593 research outputs found
Numerical Simulation of the Fouling on Structured Heat Transfer Surfaces (Fouling)
The objective of this work is to make a contribution to a good and fast prediction of the crystal growth on flat and structured heat transfer surfaces. For the numerical simulation the CFD code Fluent is used. The simulation enables an unsteady calculation of the fouling process and a realistic description of the temporal modification of the flow and temperature fields due to the continuous crystal growth. The numerical simulation of the crystal growth is based on models for the calculation of the deposition (Krause, 1993) and removal (Bohnet, 1990) mass rates. Based on experimental results of Hirsch (Bohnet et. al., 1999), a model was developed which enables the calculation of the density of the fouling layer not only as a function of the local position within the fouling layer, but also as a function of the time-dependent total thickness of the fouling layer. In addition a model was developed, that enables a realistic distribution of the heat flux along the heat transfer surface during the simulation. All models are implemented into the simulation with the help of the programming user interface of the CFD code. During the experimental and numerical investigations the operating parameters like flow rate, surface temperature, concentration of the salt solution and geometry of the flow channel are varied. The induction period and the effects of aging which occur with almost all fouling processes are not considered. Result of the numerical simulation is the prediction of the fouling resistance as function of time. In view of the complexity of the fouling process during the incrustation of heat transfer surfaces and the fact that not all influences from the used models could be considered the agreement between calculated and experimentally obtained data is satisfactory
Crystallization Fouling Of The Aqueous Two-Component System CaSO\u3csub\u3e4\u3c/sub\u3e/CaCO\u3csub\u3e3\u3c/sub\u3e
Solutions, which cause fouling problems, consist mostly of more than one single component. Up to now only few studies concerning the fouling phenomena in such multicomponent systems exist. Therefore batch and continuous experiments with the aqueous two-component system CaSO4/CaCO3 were carried out, investigating especially the influence of pH-value on the fouling behaviour. As measure for the crystalline deposit the fouling resistance Rf was used.
The composition of the obtained fouling layers were analysed by x-ray diffraction and scanning electron microscopy (SEM). Further the strength of the crystalline deposits were determined in abrasion experiments. The measured abrasion was correlated with crushing strength values.
In the fouling experiments a strong effect of the pH-value on crystallization fouling was observed. Lowest fouling tendency was seen for experiments at pH 7.0. At different pH-values the crystalline layers showed big differences in their macroscopic as well as in their microscopic structure. As it could be seen with the SEM the crystals differed in their size but also in their shape. Below pH 6.0 only calcium sulphate was detected by x-ray diffraction, which agrees with the saturation theory. At higher pH values besides calcium sulphate also calcium carbonate was found in different modifications. The different layer composition leads to different strength of the layers. Highest strength values in the crystalline upper and middle layer were measured for crystalline scales grown at pH 7.0, followed by layers at pH 6.5. At the moment it is difficult to correlate the fouling behaviour clearly to the different experimental conditions
Analyzing and Explaining Image Classifiers via Diffusion Guidance
While deep learning has led to huge progress in complex image classification
tasks like ImageNet, unexpected failure modes, e.g. via spurious features, call
into question how reliably these classifiers work in the wild. Furthermore, for
safety-critical tasks the black-box nature of their decisions is problematic,
and explanations or at least methods which make decisions plausible are needed
urgently. In this paper, we address these problems by generating images that
optimize a classifier-derived objective using a framework for guided image
generation. We analyze the behavior and decisions of image classifiers by
visual counterfactual explanations (VCEs), detection of systematic mistakes by
analyzing images where classifiers maximally disagree, and visualization of
neurons to verify potential spurious features. In this way, we validate
existing observations, e.g. the shape bias of adversarially robust models, as
well as novel failure modes, e.g. systematic errors of zero-shot CLIP
classifiers, or identify harmful spurious features. Moreover, our VCEs
outperform previous work while being more versatile
Using imprecise continuous time Markov chains for assessing the reliability of power networks with common cause failure and non-immediate repair.
We explore how imprecise continuous time Markov
chains can improve traditional reliability models based
on precise continuous time Markov chains. Specifically,
we analyse the reliability of power networks under very
weak statistical assumptions, explicitly accounting for
non-stationary failure and repair rates and the limited
accuracy by which common cause failure rates can be
estimated. Bounds on typical quantities of interest
are derived, namely the expected time spent in system
failure state, as well as the expected number of
transitions to that state. A worked numerical example
demonstrates the theoretical techniques described.
