Comparing growth rates of simulated moist and dry convective thermals

The spreading rates of convective thermals are linked to their net entrainment, and previous literature has suggested differences in spreading rates between moist and dry thermals. In this study, growth rates of idealized numerically-simulated axisymmetric dry and moist convective thermals are directly compared. In an environment with dry-neutral stratification, the increase of thermal radius with height, dR/dz, is a factor of 1.7 larger for dry compared to moist thermals. The fractional change in thermal volume ε is also greater for dry thermals within a distance of ~4 radii from the initial thermal height. Values of dR/dz are nearly constant with height for both moist and dry thermals consistent with classical theory based on dimensional analysis. The simulations are also consistent with theory relating impulse, circulation, and spreading rate for dry thermals proposed in previous papers and extended here to moist thermals, predicting they will spread less than dry thermals. Tests adding heating to dry thermals, either spread uniformly across the thermal volume or concentrated in the inner core, indicate dR/dz and ε are smaller for moist thermals because latent heating is confined mostly to their cores. Additional axisymmetric moist simulations using modified lapse rates and large eddy simulations support this analysis. Overall, these results indicate that slow spreading rates are a fundamental feature of moist thermals caused by latent heating which alters the spatial distribution of buoyancy within them compared to dry thermals.

Physico-chemical assessment of torrefied Eurasian pinecones

Background Biomass pre-treatment is gaining attention as a standalone process to improve the qualitative aspect of the lignocellulosic material. It has been gaining ground in the power station by replacing the coal with the pre-treated biomass. In this context, this paper enlightens the operating condition of carrying out the torrefaction so that the process can be made relatively more effective. The influence of physico-chemical characteristics on the heat of reaction of pyrolysis reactions, mass loss and temperature regimes are evaluated by thermogravimetry of the pre-treated samples of the pinecone; whereas, the structural transformation in the basic constituents is determined via knowing the fractional change in cellulose, hemicellulose and acid-insoluble lignin contents of the pine cone. The thermogravimetric (TGA) and differential thermal analysis (DTA) were performed to determine the physical as well as the thermal behaviour of the thermally processed biomass. The samples had undergone thermal decomposition at heating rates of 5 °C min−1, 10 °C min−1 and 15 °C min−1. Nitrogen gas was used as a purge gas for the pyrolysis of the pre-treated samples. The volumetric rate of 200 ml min−1 was pre-set for the thermal decomposition of the samples at 600 °C; whereas, the selected torrefaction temperature range varied from 210 to 250 °C. Results The heat of reaction for the pre-treated samples was found to vary from 1.04 to 1.52 MJ kg−1; whereas, it was 0.91–1.54 MJ kg−1 for the raw samples. The total annual production cost of processing 3.6 Mg of fuel in a year at a pilot scale was $ 36.72; whereas, the fiscal burden per kilogram of fuel during thermal degradation of the processed fuel was reduced by 0.08–1.5ȼ. The entropy of the system decreased with an increasing ramp rate. The exergetic gain in the system increased by 1–2%. The loss of energy during the energy-intensive processing of the pre-treated fuel was relatively low at a heating rate of 5 °C min−1. Conclusion By the physico-chemical assessment, it was determined that pinecones required the highest torrefaction temperature and time to provide the upgraded pinecones. It was concluded that the duration of the torrefaction should be at least 15 min for a temperature of 250 °C so that the chemical exergy of the system, energy yield and the energy density of the processed material are qualitatively improved. The volatile and ash contents were noticed to decrease during the torrefaction process. The least fractional change in the volatile content was estimated at 210 °C for a torrefaction time of 15 min; whereas, the ash content was minimum at 210 °C for a torrefaction time of 5 min.

Mechanical and Self-Sensing Properties of Multiwalled Carbon Nanotube-Reinforced ECCs

This paper investigates the effect of type and dosage of multiwalled carbon nanotubes (MWCNTs) on the mechanical and self-sensing properties of engineered cementitious composites (ECCs). Two types of MWCNTs (MWCNTa and MWCNTb) were employed. The tensile and flexural strengths of CNT-reinforced ECCs were improved compared with normal ECCs, while the ultimate tensile strain and midspan deflection were reduced. Compared with the dosage of MWCNTs, the type had less effect on these properties. The percolation threshold was around 0.3 wt.%. ECCs containing MWCNTs had good self-sensing ability under different loading conditions. When the midspan deflection increased from 0.1 to 0.6 mm, the fractional change in resistivity reached 9%. The dosage of MWCNTs had a significant effect on the self-sensing ability. As the MWCNT content increased, the amplitude of fractional change in resistivity decreased.

