Wind blows out fire, at the same time it spreads fire. Why this phenomenon?
K.V.SANDEEP, Rajahmundry, Andhra Pradesh
Answer 1: Firstly, consider the presence of a low velocity wind. ( say, virtually, still air). At any moment of time, this wind is just sufficient to sustain the fire. That is, it functions just as an ‘element.'
Obviously, it cannot provide any motive force (kinetic energy) for the fire (flame). Hence, the fire cannot spread. Ultimately, it blows out. Secondly, consider the presence of a high velocity wind. At any moment of time, there is not only sufficient supply of air to sustain the fire but also there is adequate air to provide motive force.
Thus, its function is two-fold: as an element and as a motive force. Hence, the fire spreads.
K.N. KRISHNA PRASAD,
Guest faculty, Building Fire Research Centre
National Engineering Institute (Autonomous)
Answer 2: When we blow out a small candle we are trying to push the hot flame away from the fuel and the heat is removed away from the fuel and thus reaction of burning gets stopped.
For bigger fires the flame is bigger and more air is to be pushed quickly in order to push the fire away from the fuel.
But when air is blown gently on small flame, fresh supply of oxygen is pushed to it for the fuel to burn and the flame may also reach new fuel because of the blowing air that would normally be out of reach.
So depending on the size of the fire and how hard the air is blown the fire becomes worse or better. Thus if the fire is small enough blowing air will put off the fire.
For bigger fires blowing air may fan the flames adding more oxygen and the fire will grow and the flame may reach new fuels nearby causing these fuels also to burn.
Retired Scientist/Engineer, ISRO
This weeks Question
Why do we feel hungry very frequently during winter?
Prabhat Kumar, New Delhi.