Saturated Pool Boiling Heat Transfer Research Articles

Corrosive effect on saturated pool boiling heat transfer characteristics of metallic surfaces with hierarchical micro/nano structures

Surface modification is of critical interest to enhance boiling heat transfer in terms of heat transfer coefficient or critical heat flux (CHF), which is significantly affected by distinct surface morphology and wettability and it can improve the efficiency and safety of equipment. Furthermore, actual service environment may cause severe corrosion to the processed structured surfaces while its consequence on boiling heat transfer is still obscure. In this article, comprehensive researches are conducted to unravel corrosive effect on metallic samples made of stainless steel (SS) and Inconel materials with microstructures. Different constructions (i.e., microgroove, microcavity and micropillar array) and characteristic dimensions (∼20, 50 μm) of microstructure, various duration time (up to 300 days) and pH values (∼7.0–8.5) of corrosive environment are compared thoroughly. Conclusions can be drawn that not all microstructures can enhance pool boiling heat transfer characteristics, especially in terms of CHF values. More importantly, CHF value of SS microgroove sample firstly increases from 60.94 to 94.09 W·cm−2 in 50 days, then decreases to 47.77 W·cm−2 in 300 days, which can be attributed to competition result between formation of hierarchical micro/nano structure with enhancing wicking capability and chemistry condition with increasing contact angle. In addition, distinct bubble dynamics during pool boiling is also analyzed. The insights obtained from this article can be used to guide surface modification method and to reveal evolvement rule of engineered metallic surface in highly corrosive and harsh boiling heating transfer environment.

A mesoscopic numerical method for enhanced pool boiling heat transfer on conical surfaces under action of electric field

The saturated pool boiling heat transfer on a conical structure surface under the action of an electric field is numerically investigated by using the lattice Boltzmann (LB) model coupled with an electric field model. A comparison study of boiling heat transfer phenomenon smooth surface and conical surface without the action of an electric field is first conducted in order to quantitatively analyze the mechanism of the electric field effect on boiling heat transfer on the conical structure surface. It is discovered that the conical structure has more active nucleation sites during the nucleate boiling regime, improving the boiling heat transfer efficiency and enhancing the critical heat flux (CHF). However, in the transition boiling stage and film boiling stage, the conical structure increases the flow resistance of the fluid on the fin surface, hindering heat transfer between the vapor and liquid and producing lower heat transfer performance than smooth surface. Based on the aforementioned findings, the boiling heat transmission on the conical structure surface is enhanced by applying an electric field. Numerical results indicate that the effect of the electric field on the boiling heat transfer performance on the conical structure surface is related to the boiling regime. In the earlier stage of the nucleation boiling regime, when an electric field is present, the onset time of bubble nucleation is slightly delayed, bubble size decreases a little, and boiling is slightly suppressed. However, the combination effect of electric field and conical structure, especially the tip effect, prevents the spread and diffusion of dry areas on the heating surface, thereby enhancing boiling heat transfer in the fully developed nucleate boiling stage. The tip effect grows more evidently in the transition boiling regime and film boiling regime, and increasing electric field intensity causes boiling to continue in the nucleate boiling regime at a higher superheat level. As a result, boiling heat transfer performance is greatly improved, and CHF steadily rises.

Saturated Pool Boiling Heat Transfer of R-141b on Al2O3 Nano-Structured Surfaces Fabricated by Dip-Coating Method

Saturated Pool Boiling Heat Transfer of R-141b on Al2O3 Nano-Structured Surfaces Fabricated by Dip-Coating Method

Optimizing additively manufactured macro-fin structure designs for enhanced pool boiling of dielectric fluids

