Water spray is widely used in fire prevention and heat radiation protection due to its high infrared (IR) attenuation.
Recently, the calculation and theoretical analysis of water spray IR attenuation have been partially solved, where
multi-scattering in the water spray, distribution of droplets, concentration of droplets and water optical constant in the
infrared atmosphere windows are considered. However, is there a minimum for spectrum transmission of water spray in
the IR atmosphere windows band? If yes, what are the water spray parameters? These questions are still to be solved.
This article combines Lognormal Monte-Carlo Method (LNMCM) and Single Diameter Monte Carlo Method (SDMCM)
to calculate water spray infrared transmission. The droplet mean diameter is studied with SDMCM when the water spray
infrared transmission reaches the minimum. The geometric deviation of water spray is calculated with LNMCM at the
same conditions. The infrared transmission is calculated and analyzed with changing droplet diameters, on the stipulation
that total water content of air keeps unchanged. In this condition, droplet concentration increases with droplet diameter
decrease. Calculation results show that there is a minimum IR transmission of water spray in the atmospheric windows
band, which means there is a maximum IR attenuation. Infrared transmission of water spray reaches the minimum when
the droplet mean diameter is 12 micron and geometric deviation is less than 1.5. According to these, it is possible to
achieve maximum infrared attenuation with limited water supplied, as long as the spray nozzles parameters are in
accordance with the results.