Search

Search Results

• 21. [Article] Continuous and rapid synthesis of nanoclusters and nanocrystals using scalable microstructured reactors

  Recent advances in nanocrystalline materials production are expected to impact the development of next generation low-cost and/or high efficiency solar cells. For example, semiconductor nanocrystal inks ...

  Citation × Citation Citation Abstract Copy Title: Continuous and rapid synthesis of nanoclusters and nanocrystals using scalable microstructured reactors Author: Jin, Hyung Dae Recent advances in nanocrystalline materials production are expected to impact the development of next generation low-cost and/or high efficiency solar cells. For example, semiconductor nanocrystal inks are used to lower the fabrication cost of the absorber layers of the solar cells. In addition, some quantum confined nanocrystals display electron-hole pair generation phenomena with greater than 100% quantum yield, called multiple exciton generation (MEG). These quantum dots could potentially be used to fabricate solar cells that exceed the Schockley-Queisser limit. At present, continuous syntheses of nanoparticles using microreactors have been reported by several groups. Microreactors have several advantages over conventional batch synthesis. One advantage is their efficient heat transfer and mass transport. Another advantage is the drastic reduction in the reaction time, in many cases, down to minutes from hours. Shorter reaction time not only provides higher throughput but also provide better particle size control by avoiding aggregation and by reducing probability of oxidizing precursors. In this work, room temperature synthesis of Au₁₁ nanoclusters and high temperature synthesis of chalcogenide nanocrystals were demonstrated using continuous flow microreactors with high throughputs. A high rate production of phosphine-stabilized Au₁₁ nanoclusters was achieved using a layer-up strategy which involves the use of microlamination architectures; the patterning and bonding of thin layers of material (laminae) to create a multilayered micromixer in the range of 25-250 µm thick was used to step up the production of phosphine-stabilized Au₁₁ nanoclusters. Continuous production of highly monodispersed phosphine-stabilized Au₁₁ nanoclusters at a rate of about 11.8 [mg/s] was achieved using a microreactor with a size of 1.687cm³. This result is about 30,000 times over conventional batch synthesis according to production rate/per reactor volume. We have elucidated the formation mechanism of CuInSe₂ nanocrystals for the development of a continuous flow process for their synthesis. It was found that copper-rich CuInSe₂ with a sphalerite structure was formed initially followed by the formation of more ordered CuInSe₂ at longer reaction times, along with the formation of Cu₂Se and In₂Se₃. It was found that Cu₂Se was formed at a much faster rate than In₂Se₃ under the same reaction conditions. By adjusting the Cu/In precursor ratio, we were able to develop a very rapid and simple synthesis of CuInSe₂ nanocrystals using a continuous flow microreactor with a high throughput per reactor volume. The microreactor has a simple design which uses readily available low cost components. It comprised an inner microtube to precisely control the injection of TOPSe into a larger diameter tube that preheated CuCl and InCl₃ hot mixture was pumped through. Rapid injection plays an important role in dividing the nucleation and growth process which is crucial in getting narrow size distribution. The design of this microreactor also has the advantages of alleviating sticking of QDs on the growth channel wall since QDs were formed from the center of the reactor. Furthermore, size-controlled synthesis of CuInSe₂ nanocrystals was achieved using this reactor simply by adjusting ratio between coordinating solvents. Semiconductors with a direct bandgap between 1 and 2eV including Cu(In,Ga)Se₂ (1.04 – 1.6eV) and CuIn(Se,S)₂ (1.04 - 1.53eV) are ideal for single junction cells utilize the visible spectrum. However, half of the solar energy available to the Earth lies in the infrared region. Inorganic QD-based solar cells with a decent efficiency near 1.5 μm have been reported. Therefore, syntheses of narrow gap IV-VI (SnTe, PbS, PbSe, PbTe), II-IV (HgTe, Cd[subscript X]Hg[subscript 1-X]Te), and III-V (InAs) QDs have attracted significant attention and these materials have potential uses for a variety of other optical, electronic, and optoelectronic applications. SnTe with an energy gap of 0.18eV at 300K can be used for IR photodetectors, laser diodes, and thermophotovoltaic energy converters. First continuous synthesis of shape-controlled SnTe nanocrystals were also accomplished in this work. SnCl₂, and TOPTe were used as reactants successfully in coordinating OA and TOP solvents. Both rod shape and dot shape SnTe nanocrystals with uniform size distributions could be obtained. A blue shift was observed from these SnTe nanocrystals. Production rate at about 5mg/min (300mg/hr) was achieved