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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]

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]

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]