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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 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]

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]

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]

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 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]

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]