15 research outputs found
Flash Gas Bypass Method for Improving Performance of an A/C System With a Microchannel Evaporator
No full textThis paper demonstrates that Flash Gas Bypass (FGB) method significantly improved the performances of the microchannel evaporator and the system, compared to the conventional direct expansion (DX) system. When system in both modes operated at the same compressor speed, the system in FGB mode produces about 13% to 18% more cooling capacity at 4% to 7% higher COP than in DX mode. When compressor speed was adjusted to maintain the same cooling capacity, COP improves 37% to 55%. Based on experimental results, a detailed microchannel heat exchanger model considering the refrigerant distribution helps to identify the two mechanisms augmenting the performances: 1) improved refrigerant quality distribution and 2) the reduction of refrigerant pressure drop across the evaporator
Scanning tunneling microscopy investigations of transition metal dichalcogenide 1t-tantalum disulfide and zigzag-shape graphene nanoribbons in ultra-high vacuum
No full textThis thesis demonstrates the studies of 1T-TaS2, a member of the transition metal dichalcogenides (TMDCs) family, and wet-chemically synthesized zigzag-shape graphene nanoribbons (GNRs) on hydrogen passivated silicon substrate through scanning tunneling microscopy (STM). Dry contact transfer allows unperturbed transfer and exfoliation of desired two-dimensional (2D) layered materials onto a desired substrate under ultra-high vacuum (UHV), to largely prevent potential solvent and atmospheric contamination.
STM imaging and scanning tunneling spectroscopy (STS) measurements provide preliminary results on lattice structure of possibly monolayer or few- layer 1T-TaS2 under room temperature. Exploration of existence of charge density waves (CDWs) in monolayer 1T-TaS2 is in progress. Past work on surface modification of another TMDC material, MoS2, using STM has shown successful removal of desired atoms to form letters on the sample surface. Hypothesis of modifying monolayer 1T-TaS2 surface structure using similar technique may result in intriguing mechanical or electronic properties.
Detailed STM imaging and STS measurements of wet chemically synthesized zigzag shape GNRs exhibit an atomic structure matching the expected molecular formula, and alternating bandgap energies of 0.4 eV and 2.68 eV, presumably due to the alternating armchair and zigzag edge terminations.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2023-08-01The student, Hanfei Wang, accepted the attached license on 2021-07-21 at 00:27.The student, Hanfei Wang, submitted this Thesis for approval on 2021-07-21 at 00:32.This Thesis was approved for publication on 2021-07-21 at 10:47.DSpace SAF Submission Ingestion Package generated from Vireo submission #17039 on 2022-01-12 at 13:05:22Made available in DSpace on 2022-01-12T22:56:14Z (GMT). No. of bitstreams: 2
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Previous issue date: 2021-07-21Embargo set by: Seth Robbins for item 121271
Lift date: 2024-01-12T22:56:20Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemAuthor requested closed access (OA after 2yrs) in Vireo ETD systemLimite
Flash gas bypass - a way to improve distribution of adiabatic two-phase refrigerant flow in headers of microchannel evaporators
No full textThis work presents an experimental and numerical study of flash gas bypass (FGB) method as a way to solve the existing problem of distribution of two phase adiabatic flow in headers of parallel flow evaporators, of typically microchannel design. Three main issues that will be explored in this thesis are: 1) separation of vapor-liquid refrigerant immediately after an expansion device in a compact T-junction and design options to enhance its performance; 2) effects of bypassing the flash vapor and header pressure drop induced flow maldistribution on heat transfer performance of microchannel evaporators; 3) periodic reverse vapor flow in microchannel evaporators used in A/C systems. These three issues were part of a project to improve distribution by separation of vapor prior to reaching inlet to each channel.
In the first part, experimental work is performed to study vapor-liquid refrigerant separation in vertical impact T-junctions using R134a and R410A as working fluids. Inlet flow rate and quality are varied in the range of 10 - 35 g/s and 10 - 25% with an intention for application in A/C systems with cooling capacities about 1.5 to 6 kW. Flow patterns in the T-junction separator are identified and characterized. It is found that liquid separation efficiency strongly depends on the flow pattern right above the impact region (junction). The efficiency deteriorates dramatically when mist turns into churn flow regime, with increasing inlet flow rate and/or quality. Then, five design modifications on the baseline T-separator are explored: inlet inclination angle, dual-inlet as pre-separation, inlet tube diameter, cross-sectional shape and location with respect to the vertical tube. The objective is to avoid or at least delay transition from mist to churn flow by reducing and changing the direction of liquid phase inertia force, and decreasing liquid and vapor force interaction.
