共查询到18条相似文献,搜索用时 187 毫秒
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基于分子动力学模拟的方法,对氮气环境中单个烷烃液滴的蒸发过程进行了模拟研究,揭示了液滴在亚临界和超临界条件下液滴蒸发特性的显著差异.对正十二烷液滴在氮气环境内的蒸发过程进行分子动力学模拟,结果表明:在超临界温度和压力条件下,液滴的温度持续上升,能够超过燃油组分的临界温度;此时,液滴与周围气相区的密度差异近乎消失,气-液相交界变得难以辨别,明显不同于亚临界条件下典型的气-液两相蒸发特征;蒸发速率随环境温度的升高而增大.在较低的压力范围内,升高环境压力能够提升液滴蒸发速率,但当压力达到一个特定值后,随着环境压力的升高蒸发速率反而会降低,同时液滴转变为超临界蒸发状态所需的最小压力随环境温度的升高而降低.对于双组分混合液滴,在亚临界环境条件下,液滴内的轻质组分优先蒸发;而在超临界环境条件下,液滴内各个组分近乎保持同步蒸发,两个燃油组分共同主导液滴的完整蒸发过程. 相似文献
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基于格子玻尔兹曼方法(Lattice Boltzmann Method, LBM)对固着在加热基板上的液滴铺展及蒸发过程进行模拟,主要研究重力场、基板润湿性以及初始环境温度对液滴铺展及蒸发过程的影响。通过预测蒸发过程中液滴与基板的接触直径变化和液滴剩余质量变化,分析液滴形状及体积变化。研究结果发现,液滴形貌及蒸发过程受重力影响较大,重力作用下液滴铺展现象明显且蒸发加快。基板的接触角越小,液滴铺展现象越明显,其接触直径越大,蒸发越快。当环境温度与基板温度相差较大时,液滴内部出现涡流,强化换热使蒸发过程加快。 相似文献
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《内燃机学报》2017,(5)
针对多组分混合燃料的喷雾过程研究了相应的液滴蒸发模型,着重于研究混合燃料的组分对其液滴蒸发特性的影响.对柴油-生物柴油混合燃料的液滴蒸发模拟,依据燃料本身的特点,分别采用连续热力学方法和离散组分法描述其中柴油和生物柴油的组成.利用所得模型,对单组分燃料、双组分燃料以及生物柴油的液滴进行了蒸发模拟,通过将液滴蒸发历史曲线与试验结果对比,发现对于这些燃料液滴的蒸发模拟结果与相应试验数据很好地吻合,证实了此混合燃料液滴蒸发模型的正确性.此外,还着重对柴油-生物柴油的混合燃料的液滴进行了蒸发模拟研究,探讨混合燃料成分对其液滴蒸发特性的影响.结果表明:轻质柴油组分在蒸发过程中优先蒸发,而相对重质的柴油组分的蒸发则相对滞后,生物柴油在混合燃料中的质量分数则在液滴蒸发过程中不断增加,随着重质组分在柴油中所占比例达到一定程度之后,生物柴油的质量分数则开始迅速减小. 相似文献
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《燃烧科学与技术》2018,(6)
采用大涡模拟方法结合多组分蒸发模型对定容燃烧室内的柴油喷雾和燃烧过程进行了数值模拟研究.在大涡模拟中,采用动态结构模型模拟亚网格应力项,基于反褶积方法在湍动能的输运方程中增加了源项,表征燃油液滴与环境气体在亚网格尺度的相互作用;在多组分蒸发模型中,采用Gamma函数描述柴油液滴组分的摩尔分布,推导了基于连续热力学方法的气相组分控制方程的大涡模拟形式.将上述模型写入到多维CFD程序KIVA-4中,通过与无蒸发燃油喷雾和有蒸发喷雾燃烧过程相关实验数据的对比,计算得到了更为真实的喷雾形态和碳烟分布等结果,表明了该模型对提高柴油喷雾燃烧过程仿真性能具有巨大潜力. 相似文献
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《International Journal of Heat and Mass Transfer》2007,50(17-18):3670-3683
Heat and mass exchanges between the two phases of a spray is a key point for the understanding of physical phenomena occurring during spray evaporation in a combustion chamber. Development and validation of physical models and computational tools dealing with spray evaporation requires experimental databases on both liquid and gas phases. This paper reports an experimental study of evaporating acetone droplets streaming linearly at moderate ambient temperatures up to 75 °C. Two-color laser-induced fluorescence is used to characterize the temporal evolution of droplet mean temperature. Simultaneously, fuel vapor distribution in the gas phase surrounding the droplet stream is investigated using acetone planar laser-induced fluorescence.Temperature measurements are compared to simplified heat and mass transfer model taking into account variable physical properties, droplet-to-droplet interactions and internal fluid circulation within the droplets. The droplet surface temperature, calculated with the model, is used to initiate the numerical simulation of fuel vapor diffusion and transport in the gas phase, assuming thermodynamic equilibrium at the droplet surface. Influence of droplet diameter and droplet spacing on the fuel vapor concentration field is investigated and numerical results are compared with experiments. 相似文献
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采用VOF(Volume of Fluid)自由表面捕捉方法对盐水液滴蒸发过程中气液界面进行追踪,建立了降压环境下单个盐水液滴的蒸发模型,并通过盐水液滴蒸发的实验数据验证了此模型。通过对盐水液滴在相变过程中的形态变化以及传热传质特性的分析,研究了液滴内部温度、速度、蒸汽分布以及液滴形态等随时间的变化情况,分析了影响盐水液滴降压蒸发过程的主要因素。结果表明:在降压蒸发过程中液滴形态变化和环境中蒸汽的分布会随速度场的变化而变化;蒸发过程中初始盐组分质量浓度越大的液滴蒸发速率越缓慢,最终能达到的液滴最低中心温度越高,且液滴中心温度回升速度越慢、回升时间也越晚;液滴初始温度对蒸发速率影响较大,初始温度越高,表面蒸发速率越快,液滴中心温度回升速度越快。 相似文献
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Numerical simulation is performed for the evaporation of a droplet impacted on a porous surface. A level-set formulation for tracking the droplet deformation is extended to include the effects of evaporation coupled to heat and mass transfer, porosity and porous drag and capillary forces. The local volume averaged conservation equations of mass, momentum, energy and vapor fraction for the porous region are simultaneously solved with the conservation equations for the external fluid region. The computations demonstrate not only the evolution of the liquid-gas interface during the whole period of droplet penetration and evaporation in a porous medium, but also the associated flow, temperature and vapor fraction fields. The effects of impact velocity, porosity and particle size on the droplet deformation and evaporation are quantified. 相似文献
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Rajneesh Bhardwaj Jon P. Longtin Daniel Attinger 《International Journal of Heat and Mass Transfer》2010,53(19-20):3733-3744
