The velocity profile of laminar MHD flows in circular conducting pipes

We present numerical simulations without modeling of an incompressible, laminar, unidirectional circular pipe flow of an electrically conducting fluid under the influence of a uniform transverse magnetic field. Our computations are performed using a finite-volume code that uses a charge-conserving formulation [called current-conservative formulation in references (Ni et al J Comput Phys 221(1):174-204, 2007, Ni et al J Comput Phys 227(1):205-228, 2007)]. Using high resolution unstructured meshes, we consider Hartmann numbers up to 3000 and various values of the wall conductance ratio c. In the limit $${c{\ll}{\rm Ha}^{-1}}$$ (insulating wall), our results are in excellent agreement with the so-called asymptotic solution (Shercliff J Fluid Mech 1:644-666, 1956). For higher values of the wall conductance ratio, a discrepancy with the asymptotic solution is observed and we exhibit regions of velocity overspeed in the Roberts layers. We characterise these overspeed regions as a function of the wall conductance ratio and the Hartmann number; a set of scaling laws is derived that is coherent with existing asymptotic analysis. © 2009 Springer-Verlag.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Turbulent effects of liquid metal flow under strong fringing magnetic fields

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Numerical simulation of a liquid-metal flow in a poorly conducting pipe subjected to a strong fringing magnetic field

Using high resolution numerical simulations, we study the flow of a liquid metal in a pipe subjected to an intense decreasing magnetic field (fringing magnetic field). The chosen flow parameters are such that our study is directly relevant for the design of fusion breeder blankets. Our objectives are to provide a detailed description of the numerical method and of the results for benchmarking purposes but also to assess the efficiency of the so-called "core flow approximation" that models liquid-metal flows under the influence of intense magnetic fields. Our results are in excellent agreement with available experimental measurements. As far as the pressure drop is concerned, they also match perfectly the predictions of the core flow approximation. On the other hand, the velocity profiles obtained in our numerical simulations show a significant departure from this approximation beyond the inflection point of the magnetic field's profile. By plotting the momentum budget of the MHD equations, we provide evidence that this discrepancy can be attributed to the role of inertia that is neglected in the core flow approximation. We also consider a case with vanishing outlet magnetic field and we briefly illustrate the transition to turbulence arising in the outlet region of the pipe. © 2011 American Institute of Physics.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Frost detection method on evaporator in vapour compression systems

To preserve food nutrients, texture, and taste, as well as to prevent its putrefaction, food is frozen and kept at around −20 °C. Refrigerators and freezers are highly energy demand systems which can suf- fer a considerable decrease in operational efficiency due to frost growth on the evaporator. Defrost pro- cesses are launched periodically to avoid the frost built-up, consuming a relevant part of the total energy demand. To control the defrost launching and to improve the energy performance of the refrigeration system an accurate measurement of the frost level is required. Many frost detecting methods are expen- sive, not feasible due to their size, or simply they cannot measure the frost stacking precisely enough to swerve mal-defrost phenomena. This study provides an accurate parameter to indirectly estimate the frost layer built-up on the evaporator. The new parameter called thermal variation easiness (TVE) was experimentally tested and validated by comparison with another frost leveling method, T method, on a walk-in freezer unit. Then the TVE was successfully tested on a multi-cold room refrigeration system, proving its applicability on both walk-in freezers run by remote condensing units and multiple cold rooms fed by a rack of compressors. The novelty of this parameter lays on its capacity to work on refrigeration facilities which are used to feed several walk-in fridges and refrigerated displays in big installations, such as supermarkets