Search

In some nuclear reactors, under accidental conditions, core debris forms a molten pool, which is later located in a core catcher. The core catcher proposed by the authors uses special refractory material to absorb enthalpy of corium so that temperatures are within 1500 K, which is possible to cool with side cooling and top flooding. Since performing a full-scale prototypic experiment is extremely challenging and complex because of the involvement of very high temperatures and the presence of radioactive materials, it is important to develop a Computational Fluid Dynamics (CFD) model capable of simulating coolability of the melt pool with the above cooling strategy. In the present work, a CFD model was developed for the above purpose and was benchmarked with experiments conducted under simulated conditions by the authors. The experiment involved the melting of about 25 L of sodium borosilicate glass at about 1473 K and cooling it in a scaled-down core catcher model. In the presence of decay heat inside the melt pool, turbulent natural convection plays an important role in the temperature distribution inside the melt pool and on the vessel walls. For this, we used different turbulence models. Comparisons among the Standard k-ε, Shear Stress Transport (SST) k-ω, and two-dimensional (2D) Large Eddy Simulation (LES) turbulence models show that SST k-ω and 2D LES turbulences are found to be in good agreement with the experimental results for the temperature distribution in the melt pool, and SST k-ω is found to be computationally less expensive than 2D LES. In general, the CFD model is capable of simulating heat transfer with phase changes inside the heat-generating melt pool. In view of this, the model can be further extended to include cooling of the melt pool in the prototype core catcher. The evolution of crust formation has been investigated in detail using a CFD model. [ABSTRACT FROM AUTHOR]

A severe accident involving core melt in a nuclear reactor is a major concern especially after Fukushima. Thus, to mitigate the effects of core melt accidents, an ex-vessel core catcher is being developed for Advanced Indian Nuclear Reactors. The core catcher design envisages using special refractory material. The cooling strategy of the core catcher is one of the key components in the design of the core catcher. Performing a full-scale prototypic experiment is extremely challenging and prohibitory due to the involvement of very high temperature and presence of radioactive materials. Therefore, a computational fluid dynamics (CFD) model capable of simulating the coolability of the melt pool is important to develop. In the present work, a two-dimensional (2D) CFD model was developed to understand the heat transfer phenomenon and solidification of the heat-generating simulant melt pool. The 2D symmetry geometry of the simulated core catcher vessel was used. The CFD model considers appropriate models for melting and solidification to understand crust formation in the melt pool and the k-ε turbulence model to resolve turbulence inside the melt pool. A decay heat of 1 MW/m3 was also considered inside the melt pool. The CFD simulation results were compared with the authors' experimental results. The experiment involved a scaled-down ex-vessel core catcher model (CCM) employing electrical heaters to simulate decay heat. The experiment was carried out by melting about 25 L of sodium borosilicate glass using a cold crucible induction furnace at about 1200°C and cooling it in the scaled-down CCM. The scaled-down CCM was strategically cooled in three phases, namely, air cooled, indirect side cooling, and complete top flooding. To overcome the complexities of simulation of the initial melt pour condition, the CFD simulation was initialized with the temperatures just after the melt pour was completed in the experiment. Similar to the experimental conditions, the CFD simulations were carried out in three phases by changing the boundary condition. Comparison of the temperatures of the melt pool by the CFD simulations and experiments at different locations gave reasonable agreement. The evolution of crust formation, melt pool temperatures, core catcher inner wall temperatures, and heat flux distribution were investigated in detail using the CFD model. [ABSTRACT FROM AUTHOR]

Core catchers are being installed in advanced light water reactors after TMI and Fukushima. The core catchers are designed to stabilise and cool molten corium within the reactor containment for a long time, reducing the risk of core meltdown. Some core catchers use sacrificial material to lower the specific volumetric heat release of the molten corium. The low density oxidic component of molten pool migrate to the top layer of the core catcher after melt-sacrificial material interaction. The high density metallic component migrate to the bottom layer. In the event that the core catcher is top flooded, this prevents metallic components from reacting with water and generate hydrogen. Post experimental analysis of the cooled stratified melt pool was performed. The experimental scaled down core catcher used reactor the prototypic sacrificial material while maintaining the actual reactor flooding conditions. About 550 kg of simulant corium was melted at about 2500 °C using heat of thermite reaction. The core catcher vessel was partially filled with sacrificial bricks. After the test, solidified debris samples from different locations of the core catcher model were analysed for density and SEM-EDS for quantitative and qualitative characterisation. Post-test analysis showed the melt inversion characteristics. [ABSTRACT FROM AUTHOR]

The primary objective of core catchers is to minimize the potential risk of core meltdown by stabilization and cooling of molten corium within the reactor containment for a prolonged period by strategically cooling it. Quenching of the high‐temperature melt by top flooding strongly depends on the timing of injection of the cooling water, that is, immediate flooding or delayed flooding. To understand this aspect, we conducted experiments in a simulated core catcher of an Indian advanced nuclear reactor. In the experiments, sodium borosilicate glass, as a mixture of corium and sacrificial material simulant, was melted at about 1200°C and cooled by water from the side of a simulated core catcher and then flooding at the top of the melt pool. Two experiments were conducted; the first one was done with immediate flooding at the top of the melt pool; the second was performed with delayed top flooding of 45 min gap. The results show that immediate flooding quenches the melt pool rapidly compared to delayed flooding. A stable crust formation at the top of the melt pool and wall adhesion is observed in the delayed flooding. It was also found that no ingression of the water in the molten pool occurred with delayed flooding. On the other hand, with immediate flooding, water was found to ingress into the melt pool due to gap formations between melt and core catcher sidewall, causing the melt to break up through eruption by ingressed water. This phenomenon resulted in the formation of sand‐type debris and in faster melt cooling. [ABSTRACT FROM AUTHOR]

