Browsing by Author "Tran, Ngoctan"

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An experimental study and empirical correlations to describe the effect of lubricant oil on the nucleate boiling heat transfer performance for R-1234ze and R-134a
(2018-10-01)
Tran, Ngoctan
;
Sheng, Shou-Ren
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
In the present work, nucleate boiling heat transfer characteristics of R-134a and R-1234ze refrigerants subject to the influence of lubricant oil on a plate surface were experimentally examined. Eight types of lubricants, including POEA68, POEA120, POEA170, POEB68, POEB120, POEB170, POEC170, and POED 150, were used for investigations; the experiments were carried out at saturated temperatures of 0 degrees C and 10 degrees C, respectively. The oil concentrations range from 0% to 10% and heat fluxes vary from 10 kW/m(2) to 70 kW/m(2). The test results for pure refrigerants are in line with the existing data and correlations. With the presence of lubricant, the heat transfer coefficient may be slightly higher or lower than those of pure refrigerants, depending on the lubricant and operating pressure. However, all the heat transfer coefficients deteriorate appreciably as compared to the pure refrigerants when the oil concentration exceeds 5%. A correlation is developed that is capable of describing the influence of lubricant oil. The proposed correlation not only can predict the influence of the lubricants of this study but also can extend to predict existing data with a good accuracy.
Effects of surface inclination and type of surface roughness on the nucleate boiling heat transfer performance of HFE-7200 dielectric fluid
(2020-02-01)
Tran, Ngoctan
;
Sajjad, Uzair
;
Lin, Rick
;
Wang, Chi-Chuan
;
機械工程學系
In the present work, the effects of surface roughness (including roughness from machined roughness and sandblasted roughness), surface inclinations, and heat fluxes on the nucleate pool boiling heat transfer of HFE-7200 dielectric liquid at a working pressure of 1 atm. are examined in detail. Five aluminum surfaces, with different roughness from 0.45 mu m to 9 mu m, are employed as test samples. The nucleate boiling phenomenon is examined with the wall-superheat temperature ranging from 5 degrees C to 30 degrees C, and surface inclinations of 0 degrees, 90 degrees, and 180 degrees, respectively. SEM images and nucleate boiling bubbles captured by a high-speed camera are utilized for further analyses. For all cases in this study, it is found that the nucleate boiling heat transfer of the sandblasted rough surfaces is superior to those of the polished and machined rough surfaces. For instance, at a wall-superheat temperature of 30 degrees C and inclination angle of 0 degrees, the heat flux is augmented up to 69.7%, and 139.3% when compared to the polished surface for the sandblasted surfaces with the roughness of 6.2 mu m, and 9 mu m, respectively. In addition, novel correlations are also proposed for predicting the nucleate pool boiling heat transfer coefficient of HEF-7200 dielectric fluid with different surface roughness, inclinations and heat fluxes. (C) 2019 Elsevier Ltd. All rights reserved.
Optimization of thermal performance of multi-nozzle trapezoidal microchannel heat sinks by using nanofluids of Al2O3 and TiO2
(2018-02-01)
Tran, Ngoctan
;
Chang, Yaw-Jen
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
In this study, a new multi-nozzle trapezoidal microchannel heat sink (MNT-MCHS) was proposed. Five substrate materials, two nanofluids with nanoparticle volume fractions, 0.1% <= phi <= 1%, and channel hydraulic diameters, 157.7 mu m <= D-h <= 248.2 mu m, were numerically examined in detail. In addition, heat fluxes in the range of 100-1450 W/cm(2) subject to inlet coolant temperature from 15 degrees C to 75 degrees C were examined in detail. A locally optimal MNT-MCHS was defined, and a novel equation was proposed for predicting the maximum temperature on the locally optimal MNT-MCHS depending on the heat flux, coolant inlet temperature, and the Reynolds number. It was found that at a Reynolds number of 900, the overall thermal resistance of a MNT-MCHS using copper as a substrate material is improved up to 76% as compared to that using stainless steel 304. The locally optimal MNT-MCHS, using TiO2-water nanofluid with phi = 1%, could dissipate a heat flux up to 1450 W/cm(2) at a Re of 900. A minimum thermal resistance in the present study is improved up to 11.6% and 36.6% in association with those of a multi nozzle MCHS and a double-layer MCHS, respectively. (C) 2017 Elsevier Ltd. All rights reserved.