增進活塞熱傳效率之數值模擬

Abstract

本文主要目的在以數值方法探討活塞內部加裝一冷卻渠道，觀察此渠道在近進口流、活塞做往復運動下以及浮力效應作用下，渠道內部流場和溫度場的變化，並討論冷卻流體對活塞高溫壁面的熱傳增益。 本研究採用葛拉金有限元素法，配合Arbitrary Lagrangian-Eulerian （ALE）座標描述方法，首先探討冷卻流體在往復運動下的流場和溫度場之變化，以瞭解這類問題的運動機制，並進而針對不同引擎的擺置方式、流場雷諾數以及浮力效應，比較其對活塞高溫壁面的熱傳增益。綜合所獲得的研究結果，當活塞運動後，會在活塞高溫壁面附近產生類似牽引和推擠流體現象，使得原本附在高溫壁面上之溫度邊界層受到擾動而被破壞和縮小，熱傳效果會有效提升。而當進口流速度及重力方向相反的狀況下，高溫流體會因為浮力效應被帶離壁面，熱傳效率有提升的現象。

The aim of this numerical analysis is to investigate the variations of flow and thermal fields in the cooling channel within the reciprocating piston, and discuss the heat transfer effect of the cooling flow on the heated crown of the piston. At first, a Galerkin finite element formulation with ALE method is adopted to investigate the variations of the flow and thermal fields induced by the reciprocation of the piston. Then, the heat transfer effect on different model set up such like different Reynolds numbers, Grasholf numbers, and piston’s relative angles from horizon, will be shown and discussed. Based on the above procedures, the results show that the reciprocating piston may draw and push the cooling flow. This phenomenon will destroy the thermal boundary layer and contract its thickness. Therefore, the heat transfer rate is enhanced remarkably. On the other hand, when inlet flow direction is set up against gravity, heated fluid will be taken away from high temperature surface by buoyancy, that way enhances heat transfer efficiency.