Results 1 - 10 of 8991
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[en] The papers deals with a numerical 3D study of natural convection in a finned Rayleigh-Be nard (RB) cubical enclosure. A single fin with a thickness of 10 pour cent of the cavity side (and a height of 50 pour cent ) is placed vertically on the bottom hot wall at TH. The working fluid is air with Prandtl number Pr = 0.71 and the Rayleigh number (Ra) varies from 103 to 105. The solid-to-fluid thermal conductivity ratio (kR) was fixed at RK = 7000, corresponding to a metal of high conductivity. The top wall is at the temperature TC < TH and the remaining four surfaces are insulated. Inside the RB enclosure, the flow structure and the temperature distribution are presented in terms of mean velocity vector plots and isotherm plots. The effects of the Rayleigh number on the mean heat transfer rate through the cold wall are presented and discussed. A correlation between the averaged Nusselt number through the top wall and Ra is proposed
[en] Highlights: •Experimental and numerical investigations on expansion tank dynamics are reported. •Investigations on natural circulation loop (NCL) performance are presented. •Role of expansion tank in loop single-phase NCL dynamics has been investigated. •A model has been proposed for expansion tank dynamics. •Role of expansion tank in hysteresis observed experimentally is explained. -- Abstract: Expansion tank is an important component of all single-phase natural circulation loops. The tank serves the twin purposes of venting the air out during the loop filling and accommodation of the swells and shrinkages of the loop fluid during the transient. In the present study, experimental investigations have been carried out in a rectangular single-phase natural circulation loop to bring out the role of expansion tank in loop dynamics. The results on loop stability and transient behavior are discussed. The experimental investigations show that there is significant amount of heat exchange between the main loop fluid and the expansion tank fluid. First, the results of experimental studies are presented. The different flow regimes observed in these experimental investigations are discussed. Next, a model is presented for taking into account the heat exchange between the main loop fluid and the expansion tank fluid. Finally, the model is applied to simulate the dynamic behavior of different single-phase natural circulation loops. The investigations carried out in this study have helped in resolving the issue of hysteresis observed in these loops.
[en] The heat line, as defined by Kimura and Bejan does not involve, explicitly, a reference temperature for the energy converted by the flow. As a result different flow patterns may be obtained for the same physical problem. In this paper the heat function is re-defined and the temperature of the cold wall, in natural convection flow problem inside cavities, is used as reference temperature. The heat function, as defined here, is also written in a generalized coordinate system and is employed to visualize the heat flow in some natural convection problems inside irregular cavities. (author)
[en] It may be stated that for the calculation of the heat transfer coefficients for natural convection cooling of large-diameter finned surfaces it is necessary to take in consideration the effects of the fin length, fin material and of the azimuthal angle around the cylinder. The present results replace earlier less accurate correlations gained from the same measurements. The new, better correlation was elaborated with the aid of the improved method of evaluation of multiple parameters of a measurement matrix of multiple dimension (one dimension for each parameter), rendering the sum of the squares error a minimum at the same time for the complete matrix. 5 references
[en] To increase the capacity of decay heat removal of a LMR system that uses a natural air circulation cooling, feasibility of heat transfer enhancement has been studied for a planar air channel by introducing a new channel configuration using radiation-convection structures of compact heat transfer surface. For the new channel configuration, the heat transfer mechanism has been investigated and design guides for the radiation-convection structure have been developed based on the investigation results. Following the developed design guides, a new radiation-convection structure has been also devised. Analysis of the air channel cooling with the new radiation structures revealed substantial heat transfer enhancement and the feasibility of the heat transfer enhancement with the new channel configuration design has been confirmed. (orig.)
[en] Highlights: • Inlet subcooling, pressure and gap size affect the Ledinegg flow instability. • Ledinegg flow instability is classified into 4 stages for narrow channels. • Empirical formula is proposed for initial power of Ledinegg flow instability. - Abstract: Ledinegg flow instability of two-phase natural circulation has been experimentally investigated based on a natural circulation device with narrow rectangular channels. When heating power reaches a certain range, the Ledinegg flow instability occurs in the system, accompanied by flow oscillation. The stability of the experimental system increases with the increasing of inlet sub-cooling, pressure and size of rectangular channels. Stability boundaries of the system are obtained by the sub-cooling number and the phase change number. It is discovered that there are 4 stages of Ledinegg flow instability in the narrow rectangular channels: condensation, ebullition, choking, and rebalance. Finally, the experience correlation is proposed for the initial power of Ledinegg flow instability in narrow rectangular channels.