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• 1. [Article] Applications of the thermal wave technique in liquid thermal conductivity measurements and flow field diagnostics

  The thermal wave technique has been explored for the use of liquid thermal conductivity measurement and flow property diagnostics in this dissertation. For liquid thermal conductivity measurements, an ...

  Citation × Citation Citation Abstract Copy Title: Applications of the thermal wave technique in liquid thermal conductivity measurements and flow field diagnostics Author: Wang, Zhefu The thermal wave technique has been explored for the use of liquid thermal conductivity measurement and flow property diagnostics in this dissertation. For liquid thermal conductivity measurements, an experimental technique based on the thermal wave approach is developed. A stainless steel strip functions as both a heating element and a sealing cover for a chamber containing a test liquid. A periodic current passing through this metal strip generates a periodic Joule heating source. An infrared detector measures the temperature response at the front surface of the stainless steel strip. The phase and magnitude of the temperature response were measured by a lock-in amplifier at various frequencies. A one-dimensional, two-layered transient heat conduction model is developed to predict the temperature response. The phase information of this temperature response shows high sensitivity to the change of thermal properties of the liquid layer and is employed to match experimental data to find the thermal properties of the test liquid. The measured thermal conductivities of water and ethylene glycol agree quite well with data from the literature and support the validity of this measurement technique. An aqueous fluid consisting of gold nanoparticles was also tested and anomalous thermal conductivity enhancement was observed. Our measurement results also showed a divergence of thermal transport behavior between nanofluids and pure liquids. This suggests a need to carefully examine the role of the measurement technique in the heat transfer experimental studies of nanofluids. For the study of flow property diagnostics, a heat transfer system with a periodic boundary condition in a steady flow field is examined. Due to linearity and homogeneity of the heat transfer system under certain conditions, a thermal wave field generated by a periodic heating flux at the boundary of the flow field may be used to detect important flow field parameters, such as the velocity gradient at the wall, and therefore, the wall shear stress. A wall shear stress sensor design with a heater and two temperature sensors on a silicon dioxide substrate is analyzed. The heater with a periodic heating source generates an oscillating temperature field which interacts with the flow field. The temperature sensors pick up the temperature response that contains information on the velocity gradient at the wall. Based on the above sensor design, a two-dimensional conjugate heat convection model is developed with a periodic heating flux at the solid/fluid interface and a linear velocity profile in the fluid domain. Two designs are studied, one with a silicon heat sink under the silicon dioxide substrate and another without the heat sink. The effects of the two main design parameters, the operating frequency and the distance between the heater and the temperature sensor, are discussed. A reasonable sensitivity of the phase information to the velocity gradient suggests a practical sensor design. A preliminary experimental test on a water channel flow has been conducted to support the concept of applying the thermal wave method to wall shear stress measurements. Title: Applications of the thermal wave technique in liquid thermal conductivity measurements and flow field diagnostics Author: Wang, Zhefu The thermal wave technique has been explored for the use of liquid thermal conductivity measurement and flow property diagnostics in this dissertation. For liquid thermal conductivity measurements, an experimental technique based on the thermal wave approach is developed. A stainless steel strip functions as both a heating element and a sealing cover for a chamber containing a test liquid. A periodic current passing through this metal strip generates a periodic Joule heating source. An infrared detector measures the temperature response at the front surface of the stainless steel strip. The phase and magnitude of the temperature response were measured by a lock-in amplifier at various frequencies. A one-dimensional, two-layered transient heat conduction model is developed to predict the temperature response. The phase information of this temperature response shows high sensitivity to the change of thermal properties of the liquid layer and is employed to match experimental data to find the thermal properties of the test liquid. The measured thermal conductivities of water and ethylene glycol agree quite well with data from the literature and support the validity of this measurement technique. An aqueous fluid consisting of gold nanoparticles was also tested and anomalous thermal conductivity enhancement was observed. Our measurement results also showed a divergence of thermal transport behavior between nanofluids and pure liquids. This suggests a need to carefully examine the role of the measurement technique in the heat transfer experimental studies of nanofluids. For the study of flow property diagnostics, a heat transfer system with a periodic boundary condition in a steady flow field is examined. Due to linearity and homogeneity of the heat transfer system under certain conditions, a thermal wave field generated by a periodic heating flux at the boundary of the flow field may be used to detect important flow field parameters, such as the velocity gradient at the wall, and therefore, the wall shear stress. A wall shear stress sensor design with a heater and two temperature sensors on a silicon dioxide substrate is analyzed. The heater with a periodic heating source generates an oscillating temperature field which interacts with the flow field. The temperature sensors pick up the temperature response that contains information on the velocity gradient at the wall. Based on the above sensor design, a two-dimensional conjugate heat convection model is developed with a periodic heating flux at the solid/fluid interface and a linear velocity profile in the fluid domain. Two designs are studied, one with a silicon heat sink under the silicon dioxide substrate and another without the heat sink. The effects of the two main design parameters, the operating frequency and the distance between the heater and the temperature sensor, are discussed. A reasonable sensitivity of the phase information to the velocity gradient suggests a practical sensor design. A preliminary experimental test on a water channel flow has been conducted to support the concept of applying the thermal wave method to wall shear stress measurements. Close

• 2. [Article] An experimental study of co-flow ammonia-water desorption in an oil-heated, microscale, fractal-like branching heat exchanger

  An experimental study was performed in which an ammonia-water solution was desorbed within a branching fractal-like microchannel array. The solution entered in the center of a disk, and flowed out radially ...

