Pressurized Icing Tunnel for Graupel, Hail and Secondary Raindrop Production

Abstract

The closed-circuit icing wind tunnel with pressure control serves for experiments on the growth and melting of ice particles, such as graupel and hailstones, and the observation of shedding of rain-sized drops by growing and melting hailstones. The pressure variation allows a better simulation of nature by duplicating all state parameters of the atmosphere and, consequently, also the correct relative speeds between test particle and air under all conditions. Thus, the hailstone growth according to different in-cloud trajectories can be simulated. The double-wall construction of the measuring and downstream calming sections provides excellent heat transfer at the outside wall and thus, minimal temperature gradients and radiative imbalances across the tunnel. This is of particular importance for the study of small particles requiring low speeds. The tunnel is 4.3 m high and has a 70 cm vertical measuring section with an inner cross section of 17.8 cm ? 17.8 cm. The operational pressure range is between laboratory pressure (?100 kPa) and 20 kPa, and the dynamic pressure can be as high as 615 Pa at 100 kPa and 430 Pa at 50 kPa air pressure. The lowest operational temperature is ?30°C, and the liquid water content can vary from 0?20 g m?3, with various drop size distributions. In order to simulate free fall, two gyrator systems are available to duplicate the complex rotational particle motions. This is preferred over modification of the parallel airflow which is required for floating of particles >0.5 mm in the presence of close restraining walls. Auxiliary measuring equipment includes two radiometric (infrared or IR) microscopes for surface temperature, a two-dimensional (2DC) Grey Scale Laser Spectrometer for droplet size distributions [15?960 ?m], by Particle Measuring Systems, Inc. (PMS), a PMS King probe for total liquid water content, various cameras (35 mm, video, high speed), Prandtl tubes and hot wire anemometers, two microcomputers (DEC LSI 11 and IBM PC-AT), and various other instruments, transducers and controllers. Examples of applications are given for graupel growth and the shedding of millimeter-sized drops by growing hailstones.