using a microreactor at a size of 1.78cm³. Title: Continuous and rapid synthesis of nanoclusters and nanocrystals using scalable microstructured reactors Author: Jin, Hyung Dae Recent advances in nanocrystalline materials production are expected to impact the development of next generation low-cost and/or high efficiency solar cells. For example, semiconductor nanocrystal inks are used to lower the fabrication cost of the absorber layers of the solar cells. In addition, some quantum confined nanocrystals display electron-hole pair generation phenomena with greater than 100% quantum yield, called multiple exciton generation (MEG). These quantum dots could potentially be used to fabricate solar cells that exceed the Schockley-Queisser limit. At present, continuous syntheses of nanoparticles using microreactors have been reported by several groups. Microreactors have several advantages over conventional batch synthesis. One advantage is their efficient heat transfer and mass transport. Another advantage is the drastic reduction in the reaction time, in many cases, down to minutes from hours. Shorter reaction time not only provides higher throughput but also provide better particle size control by avoiding aggregation and by reducing probability of oxidizing precursors. In this work, room temperature synthesis of Au₁₁ nanoclusters and high temperature synthesis of chalcogenide nanocrystals were demonstrated using continuous flow microreactors with high throughputs. A high rate production of phosphine-stabilized Au₁₁ nanoclusters was achieved using a layer-up strategy which involves the use of microlamination architectures; the patterning and bonding of thin layers of material (laminae) to create a multilayered micromixer in the range of 25-250 µm thick was used to step up the production of phosphine-stabilized Au₁₁ nanoclusters. Continuous production of highly monodispersed phosphine-stabilized Au₁₁ nanoclusters at a rate of about 11.8 [mg/s] was achieved using a microreactor with a size of 1.687cm³. This result is about 30,000 times over conventional batch synthesis according to production rate/per reactor volume. We have elucidated the formation mechanism of CuInSe₂ nanocrystals for the development of a continuous flow process for their synthesis. It was found that copper-rich CuInSe₂ with a sphalerite structure was formed initially followed by the formation of more ordered CuInSe₂ at longer reaction times, along with the formation of Cu₂Se and In₂Se₃. It was found that Cu₂Se was formed at a much faster rate than In₂Se₃ under the same reaction conditions. By adjusting the Cu/In precursor ratio, we were able to develop a very rapid and simple synthesis of CuInSe₂ nanocrystals using a continuous flow microreactor with a high throughput per reactor volume. The microreactor has a simple design which uses readily available low cost components. It comprised an inner microtube to precisely control the injection of TOPSe into a larger diameter tube that preheated CuCl and InCl₃ hot mixture was pumped through. Rapid injection plays an important role in dividing the nucleation and growth process which is crucial in getting narrow size distribution. The design of this microreactor also has the advantages of alleviating sticking of QDs on the growth channel wall since QDs were formed from the center of the reactor. Furthermore, size-controlled synthesis of CuInSe₂ nanocrystals was achieved using this reactor simply by adjusting ratio between coordinating solvents. Semiconductors with a direct bandgap between 1 and 2eV including Cu(In,Ga)Se₂ (1.04 – 1.6eV) and CuIn(Se,S)₂ (1.04 - 1.53eV) are ideal for single junction cells utilize the visible spectrum. However, half of the solar energy available to the Earth lies in the infrared region. Inorganic QD-based solar cells with a decent efficiency near 1.5 μm have been reported. Therefore, syntheses of narrow gap IV-VI (SnTe, PbS, PbSe, PbTe), II-IV (HgTe, Cd[subscript X]Hg[subscript 1-X]Te), and III-V (InAs) QDs have attracted significant attention and these materials have potential uses for a variety of other optical, electronic, and optoelectronic applications. SnTe with an energy gap of 0.18eV at 300K can be used for IR photodetectors, laser diodes, and thermophotovoltaic energy converters. First continuous synthesis of shape-controlled SnTe nanocrystals were also accomplished in this work. SnCl₂, and TOPTe were used as reactants successfully in coordinating OA and TOP solvents. Both rod shape and dot shape SnTe nanocrystals with uniform size distributions could be obtained. A blue shift was observed from these SnTe nanocrystals. Production rate at about 5mg/min (300mg/hr) was achieved using a microreactor at a size of 1.78cm³. Close

• 22. [Article] CO₂ reduction in aqueous-ionic liquid solution in microscale-based corona reactor

  Global warming problem is becoming an increasingly important environmental concern and CO₂ is considered as the major cause of global warming. Among various methods of CO₂ utilization, conversion of CO₂ ...

