A scientific view of the productivity of abrasive blasting nozzles

Abstract

ABRASIVE blasting is a sensitive issue in infrastructure refurbishment. Recent environmental laws and resulting costs have brought about a powerful incentive for improvement, but the scientific basis of abrasive blasting has remained essentially the same for over a century. Everyone knows that abrasive blasting "doesn' t take a rocket scientist." However, a rocket propulsion scientist, in fact, can bring about some much needed improvements in abrasive blasting, especially in the efficiency and productivity of blasting nozzles. For instance, the exhaust of the space shuttle main engine contains "shock diamonds," which are characteristic of supersonic flow. So does a blasting nozzle, although the diamonds are seldom visible to the naked eye. Micrometer-sized alumina particles are accelerated through the nozzles of the shuttle's solid rocket boosters (Ref 1), just as abrasive particles are accelerated through a blasting nozzle. (Actually, it is easier to analyze the shuttle booster problem because the particles are so small.) This article will explain how some of the extensive knowledge and sophistication of rocket nozzle technology (Ref 2) can be used to improve the productivity of abrasive blasting nozzles. It begins with a brief review of the history and state-of-the-art of blasting nozzles, followed by a somewhat technical explanation of bow to improve productivity. Why is the so-called venturi nozzle a better choice for abrasive blasting than the earlier straight-bore nozzle? From the field of gas dynamics (Ref 4) comes an immediate answer: just as in a rocket nozzle, it is the only way to achieve high speed flow at the nozzle exit. The nozzle must contract to a minimum area for the flow to reach the speed of sound, and then it expands to produce supersonic airspeeds. Rocket nozzles like the one shown in Fig. 2 obey the same principle, although in order to be effective at high altitudes, they expand to much larger exit diameters than the earthbound blasting nozzle. However, blasting nozzles have not benefited from any twentieth-century aerospace technology developments. For example, consider the double-venturi scheme shown in Fig. 1. It is a type of ejector nozzle that entrains outside air, but there is no