Ankit Kalani

Enhanced Flow Boiling Over Open Microchannels With Uniform and Tapered Gap Manifolds

Flow boiling in microchannels has been extensively studied in the past decade. Instabilities, low critical heat flux (CHF) values, and low heat transfer coefficients have been identified as the major shortcomings preventing its implementation in practical high heat flux removal systems. A novel open microchannel design with uniform and tapered manifolds (OMM) is presented to provide stable and highly enhanced heat transfer performance. The effects of the gap height and flow rate on the heat transfer performance have been experimentally studied with water. The critical heat fluxes (CHFs) and heat transfer coefficients obtained with the OMM are significantly higher than the values reported by previous researchers for flow boiling with water in microchannels. A record heat flux of 506W/cm 2 with a wall superheat of 26.2 � C was obtained for a gap size of 0.127mm. The CHF was not reached due to heater power limitation in the current design. A maximum effective heat transfer coefficient of 290,000W/m 2 � C was obtained at an intermediate heat flux of 319W/cm 2 with a gap of 0.254mm at 225mL/min. The flow boiling heat transfer was found to be insensitive to flow rates between 40‐333mL/min and gap sizes between 0.127‐1.016mm, indicating the dominance of nucleate boiling. The OMM geometry is promising to provide exceptional performance that is particularly attractive in meeting the challenges of high heat flux removal in electronics cooling applications. [DOI: 10.1115/1.4023574]

Evaluation of Pressure Drop Performance During Enhanced Flow Boiling in Open Microchannels With Tapered Manifolds

Boiling can provide several orders of magnitude higher performance than a traditional air cooled system in electronics cooling applications. It can dissipate large quantities of heat while maintaining a low surface temperature difference. Flow boiling with microchannels has shown a great potential with its high surface area to volume ratio and latent heat removal. However, flow instabilities and low critical heat flux (CHF) have prevented its successful implementation. A novel flow boiling design is experimentally investigated to overcome the above-mentioned disadvantages while presenting a very low pressure drop. The design uses open microchannels with a tapered manifold (OMM) to provide stable and efficient operation. The effect of tapered manifold block with varied dimension is investigated with distilled, degassed water at atmospheric pressure. Heat transfer coefficient and pressure drop results for uniform and tapered manifolds with plain and microchannel chips are presented. The OMM configuration yielded a CHF of over 500 W/cm in our earlier work. In the current work, a heat transfer coefficient of 277.8 kW/m C was obtained using an OMM design with an inlet gap of 127 lm and an exit gap of 727 lm over a 10 mm flow length. The OMM geometry also resulted in a dramatic reduction in pressure drop from 158.4 kPa for a plain chip and 62.1 kPa for a microchannel chip with a uniform manifold, to less than 10 kPa with the tapered OMM design. A tapered manifold (inlet and exit manifold heights of 127 and 727 lm, respectively) with microchannel provided the lowest pressure drop of 3.3 kPa. [DOI: 10.1115/1.4026306]

Combining liquid inertia with pressure recovery from bubble expansion for enhanced flow boiling

In this paper, we demonstrate using liquid inertia force in a taper gap microchannel geometry to provide a high level of heat dissipation capacity accompanied by a high heat transfer coefficient and low pressure drop during flow boiling. The high mass flux increases liquid inertia force and promotes vapor removal from the manifold, thereby increasing critical heat flux (CHF) and heat transfer coefficient. The tapered gap above the microchannels provides an increasing cross-sectional area in the flow direction. This gap allows bubbles to emerge from microchannels and expand within the gap along the flow direction. The bubble evaporation and expansion in tapered gap causes pressure recovery and reduces the total pressure drop. The pressure recovery increases with the increased evaporation rate at higher heat fluxes. Using a 6% taper and a moderately high inlet liquid flow Reynolds number of 1095, we have reached a CHF of 1.07 kW/cm2 with a heat transfer coefficient of 295 kW/m2 °C and a pressure drop of 30 kPa.

Enhanced Pool Boiling With Ethanol at Subatmospheric Pressures for Electronics Cooling

The growing trend in miniaturization of electronics has generated a need for efficient thermal management of these devices. Boiling has the ability to dissipate a high heat flux while maintaining a small temperature difference. A vapor chamber with pool boiling offers an effective way to provide cooling and to maintain temperature uniformity. The objective of the current work is to investigate pool boiling performance of ethanol on enhanced microchannel surfaces. Ethanol is an attractive working fluid due to its better heat transfer performance and higher heat of vaporization compared to refrigerants, and lower normal boiling point compared to water. The saturation temperature of ethanol can be further reduced to temperatures suitable for electronics cooling by lowering the pressure. Experiments were performed at four different absolute pressures, 101.3 kPa, 66.7 kPa, 33.3 kPa, and 16.7 kPa using different microchannel surface configurations. Heat dissipation in excess of 900 kW/m was obtained while maintaining the wall surface below 85 C at 33 kPa. Flammability, toxicity, and temperature overshoot issues need to be addressed before practical implementation of ethanol-based cooling systems can occur. [DOI: 10.1115/1.4024595]

