Thermal Conductivity of Sintered Capillary Structures for Heat Pipes / Condutividade Térmica de Estruturas Capilares Sinterizadas para Tubos de Calor

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Braz. J. of Develop.,Curitiba, v. 6, n. 8, p.57375- 57381 aug. 2020. ISSN 2525-8761

Thermal Conductivity of Sintered Capillary Structures for Heat Pipes

Condutividade Térmica de Estruturas Capilares Sinterizadas para Tubos de

Calor

DOI:10.34117/bjdv6n8-225

Recebimento dos originais:08/07/2020 Aceitação para publicação:14/ 08/2020

Guilherme Antonio Bartmeyer

Mestrando em Engenharia Mecânica

Universidade Tecnológica Federal do Paraná (UTFPR)

Endereço:Av. Doutor Washington Subtil Chueire, 330, CEP 84.017-220, Ponta Grossa/PR E-mail:gabartmeyer@hotmail.com

Luis Vitório Gulineli Fachini

Mestrando em Engenharia Mecânica

Endereço:Av. Doutor Washington Subtil Chueire, 330, CEP 84.017-220, Ponta Grossa/PR E-mail:luisgulineli@gmail.com

Larissa Krambeck

Doutoranda em Engenharia Mecânica Universidade Federal de Santa Catarina (UFSC)

Endereço:Caixa Postal 476, Campus Universitário, CEP 88.040-900, Florianópolis/PR E-mail:larikrambeck@hotmail.com

Davi Fusão

Doutor em Engenharia Mecânica

Endereço;Av. Doutor Washington Subtil Chueire, 330, CEP 84.017-220, Ponta Grossa/PR E-mail:davi@utfpr.edu.br

Thiago Antonini Alves

Doutor em Engenharia Mecânica

Endereço:Av. Doutor Washington Subtil Chueire, 330, CEP 84.017-220, Ponta Grossa/PR E-mail:antonini@utfpr.edu.br

ABSTRACT

The heat pipes basically consist of a metal tube sealed with a capillary structure internally that is embedded with a working fluid. In heat pipes, the most important properties to be characterized are those related to pore structure and thermal conductivity, as they are responsible for capillary pumping and heat transfer, respectively. In this research, an experimental evaluation of the thermal conductivity of a sintered copper powder structure was performed, which can be used as a capillary structure in heat pipes. An experimental workbench based on the guarded-hot-plate principle was

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developed to determinate the effective thermal conductivity of the capillary structure of sintered copper powder. The average effective thermal conductivity of the capillary structure was 15.13 W/mK, which are approximated to the theoretical value for this structure.

Keywords: Capillary Structure, Heat Pipe, Sintered Copper Powder, Thermal Conductivi. RESUMO

Os tubos de calor consistem basicamente num tubo metálico selado com uma estrutura capilar interna que é embutida com um fluido de trabalho. Nos tubos térmicos, as propriedades mais importantes a serem caracterizadas são as relacionadas com a estrutura dos poros e a condutividade térmica, pois são responsáveis pelo bombeamento capilar e pela transferência de calor, respectivamente. Nesta investigação, foi realizada uma avaliação experimental da condutividade térmica de uma estrutura em pó de cobre sinterizado, que pode ser utilizada como estrutura capilar em tubos de calor. Foi desenvolvida uma bancada de trabalho experimental com base no princípio da chapa quente-guarda para determinar a condutividade térmica efectiva da estrutura capilar do pó de cobre sinterizado. A condutividade térmica média efectiva da estrutura capilar foi de 15,13 W/mK, que são aproximados ao valor teórico para esta estrutura.

Palavras-chave: Estrutura capilar, Tubo de Calor, Pó de Cobre Sinterizado, Condutas Térmicas.

1INTRODUCTION

The heat pipe is a highly efficient heat transfer passive device that operates on a closed biphasic cycle and uses the latent heat of vaporization of the working fluid to transfer heat from small temperature gradients [1]. The heat pipes basically consist of a metal tube sealed with a capillary structure internally that is embedded with a working fluid [2].

In heat pipes, the most important properties to be characterized are those related to pore structure and thermal conductivity, as they are responsible for capillary pumping and heat transfer, respectively [3]. The effective thermal conductivity is the property of a material in transferring heat by conduction.

