Tribological Studies on Scuffing Due to the Influence of Carbon Dioxide Used as a Refrigerant in Compressors

The refrigeration and air conditioning industry has expressed a great interest in the use of carbon dioxide (CO2) as a refrigerant. CO2 is anticipated to replace HFC refrigerants, which are known to have a negative effect on the environment. The reason behind the interest in CO2 is the fact that it is a natural refrigerant, thus environmentally acceptable. Of course, such a replacement raises concerns regarding design criteria and performance due to the different thermodynamic properties of CO2 and the very different range of pressures required for the CO2 refrigeration cycle. So far, work related to CO2 has been done from a thermodynamics point of view and researchers have made significant progress developing automotive and portable air-conditioning systems that use the environmentally friendly carbon dioxide as a refrigerant. The purpose of this work is to develop an understanding of how CO2 plays a role from a tribology standpoint. More specifically, the goal of this work is to gain an understanding on how CO2 influences friction, lubrication, wear and scuffing of tribological pairs used in compressors. Work in the area of tribology related to CO2 is very limited. Preliminary work by Cusano and coworkers showed that consistent data for tests using CO2 could not be acquired nor could a satisfactory explanation be offered for the inconsistency. Their results triggered the initiation of the work presented here. In this first attempt to understand the tribological behavior of CO2 several problems were encountered. During this work we noted that its behavior, unlike conventional refrigerants, could not always be predicted. We believe that this can be attributed to the thermodynamic properties of CO2, which cannot be ignored when studying its tribological behavior. Thermodynamic Properties such as miscibility are very important when tribological testing is performed. A limiting factor with our tester was that it was not designed for CO2 testing, but for other conventional refrigerants and therefore made previously developed testing protocols non-applicable with CO2. Through a different approach and some modifications to our tester we were able to establish a protocol for testing under the presence of CO2. CO2 was then compared to R134a and the experimental results showed that it performs equally well.Air Conditioning and Refrigeration Project 13

On the stability properties of power networks with time-varying inertia

A major transition in modern power systems is the replacement of conventional generation units with renewable sources of energy. The latter results in lower rotational inertia which compromises the stability of the power system, as testified by the growing number of frequency incidents. To resolve this problem, numerous studies have proposed the use of virtual inertia to improve the stability properties of the power grid. In this study, we consider how inertia variations, resulting from the application of control action associated with virtual inertia and fluctuations in renewable generation, may affect the stability properties of the power network within the primary frequency control timeframe. We consider the interaction between the frequency dynamics and a broad class of power supply dynamics in the presence of time-varying inertia and provide locally verifiable conditions, that enable scalable designs, such that stability is guaranteed. To complement the presented stability analysis and highlight the dangers arising from varying inertia, we provide analytic conditions that enable to deduce instability from single-bus inertia fluctuations. Our analytical results are validated with simulations on the Northeast Power Coordinating Council (NPCC) 140-bus system, where we demonstrate how inertia variations may induce large frequency oscillations and show that the application of the proposed conditions yields a stable response.Comment: 14 pages, 8 figure

Optimal intervention strategies to mitigate the COVID-19 pandemic effects

Governments across the world are currently facing the task of selecting suitable intervention strategies to cope with the effects of the COVID-19 pandemic. This is a highly challenging task, since harsh measures may result in economic collapse while a relaxed strategy might lead to a high death toll. Motivated by this, we consider the problem of forming intervention strategies to mitigate the impact of the COVID-19 pandemic that optimize the trade-off between the number of deceases and the socio-economic costs. We demonstrate that the healthcare capacity and the testing rate highly affect the optimal intervention strategies. Moreover, we propose an approach that enables practical strategies, with a small number of policies and policy changes, that are close to optimal. In particular, we provide tools to decide which policies should be implemented and when should a government change to a different policy. Finally, we consider how the presented results are affected by uncertainty in the initial reproduction number and infection fatality rate and demonstrate that parametric uncertainty has a more substantial effect when stricter strategies are adopted

Analytical approximations in modeling contacting rough surfaces

A critical examination of the analytical solution presented in the classic paper o

Extracting summit roughness parameters from random Gaussian surfaces accounting for asymmetry of the summit heights

ABSTRACT The random Gaussian surface model proposed by Nayak is important to many statistical summit-based micro contact models. A Gaussian distribution is usually assumed for the summit heights as many surfaces have a Gaussian distribution of surface heights. In this work, based on Nayak's model, the skewness and kurtosis of the summit heights distribution are derived as a function of the bandwidth parameter α. The correctness of these two equations is verified using a numerical scheme that generates random Gaussian surfaces with various α values. Also, practical contact simulations are performed to demonstrate the significance of the proposed equations and also to show the error of using a Gaussian distribution versus a correct asymmetric distribution for the summit heights