  Citation × Citation Citation Abstract Copy Title: An experimental study of co-flow ammonia-water desorption in an oil-heated, microscale, fractal-like branching heat exchanger Author: Mouchka, Gregory A. An experimental study was performed in which an ammonia-water solution was desorbed within a branching fractal-like microchannel array. The solution entered in the center of a disk, and flowed out radially until discharging in to a gravity-driven separation chamber. Heat was added to the ammonia-water through a thin wall, above which flowed heat transfer oil in a separate branching fractal-like microchannel array. Such arrays have been shown to utilize the increased heat transfer coefficients seen in parallel channel arrays; however, they do so with a lower pressure drop. An experimental flow loop consisting of ammonia-water and heat transfer oil sub-loops was instrumented along with a test manifold for global measurements to be taken. Temperature, pressure, density and mass flow rate measurements permitted calculation of desorption and heat transfer characteristics. Parameters included oil mass flow rate, oil inlet temperature, and strong solution flow rate, while strong solution concentration, temperature, and weak solution pressure were kept constant. The desorber was assumed to achieve equilibrium conditions between the vapor and weak solution in the separation chamber. The exit plenum was large and acted as a flash chamber, making the assumption reasonable. The vapor mass fraction was determined from knowledge of the weak solution saturation temperature. Heat exchanger analyses (LMTD and ε-NTU) were done to determine the heat transfer characteristics of the desorber. Calculated values of UA are shown to be as high as 5.0 W/K, and desorber heat duties were measured as high as 334 W. Strong solution, at 0.30 mass fraction, was desorbed into weak solution and vapor with concentrations ranging from 0.734 to 0.964. Circulation ratios, defined as strong solution mass flow rate per unit desorbed vapor mass flow rate, varied in this study from 3.4 to 20. A method for specifying desorber operating conditions is described, in which a minimum desorber heat input per unit vapor flow rate is determined at an optimum circulation ratio. A description of how the circulation ratio behaves as a function of strong solution mass flow rate, oil flow rate, and the maximum temperature difference between oil and ammonia-water solution is shown. Title: An experimental study of co-flow ammonia-water desorption in an oil-heated, microscale, fractal-like branching heat exchanger Author: Mouchka, Gregory A. An experimental study was performed in which an ammonia-water solution was desorbed within a branching fractal-like microchannel array. The solution entered in the center of a disk, and flowed out radially until discharging in to a gravity-driven separation chamber. Heat was added to the ammonia-water through a thin wall, above which flowed heat transfer oil in a separate branching fractal-like microchannel array. Such arrays have been shown to utilize the increased heat transfer coefficients seen in parallel channel arrays; however, they do so with a lower pressure drop. An experimental flow loop consisting of ammonia-water and heat transfer oil sub-loops was instrumented along with a test manifold for global measurements to be taken. Temperature, pressure, density and mass flow rate measurements permitted calculation of desorption and heat transfer characteristics. Parameters included oil mass flow rate, oil inlet temperature, and strong solution flow rate, while strong solution concentration, temperature, and weak solution pressure were kept constant. The desorber was assumed to achieve equilibrium conditions between the vapor and weak solution in the separation chamber. The exit plenum was large and acted as a flash chamber, making the assumption reasonable. The vapor mass fraction was determined from knowledge of the weak solution saturation temperature. Heat exchanger analyses (LMTD and ε-NTU) were done to determine the heat transfer characteristics of the desorber. Calculated values of UA are shown to be as high as 5.0 W/K, and desorber heat duties were measured as high as 334 W. Strong solution, at 0.30 mass fraction, was desorbed into weak solution and vapor with concentrations ranging from 0.734 to 0.964. Circulation ratios, defined as strong solution mass flow rate per unit desorbed vapor mass flow rate, varied in this study from 3.4 to 20. A method for specifying desorber operating conditions is described, in which a minimum desorber heat input per unit vapor flow rate is determined at an optimum circulation ratio. A description of how the circulation ratio behaves as a function of strong solution mass flow rate, oil flow rate, and the maximum temperature difference between oil and ammonia-water solution is shown. Close

• 3. [Article] Flow boiling of lithium bromide-water solution in microchannels, with application to desorption

  Recent studies of flow boiling within microchannels have shown high heat fluxes, leading to successful design and testing of compact heat exchangers, cooling systems for electronics and other high efficiency, ...

