Spacecraft thermal control is entirely reliant upon radiative heat transfer for temperature regul... more Spacecraft thermal control is entirely reliant upon radiative heat transfer for temperature regulation. Current methods are often static in nature and do not provide dynamic control of radiative heat transfer. As a result, modern spacecraft thermal control systems are typically 'cold-biased' with radiators that are larger than necessary for many operating conditions. Deploying a variable radiator as a thermal control technique in which the projected surface area can be adjusted to provide the appropriate heat loss for a given condition can reduce unnecessary heat rejection and reduce power requirements. However, the radiative behavior of the apparent surface representing the expanding/collapsing radiator changes in addition to the projected surface area size. This work experimentally quantifies the spectral, directional emissivity of an apparent surface comprised of a series of V-grooves (e.g. corrugated surface), as a function of angle and highlights its emission characteristics that trend toward black behavior. The experimental setup for quantifying this apparent radiative surface behavior is described and utilized to show the influence of surface geometry, direction and wavelength. The experimental design is validated and demonstrated using fully oxidized, nearly diffuse, copper, corrugated test samples. The results presented in this work demonstrate, for the corrugated oxidized copper surfaces tested, that (1) higher emissivity values correspond to higher wavelengths in the spectral range of 2.5 to 15.4 μm (2) apparent emissivity values increase with decreasing V-groove angle resulting in less spectral variation in emissivity and greater blackbody like behavior, (3) azimuth dependence can be relatively small despite the obvious pattern associated with a corrugated surface, (4) as the V-groove angle decreases, higher emissivity values are associated with → 0° and → 90°. Results provide a foundation for future radiator design, improved spacecraft thermal control methods, and improved emissivity testing methods for patterned or angular surfaces.
This article presents a method of simulating molecular transport in capillary gas chromatography ... more This article presents a method of simulating molecular transport in capillary gas chromatography (GC) applicable to isothermal, temperature-programmed, and thermal gradient conditions. The approach accounts for parameter differences that can occur across an analyte band including pressure, mobile phase velocity, temperature, and retention factor. The model was validated experimentally using a GC column comprised of microchannels in a stainless-steel plate capable of isothermal, temperature-programmed, and thermal gradient GC separations. The parameters governing retention and dispersion in the transport model were fitted with 12 experimental isothermal separations. The transport model was validated with experimental data for three analytes using four temperature-programmed and three thermal gradient GC separations. The simulated peaks (elution time and dispersion) give reasonable predictions of observed separations. The magnitudes of the maximum error between simulated peak elution time and experiment were 2.6 and 4.2% for temperature-programmed and thermal gradient GC, respectively. The magnitudes of the maximum error between the simulated peak width and experiment were 15.4 and 5.8% for temperature-programmed and thermal gradient GC, respectively. These relatively low errors give confidence that the model reflects the behavior of the transport processes and provides meaningful predictions for GC separations. This transport model allows for an evaluation of analyte separation characteristics of the analyte band at any position along the length of the GC column in addition to peak characteristics at the column exit. The transport model enables investigation of column conditions that influence separation behavior and opens exploration of optimal column design and heating conditions.
Bulletin of the American Physical Society, Nov 23, 2015
Submitted for the DFD15 Meeting of The American Physical Society Inertial effects on heat transfe... more Submitted for the DFD15 Meeting of The American Physical Society Inertial effects on heat transfer in superhydrophobic microchannels 1 ADAM COWLEY, DANIEL MAYNES, JULIE CROCKETT, BRIAN IVERSON, Brigham Young University, BYU FLUIDS TEAM-This work numerically studies the effects of inertia on thermal transport in superhydrophbic microchannels. An infinite parallel plate channel comprised of structured superhydrophbic walls is considered. The structure of the superhydrophobic surfaces consists of square pillars organized in a square array aligned with the flow direction. Laminar, fully developed flow is explored. The flow is assumed to be non-wetting and have an idealized flat meniscus. A shear-free, adiabatic boundary condition is used at the liquid/gas interface, while a no-slip, constant heat flux condition is used at the liquid/solid interface. A wide range of Peclet numbers, relative channel spacing distances, and relative pillar sizes are considered. Results are presented in terms of Poiseuille number, Nusselt number, hydrodynamic slip length, and temperature jump length. Interestingly, the thermal transport is varied only slightly by inertial effects for a wide range of parameters explored and compares well with other analytical and numerical work that assumed Stokes flow. It is only for very small relative channel spacing and large Peclet number that inertial effects exert significant influence. Overall, the heat transfer is reduced for the superhydrophbic channels in comparison to classic smooth walled channels.
