Abstract
自然循環迴路的雙相流不穩定現象之實驗研究摘 要 在能源工程及熱傳工程應用上，雙相自然循環迴路系統是一種被廣泛應用的熱流技術之一。 本研究運用清華大學的雙相自然循環環路實驗系統，來進行環路的熱流實驗，並探討其穩 定性的特性與不穩定性的現象。當雙相自然循環迴路系統發生熱流不穩定現象時， 此環路為具有強烈的非線性效應且多變數複雜系統。此雙相流迴路系統的熱流不穩 定振盪特性，無法運用傳統線性控制系統方法來分析。本研究應用田口氏實驗分析 法及流譜識別分析技術, 來探討雙相自然循環環路內複雜的非線性熱流現象。本研 究應用田口氏實驗分析法對於進口溫度 ､加熱功率和單/雙相流區流阻大小等因子對迴 路熱流穩定性影響定性分析﹐與相關文獻研究分析結果趨勢完全符合。此田口實驗 簡單可靠的特性﹐採用十八組實驗的矩陣實驗及兩組驗證實驗﹐即可充分掌握本迴 路多變數因子極高非線性的熱流不穩定性現象。最後整合田口式實驗與雙相流譜影 像數據，來進一步探討迴路的熱流不穩定現象。在本實驗範圍 雙通道運轉模式，迴 路較易發生熱流不穩定現象。亦即未發熱的加熱段與升流段平行通道，較易造成迴 路熱流不穩定現象。因此未發熱的旁流平行通道設計，將不利於雙相自然迴路的熱流 應用。採較大單相流區流阻及較小雙相流區流阻因子水準﹐雖可改善迴路熱流不穩 定性現象﹐但單/雙相流區流阻大小並非本迴路系統熱流不穩定性現象重要因子﹐ 因此調整單/雙相流區流阻大小將無法有效改善迴路熱流不穩定性現象。Two-Phase Flow Instability Experiment in a NaturalCirculation LoopAbstractBecause of their high heat transfer capability,simplicity, and inherently safe nature, two phase naturalcirculation loops have manyindustrial applications, suchas waste heat recovery, chemical processes usingthermosiphon reboilers, and the next generation boiling waterreactor. The thermal-hydraulic instability of a two-phasenatural circulation loop, which is a complexmultivariable system with high non-linearity, wasinvestigated experimentally based on the Taguchi method.Excellent results were obtained and a broad picture of thetrends producedby the various parameters that affectinstability is presented. This study concluded that, withinthe experimental range, the inlet temperature,heatingpower and the presence of the unheated bypass channelare the three most dominant factors affecting the loopinstability. The Taguchi method is found to be veryefficient and there is a great deal of savingin the experimentalwork. The 20 runs of experiment based on the Taguchi methodsuggests that the stability of the present two-phasenatural circulation loop would be improved under the followingconditions: decreased inlet temperature, decreased thermalload, disconnected unheated bypass channel, installing thecompressible volume with a higher water level in thedownstream two-phase flow region, disconnected upstreamcompressiblevolume, increased single-phase flow restrictions,and decreased two-phase flow restrictions. Although thesame trends bear the standard method of stabilizing a boilingchannel, the restriction factors in the presentexperimentalinvestigation are found to be ineffective in dampening thethermal-hydraulic instability.( Keywords: two-phase flow, natural circulation loop,instability, Taguchi method )