Boiling—a process widely used for its good heat transferability—is limited by a phenomenon known as critical heat flux (CHF). Our experiments revealed a new CHF mechanism that is different from previously believed theories; we refer to it as "sonneting CHF." At CHF, the flow pattern changes from bubbly flow to slug/churn flow and then to an unusual reverse annular flow, leading to a significant rise in the heater surface temperature. The reverse annular flow, however, does not sustain but breaks down into a chaotic flow pattern, resulting in unprecedented quenching of the heater surface. The flow pattern shortly reverts back to bubbly flow; this entire process repeats for a few cycles, where the heater surface temperature rises and falls with amplitudes increasing in each cycle until the heater trips. The maximum removal-surface heat flux is significantly higher than the CHF. This new understanding will enable flexible and innovative boiling systems for several energy applications. [ABSTRACT FROM AUTHOR]

Flow boiling experiments were conducted in an annular channel at very low flow rates and atmospheric pressure varying the inlet subcooling from 20 to 40. critical heat flux occurred during the oscillatory flow pattern transition from slug to churn regime. Comparison of low-pressure and low-flow critical heat flux correlations with the present data has shown that these correlations are not applicable for prediction of critical heat flux under oscillatory flow pattern transition from slug to churn. Power transient experiments were also performed at low-pressure and low-flow conditions. Flow regimes and the sequence of events leading to critical heat flux were visually observed using the high-speed camera. [ABSTRACT FROM AUTHOR]

Natural convection in an internally heated porous bed of height and diameter of 450 mm and 500 mm, respectively, and superposed with the fluid layer has been experimentally investigated. The onset of natural convection in the bed is indicated by change in the rate of temperature rise within the bed. An empirical model based on local Nusselt number and local Rayleigh number has been developed. A comparison of the present model with the models in literature is made to draw out the differences between the local heat transfer of large multidimensional beds and the average heat transfer of small beds. [ABSTRACT FROM AUTHOR]

The nuclear accident at Fukushima has brought an increased research focus on nuclear safety and severe accidents. Radioactivity leakage into the environment has environmental, societal, and political impact, and further robustness of nuclear reactor design is essential. Thus, coolability and stabilization of corium within reactor containment in a severe accident scenario are important issues that need to be resolved. In this context, many new reactors have been envisaged with dedicated core catchers. In ex-vessel core catchers, corium coolability is one of the biggest concerns. Despite several efforts, melt pool coolability still needs to be understood in sufficient detail. Among the various cooling strategies, melt coolability using bottom flooding has been demonstrated to be one of the most efficient techniques so far. This paper presents the numerical and experimental study of melt pool coolability under bottom flooding with decay heat simulation. An experiment has been performed using a test section consisting of two parts: a lower part for melt retention and quenching, and an upper part for steam expansion and its outlet. To simulate the decay heat, ten radiative heaters equivalent to 10 kW were used to heat the lower part of the test section housing the melt. The experimental measurements showed that quenching of about 25 L of melt at 1200°C initial temperature took only a few minutes. The same phenomenon has been modeled using a mechanistic model. The model considers heat transfer in the melt pool, eruption in the melt pool by steam formed when water is injected at the bottom of the melt pool, and heat transfer from porous debris to a steam mixture. The model postulates the formation of crust below the melt pool when water is being inserted from the bottom. The model predicts the failure of this crust due to various stresses, resulting in an inverted cone-shaped melt eruption. The model captures the physics of this eruption cone along with the spatial variation in porosity. The model predictions have been compared with the measurements for the melt pool temperature during the cooling process. The results show that the model is able to capture quite accurately the multidimensional temperature fields in the melt pool during the cooling process. [ABSTRACT FROM AUTHOR]

Visualization experiments in upward forced convective nucleate boiling flows were carried out. The bubble growth and collapse have been measured using high-speed photography technique with distilled water under atmospheric pressure. The experiments show that the bubbles depart from nucleating sites shortly after nucleation and slide along the heater surface. The bubbles grow while sliding, attain a maximum size, then lift from the surface sometime during condensation, and quickly vanish in the bulk liquid. Parametric studies show that bubble diameter and departure frequency increases with an increase in heat flux, a decrease in subcooling, and a decrease in mass flux. [ABSTRACT FROM PUBLISHER]

The visualization experiments for measuring departure diameter and frequency of vapor bubbles in the saturated nucleate boiling of distilled water have been carried out at atmospheric pressure. The experiments were carried out for an isolated single cylindrical tube and that for a tube bundle under pool and cross flow boiling conditions. The test setup was a stainless steel cuboidal tank of dimensions 400 mm ×  200 mm ×  180 mm provided with a 5  ×  3 staggered tube bundle with an equilateral triangular pitch of 1.95. The departure diameter and frequency were measured using a high-speed camera to understand the parametric behavior of the bubble departure characteristics in a tube bundle under pool and cross flow condition. The range of operating conditions was: (1) heat flux q ″ = 8 − 28 k W / m 2 , (2) wall superheat Δ T s a t = 7 − 22 K , (3) inlet mass flux G = 120 − 303.5 kg / m 2 s . The departure diameter and frequency were found to increase with an increase in the heat flux, but decrease with an increase in the mass flow rate. The local departure diameter and frequency were observed to increase as the neighboring tubes in the bundle were successively heated. Empirical correlations have been developed for departure diameter and frequency for saturated nucleate boiling under cross flow conditions incorporating the effect of the tube bundle. The correlations were found to predict experimental data within acceptable limit. [ABSTRACT FROM AUTHOR]

