本文選題：低壓蒸汽冷凝 + 液滴動態(tài)特性　； 參考：《大連理工大學(xué)》2015年博士論文

【摘要】：蒸氣冷凝作為典型的相變現(xiàn)象,普遍存在于自然界與日常生活中,同時作為高效的傳熱形式被廣泛應(yīng)用在工業(yè)生產(chǎn)中。其中,低壓蒸汽冷凝在低品位余熱回收領(lǐng)域具有重要的應(yīng)用,如低壓蒸餾、低溫多效海水淡化、低溫?zé)岜煤蜔峁芗夹g(shù)等。隨著蒸汽壓力下降,界面?zhèn)鬟f阻力顯著升高,降低了冷凝過程的傳熱性能。與膜狀冷凝相比,滴狀冷凝可以有效地降低表面凝液引起的導(dǎo)熱熱阻,而且液滴動態(tài)行為促進汽液界面的傳遞過程,成為了低壓蒸汽冷凝傳熱的理想強化策略。本文結(jié)合實驗觀測、數(shù)值模擬和理論分析,系統(tǒng)地研究了低壓蒸汽滴狀冷凝過程液滴動態(tài)特性、尺寸分布及演化規(guī)律和表面溫度演化特征,揭示了蒸汽滴狀冷凝傳熱微觀機理；利用超疏水表面界面效應(yīng)促進液滴合并彈跳,實現(xiàn)了低壓蒸汽冷凝傳熱強化。設(shè)計搭建了低壓蒸汽冷凝傳熱實驗系統(tǒng),考察了蒸汽壓力對滴狀冷凝傳熱性能和液滴動態(tài)行為的影響規(guī)律。結(jié)果表明,隨著蒸汽壓力下降,滴狀冷凝傳熱系數(shù)先緩慢降低而后迅速下降。結(jié)合液滴傳熱模型,分析了低壓蒸汽冷凝傳熱控制機理,表明液滴生長受汽液界面?zhèn)鬟f過程的影響隨著壓力降低而加強。實驗發(fā)現(xiàn)了低壓蒸汽冷凝中存在液滴脫落滯后現(xiàn)象,延緩了表面更新頻率,揭示了液滴運動與宏觀傳熱間的內(nèi)在關(guān)聯(lián)。通過引入液滴動態(tài)特性,修正了經(jīng)典滴狀冷凝傳熱模型,準確地預(yù)測了蒸汽壓力和表面過冷度對滴狀冷凝傳熱特性的影響規(guī)律。實驗觀測了不同蒸汽壓力下液滴瞬態(tài)尺寸分布演化與穩(wěn)態(tài)尺寸分布規(guī)律,揭示了蒸汽壓力對液滴生長特性的影響規(guī)律。初始冷凝液滴呈現(xiàn)出由正態(tài)分布到雙峰分布再到指數(shù)分布的瞬態(tài)尺寸演化特征,隨著蒸汽壓力降低,核化點密度減小且液滴生長速率減慢。與常壓蒸汽相比,低壓條件下冷凝表面小液滴密度減小,而大液滴出現(xiàn)頻率升高,增加了冷凝表面液滴的平均尺寸。利用液滴脫落動態(tài)滯后模型,解釋了液滴動態(tài)行為降低動態(tài)滯后角的現(xiàn)象；從局部能壘角度出發(fā),闡明了液滴非連續(xù)性脫落行為的物理機制。利用紅外熱成像技術(shù)觀測了低壓蒸汽冷凝中液滴表面溫度分布與演化特征,直觀地獲得了液滴運動引起的微細傳熱現(xiàn)象。結(jié)果表明,冷凝液滴表面溫度呈現(xiàn)中間高邊緣低的分布特征。蒸汽冷凝過程與液滴生長行為和界面演化密切相關(guān),與微液滴的傳熱不同,大液滴通過運動更新汽液界面促進蒸汽發(fā)生快速冷凝,揭示了液滴行為引起的界面演化對宏觀傳熱影響的內(nèi)在機制。與超疏水表面相比,疏水表面更有利于液滴核化生長,表面溫度分布更不均勻,液滴運動引起的表面溫度波動更劇烈。結(jié)合模型分析、SE模擬和可視化實驗,從蒸汽初始核化入手,考察了表面結(jié)構(gòu)和冷凝條件對初始核化液滴及微液滴潤濕模式的影響規(guī)律。根據(jù)冷凝液滴的跨尺度生長特性和超疏水表面結(jié)構(gòu)特征,提出了表面結(jié)構(gòu)對液滴生長的空間限制效應(yīng)。利用V形納米結(jié)構(gòu)控制液滴初始核化形態(tài)和位置,揭示了超疏水表面上冷凝液滴的潤濕轉(zhuǎn)變機理。利用浸潤因子來描述冷凝液滴的浸潤程度,模型計算結(jié)果很好地預(yù)測了液滴表觀接觸角和滯后角的變化規(guī)律。結(jié)果表明,當液滴初始核化尺寸與納米結(jié)構(gòu)尺寸具有可比性時,核化傾向于在靠近納米柱頂端的位置發(fā)生,從而形成懸掛模式的液滴,有利于合并誘導(dǎo)彈跳；隨著表面過冷度增加,液滴核化尺寸大大減小,蒸汽在納米結(jié)構(gòu)間隨機發(fā)生核化,形成了浸潤模式的冷凝液滴,導(dǎo)致表面的超疏水性失效。通過控制氧化刻蝕時間制備了四種不同納米結(jié)構(gòu)的超疏水表面,實驗考察了表面結(jié)構(gòu)和表面過冷度對液滴合并彈跳行為的影響規(guī)律。隨著納米結(jié)構(gòu)長度和間距的增加,液滴彈跳尺寸增加,而彈跳頻率下降。隨著表面過冷度提高,冷凝液滴在納米結(jié)構(gòu)內(nèi)的浸潤程度增加,合并初始彈跳速度減小,最佳合并彈跳尺寸增大。將冷凝液滴的潤濕特性引入液滴合并誘導(dǎo)彈跳模型,分析了納米結(jié)構(gòu)尺寸和表面過冷度對液滴合并初始彈跳速度的影響機理,結(jié)果表明,蒸汽冷凝環(huán)境中液滴的動態(tài)行為由納米表面結(jié)構(gòu)、冷凝條件和液滴尺寸共同決定。實驗發(fā)現(xiàn)了超疏水表面冷凝液滴潤濕模式轉(zhuǎn)變的不可逆性和傳熱性能隨過冷度變化的單向性。利用超疏水表面實現(xiàn)了蒸汽在低過冷度范圍內(nèi)的冷凝傳熱強化,傳熱系數(shù)明顯高于相同條件下膜狀冷凝,甚至超過了光滑疏水表面的滴狀冷凝傳熱性能。通過對比三種超疏水表面的傳熱性能可以看出,合理地設(shè)計和優(yōu)化表面結(jié)構(gòu)可以拓寬超疏水表面的強化傳熱區(qū)間,為低壓蒸汽冷凝傳熱強化提供了實驗基礎(chǔ)和指導(dǎo)原則。
[Abstract]:As a typical vapor condensation phase transition phenomenon, widely exist in nature and daily life, at the same time as the heat transfer efficiency is widely used in industrial production. The low pressure steam condensate has an important application in the field of low grade waste heat recovery, such as low pressure distillation, low-temperature multi effect desalination technology, low temperature heat pump and heat pipe with the steam pressure drop, interfacial transfer resistance increased significantly reduces the heat transfer performance of the condensation process. Compared with the film condensation, dropwise condensation can effectively reduce the thermal resistance of the surface caused by the condensate, and the droplet transfer process to promote the dynamic behavior of the liquid vapor interface, a low-pressure steam condensation heat transfer enhancement ideal strategy. Based on the experimental observation, numerical simulation