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We report experimental and numerical studies of combined natural and magnetic convection of a paramagnetic fluid inside a cubical enclosure heated from below and cooled from above and subjected to a magnetic field gradient. Values of the magnetic field gradient are in the range 9 |grad|b 0 | 2 | 900 T 2 /m for imposed magnetic field strengths in the center of the superconducting magnet bore of 1 |b 0 | max 10 T. Very good agreement between experiments and simulation is obtained in predicting the integral heat transfer over the entire range of working parameters (i.e., thermal Rayleigh number 1. Natural convection for heated-from-below configurations serves as a paradigm for a wide range of environmental, astrophysical, and industrial applications [1]. An interesting case of thermal convection is when the working fluid becomes magnetized in the presence of an external magnetic field. In such a case, in addition to gravity, the magnetization force is also important. By changing the strength and orientation of the imposed magnetic field, additional possibilities for affecting flow and heat transfer can be investigated. Possible areas of application where magnetic fields can be used to control flow and heat transfer include control of the growth rate and microstructure of materials [2] or protein crystals [3]. Starting from the pioneering work of Braithwaite et al. [4], the potential for magnetically controlled convection in paramagnetic, or even in ordinary (mainly diamagnetic), fluids (water) has been investigated both experimentally and numerically, e.g., [5][6][7][8][9][10][11][12]. A major contribution of these studies was in providing the integral heat transfer (Nusselt number) behavior under strong magnetic field gradients and in reporting some basic flow visualisations. All these studies addressed steady laminar flow regimes. The goal of our present investigation is to extend the range of working parameters toward transitional or potentially turbulent flow regimes. This is achieved, first, by employing significantly larger magnetic field gradients generated by state-of-the-art superconducting helium-free magnets (up to 900 T 2 /m for magnetic field strength of 10 T, in contrast to up to 200 T 2 /m for magnetic field strength of 5 T in previous studies); second, by using fluids with lower Prandtl numbers; and, finally, by performing a series of three-dimensional time-dependent simulations that revealed detailed insights into the dynamics and spatial reorganization of flow and thermal structures. Strong magnetic field gradients are generated by the superconducting helium-free magnet shown in Fig. 1. The calculated distributions of the magnetic field and resulting gradients for the upper limit of working conditions are shown in Fig. 2. The cubical enclosure can be positioned at different locations along the vertical axis of the superconducting magnet and combined effects (subtractive or additive) of the gravitational and magnetization forces can be generated. The suppression of the flow and heat transfer can be...

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A tendency to increase the importance of so-called dispersed generation, based on the local energy sources and the working systems utilizing both the fossil fuels and the renewable energy resources is observed nowadays. Generation of electricity on industrial or domestic scale together with production of heat can be obtained for example through employment of the ORC systems. It is mentioned in the EU directive 2012/27/EU for cogenerative production of heat and electricity. For such systems the crucial points are connected with the heat exchangers, which should be small in size but be able to transfer high heat fluxes. In presented paper the prototype microjet heat exchanger dedicated for heat recovery systems is introduced. Its novel construction is described together with the systematical experimental analysis of heat transfer and flow characteristics. Reported results showed high values of the overall heat transfer coefficient and slight increase in the pressure drop. The results of microjet heat exchanger were compared with the results of commercially available compact plate heat exchanger.