Critical Heat Flux Enhancement Research Articles

Cooling time and CHF enhancement of reactor vessel cooling process by flame sprayed porous coating in different orientations

External reactor vessel cooling (ERVC) is an effective strategy to achieve in-vessel retention (IVR) under severe nuclear plant accidents. Earlier transition from film boiling to nucleate boiling can lead to faster cooling and the critical heat flux (CHF) of the reactor pressure vessel (RPV) determines the success of IVR-ERVC. In this study, a porous coating of 316L was prepared on the surface of SA508 steel by flame spraying. Quenching experiments of plain surface and coating surface were performed to investigate the enhancement of reactor vessel cooling process at atmospheric pressure in saturated deionized water by using a variable orientation quenching device. The orientations of all experimental surfaces were varied between 0°(downward) and 90°(vertical). The experimental results show that the cooling time of coating surface was reduced by more than 56% compared with plain surface, which is mainly due to the reduction of duration in film boiling phase and the early appearance of Leidenfrost point (LFP). The CHF of coating surface achieved about 30% enhancement compared with plain surface in all orientations. In addition, the cooling time decreased and CHF increased with the increase of orientation because the bubbles separation was easier in the larger orientation angle. The visualization of cooling process was conducted in the vertical orientation and demonstrated that there were more nucleation sites of bubbles and the bubbles grew and separated faster in coating surface.

Critical heat flux characteristics of flow boiling on a heater rod under inclined and rolling conditions

Offshore floating nuclear power plants have been developed to supply power to remote regions. Since they are operated in an ocean environment, the thermal–hydraulic phenomena in the reactor core may be different from those in the land-based reactors. However, the experimental study on critical heat flux (CHF) under ocean condition is limited and its mechanism is unclear. In this study, an experiment was performed to measure CHF in a single heater rod under inclined and rolling conditions. To create the required environment to simulate the conditions of inclination and rolling, a rolling platform was designed and constructed. It could provide an inclination of up to 45° and rolling of up to 45° with a minimum period of 6 s. In the CHF test loop, the experimental conditions were set up to correspond to pressurized water reactor conditions, using the fluid-to-fluid scaling method. The working fluid used was R134a. The test section was a concentric annular channel composed of a heater rod and an unheated tube housing. The result of the inclination experiment suggested that in the most cases, the CHF was enhanced when compared with the vertical condition. In a limited number of cases of low-pressure and high-flow conditions, the CHF was degraded as compared to the vertical condition. These variations were proportional to the inclination angle and inversely proportional to the mass flux. From the results of the rolling experiment, a precursor of CHF was observed with a large fluctuation in temperature before the CHF, and its trigger was related to the CHF value of the inclined case. Based on this observation, the overall relations between the vertical, inclined, and rolling CHF values were derived. It was concluded that the rolling motion weakened the CHF enhancement and degradation effects of the inclination.

Electrowetting Modulated Enhancement of Critical Heat Flux in Pool Boiling

Electrowetting Modulated Enhancement of Critical Heat Flux in Pool Boiling

Electrowetting Modulated Enhancement of Critical Heat Flux in Pool Boiling

Electrowetting Modulated Enhancement of Critical Heat Flux in Pool Boiling

Enhancement of Pool-Boiling Critical Heat Flux with Structure Focused on Liquid Film Retention Function

Enhancement of Pool-Boiling Critical Heat Flux with Structure Focused on Liquid Film Retention Function

Unified descriptor for enhanced critical heat flux during pool boiling of hemi-wicking surfaces

Boiling heat transfer is dictated by interfacial phenomena at the three-phase contact line where vapor bubbles form on the surface. Structured surfaces have shown significant enhancement in critical heat flux (CHF) during pool boiling by tailoring interfacial phenomena. This CHF enhancement has been primarily explained by two structural effects: roughness, which extends the contact line length at the bubble base, and wickability, the ability to imbibe liquid through surface structures by capillary pumping. In this work, we show that CHF enhancement on structured surfaces cannot be described by roughness or wickability alone. This result was confirmed using systematically designed micropillar surfaces with controlled roughness and wickability. Further, we performed a scaling analysis and derived a unified descriptor, which represents the combined effects of thin film density and volumetric wicking rate. This unified descriptor shows a reasonable correlation with CHF values with our experiments and literature data. This work provides important insights in understanding the role of surface structures on CHF enhancement, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.

