An experimental study on micro-scale flow boiling heat transfer

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.New experimental flow boiling heat transfer results in micro-scale tubes are presented in this paper. The experimental data were obtained in a horizontal 2.32 mm I.D. stainless steel tube with heating length of 464 mm, R245fa as working fluid, mass velocities ranging from 50 to 500 kg/m2s, heat flux from 5 to 55 kW/m2, exit saturation temperatures of 22, 31 and 41 oC, and vapor qualities from 0.05 to 0.99. Flow pattern characterization was also performed from images obtained by high speed filming. Heat transfer coefficient results from 2 to 6 kW/m2K were measured. It was found that the heat transfer coefficient is a strong function of the heat flux, mass velocity and vapor quality. The experimental data were compared against the following micro-scale flow boiling predictive methods from the literature: Bertsch et al. (2008), Saitoh et al. (2007), Kandlikar and Balasubramanian (2004), Zhang et al. (2004), Thome et al. (2004) and Liu and Winterton (1991). Although not satisfactory, Thome et al. (2004) worked the best when predicting the present database.This study is funded under contract numbers 05/60031-0, 06/52089-1 and 07/53950-5 by FAPESP (The State of São Paulo Research Foundation, Brazil)

Atividade física e Lúpus Eritematoso Sistêmico (LES) : revisão crítica da literatura

Orientador : Luciano Lacerda GurskiTrabalho de Conclusão de Curso (Especialização) - Universidade Federal do Paraná, Setor de Ciências Biológicas, Curso de Especialização em Fisiologia do Exercício.Resumo : O Lúpus Eritematoso Sistêmico (LES) é uma doença inflamatória crônica autoimune, que pode ter diversas consequências à saúde por atacar o organismo de forma sistêmica e cujo tratamento normalmente envolve uso de corticóides trazendo diversas consequências para a qualidade de vida. Neste contexto buscou-se respostas para a seguinte problemática: quais as possíveis relações entre a atividade física e o tratamento de LES? Para isto realizou-se uma revisão sistemática em artigos acadêmicos disponíveis no portal de periódicos CAPES, utilizando como descritores os termos "lúpus" e "atividade física", resultando em 59 artigos, dos quais foram selecionados 6 que indicavam relações com a temática. Após análise qualitativa foram categorizados quanto à relação com aptidão física e qualidade de vida. Os resultados indicam que o pacientes com LES tiveram piores resultados em todos os testes, com índices menores de força muscular, resistência física e qualidade de vida, ressaltam a importância do desenvolvimento de atividade física como coadjuvante no tratamento

COMPARATIVE ANALYSES OF THE THERMAL PERFORMANCE OF REFRIGERANTS R134A, R245fa, R407C AND R600a DURING FLOW BOILING IN A MICROCHANNELS HEAT SINK

A comparative study of the performance of of refrigerants R134a, R407C, R245fa and R600a during flow boiling was performed for a 123x494 µm2 heat sink composed of 50 parallel rectangular microchannels. Heat transfer experimental results for heat fluxes up to 310 kW/m2, mass velocities from 300 to 800 kg/(m2 s), liquid subcoolings of 5 and 10 °C and saturation temperature close to 30 ºC were obtained. Global heat transfer coefficients (footprint) up to 10 kW/(m2 °C) were found. The liquid superheating necessary for the onset of nucleate boiling (ONB) was also characterized, and the fluids R245fa and R407C presented the highest and lowest, respectively, superheating to trigger the boiling process. Moreover, for a fixed averaged vapor quality, the average effective heat transfer coefficient increases with increasing mass velocity and liquid subcooling. The refrigerants R600a and R407C presented the highest and the lowest heat transfer coefficients, respectively. Five heat transfer predictive methods from literature provided accurate predictions of the data for R134a, R245fa and R600a, capturing most of the data trends. No one method provided accurate predictions of the heat transfer coefficient data of R407C

Heat transfer correlation for flow boiling in small to micro tubes

This article is available open access under a Creative Commons license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Copyright © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.There is a large discrepancy in the open literature about the comparative performance of the existing macro and microscale heat transfer models and correlations when applied to small/micro flow boiling systems. This paper presents a detailed comparison of the flow boiling heat transfer coefficient for R134a in stainless steel micro tubes with 21 macro and microscale correlations and models. The experimental database that was used in the comparison includes the data for 1.1 and 0.52 mm diameter tubes, mass flux range of 100–500 kg/m2 s and system pressure range 6–10 bar obtained in the course of this study. The effect of the evaporator heated length on the comparative performance of the correlations and models was investigated using three different lengths of the 1.1 mm diameter tube (L = 150, 300 and 450 mm). This comparative study demonstrated that none of the assessed models and correlations could predict the experimental data with a reasonable accuracy. Also, the predictability of most correlations becomes worse as the heated length increases. This may contribute in explaining the discrepancy in the comparative performance of the correlations from one study to another. A new correlation is proposed in the present study based on the superposition model of Chen. The database used in developing the correlation consists of 5152 data points including the current experimental data and data obtained previously with the same test rig, fluid and methodology for tubes of diameter 4.26, 2.88, 2.01 mm. The new correlation predicted 92% of the data within the ±30% error bands with a MAE value of 14.3%

THE ADVANTAGES OF EVAPORATION IN MICRO-SCALE CHANNELS TO COOL MICROELETRONIC DEVICES

In this paper, the importance of the development of new high power density thermal management systems for electronic devices is assessed. It is described the new heat sink technologies under development to be used in the cooling of microprocessors. The main difficulties to be overcome before the spreading of one specific heat sink configuration are identified. At the end, it is concluded that a heat sink based on flow boiling in micro-scale channels is the most promising approach

