104,110 research outputs found
Volume and surface propellant heating in an electrothermal radio-frequency plasma micro-thruster
The temporal evolution of neutral gas temperature over the first 5 min of operation for an electrothermal radio-frequency micro-thruster with nitrogen (N2) propellant was measured using rovibrational band matching of the second positive N2 system. Three distinct periods of gas heating were identified with time constants of Ï 1â=â8âĂâ10â»â” s, Ï 2â=â8âs, and Ï 3â=â100âs. The fast heating (Ï 1) is attributed to volumetric heating processes within the discharge driven by ion-neutral collisions. The slow heating (Ï 3) is from ion neutralization and vibrational de-excitation on the walls creating wall heating. The intermediate heating mechanism (Ï 2) is yet to be fully identified although some theories are suggested.This research was partially funded by the Australian
Space Research Program (APT project) and the Australian
Research Council Discovery Project (No. DP140100571)
A blackbox optimization of volumetric heating rate for reducing the wetness of the steam flow through turbine blades
This paper proposes to use a blackbox optimization to obtain the optimal volumetric heating required to reduce the wetness at the last stages of steam turbines. For this purpose, a global multiobjective optimization is utilized through the automatic linking of genetic algorithm and CFD code, where the blackbox function evaluations are performed by CFD runs. The logarithm of number of droplets per volume (LND), the droplet average radius (DAR), and the integral of local entropy (ILE) at the end of the cascade (after the condensation location) are minimized, while the volumetric heating rate is the optimization parameter. The EulerianâEulerian approach is implemented to model the two-phase wet steam turbulent flow and the numerical results are validated against well-established experiments. Since higher volumetric heating rates reduce DAR and LND, while increase ILE, according to optimization results, there is an optimum for the volumetric heating rate to reach the best performance of steam turbines. For case studies presented in this work, the optimal volumetric heating rates of 5.21x10^8 and 4.67x10^8 W/m^2 are obtained for two different cases of supersonic and subsonic outlets, respectively. Particularly, these rates improve DAR by 45.7% and 57.5%, and LND by 6.0% and 7.8% for respective cases
On red shifs in the transition region and corona
We present evidence that transition region red-shifts are naturally produced
in episodically heated models where the average volumetric heating scale height
lies between that of the chromospheric pressure scale height of 200 km and the
coronal scale height of 50 Mm. In order to do so we present results from 3d MHD
models spanning the upper convection zone up to the corona, 15 Mm above the
photosphere. Transition region and coronal heating in these models is due both
the stressing of the magnetic field by photospheric and convection `zone
dynamics, but also in some models by the injection of emerging magnetic flux.Comment: 8 pages, 9 figures, NSO Workshop #25 Chromospheric Structure and
Dynamic
Diagnosing the time-dependence of active region core heating from the emission measure: I. Low-frequency nanoflares
Observational measurements of active region emission measures contain clues
to the time-dependence of the underlying heating mechanism. A strongly
non-linear scaling of the emission measure with temperature indicates a large
amount of hot plasma relative to warm plasma. A weakly non-linear (or linear)
scaling of the emission measure indicates a relatively large amount of warm
plasma, suggesting that the hot active region plasma is allowed to cool and so
the heating is impulsive with a long repeat time. This case is called {\it
low-frequency} nanoflare heating and we investigate its feasibility as an
active region heating scenario here. We explore a parameter space of heating
and coronal loop properties with a hydrodynamic model. For each model run, we
calculate the slope of the emission measure distribution . Our conclusions are: (1) low-frequency nanoflare heating is
consistent with about 36% of observed active region cores when uncertainties in
the atomic data are not accounted for; (2) proper consideration of
uncertainties yields a range in which as many as 77% of observed active regions
are consistent with low-frequency nanoflare heating and as few as zero; (3)
low-frequency nanoflare heating cannot explain observed slopes greater than 3;
(4) the upper limit to the volumetric energy release is in the region of 50 erg
cm to avoid unphysical magnetic field strengths; (5) the heating
timescale may be short for loops of total length less than 40 Mm to be
consistent with the observed range of slopes; (6) predicted slopes are
consistently steeper for longer loops
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Impact of temperature on the pullout of reinforcing geotextiles from unsaturated silt
This study investigates the thermal soil-geosynthetic interaction mechanisms of reinforcing geotextiles confined in compacted silt that may be encountered when using mechanically-stabilized earth (MSE) walls as geothermal heat sinks. A thermo-mechanical geosynthetic pullout device was used that incorporates standard components for geosynthetic pullout or creep testing but also heating elements at the top and bottom of the soil box to apply boundary temperatures and dielectric sensors embedded in the soil layer to monitor distributions in temperature and volumetric water content. Two test series were performed: the first involves monotonic pullout of woven polypropylene geotextiles after reaching steady-state conditions under different boundary temperatures without a seating load, and the second involves monotonic pullout of woven polyethylene-terephthalate geotextiles after reaching steady-state conditions under different boundary temperatures with a seating pullout load. The results indicate that the pullout resistance of both geotextiles decreased with increasing temperature. Although heating led to drying of the unsaturated silt layers as expected, measurements from the second test series indicate accumulation of water at the silt-geotextile interface. An effective stress analysis considering thermal softening of soils indicates that the increase in effective saturation at the silt-geotextile interface was the cause of the decrease in pullout resistance with heating
Modelling the thermo-mechanical volume change behaviour of compacted expansive clays
Compacted expansive clays are often considered as a possible buffer material
in high-level deep radioactive waste disposals. After the installation of waste
canisters, the engineered clay barriers are subjected to
thermo-hydro-mechanical actions in the form of water infiltration from the
geological barrier, heat dissipation from the radioactive waste canisters, and
stresses generated by clay swelling under almost confined conditions. The aim
of the present work is to develop a constitutive model that is able to describe
the behaviour of compacted expansive clays under these coupled
thermo-hydro-mechanical actions. The proposed model is based on two existing
models: one for the hydro-mechanical behaviour of compacted expansive clays and
another for the thermo-mechanical behaviour of saturated clays. The elaborated
model has been validated using the thermo-hydro-mechanical test results on the
compacted MX80 bentonite. Comparison between the model prediction and the
experimental data show that this model is able to reproduce the main features
of volume changes: heating at constant suction and pressure induces either
expansion or contraction; the mean yield stress changes with variations of
suction or temperature
Investigating the time-dependent behaviour of Boom clay under thermo-mechanical loading
Among the various laboratory studies to investigate the
Thermo-Hydro-Mechanical (THM) behaviour of Boom clay, relatively few were
devoted to the time dependent behaviour, limiting any relevant analysis of the
long-term behaviour of the disposal facility. The present work aims at
investigating the time-dependent behaviour of Boom clay under both thermal and
mechanical loading. High-pressure triaxial tests at controlled temperatures
were carried out for this purpose. The tests started with constant-rate thermal
and/or mechanical consolidation and ended with isobar heating and/or isothermal
compression at a constant stress rate or by step loading. Significant effects
of temperature as well as of compression and heating rates were observed on the
volume change behaviour. After being loaded to a stress lower than the
pre-consolidation pressure (5 MPa) at a low temperature of 25\degree C and at a
rate lower than 0.2 kPa/min, the sample volume changes seemed to be quite
small, suggesting a full dissipation of pore water pressure. By contrast, after
being subjected to high loading and heating rates (including step loading or
step heating), the volume changes appeared to be significant, particularly in
the case of stresses much higher than the pre-consolidation pressure. Due to
low permeability, full consolidation of Boom clay required a long period of
time and it was difficult to distinguish consolidation and creep from the total
volume change with time
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