  Citation × Citation Citation Abstract Copy Title: CO₂ reduction in aqueous-ionic liquid solution in microscale-based corona reactor Author: Miao, Yu Global warming problem is becoming an increasingly important environmental concern and CO₂ is considered as the major cause of global warming. Among various methods of CO₂ utilization, conversion of CO₂ to value added chemical products is the most attractive. In this study, a microscale-based corona reactor is introduced for reduction of CO₂. Two kinds of solvent were used in this study for absorbing CO₂: DI-water and ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). The latter one has a much higher solubility of CO₂. After saturated with CO₂, solution was introduced into the microreactor built around the concept of corona discharge. The corona was created through a significant potential difference between two graphite electrodes. The current that passed through two electrodes acted as a catalytic agent for the reduction of CO₂. The experiments were conducted at room temperature and at steady state. The ranges of the operating conditions were: mean residence time 5 to 100 (sec), thickness of spacer 200 and 500 (μm), and voltage applied across the reactor 20 and 22.5 (V). Reactions happened in the bulk of the reactor and five main products were detected at the outlet stream: i) formic acid (HCOOH), ii) formaldehyde (HCHO), iii) methanol (CH₃OH), iv) methane (CH₄) and v) hydrogen (H₂). Among these compounds, formic acid, formaldehyde and methanol are intermediate products. The conversion of CO₂ in aqueous solution can reach as high as 94.8% at mean residence time of 100 sec. Although in ionic liquid solution the conversion of CO₂ is much lower (19.3% at mean residence time of 100 sec), consumption of CO₂ in ionic liquid is 6-7 times larger than that in water when generating same volume of products. A mathematical model reflecting geometry and flow conditions inside the microreactor was developed to simulate the process of CO₂ reduction. The model was solved numerically using COMSOL Multiphysics software package. The simulated results were optimized to fit the experimental data using COMSOL-Matlab LiveLink software package. Primary reaction rate constants for CO₂ reduction were predicted. The mathematical model was found to explain the experimental data pretty well. Title: CO₂ reduction in aqueous-ionic liquid solution in microscale-based corona reactor Author: Miao, Yu Global warming problem is becoming an increasingly important environmental concern and CO₂ is considered as the major cause of global warming. Among various methods of CO₂ utilization, conversion of CO₂ to value added chemical products is the most attractive. In this study, a microscale-based corona reactor is introduced for reduction of CO₂. Two kinds of solvent were used in this study for absorbing CO₂: DI-water and ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). The latter one has a much higher solubility of CO₂. After saturated with CO₂, solution was introduced into the microreactor built around the concept of corona discharge. The corona was created through a significant potential difference between two graphite electrodes. The current that passed through two electrodes acted as a catalytic agent for the reduction of CO₂. The experiments were conducted at room temperature and at steady state. The ranges of the operating conditions were: mean residence time 5 to 100 (sec), thickness of spacer 200 and 500 (μm), and voltage applied across the reactor 20 and 22.5 (V). Reactions happened in the bulk of the reactor and five main products were detected at the outlet stream: i) formic acid (HCOOH), ii) formaldehyde (HCHO), iii) methanol (CH₃OH), iv) methane (CH₄) and v) hydrogen (H₂). Among these compounds, formic acid, formaldehyde and methanol are intermediate products. The conversion of CO₂ in aqueous solution can reach as high as 94.8% at mean residence time of 100 sec. Although in ionic liquid solution the conversion of CO₂ is much lower (19.3% at mean residence time of 100 sec), consumption of CO₂ in ionic liquid is 6-7 times larger than that in water when generating same volume of products. A mathematical model reflecting geometry and flow conditions inside the microreactor was developed to simulate the process of CO₂ reduction. The model was solved numerically using COMSOL Multiphysics software package. The simulated results were optimized to fit the experimental data using COMSOL-Matlab LiveLink software package. Primary reaction rate constants for CO₂ reduction were predicted. The mathematical model was found to explain the experimental data pretty well. Close

• 23. [Article] Fluidic and thermal modeling for the high production rate synthesis of high quality nanoparticles

  Advances of colloidal nanomaterials for societal benefit have been hampered by the high cost and low quality of nanoparticles (NPs). The production of high quality nanoparticles within colloidal suspension ...

