An experimental study of the formation and evolution of frazil ice.

摘要

Despite the negative effects of frazil ice formation on a variety of hydraulic structures, there is a lack of fundamental understanding of several key aspects of frazil formation and evolution. The purpose of this research was to investigate the effects of a variety of thermal and hydraulic parameters on the formation and evolution of frazil ice. Laboratory experiments were conducted in a counter-rotating flume that was equipped with a high precision thermometer and a digital image acquisition system. Three groups of experiments were conducted to study the effects of air temperature, water velocity, bottom roughness and turbulence intensity.; A constant temperature anemometer equipped with a conical hot-film probe was used to quantify the velocity and turbulence intensity distributions within the counter-rotating flume after first calibrating the wall speeds in order to reduce secondary circulation. By using five sets of bed plates with varying roughness, five corresponding flow conditions were created whereby the velocity distributions were nearly identical while the turbulence intensity increased with increasing roughness. These conditions provided a means of experimentally investigating the effects of turbulence intensity on frazil formation and evolution.; Based on the subsequent series of experiments it was concluded that the frazil particle size distributions followed a lognormal distribution regardless of the experimental parameters tested, and that the turbulence intensity of the water had a significant impact on the average size and standard deviation of the frazil particles. An empirical equation was developed to describe the variation of the mean and standard deviation of the frazil particles as a function of both time and turbulence intensity. The effects of air temperature, water velocity, bottom roughness and turbulence intensity on a wide range of parameters including cooling rate, the time of principal supercooling, the maximum degree of supercooling, secondary nucleation and flocculation among others has been investigated and presented herein.