This paper presents the results of the saturated pool boiling heat transfer performances of macrostructured fins fabricated by Selective Laser Melting, a metal additive manufacturing technique. A plain surface, two-dimensional triangular and rectangular fins, and three-dimensional pin and segregated fin array designs investigated in this study were fabricated using AlSi10Mg. A dielectric fluid, HFE-7100, was used as the cooling medium in a water-cooled thermosyphon to test the structures under atmospheric conditions. A macro-fin design optimization approach was developed by experimentally characterizing the boiling performances and bubble dynamics in fin arrays with different fin parameters such as fin topology, angle, and height. From the results obtained, this study optimized and tuned the macrostructure design to eliminate the detrimental bubble dynamics that were identified through high-speed visualization. Following a systematic study of the boiling characteristics of two-dimensional triangular and rectangular fins, and three-dimensional pin fins, the segregated fin array was developed to tackle the unfavorable bubble dynamics in these conventional fin geometries to enhance both critical heat flux and heat transfer coefficient. Various performance metrics were used to compare and evaluate the boiling performances of different fin topologies. From our studies, the segregated fin specimen (Seg-3) was determined to achieve the best balance between critical heat flux and heat transfer coefficient enhancements, attaining a critical heat flux of 64.19 W/cm2 and a maximum heat transfer coefficient of 2.22 W/cm2·K, enhancing critical heat flux and heat transfer coefficient by up to 97.2 % and 43.1 % as compared to the plain surface, respectively. Furthermore, Seg-3 has achieved boiling enhancements with only 334 mm2 and 50.6 mm3 of surface area and fin volume, respectively, a substantial surface area and fin volume reduction as compared to conventional macro-fin geometries with the same fin height. In all, this work not only provides fundamental insights into the bubble dynamics in various macro-fin structures and their influence on the boiling performance, but it also presents a macro-fin design optimization strategy that can be adopted for other immersion cooling applications.

Influence of smooth heater size on critical heat flux and heat transfer coefficient of saturated pool boiling heat transfer

In order to study the influence of smooth heater size on the performance of pool boiling heat transfer, saturated boiling experiments of FC-72 were conducted on eight sized smooth silicon chips (the test surfaces are denoted as S5 ∼ S30, and their dimensionless dimensions are in the range of Lh/Lc = 7 ∼ 42). The bubble dynamics during boiling were observed with a high-speed camera. The results indicated that in natural convection regime, the heat transfer coefficient decreases with increasing heater size at a given heat flux. Besides, the larger surfaces enter into the nucleate boiling regime earlier. In a fully developed nucleate boiling regime, S5 surface displays the optimal heat transfer performance. The reality is that no vapor columns are produced even in the high heat flux range due to its small bubble departure diameter and high bubble departure frequency. For S8 ∼ S15 surfaces, the wall temperatures increase continuously with the increase of heat flux, while S20 ∼ S30 surfaces present a trend of first keeping constant and then increasing. The active nucleation site densities on S15 ∼ S30 surfaces follow the law of Na = 0.45q2 in the whole nucleate boiling regime. By contrast, for S5 ∼ S12 surfaces, the nucleation site densities abide by this law only in a fully developed nucleate boiling regime. In addition, the departure diameter and departure frequency of bubbles increase with the increase of heat flux for all surfaces. It is observed that with the increase of smooth heater size, CHF presents a changing trend of first increasing (in range of Lh ≤ λD, λD is the most dangerous Taylor wavelength), then decreasing (in range of λD < Lh ≤ 3λD) and finally leveling off (in range of Lh > 3λD). Accordingly, the CHF value maximizes at the heater size Lh = λD. In the segment of decreased CHF, the decline rate of CHF in range of λD < Lh ≤ 2λD is larger than that in range of 2λD < Lh ≤ 3λD. Meanwhile, the influencing mechanisms of heater size on CHF were explained by consideration into bubble patterns and behavior.

Saturated pool boiling heat transfer enhancement of R245fa based on the surface covered by sintered copper powder with and without nanostructure

The pool boiling characteristics on the enhanced surfaces with sintered copper powder coatings with and without nano-treatment were experimentally investigated. Three kinds of enchanted surfaces formed by sintered porous layers with copper powder particles and their further nano-treated surfaces were adopted as the test surfaces. The thermal oxidation process was used to form nanostructures with super hydrophilic characteristic on the surface of sintered copper particles. Refrigerant R245fa was selected as the test fluid due to the potential the application in electronic cooling field. The saturation temperature of boiling experiments was maintained about 20 °C. The result showed that the boiling heat transfer performance was effectively improved by coating sintered copper powder porous layer on the plain copper surface, and the maximum heat transfer coefficient and CHF value enhancement could reach 151% and 160% of the plain one, respectively. The enhanced surfaces no matter with or without nano-treatment could trigger the nucleation process at the wall superheats lower than 12 °C rather than the value as high as 30 °C for plain copper surface. After nano-treatment, the heat transfer enhancement for sintered porous surfaces was further improved at medium and low heat flux, where the bubble nucleation or detachment remained robust. The CHF peaked as 140% of untreated surface, while the heat transfer coefficient increased to 108% at most.

