Based on observations of flows, an outline of the blocking phenomenon of snow/water mixture flow in closed channels was given. It was confirmed that a characteristic blocking occurred due to the property of snow in the mixture flow, i.e. a tendency of snow blocks to adhere to each other and to form a densified plug in the channel compressed by the water pressure. Since the blocking was revealed to originate from the stagnation of the snow cluster, the criterion at on orifice in a horizontal pipe was investigated experimentally. With pipe diameter
D=47∼90 mm and the areal contraction ratio
K=0.068∼0.27, the dependence of the critical flow velocity
Vst (below which snow clusters were arrested at the orifice) on
D, on
K, on snow volume fraction
C in discharge, and on the property of snow was made clear as follows.
(1) When
C<
C*, where
C* is 0.15 and 0.12 for granulated snow and fresh snow respectively,
Vst is inversely proportional to
D and independent of
C. When
C>
C*, snow clusters almost fill the pipe and
Vst is inversely proportional to
D and proportional to
C. While, the influence of
K on
Vst is not significant over the whole experimental range of
C.
(2) Fresn snow is apt to stagnate, and the critical values of
DVst with
C<
C* and
DVst/
C with
C>
C* for fresh snow are 1.4 and 2 times the corresponding ones for granulated snow.
(3) Blocking is avoidable by applying the critical values of
DVst or
DVst/
C obtained in this work as the criterion of stagnation.
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