nanobubbles ) formed within the nanorod interstices and micrometer-scale defects (voids) that form on the film surface during nanorod deposition. For such a coupled system, the interconnected nanoscale gas cavities stabilize (or feed) bubble nucleation at the microscale defect sites. This is distinct from conventional-scale boiling surfaces, since for the nanostructured surface the bubble nucleation stability is provided by features with orders-of-magnitude smaller scales than the cavity-mouth openings.

Scripta Materialia, 2008

ABSTRACT The prediction of the strength of complicated structures based on material test data is ... more ABSTRACT The prediction of the strength of complicated structures based on material test data is important for high-power density microelectromechanical systems. Probalistic analysis is used to predict the strength of pressure-tested microfabricated silicon structures from simpler test specimens. The predictions are found to be non-conservative. This is probably due to interactions between the etching process and the structural geometry, which changes the flaw distribution between the two statistical populations. In both cases secondary smoothing etches recover the strength.

Physics of Fluids, 2006

The existence of hydrodynamic cavitation in the flow of de-ionized water through micro-Venturis h... more The existence of hydrodynamic cavitation in the flow of de-ionized water through micro-Venturis has been witnessed in the form of traveling bubble cavitation and fully developed streamer bubble/ supercavitation, and their mechanisms have been discussed. High-speed photography and flow visualization disclose inchoate cavitation bubbles emerging downstream from the micro-Venturi throat and the presence of a single streamer bubble/supercavity, which is equidistant from the micro device walls. The supercavity initiates inside the diffuser section and extends until the microchannel exit and proceeds to bifurcate the incoming flow. This article strives to provide numerical data and experimental details of hydrodynamic cavitation taking place within micro-Venturis.