Experimental and mathematical study of the spray flash evaporation phenomena

Abstract

Spray flash evaporation is a promising way that potentially enables various renewable energy sources to be employed for desalination. This paper specifically studies the flash evaporation phenomenon of a hot water jet sprayed into a low-pressure chamber. Special considerations are rendered to the flash evaporation process under lower temperature differences which evolves higher thermal efficiency and better low-grade heat utilization for desalination. An experimental setup has been developed and temperature profiles of the sprayed water in the axial direction are mapped. Under low flow velocity conditions, the water jet shatters into droplets due to the flash atomization effect. Such a shattering phenomenon can be captured and quantified by a mathematical model based on droplet analysis. Applying this model, the temperature profiles are translated into the mean spray droplet diameters. The effect of different variables on the flash evaporation process is further investigated. Key results revealed that the mean droplet diameters of the spray are several orders of magnitudes smaller than the nozzle diameter. Such fine droplets allow complete evaporation to be accomplished within 50 cm from the nozzle exit, enabling a compact evaporator design. Additionally, higher initial temperature differences and higher flow velocities reduce the mean droplet diameter to be smaller than 300 μm, and the corresponding vertical distance required to complete the evaporation process is shorter than 10 cm.