Fluid models of congestion collapse in overloaded switched networks

We consider a switched network (i.e. a queueing network in which there are constraints on which queues may be served simultaneously), in a state of overload. We analyse the behaviour of two scheduling algorithms for multihop switched networks: a generalized version of max-weight, and the α-fair policy. We show that queue sizes grow linearly with time, under either algorithm, and we characterize the growth rates. We use this characterization to demonstrate examples of congestion collapse, i.e. cases in which throughput drops as the switched network becomes more overloaded.We further show that the loss of throughput can be made arbitrarily small by the max-weight algorithm with weight function f (q) = q[superscript α] as α→0.National Science Foundation (U.S.) (Career CNS-0546590

Magnetic Resonance Imaging of Water Freezing in Packed Beds Cooled from Below

Full-field quantitative visualization of freezing interfaces requires the introduction of high resolution noninvasive methods. Magnetic Resonance Imaging (MRI) is a versatile tool for mapping the distribution of liquids (primarily water) in three-dimensional space, and is the only practical solution in systems that are strongly refracting or opaque to visible light. MRI is employed to visualize freezing in water-saturated packed beds consisting of spherical beads cooled from below. Imaging of the stagnant interstitial water is accomplished by exploiting the strong contrast in MRI signal between interstitial ice and liquid water. Our implementation of MRI allows fully three-dimensional reconstruction of the solidification front and adequate time resolution to quantify the freezing of the pore water. The wall effect, as expressed by the ratio of bed to bead diameter, is examined with respect to the shape and propagation rate of the freezing interface. MRI can.be effective only in media that do not affect the imposed magnetic fields. In heat transfer applications, extra provisions in terms of design and choice of materials of the test section are np.~essary to accommodate the special environment of the MRI scanner.Air Conditioning and Refrigeration Center Project 5

The Interaction Between the Substrate and Frost Layer Through Condensate Distribution

Microscopic observations of frost deposition on a variety of substrates having different contact angles, (polytetrafluoroethylene PTFE, kapton, glass and others) allow the quantification of substrate effects on frost structure during inception and growth. The deposition of water vapor at the beginning of the frosting process on a clean glass substrate is found to be as condensate (condensation frosting) rather than as ice (ablimation frosting) for a substrate temperatures above -33??C and an absolute humidity above 0.15 g/kg. The inception of "condensation frosting" (the condensation period and early frost growth period) is further examined microscopically as a function of air and substrate temperatures, absolute humidity, and substrate contact angle. The water distribution on the substrate at the end of the condensation period is found to be strongly dependent on substrate temperature, humidity ratio, and substrate contact angle. Colder substrates result in smaller more uniform droplets and substrates with lower contact angles result in shorter, larger diameter droplets with a larger percentage of the substrate covered. The effective density of the condensate on hydrophobic substrates is found to be lower than that on hydrophilic substrates. The structure and form of the ice immediately after freezing is substrate dependent. High-speed imaging of the freezing process is used to study the propagation of the freezing front in a droplet. The images show that a protrusion is formed at the top of the droplets during freezing. From observations, this protrusion is hypothesized to result from the convective condition at the droplet surface and the difference in specific volume between liquid and solid water. Additionally, the apparent ejection of water vapor during freezing of a droplet on a hydrophobic substrate was observed. This ejection of water vapor is thought to be caused by the wanning of the droplet caused by the release of latent heat. In contrast to trends observed during the early growth period, the growth rate of mature frost is found to decrease with substrate contact angle while frost density is found to increase. This behavior is explained in terms of the effect of substrate contact angle on the structure and form of the incipient frost, which constitutes the initial condition for further (mature) frost growth. A higher conductivity layer is formed on the hydrophilic than on the hydrophobic substrate. A model relating crystal orientation to conductivity is used to simulate the frost growth rate and density on the two different substrates and match the experimental data. Using similar reasoning, the higher conductivity frost formed on colder substrates is also explained.Air Conditioning and Refrigeration Project 10

Critical Heat Flux of CO2 in a Microchannel at Elevated Subcritical Pressures

The connection between chlorofluorocarbons and ozone depletion has prompted a resurgence in the study of environmentally friendly refrigerants for heat exchanger applications. Recent studies have shown compact heat exchangers using a transcritical CO2 cycle optimized in a microchannel geometry are competitive with baseline units in terms of physical dimension and thermal performance. However, the critical heat flux may occur at lower-than-expected vapor qualities in the heat absorption portion of the cycle, significantly affecting heat exchanger performance. The purpose of this study is to determine the critical heat flux and associated vapor quality under conditions simulating compact heat exchanger conditions. Non-intrusive wall temperature measurements, in conjunction with standard transducer measurements and flow visualization, were used to detect critical heat flux phenomenon. The vapor qualities associated with the critical heat flux were determined to be lower than would be expected in larger-diameter channels, and the critical heat flux was lower than predicted by the Shah correlation.Air Conditioning and Refrigeration Project 10