Critical Heat Flux In Pool Research Articles

A comprehensive review on pool boiling heat transfer using nanofluids

Nanofluids are suspensions of nanoparticles with small concentration spread in base fluids such as water, oil and ethylene glycol. Nanofluid boiling is an important research area which provides many chances to explore new frontiers but also poses great challenges. Over the last decade, various studies have been carried out on pool boiling of nanofluids for the enhancement of critical heat flux which is otherwise limited by the use of base fluids. Several efforts have been made in the literature on nanofluid boiling, however, data on the boiling heat transfer coefficient and the critical heat flux have been unpredictable. Current study is a review of the status of research work on effects of nanofluids on heat transfer coefficient and critical heat flux. An emphasis is put in a review form on the recent progresses in nanofluid heat transfer coefficient and critical heat flux of pool boiling. This study also focuses on advancements in nanofluids, their properties and various parameters affecting boiling critical heat flux and heat transfer coefficient. At the end correlations used by different researchers to find out the critical heat flux and heat transfer coefficient are listed.

Observation of the mechanism triggering critical heat flux in pool boiling of saturated water under atmospheric pressure

We observed the dynamics of dry patches underneath massive bubbles during the pool boiling of saturated water under atmospheric pressure, and measured the associated temperature distribution. We synchronized the observations both spatially and temporally using high-speed total reflection and infrared thermometry techniques. The observations presented in this paper provide evidence that the critical heat flux phenomenon is triggered during the rewetting of large dry patches with periphery temperatures that are much lower than minimum film boiling temperature, so-called Leidenfrost point. As the liquid meniscus advanced toward the dry patch, numerous secondary bubbles nucleated and impeded the flow of liquid toward the dry patch. This prevented the liquid from rewetting the dry patch. The key physical mechanism for triggering CHF is initiated when the line density of the secondary bubbles nucleating at the periphery of a shrinking dry patch underneath a departing mushroom bubble exceeds a critical value. These bubbles completely block the liquid inflow into the dry patch and prevent rewetting, eventually causing the dry patch to expand irreversibly. We used our experimental data to determine an empirical value of the critical line nucleation site density.

The suppression effect of easy-to-activate nucleation sites on the critical heat flux in pool boiling

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/cm2 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.

Predicting the critical heat flux in pool boiling based on hydrodynamic instability induced irreversible hot spots

A new model, based on the experimental observation reported in the literature that CHF is triggered by the Irreversible Hot Spots (IHS), has been developed to predict the Critical Heat Flux (CHF) in pool boiling. The developed Irreversible Hot Spot (IHS) model can predict the CHF when boiling methanol on small flat surfaces and long horizontal cylinders of different sizes to within 5% uncertainty. It can also predict the effect of changing wettability (i.e. contact angle) on CHF to within 10% uncertainty for both hydrophilic and hydrophobic surfaces. In addition, a linear empirical correlation has been developed to model the bubble growth rate as a function of the system pressure. The IHS model with this linear bubble growth coefficient correlation can predict the CHF when boiling water on both flat surfaces and long horizontal cylinders to within 5% uncertainty up to 10 bar system pressure, and the CHF when boiling methanol on a flat surface to within 10% uncertainty up to 5 bar. The predicted detailed bubble grow and merge process from various sub-models are also in good agreement with the experimental results reported in the literature.

Effect of surface roughness on pool boiling heat transfer of water on hydrophobic surfaces

The effect of surface roughness on the pool boiling of water on hydrophobic surfaces was investigated over the entire boiling regime, from the onset of nucleate boiling to film boiling. The hydrophobic surfaces were fabricated by coating a polytetrafluoroethylene layer on 10 × 10 mm2 copper surfaces roughened by various sandpapers. The average roughness of the hydrophobic surfaces ranged from 0.042 to 1.54 μm and the corresponding contact angles varied from 116° to 153°. Boiling heat transfer coefficients (BHTC) for the rough surfaces were initially high, but decreased rapidly as the heat flux increased, whereas the BHTCs for the smooth surfaces slowly increased to reach their maximum values and then decreased. The variation in BHTC was explained in terms of the number of nucleation sites and vapor blanketing on the boiling surface, which were related to both the roughness and contact angle. An empirical critical heat flux (CHF) correlation, which showed a 12.2% mean absolute error with the present data, was proposed. Inclusion of the current CHF data for the hydrophobic region provided a complete understanding of the effect of surface roughness and wettability on the CHF in pool boiling of water.

Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

ABSTRACTSurfaces with spatial wettability patterns have been proven to enhance heat transfer coefficient and critical heat flux in pool boiling. To understand the physical mechanism behind this phenomenon and obtain the correlation among some critical parameters (bubble departure frequency, bubble size, nucleation site density, surface tension), pool boiling experiments were conducted. A Pyrex glass with a layer of indium-tin-oxide was used as the substrate. Hydrophobic patterns will serve as nucleation sites. Experiments were conducted in deionized water under atmospheric pressure at a relatively low heat flux. The processes of nucleation, growth, and departure of individual bubbles were visualized by using a high speed camera through the bottom of the heater surface. It has been found that the patterned surface performed the best in heat transfer for subcooled pool boiling when compared with hydrophilic and hydrophobic surfaces. The nucleation site density of the biphilic surface was much higher, when compared with that of the homogeneous surface. The individual bubbles always nucleate on the edge of the hydrophobic and hydrophilic area, and then move onto the hydrophobic pattern. Most of the individual bubbles detach from the wettability patterned surface in the diameter range from 300 µm to 450 µm (around 77.3%). The bubble departure periods scatter in the range from 80 ms to 1500 ms.

Time effect on wetting transition of smart surface and prediction of the wetting transition for critical heat flux in pool boiling

A smart surface that is a TiO2-coated surface (TCS) is a hydrophobic surface initially, but becomes a hydrophilic surface when heated. Therefore, such a surface can be used to enhance both boiling heat transfer (BHT) and critical heat flux (CHF) in pool boiling. In the present study, the time effect of the wetting transition of TCS was focused on. The CHF on TCS was enhanced more when the holding time of the heat flux in high-temperature regime was increased. By observing changes in contact angles on TCS through heat treatment in air, it was found that the wetting transition was affected not only by the temperature, but also by the time. Thus, a variation of the receding contact angle was correlated in the form of an exponential function. The suggested empirical correlation includes temperature and time, and it describes the transition of the receding contact angle. The correlation was also used to predict the CHF on TCS in pool boiling. As a result, CHFs on TCS could be explained using the correlation.

Effect of Thermophysical Properties of the Heater Substrate on Critical Heat Flux in Pool Boiling

While the role of the liquid properties, surface morphology, and operating conditions on critical heat flux (CHF) in pool boiling is well investigated, the effect of the properties of the heater material is not well understood. Previous studies indicate that the heater thickness plays an important role on the CHF phenomenon. However, beyond a certain thickness, called the asymptotic thickness, the local temperature fluctuations on the heater surface caused by the periodic bubble ebullition cycle are evened out, and the CHF is not influenced by further increasing the thickness. In the present work, data from literature and pool boiling experiments conducted in this study with seven substrates—aluminum, brass, copper, carbon steel, Monel 400, silver, and silicon—are used to determine the effect of the thermophysical property of the material on CHF for thick heaters that are used in industrial pool boiling applications. The results indicate that the product of density (ρ) and specific heat (cp) represents an important substrate property group that affects the CHF, and that the thermal conductivity is not an important parameter. A well-established force-balance-based CHF model (Kandlikar model) is modified to account for the thermal properties of the substrate. The predicted CHF values are within 15% of the experimental results.

