Experimental investigation of spray formation as affected by sprinkler geometry

Abstract

A preliminary study at low water discharge pressures was conducted to investigate the fire sprinkler spray formation as affected by sprinkler geometry using a laser-based shadow imaging system. A sprinkler typically consists of the following components: a deflector with tines and slots, a boss above the deflector and sprinkler frame arms. Prototypes of these components were fabricated to evaluate their effects on the spray formation process. The water sheet thickness formed on non-slotted deflectors, slot spray discharge angle, sheet breakup distance from the deflector perimeter, water flux and drop size distributions were measured. It was found that deflector diameter and boss structure have little impact on drop size and sheet breakup distance. However, wider slots form larger drops. At constant operating pressure, the slot spray discharge angle is insensitive to the slot width, but sensitive to the boss that helps directing the slot spray toward the sprinkler centerline. The frame arm tends to produce a vertical spray sheet downstream of the frame arm, which increases the complexity of overall spray formation. An integral model was developed to calculate the development of water sheet thickness and speed on the deflector for different degrees of viscous effect exerted by the deflector. An empirical correlation was also established to estimate the spray flux fraction discharging from a deflector slot. The above measurements and observations are useful for the development of a spray formation model for fire sprinklers.