Manninen’s mixture model for conjugate conduction and mixed convection heat transfer of a nanofluid in a rotational/stationary circular enclosure

Purpose This study aims to investigate numerically the problem of conjugate conduction and mixed convection heat transfer of a nanofluid in a rotational/stationary circular enclosure using a two-phase mixture model. Design/methodology/approach Hot and cold surfaces on the wall or inside the enclosure (heater and cooler) are maintained at constant temperature of Th and Tc, respectively, whereas other parts are thermally insulated. To examine the effects of various parameters such as Richardson number (0.01 = Ri =100), thermal conductivity ratio of solid to base fluid (1 = Kr = 100), volume fraction of nanoparticle (0 = φ = 0.05), insertion of conductive covers (C.Cs) around the heater in a different shape (triangular, circular or square), segmentation and arrangement of the conductive blocks (C.Bs) and rotation direction of the enclosure on the flow structure and heat transfer rate, two-dimensional equations of mass, momentum and energy conservation, as well as volume fraction, are solved using finite volume method and Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm. Findings The results show that inserting C.C around heater can increase or decrease heat transfer rate, and it depends on thermal conductivity ratio of solid to pure fluid. Also, it is found that by the division of C.B and location of its portions in a horizontal configuration, heat transfer rate reduces. Moreover, it is observed that external heating and cooling of the enclosure causes enhancement of heat transfer relative to that of internal heating and cooling. Finally, results illustrate that under the condition that cylinders rotate in the same direction, the heat transfer rate increases as compared to those that rotate in the opposite direction. Hence rotation direction of cylinders can be used as a desired parameter for controlling heat transfer rate. Originality/value A comprehensive report of results for the problem of conjugate conduction and mixed convection heat transfer in a circular cylinder containing different shapes of C.C, conducting obstacle and heater and cooler has been presented. An efficient numerical technique has been developed to solve this problem. The achievements of this paper are purely original, and the numerical results were never published by any researcher.

Investigación de la resistencia térmica de ladrillos de arcilla perforados mediante modelos numéricos = A thermal resistance investigation of red colored perforated clay bricks by numerical modeling

ResumenUno de los factores más importantes que afectan el comportamiento térmico de las paredes exteriores de la construcción es la conductividad térmica de ladrillos de arcilla huecos perforados horizontalmente que son ampliamente utilizados en muchos edificios en nuestro país. Los ladrillos que se encuentran comúnmente en las paredes exteriores tienen dimensiones de 13.5x19x19 cm. En este estudio, se eligieron para ser analizados dos tipos diferentes de ladrillos. Un tipo es un horizontal ladrillo hueco perforado estándar de esas dimensiones y el otro tipo es un ladrillo horizontal perforado hueco con las mismas dimensiones pero con sytropor instalado en algunos de los huecos. El efecto conjunto de la conducción y la transferencia de calor por convección natural en este tipo de ladrillo se estudió numéricamente para calcular la conductividad térmica general de los ladrillos y los demás aspectos tales como la producción y el diseño del ladrillo. La energía, el impulso, y las ecuaciones de transferencia de masa asociadas con los modelos de ladrillo se han resuelto numéricamente mediante el empleo del software comercial llamado ANSYS. La distribución de la velocidad del aire en los huecos y de la distribución típica de temperatura se muestran en las figuras, y se han determinado la conductividad térmica y la función de la diferencia de temperatura, y los resultados de conductividad térmica se compararon con los indicados en las normas. Los resultados muestran que las conductividades térmicas de los ladrillos con y sin sytropor son casi la mitad de los que figuran en las normas. Por lo tanto, se puede decir que los valores dados en la norma se consideran extremadamente conservadores. Los resultados también muestran que la convección natural que ocurre en las cavidades de aire afecta a la conductividad térmica por 0,046% y 0,068% en los casos de con y sin sytropor, respectivamente. AbstractOne of the most important factors affecting the thermal behavior of building exterior walls is the thermal conductivity of red fired horizontally perforated hollow clay bricks which are widely used in many buildings in our country. The bricks commonly encountered in the exterior walls have dimensions of13.5x19x19cm. In this study, two different types of the bricks were chosen to be analyzed. One type is a 13.5x19x19cm horizontally perforated standard hollow brick and the other type is a 13.5x19x19cm horizontally perforated hollow brick with sytropor board installed in some of the hollows. The conjugate conduction and natural convection heat transfer in these brick types was studied numerically to compute the overall thermal conductivity of the bricks and the further aspects such as the brick production and design were also investigated. The energy, the momentum, and the mass transfer equations associated with the brick models were solved numerically by employing the commercial software called ANSYS. The air velocity distribution in hollows and the typical temperature distribution were shown in figures, and the thermal conductivity as a function of temperature difference were determined and the thermal conductivity results were compared with those given in the standards. The results show that the thermal conductivities of the bricks with and without sytropor board are almost half of those given in the standards. Therefore, it can be said that the values given in the standard are considered to be extremely conservative. The results also show that the natural convection occurring in air cavities affects the thermal conductivity by 0.046% and 0.068% in cases of with and without sytropor board, respectively.

Thermal Analysis of Irradiated Fuel Subassemblies and Fuel Pins During Storage in Concrete Pits of Head-End Facility

Irradiated fuel subassembly (SA)/fuel pins, with significant decay heat are transported from reactor and stored in hot cells (HCs) before reprocessing. During transportation they are heavily shielded and no forced cooling is provided. The HCs are made of concrete structures, the outer surfaces of which are force cooled. During these processes, the fuel pin clad temperature and concrete temperatures are to be limited within specific safety limits. These temperatures are function of the decay power and geometric details of surrounding structures. To predict these temperatures, three-dimensional conjugate conduction–convection–radiation heat transfer analysis has been carried out. For this purpose, the computational fluid dynamics (CFD) code STAR-CD has been utilized, wherein individual fuel pins, steel cans, hexagonal wrapper, lead shielding blocks, and concrete structures have been considered in detail. Based on parametric studies pertaining to fuel pin transportation, it is established that for a decay power of 150 W, natural convection is adequate with maximum clad temperature of 686 K. From the studies related to storage in HCs, it is seen that nine fast breeder test reactor (FBTR) SA can stored in hot cell-1 (HC-1), with a decay power of 31.3 W per SA, to respect the temperature limits. For 3 prototype fast breeder reactor (PFBR) cans and 2 FBTR cans stored in hot cell-3 (HC-3), a decay power of 12.5 W per FBTR can and 44 W per PFBR can, can be handled without exceeding temperature limits.