The objective of this work is to investigate the coupling of fluid dynamics, heat transfer and mass transfer during the impact and evaporation of droplets on a heated solid substrate. A laser-based thermoreflectance method is used to measure the temperature at the solid–liquid interface, with a time and space resolution of 100 μs and 20 μm, respectively. Isopropanol droplets with micro- and nanoliter volumes are considered. A finite-element model is used to simulate the transient fluid dynamics and heat transfer during the droplet deposition process, considering the dynamics of wetting as well as Laplace and Marangoni stresses on the liquid–gas boundary. For cases involving evaporation, the diffusion of vapor in the atmosphere is solved numerically, providing an exact boundary condition for the evaporative flux at the droplet–air interface. High-speed visualizations are performed to provide matching parameters for the wetting model used in the simulations. Numerical and experimental results are compared for the transient heat transfer and the fluid dynamics involved during the droplet deposition. Our results describe and explain temperature oscillations at the drop–substrate interface during the early stages of impact. For the first time, a full simulation of the impact and subsequent evaporation of a drop on a heated surface is performed, and excellent agreement is found with the experimental results. Our results also shed light on the influence of wetting on the heat transfer during evaporation. 相似文献
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亚临界和超临界压力下燃料液滴的蒸发特性 总被引:1,自引:0,他引:1
以实际气体状态方程为基础,建立了适用于高压下的导热系数、扩散系数等物性参数的计算方法,并将高压汽液相平衡、混合物临界点以及蒸发焓的概念引入到液滴表面的传热传质过程中,以此为基础建立了单个燃料液滴的高压蒸发模型.研究了亚临界和超临界压力下壬烷液滴在氮气中的蒸发过程及其物理控制因素,重点探讨了超临界压力下液滴蒸发过程中液滴表面自亚临界状态向超临界状态的迁移过程及迁移条件.结果表明,在亚临界压力下,液滴蒸发始终受相变控制.在超临界压力下,当液滴表面由燃料和环境气体组成的混合物达到其临界点时,液滴表面将发生自亚临界状态向超临界状态的迁移.在液滴表面迁移之后,液滴表面消失,燃料自高浓度的燃料核心向远方场的扩散过程不受相变控制.另外,随着环境温度的升高液滴表面发生迁移所需的最低环境压力逐渐降低. 相似文献
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G. Miliauskas 《International Journal of Heat and Mass Transfer》2003,46(21):4119-4138
The study presents the mathematical model of unsteady heat transfer in evaporating semitransparent droplets of non-isothermal initial state and the numerical research method, evaluating selective radiation absorption and its influence on the interaction of transfer processes. The relation of the transfer processes inside droplets and in their surroundings and the necessity of thorough research of these processes are substantiated. When modeling the combined energy transfer in water droplets, the evaluation of thermoconvective stability in evaporating semitransparent liquid droplets is presented; the influence of the droplet initial state on its heating and evaporation process is investigated. The influence of heat transfer peculiarities on the change of the evaporating droplet state is indicated. Main parameters, which decide the peculiarities of the interaction of unsteady transfer processes in droplets and their surroundings, are discussed. The results of the numerical research are compared to the known results of the experimental studies of water droplet temperature and evaporation rate. 相似文献
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The coupling model of flow and heat and mass transfer for gas-spray droplet two-phase flow has been developed to simulate the evaporating spray in cross-flow. The correlations used for describing the droplet evaporation and motion in convective flow have been compared. The comparisons of calculated results show that the different correlations for determining Nusselt number and Sherwood number impose a significant influence on the lifetime of droplet. The modification of Nusselt number and Sherwood number with regard to the heat and mass boundary around the droplet is of great importance, while different mixing laws for mixture properties and different drag coefficient equations only demonstrate a slight effect on the evaporation characteristics of droplet. The characteristics of spray droplets and cross-flow in terms of both evaporation and motion are obtained. The secondary flow phenomenon is observed in the simulation results and contributes to achieving a more even distribution of temperature and an improved mixing effect of the vapor and cross-flow. 相似文献
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先从对流传质基本理论出发得到了单雾滴的蒸发公式,并用氮气流中正庚烷雾滴的蒸发验证了其正确性。后将该公式应用到急冷油雾滴蒸发过程计算,用Matlab6.5编程,数值计算方法求解。计算过程中急冷油物性参数随温度变化的关系式经过数据拟合获得,然后输入计算程序。最后得到了急冷油雾滴蒸发直径、温度变化曲线和雾滴内部温度分布曲线。 相似文献