可調整式入口對直管混合對流之熱傳效率最佳化研究

Abstract

本研究以數值方法分析可壓縮流在三維四面加熱垂直方管中混合對流之流動與熱傳機制，並提出可調整式入口將混合對流熱傳效率最佳化。為了模擬三維可壓縮流流場現象，Roe Scheme，preconditioning，dual time stepping與LUSGS(Lower Upper Symmetric Gauss Seidel)法被用來求解統御方程式，且由於流場溫差超過30K，因此不使用Boussinesq assumption。 過去在模擬直管混合對流時，會在入口給定均勻等速以模擬強制對流並與熱浮力產生之自然對流形成混合對流。過去研究指出:若入口速度給定不足，由於入出口質量守恆，出口處會形成回流，進而影響熱傳效果。本研究以可調整式入口取代入口完全由強制對流提供流體的流動機制，在入口靠近加熱壁面處加入自然吸入流體的開口區，補足不足的流量來解決回流問題，並加入薄壁面區並調整寬度改變流場速度以達到熱傳最佳化。結果顯示開口區同時受到浮力效應與強制對流的影響，壁面附近的開口區速度加快使得加熱壁面的熱傳上升。加入薄壁面使得開口區口徑下降並使開口區速度上升、熱傳效果增強。本研究進而得出熱傳效果最佳的開放區域口徑大小，並得出欲使用本熱傳增強方法所需之最小入口雷諾數。與之前的研究比較，本方法有效地消除回流現象並將熱傳效率大幅提升百分之三十。

This article is dedicated to a new method for improving heat transfer of mixed convection from the numerical perspective. An adjustable inlet boundary is proposed in the new method to substitute an inlet that is completely occupied by the amount of fluid provided by forced convection included in mixed convection. According to this new method, the flow reversal occurring in the large Richardson number situation can be eliminated significantly, which results in an improvement in the heat transfer rate of mixed convection. In order to investigate a compressible flow problem, this study focus on four methods, i.e. Roe scheme, preconditioning, dual time stepping, and the LUSGS (Lower-Upper Symmetric-Gauss-Seidel), to solve the governing equations. Except for the aperture occupied by the mount of fluid provided by forced convection, this study simulates the non-reflecting boundary conditions in other open boundaries. Based on our previous study which has successfully explored a phenomenon of flow reversal, this study intends to enhance the heat transfer rate under this circumstance, and the result has shown a significant improvement, by 30 percentage, comparing with the previous study. Also, a criterion of estimation the necessity of forced convection included in mixed convection is proposed for enhancing heat transfer rate. Present results and existing results have good agreement.