Fiber-reinforced composites have become the preferred material in the fields of aviation and aerospace because of their high-strength performance in unit weight. The composite components are manufactured by near net-shape and only require finishing operations to achieve final dimensional and assembly tolerances. Milling and grinding arise as the preferred choices because of their precision processing. Nevertheless, given their laminated, anisotropic, and heterogeneous nature, these materials are considered difficult-to-machine. As undesirable results and challenging breakthroughs, the surface damage and integrity of these materials is a research hotspot with important engineering significance. This review summarizes an up-to-date progress of the damage formation mechanisms and suppression strategies in milling and grinding for the fiber-reinforced composites reported in the literature. First, the formation mechanisms of milling damage, including delamination, burr, and tear, are analyzed. Second, the grinding mechanisms, covering material removal mechanism, thermal mechanical behavior, surface integrity, and damage, are discussed. Third, suppression strategies are reviewed systematically from the aspects of advanced cutting tools and technologies, including ultrasonic vibration-assisted machining, cryogenic cooling, minimum quantity lubrication (MQL), and tool optimization design. Ultrasonic vibration shows the greatest advantage of restraining machining force, which can be reduced by approximately 60% compared with conventional machining. Cryogenic cooling is the most effective method to reduce temperature with a maximum reduction of approximately 60%. MQL shows its advantages in terms of reducing friction coefficient, force, temperature, and tool wear. Finally, research gaps and future exploration directions are prospected, giving researchers opportunity to deepen specific aspects and explore new area for achieving high precision surface machining of fiber-reinforced composites.
A major portion of the worldwide emissions arise from mobile air-conditioning systems with hydrofluorocarbon refrigerant as working substance and which is one of major cause for the greenhouse effect. R134a refrigerant having GWP of 1400 has been extensively used in car air conditioning. To reduce greenhouse gas emissions, the current R134a refrigerant must be phase out as per Kigali Amendment. The present study deals with cooling load calculation of car model by heat balance method as per ASHRAE standard using local climate condition. Further, thermodynamic analysis of R1234yf as an alternate refrigerant to R134a has been carried out for automobile air conditioning system. The required properties of refrigerants are extracted from Engineering Equation Software. The thermodynamic analysis is carried out to study the effect of operating parameters viz. condensing temperature, evaporating tempera-ture, degree of superheating and degree of subcooling on COP, EDR, exergy efficiency and entropy generation. The previous literature reports mainly focus on separate study of either cooling load calculation or energy analysis or exergy analysis of R1234yf and R134a for au-tomobile air conditioning system, while this paper presents the comprehensive study of new low GWP R1234yf as an alternate refrigerant to R134a in automobile air conditioning system with cooling load calculation including the concept of energy, entropy and exergy analysis. The percentage difference in COP between R134a and R1234yf system varies from 2.44 % to 4.78 % while percentage difference in EDR varies from 6.79 % to 2.87 % when evaporating temperature varied from -10 °C to 10 °C. With 12 °C of superheating at compressor inlet, the COP of R134a is 3.9 whereas COP of R1234yf is 3.75, which makes 3.85 % lower than that of R134a. The R1234yf has 4.78 % lower value of exergy efficiency as compared to that of R134a at evaporating temperature of -10 °C and it is found that maximum exergy destruction takes place in compressor.
D. Bashlakov, O. Kvitnitskaya, S. Aswartham
et al.
Compound TiSe2 has received much attention among the transition metal chalcogenides because of its thrilling physical properties concerning atypical resistivity behavior, the emergence of charge density wave (CDW) state, induced superconductivity, etc. Here, we report the discovery of a new feature of TiSe2, namely, the observation of resistive switching in voltage biased point contacts (PCs) based on TiSe2 and its derivatives doped by S and Cu (TiSeS, CuxTiSe2). The switching occurs between a low resistive mainly “metallic-type” state and a high resistive “semiconducting-type” state when a bias voltage is applied (usually < 0.5 V), and reverse switching occurs when a voltage of opposite polarity is applied (usually < 0.5 V). The difference in resistance between these two states can reach up to two orders of magnitude at room temperature. The origin of this effect can be attributed to the variation of stoichiometry in the PC core due to the drift/displacement of Se/Ti vacancies under a high electric field. Additionally, we demonstrated that heating occurs in the PC core, which can facilitate the electric field-induced effect. At the same time, we did not find any evidence for CDW spectral features in our PC spectra for TiSe2. The observed resistive switching allows proposing TiSe2 and their derivatives as promising materials, e.g., for non-volatile resistive random access memory (ReRAM) engineering.