To understand the performance of a venturi scrubber operating in self-priming mode, the present study develops a computational model to simulate the interacting three phase flow occurring in the venturi scrubber. The computational model is based on an Euler- Lagrangian framework where parcels of droplets and aerosols are tracked throughout the flow domain. The model incorporates the effect of droplet collision and increased momentum source in the regions of high liquid concentration. Additionally, the droplet inertial effects on the shear induced multi-jet breakup is included to predict the evolution of the droplet size distribution. The predicted local liquid loading, pressure drop and the collection efficiency were found to be in good match with the cited experimental values. The jet penetration and the droplet mean diameter is combined to form a unique dimensionless number, called the Performance Parameter, to quantify the multi-jet characteristics in the throat section of the scrubber. Higher values of Performance Parameter corresponded to higher efficiencies of the scrubber, hence serves as an analytical tool to assess the performance of the scrubber. Based on the parametric study for different throat gas velocities, liquid injection velocities, changing throat length and changing orifice diameter, the applicability of such a scrubber to self-priming mode is discussed. Further, for transients incurring lower gas velocities and lower operating height, the advantages of multi-stage injection and variable injection diameter are presented. [ABSTRACT FROM AUTHOR]

The boiling models use departure diameter and frequency in closure relations for the calculation of nucleate boiling heat flux. These parameters are normally derived from empirical correlations which depend heavily on experiments. While these parameters are studied mostly for saturated conditions, there is not sufficient data for the values of departure diameter and frequency in subcooled boiling. In this work, the bubble departure characteristics, i.e. the departure diameters and frequency have been measured using high speed visualization experiments with subcooled demineralized water at atmospheric pressure for nucleate pool boiling conditions. The water pool dimensions were 300  mm × 135  mm × 250  mm with four different heating elements to carry out the parametric studies of bubble departure behavior. The considered parameters were heater surface roughness, heater geometry and heater inclination along with the experimental conditions like degree of subcooling (Δ T sub = 5−20 K), superheat (Δ T sat = 1−10 K) and the heat flux. The departure diameters and frequencies were directly measured from the images captured. It was intended to generate the subcooled nucleate pool boiling data under a wide range of conditions which are not present in the literature. The departure diameter was found to increase with the wall superheat, heater size and the inclination angle while the liquid subcooling and surface roughness produced a damping effect on the diameter. The departure frequency was found to increase with the wall superheat and the inclination angle, but decreases with an increase in the heater size. The frequency increases with the degree of subcooling except very close to the saturation, and is unaffected by the surface roughness beyond a certain superheat value. [ABSTRACT FROM AUTHOR]

Abstract: The quenching characteristics of a volumetrically-heated particulate bed composed of radially stratified sand layers were investigated experimentally in the POMECO facility. The sand bed simulates the corium particulate debris bed which is formed when the molten corium released from the vessel fragments in water and deposits on the cavity floor during a postulated severe accident in a light water reactor (LWR). The electrically-heated bed was quenched by water from a water column established over top of it, and later also with water coming from its bottom, which was circulating from the water overlayer through downcomers. A series of experiments were conducted to reveal the effects of the size of downcomers, and their locations in the bed, on the quenching characteristics of the radially stratified debris beds. The downcomers were found to significantly increase the bed quenching rate. To simulate the non-condensable gases generated during the MCCI, air and argon were injected from the bottom of the bed at different flow rates. The effects of gas flow rate and its properties on the quenching behaviour were observed. The results indicate that the non-condensable gas flows reduce the quenching rate significantly. The gas properties also affect the quenching characteristics. [Copyright &y& Elsevier]

• Bubble departure diameter and nucleation site density models are recalibrated for high pressure conditions. • Theoretical and experimental basis for boiling and momentum closures are discussed. • CFD model is developed and DNB is predicted within 15% absolute error and the error is less than 6% under high subcooled conditions. • Sensitivity analysis is performed to understand the impact of boiling and momentum closures. • The effect of mass flux, inlet subcooling and exit quality on DNB prediction is studied in detail. In this study, Departure from Nucleate Boiling (DNB) is investigated in vertical tubes under high pressure and high mass flux conditions using two-fluid Eulerian approach coupled with improved wall boiling model. It is essential to determine the near wall void fraction accurately in order to predict DNB. After accessing the theoretical and experimental basis of the correlations used for bubble departure diameter and nucleation site density, existing correlations are recalibrated to capture the phenomena at high pressure conditions. Sensitivity analysis is performed to analyze the impact of boiling and momentum closures. The effect of mass flux, inlet temperature, and exit quality on DNB is studied. The proposed model predicted DNB within 15%. This result indicates that CFD is a promising tool for predicting DNB. [ABSTRACT FROM AUTHOR]

