What design feature lowers the pressure but increases the velocity of gas in a burner tube?

The design feature that effectively lowers the pressure while increasing the velocity of gas in a burner tube is the Venturi. The Venturi effect is based on the principle of fluid dynamics, where a fluid's velocity increases as it passes through a constricted section of pipe. When gas enters the narrower section of a Venturi tube, its speed increases due to the conservation of energy; as the gas speeds up, the pressure decreases.

This principle is crucial in gas burners because it allows for the proper mixing of gas with air, ensuring complete combustion and efficient operation. The design of a Venturi enables not just the acceleration of gas flow but also a reduction in pressure, which is vital for creating the right conditions in a burner system to achieve optimal performance.

In contrast, the other options refer to different functions and mechanics that do not necessarily achieve the same result. A diffuser, for instance, is used to smoothly distribute gas or air but does not have the same effect on pressure and velocity as a Venturi. A siphon operates on gravity and pressure gradients to move liquids, while an orifice controls flow but does not inherently involve a reduction in pressure with an increase in velocity like a Venturi does.