Effects of electric field on pool boiling heat transfer over microstructured surfaces under different liquid subcoolings

• The smooth surface's CHF enhancement by electric field ranges from 15.2% to 22.5%. • Microstructured surfaces’ CHF increment by electric field ranges from −3.9% to 19.2%. • The CHF of PF50-60 is decreased by the electric field at ΔTsub = 25 and 35 K. • The increase of CHF by electric field is closely related to the field trap effect. The uniform electric field is an effective method to improve boiling heat transfer performances, especially via breaking the mushroom bubbles and preventing dry-out; microstructured surfaces remarkably enhance both heat transfer coefficients and critical heat flux in boiling process. In the present study, the coupling effect of a uniform electric field and the microstructured surfaces on pool boiling in FC-72 at the liquid subcoolings (Δ T sub ) of 5 K, 15 K, 25 K, and 35 K was studied. Four kinds of the microstructured surfaces with different sizes of micro-pin-fins were fabricated and tested, and compared with a reference smooth surface. It was found that the critical heat flux (CHF) enhancement of the smooth surface due to the electric field ranges from 15.2% to 22.5% and increases with liquid subcooling. For the microstructured surfaces, the effects of electric field on CHF are closely related to the liquid subcooling and size of micro-pin-fins. For the surfaces with fin width of 30 μm, the increase of CHF due to the electric field is not sensitive to the liquid subcooling; rather, for the surfaces with fin width of 50 μm, the electric field CHF enhancement decreases with the liquid subcooling. The enhancement of CHF due to the electric field can be explained by the competition of a reduction of the Taylor length and the break of the vapor layer (beneficial to the CHF enhancement) and the field trap effect (not conducive to CHF improvement). The field trap effect on the surface with fin width of 50 μm becomes very strong at Δ T sub = 25 and 35 K; as a result, under these two conditions the CHF is reduced under the action of the electric field.

Microflow-Enhanced Bubble Dynamics Along With Gradient Porous Surfaces

Abstract Boiling heat transfer has been a popular topic for decades because of its ability to remove a significant amount of thermal energy while maintaining a low wall superheat during the liquid phase change. Such boiling mechanisms can be tailored by engineering new boiling substrates through surface wettability modification and/or microscale feature installation. Here, we create new types of heterogeneous boiling surfaces that integrate vertical gradient micropores on macroscale fins by using a template-free electrodeposition method. The gradient morphology and corresponding gradient wettability simultaneously enable bubble nucleation on the top pores and capillary wicking through the bottom pores. With these unique wetting characteristics, we find that the gradient pores installed at the trench bottom demonstrate the most significant boiling enhancement in critical heat flux and heat transfer coefficients by 160% and 600%, respectively. This enhancement can be attributed to the microflow-enhanced nature of bubble departures around the fins while isolating bubble nucleation and liquid supply through gradient pores. These results provide fundamental insights into boiling mechanisms using porous media and the potential for future works that can optimize the design of multidimensional heterogeneous surfaces to engineer flow patterns and boiling mechanisms accordingly.