Evaluating the Self-Sensing Ability of Cement Mortars Manufactured with Graphene Nanoplatelets, Virgin or Recycled Carbon Fibers through Piezoresistivity Tests

This paper presents the resistivity and piezoresistivity behavior of cement-based mortars manufactured with graphene nanoplatelet filler (GNP), virgin carbon fibers (VCF) and recycled carbon fibers (RCF). GNP was added at 4% of the cement weight, whereas two percentages of carbon fibers were chosen, namely 0.05% and 0.2% of the total volume. The combined effect of both filler and fibers was also investigated. Mortars were studied in terms of their mechanical properties (under flexure and compression) and electrical resistivity. Mortars with the lowest electrical resistivity values were also subjected to cyclic uniaxial compression to evaluate the variations in electrical resistivity as a function of strain. The results obtained show that mortars have piezoresistive behavior only if they are subjected to a prior drying process. In addition, dry specimens exhibit a high piezoresistivity only when loaded with 0.2 vol.% of VCF and 0.4 wt.% of GNP plus 0.2 vol.% RCF, with a quite reversible relation between their fractional change in resistivity (FCR) and compressive strain.

Electrical and piezoresistive properties of cement composites with carbon nanomaterials

This study investigates the effect of nanomaterials on the piezoresistive sensing capacity of cement-based composites. Three different nanomaterials—multi-walled carbon nanotubes, graphite nanofibers, and graphene oxide—were considered along with a plain mortar, and a cyclic compressive test was performed. Based on a preliminary test, the optimum flowability was determined to be 150 mm in terms of fiber dispersion. The electrical resistivity of the composites substantially decreased by incorporating 1 wt% multi-walled carbon nanotubes, but only slightly decreased by including 1 wt% graphite nanofibers and graphene oxide. This indicates that the use of multi-walled carbon nanotubes is most effective in improving the conductivity of the composites compared to the use of graphite nanofibers and graphene oxide. The fractional change in resistivity of the composites with nanomaterials exhibited similar behavior to that of the cyclic compressive load, but partial reversibility in fractional change in resistivity was obtained beyond 60% of the peak load. A linear relationship between the fractional change in resistivity and cyclic compression strain (up to 1500 με) was observed in the composites with multi-walled carbon nanotubes, and the gauge factor was found to be 166.6. It is concluded that cement-based composites with 1 wt% multi-walled carbon nanotubes can be used as piezoresistive sensors for monitoring the stress/strain generated in concrete structures.

Pressure sensitivity of multiscale carbon-admixtures–enhanced cement-based composites

The pressure-sensitive cement-based composites added with multiscale carbon materials, that is, carbon blacks, carbon fibers, and carbon nanotubes are investigated. In the article, the sensing property of cement-based composites with seven different proportions of carbon blacks, carbon fibers, and carbon nanotubes under cyclic loading is discussed and then the optimized formula among these seven proportions is chosen to investigate the influences of temperature and saturation degree on its sensing properties. In addition, the maximum perceivable frequency of multiscale carbon-admixtures–enhanced cement-based composite is obtained from the experimental results. The results indicate that the fractional change in resistance of the cement-based sensing composites increases at first and then decreases with the increase of temperature, but decreases with the increase of humidity. Additionally, the fractional change in resistance has a decrease with the increase of loading frequency, and the cement-based sensing composites prepared can perceive the biggest loading frequency of 0.5 Hz.

Discrete Vibration Stability Analysis With Hydromechanical Specific Energy

Drill-bit vibrations and bit wear have been identified as the two major causes for premature polycrystalline diamond-compact (PDC) bit failure and difficulty in accurately predicting PDC bit performance. The objective of this paper is to present a new approach to drilling optimization by developing an algorithm that defines and generates a constrained stable rotary speed (RPM)–weight-on-bit (WOB) working domain for a given system as opposed to the traditional RPM–WOB charts. The algorithm integrates the dynamic-stability model for bit vibrations with the bit-performance model for degraded bits. This study addresses the issues of dynamic-bit stability under torsional and lateral vibrations coupled with bit wear. The approach presented in this paper involves performing two separate analyses: vibration stability and bit-wear performance analysis. The optimum operating conditions are estimated at each depth of the drilling interval, taking into consideration the effect of bit wear and bit vibrations. Because the bit wears continuously while penetrating the rocks, discretization of depth is necessary for effective simulation. Discretization is done by dividing the drilling interval into subintervals of the desired length. Vibration-stability analysis and bit-wear performance analysis are preformed separately at every subinterval and then integrated over the discrete interval. For every subinterval, a WOB–RPM domain is determined within which the given system is dynamically stable (for vibrations), and the bit wear does not exceed the maximum allowable wear (MAW) for the section of the drilling interval selected. A unique concept to relate the fractional change in hydromechanical specific energy (HMSE) to the fractional change in bit wear has also been put forward that further constraints the WOB–RPM stable working domain. The new coupled vibration-stability chart, including the maximum rate of penetration (ROP), narrows down the conventional chart and provides different regions of operational stability. It has also been found that as the compressive strength of the rock increases, the bit-gauge friction factor also increases, which results in a compressed or reduced allowable working domain, both from the vibration-stability analysis and bit-performance analysis. Simple guidelines have been provided using the new stability domain chart to estimate the operating range for real-time optimization.