A lattice Boltzmann investigation of the saturated pool boiling heat transfer on micro-cavity/fin surfaces

Saturated pool boiling heat transfer on micro-cavity and micro-fin surfaces is examined by a mesoscopic phase change lattice Boltzmann method. The important interfacial processes and boiling heat transfer performance are explored concerning the effects of micro-structure configurations, specifically fin and cavity, and micro-structure parameters, including fin/cavity shape, height, length, and spacing between fins/cavities. It is discovered that both the micro-cavity and micro-fin surfaces are conducive to bubble nucleation and can enhance nucleate boiling heat transfer (NBHT) when compared with the smooth surface. By comparing fin and cavity surfaces, it is found that micro-cavity is more conducive to bubble nucleation, whereas micro-fin is more conducive to bubble departure. As a result, micro-cavity surface has a higher NBHT while a micro-fin surface has a higher critical heat flux (CHF). The saturated pool boiling heat transmission is significantly influenced by the micro-structure parameters as well, i.e., the boiling on the rectangular cavity/fin surfaces has an earlier nucleation time while that on the conical surfaces has a faster bubble escape speed. The mass of residual bubble left over after the bubble department increases with cavity/fin height, which leads to the advance of CHF. On the other hand, the CHF increases as the distance between micro-structures. Additionally, with the increase in micro-structure length, the CHF increases for the micro-cavity surface whereas decreases for the micro-fin surface. Finally, a series of fitting equations between CHF and the micro-structure parameters are presented and an improved hybrid surface is developed based on the theoretical predictions.

Mesoscopic study on effect of electric field and heater characteristics on saturated pool boiling heat transfer

The phase change lattice Boltzmann (LB) model combined with the electric field model is employed to investigate the heat transfer performance of saturated pool boiling. Particular attention is paid to the influence of heater surface wettability and heater length on bubble behaviors, including generation, merging, and fracture during boiling in a uniform electric field. Moreover, the effects of the bubble behavior on heat transfer performance are also investigated. The study results indicate that the enhancement of boiling heat transfer by the electric field is dependent on both the heater length and the wettability. In the case of a hydrophilic surface, when the heater length &lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$L_H^*\leqslant 6.25$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M5.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, the bubble interaction force generated on the heater surface during boiling is weak due to the small size of the heater. Thus the effect of a uniform electric field on the bubble dynamic behaviors is mainly manifested by reducing the bubble size. As a result, the whole boiling phase is suppressed in this case. In the case of &lt;inline-formula&gt;&lt;tex-math id="M6"&gt;\begin{document}$6.25 &lt; L_H^*\leqslant9.375$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M6.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M6.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, the uniform electric field enhances the critical heat flux (CHF), and the enhancement degree increases with electric field strength increasing. This can be attributed to the longer heater providing sufficient space for bubble generation, resulting in increased bubble nucleation sites and stronger interaction forces between bubbles. On the other hand, the distance between adjacent bubbles increases with the heater length increasing,thus further contributing to the improved CHF percentage. When &lt;inline-formula&gt;&lt;tex-math id="M7"&gt;\begin{document}$L_H^*&gt;9.375$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M7.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230341_M7.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, the rewetting resistance increases with heater length increasing. So the vapor generated in the boiling process is prone to be closely adhered to the heating surface under the action of electric field force, forming a thin layer of vapor on the heater surface. The vapor not only increases the heat transfer thermal resistance between the solid and the fluid but also creates no vortex near the bubble. This is not conducive to the movement of the bubble to the middle of the heater, thereby slowing down the heat mass exchange between the hot fluid on the heating surface and the colder fluid on both sides. As a result, the improved percentage of CHF decreases gradually with the increase in the heater length. In the case of hydrophobic surfaces, the increased percentage of CHF initially increases with heater length increasing and then decreases. However, comparing with the hydrophilic surface, the increase of the heater source length corresponds to the beginning of the decrease of critical heat flux.

