Convection Loop Research Articles

Corrosion of ferrous alloys exposed to thermally convective Pb-17 at% Li

A type 316 stainless steel thermal convection loop with type 316 stainless steel coupons and a Fe-9Cr-1Mo steel loop containing Fe-12Cr-1MoVW steel specimens circulated molten Pb-17 at% Li at a maximum temperature of 500°C. Specimens were exposed for more than 6000 h. Mass loss and surface characterization data were compared for these two alloys. At any particular exposure time, the corrosion of type 316 stainless steel by Pb-17 at% Li was more severe, and of a different type than that of similarly exposed Fe-12Cr-1MoVW steel. The austenitic alloy suffered nonuniform penetration and dissolution by the lead-lithium, whereas the Fe-12Cr-1MoVW steel tended to be more uniformly corroded. The presence of a ferritic layer on the type 316 stainless steel, and its susceptibility to spalling during specimen cleaning, were shown to be important in evaluating the data for this steel and in comparing corrosion losses for the two types of alloys. A model for the nonuniform penetration of type 316 stainless steel by Pb-17 at% Li was suggested.

Manganese adsorption on nickel and other structural materials in a sodium flow

Operating experience with sodium-cooled fast reactors has shown that very major complications are caused by the deposition of the radionuclide /sup 54/Mn on the surfaces of the piping and equipment of the primary circuit. The experiments were performed on a convection loop with sodium and with a source of radioactive manganese. Flat specimens of austenitic chromium-nickel steels and alloys as well as flat specimens of molybdenum and nickel were studied. The deposition of manganese on the surface of the specimens was measured by the ..gamma..-radiation of /sup 54/Mn, for which we used a ..gamma..-ray spectrometer with a semiconductor detector. The absolute /sup 54/Mn content in the deposits was determined by a comparison of the activity of the specimen with the activity of a reference source of /sup 54/Mn radiation. The measuring error did not exceed +/- 9%. The /sup 54/Mn content in the sodium was estimated from the activity of sodium samples from the loop as well as from the decrease in the activity of segments of the loop piping after the sodium had been drained. The largest deposit was detected on nickel and ferrite steels were in second place. For steels and alloys with an austenitic structure the /supmore » 54/Mn deposition is even smaller and depends on the nicikel content in the steel, growing as that content increases. The smallest deposition of /sup 54/Mn was observed on molybdenum.« less

Ion transport and the vibrating probe

The theory of ion transport in the vicinity of a vibrating probe is developed. It is shown that the convection loops produced by the probe will not affect the electrical current density, assuming that the action of the probe does not affect the sources of the current in the biological system. However, the convection loops will significantly alter the ion concentration gradients in the unstirred layer near a tissue or cell surface. The concentration gradients within each convection loop will be reduced, while the concentration gradients between the loops and outside of the loops will be increased relative to the gradients existing without the probe. As a consequence, the electrical potential gradients can be changed relative to the potential gradients existing in the absence of the convection caused by the probe. If the mobility of the ion species carrying the electrical current is greater than the average ion mobility in the medium, then a decrease in ion concentration gradient will be accompanied by an increase in electrical potential gradient, while an increase in concentration gradient will be accompanied by a decrease or even a reversal of electrical potential gradient. Thus, the electrical potential gradient measured by the probe will depend on the concentration gradient in the vicinity of the probe, which will depend in turn on the spatial relation of the convection loops to the probe. An example of the effect of the convection loops on ion concentration and electrical potential is obtained from the theory via a numerical computer calculation. Experimental tests of this theory are discussed.

Nonlinear dynamics of a convection loop: A quantitative comparison of experiment with theory

The flow in a loop of fluid, heated at constant flux over the bottom half and cooled at constant temperature over the top half, is shown to be qualitatively and quantitatively described by the nonlinear dynamics of the Lorenz model over a wide range of parameters encompassing three different chaotic flow regimes. The boundaries of the five flow regimes are determined experimentally and compared with a scaling relationship which relates the physical parameters of the loop and working fluid to the parameters of the Lorenz model. The characteristics of the three chaotic flow regimes are also discussed.