Two-phase frictional pressure drop in horizontal micro-scale channels: experimental data analysis and prediction method development

An investigation was conducted on the effects of fluid refrigerant and channel geometry on the frictional pressure drop during two-phase flow inside microchannels. Experimental results for two-phase frictional pressure drop were obtained for the refrigerants R134a, R1234ze(E), R1234yf and R600a in a circular channel and for R134a in square and triangular channels. The experiments were performed for mass velocities from 100 to 1600 kg m-2 s-1, saturation temperatures of 31 and 41°C, and vapor qualities from 0.05 to 0.95. The experimental data have been analyzed focusing on the effects of the geometry and fluid on the two-phase pressure drop. Then, the data were compared with the most quoted predictive methods from literature. Based on the broad database obtained, a new method for prediction of the two-phase frictional pressure drop was proposed. The new method provided accurate predictions of the database, predicting 89% of the results within an error band ±20%.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 also thank Prof. Renato Goulart Jasinevicius for the support in obtaining the profiles of the test sections and measuring the surface roughness. The authors are also grateful to Honeywell for supplying the low GWP refrigerants R1234ze(E) and R1234yf

Comparisons of experimental results and prediction methods of supercritical CO2 cooling heat transfer and pressure drop in macro- and micro-scale channels

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.Comparisons of heat transfer and pressure drop experimental data and correlations for supercritical CO2 cooling are presented in this article. First, the physical and transport properties of CO2 at supercritical conditions are discussed and then their influence on heat transfer and pressure drop. Then, comparison and analysis relative to the available heat transfer and pressure drop correlations for supercritical CO2 cooling were done where possible. Noting the lack of all pertinent experimental details required to use the data published in many of the available studies, comments are given on how to reduce and present supercritical CO2 experimental data properly in the future. Simulations by the available heat transfer correlations were performed and the predicted results were compared with each other. Based on the comparisons and analysis, it is recommended that further efforts be made to develop improved heat transfer methods for supercritical CO2 cooling based on a more accurate database in the future. To achieve this, more careful experiments should be done in both macro- and microchannels over a wide range of test parameters, including the effect of oil. In addition, several experimental studies show that the Blasius equation works well for pressure drop of CO2 cooling in the supercritical region. More careful experimental data are still needed to further validate this conclusion.vk201

Experimental and numerical simulation of single-phase flow in a micro heat spreader applied to the cold start of internal combustion engines fueled with ethanol

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.This work presents the results of numerical simulations and experimental evaluation of a micro heat spreader applied to the cold start of internal combustion engines fuelled with ethanol. At low temperatures (below 11 °C), using only ethanol as fuel, engines are unable to start. So, it was adopted as solution in Brazil the use of gasoline to assist the first start. The gasoline is contained in an additional small reservoir implying on safety concerns. Additionally, the use of gasoline causes an increase of emissions compared with the use of only ethanol. Therefore, in the present study a micro heat spreader containing an electrical heater was developed in order to heat up the ethanol and permit the engine start under ambient temperatures down to -10 °C. Based on this, numerical simulations were performed using Computational Fluid Dynamics (CFD) software to predict the thermal behaviour of the device. Then, based on these simulations, a micro heat spreader was fabricated and experimentally evaluated. Good agreement between experimental and simulated results was obtained. Based on the initial results, the heat spreader seems to attend the requirements of its application.dc201

Simulation of Boiling Heat Transfer at Different Reduced Temperatures with an Improved Pseudopotential Lattice Boltzmann Method

[Abstract] The pseudopotential Lattice Boltzmann Method has attracted much attention in the recent years for the simulation of boiling heat transfer. Many studies have been published recently for the simulation of the bubble cycle (nucleation, growth and departure from a heated surface). This paper puts forward two-dimensional simulations of bubble nucleation, growth and departure using an improved pseudopotential Lattice Boltzmann Model from the literature at different reduced temperatures, Tr=0.76 and Tr=0.86. Two different models using the Bhatnagar–Gross–Krook (BGK) and the Multiple-Relaxation-Time (MRT) collision operators with appropriate forcing schemes are used. The results for pool boiling show that the bubbles exhibit axial symmetry during growth and departure. Numerical results of departure diameter and release period for pool boiling are compared against empirical correlations from the literature by varying the gravitational acceleration. Reasonable agreement is observed. Nucleate boiling trends with heat flux are also captured by the simulations. Numerical results of flow boiling simulations are compared by varying the Reynolds number for both reduced temperatures with the MRT model. It was found that the departure diamenter and release period decreases with the increase of the Reynolds number. These results are a direct effect of the drag force. Proper conclusions are commented at the end of the paper.Brasil. Conselho Nacional de Desenvolvimento Científico e Tecnológico; 304972/2017-7Brasil. Coordenação de aperfeiçoamento de pessoal de nivel superior; 001Fundação de Amparo à Pesquisa do Estado de São Paulo; 2016/09509-1Fundação de Amparo à Pesquisa do Estado de São Paulo; 2018/09041-5All the authors fully acknowledge the support provided by CNPq (National Council for Scientific and Technological Development, process 304972/2017-7), CAPES (Coordination for the Improvement of Higher Education Personnel, Finance Code 001) and FAPESP (São Paulo Foundation for Research Support, 2016/09509-1 and 2018/09041-5)