實驗與理論探討上下振動式ㄇ型管道內混合對流之研究

Abstract

燃燒室內之活塞，其頂部直接與汽缸內之火焰接觸，除了承受爆炸瞬間的極大壓力外，亦 從燃氣中吸收了大量的熱量，若活塞頂部溫度過高，又不能及時冷卻，將造成活塞燒損或龜裂。 由先前的研究（93-2212-E-009-010、94-2212-E-009-006、95-2221-E-009-172、96-2221-E-009-059） 已可發現，如果活塞的散熱管道內只有強制對流存在，而不考慮因為活塞高溫所產生的自然對 流情況下，由數值模擬計算所得的散熱效率，和由實驗設備所量測出來的數據極為吻合，表示 數值模擬之程式已發展的相當完善。 但考慮到引擎的運轉狀況多為直立式，爆炸的高溫使活塞散熱管道內的自然對流效應強 烈，散熱管道內流體的數值模擬必須包含強制對流及自然對流，產生極為複雜的混合對流狀 況，且活塞往復運轉本身即為和時間有關的暫態熱傳，方程式及流場都是極為複雜的物理現 象。因此，本研究將考慮自然對流與強制對流共存狀態進行數值模擬，並將所得結果和先前的 研究比較差異。另，接續上述的數值模擬研究，希望在散熱渠道的高溫面上加入多孔性介質， 以增強高溫面的散熱效率。研究的最後階段，可希望藉由實驗的方法來取得實際的流場數據， 以驗證數值計算的結果。 燃燒室內之活塞，其頂部直接與汽缸內之火焰接觸，除了承受爆炸瞬間的極大壓力外，亦 從燃氣中吸收了大量的熱量，若活塞頂部溫度過高，又不能及時冷卻，將造成活塞燒損或龜裂。 由先前的研究（93-2212-E-009-010、94-2212-E-009-006、95-2221-E-009-172、96-2221-E-009-059） 已可發現，如果活塞的散熱管道內只有強制對流存在，而不考慮因為活塞高溫所產生的自然對 流情況下，由數值模擬計算所得的散熱效率，和由實驗設備所量測出來的數據極為吻合，表示 數值模擬之程式已發展的相當完善。 但考慮到引擎的運轉狀況多為直立式，爆炸的高溫使活塞散熱管道內的自然對流效應強 烈，散熱管道內流體的數值模擬必須包含強制對流及自然對流，產生極為複雜的混合對流狀 況，且活塞往復運轉本身即為和時間有關的暫態熱傳，方程式及流場都是極為複雜的物理現 象。因此，本研究將考慮自然對流與強制對流共存狀態進行數值模擬，並將所得結果和先前的 研究比較差異。另，接續上述的數值模擬研究，希望在散熱渠道的高溫面上加入多孔性介質， 以增強高溫面的散熱效率。研究的最後階段，可希望藉由實驗的方法來取得實際的流場數據， 以驗證數值計算的結果。

The top surface of piston directly contacts with flame in the cylinder of engine. Not only the huge pressure that the piston should be affected, it also absorbs large amount of heat that the effects of huge pressure and extremely high temperature on the top surface of piston could cause the piston to be broken and crashed. If the temperature of piston is very high and the cooling efficiency is lower, the piston will be broke down or crushed. From the previous research, the forced convection was present in the cooling channel and the nature convection caused by the high temperature from the piston was not taken into consideration. The numerical simulation data were well matched with the results measured by experimental works. However, most engines are installed vertically, the natural convection induced by the high temperature situation of the piston should be taken into consideration during analyzing process. Therefore, the aim of this project investigates phenomena of co-existence of forced and natural convection in a reciprocating ㄇ shape channel numerically and experimentally. First year, a numerical situation of a mixed convection in a vertically installed reciprocating ㄇ shape channel is executed, the directions of the reciprocating velocity of the channel parallel and opposite to that of the gravity are considered, respectively. Second year, for enhancing heat transfer rates of the piston, porous medium is installed on the heat top surface, and analyze the heat transfer mechanisms of the porous medium. Last year, an experimental work relating with the above numerical study is constructed. Due to the co-existence of force and natural convection, the magnitude of heat energy is not huge and it must be careful to control heating process.