Flow Pattern in Diesel Nozzle Spray Holes

Abstract

The paper presents a study of the flow in spray holes of 0.2 to 2.5 mm diameter and shows how the changes in cavitation pattern affect the appearance of the jet. The influence of the cavitation number, Reynolds number, the upstream edge sharpness, and the length/diameter ratio is investigated. A cavity first formed near the upstream corner, but soon caused the jet to leave the wall altogether so that only the upstream corner had any effect on the flow. Under noncavitating conditions the emerging jet had a ruffled appearance, but under conditions when the jet had left the wall, it emerged smooth and glass-like. The glass-like stage could only be obtained with very accurately made spray holes, and any disturbance upstream, such as occurs in actual Diesel nozzles, caused the jet to appear ruffled at all times. The discharge coefficient was found to vary with Reynolds number and cavitation number and a contour map covering Reynolds number of 1000 to 20 000 and cavitation number of 0·2 to 100 is presented.