1,721,063 research outputs found
Development and Validation Of Resistance-Capacitance Model (RCM) For Phase Change Material (PCM) Embedded In 3D Periodic Structures
No full textA Comprehensive Evaluation of Regression Uncertainty and the Effect of Sample Size on the AHRI-540 Method of Compressor Performance Representation
Get PDFAHRI-540 is the current standard defining the methods for representing compressor performance data. While this standard is widely used across the industry, multiple factors contribute to inaccuracies in data representation including measurement uncertainty, regression uncertainty, compressor to compressor variation, and operation outside of the normal operating envelope (extrapolation). In addition, the number and location of points in the operating envelop also affects the accuracy of the resulting 10-coefficient polynomial. The measurement uncertainty is well known and can be factored into the data reduction. However, the measurement uncertainty is generally not propagated into the regression uncertainty and hence the overall uncertainty in prediction using the polynomial is not known. This uncertainty also changes according to the number of samples used for developing the polynomial. Â As a first step of the evaluation, a regression uncertainty analysis was conducted using a Monte Carlo simulation method. Results showed that the average uncertainty in mass flow rate prediction can be as high as 4% and that in power prediction can be as high as 5%. The worst case maximum absolute error in predicted mass flow rate across all data sets was 17% and that for power was 9%. Error in predicted power and mass flow rate is higher for larger capacity compressors. For most compressors, the high errors occur in the region of the envelope with low suction and low discharge dew point temperatures. Â A study of sampling considering different sample sizes and multiple sampling methods was conducted. Two additional methods of compressor performance representation were also analyzed. This analysis was presented with several challenges, particularly since the compressor operating envelope is a non-rectangular domain. A sampling method using Latin Hypercube Design (LHS) and a proposed alternative sampling method based on polygonal design of experiments (PDOE) were evaluated. The resulting models were validated against a measured data set of more than 600 points encompassing the operating envelope for each compressor. In general, both the LHS and PDOE methods yielded similar errors in mass flow rate for samples sizes of 12, 14 and 16. Thus, for mass flow rate, it is possible to build a model with 12 systematically selected test points. For power prediction, the average error for the LHS and PDOE methods using AHRI540 and two other methods was lower than 2% for all sample sizes
Enhanced Integer Permutation based Genetic Algorithm for Optimization of Tube-Fin Heat Exchanger Circuitry with Splits and Merges
Get PDFTube-fin heat exchangers (HXs) are widely used in air-conditioning and heat pump applications. The performance of these heat exchangers is strongly influenced by the refrigerant circuitry. Studies have proved that by optimizing the refrigerant circuitry, the performance of HXs can be significantly improved. In our previous research, an Integer Permutation based Genetic Algorithm (IPGA) was developed to obtain the optimal circuitry designs. Our previous research showed that IPGA demonstrates superior capability to obtain better refrigerant circuitries with lower computational cost than the other methods in literature. And the optimal circuitry designs obtained from IPGA are manufacturable with the available tooling. However, the IPGA developed previously cannot generate designs with splitting and merging of circuits. To remedy this limitation, a new chromosome which can represent circuitry with splitting and merging of circuits is developed. In addition to the six genetic operators implemented previously, two new genetic operators are developed to generate splits and merges. As a result, the enhanced IPGA can explore the solution space more thoroughly than the previous IPGA. A case study using an evaporator from an A-type indoor unit shows that, given the similar capacity improvements obtained from the enhanced IPGA compared with the previous IPGA, the refrigerant pressure drop reduction obtained from the enhanced IPGA is 26.5% compared against 1.0% pressure drop reduction from the previous IPGA. The benchmark of the enhanced IPGA with other methods in literature demonstrates that the enhanced IPGA can generate circuitry designs with performance superior to those obtained from other methods
Standardized Polynomials for Fast Evaluation of Refrigerant Thermophysical Properties
Get PDFSteady state and dynamic simulation and optimization are a key step in the design of heating, ventilation, air-conditioning and refrigeration (HVAC&R) systems. It is well known that the computation time in such simulations is dominated by the refrigerant thermophysical property calculations. These calculations generally involve calculating all thermophysical properties given one or two independent parameters. Refrigerant thermophysical properties are typically calculated using some fundamental equations of state (EOS). The NIST REFPROP database is an industrially accepted standard for EOS implementation. Due to the iterative nature of the EOS calculations in REFPROP, the computation time is significant and sometimes not acceptable for optimization of HVAC&R systems. In this paper, a comprehensive approach for speeding up thermophysical property calculations is presented, including the functional forms as well as the implementation aspects. A set of polynomial functional forms are presented that allow for approximation of all the thermophysical properties in all the regions for a particular refrigerant (pure fluid or a blend) of interest. The polynomials can be easily scaled to make a judicious trade-off between computation time and accuracy. Analyses for refrigerants such as R1234yf, R32, R410A, R407C and R407F are presented. Using the proposed curve fits, the saturation properties for any refrigerant can be evaluated using less than 42 floating point operations (flops) and the flash calculations with less than 300 flops per property. The mean absolute error in predicted saturation properties is 0.001% and that of flash calculations is within 0.05%. Overall the individual property calculations are 100-5000 times faster than NIST REFPROP resulting in component and system simulation speed up factor of more than 100 for refrigerant blends. The use of two standardized and scalable functional forms for approximating all properties for all refrigerants of interest facilitates easy and robust implementation on a variety of steady state and transient simulation platforms as well as on hardware since limited data needs to be stored
