Measurements of Radiation-Induced Condensational Growth of Cloud/Mist Droplets

Cloud droplet evolution and the associated impact of thermal radiation are being investigated experimentally and theoretically. The droplet size range of interest is 20 to 80 µm, the so-called condensation-coalescence bottleneck regime in which growth by normal (non-radiatiatively augmented) condensational growth in a saturated environment via solute and surface tension effects has essentially ceased (at around 20 µm) but coalescence growth via turbulence-induced collision has not yet began to operate. Theoretically it has been shown [1] that net thermal radiation away from cloud droplets, say, to a colder upper layer (outer space or higher cloud layer), can continue to induce condensational growth of cloud droplets into and through the bottleneck regime in time scales on the order of an hour and thereby perhaps help explain how precipitation forms in warm rain clouds. We have begun experiments to study this effect. We have developed a laboratory apparatus to measure thermal radiation-induced growth of mist droplets that are approximately the same size as stable cloud droplets. The apparatus consists of a flow tube that passes saturated mist at essentially constant (atmospheric) pressure and near or just below room temperature (~ 290 K) through a radiative cooling section. An optical droplet analyzer is used to measure droplet size distributions at the entrance and exit of the radiative cooling section. Radiative cooling of the mist droplets is induced by cooling the outer anodized aluminum tube wall with liquid nitrogen to create radiative sink temperatures ranging from 245 to 265 K. In the radiative section, convective cooling is essentially eliminated by the use of an annular shield flow of air that is at the same temperature as the mist. An ultrasonic-piezoelectric mist generator is used with de-ionized water to eliminate any solute effect. The mist flow is diluted by mixing it with an air stream before feeding into the flow tube. Mist flow velocity and Reynolds number are low enough to induce laminar flow in the radiative section but high enough that the flow is hydrodynamically developing over most of the radiative section. The mist is optically thin radially so that it remain essentially isothermal radially. The energy lost by radiation is small compared with the thermal capacity of the mist such that the mist flow remains essentially isothermal axially also. The mist generator produces droplets that are too small (0 to 10 µm) to be representative of clouds and too small (too optically thin individually) for appreciable radiative cooling to occur. Therefore the mist flow is conditioned before it enters the radiative cooling section. The mist is pre-cooled mostly by conduction-convection to cooled internal surfaces to induce a limited amount of droplet growth. The precooled mist then passes through flow straightening heat sinks to dampen any turbulence and establish uniform initial velocity and temperature profiles for entering the radiative cooling section. In the radiative cooling section, the mist flows through a 74-mm diameter polyethylene tube. Thin polyethylene film (relatively transparent to infrared radiation) is used to form the barrier between the mist flow and the annular convective shield flow of air around it. The annular air flow has a 4.5-mm annulus and is precooled to match the temperature of the mist flow. The Reynolds number of the annular flow is maintained high enough that the thermal entry region boundary layer formed by the cooled aluminum outer tube wall does not reach the polyethylene tube and thereby convectively cool the mist flow. During the radiative cooling experiments the heat sink (tube wall) is maintained at essentially uniform temperature in the axial direction. Tube wall temperature is measured using five thermocouples distributed on the inner surface of the annular aluminum tube wall. The differences in tube-wall temperatures are less than 1 K in most experiments. The figure shows an example of preliminary results. The top figure shows the conditioned droplet size distribution before radiative cooling of a saturated mist at 290 K. The bottom figure shows the size distribution after radiative cooling of the optically thin mist to a radiative heat sink at 245 K for a flow residence time of one minute. The development of a size mode above 20 µm, within the condensation-coalescence bottleneck, via radiation-induced condensational growth is evident. Additional preliminary measurements have been made for a range of radiative fluxes and flow rates. Radiative flux has been varied from 80 to 180 W/m² and droplet residence time in the radiative section varied from 30 to 120 seconds. It is observed that droplet sizes increase significantly upon experiencing radiative cooling. For example, with 150 W/m² radiative flux, the volume-average diameter increases up to ten µm after approximately one minute of residence time in the radiative cooling section. The magnitude of these growth rates is consistent with previously published theoretical predictions [1]. Theoretical analysis of these experimental results and comparison with them is in progress. In summary, experimental evidence is being gathered that shows droplet radiation to a remote, cold radiative sink can significantly augment cloud droplet growth in the 20-µm size regime in time scales that could be relevant to cloud dynamics and stability.

[1] Brewster, M. Q., "Evaporation and condensation of water mist/cloud droplets with thermal radiation,” International Journal of Heat and Mass Transfer 88 (2015) 695-712.