  Citation × Citation Citation Abstract Copy Title: Fluidic and thermal modeling for the high production rate synthesis of high quality nanoparticles Author: Peterson, Daniel Alan Advances of colloidal nanomaterials for societal benefit have been hampered by the high cost and low quality of nanoparticles (NPs). The production of high quality nanoparticles within colloidal suspension has two related concerns; (1) current high production rates methods of synthesizing nanoparticles result in a larger range of particle size, which requires expensive and time consuming separation steps resulting in high costs; and (2) the thermal and concentration gradients within batch processes used to scale-up colloidal NP synthesis results in products of varying sizes. Continuous flow microreactors provide a means to minimize these gradients during the synthesis of colloidal NPs thereby providing the potential to produce size-controlled suspensions at higher production rates compared to conventional batch reactors. In this dissertation, a number of microreactor mixing strategies are investigated. In addition, efforts are made to model the thermal profile caused by the conversion of microwave energy within a continuous flow scenario. Based on learnings, efforts are made to redesign flow applicators to maximize energy absorption with minimal thermal gradient. Within this dissertation, concentration gradients are controlled through the use of different mixing schemes within the various microreactor setups. It is demonstrated that the ability to control the mixing characteristics provides the ability to tune the NP size. T-mixing, interdigital mixing and reverse oscillatory flow mixing are all modeled and evaluated for mixing time. Mixing quality and mixing time metrics are defined and used for comparison of these methods. The scalability of these methods is explored in order to show methods which can maintain small particle size distributions at high production rates. In particular, a new reverse oscillatory flow (ROF) mixing system is developed for high rate NP synthesis. The relatively large size (460 µm by 152 mm channel) produces high production rates of nanoparticles while maintaining quality mixing through a novel mixing method. The ROF system is shown to produce CdS nanoparticles at a production rate of 115.7 g/hour with a coefficient of variation down to 19%. The size distributions of this method are comparable to other methods with production rates from 1% to 10% of the ROF method. In colloidal NP syntheses, thermal gradients are controlled by the time scale for heating. Here, a single mode microwave system is designed and developed for rapidly heating the reacting flux. Rapid heating minimizes the thermal gradients within the solution during synthesis, thereby shortening the nucleation time scale and providing opportunity for burst nucleation. A model is developed to simulate microwave heating which is verified over a range of operating parameters; flow rates (15 to 40 mL/min), microwave power (150 to 300 Watts) and salinity (1 to 5 g/L). Experimental results show model predictions of the temperature profile within 16.8% for all cases considered (averaged absolute error reported). Title: Fluidic and thermal modeling for the high production rate synthesis of high quality nanoparticles Author: Peterson, Daniel Alan Advances of colloidal nanomaterials for societal benefit have been hampered by the high cost and low quality of nanoparticles (NPs). The production of high quality nanoparticles within colloidal suspension has two related concerns; (1) current high production rates methods of synthesizing nanoparticles result in a larger range of particle size, which requires expensive and time consuming separation steps resulting in high costs; and (2) the thermal and concentration gradients within batch processes used to scale-up colloidal NP synthesis results in products of varying sizes. Continuous flow microreactors provide a means to minimize these gradients during the synthesis of colloidal NPs thereby providing the potential to produce size-controlled suspensions at higher production rates compared to conventional batch reactors. In this dissertation, a number of microreactor mixing strategies are investigated. In addition, efforts are made to model the thermal profile caused by the conversion of microwave energy within a continuous flow scenario. Based on learnings, efforts are made to redesign flow applicators to maximize energy absorption with minimal thermal gradient. Within this dissertation, concentration gradients are controlled through the use of different mixing schemes within the various microreactor setups. It is demonstrated that the ability to control the mixing characteristics provides the ability to tune the NP size. T-mixing, interdigital mixing and reverse oscillatory flow mixing are all modeled and evaluated for mixing time. Mixing quality and mixing time metrics are defined and used for comparison of these methods. The scalability of these methods is explored in order to show methods which can maintain small particle size distributions at high production rates. In particular, a new reverse oscillatory flow (ROF) mixing system is developed for high rate NP synthesis. The relatively large size (460 µm by 152 mm channel) produces high production rates of nanoparticles while maintaining quality mixing through a novel mixing method. The ROF system is shown to produce CdS nanoparticles at a production rate of 115.7 g/hour with a coefficient of variation down to 19%. The size distributions of this method are comparable to other methods with production rates from 1% to 10% of the ROF method. In colloidal NP syntheses, thermal gradients are controlled by the time scale for heating. Here, a single mode microwave system is designed and developed for rapidly heating the reacting flux. Rapid heating minimizes the thermal gradients within the solution during synthesis, thereby shortening the nucleation time scale and providing opportunity for burst nucleation. A model is developed to simulate microwave heating which is verified over a range of operating parameters; flow rates (15 to 40 mL/min), microwave power (150 to 300 Watts) and salinity (1 to 5 g/L). Experimental results show model predictions of the temperature profile within 16.8% for all cases considered (averaged absolute error reported). Close

