Flow boiling heat transfer of R134a and low GWP refrigerants in a horizontal micro-scale channel

The present paper presents an investigation of the effects of the refrigerant type on the heat transfer coefficient during flow boiling inside micro-scale channels. Experimental results for R134a, R1234ze(E), R1234yf and R600a for flow boiling in a circular channel with internal diameter of 1.1 mm are presented. The experimental database comprises 3409 data points covering mass velocities ranging from 200 to 800 kg/m²s, heat fluxes from 15 to 145 kW/m², saturation temperatures of 31 and 41°C, and vapor qualities from 0.05 to 0.95. The experimental data were parametrically analysed and the effects of the experimental parameters (heat flux, mass velocity, saturation temperature and working fluid) identified. Subsequently, the experimental data were compared against the most quoted predictive methods from literature, including macro and micro-scale methods. Based on the broad database obtained in the present study, an updated version of the predictive method of Kanizawa et al. [1] was proposed. The updated version provided accurate predictions of the present experimental database, predicting more than 97% and 86% of the results within error bands of ±30 and ±20%, respectively.The authors gratefully acknowledge FAPESP (The State of São Paulo Research Foundation, Brazil) for the financial support under contract numbers 2010/17605-4 and 2011/50176-2 and CNPq (The National Council for Scientific and Technological Development, Brazil) for the financial support under Contract Numbers nº476763/2013-4 and 303852/2013-5. The technical support given to this investigation by Mr. José Roberto Bogni is also appreciated and deeply recognized. The authors are also grateful to Honeywell for supplying the low GWP refrigerants R1234ze(E) and R1234yf

Theoretical and experimental study on flow pattern, void fraction and pressure drop during air-water two-phase upward crossflow through tube bundles

