生質柴油預混醇類之微爆機制研究

Abstract

一般生質柴油混合醇類之液滴燃燒時，通常在過程中皆會有微爆之情形發生，然而在本文中卻發現有非微爆使液滴燒完的現象存在。因此本研究主要針對生質柴油預混碳數較低之醇類(甲醇、乙醇、異丙醇)液滴，利用懸掛以及自由液滴法，探討其燃燒時所發生之微爆機制。經由實驗結果顯示，使用懸掛液滴法將液滴打上懸掛線時，會觀察到液滴表面有產生許多小液泡，經由碰撞並結合後形成大顆之不均勻液泡，且隨著環境相對濕度上升，液滴內所產生之液泡尺寸也會變大。而在相同相對濕度下，甲醇混合液滴會產生較大的液泡，其次為乙醇，最後為異丙醇，推測使此種原先互溶之混合液滴內部形成不均勻液泡，是因混合具有吸濕性之低碳數醇類，使得液滴表面吸收環境中水份並產生相分離所造成。 本文亦發現在懸掛液滴法下，混合醇類之液滴其內部相分離液泡，經由靜置過程後會慢慢消失，液滴也會恢復成互溶狀態。對於混合最易吸水之甲醇液滴而言，會因其內部液泡較大而需2.5分鐘左右之靜置時間，而混合乙醇以及異丙醇液滴則約需1.5分鐘。而液滴所形成之相分離液泡也是影響燃燒時微爆發生與否之主因，若經由靜置使液滴內部液泡消失，則點燃時即不會有微爆發生。 實驗最後證實，生質柴油混合醇類液滴所發生之微爆現象，並非是因兩者沸點差異過大所形成之均勻成核造成，而是因混合吸濕性醇類使液滴形成不均勻現象，並在燃燒時產生非均勻成核所導致，且此種情形在醇類混合其他不溶於水之油料時亦會發生，故若希望液滴在燃燒過程中產生微爆並提升燃燒速率，則可在油料中添加易吸濕之醇類來達到此效果。

Microexplosions have been reported to occur during the combustion process of mixtures of biodiesel and alcohol fuels. However, we have found that they can be prevented if the standing time of the fuel sample is sufficiently long. In the experiments based on suspending droplets and free droplets, the mechanisms of microexplosions were studied by investigating the burning process of droplets composed of various proportions of low-C alcohols and biodiesel. Before ignition, we found that many heterogeneous sites, which appeared similar to drops, emerged from the droplet surface and subsequently merged to a bigger one and deposited at the bottom of the droplet. The diameter of this heterogeneous drop increased with the humidity of the ambience. At the same humidity, the experiments showed that the merged heterogeneous drop of biodiesel/methanol was largest and the smallest one was generated by the biodiesel/isopropanol; the one in-between was generated by biodiesel/ethanol. These heterogeneous sites were likely caused by the hygroscopic characteristics of alcohol fuels, which were supposed to mix well with the fuel of biodiesel and treated as a miscible blend in the first place. However, the miscibility was destroyed by absorbed water and phase separation occurred. On the other hand, the interior heterogeneous drop vanished after a sufficient standing time, and consequently microexplosion were not found during the whole combustion process. For the mixture fuel with methanol, which has higher tendency of water absorption, about 2.5 minutes of standing time was needed to remove the heterogeneous drop. Nonetheless, only ~1.5 minutes was measured for the mixture fuel with isopropyl, which has lower propensity to absorb water. The heterogeneous site inside the blended fuel droplet was concluded to be the primary cause of microexplsion during combustion process. In contrast, elimination of the heterogeneous site in the test droplets can prohibit the appearance of microexplosion.