相近平面邊界之單圓柱層流與紊流之三維時變流場與熱傳分析

Three-Dimensional Laminar and Turbulent Unsteady Flow Field and Heat Transfer Analysis of a Circular Cylinder near the Plane Boundary

林冠瑋(Guan-Wei Lin)

指導教授 ： 謝宜宸

國立虎尾科技大學/工程學院/機械與機電工程研究所/碩士(2015年)

摘要

本文主要以Comsol 分析軟體進行數值模擬，研究於三維模型流場中空氣
流經一熱圓柱與相鄰熱絕緣邊界之流場與熱傳變化。在數值模擬上以非穩態、不可壓縮流體及熱絕緣為基本的物理假設，再配合層流與標準紊流模式建立三維流場模型。本研究為雷諾數(Re)介於1300∼4100、間距比(G/D)介於0.1∼1.2之流場，從中比較當流體在不同條件時所產生的不同現象以及在標準k–ε 紊流模式與層流模式中的差異性。
由結果可觀察到在不同雷諾數下，層流與紊流模式在各間距比之速度分佈
皆無太大差異。圓柱後方渦度變化會隨著雷諾數與間距比的上升而逐漸產生明顯的渦街(vortex street) ，在渦流中心會產生較大的負壓，會隨著靠近出口而減弱，在紊流模式下流體較容易受邊界層剪切影響，後方渦流較長且強度較弱，負壓值也較小。升力以及阻力的部分在低雷諾數與間距比時較無明顯振盪，在紊流模式下會更加明顯。在熱傳方面，在任何雷諾數下，間距比0.1 時會有較好的散熱效果，因間隙速度較大，帶走的能量也會越多，其圓柱表面平均紐塞數也會隨著間距比上升而下降，熱傳效果也越差。

關鍵字

有限元素 ； 流場分析 ； 紐塞數 ； 圓柱 ； 雷諾數 ； 間距比

並列摘要

In this study, used Comsol software to numerical simulation, research on the three-dimensional model of the flow field, when fluid flows through a heat-insulated
boundary and heat cylindrical, observed the flow field and heat transfer changes. With unsteady and incompressible fluid and thermal insulation for the basic physical assumption, used laminar and turbulent flow model to establish a standard three-dimensional flow model. The conditions for Reynolds number between 1300 and 4100, gap ratio from 0.1 to 1.2. Comparison different phenomena when the fluid produced in different conditions, as well as the k-ε turbulence model and laminar flow pattern differences.
From the results can be observed at different Reynolds numbers, Laminar and turbulent flow mode in the gap ratio of the velocity distribution are not much
differences. The cylindrical rear, the vortices increase as the Reynolds number and gap ratio gradually significantly, the center of the vortex will have a large negative pressure, it will be weaken when close to the exit, In the turbulent mode, easily affected by the boundary layer shear, the vortex become longer and weaken and the negative pressure is smaller. The lift and drag at low Reynolds number and gap ratio without obvious oscillation, in the turbulent flow mode will become more apparent. In heat transfer, have better heat dissipation when gap ratio in 0.1 because the high gap velocity. The average number of cylindrical surface Nusselt number also increased by gap ratio decreases.

並列關鍵字

Finite element ； Flow field analysis ； Nusselt number ； Cylinder ； Reynolds number ； gap ratio

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