Highlights • Pressure profile in submerged venturi scrubber has been predicted. • Injection characteristics of submerged venturi scrubber are discussed. • Iodine retention in venturi scrubber is evaluated. Abstract Post Fukushima, many utilities are planning to install the Filtered Containment Venting System (FCVS) to protect the nuclear reactor containment under severe accident scenarios. FCVS aims at depressurizing the reactor containment and filtering the radioactive gas leaving the containment boundary under such scenarios. A key component of FCVS is the venturi scrubber. The hydrodynamics of the fluid flow inside the FCVS is complex involving multi-phase and multi-component systems with droplets formed by entrainment of high velocity gas stream with liquid jet. The size of droplets and their trajectories are critical with regard to decontamination of iodine which combines with the droplets inside the venturi. Thus, understanding the injection characteristics, hydrodynamics and absorption of iodine vapour with the droplets in the venturi scrubber is very important. There are very limited studies on this aspect. In view of this, a CFD simulation study has been performed in this work for understanding of venturi scrubber operation and performance in self-priming mode. The interaction of the continuous gas phase and droplets is assumed to occur through drag force. The gas flow rate is varied from 1700 to 3400 lpm for two submergence heights of 3 m and 4 m. The liquid loading is estimated from pressure drop at the nozzles observed in the experiments. The retention of iodine in venturi scrubber is calculated assuming instantaneous mass transfer at gas droplet interface. The mass transfer coefficient is evaluated from Steinberg and Treybal correlation. Our results indicate that the liquid loading to the scrubber increases at lower flow rates of 1700 lpm gas flow and further decreases with the increase in gas flow. The two phase pressure profile predicted is close to measured values for each case. The pressure decreases in the converging section; decreases sharply in the throat section due to liquid injection as droplets and shows negligible recovery in the diverging section of the venturi scrubber. The retention of iodine in the venturi scrubber increases with the increase in gas flow rate in the venturi scrubber. This indicates that the participation of venturi scrubber in the iodine retention increases as the air flow rate increases. [ABSTRACT FROM AUTHOR]

The objective of the present numerical study is to investigate the thermal-hydraulic characteristics of various circular (plain circular fin, crimped spiral fin, serrated fin) and plate fins [plain plate fin, wavy fin and fin with punched delta winglet pair (DWP)]. The geometrical parameters are, tube outer diameter (OD) = 7 mm, fin spacing (S) = 3–7 mm, fin height ( h f ) = 5 mm, number of tube rows ( N r ) = 2, transverse tube pitch ( P t ) = 23 mm and longitudinal tube pitch ( P l ) = 18 mm. The Reynolds number based on hydraulic diameter has been considered in the range of 2500 ⩽ Re h ⩽ 4000. The circular fins outdo the plate fin in terms of the heat transfer coefficient and the heat transfer per unit pumping power for the circular fins has been found to be 140–170% higher as compared to the plate fins for a fixed Reynolds number. Local Nusselt number around the periphery of the tubes showed that fin patterns affect the wake region formation and flow separation on the tube surface. It has been observed that heat transfer coefficient decreases with an increase in the fin spacing due to increase in bypass flow rate through the heat exchanger at a constant Reynolds number. The heat transfer per unit pumping power showed an increase of more than 100% with an increase in the fin spacing from 3 to 6 mm. The flow patterns have been studied in detail for serrated fin, crimped fin and fin with DWP. The vortices generated by the fins merge with the horseshoe vortices and enhances the heat transfer in the domain. The fin efficiency has been found to be in the range of 77–83% for the circular fins and 55–66% for the plate fins for a range of 2500 ⩽ Re h ⩽ 4000. Only one circular fin design i.e., crimped fin provides higher volume goodness factor as compared to the plate fins. In general it has been observed that more compact heat exchanger can be designed using plate fins but at a higher pumping power cost. [ABSTRACT FROM AUTHOR]

The Passive Moderator Cooling System (PMCS) of the Advanced Heavy Water Reactor is designed to remove heat from the moderator passively in case of an extended station black out condition (SBO). The hot heavy-water moderator inside the Calandria rises upward due to buoyancy, cooled in a shell and tube heat exchanger (located within the loop) and returns back to the Calandria, completing a natural circulation loop. The heat exchanger, in turn, is cooled by water from a Gravity Driven Water Pool (GDWP); forming a second loop. This coupled natural circulation loop system provides sufficient cooling to prevent the increase of moderator temperature inside the Calandria vessel beyond safe limits during SBO. The feasibility of such a system should be assessed before it is implemented in the reactor. Thus, a scaled test facility was set up to simulate the thermal hydraulic characteristics of the PMCS. A set of time varying power experiments were performed, which capture the flow initiation from rest phenomena and the multidimensional natural convection flow in a coupled natural circulation system. Next, the experimental geometry was simulated using the three-dimensional computational fluid dynamics code (OpenFoam 2.2.0), which predicted temperature and flow distribution inside the system. The CFD results agree well with the experimental data within ±6%. The flow initiation phenomena shows that the initial flow generated recirculates within the Calandria, after ~900 s the flow is able to come out of the Calandria. The experiments show a time lag between the starting of the primary and secondary loop circulation. This work demonstrates the experimental and computational capability to understand and design an effective Passive Moderator Cooling System. [ABSTRACT FROM AUTHOR]

