Ice accretion on fixed cylinders

A comparison of ice accretions obtained on fixed and on rotating deposits was expected to provide a better understanding of the basic accretion mechanisms as well as being useful in many practical applications. Therefore, the morphological and physical properties of ice deposits accreted in a wind tunnel on fixed cylinders have been studied at temperatures, liquid water contents and air speed ranges previously tested for rotating cylinders. The deposits obtained are studied as to their external shape and internal morphology, crystallographic properties and local density distributions. The external shape is mainly dependent on the growth regime (dry or wet-spongy) and the wind speed. In dry growth and high wind speed the experimental profile is lenticular in agreement with prediction; however, at lower wind speeds, the growth rate at the stagnation line is reduced with respect to the flanks, and the deposit develops a central depression. In wet or spongy growth, run back is effective at high wind speeds, while shedding becomes important at lower speeds. Observation of the crystal structure of fixed deposits shows that grain size and orientation depend on the growth regime and temperature in a similar way as with the rotating cylinders. However, the grain elongation is more pronounced in the fixed deposits, especially at lower temperatures. The local density distribution of the deposits is measured from X-ray micrographs. The stagnation line density is maximum or close to the maximum at impact speeds of 26 and 10 ms-1, while in deposits obtained at 5 and 2.5 ms-1 it drops to remarkably lower values. The deposit profiles are compared with previous model investigations, and information is provided on the behaviour of some growth parameters which should be introduced into the model in order to simulate the experimental process better. The angular profile of the local density is also compared with predictions by previous investigators. It is shown that the best representation of the experimental points is achieved by calculating the mean volume radius of droplets at the collection angle ? and the corresponding impact speed, and using a new relationship between ice density and the Macklin parameter.