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    A mechanistic model for nucleate boiling heat transfer performance with lubricant-refrigerant mixture
    (2020-10-01)
    Hung, Jung-Tsung
    ;
    Chen, Yu-Kai
    ;
    Shih, Hou-Kuang
    ;
    Wang, Chi-Chuan
    ;
    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.
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    Numerical investigation of thermal and hydraulic performance of shell and plate heat exchanger
    (2020-02-25)
    Abbas, Ali
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    Lee, Howard
    ;
    Sengupta, Mash
    ;
    Wang, Chi-Chuan
    ;
    機械工程學系
    Detailed single-phase performance of the shell and plate heat exchangers is presented in this study with corresponding chevron angles (beta) of 45 degrees and 75 degrees, respectively. Unlike those conventional plate heat exchanger, the thermofluids characteristics perform quite differently between shell and inner channel. For a larger chevron angle (75 degrees), the friction factor is comparable amid shell and inner channel. Yet the friction factor in the shell channel is much lower than that in inner channel at a smaller chevron angle (45 degrees). The shell channel contains more unidirectional flow pattern while some flow re-circulation may occur at the entrance/exit port of the inner channel. The flow pattern and temperature distribution in the shell channel are generally more uniform than those in inner channel. The shell channel with beta = 45 degrees shows a better overall performance for its much lower friction factor when the Reynolds number is low. The heat transfer performance for shell channel exceeds the inner channel appreciably when the chevron angle is high (75 degrees). The heat transfer performance depends on the corrugation aspect ratio, and an optimum ratio is derived. For the inner channel, the optimum value is around 0.75 while it is 0.53 for the shell channel.
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    A novel micro-channel heat sink with trapezoid drainage for enhancing condensation heat transfer of dielectric fluid
    (2019-09-01)
    Chu, Wen-Xiao
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    Chen, Chuan-Yu
    ;
    Liao, Ying-Hao
    ;
    Wang, Chi-Chuan
    ;
    機械工程學系
    This study experimentally investigates the condensation of dielectric fluid HFE-7100 in a micro-channel heat sink with the hydraulic diameter of 1.21 mm. Tests are conducted at a fixed pressure of 110 kPa, with vapor mass quality (x(in)) ranging from 0.1 to 0.9, mass fluxes (G) from 150 to 250 kg.m(-2).s(-1), inclined angle ( theta) from -90 degrees to +90 degrees. A novel trapezoid drainage design is proposed to enhance the condensation heat transfer by effectively entraining condensation film. When G is lower than 200 kg.m(-2).s(-1), the two phase heat transfer coefficient (h(tp)) for the trapezoid drainage design exceeds the conventional rectangular micro-channel heat sink by 10-26% while the corresponding pressure drop is about 22-45% lower. When G is increased to 250 kg.m(-2).s(-1), the drainage design can still dramatically decrease the pressure drop, however, it shows a negative effect on h(tp) when the x(in) is greater than 0.2. With inclined arrangements, the pressure drop would dramatically increase due to gravity effect. Besides, at the conditions of theta = -45 degrees and -90 degrees, the trapezoid drainage channel can improve the h(tp) by 43-45% when G is lower than 200 kg.m(-2).s(-1). On the contrary, with the arrangement of theta = +45 degrees and +90 degrees, the trapezoid drainage channels may decrease the h(tp) by 8-15%. This phenomenon is especially pronounced at a high z i p. In addition, the heat transfer enhancement and deterioration are both analyzed based on stress distribution at vapor phase and condensation film.
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    Condensing heat transfer coefficients of R134a in smooth and grooved multiport flat tubes of automotive heat exchanger: An experimental investigation
    (2019-05-01)
    Ammar, Syed Muhammad
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    Abbas, Naseem
    ;
    Abbas, Saleem
    ;
    Ali, Hafiz Muhammad
    ;
    Hussain, Iftikhar
    Condensation heat transfer coefficient of R134a was obtained in multi-port micro channel smooth and grooved flat tubes of automotive heat exchanger. The test consisted of two smooth and one grooved tube. The test range covered mass flux 490-1600 kg/m(2) s, heat flux 5.5-19 kW/m(2) and saturation temperature 51-68 degrees C. Results revealed that condensation heat transfer coefficient increased with an increment of mass flux, vapor quality and decreased with the increase of saturation temperature. The effect of heat flux on heat transfer coefficient was negligible. The grooved tube showed larger heat transfer coefficient than that of smooth tubes. A comparison of the present data of flat tube with existing correlations revealed that the heat transfer coefficient was not reasonably predicted, therefore new correlation was developed for heat transfer coefficient that predicted 86% data within +/- 30% with a mean error of 16%. (C) 2019 Elsevier Ltd. All rights reserved.
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    The suppression effect of easy-to-activate nucleation sites on the critical heat flux in pool boiling
    (2018-07-01)
    Liu, Yi
    ;
    Lu, Ming-Chang
    ;
    Xu, Dongyan
    ;
    機械工程學系
    ;
    Department of Mechanical Engineering
    We report experimental studies of the role of easy-to-activate nucleation sites in pool boiling on hybrid hydrophobic-hydrophilic surfaces. Microstructured surfaces with controlled wettability and nucleation sites are designed and fabricated to investigate their individual and collective effects on the boiling heat transfer performance. Our results reveal that excellent critical heat flux (CHF) up to 231.5 W/cm(2) is attainable only when the surface has good wettability but is free of easy-to-activate nucleation sites. However, with the presence of easy-to-activate nucleation sites on the boiling surface, only limited CHFs can be achieved irrespective of the surface wettability, suggesting a suppression effect of nucleation sites on the CHF. Based on the previous experimental observation of the dynamic network-like wetting pattern at high heat fluxes, a wetting-deterioration mechanism is proposed to explain the suppression effect of easy-to-activate nucleation sites on the CHF. Easy-to activate nucleation sites continuously generate bubbles that pose a large flow resistance for the liquid to wet the hot surface and thus lead to early dry-out.
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    Enhanced immersion cooling using two-tier micro- and nano-structures
    (2018-02-25)
    Hsu, Ya-Tzu
    ;
    Li, Jia-Xiong
    ;
    Lu, Ming-Chang
    ;
    機械工程學系
    ;
    Department of Mechanical Engineering
    Continual increases in the functionality and miniaturization of electronic devices have resulted in a rapid increase in the power density of such devices. Thus, an efficient cooling technology is required to maximize heat dissipation and prevent electronic failure. Immersion cooling is a promising technique for the thermal management of high-power-density electronics. However, common working fluids in immersion cooling have high global warming potential, and the heat transfer performance of immersion cooling requires improvement to achieve efficient cooling of state-of-the-art high-power-density electronics. In this study, Novec 649, which has low global warming potential and a low boiling point, was applied as a working fluid for immersion cooling. A Si nanowire (SiNW) array, Si micropillar (SiMP) array, and Si nanowires on a Si micropillar (SiNW/MP) two-tier structure were employed to enhance boiling performance. The SiMP surface exhibited the highest critical heat flux (CHF) of 23.5 +/- 1.3 W/cm(2), whereas the SiNW surface exhibited the lowest CHF but a relatively high heat transfer coefficient (HTC). The SiNW/MP surface exhibited the highest HTC of 23611.7 +/- 1586.2 W/m(2) K and a relatively large CHF of 17.4 +/- 1.2 W/cm(2). Compared with a plain SiO2 surface, the CHF and HTC of the SiNW/MP two-tier structure could be enhanced by 30% and 455%, respectively. These results suggest that the SiNW/MP surface is effective for enhancing immersion cooling. (C) 2017 Elsevier Ltd. All rights reserved.