In the second part, effects of FGB on system performances are experimentally investigated by implementing FGB method into an R134a A/C system with a single-pass microchannel evaporator. Compared to the baseline A/C system with the identical components and operating at the same compressor speed, FGB increased the cooling capacity and COP by up to 18% and 7%, respectively. For the case of matched cooling capacity with the baseline larger COP improvements were achieved in FGB system. Two main benefits of FGB approach were identified: 1) improved refrigerant distribution; 2) reduced refrigerant-side pressure drop. To systematically explore FGB effects, parametric analysis based on an experimentally validated evaporator model is performed with emphasis on pressure drop and heat transfer. Flow maldistribution induced by header pressure drop is found to be an issue. Results reveal that the outlet header pressure drop should be limited below approximately 30% of the entire evaporator pressure drop, to keep capacity degradation within 3%.
The last part work presents the phenomenon of periodic reverse flow and associated boiling fluctuation found in experiments with a parallel microchannel evaporator. A simultaneous flow visualizations and measurements reveal that synchronized oscillations of the evaporator inlet pressure and pressure drop are related to this phenomenon. Three potential impacts of flow reversal on evaporator performance are identified. Then, venting reversed vapor method is incorporated in existing FGB system. An experimental comparison of the system with new approach to a FGB system revealed that the vapor venting provided a 5% increase of cooling capacity and 3% of COP when operated at identical test conditions, while the maximum COP improvement was approximately 12% at matched capacities. In addition, the periodic reverse vapor flow is characterized and quantified through this method. Both its average flow rate and oscillation amplitude increase with average heat flux, while the oscillation period is reduced. Compared with total refrigerant flow rate supplied to the evaporator, average reverse vapor flow is in the range 2% to 8% at the conditions explored. Flow visualization within one of microchannels in the evaporator infers that the microchannel repeats the transient flow patterns with two stages: liquid rewetting stage, and transient annular evaporating/dewetting stage. The decrease in oscillation period with heat flux is mainly due to the shortened time interval for the annular film evaporating/dewetting stage.Item withdrawn by Mark Zulauf ([email protected]) on 2013-09-06T20:17:22Z
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University of Illinois Theses & Dissertations (ID: 1)
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Thermodynamic and Economic Analysis of Reheat Transcritical Organic Rankine Power Cycle Using a Low Temperature Geothermal Heat Source
No full textAnalysis of a Reheat Carbon Dioxide Transcritical Power Cycle Using a Low Temperature Heat Source
No full textThermodynamic analysis of performance improvement by reheat on the CO2 transcritical power cycle
No full textExergy Analysis of Combined Cycle of Air Separation and Natural Gas Liquefaction
No full textThis paper presented a novel combined cycle of air separation and natural gas liquefaction. The idea is that natural gas can be liquefied, meanwhile gaseous or liquid nitrogen and oxygen are produced in one combined cryogenic system. Cycle simulation and exergy analysis were performed to evaluate the process and thereby reveal the influence of the crucial parameter, i.e., flow rate ratio through two stages expanders β on heat transfer temperature difference, its distribution and consequent exergy loss. Composite curves for the combined hot streams (feeding natural gas and recycled nitrogen) and the cold stream showed the degree of optimization available in this process if appropriate β was designed. The results indicated that increasing β reduces temperature difference and exergy loss in heat exchange process. However, the maximum limit value of β should be confined in terms of minimum temperature difference proposed in heat exchanger design standard and heat exchanger size. The optimal βopt under different operation conditions corresponding to the required minimum temperature differences was investigated
Two-Phase Refrigerant Maldistribution in the Vertical Header and its Effect on the Heat Exchanger Performance as Evaporator in Heat Pump Mode
No full textCost Benefit Analysis of Waste Heat to Power Option for Multistage Air Compressor
No full textThis paper summarizes results of a study conducted to minimize total cost of ownership of multistage air compressors by integrating it with compact and efficient off-the-shelf organic rankine power cycle units to recover low grade waste heat from inter-stage coolers with subsequent conversion to power. The paper also highlights challenges faced by the integration and provides guidance for future cost and technology targets for key components to make it a commercial scale reality. Various schemes for vaporization of the working fluid including direct and indirect as well as full or partial were explored. Also, in order to better understand interaction between cycle efficiency and capital cost of key components, design as well as operating parameters including evaporator approach temperature, compression stage suction temperature, number of compression stages and cooling water supply temperature were investigated. Configuration, size and hence the cost of evaporator/ inter-stage cooler was found to be one of the major factors governing the overall cost. Impact of various operating modes including turn-down and seasonal variations were also studied. Air flow and final discharge pressure from the multistage air compressor were kept constant throughout the study to facilitate a fair comparison.</jats:p
Effect of Flash Gas Bypass on the Performance of R134a Mobile Air-Conditioning System with Microchannel Evaporator
No full text
