440 research outputs found
Theoretical and experimental analysis of the compaction process in a tapered screw press
PhD ThesisEarlier research reported in the literature of plastic extrusion pointed out the
surperiority of a tapered screw for developing high pressure in a short barrel. But
the conveying process in the tapered screw and how pressure was generated were
not explained.
This work is intended to provide a thorough understanding of the conveying
and compaction processes by a tapered screw. A complete theory of the performance
of a tapered screw conveyor was developed, based on the conventional
approach which assumes 'plug flow'
,
but with the following two modifications:
the compressibility of the material was taken into account by using a pressure
/density relationship which makes it possible to simulate the density
change with pressure during compaction.
ii. the effect of the slip of material along the screw channel wall was taken into
account in the analysis by using a slip factor Po, which modifies the down
channel velocity. Slip, by reducing the down channel velocity, leads to a
reduction in the axial velocity and the mass flow rate of the extruder at any
speed of rotation.
The resulting equations express the relationship between the performances of
the tapered screw (mass flow rate and pressure build up) and the screw geometries
and frictional conditions.
A one dimensional differential equation obtained for the pressure build up
indicates that low friction between the material and the screw, and high friction
between the material and the barrel can help the pressure generation process. A
method measuring accurately the coefficients of friction between the material to
be compacted and both the screw and the barrel is proposed.
An experimental rig was designed and developed to measure the pressure in a
tapered screw conveyor for different mass flow rates at four speeds of revolution in
order to confirm the theory. A proposal is made for a better design of inlet to a
screw conveyor.
Comparisons were made between tapered and standard screw extruders with
the same length, which show that the tapered screw is capable of generating a much
higher pressure than the standard screw under the same operationing conditions. The experimental results indicate that the predicted pressures agree well with
the measured ones.Chinese government, and British Government; ORS Award (1988-1990
Finite-dimensionality of attractors for wave equations with degenerate nonlocal damping
In this paper we study the fractal dimension of global attractors for a class
of wave equations with (single-point) degenerate nonlocal damping. Both the
equation and its linearization degenerate into linear wave equations at the
degenerate point and the usual approaches to bound the dimension of the
entirety of attractors do not work directly. Instead, we develop a new process
concerning the dimension near the degenerate point individually and show the
finite dimensionality of the attractor.Comment: 33 page
Hybrid Topological Superconductivity and Hinge Majorana Flat Band in Type-II Dirac Semimetals
Type-II Dirac semimetals (DSMs) have a distinct Fermi surface topology, which
allows them to host novel topological superconductivity (TSC) different from
type-I DSMs. Depending on the relationship between intra- and inter-orbital
electron-electron interactions, the phase diagram of superconductivity is
obtained in type-II DSMs. We find that when the inter-orbital attraction is
dominant, an unconventional inter-orbital intra-spin superconducting (SC) state
( and pairing channels of point group) is realized,
yielding hybrid TSC, i.e., first- and second-order TSC exists at the same time.
Further analysis reveals the Majorana flat bands on the -directed hinges,
which penetrate through the whole hinge Brillouin zone and link the projections
of the surface helical Majorana cones at time-reversal-invariant momenta. These
higher-order hinge modes are symmetry-protected and can even host strong
stability against finite rotation symmetry-breaking order. We suggest
that experimental realization of these findings can be explored in transition
metal dichalcogenides
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