  Citation × Citation Citation Abstract Copy Title: Flow boiling of lithium bromide-water solution in microchannels, with application to desorption Author: Fackrell, Kurt Recent studies of flow boiling within microchannels have shown high heat fluxes, leading to successful design and testing of compact heat exchangers, cooling systems for electronics and other high efficiency, miniature components. An experimental study of flow boiling of a concentrated salt solution in microchannels is presented. Experiments examined flow in single, circular microchannels, with a length of 25 mm and diameters of 0.127 mm and 0.25 mm, and in arrays of five rectangular microchannels, each with a length of 8.5 mm and a hydraulic diameter of 0.133 mm. Inlet concentrations of lithium bromide were varied from 47% to 57% by mass, heat inputs from 2.2 to 7.0 W per channel, and flow rates from 0.60 to 0.80 g/min per channel. Single channel tests yielded as much as 0.05 g/min of water vapor for the conditions examined. If an array of channels could produce vapor at a similar rate, it may be possible to apply microchannel flow boiling in a compact desorber or evaporator. Unequal flow distribution in the microchannel arrays limited testing to heat input rates too low to verify array desorption. Infrared visualization of flow at the exit of the microchannel was performed to document solution exit condition based on flow rate, heat input rate, and lithium bromide concentration. Visualization identified four basic exit conditions: steady dripping at low heat input rates, unsteady dripping with occasional vapor at moderate heat input rates, continuous droplet spraying at high heat input rates and flow rates, and long period oscillation between unsteady dripping and droplet spraying at high heat input rates and low flow rates. Title: Flow boiling of lithium bromide-water solution in microchannels, with application to desorption Author: Fackrell, Kurt Recent studies of flow boiling within microchannels have shown high heat fluxes, leading to successful design and testing of compact heat exchangers, cooling systems for electronics and other high efficiency, miniature components. An experimental study of flow boiling of a concentrated salt solution in microchannels is presented. Experiments examined flow in single, circular microchannels, with a length of 25 mm and diameters of 0.127 mm and 0.25 mm, and in arrays of five rectangular microchannels, each with a length of 8.5 mm and a hydraulic diameter of 0.133 mm. Inlet concentrations of lithium bromide were varied from 47% to 57% by mass, heat inputs from 2.2 to 7.0 W per channel, and flow rates from 0.60 to 0.80 g/min per channel. Single channel tests yielded as much as 0.05 g/min of water vapor for the conditions examined. If an array of channels could produce vapor at a similar rate, it may be possible to apply microchannel flow boiling in a compact desorber or evaporator. Unequal flow distribution in the microchannel arrays limited testing to heat input rates too low to verify array desorption. Infrared visualization of flow at the exit of the microchannel was performed to document solution exit condition based on flow rate, heat input rate, and lithium bromide concentration. Visualization identified four basic exit conditions: steady dripping at low heat input rates, unsteady dripping with occasional vapor at moderate heat input rates, continuous droplet spraying at high heat input rates and flow rates, and long period oscillation between unsteady dripping and droplet spraying at high heat input rates and low flow rates. Close

• 4. [Article] Experimental study of two-phase gas-liquid flow in a microscale fractal-like branching flow network

  Two-phase gas-liquid flows in microscale fractal-like branching channel flow networks were experimentally studied to assess the validity of existing void fraction correlations and flow regimes based on ...