Saturated pool boiling heat transfer of HFE-7100 on sintered copper powder and wire mesh microporous surfaces: A comparison study

Pool boiling heat transfer enhancement is of great significance for the thermal management of high-heat-flux electronics. In this paper, two types of microporous coating surfaces, including sintered spherical copper powder and wire mesh microporous surfaces, were fabricated, and a comparison study on the boiling performance was experimentally conducted in saturated HFE-7100 dielectric fluid in order to further evaluate the performance difference and reveal the main mechanisms affecting the variation trend. The results showed that aided by the increased nucleation sites and capillary force, the boiling heat transfer performance of the sintered microporous coating surfaces was remarkably higher than the polished copper surface. Among them, sintered 300 in.−1 wire mesh surface demonstrated the best performance which presented an outstanding critical heat flux of 48.95 W/cm2 with a corresponding heat transfer coefficient of 2.2 W/cm2·K, increasing by up to 81.50 % and 144.44 % compared to the polished copper surface, respectively. Additionally, for the two different microporous surfaces, the heat transfer coefficient of sintered copper powder surface was slightly better at low heat fluxes. On the contrary, the sintered wire mesh surface exhibited a higher critical heat flux and a heat transfer coefficient at high heat fluxes. Finally, the experimental results were agreed well with those obtained from the critical heat flux prediction model, which provides a possible analytical model to evaluate the boiling performance of microporous surfaces.

Effect of T-shaped micro-fins on pool boiling heat transfer performance of surfaces

The saturated pool boiling heat transfer performance on the surface of T-shaped micro-fins was investigated with deionized water as working medium. Ten different T-shaped micro-fins surfaces were fabricated by machining method and the pool boiling experiments were carried out. By comparing the influence of varios parameters on the boiling heat transfer performance, the results showed that reducing the width of upper fin, increasing the gap or height of upper fin can improve the boiling heat transfer coefficient. With the increase of the width of the lower fins, the boiling heat transfer performance showed a different trend, that is, it increased at first and then decreased. The maximum boiling heat transfer coefficient of the surface S1 to the smooth surface was 184.7%. The visualization results showed that the merging of bubbles on the outside of the channel surface could strengthen the vertical and horizontal replenishment of the liquid to dry spot in the channel, thereby delaying the occurrence of critical heat flux and effectively reducing the wall temperature. The simplified semi-analytical model to determine the total heat flux of the micro channeal was proposed, which provided a heat flux prediction with an error of ± 40%.

Effect of inclination angle of lower tube on saturated pool boiling heat transfer of upper tube

An experimental study was conducted to examine the effects of the inclination angle and heat flux of the tube located at the bottom of the vertically arranged tube bundle on the pool boiling heat transfer of the upper tube. In the experiment, two stainless steel tubes submerged in saturated water at atmospheric pressure were used. The angle of inclination varied from 0° to 90° at 15° intervals. When the inclination angle of the lower tube is increased, the length of the lower tube projected onto the upper tube decreases, and thus the heat transfer coefficient tends to decrease. However, when the angle of the lower tube is between 45° to 75°, bubbles gather in the narrow area, resulting in a strong flow, which tends to increase the heat transfer coefficient. When the heat flux of the lower tube was high and the heat flux of the upper tube was low, an increase in the heat transfer coefficient of the upper tube was clearly observed.

Numerical simulation of bubble dynamics and heat transfer in the 2D saturated pool boiling from a circular surface