This work investigates the electrochemical dynamics and performance of additively manufactured co... more This work investigates the electrochemical dynamics and performance of additively manufactured composite electrolytes for resistive switching. Devices are comprised of a Ag/AgI-Al2O3/Pt stack, where the solid state electrolyte is additively manufactured using extrusion techniques. AgI-Al2O3 composite electrolytes are characterized by X-ray diffraction and electrochemical impedance spectroscopy. The ionic conductivities of the electrolytes were measured for different concentrations of Al2O3, observing a maximum conductivity of 4.5 times the conductivity of pure AgI for composites with 20 mol% Al2O3. There was little change in activation energy with the addition of Al2O3. Setting the Ag layer as the reference electrode and the Pt layer as the working electrode, a high conductivity state was achieved by applying a positive voltage to electrochemically establish an electrically conducting Ag filament within the solid state AgI-Al2O3 electrolyte. The low conductivity state was restored by reversing this applied voltage to electrochemically etch the newly grown Ag filament. Pure AgI devices switch between specific electrical resistivity states that are separated by five orders of magnitude in electrical conductivity. Endurance tests find that the AgI resistive switches can transition between a low and high electrical conductivity state over 8,500 times. Composite AgI-Al2O3 resistive switches formed initial Ag filaments significantly faster and also demonstrated two orders of magnitude separation in resistivity when cycling for 1,600 cycles.
International Journal of Heat and Mass Transfer, Mar 1, 2023
Spacecraft thermal control is entirely reliant upon radiative heat transfer for temperature regul... more Spacecraft thermal control is entirely reliant upon radiative heat transfer for temperature regulation. Current methods are often static in nature and do not provide dynamic control of radiative heat transfer. As a result, modern spacecraft thermal control systems are typically 'cold-biased' with radiators that are larger than necessary for many operating conditions. Deploying a variable radiator as a thermal control technique in which the projected surface area can be adjusted to provide the appropriate heat loss for a given condition can reduce unnecessary heat rejection and reduce power requirements. However, the radiative behavior of the apparent surface representing the expanding/collapsing radiator changes in addition to the projected surface area size. This work experimentally quantifies the spectral, directional emissivity of an apparent surface comprised of a series of V-grooves (e.g. corrugated surface), as a function of angle and highlights its emission characteristics that trend toward black behavior. The experimental setup for quantifying this apparent radiative surface behavior is described and utilized to show the influence of surface geometry, direction and wavelength. The experimental design is validated and demonstrated using fully oxidized, nearly diffuse, copper, corrugated test samples. The results presented in this work demonstrate, for the corrugated oxidized copper surfaces tested, that (1) higher emissivity values correspond to higher wavelengths in the spectral range of 2.5 to 15.4 μm (2) apparent emissivity values increase with decreasing V-groove angle resulting in less spectral variation in emissivity and greater blackbody like behavior, (3) azimuth dependence can be relatively small despite the obvious pattern associated with a corrugated surface, (4) as the V-groove angle decreases, higher emissivity values are associated with → 0° and → 90°. Results provide a foundation for future radiator design, improved spacecraft thermal control methods, and improved emissivity testing methods for patterned or angular surfaces.
Increased sensor sensitivity is important for enabling detection of low concentrations of target ... more Increased sensor sensitivity is important for enabling detection of low concentrations of target analyte. Being able to quickly and accurately detect low concentrations of proteins at point-of-care allows for results to be analyzed more easily and effectively. Vertically-aligned carbon nanotubes (VACNTs) were patterned into interdigitated electrodes (IDEs) and then functionalized with the representative protein streptavidin that was functionalized on the VACNT surfaces by covalent bonding (with EDC/NHS). Further detection was observed by binding biotin to the streptavidin. Fluorescence microscopy enabled the optimization of protein loading on VACNTs. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the electrodes and monitor the associated changes with the addition of streptavidin and biotin. A unique change in impedance is observed, which allows for monitoring the quantity of target analyte bound to the VACNT electrode surface with an observed 14x increase in resistance.
