There are many different kinds of burners. We’re mainly going to focus on the inshot type burner as seen above. The inshot burners are in most modern furnaces. A combustion blower is used to pull the flames through the burner tubes into the heat exchanger.
Burners are used to prepare the air:gas mixture for combustion. Gas is shot through one side, and as gas flows through at a relatively high speed, it pulls primary air into the burners with it. The quick flow of gas pulling air in with it is called the venturi effect.
Think of a spray-paint can. Venturi speeds up airflow by using a reduction in the size of a passage that gases pass through causing the gas to speed up.
A partially clogged orifice can cause a flashback of the flame as not enough primary air is being drawn in, so ignition happens in the mixing area.
After ignition happens correctly in the burners, the flames are pulled through the heat exchanger with the assistance of the inducer.