|Alternative Title||Numerical Simulation Research on the Superheated Flow and Flash-Boiling Atomization|
|Place of Conferral||北京|
|Keyword||过热流动，闪急沸腾雾化， 气泡生长模型，过热液滴破碎模型， 过热相变模型|
根据过热流动和闪急沸腾喷雾过程的主要流动特点，本文分别利用一维气泡生长模型、拉格朗日粒子方法和连续欧拉方法对过热流体中的气泡生长过程、外部闪蒸喷雾过程和过热管道内部流动过程进行了一系列的数值模拟研究工作。我们首先对过热流体中的气泡生长过程进行了详尽研究，运用直接数值离散方法和基于动态热边界层的数值方法对处于常压低过热度、常压中高过热度和低压中等过热度条件下的过热液体中的气泡生长状况进行了数值求解和模型验证工作。通过数值模拟计算获得了在相同过热条件下气泡在过热液体和过热液滴两种情形下的生长规律，并进行了对比研究。通过对过热液滴中的气泡生长动力学方程的小扰动分析，得到了过热液滴-气泡系统中的扰动发展方程，并对水、正戊烷、正己烷等三种液体在不同过热状态下的破碎情况进行了数值预测分析。对过热液滴的蒸发机制进行了研究，并初步探讨了液滴破碎与蒸发过程之间的关系。针对现有的破碎模型对过热液滴破碎过程描述不够清晰、 并且缺少足够的理论基础的现状，在前期研究的基础上发展了两种破碎模型来反映不同过热度下的过热液滴破碎特征：在低过热度下，基于现有的气动力破碎模型，将气泡的存在对液滴受力的影响考虑进去，发展出了过热条件下的液滴气动力破碎模型；在高过热度下，利用过热液滴-气泡系统中的扰动增长率发展模型来预测过热液滴的临界破碎状态，并利用能量守恒原理和能量转换系数来对子液滴特性参数进行计算，建立了一个考虑了液滴“微爆”效应的热力学破碎模型。针对复杂的过热流动现象，在开源平台上搭建了一个基于 VOF 方法的求解器，在其构造过程中考虑了可压缩效应、不可冷凝气体输运以及气液相变过程，并通过推导将多组分能量输运方程改造成了以温度为变量的输运方程形式。针对当前的过热相变模型大多基于均匀气泡数密度假设的现状，在 VOF 求解器中发展了基于气泡成核与输运过程的过热相变模型，在其构造过程中分别采用了异质成核模型、气泡数密度输运模型、过热气泡生长速率模型以及气液交界面密度估算公式来对应相关物理过程，同时研究了过热相变模型中的参数取值对数值模拟结果的影响规律， 并根据过热管道内部和外部的相变特征改进了原始的ELSA 模型，从而实现对过热管道内外流场的统一预测。 度下的过热液滴破碎特征：在低过热度下，基于现有的气动力破碎模型，将气泡的存在对液滴受力的影响考虑进去，发展出了过热条件下的液滴气动力破碎模型；在高过热度下，利用过热液滴-气泡系统中的扰动增长率发展模型来预测过热液滴的临界破碎状态，并利用能量守恒原理和能量转换系数来对子液滴特性参数进行计算，建立了一个考虑了液滴“微爆”效应的热力学破碎模型。针对复杂的过热流动现象，在开源平台上搭建了一个基于 VOF 方法的求解器，在其构造过程中考虑了可压缩效应、不可冷凝气体输运以及气液相变过程，并通过推导将多组分能量输运方程改造成了以温度为变量的输运方程形式。针对当前的过热相变模型大多基于均匀气泡数密度假设的现状，在 VOF 求解器中发展了基于气泡成核与输运过程的过热相变模型，在其构造过程中分别采用了异质成核模型、气泡数密度输运模型、过热气泡生长速率模型以及气液交界面密度估算公式来对应相关物理过程，同时研究了过热相变模型中的参数取值对数值模拟结果的影响规律， 并根据过热管道内部和外部的相变特征改进了原始的ELSA 模型，从而实现对过热管道内外流场的统一预测。
通过上述研究工作， 我们获得了关于过热流动和闪急沸腾喷雾现象的丰富研究成果。首先通过数值计算发现了气泡在过热液体和过热液滴两种物理模型下的不同生长规律：在低过热度和较大的液滴直径情形下，气泡在过热液体和过热液滴中的生长规律基本相同；但在高过热度和较小的液滴直径的情形下，过热液滴中的气泡会出现二次加速生长现象，出现这种现象的主要原因是处于惯性生长阶段的气泡在生长到与液滴初始尺寸相当时，其所受惯性力会有明显的放大作用。过热液滴-气泡系统中的扰动发展方程为一元三次方程，方程的实数解表征了在气泡不同生长时刻系统内的扰动增长情况。通过对扰动率的时间积分，可以获得系统扰动量的变化情况，当其满足临界破碎准则时，过热液滴-气泡系统发生破碎，这对于建立合适的过热液滴破碎模型具有重要意义。 过热液滴的破碎模型和蒸发模型对于可靠的闪急沸腾喷雾模拟来说缺一不可，能量转换系数的取值大小会影响到喷雾流场的计算结果，对于本文算例来说 0.05-0.1之间的取值较为合适。 通过与实验结果的对比研究，验证了本文发展的过热液滴破碎模型能够较好的捕捉不同喷管构型和不同过热度下地闪急沸腾喷雾现象，并可以用于高过热度下的喷雾坍缩现象研究。 本文研究认为喷雾结构附近的卷吸结构随着液滴过热程度的提高而迅速增强， 并造成邻近油束乃至整个油束结构之间的相互干扰过程，是导致多喷嘴喷雾结构在高过热度下出现坍缩现象的主要原因。通过大量的数值计算，验证了本文发展的过热相变模型能够对不同过热状态下的管道出口流量和管道内部的压力分布、气泡含量分布做出很好的预测，并且能够较为合理地预测过热管道内部的成核、蒸发等不同的过热流动现象，从而为流场分析工作提供很好的支撑。在过热相变模型中， 异质成核因子的大小对不同过热状态下的管道内部流场计算有重要影响，其大小能在很大程度上决定过热流场的成核水平，并决定成核过程发生的临界过热度，本文建议的合理取值为 1e-6。通过数值计算发现在不同过热度下小长径比的过热管道分别表现出两种不同的内部流场特征： 在低过热度下，管道内的过热液体整体处于亚稳定状态，表现出典型的过冷空化流动特征；在流场过热度超过某一临界值后，过热管道内部会有成核过程的发生，流场表现出典型的外部沸腾特征。对于大长径比的管道，当其来流温度接近来流压力所对应的饱和温度时，过热管道内部将完全气化，进入到所谓的内部闪急沸腾状态。
