Thank you for your question about underwater cutting torches. The first thing you must realize is that a flame requires a combustible substance and an oxidizer. In the case of a candle flame, the combustible substance is the heated wax which rises up the candle wick and is vaporized by the heat of the flame. The oxidizer is the oxygen in the surrounding atmosphere. These two ingredients can sustain a candle flame.
In the case of an underwater torch, both the combustible substance and the oxidizer must be supplied by hoses leading down to the torch, since no free oxygen is available underwater. The combustible gas, which might be hydrogen or acetylene, depending on the application, can be combined with oxygen gas (the oxidizer) to produce an underwater flame at the tip of the torch.
In order to maintain a steady flame, it is necessary to have a stable bubble around the flame which the exhaust gases from the combustion (typically water vapor and carbon dioxide) cannot maintain by themselves. The gas must be supplied at a high enough pressure to overcome the pressure exerted by the water. An underwater torch must therefore have a separate means for sustaining a bubble around the work location. This is accomplished by separate jets of compressed air introduced around the tip of the torch to protect the flame from the surrounding water by forming a rather large bubble enclosing the flame and the region directly adjacent to the work location. As you might imagine, a great deal of skill is required for divers who perform this kind of work.
Ph.D. Yale University (1959)
West Middle School