Dr C Yap | National University of Singapore (original) (raw)

Papers by Dr C Yap

IFAC Proceedings Volumes, 2005

Proceedings of the 2012 IEEE 14th Electronics Packaging Technology Conference, EPTC 2012, 2012

ABSTRACT Flow boiling instabilities induce mechanical vibration in the system and deteriorate the... more ABSTRACT Flow boiling instabilities induce mechanical vibration in the system and deteriorate the heat transfer performances, for example- premature dryout, critical heat flux limitation etc. The two phase microgap heat sink has novel potential to mitigate these undesirable flow boiling instabilities and flow reversal issues inherent with two phase microchannel heat sink. This work is an experimental study of boiling instabilities in microgap heat sink for different microgap depths ranging from 80μm-1000μm, mass fluxes from 390kg/m2s-900kg/m2s, heat fluxes up to 85W/cm2 and different microgap surface roughnesses, Ra=0.6-1μm. A series of systematic experiments have been carried out to investigate the inlet pressure and wall temperature oscillations during two phase flow boiling condition under uniform heating, with deionized water as a cooling liquid. Experimental result shows that pressure oscillation increases with the decreasing microgap depth. Temperature oscillation is observed lower for smaller gap than larger gap up to a certain heat flux condition before the dryout phase. In addition, inlet pressure instabilities increase with increasing heat flux and decreasing mass flux. Moreover, surface roughness has an adverse effect on the inlet pressure instability at larger depth microgap heat sink and inlet pressure fluctuation increases with increasing surface roughness.

ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE), 2012

Flow boiling in microgap heat sink is very attractive for high-performance electronics cooling du... more Flow boiling in microgap heat sink is very attractive for high-performance electronics cooling due to its high heat transfer rate and easy fabrication process. In absence of thermal interface material between the active electronic component and a microgap cold plate, significant reduction in interface thermal resistance and enhancement in heat transfer rate can be achieved. In earlier studies by these authors, encouraging results have been obtained using microgap heat sink as it can potentially mitigate flow instabilities, flow reversal and maintain uniform wall temperatures over the heated surface. So, more work should be carried out to advance the fundamental understanding of the two-phase flow heat transfer associated with microgap heat sink and the underlying mechanisms. In this study, local flow boiling phenomena in different microgap sizes have been investigated experimentally. Experiments are performed in silicon based microgap heat sink having microgap depth ranging from 80 μm to 500 μm, using deionized water with 10 °C subcooled inlet temperature. The effects of mass flux and heat flux on heat transfer coefficient and pressure drop characteristics are examined by using different mass fluxes ranging from 400 kg/m2s to 1000 kg/m2s and effective heat flux varying from 0 to 100 W/cm2. Apart from these experimental investigations, simultaneous high speed visualizations are conducted to observe and explore the mechanism of flow boiling in microgap. Confined slug and annular boiling are observed as the two main heat transfer mechanisms in microgap. Moreover, experimental results show that flow boiling heat transfer coefficients are dependent on gap size, and the lower the gap size, higher the heat transfer coefficient.

International Journal of Multiphase Flow, 2012

Heat Transfer: Volume 2, 2008

CPUs with high clock rates can dramatically increase heat dissipation within their encapsulation ... more CPUs with high clock rates can dramatically increase heat dissipation within their encapsulation due to internal Joule heat from the transistors. The conventionally used air cooling systems for CPUs, such as the aluminum or copper extruded heat sink types, have severe heat transfer “bottlenecks” due to high thermal resistances and they easily reach their thermal design limits (TDL). Alternative cooling devices such as heat pipes and liquid cooling tends to have externally attached radiator/condenser and/or pump and such designs are cumbersome. This paper describes the modeling, design, and testing of a compact (about the size of the Intel stock cooler, diameter: 96mm, height: 50mm), fully integrated, orientation-free, evaporator-condenser device for CPU cooling, with excellent attributes of low thermal resistance from phase change phenomena and minimal vapor pressure drop. The prototype fabricated is designed to reject 200 W (twice the capacity of conventional heat sinks). It is made of copper and uses distilled water as the working fluid. The working fluid boils inside a porous structure clad evaporator and is transported radially to nearby air-cooled condenser sections; this unique arrangement minimizes space while providing adequate area for air convection. Testing was done by subjecting it to varying heat loads and air flow rates. A best performance of 0.206 K/W of the device’s thermal resistance was achieved at a fan air flow rate of 34.5 CFM under 203 W of cooling load, and moreover, these results are in good agreement with the simulation. Further improvement of the current design could yield significantly better performance as the device has yet to reach its full potential, especially with regard to the design of its air-cooled curvilinear fins and boiling enhancement.Copyright © 2008 by ASME

