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Experimental study on the CHF enhancement effect of nanofluids on the oxidized low carbon steel surface
Abstract
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.
... Detailed descriptions of the experimental setup were reported in publications of our group (e.g., [17]). Hence, only a brief overview will be presented here. ...
... Energies 2024,17, 5849 ...
The nucleation and growth of bubbles on homogeneous wetting surfaces have been extensively studied, but the intricate dynamics on hybrid wetting surfaces remain under-explored. This research aims to elucidate the impact of hybrid wettability on pool boiling heat transfer efficiency, specifically under downward-facing heating conditions. To this end, a series of hybrid wettability surfaces with varying hydrophilic and hydrophobic configurations are meticulously fabricated and analyzed. The study reveals distinctive interfacial phenomena occurring at the boundary between hydrophilic and hydrophobic regions during the boiling process. Experimental results indicate that surfaces with a higher proportion of hydrophilic to hydrophobic interfaces exhibit reduced superheat requirements and enhanced boiling heat transfer coefficients for equivalent heat flux densities. Furthermore, the rewetting characteristics of hybrid wettability surfaces are identified as pivotal factors in determining their critical heat flux (CHF). This investigation underscores the potential of hybrid wettability surfaces to optimize pool boiling heat transfer, offering valuable insights for the design and en-hancement of heat exchangers and other thermal management systems.
... Furthermore, they have found HTC for 0.1% hybrid nanofluid to be lower than other nanofluids under the same heat flux and have attributed this to a reduction in surface roughness and nucleation site density owing to an increased deposition of nanoparticles on the boiling surface in a nanofluid with higher concentration of nanoparticles. Wang et al. [85] are the latest to have researched on the effect of Al 2 O 3 nanoparticles on the CHF of low-carbon steel SA508, a crucial component of the nuclear reactor vessel. They have employed a 1 mm thick downwardfacing test section (25 Â 25 mm 2 ) with 15 degrees inclination to the horizontal as the heating element in all experiments. ...
... Variation in CHF values with boiling time for water and nanofluid[85]. ...
... In other words, CHF is the maximum heat flux that one surface can handle before experiencing a transition from nucleate boiling to film boiling [11]. When the heat transfer system, especially cooling systems, cannot dissipate the produced heat load effectively, a temperature increase happens, leading to a thermal runaway, failure, or low-efficiency condition [12,13]. By accurately predicting CHF under various conditions, our model aids in the design and optimization of cooling systems, ensuring they operate within safe thermal margins while maximizing heat dissipation. ...
The rising demand for advanced energy systems requires enhanced thermal management strategies to maximize resource utilization and productivity. This is quite an important industrial and academic trend as the efficiency of energy systems depends on the cooling systems. This study intends to address the critical need for efficient heat transfer mechanisms in industrial energy systems, particularly those relying on pool boiling conditions, by mainly focusing on Critical Heat Flux (CHF). In fact, CHF keeps a limit in thermal system design, beyond which the efficiency of the system drops. Recent research materials have highlighted nanofluids’ superior heat transfer properties over conventional pure fluids, like water, which makes them a considerable substitution for improving CHF in cooling systems. However, the broad variability in experimental outcomes challenges the development of a unified predictive model. Besides, Machine Learning (ML) based prediction has shown great accuracy for modeling of the designing parameters, including CHF. Utilizing ML algorithms—Cascade Forward Neural Network (CFNN), Extreme Gradient Boosting (XGBoost), Extra Tree, and Light Gradient Boosting Method (LightGBM)— four predictive models have been developed and the benchmark shows CFNN’s superior accuracy with an average goodness of fit of 89.32%, significantly higher than any available model in the literature. Also, the iterative stability analysis demonstrated that this model with a 0.0348 standard deviation and 0.0268 mean absolute deviation is the most stable and robust method that its performance minorly changes with input data. The novelty of the work mainly lies in the prediction of CHF with these advanced algorithm models to enhance the reliability and accuracy of CHF prediction for designing purposes, which are capable of considering many effective parameters into account with much higher accuracy than mathematical fittings. This study not only explains the complex interplay of nanofluid parameters affecting CHF but also offers practical implications for the design of more efficient thermal management systems, thereby contributing to the broader field of energy system enhancement through innovative cooling solutions.
