熱管做為熱傳增強元件之金屬氫化物儲氫容器吸氫熱流模擬研究; Simulation of Heat and Mass Transfer during hydrogen absorption in a storage canister Equipped with a Heat Pipe

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Title:	熱管做為熱傳增強元件之金屬氫化物儲氫容器吸氫熱流模擬研究;Simulation of Heat and Mass Transfer during hydrogen absorption in a storage canister Equipped with a Heat Pipe
Authors:	陳元任;Yuan-Jen Chen
Contributors:	機械工程研究所
Keywords:	熱管;金屬氫化物;模擬;吸氫;儲氫容器;Heat pipe;storage Canister;hydrogen;absorption;simulation
Date:	2011-07-27
Issue Date:	2012-01-05 12:33:03 (UTC+8)
Abstract:	本文為LaNi5儲氫罐之吸氫過程進行建模與數值模擬研究，儲氫罐內部分成上下兩種結構，上層為提供儲氫合金因為吸氫膨脹所預留的膨脹區，下層則由儲氫合金構成的多孔性介質區。本文對儲氫合金物理特性進行適當的假設，以能量方程式描述合金吸氫時放熱反應所造成的儲氫罐溫度變化情形。氫氣與合金質量的變化以連續方程式描述。至於氫氣流場的描述，在多孔性介質的合金區與膨脹區分別使用布里克曼-佛許海默與那維爾-史托克斯方程式。最後再藉由COMSOL Mutilphysics 3.5a 利用有限元素法進行數值模擬分析。模擬結果並與文獻實驗比對，以驗證模型的正確性。由模擬分析得知，添加熱管與內鰭片的儲氫罐在相同條件下整體吸氫速率較中空圓柱型儲氫罐快。原因為中空圓柱型儲氫罐中心的熱在吸氫過程中較不易傳出，所以其吸氫平衡壓較高，因而不易進行吸氫反應。添加熱管與內鰭片的儲氫罐整體散熱能力較佳，故整體吸氫速率較快。由整個吸氫過程的模擬分析得知，氫氣在膨脹區大致上會有四種流場分佈情形，主要原因為入口的氫氣慣性力與浮力抗衡所造成的結果。至於參數分析的部份，固定鰭片厚度而比較不同半徑長度的影響，發現鰭片半徑越長的儲氫罐整體吸氫速率越快；而固定鰭片半徑長度而比較不同厚度的影響，發現鰭片厚度越厚的其整體吸氫速率越快。藉由本模擬亦可得知熱管的熱傳極限需求，有助於儲氫罐設計製做時熱管的選取，如果需求熱傳量超過熱管的熱傳極限，則熱管只對後期的吸氫速率有些微幫助。 A simulation study of the hydrogen absorption processes using LaNi5 hydrogen storage alloy is presented. The hydrogen storage canister is assumed to comprise two physical domains, which includes an expansion volume atop an alloy bed. The expansion volume provides the spare space which allow for alloy to expand during the absorption processes. The alloy bed is treated as a porous medium. The energy equations are used to describe the temperature changes induced by the heat release during the hydriding processes which causes the alloy's temperature to change with time. The continuity equations are adopted to describe the mass balance between the alloy and hydrogen. The Brinkman-Forchheimer and Navier-Stokes equations are used to describe the gas flow within the porous medium and expansion volume, respectively. The mathematical model developed is solved using a finite element code, COMSOL Multiphysics 3.5a. To verify the model, results from simulation are compared with experimental data. Results show that in the same conditions, the canister with heat pipe and fins absorbs hydrogen faster than the hollow one. This is because the heat pipe and fins help transfer heat inside out, so the equilibrium pressures of the alloy would be lower and the absorbing rates would be higher in the finned canister than in the hollow one. Results also show the entire period may be divided into four stages according to the hydrogen flow patterns, which is the outcome of the competition between the inertia and buoyancy forces in the expansion volume. Parameter analyses are also performed on the absorption rates. Results show the larger the diameter and thickness of the fins, the faster the absorption rates. The model can be applied to evaluate the request for the power limit of the instored heat pipe. The heat pipe would work when the heat transfer rates through the pipe are less than the power limit. Otherwise, the heat pipe would increase the absorption rates only in the very final stage of the hydriding processes.
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