289 research outputs found
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The matching of a "one-dimensional" numerical simulation and experiment results for low viscosity Newtonian and non-Newtonian fluids during fast filament stretching and subsequent break-up
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The Effect of Inkjet Ink Composition on Rheology And Jetting Behaviour
This work presents recent results on the way linear and non linear viscoelastic properties of the fluids affect the jetting
mechanism. Recent progress on quantitative characterising both high frequency linear (LVE) and non-linear (NLVE) viscoelasticity
of fluids allows fluids to be assessed for their jettability before using such materials in a DoD print head. In term of linear viscoelastic measurements, the Piezo Axial Vibrator (PAV) was used to probe the rheology of the fluids on a frequency range
between 10Hz and 10000Hz. A filament stretching apparatus, called the “Cambridge Trimaster”, was used in combination with
high speed cinematography, to characterize the fluids high speed stretching and break-up behaviour. The series of fluids investigated here consist in dilutions of mono disperse polystyrene with different molecular weight (110, 210, 306 and 488 kg/mol respectively) diluted in diethyl phthalate. The choice of polymer weights and concentrations were chosen to match both the
complex viscosity and the LVE. However, non linear rheological data experiments exhibit differences in the fluid relaxation time
and filament break-up mechanism. Ultra-high speed cinematography of DoD jetting events were correlated with filament break-up experiments and demonstrated that fluid rheology provides valuable information on the jetting quality of the fluids
Langley Mach 4 scramjet test facility
An engine test facility was constructed at the NASA Langley Research Center in support of a supersonic combustion ramjet (scramjet) technology development program. Hydrogen combustion in air with oxygen replenishment provides simulated air at Mach 4 flight velocity, pressure, and true total temperature for an altitude range from 57,000 to 86,000 feet. A facility nozzle with a 13 in square exit produces a Mach 3.5 free jet flow for engine propulsion tests. The facility is described and calibration results are presented which demonstrate the suitability of the test flow for conducting scramjet engine research
The use of fibre optic probes for flow monitoring within a small-scale tube
This paper demonstrates the effectiveness of using fibre optic micro-probes for the measurement of dispersion and mixing
in continuous flow within small-scale tubes under oscillatory flow conditions.
The experimental data was modelled and compared either by three different well-known non-ideal models: a) tanks-inseries
(with no backflow); b) differential backmixing; c) stagewise backmixing, and by one two-parameter flow model
consisting of a plug flow and a stirred tank reactor in series. Model parameters were found by fitting the theoretical response
with experimental data in both Laplace and time domains by different methods.
In addition, specific results are presented relating to a small scale tube provided with smooth periodic constrictions (SPCs),
the basic element of a novel screening reactor presented by Harvey et al. (Proceedings of the ECCE-4, Granada (2003) 0-6.4-
004). The unsteady tracer injection technique was used at different oscillation conditions, with oscillation frequencies from 0 to
20 Hz and amplitudes from 0 to 3 mm (centre-to-peak). An intermediate mixing behaviour (between plug flow and stirred tank
reactor) was achieved in that range of oscillation frequencies and amplitudes. Dispersion was found to be dependent on the
oscillation conditions (amplitude and frequencies) and related with the fluid backflow and with the breaking of flow symmetry.
The discrete (stagewise) backmixing model was considered as the best model representing residence time behaviour in the
small-scale tube.Fundação para a Ciência e a Tecnologia (FCT
Residence times and mixing of a novel continuous oscillatory flow meso reactor
A novel meso reactor based on oscillatory flow technology (Harvey et al., 2001) has been
recently presented in Harvey et al. (2003) as a new technology for reaction engineering and
particle suspension applications. Due to the demonstrated enhanced performances for fluid micro
mixing and suspension of catalyst beads and to the small volume of the reactor, this novel
miniature reactor is suitable for applications at specialist chemical manufacture and high
throughput screening. Furthermore, a high control of environment conditions (e.g. mixing
intensity, temperature) coupled with an online monitoring turns this reactor suitable for smallscale
applications to the bioengineering field, such as for fast parallel bioprocessing tasks.
This work concerns with the fluid dynamics characterisation of a novel miniature reactor.
Experimental results using state-of-art fibre-optic technology is used in order to demonstrate that
an accurate control of the residence time distribution (RTD) of liquid and solid phases can be
achieved within this reactor as well as enhanced (oxygen) mass transfer rates. Furthermore,
numerical simulations using Fluent ® software will be presented where simulated RTDs agrees
with the experimental results.
The meso reactor unit consists of 4.4 mm internal diameter and 35 cm long jacketed glass tubes,
with a unit volume of 4.5 ml and provided with smooth periodic constrictions (SPCs), with an
average baffle spacing of 13 mm. The internal diameter at the constricted zone (baffle internal
diameter) is 1.6 mm, leading to a reduction of the baffle free are of 87 %. This unit is able to
support batch or continuous operations mode, simply by configuring the tubes in parallel or in
series, according to the intended application. Mixing is achieved by oscillating the fluid at the
bottom or the top of the reactor by means of a piston pump, using oscillation amplitudes and
frequencies ranging from 0 to 4 mm centre-to-peak and 0 to 25 Hz, respectively.
