FLAME-SPEED DEMONSTRATIONS W. E. THRUN Valparaiso University, Valparaiso, Indiana
FLAME-propagation demonstrations for first-year classes in chemistry can be conveniently performed with simple apparatus. Three lengths (15 ft.) of glass tubing of 2-cm. bore are laid end-to-end on the table and connected with tape. A Meker burner having an outlet of about the same diameter as the glass tubing is used to give the gas-air mixtures. In order to get a flame of about maximum speed the gas supply is adjusted with the air-port wide open until the flame is judged to be a t maximum efficiency. The flame is then pinched off with a clamp or by kinking the rubber tubing, and then the burner with the gas-air mixture flowing through i t is held against one end of the glass tube for about 30 seconds. This end of the tube is then closed with a stopper. The assistant ignites the mixture and with a stopwatch takes the time for the flame to travel the length of the tube. The time for a rich mixture is taken in the same way except that the air-port of the burner has been closed about halfway. For a lean mixture the air-port is wide open but the gas supply is cut down so that the flame begins to flicker. More time will be required to displace the air from the tube. That there is a partial vacuum behind the flame due to the condensation of the water formed may be shown by inserting a stopper holding a bent tube into the end of the ignition tube after ignition of the mixture and allowing water to be sucked up through the bent tube.
When both ends of the tube are left open the flame surges and its velocity is accelerated, especially for the last third of its course. Other variations suggest themselves to the teacher. A circle of wire gauze inserted a t one of the joints will stop the progress of the flame, thus demonstrating the principle of the miner's safety lamp. If the air-port is allowed to draw in oxygen instead of air it can be shown that the flame speed is increased. When ethylene is used a lean mixture will give a luminous, peculiarly shaped, fuzzyappearing flame. For natural gas which had been enriched the following results were obtained: maximum, 6.8 secs.; rich, 13.6secs.; lean, 13.6 secs. For ethylene the following results were obtained: maximum, 7 secs.; rich, 14.6 secs.; lean, 22 secs. With these demonstrations the teacher can show that the theoretically required amount of air or oxygen furnishes a combustible mixture in which the maximum flame speed is attained, and that the flame speed depends upon the kind of gas being bnmed. He can call attention to the fact that the burner has here a function similar to that of the carburetor. He can explain why violent gas explosions sometimes occur in the kitchen gas oven when the flame has been turned too low and goes out and an attempt to relight i t is made without proper ventilation. Students observe these demonstrations with great interest and generally learn the principles involved.
