In vent design guides there is no reference to the shape of the vent, only to its geometrical area. This work was carried out on the commonly used circular and square vent shapes for the same geometrical area, in order to investigate whether vent shape was a significant parameter in vent design. The work was carried out in a 0.01 m3 cylindrical explosion vessel with an L/D of 2.8 with end wall ignition opposite the vent, which is close to the limit of applicability of US and European vent design guides for compact vessels. The vents were located on the centreline of the vessel and were compared under free venting conditions. The work was carried out for the most reactive gas mixtures of methane/air and ethylene/air. The impact of the vent shape was mainly on the external explosion and was significant at low Kv=V2/3//Av, with a reduced overpressure for square vents of about 30%. The effect of the vent shape was greater for ethylene/air and occurred at all Kv tested from 3.6 to 10.9. There were two contributory factors to the effect, the change in the discharge coefficient of the vent with vent shape and the greater entrainment of external air into square jets, which caused the jet to spread faster and have lower flame speeds and lower external overpressures.