In order to improve the combustion efficiency as well as reduce the emission of pollutants, it is necessary to optimize the main flow mechanisms involved in the combustion, such as fuel atomization, droplet evaporation, oil-gas mixing and chemical reaction. As an effective way to improve the quality of fuel atomization, flash-boiling atomization has gained more and more application in internal combustion engines. During the spray development, the superheated droplet will break up completely due to the continuous growth of the inner bubbles and produce many tiny droplets. This effect can not only effectively improve the macroscopic characteristics of the spray flow field: shorten the length of the liquid core zone, increase the spray expansion angle, shorten the penetration distance of the droplet, but also can enhance the microscopic characteristics of the droplet, including reducing the droplet size, improving the droplets’ evaporation rate, promoting the rapid mixing of fuel and air. However, there are still some practical problems in the application of flash-boiling spray. For example, when near saturation state, the flow field inside the nozzle will be unstable. Under high superheated condition, the multi-hole spray structure may have severe spray collapse phenomenon, and the penetration depth of the spray structure will increase rapidly or even cause spray-wall impingement. Therefore, it is necessary to study the flash-boiling atomization as well as the superheated flow phenomenon inside the nozzle.
The flash-boiling spray is a complex physical process. At present, the main research work is conducted through experimental investigation, including High-Speed Cameras, Schlieren, Mie Scattering, Laser Induced Fluorescence, Laser Doppler Velocimetry, Phase Doppler Velocimetry, and so on. Using the above experimental apparatus, the characteristics of spray flow field are observed and analyzed, such as penetration depth and width, droplet size and velocity distribution, vapor concentration distribution. However, the physical mechanism behind theses phenomenon is still not clear. Meanwhile, there is a huge demand in product design about how to effectively utilize the flash-boiling atomization. Therefore, it is necessary to carry out numerical simulation research in this area. At the same time, most of the existing models about superheated flow and flash-boiling atomization are full of phenomenological description and lack of physical background. Therefore, it is necessary to carry out targeted research to meet the engineering and research needs.
