Abstract: The main component of vehicle internal combustion engine and aircraft engine fuel is n-heptane. Combustion of liquid fuel belongs to diffusive combustion. The combustion process is composed of atomization, evaporation and diffusive combustion, and the evaporation process is vitally important, which critically determines the combustion speed of fuel droplet. Hence it is essential to optimize this process. In this paper, the evaporation of n-heptane droplet was evaluated from the perspective of second law of thermodynamics. A two-dimensional quasi-steady state model of the evaporation process was proposed, and the entropy production of the process was solved by integration of the volumetric entropy production rate in the whole domain. The viscous, conductive, mass transfer entropy production was investigated and compared, with the help of viscous and conductive entropy production rate equations reported in literatures and mass transfer entropy production rate equation derived in this paper, after numerical simulation of the evaporation process with Fluent software. Also, the relative total entropy production was defined. Conductive entropy production was the main part of the total entropy production of n-heptane evaporation in the low-Re air flow. With increasing Re and temperature of airflow, relative total entropy production decreased. In other words, the evaporation process produced less entropy relatively and it became easier, which from a thermodynamics perspective was optimal.

Key words: atomization combustion, droplet evaporation, mass transfer, thermodynamics, numerical simulation, entropy production

摘要： 车用内燃机、航空发动机燃料的主要成分为正庚烷液体。液体燃烧属于扩散型燃烧，燃料先雾化成油雾，然后蒸发，燃油蒸气在气态扩散火焰中燃烧。油雾蒸发是液体燃烧中的重要过程，是决定液体燃烧的燃烧速率的关键步骤。建立了一个二维准稳态的液滴蒸发数值模型，分析了液滴蒸发的熵产，推导出了三维笛卡尔坐标系下的传质熵产公式。基于Fluent软件进行了数值模拟，结果显示低Reynolds数空气来流中的液滴蒸发过程的熵产主要由导热熵产组成，且液滴蒸发越容易，相对总熵产越小，从热力学角度来看其液滴蒸发过程更优。

关键词: 雾化燃烧, 液滴蒸发, 传质, 热力学, 数值模拟, 熵产