Method for Measuring Cooling Efficiency of Water Droplets Impinging onto Hot Metal Discs

Abstract

The present work outlines a method for measuring the cooling efficiency of droplets impinging onto hot metal discs in the temperature range of 85 • C to 400 • C, i.e., covering the boiling regimes experienced when applying water to heated objects in fires. Stainless steel and aluminum test discs (with 50-mm diameter, 10-mm thickness, and a surface roughness of Ra 0.4 or Ra 3) were suspended horizontally by four thermocouples that were used to record disc temperatures. The discs were heated by a laboratory burner prior to the experiments, and left to cool with and without applying 2.4-mm diameter water droplets to the discs while the disc temperatures were recorded. The droplets were generated by the acceleration of gravity from a hypodermic injection needle, and hit the disc center at a speed of 2.2 m/s and a rate of 0.02 g/s, i.e., about three droplets per second. Based on the recorded rate of the temperature change, as well as disc mass and disc heat capacity, the absolute droplet cooling effect and the relative cooling efficiency relative to complete droplet evaporation were obtained. There were significant differences in the cooling efficiency as a function of temperature for the two metals investigated, but there was no statistically significant difference with respect to whether the surface roughness was Ra 0.4 or Ra 3. Aluminum showed a higher cooling efficiency in the temperature range of 110 • C to 140 • C, and a lower cooling efficiency in the temperature range of 180 • C to 300 • C compared to stainless steel. Both metals gave a maximum cooling efficiency in the range of 75% to 85%. A minimum of 5% cooling efficiency was experienced for the aluminum disc at 235 • C, i.e., the observed Leidenfrost point. However, stainless steel did not give a clear minimum in cooling efficiency, which was about 12-14% for disc temperatures above 300 • C. This simple and straightforward technique is well suited for assessing the cooling efficiency of water droplets impinging onto heated metal objects. The test rig also worked well for demonstrating droplet boiling regimes and water droplet cooling efficiency to fire safety engineering students.

Influence of Acetone and Sodium Chloride Additives on Cooling Efficiency of Water Droplets Impinging onto Hot Metal Surfaces

“…Their average temperature was recognized as the disc temperature. The experimental setup and method is described in detail in [5] and the principal setup is shown in Figure 2. The experimental procedure was as follows: The metal disc was first heated by a butane burner to about 430 °C.…”

“…During this period, the setup frame was covered by a fine mesh screen to prevent any air drafts influencing the measurements. Dry-cooling was also recorded after a series of droplet cooling experiments, with no The experimental setup and method is described in detail in [5] and the principal setup is shown in Figure 2.…”

“…To study droplet cooling efficiency with acetone and NaCl (salt) additives, aluminum (EN AW-6082) discs of 50 mm diameter and 10 mm thickness, surface roughness Ra 0.4 and thermal conductivities of about 170 W/m•K were used. The choice of a material with much higher thermal conductivity than stainless steel was made to obtain results with less scattering [5]. The choice of a smooth surface (Ra 0.4) was made based on discrepancies in the literature as to the effect of surface roughness on cooling efficiency.…”

“…However, very few studies document cooling efficiency as a function of exposed metal temperature for representative water droplet sizes, which also includes all the relevant droplet cooling/evaporation regimes. The few studies found in the literature are generally devoted to horizontal objects [5].…”

“…They found cRPT to be most conspicuous at O 2 concentrations above 21 mole%. Bjørge et al [5] presented a simple and straightforward method for obtaining the cooling efficiency of droplets impinging onto hot metal discs in the temperature range of 85 • C to 410 • C, covering all the boiling regimes experienced when water droplets are applied to hot metal objects.…”