Enhancement of critical heat flux (CHF) in pool boiling with anodized copper surfaces

Present work reports the creation of Cu(OH)2 nanoneedles on a copper surface using an anodization process for extending critical heat flux (CHF) limit in pool boiling applications. Three anodized samples have been prepared by applying three different current densities. Surface morphology and wetting characteristics of the anodized surfaces are presented. The characteristics of nanoneedles depend upon the applied current density. The formation of Cu(OH)2 nanoneedles during the anodization process is confirmed by phase analysis using XRD confirms. The formation of Cu(OH)2 nanoneedles increases the wettability and wickability of the anodized surfaces compared to the plain surface. At high current density, the anodized surfaces show nearly super hydrophilic behaviour. Due to enhanced wettability, the wall superheat required for nucleation is more, and nucleation site density is less for anodized copper surfaces. Bubble departure size is also more for anodized surfaces compared to the plain copper surface. The anodized surfaces show a higher CHF value compared to the plain copper surface. Rewetting of the boiling surface occurs by capillary wicking in the anodized surfaces, which delays the dryout phenomena resulting in CHF enhancement. A maximum enhancement of 39.74% in CHF is observed for the C-9 anodized surface compared to the plain copper surface.

Evaluation of nanofluids for enhancing critical heat flux for in-vessel retention during sever reactor accidents

ABSTRACT In-vessel retention (IVR) of a molten core through external cooling of the reactor vessel is a severe accident management strategy at nuclear power plants. The enhancement of critical heat flux (CHF) is one of the measures to improve the success probability of IVR. A nanofluid, in which nanoparticles are dispersed in a liquid, is known to reliably enhance CHF, mainly under pool boiling conditions. However, to apply nanofluids to IVR, it is necessary to clarify the effect of CHF enhancement under convection boiling similar to that in an actual IVR event. Therefore, CHF tests were conducted using a rectangular test section 50 mm wide × 50 mm deep × 600 mm long under conditions simulating severe accidents. From the CHF data obtained for various nanofluid parameters under typical severe accident conditions, each parameter’s effect on CHF enhancement was clarified. It was found that CHF was improved by 50% or more by a nanofluid as compared with deionized water. Furthermore, the CHF of nanofluid obtained in the tests simulating the actual severe accident conditions increased from 30% to 50% compared with that of deionized water, suggesting that CHF can be enhanced by nanofluids even in an actual plant.

Critical heat flux enhancement in microgravity conditions coupling microstructured surfaces and electrostatic field

We run pool boiling experiments with a dielectric fluid (FC-72) on Earth and on board an ESA parabolic flight aircraft able to cancel the effects of gravity, testing both highly wetting microstructured surfaces and plain surfaces and applying an external electric field that creates gravity-mimicking body forces. Our results reveal that microstructured surfaces, known to enhance the critical heat flux on Earth, are also useful in microgravity. An enhancement of the microgravity critical heat flux on a plain surface can also be obtained using the electric field. However, the best boiling performance is achieved when these techniques are used together. The effects created by microstructured surfaces and electric fields are synergistic. They enhance the critical heat flux in microgravity conditions up to 257 kW/m2, which is even higher than the value measured on Earth on a plain surface (i.e., 168 kW/m2). These results demonstrate the potential of this synergistic approach toward very compact and efficient two-phase heat transfer systems for microgravity applications.

Critical heat flux enhancement using composite porous structure produced by selective laser melting

• Uniform porous structures (with single pore size) and Composite porous structures（with two different pore sizes) are fabricated by selective laser melting. • Composite porous structure has the best heat transfer performance. • Critical heat flux enhancement of up to 3.2 times is achieved. • Gas-liquid flow characteristics of the composite porous structure is analyzed. Cross scale composite porous structures have been proved to have a better effect on enhanced boiling heat transfer. Accurate control of composite pore size and porous structure thickness is important for the study of enhanced boiling heat transfer. In this study, composite porous structures were manufactured with selective laser melting (SLM) technique from aluminum alloy (AlSi10Mg) powder. The heat transfer characteristics of a composite porous structure with composite pores (0.6 mm small pores and 2 mm large pores) for pool boiling under atmospheric pressure were investigated. A comparative analysis was performed with the uniform porous structure (0.6 mm pores) and the composite porous structure. The results shown that the composite porous structure can significantly increase the critical heat flux (CHF) compared to the plain copper surface. The CHF of the composite porous structure was higher than that of the uniform porous structure with the same height. As the height increases, the nucleate boiling heat transfer performance with composite porous structures will decrease, due to the larger gas–liquid resistance as a result of the height increment. In the range of 1.5–4.5 mm studied in this research, the 1.5 mm high composite porous structure achieves the maximum increase in CHF. The maximum CHF was 346.6 W/cm 2 , which was 3.2 times that of the plain surface. The large pores in the composite porous structure can play a role of shunting to reduce the escape resistance of the bubbles, which can further enhance CHF.

