Influence of flow parameters in the dual nozzle CO2-based vortex tube cooling system during turning of Ti-6Al-4V

Dual nozzle vortex tube cooling system (VTCS) is developed to improve the machinability of Ti-6Al-4V where cold-compressed CO2 gas is used as a coolant. The cooling effect is produced by the process of energy separation in the vortex tube and the coolant is supplied into the machining zone to remove the generated heat in machining. In this study, the responses such as cutting force (Fz), cutting temperature (Tm), and surface roughness (Ra) are analyzed by considering coolant inlet pressure, cold fraction, and nozzle diameter as input variables. Further optimization is performed for the input variables using the genetic algorithm technique, and the results at optimum conditions are compared with those of dry cutting. From the results, lower cutting force is observed at lower coolant pressure and cold fraction and higher nozzle diameter. The cutting temperature is minimized by increasing coolant pressure and cold fraction and by decreasing nozzle diameter. A better surface finish is observed at high coolant pressure and cold fraction and lower nozzle diameters. It is observed from the response surface method (RSM) that the coolant pressure is most significantly affecting all the responses. At optimum conditions, the cutting temperature and surface roughness are 35.6% and 66.14%, respectively, lower than dry cutting due to the effective cooling and lubricating action of the CO2 gas, whereas cutting force observed under the VTCS is 18.6% higher than that of dry cutting because of the impulse force of the coolant VTCS and thermal softening of the workpiece in dry cutting.

Magnetic Exchange Interactions in Binuclear and Tetranuclear Iron (III) Complexes Described by Spin-Flip DFT and Heisenberg Effective Hamiltonians

Low-energy spectra of single-molecule magnets (SMMs) are often described by the Heisenberg Hamiltonian. Within this formalism, exchange interactions between magnetic centers determine the ground-state multiplicity and energy separation between the ground and excited states. In this contribution, we extract exchange coupling constants (J) for a set of iron (III) binuclear and tetranuclear complexes from all-electron calculations using non-collinear spin-flip time-dependent density functional theory (NC-SF-TDDFT). For the series of binuclear complexes with J-values ranging from -6 to -132 cm−1 , our benchmark calculations using the short-range hybrid LRC-ωPBEh functional and 6-31G(d,p) basis set agree well (mean absolute error of 4.7 cm−1) with the experimentally derived values. For the tetranuclear SMMs, the computed J constants are within 6 cm−1 from the values extracted from the experiment. We explore the range of applicability of the Heisenberg model by analyzing the radical character in the binuclear iron (III) complexes using natural orbitals (NO) and their occupations. On the basis of the number of effectively unpaired electrons and the NO occupancies, we attribute larger errors observed in strongly anti-ferromagnetic species to an increased ionic character. The results illustrate the efficiency of the spin-flip protocol for computing the exchange couplings and the utility of the NO analysis in assessing the validity of effective spin Hamiltonians.

Magnetic Exchange Interactions in Binuclear and Tetranuclear Iron (III) Complexes Described by Spin-Flip DFT and Heisenberg Effective Hamiltonians

Low-energy spectra of single-molecule magnets (SMMs) are often described by the Heisenberg Hamiltonian. Within this formalism, exchange interactions between magnetic centers determine the ground-state multiplicity and energy separation between the ground and excited states. In this contribution, we extract exchange coupling constants (J) for a set of iron (III) binuclear and tetranuclear complexes from all-electron calculations using non-collinear spin-flip time-dependent density functional theory (NC-SF-TDDFT). For the series of binuclear complexes with J-values ranging from -6 to -132 cm−1 , our benchmark calculations using the short-range hybrid LRC-ωPBEh functional and 6-31G(d,p) basis set agree well (mean absolute error of 4.7 cm−1) with the experimentally derived values. For the tetranuclear SMMs, the computed J constants are within 6 cm−1 from the values extracted from the experiment. We explore the range of applicability of the Heisenberg model by analyzing the radical character in the binuclear iron (III) complexes using natural orbitals (NO) and their occupations. On the basis of the number of effectively unpaired electrons and the NO occupancies, we attribute larger errors observed in strongly anti-ferromagnetic species to an increased ionic character. The results illustrate the efficiency of the spin-flip protocol for computing the exchange couplings and the utility of the NO analysis in assessing the validity of effective spin Hamiltonians.

Controlling the Pump-Probe Optical Response in Asymmetric Tunneling-Controlled Double Quantum Dot Molecule—Metal Nanoparticle Hybrids

In the present work, we investigate the modified nonlinear pump-probe optical properties due to the excitonic–plasmonic interaction of a double semiconductor quantum dot (SQD) molecule coupled to a metal nanoparticle (MNP). More specifically, we study the absorption and the dispersion spectra of a weak electromagnetic field in a hybrid structure with two counterparts, a molecule of two coupled SQDs, and a spherical MNP driven by a field of high intensity. We solve the relevant density matrix equations, calculate the first-order optical susceptibility of the probe field in the strong pumping regime, and investigate the way in which the distance between the two counterparts modifies the optical response, for a variety of values of the physical constants of the system, including the pump-field detuning, the tunnelling rate, and the energy separation gap associated with the excited states of the coupled SQDs.

Numerical Investigation of the Secondary Swirling in Supersonic Flows of Various Nature Gases

Despite the application of vortex tubes for cooling, separating gas mixtures, vacuuming, etc., the mechanism of energy separation in vortex tubes remains an object of discussion. This paper studies the effect of secondary swirling in supersonic flows on the energy separation of monatomic and diatomic gases. The approach used is a numerical solution of the Reynolds-averaged Navier-Stokes equations, closed by the Reynolds Stress Model turbulence model. The modelling provided is for a self-vacuuming vortex tube with air, helium, argon, and carbon dioxide. According to the results of the calculations, the effect of secondary swirling is inherent only in viscous gases. A comparison was made between obtained total temperature difference, the level of secondary swirling and power losses on expansion from the nozzle, compression shocks, friction, turbulence, and energy costs to develop cascaded swirl structures. Our results indicate that helium and argon have the highest swirling degree and, consequently, the highest energy separation. Moreover, it can be concluded that the power costs on the development of cascaded vortex structures have a significant role in the efficiency of energy separation.