Increased sensitivity of electrochemical sensors is important for detection of low analyte concen... more Increased sensitivity of electrochemical sensors is important for detection of low analyte concentrations. A unique flow-through sensor is demonstrated by depositing nanostructured platinum catalyst onto high aspect ratio, porous membranes of vertically-aligned carbon nanotubes (CNTs). Convective mass transfer enhancement was shown to improve the platinumnanowire-coated CNT (PN-CNT) sensor performance in amperometric sensing of hydrogen peroxide (H2O2). Over 90% of the H2O2 was oxidized as it passed through the PN-CNT sensor, even for low concentrations in the range of 50 nM to 500 µM. This effective utilization of the analyte in detection demonstrates the utility of exploiting convection in electrochemical sensing. At a 100 µL s-1 flow rate, a sensitivity of 24,300 µA mM-1 cm-2 was achieved based on the frontal projected area, with a 0.03 µM limit of detection and a linear sensing range of 0.03-500 µM. Glucose oxidase was also functionalized unto the surfaces of PN-CNT sensors by polymer entrapment to enable detection of low glucose concentrations.
Of the mechanisms to improve efficiency for solar-thermal power plants, one of the most effective... more Of the mechanisms to improve efficiency for solar-thermal power plants, one of the most effective ways to improve overall efficiency is through power cycle improvements. As increases in operating temperature continue to be pursued, supercritical CO₂ Brayton cycles begin to look more attractive despite the development costs of this technology. Further, supercritical CO₂ Brayton has application in many areas of power generation beyond that for solar energy alone. One challenge particular to solar-thermal power generation is the transient nature of the solar resource. This work illustrates the behavior of developmental Brayton turbomachinery in response to a fluctuating thermal input, much like the short-term transients experienced in solar environments. Thermal input to the cycle was cut by 50% and 100% for short durations while the system power and conditions were monitored. It has been shown that despite these fluctuations, the thermal mass in the system effectively enables the Brayton cycle to continue to run for short periods until the thermal input can recover. For systems where significant thermal energy storage is included in the plant design, these transients can be mitigated by storage; a comparison of short- and long-term storage approaches on system efficiency is provided. Also, included in this work is a data set for stable supercritical CO₂ Brayton cycle operation that is used to benchmark computer modeling. With a benchmarked model, specific improvements to the cycle are interrogated to identify the resulting impact on cycle efficiency and loss mechanisms. Status of key issues remaining to be addressed for adoption of supercritical CO₂ Brayton cycles in solar-thermal systems is provided in an effort to expose areas of necessary research.
Bulletin of the American Physical Society, Nov 20, 2017
Submitted for the DFD17 Meeting of The American Physical Society The effect of channel height on ... more Submitted for the DFD17 Meeting of The American Physical Society The effect of channel height on bubble nucleation in superhydrophobic microchannels due to subcritical heating 1 ADAM COWLEY, DANIEL MAYNES, JULIE CROCKETT, BRIAN IVERSON, Brigham Young Univ-Provo-This work experimentally investigates the effects of heating on laminar flow in high aspect ratio superhydrophobic (SH) microchannels. When water that is saturated with dissolved air is used, the unwetted cavities of the SH surfaces act as nucleation sites and air effervesces out of solution onto the surfaces. The microchannels consist of a rib/cavity structured SH surface, that is heated, and a glass surface that is utilized for flow visualization. Two channel heights of nominally 183 and 366 µm are considered. The friction factor-Reynolds product (fRe) is obtained via pressure drop and volumetric flow rate measurements and the temperature profile along the channel is obtained via thermocouples embedded in an aluminum block below the SH surface. Five surface types/configurations are investigated: smooth hydrophilic, smooth hydrophobic, SH with ribs perpendicular to the flow, SH with ribs parallel to the flow, and SH with both ribs parallel to the flow and sparse ribs perpendicular to the flow. Depending on the surface type/configuration, large bubbles can form and adversely affect fRe and lead to higher temperatures along the channel. Once bubbles grow large enough, they are expelled from the channel. The channel size greatly effects the residence time of the bubbles and consequently fRe and the channel temperature.