According to the main flow characteristics of superheated flow and flash-boiling atomization, the one-dimensional bubble growth model, Lagrange particle tracking method and continuous Euler method are used to simulate the bubble growth in superheated fluid, the external flash-boiling spray and the internal nozzle flowfield, respectively. Firstly, the bubble growth in superheated fluid was studied in detail. Two numerical solutions, including the finite difference method and the dynamic thermal boundary layer method, were derived and numerically validated with experimental data under various superheat conditions, which contains low superheat degree at atmospheric pressure, medium superheat degree at sub-atmospheric pressure, and medium superheat degree at low pressure. Through numerical simulation, the bubble growth law in superheated liquid and superheated droplet under the same superheated condition was solved and compared. Based on the small disturbance analysis, the perturbation development equation in the superheated droplet-bubble system was obtained. The numerical prediction and analysis of the superheated droplet breakup under different superheated conditions was carried out. The evaporation mechanism of superheated droplets was studied, and its relationship with droplet breakup mechanism was discussed in thorough. In view of the fact that the existing breakup models were not clear enough to describe the breakup process of superheated droplets and lack of sufficient theoretical basis, two new droplet breakup models were developed to reflect the breakup characteristics of superheated droplets under different superheat degrees. Under low-superheat condition, the existing breakup models dominated by aerodynamic force was modified to take the influence of inner bubble existence on droplet in consideration. Under high-superheat condition, a thermodynamic breakup model considering the "micro explosion" effect of superheated droplet was established. In this model, the breakup criterion of superheated droplets was predicted by the perturbation growth equation in the superheated droplet-bubble system, and the characteristic parameters of child droplets were calculated through the energy conservation principle and energy conversion coefficient. To simulate at the complex superheated flow phenomenon inside the nozzle, a VOF-based solver was built on the open source platform, which could deal with the compressible effect, non-condensable gas transport and gas-liquid phase change phenomenon. And the multiphase energy transport equation was transformed into a simplified temperature transport equation. Since most of the current superheated phase change models are built on the assumption of uniform bubble number density, a new superheated phase change model based on bubble nucleation and transport process was developed in the VOF-based solver. In its construction, heterogeneous nucleation model, bubble number density transport model, superheated bubble growth rate model and gas-liquid interface density estimation formula were used to correspond to the relevant physical phenomenon. At the same time, the influence of the model parameters on the numerical simulation results was studied. According to the phase change characteristics of the superheated nozzle flow, the original ELSA model was improved to realize the unified prediction of the internal and external flow field of the superheated nozzle.
Through the above research work, we have obtained rich research results about the phenomenon of superheated flow and flash-boiling atomization. Firstly, different bubble growth laws in superheated liquid and superheated droplet were found through numerical calculation: under low-superheat or large-droplet-diameter condition, the bubble growth laws in both cases are basically identical; under high-superheat or small-droplet-diameter condition, the bubble growth in superheated liquid droplet will experience second acceleration process. The main reason for this phenomenon is that the force exerted on the bubble in the inertial growth stage will be magnified obviously when it grows to be equivalent to the initial size of the droplet. The perturbation development equation in the superheated droplet bubble system is a cubic equation with one variable. The real number solution of the equation represents the growth of the disturbance in the system. The magnitude of the disturbance can be obtained by integrating the disturbance rate by the time step. When the critical breaking criterion met, the superheated droplet bubble system will break up into many small droplets. It is of significance of establishing an appropriate model to describe the superheated droplet breakup during bubble growth. The breakup model and evaporation model of superheated droplets were indispensable to conduct reliable simulation of flash-boiling atomization. The energy conversion factor will affect the calculation results of the spray flow field. For this paper, the value between 0.05-0.1 was appropriate. Through the comparison with the experimental results, it is verified that the superheated droplet breakup model developed in this paper can better capture the flash-boiling phenomenon of different nozzle configurations under different superheat conditions, and can be applied to study of spray collapse phenomenon under high superheat. According to our research, the entrainment structure near the spray tip developed rapidly with the increase of droplet superheat, which would cause the interference between adjacent plumes under medium superheat degree, and even highly violent interfaces between the whole plumes under high superheat degree. Such explanation might be the main reason leading to the spray collapse phenomenon of multi-hole spray structure under high superheat degree. Through thorough numerical simulations, it was verified that the superheated phase change model developed in this paper could make a good prediction of the flow rate at the nozzle outlet, the distribution of the pressure and bubble content under different superheat conditions. And the new model can reasonably predict the different superheated flow phenomenon, such as nucleation and evaporation in the nozzle, which was very helpful to the flow field analysis. In the superheated phase change model, the heterogeneous nucleation factor was very important to the calculation of the flow field inside the nozzle, which could largely determine the nucleation level of superheated flow field and determine the critical superheat degree of nucleation process, and its reasonable value suggested in this paper was 1e-6. Through numerical calculation, it was found that the superheated nozzle with small length-diameter ratio has two different internal flow field characteristics under different superheat conditions. Under low superheat degree, the superheated liquid in the nozzle was still in metastable state, and the internal flow field showed up typical characteristics of subcooled cavitation flow; when the superheat degree exceeds a certain value, there would be nucleation process in the superheated nozzle, and the flow field would show up the typical characteristics of external flash-boiling. For the nozzle with large length-diameter ratio, when the inlet temperature was close to the saturation temperature corresponding to the incoming pressure, the superheated liquid inside the nozzle would completely vaporize and the flow field would enter the so-called internal flash-boiling state.
|李亚超. 过热流动和闪急沸腾雾化过程的数值模拟研究[D]. 北京. 中国科学院大学,2020.|
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