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Experimental study on CHF enhancement of different oxidized surfaces of low carbon steel in nanofluid
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An experimental study was carried out to investigate the effects of heat transfer surface orientation and the solid–liquid
contact angle on the boiling heat transfer and critical heat flux (CHF) in water pool boiling using a smooth heat-transfer
surface under atmospheric pressure. The orientation angle was ranged from 0° (up-facing horizontal position) to 180° (down-facing
horizontal position) with a pace of 45°. The three kinds of heat transfer surfaces having different solid–liquid contact angles
were the normal surface with a contact angle of 55°, the hydrophilic surface with a contact angle of 30° and the superhydrophilic
surface with a contact angle of 0°. The experimental results indicate that orientation and contact angle have complex, coupling
effects on heat transfer and CHF. A predicting correlation for the CHF which takes the effects of both orientation and contact
angle into account is established. The predicting correlation agrees reasonably well with the experimental data.
Studies about using nanofluids to enhance the Critical Heat Flux (CHF) of In-vessel Retention (IVR) strategy in the third-generation reactor have been conducted extensively and show a significant CHF enhancement effect. However, low carbon steel SA508 used in the reactor vessel is easy to oxidize and the oxidation can lead to changes in the surface properties which may affect the CHF enhancement effect of nanofluids. In this study, pool boiling CHF experiments with low carbon steel SA508 surfaces were conducted in distilled water and nanofluids under different boiling time to investigate the CHF enhancement effect of nanofluids under low carbon steel surface oxidization condition. CHF in distilled water increases rapidly with boiling time due to the rapid surface oxidation during the boiling and the increase ratio can be nearly 2 due to the surface oxidation. CHF in nanofluids is stable and independent of boiling time. The difference between CHF in nanofluids and CHF in distilled water decrease to 17% under the longest boiling time conditions due to the surface oxidation. The deposition layer of nanoparticles on the surface leads to the capillary wicking and decrease in the nucleation site and thus, CHF is enhanced. This study is of great significance for exploring the actual effect of nanofluids on the CHF enhancement of IVR strategy.
External reactor vessel cooling (ERVC) is the key technology for In-Vessel Retention (IVR) to ensure the safety of a nuclear power plant (NPP) under severe accident conditions. The thermal margin of nucleate boiling heat transfer on the reactor pressure vessel (RPV) lower head is important for ERVC and of wide concern to researchers. In such boiling heat transfer processes, the reactor vessel wall inclination effect on the heat transfer coefficient (HTC) and critical heat flux (CHF) should be considered. In this study, experiments were performed to investigate the effects of surface orientation and heater material on the HTC and CHF of nucleate boiling. Copper (Cu), stainless steel (SS) as well as prototype material (SA508) of the reactor pressure vessel in the nuclear power plant were used to perform boiling tests under atmosphere pressure, respectively. The orientation angle of all boiling surfaces were varied between 0° (upward) and 180° (downward). The experimental results show that the HTC is enhanced for the downward heater surfaces (φ>90°) under the low heat flux conditions and the enhancement ratio decreases with the increase of the heat flux. In addition, the relationship of measured CHF value with the orientation angle was obtained and it shows that the CHF value changes little as the inclination angle increases from 0° to 120°, but it decreases rapidly as the orientation angle increases towards 180° for all boiling surfaces. The material effect, which indicated by thermal effusivity ρcpk, on CHF is also investigated that the surface with larger thermal effusivity has the higher CHF value. In addition, the SA508 surface has the highest CHF value comparing with those of Cu surface and SS surface due to surface corrosion in the boiling process. Based on the experimental data, a correlation for CHF prediction is developed which includes the effects of surface orientation, thermal effusivity and corrosion.
The present paper is an overview of the latest developments regarding the application of nanofluids in boiling critical heat flux by means of both pool-and convective-flow boiling. Boiling heat transfer is a significant field in thermal engineering systems, and it is especially used for boiling in power plants as well as the cooling of nuclear reactors and high-tech electronic systems. This concise review contains efforts to show how nanofluids could play an essential role in achieving high heat flux with small temperature differences during the boiling process which , in turn, improves the critical heat flux (boiling crisis) for such an operation and makes the heat-exchange system's performance safer and more durable. It is also expected that this work could be a helpful new reference guide that will allow investigators to update their knowledge on the topic of boiling critical heat flux using nanofluids. In addition, this work contains concise recommendations for future study directions.
In vessel retention (IVR) by passive external reactor vessel cooling (ERVC) under severe accidents is a feasible approach to retain radioactive core melt within the reactor vessel. The effectiveness of IVR by external reactor vessel cooling strongly depends on the critical heat flux (CHF). As long as the local heat flux does not exceed the local CHF in the vessel, the lower head can be cooled sufficiently to prevent its failure. In this study, the FIRM subcooled flow boiling facility conducted by State Nuclear Power Technology Research & Development Center (SNPTRD) was built to simulate the IVR-ERVC condition for Chinese Advanced Passive 1400MWe PWR, as well as to verify and validate the safety margin of CHF during severe accidents. To investigate the effects of the heater surface material, coolant additives and other thermal hydraulic parameters such as flow rate and subcooling on CHF, flow boiling CHF experiments using a full scale 2-D curved test section under atmospheric pressure with SA508 Gr3 carbon steel heater as well as coolant additives of trisodium phosphate (TSP, Na3PO4) and boric acid (BA, H3BO5) with different concentrations were performed. The results showed that SA508 material displayed quite different CHF behavior in comparison with other materials such as stainless steel, copper and aluminum. It showed that higher CHF value and the test heater surfaces were changed significantly. The test heater surface change was due to the corrosion of SA508, and the rate of corrosion increased with boiling time. CHF values showed a little reduction with increasing the concentrations of BA. In case of TSP, CHF values were enhanced with lower concentrations of 500ppm and 1000ppm due to the increased wettability of coolant, while it was reduced with 3500ppm probably due to the instability of two phase flow and the preventing effect on SA508 corrosion. CHF values for the mixed solutions of BA and TSP showed the similarity with the case of TSP, which indicated that TSP played more important effect on CHF behavior.
