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Access type: Open accessSubject: search.filters.subject.Heat transfer coefficient

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    Investigation of the two-phase convective boiling of HFO-1234yf in a 3.9 mm diameter tube
    (2013-10-01)
    Lu, Ming-Chang
    ;
    Tong, Jing-Rei
    ;
    Wang, Chi-Chuan
    ;
    機械工程學系
    ;
    Department of Mechanical Engineering
    In this study, the influences of heat flux and mass flux on the two-phase convective boiling heat transfer performance are reported for refrigerants HFO-1234yf and HFC-134a in a 3.9 mm smooth diameter tube. Tests are performed with a saturation temperature of 10 degrees C. It is found that at lower vapor quality region the nucleate boiling is the dominant heat transfer mechanism while the convective evaporation mechanism takes control at the higher vapor quality region. Both HFC-134a and HFO-1234yfshows similar trend and the difference in heat transfer coefficient between HFO-1234 and HFC-134a is quite small. The comparable heat transfer performance between HFC-134a and HFO-1234yf is attributed to similar physical properties and nucleate boiling contribution. The present test results are in line with some existing reports but are inconsistent with one other study having a tube diameter of 1.1 mm. It is found that the departure of heat transfer coefficients between the available publications is mainly attributed to the different flow phenomena caused by the difference of the channel size and channel geometry. A noticeable deterioration of the heat transfer coefficient for HFO-1234yf is encountered in the microchannel. The pressure drops for HFC-134a is about 5-15% higher than that of HFO-1234yf. (C) 2013 Elsevier Ltd. All rights reserved.
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    An overview of the effect of lubricant on the heat transfer performance on conventional refrigerants and natural refrigerant R-744
    (2012-09-01)
    Wang, Chi-Chuan
    ;
    Hafner, Armin
    ;
    Kuo, Cheng-Shu
    ;
    Hsieh, Wen-Der
    ;
    機械工程學系
    This review provides an overview of the lubricant on the heat transfer performance, including nucleate boiling, convective boiling, shell side condensation, forced convective condensation, and gas cooling, for conventional refrigerants and natural refrigerant R-744. Various parameters affecting the heat transfer coefficient subject to lubricant, such as oil concentration, heat flux, mass flux, vapor quality, geometric configuration, saturation temperature, thermodynamic and transport properties are discussed in this overview. It appears that the effect of individual parameter on the Prod. Type: FTP heat transfer coefficient may be different from studies to studies. This is associated with the complex nature of lubricant and some compound effect accompanying with the heat transport process. In this review, the authors try to summarize the general trend of the lubricant on the heat transfer coefficient, and to elaborate discrepancies of some inconsistent studies. The lubricant can, increase or impair the heat transfer performance depending on the oil concentration, surface tension, surface geometry, and the like. For the condensation, it is more well accepted that the presence of lubricant normally will impair the heat transfer performance due to deposited oil film. However, the deterioration is comparatively smaller than that in nucleate/convective boiling. For the effect of lubricant on R-744 with convective evaporation, the general behavior is in line with the convectional refrigerant. For gas cooling, the lubricant cast significant effect on heat transfer coefficient especially for a higher mass flux or at a smaller diameter tube. (c) 2012 Elsevier Ltd. All rights reserved.
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    On the heat transfer correlation for membrane distillation
    (2011-04-01)
    Wang, Chi-Chuan
    ;
    機械工程學系
    ;
    Department of Mechanical Engineering
    The present study examines the heat transfer coefficients applicable for membrane distillation. In the available literatures, researchers often adopt some existing correlations and claim the suitability of these correlations to their test data or models. Unfortunately this approach is quite limited and questionable. This is subject to the influences of boundary conditions, geometrical configurations, entry flow conditions, as well as some influences from spacer or support. The simple way is to obtain the heat transfer coefficients from experimentation. However there is no direct experimental data for heat transfer coefficients being reported directly from the measurements. The main reasons are from the uncertainty of permeate side and of the comparatively large magnitude of membrane resistance. Additional minor influence is the effect of mass transfer on the heat transfer performance. In practice, the mass transfer effect is negligible provided the feed side temperature is low. To increase the accuracy of the measured feed side heat transfer coefficient, it is proposed in this study to exploit a modified Wilson plot technique. Through this approach, one can eliminate the uncertainty from permeate side and reduce the uncertainty in membrane to obtain a more reliable heat transfer coefficients at feed side from the experimentation. (C) 2010 Elsevier Ltd. All rights reserved.
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    Effect of flow deflector on the flux improvement in direct contact membrane distillation
    (2010-04-01)
    Liu, Chu-Lien
    ;
    Chen, Yu-Feng
    ;
    Sheu, Wen-Junn
    ;
    Wang, Chi-Chuan
    ;
    機械工程學系
    This study experimentally investigated the influence of flow deflector on the performance of direct contact membrane distillation A total of seven modules, including a smooth membrane module and six flow deflector modules were fabricated and tested Effects of the number of flow deflector, deflector height, and the placement of deflectors are examined The smooth module reveals a comparatively abrupt change of water flux when the inlet velocity is around 0.25 m s(-1), and a plateau is seen for the water flux ratio vs. the Reynolds number for modules containing flow deflectors Depending on the arrangement of flow deflectors. the maxima occur at a Reynolds number of 500 for D1 and 650 for the others. The plateau for module having more flow deflectors has shifted toward a lower Reynolds number while the pressure drop ratio does not reveal such a plateau For the same pumping power, the test results show that the water flux for smooth module are smaller than that of all flow deflector modules, and it is applicable to all temperatures. It is also found that modules with more deflectors become more and more competitive at a higher feed temperature but are less effective when the velocity is high (C) 2009 Elsevier B V. All rights reserved.