Near-Field Thermal Losses of Heated Water Sprays from Disc-Core Hollow Cone Nozzles

<b><sc>Highlights</sc></b> <list list-type=bullet><list-item> Six different disc-core hollow cone nozzles were evaluated for thermal and evaporative losses. </list-item><list-item> Latent heat of vaporization and convection were major contributors to thermotherapy. </list-item><list-item> Thermal energy was primarily at the center of the tested hollow cone nozzles. </list-item><list-item> Study developed an empirical equation for heat loss from hollow cone sprays. </list-item></list> <b><sc>ABSTRACT.</sc></b> This study evaluated the thermal efficiency of disc-core hollow cone nozzles in delivering heated sprays for thermotherapy in tree fruit crops. Previous laboratory-scale studies have shown that thermotherapy applied through spray can aid in insect pest control, but there is significant heat loss within 300 mm of the spray distance. This study was thus undertaken to understand and resolve the thermal and evaporative losses for six different disc-core hollow cone nozzle combinations in ambient conditions. The first experiment quantified droplet size distribution of each nozzle under both heated and non-heated spray conditions. The second experiment evaluated the change in collected spray volume and spray pattern between heated and non-heated sprays at 250 mm spray distance from the nozzle tip. The third experiment evaluated the thermal profile of the heated spray for each nozzle by moving an array of thermocouples vertically from 30 to 300 mm from the nozzle tip. Results indicate that smaller droplets formed in the atomization process are subjected to immediate evaporation, in the ranges of 11.0% to 60.4%, upon exiting the nozzle in a heated spray. The initial evaporative losses caused latent heat loss as well and significantly reduced the spray plume bulk temperature from 100.5°C–115.3°C to 38.8°C–49.4℃ within 30 mm spray distance from the nozzle tip. One of the key study outcomes is an exponential empirical equation developed to understand reduction in bulk dimensionless temperature with respect to spray distance for disc-core nozzles. Overall, while convective heat transfer is likely responsible for heat loss from the spray liquid after the nozzle exit, considerable thermal energy is lost to latent heat of vaporization at the nozzle exit.