在實際的熱傳問題中，總是存在許多無法直接經由感測器量測或者藉由精密計算之理論而獲得的資訊。例如在機車引擎中我們無法得知引擎內部實際產生熱通量為何，為了求取上述情況之物理量，必須借助於其他可容易取得或量測之條件，進一步得到待求之物理現象。此類之問題研究稱之為反算問題(Inverse Problem)或是逆問題。換句話說，反算問題就是如何根據具體物理行為和實際可以提供的量測資訊來訂定出準確的逆數學模型，使其可穩定且準確估測出所欲求解之未知條件。
本論文可分為兩個章節，均在探討反算法應用於散熱系統中未知熱通量之預測，在第二章與第三章中吾人探討與鰭片散熱的相關的反算問題，吾人皆以套裝軟體CFD-RC為基礎，利用共軛梯度法(CGM)來分別研究一二維不規則形狀鰭片及三維汽缸的逆向熱傳導問題。
在第二章中，問題為一二維不規則形狀鰭片模型，是參考機車引擎汽缸外部鰭片之剖面型狀設計，鰭片底部邊界為一未知暫態熱通量邊界，本章利用共軛梯度法(CGM)，藉由已知邊界條件，配合模擬溫度感測器測量鰭片表面一點溫度來預測未知的邊界熱通量。
在第三章中，進一步對機車引擎汽缸研究，吾人建一三維機車引擎汽缸之物理模型。利用共軛梯度法(CGM)來進行汽缸內部未知邊界熱通量預測，藉由已知邊界條件，配合模擬紅外線熱像儀量測汽缸外部表面之溫度分佈來預測汽缸內部未知邊界熱通量。

In the realistic thermal problems, there exists unknown information which is difficult to obtain directly through sensor measurements or numerical methods. For instance, heat flux intensity inside the cylinder. In order to obtain those unknown physical quantities, we need to utilize some known measurement or acquirable condition to determine the unknown physical phenomena. This technique is called the inverse method which can be used to solve many kinds of industry problems.
The present thesis divides into two chapters; the techniques of the Conjugate Gradient Method (CGM) and a general purpose commercial code CFD- ACE+ is successfully developed and examined in this thesis based on the simulated measured temperature distributions on the surface by infrared thermography.
In chapter two, an inverse problem is examined by an iterative regularization method, i.e. the Conjugate Gradient Method (CGM), and commercial package CFD-ACE+ to estimate the unknown boundary heat flux of a two-dimensional irregular shape fin, based on one sensor measurements with time. The irregular shape fin is modeled as the fin of motorcycle cylinder.
In chapter three, a three-dimensional inverse heat conduction problem is considered by using the Conjugate Gradient Method (CGM) and the general purpose commercial code CFD-ACE+ to estimate the inside unknown heat fluxes of an engine cylinder based on the simulated measured temperature distributions on surface by the infrared thermography.

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