What device is used for the introduction of flame by combining a fuel and air mixture with heat?

The correct answer is a burner. A burner is specifically designed to mix fuel and air and provide a source of ignition, which results in the controlled combustion of the fuel. This process is essential in a variety of applications, including heating systems and industrial processes, where a consistent and efficient flame is necessary.

In practical terms, a burner typically features various components such as nozzles and valves that help regulate the flow of both the fuel and the air, ensuring they mix properly before ignition occurs. By achieving the right mixture and heat source, the burner is able to create a flame suited for the intended purpose, such as heating a space or powering an engine.

While other devices mentioned, such as igniters, combustors, and furnaces, have important roles in flame generation or heat exchange processes, they do not primarily focus on the combination of fuel and air for creating a flame in the same way that a burner does. For instance, an igniter initiates combustion but does not sustain it; a combustor refers to a container where combustion takes place, and a furnace is typically a larger system that incorporates a burner for heating but has a broader function.