Simultaneous Measurement of Condensation and Thermal Accommodation Coefficients

同时测量冷凝和热调节系数

• 批准号: 0094909
• 负责人: Norihiko Fukuta
• 金额: $ 27.75万
• 依托单位: University of Utah
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-01-01 至 2004-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0094909&HistoricalAwards=false
• 关键词: Simultaneous; Measurement; Condensation; Thermal; Accommodation

As a cloud droplet grows by condensation, water molecules flow from the vapor surrounding the droplet to its surface and the heat released by condensation flows away from the droplet. Classical theory treats the environment of the drop as a continuum, so that the heat and mass transfer are described by the diffusion equation. This is a valid approximation for drops that are much larger than the molecular mean free path, which is about 0.06 mm for normal sea level conditions. However, newly formed cloud droplets have sizes typically between 0.1 and 1 mm, for which the classical, continuum theory must be modified to include gas kinetic effects. The modification introduces two new parameters that characterize the transfer of heat and mass, the accommodation coefficient and the condensation coefficient, denoted respectively by a and b. The accommodation coefficient may be thought of as the fraction of the molecules bouncing off the surface of a drop that have acquired the temperature of the drop. The condensation coefficient may be described as the fraction of the molecules hitting the surface of the drop that stick to it. In general, a and b are not equal and must be determined experimentally, because there is no known way to derive their values theoretically. Though there have been previous attempts to determine the coefficients, the results are quite variable. There are several reasons for this. First, it is no easy matter to measure the rate of growth of a micron-sized droplet in controlled conditions. Second, a and b are linked in such a way that a given droplet growth rate can be explained by many combinations of a and b. Third, there is evidence that the values of the coefficients may be influenced by small amounts of impurities in the water. This project aims to determine a and b separately by taking advantage of their different dependence on atmospheric pressure. The rates of growth of small droplets are observed under different conditions of temperature and pressure by illuminating them with a laser and employing the known peaks in the Mie scattering curve for water droplets to determine drop size. Experiments with drops formed on aerosols of different chemical composition will indicate the possible influence of contaminants on a and b. There is already some evidence that impurities lower the values of these coefficients. The effect of lower values is to reduce the growth rates of small droplets. A consequence of smaller droplet size is a greater reflectivity of the cloud for solar radiation. Therefore pollution aerosols may have the effect not only of increasing the number of small cloud droplets but also retarding their early growth by condensation. The correct treatment of the effects of clouds on solar radiation may therefore require accurate knowledge of the accommodation and condensation coefficients.

当云滴通过凝结而生长时，水分子从水滴周围的蒸气流到其表面，并且凝结释放的热量从水滴流走。 经典理论将液滴的环境视为连续体，因此传热传质由扩散方程描述。 对于远大于分子平均自由程（正常海平面条件下约为 0.06 毫米）的液滴来说，这是一个有效的近似值。 然而，新形成的云滴的尺寸通常在 0.1 至 1 毫米之间，为此必须修改经典的连续介质理论以包括气体动力学效应。 该修改引入了两个表征热和质量传递的新参数，即调节系数和冷凝系数，分别用a和b表示。 调节系数可以被认为是从液滴表面弹回并获得液滴温度的分子的分数。 冷凝系数可以描述为撞击液滴表面并粘附在其上的分子的分数。 一般来说，a 和 b 不相等，必须通过实验确定，因为理论上没有已知的方法来推导它们的值。 尽管之前已经尝试过确定系数，但结果差异很大。 这有几个原因。 首先，在受控条件下测量微米级液滴的生长速度并非易事。 其次，a 和 b 的联系方式使得给定的液滴生长速率可以通过 a 和 b 的许多组合来解释。 第三，有证据表明系数值可能会受到水中少量杂质的影响。 该项目旨在利用a和b对大气压的不同依赖性来分别确定它们。 通过用激光照射小液滴并利用水滴的米氏散射曲线中的已知峰来确定液滴尺寸，在不同的温度和压力条件下观察小液滴的生长速率。 对不同化学成分的气溶胶上形成的液滴进行的实验将表明污染物对 a 和 b 的可能影响。 已经有一些证据表明杂质会降低这些系数的值。 较低值的效果是降低小液滴的生长速率。 液滴尺寸较小的结果是云对太阳辐射的反射率较高。 因此，污染气溶胶不仅可以增加小云滴的数量，还可以通过凝结来延缓它们的早期生长。 因此，正确处理云对太阳辐射的影响可能需要准确了解调节和凝结系数。