【摘要】：水平管降膜蒸發(fā)器和傳統(tǒng)的換熱器相比,具有傳熱溫差小、傳熱系數高、可低溫傳熱等優(yōu)點,廣泛應用在海水淡化、食品加工、制冷工程、石油冶煉和化學工程等眾多領域。本文研究的是真空狀態(tài)下水平單管降膜蒸發(fā)傳熱傳質過程,該過程中管內是蒸汽冷凝過程,管外是降膜蒸發(fā)過程。管內管外都屬于氣液兩相流范疇,且都存在相變潛熱交換,情況較為復雜。建立水平管降膜蒸發(fā)冷凝管內外換熱模型,對管內蒸汽冷凝換熱熱阻、管壁導熱熱阻、污垢熱阻和管外降膜蒸發(fā)換熱熱阻進行了分析計算,分析計算出在四個熱阻中管外降膜蒸發(fā)換熱熱阻占總熱阻比重最大,且是管內的兩倍左右,因此研究管外降膜蒸發(fā)換熱機理對于水平管降膜蒸發(fā)器的換熱性能的提高具有重要意義。建立了水平管降膜蒸發(fā)器管外物理模型和單元仿真模型,加載了單元仿真模型的傳熱邊界條件,利用VOF模型以及ANSYS FLUENT自帶蒸發(fā)冷凝模型對不同噴淋密度和傳熱溫差下的管外降膜蒸發(fā)流動傳熱傳質情況進行了數值模擬,得到不同噴淋密度下的管外降膜蒸發(fā)傳熱系數,與相同條件,同種工況下的實驗數據進行對比,發(fā)現實驗結果與數值模擬結果相差在15%之內,驗證了所建立的數學模型以及VOF模型加蒸發(fā)冷凝模型模擬管外降膜蒸發(fā)傳熱傳質的準確性。模擬結果顯示隨著噴淋密度增大,傳熱溫差增大,噴淋密度由0.013 s)kg/(m?增大到0.058 s)kg/(m?,管外降膜蒸發(fā)換熱系數由716 2k)W/(m?增大到2698k)W/(m2?,增大了4倍左右。對真空條件下水平管外降膜蒸發(fā)過程中液膜厚度以及流場流動換熱特性進行了數值模擬及分析,具體包括:液膜內液體x方向速度分布規(guī)律、液膜內液體y方向速度分布規(guī)律以及水平管圓周不同位置液膜厚度分布規(guī)律,還研究了不同噴淋量下的液膜厚度分布。研究發(fā)現,在最開始液體與管壁發(fā)生撞擊,液膜發(fā)生堆積所以液膜厚度較厚,然后在θ取5o~30o范圍內時,液膜在管壁表面鋪展開來,液膜厚度逐漸變薄,且降低幅度較大;然后在θ取30o~90o范圍內時由于重力以及管壁表面剪切力作用,液膜厚度逐漸緩慢減小,且液膜厚度達到最小值;θ為90o~155o時液膜厚度有較小幅度的增大;θ為155o~180o時由于管壁兩側液膜合流相互撞擊以及液膜脫離管壁,液膜在此范圍內出現堆積,液膜厚度急速增大。因此,噴淋密度為0.4 kg/(m·s)、蒸發(fā)溫度為80℃、管壁溫度為85℃時水平單管降膜蒸發(fā)液膜最薄位置出現在90°,液膜最薄為0.001m,液膜最厚的位置出現在θ為180°的位置為0.006m,是最薄液膜厚度的6倍。隨著噴淋量增加,液膜厚度增厚發(fā)生在沿管壁圓周方向,但是不同區(qū)域厚度增加幅度會有不同,0o~90o區(qū)間厚度增加比90o~180o更為明顯,產生這種現象可能是由于噴淋量增加,增強了換熱,強化了傳質過程的發(fā)生。
[Abstract]:Compared with the traditional heat exchanger, the horizontal tube falling film evaporator has the advantages of small heat transfer temperature difference, high heat transfer coefficient and low temperature heat transfer. It is widely used in many fields such as seawater desalination, food processing, refrigeration engineering, petroleum smelting and chemical engineering. In this paper, the heat and mass transfer process of falling film evaporation of horizontal single tube in vacuum state is studied, in which steam condensation process is in the tube and falling film evaporation is in the outer part of the tube. Both inside and outside the tube belong to the category of gas-liquid two-phase flow, and there is phase transition latent heat exchange, which is more complex. The internal and external heat transfer model of falling film evaporation condensing tube in horizontal tube is established. The heat transfer resistance of steam condensation, heat conduction of pipe wall, fouling heat resistance and outside falling film evaporation heat transfer resistance of tube are analyzed and calculated. It is analyzed and calculated that the heat transfer resistance of falling film evaporation outside the tube accounts for the largest proportion of the total thermal resistance in the four thermal resistances, and is about twice as much as that in the tube. Therefore, it is of great significance to study the heat transfer mechanism of falling film evaporation outside the tube for improving the heat transfer performance of horizontal tube falling film evaporator. The physical model and unit simulation model of horizontal tube falling film evaporator are established, and the heat transfer boundary conditions of the unit simulation model are loaded. The heat and mass transfer of falling film evaporation flow under different spray density and heat transfer temperature difference was numerically simulated by using VOF model and ANSYS FLUENT evaporation condensation model, and the heat transfer coefficient of falling film evaporation outside tube under different spray density was obtained. Compared with the experimental data under the same conditions and the same working conditions, it is found that the difference between the experimental results and the numerical simulation results is less than 15%. The accuracy of the established mathematical model and VOF model plus evaporative condensation model to simulate the heat and mass transfer of falling film evaporation outside the tube is verified. The simulation results show that the heat transfer temperature difference increases with the increase of spray density, and the spray density is from 0.013 s) kg/ (m? When it increases to 0.058 s) kg/ (m 鈮，

本文編號：2472354