Regioselective glycosylation is a chemical challenge, leading to multistep syntheses with protecting group manipulations, ultimately resulting in poor atom economy and compromised sustainability. Enzymes allow eco-friendly and regioselective bond formation with fully deprotected substrates in a single reaction. For the selective glucosylation of silibinin, a pharmaceutical challenged with low solubility, enzyme engineering has previously been employed, but the resulting yields and kcat were limited, prohibiting the application of the engineered catalyst. Here, we identified a naturally regioselective silibinin glucosyltransferase, UGT706F8, a family 1 glycosyltransferase from Zea mays. It selectively and efficiently (kcat = 2.1 ± 0.1 s–1; KM = 36.9 ± 5.2 μM; TTN = 768 ± 22) catalyzes the quantitative synthesis of silibinin 7-O-β-d-glucoside. We solved the crystal structure of UGT706F8 and investigated the molecular determinants of regioselective silibinin glucosylation. UGT706F8 was the only regioselective enzyme among 18 glycosyltransferases found to be active on silibinin. We found the temperature optimum of UGT706F8 to be 34 °C and the pH optimum to be 7–8. Our results indicate that UGT706F8 is an efficient silibinin glycosyltransferase that enables biocatalytic production of silbinin 7-O-β-d-glucoside.
F. Hernández, P. Pereslavtsev, Guangming Zhou
et al.
Abstract From 2014 to 2020, the Pre-Conceptual Design phase (PCD) of the EU DEMO has taken place. The activities in the PCD phase differ from past exercises in their strong Systems Engineering methodology, as well as for the pragmatic approach in their technology choices. The Helium Cooled Pebble Bed (HCPB) is one of the 2 candidates as driver blanket for the EU DEMO in the PCD phase. Several design iterations have been required during the PCD phase in order to adjust the design to the current demanding DEMO requirements, to the very challenging systems integration and to the need to keep near-term technologies. To this respect, the design has evolved to a so-called fuel-breeder pin architecture built in single-module segments. The pins are filled with a pebble bed of a ceramic breeder mixture of Li4SiO4 + 35 mol % Li2TiO3 (60% 6Li) and are embedded in prismatic blocks of Be12Ti acting as neutron multiplier. He gas at 8 MPa is used as coolant with a temperature window of 300–520 °C. This architecture has proven to achieve a large tritium breeding performance (≈1.20), a remarkably low plant circulating power (
N. Contessi Negrini, A. Angelova Volponi, P. Sharpe
et al.
Engineering cytocompatible hydrogels with tunable physico-mechanical properties as a biomimetic three-dimensional extracellular matrix (ECM) is fundamental to guide cell response and target tissue regeneration or development of in vitro models. Gelatin represents an optimal choice given its ECM biomimetic properties; however, gelatin cross-linking is required to ensure structural stability at physiological temperature (i.e., T > Tsol-gel gelatin). Here, we use a previously developed cross-linking reaction between tetrazine (Tz)- and norbornene (Nb) modified gelatin derivatives to prepare gelatin hydrogels and we demonstrate the possible tuning of their properties by varying their degree of modification (DOM) and the Tz/Nb ratio (R). The percentage DOM of the gelatin derivatives was tuned between 5 and 15%. Hydrogels prepared with higher DOM cross-linked faster (i.e., 10-20 min) compared to hydrogels prepared with lower DOM (i.e., 60-70 min). A higher DOM and equimolar Tz/Nb ratio R resulted in hydrogels with lower weight variation after immersion in PBS at 37 °C. The mechanical properties of the hydrogels were tuned by varying DOM and R by 1 order of magnitude, achieving elastic modulus E values ranging from 0.5 (low DOM and nonequimolar Tz/Nb ratio) to 5 kPa (high DOM and equimolar Tz/Nb ratio). Human dental pulp stem cells were embedded in the hydrogels and successfully 3D cultured in the hydrogels (percentage viable cells >85%). An increase in metabolic activity and a more elongated cell morphology was detected for cells cultured in hydrogels with lower mechanical properties (E < 1 kPa). Hydrogels prepared with an excess of Tz or Nb were successfully adhered and remained in contact during in vitro cultures, highlighting the potential use of these hydrogels as compartmentalized coculture systems. The successful tuning of the gelatin hydrogel properties here developed by controlling their bioorthogonal cross-linking is promising for tissue engineering and in vitro modeling applications.