Condensation heat transfer is significant in many applications such as such as desalination, ener... more Condensation heat transfer is significant in many applications such as such as desalination, energy conversion [1], atmospheric water harvesting [2, 3], electronics cooling, and other high heat flux applications [4]. However, condensate on the surface adds a thermal resistance that limits condensation rates. The rate of condensation heat transfer is inversely proportional to the diameter of the condensate drops [5]. In industrial condensing systems, the resistance is minimized by removing the condensate via gravity or a vapor shear, but the minimum size of droplet removal is typically on the order of the capillary length of the condensate, about 2.7 mm for water.
International Journal of Heat and Mass Transfer, 2019
This work experimentally explores sub-boiling pool nucleation on micro-structured superhydrophobi... more This work experimentally explores sub-boiling pool nucleation on micro-structured superhydrophobic surfaces. All surfaces tested were submerged in a 20 mm deep pool of water and heated from below to maintain a constant surface temperature, while the side walls of the pool were insulated, and the top was covered. Three thermocouples positioned in the pool obtain the average pool temperature. A heat flux sensor is placed directly beneath the surface to measure the heat flux supplied to the pool. Free convection heat transfer coefficients are obtained for the sub-boiling temperature range of 40-90 • C. Six surface types are studied: smooth hydrophilic, smooth hydrophobic, superhydrophobic with rib/cavity structures, superhydrophobic with rib/cavity structures and additional sparsely spaced ribs to close off the cavities, circular posts, and circular holes. It is found that structured superhydrophobic surfaces provide cavities for nucleation to occur. More dissolved air effervesces from the water as the surface temperature increases due to an increased level of supersaturation and convection. The nucleation leads to large air bubble formations that reduce the overall convection coefficient when compared to the smooth surfaces. For the rib/cavity structured surfaces, the bubbles form in an anisotropic manner and are aligned with the surface structure. More bubbles are observed on the superhydrophobic surfaces where the cavities are bounded. Since water's ability to dissolve air is dependent on temperature, heat and mass transfer cannot be treated independently on any of the superhydrophobic surfaces studied here.
Piezoelectric fans have emerged as a viable alternative for electronics cooling applications requ... more Piezoelectric fans have emerged as a viable alternative for electronics cooling applications requiring low input power and noiseless operation. A piezoelectric fan is a cantilever actuated by a piezoelectric ceramic material bonded to it. The fan oscillates back and forth creating airflow when an alternating electric field is applied to this bonded piezoelectric ceramic. Forced convection induced by such an oscillating fan in an enclosure is numerically investigated. The computational model is capable of sustaining deforming fluid cells that allow large boundary movement. The moving wall boundary, modeled as large-amplitude beam deflection, initiates flow in the fluid domain which enhances convection to varying extents depending on the heat source-to-fan distance and beam deflection amplitude. The effects of these parameters on heat transfer are studied. Transition between distinct convection patterns is observed with changes in the parameters. Results are validated against experimental measurements, with good agreement.
Miniaturization of gas chromatography (GC) instrumentation is of interest because it addresses cu... more Miniaturization of gas chromatography (GC) instrumentation is of interest because it addresses current and future issues relating to compactness, portability and field application. While incremental advancements continue to be reported in microchip GC, the current performance is far from acceptable. This lower performance compared to conventional GC is due to factors such as pooling of the stationary phase in corners of non-cylindrical channels, adsorption of sensitive compounds on incompletely deactivated surfaces, shorter column lengths and less than optimum interfacing to injector and detector. In this work, a microchip GC system was developed that solves the latter challenge, i.e., microchip interfacing to injector and detector. A microchip compression clamp was constructed that seals injector and detector fused silica interface tubing to inlet and outlet ports of the microchip channels with minimum extra-column dead volume, and that allows routine operation at least up to 300 ºC. The compression clamp was constructed of a low expansion alloy, Kovar™, to minimize leaking due to thermal expansion mismatch at the interface during repeated thermal cycling. A 5.9 m channel with a cross-section that approximately matches a 100 µm i.d. cylindrical fused silica column was fabricated in a silicon wafer using wafer bonding and deep reactive ion etching (DRIE) and coated statically with a 1% vinyl, 5% phenyl, 94% methylpolysiloxane stationary phase. High temperature separations of C10-C40 n-alkanes and a commercial diesel sample were demonstrated using the system under both temperature programmed GC (TPGC) and thermal gradient GC (TGGC) conditions. TGGC analysis of a complex essential oil sample was also demonstrated.
