本文選題：破裂頻率 + 湍流渦�。� 參考：《湘潭大學(xué)》2017年碩士論文

【摘要】：湍流分散體系(如氣-液、液-液等)中流體流動時伴隨發(fā)生的流體顆粒(以下簡稱流粒,氣泡或液滴)的破裂現(xiàn)象通常決定了流粒在流場中的分散狀態(tài)、粒徑分布和相界面積,因而對整個體系的傳質(zhì)、傳熱和反應(yīng)性能有著重要影響。深入理解湍流中流粒的破裂機理并構(gòu)建出相應(yīng)的理論模型可為多相反應(yīng)器的設(shè)計、優(yōu)化和工業(yè)放大提供分散相粒徑分布和相界面積分布方面的重要理論依據(jù)。本文通過對以往破裂頻率模型進行分析,提出了前人模型中采用的“流粒尺寸總是落在慣性子區(qū)”和“小于或等于流粒尺寸的湍流渦才能引起流粒破裂”的假設(shè)具有明顯不合理性(二者結(jié)合,相當于認為只有慣性子區(qū)的湍流渦才對流粒破裂有貢獻)。此外,以往破裂頻率模型大都是借鑒氣體分子反應(yīng)速率的建模方式,以流粒與湍流渦的碰撞頻率與破裂概率的乘積來獲得流粒的破裂頻率。但由于受湍流脈動隨機性的影響,湍流渦的壽命時間也具有一定的隨機性,這使得湍流渦與流粒之間的碰撞較氣體分子情形要更為復(fù)雜。前人模型由于是直接照搬氣體分子碰撞模式,因而無法體現(xiàn)湍流渦壽命時間對流粒與湍流渦碰撞的影響。不同于前人模型,本文將對流粒破裂有貢獻的湍流渦的分布區(qū)域擴展至了整個湍流能譜范圍(即同時包括含能渦子區(qū)、慣性子區(qū)以及耗散子區(qū));進而在全能譜分布函數(shù)的基礎(chǔ)上和考慮湍流渦壽命時間的前提下,提出從構(gòu)建湍流渦與流粒的碰撞概率模型以及引入臨界破裂速度的思路來推導(dǎo)流粒破裂頻率模型。通過引入湍流渦與流粒的相對速度、碰撞角度、碰撞自由程等反映碰撞物理過程的參數(shù),并考慮流粒與湍流渦的初始相對距離、湍流渦壽命時間內(nèi)渦旋所能運動的相對距離對碰撞的影響,本文獲得了可導(dǎo)致流粒破裂的碰撞頻率及其相應(yīng)的流粒破裂頻率模型、子尺寸分布模型。本文提出的破裂頻率模型和子尺寸分布模型的預(yù)測結(jié)果表明:流粒破裂頻率隨著湍流動能耗散速率的增加而增大,但隨著流粒尺寸的增大,流粒破裂頻率呈現(xiàn)出先增大后降低的趨勢。越高的湍流動能耗散速率對應(yīng)著湍流渦傳遞給流粒的能量越多,因此流粒發(fā)生破裂的頻率越大;此外,在湍流動能耗散速率較低時,液滴子尺寸分布以等尺寸破裂概最高,但當湍流動能耗散速率增大時,非等尺寸破裂概率逐漸增大,液滴子尺寸分布曲線變得平坦;表面張力越小,流粒越易發(fā)生破裂,相應(yīng)的流粒破裂頻率越大、非等尺寸破裂概率增大;模型預(yù)測結(jié)果與實驗數(shù)據(jù)表現(xiàn)出一致的演變趨勢,且與攪拌槽中液滴累積尺寸分布吻合良好。
[Abstract]:In turbulent dispersion systems (such as gas-liquid, liquid-liquid, etc.), the breakup of fluid particles (hereinafter referred to as flow particles, bubbles or droplets) associated with fluid flow usually determines the dispersion state of flow particles in the flow field. The particle size distribution and phase boundary area play an important role in the mass transfer, heat transfer and reaction performance of the whole system. A thorough understanding of the fracture mechanism of particles in turbulent flow and the construction of corresponding theoretical models can provide an important theoretical basis for the design of multiphase reactors, optimization and industrial amplification of dispersed phase particle size distribution and phase interface integral distribution. Based on the analysis of the previous rupture frequency model, The assumption that the particle size always falls in the inertial sub-region and the turbulent vortex smaller than or equal to the particle size can cause the flow particle rupture is obviously unreasonable. It is considered that only turbulent vortices in the inertial subregion contribute to the rupture of convective particles. In addition, most of the previous fracture frequency models were based on the gas molecular reaction rate model, and the fracturing frequency was obtained by using the product of collision frequency and rupture probability of flow particle and turbulent vortex. However, due to the randomness of turbulence pulsation, the lifetime of turbulent vortices is also stochastic, which makes the collision between turbulent vortices and particles more complicated than the case of gas molecules. Because the previous model is a direct model of gas molecule collision, it can not reflect the impact of turbulent vortex lifetime time convection particle and turbulent vortex collision. Different from previous models, this paper extends the distribution region of turbulent vortices which contribute by convection particle rupture to the whole turbulent energy spectrum (that is, including the energetic vortex subregion). The inertial subregion and the dissipative subregion are further considered on the basis of the omnipotent spectrum distribution function and the turbulent vortex lifetime time. In this paper, the probability model of collision between turbulent vortex and particle and the method of introducing critical rupture velocity are proposed to deduce the model of particle rupture frequency. By introducing the relative velocity of turbulent vortex and particle, collision angle, collision free path and other parameters to reflect the physical process of collision, the initial relative distance between particle and turbulent vortex is considered. The effect of relative distance of vortex motion on the collision during the turbulent vortex lifetime is studied. In this paper, the collision frequency and the corresponding particle rupture frequency model, the sub-size distribution model, are obtained. The prediction results of the fracture frequency model and the sub-size distribution model proposed in this paper show that the particle rupture frequency increases with the increase of turbulent kinetic energy dissipation rate, but with the increase of particle size. The flow particle rupture frequency increased first and then decreased. The higher the turbulent kinetic energy dissipation rate is, the more energy the turbulent vortices transmit to the flow particles, so the higher the frequency of the flow particles rupture is, in addition, when the turbulent kinetic energy dissipation rate is lower, the droplet size distribution is the highest in the same size fracture. However, when the turbulent kinetic energy dissipation rate increases, the non-equidimensional fracture probability increases gradually, and the droplet size distribution curve becomes flat, the smaller the surface tension, the more prone the flow particle is to rupture, and the higher the corresponding flow particle rupture frequency is. The prediction results of the model are consistent with the experimental data and agree well with the cumulative size distribution of the droplet in the stirred tank.
【學(xué)位授予單位】：湘潭大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TQ021.1

【參考文獻】

相關(guān)期刊論文 前1條

1 林宗虎,王棟,王樹眾,林益;近期多相流基礎(chǔ)理論研究綜述[J];西安交通大學(xué)學(xué)報;2001年09期

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本文編號：1841557