Cooling efficiency of water droplets impinging on a hot stainless steel surface

Abstract

The behavior of water droplets applied to hot metal surfaces depends strongly on the metal surface temperature. Four distinct boiling regimes, film evaporation, nuclear boiling, transition boiling and film boiling are observed, with large differences in heat exchange between surface and the droplet. The purpose of the present work was to study water droplet cooling efficiency as a function of temperature for stainless steel in these boiling regimes. Stainless steel (AISI 316) discs (50 mm diameter and 10 mm thickness) were preheated to 430°C and cooled to 80°C by water droplets. Droplet of 2,5 mm, 3,2 mm and 3,7 mm diameter were released from 25 cm, 50 cm and 100 cm height, respectively resulting in impingement speeds of 2,2 m/s, 3,1m/s and 4,4 m/s. The hot metal surface was aligned in 3 positions, i.e. horizontal, 30° or 60° inclination. These combinations resulted in 27 experimental data series, each consisting of five droplet tests cooling the disc and sandwiched between two reference tests without releasing water droplets. The water flow was carefully tuned to 0,023 g/s for all the droplet experiments to compare cooling efficiency based on the same water application rate. Four thermocouples were used for recording temperature versus time within the stainless steel disc. Based on the differences in cooling rate with and without droplets cooling the metal disk, disk mass and heat capacity, the absolute cooling rate of the droplets were obtained. Comparing this cooling rate to the heat flow required to completely evaporate the droplets gave the dimensionless cooling efficiency. Evenly across the results, smallest droplets were observed to have a higher cooling efficiency in a wider temperature range, while the largest droplets demonstrated a higher peak efficiency (i.e. higher boiling crisis), which was recorded to be between approximately 64% and 85% respectively for the different configurations. Based on most of the results, the droplet cooling efficiency increased with increasing impact velocity, due to higher momentum impact and a larger contact area. However, regarding the horizontal stainless steel disc setup – middle-sized and smallest droplets, the effect of bouncing was less prominent, and droplets had a higher ability to be attached to the surface (i.e. lower Weber number). The droplet cooling efficiency decreased with increased inclination for the lowest impact velocity (2,2 m/s), due to a lower amount of possible directions for reflecting disintegrated droplets. For higher droplet velocities, the droplets resulted in higher cooling efficiency for 30o inclination. Similar to the results published by other workers, a maximum in heat transfer was observed at about 160 – 220°C. Above this temperature, the cooling efficiency was reduced to about 10 % for temperatures in the film boiling region (i.e. temperatures above 290 - 300°C). This indicates that firewater must be applied before the metal becomes too hot, else the water spray cooling will be very limited. It is recommended to study this topic further, either on a larger scale or by altering several parameters, i.e. droplet sizes, surface roughness and impact velocity.