Isentropic process

In thermodynamics, an isentropic process is an idealized thermodynamic process that is both adiabatic and reversible. The work transfers of the system are frictionless, and there is no net transfer of heat or matter. Such an idealized process is useful in engineering as a model of and basis of comparison for real processes. This process is idealized because reversible processes do not occur in reality; thinking of a process as both adiabatic and reversible would show that the initial and final entropies are the same, thus, the reason it is called isentropic (entropy does not change). Thermodynamic processes are named based on the effect they would have on the system (ex. isovolumetric: constant volume, isenthalpic: constant enthalpy). Even though in reality it is not necessarily possible to carry out an isentropic process, some may be approximated as such. The word "isentropic" can be interpreted in another way, since its meaning is deducible from its etymology. It means a proces

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Exploration of micro-acoustic spectrometry techniques

Florian Fernand Hartmann

This master’s thesis aimed at exploring the field of micro-acoustic spectrometry using surface acoustic waves. The objectives of this project were to design and fabricate a platform for the detection and characterization of bioentities. Design of a delay line based on the experience and knowledge of the ANEMS group and on investigation of existing solutions was performed. Different configurations and types of transducers were simulated to come up with some meaningful designs for fabrication. The devices were fabricated in the clean rooms at EPFL with a specific process flow used for the suspended structures. Extensive simulation of bidirectional and unidirectional transducers were carried out in order to understand their behavior and to be able to use them as the elements of the sensing platform. Setbacks prevented the completion of the fabrication and no real measurements could be done before the end of this project, but the simulations gave nice perspectives for the targeted application. The potential of surface acoustic waves to detect modifications in the elasticity or the density of a material could be witnessed in the last simulations.

2022

EPFL's District Heating Heat Pump - Performance Analysis for DIfferent Working Fluids

Sylvain Clément

The present case study is multi-criteria performance analysis of EPFL’s heat pump for different working fluids. The heat pump is based on a 2-stage vapour compression cycle which is modelled using the software Belsim VALI for 7 low-GWP refrigerants. The simulation of the heat pump over different typical periods allowed to estimate the yearly mean COP of the heat pump. R-600a, R-600 and R-717 are the refrigerants showing the higher thermal efficiencies (COP = 5.5-5.7) and thereby also the higher environmental and economical performances. According to the TEWI indicator, the leakages of low-GWP refrigerants are negligible compared to the CO2-eq. emissions from the electricity generation. The decrease in the isentropic efficiency when the heat pump does not work at its maximum capacity strongly affect the COP. Therefore it is recommended to implement a heat storage system to increase the number of operating hours without isentropic efficiency decrease.

2021

Experimental and Numerical Investigation of a Small-Scale Steam Turbine

Diya Achi

This paper focuses on a small-scale radial steam turbine which is part of a fan-turbine unit (FTU) realizing the anode off-gas recirculation of a Solid Oxide Fuel Cell (SOFC). Due to the small size of the turbine (15mm diameter), it is not possible to determine its power by directly measuring the torque. Therefore, the performance parameters are evaluated using thermodynamics. However, heat transfers within the FTU have a non-negligible impact on the evaluation of the turbine power and total-to-total isentropic efficiency. New temperature and pressure measurements were thus conducted to refine the experimental results and the numerical model. The analysis of the experimental and numerical results highlights the significant influence of heat transfers, and provides a range for the turbine power and total-to-total isentropic efficiency, which are on the order of 34W and 39%, respectively. Furthermore, a heat investigation was carried out on the FTU in order to localise and determine the heat losses to the environment by using a simplified model. Through investigation, the primary source of heat loss was located at the turbine outlet.

2020

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