Many advanced nuclear reactors adopt methodologies of passive safety systems based on natural forces such as gravity. In one of such system, the decay heat generated from a reactor is removed by isolation condenser (ICs) submerged in a large water pool called the Gravity Driven Water Pool (GDWP). The objective of the present study was to design an IC for the passive decay heat removal system (PDHRS) for advanced nuclear reactor. First, the effect of inclination of IC tube on three dimensional temperature and flow fields was investigated inside a pilot scale (10 L) GDWP. Further, the knowledge of these fields has been used for the quantification of heat transfer and thermal stratification phenomenon. In a next step, the knowledge gained from the pilot scale GDWP has been extended to design an IC for real size GDWP (∼10,000 m 3 ). Single phase CFD simulation using open source CFD code [OpenFOAM-2.2] was performed for different tube inclination angles ( α ) (w.r.t. to vertical direction) in the range 0° ⩽ α ⩽ 90°. The results indicate that the heat transfer coefficient increases with increase in tube inclination angle. The heat transfer was found to be maximum for α = 90° and minimum for α = 15°. This behavior is due to the interaction between the primary flow (due to pressure gradient) and secondary flow (due to buoyancy force). The primary flow enhanced the fluid sliding motion at the tube top whereas the secondary flow resulted in enhancement in fluid motion along the circumference of tube. As the angle of inclination ( α ) of the tube was increased, the secondary flow became dominant and resulted into enhanced heat transfer near the tube bottom. Three different heat transfer regimes were identified in the transient period: conduction (0 < t < 0.4 s), quasi-steady (0.4 s < t < 10 s) and fluctuating period (10 s < t < 100 s). The effect of inclination angle ( α ) was more pronounced in the fluctuating period. The natural convection and heat transfer in the regime of laminar-turbulent transition was studied in the presence of longitudinal vortices. The heat transfer was enhanced in the transition region due to vortices formation and it was reduced in the turbulent regime due to decay of vortices. [ABSTRACT FROM AUTHOR]

To mitigate severe accidents in nuclear reactors, the present research sheds light on the melt-coolability behavior of corium with hypothetical experiments that have been performed at two different nozzle diameters under bottom flooding conditions. In this research, a simulant material CaO-Fe2O3 powder mixture was melted and poured into the test section that was embedded in the test facility (using a bottom pouring furnace instead of a tiltable furnace). Then, from the bottom of the melt pool, water was flooded through a nozzle at a pressure of 0.70 bar and a water flow rate of 12 liters per minute. Because of the interaction between the water and melt, the melt quenched and converted into fine porous debris, and the temperature history was recorded using 12 K-type thermocouples connected to a data acquisition system. The average quenching time and porosity of the debris were affected by variations in the nozzle diameter. This research will help in understanding real core-melt accidents that generally occur in nuclear power plants. [ABSTRACT FROM AUTHOR]

Concentration solar power (CSP) systems convert solar radiation to heat and use heat engines to convert the heat to electricity. The solar receiver over which the solar radiation is concentrated and converted to heat is the most important part of the CSP. To attain maximum efficiency, the receiver in the CSP systems needs to be coated with an efficient selective solar absorber coating. In recent years, a lot of research has been focused on solar selective coatings. This has resulted in the synthesis of novel coatings that have high thermal and chemical stability, long term durability, and excellent solar selectivity making them suitable for solar thermal applications. This report reviews various solar selective coatings based on transition metals and their compounds. Various failure mechanisms are discussed in detail along with suggested prevention methods. Several thermal stability and durability tests are reported with their benefits and limitations. The effect of long-term durability on the levelized cost of coating is also discussed. Finally, we list some excellent systems and explore different ways of improving the thermal stability for SSCs, thus providing a reference for the design and optimization of new SSCs. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate thermal–hydraulic characteristics of an air cooled condenser and optimized the design in terms of heat transfer per unit pumping power, area goodness factor, volume goodness factor and material requirement using the 3D numerical simulations. The annular-finned tubes have been considered in this study, which is one of the most common type of fin used for the design of air cooled heat exchangers and known to provide a high heat transfer coefficient per unit pressure drop. The effect of tube shape (elliptical and round), tube diameter [7–24 mm (circular), 30 × 10–30 × 20 mm (elliptical)], fin spacing (2–10 mm), number of rows (2–10), fin height (5–10 mm), air frontal velocity (4.76–6.32 m/s), transverse tube pitch (36.8–44 mm) on the thermal–hydraulic performance has been studied. It is observed that, with an increase in the fin spacing, the heat transfer coefficient increases (by 35–40%) at a constant inlet velocity and the pressure drop decreases (by 60–80%). The frontal area requirement increases by 100–150% with an increase in the fin spacing for the same heat removal capacity. As the row number is increased, the heat transfer coefficient increases initially for N r < 4 by 7–8%, and for N r > 4, the heat transfer coefficient decreases by 23%. An increase in the tube pitch increases the heat transfer coefficient per unit pressure drop by 40%. The fin efficiency is found to decrease with an increase in the Reynolds number, fin spacing and number of tube rows. In the last section optimization of the design has been performed based on Taguchi method and numerical results obtained. It has been observed that the number of tube rows must be kept between 2–4, fin spacing 3–5 mm, tube pitch around 40 mm, and fin height 5 mm for better performance of the condenser. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate the transient 3D numerical simulations of natural convection of air around a circular finned tube (24.9 mm OD ) kept in a small chimney. The annular plain fins are considered in this study. The effects of fin spacing to fin diameter ratio (0.057 mm ⩽ S / D f ⩽ 0.24 mm), chimney height (400–1000 mm) and ambient to surface temperature difference (10 K ⩽ T s ⩽ 65 K) on the heat transfer and the driving force have been investigated. The results are presented in terms of the temperature contours, velocity vectors, heat transfer and the driving force. It has been found that the heat transfer coefficient increases with an increase in the fin spacing upto an optimum value ( S = 8 mm) for all the fin geometries, and beyond S = 8 mm, the heat transfer coefficient decreases. The separation of the thermal boundary layer with a variation in the fin spacing and its effects on the heat transfer and driving force has been shown. For a fixed fin spacing, the heat transfer rate, heat transfer coefficient and the driving force increases with an increase in the fin diameter, however, for D f > 41 mm, the rate of increase in the heat transfer coefficient reduces. The heat transfer coefficient increases with an increase in the chimney height and it has been found that the effect of chimney height on the heat transfer coefficient is a resultant effect of the air outlet temperature and the driving force generated by the chimney. The base to ambient temperature difference has been varied to observe the temperature sensitivity on the heat transfer coefficient and flow patterns. In the last section, various circular and elliptical tube designs have been investigated, and it is found that, the elliptical tube with minimum ellipticity ( b / a = 0.33) and circular tube with smallest diameter (7 mm) provides better heat transfer coefficient than the other circular and elliptical designs. [ABSTRACT FROM AUTHOR]