... However, the nanofluids exhibited increased CHF and HTC at different flow rates. The presence of nanoparticles in the nanofluids facilitated the replenishment of the dry area under the bubbles due to the wicking effect [32,33]. Moreover, these nanoparticles delayed the coalescence of bubbles (Fig. 6d) [19,20], thereby boosting CHF. ...
... The boiling performance was discovered to be enhanced by using NFs in many studies [10,11], and several recent studies have been conducted to assess the boiling behavior of various NFs [12]. For example, the use of Al 2 O 3-dispersed MNF in the PB of in-vessel retention was shown to improve the CHF during the boiling period [13]. Developments in the NF field allowed the discovery of new possible types of NFs, such as HNFs, by combining two types of NPs in their preparation, providing more advantages for their adjustable and advanced thermophysical properties [14]. ...
This study reports an experimental investigation of pool boiling (PB) heat transfer performance of hybrid (two types of particles) and mono (single-particle) nanofluids consisting of Boron nitride (BN) and Silicon dioxide (SiO2) nanoparticles (NPs). While hybrid nanofluids (HNFs) were prepared in a total particle concentration of 0.05 vol.% with four different percentages of these two types of NPs (are 0.01/0.04, 0.02/ 0.03, 0.03/0.02, and 0.04/0.01 (BN vol.%/SiO2 vol.%)), two mono nanofluids (MNFs) of BN and SiO2 nanoparticles were prepared at the same total concentration of 0.05 vol.% for each NP type. Both nanofluids (NFs) were prepared in the base fluid (BF), which is the mixture of 15 vol.% of ethylene glycol (EG) and 85 vol.% of distilled water (DW). Then, the boiling heat transfer performance of these MNFs and HNFs was assessed by experimentation in a pool boiling test rig. The obtained results demonstrated good improvements in critical heat flux (CHF) and burnout heat flux (BHF) of both types of NFs. The CHF increased by up to 80% for BN-based MNF and up to 69% for HNF at 0.04 vol.% BN, which is the maximum percentage of BN into HNF, while the lowest improvement in CHF was 48% for the SiO2-based MNF compared to the BF. Similarly, the BHF was found to increase with the increasing in the loading of BN nanoparticles and a maximum enhancement of BHF of 103% for BN-based MNF was observed. These HNFs and MNFs exhibited significantly improved pool boiling heat transfer performance compared to this BF, and it became lower by increasing the percentage of SiO2 NPs in the HNFs.
... Other secondary effects have been investigated, such as heating element boiling time [9,18,29], boiling at different heat fluxes [18,29], and boiling in different shapes, such as wires, ribbons, flat surfaces, etc. [27,30]. Those characteristics also have secondary effects on NF pool boiling CHF. ...
Nanofluid (NF) pool boiling experiments have been conducted widely in the past two decades to study and understand how nanoparticles (NP) affect boiling heat transfer and critical heat flux (CHF). However, the physical mechanisms related to the improvements in CHF in NF pool boiling are still not conclusive due to the coupling effects of the surface characteristics and the complexity of the experimental data. In addition, the current models for pool boiling CHF prediction, which consider surface microstructure characteristics, show limited agreement with the experimental data and do not represent NF pool boiling CHF. In this scenario, artificial intelligence tools, such as machine learning (ML) regressor models, are a very promising means of solving this nonlinear problem. This study focuses on creating a new model to provide more accurate NF pool boiling CHF predictions based on pressure, substrate thermal effusivity, and NP size, concentration, and effusivity. Three ML models (supporting vector regressor—SVR, multi-layer perceptron—MLP, and random forest—RF) were constructed and showed good agreement with an experimental database built from the literature, with MLP presenting the highest mean R2 score and the lowest variability. A systematic methodology for optimizing the ML models is proposed in this work.