Effect of Nanofluids on the Enhancement of Boiling Heat Transfer: A Review

Boiling heat transfer can play a vital role in the two-phase flow applications. The analysis of the boiling hat transfer enhancement is of importance in such applications and the enhancement can be mostly conducted by using various active and passive techniques. One type of passive techniques is the enhancement of heat transfer by nanofluids. This article presents an extensive review on the effect of different nanofluids on the enhancement of heat transfer coefficient (HTC) and critical heat flux (CHF) for both pool as well as flow boiling. Nanoparticles addition to a working fluid is done arbitrarily to improve the thermophysical properties which in turn improves heat transfer rate. Numerous works have been done in the studies on nanofluid boiling. Among various nanoparticles, the most frequently used nanoparticles are Al2O3 and TiO2. In the case of binary nanoparticles, the most commonly used combination is Al2O3 and TiO2. After reviewing the relevant literatures, it is found that for pool boiling, the maximum HTC is increased to 138% for TiO2 nanoparticles and the maximum CHF is increased to 274.2% for MWCNTs. Conversely, in flow boiling the maximum HTC is increased to 126% for ZnO nanoparticles and the maximum CHF increased to as 100% for GO nanoparticles. In addition, when two or more nanoparticles in succession or binary nanofluids are used the CHF in pool boiling increased up to 100% for Al2O3 and TiO2 as well as the CHF in flow boiling increased up to 100% for Al2O3, ZnO, and Diamond. Though the information of the coefficient of heat transfer and the critical heat flux varied for different nanofluids and vary from experiment to experiment for each of the nanofluids. This variation happens because the coefficient of heat transfer and the critical heat flux in boiling is dependent upon several factors.

Method for calculating the critical heat flux at pool boiling on structured surfaces

A method of the critical heat flux enhancements at pool boiling using rough structures of various regular and irregular geometries is investigated. The experimental data are compared, and the critical heat flux is calculated since the V. V. Yagov model, which considers the additional inflow of liquid into the evaporation zone due to the action of capillary forces in the porous space. Based on the comparison of experimental and calculated data, it is concluded that the model is not universal for all microstructured surfaces. Recommendations for the calculation are given.

Flow boiling critical heat flux enhancement via different-mode-interacting boiling in narrow gaps

Flow boiling heat transfer has the potential to achieve the required phase-change performance cooling in narrow gaps. To further improve the critical heat flux (CHF) and heat transfer coefficient (HTC) in flow boiling, a method of different-mode-interacting boiling (DMIB) involving the interaction between neighboring boiling of different heat flux areas or different modes of nucleate and film boiling was utilized. We employed a nonuniform heating plate, in which copper and Polytetrafluoroethylene (PTFE) with different thermal conductances were alternately arranged near the upper surface to achieve a spontaneous nonuniform temperature distribution. Experiments were performed using a square heating surface (10 mm × 10 mm) with two material widths (W) of different materials. The results were compared with a uniform plate using deionized water as working fluid under the conditions of inlet subcooling ΔTsub = 2.0, 10, 20, and 30 K, flow velocity v = 0,1, 0.2, and 0.4 m/s, and gap size h = 1.0, 2.0, and 5.0 mm. The nonuniform surfaces of W = 0.5 and 1.0 mm are found to significantly enhance the CHF at all values of v, ΔTsub, and h. The enhancement of CHF and HTC was owing to the high efficiency of the DMIB and reasonable liquid supply on the nonuniform surfaces. In addition, v and ΔTsub had a significant effect on the CHF and HTC, whereas the trends were identical. The reduction in v and ΔTsub could increase the ratios of CHF, where the highest enhancement ratio achieved was 89% at h = 1.0 mm, ΔTsub = 2.0 K, and v = 0.1 m/s. Moreover, increments in v and ΔTsub led to higher HTC enhancements, whereas the highest ratio increased by a factor of 12 at h = 5.0 mm, ΔTsub = 30 K, and v = 0.4 m/s.