In case of heavy water moderated nuclear reactors, the decay heat of a fuel bundle can be transferred to the moderator available outside calandria vessel. Radiative heat transfer accounts for significant amount of total energy exchange from the fuel bundle during LOCA (loss of coolant accident) situation. In order to simulate one channel of an advanced reactor comprising 54 nuclear rods of 3.5 m length, scaling analysis has been used to scale it down to an 18 rod bundle facility of 1 m test length. The experimental analysis, in order to examine the contribution of energy transfer by thermal radiation and natural convection in removing decay heat of fuel/heater rods using moderator as a heat sink, is presented. It focuses mainly on the role of moderator in removal of decay heat, for situation with air alone in the annular gap between pressure tube and calandria tube coupled with flowing and standstill/boiling moderator situations. Four experimental tests have been presented in this paper. It is seen that, 80% of heat energy is transferred from the heater bundle of rods to the enclosing pressure tube by means of radiation mode, out of this 20–45% of energy is carried away by the moderator. It depends mainly on the resistances involved in the heat flow path and the test set up conditions. [ABSTRACT FROM AUTHOR]

The recent Fukushima accident has raised strong concern and apprehensions about the safety of reactors in case of a prolonged Station Black Out (SBO) continuing for several days. In view of this, a detailed study was performed simulating this condition in Advanced Heavy Water Reactor. In this study, a novel concept of moderator cooling by passive means has been introduced in the reactor design. The Passive Moderator Cooling System (PMCS) consists of a shell and tube heat exchanger designed to remove 2 MW heat from the moderator inside Calandria. The heat exchanger is located at a suitable elevation from the Calandria of the reactor, such that the hot moderator rises due to buoyancy into the heat exchanger and upon cooling from shell side water returns to Calandria forming a natural circulation loop. The shell side of the heat exchanger is also a natural circulation loop connected to an overhead large water reservoir, namely the GDWP. The objective of the PMCS is to remove the heat from the moderator in case of an SBO and maintaining its temperature below the permissible safe limit (100 °C) for at least 7 days. The paper first describes the concept of the PMCS. The concept has been assessed considering a prolonged SBO for at least 7 days, through an integrated analysis performed using the code RELAP5/MOD3.2 considering all the major components of the reactor. The analysis shows that the PMCS is able to maintain the moderator temperature below boiling conditions for 7 days. [ABSTRACT FROM AUTHOR]

In water‐cooled nuclear reactors, the maximum power which can be extracted from the core is limited by critical heat flux (CHF). CHF in the high‐quality region is known as dryout. In advanced nuclear reactors, the coolant flow occurs solely by virtue of natural circulation; however, instabilities may occur during off‐normal operations. This may lead to premature dryout due to lower coolant flow rates seen by the heater during such oscillations. This paper describes the experimental investigation on the effect of flow oscillations on the CHF with the time period of 120 s, which is observed typically in the large‐scale natural circulation system. Based on observations made with respect to temperature transient, the continuous dryout is preceded by the transient dryout for higher flow oscillations. But as flow fluctuation decreases, the transient dryout phenomenon is found to disappear. The applicability of the look‐up table to predict CHF under oscillatory flow conditions using suitable correction factors (CFs) for premature dryout has been evaluated. CFs for the CHF under oscillations suggested by previous authors have been compared. The maximum possible degradation in CHF value suggested by previous authors has been found to agree with the present experimental data. Percentage fluctuation in heat transfer coefficient (HTC) at fully developed annular flow conditions has been evaluated, and it is found that fluctuation in HTC is in phase with the fluctuation in flow. [ABSTRACT FROM AUTHOR]

In this paper, a review is presented on the experimental investigations and the numerical simulations performed to analyze the thermal-hydraulic performance of the air-cooled heat exchangers. The air-cooled heat exchangers mostly consist of the finned-tube bundles. The primary role of the extended surfaces (fins) is to provide more heat transfer area to enhance the rate of heat transfer on the air side. The secondary role of the fins is to generate vortices, which help in enhancing the mixing and the heat transfer coefficient. In this study, the annular and plate fins are considered, the annular fins are further divided into four categories: (1) plane annular fins, (2) serrated fins, (3) crimped spiral fins, (4) perforated fins, and similarly for the plate fins, the fin types are: (1) plain plate fins, (2)wavy plate fins, (3) plate fins withDWP, and (4) slit and strip fins. In Section 4, the performance of the various types of fins is presented with respect to the parameters: (1) Reynolds number, (2) fin pitch, (3) fin height, (4) fin thickness, (5) tube diameter, (6) tube pitch, (7) tube type, (8) number of tube rows, and (9) effect of dehumidifying conditions. In Section 5, the conclusions and the recommendations for the future work have been given. [ABSTRACT FROM AUTHOR]