The use of nanofluids, which are aqueous fluids containing nanoparticles, facilitates the ability to increase the critical heat flux (CHF) of a system. This study conducted pool boiling experiments at 50 °C and atmospheric pressure to examine the CHF features of CuO nanoparticles, Al2O3 nanoparticles, and a hybrid of Al2O3 and CuO colloids. Ni–Cr wire is used as the primary heating element. The concentrations of nanoparticles in the base fluid varied between 0.1 and 0.5 percent by weight, with each increase representing a doubling of the previous value. From a low of roughly 2%, the critical heat flow climbed by around 19% in each of the test conditions. The appearance of a nanofluid on the heating surface, which enhances surface wettability, is associated with an increase in CHF. The accumulation of nanoparticles on the surface of the heater also contributes to the increase in CHF. After attaining a peak at 17% at 0.2 weight%, the rate of CHF enhancement shown by hybrid nanofluid began to fall. These findings have important applications in thermal management systems used in nuclear reactors, electronics cooling, and other high-heat flow scenarios.
This investigation reports on the experimental outcomes of the pool boiling heat transfer characteristics, specifically on the downward heated surface, concerning reverse osmosis water and γ -Fe 2 O 3 nanofluids. To conduct the pool boiling experiments, γ -Fe 2 O 3 nanofluids were prepared with variable concentrations ranging from 2 mg/L to 10 mg/L. Analysis of the experimental data revealed that a concentration of 5 mg/L yielded the greatest enhancement effect on critical heat flux (CHF), with an increase of 13.5 %. However, the results also indicated that excessively high concentrations of nanofluid had a negative impact on CHF enhancement. The impact of nanofluids on heat transfer performance was investigated by analyzing the observed bubble behavior during the boiling process, measuring the drop angle and surface roughness post-experiment, and characterizing the heated surface morphology via scanning electron microscopy (SEM). Through these methods, the underlying mechanism behind the impact of nanofluids on heat transfer performance was identified and analyzed.
This paper reports an experimental investigation of the heat transfer features of new and recycled Alumina (Al2O3) nanofluids (NFs) in the pool boiling (PB) system. The mixture of ethylene glycol (EG) and distilled water (DW) is selected as the base fluid (BF), and NFs samples of two low concentrations (0.01 and 0.05 vol.%) of Al2O3 nanoparticles were prepared. Furthermore, the characteristics of the prepared NFs are evaluated to investigate the heat transfer performance as well as the reusability of the NFs for long-term applications and recycling consideration. Although there have been a large number of boiling studies with NFs, the current study is the first of its kind that addresses the mentioned operation conditions of recycling NF samples. The results are compared with the relevant BF in terms of properties, critical heat flux (CHF), burnout heat flux (BHF), and the convection coefficient of the Al2O3 NFs in the PB system. The results showed good enhancements in both CHF and BHF of these NFs yielding up to 60% and 54% for BHF at 0.05 vol.%, respectively. The reusage of the previously used (recycled) Al2O3 NF showed a considerable increase in heat transfer performance compared to base fluids but slightly lower than the newly prepared one. The results of the reused nanofluids demonstrate the great prospects of their recyclability in heat transfer systems and processes such as in pool boiling.
The Critical Heat Flux (CHF) of water with dispersed alumina nanoparticles was measured for the geometry and flow conditions relevant to the In-Vessel Retention (IVR) situation which can occur during core melting sequences in certain advanced Light Water Reactors (LWRs). CHF measurements were conducted in a flow boiling loop featuring a test section designed to be thermal-hydraulically similar to the vessel/insulation gap in the Westinghouse AP1000 plant. The effects of orientation angle, pressure, mass flux, fluid type, boiling time, surface material, and surface state were investigated. Results for water-based nanofluids with alumina nanoparticles (0.001% by volume) on stainless steel surface indicate an average 70% CHF enhancement with a range of 17% to 108% depending on the specific flow conditions expected for IVR. Experiments also indicate that only about thirty minutes of boiling time (which drives nanoparticle deposition) are needed to obtain substantial CHF enhancement with nanofluids.
