•New measurement methods of outer wall temperature and heat flux were introduced with good verification results.•Pool boiling regimes were obtained as the representation of heat transfer ...enhancement.•Three previous correlations which apply to the annular flow pattern were assessed with the experimental results.•A modified correlation was proposed to predict the condensation heat transfer coefficients.
In order to investigate the heat transfer and resistance performance of high-pressure steam in-tube condensation with pool boiling outside the tube, experiments were conducted on a short horizontal tube with a 25mm outer diameter and 3mm wall thickness. The test parameters were in the range of 4–10MPa pressure, 400–1000kg/m2s mass velocity, and 0.1–0.9 steam quality. New measurement methods of wall temperature and heat flux were introduced. It is found that the outer wall temperature distribution is not uniform in the circumferential direction, and the wall temperature and heat flux increase with pressure, mass velocity, and steam quality. The condensation heat transfer coefficient and frictional pressure drop increase with the mass velocity and steam quality, but decrease with pressure. In addition, the pool boiling regimes were captured by a high-speed camera. The experimental condensation heat transfer coefficients were compared with available existing correlations, and the comparison results were found to be unsatisfactory. Finally, a modified condensation correlation from Akers et al. was proposed for high-pressure steam in-tube condensation with pool boiling outside in smooth horizontal tubes.
•Pool boiling heat transfer enhancement with biosurfactant particles deposited surfaces has been reported.•Surface morphology of the surfactant deposited surfaces has been modified.•Surfactant ...treated surfaces show higher roughness and better wettability.•The modified surfaces show 188% enhancement HTC and 152% enhancement in CHF.
The present work reports an experimental investigation of pool boiling of deionized water on a surfactant particle deposited heating surfaces. Heating surfaces made of copper are prepared by depositing surfactant particles through pool boiling of biosurfactant (Rhamnolipid) solutions at different concentrations (100–400 ppm of surfactant particles). Pool boiling experiments of deionized water have been performed with surfactant deposited heating surfaces, and results have been compared with the pool boiling of deionized water on a plain heating surface. Characterization of surfactant deposited heating surfaces indicates the presence of other elements like carbon, oxygen, silicon, chlorine, and potassium. The roughness of surfactant treated surfaces are 4–10 times more than the fresh heating surface, and their wettabilities are more than the fresh heating surface. Surfactant treated surfaces show a maximum 188% enhancement in heat transfer coefficient and 152% enhancement in critical heat flux as compared to the fresh heating surface. Porous structures are formed on the surfactant treated surfaces during boiling with the deionized water, which helps in heat transfer and critical heat flux enhancement through capillary wicking effects at high heat flux. However, no heat transfer enhancement has been observed when fresh pool boiling experiments of biosurfactant solutions have been performed with the surfactant deposited heating surfaces.
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Understanding nucleate pool boiling heat transfer and, in particular the accurate prediction of conditions that can lead to critical heat flux, is of the utmost importance in many industries. Due to ...the safety issues related to the nuclear power plants, and for the efficient operation of many heat transfer units including fossil fuel boilers, fusion reactors, electronic chips, etc., it is important to understand this kind of heat transfer. In this paper, a comprehensive review of analytical and numerical work on nucleate pool boiling heat transfer is presented. In order to understand this phenomenon, existing studies on boiling heat transfer coefficient and boiling heat flux are also discussed, as well as characteristics of boiling phenomena such as bubble departure diameter, bubble departure frequency, active nucleation site density, bubble waiting and growth period and their impact on pool boiling heat transfer.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
In this study, pool boiling from micro-pillar modified surface has been simulated numerically by a 3-D lattice Boltzmann method. Effects of geometries and wettability of micro-pillar on boiling heat ...transfer performance were also systematically evaluated. Result showed that compared within micro-pillar surface, heat flux of cubic micro-pillar surface was the highest with the lowest wall temperature. In addition, compared to hydrophilic condition, Heat flux of cubic micro-pillar surface with hydrophobic wettability increased by 98.3%. This is because hydrophobic wettability influenced nucleation site density, vapor-liquid-flow field and heat transfer performance much more than cubic shaped geometry. Finally, heat flux of cubic micro-pillar surface with hybrid wettability increased by 430.7% compared to pure hydrophilic wettability. That is due to optimal hybrid wettability surface could control nucleate site location, restrict bubble growth, and increase obviously heat transfer performance.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•Highly nanotextured “fractal-like” nanostructures on nickel-chrome wires improved the pool boiling performance.•Vapor bubbles from wire Joule heating formed, whose buoyant rise removed heat and ...promoted cooling.•Hydrophobic texturing is desirable for electronics cooling, which demands low surface temperatures.•Fractal-like hydrophilic surfaces increased the number of nucleation sites and the critical heat flux.
Highly nanotextured surfaces fabricated by electroplating are demonstrated in pool-boiling applications. Nickel-chrome wires were electroplated with copper and then annealed to cuprous oxide before being subject to Joule heating in a water bath. Vapor bubbles formed whose buoyant rise removed heat and promoted cooling. Hydrophobic and hydrophilic nanotextured surfaces could be tuned by varying the electroplating time. A hydrophobic surface enhanced bubble dynamics to locally decrease the surface temperature of the wire, which, in turn, enhanced superheat and the effective heat transfer coefficient. Conversely, a hydrophilic surface, characterized by a “fractal-like” surface decorated with numerous nucleation sites, increased the overall heat removal and thus the critical heat flux. These nanotextured surfaces were characterized by scanning electron microscopy and their pool boiling dynamics were visualized with a high-speed CCD camera. Theoretical heat-transfer estimates compared well with experimental data.