International Journal of Heat and Mass Transfer, 2013

ABSTRACT Understanding the influence of surface characteristics on flow boiling heat transfer beh... more ABSTRACT Understanding the influence of surface characteristics on flow boiling heat transfer behavior in microgap is necessary to enhance the performance of microgap heat sink. The influences of surface roughness on flow boiling heat transfer, pressure drop and instability in microgap heat sink are experimentally investigated. Flow boiling experiments are conducted over silicon microgap heat sink of three different microgap dimensions namely 500 μm, 300 μm and 200 μm. The original silicon surface of surface roughness, Ra = 0.6 μm is modified to Ra = 1.0 μm and 1.6 μm to examine the effect of surface finish. These studies are carried out with the inlet deionized water temperatures 91 °C at two different mass fluxes, G = 390 kg/m2 s and 650 kg/m2 s and imposed effective heat flux, View the MathML source ranging from 0 W/cm2 to 85 W/cm2. High speed flow visualizations are conducted simultaneously along with experiments to explore the bubble behavior in microgap heat sink. The results of this study show that bubble nucleation site density as well as heat transfer coefficient increases with the increase of surface roughness and pressure drop is independent of surface roughness in microgap heat sink. Moreover, rougher surface maintains lower and uniform wall temperature over the heated surface. However, surface roughness has an adverse effect on the inlet pressure instability and inlet pressure fluctuation increases with increasing surface roughness at larger microgap heat sink.

International Journal of Heat and Mass Transfer, 2013

2011 IEEE 13th Electronics Packaging Technology Conference, EPTC 2011, 2011

ABSTRACT Hotspots can be generated by non-uniform heat flux condition over the heated surface due... more ABSTRACT Hotspots can be generated by non-uniform heat flux condition over the heated surface due to higher packaging densities and greater power consumption of high-performance computing technology in military systems designs. Because of this hotspot within a given chip, local heat generation rate exceed the average value on the chip and increase the peak temperature for a given total power generation which degrades the reliability and performance of equipments. Two phase microgap cooling technology is promising to minimization of temperature gradient and reduction of maximum temperature over the heated surface of the device because of unique boiling mechanism in microgap: confined flow and thin film evaporation. The present study aims to experimentally investigate the applicability of microgap cooling technology for minimizining temperature gradient and mitigating hotspots from the heated surface of electronic device. Experiments are performed in silicon based microgap heat sink having a range of gap dimension from 200 µm – 400 µm. Encouraging results have been obtained using microgap channel cooler for hotspots mitigation as it maintain uniform and low wall temperature over the heated surface.

Evaluated here is the performance of current near-wall and turbulence models in computing turbule... more Evaluated here is the performance of current near-wall and turbulence models in computing turbulent heat and momentum transfer in recirculating and impinging flows. The effect on the computations of a novel wall-damping term in the energy dissipation rate equation is also assessed. Computations of the flow downstream of an abrupt pipe expansion (the principal test flow) are compared with detailed

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

Abstract A stable flow boiling operation is key to enhancing the two-phase cooling performance of... more Abstract A stable flow boiling operation is key to enhancing the two-phase cooling performance of microchannel heat sinks. To this end, a novel heat sink is developed which integrates a 300 μm × 600 μm straight microchannel array in the upstream region with a 25 mm × 600 μm microgap channel in the downstream region of a 25 mm × 25 mm copper heat sink. Flow boiling experiments are conducted using de-ionized water at 5 different mass fluxes in the range of 100–399 kg/m2 s, supplied at a fixed inlet temperature of 85.5 °C. The downstream heat transfer coefficient in the microgap section shows an M-shaped profile with increasing heat flux and vapor quality. Stable boiling conditions are prevalent across a large span of operating heat flux. Instabilities are experienced only for a short range of low heat flux following ONB. The stabilizing effect is attributed to the larger flow cross-section area offered by the downstream microgap section that allows expanding vapor bubbles to evacuate with lesser hindrance. Flow visualization reveals that a stable annular flow regime is established at moderate to high heat flux during which the stable boiling operation is observed. Thin film evaporation taking place during annular flow conditions results in an increasing trend in the downstream heat transfer coefficient from moderate to high heat flux. Pressure drop associated with the hybrid heat sink is found to be modest and reaches a maximum of 6 kPa at the highest mass flux of 399 kg/m2 s. A brief comparison of this heat sink is made with its straight microchannel and microgap heat sink counterparts. The hybrid heat sink shows a heat transfer performance that is superior to the microgap heat sink while poorer than the straight microchannel heat sink although it offers a better boiling stability than the straight microchannel heat sink. It however lowers the pressure drop compared to the straight microchannel heat sink.

