Leonhard Meyer

Turbulent convective heat transfer from rough surfaces with two-dimensional rectangular ribs

Abstract Artificial roughness is often used in nuclear reactors to improve the thermal performance of the fuel elements. Although these are made up of clusters of rods, the experiments to measure the heat-transfer and friction coefficients of roughnesses are performed with single rods contained in smooth tubes. The paper illustrates a new transformation method to obtain data applicable to reactor fuel elements from these annulus experiments. New experimental friction data are presented for ten rods, each with a different artificial roughness made up of two-dimensional rectangular ribs. For each rod four tests have been performed, each in a different outer smooth tube. For two of these rods, each for two different outer tubes, heat-transfer data are also given. The friction and heat-transfer data, transformed with the present method, are correlated by simple equations. In the paper, these equations are applied to a case typical for a Gas Cooled Fast Reactor fuel element.

Experimental investigation of turbulent transport of momentum and energy in a heated rod bundle

Turbulent air flow in a central channel of heated 37-rod bundles with triangular array at two different pitch-to-diameter ratios (P/D=1.12 and P/D=1.06) was investigated. Measurements were performed with a hot-wire probe with x-wires and an additional temperature wire. Time mean velocities, time mean fluid temperatures, wall shear stresses and wall temperatures, turbulent quantities such as the turbulent kinetic energy, all Reynolds stresses and all turbulent heat fluxes were measured at two different pitch-to-diameter ratios in a central channel of the bundle. It is shown that with decreasing gap width the turbulence field in rod bundles deviates significantly from that in a circular tube. Also, data on the power spectral density functions of the velocity and temperature fluctuations are presented. These data show the existence of large-scale periodic fluctuations of velocity and temperature in the gap region of two adjacent rods. These fluctuations are responsible for the high intersubchannel heat and momentum exchange. Spectral measurements with two hot wire probes imply a distinct similarity of motion of vortices in adjacent subchannels of the bundle.

Large-scale turbulence phenomena in compound rectangular channels

Abstract In this article we investigate turbulent flow of air through compound rectangular channels to experimentally investigate the turbulence phenomena in compound channels. Detailed experimental data of axial mean velocity, wall shear stresses, five of six Reynolds stresses, auto- and cross-spectral densities, and two-point space correlations were measured by hot-wire anemometry in 18 geometrical configurations. The symmetry of the present flow appears to be better than that of previous measurements and the range of measurments is more extensive. The most interesting result is the existence of a quasi-periodic large-scale turbulence structure in most of the geometries investigated. This structure is stationary and independent of the axial position in the channel. It exists in any longitudinal slot or groove in a wall or a connecting gap between two flow channels, provided its depth is more than approximately twice its width. The frequency of this flow oscillation is determined by the geometry of the slot and is linearly dependent on the bulk velocity.

Characteristics of turbulent velocity and temperature in a wall channel of a heated rod bundle

Turbulent air flow in a wall subchannel of a heated 37-rod bundle (PD = 1.12, WD = 1.06) was investigated. Measurements were performed with a hot-wire probe with x-wires and a temperature wire. The mean velocity, mean fluid temperature, wall shear stress and wall temperature, turbulent quantities such as the turbulent kinetic energy, Reynolds stresses, and turbulent heat fluxes were measured and are discussed with respect to data from isothermal flow in a wall channel and heated flow in a central channel of the same rod bundle. Also, data on the power spectral densities of the velocity and temperature fluctuations are presented. These data show the existence of large-scale periodic fluctuations of velocity and temperature in the gap region between two adjacent rods or between rods and the wall. These fluctuations are responsible for the high intersubchannel heat and momentum exchange.

Investigations of Sloshing Fluid Motions in Pools Related to Recriticalities in Liquid-Metal Fast Breeder Reactor Core Meltdown Accidents

This paper reports that analyses of unprotected loss-of-flow accidents for medium-size cores of current liquid-metal fast breeder reactors have shown that the accident proceeds into a transition phase where further meltdown is accompanied by recriticalities and secondary excursions. Assuming very pessimistic conditions concerning fuel discharge and blockage formation, a neutronically active whole-core pool of molten m material can form. Neutronic or thermohydraulic disturbances may initiate a special motion pattern in these pools, called centralized sloshing, which can lead to energetic power excursions. If such a whole-core pool is formed, its energetic potential must be adequately assessed. This requires sufficiently correct theoretical tools (codes) and proper consideration of the fluid-dynamic and thermo-hydraulic conditions for these pools. A series of experiments has been performed that serves as a benchmark for the SIMMER-II and the AFDM codes in assessing their adequacy in modeling such sloshing motions. Additional phenomenologically oriented experiments provide deeper insight into general motion patterns of sloshing fluids while taking special notice of asymmetries and obstacles that exist in such pools.