When nanofluids are boiled, nanoparticles are deposited on the heater surface, causing a significant critical heat flux (CHF) enhancement. The authors examined the effect of the surface wettability and the capillarity of the nanoparticle deposition layer on CHF. It is well known that the deposition of nanoparticles changes the surface wettability, but it also causes capillary wicking on a porous surface, whereby the supplied liquid effectively delays the irreversible growth of a dry patch. This study demonstrates that the outstanding CHF enhancement in nanofluids is the consequence of both the improved surface wettability and the capillarity of the nanoparticle deposition layer.
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.
Heat transfer performance of downward-facing flat heaters is summarized in this paper. The combined effects of material, orientation, dimension and pressure are considered. According to the overall trends, the results show complicated tendencies in which the parameters are interrelated. With the heat transfer characteristics, some CHF trends that have been observed in previous studies are analyzed. Some transitional phenomena occur with orientation, and a related analysis is conducted using a dry-spot concept. Various environments like pressurized conditions are considered in this study, including real accidental conditions. For the material effect, a real material, carbon steel, used for the severe accident mitigation strategies is considered. The overall heat transfer performance at the most critical locations of the severe accident strategies can be extracted from the results.
The in-vessel retention (IVR) strategy is an effective method to keep the safety of reactor core by external reactor vessel cooling (ERVC) when severe accident occurs. One of the most important concerns is the characteristics of the critical heat flux (CHF) on the outside wall of reactor lower head. In this work, several influence factors on saturated pool boiling CHF for downward facing heating surface were studied, including the type of working fluid, orientation angle and wettability of heating surface. Experimental results indicated that CHF increases with increase of orientation angle of heating surface. It also indicated that both Al2O3/H2O and SiO2/H2O nanofluids could improve CHF dramatically compared to deionized water (maximum enhancement rate is 55.5%), but the CHF value of two types of nanofluids is similar to each other, which means type of nanoparticles may has just little influence on CHF. Besides, mechanism of nanofluids enhancing CHF were investigated by measuring the frequency of vapor waves on different heating surface (with and without porous layer). Results showed that the frequency of vapor waves decreases with the increase of heating surface wettability. Furthermore, a new CHF predicted correlation which could fit experimental data well and with excellent applicability was proposed based on experimental results.
CHF of downward-facing flat plate have been measured at various angle conditions in this study. Flat plates were heated in a saturated pool of water under atmospheric pressure. Among various factors, widths and lengths have been assessed for inclined flat rectangular surfaces, from horizontally downward to vertical positions. A high-speed camera was used to analyze the effects. Heaters showed no effect of length, but as width increased, the thermal limit value decreased accordingly. Between relatively narrow and wide widths, vapor motions were different, as shown in the pictures captured by the high-speed camera. As the width increased continuously, there was a width scale beyond which the results converged into each other. In addition, CHF values for the slightly tilted surfaces (75 deg in this study) showed higher results than that of vertical positions for 100 mm-length of stainless steel.