FIELD: information technology. SUBSTANCE: invention can be used normalising temperature of electr... more FIELD: information technology. SUBSTANCE: invention can be used normalising temperature of electronic components, particularly central processing units (CPU) of modern computers, which are meant for installation in street conditions or in rooms with unfavourable ambient conditions: high dust content, high humidity, as well as at high temperatures. The cooling device for electronic components has a housing on the outer surface of one of the vertical walls, which are the heatsink, of which there are heat-scattering fins, and inside the housing there is a base heat-transfer unit meant for contact with the heat-loaded electronic components; the base heat-transfer unit is connected to heat pipes with a capillary-porous structure, which are connected to the heatsink; part of the heat pipes lies above the base heat-transfer unit (ascending heat pipes) and the other part lies below the base heat-transfer unit (descending heat pipes); the pore radius of the inner structure of the ascending and descending heat pipes is different and is selected based on a condition of compensation for the effect of gravitational force on heat transfer characteristics; the pore radius of the inner structure of the ascending and descending heat pipes is selected from the following condition: where ρ is the density of the heat carrier; g is gravitational acceleration; R H is the pore radius in the capillary structure of descending heat pipes; R B is the pore radius in the capillary structure of ascending heat pipes; h B is the height of the heat-removing part of the ascending heat pipes over the base; h H is the height of the heat-removing part of the descending heat pipes below the base. EFFECT: high uniformity of the temperature field on the height of the heatsink and low temperature in region of connection of the outlet part of the heat pipes while simplifying the design of the cooling device, which improves functional capabilities of the device and increases its reliability. 10 cl, 4 dwg

Pediatric Nephrology, 2009

Modern Physics Letters B, 2010

This research paper presents a study of boiling heat transfer from longitudinal rectangular and s... more This research paper presents a study of boiling heat transfer from longitudinal rectangular and square pin finned surfaces immersed in saturated water at low vapor pressures of 2 and 9 kPa. Conventional boiling analysis, which is based on the nominal surface area of the heater, was compared with a boiling analysis that considers the total…

Journal of Heat Transfer, 2011

This study presents a generalized confined boiling correlation applicable for various working flu... more This study presents a generalized confined boiling correlation applicable for various working fluids and operating conditions. A dimensionless parameter, Bond number, has been incorporated into the correlation to include the effects of confinement in the ebullition process of boiling. The proposed correlation is compared with an existing correlation based on their capability in predicting confined boiling data from the literature. A phenomenon of heat transfer coefficient stagnation is found for boiling in narrow spaces despite an increase in heat flux. Results show that the proposed correlation entails an excellent agreement with experimental data, and the predictions have a reasonably low mean absolute error of 17.3% for the entire database.

Journal of Heat Transfer, 2010

This research paper presents a study of boiling heat transfer from longitudinal rectangular-finne... more This research paper presents a study of boiling heat transfer from longitudinal rectangular-finned surfaces immersed in saturated water at low vapor pressures. Finned surfaces with assorted fin spacing, fin thicknesses, and fin heights on a copper based surface have been investigated. All the finned surfaces were found to increase both boiling heat transfer coefficients and critical heat fluxes. An optimal fin thickness was found for a configuration, and heat transfer coefficients have been obtained at the pressures. Factors affecting the boiling characteristics have been identified and the optimal enhancement requires a balance of the active nucleation sites, bubble flow resistance, natural convection, thin film evaporation, liquid superheating, heat transfer area, bubble coalescence, and liquid reflux resistance. High speed visualization of vapor plug and vapor film generation on the boiling surfaces has revealed significant insights into the boiling mechanisms at low saturation p...

Journal of Electronic Packaging, 2005

The friction factors of single-phase microchannel with various roughness elements (rectangular, t... more The friction factors of single-phase microchannel with various roughness elements (rectangular, triangular, elliptical) were investigated numerically. The two-dimensional numerical solution shows significant influence of surface roughness including the height, size, and spacing of the roughness elements on the Poiseuille number. The fluid temperature effects are also investigated numerically for the rough channels.

