Multiple steady states of exhaust airflow in a multi-branch tunnel with the combined effects of buoyancy and fan power

The multi-branch tunnel has multiple routes for smoke extraction and air supply, consequently its ventilation and smoke control modes could have multiple solutions owing to the competitive effects of buoyancy and fan power. For the smoke control of a multi-branch tunnel, the mass and energy balance equations for every possible flow pattern were established, and the corresponding multiple solutions were obtained using mathematical methods. The results demonstrate that, even though the exhaust ventilation mode and fan type have been determined in accordance with the anticipation, multiple states of exhausted flow remain and the operation point of the fans will drift away from the design accordingly. This could cause a totally different direction of smoke route from the anticipated one. It is also shown that the type of fan has significant effects on the existence of multiple solutions.