Spacecraft thermal control is entirely reliant upon radiative heat transfer for temperature regul... more Spacecraft thermal control is entirely reliant upon radiative heat transfer for temperature regulation. Current methods are often static in nature and do not provide dynamic control of radiative heat transfer. As a result, modern spacecraft thermal control systems are typically 'cold-biased' with radiators that are larger than necessary for many operating conditions. Deploying a variable radiator as a thermal control technique in which the projected surface area can be adjusted to provide the appropriate heat loss for a given condition can reduce unnecessary heat rejection and reduce power requirements. However, the radiative behavior of the apparent surface representing the expanding/collapsing radiator changes in addition to the projected surface area size. This work experimentally quantifies the spectral, directional emissivity of an apparent surface comprised of a series of V-grooves (e.g. corrugated surface), as a function of angle and highlights its emission characteristics that trend toward black behavior. The experimental setup for quantifying this apparent radiative surface behavior is described and utilized to show the influence of surface geometry, direction and wavelength. The experimental design is validated and demonstrated using fully oxidized, nearly diffuse, copper, corrugated test samples. The results presented in this work demonstrate, for the corrugated oxidized copper surfaces tested, that (1) higher emissivity values correspond to higher wavelengths in the spectral range of 2.5 to 15.4 μm (2) apparent emissivity values increase with decreasing V-groove angle resulting in less spectral variation in emissivity and greater blackbody like behavior, (3) azimuth dependence can be relatively small despite the obvious pattern associated with a corrugated surface, (4) as the V-groove angle decreases, higher emissivity values are associated with → 0° and → 90°. Results provide a foundation for future radiator design, improved spacecraft thermal control methods, and improved emissivity testing methods for patterned or angular surfaces.
This article presents a method of simulating molecular transport in capillary gas chromatography ... more This article presents a method of simulating molecular transport in capillary gas chromatography (GC) applicable to isothermal, temperature-programmed, and thermal gradient conditions. The approach accounts for parameter differences that can occur across an analyte band including pressure, mobile phase velocity, temperature, and retention factor. The model was validated experimentally using a GC column comprised of microchannels in a stainless-steel plate capable of isothermal, temperature-programmed, and thermal gradient GC separations. The parameters governing retention and dispersion in the transport model were fitted with 12 experimental isothermal separations. The transport model was validated with experimental data for three analytes using four temperature-programmed and three thermal gradient GC separations. The simulated peaks (elution time and dispersion) give reasonable predictions of observed separations. The magnitudes of the maximum error between simulated peak elution time and experiment were 2.6 and 4.2% for temperature-programmed and thermal gradient GC, respectively. The magnitudes of the maximum error between the simulated peak width and experiment were 15.4 and 5.8% for temperature-programmed and thermal gradient GC, respectively. These relatively low errors give confidence that the model reflects the behavior of the transport processes and provides meaningful predictions for GC separations. This transport model allows for an evaluation of analyte separation characteristics of the analyte band at any position along the length of the GC column in addition to peak characteristics at the column exit. The transport model enables investigation of column conditions that influence separation behavior and opens exploration of optimal column design and heating conditions.
Bulletin of the American Physical Society, Nov 23, 2015
Submitted for the DFD15 Meeting of The American Physical Society Inertial effects on heat transfe... more Submitted for the DFD15 Meeting of The American Physical Society Inertial effects on heat transfer in superhydrophobic microchannels 1 ADAM COWLEY, DANIEL MAYNES, JULIE CROCKETT, BRIAN IVERSON, Brigham Young University, BYU FLUIDS TEAM-This work numerically studies the effects of inertia on thermal transport in superhydrophbic microchannels. An infinite parallel plate channel comprised of structured superhydrophbic walls is considered. The structure of the superhydrophobic surfaces consists of square pillars organized in a square array aligned with the flow direction. Laminar, fully developed flow is explored. The flow is assumed to be non-wetting and have an idealized flat meniscus. A shear-free, adiabatic boundary condition is used at the liquid/gas interface, while a no-slip, constant heat flux condition is used at the liquid/solid interface. A wide range of Peclet numbers, relative channel spacing distances, and relative pillar sizes are considered. Results are presented in terms of Poiseuille number, Nusselt number, hydrodynamic slip length, and temperature jump length. Interestingly, the thermal transport is varied only slightly by inertial effects for a wide range of parameters explored and compares well with other analytical and numerical work that assumed Stokes flow. It is only for very small relative channel spacing and large Peclet number that inertial effects exert significant influence. Overall, the heat transfer is reduced for the superhydrophbic channels in comparison to classic smooth walled channels.