and theoretical analysis, systematic study of the low pressure steam dropwise condensation process of droplet size distribution and dynamic characteristics, evolution and table The surface temperature evolution of steam dropwise condensation heat transfer mechanism was revealed; promote the droplet with bouncing through the super hydrophobic surface interface effect, a low pressure steam condensation heat transfer enhancement. Design of low pressure steam condensation heat transfer experiment system, this study investigated the influences of steam pressure on dropwise condensation heat transfer performance and droplet dynamic behavior. The results show that the steam pressure drop, dropwise condensation heat transfer coefficient decreases slowly at first and then decreased rapidly. The combination of droplet heat transfer model, analyzed the control mechanism of low pressure steam condensation heat transfer, showed that the droplet growth affected by the liquid vapor interface transfer process enhanced with lower pressure. The experimental results showed that the droplet falling behind the phenomenon of low pressure steam in the condensing surface, delaying the update frequency, reveals the inherent relationship between the macroscopic droplet movement and heat transfer. By introducing the droplet dynamic characteristics, modified by Classic dropwise condensation heat transfer model, accurately predict the steam pressure and surface subcooling on dropwise condensation heat transfer characteristics. The influence of different steam pressure drop size distribution and transient steady state size distribution of the observation experiment, the influences of steam pressure on the growth characteristics of droplets. The initial droplet condensation by showing a normal distribution to Shuangfeng and then to the size distribution transient exponential distribution characteristics of evolution, with the steam pressure decreases, the nucleation site density decreases and the droplet growth rate slowed down. Compared with the normal pressure steam, reducing liquid surface condensation density drops under low pressure, and large drop frequency increased, increased the average size the condensation droplet surface. The droplet dynamic hysteresis model, explain the droplet dynamic behavior to reduce the dynamic lag angle; the energy barrier from the local perspective, the non continuous droplet removal The physical mechanism of falling behavior. Observation of low pressure steam condensation on droplet surface temperature distribution and evolution characteristics of the infrared thermal imaging technology, intuitive access to micro droplet movement caused by heat transfer phenomena. The results show that the condensation of the droplet surface temperature distribution showed the middle edge of the low. Closely related to the steam condensation process and droplet the growth behavior and interface evolution, different heat transfer and micro droplet, droplet movement by updating the liquid vapor interface promote rapid steam condensation, the internal mechanism of evolution reveals the droplet behavior caused by interfacial effects on macroscopic heat transfer. Compared with the super hydrophobic surface, the hydrophobic surface is more conducive to the growth of droplet nucleation, surface the temperature distribution is more