Ultrasonic flow rate measurement of low speed non-isothermal flows

In this study, a performance evaluation of a non-intrusive ultrasonic flowmeter is carried out for slow, non-isothermal flows. The purpose of the study is to investigate the adequacy of the above meter for flow rate measurements in thermal convection loops. The flowmeter is found to be useful only for part of the range of interest.

The Effectiveness of Aluminized Impregnated Drapery on Window Energy Savings

Interior environments are directly affected by the transmission of heat energy through unprotected windows. At the point of transmission at the window plane, various methods of creating a barrier can be constructed to obstruct the energy transmission by modifying the air convection loops that pass in front of the window. Drapery is likely to be the most widely accepted and applied method of shielding the window. There are endless varieties of drapery fabric that are available for use for drapery construction. The drapery used in this research was “Meta–lon” aluminized drapery because of the advertised energy–saving quality. When tested in the Test Unit Controlled Environment (TUCE), the Metalon drapery provided a barrier for the transmission of heat to the interior air volume of the TUCE. The drapery displayed the ability to transmit less of the absorbed heat from the fabric than conventional drapery. As a result of lower heat transmission rates than ordinary drapery fabric, the air plane on the interior room side of the drapery absorbed less transmitted energy. Therefore, the natural convective loops that normally disperse the heat energy were cooler than those tested with the application of the conventional drapery. The use of the aluminum impregnated drapery can provide energy savings when combined with proper drapery management (opening and closing drapery at the proper time in the day according to needs).

The optimum configuration of natural convection loops

A theoretical study is presented of the influence of loop configuration on natural convection loops (thermosyphons) heated from below and cooled from above. The loops consist of two insulated vertical branches with two horizontal connections, one of which is immersed in a constant temperature heat source and the other in a constant temperature heat sink. A one-dimensional model is utilized to determine the effects of the conduit's cross-sectional area and the aspect ratio for an optimum loop configuration. The coupled momentum and energy equations are solved and the heat transfer rates, mean velocities and temperature distributions are obtained for both laminar and turbulent flows. It is found that the influences of the aspect ratio and cross-sectional area of the loop can be combined into a single dimensionless parameter P 0 = C 3(1 + A) (W r Pr) C4 Gr , where Pr, Gr r , A, r and W are the Prandtl number, the Grashof number based on the tube radius, the aspect ratio, the tube radius and the length of the heat transfer sections respectively, and C 3 and C 4 are constants. The criterion of optimum loop configuration for maximum heat transfer rate is determined in terms of this parameter. The variation in loop configuration results in opposite effects for plus and minus variations from the optimum value of P 0. For loops with P 0 less than the critical value the heat transfer rate can be enhanced by increasing the length of the heated and the cooled sections. If P 0 is greater than the critical value, then the temperature of the working fluid will approach the wall temperature along the heat transfer sections. Further increase in the length of the heated and the cooled sections will result in a lower heat transfer rate.

Chaotic Flow Regimes in a Convection Loop

Chaotic Flow Regimes in a Convection Loop

Mass transfer behavior of a modified austenitic stainless steel in lithium

An austenitic stainless steel that was developed to resist neutron damage was exposed to lithium in the high-temperature part of a thermal convection loop for 6700 h. Specimens of this Prime Candidate Alloy (PCA) composed of 65.0 Fe-15.9 Ni-14.0 Cr-1.9 Mo-1.9 Mn-0.4 Si-0.3 Ti-0.05 C (wt %) were exposed at 600 and 570°C in both solution annealed and cold worked forms. The dissolution process was found to be similar to other austenitic alloys in flowing lithium: weight losses of PCA eventually became linearly proportional to exposure time with the specimen surfaces exhibiting porous layers depleted in nickel and chromium. However, the measured weight losses and dissolution rates of these PCA specimens were higher than those of type 316 stainless steel exposed under similar conditions and can be attributed to the higher nickel concentration of the former alloy. The effect of cold work on dissolution rates was less definitive, particularly at 570°C. At longer exposure times, the annealed PCA specimen exposed at 600°C suffered greater dissolution than the cold worked material, while no effect of prior deformation was observed by analysis of the respective surfaces.

