In the present study, we consider the air entrainment into a suction pipe which is vertically inserted down into a suction sump across a mean free-water surface. This configuration is often referred to as the “vertical wet-pit pump”, and has many practical advantages in construction, maintenance and operation. In particular, we focus our concern upon the critical submergence depth
Sc, which is one of the prime and conventional indicators for the air-entrainment occurrence. By a systematic approach, we experimentally investigate the influences of kinetic and geometric parameters upon
Sc. As the kinetic parameters, we consider the Reynolds number
Re and the Weber number
We, in addition to the Froude number
Fr, on such a basis as
Fr is not much larger than unity in many actual cases. As the geometric parameters, we consider back clearance
X, sump breadth
B and bottom clearance
Z. Here, all parameters are non-dimensionalised by the outside diameter
D and the intake velocity
Vi of the suction pipe. As a result, we reveal the effects of such six parameters upon
Sc. The
We effect, namely, the surface-tension effect can be ignored at
We > 12. And, the
Re effect, namely, the viscous effect becomes negligibly small at
Re > 3×10
4. Under such conditions for
We and
Re, we could consider only the
Fr effect, namely, the gravitational effect. Concerning the
X/D and
B/D effects,
Sc/D attains the maximum at a certain
X/D or
B/D. On the other hand, the
Z/D effect is monotonic, and becomes small at
Z/D > 2.5. Some aspects of these geometric effects can be evaluated by a local-
Froude-number effect on the basis of the global relation between
Sc/D and
Fr. And, the other aspects is necessarily considered to be related with the flow structure in the suction sump.
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