In this paper, the saturated pool boiling heat transfer from a heated circular surface is investigated numerically based on the popular lattice Boltzmann phase-change method. The crucial interfacial phenomena, including the bubble growth, departure, coalescence, and rising under different wall superheats, wettabilities, and heater sizes are studied, and the corresponding heat transfer characteristics within these bubble dynamics are also revealed. The numerical experiments indicate that at low wall superheats the bubble nucleation occurs only at the top of the heated circle. With the increase in the wall superheat, the sides and the bottom of the heated circle also become nucleate sites. As the wall superheats continue to increase, more and more nucleate sites are activated so that the heated circle is wrapped by the vapor film early in the boiling process. And the average heat flux varies periodically with time corresponding to bubble dynamics in nucleation boiling regime. It reaches a steady state of film boiling regime after the stable vapor film is generated from the heated circular. It is also found that hydrophobic surface is conducive to the onset of boiling. However, it leads to a low critical heat flux and incurs film boiling at a lower wall superheat, which is also observed in both experimental and numerical studies. On the other hand, the boiling regimes can undergo the transition from nucleate boiling regime to film boiling regime at the same wall superheat with the increase in the heater size. In addition, under various wall superheats and wettabilities, the bubble departure diameters from the circular surface are smaller than those from the flat surface, while the bubble departure periods from the circular surface are less than those from the flat surface one. Finally, the saturated boiling curves from the circular surface for different wall wettabilities, heater sizes, liquid–vapor density ratios, and heating modes are also achieved.

Influence of hydrodynamic interactions among multiple bubbles on convective heat transfer in nucleate boiling

To clear the effects of the hydrodynamic interactions among neighboring vapor bubbles on the saturated pool boiling heat transfer, artificial nucleation sites with several cavity arrangements were manufactured on the substrate surface and measured liquid temperatures near heated substrate by a miniature thermocouple probe with 10 kHz. The surface temperature of heated substrate was measured by an infrared camera with 1 kHz and bubble dynamics were simultaneously visualized by a high-speed video camera with 5 kHz and 10 kHz, respectively. The liquid temperature fluctuations, measured with micro thermocouple 100 μm and 10 μm above the substrate surface, were compared among each other and with the transient temperature fields obtained through IR measurements. Results showed that the hydrodynamic interaction among neighboring bubbles causes the liquid convection, that was only detected 100 µm above the boiling surface. Generally, the degree of liquid temperature fluctuations at 10 μm height and different arrangements of nucleation sites was under 1 K. Here the superheated sublayer in the vicinity of the boiling surface was generally not affected by the convection. Only when the coalescence from two neighboring nucleation sites occurred and the liquid-vapor interface nearly reached the substrate, a steep temperature drop over 2 K was detected. It was concluded that the viscous sublayer stably exists during boiling of water in lower heat flux conditions. To enhance the convective heat transfer in nucleate boiling, coalesced bubble needs to peel off the viscous sublayer during its deformation. Based on the measurements in this study, this peel-off effect only happens when the coalescence point is less than 1 mm above the heated surface.

Saturated pool boiling heat transfer of acetone and HFE-7200 on modified surfaces by electrophoretic and electrochemical deposition

Boiling heat transfer intensification is of big relevance to energy conversion and conservation, materials and resources saving, and electronics cooling. This work aims to enhance saturated pool boiling of well-wetting liquids, i.e., acetone and HFE-7200 on nanoparticles-deposited surfaces by electrophoretic deposition and on microporous foam surfaces by electrochemical deposition. The electrophoretic-deposited surfaces enhance the heat transfer coefficient of acetone and HFE-7200 by up to 70% and 190%, respectively. However, the critical heat flux is not improved on electrophoretic-deposited surfaces. The electrochemical-deposited surfaces increase the boiling heat transfer coefficient by up to 370% and the critical heat flux by more than 30%. Bubble dynamics were visualized simultaneously. The bubble departure diameter from experiments can be predicted by a dynamic force balance model within a ±20% error band. A mechanistic heat transfer model was proposed for modified porous surfaces, including not only the heat fluxes from microlayer evaporation and transient conduction but also the heat flux from micro-convection due to liquid agitation and entrainment by growing and departing bubbles. The mechanistic heat transfer model can predict experimental pool boiling curves of acetone and HFE-7200 on electrophoretic-deposited and electrochemical-deposited surfaces relatively well, especially for the isolated bubble regime where most bubbles are isolated and bubble coalescence is not intensive. Besides, the critical heat flux of a modified surface can be estimated if the initial (maximum) wicked volume flux on the structured surface relative to the smooth surface is considered.