This work investigates the electrochemical dynamics and performance of additively manufactured co... more This work investigates the electrochemical dynamics and performance of additively manufactured composite electrolytes for resistive switching. Devices are comprised of a Ag/AgI-Al2O3/Pt stack, where the solid state electrolyte is additively manufactured using extrusion techniques. AgI-Al2O3 composite electrolytes are characterized by X-ray diffraction and electrochemical impedance spectroscopy. The ionic conductivities of the electrolytes were measured for different concentrations of Al2O3, observing a maximum conductivity of 4.5 times the conductivity of pure AgI for composites with 20 mol% Al2O3. There was little change in activation energy with the addition of Al2O3. Setting the Ag layer as the reference electrode and the Pt layer as the working electrode, a high conductivity state was achieved by applying a positive voltage to electrochemically establish an electrically conducting Ag filament within the solid state AgI-Al2O3 electrolyte. The low conductivity state was restored by reversing this applied voltage to electrochemically etch the newly grown Ag filament. Pure AgI devices switch between specific electrical resistivity states that are separated by five orders of magnitude in electrical conductivity. Endurance tests find that the AgI resistive switches can transition between a low and high electrical conductivity state over 8,500 times. Composite AgI-Al2O3 resistive switches formed initial Ag filaments significantly faster and also demonstrated two orders of magnitude separation in resistivity when cycling for 1,600 cycles.
International Journal of Heat and Mass Transfer, Mar 1, 2023
Spacecraft thermal control is entirely reliant upon radiative heat transfer for temperature regul... more Spacecraft thermal control is entirely reliant upon radiative heat transfer for temperature regulation. Current methods are often static in nature and do not provide dynamic control of radiative heat transfer. As a result, modern spacecraft thermal control systems are typically 'cold-biased' with radiators that are larger than necessary for many operating conditions. Deploying a variable radiator as a thermal control technique in which the projected surface area can be adjusted to provide the appropriate heat loss for a given condition can reduce unnecessary heat rejection and reduce power requirements. However, the radiative behavior of the apparent surface representing the expanding/collapsing radiator changes in addition to the projected surface area size. This work experimentally quantifies the spectral, directional emissivity of an apparent surface comprised of a series of V-grooves (e.g. corrugated surface), as a function of angle and highlights its emission characteristics that trend toward black behavior. The experimental setup for quantifying this apparent radiative surface behavior is described and utilized to show the influence of surface geometry, direction and wavelength. The experimental design is validated and demonstrated using fully oxidized, nearly diffuse, copper, corrugated test samples. The results presented in this work demonstrate, for the corrugated oxidized copper surfaces tested, that (1) higher emissivity values correspond to higher wavelengths in the spectral range of 2.5 to 15.4 μm (2) apparent emissivity values increase with decreasing V-groove angle resulting in less spectral variation in emissivity and greater blackbody like behavior, (3) azimuth dependence can be relatively small despite the obvious pattern associated with a corrugated surface, (4) as the V-groove angle decreases, higher emissivity values are associated with → 0° and → 90°. Results provide a foundation for future radiator design, improved spacecraft thermal control methods, and improved emissivity testing methods for patterned or angular surfaces.
Increased sensor sensitivity is important for enabling detection of low concentrations of target ... more Increased sensor sensitivity is important for enabling detection of low concentrations of target analyte. Being able to quickly and accurately detect low concentrations of proteins at point-of-care allows for results to be analyzed more easily and effectively. Vertically-aligned carbon nanotubes (VACNTs) were patterned into interdigitated electrodes (IDEs) and then functionalized with the representative protein streptavidin that was functionalized on the VACNT surfaces by covalent bonding (with EDC/NHS). Further detection was observed by binding biotin to the streptavidin. Fluorescence microscopy enabled the optimization of protein loading on VACNTs. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the electrodes and monitor the associated changes with the addition of streptavidin and biotin. A unique change in impedance is observed, which allows for monitoring the quantity of target analyte bound to the VACNT electrode surface with an observed 14x increase in resistance.
