Evolution characteristics of vertical hydrogen jet flame length at sub-atmospheric pressure

In this work, an experimental investigation about the hydrogen jet flame was carried out at sub-atmospheric pressure. The atmospheric pressures range from 40 kPa to 100 kPa. And the effects of volume flow rates (from 10SLPM to 25SLPM) and nozzle diameters (including 2 mm and 4 mm) were considered. The universality of the scale parameters given in previous studies were further confirmed at sub-atmospheric pressure. The critical Froude number for the transition from buoyancy-controlled to momentum-controlled jet flame was adjusted to the value of 30. Moreover, it can be found that the flame length at sub-atmospheric pressure is nearly independent of Froude number. The excellent correlation between the dimensionless heat release rate to the 0.4 power and the hydrogen jet flame length can be obtained at sub-atmospheric pressure. Ambient density was introduced into the dimensionless heat release rate to explain the change of atmospheric pressure. Through the formal transformation of dimensionless heat release rate, the quantitative correlation between flame length and mass flow rate, ambient density and temperature was presented. This paper proposes a hydrogen jet flame theory at sub-atmospheric pressure considering continuous pressure levels. Current results also provide a valuable reference for developing new standards to prevent hydrogen jet flames at high altitudes.