  Citation × Citation Citation Abstract Copy Title: Experimental study of two-phase gas-liquid flow in a microscale fractal-like branching flow network Author: Kwak, Younghoon Two-phase gas-liquid flows in microscale fractal-like branching channel flow networks were experimentally studied to assess the validity of existing void fraction correlations and flow regimes based on superficial gas and liquid velocities. Void fractions were assessed using two different methods. First, void fraction data were acquired using a High-Speed-High-Resolution (HSHR) camera and computed by area-based two-dimensional image analysis. Void fraction data were also computed using a slip ratio, defined as gas velocity over liquid velocity. Liquid velocity represents the bulk-averaged liquid velocity as determined by microscale particle image velocimetry (micro-PIV). Gas velocity was determined by averaging gas-liquid interface velocities made at the channel centerline. The fractal-like branching channel flow network has five bifurcation levels of different channel widths varying from 400 µm to 100 µm with a fixed channel depth of 250 µm. Each downstream width decreases by 30% whereas the downstream lengths increase by 40%. The total flow length through a single path is approximately 18 mm. Filtered air and deionized water were used as the gas and liquid working fluids, respectively. Mass flow rates of air and water into each k=0 branch were varied from 0.3 g/min to 2.5 g/min and from 5.2x10⁻⁵ g/min to 1.3x10⁻² g/min, respectively. These flow rates yielded superficial air and water velocities through the same branch level between 0.007 m/s and 1.8 m/s and between 0.05 m/s and 0.42 m/s, respectively. For each branching level, due to an increase in flow area, the superficial liquid and gas flow rates change. A two-phase flow regime map was generated for each level of the fractal-like branching flow network and compared to maps developed using the Taitel and Dukler (1976) model and to maps presented in Chung and Kawaji (2004). Flow regime transitions are well predicted with the Taitel and Dukler (1976) model for each branching level. Void fraction assessed using the slip ratio shows very good agreement with the homogeneous void fraction model for all branching levels. On the other hand, void fraction determined by area-based two-dimensional image analysis shows better agreement with the void fraction correlation of Zivi (1964). Title: Experimental study of two-phase gas-liquid flow in a microscale fractal-like branching flow network Author: Kwak, Younghoon Two-phase gas-liquid flows in microscale fractal-like branching channel flow networks were experimentally studied to assess the validity of existing void fraction correlations and flow regimes based on superficial gas and liquid velocities. Void fractions were assessed using two different methods. First, void fraction data were acquired using a High-Speed-High-Resolution (HSHR) camera and computed by area-based two-dimensional image analysis. Void fraction data were also computed using a slip ratio, defined as gas velocity over liquid velocity. Liquid velocity represents the bulk-averaged liquid velocity as determined by microscale particle image velocimetry (micro-PIV). Gas velocity was determined by averaging gas-liquid interface velocities made at the channel centerline. The fractal-like branching channel flow network has five bifurcation levels of different channel widths varying from 400 µm to 100 µm with a fixed channel depth of 250 µm. Each downstream width decreases by 30% whereas the downstream lengths increase by 40%. The total flow length through a single path is approximately 18 mm. Filtered air and deionized water were used as the gas and liquid working fluids, respectively. Mass flow rates of air and water into each k=0 branch were varied from 0.3 g/min to 2.5 g/min and from 5.2x10⁻⁵ g/min to 1.3x10⁻² g/min, respectively. These flow rates yielded superficial air and water velocities through the same branch level between 0.007 m/s and 1.8 m/s and between 0.05 m/s and 0.42 m/s, respectively. For each branching level, due to an increase in flow area, the superficial liquid and gas flow rates change. A two-phase flow regime map was generated for each level of the fractal-like branching flow network and compared to maps developed using the Taitel and Dukler (1976) model and to maps presented in Chung and Kawaji (2004). Flow regime transitions are well predicted with the Taitel and Dukler (1976) model for each branching level. Void fraction assessed using the slip ratio shows very good agreement with the homogeneous void fraction model for all branching levels. On the other hand, void fraction determined by area-based two-dimensional image analysis shows better agreement with the void fraction correlation of Zivi (1964). Close

• 5. [Article] Characterization of flow boiling in a fractal–like branching microchannel network

  Based on experimental investigations the present study evaluates flow instability and void fraction in a fractal-like branching microchannel network with rectangular cross-section. The hydraulic diameter ...

  Citation × Citation Citation Abstract Copy Title: Characterization of flow boiling in a fractal–like branching microchannel network Author: Edward, Lee M. Based on experimental investigations the present study evaluates flow instability and void fraction in a fractal-like branching microchannel network with rectangular cross-section. The hydraulic diameter of the channels ranged from 308μm at the inlet to 143μm at the outlet. The flow network used is characterized by set branching ratios for channel length and width of Li/sqrt(2) and Li*sqrt(2), respectively, and features 5 branching levels. Test were performed using DI degassed water heated to an 88°C inlet temperature with a mass flow rate of 10g/min. Heat fluxes of 1.76 W/cm² and 2.64 W/cm² were applied to the test device for the given flow rate. Finally, a control valve directly upstream of the test device was throttled until the pressure drop across the valve was roughly 100 times larger than the pressure drop across the test device. For the cases with and without throttling at the inlet, results for inlet pressure oscillation and vapor activity at the inlet and exit of the test device are compared. Frequency of inlet pressure oscillations and the upstream flow of vapor are correlated. In addition time averaged void fraction is presented for each branching level for the 10g/min, 1.76 W/cm² case with and without throttling and is compared to predictions from a 1-D model. Title: Characterization of flow boiling in a fractal–like branching microchannel network Author: Edward, Lee M. Based on experimental investigations the present study evaluates flow instability and void fraction in a fractal-like branching microchannel network with rectangular cross-section. The hydraulic diameter of the channels ranged from 308μm at the inlet to 143μm at the outlet. The flow network used is characterized by set branching ratios for channel length and width of Li/sqrt(2) and Li*sqrt(2), respectively, and features 5 branching levels. Test were performed using DI degassed water heated to an 88°C inlet temperature with a mass flow rate of 10g/min. Heat fluxes of 1.76 W/cm² and 2.64 W/cm² were applied to the test device for the given flow rate. Finally, a control valve directly upstream of the test device was throttled until the pressure drop across the valve was roughly 100 times larger than the pressure drop across the test device. For the cases with and without throttling at the inlet, results for inlet pressure oscillation and vapor activity at the inlet and exit of the test device are compared. Frequency of inlet pressure oscillations and the upstream flow of vapor are correlated. In addition time averaged void fraction is presented for each branching level for the 10g/min, 1.76 W/cm² case with and without throttling and is compared to predictions from a 1-D model. Close

• 6. [Article] Microscale thermal management utilizing vapor extraction from a fractal-like branching heat sink

  It is hypothesized that increases in pressure drop due to vapor generation during boiling in microchannels can be reduced by extraction of vapor at its point of inception. Ultimately, this local vapor ...

