175 research outputs found
Development and Validation Of Resistance-Capacitance Model (RCM) For Phase Change Material (PCM) Embedded In 3D Periodic Structures
No full textA Generalized Effectiveness-NTU Based Variable Geometry Microchannel Heat Exchanger Model
No full textMicrochannel heat exchangers (MCHXs) are now widely used in the HVAC&R industry. Similar to tube-fin heat exchangers used mainly in the indoor units, the innovative variable geometry microchannel heat exchanger will further reduce its compact size and enhance its performance. Variable geometry refers to a heat exchanger with different tube and fin surfaces within the same core with one or more tube banks. A comprehensive literature review reveals that there is no modeling approach in the literature that can handle such arbitrary geometry. This paper introduces a new microchannel heat exchanger model which accounts for variable port, tube and fin geometry, variable tube and fin location and variable number of tubes per bank and variable fin density. This model adopts a port-by-port calculation approach on air side and the refrigerant side. The model is based on three-stream NTU method to account for variable fin type, height and air flow on top and bottom of a given tube. The heat exchanger location is based on a Cartesian grid which would account for the air propagation through multiple banks with variable geometry and fin location. Since the basic heat exchange calculation is performed at the port-level, the model lends itself very easily to account for port-level refrigerant flow maldistribution. In addition, the decrease in air-side heat transfer coefficient along the depth in the direction of the air flow can also be accounted for. Empirical correlations from literature are used for local heat transfer coefficient, pressure drop and void fraction calculations. The model is validated against experimental data for standard geometry microchannel condenser
Plate Heat Exchanger Optimization Using Different Approximation Assisted Multiobjective Optimization Techniques
No full textThis paper presents a comparison between different multiobjective optimization approaches that can be used to optimize the design of thermal equipment. Plate heat exchanger is taken as case study to apply different optimization techniques. The thermal-hydrodynamic characteristics of single phase turbulent flow in chevron-type plate heat exchangers with sinusoidal-shaped corrugations have been used in this paper. The computational domain contains a corrugation channel and the simulations adopted the shear-stress transport (SST) κ-ω model as the turbulence model. Two different approximation assisted optimization approaches are tested. Offline approximation assisted optimization, and online approximation assisted optimization are compared to optimize plate heat exchanger design. For both approximation techniques (offline and online), design optimization is performed using multiobjective genetic algorithm based on meta-models that are built to represent the entire design space. In offline approximation, globally accurate meta-models are built which requires adding more samples. However in online approximation assisted optimization, samples are added just to improve the metamodels performance in the expected optimum region. Approximated optimum designs are validated using computationally expensive actual CFD simulations. Finally, a comparison between offline and online approximation assisted optimization is presented with guidelines to apply both approaches in the area of heat exchanger design optimization. The methods presented in this paper are generic and can be applied to optimize different types of heat exchangers, electronic cooling devices and other thermal system components
Effectiveness of Entransy Dissipation Metric and Entropy Generation Units in The Design of Fin-Tube Heat Exchangers
No full textSeveral techniques and metrics based on the Second Law of Thermodynamics have been used in the past for the analysis of heat exchangers. The terms used for these techniques include irreversibility analysis, entropy generation minimization, exergy analysis and thermodynamic efficiency. Entransy is a recently developed concept reflecting the heat transfer potential, rather than the ability to convert heat to work. Entransy is transferred along with heat flux in the heat transfer process, and subsequently dissipates. The entransy dissipation extremum principle is applicable to heat transfer enhancement. Entropy on the other hand is a thermodynamic state-based quantity. This study focuses on the comparison of entransy dissipation and entropy generation units in the context of optimizing the widely used fin-tube heat exchanger. Local entransy balance equations are established and implemented in a finite-volume based fin-tube heat exchanger model. The model can then calculate the entransy dissipation in each control volume, as well as the total dissipation for the entire heat exchanger. Parametric study about two heat exchangers, one undergoing only single phase heat transfer (water coil) and the other undergoing both single phase and two-phase heat transfer (R134a evaporator) are conducted without water condensation on the air side
New Generation of Air Cooled Heat Exchanger 1 kW Module Design Optimization
No full textThe objective of this paper is to evaluate and optimize the performance of 1 kW integrated heat exchanger module for new generations of air cooled heat exchangers. The first objective is to minimize the ratio of the header frontal area to the entire heat exchanger frontal which will help to reduce the header size. The second objective is to minimize the pressure drop for the entire heat exchanger, i.e., inside the inlet and outlet headers in addition to pressure drop inside the tubes. A three step approach is proposed. First step involves selecting the header design based on previous header optimization studies and then simulating the header using a new 3D CFD simulation approach. Second step includes solving the heat exchanger using information from the header simulation that accounts for the variation in refrigerant mass flow rate inside the tubes and obtain the performance for the entire heat exchanger. Finally, a solver is used to evaluate the overall module performance. Three different headers are investigated with different header height and size ratio. Then parametric studies are conducted to explore the effect of header size ratio on the optimum designs. Lastly, design guidelines to optimize the integrated heat exchange module are provided based on the study results
Securitization and mortgage default
No full textThe academic literature, the popular press, and policymakers have all debated securitization's contribution to the poor performance of mortgages originated in the run-up to the recent crisis. Theoretical arguments have been advanced on both sides, but the lack of suitable data has made it difficult to assess them empirically. The author examines this issue by using a loan-level data set from LPS Analytics, covering approximately two-thirds of the mortgages originated in 2005 and 2006, and including both securitized and nonsecuritized loans. ; The author finds evidence that privately securitized loans do indeed perform worse than observably similar, nonsecuritized loans. Moreover, this effect is strongest in prime mortgage markets, which have not been studied in the previous literature. For example, a typical prime loan becomes delinquent at a 20 percent higher rate if it is privately securitized, ceteris paribus. This is consistent with the existence of adverse selection; that is, that lenders used information not available to investors to securitize loans that were riskier than they otherwise appeared. By contrast, for subprime mortgages, the impact of private securitization is concentrated in low or no-documentation loans; this latter result is consistent with previous work such as Keys et al. (2009).Mortgage-backed securities ; Default (Finance)
A Comprehensive Evaluation of Regression Uncertainty and the Effect of Sample Size on the AHRI-540 Method of Compressor Performance Representation
No full textAHRI-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
Role of Risk Stratification and Genetics in Sudden Cardiac Death
No full textSudden cardiac death (SCD) is a major public health issue due to its increasing incidence in the general population and the difficulty in identifying high-risk individuals. Nearly 300,000-350,000 patients in the United States and 4- to 5 million patients in the world die from SCD. Coronary artery disease and advanced heart failure are the main etiology for SCD. Ischemia of any cause precipitates lethal arrhythmias, and ventricular tachycardia and ventricular fibrillation are the most common lethal arrhythmias precipitating SCD. Pulse-less electrical activity, brady-arrhythmia and electromechanical dissociation also result in SCD. Most sudden cardiac deaths occur out-of-the-hospital setting, so it is difficult to estimate the public burden, which results in overestimating the incidence of SCD. The insufficiency and limited predictive value of various indicators and criteria for SCD result in the increasing incidences. As a result, there is a need to develop better risk stratification criteria and find modifiable variables to decrease the incidence. Primary and secondary prevention and treatment of SCD need further research. This critical review is focused on the etiology, risk factors, prognostic factors and importance of risk stratification of SCD.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
Enhanced Integer Permutation based Genetic Algorithm for Optimization of Tube-Fin Heat Exchanger Circuitry with Splits and Merges
No full textTube-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
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