Question

why the burning smoke in insencestick move in a straight path

Answer 1

The chemical reaction(s) that occur while an incense stick burns releases molecules that move with some fixed velocity. A large number of these molecules form what we see as the smoke rising. Now when the smoke rises (assuming it to be an ideal gas) it follows streamline flow. That is the molecule move up in layers while constantly losing energy. The streamline flow continues till the molecules have enough kinetic energy to maintain the aerodynamic layer movement. As soon as they have lost a certain amount of their kinetic energy, they start moving slowly and lose this layer movement and the surrounding air molecules which move at a relatively faster speed now disturb the gas and act as an obstacle and thus forms the turbulent flow which we see as dispersion of smoke.