215 research outputs found
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Fanno flows in micro-channels: an enhanced quasi-2D laminar numerical model for gas flow
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Review of Flow Boiling Patterns in Microchannels
Engineering and Physical Sciences Research Council of the UK under grant EP/T033045/1
Flow patterns in upward two-phase flow in small diameter tubes
Two-phase flow in small tubes and channels is becoming a common phenomenon in industrial processes. However, the study of two-phase flow regimes in small tubes is still at its infancy. The previous studies are reviewed and discussed in the literature section. The problems and inconsistencies encountered in the earlier studies are presented and discussed. The experimental facility is introduced in the chapters that follow. They include a section on the design of the experimental system and the test sections, the selection of the experimental parameters and the introduction of the purposely-developed programs to control the experiments and collect and process the data. The methodology of the calibration and the uncertainty analysis, the problems encountered and their solutions and the single-phase validation experiments are also described. In this project we studied the effect of tube diameter and fluid flow parameters on flow patterns in small tubes using R134a as the working fluid. The tested tube diameters were 1.10, 2.01, 2.88 and 4.26 mm; the fluid pressures were 6, 10 and 14 bar; the liquid and gas superficial velocities covered a range of 0.04-5.0 m/s and 0.01-10.0 m/s respectively. The observed flow patterns included bubbly, dispersed bubble, confined bubble, slug, chum, annular and mist flow. Twelve integrated flow maps are sketched in this report. The obtained results were compared with earlier experiments by other workers and with existing models, with obvious differences in the prediction of the transition boundaries. A set of new models and correlations were developed, based on the new data for boiling R134a presented in this thesis, to predict the effect of tube diameter and fluid properties on the transition boundaries. Some also agreed with the limited data available from earlier studies for adiabatic air-water flow in small to normal size tubes.EThOS - Electronic Theses Online ServiceLondon South Bank UniversityGBUnited Kingdo
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Flow boiling in plain and porous coated microchannels
Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 kg/m2s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 160.7 kW/m2. HFE-7200 was used as the working fluid. Flow visualisation results and SEM surface analyses are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide, while shallow but larger cavities up to 6 µm were found on the plain copper channels. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern evolution occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux. Microchannel flow boiling heat transfer is enhanced by up to 43.5 % at low heat fluxes where the nucleate boiling mechanism is dominant. Heat transfer enhancement diminishes with further increase in heat flux to 13.2 %, potentially due to nucleate boiling suppression with flow regime transition.TMD Technologies Ltd
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