Operating temperature | EPFL Graph Search

An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the maximum operating temperature (or peak operating temperature). Outside this range of safe operating temperatures the device may fail. It is one component of reliability engineering. Similarly, biological systems have a viable temperature range, which might be referred to as an "operating temperature". Ranges Most semiconductor devices are manufactured in several temperature grades. Broadly accepted grades are: *Commercial: 0 ° to 70 °C *Industrial: −40 ° to 85 °C *Military: −55 ° to 125 °C Nevertheless, each manufacturer defines its own temperature grades so designers must pay close attention to actual da

Heat Integration and Exergy Analysis of Reactors

Abdul Ghani Soufi

In the context of a semester project conducted at the LENI Lab at EPFL, a study on reactors integration and the effect of the operating conditions of the reactor and the reaction, on the T-H profile was carried out with the help of an Exergy Analysis. At the end, a real temperature enthalpy profile inside the reactor was drawn and analyzed and finally a recommendation was proposed on a systematic decrease in reaction’s operating temperature, which gave a gain in exergy delivered for the exothermic reaction used in this project.

2013

Physical Mechanisms governing Self-Excited Pressure Oscillations in Francis Turbines

Andres Müller

The importance of renewable energy sources for the electrical power supply has grown rapidly in the past decades. Their often unpredictable nature however poses a threat to the stability of the existing electric grid. Hydroelectric powerplants play an important role in regulating the integration of renewable energy sources into the network by supplying on-demand load balancing as well as primary and secondary power network control. Therefore, the operating ranges of hydraulic machines has to be continuously extended, which potentially produces undesirable flow phenomena involving cavitation. An example is the formation of a gaseous volume in the swirling flow leaving a Francis turbine runner at off-design operating conditions. At high load, this so called vortex rope is shaped axisymmetrically and may enter a self-excited oscillation, measurable through significant fluctuations of the pressure throughout the system and the mechanical torque transferred to the generator. The main objective of the present work is the identification of the physical mechanisms governing this self-sustained, unstable behavior by measurement. Furthermore, the key parameters of numerical approaches using one-dimensional hydroacoustic flowmodels or CFD require experimental validation. For this purpose, the measurements provide a comprehensive data base of various flow and system parameters at varying operating conditions. Two test cases are studied, a small scale hydraulic circuit with a micro-turbine as well as a reduced scale physical model of an existing Francis turbine. On the first test case, the study of the flow rate fluctuations up- and downstream of the oscillating vortex rope in the draft tube, together with the volume of the cavity, revealed the destabilizing effect of the flow swirl in the draft tube inlet. The second test case accurately simulates the behavior of an actual hydraulic power plant. Investigations range from a local study of the flow field in the draft tube cone bymeans of LDV, PIV, high speed visualization and wall pressure measurements to a global analysis, considering the response of the hydraulic and mechanical system to the excitation by the vortex rope oscillation. Among the main observations is a periodical variation of the flow swirl in the draft tube, synchronized with the pressure oscillations. This is likely to be caused by a cyclically appearing volume of cavitation on the runner blades, modifying the relative flow angle at the outlet. The interaction of the blade cavitation and the vortex rope oscillation via the flow swirl is found to play a crucial role in the occurrence of self-excited pressure oscillations in Francis turbines.

EPFL2014

Modelling vapour expansion of extruded cereals

Laurent Lach

The vapour expansion of extruded cereals is a versatile technique used in the food processing industry to produce a wide variety of light, crisp & crunchy products such as snacks, breakfast cereals and pet foods. The range of textures that can be produced depends on a complex interaction of a many parameters controlling the expansion phase making the development and optimisation of the process a difficult task involving many trials. This thesis is aimed at developing a numerical model of vapour expansion of extruded cereal to improve our understanding of the physical process and to help speed up the development of new products. Rather than try to model the growth of each bubble explicitly a more economical "micro-macro" approach was developed involving the coupling of a 1D model for single bubble growth with a CFD code for modeling the bulk fluid flow. Several bubble growth models have previously been developed and coupled with simplified models for the flow inside the die, but none of them have succeeded in predicting even the right trend in the observed dependence of the expansion with operating conditions. A microscopic model was first developed along similar lines to those described in the literature with some improvements and then coupled to different macroscopic flow models. Although the process requires at least a 2D coupling to properly capture the full behaviour of vapour expansion, considerable insight was gained by coupling with a 1D compressible macroscopic flow model assuming isotropic expansion in order to predict the evolution of the extrudate outside the die. In particular the 1D model predicted the growth and maximum extrudate diameter in qualitative agreement with experimental measurements and showed for the first time why the expansion is observed to be stronger with lower water content or lower temperature. It showed for example that increasing the water content influences the expansion more by changing the partition between lateral and longitudinal expansion than by changing the degree of volume expansion itself. In other words, the increased water content decreases the viscosity and increases the nucleation pressure so the expansion occurs sooner and more rapidly and hence spends more time expanding inside the die where it can only accelerate. There is thus less bubble growth remaining for lateral expansion by the time it reaches the die outlet. For very low moisture content the model predicts a weak reduction in lateral expansion due to the reduced availability of water for vaporization. The resulting peak was also observed experimentally. Two types of 2&3D coupling with the Fluent CFD code were developed and tested: 1) where the equations for the microscopic model are solved in Eulerian form as User Defined Scalar (UDS) equations and 2) where the microscopic model equations are solved in Lagrangian form along stream lines computed using the Discrete Phase Model (DPM). In both cases the free-surface expansion of the extrudate from the die is simulated with the Volume of Fluid (VOF) capability in Fluent. Unfortunately neither approach could be made to converge stably due to numerical problems that could not be rectified due to the closed source nature of Fluent. A similar problem with the 1D model was solved by deriving a more intimate coupling between the micro and macro equations to reduce their order.

EPFL2006