  Citation × Citation Citation Abstract Copy Title: Microscale thermal management utilizing vapor extraction from a fractal-like branching heat sink Author: Apreotesi, Mario A. It is hypothesized that increases in pressure drop due to vapor generation during boiling in microchannels can be reduced by extraction of vapor at its point of inception. Ultimately, this local vapor extraction decreases the pressure drop required to drive the flow through a fractal-like branching, microchannel flow network within the heat sink. Indeed, by lowering the overall flow rate by vapor extraction the pressure drop can, in principle, be lowered to that of single-phase flow. In the current study, the feasibility of vapor extraction and its influence on the pressure drop across the microchannels are investigated. The concept also has the potential to separate flow independent of heat sink orientation or the influence of gravity. The fractal-like flow network used here is one that has been previously shown to reduce pressure drop and yield a more uniform surface temperature distribution for single-phase flows than that observed in parallel microchannel flow networks. The disk shaped heat sink was covered with two porous Nylon membranes with an average pore size of 0.45 microns that were backed with a porous aluminum block. Deionized, degassed water was used as the working fluid. A theoretical model was developed to predict the pressure drop across the flow network as a function of inlet mass flow rate, heat input, and pressure difference driving extraction across the membranes. Results of the model predictions are presented and discussed in both dimensional and non-dimensional format. Model predictions were used to discuss the trends and physical implications of local vapor extraction. It was observed that conditions existed where further increases in the membrane pressure difference had no influence vapor extraction and network pressure drop values. Experimental data was also collected and analyzed in order to assess the validity of the theoretical model. The predictive model and experimental data indicated that the two-phase network pressure drop is reduced by locally extracting vapor from the heat sink. The network pressure drop was reduced by as much as 80% theoretically and 50% experimentally in the predictive model and experimental data, respectively, for similar test conditions. A sensitivity analysis was performed on several parameters used in the model in an attempt to help explain some of the discrepancies observed between the experimental results and model predictions. Suggestions are made to improve the theoretical model pressure predictions and to investigate the influence of vapor extraction on the heat transfer coefficient values. Title: Microscale thermal management utilizing vapor extraction from a fractal-like branching heat sink Author: Apreotesi, Mario A. It is hypothesized that increases in pressure drop due to vapor generation during boiling in microchannels can be reduced by extraction of vapor at its point of inception. Ultimately, this local vapor extraction decreases the pressure drop required to drive the flow through a fractal-like branching, microchannel flow network within the heat sink. Indeed, by lowering the overall flow rate by vapor extraction the pressure drop can, in principle, be lowered to that of single-phase flow. In the current study, the feasibility of vapor extraction and its influence on the pressure drop across the microchannels are investigated. The concept also has the potential to separate flow independent of heat sink orientation or the influence of gravity. The fractal-like flow network used here is one that has been previously shown to reduce pressure drop and yield a more uniform surface temperature distribution for single-phase flows than that observed in parallel microchannel flow networks. The disk shaped heat sink was covered with two porous Nylon membranes with an average pore size of 0.45 microns that were backed with a porous aluminum block. Deionized, degassed water was used as the working fluid. A theoretical model was developed to predict the pressure drop across the flow network as a function of inlet mass flow rate, heat input, and pressure difference driving extraction across the membranes. Results of the model predictions are presented and discussed in both dimensional and non-dimensional format. Model predictions were used to discuss the trends and physical implications of local vapor extraction. It was observed that conditions existed where further increases in the membrane pressure difference had no influence vapor extraction and network pressure drop values. Experimental data was also collected and analyzed in order to assess the validity of the theoretical model. The predictive model and experimental data indicated that the two-phase network pressure drop is reduced by locally extracting vapor from the heat sink. The network pressure drop was reduced by as much as 80% theoretically and 50% experimentally in the predictive model and experimental data, respectively, for similar test conditions. A sensitivity analysis was performed on several parameters used in the model in an attempt to help explain some of the discrepancies observed between the experimental results and model predictions. Suggestions are made to improve the theoretical model pressure predictions and to investigate the influence of vapor extraction on the heat transfer coefficient values. Close

• 7. [Article] Design of a single lamina for a microchannel dialyzer with a focus on bubble removal

  A microchannel-based hemodialyzer offers a novel approach to hemodialysis practice and holds many promises for improving dialysis treatment efficiency and quality of life for kidney patients. The hollow ...