Nanofluid boiling is an important research area of nanofluids, which provides many opportunities to explore new frontiers but also poses great challenges. This paper presents a comprehensive review on the nanofluid heat transfer (HT) and critical heat flux (CHF) of pool boiling and flow boiling. The research results in the literature previously reviewed are briefly summarized. An emphasis is put on the recent progresses in the nanofluid HT and CHF of pool boiling and flow boiling. It is also included comparing developments and research results of the nanofluid HT and CHF between pool boiling and flow boiling. The important achievements, inconsistence, and contradictions of the existing research results are identified and discussed in detail. Topics worthy of attention for future studies are suggested.
In this study, the flow boiling critical heat flux (CHF) using graphene oxide (GO)/water nanofluid was investigated under low pressure and low flow conditions. The 0.01 vol.% GO/water nanofluid is prepared for CHF enhancement test because recently, there are a lot of interests about graphene as an exceptional heat conduction material for thermal management and GO nanoparticles are more dispersed in water than graphene nanoparticles in terms of hydrophilicity. All experiments were carried out for round tubes with 1/2 in. diameter and 0.5 m heating length under low pressure and low flow (LPLF) at two fixed inlet temperatures (25 and 50 degrees C) and at four different mass fluxes (100, 150, 200 and 250 kg/m(2) s). It was found that the CHF of the GO/water nanofluid was more enhanced up to similar to 100% than the CHF of water as a base fluid. The causes of CHF enhancement were investigated through macroscopic observations, SEM observations and measurement of contact angles of the heated surfaces with depositions. Liquid film thickness affected by evaporation, entrainment and deposition mass transfer can be closely linked with wettability and GO properties.
In recent years, nanofluids have been attracting significant attention in the heat transfer research community. These fluids are obtained by suspending nanoparticles having sizes between 1 and 100 nm in regular fluids. It was found by several researchers that the thermal conductivity of these fluids can be significantly increased when compared to the same fluids without nanoparticles. Also, it was found that pool boiling critical heat flux increases in nanofluids. In this paper, our objective is to evaluate the impact of different nanoparticle characteristics including particle concentration, size and type on critical heat flux experimentally at saturated conditions. As a result, this work will document our experimental findings about pool boiling critical heat flux in different nanofluids. In addition, we will identify reasons behind the increase in the critical heat flux and present possible approaches for analytical modeling of critical heat flux in nanofluids at saturated conditions.
Nanofluids, colloidal dispersions of nanoparticles, exhibit a substantially higher critical heat flux (CHF) compared to water. As such, they could be used to enhance the in-vessel retention (IVR) capability in the severe accident management strategy implemented by certain light-water reactors. It is envisioned that, at normal operating conditions, the nanofluid would be stored in dedicated storage tanks, which, upon actuation, would discharge into the reactor cavity through injection lines. The design of the injection system was explored with risk-informed analyses and computational fluid dynamics. It was determined that the system has a reasonably low failure probability, and that, once injected, the nanofluid would be delivered effectively to the reactor vessel surface within seconds. It was also shown analytically that the increase in decay power removal through the vessel using a nanofluid is about 40%, which could be exploited to provide a higher IVR safety margin or, for a given margin, to enable IVR at higher core power. Finally, the colloidal stability of a candidate alumina-based nanofluid in an IVR environment was experimentally investigated, and it was found that this nanofluid would be stable against dilution, exposure to gamma radiation, and mixing with boric acid and lithium hydroxide, but not tri-sodium phosphate.
Nanofluids are colloidal dispersions of nanoparticles in homogenous base fluids. Previous studies have shown that nanofluids can increase pool boiling critical heat flux (CHF) by forming a porous deposition on the heated surface. However, questions remain whether nanoparticles can further enhance the CHF on a passively engineered heat transfer surface, such as a sandblasted metal plate. In this study, three water-based nanofluids (diamond, zinc oxide and alumina) were used to modify sandblasted stainless steel 316 plate heaters via boiling induced deposition. The pool boiling CHF of these pre-coated heaters increased by up to 35% with respect to that of the bare, sandblasted heaters. The enhancements are highest for alumina and zinc oxide nanofluids. Detailed surface characterization of these pre-coated heaters showed different surface morphology depending on the type of nanofluids used. Additionally, the deposited nanoparticles layers were found to alter the wettability of the heaters. Contact angle measurement provided quantitative data to determine possible CHF enhancement based on existing correlations.
Boiling heat transfer characteristics of nano-fluids with nano-particles suspended in water are studied using different volume concentrations of alumina nano-particles. Pool boiling heat transfer coefficients and phenomena of nano-fluids are compared with those of pure water, which are acquired on a smooth horizontal flat surface (roughness of a few tens nano-meters). The experimental results show that these nano-fluids have poor heat transfer performance compared to pure water in natural convection and nucleate boiling. On the other hand, CHF has been enhanced in not only horizontal but also vertical pool boiling. This is related to a change of surface characteristics by the deposition of nano-particles. In addition, comparisons between the heat transfer data and the Rhosenow correlation show that the correlation can potentially predict the performance with an appropriate modified liquid-surface combination factor and changed physical properties of the base liquid.