Browsing by Author "Chen, Yu-Kai"

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A mechanistic model for nucleate boiling heat transfer performance with lubricant-refrigerant mixture
(2020-10-01)
Hung, Jung-Tsung
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Chen, Yu-Kai
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Shih, Hou-Kuang
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Wang, Chi-Chuan
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機械工程學系
The present study proposes a rationally based model to investigate the influence of lubricant on the nucleate boiling characteristics of refrigerant-lubricant mixtures. This model is developed based on site-activation boiling mechanism along with detailed physical parameters in association with thermal and hydrodynamic processes, including waiting period for bubble incipience, bubble growth and departure period. Yet, the model also encompasses the polymer adsorption theory and energy gap concept to address the effect of lubricant with different chemical structure and physical property on the heat transfer phenomenon during each period of nucleate boiling individually. In addition, by using the partition function with Boltzmann energy distribution in different energy state, bubble density can be expressed in an analytic form to facilitate the calculation of heat transfer coefficient. Based on the proposed model, the presence of lubricant appreciably changes interfacial energy upon metal-liquid and liquid-bubble interfaces. The lubricant prefers lying on metal surface, thereby influencing surface coverage concentration when bubble is initiating. Such lubricant-rich layer near metal surface significantly alters the waiting period of nucleate boiling process, bubble size, growth and departure time, bubble density and superheat on heating surface. In essence, the presence of lubricant dramatically influences heat transfer performance. The proposed model is validated against some recent test data for R-134a/POE, R-1234ze/POE and R-134a/PVE refrigerant/lubricants mixtures. (C) 2020 Elsevier Ltd. All rights reserved.
Periodic Modeling and Analysis of Bifurcation Dynamics for Switching Converters
(2009)
Song, Chau-Chung
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Chen, Yu-Kai
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Liaw, Der-Cherng
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電機工程學系
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Department of Electrical and Computer Engineering
In this paper, the nonlinear dynamics of PWM-type converters is studied. The sampled-data approach is applied to the periodic modeling and stability analysis of switching converters. The period-one and period-two modeling for the converter dynamics is proposed to predict the occurrence of the corresponding bifurcations and used to analyze the stability of converter circuits. The example study of buck converters is presented to demonstrate the feasibility of the methodological approach proposed in this paper. Applying to buck converters, the period-doubling bifurcation is found to exhibit as the input voltage varies, which might produce a series of period-doubling bifurcations and then result in a chaotic motion. Furthermore, the system stability of buck dynamics is studied and evaluated.
Total-FactorEnergy and Emission Efficiencies ofAssociation of Southeast Asian Nationsand Other Asian Economies
(1970-01-01)
Hu, Jin-Li
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Honma, Satoshi
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Chen, Yu-Kai
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交大名義發表
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National Chiao Tung University
This research uses the slack-based measure data envelopment analysis to compute the energy and emission efficiencies of Association of Southeast Asian Nations (ASEAN) and other Asian economies during the 2001-2017 period, employs the Malmquist productivity index to check for the main source of efficiency score changes due to technical changes, and then applies panel Tobit regressions to determine the factors explaining the efficiencies. Our empirical results show that the energy efficiency scores of ASEAN economies had been catching up to other Asian economies after the Global Financial Crisis, whereas the emission efficiency scores of various ASEAN economies had been falling behind other Asian economies over the same period. The main source of efficiency score changes over time is efficiency changes and not technical changes. Decreases in the fossil fuel ratio of net electricity generation and the secondary industry ratio within total industry improve both lead to improvements in energy and emission efficiencies.