• 24. [Article] Single Phase and Flow Boiling Heat Transfer and Flow Characterization in Microscale Pin Fin Heat Sinks

  The ever increasing requirements for heat dissipation in various thermal management applications such as computer chip cooling and high power electronics have necessitated the need for novel thermal management ...

  Citation × Citation Citation Abstract Copy Title: Single Phase and Flow Boiling Heat Transfer and Flow Characterization in Microscale Pin Fin Heat Sinks Author: Rasouli, Erfan The ever increasing requirements for heat dissipation in various thermal management applications such as computer chip cooling and high power electronics have necessitated the need for novel thermal management techniques. Thermal management using heat sinks with microscale features is amongst the prominent techniques developed over the past two decades. In this dissertation, single and phase change heat transfer and pressure drop through one such heat sink, namely microscale pin fin heat sinks (µPFHS), is examined experimentally. In particular, effects of pitch-to-diameter and aspect ratio variations are studied on the thermofluidic performance of studied µPFHSs. Single phase heat transfer and pressure drop of two distinct fluids, liquid nitrogen and Performance Fluid (PF5060) are characterized experimentally through the µPFHSs with staggered diamond shape pin fins. The LN₂ and PF5060 experiments' Reynolds number (Re_Dh, based on pin fin hydraulic diameter) is in range of 108-570 and 8-462, respectively. Results are presented in a non-dimensional form in terms of the friction factor (f), Nusselt (Nu), and Reynolds numbers and are compared with the predictions of existing correlations in the literature for micro pin fin heat sinks. Heat sinks with the higher pitch ratio (coarser array) not only show lower pressure drops at a fixed Re_Dh, but also enhance significantly heat transfer rate when compared against the heat sink of the same pin fin size but denser arrangement. Flow visualization experiments using an infrared camera on PF5060 single phase tests are performed to understand the counter-intuitive trends seen in the global results. Flow through heat sinks with the same aspect ratio but larger pitch ratio exhibit unsteady vortex shedding in the wake region of pin fins, which markedly enhances convective heat transfer rate. Existing correlations developed for µPFHSs (such as that by Prasher et al. [1] and Koşar and Peles [2]) are capable of predicting the f and Nu data with good agreement only in the absence of vortex shedding, while the unsteady flow past the transition Re_Dh results in poor comparison of correlations with experimental data. A comparison of the experimental Nu data of PF5060 (Pr≅12.2) with the data of LN₂ (Pr≅1.9) shows significant change between the slopes of the curves of two fluids only in the heat sinks without vortex shedding. In the heat sinks with unsteady vortex shedding, the Nu_Dh curves show significantly decreased dependency on Pr number. Consequently, separate correlations are developed for predicting Nu in the case with and without unsteady vortex shedding using data from two distinct fluids and four PFHS geometries over a range of Re_Dh from 8 to 643. Given the clear heat transfer enhancement that occurs for certain pitch ratio designs of PFHSs in single phase flows, flow boiling experiments with PF5060 are performed to clarify whether additional changes to the pressure drop and two-phase heat transfer coefficient occur upon the introduction of the unsteady vortex shedding. Subcooled (∆T_sub=12.5℃) and saturated flow boiling of PF5060 through the micro pin fins are investigated. The heat sinks are tested at three constant mass fluxes of 30, 60, and 100 kg/m².s with heat fluxes ranging from 1.1 to 17.8 W⁄(cm² ) based on the planform area of the heat sinks. Flow regimes are studied with high speed imaging. Nucleate boiling heat transfer is the dominant mechanism for exit vapor qualities less than 0.5; at higher qualities annular film evaporation becomes dominant. The salient effect of unsteady vortex shedding is in elimination of wall temperature overshoot. In nucleate boiling regime, the heat sinks with unsteady flow flapping show higher two-phase heat transfer coefficients. The predictions of existing correlations for h_tp in literature are not in good agreement with the experimental data (MAE>30%) and show a systematic deviation