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 severe accident mitigation approaches that aim to achieve In-Vessel Retention (IVR) the decay heat is removed from the corium by conduction through the Reactor Pressure Vessel (RPV) wall, and by flow boiling on the outer surface of the RPV. The boiling Critical Heat Flux (CHF) limit must not be exceeded to prevent RPV failure. Previous studies for prediction of CHF in IVR were predominantly based on data for stainless-steel heaters and de-ionized (DI) water coolant. However, the RPV is made of low-carbon steel, and its surface has an oxide layer that results from pre-service heat treatment as well as oxidation during service; this oxide layer renders the surface much more hydrophilic and rough with respect to an un-oxidized stainless-steel surface, which can have a significant influence on boiling heat transfer. In this study, test heaters were fabricated from low-carbon steel (grade 18MnD5), pre-oxidized in a controlled, high-temperature, humid-air environment, reproducing the prototypical surface oxides present on the outer surface of the RPV. The heaters were then tested in a flow boiling loop using the IVR water chemistry, i.e., DI water with addition of boric acid and sodium tetra-borate. CHF was measured in the range of pressures (100–440 kPa), mass fluxes (180–2450 kg/m² s), inclination angles (30–90°) and equilibrium qualities (from −0.020 to +0.034) encompassing the IVR conditions. Up to 70% enhancement in CHF values was observed for pre-oxidized, low-carbon steel heaters in comparison to the stainless-steel control heaters. The effect of water chemistry on the CHF was found to be marginal. An empirical correlation fitting the CHF data for pre-oxidized, low-carbon steel surfaces with IVR water chemistry is also presented.
In previous research on nuclear reactors, the effect of irradiation on heat transfer has rarely been studied. We investigated the electron beam irradiation effect on downward-facing flow boiling heat transfer and critical heat flux. All the critical heat flux values in flow boiling decreased after irradiation, particularly at low doses, whereas heat transfer coefficient almost remained the same. Low dose electron beam irradiation substantially increased the nucleation sites on the heated copper surface, and the critical heat flux was inversely related to the nucleation site density in downward-facing flow boiling.
Carbon steel is used as a heating material in severe accident mitigation components, such as in in-vessel retention through the external reactor vessel cooling (IVR-ERVC) and core catchers strategies, since the reactor pressure vessel and some core catchers in nuclear power plants are made of this material. During a severe accident, molten corium is located on the down plenum or core catcher facility, depending on the strategy; through coolant flow at the downward face that is induced by natural circulation, decay heat is removed. The effect of carbon steel in downward-facing conditions has been considered in previous studies, and the critical heat flux (CHF) enhancement was mainly explained by surface morphology changes induced by corrosion. In this study, SA508 Gr.3 Cl.1 has been used in various orientation and width conditions. The effect of width was evaluated; the burnout point tended to decrease with increasing width. For the assessment of the effect of heater material, the phenomena could be divided into two conditions: narrow and wide. As width size decreased, the CHF enhancement ratio increased for carbon steel compared with stainless steel heaters, especially at high inclination angles. At low angles, the effects of both width and material tended to decrease. The effect of oxidation was assessed with various boiling schemes, static contact angles and dissolved oxygen concentrations in the coolant used for the experiments. Based on the test results, other contributing factors, such as the thermal properties of carbon steel, should also be considered to explain the enhancement.
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.
In-vessel retention of molten corium through external reactor vessel cooling (IVR-ERVC) is a severe
accidents management strategy. FIRM(Key Factor of Improving ERVC-CHF experiMent) facility which
is a two dimensional full scale model is built to investigate CHF behavior with SA508 Grade 3 Class 1 as
surface material of the heater.
CHF behaviors with SA508 steel (that is the prototype surface material of RPV) as the surface material
are quite different from those with copper or stainless steel as the surface material. By far there is few
experimental studies especially of the full scale consider the surface material effect. CHF behaviors are
investigated at atmospheric pressure in deionized water a with real surface material in present study in
FIRM facility. Preliminary results are obtained and influences of key factors like inclining angle of the
surface on CHF are discussed. The aging effect of SA508 steel in DI water is invstigated. The test result
will provide a profound understanding of CHF behaviors with real RPV material under IVR-ERVC
condition.
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.
