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Master's Dissertation
DOI
10.11606/D.18.2011.tde-01092011-153114
Document
Author
Full name
Daniel Felipe Sempertegui Tapia
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Carlos, 2011
Supervisor
Committee
Ribatski, Gherhardt (President)
Passos, Julio Cesar
Silva, Maíra Martins da
Title in Portuguese
Estudo teórico-experimental dos padrões de escoamento durante a evaporação convectiva no interior de canais com diâmetro reduzido
Keywords in Portuguese
Escoamento bifásico
Evaporação convectiva
Microcanais
Padrões de escoamento
Abstract in Portuguese
Em linhas gerais, esta dissertação de mestrado envolve o estudo de padrões de escoaomento durante a ebulição convectiva em microcanais. Resultados experimentais foram levantados para um tubo com diâmetro de 2,32 mm durante a evaporação convectiva dos refrigerantes R134a e R245fa. Para a investigação, técnicas experimentais e de análise foram desenvolvidas. A caracterização dos padrões de escoamento envolveu o tratamento simultâneo de sinais provenientes dos seguintes dispositivos: um par diodo/sensor-laser tendo um tubo transparente entre eles no interior do qual ocorre o escoamento bifásico; um transdutor de pressão piezo-elétrico de tamanho reduzido com o objetivo de determinar a variação local da pressão do escoamento; e de um micro-termopar em contato com o fluido refrigerante. A técnica de tratamento de dados utilizada envolve a aglomeração progressiva de dados que apresentem características médias similares através do algoritmo k-means. Os sinais de pressão, intensidade de radiação e temperatura foram adquiridos simultaneamente a uma freqüência de 25 kHz. Imagens simultâneas do escoamento bifásico a uma velocidade de captura em torno de 10.000 imagens/s foram levantadas através de uma câmera de filmagem rápida (até 100.000 imagens/s), e os padrões de escoamentos observados contrastados aos resultados fornecidos pelo método proposto. Baseado nesta análise, mapas de escoamento foram propostos, os quais incorporaram não apenas critérios subjetivos como a visualização, mas também objetivos como as variações transientes da pressão local do escoamento e da morfologia do escoamento através do seu efeito na dispersão da radiação emitida pelo foto-diodo. Os resultados previstos pelo método objetivo apresentam concordância razoável com os dados caracterizados com base em visualizações. Adicionalmente, características de bolhas alongadas foram determinadas.
Title in English
Experimental study of the two-phase flow patterns during convective boiling in microchannels
Keywords in English
Convective boiling
Flow pattern
Microchannels
Two-phase flow
Abstract in English
The present research has been focused on the study of flow patterns inside channels of diameter less than 3 mm during the convective evaporation of refrigerants such as R134a and R245fa. For the investigation of such topics, experimental techniques and methods of analysis of results were developed. A broad database was gathered in an experimental test facility. The characterization of flow patterns involved the simultaneous processing of signals from the following devices: a pair diode / laser-sensor having a transparent tube between them, within which occurs the two-phase flow; a micro piezoelectric pressure transducer to determine the local variation of pressure of the flow and a micro-thermocouple fixed within the fluid. The technique used in data processing involves the gradual agglomeration of data having similar average characteristics; this method was developed based on the k-means clustering algorithm. The signals from the transducers were acquired simultaneously at a frequency of 25 kHz. The program for the acquisition and for processing of the signals was developed using LabView. Simultaneous images of two-phase flow at a speed of capture around 10,000 images / s were obtained through a high speed camera and the observed flow patterns were contrasted to the results provided by the objective method. Based on this analysis, flow maps were proposed, which incorporate not only subjective criteria such as visualization, but also objective criteria like the transient variations of local pressure of the flow, temperature of the fluid and the effect of the flow morphology based on the dispersion of light which effect was captured by the photo-diode. The maps obtained by the objective method were compared against flow pattern segregated based on visualization and a reasonable agreement was obtained. Besides the elongated bubble characteristics were determined.
 
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Daniel_F_S_Tapia.pdf (2.22 Mbytes)
Publishing Date
2011-09-05
 
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