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)

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

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

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

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

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%

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

Cipreste para madeira: alto incremento volumétrico com material genético apropriado.

Cupressus lusitanica Mill. é uma das espécies florestais de alto potencial como fonte de madeira para usos múltiplos no Brasil, principalmente em forma de madeira serrada para marcenaria. A espécie apresenta bom desenvolvimento na Serra da Mantiqueira, no sul de Minas Gerais, a mais de 1.000m de altitude, onde o clima é frio, com alta umidade. Visando aumentar a sua produtividade, foram introduzidas progênies de matrizes selecionadas de diversas procedências colombianas e produzidas em um pomar clonal instalado em Popayan, Colômbia. Esse material foi plantado na forma de teste de progênie, juntamente com uma testemunha comercial, além de progênies de várias matrizes selecionadas no local. Aos dez anos de idade, as progênies das matrizes selecionadas localmente não diferiram da testemunha quanto ao incremento volumétrico, retidão do fuste e diâmetro dos ramos, mas tiveram sobrevivência significativamente menor. Apesar de a média das notas para o diâmetro dos ramos das progênies de matrizes selecionadas não ter diferido estatisticamente da testemunha comercial, quatro das seis progênies apresentaram ramos mais finos que a testemunha e esse caractere apresentou correlação inversa com a sobrevivência (árvores com ramos mais finos tiveram menor capacidade de sobrevivência) em povoamentos fechados. Por outro lado, as progênies colombianas apresentaram incremento volumétrico significativamente maior, mas com fustes mais tortuosos e ramos mais espessos do que a testemunha. Não houve diferença quanto à sobrevivência entre as famílias colombianas e a testemunha. Entre as progênies colombianas, as de Caldas apresentaram sobrevivência e incremento volumétrico significativamente menores do que as de Medellín. As progênies de Medellín e Caldas tiveram fustes significativamente mais tortuosos do que as de Guarne e Tausa. O maior incremento volumétrico e os ramos mais finos entre os materiais introduzidos foram obtidos com a progênie de Tausa

Surface effects in flow boiling of R134a in microtubes

This is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2011 ElsevierThe inner surfaces of microtubes may be influenced strongly by the process of making them due to manufacturing difficulties at these scales compared to larger ones, e.g. the surface characteristics of a seamless cold drawn tube may differ from those of a welded tube. Accordingly, flow boiling heat transfer characteristics may vary. In addition, there is no common agreement between researchers on the criteria of selecting tubes for flow boiling experiments. Instead, tubes are usually ordered from commercial suppliers, in many cases without taking into consideration the manufacturing method and its effect on the heat transfer process. This may explain some of the discrepancies in heat transfer characteristics which are found in the open literature. This paper presents a comparison between experimental flow boiling heat transfer results obtained using two different metallic tubes. The first one is a seamless cold drawn stainless steel tube of 1.1 mm inner diameter while the second is a welded stainless steel tube of 1.16 mm inner diameter. Both tubes have a heated length of 150 mm and the flow direction is vertically upwards. The tubes were heated using DC current. Other experimental conditions include: 8 bar system pressure, 300 kg/m2 s mass flux, about 5K inlet sub-cooling and up to 0.9 exit quality. The results are presented in the form of local heat transfer coefficient versus local quality and axial distance. Also, the boiling curves of the two tubes are discussed. The results show a significant effect of tube inner surface morphology on the heat transfer characteristics