Increased sensitivity of electrochemical sensors is important for detection of low analyte concen... more Increased sensitivity of electrochemical sensors is important for detection of low analyte concentrations. A unique flow-through sensor is demonstrated by depositing nanostructured platinum catalyst onto high aspect ratio, porous membranes of vertically-aligned carbon nanotubes (CNTs). Convective mass transfer enhancement was shown to improve the platinumnanowire-coated CNT (PN-CNT) sensor performance in amperometric sensing of hydrogen peroxide (H2O2). Over 90% of the H2O2 was oxidized as it passed through the PN-CNT sensor, even for low concentrations in the range of 50 nM to 500 µM. This effective utilization of the analyte in detection demonstrates the utility of exploiting convection in electrochemical sensing. At a 100 µL s-1 flow rate, a sensitivity of 24,300 µA mM-1 cm-2 was achieved based on the frontal projected area, with a 0.03 µM limit of detection and a linear sensing range of 0.03-500 µM. Glucose oxidase was also functionalized unto the surfaces of PN-CNT sensors by polymer entrapment to enable detection of low glucose concentrations.
Of the mechanisms to improve efficiency for solar-thermal power plants, one of the most effective... more Of the mechanisms to improve efficiency for solar-thermal power plants, one of the most effective ways to improve overall efficiency is through power cycle improvements. As increases in operating temperature continue to be pursued, supercritical CO₂ Brayton cycles begin to look more attractive despite the development costs of this technology. Further, supercritical CO₂ Brayton has application in many areas of power generation beyond that for solar energy alone. One challenge particular to solar-thermal power generation is the transient nature of the solar resource. This work illustrates the behavior of developmental Brayton turbomachinery in response to a fluctuating thermal input, much like the short-term transients experienced in solar environments. Thermal input to the cycle was cut by 50% and 100% for short durations while the system power and conditions were monitored. It has been shown that despite these fluctuations, the thermal mass in the system effectively enables the Brayton cycle to continue to run for short periods until the thermal input can recover. For systems where significant thermal energy storage is included in the plant design, these transients can be mitigated by storage; a comparison of short- and long-term storage approaches on system efficiency is provided. Also, included in this work is a data set for stable supercritical CO₂ Brayton cycle operation that is used to benchmark computer modeling. With a benchmarked model, specific improvements to the cycle are interrogated to identify the resulting impact on cycle efficiency and loss mechanisms. Status of key issues remaining to be addressed for adoption of supercritical CO₂ Brayton cycles in solar-thermal systems is provided in an effort to expose areas of necessary research.
Bulletin of the American Physical Society, Nov 20, 2017
Submitted for the DFD17 Meeting of The American Physical Society The effect of channel height on ... more Submitted for the DFD17 Meeting of The American Physical Society The effect of channel height on bubble nucleation in superhydrophobic microchannels due to subcritical heating 1 ADAM COWLEY, DANIEL MAYNES, JULIE CROCKETT, BRIAN IVERSON, Brigham Young Univ-Provo-This work experimentally investigates the effects of heating on laminar flow in high aspect ratio superhydrophobic (SH) microchannels. When water that is saturated with dissolved air is used, the unwetted cavities of the SH surfaces act as nucleation sites and air effervesces out of solution onto the surfaces. The microchannels consist of a rib/cavity structured SH surface, that is heated, and a glass surface that is utilized for flow visualization. Two channel heights of nominally 183 and 366 µm are considered. The friction factor-Reynolds product (fRe) is obtained via pressure drop and volumetric flow rate measurements and the temperature profile along the channel is obtained via thermocouples embedded in an aluminum block below the SH surface. Five surface types/configurations are investigated: smooth hydrophilic, smooth hydrophobic, SH with ribs perpendicular to the flow, SH with ribs parallel to the flow, and SH with both ribs parallel to the flow and sparse ribs perpendicular to the flow. Depending on the surface type/configuration, large bubbles can form and adversely affect fRe and lead to higher temperatures along the channel. Once bubbles grow large enough, they are expelled from the channel. The channel size greatly effects the residence time of the bubbles and consequently fRe and the channel temperature.
