A new formulation for unsteady heat transfer of oscillatory flow in a circular tube

원관내 왕복유동에서 비정상 열전달 관계식의 공식화

Heat Transfer with periodic fluctuation of fluid temperature caused by oscillatory flow or compression expansion can be out of phase with balk fluid-wall temperature difference. Newton's law of convection is inadequate to describe this phenomenon. In order to solve this problem the concept of the complex Nusselt number has been introduced by severla researchers. The complex Nusselt number expresses out of phase excellently while the first harmonic is dominant in the variations of both fluid-wall temperature difference and heat flux. However, in the case of oscillatory flow with non-linear wall temperature distribution, the complex Nusselt number is not appropriate to predict the heat transfer phenomena since the higher order harmonic components appear in periodic temperature variation. Analytic solutions to the heat transfer with an sinusoidal well temperature distribution were obtained to investagate the effect of non-linear wall temperature distribution. A new formula considering the thermal boundary layer was suggested based on the solutions. A comparison was also made with the complex Nusselt number. It was verified that the new formula describes well the heat transfer of oscillating flow even if the first harmonic component is not dominant in the fluid-wall temperature difference.

왕복유동에 의한 순간 열전달현상은, 왕복유동의 두가지 특성인 왕복주파수와 왕복거리를 나타내는 .betha.와 .gamma.에 의하여 결정되고, 그 특성에 따라 세영역으로 나누어짐을 확인하였다. 영역 I에서는 일방향유동에서와 마찬가지로 열유속이 평균온도차에 비례하고, 영역 II에서는 열유속과 평균온도차간에 약 45.deg.의 위상차가 발생하지만 모두 1차 조화성분이 주로 나타났다. 한편 영역 III의 경우에는 위상차가 생길뿐만 아니라 평균온도차에 고차 조화성분이 나타났다. 기존의 방법인 복소 Nusselt수를 고차 조화성분까지 확장하면 열유속을 평균온도차로 나타낼 수 있지만 각 조화성분의 정보를 다 알아야하므로 실제 적용이 불가능하였다.