Capillary structures of sintered metal powders are made from the sintering process, where the powder particles melt resulting in a continuous medium of flow. Heating the casing tube with the metallic powder particles and the aid of a mandrel performs the sintering. In this way, the metallic powder particles sinter between them and the inner wall of the tube [4]. Sintered capillary structures have a high capillary pumping, low pores, and a good thermal conductivity [5].

In this context, an experimental evaluation of the thermal conductivity of a sintered copper powder structure was performed, which can be used as a capillary structure in heat pipes.

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2METHODOLOGY

An experimental workbench based on the guarded-hot-plate principle, an adaptation of NBR 15220 - Part 04 [6], was developed to determinate the effective thermal conductivity of the capillary structure of sintered copper powder.

2.1 COPPER POWDER

The sintered capillary structure will be fabricated from an XF copper powder obtained by gas atomization. A micrograph of the copper particles with a magnification of 500x was obtained from the Scanning Electron Microscopy (SEM) and is presented in Fig. 1. The volume-based average particle diameter was 33 μm. More details about the characterization of the copper powder can be obtained in [7].

Fig. 1. SEM micrograph of the copper powder (500x)

2.2 SAMPLES FOR THERMAL CONDUCTIVITY EVALUATION

Three samples (A, B, and C) were sintered with a squared face of 37.5 mm and area of 1,406.25 mm². However, the samples had different thicknesses, which were 13.0 mm, 8.4 mm, and 5.0 mm respectively for samples A, B, and C. The sintering procedure is the same that was applied to the heat pipes subsequently. The former of sample B (made of stainless steel) and Sample B are presented in Fig. 2 and Fig 3, respectively.

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Braz. J. of Develop.,Curitiba, v. 6, n. 8, p.57375- 57381 aug. 2020. ISSN 2525-8761 Fig. 2. Former in stainless

steel for Sample B

Fig. 3. Sample B for the thermal conductivity test

2.3 EXPERIMENTALAPPARATUS

The experimental workbench, shown in Fig. 4, consisted of a laptop (DellTM_{), a} uninterruptible power supply (NHSTM), a data logger (AgilentTM 34970A with 20 channels), two power supply units (PolitermTM_{16E), a controlled automated system controlled by an Arduino}TM for the hot plate, an ultrathermostatized bath (SOLABTM SL-130), and a flowmeter (Omega

EngineeringTM FL-2051).

Fig. 4. Experimental workbench

The determination of the thermal conductivity involves the measurement of the average temperature gradient established on the sample, from certain heat flow, under steady state conditions. In this way, the testing section was composed of four elements. A hot plate (HP1) that promotes the heat flow to the sintered sample by a power dissipated in two cartridge resistances. A hot plate (HP2) heated by two cartridge resistors and controlled by an ArduinoTM_{system, which} ensures that all the flow in heat will be directed towards the sample. The sintered sample with dimensions previously mentioned. And, finally, a cold plate (CP) that promotes the cold face to the sample by the ultrathermostatized bath. The entire test section is isolated from the external environment by a 3MTM MTI PolyfabTM aeronautical insulation.

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For the evaluation of the temperatures, T-type thermocouples Omega EngineeringTM were used. These temperature sensors were allocated in roles inside the samples, one on the hot surface and another on the cold surface, Th and Tc, respectively. Fig. 5 shows the test section in detail.

Fig. 5. Test section

2.4EXPERIMENTALPROCEDURE

To perform the tests, the sample was carefully mounted in the test section, the temperature of the cold plate was maintained at 16.0°C ± 0.2°C by the ultrathermostated bath, the data acquisition system was activated, the power supplies were connected and adjusted for dissipation of desired powers. The tests were repeated three times for each sample and the errors were compared taking into account that the difference between the mean values of each thermocouple was less than 0.5°C. The tests were carried out for increasing thermal loads of 10 W, ranging from 10 to 30 W. Each thermal load was maintained for 30 (thirty) minutes, where the quasi-permanent operating condition was reached. Data were acquired every 10 (ten) seconds, recorded by the software

AgilentTM Benchlink Data Logger 3.