Experimental Investigation of Flow Boiling Performance of Open Microchannels With Uniform and Tapered Manifolds (OMM)

Boiling can provide orders of magnitude higher cooling performance than a traditional air cooled system especially related to electronics cooling application. It can dissipate large quantities of heat while maintaining a low surface temperature difference. Flow boiling with microchannels has shown a lot of potential due to its high surface area to volume ratio and latent heat removal. Flow instabilities and early critical heat flux have however prevented its successful implementation. A novel flow boiling design is experimentally investigated to overcome the above mentioned disadvantages while presenting a very low pressure drop. The design uses open microchannels with a tapered manifold (OMM) to provide stable and efficient operation. Distilled, degassed water at atmospheric pressure was used as the fluid medium. Effect of tapered block with varied dimension is investigated. Pressure drop data for uniform and tapered manifold for plain and microchannel chip are presented. A maximum heat flux of 281 W/cm 2

Pool Boiling of FC-87 Over Microchannel Surfaces at Atmospheric Pressure

Advances in technology and trends towards higher processing speeds have generated a greater need for thermal management. Two-phase cooling (boiling) has the ability to dissipate large amounts of heat and is attractive because of lower mass flow requirement and uniform substrate temperature. Further improvements can be obtained through passive surface enhancements. The objective of this work is to investigate the effect of microchannel surfaces on pool boiling performance at atmospheric pressure with FC-87. Being a dielectric fluid with a low normal boiling point, FC-87 has desirable characteristics for an electronics cooling fluid. A maximum heat flux of 550 kW/m 2 at a wall superheat of 37°C was obtained with the microchannel surface. Surface area increase is noted as the primary reason for the enhanced performance for FC-87 on microchannel surfaces.

Pool Boiling Heat Transfer Over Microchannel Surfaces With Ethanol at Atmospheric Pressure

The growing trend in miniaturization has brought forth boiling as an important research topic for augmentation of heat transfer in electronic cooling application. Pool boiling has the ability to dissipate a large quantity of heat while maintaining a small temperature difference. The heat dissipation capacity of pool boiling is further augmented through the use of enhanced microchannel surfaces. The objective of this work is to investigate the pool boiling performance of various microchannel structures using ethanol as the fluid medium. Ethanol has higher heat of vaporization compared to refrigerants and lower saturation temperature compared to water. Hence it has the potential to be used as an alternative cooling medium. This investigation will focus on experimentally studying the effect of enhanced surface structures on pool boiling of ethanol at atmospheric pressure.Copyright

A Photographic Study of Flow Boiling Stability on a Plain Surface With Tapered Manifold

Flow boiling in microchannels offers many advantages such as high heat transfer coefficient, higher surface area to volume ratio, low coolant inventory, uniform temperature control and compact design. The application of these flow boiling systems has been severely limited due to early critical heat flux (CHF) and flow instability. Recently, a number of studies have focused on variable flow cross-sectional area to augment the thermal performance of microchannels. In a previous work, the open microchannel with manifold (OMM) configuration was experimentally investigated to provide high heat transfer coefficient coupled with high CHF and low pressure drop. In the current work, high speed images of plain surface using tapered manifold are obtained to gain an insight into the nucleating bubble behavior. The mechanism of bubble nucleation, growth and departure are described through high speed images. Formation of dry spots for both tapered and uniform manifold geometry is also discussed.Copyright

Preliminary Results of Pressure Drop Modeling During Flow Boiling in Open Microchannels With Uniform and Tapered Manifolds (OMM)

Flow boiling with microchannel can dissipate high heat fluxes at low surface temperature difference. A number of issues, such as instabilities, low critical heat flux (CHF) and low heat transfer coefficients, have prevented it from reaching its full potential. A new design incorporating open microchannels with uniform and tapered manifold (OMM) was shown to mitigate these issues successfully. Distilled, degassed water at 80 mL/min is used as the working fluid. Plain and open microchannel surfaces are used as the test section s. Heat transfer and pressure drop performance for uniform and tapered manifold with both the surfaces are discussed. A low pressure drop of 7.5 kPa is obtained with tapered manifold and microchannel chip at a heat flux of 263 W/cm 2 without reaching CHF. The pressure drop data is further compared with the homogenous model and the initial results are presented.

Pool Boiling Heat Transfer With Binary Mixture on Open Microchannel Surface

Two-phase cooling is considered an attractive option for electronics cooling due to its ability to dissipate large quantities of heat. In recent years, pool boiling has shown tremendous ability in high heat dissipation applications. Researchers have used various fluid medium for pool boiling including water, alcohol, refrigerants, nanofluids and binary mixture. In the current work, binary mixture of water with ethanol was chosen as the working fluid. Plain copper chip was used as the boiling surface. Effect of various concentrations of binary mixture was investigated. A maximum heat flux of 1720 kW/m2 at a wall superheat of 28°C was recorded for 15% ethanol in water. It showed a 1.5 fold increase in CHF over pure water.© 2013 ASME