A Review of State of the Art in Modeling of Air-to-Refrigerant Heat Exchangers for HVAC&R Applications
Get PDFAir-to-refrigerant heat exchangers are a key component in all air-conditioning, heat pump and refrigeration systems. The most common of types of air-to-refrigerant heat exchangers are tube-fin and microchannel heat exchangers. There has always been a great emphasis on understating the underlying physics and improving the performance of these heat exchangers. More recently, researchers have been investigating the use of small hydraulic diameter flow channels as well as novel heat transfer surfaces for use in such heat exchangers. The novel designs not only include shape optimized tubes, but also tube bundles with varying tube and fin geometries. In order to design optimum heat exchangers for a given application, it is crucial to use a reliable thermal-hydraulic model to evaluate the performance of air-to-refrigerant heat exchangers. In the last two decades, significant strides have been made in modeling of tube-fin and microchannel heat exchangers. The different modeling techniques include the use of performance maps, LMTD and epsilon-NTU based methods and fully discretized finite volume approaches. In terms of accuracy, the finite volume models are by far the preferred ones. The goal of this paper is to present the state of the art in finite volume modeling of air-to-refrigerant heat exchangers and to highlight research that stretches the boundaries of conventional heat exchanger modeling methods. Â The review starts out with a comprehensive survey of finite volume models in the literature and their capabilities to account for the various underlying physical phenomenon. High level modeling paradigms are derived and the best practices are highlighted. The various methods of geometry and circuitry representation and solution methodologies are summarized. Majority of these models rely on empirical correlations for local heat transfer and pressure drop evaluations. The use of such correlations has its own challenges and the lessons learned from the literature in this context are highlighted. Air and refrigerant flow maldistribution, especially in microchannel heat exchangers, is a critical phenomenon that needs to be accounted for in such models. Refrigerant flow maldistribution models in the literature range from user-specified quality and mass flow distribution profiles to more sophisticated methods that use CFD-based co-simulation techniques. The different techniques for handling dehumidifying conditions, such as those in an evaporator, are summarized. Finite volume models can be computationally expensive, especially when used as a part of a system simulation. The different methods used to speed up individual HX simulations are reviewed. Lastly, recent literature on optimization of air-to-refrigerant heat exchangers is presented. The review concludes with some thoughts on what the future of air-to-refrigerant heat exchanger design and optimization might be
Single and Multiresponse Adaptive Design of Experiments with Application to Design Optimization of Novel Heat Exchangers
Get PDFEngineering design optimization often involves complex computer simulations.
Optimization with such simulation models can be time consuming and sometimes
computationally intractable. In order to reduce the computational burden, the use of
approximation-assisted optimization is proposed in the literature. Approximation
involves two phases, first is the Design of Experiments (DOE) phase, in which
sample points in the input space are chosen. These sample points are then used in a
second phase to develop a simplified model termed as a metamodel, which is
computationally efficient and can reasonably represent the behavior of the simulation
response. The DOE phase is very crucial to the success of approximation assisted
optimization.
This dissertation proposes a new adaptive method for single and multiresponse
DOE for approximation along with an approximation-based framework for multilevel
performance evaluation and design optimization of air-cooled heat exchangers.
The dissertation is divided into three research thrusts. The first thrust presents a new
adaptive DOE method for single response deterministic computer simulations, also
called SFCVT. For SFCVT, the problem of adaptive DOE is posed as a bi-objective
optimization problem. The two objectives in this problem, i.e., a cross validation error
criterion and a space-filling criterion, are chosen based on the notion that the DOE
method has to make a tradeoff between allocating new sample points in regions that
are multi-modal and have sensitive response versus allocating sample points in
regions that are sparsely sampled. In the second research thrust, a new approach for
multiresponse adaptive DOE is developed (i.e., MSFCVT). Here the approach from
the first thrust is extended with the notion that the tradeoff should also consider all
responses. SFCVT is compared with three other methods from the literature (i.e.,
maximum entropy design, maximin scaled distance, and accumulative error). It was
found that the SFCVT method leads to better performing metamodels for majority of
the test problems. The MSFCVT method is also compared with two adaptive DOE
methods from the literature and is shown to yield better metamodels, resulting in
fewer function calls.
In the third research thrust, an approximation-based framework is developed for
the performance evaluation and design optimization of novel heat exchangers. There
are two parts to this research thrust. First, is a new multi-level performance evaluation
method for air-cooled heat exchangers in which conventional 3D Computational
Fluid Dynamics (CFD) simulation is replaced with a 2D CFD simulation coupled
with an e-NTU based heat exchanger model. In the second part, the methods
developed in research thrusts 1 and 2 are used for design optimization of heat
exchangers. The optimal solutions from the methods in this thrust have 44% less
volume and utilize 61% less material when compared to the current state of the art
microchannel heat exchangers. Compared to 3D CFD, the overall computational
savings is greater than 95%
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