CHF enhancement of downward-facing saturated pool boiling on the SCGS-modified surfaces with multi-scale conical pin fin structures

Enhancing the critical heat flux (CHF) of external reactor vessel cooling (ERVC) to achieve the strategy of in-vessel retention (IVR) has been deemed as an essential requirement for the security of modern commercial PWR nuclear power plants. A new type of multi-scale conical pin fin structures had been designed to meet this need. These conical structures of four different axial heights (1.0, 1.5, 2.0 & 2.5mm) and similar radial geometric parameters were fabricated on four heating surfaces of copper substrates by supersonic cold gas spray technique (SCGS), and the conical pin fin structures were combined with a thin layer of micro porous structures. Transient-state (quenching) and steady-state (heating) pool boiling experiments had been respectively conducted on 6 different downward-facing surfaces under different inclination angles including 5°, 30°, 45°, 60°, 75° & 90° (in vertical), which included above-mentioned four pin fin surfaces, a plain surface and a coated surface covered with a 1mm-thick layer of uniform porous coating fabricated by SCGS with copper particles. The experimental results showed that the CHF increased with the inclination angles. The CHF of all the SCGS-modified surfaces were significantly enhanced compared with those of the plain surface. The maximum CHF value and the maximum CHF enhancement belonged to the 2.0mm-height pin fin surface whose CHF values were more than 1.7 times of those of the plain surface, and up to 2.1 times in 90°. The 1.0mm pin fin surface had the greatest performance in boiling heat transfer coefficient (HTC) and its CHF-enhancing performance also greatly exceeded that of 1.0mm uniform porous coating surface. Remarkable performances of multi-scale conical pin fin structures had been experimentally confirmed, hence the safety margin of IVR-ERVC will be greatly enhanced after adopting them. In addition, the CHF enhancement mechanism of the multi-scale conical pin fin structures was analyzed.

Study on boiling heat transfer of surfactant solution on grooved surface

In the present study, the coupling effect of wire-cutting grooved surface and surfactant (Sodium dodecyl sulfate, SDS) in pool boiling heat transfer is studied experimentally. Based on visualization, the bubble growth at the onset of bubble formation is measured, and the results are found to yield the reported empiric laws in the functional form of heat diffusion growth. In addition, the surfactant is found to improve the bubble nucleation on the heated surface with a shorter growth time and smaller departure diameter. Moreover, the grooved surface is also easier to nucleate than the smooth one, that is, the onset of bubble formation on the grooved surface is advanced, and thus the coupling effect further enhances the bubble nucleation. For the heat transfer, the SDS solution presents good heat transfer performance at low and medium heat flux in comparison to the DI water, and the coupling effect with the grooved surface would lead to further enhancement, by which the heat transfer coefficient (HTC) is maximally increased by about 238%. In addition, there are three main forms of bubbles found during the boiling of SDS solution, including small bubbles, discrete bubbles and large bubbles. The discrete bubbles are found to be formed by the entrainment of the upper foam due to the liquid convection. At relatively high heat flux, there is bubble crowding in the boiling of surfactant solution, accompanied by the expansion and contraction of bubble clusters and significant fluctuation of near-wall temperature. Furthermore, the bubble crowding seems to cause the premature dryout and the low critical heat flux (CHF), and could be attributed to the good nucleation ability and the repulsion between bubbles, which also result in the heat transfer enhancement of SDS solution in low and medium heat flux. Therefore, the enhancement of HTC and CHF cannot be achieved simultaneously in the present method.