Interestingly, the number of iterations required for
convergence is observed to be much lower than current
theoretical bounds
Mobile Telephony Access and Usage in Africa
This paper uses data from nationally representative household surveys conducted in 17 African countries to analyse mobile adoption and usage. The paper shows that countries differ in their levels of ICT adoption and usage and also in factors that influence adoption and usage. Income and education vastly enhance mobile adoption but gender, age and membership of social networks have little impact. Income is the main explanatory variable for usage. In terms of mobile expenditure the study also finds linkages to fixed-line, work and public phone usages. These linkages need, however, to be explored in more detail in future. Mobile expenditure is inelastic with respect to income, ie the proportion of mobile expenditure to individual income increases less than1% for each1% increase in income. This indicates that people with higher income spend a smaller proportion of their income on mobile expenditure compared to those with less income. The study provides tools to identify policy intervention to improve ICT take-up and usage and defines universal service obligations based on income and monthly usage costs. It helps to put a number to what can be expected from lower access and usage costs in terms of market volume and number of new subscribers. Linking this to other economic data such as national household income and expenditure surveys and GDP calculation would allow forecast of the economic and social impact of policy interventions. Key policy interventions would be regulatory measures to decrease access and usage costs, rural electrification and policies to increase ICT skills of pupils and teachers
Spurious Features Everywhere -- Large-Scale Detection of Harmful Spurious Features in ImageNet
Benchmark performance of deep learning classifiers alone is not a reliable
predictor for the performance of a deployed model. In particular, if the image
classifier has picked up spurious features in the training data, its
predictions can fail in unexpected ways. In this paper, we develop a framework
that allows us to systematically identify spurious features in large datasets
like ImageNet. It is based on our neural PCA components and their
visualization. Previous work on spurious features of image classifiers often
operates in toy settings or requires costly pixel-wise annotations. In
contrast, we validate our results by checking that presence of the harmful
spurious feature of a class is sufficient to trigger the prediction of that
class. We introduce a novel dataset "Spurious ImageNet" and check how much
existing classifiers rely on spurious features
Microplastic fibers affect dynamics and intensity of CO2 and N2O fluxes from soil differently
Microplastics may affect soil ecosystem functioning in critical ways, with previously documented effects including changes in soil structure and water dynamics; this suggests that microbial populations and the processes they mediate could also be affected. Given the importance for global carbon and nitrogen cycle and greenhouse warming potential, we here experimentally examined potential effects of plastic microfiber additions on CO2 and N2O greenhouse gas fluxes. We carried out a fully factorial laboratory experiment with the factors presence of microplastic fibers (0.4% w/w) and addition of urea fertilizer (100âmgâNâkgââ1) using one target soil. The conditions in an intensively N-fertilized arable soil were simulated by adding biogas digestate at the beginning of the incubation to all samples. We continuously monitored CO2 and N2O emissions from soil before and after urea application using a custom-built flow-through steady-state system, and we assessed soil properties, including soil structure. Microplastics affected soil properties, notably increasing soil aggregate water-stability and pneumatic conductivity, and caused changes in the dynamics and overall level of emission of both gases, but in opposite directions: overall fluxes of CO2 were increased by microplastic presence, whereas N2O emission were decreased, a pattern that was intensified following urea addition. This divergent response is explained by effects of microplastic on soil structure, with the increased air permeability likely improving O2 supply: this will have stimulated CO2 production, since mineralization benefits from better aeration. Increased O2 would at the same time have inhibited denitrification, a process contributing to N2O emissions, thus likely explaining the decrease in the latter. Our results clearly suggest that microplastic consequences for greenhouse gas emissions should become an integral part of future impact assessments, and that to understand such responses, soil structure should be assessed
A robust Bayesian analysis of the impact of policy decisions on crop rotations.
We analyse the impact of a policy decision on crop rotations, using the imprecise land use model that was developed by the authors in earlier work. A specific challenge in crop rotation models is that farmerâs crop choices are driven by both policy changes and external non-stationary factors, such as rainfall, temperature and agricultural input and output prices. Such dynamics can be modelled by a non-stationary stochastic process, where crop transition probabilities are multinomial logistic functions of such external factors. We use a robust Bayesian approach to estimate the parameters of our model, and validate it by comparing the model response with a non-parametric estimate, as well as by cross validation. Finally, we use the resulting predictions to solve a hypothetical yet realistic policy problem
What are the effects of climate change on agriculture in North East Central Europe?
Global and climate changes influence the basic conditions for agriculture and so there is not only a demand for a consequent climate protection but also for an adaptation of agriculture to these global changing conditions. For the whole "Maerkisch-Oderland" district (60x40 km) within the moraine landscape of North-East-Germany mainly used for agriculture water balance, nitrogen and sulphur loads as well as crop yields are calculated for two land use and climate scenarios. The comparison between the Scenario2050 and the Scenario2000 reveals significant changes of the water balance (decrease in percolation water, increase in actual evapotranspiration) as well as the concentration of the examined nitrogen in the percolation water. For the study region the crop yields decrease only slightly if the CO2 fertilizing effect is taken into account. Adaptation measures in reaction to the changing climate conditions for an economically secured and sustainable agriculture are recommended.climate change impact assessment, water balance, nitrogen load, crop yield, moraine landscape, Environmental Economics and Policy, Farm Management,
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