depending on the µPFHSs dimensions. Title: Single Phase and Flow Boiling Heat Transfer and Flow Characterization in Microscale Pin Fin Heat Sinks Author: Rasouli, Erfan The ever increasing requirements for heat dissipation in various thermal management applications such as computer chip cooling and high power electronics have necessitated the need for novel thermal management techniques. Thermal management using heat sinks with microscale features is amongst the prominent techniques developed over the past two decades. In this dissertation, single and phase change heat transfer and pressure drop through one such heat sink, namely microscale pin fin heat sinks (µPFHS), is examined experimentally. In particular, effects of pitch-to-diameter and aspect ratio variations are studied on the thermofluidic performance of studied µPFHSs. Single phase heat transfer and pressure drop of two distinct fluids, liquid nitrogen and Performance Fluid (PF5060) are characterized experimentally through the µPFHSs with staggered diamond shape pin fins. The LN₂ and PF5060 experiments' Reynolds number (Re_Dh, based on pin fin hydraulic diameter) is in range of 108-570 and 8-462, respectively. Results are presented in a non-dimensional form in terms of the friction factor (f), Nusselt (Nu), and Reynolds numbers and are compared with the predictions of existing correlations in the literature for micro pin fin heat sinks. Heat sinks with the higher pitch ratio (coarser array) not only show lower pressure drops at a fixed Re_Dh, but also enhance significantly heat transfer rate when compared against the heat sink of the same pin fin size but denser arrangement. Flow visualization experiments using an infrared camera on PF5060 single phase tests are performed to understand the counter-intuitive trends seen in the global results. Flow through heat sinks with the same aspect ratio but larger pitch ratio exhibit unsteady vortex shedding in the wake region of pin fins, which markedly enhances convective heat transfer rate. Existing correlations developed for µPFHSs (such as that by Prasher et al. [1] and Koşar and Peles [2]) are capable of predicting the f and Nu data with good agreement only in the absence of vortex shedding, while the unsteady flow past the transition Re_Dh results in poor comparison of correlations with experimental data. A comparison of the experimental Nu data of PF5060 (Pr≅12.2) with the data of LN₂ (Pr≅1.9) shows significant change between the slopes of the curves of two fluids only in the heat sinks without vortex shedding. In the heat sinks with unsteady vortex shedding, the Nu_Dh curves show significantly decreased dependency on Pr number. Consequently, separate correlations are developed for predicting Nu in the case with and without unsteady vortex shedding using data from two distinct fluids and four PFHS geometries over a range of Re_Dh from 8 to 643. Given the clear heat transfer enhancement that occurs for certain pitch ratio designs of PFHSs in single phase flows, flow boiling experiments with PF5060 are performed to clarify whether additional changes to the pressure drop and two-phase heat transfer coefficient occur upon the introduction of the unsteady vortex shedding. Subcooled (∆T_sub=12.5℃) and saturated flow boiling of PF5060 through the micro pin fins are investigated. The heat sinks are tested at three constant mass fluxes of 30, 60, and 100 kg/m².s with heat fluxes ranging from 1.1 to 17.8 W⁄(cm² ) based on the planform area of the heat sinks. Flow regimes are studied with high speed imaging. Nucleate boiling heat transfer is the dominant mechanism for exit vapor qualities less than 0.5; at higher qualities annular film evaporation becomes dominant. The salient effect of unsteady vortex shedding is in elimination of wall temperature overshoot. In nucleate boiling regime, the heat sinks with unsteady flow flapping show higher two-phase heat transfer coefficients. The predictions of existing correlations for h_tp in literature are not in good agreement with the experimental data (MAE>30%) and show a systematic deviation depending on the µPFHSs dimensions. Close

• 25. [Article] CO₂ Reduction in Microscale-Based Corona Reactor : Experiments and Modeling

  Global warming is becoming an increasingly important environmental concern and CO₂ is considered as the major cause of global warming. Creating useful applications for CO₂ would generate alternatives to ...

