expanding pipe; diffuser; expander; reducer
Technical term in engineering contexts for a pipe section that changes diameter to expand or reduce flow cross-section. The English glosses cover different functional aspects: an expanding pipe increases diameter, a diffuser slows flow and increases pressure, an expander is a tool or device for enlarging, and a reducer decreases diameter. In Japanese, 拡大管 can refer to any of these depending on context.
An expanding pipe is used in this piping system.
In the design of a diffuser, preventing flow separation is important.
Compound of 拡大 (kakudai, 'enlargement') + 管 (kan, 'pipe'). The term is a straightforward technical coinage.