  Citation × Citation Citation Abstract Copy Title: Design of a single lamina for a microchannel dialyzer with a focus on bubble removal Author: Mohammadi, Mahshid A microchannel-based hemodialyzer offers a novel approach to hemodialysis practice and holds many promises for improving dialysis treatment efficiency and quality of life for kidney patients. The hollow fiber hemodialyzer, a conventional dialysis device, has certain limitations including non-uniformity of the dialysate flow path which necessitates the use of a high dialysate flow rate. In the microchannel-based design, successive stacked layers alternate between blood flow and dialysate flow. A flat porous membrane between these layers allows for the transport of toxins from the blood side to the dialysis fluid side. This design improves mass transfer characteristics and enables the use of lower dialysate to blood flow rate ratios suitable for a home hemodialysis system. One of the current issues in microchannel dialyzer is the emergence of bubbles through blood-outgassing and air ingress. This dissertation provides an in depth study of the bubble pinning phenomenon and the impact of the geometrical design of a single lamina of the microchannel dialyzer on bubble removal. Our primary method to facilitate bubble mitigation in the system is increasing the surface hydrophilicity by applying polyethylene oxide (PEO) coating on internal surface of polycarbonate laminas. The effects of contact angle hysteresis and contact line (pinning) forces as well as the dewetting velocity on bubble stagnation have been studied. A theoretical model that estimates the required pressure drop along the length of a stagnant bubble to overcome pinning forces to stimulate motion has been developed. The model has been validated by experimental results. The model demonstrates that the use of highly hydrophilic surfaces is a robust approach to reducing both pinning and capillary forces and subsequently bubble clogging in microfluidic systems. The geometrical design must exhibit low resistance for bubble mitigation while maintaining a relatively uniform flow distribution between channels for efficient mass transfer. A single lamina of the microchannel dialyzer under development at OSU is comprised of an inlet manifold, an outlet manifold and a parallel microchannel array; the manifolds feature a micropost arrangement that supports the membrane. The overall manifold geometry was designed for uniform flow distribution while the suggested arrangement of microposts inside the manifolds was aimed at exhibiting low bubble retention. For all the experiments in this dissertation, water has been used as the working fluid; however, for the selection of the microchannel size and velocity conditions, considerations for preventing blood hemolysis under shear stress were taken into account. Furthermore, the anticipated differences between an air-water system and an air-blood system have been discussed. Title: Design of a single lamina for a microchannel dialyzer with a focus on bubble removal Author: Mohammadi, Mahshid A microchannel-based hemodialyzer offers a novel approach to hemodialysis practice and holds many promises for improving dialysis treatment efficiency and quality of life for kidney patients. The hollow fiber hemodialyzer, a conventional dialysis device, has certain limitations including non-uniformity of the dialysate flow path which necessitates the use of a high dialysate flow rate. In the microchannel-based design, successive stacked layers alternate between blood flow and dialysate flow. A flat porous membrane between these layers allows for the transport of toxins from the blood side to the dialysis fluid side. This design improves mass transfer characteristics and enables the use of lower dialysate to blood flow rate ratios suitable for a home hemodialysis system. One of the current issues in microchannel dialyzer is the emergence of bubbles through blood-outgassing and air ingress. This dissertation provides an in depth study of the bubble pinning phenomenon and the impact of the geometrical design of a single lamina of the microchannel dialyzer on bubble removal. Our primary method to facilitate bubble mitigation in the system is increasing the surface hydrophilicity by applying polyethylene oxide (PEO) coating on internal surface of polycarbonate laminas. The effects of contact angle hysteresis and contact line (pinning) forces as well as the dewetting velocity on bubble stagnation have been studied. A theoretical model that estimates the required pressure drop along the length of a stagnant bubble to overcome pinning forces to stimulate motion has been developed. The model has been validated by experimental results. The model demonstrates that the use of highly hydrophilic surfaces is a robust approach to reducing both pinning and capillary forces and subsequently bubble clogging in microfluidic systems. The geometrical design must exhibit low resistance for bubble mitigation while maintaining a relatively uniform flow distribution between channels for efficient mass transfer. A single lamina of the microchannel dialyzer under development at OSU is comprised of an inlet manifold, an outlet manifold and a parallel microchannel array; the manifolds feature a micropost arrangement that supports the membrane. The overall manifold geometry was designed for uniform flow distribution while the suggested arrangement of microposts inside the manifolds was aimed at exhibiting low bubble retention. For all the experiments in this dissertation, water has been used as the working fluid; however, for the selection of the microchannel size and velocity conditions, considerations for preventing blood hemolysis under shear stress were taken into account. Furthermore, the anticipated differences between an air-water system and an air-blood system have been discussed. Close

• 8. [Article] Development of a microchannel device for adsorption cooling application

  Single-layer and three-layer microchannel adsorption devices were fabricated by procedure that was newly developed. Aluminum was chosen for metal housing while the adsorption pair was silica gel and water. ...

