降温速率对三种类型水滴冻结过程影响的实验研究

Experimental study of the impact of cooling rate on the freezing process of three kinds of water drops

大量实验研究表明,水滴的异质冻结既与所含冰核有关,又包含一定的随机过程。本研究一方面对已有的实验研究结果进行验证,同时寻找水滴冻结与温度及时间的定量关系,检验随机假设和奇点假设的相互作用。选用雨水、纯净水和瓶装饮用水3种水样,采用中国气象科学研究院的均匀水滴冻结实验装置,对3类水样做了不同的恒定降温速率实验,并对雨水进行了恒温实验。结果表明,总体来看,随温度降低,降温速率慢的液滴累加冻结比例高于降温速率快的液滴,并且在纯净水和瓶装饮用水中更明显;降温速率越大,平均冻结温度和中值冻结温度越低,不同的是雨水的值跟降温速率成对数关系,而瓶装饮用水的值与降温速率成线性关系(纯净水因为只有两种降温速率,故不能确定是哪一种关系);不同水样的冻结几率随温度的降低指数增大,而瓶装饮用水冻结几率函数中指数前的系数随降温速率的增大指数减小;恒温阶段,雨水的冻结几率随时间指数衰减,在恒温的前2 min衰减较快,之后减慢,并且水滴冻结在前2 min发生较多,之后变的很少,15 min之后则没有冻结事件发生。该现象可以用随机假设和奇点假设结合起来解释,当温度达到或低于特征温度时,水分子在冻结核上的聚合需要一段时间,所以造成了在恒温阶段出现水滴冻结的现象。水滴冻结是两种假设的共同作用。

Large number of the previous experimental researches have indicated that heterogeneous freezing is caused by nucleus contained in drops and stochastic process both.In this research,experiments are designed to verify the previous experiment results.On the other hand,aquantitative relationship of the freezing phenomenon of water drops with temperature and time is also studied.Besides,the interaction of the stochastic hypothesis with the singular hypothesis is also examined.The constant cooling rate experiments for rainwater,pure water and bottled water,and the constant temperature experiment for rainwater only,were conducted using a freezing experimental device of uniform drops designed by Chinese Academy of Meteorological Sciences.According to the results,it is seen that while the temperature decreasing,the accumulative freezing fraction in the slow cooling rate experiment is larger than that in the fast cooling rate experiment in general.This phenomenon is more obvious for pure water and bottled water.The faster cooling rate is corresponding to the lower average freezing temperature and median freezing temperature.However,the relationship between freezing temperature and cooling rate in rainwater is logarithmic while their relationship is linear in bottled water（there are only two rates of cooling in pure water experiment,so it is not certain to belong to which relationship）.The probability of freezing increases exponentially with the decreasing temperature in each water sample,and in the probability of freezing function,the exponential coefficient decreases exponentially with the increasing cooling rate in bottled water;during the constant temperature period,the probability of freezing decays exponentially with time in rainwater.The decay is faster in first two minutes,and becomes slower later.No freezing event occurs after 15 minutes.This phenomenon can be explained by combining the stochastic hypothesis and singular hypothesis as follows：when the temperature of drops reaches or is lower than hte characteristic temperature,water molecules need a period of time to aggregate on freezing nucleus,which lead the freezing of drops to occurring in constant temperature period.Freezing of drops is caused by a combined effect of the two hypotheses.