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

The Korean Society of Mechanical Engineers (대한기계학회)
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The Effect of Impact Velocity on Droplet-wall Collision Heat Transfer Above the Leidenfrost Point Temperature

Leidenfrost 지점 온도 이상에서 액적-벽면 충돌 열전달에 대한 충돌 속도의 영향

  • Park, Jun-seok (Dept. of Nuclear Engineering, Kyung Hee Univ.) ;
  • Kim, Hyungdae (Dept. of Nuclear Engineering, Kyung Hee Univ.) ;
  • Bae, Sung-won (Thermal-Hydraulics Safety Research Division, Korea Atomic Energy Research Institute) ;
  • Kim, Kyung Doo (Thermal-Hydraulics Safety Research Division, Korea Atomic Energy Research Institute)
  • 박준석 (경희대학교 원자력공학과) ;
  • 김형대 (경희대학교 원자력공학과) ;
  • 배성원 (한국원자력연구원 열수력안전연구부) ;
  • 김경두 (한국원자력연구원 열수력안전연구부)
  • Received : 2015.01.19
  • Accepted : 2015.05.25
  • Published : 2015.07.01
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Abstract

Single droplet-wall collision heat transfer characteristics on a heated plate above Leidenfrost temperature were experimentally investigated considering the effects of impact velocity. The collision characteristics of the droplet impinged on the heated wall and the changes in temperature distribution were simultaneously measured using synchronized high-speed video and infrared cameras. The surface heat flux distribution was obtained by solving the three-dimensional transient heat conduction equation for the heated substrate using the measured surface temperature data as the boundary condition for the collision surface. As the normal impact velocity increased, heat transfer effectiveness increased because of an increase in the maximum spreading diameter and a decrease in the vapor film thickness between the droplet and heated wall. For We < 30, droplets stably rebounded from a heated wall without breakup. However, the droplets broke up into small droplets for We > 30. The tendency of the heat transfer to increase with increasing impact velocity was degraded by the transition from the rebounding region to the breakup region; this was resulted from the reduction in the effective heat transfer area enlargement due to the breakup phenomenon.

Leidenfrost 온도 이상으로 가열된 벽면과 충돌하는 액적의 속도가 열전달 특성에 미치는 영향에 관한 실험 연구를 수행하였다. 동기화된 초고속 가시화 카메라와 적외선 카메라를 이용하여 벽면과 충돌하는 액적의 충돌 특성과 충돌면의 온도 분포를 측정하였다. 획득한 표면온도 분포를 충돌면의 경계 조건으로 이용하여 가열 벽면의 3차원 비정상 열전도 수치해석을 통해 표면 열유속 분포를 얻었다. 수직방향 충돌속도가 증가할수록 최대 액막 직경이 증가하고 가열 벽면과 액막 사이에 존재하는 증기막의 두께가 감소하여 열전달 효율이 증가하였다. 액적은 웨버수가 30보다 작은 경우 되튐현상이 발생하였으며, 큰 경우 작은 액적들로 분쇄되어졌다. 충돌속도에 의한 열전달량의 증가 경향이 되튐영역에서 분쇄영역에서 가면서 약화되었으며, 이는 분쇄현상에 의해 유효 열전달 면적의 확대 효과가 저감되었기 때문으로 해석된다.

Keywords

References

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