FogFS: A Fog File System For Hyper-Responsive Mobile Applications

Hyper-responsive mobile applications}, such as augmented reality and online games, require ultra-low latency access to back-end services and data at runtime. While fog computing tries to meet such latency requirements by placing corresponding back-end services and data closer to clients, for e.g., within an access network, assuming a fixed back-end server throughout execution is problematic due to user mobility. A more flexible approach is thus required to allow for adapting to changes in network conditions when users roam, by relocating back-end services and data to closer available infrastructure. Support for real-time migration of software services exists, however, migrating associated disk state remains a bottleneck. This paper presents FOGFS, a fog file system that employs intelligent snapshotting, migration and synchronization mechanisms to speed up the migration of an application‘s disk state between different edge locations at runtime. The experimental evaluation of our prototype implementation reveals that the attainable speed-up is as much as 3. 3 x compared to conventional migration approaches

Morphology-influenced wetting model of nanopore structures

Abstract Understanding the wetting behavior of nanostructures is important for surface design. The present study examined the intrinsic wettability of nanopore structures, and proposed a theoretical wetting model. Using this model, it was found that the wetting behavior of nanopore structures depends on the morphology of a surface. To accurately predict the wetting behavior of nanopore structures, correction factors were introduced. As a result, the proposed wetting model can be used to predict the wettability of nanopore structures for various engineering purposes.</jats:p

Adaptation of the Cerebrocortical Circulation to Carotid Artery Occlusion Involves Blood Flow Redistribution between Cortical Regions and is Independent of eNOS

Cerebral circulation is secured by feed-forward and feed-back control pathways to maintain and eventually reestablish the optimal oxygen and nutrient supply of neurons in case of disturbances of the cardiovascular system. Using the high temporal and spatial resolution of laser-speckle imaging we aimed to analyze the pattern of cerebrocortical blood flow (CoBF) changes after unilateral (left) carotid artery occlusion (CAO) in anesthetized mice in order to evaluate the contribution of macrovascular (Willis circle) vs. pial collateral vessels as well as that of endothelial nitric oxide synthase (eNOS) to the cerebrovascular adaptation to CAO. In wild-type mice CoBF reduction in the left temporal cortex started immediately after CAO, reaching its maximum (-26%) at 5-10 s. Thereafter, CoBF recovered close to the pre-occlusion level within 30 s indicating the activation of feed-back pathway(s). Interestingly, the frontoparietal cerebrocortical regions also showed CoBF reduction in the left (-17-19%) but not in the right hemisphere, although these brain areas receive their blood supply from the common azygos anterior cerebral artery in mice. In eNOS-deficient animals the acute CoBF reduction after CAO was unaltered, and the recovery was even accelerated as compared to controls. These results indicate that (i) the Willis circle alone is not sufficient to provide an immediate compensation for the loss of one carotid artery, (ii) pial collaterals attenuate the ischemia of the temporal cortex ipsilateral to CAO at the expense of the blood supply of the frontoparietal region, and (iii) eNOS, surprisingly, does not play an important role in this CoBF redistribution

Tribology of Unfilled and Filled Polymeric Surfaces in Refrigerant Environment for Compressor Applications

Recent changes in environmental laws requiring the transition from known ozone depleting chlorofluorocarbon (CFC) refrigerants such as R-22 to safer alternative refrigerants such as fluorocarbon based R- 134A and R-410A, have necessitated other changes in the refrigeration systems as well. Refrigeration compressor oils have also been replaced in order to be miscible with the alternative refrigerants. These alternative refrigerants and oils have changed the tribological characteristics of compressor critical contacts, and in some cases have led to an increase in failure rates. Much research has been conducted on the compressor contacts with alternative refrigerants and oils to understand the tribological impacts. Polymers have seen very limited study related to compressor tribocontacts in the presence of refrigerants. Polymers have a self lubricating effect by transferring material to the metal counterface. This is an important group of materials for tribological applications, especially blended polymers, which often have enhanced mechanical and low friction properties. Most of the literature on polymer tribology is conducted at speeds and loads significantly lower than typical compressor conditions. At these low speeds and applied loads, varying degrees of polymer transfer films are reported on the metal counterfaces. It is postulated that coherent transfer films are necessary for reduced wear. The current study looks at the tribological response of polymer/metal contacts in the presence of refrigerant versus ambient air under conditions simulating refrigeration compressors. Ten different polymers are employed as potential compressor bearing materials; four unfilled polymers and six blended polymers. Friction coefficient, wear, and surface topography were evaluated at a 60??C system temperature, 25 psi R-134A atmosphere (or ambient air), 2.4 m/s sliding velocity, and 45 or 225 N applied loads. Polymers were tested against cast iron disks of roughness 0.3 to 0.5 ??m Rq. Experiments conducted in R-134A show slightly favorable friction and wear characteristics to experiments conducted in ambient air. All blended polymers have good tribological characteristics. PEEK and polyimide in both unfilled and blended forms exhibit minimal wear and do not adversely affect the metal disks. These polymers show promise for compressor bearing materials. Representative testing in starved lubricant conditions shows decreased polymer friction and wear. This study also shows that although coherent, uniform films are not produced under compressor-like conditions, as evidenced by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX), the tested polymers still have favorable tribological properties.Air Conditioning and Refrigeration Project 14