A Theoretical Study of Dropwise Condensation

1975 ◽  
Vol 97 (1) ◽  
pp. 72-78 ◽  
Author(s):  
Hiroaki Tanaka
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Size Distribution ◽  
General Expression ◽  
Drop Size ◽  
Theoretical Study ◽  
Dropwise Condensation ◽  
Fundamental Equations ◽  
The Heat Transfer Coefficient

The so-called steady dropwise condensation consists of the transient dropwise condensation occurring repeatedly from place to place on the tracks of departing drops. By taking statistical and geometrical conditions into account, the author derives fundamental equations describing the process of this transient dropwise condensation. By solving these equations, the existence of a universal drop-size distribution is predicted. Further, by introducing a model for the cycle of drop departure, a general expression for the heat-transfer coefficient under the so-called steady dropwise condensation is obtained.

Experimental Study and Visualization of Dropwise Condensation on SUS316 Surface Under Non-Condensable Gas Condition

2016 ◽  
Author(s):  
Jae Young Choi ◽  
Yong Hoon Jeong ◽  
Noriyuki Watanabe
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Heat Exchanger ◽  
Transfer Coefficient ◽  
Surface Coating ◽  
Drop Size ◽  
Dropwise Condensation ◽  
Light Water ◽  
Cooling Surface ◽  
The Heat Transfer Coefficient

The passive safety features of nuclear power plant against station blackout (SBO) and intact containment integrity are the main key issues after Fukushima accident. As a corresponding safety system, passive containment cooling system (PCCS) received attention as one of the candidate systems applying to advanced light water reactors. Next generation of light water reactor, AP-1000 and ESBWR have suggested their own PCCS design. However, PCCS have difficulty in its heat exchanger volume due to low heat transfer coefficient of condensation under the presence of non-condensable gas condition. Several attempts had been studied worldwide to enhance the heat transfer coefficient of PCCS and this paper focused on dropwise condensation, which has much higher heat transfer coefficient than those found with filmwise condensation. Historically, surface coating or applying organic promoter on the cooling surface were typically used to induce dropwise condensation, but those method had disadvantage of their duration time. In general, surface coating and organic promoter were considered to have few years and few days of their duration, respectively. Therefore, an aim of our experiment was to determine whether SUS316 without any surface treatment is able to utilize dropwise condensation for PCCS heat exchanger. Following studies were compared to the filmwise condensation on SUS316 with same condition and also the experiment results reported by other researchers. The overall results determined how much the heat transfer coefficient was enhanced. To analyze the behavior of dropwise condensation, visualized images of cooling surface with droplets were graphically processed. All experiments were conducted on 13mm diameter of vertical-oriented flat surface with 6mm thickness. Air concentration (non-condensable concentration) was estimated by the partial pressure ratio of steam and air. Subcooled temperature, the difference between steam saturated temperature and surface temperature, was tested from 1 to 30 degree Celsius. High-speed camera visualized the condensate on the cooling surface with several magnification and frame speed. All measurement was measured after the whole system reached to equilibrium state and sustained it more than 30 minutes. Each data was recorded for 60 seconds and time-averaged its measurement. Experiment results indicated that SUS316 surface without any surface treatment could sustain dropwise condensation phase over 12 hours of experiment under low subcooled temperature below approximately 20 degree Celsius. Also, dropwise condensation had around 3 to 4 times enhanced heat transfer coefficients than those of filmwise condensation. The result of dropwise condensation with pure steam condition was well proportional to the power of subcooled temperature. Otherwise, the result with non-condensable gas showed heat transfer coefficient, shaded in high uncertainty of systematic error, seems to be decreased in low subcooled temperature under 5 degree Celsius. Higher sensitivity of non-condensable concentration upon heat transfer coefficient was observed on dropwise condensation more than filmwise condensation. The variation of drop-size distribution with the periodic time passage after surface sweeping was observed and patterns of drop-size distribution were repeated after the surface sweeping regardless of experimental conditions. The study provides feasibility and benefit of the utilization of dropwise condensation to PCCS heat exchanger if the system has designed to be operated within low subcooled temperature.