uniform, the surface temperature fluctuation of the droplet movement caused more severe. Combined with the model analysis, SE simulation and visualization experiments, starting from the initial nuclear steam, the effect of surface structure and Influence of condensing conditions on initial droplet nucleation and droplet wetting mode. According to the cross scale and growth characteristics of super hydrophobic surface structure characteristics of condensing droplets, the surface structure on the droplet growth space limiting effect. Use of V shaped nano structure control of droplet initial nucleation over the shape and position. Hydrophobic condensation on the surface of droplet wetting transition mechanism was revealed. The infiltration factor to describe the infiltration degree of condensing droplets, the calculation results of the model well predicted the droplet apparent contact angle and hysteresis angle variation. The results show that when the initial droplet nucleation size and nano structure size comparable when the nucleation tend to occur near the nano column top position, thereby forming a droplet suspension mode, to merge with the surface induced bounce; increasing the degree of supercooling, droplet nucleation size is greatly reduced, the steam in nano structure Random nucleation, formation of condensate liquid infiltration mode, leading to failure of super hydrophobic surface. The super hydrophobic surface of four different nanostructures were prepared by controlled oxidation etching time, the influences of surface structure and surface subcooling on the droplet with bouncing behavior. With the increase of nano structure the length and spacing of the droplet size increases and bounce, bounce frequency decreased. With the increase of surface subcooling and condensation droplets in the nano structure within the degree of infiltration increased with initial jumping speed decreases with increasing of the size of the spring. The best wetting characteristics of condensing droplets into the droplet coalescence induced spring model, analysis the influence mechanism of nano structure, size and surface subcooling on droplet with initial hopping velocity. The results show that the dynamic behavior of steam condensation environment drops by nano structure on the surface of a liquid, condensation and Drop size determined. The experimental results showed that the one-way irreversibility and heat transfer performance change of super hydrophobic surface condensation droplet wetting mode varies with the degree of supercooling. The super hydrophobic surface to achieve the steam condensation heat transfer at low undercooling range enhanced heat transfer coefficient was significantly higher than that in the same conditions of film condensation. Even more than the smooth hydrophobic surface of dropwise condensation heat transfer performance. By comparing the three kinds of super hydrophobic surface heat transfer performance can be seen, the reasonable design and optimization of surface structure can broaden the super hydrophobic surface to enhance the heat transfer range, provides the experimental basis and guiding principles for low pressure steam condensation heat transfer enhancement.

【學(xué)位授予單位】：大連理工大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TQ021.3

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