6 research outputs found
Formation and selection of highly nonlinear microstructure during directional solidification
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1991.Includes bibliographical references (leaves 271-286).by Narayanan Ramprasad.Ph.D
Fundamentals of Multiphase Flow
The subject of multiphase flows encompasses a vast field, a host of different
technological contexts, a wide spectrum of different scales, a broad range of
engineering disciplines and a multitude of different analytical approaches.
Not surprisingly, the number of books dealing with the subject is voluminous.
For the student or researcher in the field of multiphase flow this broad
spectrum presents a problem for the experimental or analytical methodologies
that might be appropriate for his/her interests can be widely scattered
and difficult to find. The aim of the present text is to try to bring much
of this fundamental understanding together into one book and to present
a unifying approach to the fundamental ideas of multiphase flows. Consequently
the book summarizes those fundamental concepts with relevance to
a broad spectrum of multiphase flows. It does not pretend to present a comprehensive
review of the details of any one multiphase flow or technological
context though reference to books providing such reviews is included where
appropriate. This book is targeted at graduate students and researchers at
the cutting edge of investigations into the fundamental nature of multiphase
flows; it is intended as a reference book for the basic methods used in the
treatment of multiphase flows
Yield-stress drops
The behaviour of viscoplastic drops during formation and detachment from a
capillary nozzle, free-fall, impact on a solid substrate and subsequent spreading are
investigated experimentally by high-speed imaging. Drop dynamic behaviour is an
integral component of many contemporary industrial processes ranging from fuelinjection
systems in combustion engines to spray coating, agrochemical and
pharmaceutical delivery, fire extinguishment and ink-jet printing. Yield-stress fluids are
commonly used nowadays in products ranging from mayonnaise to hair-gel. It is hoped
that through understanding the dynamics of viscoplastic fluids, additional spray
applications can be developed that will help to advance and optimise industrial processes.
Viscoplastic fluids exhibit shear-thinning behaviour when the applied stress exceeds a
certain threshold value, called the yield-stress. Below this threshold however, the fluid
behaves like an elastic solid. By comparing the behaviour of viscoplastic drops with both
Newtonian and shear-thinning fluids, yield-stress is shown to be capable of altering
detachment behaviour, drop shape during free-fall, impact morphology and the final sessile
shape of drops after spreading. For drops attached to the end of a capillary tube, growth
continues until a maximum supportable tensile stress is reached in the drop neck. After
this critical point, drops become unstable and detach. The critical break-up behaviour of
low yield-stress drops is found to be similar to those of Newtonian and shear-thinning
fluids. Above a threshold value however, characterised in terms of the ratio between
yield-stress magnitude and capillary pressure, yield-stress forces exceed surface tension
forces and the maximum tensile stress achievable in the drop neck at critical stability is
governed by the extensional yield-stress, established using the von Mises criterion. This
threshold value can also be used to characterise equilibrium drop shapes during free-fall.
Whereas Newtonian, shear-thinning and low yield-stress fluids form near spherical
equilibrium drop shapes, fluids above a threshold value become increasingly more prolate
as the yield-stress increases. Upon impact, viscoplastic drops can exhibit central peaks at
the end of inertial spreading. The influence of yield-stress magnitude on impact
behaviour is qualitatively established by measuring the size of these peaks. Peaks indicate
that deformation during impact is localized and within a threshold radius, shear stresses
will not be large enough to overcome the yield-stress, therefore fluid within this region
will not deform from the drop shape prior to impact. After impact, spreading will be
dependent on the surface energy. Again, the ratio of the yield-stress magnitude to the
capillary pressure can be used to characterise the final sessile drop shape. Whilst the
equilibrium contact angle of Newtonian, shear-thinning and low yield-stress drops is
independent of the yield-stress magnitude, above a threshold value, contact angles vary as
a function of yield-stress magnitude. Whilst the research presented in this thesis
highlights how fluid yield-stress can influence drop dynamics, some results are only
qualitative. To establish more quantitative results, computational fluid dynamics methods
should be used to examine viscoplastic drop dynamics. This research should focus
primarily on impact behaviour, an aspect that has not received much attention previously.
Modelling shear-thinning and viscoplastic fluid behaviour can be achieved by
incorporating the relevant rheological models into the flow equations and examining
impact morphology using a volume of fluid method. Numerical results can then be
directly compared with the experimental results. Useful further experimentation could
examine the relaxation behaviour of diamagnetically levitated viscoplastic drops. The
results from this work could provide further insight into what rheological model best
describes viscoplastic behaviour for shear-stresses below the yield-point
Bibliography of Lewis Research Center technical publications announced in 1984
This compilation of abstracts describes and indexes the technical reporting that resulted from the scientific and engineering work performed and managed by the Lewis Research Center in 1984. All the publications were announced in the 1984 issues of STAR (Scientific and Technical Aerospace Reports) and/or IAA (International Aerospace Abstracts). Included are research reports, journal articles, conference presentations, patents and patent applications, and theses