O presente trabalho envolve um estudo teórico e experimental do escoamento bifásico externo a banco de tubos. Inicialmente, apresenta-se uma ampla revisão da literatura sobre padrões de escoamento, fração de vazio e perda de pressão, durante escoamentos monofásicos e bifásicos externos a banco de tubos. Nesta análise são também descritos os métodos de previsão destes parâmetros. Verificam-se diferenças significativas entre as estimativas proporcionadas por eles, fato que indica a inexistência de métodos generalizados. Posteriormente é apresentada uma descrição detalhada da bancada experimental projetada e construída durante o doutoramento. O aparato completo compõe-se da seção de testes, circuito de água, sistema de compressão e condicionamento de ar, e seções de injeção dos fluxos e condicionamento do escoamento. A seção de testes consiste em um banco de tubos distribuídos segundo configuração triangular normal, com os tubos apresentando diâmetro externo de 19,1 mm, comprimento de 381 mm, e espaçamento transversal de 24 mm. Os experimentos foram realizados para escoamento vertical ascendente de misturas água-ar e velocidades superficiais da fase líquida e gás de 0,020 a 1,500 m/s e de 0,10 a 10,00 m/s, respectivamente. Neste estudo foram desenvolvidas técnicas inéditas para determinação experimental da fração de vazio superficial no interior do banco de tubos baseadas em sistemas óptico e de sensoriamento capacitivo. Os padrões de escoamento foram identificados subjetivamente através de visualização de imagens e vídeos do escoamento, e objetivamente com o auxílio do método de agrupamento de dados k-means utilizando parâmetros baseados no sinal de perda de pressão e do sensoriamento capacitivo. Identificou-se subjetivamente os padrões de escoamento bolhas, bolhas dispersas, bolhas grandes, agitante, intermitente e anular. Constatou-se equivalência entre os padrões de escoamento identificados através dos métodos objetivo e subjetivo. Resultados experimentais para fração de vazio foram obtidos através de técnicas óptica e capacitiva. Constatou-se que o traçador rodamina B utilizado no método óptico altera as condições do escoamento, ainda que em concentrações reduzidas. A partir dos resultados obtidos com o sensoriamento capacitivo estimou-se a fração de vazio para o padrão bolhas. Resultados para a parcela friccional da perda de pressão também foram levantados. Constata-se o incremento da fração de vazio e da parcela friccional da perda de pressão com as velocidades superficiais das fases líquida e gás. Os resultados para fração de vazio foram comparados com métodos de previsão da literatura, e de maneira geral os métodos preveem as tendências dos resultados experimentais apenas para vazões de líquido reduzidas. Analogamente, os resultados para perda de pressão foram comparados com estimativas segundo métodos da literatura, concluindo que os métodos não preveem satisfatoriamente os resultados obtidos. Desta forma, foram propostos novos métodos de previsão para padrões de escoamento, fração de vazio e parcela friccional da perda de pressão, desenvolvidos a partir de análises dos mecanismos dominantes do escoamento, e adotando parâmetros adimensionais para correlacionar os dados. Os métodos propostos preveem satisfatoriamente os resultados experimentais deste estudo e da literatura para escoamentos bifásicos água-ar.The present thesis concerns a theoretical and experimental study of external two-phase flows across tube bundles. Initially, a comprehensive literature review covering flow patterns, void fraction and pressure drop for single and two-phase flows across tubes bundle is presented. The review also describes predictive methods for these parameters. A comparison of these methods reveals reasonable disagreement among their predictions, indicating the absence of generalized methods. Subsequently, the apparatus and instrumentation designed and built to obtain the experimental data are described. The experimental apparatus comprises the test section, a water loop, air compression and conditioning systems, and sets for fluid flow injections and conditioning. The test section is a normal triangular tube bundle, with 19.1 mm OD tubes, 381 mm long and transversal pitch of 24 mm. The experiments were performed for air-water upward vertical flow, for superficial liquid and gas velocities ranging from 0.020 to 1.500 m/s and 0.10 to 10.00 m/s, respectively. Innovative techniques to evaluate the void fraction within the bundle were developed based on capacitive and optical methods. The flow patterns were identified subjectively and objectively by k-means clustering method, using as clustering parameters the pressure drop and the capacitive signals. Bubbles, dispersed bubbles, large bubbles, churn, intermittent and annular flow patterns were identified subjectively. The data groups identified by the objective method are representative of the flow patterns. Void fraction measurements were obtained for bubbly flow using both techniques (optical and capacitive). The void fraction data based on the optical method had its experimental range limited due to changes in the flow characteristics caused by the addition of the fluorescent dye Rhodamine B. The experimental results indicate that the void fraction increases with increasing the superficial velocities of both phases. In general, the void fraction predictive methods available in the literature capture the trends of the experimental results only for reduced liquid flow rates. According to the experimental results, the frictional pressure drop increases asymptotically with increasing the flow rates of both phases. None of the predictive methods from literature evaluated in the present study predicted satisfactorily the experimental results. Methods for prediction of flow patterns, void fraction and frictional pressure drop parcel were also developed in the present study. These methods provided reasonable predictions of the experimental results obtained in the present study, and also from the literature for air and water flows across tube bundles

Theoretical and experimental study on flow pattern identification and pressure drop during single and two-phase flow in tubes with twisted tape inserts