IFAC Proceedings Volumes, 2005

Proceedings of the 2012 IEEE 14th Electronics Packaging Technology Conference, EPTC 2012, 2012

ABSTRACT Flow boiling instabilities induce mechanical vibration in the system and deteriorate the... more ABSTRACT Flow boiling instabilities induce mechanical vibration in the system and deteriorate the heat transfer performances, for example- premature dryout, critical heat flux limitation etc. The two phase microgap heat sink has novel potential to mitigate these undesirable flow boiling instabilities and flow reversal issues inherent with two phase microchannel heat sink. This work is an experimental study of boiling instabilities in microgap heat sink for different microgap depths ranging from 80μm-1000μm, mass fluxes from 390kg/m2s-900kg/m2s, heat fluxes up to 85W/cm2 and different microgap surface roughnesses, Ra=0.6-1μm. A series of systematic experiments have been carried out to investigate the inlet pressure and wall temperature oscillations during two phase flow boiling condition under uniform heating, with deionized water as a cooling liquid. Experimental result shows that pressure oscillation increases with the decreasing microgap depth. Temperature oscillation is observed lower for smaller gap than larger gap up to a certain heat flux condition before the dryout phase. In addition, inlet pressure instabilities increase with increasing heat flux and decreasing mass flux. Moreover, surface roughness has an adverse effect on the inlet pressure instability at larger depth microgap heat sink and inlet pressure fluctuation increases with increasing surface roughness.

ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE), 2012

Flow boiling in microgap heat sink is very attractive for high-performance electronics cooling du... more Flow boiling in microgap heat sink is very attractive for high-performance electronics cooling due to its high heat transfer rate and easy fabrication process. In absence of thermal interface material between the active electronic component and a microgap cold plate, significant reduction in interface thermal resistance and enhancement in heat transfer rate can be achieved. In earlier studies by these authors, encouraging results have been obtained using microgap heat sink as it can potentially mitigate flow instabilities, flow reversal and maintain uniform wall temperatures over the heated surface. So, more work should be carried out to advance the fundamental understanding of the two-phase flow heat transfer associated with microgap heat sink and the underlying mechanisms. In this study, local flow boiling phenomena in different microgap sizes have been investigated experimentally. Experiments are performed in silicon based microgap heat sink having microgap depth ranging from 80 μm to 500 μm, using deionized water with 10 °C subcooled inlet temperature. The effects of mass flux and heat flux on heat transfer coefficient and pressure drop characteristics are examined by using different mass fluxes ranging from 400 kg/m2s to 1000 kg/m2s and effective heat flux varying from 0 to 100 W/cm2. Apart from these experimental investigations, simultaneous high speed visualizations are conducted to observe and explore the mechanism of flow boiling in microgap. Confined slug and annular boiling are observed as the two main heat transfer mechanisms in microgap. Moreover, experimental results show that flow boiling heat transfer coefficients are dependent on gap size, and the lower the gap size, higher the heat transfer coefficient.

International Journal of Multiphase Flow, 2012

Heat Transfer: Volume 2, 2008

CPUs with high clock rates can dramatically increase heat dissipation within their encapsulation ... more CPUs with high clock rates can dramatically increase heat dissipation within their encapsulation due to internal Joule heat from the transistors. The conventionally used air cooling systems for CPUs, such as the aluminum or copper extruded heat sink types, have severe heat transfer “bottlenecks” due to high thermal resistances and they easily reach their thermal design limits (TDL). Alternative cooling devices such as heat pipes and liquid cooling tends to have externally attached radiator/condenser and/or pump and such designs are cumbersome. This paper describes the modeling, design, and testing of a compact (about the size of the Intel stock cooler, diameter: 96mm, height: 50mm), fully integrated, orientation-free, evaporator-condenser device for CPU cooling, with excellent attributes of low thermal resistance from phase change phenomena and minimal vapor pressure drop. The prototype fabricated is designed to reject 200 W (twice the capacity of conventional heat sinks). It is made of copper and uses distilled water as the working fluid. The working fluid boils inside a porous structure clad evaporator and is transported radially to nearby air-cooled condenser sections; this unique arrangement minimizes space while providing adequate area for air convection. Testing was done by subjecting it to varying heat loads and air flow rates. A best performance of 0.206 K/W of the device’s thermal resistance was achieved at a fan air flow rate of 34.5 CFM under 203 W of cooling load, and moreover, these results are in good agreement with the simulation. Further improvement of the current design could yield significantly better performance as the device has yet to reach its full potential, especially with regard to the design of its air-cooled curvilinear fins and boiling enhancement.Copyright © 2008 by ASME