Spray Cooling on Enhanced Surfaces: A Review of the Progress and Mechanisms

Abstract The rapid development of high-power electronic, energy, and propulsion systems has led us to the point where the performances of these systems are limited by their cooling capacities. Current electronics can generate heat fluxes up to 10–100 W/cm2, and heat flux over 1000 W/cm2 needs to be dissipated with a minimum coolant flow rate in next-generation power electronics. The multiple efficient heat transfer mechanisms have made spray cooling a high heat flux, uniform and efficient cooling technique proven effective in various applications. However, the cooling capacity and efficiency of spray cooling need to be further improved to meet the demands of next-generation ultrahigh-power applications. Engineering of surface properties and structures, which is enabled by state-of-the-art manufacturing techniques, can fundamentally affect the liquid–wall interactions in spray cooling, thus becoming the most promising way to enhance spray cooling. However, the mechanisms of surface-enhanced spray cooling are diverse and ambiguous, causing a lack of clear guiding principles for engineered surface design. Here, the progress in surface engineering-enhanced spray cooling is reviewed for surface structures of millimeter, micrometer, and nanometer scales and hierarchical structured surfaces, and the performances from the reviewed literature are evaluated and compared. The reviewed data show that spray cooling can achieve a critical heat flux (CHF) above 945.7 W/cm2 and a heat transfer coefficient (HTC) up to 57 W/cm2K on structured surfaces without the assistance of secondary gas flow and a CHF and an HTC up to 1250.1 W/cm2 and 250 W/cm2K, respectively, on a smooth surface with the assistance of secondary gas flow. A CHF enhancement up to 110% was achieved on a hybrid micro- and nanostructured surface. A clear map of enhancement mechanisms related to the scales of surface structures is proposed, which can help the design of engineered surfaces in spray cooling. Some future concerns are proposed as well. This work helps the understanding and design of engineered surfaces in spray cooling and provides insights for interdisciplinary applications of heat transfer and advanced engineering materials.

Numerical investigation on the effects of non-gaussian random and regular textured rough surface on critical heat flux

Modified surfaces with rough structure have been widely employed to enhance the critical heat flux (CHF) in the heat transfer engineering. Since any structure diagram in its intersecting surface can be treated as spectrum/signal in 2D dimensions, and any spectrum in time domain and in frequency domain can be transformed into sine transforms by the Fourier. Thus, in this article, the effects of the non-gaussian random rough surface and the ordered sine function structured surface were investigated on the CHF enhancement using the fast Fourier transform (FFT) method and the hybrid thermal lattice Boltzmann method (LBM). It finds out that for the non-gaussian random rough surface, CHF is enhanced with decreasing the skewness of the surface. The enhancement seems limited with further reduced the skewness. Besides, the CHF enhancement will be improved by increasing surface wettability. For regular textured surfaces, increasing either the amplitude or the periodicity of the surface structures initially increases the CHF and then the enhancement is impeded with further increase. It concludes that there exists an optimal design of the surface morphology with either non-gaussian random structure or the ordered sine function structure for the CHF enhancement in pool boiling.

Burnout Investigation of Small Diameter Tubes Immersed in Nanofluids

This paper deals with research into pool boiling critical heat flux (CHF) of water–Al2O3, water–TiO2 and water–Cu nanofluids on horizontal stainless steel tubes. The experiments were conducted under atmospheric pressure. Nanoparticles were tested at concentrations of 0.001%, 0.01%, 0.1% and 1% by weight. Ultrasonic vibration was used in order to stabilize the dispersion of the nanoparticles. Although dispersants were not used to stabilize the suspension, the solutions tested showed satisfactory stability. Experimental measurements were performed with stainless steel tubes of three outside diameters: 1.6, 3 and 5 mm. Enhancement of CHF was observed to be independent of the concentration and material of the nanoparticles and tube diameter, with simultaneous heat transfer degradation. Built up during the boiling process, nanolayers improve substantially the heating surface wettability. A correlation is suggested for the CHF prediction during pool boiling of nanofluids.