| OCR Text |
Show lUI TYPE TUBE The first tests of u-type/63TFB showed that the burner could be operated with three stages of combustion air on natural gas and only two stages of combustion air with propane and butane. One of the restrictions is a strong tube vibration due to the flame instability in the nozzle cone. The following is a basic explanation of that phenomenon. It is our observation that an initial ignition zone is located at the nozzle's outlet ring. It can be the ring's whole surface or just a zone around the grounding lug. The mixture of gas flow with the primary air stream is being ignited on that ignition ring. Having three stages of combustion air, the ratio between primary, secondary, and tertiary air is approximately 17%, 50%, and 33% (Fig.4). Natural gas operation is characterized by quiet combustion because the ignition ring is a stabile igniter of the gas/primary air mixture. Propane and butane operation gives a noisy combustion at any excess of combustion air. Noise is caused by flame vibration between nozzle and radiant tube due to the colder nozzle condition at higher air to gas ratio, which is equal to approximately (25-31):1. Having two stages of combustion air, the ratio between primary and secondary air is approximately 25% and 75%. It produces a noisy combustion at any excess air rate due to a flame penetration into the cone. Increasing dimension 'A' from %" to %" (10-20mm) gives the opportunity to burn with acceptable noise. with that nozzle set-up the mixture is rich enough and there is no ignition inside the cone, only on the ignition ring surface. Propane and butane produces quiet combustion at two stages of combustion air. This is because the gas/air ratio inside of the cone is conducive to forming a mixture which ignites on the ignition ring's surface without flame movement back to the nozzle cone. -13- |