  Citation × Citation Citation Abstract Copy Title: Development of a microchannel device for adsorption cooling application Author: Asumpinpong, Kasidid Single-layer and three-layer microchannel adsorption devices were fabricated by procedure that was newly developed. Aluminum was chosen for metal housing while the adsorption pair was silica gel and water. Each adsorption layer contained two layers of 250 μm-thick silica gel. Both silica gel layers opened to the same layer of adsorption channels. The layers of air flow channels were placed between two adsorption layers, including over the top and below the bottom adsorption layers. Each layer of adsorption channels and air flow channels contained array of six microchannels. Each microchannel was 400 μm tall, 5.8 mm wide and 4.4 cm long. The device was bonded by B-staged epoxy thin film at moderate temperature of 120°C. The overall dimensions of the fabricated device were 5 cm x 5 cm x 10.2 mm for the three-layer device. A maximum coefficient of performance (COP) of 0.506 was achieved at the evaporator temperature of 22°C, adsorption temperature of 23°C and desorption temperature of 50°C. The highest specific cooling power (SCP) obtained was 601 W/kg silica gel when desorption temperature was increased to 70°C The overall performance of the device improved slightly when the air flow rate was increased. The COP increased by a factor of 2.4 when the number of adsorption layers increased from 1 to 3. The results from simulation model agreed with the experimental results and also showed that both COP and SCP decreased drastically when evaporator temperature was far below the adsorption temperature. Title: Development of a microchannel device for adsorption cooling application Author: Asumpinpong, Kasidid Single-layer and three-layer microchannel adsorption devices were fabricated by procedure that was newly developed. Aluminum was chosen for metal housing while the adsorption pair was silica gel and water. Each adsorption layer contained two layers of 250 μm-thick silica gel. Both silica gel layers opened to the same layer of adsorption channels. The layers of air flow channels were placed between two adsorption layers, including over the top and below the bottom adsorption layers. Each layer of adsorption channels and air flow channels contained array of six microchannels. Each microchannel was 400 μm tall, 5.8 mm wide and 4.4 cm long. The device was bonded by B-staged epoxy thin film at moderate temperature of 120°C. The overall dimensions of the fabricated device were 5 cm x 5 cm x 10.2 mm for the three-layer device. A maximum coefficient of performance (COP) of 0.506 was achieved at the evaporator temperature of 22°C, adsorption temperature of 23°C and desorption temperature of 50°C. The highest specific cooling power (SCP) obtained was 601 W/kg silica gel when desorption temperature was increased to 70°C The overall performance of the device improved slightly when the air flow rate was increased. The COP increased by a factor of 2.4 when the number of adsorption layers increased from 1 to 3. The results from simulation model agreed with the experimental results and also showed that both COP and SCP decreased drastically when evaporator temperature was far below the adsorption temperature. Close

• 9. [Article] Study of a constrained-film bubble absorber under cycle operating conditions

  An experimental and numerical study of absorption of ammonia vapor bubbles into a constrained thin film of ammonia-water solution is presented in the context of a potential reduction in size of a heat-actuated ...