Investigations on severe accident phenomena help us in understanding the realistic accidental phenomena for the assessment of associated risk. The societal impact of radiological leakage to the environment has demanded further robustness in the line of defence of nuclear safety. Thus, to ensure the cooling and stabilization of corium within reactor containment in case of severe accident scenarios, many new reactors have been envisaged with core catcher. In this regard, corium coolability still remains an unresolved issue in spite of several efforts being taken towards its understanding. After studying the various cooling strategies, it has been demonstrated that melt coolability using bottom flooding of water is one of the most efficient techniques so far. To study the effect of decay heat on melt pool coolability under bottom flooding condition, two experiments have been performed in this paper; one without the decay heat and the other with decay heat. The test section used for carrying out these experiments consisted of two parts viz. lower part for retaining the melt from furnace, water inlet and melt quenching, and upper part for steam expansion and its outlet. The total height of the test section was 1400 mm and was made of 33 mm thick carbon steel. Total six stainless steel nozzles of diameter 12 mm were used for injecting water at the bottom of the melt pool. The lower part was surrounded by 10 radiative heaters to simulate decay heat of 10 kW which corresponds to 0.5 MW/m 3 . The experiments showed that quenching of about 25 l of melt at initial temperature of nearly 1200 °C took only a few minutes in both the cases. The debris formed in the experiments ranged between 0.5 mm fine particles to 50 mm porous chunks. The measured porosities were nearly 51% without decay heat and nearly 67% with decay heat. This paper presents the comparative experimental study of melt coolability using bottom flooding without and with decay heat scenarios. The experimental measurements show that the quenching behaviour of melt pool is not affected by decay heat, although stabilization of debris temperatures to saturation temperature takes a bit longer duration as compared to without decay heat scenarios. [ABSTRACT FROM AUTHOR]

Many advanced designs of nuclear reactors adopt a methodology of passive safety systems in order to avoid the occurence of a severe accident. In one of such systems, the decay heat generated from a reactor is transferred by natural circulation into large pool of water called the Gravity Driven Water Pool (GDWP). Three-dimensional (3D) convection flows develop, which in turn affect the heat transfer process and hence the temperature pattern. The heat transfer process can get compromised by the possible stratification of the temperature. Further, the material of construction of GDWP may have certain temperature limitations and puts bounds on the extent of stratification. The objective of this study is to investigate the 3D natural circulation phenomenon in GDWP. The present work is in continuation of our earlier work Gandhi et al. [1,2] where the natural convection has been analyzed in 0.025 and 0.21 m 3 vessels. Now, we have reported the simulation for 9247 m 3 GDWP tank. Single phase CFD simulations using open source CFD code [OpenFOAM-1.6] have been performed for a geometry (ID = 12 m, OD = 50 m and height (H T ) = 5 m). In order to reduce the thermal stratification, various geometrical modifications have been incorporated on the heat exchanger design, such as (1) distributing the heat transfer area of heat exchanger among single, double and multiple heat sources (2) to optimize the location of heat exchanger inside the GDWP (3) provision of passive elements such as draft tubes (single or concentric multiple) around the heat source at the center, which can act as a chimney. A detailed CFD analysis of three dimensional temperature and velocity distribution in the secondary side of GDWP has confirmed the mitigation of thermal stratification phenomenon by optimizing the distribution and position of heat exchangers. Passive draft tubes also result in significant enhancement in natural circulation and hence reduction in thermal stratification. [ABSTRACT FROM AUTHOR]

The objective of this work is to investigate the transient 3D numerical simulations of natural convection of air around a circular cylinder (76.2 mm OD ) enclosed in a box of 1000 mm × 600 mm × 1200 mm for a Rayleigh number of 1.3 × 10 6 . The 2D numerical simulations have also been performed and the comparison between the 2D and 3D simulations has been presented in terms of the Nusselt number. The effect of clearance between the top wall and the cylinder (0.2 ⩽ H ∗ / D ⩽ 2.3) on the flow pattern has also been investigated for the case of conducting ceiling. The flow becomes 3D, unstable and oscillating when the H ∗ / D ratio is 0.2, and as the H ∗ / D ratio is increased to 0.4, 1 and then to 2.3; the flow becomes 2D and stable. The studies of Cesini et al. (1999) [5] and Newport et al. (2001) [7] have also been analyzed by 2D and 3D numerical simulations. The flow shows 3D, unstable and oscillatory behavior in the case of Cesini et al. (1999) [5] due to the wall-cylinder interaction. However, the flow was found to be 2D and stable for the case of Newport et al. (2001) [7]. A comparison between our results and their numerical and experimental results has been presented. The time varying behavior of the surface averaged Nusselt number has been estimated, and it was found that, the time to reach the steady state for the flow depends on the aspect ratio of the geometry and 3D nature of the natural convection. [ABSTRACT FROM AUTHOR]

