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Model-based analysis of the system reliability under accelerated life tests for the fuel supply of an oil burner
For the assessment of reliability of technical products, time consuming endurance tests are used with and without accelerated life testings. In the accelerated life testings, a stress factor is imposed to reduce the failure time of the component. The stress factor can be an increased utilization rate or increased operating temperature. The diversity and complexity of liquid fuels for vehicles and domestic oil heaters has increased recently due to the use of biogenic components. The topic of fuel stability and aging is the subject of current research. At OWI Science for Fuels gGmbH, test methods for application-related examinations were developed for this purpose. The test time could be reduced significantly by the imprint of physical stress. A research question that had only been investigated secondarily was the relationship between the quality characteristics of the fuel-carrying components, the aging of fuel and components, and the failure behavior. The question is taken up in this work. The objective of this work is to describe a model-based method for the analysis and simulation of aging processes in technical systems.
Impact of Solid Body Emissivity on Radiative Heat Transfer Efficiency in Furnaces – a Numerical Study
In this paper a theoretical approach is proposed and applied for quantifying the relevant radiation heat fluxes between the furnace walls, the internal surfaces and the flue gas. Numerical fluid dynamics calculations (CFD) are performed using a commercial code in which different wall emissivities are applied. To consider the spectral properties of the gas phase, the exponential wide band model (EWBM) was implemented into the CFD solver. Simulations results with a simplified gray radiation model and the EWBM clearly show that the simplified model leads to a considerable overestimation of the radiative heat transfer. Furthermore it can be shown, that for certain gas atmospheres the widely used gray radiation model is not suitable to identify the effect of high emissivity coatings on radiative heat transfer.
Primary Energy Savings of HT-PEM CHP Units
The electrical efficiency of a combined heat and power (CHP) plant and the relation of power and heat demand have an important impact on primary energy savings. In a target application for domestic energy supply with electrical power up to 100 kW, high temperature proton exchange membrane (HT-PEM) fuel cells promise a higher electrical efficiency in comparison to competing technologies. Due to thermal and mechanical limitations, fuel cells are commonly scaled up modularly. The benefit of a modular concept is that the operating point of each module can be controlled separately in order to optimize the complete CHP plant. In this paper we describe the primary energy savings of a modular CHP plant based on HT-PEM fuel cells by numerical simulation. The demand side is modeled using reference load profiles for 10 multi-family buildings. A system model is set up and the control strategy is presented. With the modular CHP concept primary energy savings up to 20% compared to separated generation (boiler and power plant) could be demonstrated. Furthermore, a major benefit of the modular concept is that the primary energy relation increases only slightly with increasing installed power, due to a stable degree of coverage for electrical demand at higher installed capacities. This characteristic also helps to optimize degradation behavior.DOI: 10.1016/j.applthermaleng.2016.05.055
Physical model based reliability analysis
In a fuel supply system of a conventional household oil heating system the fuel degrades over time depending on the system temperature, the tank volume and the flow rates. The test fuel is a blend of 80 vol.% conventional domestic heating oil and 20 vol.% fatty acid methyl ester from rapeseed oil feedstock. The fuel will increase the deposits in the fuel supply which lead to system failures. In this paper a model based approach is introduced for the analysis of accelerated life testings. The impact of the tank volume on the failure time is analyzed. A well balanced physical model is set up for stochastic simulation. The model contains short term dynamic and long term transient degradation effects that have an important impact on the system reliability. The degradation effects of the components and the used fuel cause a system failure over time. A system failure is detected when the volume flow of the fuel supply declines. The analysis shows that the typical variances of characteristic parameters lead to a significant distribution of failure time. Furthermore, the functional relation between tank volume as stress factor and failure time is described.