Large eddy simulation and measurement of the structure of turbulence in two rectangular channels connected by a gap

Theoretical and experimental investigations of the structure of turbulent flow have been performed in a channel consisting of two rectangular channels which are connected by a gap near a wall. Hot-wire experiments showed that very high Reynolds stresses occur close to the gap. Large eddy simulations confirmed the experimental results and showed that large scale streamwise vortices move within the gap along the whole length of the channel.

A comparison of the heat transfer and friction factor performance of four different types of artificially roughened surface

Abstract A comparison is made of the heat transfer and friction factor performance of four different types of artificially roughened surface. Each surface has near-optimum thermal performance for its own particular type of roughness. These roughened surfaces have been developed and tested as part of nuclear reactor research programmes with the object of improving the rate of heat removal from fuel pins in gas-cooled reactors. The empirical data are taken from single pin tests using gaseous coolants. The comparison of thermal performance is made on the basis of transformed data, with some extrapolation using roughness parameter techniques. A discussion is included on other factors which may be important in the selection of an artificially roughened surface for practical use.

Thermohydraulic characteristics of single rods with three-dimensional roughness

Abstract An investigation has been carried out into the heat transfer and friction characteristics of threedimensional roughnesses consisting of uniformly distributed protrusions with sharp edges. The results of measurements of the velocity and temperature distributions in rough annuli are used to analyze data obtained from measurement of the pressure drop and heat transfer on seven single rods with different roughnesses contained in up to four smooth outer tubes. The results show that three dimensional roughnesses, within a certain range of rib parameters, produce higher friction factors and Stanton numbers than two-dimensional roughnesses. For such roughnesses the law of the wall does not hold as regards velocity and temperature distributions. Correlations for determination of friction factors and Stanton numbers in different annuli are given in the paper. A simple method of transforming the Stanton number measured in an annulus into an arbitrary annular cross section is presented.

Low-pressure corium dispersion experiments with simulant fluids in a scaled annular cavity

Abstract Experiments were performed in a scaled annular cavity design, to investigate melt dispersal from the reactor pit when the reactor pressure vessel (RPV) lower head fails at low system pressure of less than 2 MPa. The fluid dynamics of the dispersion process was studied using model fluids, water, or bismuth alloy instead of corium, and nitrogen or helium instead of steam. The effects of different breach sizes and locations and different failure pressures on the dispersion were studied, specifically by testing central holes, lateral holes, horizontal rips, and complete unzipping of the bottom head. With holes at the base of the bottom head, the most important parameters governing the dispersion of melt are the hole size and the burst pressure. The fraction dispersed into the reactor compartments increases with larger holes and higher pressures. Values up to 76% have been found for both melt simulant liquids, water, and metal. With lateral breaches the liquid height in the lower head relative to the upper and lower edge of the breach is an additional parameter for the dispersion process, and usually not all the liquid is discharged out of the RPV. The liquid fraction entrained out of the RPV can be higher with a small breach than with a large one because of the longer blowdown time. With lateral failures, maximum dispersed fractions of 50% were found with water as melt simulant and less than 1% with liquid metal. It follows from similarity considerations that the results from the liquid metal tests represent the lower bound for the dispersed melt fractions; however, they are probably closer to the expected values than the results from the water tests, which represent the upper bound. So, significantly less dispersion of melt can be expected for lateral breaches at pressures below 2 MPa, probably less than 10%. If higher dispersion occurs, due to higher pressure at failure or with failures near the bottom center, simple devices to reduce the dispersion out of the cavity may be feasible.

QUEOS, an experimental investigation of the premixing phase with hot spheres

Abstract A total of 60 experiments with hot spheres, to investigate the premixing phase of a steam explosion, have been performed in the QUEOS facility at Forschungszentrum Karlsruhe (FZK). Here, the data of seven experiments are presented using three types of spheres at 1800 K and total volumes of ≈2, and 4l, respectively. The sphere jet plunging into the water had a diameter of 10 cm and a length between 60 and 120 cm. The average solid volume fraction was ≈25%. These relatively long jets and high particle volume fractions show a different behavior in the water, compared to the 18 cm wide short pours of a first series of QUEOS experiments. They are also unlike experiments performed in other facilities with much lower particle volume fractions. High speed films were taken, pressures, water temperatures and the steaming rate were measured.