  Citation × Citation Citation Abstract Copy Title: CO₂ Reduction in Microscale-Based Corona Reactor : Experiments and Modeling Author: Miao, Yu Global warming is becoming an increasingly important environmental concern and CO₂ is considered as the major cause of global warming. Creating useful applications for CO₂ would generate alternatives to merely venting CO₂ to the atmosphere, and decreasing the carbon intensity of human activities. Among various methods of CO₂ utilization, conversion of CO₂ to value-added chemical products is the most attractive. In this study, microtechnology and application of corona discharge are combined and introduced into CO₂ reduction process. Experiments were conducted before the design and manufacture of the microreactor to determine the configuration of the reactor, products of reaction and effect of active volume ratio: (1) it is proved that through-gas corona discharge are much simpler to implement than through-liquid, especially those with high solubility of CO₂; (2) the V-I curve for the corona discharge reactors is characterized by a transition from a high voltage low current (which we call spark discharge) to a low voltage high current state (the corona discharge state), with this transition used as a diagnostic of the reactor operation in the corona discharge mode even when direct observation of the discharge is not practical; (3) the products for the reduction of dry CO₂ are carbon monoxide (CO) and oxygen (O₂), while for the reduction of wet CO₂, methane (CH4) is also formed in addition to these two products; (4) a larger active volume ratio results in higher conversion of CO₂ to products. Based on the experimental results listed above, two multi-discharge microscale-based corona reactors were designed and manufactured. We found that for a needle-to-plate gap of 110μm, at the voltage of 0.840kV and current of 0.62mA, a flow of CO₂ and H₂O mixture (flow rate of CO₂ = 50sccm, CO₂-to-H₂O molar ratio = 1:2) can result in 5.5~6% conversion of CO₂ (with 40~50% conversion within the active volume of the reactor) with energy efficiency of 85~95%. The influence of the three main factors, namely the power applied to the reactor (specific points of the V-I curve used), flow rate of CO₂, and CO₂-to-H₂O molar ratio, on the performance and energy efficiency of the reactor were investigated. It was found that (1) the glow regime (or corona regime) is the optimal operation regime for this process from both conversion and energy efficiency perspectives, with higher current in this regime resulting in higher CO₂ conversion; (2) lower flow rate of CO₂ can result in higher conversion with lower energy efficiency, and conversely, the highest energy efficiency is achieved at the highest flow rate; (3) the conversion of CO₂ increases as the CO₂-to-H₂O molar ratio decreases, but the highest energy efficiency is achieved when this ratio matches the ratio of stoichiometric numbers. A numerical model of the process reflecting the geometry, momentum balance, material balance and kinetics inside the reactor was developed to help understand the chemical reaction process. The reaction scheme was modelled as being driven by the initial cleavage of a CO₂ or H₂O molecule caused by collision with energetic electrons to produce CO+O or OH+H, followed by cascaded spontaneous reactions that yield the products. The reaction kinetics were approached as following pseudo-Arrhenius laws with a pre-exponential term k₀ and an exponential term, but rather than modelling the exponential term as dependent on temperature, the term was modelled as depending on the applied electrical potential in the corona discharge (i.e., rather than dependent on e^(-EA/RT) it was modelled as dependent on e^(-EA/βVF), where β is an effectiveness parameter, V is the applied electrical potential to the discharge, and F is Faraday's constant). A refined parameter n that defines the fraction of CO intermediate that either further fragment or remain as CO in the product stream, termed the kinetic parameter in this work, was also employed. Optimization of the numerical model was applied to extract kinetic parameters for the CO₂ reduction process in the multi-point corona discharge with good agreement between simulated results and experimental data. The values of the final refined parameters were: for the initial dissociation of CO₂, the final values of the refined parameters were k₀,₁ = 3.543±0.071×10¹⁰sec⁻¹ and the electrical potential effectiveness parameter was β₁ = 2.181±0.044×10¹, and for the dissociation of H₂O, the parameters were k₀,₁ = 1.266±0.025×10⁸sec⁻¹, β4 = 8.403±0.168, and the kinetic parameter