水平圓管滴淋蒸發之熱傳分析與兩相流場模擬

Analysis of Falling Film Evaporation on Horizontal Tubes and Numerical Simulation of Two Phase Flow

王世民(Shih-Ming Wang)

指導教授 ： 簡良翰

國立臺北科技大學/機電學院/冷凍空調工程系所/碩士(2005年)

摘要

本研究水平圓管滴淋薄膜蒸發，利用實驗與數值模擬之兩種方法探討滴淋式蒸發器之熱傳性能。實驗方面以三根水平圓管進行R134a之滴淋蒸發熱傳實驗，並觀測暫態滴淋蒸發模式與模擬流場做印證，實驗結果與文獻中之經驗公式吻合。在數值模擬方面，將幾何模型分為冷媒分配器之單相流場模擬、與滴淋蒸發測試室之兩相氣液流場模擬兩部份。
模擬冷媒分配器以穩態重整化群紊流(RNG k-ε)模式計算，在1.6g/s—6.4g/s的三種冷媒入口流量下，改變分配器內管圓孔尺寸與外管縫隙寬度，將模擬所得之速度與壓力流場分佈作為設計分配器之參考。冷媒分配器模擬結果發現，改變供給系統流量，分配器內管之孔間距與孔徑比例應隨著流率增加而適度降低；外管縫隙寬度受限內管速度與壓力流場分佈，因此縫隙寬度設計必須依據內管滴淋圓孔尺寸做適當調整。在模擬測試室滴淋蒸發兩相氣液流場分佈時，將蒸發現象簡化為熱質傳遞模型，由FLUENT軟體以UDF方式引入蒸發模型；氣液界面以三維尤拉格點不相溶流體之體積分率，並搭配連續線性界面重建與連續表面力(VOF-CSF)算法，模擬氣液界面所造成液膜波動。滴淋蒸發模擬結果發現，蒸發器負載應隨供給流率做適當調整，以避免管集表面過熱發生；提高供給蒸發器流量可減緩管集表面乾枯現象。模擬結果顯示液體不易浸濕表面而呈乾枯現象，此與實驗結果有所差異，也因此模擬之蒸發熱傳係數遠低於實驗值。

關鍵字

滴淋式蒸發器 ； 冷媒分配器 ； 兩相流 ； 薄膜蒸發 ； 計算流體力學 ； 氣液界面 ； 連續表面力

並列摘要

Numerical simulations and experiments of spray type (falling film) evaporators were conducted in the present study. The present falling film evaporation tests were performed on an apparatus having three smooth horizontal tubes using R-134a. Flow visualization was also conducted using a high speed camera system on this experiment. The test results reasonably agree with predictions of two empirical correlations in the literature. The numerical simulation includes: single phase flow simulation of liquid distributor, and two phase flow simulation on evaporating tubes.
The liquid distributor consists of two tubes. The inner tube has a series of pores and the outer tube has a slit on the bottom. Velocity distributions and pressure drops are compared with various geometric parameters including pore size, pitch and gap. The simulation reveals that the pore size and the gap between the two tubes are important factors for 1.6 g/s—6.4 g/s flow rates. For the two phase flow simulation in the evaporator, a simplified model of evaporation was written in a computer code and linked to the commercial computational fluid dynamic simulation code, FLUNET. The Continuum Surface Force Model was used for the liquid-vapor interface simulation, and the VOF model was selected as the two phase model assuming liquid and vapor as two separated fluids. The simulations indicate that the dry out problem can be relieved by increasing the inlet flow rate or decreasing heat input. However, the two-phase flow simulation over estimated the percentage of dry out regions as compared with experimental observations. As a result, the evaporation heat transfer coefficient is under-estimated by the simulation.

並列關鍵字

Falling Film Evaporator ； Refrigerant Distributor ； Two Phase Flow ； Computational Fluid Dynamics ； Continuum Surface Force

參考文獻

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