Flash gas bypass - a way to improve distribution of adiabatic two-phase refrigerant flow in headers of microchannel evaporators

Two-phase flow;Boiling heat transfer;Microchannel heat exchanger;Air-conditioning system

Tuo, Hanfei

关键词: Two-phase flow;    Boiling heat transfer;    Microchannel heat exchanger;    Air-conditioning system;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/46717/Hanfei_Tuo.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This 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.