Shortcut model for predicting refrigeration cycle performance

Compression refrigeration systems are very widely used to provide cooling to sub-ambient processes. The power demand of the cycle depends strongly on the temperature at which cooling is required, the temperature at which the refrigerant is condensed, as well as the type of refrigerant being used. At lower temperatures (typically lower than -40 °C), complex refrigeration schemes, such as cascaded refrigeration cycles, may be needed, increasing the complexity of the models used to predict the power requirements for a given cooling demand. This work proposes a simple model for predicting the power consumption of such complex cycles, based on regression of more rigorous process simulation models. A simple linear refrigeration model which relates the actual power demand of a refrigeration cycle to the ideal performance (i.e. the Carnot cycle) is developed. The model predicts the power demand prior to design of the refrigeration scheme given the condensing and evaporation temperatures of the refrigerant. The model predictions are shown to be in good agreement with those of more accurate simulation models. Case studies demonstrate the validity of the refrigeration model. The predicted power demand is shown to be within 10 % of that of Branan (2005). The simplicity of the model enables its use for optimizing the design conditions of a complex refrigeration cycle and/or the associated processing conditions

Optimizing Cycle Exercise Performance During Normobaric Hypoxia Exposure

Introduction: The purpose of the present study was to examine whether implementing factors of OPTIMAL Theory: Enhanced Expectancies (EE), Autonomy Support (AS), and External Focus (EF) during a cycle exercise bout at a simulated altitude of 21,000 feet elevation had an effect on exercise performance and EPOC response in comparison to a control condition. Methods: Sixteen participants (n = 8 women, n = 8 men) completed resting oxygen measurements (resting metabolic rate) between 6:00 A.M. and 8:00 A.M. Cycle exercise to fatigue at a constant workload was performed (100 W) while breathing air with reduced oxygen content to simulate exercising at altitude (9.4% fraction of oxygen, equivalent of 6401 m above sea level). All participants performed under two conditions, an optimized and a control condition. The order of conditions were counterbalanced. Following cycle to fatigue protocol, participants were reconnected to the metabolic analysis system and instructed to sit quietly until they returned to their baseline oxygen values (EPOC duration). EPOC magnitude was determined by adding up the net oxygen consumption for every minute during the EPOC duration. Data analysis consisted of paired t-tests. Results: In summary, the results of this study reveal that cycle exercise performance between both conditions was significant, p = .03. Performance outcome measures included duration of cycle exercise to fatigue and mean watts (W). Participants were able to cycle longer in the optimized condition relative to the control, with exercise carried out at the same absolute workload. EPOC duration and magnitude in participants (N = 16) who performed cycling exercise at 100 W under simulated altitude of 6401 m (21,001 ft) to fatigue, resulted in no statistically significant difference between the following optimized and control conditions. Therefore, despite longer cycle exercise duration in the optimized condition, EPOC duration and magnitude in both conditions was not significantly different. Discussion: The present findings adds to evidence that key variables in the OPTIMAL theory influence energy expenditure, enhance movement efficiency, and reduce oxygen consumption. To the best of our knowledge, this is the first study to investigate aerobic exercise performance and EPOC response where all three variables in OPTIMAL theory are applied consecutively during exercise. Thus, further investigation is necessary to examine the physiological parameters of other exercise intensities to asses if similar results are produced

Global linear-irreversible principle for optimization in finite-time thermodynamics

There is intense effort into understanding the universal properties of finite-time models of thermal machines---at optimal performance---such as efficiency at maximum power, coefficient of performance at maximum cooling power, and other such criteria. In this letter, a {\it global} principle consistent with linear irreversible thermodynamics is proposed for the whole cycle---without considering details of irreversibilities in the individual steps of the cycle. This helps to express the total duration of the cycle as $\tau \propto {\bar{Q}^2}/{\Delta_{\rm tot} S}$, where $\bar{Q}$ models the effective heat transferred through the machine during the cycle, and $\Delta_{\rm tot} S$ is the total entropy generated. By taking $\bar{Q}$ in the form of simple algebraic means (such as arithmetic and geometric means) over the heats exchanged by the reservoirs, the present approach is able to predict various standard expressions for figures of merit at optimal performance, as well as the bounds respected by them. It simplifies the optimization procedure to a one-parameter optimization, and provides a fresh perspective on the issue of universality at optimal performance, for small difference in reservoir temperatures. As an illustration, we compare performance of a partially optimized four-step endoreversible cycle with the present approach.Comment: 13 pages, one figure, main results unaltered, discussion on mapping to endoreversible model adde

Improved molecular sorbent trap for high-vacuum systems

Closed cycle refrigeration loop in which trays holding molecular sorbent are made to serve as cooling baffles improves the performance of high vacuum systems. High performance is obtained with almost no decrease in pumping speed

Brayton cycle 3.2-inch radial compressor performance evaluation

Brayton cycle 3.2 inch radial compressor performance evaluation over wide range of Reynolds number

Subatmospheric Brayton-cycle Engine Program Review

A solar energy powered electrical generator utilizing a Subatmospheric Brayton cycle engine is examined. The generator consists of a subatmospheric, Brayton-cycle engine and a permanent magnet (PM) alternator. The electrical power is generated by an alternator driven directly by the Brayton-cycle engine rotating group. Features that enhance reliability and performance include air foil bearings on both the Brayton-cycle engine rotating group and the PM alternator, an atmospheric-pressure solar receiver and gas-fired trim heater, and a high temperature recuperator. The subatmospheric Brayton-cycle engine design is based on that of the gas fired heat pump engine

The Statistical Loop Analyzer (SLA)

The statistical loop analyzer (SLA) is designed to automatically measure the acquisition, tracking and frequency stability performance characteristics of symbol synchronizers, code synchronizers, carrier tracking loops, and coherent transponders. Automated phase lock and system level tests can also be made using the SLA. Standard baseband, carrier and spread spectrum modulation techniques can be accomodated. Through the SLA's phase error jitter and cycle slip measurements the acquisition and tracking thresholds of the unit under test are determined; any false phase and frequency lock events are statistically analyzed and reported in the SLA output in probabilistic terms. Automated signal drop out tests can be performed in order to trouble shoot algorithms and evaluate the reacquisition statistics of the unit under test. Cycle slip rates and cycle slip probabilities can be measured using the SLA. These measurements, combined with bit error probability measurements, are all that are needed to fully characterize the acquisition and tracking performance of a digital communication system

Instruction fetch architectures and code layout optimizations

The design of higher performance processors has been following two major trends: increasing the pipeline depth to allow faster clock rates, and widening the pipeline to allow parallel execution of more instructions. Designing a higher performance processor implies balancing all the pipeline stages to ensure that overall performance is not dominated by any of them. This means that a faster execution engine also requires a faster fetch engine, to ensure that it is possible to read and decode enough instructions to keep the pipeline full and the functional units busy. This paper explores the challenges faced by the instruction fetch stage for a variety of processor designs, from early pipelined processors, to the more aggressive wide issue superscalars. We describe the different fetch engines proposed in the literature, the performance issues involved, and some of the proposed improvements. We also show how compiler techniques that optimize the layout of the code in memory can be used to improve the fetch performance of the different engines described Overall, we show how instruction fetch has evolved from fetching one instruction every few cycles, to fetching one instruction per cycle, to fetching a full basic block per cycle, to several basic blocks per cycle: the evolution of the mechanism surrounding the instruction cache, and the different compiler optimizations used to better employ these mechanisms.Peer ReviewedPostprint (published version