Layer-by-layer carbon nanotube coatings for enhanced pool boiling heat transfer on metal surfaces

Micro/nanotextured coatings have enabled the manipulation of thermal characteristics in pool boiling heat transfer such as the heat transfer coefficient (HTC) and critical heat flux (CHF) because of the ability to optimize bubble formations and departures. However, fabricating such coatings on substrates involves high cost-bulky setup, and is limited by the materials and adhesion properties. Herein, we report layer-by-layer (LbL)-assembled polyethylenimine (PEI)-multi-walled carbon nanotube (MWCNTs) coatings on stainless steel (SS316) to enhance HTC and CHF in pool boiling heat transfer. LbL-assembled PEI-MWCNTs coatings (10, 20, 40 bi-layers) on SS316 were fabricated, whereas bare SS316 substrate and vacuum-filtered MWCNTs coating were prepared as controls. Because of the nano-cavities and the inner-nanoporous structures, LbL coatings showed significant enhancement of HTCs and CHFs compared to bare SS316. Furthermore, denser networks of MWCNTs due to electrostatic bonding and hydrophilic nature of PEI in the LbL coatings could enhance the HTCs and CHFs compared to vacuum-filtered MWCNTs coating. LbL-assembled PEI-MWCNTs of 20 bi-layers showed the highest improvement in HTC, whereas the 40 bi-layers coatings achieved the best enhancement ratio of CHF, (∼147%). Further development of the LbL-assembled coatings on metal surfaces would enable potential applications for thermal management, from micro/nanoscale platforms to macroscale systems.

Pool boiling with high heat flux enabled by a porous artery structure

A porous artery structure utilizing the concept of “phase separation and modulation” is proposed to enhance the critical heat flux of pool boiling. A series of experiments were conducted on a range of test articles in which multiple rectangular arteries were machined directly into the top surface of a 10.0 mm diameter copper rod. The arteries were then covered by a 2.0 mm thickness microporous copper plate through silver brazing. The pool wall was fabricated from transparent Pyrex glass to allow a visualization study, and water was used as the working fluid. Experimental results confirmed that the porous artery structure provided individual flow paths for the liquid supply and vapor venting, and avoided the detrimental effects of the liquid/vapor counter flow. As a result, a maximum heat flux of 610 W/cm2 over a heating area of 0.78 cm2 was achieved with no indication of dryout, prior to reaching the heater design temperature limit. Following the experimental tests, the mechanisms responsible for the boiling critical heat flux and performance enhancement of the porous artery structure were analyzed.

Simultaneous observation of dynamics and thermal evolution of irreversible dry spot at critical heat flux in pool boiling

It is generally accepted that the dry area underneath growing bubbles plays a vital role in understanding and modelling nucleate boiling heat transfer phenomena, including the critical heat flux (CHF). This study presents an investigation of the nucleate boiling phenomena under various surface heat flux conditions using a high-speed infrared thermometry technique called DEPIcT. The observation results for the liquid–vapour phase distribution on a boiling surface showed the formation, coalescence, and dynamic behaviours of dry spots on a boiling surface, and as the surface heat flux increased, the frequency and density of the dry spots significantly increased. Intensive nucleation behaviour near the triple contact line of a dry spot interrupting the wetting of the liquid was observed at a high heat flux. The liquid–vapour phase distribution and the temperature evolution of the dry spot area indicated the existence of an irreversible dry spot, which leads to surface overheating and consequent CHF triggering by rapid spreading on a boiling surface. The surface temperature initiating the formation of the irreversible dry spot was measured to 134°C, which is far lower than the Leidenfrost temperature and the maximum liquid-contact temperature reported by other studies. To provide qualitative explanations of the CHF triggering mechanism related to the irreversible dry spot, a conceptual boiling curve was presented. It can consider the hydrodynamic and thermal effects postulated on the boiling surface in the time-varying manner of dry spots and their re-wetting.