International Journal of Heat and Mass Transfer, 2013

ABSTRACT Understanding the influence of surface characteristics on flow boiling heat transfer beh... more ABSTRACT Understanding the influence of surface characteristics on flow boiling heat transfer behavior in microgap is necessary to enhance the performance of microgap heat sink. The influences of surface roughness on flow boiling heat transfer, pressure drop and instability in microgap heat sink are experimentally investigated. Flow boiling experiments are conducted over silicon microgap heat sink of three different microgap dimensions namely 500 μm, 300 μm and 200 μm. The original silicon surface of surface roughness, Ra = 0.6 μm is modified to Ra = 1.0 μm and 1.6 μm to examine the effect of surface finish. These studies are carried out with the inlet deionized water temperatures 91 °C at two different mass fluxes, G = 390 kg/m2 s and 650 kg/m2 s and imposed effective heat flux, View the MathML source ranging from 0 W/cm2 to 85 W/cm2. High speed flow visualizations are conducted simultaneously along with experiments to explore the bubble behavior in microgap heat sink. The results of this study show that bubble nucleation site density as well as heat transfer coefficient increases with the increase of surface roughness and pressure drop is independent of surface roughness in microgap heat sink. Moreover, rougher surface maintains lower and uniform wall temperature over the heated surface. However, surface roughness has an adverse effect on the inlet pressure instability and inlet pressure fluctuation increases with increasing surface roughness at larger microgap heat sink.

International Journal of Heat and Mass Transfer, 2013

2011 IEEE 13th Electronics Packaging Technology Conference, EPTC 2011, 2011

ABSTRACT Hotspots can be generated by non-uniform heat flux condition over the heated surface due... more ABSTRACT Hotspots can be generated by non-uniform heat flux condition over the heated surface due to higher packaging densities and greater power consumption of high-performance computing technology in military systems designs. Because of this hotspot within a given chip, local heat generation rate exceed the average value on the chip and increase the peak temperature for a given total power generation which degrades the reliability and performance of equipments. Two phase microgap cooling technology is promising to minimization of temperature gradient and reduction of maximum temperature over the heated surface of the device because of unique boiling mechanism in microgap: confined flow and thin film evaporation. The present study aims to experimentally investigate the applicability of microgap cooling technology for minimizining temperature gradient and mitigating hotspots from the heated surface of electronic device. Experiments are performed in silicon based microgap heat sink having a range of gap dimension from 200 µm – 400 µm. Encouraging results have been obtained using microgap channel cooler for hotspots mitigation as it maintain uniform and low wall temperature over the heated surface.

Evaluated here is the performance of current near-wall and turbulence models in computing turbule... more Evaluated here is the performance of current near-wall and turbulence models in computing turbulent heat and momentum transfer in recirculating and impinging flows. The effect on the computations of a novel wall-damping term in the energy dissipation rate equation is also assessed. Computations of the flow downstream of an abrupt pipe expansion (the principal test flow) are compared with detailed

International Journal of Heat and Mass Transfer

International Journal of Heat and Mass Transfer

Abstract A stable flow boiling operation is key to enhancing the two-phase cooling performance of... more Abstract A stable flow boiling operation is key to enhancing the two-phase cooling performance of microchannel heat sinks. To this end, a novel heat sink is developed which integrates a 300 μm × 600 μm straight microchannel array in the upstream region with a 25 mm × 600 μm microgap channel in the downstream region of a 25 mm × 25 mm copper heat sink. Flow boiling experiments are conducted using de-ionized water at 5 different mass fluxes in the range of 100–399 kg/m2 s, supplied at a fixed inlet temperature of 85.5 °C. The downstream heat transfer coefficient in the microgap section shows an M-shaped profile with increasing heat flux and vapor quality. Stable boiling conditions are prevalent across a large span of operating heat flux. Instabilities are experienced only for a short range of low heat flux following ONB. The stabilizing effect is attributed to the larger flow cross-section area offered by the downstream microgap section that allows expanding vapor bubbles to evacuate with lesser hindrance. Flow visualization reveals that a stable annular flow regime is established at moderate to high heat flux during which the stable boiling operation is observed. Thin film evaporation taking place during annular flow conditions results in an increasing trend in the downstream heat transfer coefficient from moderate to high heat flux. Pressure drop associated with the hybrid heat sink is found to be modest and reaches a maximum of 6 kPa at the highest mass flux of 399 kg/m2 s. A brief comparison of this heat sink is made with its straight microchannel and microgap heat sink counterparts. The hybrid heat sink shows a heat transfer performance that is superior to the microgap heat sink while poorer than the straight microchannel heat sink although it offers a better boiling stability than the straight microchannel heat sink. It however lowers the pressure drop compared to the straight microchannel heat sink.