A. illustrating the steps involved in the growth of few layer graphene over copper substrate using PECVD followed by Fe catalyst deposition using RF magnetron sputtering. After which, CNT is grown ...over the Graphene coated copper using PECVD technique under CMOS compatible conditions. Finally, Graphene/CNT heterostructure is obtained. B. Illustrates the characterisation (FESEM) details and pool boiling characteristics (CHF and Heat transfer coefficient). Both CHF and heat transfer coefficient increases in the case of Gr/CNT heterostructures as compared to bare copper and Graphene coated copper substrates.
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•Growing seamless Graphene/CNT heterostructures on Cu using PECVD technique.•Role of Graphene based intermediate layer in promoting boiling heat transfer.•Synergic effect of Graphene/CNT network in influencing CHF and HTC values.•Qualitative bubble dynamics of plain copper and Gr/CNT heterostructures.•Graphene/CNT coated surface was superior in performance with higher CHF and HTC.
Nanostructures are one of the promising options when it comes to the overall enhancement of boiling systems. Growing carbon nanotubes (CNT) over the Graphene (Gr) coated copper substrates is believed to be one such approach. This study attempts to deliver an insight into this aspect using Gr/CNTs heterostructures, over the copper substrate, using FC-72 as working fluid. PECVD technique was used to grow both the few layer Graphene (FLG) as well as CNT over the copper substrate. The as fabricated structures were characterised using SEM, TEM, Raman Spectroscopy, AFM and contact angle measurement. Through detailed experimentation, it was observed that the Gr/CNT heterostructures improve both the heat transfer coefficient (155%) and the critical heat flux (40%) of the surface as compared to bare copper surface. Most importantly, the boiling incipience superheat was found to reduce drastically (62%) as compared to bare copper surface, which is favourable for electronics cooling application. Increased nucleation sites, capillary effect, lateral heat transfer through Graphene intermediate layer and improved bubble dynamics caused these enhancements. The improved thermal transport properties of the surface as a result of Graphene based intermediate layer and the surface properties due to CNT produced a synergic effect to improve the boiling performance as a whole.
In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid ...takes place on the external surfaces of tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate pool boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the experimental evaluation of nucleate pool boiling heat transfer correlations for R245fa and its possible replacement R1233zd(E) in ORC applications. The nucleate boiling HTC on a horizontal tube and the electrical power generation of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained measurement results, regarding heat transfer characteristics as well as power output, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and saturation pressure, and the lower viscosity. The HTC for R245fa is up to 43.2% higher in comparison to R1233zd(E). The experimental HTC are compared to selected nucleate pool boiling HTC correlations. The evaluation reveals that correlations according to Cooper and Gorenflo et al. show the slightest mean absolute deviations between 4.75% and 15.65% for both working fluids.
•An evaluation of HTC correlations is presented to assess their prediction accuracy.•Nucleate pool boiling HTC and power of scroll expander are simultaneously measured.•R245fa performs better than R1233zd(E) at equal saturation temperatures.•Cooper and Gorenflo correlations show the slightest deviations.
We use the phase field method to track the gas–liquid interface based on the gas–liquid two-phase flow in the pool boiling process, and study the bubble nucleation, growth, deformation, departure and ...other dynamic behaviors on the heating surface under microgravity. By simulating the correlation between liquid undercooling and bubble dynamics, we find that the bubble growth time increases with the increase of liquid undercooling, but the effect of liquid undercooling on bubble height is not significant. Meanwhile, the gas–liquid–solid three-phase contact angle and the gravity level will also have an effect on the bubble growth time and bubble height. With the increase of the contact angle, the bubble growth time and bubble height when the bubble departs also increase. While the effect of gravity level is on the contrary, the smaller the gravity level is, the larger the bubble height and bubble growth time when the bubble separates.
Pool boiling on nano-textured surfaces Jun, Seongchul; Sinha-Ray, Suman; Yarin, Alexander L.
International journal of heat and mass transfer,
07/2013, Letnik:
62
Journal Article
Recenzirano
Pool boiling on nano-textured surfaces was studied experimentally and theoretically for ethanol and water as working fluids. The nano-textured surfaces were copper platelets covered with ...copper-plated electrospun nanofibers. In addition, for comparison pool boiling on the corresponding bare copper surfaces was experimentally studied. The results revealed that the heat flux and heat transfer coefficient in boiling on the nano-textured surfaces were about 3–8 times higher than those on the bare copper surfaces. This stems from the fact that nano-textured surfaces promote bubble growth by increasing the average temperature of fluid surrounding growing bubbles, as our experimental and theoretical results show. Hence, nano-textured surfaces comprised of copper-plated nanofibers facilitate bubble growth rate and, thus, increase bubble detachment frequency. On the other hand, the critical heat flux (CHF) on the nano-textured surfaces was found to be very close to its counterpart on the bare copper surfaces. However, the heat flux on the nano-textured surfaces in transition boiling was significantly higher than on the bare copper ones, since the presence of nanofibers prevented bubble merging and delayed formation of vapor film. The wall temperature is about 10°C lower on the nano-textured surfaces. In transitional boiling the heat flux on nanofiber mats reduces much slower than on the bare surfaces.
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