Numerical study on saturated pool boiling heat transfer in presence of a uniform electric field using lattice Boltzmann method

For the industrial equipment with boiling, critical heat flux (CHF) is an important parameter that determines its upper limit of heat load for safe operation. Therefore, increasing CHF is of great importance in engineering field. Previous studies proved that it’s an effective way to enhance boiling heat transfer and CHF by electrohydrodynamics (EHD). However, it is difficult to investigate the influence mechanism of the electric field on the bubble dynamics and boiling heat transfer because of the limit of experimental technical methods. Thus, in this paper, a two-dimensional lattice Boltzmann model was established by coupling the pseudopotential model with phase-change model and electric field model to meet the lack of studies on simulating the pool boiling in presence of an electric field. After validating the rationality of the two-dimensional lattice Boltzmann model established at present study, the heat transfer during nucleate boiling and film boiling under a uniform electric field were studied in detail. In addition, the influence of electric field intensity on boiling curves and CHF was investigated quantitatively. In the partial nucleate boiling regime, a uniform electric field had limited influence on bubble dynamics and boiling heat transfer. In the fully developed nucleate boiling regime, increasing electric field intensity could enhance boiling heat transfer obviously. In the film boiling regime, increasing electric field intensity could slightly enhance averaged heat flux at first. After the electric field intensity reached a certain value, averaged heat flux increased rapidly. Increasing electric field intensity could expand the nucleate boiling regime, and enhance CHF and the wall temperature at CHF point.

Experimental investigation on saturated pool boiling CHF for downward facing heating surface with different sizes and aspect ratio

IVR-ERVC (In-Vessel Retention – External Reactor Vessel Cooling) is a key severe accident management for keeping the integrity of the reactor pressure vessel (RPV) lower head. One of most important topics is the characteristics of the CHF (Critical Heat Flux) on the outside wall of RPV lower head. Some experiments had been conducted to investigate the effect of the orientation on characteristics of the boiling heat transfer and CHF for downward facing heating plate. While the size of the heating plate used may have an appreciable effect on the characteristics of the CHF. In this work, experiments were carried out to investigate the effects of the heater surface area and length-width ratios on saturated pool boiling and CHF for downward facing heating surface. Experimental results indicated that the inclined angle has little effect on the saturated pool boiling heat transfer, and the wall superheat of triggering of CHF was greater for the larger inclined angle. For the same aspect ratio of the heating surface, the CHF decreased with the increase of the heating surface area for the same surface orientation. It also indicated that the CHF was influenced by the bubble escaping length and wetted perimeter complicatedly, and the short edge effect had an appreciable influence on CHF based on present experimental work. Meanwhile, a modified El-Genk and Guo CHF correlation was proposed based on CHF data with the same aspect ratio of the heating surface, which could consider the effect of heating surface area on pool boiling CHF.

Experimental study of saturated pool boiling heat transfer with FeCrAl- and Cr-layered vertical tubes under atmospheric pressure

This study compares the nucleate boiling heat transfer coefficient (NBHTC) and critical heat flux (CHF) of two candidate coatings, FeCrAl and Cr, being considered for accident-tolerant fuel (ATF) cladding applications. To form an intrinsic surface roughness, the tube surfaces were initially grinded with 800 and 60 grit sandpapers, and then, FeCrAl and Cr were physically deposited using the direct current magnetron sputtering technique. The FeCrAl- and Cr-layered surfaces became hydrophilic on the lumped nanostructures and superhydrophilic on the particulate nanostructures, respectively. When subjected to the pool boiling conditions of vertically-oriented tubes, the NBHTC and CHF of the FeCrAl-layered tube increased by 24% and 34%, respectively, with the exception of the similar NBHTC generated by the 60 grit sandpaper. In contrast, the NBHTC of the Cr-layered tube decreased by 15% due to the suppressed nucleation on the particulate nanostructures, while CHF increased by 27%. The CHF enhancement was analyzed based on the liquid spreading behavior of a water droplet from the morphological changes. The capillary flow rate, which is a product of the liquid spreading rate, us, and arithmetic roughness height, Ra, resulted in a more accurate prediction of the CHF than the equilibrium contact angle. Additionally, the potential boiling performance of the FeCrAl and Cr coatings were discussed by comparing the pool boiling results of the vertical tube and horizontal plate orientations.