  Citation × Citation Citation Abstract Copy Title: Study of a constrained-film bubble absorber under cycle operating conditions Author: Cardenas, Ruander An experimental and numerical study of absorption of ammonia vapor bubbles into a constrained thin film of ammonia-water solution is presented in the context of a potential reduction in size of a heat-actuated heat pump component. A large-aspect-ratio channel with a depth of 600 µm restricts the thickness of the weak solution film, while ammonia vapor bubbles are injected from a porous wall. Experiments are performed at a nominal system pressure of 6.2 bar absolute and at an inlet weak solution temperature of 75ºC. A counter-flowing coolant in a minichannel removes the generated heat of absorption. The mass flow rate of the weak solution, vapor flow rate, coolant inlet temperature, and mass flow rate of the coolant solution are varied. Two absorber channel geometries are considered: 1) a smooth 600 µm channel, and 2) a stepped geometry that has 2-mm deep trenches across the width of one of the channel walls. The 1-D, steady state species and energy transport equations, are solved for the smooth-channel absorber to yield, along the length of the channel, concentration and temperature profiles of the solution stream and the temperature profile of the coolant fluid stream. Experimental results indicate that overall heat transfer coefficients vary from 700 W/m²-K to 2,300 W/m²-K, while the mass transfer conductances range from 0.024 kg/s-m² to 0.24 kg/s-m². The coolant inlet temperature has a significant effect on the mass transfer rates. At the highest inlet coolant temperature of 58ºC, up to 1.5 g/min of vapor is at best absorbed into 35 g/min of weak solution for the smooth absorber plate, while at the lowest coolant temperature of 30ºC, up tp 3 g/min of vapor is absorbed in 35 g/min of weak solution with the same absorber plate. For the stepped absorber geometry, only 1 g/min of vapor is absorbed into 35 g/min of weak solution for the highest coolant temperature of 58ºC, while for the lowest coolant temperature of 30ºC, 5 g/min of vapor is absorbed in 40 g/min of weak solution. Trends of local variation of temperature and convected vapor from the numerical parametric study complement experimental results and provide further insight into the performance of the absorber. Based on the experimental results, a preliminary size estimate for the absorber to operate in a cooling cycle with a 6 kW evaporator load is provided. Other considerations such as strong solution exit subcooling and porous plate pressure drop are also addressed. Title: Study of a constrained-film bubble absorber under cycle operating conditions Author: Cardenas, Ruander An experimental and numerical study of absorption of ammonia vapor bubbles into a constrained thin film of ammonia-water solution is presented in the context of a potential reduction in size of a heat-actuated heat pump component. A large-aspect-ratio channel with a depth of 600 µm restricts the thickness of the weak solution film, while ammonia vapor bubbles are injected from a porous wall. Experiments are performed at a nominal system pressure of 6.2 bar absolute and at an inlet weak solution temperature of 75ºC. A counter-flowing coolant in a minichannel removes the generated heat of absorption. The mass flow rate of the weak solution, vapor flow rate, coolant inlet temperature, and mass flow rate of the coolant solution are varied. Two absorber channel geometries are considered: 1) a smooth 600 µm channel, and 2) a stepped geometry that has 2-mm deep trenches across the width of one of the channel walls. The 1-D, steady state species and energy transport equations, are solved for the smooth-channel absorber to yield, along the length of the channel, concentration and temperature profiles of the solution stream and the temperature profile of the coolant fluid stream. Experimental results indicate that overall heat transfer coefficients vary from 700 W/m²-K to 2,300 W/m²-K, while the mass transfer conductances range from 0.024 kg/s-m² to 0.24 kg/s-m². The coolant inlet temperature has a significant effect on the mass transfer rates. At the highest inlet coolant temperature of 58ºC, up to 1.5 g/min of vapor is at best absorbed into 35 g/min of weak solution for the smooth absorber plate, while at the lowest coolant temperature of 30ºC, up tp 3 g/min of vapor is absorbed in 35 g/min of weak solution with the same absorber plate. For the stepped absorber geometry, only 1 g/min of vapor is absorbed into 35 g/min of weak solution for the highest coolant temperature of 58ºC, while for the lowest coolant temperature of 30ºC, 5 g/min of vapor is absorbed in 40 g/min of weak solution. Trends of local variation of temperature and convected vapor from the numerical parametric study complement experimental results and provide further insight into the performance of the absorber. Based on the experimental results, a preliminary size estimate for the absorber to operate in a cooling cycle with a 6 kW evaporator load is provided. Other considerations such as strong solution exit subcooling and porous plate pressure drop are also addressed. Close

• 10. [Article] Adapting the Methylene Blue Sulfide Detection Method for Use in a Microfluidic in-situ Monitoring Device

  Sulfide is an important species to measure in environmental water samples. The spectrophotometric detection method for reduced sulfur species (sulfide) based on generation of methylene blue was adapted ...

  Citation × Citation Citation Abstract Copy Title: Adapting the Methylene Blue Sulfide Detection Method for Use in a Microfluidic in-situ Monitoring Device Author: Monares, Shane W. Sulfide is an important species to measure in environmental water samples. The spectrophotometric detection method for reduced sulfur species (sulfide) based on generation of methylene blue was adapted to work in an apparatus utilizing microfluidic PDMS mixing and observation cells with flow channels in the 100 μm range. The benefits of the system include the use of low volumes of reagents and sample, generation of low volumes of waste, and the possibility of adaptation to in-situ measurements with a submergible, microfluidic sensing device. The calibration equation for the detection of sulfide with this method is [S(-II)] = (Absorbance – 0.0147)/0.0045, where the sulfide concentration is in micromolar. The detection limit for this method was found to be 1 μM, which is comparable to conventional methods. Topics explored include: sulfide standard stability and protection from loss due to oxidation and volatilization, preparation and stability of a single reagent with multiple components, microfluidic apparatus development, and the calibration of the method for sulfide concentrations. Title: Adapting the Methylene Blue Sulfide Detection Method for Use in a Microfluidic in-situ Monitoring Device Author: Monares, Shane W. Sulfide is an important species to measure in environmental water samples. The spectrophotometric detection method for reduced sulfur species (sulfide) based on generation of methylene blue was adapted to work in an apparatus utilizing microfluidic PDMS mixing and observation cells with flow channels in the 100 μm range. The benefits of the system include the use of low volumes of reagents and sample, generation of low volumes of waste, and the possibility of adaptation to in-situ measurements with a submergible, microfluidic sensing device. The calibration equation for the detection of sulfide with this method is [S(-II)] = (Absorbance – 0.0147)/0.0045, where the sulfide concentration is in micromolar. The detection limit for this method was found to be 1 μM, which is comparable to conventional methods. Topics explored include: sulfide standard stability and protection from loss due to oxidation and volatilization, preparation and stability of a single reagent with multiple components, microfluidic apparatus development, and the calibration of the method for sulfide concentrations. Close