Experimental studies using the Particle Image Velocimetry (PIV) technique (Harvey et al., 2003)
showed that different fluid mechanics are originated at different oscillation conditions
(oscillation amplitudes and frequencies). A plug flow or a stirred tank behaviour can be obtained
just by controlling the oscillation conditions. At low oscillatory Reynolds numbers (Reo), e.g. 10
to 100, the formation of axisymmetric eddies detached from the constrictions is coupled with low
axial velocities and makes it possible to continuously operate the reactor in a plug flow mode.
Increasing the Reo to values higher than 100, the eddy symmetry is broken and a complete
mixing state is achieved inside the meso reactor. Low oscillation amplitudes must be used if
axial dispersion is intended to be minimized, namely at plug flow setup.
Through an overall oscillation cycle, changes of the location of the main flow stream from near
the wall to the centre of each cavity and vice-versa was observed and is expected to lead to high
mass and heat transfer rates (Perry, 2002). Due to the observed high radial velocities, narrow
residence times distributions are expected to be obtained (Perry, 2002). Also high axial
circulation rates were also observed at high Reos (above 100) and it was proved to lead to an
enhanced performance on catalyst beads suspension. The relation of this fluid mechanics with
the real performance of this novel meso reactor will be demonstrated.
Tracer injection technique is applied to perform RTD studies inside a single SPC tube of the
meso reactor. Spectroscopy UV/VIS technique is used to measure the concentration of a
coloured tracer at the inlet and outlet (at continuous mode) or at the bottom and the top of the
tube (at batch mode). A fibre optic apparatus is employed in order to obtain highly accurate
online measurements of the UV/VIS absorbance. Mixing times are calculated for experiments at
batch mode. Different flow rates are used to determine the effect of the flow rate over the RTD at
continuous operation and axial dispersion is presented by the Bodenstein number, Bo.
Determination of KL.a values is achieved by online measurement of the oxygen concentration
using a special fibre optic probe. The working tip of the probe was dip-coated with a ruthenium
complex immobilised in a sol-gel matrix. This complex is excited to fluorescence by a blue led
(470 nm outpuk peak) and the level of the fluorescence is inversely related to the concentration
of the oxygen through the Stern-Volmer equation (Wang et al., 1999), which is measured by the
fibre-optic apparatus. Retention of solid phases (e.g. catalyst beads and yeast cells) inside the
meso reactor will also be tested.
Further studies using the Computation Fluid Dynamics (CFD) technique will be presented where
accurate prediction of the distribution of residence times is achieved. The use of the distributionfunctions
permits to classify the flow behaviour inside this novel meso reactor patterns and to
calculate mixing efficiencies and axial dispersion coefficients (expressed by the Bo number) at
different oscillation conditions.
A simple 2-D axisymmetric laminar model showed good agreement with flow patterns
visualisations using PIV for Reo below 100 but a 3-D model with a very fine mesh was required
to simulate breakage of axisymmetry. Consequently, 3-D models based on laminar and Large
Eddy Simulations (LES) will be used to maximize the matching of RTD at higher oscillation
conditions. Main intended application of CFDs to this novel meso reactor is the design of a meso
reactor unit, which could operate at the best oscillation conditions and flow rate for cell cultures
and biocatalyst applications
Residence times and mixing of a novel continuous oscillatory flow screening reactor
This paper is concernedwith the fluidmechanics andmixing performance of a novel oscillatory flow screening reactor. Using fibre
optic probes, a mixing coefficient k_m is determinedfor the system as a function of the applied fluid oscillation frequency and amplitud.
In a continuous operation mean residence time and a backmixing coefficient g are estimatedas a function of the oscillation conditions.
Finally, in order to compare data with numerical simulations steady state flow data are also included.
The screening reactor presented an intermediate mixing behaviour throughout all the studied range of oscillation amplitudes (0–3mm
centre-to-peak) and frequencies (0–20 Hz). The backmixing was foundto be highly dependent of the oscillation frequency and amplitud.
Nevertheless, a stronger effect of the oscillation amplitude over the axial dispersion was detected presumably due to the increase of the
mixing length. On the other hand, the increase of the oscillation frequency was concluded to have the increase in the radial mixing rates
as the main effect. Thus, it was possible to achieve a decrease in the axial dispersion with the screening reactor using oscillatory flow,
when compared to the laminar steady flow in a plain tube with the same mean internal diameter.Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/6954/2001
Enhanced mass transfer rates of a novel oscillatory flow screening reactor
A novel continuous small-scale reactor based on the oscillatory flow technology (Harvey
et al 2001) is being developed for application to specialist chemical manufacture and high
throughput continuous screening (Harvey et al 2003). This novel reactor is able to perform
continuous multiphase reactions, including those involving the suspension of catalyst beads.
