Abstract In the present work, 3-D numerical simulations have been carried out to understand the flow patterns and heat transfer on the jacket side of a stirred vessel under mixed convection conditions. Various parameters have been varied (1 ≤ Uin ≤ 20 m/s, 30 ≤ ΔT ≤ 60 K, 30 ≤ Jg ≤ 150 mm, Di = 1 and 3 m) to understand their effect on the heat transfer and pressure drop on the jacket side. SST k-ω model has been used for the numerical simulations because of its capability to predict natural convection heat transfer and low Re forced convection heat transfer. It was found that, the heat transfer coefficient increases by 40-80% by increasing the inlet fluid velocity from 1 to 5 m/s. The pressure drop increases by 120-200% and heat transfer per unit pumping power decreases by 60-75% as we increase the velocity from 1 to 5 m/s. It has also been observed that the flow inside the jacket is 3-D, nonuniform and has dead zones and hot pockets at various locations. The effects of inlet flow impinging have also been studied by varying inlet diameter and removing inlet pipe. Impinging by inlet flow on jacket wall contributes about 10-20% to heat transfer and 10-25% to pressure drop. Due to the presence of recirculating flow zones, dead zones, and impinging effect, existing correlations are not suitable for the estimations of heat transfer in large jackets. Large differences have been observed between the 3-D CFD predictions and the widely used empirical correlations. Further, the 3-D CFD predictions have been shown to be comparable to the experimental data available in the published literature. Based on simulations performed in the present work and also validated by experimental data, a new correlation has been proposed which is expected to be useful to design engineers, particularly for those, having limited computational facility.

Soon after the discovery of fission, large number of nuclear reactors was deployed across the globe. These reactors utilize active components in their design during normal operations as well as accidental conditions. Even though these reactors were having provisions of redundancy and defense in depth, still the reliability of the active components cannot be assured. Further, after the accidents like Three Mile Island, Chernobyl, and most recently Fukushima, lot of emphasis is being given to employ passive components in reactor designs for safety purposes. In view of this, all the advanced reactors (Gen-III and III+) are being designed with multiple passive safety features such as passive residual heat removal system (PRHRS) and passive moderator cooling system (PMCS). All these passive safety systems are based on natural forces (Natural Convection) such as gravity. Before the incorporation of these new passive systems in reactor design, these need to be rigorously assessed in a scaled integral experimental test facility, which is having its own challenges and limitations. In this context, there has been a parallel development because of the increasing computational resources in the past three decades. It has now become possible to analyze the performance of these systems using computational fluid dynamics (CFD). Previously these systems were analyzed using one-dimensional computational tools such as RELAP and CATHARE. However, these tools were not able to provide detailed three-dimensional flow patterns and also utilize empirical correlations in their modeling. On the contrary, CFD gives detailed flow and temperature fields inside the components, which is not possible even to measure experimentally due to the limitations of the measurement techniques. However, most of the conventional CFD models were developed for the simulation of forced flows (high Re Pump driven). On contrast, flows in natural convection based systems; the flows are generated through the density difference caused by temperature gradients. Thus, the temperature and velocity fields in the natural convection systems are coupled and offer major challenges in CFD simulations. In this chapter, an effort has been made to assess the performance of different Passive safety systems using CFD, with emphasis lying on the modeling of turbulent natural convection and near-wall heat transfer. Also the CFD models were validated using experimental data generated in scaled test facilities. This chapter includes the simulation of the following four passive systems: (1) PRHRS, (2) PMCS, (3) air-cooled condenser, and (4) venturi scrubber. Finally, the effect of various geometrical parameters has been studied to optimize the design of these passive systems.

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. (C) 2017 Elsevier Ltd. All rights reserved.

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/Df <= 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. (C) 2015 Elsevier Ltd. All rights reserved.

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 x 10-30 x 20 mm (elliptical)1, 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. (C) 2015 Elsevier Ltd. All rights reserved.

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 with DWP, 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.

The objective of this work is to investigate the transient 3D numerical simulations of natural convection of air around a circular cylinder (76.2mm OD) enclosed in a box of 1000mm×600mm×1200mm for a Rayleigh number of 1.3×106. 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.