Supersonically sprayed reduced graphene oxide film to enhance critical heat flux in pool boiling

Reduced graphene oxide (rGO) flakes were supersonically sprayed onto a copper substrate, which was used to enhance the heat transfer between the cooling liquid and heat source in pool boiling. The results show that both the critical heat flux at which film bubbles are formed and the effective heat transfer coefficient increased substantially. The surface temperature of the heat source decreased substantially during efficient heat transfer. The higher wettability of the rGO film increased the cohesion of the cooling liquid with the heated surface. The roughened surface of the rGO film also facilitated the formation of small-scale bubbles that enhanced mixing and thus increased the rate of heat transfer. The rGO film was characterized using scanning electron microscopy, atomic force microscopy, and optical profiling.

Development of a critical heat flux mapping method for fin-structured surfaces

ABSTRACTIn this study, we performed critical heat flux (CHF) experiments using structured surfaces to validate the parameter effects and understand their physical meanings. Experimental results showed that the CHF has a peak value as the fin geometry changes. Fins with height of 0.5 mm produced the largest CHF, 1.7 MW/m2, and fins longer than 2 mm reduced the CHF values. To explain the results, a CHF mapping method was developed describing the liquid supply-side and demand-side limits. The liquid demand-side limit is governed by the heat removal capability, mainly the nucleate boiling, calculated using the hot spot model. We consider three liquid supply-side limits restricting the liquid supply to the heating surface: capillary limit and counter-current flow limitations (CCFLs). The capillary limit is determined by balancing the capillary pressure and viscous dissipation in the liquid film on the fin side. The CCFL in the structure is calculated using the Wallis correlation and the CCFL in the free volume limits the liquid downward flow by the vapor jetting from the heating surface. The CHF map for our experimental results successfully describes the CHF trend of the structured surfaces. As a result, we concluded that CHF mapping method is an effective means of explaining CHF in pool boiling.

A CHF model for saturated pool boiling on a heated surface with micro/nano-scale structures

A theoretical model is developed to predict the critical heat flux in pool boiling on a heated surface with micro/nano structures based on a force balance analysis, with effects of the capillary wicking force and modification of the critical instability wavelength taken into consideration. An analytical expression for CHF of a heated surface with micro/nano-size structures is obtained in terms of the surface roughness factor, the solid fraction as well as the contact angle explicitly. It is shown that geometric parameters of these micro/nano structures influence the ability of liquid spreading on a hydrophilic surface, effectively supplying liquid to the heated surface, and thus delaying the occurrence of the CHF. The micro/nano-sized cavities provide additional sites for vapor formation, altering the critical distance between vapor columns and thus reducing the critical instability wavelength. The critical wavelength for the occurrence of critical heat flux is obtained based on the change in surface free energy. The predicted values of CHF on heated surfaces with micro/nano-size structures in pool boiling obtained from the present model are found in agreement with existing experimental data.

F123 ステップ状加熱時におけるハニカム多孔質体による飽和プール沸騰限界熱流束の向上(OS9 熱・流動(2))

Several studies have been performed to make clear the transient boiling heat transfer during the exponential heat generation which is occurred in reactivity accident of a nuclear reactor. These researches have been focused on the mechanism of the phenomena mainly, not on the enhancement of the transient boiling heat transfer. In a previous study, we proposed a method of CHF enhancement under steady- state conditions using honeycomb porous plate. The CHF was shown experimentally to be enhanced to more than twice that of a plain surface using honeycomb porous plate. The enhancement is considered to result from the capillary supply of liquid onto the heated surface and the release of generated vapor through the channels. In the present paper, enhancement of the transient critical heat flux in pool boiling by the attachment of a honeycomb-structured porous plate on a heated wire is investigated experimentally using water under saturated boiling conditions.

