In a wood fire, the wood is heated by lightning, burning match stick, friction or a focused point of sunlight. Decomposition of a portion of the cellulose material happens when the wood reaches about 260 deg Celsius.
Decomposed material degenerates to volatile gases that are commonly termed smoke. Smoke is composed of hydrogen, carbon and oxygen. At this stage, the compound molecules disintegrate and the atoms undergo recombination to form water, carbon dioxide and other gases. This process is called burning. Burning emits light. Flame is a product of light through heat and the effect is called incandescence. Colors of the flame depend on the material being used for burning and the temperature. Hottest part or the base of the flame is blue and the top of the flame is orange or yellow in color.
How a flame burns is determined by the Earth’s gravitational force. Fire is a chemical reaction of oxygen with fuel like wood or petrol etc. Presence of Oxygen and heating of fuel to the ignition temperature is required for combustion to happen. When Combustion occurs, oxygen in the air is consumed accompanied by production of other gases like carbon dioxide with transformation of energy into heat and light. Transformed energy heats up the gases produced, increasing the gases temperature than the environment. Hot gases expand and possess lower density than the atmosphere surrounding it and hence the gases move upwards. Fire burns upwards as the heated gases carry energy and move upwards.
Self perpetuating nature of fire ensures that the combustion requirement of ignition temperature is always met as long as fuel and oxygen is available. Char or Charcoal is the remaining portion of the wood material that contains unburnable minerals and pure carbon. Charcoal fire emits no smoke as it is the residue of the wood as carbon without the gases.
Normal flame assumes the shape of a tear drop as a result of upward convection of air and buoyancy supported by gravity. In the case of no gravity no convection current is present and buoyancy would be zero forcing the flame to assume the shape of a sphere. Flame would not burn longer as there is no upward movement and oxygen in the surrounding would be consumed quickly.