On the effects of viscous dissipation and pressure work in free convection loops

The effects of viscous dissipation and pressure work are examined theoretically for laminar free convection loops. The appropriate governing equations are derived. Whereas previous work has considered only dissipation effects, the present paper shows that dissipation and pressure work effects are of comparable magnitude and must be considered together. Analytical solutions are presented for several open and closed loops. Both constant flux and constant temperature heating conditions are examined. Viscous dissipation and pressure work effects are found to have opposing influences on the flow in a loop. The former can enhance a flow for certain heating orientations, but the latter is usually dominant and retards a flow.

Convective heat transfer enhancement produced by secondary heat sources in a liquid nitrogen pool

This paper studies the influence of secondary heat sources on the convective heat transfer from a vertical cylindrical heater immersed in a LIN pool. Two types of secondary heat source have been used. In the first case, the heat leak through the vessel walls into the pool was varied without causing nucleation, so as to retain convective heat transfer. It was found that the convective flow loops induced in the pool produced a small enhancement of the heat transfer from the cylindrical heater. In the second case, a small horizontal heater loop was introduced into the pool below the cylindrical heater. The secondary heat flux used was such that boiling occurred at the loop to give rising bubbles surrounding the cylindrical heater. This produced a large enhancement of the convective heat transfer from the cylindrical heater. The enhancement ratio is presented as a function of the power supplied to the loop heater.

Two-Dimensional Study of Heat Transfer and Fluid Flow in a Natural Convection Loop

A study has been made of the heat transfer and fluid flow in a natural convection loop. Previous studies of these systems have utilized a one-dimensional approach which requires a priori specifications of the friction and the heat-transfer coefficients. The present work carries out a two-dimensional analysis for the first time. The results yield the friction and the heat-transfer coefficients and give their variation along the loop with the Graetz number as a parameter. Comparison is also made with experimental data for the heat flux and good agreement is obtained.

Etude de la corrosion de deux aciers ferritiques par le plomb liquide circulant dans un thermosiphon; recherche d'un modele

In the field of the investigations concerning direct contact for Molten Salts Reactors, a study of the behaviour of ferritic steels in flowing liquid lead has been untertaken. For this purpose, two thermal convection loops were constructed, respectively of Chromesco 3 and EM 12 steels. They operated during 3000 h, without plugging, with a hot leg temperature of 550°C, a ΔT of 80°C and a lead flow velocity of 0.1 m/s. It appears that the hot leg corrosion and the associated deposition in the cold leg are significantly more pronounced for Chromesco 3 compared to EM 12 steel.A unsophisticated model, only taking into account the phenomena of dissolution-deposition, liquid state diffusion and transport of the dissolved metal in liquid lead allows to explain qualitatively the results obtained with Chromesco 3 steel. Concerning EM 12 steel, the formation of a protective chromium rich oxide layer is probably involved in the corrosion mechanism.

Numerical analysis of the stability and transient behavior of natural convection loops

A general numerical model is developed for analyzing the transient thermal-hydraulic behavior of a single-phase, toroidal, natural convection loop (thermosyphon) connected to a pressurizer. The thermosyphon torus is positioned vertically, with heat sources and sinks distributed over its lower and upper sections, respectively. A set of three basic simultaneous time-dependent conservation equations for 1-dimensional flow is solved, coupled with a lateral momentum equation in one of the volume cells interacting with an external pressurizer. The unknown dependent variables are the mass velocity, enthalpy, pressure and the lateral mass velocity into/from the pressurizer. Other fluid thermodynamic and transport properties are accurately determined using tabulated steam/water table module incorporated in the model. A parametric study is performed over a wide range of operational parameters and angular displacements of the heated and cooled sections, which was impracticable in previous studies. Steady-state, transient behavior, and instability characteristics are studied and discussed. The main conclusions obtained are; (i) The prediction of dynamic instability largely depends on the assumptions related to the heat transfer coefficient and friction factor and their transient dependence on local and instantaneous thermodynamic and transport properties, (ii) In the turbulent flow regime the instability threshold qas found to be sensitive to cooling section wall temperature, (iii) Rotation of the heated/cooled sections in the vertical plane by angles of θ 0>20° about the horizontal axis θ 0= 0 , stabilizes the system's operation for all heat fluxes and wall temperatures. Good agreement is obtained between the present predictions and experimental results available in the literature.