Effect of Electric Field Generated by Microsized Electrode on Pool Boiling

This paper describes the results from an experimental study of the effect of an electrohydrodynamic (EHD) and an electrostatic force on the critical heat flux in pool boiling. The dielectric liquid of HFE-7100 (3M Ltd.), which has a boiling point of 59.8 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, was selected as a working fluid. In order to generate high electric fields with the application of low voltages, a micropatterned electrode with an array of planar comb fingers was made by microelectromechanical systems technology. The electrode had ten slits whose width was 400 μm between electrode pairs so as to remove vapor bubbles from a heat transfer surface by the EHD effect. A cooling device fabricated with the electrode was evaluated by different applied voltages. As a result, the application of voltage enhanced heat transfer.

EHD Conduction-Driven Enhancement of Critical Heat Flux in Pool Boiling

When a liquid undergoes pool boiling, there is a critical heat flux at which the departing vapor bubbles start to inhibit the flow of fresh liquid to the heated surface. A number of methods to enhance the critical heat flux of pool boiling have been considered, including electrohydrodynamic (EHD) enhancement. In these studies, the heater surface has generally been electrically grounded, and a high-voltage electrode has been placed a short distance away from the heater surface to create a high intensity field near the heater. The resulting configuration produces a dielectrophoretic force on the vapor bubbles to promote vapor removal. However, the high-voltage electrode can also act as a physical barrier to the incoming liquid and the departing vapor. This paper considers a different technique for the EHD enhancement of critical heat flux. EHD conduction pumping-type electrodes are embedded into the heater surface and used to pump liquid along the heated surface. The embedding of this pumping technique into the heater thereby promotes the wetting of the surface and subsequently delivers sizable enhancements to the heat transfer. The effect of a variety of applied voltages on the boiling curve and the critical heat flux point are quantified. An analytical model is developed to explain the enhancement effect.

A theoretical CHF model for downward facing surfaces and gaps under saturated boiling

A theoretical model is developed to predict the critical heat flux (CHF) based on the description of the hydrodynamic behavior of the vapor-liquid interface of a bubble at the heater surface leading to the initiation of CHF condition under saturated boiling. The CHF model considering heater size and gap size effects is developed to account for surface orientation effect, gap size effect, pressure effect, and contact angle effect for downward facing surfaces and gaps with orientations of 0° (horizontal downward facing position) to 90° (vertical position). The CHF in pool boiling and gap boiling was well predicted by the model for different effects. And the CHF model was extended to predict the CHF of the reactor-scaling hemispherical surface. The results indicated that the CHF model could also well predict the experimental data on large scale hemispherical surface. The present work is instructive for the safety analysis of the lower head of PWR in case of core meltdown during severe accident.

CRITICAL HEAT FLUX DURING FLOW BOILING IN MINI AND MICROCHANNEL-A STATE OF THE ART REVIEW

A state of the art review of critical heat flux during flow boiling through mini and microchannels has been provided based on the open literature. This review mainly examines three aspects, namely the experimental investigations, the available correlations and the state of prediction using those correlations and finally the proposed physical mechanisms as well as the theoretical models. Before discussing the specific literature on microchannels, a brief overview of critical heat flux for pool and flow boiling is provided. The review has been concluded with a summary of the available information on this topic and the need for future research.

Critical heat flux of pool boiling on Si nanowire array-coated surfaces

Pool boiling of saturated water on plain Si surfaces and surfaces covered with a dense array of Si nanowires (SiNWs) has been studied. Measured CHF and heat transfer coefficient (HTC) values of about 223 ± 5.61 W/cm 2 and 9 ± 1.60 W/cm 2 K, respectively, on the nanowire array-coated surface are among the highest reported in boiling heat transfer. Meanwhile, the CHFs on both nanowire-coated and Plain Si surfaces show a similar heater size dependence – the CHF increases as heater size decreases. The measured CHFs on both types of surfaces are approximately following the prediction of the hydrodynamic theory assuming the Helmholtz wavelength equal to the corresponded heater length. It suggests that the CHFs on both types of surfaces might be limited by pool hydrodynamics.