AbstractIn this study, the tribological properties of drawn PTFE were tested on a CSM tribometer at room temperature and cryogenic temperature. The orientation degree and worn surface morphology of drawn PTFE were studied and correlated with the friction and wear behaviors. It was found that the sample with low friction coefficient corresponded to high orientation degree in the parallel direction at room temperature, while the friction coefficient barely changed with increasing draw ratio in the perpendicular direction. The friction coefficient of drawn PTFE at liquid nitrogen temperature is higher than that at room temperature in the perpendicular direction, while the two values are almost undistinguishable within error range in the parallel direction. The wear volume of drawn PTFE at liquid nitrogen temperature is lower than that at room temperature in both perpendicular and parallel directions due to the mitigation of adhesive wear of drawn PTFE under liquid nitrogen temperature.
G. C. Soares, D. Learmonth, Mariana C Vallejo
et al.
Sterilization of implantable medical devices is of most importance to avoid surgery related complications such as infection and rejection. Advances in biotechnology fields, such as tissue engineering, have led to the development of more sophisticated and complex biomedical devices that are often composed of natural biomaterials. This complexity poses a challenge to current sterilization techniques which frequently damage materials upon sterilization. The need for an effective alternative has driven research on supercritical carbon dioxide (scCO2) technology. This technology is characterized by using low temperatures and for being inert and non-toxic. The herein presented paper reviews the most relevant studies over the last 15 years which cover the use of scCO2 for sterilization and in which effective terminal sterilization is reported. The major topics discussed here are: microorganisms effectively sterilized by scCO2, inactivation mechanisms, operating parameters, materials sterilized by scCO2 and major requirements for validation of such technique according to medical devices' standards.
ABSTRACT Polyethylene-based modification for asphalt is more and more widely used in paving engineering to deal with growing rutting distress on road pavement. The internal structure of polyethylene (PE) and the resulting asphalt are of interest due to their great influences on performance of pavement. This study investigated the correlation between polyethylene structure and asphalt performance. The polyethylene considered in this paper incorporates high-density polyethylene (HDPE), medium-density polyethylene (MDPE), low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE). The influence of various PEs on rheological properties of asphalt was investigated by SHRP (Strategic Highway Research Program) method. Compatibility was also evaluated by rheological criterion and microscopic characterisation. The results indicated that modulus, G*/sinδ and viscosity of MDPE modified asphalt are largest among studied samples and it is a good choice for asphalt modification from the perspective of rutting resistance. LLDPE modified asphalt showed the preferable low temperature performance and LDPE is most compatible with asphalt. Higher branched degree of PE improves the low temperature performance of asphalt, but it reduces high temperature performance. A reduction in MFI (melt flow index) facilitates improvement of rutting resistance performance. Unfortunately, low MFI makes PE difficult to be dispersed and leads to poor compatibility with asphalt.
A spherical experimental device placed in a cylindrical hall of diameter 43 m and height 44 m was used to measure the radiation signal. Photomultiplier tubes (PMTs), which is used to receive the signal, were installed on the exterior surface of the sphere. The PMTs generated heat at 200 kW during operation. To maintain the surface temperature of the sphere at (21±1) ℃, a method for high-precision temperature control by immersing the sphere in circulated water and distributing water up and down simultaneously was proposed. A simulation method using computational fluid dynamics was applied to analyze the temperature field of the water. The results showed that the method of distributing water up and down simultaneously can achieve a precise local temperature control in large spaces. Owing to the limited flow channels in the irregular cavity, the angle of inclination for both upward and downward water distributors of the circulating system was adjusted to improve the performance of temperature control locally. The increase in the area with a low temperature using the upper water distributors can be up to 20%. To reduce the vertical temperature difference of the water, the water flow ratio of the upper and lower water distributors was increased. The temperature requirement can be met when the water flow ratio is 2.5:1.The water volumes that are effectively cooled are the porous media layer and outer water layer. To obtain temperature profiles with an accuracy of (21±1) ℃ under various heating intensities, the heat flux of the heating sources was determined. The variation ranges of the low-temperature volumes around the upper water distributors differed from that around lower water distributors of the inner water layer. The low-temperature volumes around the water distributors at the top decreased by 41%, while the volumes around the distributors at the bottom decreased by 62%. However, the low-temperature volumes around the lower water distributors were larger than those at the upper water distributor, regardless of the amount of heat flux from the heating sources.
