Transactions of the Korean Society of Mechanical Engineers B (대한기계학회논문집B)

The Korean Society of Mechanical Engineers (대한기계학회)
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Onset of Natural Convection in Transient Hot Wire Device for Measuring Thermal Conductivity of Nanofluids

비정상열선법을 이용한 나노유체 열전도도 측정 시 자연대류 개시점에 대한 연구

  • Lee, Seung-Hyun (School of Mechanical and Aerospace Engineering, Korea Aerospace Univ.) ;
  • Kim, Hyun-Jin (School of Mechanical and Aerospace Engineering, Korea Aerospace Univ.) ;
  • Jang, Seok-Pil (School of Mechanical and Aerospace Engineering, Korea Aerospace Univ.)
  • 이승현 (한국항공대학교 항공우주 및 기계공학부) ;
  • 김현진 (한국항공대학교 항공우주 및 기계공학부) ;
  • 장석필 (한국항공대학교 항공우주 및 기계공학부)
  • Received : 2010.09.29
  • Accepted : 2010.12.07
  • Published : 2011.03.01
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Abstract

We perform a numerical study to determine the time of onset of natural convection in a transient hot wire (THW) device for measuring the thermal conductivity of nanofluids. The samples used in this simulation are water-based $Al_2O_3$ nanofluids with volume fractions of 1%, 4%, and 10%, and the properties are calculated by theoretical models and experimental correlations. The THW apparatus using coated wire is modeled by the control-volume-based finite difference method, and the start of natural convection is determined by observing the temperature rise of the wire under a gravity field. The onset time is 11.5 s for water and 41.6 s for water-based $Al_2O_3$ nanofluids predicted by Maxwell thermal conductivity model with a 10% volume fraction. We confirm that the onset time of natural convection of nanofluids in the cylinder increases with the nanoparticle volume fraction. We suggest a correlation for predicting the onset time on the basis of the numerical results. Finally, it is shown that the measurement error due to natural convection is negligible if the measurement using the transient hot wire method is completed before the onset of natural convection in the base fluid.

본 논문에서는 비정상열선법을 이용한 나노유체의 열전도도 측정시, 자연대류 개시점을 수치적 방법을 통하여 파악해 보았다. 측정 유체는 부피비 1, 4, 10% 를 갖는 물-기반 알루미나 나노유체이고, 이에 대한 물성치는 기존 이론모델 및 실험적 상관관계식을 이용하여 계산하였다. 비정상열선법 장치는 FDM 방식으로 모델링 되었으며, 자연대류의 개시점은 중력장하의 열선의 온도변화를 관찰함으로써 파악하였다. 자연대류의 개시점은 물의 경우 11.5 초이고, 10% 부피비에서 Maxwell 모델로 열전도도를 예측한 알루미나 나노유체인 경우 41.6 초로 계산되었다. 특히 부피비가 증가할수록 자연대류가 늦게 발생함을 확인하였으며, 계산된 결과를 이용하여 비정상열선법의 실린더 내부에서 나노유체의 자연대류 개시점을 예측할 수 있는 관계식을 제시하였다. 또한 비정상열선법으로 열전도도를 측정할 때, 기본유체의 자연대류 발생시점 이전에 측정이 이루어진다면 나노유체의 열전도도 측정시 자연대류에 의한 측정오차는 무시할 수 있음을 확인하였다.

Keywords

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