n = 0.50±0.010. Title: CO₂ Reduction in Microscale-Based Corona Reactor : Experiments and Modeling Author: Miao, Yu Global warming is becoming an increasingly important environmental concern and CO₂ is considered as the major cause of global warming. Creating useful applications for CO₂ would generate alternatives to merely venting CO₂ to the atmosphere, and decreasing the carbon intensity of human activities. Among various methods of CO₂ utilization, conversion of CO₂ to value-added chemical products is the most attractive. In this study, microtechnology and application of corona discharge are combined and introduced into CO₂ reduction process. Experiments were conducted before the design and manufacture of the microreactor to determine the configuration of the reactor, products of reaction and effect of active volume ratio: (1) it is proved that through-gas corona discharge are much simpler to implement than through-liquid, especially those with high solubility of CO₂; (2) the V-I curve for the corona discharge reactors is characterized by a transition from a high voltage low current (which we call spark discharge) to a low voltage high current state (the corona discharge state), with this transition used as a diagnostic of the reactor operation in the corona discharge mode even when direct observation of the discharge is not practical; (3) the products for the reduction of dry CO₂ are carbon monoxide (CO) and oxygen (O₂), while for the reduction of wet CO₂, methane (CH4) is also formed in addition to these two products; (4) a larger active volume ratio results in higher conversion of CO₂ to products. Based on the experimental results listed above, two multi-discharge microscale-based corona reactors were designed and manufactured. We found that for a needle-to-plate gap of 110μm, at the voltage of 0.840kV and current of 0.62mA, a flow of CO₂ and H₂O mixture (flow rate of CO₂ = 50sccm, CO₂-to-H₂O molar ratio = 1:2) can result in 5.5~6% conversion of CO₂ (with 40~50% conversion within the active volume of the reactor) with energy efficiency of 85~95%. The influence of the three main factors, namely the power applied to the reactor (specific points of the V-I curve used), flow rate of CO₂, and CO₂-to-H₂O molar ratio, on the performance and energy efficiency of the reactor were investigated. It was found that (1) the glow regime (or corona regime) is the optimal operation regime for this process from both conversion and energy efficiency perspectives, with higher current in this regime resulting in higher CO₂ conversion; (2) lower flow rate of CO₂ can result in higher conversion with lower energy efficiency, and conversely, the highest energy efficiency is achieved at the highest flow rate; (3) the conversion of CO₂ increases as the CO₂-to-H₂O molar ratio decreases, but the highest energy efficiency is achieved when this ratio matches the ratio of stoichiometric numbers. A numerical model of the process reflecting the geometry, momentum balance, material balance and kinetics inside the reactor was developed to help understand the chemical reaction process. The reaction scheme was modelled as being driven by the initial cleavage of a CO₂ or H₂O molecule caused by collision with energetic electrons to produce CO+O or OH+H, followed by cascaded spontaneous reactions that yield the products. The reaction kinetics were approached as following pseudo-Arrhenius laws with a pre-exponential term k₀ and an exponential term, but rather than modelling the exponential term as dependent on temperature, the term was modelled as depending on the applied electrical potential in the corona discharge (i.e., rather than dependent on e^(-EA/RT) it was modelled as dependent on e^(-EA/βVF), where β is an effectiveness parameter, V is the applied electrical potential to the discharge, and F is Faraday's constant). A refined parameter n that defines the fraction of CO intermediate that either further fragment or remain as CO in the product stream, termed the kinetic parameter in this work, was also employed. Optimization of the numerical model was applied to extract kinetic parameters for the CO₂ reduction process in the multi-point corona discharge with good agreement between simulated results and experimental data. The values of the final refined parameters were: for the initial dissociation of CO₂, the final values of the refined parameters were k₀,₁ = 3.543±0.071×10¹⁰sec⁻¹ and the electrical potential effectiveness parameter was β₁ = 2.181±0.044×10¹, and for the dissociation of H₂O, the parameters were k₀,₁ = 1.266±0.025×10⁸sec⁻¹, β4 = 8.403±0.168, and the kinetic parameter n = 0.50±0.010. Close