A presente dissertação trata de um estudo teórico-experimental sobre escoamento monofásico e bifásico no interior de tubos com fitas retorcidas. Esta técnica tem sido utilizada há várias décadas para a intensificação de troca de calor para escoamento monofásico e evaporação convectiva no interior de tubos. No entanto, com sua utilização, o aumento do coeficiente de troca de calor é acompanhado pelo incremento da perda de pressão. Portanto a compreensão dos fenômenos relacionados aos incrementos da perda de pressão e troca de calor são fundamentais para engenheiros projetistas. Neste estudo, inicialmente, é apresentada uma extensa revisão bibliográfica sobre padrões de escoamento, modelos para estimativa de fração de vazio e metodologias para previsão da perda de pressão em tubos com e sem fitas retorcidas para escoamentos bifásicos e monofásicos. Foram realizados experimentos em bancada experimental utilizando seção de testes com diâmetro interno de 15,9 mm e dois metros de comprimento com fitas apresentando razões de retorcimento de 3, 4, 9, 14 e , a última correspondente ao tubo sem inserto. Os experimentos foram executados para o refrigerante R134a, velocidades mássicas entre 75 e 250 kg/m²s, e títulos de vapor entre 5 e 95% no caso de escoamentos bifásicos. Resultados foram levantados para velocidades mássicas entre 100 e 450 kg/m²s para escoamentos monofásicos. Durante os ensaios foram levantados resultados de perda de pressão e identificados subjetivamente os respectivos padrões de escoamento. Como resultado final deste estudo é proposta uma correlação para previsão da perda de pressão durante o escoamento no interior de tubos com fitas retorcidas.A theoretical and experimental study on single and two-phase flow inside tubes with twisted tapes inserts is presented. Twisted tape inserts have been used for decades as a technique of heat transfer enhancement. However, their heat transfer enhancement is accompanied by an increase of the pressure drop. Therefore, understanding the pressure drop and heat transfer mechanisms is fundamental for heat exchanger designers in order to optimize these devices when using twisted-tape inserts. Based on these aspects the present study concerns single and two-phase flows inside tubes with twisted tapes. Experiments were performed in an experimental apparatus for a 15.9 mm ID and twisted tape with twist ratios of 3, 4, 9, 14 and , the last one corresponding to a plain tube without tape. For two-phase flow, experiments were conducted for R134a, mass velocities from 75 to 250 kg/m²s and vapor qualities from 5 to 95%. In case of single-phase flow, experiments were performed for mass velocities from 100 to 450 kg/m²s. The experimental campaign was focused on pressure drop and flow pattern subjective identification. Additionally an extensive literature review on flow patterns, superficial void fraction, and methodologies for prediction of the pressure drop in plain tubes with and without twisted-tape inserts during single and two-phase flow is presented. As final result, a correlation for prediction of pressure drop during flow inside tubes with twisted tape inserts is proposed

Two-phase pressure drop during upward cross flow in triangular tube bundle

This paper presents experimental results for pressure drop during air-water upward two-phase flow across horizontal triangular tube bundle. It is estimated that more than half of shell-and-tube heat exchangers in industry operate under two-phase flow conditions in the shell side. However the number of research and publications focused on external flow is considerably reduced when compared to intube flow. This study addresses experimental results for pressure drop during external flow of air and water mixtures across a triangular tube bundle counting with 19 mm OD tubes and 24 mm transverse pitch, for superficial velocities up to 0.553 and 10 m/s for water and air, respectively. For reduced mass velocities, the gravitational pressure drop parcel is dominant, consequently the adoption of an appropriate methodology for void fraction estimative is essential for accurate estimative of the gravitational pressure drop parcel. The experimental results are compared with predictive methods available in the open literature, and an analysis of this comparison is presented

Experimental flow boiling heat transfer in a small polyimide channel

New experimental flow boiling heat transfer results averaged along a very low thermal conductivity polyimide channel are presented. The experimental setup consists of a horizontal, 285 mm-long, 2.689 mm inner diameter polyimide tube, heated by a counter-current, external water flow. R245fa is the working fluid, at mass fluxes ranging from 100 to 500 kg m−2 s−1, heat fluxes from 15 to 55 kW m−2, and three saturation temperatures: 35, 41 and 47 °C. Heat transfer coefficient results are presented averaged over the whole polyimide channel for mean vapor qualities ranging from 0.05 to 0.80. High-speed flow visualization was possible downstream of the polyimide tube. Parametric analysis of the results revealed a strong dependence of the heat transfer coefficients on mass flux and vapor quality, and mild influence of the heat flux and the saturation temperature. The experimental database is compared with results obtained for metallic tubes of comparable size in similar experimental facilities. Last, experimental results are contrasted with several heat transfer methods, of which a convective model for annular flow well predicts the heat transfer coefficient values and convective boiling trends