On the stability and flow reversal of an asymmetrically heated open convection loop

Experimental results are reported for a U-shaped, free convection loop. The top of the loop is open to an isothermal reservoir. The horizontal leg and one vertical leg are heated at rates Q1 and Q2, respectively. The loop is filled either with water or a water-saturated porous medium. Symmetric heating and asymmetric heating favouring the ascending leg of the loop both yield stable flows. Asymmetric heating favouring the descending leg leads to stable flows when the ratio Q1/Q2 is above a critical value. Below this critical value, the flow is observed to oscillate with increasing amplitude until the direction of flow in the loop undergoes a reversal. A steady flow follows the reversal. Analytical results include a stability analysis and time-dependent, one-dimensional numerical calculations, both of which compare favourably with experiment.The disturbance amplification mechanism is explained in terms of thermal anomalies which move through the loop with the material motion of the fluid. Since the heating and buoyancy generation processes are in phase in the heated, descending leg, a thermal anomaly can amplify as it flows through that leg. As the anomaly moves through the ascending leg, it initiates a subsequent anomaly of opposite sign in the descending leg. The result is an oscillating flow which, under appropriate conditions, can amplify.

Transient and steady behavior of an open, symmetrically-heated, free convection loop

Experiments and analyses are reported for an open, free convection loop. The loop is U-shaped with the lower segment heated; the vertical legs are adiabatic and are connected to an isothermal reservoir. The loop is filled with water or a water-saturated porous medium. Experimental results include the starting transients and the friction factors and heat transfer rates at steady state for flow Reynolds numbers from 4 to 1000. Oscillations are observed with the onset of boiling. Single-phase stability analyses confirm the unstable rest states and the stable steady-states observed in the experiments, and reveal a conditional instability of the steady states. Numerical simulations of the starting transients are obtained and are compared with experiment. Results are applicable to geothermal, solar, and industrial open-loop thermosyphons.

The transient, steady state and stability behavior of a thermosyphon with throughflow

A study has been made of the flow, heat transfer and stability of a natural convection loop when there is an addition and withdrawal of fluid. The loop is a toroid that is oriented in a vertical plane and is heated over the lower half and cooled, by maintaining a constant wall temperature, on the upper half. The results include stable, as well as unstable configurations and also reveal multiple solutions.

Steady-state analysis of the convective loop

Steady-state analysis of the convective loop

Heat transfer by a free convection loop embedded in a heat-conducting solid

An open, free convection loop embedded near the surface of a heat-conducting solid is examined numerically. The loop is in the form of a half-torous with duct diameter d and toroidal major radius D2. The solid is heated uniformly from below. Critical Rayleigh numbers (Rac) for the onset of motion are determined. Velocities, exit temperatures, and convective heat transfer rates are determined for dD values of 10−2, 10−3 and 10−4 and for Rayleigh number ratios (RaRac from 1 to 360. The Prandtl number is 2.8 and the fluid/solid thermal conductivity ratio is kfks = 0.133. Results are compared with an analytical solution {7} for a high-conductivity solid, i.e. kfks → 0.

Control of Beryllium-7 in liquid lithium

Radiation fields created by the production of 7Be in lithium of the Fusion Materials Irradiation Test (FMIT) Facility can be sufficiently high to prevent contact maintenance of system components. Preliminary experiments have shown that 7Be will adhere strongly to the FMIT piping and components and a good control method for 7Be must be develop, ed. The initial experiments have been conducted in static stainless steel capsules and a Modified Thermal Convection Loop (MTCL). The average lithium film thickness on stainless steel was found to be 11 μm in the temperature range 495°–571°K from the capsule experiments. The short-term experiments showed that 7Be migrates to the capsule wall and the long-term experiment indicated that 7Be actually penetrates into the stainless steel wall. The diffusion coefficient for 7Be in stainless steel at 543°K was calculated to be 5.31 × 10−15 cm2/sec. The cold leg of the MTCL picked up much of the 7Be activity released into the loop. The diffusion trap, located in the cold leg of the MTCL, was ineffective in removing 7Be from lithium, at the very slow flow rates (< 3.79 × 10−4 m3/s) used in the MTCL. Pure iron has been shown to be superior to cobalt and nickel as a getter material for 7Be.