Extensive potential use in chemistry, biological and pharmaceutical laboratories is envisaged.
Optimum operation conditions for applications in the bioengineering field depend on at
least four parameters: 1) fluid mixing, 2) residence time characteristics, 3) particle suspension
and 4) (oxygen) mass transfer rates. This work particularly concerns the establishment of the
operation conditions in relation to oxygen mass transfer rates, KL.a. Furthermore, KL.a values
are correlated with the fluid mixing, axial dispersion coefficients and finally with the fluid
mechanics observed experimentally by particle image velocimetry (PIV) technique and
numerically simulated using Fluent® software.
The screening reactor is composed of a 35 cm length glass jacketed tubes (Figure 1),
provided with smooth periodic constrictions (SPCs). The internal diameter of the tube is 4.4
mm. The diameter in the constricted zone (the baffle internal diameter) is about 1.6 mm,
representing an 87 % reduction in the cross-sectional area. Mixing intensity is controlled by
setting up the frequency and the amplitude (centre-to-peak) of the fluid oscillations. Typical
oscillation frequencies and amplitudes are from 0 to 20 Hz and from 0 to 4 mm, respectively.
A screening arrangement based on some SPC tubes (say 10 to 20) placed at different
configurations (serial or parallel) makes this novel reactor suitable for parallel processing
and/or for sequential reactions procedures.
State-of-the-art fibre-optical technology was used for on-line monitoring of the oxygen
concentration inside the screening reactor by using a special fibre optical micro-probe. The
working tip of the probe was dip-coated with a ruthenium complex immobilised in a sol-gel
matrix. This complex was excited to fluorescence by a blue led (≈ 470 nm output peak) and
the level of such fluorescence is inversely related with the concentration of the oxygen
through the Stern-Volmer equation (Wang et al., 1999). Continuous fluorescence levels are
accurately measured by an UV/VIS/NIR multi-channel spectrometer.
Numerical simulations by the computational fluid dynamics (CFD) technique, using
Fluent ® software, permitted us to conclude that either near plug flow or stirred tank
behaviour can be approached in a single SPC tube, by controlling the fluid oscillation
conditions, i.e. the oscillation frequency and/or amplitude. For oscillatory Reynolds numbers
(Reo) between 10 and 100, the formation of axisymmetric vortex rings leads to a good radial
mixing of the fluid and to low axial dispersions, which suggests that a performance near a
plug flow reactor is achieved. For Reo above 100 the high intensity and asymmetry of the
vortex rings leads to an increase of the axial dispersion and the fluid behaviour approach that
of a stirred tank. All these results were experimentally validated by PIV observations.
High radial rates of flow exchange were numerically simulated and experimentally
observed during a complete oscillation cycle, coupled with a high degree of velocity
gradients. Thus, enhancements of heat and mass transfer rates are expectable within this novel
screening reactor. Further, an increase of bubbles breakage is also expected (leading to a
decrease of bubble diameter, i.e. an increase of the specific bubble area, a, and also of the gas
hold-up), conducting to a significant improvement of oxygen mass transfer. This will be
demonstrated with the present work
Molecular physics of a polymer engineering instability: Experiments and computation
Entangled polymer melts exhibit a variety of flow instabilities that limit production rates in industrial applications. We present both experimental and computational findings, using flow of monodisperse linear polystyrenes in a contraction-expansion geometry, which illustrate the formation and development of one such flow instability. This viscoelastic disturbance is observed at the slit outlet and subsequently produces large-scale fluid motions upstream. A numerical linear stability study using the molecular structure based Rolie-Poly model confirms the instability and identifies important parameters within the model, which gives physical insight into the underlying mechanism. Chain stretch was found to play a critical role in the instability mechanism, which partially explains the effectiveness of introducing a low-molecular weight tail into a polymer blend to increase its processability
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Application of NDT thermographic imaging of aerospace structures
This work aims to address the effectiveness and challenges of Non-Destructive Testing (NDT) inspection and improve the detection of defects without causing damage to the material or operator. It focuses on two types of NDT methods; pulsed thermography and vibrothermography. The paper also explores the possibility of performing automated aerial inspection using an unmanned aerial vehicle (UAV) provided with a thermographic imaging system. The concept of active thermography is discussed for inspecting aircraft CFRP panels along with the proposal for performing aerial inspection using the UAV for real time inspection. Static NDT results and the further UAV research indicate that the UAV inspection approach could significantly reduce the inspection time, cost, and workload, whilst potentially increasing the probability of detection
The fluid mechanics relating to a novel oscillatory flow micro reactor
Fundação para a Ciência e a Tecnologia (FCT) - scholarship SFRH/BD/6954/2001
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