Results 1 - 10 of 12
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[en] An experimental apparatus was constructed for the measurement of thermal contact resistance in a nuclear reactor fuel elements. A special heating arrangements were made in order to obtain a measurable temperature drop in a stainless steel rod of a diameter of about 40 mm. Preliminary runs, show a relation between the heating rate and the thermal contact resistance. (authors) 1 ref, 2 figs
[en] This paper reviews several R and D activities associated with the subject of passive cooling systems, conducted by the N.R.C.Negev thermohydraulic group. A short introduction considering different types of thermosyphons and their applications is followed by a detailed description of the experimental work, its results and conclusions. An ongoing research project is focused on the evaluation of the external dry air passive containment cooling system (PCCS) in the AP-600 (Westinghouse advanced pressurized water reactor). In this context some preliminary theoretical results and planned experimental research are for the fature described
[en] Many research reactors in the world are pool type reactors that use MTR fuel elements. Those reactors are usually safe because of the large amount of water covering the reactor core and have a large cooling potential in case of an accident (Loss Of Coolant Accident - LOCA or Loss Of Flow Accident - LOFA). However, for high power levels or in a case of large breakage of one of the reactor main circulation tubes or one of the irradiation tubes, the fuel elements at the bottom of the reactor pool, will finally be exposed to the air. In that case, as long as the bottom of the fuel immersed in water there is a cooling potential to the exposed part of the fuel elements by longitudinal heat conductance and due to the boiling at the bottom part of the fuel elements. Those two mechanisms are important to assure core integrity during the accident and they were investigated in the past in few references [1-3] . In order to investigate the cooling potential of partially immersed heat generated plates, an experimental setup have been constructed. This paper present the results obtained in a set of experiments using an electrically heated plates simulating the fuel element heat fluxes, corresponding to the decay power in the fuel. Those experiments were performed to demonstrate the effectiveness of the boiling at the bottom of the fuel element as a cooling mechanism to the exposed zone
[en] Reflooding of a heated core is a complex phenomenon generally encountered in a large-break loss of coolant accident (LOCA) scenario, some times after the accumulator injection. The emergency core cooling liquid penetrates into the core and start cooling the hot rods. In Top - down rewetting, a liquid film flows down along tile wail. At the quench front, the liquid is violently sputtered off the wall and a spray of droplets is injected into the vapor field. 'The droplets bounce against the pipe walls. In Power reactor, rod bundle geometry is very prevalent. This study presents a new experimental apparatus that enable examination and validation of top-down rewetting in rod bundle geometry
[en] The present work deals with one of the flow boiling crises, the Onset of Flow Instability (OFI), in a channel in parallel to other channels of flow system. An experiment was performed with water flowing in a 8 mm diameter 1 m length, vertical tube at atmospheric pressure. Emphasis was put on the post dry out phase of that phenomenon in order to characterize the heat transfer during that phase and the recovery of the flow channel after the power to the channel was shut down. The flow rate and the channel wall temperature were measured during the dry out phase and during the recovery of the channel. It was shown that the channel and flow through the flow channel were recovered immediately when the power was shut despite the wall temperature was reached a temperature of about 400 degrees C
[en] It is shown that the implantation of protons in electrooptical substrates enables the construction of 3D structures with submicron features that are both conductive and photoconductive embedded in amorphized regions that possess reduced refractive index. The conductivity and photoconductivity are attributed to the transformation of the material into a degenerate semiconductor due to the formation of high concentration of OH- complexes that are created by the bonding of the implanted H+ ions to the O-2 ions of the lattice. It is argued that these results extend significantly the capabilities of integrated photonic circuits and devices fabricated by Refractive Index Engineering by ion implantations. (orig.)
[en] Recent studies of the minute morphology of the skin by optical coherence tomography revealed that the sweat ducts in human skin are helically shaped tubes, filled with a conductive aqueous solution. This, together with the fact that the dielectric permittivity of the dermis is higher than that of the epidermis, brings forward the supposition that as electromagnetic entities, the sweat ducts could be regarded as low Q helical antennas. The implications of this statement were further investigated by electromagnetic simulation and experiment of the in vivo reflectivity of the skin of subjects under varying physiological conditions (Feldman et al 2008 Phys. Rev. Lett. 100 128102). The simulation and experimental results are in a good agreement and both demonstrate that sweat ducts in the skin could indeed behave as low Q antennas. Thus, the skin spectral response in the sub-Terahertz region is governed by the level of activity of the perspiration system and shows the minimum of reflectivity at some frequencies in the frequency band of 75-110 GHz. It is also correlated to physiological stress as manifested by the pulse rate and the systolic blood pressure. As such, it has the potential to become the underlying principle for remote sensing of the physiological parameters and the mental state of the examined subject.
[en] The present study deals with the effects of wall geometry on the fluid flow and heat transfer in an annulus. Rectangular-rib structural wall roughness on the inner wall is considered. This roughness is characterized by rib height, width and pitch. A detailed parametric investigation of the effect of rib width-to-height ratio on the friction coefficient is performed for different rib pitches. This ratio varies from 0.125 to 6.0. Various rib pitches are considered, from those corresponding to a smooth wall, and up to the pitch-to-rib-height ratio of 16. Examples of various rib heights are also shown. The hydraulic diameter and the flow Reynolds number serve as additional parameters. Numerical simulations are performed using a modified k-ε turbulence model, which makes possible the prediction of flow separation, reattachment and adverse pressure gradients. The model is validated by comparing its results with experimental and numerical results for overall and local flow parameters reported in the literature. The results of the present study indicate that for the given hydraulic diameter of the annulus and flow Reynolds number, the pitch that corresponds to maximal flow resistance depends on the rib width-to-height ratio. It appears that when the width-to-height ratio decreases, this optimal pitch approaches an asymptotic value. On the other hand, the optimum rib distance-to-height ratio remains almost constant. These findings are discussed in context of the available literature. Generalization of the results is attempted through the use of dimensionless groups based on the flow and geometrical parameters of the systems. A correlation for the friction factor is suggested.
[en] The present work deals with natural convection on a vertical flat plate, where one side of the plate is maintained at a uniform temperature - Ta, and the other side of the plate is exposed to an environment of constant temperature -T∞. The plate is consisted of several layers of conductive and non-conductive materials such that the series thermal resistance can be expressed as an equivalent heat transfer coefficient heq 1/Σ(ki/di). It is also assumed a negligible axial conduction, which can be neglected. The present investigation treats the heat transfer problem in the laminar zone in air (Pr∼1). The wall effective heat transfer coefficient is in the range of 4.3 to 11.5 W/m2 - deg C. An experimental apparatus was constructed to confirm the heat transfer features predicted analytically in a previous work. The local experimental Nusselt number was correlated with the modified Rayleigh number, for the laminar range (authors)
[en] We find that the anomalous behavior of optical spatial screening solitons observed in the high-symmetry paraelectric phase is a consequence of nonlinear dielectric effects. These, coupled to space charge in saturated conditions, change the effective optical nonlinearity even far from the phase-transition regime