Heat transfer by natural convection in a cavity is encountered in various industrial applications, such as heating and cooling of living spaces, fire in building, solar thermal collector systems, electronic and photovoltaic cooling devices, thermosiphon heat exchangers, passive decay heat removal systems, air condensers, etc. In such systems, the cooler (heavier) fluid naturally moves towards the bottom of the tank while the hotter lighter fluid rises towards the top. These flows result into certain extent of non-uniformity (or stratification) of temperature/concentration depending upon the intensity of flow. We have carried out flow (using particle image velocimetry) and temperature (using thermocouples) measurements in a rectangular tank (0.8×0.6×0.6m3) fitted with a central tube (forming the heat transfer surface). In order to reduce the stratification, the effects of various internals have been examined. These include (a) changing the ratio of area (heat transfer) to volume (aspect ratio), which is achieved by insulating the heating tube with the help of teflon, (b) introduction of draft tube concentric to the heat transfer tube, and (c) provision of non-conducting or highly conducting fins attached to the tube at different axial locations. Additionally, computational fluid dynamic (CFD) simulations of these systems were performed using the commercial software FLUENT-6.3. The extent of stratification has been investigated for a wide range of Rayleigh numbers (4.34×1011≤Ra≤2.59×1014). In addition, the flow information obtained from PIV was analyzed for insights into the dynamics of turbulent flow structures. For this purpose, we have used the signal processing technique of Discrete Wavelet Transform (DWT). From the analysis, we were able to estimate the size, velocity and energy distribution of turbulent structures. This detailed knowledge was employed in surface renewal type of theories for the estimation of rates of heat transfer. A good agreement was observed between the predicted and the experimental values of heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this study was to determine the sphingolipid (SL) profile in cells defective in autophagy protein ATG-7 and overall cell death after photodynamic therapy (PDT) with the photosensitizer Pc 4. MCF-7 human breast cancer cells with downregulated ATG-7 and their scrambled controls (Scr) were used. Exposure of ATG-7 knockdown cells to PDT led to increased cell killing. PDT evoked an early (2h) greater global increase in ceramides in ATG-7 defective cells compared to Scr cells. The total increases in dihydroceramide (DHceramide) were significant at 2 and 24h in both cell types post-PDT. The levels of sphingosine-1-phosphate (S1P) and sphingosine were decreased below resting levels at both time points irrespective of the cell type. The data imply that ceramide might be a marker of ATG-7 deficiency in cells sensitized to PDT. [Copyright &y& Elsevier]

• Simulation of reactor and 1/10th scaled model of Passive Moderator Cooling system. • 1D numerical simulation using RELAP5/MOD3.2 of coupled natural circulation loop. • Validation of scaling philosophy was performed for a transient scenario. • Comparison of simulation results with experimental data. • Design optimization studies of Passive Moderator Cooling system. The passive moderator cooling system (PMCS) for the Advanced Heavy Water Reactor is one of the passive systems incorporated to enhance safety of the reactor. The PMCS comprises of a coupled natural circulation loop, with a Gravity-driven-water pool as the ultimate sink and a shell and tube heat exchanger as the coupling component. Previously (Pal et al., 2016), a scaled experimental facility for PMCS was setup. In this work, validity of scaling theory was verified using 1D thermal-hydraulic code (RELAP5/MOD3.2) and simulations were compared with experimental data. Finally, PMCS was simulated for various geometrical parameters, e.g. elevation difference and pipe diameters, in an attempt to optimize the design. A good agreement with experimental data (within ± 8%), except for initial 2 h, was observed in RELAP/MOD3.2 code simulation to scaled experimental facility because of internal recirculations within Calandria being more prominent than overall circulatory flows in the passive loops. [ABSTRACT FROM AUTHOR]

There have been continuous efforts by researchers and power producing companies to reduce various costs involved in solar power tower (SPT) plants. Economically deploying conventional SPT plants for industries using thermal or thermo-chemical processes which need temperatures >1100 K could be challenging. Here, the overall economics of deployment of conventional SPT could go unfair as such design needs larger heliostat field, therefore costlier stiffer heliostats to reach high concentration ratio (CR). These challenges can be solved by beam-down SPT which uses secondary reflector mounted on tower top and receiver cum secondary concentrator on the ground could achieve desired CR, is one potential candidate to save on tower construction and pumping costs. Using beam-down SPT heat can be made available at the ground which opens an option of extracting the heat using natural circulation loop (NCL). The current paper explores the new proposed configuration of NCL in terms of understanding thermal hydraulics using 3-D CFD simulations. Further, it also incorporates optimization of the proposed design configuration using formulated heat transfer model. The optimized geometry is simulated using 3-D CFD simulation which gave the desired rating. [ABSTRACT FROM AUTHOR]

Passive systems are being examined for the future Advanced Nuclear Reactor designs. One of such concepts is the Passive Moderator Cooling System (PMCS), which is designed to remove heat from the moderator in the Calandria vessel passively in case of an extended Station Black Out condition. The heated heavy-water moderator (due to heat transferred from the Main Heat Transport System (MHTS) and thermalization of neutrons and gamma from radioactive decay of fuel) rises upward due to buoyancy, gets cooled down in a heat exchanger and returns back to Calandria, completing a natural circulation loop. The natural circulation should provide sufficient cooling to prevent the increase of moderator temperature and pressure beyond safe limits. In an earlier study, a full-scale 1D transient simulation was performed for the reactor including the MHTS and the PMCS, in the event of a station blackout scenario ( Kumar et al., 2013 ). The results indicate that the systems remain within the safe limits for 7 days. However, the flow inside a geometry like Calandria is quite complex due to its large size and inner complexities of dense fuel channel matrix, which was simplified as a 1D pipe flow in the aforesaid analysis. In the current work, CFD simulations are performed to study the temperature distributions and flow distribution of moderator inside the Calandria vessel using a three-dimensional CFD code, OpenFoam 2.2.0. First, a set of steady state simulation was carried out for a band of inlet mass flow rates, which gives the minimum mass flow rate required for removing the maximum heat load, by virtue of prediction of hot spots inside the Calandria. Next, a pseudo transient simulation was carried out by taking the power and mass flow rate at different time instants to the Calandria from the 1D analysis, as an input to the 3D steady state simulations. The results indicate that temperature hotspots are located at the top center of the fuel channel matrix depending on the operating regime. Furthermore, it was observed that for a period of 7 days the temperatures are well within the safe limits. [ABSTRACT FROM AUTHOR]