Condensation heat transfer is significant in many applications such as such as desalination, ener... more Condensation heat transfer is significant in many applications such as such as desalination, energy conversion [1], atmospheric water harvesting [2, 3], electronics cooling, and other high heat flux applications [4]. However, condensate on the surface adds a thermal resistance that limits condensation rates. The rate of condensation heat transfer is inversely proportional to the diameter of the condensate drops [5]. In industrial condensing systems, the resistance is minimized by removing the condensate via gravity or a vapor shear, but the minimum size of droplet removal is typically on the order of the capillary length of the condensate, about 2.7 mm for water.
International Journal of Heat and Mass Transfer, 2019
This work experimentally explores sub-boiling pool nucleation on micro-structured superhydrophobi... more This work experimentally explores sub-boiling pool nucleation on micro-structured superhydrophobic surfaces. All surfaces tested were submerged in a 20 mm deep pool of water and heated from below to maintain a constant surface temperature, while the side walls of the pool were insulated, and the top was covered. Three thermocouples positioned in the pool obtain the average pool temperature. A heat flux sensor is placed directly beneath the surface to measure the heat flux supplied to the pool. Free convection heat transfer coefficients are obtained for the sub-boiling temperature range of 40-90 • C. Six surface types are studied: smooth hydrophilic, smooth hydrophobic, superhydrophobic with rib/cavity structures, superhydrophobic with rib/cavity structures and additional sparsely spaced ribs to close off the cavities, circular posts, and circular holes. It is found that structured superhydrophobic surfaces provide cavities for nucleation to occur. More dissolved air effervesces from the water as the surface temperature increases due to an increased level of supersaturation and convection. The nucleation leads to large air bubble formations that reduce the overall convection coefficient when compared to the smooth surfaces. For the rib/cavity structured surfaces, the bubbles form in an anisotropic manner and are aligned with the surface structure. More bubbles are observed on the superhydrophobic surfaces where the cavities are bounded. Since water's ability to dissolve air is dependent on temperature, heat and mass transfer cannot be treated independently on any of the superhydrophobic surfaces studied here.
Piezoelectric fans have emerged as a viable alternative for electronics cooling applications requ... more Piezoelectric fans have emerged as a viable alternative for electronics cooling applications requiring low input power and noiseless operation. A piezoelectric fan is a cantilever actuated by a piezoelectric ceramic material bonded to it. The fan oscillates back and forth creating airflow when an alternating electric field is applied to this bonded piezoelectric ceramic. Forced convection induced by such an oscillating fan in an enclosure is numerically investigated. The computational model is capable of sustaining deforming fluid cells that allow large boundary movement. The moving wall boundary, modeled as large-amplitude beam deflection, initiates flow in the fluid domain which enhances convection to varying extents depending on the heat source-to-fan distance and beam deflection amplitude. The effects of these parameters on heat transfer are studied. Transition between distinct convection patterns is observed with changes in the parameters. Results are validated against experimental measurements, with good agreement.
Miniaturization of gas chromatography (GC) instrumentation is of interest because it addresses cu... more Miniaturization of gas chromatography (GC) instrumentation is of interest because it addresses current and future issues relating to compactness, portability and field application. While incremental advancements continue to be reported in microchip GC, the current performance is far from acceptable. This lower performance compared to conventional GC is due to factors such as pooling of the stationary phase in corners of non-cylindrical channels, adsorption of sensitive compounds on incompletely deactivated surfaces, shorter column lengths and less than optimum interfacing to injector and detector. In this work, a microchip GC system was developed that solves the latter challenge, i.e., microchip interfacing to injector and detector. A microchip compression clamp was constructed that seals injector and detector fused silica interface tubing to inlet and outlet ports of the microchip channels with minimum extra-column dead volume, and that allows routine operation at least up to 300 ºC. The compression clamp was constructed of a low expansion alloy, Kovar™, to minimize leaking due to thermal expansion mismatch at the interface during repeated thermal cycling. A 5.9 m channel with a cross-section that approximately matches a 100 µm i.d. cylindrical fused silica column was fabricated in a silicon wafer using wafer bonding and deep reactive ion etching (DRIE) and coated statically with a 1% vinyl, 5% phenyl, 94% methylpolysiloxane stationary phase. High temperature separations of C10-C40 n-alkanes and a commercial diesel sample were demonstrated using the system under both temperature programmed GC (TPGC) and thermal gradient GC (TGGC) conditions. TGGC analysis of a complex essential oil sample was also demonstrated.
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