Effect of Open Micro-Channels on External Condensation Heat Transfer

2016 ◽  
Author(s):  
Brandon Hulet ◽  
Andres Martinez ◽  
Melanie Derby ◽  
Amy Rachel Betz
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Film Thickness ◽  
Transfer Coefficient ◽  
Heat Fluxes ◽  
Condensation Heat Transfer ◽  
Dropwise Condensation ◽  
Micro Channels ◽  
Lower Heat ◽  
The Heat Transfer Coefficient

This research experimentally investigates the heat transfer performance of open-micro channels under filmwise condensation conditions. Filmwise condensation is an important factor in the design of steam condensers used in thermoelectric power generation, desalination, and other industrial applications. Filmwise condensation averages five times lower heat transfer coefficients than those present in dropwise condensation, and filmwise condensation is the dominant condensation regime in the steam condensers due to a lack of a durable dropwise condensation surface. Film thickness is also of concern because it is directly proportional to the condenser’s overall thermal resistance. This research focuses on optimizing the channel size to inhibit the creation of a water film and/or to reduce its overall thickness in order to maximize the heat transfer coefficient of the surface. Condensation heat transfer was measured in three square channels and a plane surface as a control. The sizes of the square fins were 0.25 mm; 0.5 mm; and 1 mm, and tests were done at a constant pressure of 6.2 kPa. At lower heat fluxes, the 0.25mm fins perform better, whereas at larger heat fluxes a smooth surface offers better performance. At lower heat fluxes, droplets are swept away by gravity before the channels are flooded. Whereas, at higher heat fluxes, the channels are flooded increasing the total film thickness, thereby reducing the heat transfer coefficient.

scholarly journals Effect of Contact Angle on Steam Dropwise Condensation: A Simulation Approach

2012 ◽  
Vol 2012 ◽  
pp. 1-7
Author(s):  
Milad Nahavandi ◽  
Arjomand Mehrabani-Zeinabad
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Distribution Coefficient ◽  
Size Distribution ◽  
Atmospheric Pressure ◽  
Simulated Data ◽  
Condensation Process ◽  
Dropwise Condensation

Dropwise condensation process on surface of a vertical polytetrafluouroethylene (PTFE) plate at atmospheric pressure was simulated. Comparison of simulated data with experimental and theoretical results indicates that performed simulation results confirm experimental data, although they deviated from existing proposed correlations. For calculation of heat transfer coefficient and droplets size distribution, simulation of condensation process over vertical copper and PTFE surfaces at atmospheric pressure was performed. By considering the effect of contact angle on heat transfer resistances of droplets, the gained data were optimized in order to evaluate droplets size distribution coefficient. This distribution coefficient was used in a new correlation for prediction of heat transfer coefficient for dropwise condensation process. Comparison of experimental results with the correlation shows a good agreement, 11% relative error.

Effect of Micro-Structured Surface on Dropwise Condensation Heat Transfer

2013 ◽  
Author(s):  
Atsushi Tokunaga ◽  
Masaki Mizutani ◽  
Gyoko Nagayama ◽  
Takaharu Tsuruta
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Hydrophobic Surface ◽  
Condensation Heat Transfer ◽  
Dropwise Condensation ◽  
Mems Technology ◽  
Phase Change Phenomena ◽  
The Heat Transfer Coefficient ◽  
Condensation Heat Transfer Coefficient

The micro/nano scale phase change phenomena become more and more important because the MEMS technology develops rapidly in the fields of electro- and bio-devices [1][2] and the MEMS enable us to control the surface wettability. In the dropwise condensation on the hydrophobic surface, the heat transfer coefficient is determined by the departing droplet size. In our previous paper, it was found that the droplets in radius around 7 μm made more significant contribution to the condensation heat transfer under the low-pressure conditions. That is, when the smaller droplets less than 7 μm cover the condensing surface, the higher condensing heat flux would be achieved than that of the ordinary dropwise condensation. However, it is still very difficult to keep the droplets to be continuous condensed within 7 μm at the surface. A challenging work has been conducted to fabricate a droplets exclusion structure on the condensing surface for the purpose of the enhancement of condensation heat transfer in our previous experiment [3]. By using the MEMS technology, we made the hybrid-condensing surface with hydrophobic and hydrophilic patterns in order to remove the grown droplets effectively. It was found that the hybrid-surface has a possibility to increase the condensation heat transfer coefficient but its drainage-ability of the condensate has the limitation due to the occurrence of the flooding over the surface structures. In order to reduce the flooding at the hydrophobic area, in this study, the new design of the condensing surface has been proposed and the condensation heat transfer coefficient is evaluated.