FIELD: information technology. SUBSTANCE: invention can be used normalising temperature of electr... more FIELD: information technology. SUBSTANCE: invention can be used normalising temperature of electronic components, particularly central processing units (CPU) of modern computers, which are meant for installation in street conditions or in rooms with unfavourable ambient conditions: high dust content, high humidity, as well as at high temperatures. The cooling device for electronic components has a housing on the outer surface of one of the vertical walls, which are the heatsink, of which there are heat-scattering fins, and inside the housing there is a base heat-transfer unit meant for contact with the heat-loaded electronic components; the base heat-transfer unit is connected to heat pipes with a capillary-porous structure, which are connected to the heatsink; part of the heat pipes lies above the base heat-transfer unit (ascending heat pipes) and the other part lies below the base heat-transfer unit (descending heat pipes); the pore radius of the inner structure of the ascending and descending heat pipes is different and is selected based on a condition of compensation for the effect of gravitational force on heat transfer characteristics; the pore radius of the inner structure of the ascending and descending heat pipes is selected from the following condition: where ρ is the density of the heat carrier; g is gravitational acceleration; R H is the pore radius in the capillary structure of descending heat pipes; R B is the pore radius in the capillary structure of ascending heat pipes; h B is the height of the heat-removing part of the ascending heat pipes over the base; h H is the height of the heat-removing part of the descending heat pipes below the base. EFFECT: high uniformity of the temperature field on the height of the heatsink and low temperature in region of connection of the outlet part of the heat pipes while simplifying the design of the cooling device, which improves functional capabilities of the device and increases its reliability. 10 cl, 4 dwg

Pediatric Nephrology, 2009

Modern Physics Letters B, 2010

This research paper presents a study of boiling heat transfer from longitudinal rectangular and s... more This research paper presents a study of boiling heat transfer from longitudinal rectangular and square pin finned surfaces immersed in saturated water at low vapor pressures of 2 and 9 kPa. Conventional boiling analysis, which is based on the nominal surface area of the heater, was compared with a boiling analysis that considers the total…

Journal of Heat Transfer, 2011

This study presents a generalized confined boiling correlation applicable for various working flu... more This study presents a generalized confined boiling correlation applicable for various working fluids and operating conditions. A dimensionless parameter, Bond number, has been incorporated into the correlation to include the effects of confinement in the ebullition process of boiling. The proposed correlation is compared with an existing correlation based on their capability in predicting confined boiling data from the literature. A phenomenon of heat transfer coefficient stagnation is found for boiling in narrow spaces despite an increase in heat flux. Results show that the proposed correlation entails an excellent agreement with experimental data, and the predictions have a reasonably low mean absolute error of 17.3% for the entire database.

Journal of Heat Transfer, 2010

This research paper presents a study of boiling heat transfer from longitudinal rectangular-finne... more This research paper presents a study of boiling heat transfer from longitudinal rectangular-finned surfaces immersed in saturated water at low vapor pressures. Finned surfaces with assorted fin spacing, fin thicknesses, and fin heights on a copper based surface have been investigated. All the finned surfaces were found to increase both boiling heat transfer coefficients and critical heat fluxes. An optimal fin thickness was found for a configuration, and heat transfer coefficients have been obtained at the pressures. Factors affecting the boiling characteristics have been identified and the optimal enhancement requires a balance of the active nucleation sites, bubble flow resistance, natural convection, thin film evaporation, liquid superheating, heat transfer area, bubble coalescence, and liquid reflux resistance. High speed visualization of vapor plug and vapor film generation on the boiling surfaces has revealed significant insights into the boiling mechanisms at low saturation p...

Journal of Electronic Packaging, 2005

The friction factors of single-phase microchannel with various roughness elements (rectangular, t... more The friction factors of single-phase microchannel with various roughness elements (rectangular, triangular, elliptical) were investigated numerically. The two-dimensional numerical solution shows significant influence of surface roughness including the height, size, and spacing of the roughness elements on the Poiseuille number. The fluid temperature effects are also investigated numerically for the rough channels.