The critical heat flux (CHF) in the vicinity of an inclination angle of 90° for the reactor vessel lower head external wall was measured on a downward facing curved surface. Two test sections having radii of curvature 0.15 m and 0.5 m were used. The objective was to investigate the effect of heater material and the combined effect of the heater material and additives on flow boiling CHF to assess the CHF enhancement under accident conditions. The heater material SA508 (low alloy steel) and the additive solutions of boric acid and tri-sodium phosphate (TSP, Na3PO4·12H2O) were used. An enhancement of CHF with the SA508 heater was confirmed in comparison with stainless steel reference heaters, which have negligible steel oxidation. As a result of the combined effect tests, the CHF with a TSP solution was reduced and the CHFs with a boric acid and a mixed solution (boric acid and TSP) were enhanced in comparison with the deionized water reference case. The CHF results are discussed in terms of steel oxidation according to the pH of the working fluid. Steel oxidation is also affected by local flow conditions as shown in the R = 0.5 m tests in which the boric acid and mixed solution had negligible effects on CHF enhancement. Under a relatively high concentration of boric acid (2.5 wt%), additive deposition as well as steel oxidation were observed and resulted in CHF enhancement.
As a novel strategy to improve heat transfer characteristics of fluids by the addition of solid particles with diameters below 100 nm, nanofluids exhibit unprecedented heat transfer properties and are being considered as potential working fluids to be used in high heat flux systems such as electronic cooling systems, solar collectors, heat pipes, and nuclear reactors. The present paper reviews the state-of-the-art nanofluid studies on such topics as thermo-physical properties, convective heat transfer performance, boiling heat transfer performance, and critical heat flux (CHF) enhancement. It is indicated that the current experimental data of nanofluids thermal properties are neither sufficient nor reliable for engineering applications. Some inconsistent or contradictory results related to thermo-physical properties, convective heat transfer performance, boiling heat transfer performance, and CHF enhancement of nanofluids are found in data published in the literature. No comprehensive theory explains the energy transfer processes in nanofluids. To bridge the research gaps for nanofluids' engineering application, the urgent work are suggested as follows. (1) Nanofluid stability under both quiescent and flow conditions should be evaluated carefully; (2) A nanofluid database of thermo-physical properties, including detailed characterization of nanoparticle sizes, distribution, and additives or stabilizers (if used), should be established, in a worldwide cooperation of researchers; (3) More experimental and numerical studies on the interaction of suspended nanoparticles and boundary layers should be performed to uncover the mechanism behind convective heat transfer enhancement by nanofluids; (4) Bubble dynamics of boiling nanofluids should be investigated experimentally and numerically, together with surface tension effects, by considering the influences of nanoparticles and additives if used, to identify the exact contributions of solid surface modifications and suspended nanoparticles to CHF enhancement in boiling heat transfer. Once we acquire such details about the above key issues, we will gain more confidence in conducting application studies of nanofluids in different areas with more efficiency.
In this study, pool boiling CHF experiments under atmospheric pressure with SA508 test heater was performed. SA508 is the material of the reactor pressure vessel in a nuclear power plant. Therefore, the CHF behavior of SA508 material is important for the reactor pressure vessel integrity at accident, but there are few studies about that. SA508 material showed very different CHF behavior from other cases (stainless steel). It showed very higher CHF value and the test heater surface was changed significantly. This test heater surface change is due to corrosion of SA508, and the rate of corrosion increases with boiling time. The corrosion product is analyzed as magnetite (Fe3O4), and magnetite has been used as one of nanofluid for CHF enhancement. The size of magnetite produced on the test heater surface is 20–140 nm, that is, it is nanoscale. Therefore, the CHF enhancement mechanism of SA508 material is similar to the case using other test heater (like stainless steel) with magnetite nanofluid. Additives like TSP, Al2O3 and CNT nanoparticles are also tested, but they did not show the CHF enhancement effect. In case of TSP, it showed CHF decrease effect due to preventing effect on SA508 corrosion.
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.
The pool boiling characteristics of dilute dispersions of alumina, zirconia and silica nanoparticles in water were studied. Consistently with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). Buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improves the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. A review of the prevalent CHF theories has established the nexus between CHF enhancement and surface wettability changes caused by nanoparticle deposition. This represents a first important step towards identification of a plausible mechanism for boiling CHF enhancement in nanofluids.
Effect of oxide layer thickness on the pool boiling critical heat flux of pre-oxidized RPV material
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