scholarly journals Enhanced condensation heat transfer using porous silica inverse opal coatings on copper tubes

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Solomon Adera ◽  
Lauren Naworski ◽  
Alana Davitt ◽  
Nikolaj K. Mandsberg ◽  
Anna V. Shneidman ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Porous Silica ◽  
High Heat ◽  
Condensation Heat Transfer ◽  
Inverse Opal ◽  
Dropwise Condensation ◽  
High Heat Transfer ◽  
The Heat Transfer Coefficient

AbstractPhase-change condensation is commonplace in nature and industry. Since the 1930s, it is well understood that vapor condenses in filmwise mode on clean metallic surfaces whereas it condenses by forming discrete droplets on surfaces coated with a promoter material. In both filmwise and dropwise modes, the condensate is removed when gravity overcomes pinning forces. In this work, we show rapid condensate transport through cracks that formed due to material shrinkage when a copper tube is coated with silica inverse opal structures. Importantly, the high hydraulic conductivity of the cracks promote axial condensate transport that is beneficial for condensation heat transfer. In our experiments, the cracks improved the heat transfer coefficient from ≈ 12 kW/m2 K for laminar filmwise condensation on smooth clean copper tubes to ≈ 80 kW/m2 K for inverse opal coated copper tubes; nearly a sevenfold increase from filmwise condensation and identical enhancement with state-of-the-art dropwise condensation. Furthermore, our results show that impregnating the porous structure with oil further improves the heat transfer coefficient by an additional 30% to ≈ 103 kW/m2 K. Importantly, compared to the fast-degrading dropwise condensation, the inverse opal coated copper tubes maintained high heat transfer rates when the experiments were repeated > 20 times; each experiment lasting 3–4 h. In addition to the new coating approach, the insights gained from this work present a strategy to minimize oil depletion during condensation from lubricated surfaces.

Evaporation of lignin-lean black liquor

TAPPI Journal ◽  
2015 ◽  
Vol 14 (7) ◽  
pp. 441-450
Author(s):  
HENRIK WALLMO, ◽  
ULF ANDERSSON ◽  
MATHIAS GOURDON ◽  
MARTIN WIMBY
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Black Liquor ◽  
Salt Content ◽  
Solubility Limit ◽  
Lignin Removal ◽  
Kraft Black Liquor ◽  
Black Liquors ◽  
The Heat Transfer Coefficient

Many of the pulp mill biorefinery concepts recently presented include removal of lignin from black liquor. In this work, the aim was to study how the change in liquor chemistry affected the evaporation of kraft black liquor when lignin was removed using the LignoBoost process. Lignin was removed from a softwood kraft black liquor and four different black liquors were studied: one reference black liquor (with no lignin extracted); two ligninlean black liquors with a lignin removal rate of 5.5% and 21%, respectively; and one liquor with maximum lignin removal of 60%. Evaporation tests were carried out at the research evaporator in Chalmers University of Technology. Studied parameters were liquor viscosity, boiling point rise, heat transfer coefficient, scaling propensity, changes in liquor chemical composition, and tube incrustation. It was found that the solubility limit for incrustation changed towards lower dry solids for the lignin-lean black liquors due to an increased salt content. The scaling obtained on the tubes was easily cleaned with thin liquor at 105°C. It was also shown that the liquor viscosity decreased exponentially with increased lignin outtake and hence, the heat transfer coefficient increased with increased lignin outtake. Long term tests, operated about 6 percentage dry solids units above the solubility limit for incrustation for all liquors, showed that the heat transfer coefficient increased from 650 W/m2K for the reference liquor to 1500 W/m2K for the liquor with highest lignin separation degree, 60%.

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