An experiment and analysis of the water backflow mechanism of a submerged horizontal exhaust pipe was conducted to make marine diesel engine exhaust under water without seawater backflow accident. According to the current studies of the counter current flow limitation (CCFL)

the ratio of gas phase inertial force to liquid phase gravity was the main physical mechanism of water backflow in underwater exhaust pipe. To this end

a verification test was carried out by putting an exhaust pipe with inner diameter of 90 mm in the place of 3 m underwater. Results show that：the critical superficial gas velocities in exhaust pipe to prevent water backflow is within a certain range related to the length of water remaining in exhaust pipe. With the remaining water length increasing from 0.3 m to 1.8 m

the critical superficial gas velocity decreases from 13.0 m/s to 7.8 m/s

and accordingly the critical gas phase Wallis number decreases from 0.72 to 0.55. Then

based on the exhaust parameters of a real diesel engine

the effects of the key factors such as the exhaust pipe diameter

underwater depth of exhaust outlet and exhaust temperature on the critical superficial gas velocity were calculated and analyzed according to the critical gas phase Wallis number of 0.60. Results indicate that the critical superficial gas velocity increases with the increasing exhaust pipe diameter

meanwhile

the underwater depth of exhaust outlet and the exhaust temperature also have a certain effect on the critical superficial gas velocity because they are related with the exhaust density.