Sections

Advertisement

University Scientists Explore Fire Whirls

Last March residents of the Los Angeles foothills were forced to take to their roof-tops with garden hoses to fight walls of fire which razed 10,000 acres. A fire storm in Dorchester last May raged for an entire day before the Boston fire department could get it under control.

Both these holocausts were caused by winds which whipped the fires into swirling columns called fire whirls, or vortexes. These whirls speed up the burning rate and spread fires by picking up flaming bits and dropping them outside the previous boundaries of the fire. They occur in almost every fire, and occasionally grow as large as 200 feet across and 1000 feet high, producing winds of 200 miles per hour.

For the past year, a Harvard graduate student has been experimenting with fire whirls in order to learn how to predict their occurrence and properties. This study, which is being done by Shuh-Jing Ying 4G, working under Howard W. Emmons, Gordon McKay Professor of Mechanical Engineering, is the first large scale research on fire whirls. Eventually such research may help fire fighters to eliminate the threat of fire storms and other natural vortexes, like tornadoes and hurricanes.

Height: Eighteen Feet

Advertisement

Ying's apparatus, located in the New Engineering Sciences Laboratory on Oxford Street, consists simply of a platform surrounded by a wire window screen ten feet high, eight feet in diameter. Ying is the first to use a rotating screen, which gives a symmetrical distribution to the air currents. He places one or more cans of acetone in the center of the platform and ignites the fuel. When the screen revolves slowly, the draft whips the acetone fires into a whirl up to 18 feet high--only two feet lower than the ceiling of the building. (The scale attained in the New Engineering Sciences Laboratory, which was dedicated in June, 1963, would have been impossible in the old wooden engineering building.)

It takes only a slight draft to start a natural vortex. The vortex then increases the wind speed, which, in turn, enlarges the whirl. In the laboratory the vortex increases the velocity of the air in propotion to the screen's radius. With a screen speed of four rotations per second, Ying has produced winds up to seven miles per hour near the whirl. While the rotating screen sucks air outwards, the rising heat of the fire pulls air toward the flame; at speeds faster than four revolutions per second, the screen's pull is greater than that of the fire and the whirl degenerates into a normal short flame.

Temperature: 3200* F

In December Ying perfected a method of determining the temperature distribution of vortexes. Since the flame wiggles, a simple thermometer was of no use, so Ying and Emmons swung a long tungsten live wire through the flame. In this way they obtained data which could be plugged into a computer program to give the temperature distribution. Ying discovered the highest temperature, 3200 degrees Farenheit, several inches from the center of the whirl.

So far, the project has cost about $30,000, which has been paid by the National Science Foundation and the University. In January the NSF granted an additional $30,000 to allow Ying and other graduate students two more years' work on this and another project. But, according to Emmons, the present research is still a long way from yielding practical data.

The phenomenon of fire whirls was discovered accidentally by the Allies during World War II. The vortexes enormous destructive power was thes demonstrated in a raid on Dresdes, Germany, where a special bombing pattern was purposely used to facil the formation of whirls, which killed over 100,000 people.

Although many observers criticized the uselessness and the barbarity of the raid, the knowledge of fires it uncovered has been used and "built upon." Two weeks ago, a fire begun by US planes in Vietnam raged for over day and killed over 200 Vietnamese

Advertisement

Advertisement