Heating and ventilation. Air conditioning, Low temperature engineering. Cryogenic engineering. Refrigeration
The artificial freezing method is extensively used in the reinforcement of engineering strata in various regions for shaft excavation and subway connection channels. In this study, representative rock and soil strata from different regions were subjected to low-temperature physical and mechanical performance tests. The results show that, compared with Cretaceous and Jurassic rock and soil strata, deep topsoil and shallow coastal topsoil have high water content, low thermal conductivities, high frost heave rates, and high freezing temperatures. In addition, the results show that, as the curing temperature decreases, the uniaxial compressive strengths and elastic moduli of deep topsoil and shallow coastal topsoil increase almost linearly. The strength of the sandy soil strata is the highest, followed by the cohesive soil strata, and the strength of the mucky soil and the calcareous clay is the lowest. The strength of the frozen wall and the waterproof requirements must both be taken into account in the freezing design. Deep Cretaceous and Jurassic rocks can have high strength of more than 5 MPa under normal temperature conditions. An increase in the uniaxial compressive strength and elastic modulus with decreasing curing temperature is mainly manifested within the range from the normal temperature to −10°C. The strength can reach more than 10 MPa at −10°C, and only the strength requirements of the frozen wall need to be considered in the freezing design. At low temperatures, deep topsoil and shallow coastal topsoil are dominated by the form of compression failure. The average failure strain at −10°C is typically greater than 5%. When excavating the strata, it is essential to pay attention to the effect of creep. The failure strain of deep Cretaceous and Jurassic rocks is between 1% and 2%, and the breaking and sudden collapse of surrounding rocks should be prevented.
Nowadays, the application of renewable energy sources is one of the primary goals to contribute to low greenhouse gas emissions and low energy cost. Geothermal energy is one of the most attractive renewable energy sources that can be exploited at low, medium and high enthalpy. Absorption chillers are cooling machines that can be powered by a low-grade geothermal energy source. The geofluid leaving the power plant can be used to provide heat for the desorber before it is reinjected to the reinjection well. In this work, a thermodynamic analysis of parallel flow double effect water/LiBr absorption chiller driven by geothermal energy is presented. The effects of many design parameters such as temperature, mass flow rate, and heat exchanger size on the coefficient of performance (COP) and cooling load are investigated. An Engineering Equation Solver (EES) is used for that purpose. The results are presented in graphical forms to show the chiller performance behavior under different operating conditions. The results show that COP and cooling load can reach about 1.43 and 420 kW for certain operating and design conditions.
Abstract Ti-(24-26)Nb-(2-4)Hf at.% alloys were designed by assuming that hafnium has a similar effect to zirconium in the Ti-Nb-Zr system. Alloy specimens were produced using vacuum arc melting and subsequently hot-rolled. Uniaxial tensile testing was then performed both at ambient temperature and in liquid nitrogen at −196°C. While the alloys showed no obvious superelastic behaviour, they exhibited pronounced strain hardening and could achieve high elongations before failure (>30% engineering strain). Post-mortem examination revealed that the mechanism of strain hardening was extensive {332} and/or {211} deformation twinning. Twinning was found to be more prevalent in alloys with 2 at.% Hf compared to those with 4 at.%. The cryogenic temperature deformation also promoted deformation twinning when compared to ambient temperature results. As is the case with other metastable β -Ti alloys, maintaining control over the precipitation of ω phases was found to be crucial for attaining desirable mechanical behaviour. Further, microstructural engineering and alloying may be used to develop strong, lightweight alloys based on the Ti-Nb-Hf system with beneficial strain hardening characteristics for energy absorption, cryogenic and biomedical applications.
Cylindrical fin-tube is a new type of heat exchanger, and its mathematical calculation model has not been researched sufficiently. This study establishesa row-parameter model to realize the numerical calculation of cylindrical fin-tube heat exchangers, i.e., the exchanger is divided into a network of single-row heat exchange tubes andeach row is regarded as a heat exchanging unit. The numerical method presents an iterative solution of heat transfer equations using dichotomy method. The convergence problem in the iterative solution of single-row tubes was discussed in detail. Two iterative criteria were mentioned to guarantee the correct lower limit of the iteration variable (inlet temperature of water of each row). The algorithm was verified, and the results showed that the lower limit of the iteration variable is a linearly increasing function of the inlet water temperature of the heat exchanger. Moreover, for pipelines close to the water outlet of the heat exchanger, the lower limit temperature is higher than the total inlet temperature, which means that the algorithm has practical physical meaning, and could avoid the iteration into the unreasonable interval.
Heating and ventilation. Air conditioning, Low temperature engineering. Cryogenic engineering. Refrigeration