Pool boiling heat transfer of N-pentane on micro/nanostructured surfaces

In the present study, one type of uniformly nanostructured surface (NPDS) was modified by electrophoretic deposition. Two kinds of micro/nanostructured surfaces (FLS1 and FLS2) were fabricated on copper surfaces by femtosecond laser processing. The micro/nanostructured surfaces were further modified by electrophoretic deposition. Afterwards, composite micro/nanostructured surfaces (CS1 and CS2) were developed. Saturated pool boiling heat transfer of the modified surfaces was investigated experimentally. An organic fluid, n-pentane was chosen as the working liquid. Heat transfer coefficient and critical heat flux (CHF) of smooth and micro/nanostructured surfaces were studied. The results showed that the heat transfer coefficient (HTC) of all structured surfaces increased obviously with a notable decrease of wall superheat at CHF compared to the smooth surface, which was attributed to increments in nucleation site density and heat transfer area. The CHF of femtosecond laser processed surfaces was also increased compared with the smooth surface due to a much higher liquid spreading ability, while a uniformly nanostructured surface has no augmentation in CHF. Composite micro/nanostructured surfaces show the best heat transfer performance among all tested surfaces, and the critical heat flux and heat transfer coefficient were increased by more than 60% and 300% over the smooth surface, respectively. The liquid spreading ability of n-pentane on the tested surfaces was measured. For the well wetting liquid, the liquid spreading ability of the heated surface, instead of the wettability, is the main factor for CHF enhancement. It is suggested that a surface with multiscale structures can be an efficient way for boiling heat transfer enhancement.

Pool-boiling enhancement using multilevel modulated wick

Main challenges in saturated pool-boiling heat transfer are limited Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC), caused by the counterflow of liquid and vapor over the heated surface. Using multilevel modulated wicks, i.e., monolayer, columnar, and mushroom post wicks, we control the liquid and vapor flow for efficient phase separation, thereby improving the CHF and HTC. The wicks are fabricated using multi-step sintering process using 200 μm copper particles, and the pool boiling uses n-pentane at ambient pressure. The monolayer wicks without and with the mushroom post structure provide 20% and 87% CHF enhancements, respectively, compared to the plain surface. It is found that the CHF enhancement of the mushroom wick is attributed to its pitch distance, 3.5 mm, which effectively reduce the hydrodynamic instability (Rayleigh-Taylor) wavelength. The further reduction of the pitch distance, 1 mm, results in the 250% CHF improvement in agreement with the theory. The columnar and mushroom posts with monolayer increase HTC by tenfold, compared to the plain surface, due to the reduced conduction path through the thin monolayer wick under the controlled vapor region using the columnar and mushroom post wick (vapor chamber-like environment in pool boiling). The multilevel wick design provides fundamental insights into simultaneous CHF and HTC enhancements with potential use in advanced thermal management systems.

Effects of orientation and structure geometry on boiling heat transfer for downward facing IGTAC surfaces

External Reactor Vessel Cooling has been a key severe accident management strategy to prevent the damage of the reactor vessel. Saturated pool boiling heat transfer coefficients and critical heat flux (CHF) were measured from the downward facing structured surface of interconnected-grooves with triangular array cavities (IGTAC surface) in deionized water to enhance the CHF. Four IGTAC surfaces were investigated at inclination angles of 5°, 30°, 45°, 60° and 90° (vertical). The experimental results indicated that the CHF increases with the increasing inclination angle. The highest CHF at inclination angle of 5° on downward facing IGTAC surface could reach 1258.4 kW/m2, which was over 2 times of that on a plain surface. The CHF on IGTAC surface did not appear at larger inclination angles when the heat flux reached 2400 kW/m2, which is the limit of the experimental apparatus. The IGTAC surfaces can form a liquid-vapor conversion path with the cavities as stable nucleation sites and the interconnected grooves as the cooling water supply pathways, which results in a significant boiling heat transfer enhancement and CHF increase. It is shown that deeper groove depth and larger cavity diameter will enhance the nucleate boiling heat transfer (NBHT) and CHF on IGTAC surfaces within the current dimension parameter range.