2.5DATAREDUCTION

The Fourier Law governs the phenomenon of heat transfer by thermal conductivity. From the experimental tests, the thermal conductivity of the capillary structure, kcs, can be calculated by:

( ) cs C h c q k A T T = −  (1)

where, q is the heat flow rate [W], δ is the distance between the thermocouples [m], AC is the cross-sectional area of the sample [m2], Th and Tc are the hot surface and cold surface temperatures [K] of the sintered sample, respectively.

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Alexander (1972) apud [8] developed a model for the calculation of effective thermal conductivity based on experimental data of sintered materials of nickel and stainless steel. This model is represented by the expression:

( )0 53 1 . cs k k k k −   = _ _       (2)

where, kα and kβ are the thermal conductivities of the solid and the fluid, respectively, and ε is the porosity of the sample. This theoretical model was validated in the comparisons of [9] for copper sintered. So, this model will be used for the comparison of the experimental data in this research.

3RESULTSANDDISCUSSION

The effective thermal conductivity of the sintered copper capillary structure is presented in Table 1. It was calculated based on the experimental results for three different samples. The average effective thermal conductivity of the capillary structure is 15.13 W/mK.

TABLEI

EXPERIMENTAL RESULTS OF THE EFFECTIVE THERMAL CONDUCTIVITY

Sample kcs [W/mK]

A 16.52 B 13.60 C 15.29 Average 15.13

The theoretical model can be calculated, 15.55 W/mK, since the average porosity of this sintered structure is 55.03% [7]. As a result, the experimental value of the effective thermal conductivity of the capillary structure is close to the theoretical value. The difference is 2.89% that validates the proposed experimental apparatus to determine the effective thermal conductivity.

4CONCLUSION

In this research was performed an experimental evaluation of the thermal conductivity of a sintered copper powder structure, which can be used as a capillary structure in heat pipes. An experimental study based on the guarded-hot-plate principle, an adaptation of NBR 15220 - Part 04, was used for the thermal conductivity determination. The experimental results showed that the average thermal conductivity of the copper powder capillary structure was 15.13W/mK, which are approximated to the theoretical value for this structure.

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ACKNOWLEDGEMENTS

The present work was carried out with the support of CAPES, CNPq, PROPPG/UTFPR, DIRPPG/UTFPR/ Ponta Grossa, PPGEM/UTFPR/Ponta Grossa, DAMEC/ UTFPR/Ponta Grossa, and LabMPEE/DAMEC/UTFPR/ Ponta Grossa.

REFERENCES

[1] D. A. Reay, P. A. Kew, R. J. McGlen, Heat Pipe: Theory, Design and Applications (Butterworth-Heinemann, 2014).

[2] P. H. D. Santos, L. Krambeck, T. Antonini Alves, Experimental Analysis of a Stainless Steel Heat Pipe, International Journal of Science and Advanced Technology, Vol. 4, pp. 17-22, 2014.

[3] X. Wang, Y. Tang, P. Chen, Investigation into Performance of a Heat Pipe with Micro Grooves Fabricated by Extrusion-Ploughing Process, Energy Conversion and Management, Vol. 50, pp. 1384-1388, 2009.

[4] R. M. German, Porous Metallurgy Science (2nd edition, Princeton, N.J.: Metal Powder Industries Federation, 1994).

[5] M. Khalili, M. B. Shafili, Experimental and numerical investigation of the thermal performance of a novel sintered-wick heat pipe, Applied Thermal Engineering, Vol. 94, pp. 59-75, 2016.

[6] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 15220: Desempenho térmico de edificações. Rio de Janeiro: ABNT, 2003. 8 p.

[7] G. A. Bartmeyer, L. Krambeck, R. C. Silva, D. Fusão, T. Antonini Alves, Characterization of a Copper Power for Heat Pipe Wicks, International Journal of Advanced Engineering Research and Science (ISSN: 2349-6495(P) | 2456-1908(O)), Vol. 5, n. 10, pp.52-54, 2018.

[8] N. Atabaki, B. R. Baliga, Effective Thermal Conductivity of Water-Saturated Sintered Powder-Metal Plates, Heat and Mass Transfer, Vol. 44, n. 1, pp. 85-99, 2007.

[9] J. P. F. Mera, Análise da transferência de calor em meios de porosidade variável para tubos de calor, M.Sc. dissertation, Dept. Mech. Eng., Universidade Federal de Santa Catarina, Florianópolis, SC, 2011.