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Mixed convection heat transfer characteristics from heat source located symmetrically inside square enclosure and cooled by Al2O3/water-based nanofluid flow was experimentally investigated. The configuration was subjected to high levels of natural convection and low rates of nanofluid flow. The nanofluid thermophysical properties were characterized using the available correlations in the literatures except the viscosity which was measured and correlated in terms of the nanoparticles loading ratios. Comparative analysis indicated that the application of nanofluid could not guarantee heat transfer enhancement in configurations dominated by natural convection. Exception heat transfer enhancement was only found when very low nanoparticles loading ratio was applied. Instead, heat transfer degradation was found especially in the cases of highest nanoparticles loading ratios. Alternatively, heat transfer enhancement was observed when the forced convection effect was substantial at the highest nanofluid flow rate. The present conclusions were justified and correlated to the findings reported in the literature. [ABSTRACT FROM AUTHOR]

The problem of unsteady conjugate natural convection and entropy generation within a semicircular porous cavity bounded by solid wall of finite thickness and conductivity has been investigated numerically. The governing partial differential equations with the corresponding initial and boundary conditions have been solved by the finite difference method using the dimensionless stream function, vorticity, and temperature formulation. Numerical results for the isolines of the stream function, temperature, and the local entropy generation due to heat transfer and fluid friction as well as the average Nusselt and Bejan numbers, and the average total entropy generation and fluid flow rate have been analyzed for different values of the Rayleigh number, Darcy number, thermal conductivity ratio, and the dimensionless time. It has been found that low values of the temperature difference reflect the entropy generation, mainly in the upper corners of the cavity, while for high Rayleigh numbers, the entropy generation occurs also along the internal solid-porous interface. A growth of the thermal conductivity ratio leads to an increase in the average Bejan number and the average entropy generation due to a reduction of the heat loss inside the heat-conducting solid wall. [ABSTRACT FROM AUTHOR]

Natural convection during solidification of liquids is known to impact the freezing characteristics and also lead to defect formation. In this study, we report the findings of real-time interferometric observation of bottom-cooled solidification of pure water in a cubical cavity. The results show first quantitative evidence of full-field thermal history during solidification, clearly depicting the anomalous expansion of water below 4 °C. Furthermore, based on the strength of natural convection, characterized by the Rayleigh number, we identify and report four distinct regimes of solidification, namely--conduction dominated, early convection, front instability, and sustained convection. A critical Rayleigh number that initiates instability in the solidifying front has been proposed, which is significantly different from conventional calculations of Rayleigh number relating to the initiation of flow. The study shows full-field quantitative evidence of a well-known phenomenon and provides a further understanding of flow driven nonhomogeneities in the solidifying interfaces. [ABSTRACT FROM AUTHOR]

In this study, a new coupled surface shape design (SSD) methodology named direct design method is presented for the solution of problems containing different types of convection heat transfer in which a specific distribution of either heat flux or temperature is given instead of the shape of a boundary. In the proposed method, the governing equation, without using any mathematical transformation for the physical domains, is manipulated so that the grid generation, solving fluid flow, and heat transfer as well as shape updating can all be carried out simultaneously. Five different inverse shape design problems containing different types of convection heat transfer are solved by the proposed method. All the problems are also solved using the ball-spine algorithm (BSA), which is a recently developed de-coupled algorithm, for the sake of comparison. In all problems, the effects of using different under-relaxation parameters are investigated and the capability of both approaches is compared with each other. The results show that the proposed coupled method can solve the problems better than the BSA in the sense that the direct design method converges sooner than the BSA when the same under-relaxation parameter is used for both methods. Also, it is shown that the computational cost of solving a SSD problem using the direct design method is slightly greater than solving an analysis problem. [ABSTRACT FROM AUTHOR]

In highly efficient converter systems, the power consumption of the cooling system (fans) significantly influences the total system efficiency as well as the power density. This paper investigates the potential of free convection cooled heat sinks and compares their performance to forced convection cooled heat sinks on the basis of their volume and power consumption. Underlying theoretical concepts for both types of cooling systems are summarized and their application in an optimization procedure is presented. The theoretical concepts are validated with prototype heat sinks. Finally, at an example of a half-bridge converter, it is shown that by free convection cooling, not only the efficiency but also the power density can be increased. In this case, by paralleling semiconductor devices the losses decrease approximately by a factor of three while simultaneously the heat sink volume can be reduced by approximately 50%. [ABSTRACT FROM AUTHOR]

The mass transfer of the system CO2–water was measured up to a pressure of 360 bar at a temperature of 25 °C and the results were compared to empirical predictions. The influence of a surfactant on the mass transfer was investigated.A Schlieren method and a three-mode magnetic suspension balance connected to an optical cell were used to analyse the mass transfer.A good simulation of the experimental values was achieved by linear superposition of free convection and Marangoni convection. The surfactant enforces the Marangoni convection while the solubility of CO2 stayed unaffected. [Copyright &y& Elsevier]