Indice adiabatique | EPFL Graph Search

En thermodynamique, l'indice adiabatique d'un gaz (corps pur ou mélange), aussi appelé coefficient adiabatique, exposant adiabatique ou coefficient de Laplace, noté \gamma, est défini comme le rapport de ses capacités thermiques à pression constante (isobare) C_P et à volume constant (isochore) C_V : \gamma = \frac{C_P}{C_V} Le coefficient de Laplace se définit également à partir des capacités thermiques molaires \bar C_P et \bar C_V si la transformation concerne n moles de gaz, ou des capacités thermiques massiques (ou spécifiques) c_P et c_V si la transformation concerne une masse m de gaz : :C_P = n , \bar C_P = m , c_P :C_V = n , \bar C_V = m , c_V \gamma = {\bar C_P \over \bar C_V} = {c_P \over c_V} Cette grandeur sans dimension apparaît notamment dans la loi de Laplace : pour une transformation adiabatiqu

Magnetic and structural properties of Ni-substituted magnetoelectric Co4Nb2O9

Virgile Yves Favre, Hadi Papi, Henrik Moodysson Rønnow, Ivica Zivkovic

The magnetic and structural properties of polycrystalline Co4-xNixNb2O9 (x = 1, 2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x = 1, the compound crystallizes in the trigonal P (3) over bar c1 space group. Below T-N = 31 K it develops a weakly noncollinear antiferromagnetic structure with magnetic moments in the ab plane. The compound with x = 2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T-C = 47 K. For this compound a weakly noncollinear ferrimagnetic structure with two possible configurations in the ab plane was derived from neutron diffraction study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provides further information on the magnetic structure of the compounds.

2019

Lagrangian analysis of bio-inspired vortex ring formation

Mrudhula Baskaran, Karen Ann J Mulleners

Pulsatile jet propulsion is a highly energy-efficient swimming mode used by various species of aquatic animals that continues to inspire engineers of underwater vehicles. Here, we present a bio-inspired jet propulsor that combines the flexible hull of a jellyfish with the compression motion of a scallop to create individual vortex rings for thrust generation. Similar to the biological jetters, our propulsor generates a nonlinear time-varying exit velocity profile and has a finite volume capacity. The formation process of the vortices generated by this jet profile is analysed using time-resolved velocity field measurements. The transient development of the vortex properties is characterised based on the evolution of ridges in the finite-time Lyapunov exponent field and on local extrema in the pressure field derived from the velocity data. Special attention is directed toward the vortex merging observed in the trailing shear layer. During vortex merging, the Lagrangian vortex boundaries first contract in the streamwise direction before expanding in the normal direction to keep the non-dimensional energy at its minimum value, in agreement with the Kelvin–Benjamin variational principle. The circulation, diameter and translational velocity of the vortex increase due to merging. The vortex merging takes place because the velocity of the trailing vortex is higher than the velocity of the main vortex ring prior to merging. The comparison of the temporal evolution of the Lagrangian vortex boundaries and the pressure-based vortex delimiters confirms that features in the pressure field serve as accurate and robust observables for the vortex formation process.

2022

Two-Stage Heat Pump using Oil-Free Turbocompressors – System Design and Simulation

Adeel Javed, Jürg Alexander Schiffmann

The combination of multi-stage heat pump cycles with small-scale oil-free turbocompressor technology running on gas bearings could be a promising way to increase performance in domestic and commercial heat pumps. This paper presents a novel two-stage heat pump system with two heat sources at two different temperature levels using two separate turbocompressors rotating on gas bearings optimized for R134a. The system allows integration of unused heat sources, e.g. solar thermal or waste heat, into heat production with a minimal loss of exergy. The cycle comprises an evaporator for the first heat source, a condenser as heat sink, an open economizer with integrated heat exchanger for the second heat source, and a tube-in-tube suction line heat exchanger (SHX) in the high-pressure for superheating and subcooling. The aim of this study is to evaluate theoretically the performance of this heat pump cycle using a system model programmed in the software EES (Engineering Equations Solver). The simulation assumes steady-state, negligible pressure drops and heat losses, and adiabatic expansion processes. The superheating in the evaporator and the SHX is 5°C, and there is no subcooling in the condenser. The heat exchangers are modeled using effectiveness-NTU models. At the design point, the heating capacity of the condenser is set to 6.5 kW and provides hot water of 55°C. The first heat source is brine of 5°C. The second heat source is water of 30°C and has been designed to provide up to 30% of the total condenser heat capacity. The two turbocompressors are designed specifically to meet the heat pump design point. Presently, one-dimensional (1D) compressor maps are used in the heat pump model. Simulation results show that coefficient of performance (COP) improvements of 20% to 30% are achievable, depending on the source temperature levels of the heat pump cycle and the amount of second heat source added to the system. The COP increases with higher source temperatures, higher second heat source capacity, and lower sink temperature. The pressure ratios are defined by the imposed temperature levels. The mass flow rate of the refrigerant in the first stage is mainly determined by the second heat source capacity, and in the second stage by the heat capacity of the condenser. In future work, this novel heat pump concept will be tested experimentally.

2016

Two-Stage Heat Pump using Oil-Free Turbocompressors – System Design and Simulation

Adeel Javed, Jürg Alexander Schiffmann

2016