Dryout in serpentine evaporators

Waterside on-load corrosion failures in power station evaporators have resulted from dryout along the upper surfaces of the tubes in serpentine arrangements.

It is shown that this type of dryout can occur at intermediate qualities over a wide range of flow rates and pressures for heat fluxes much less than in vertical tubes.

A type of flow which is here called “surge flow” has been identified as being particularly likely to lead to dryout, because the top surfaces are wetted only intermittently by waves or plugs of frothy liquid.

A method is being developed for assessing the likelihood of dryout under surge flow conditions. It correctly predicts the dryout boundaries in a pressurized steam/water facility.     

The calculation of dryout in a rod bundle

A model of annular two-phase flow is used to calculate dryout on the assumption that dryout occurs when the liquid flowrate in the film on the solid surfaces becomes equal to zero. To enable the calculation to be performed, the processes of entrainment and deposition of liquid droplets must be adequately described. The rod bundle is divided, for calculational purposes, into rod centred subchannels, and the liquid flows in the liquid films and as droplets in each subchannel are calculated. The agreement between experiment and calculation for dryout power is encouraging.     

Convective boiling of halocarbon refrigerants flowing in a horizontal copper tube – an experimental study

An experimental study of convective boiling of refrigerants R-22, R-134a and R-404A in a 12.7 mm internal diameter, 2 m long, horizontal copper tube has been performed. Experiments involved a relatively wide range of operational conditions. Experiments were performed at the evaporating temperatures of 8°C and 15°C. Quality, mass velocity and heat flux varied in the following ranges: 5% to saturated vapor, 50–500 kg/(s m²); and 5–20 kW/m². Effects of these physical parameters over the heat transfer coefficient have been investigated. High quality experiments were also performed up to the point of the tube surface dryout, a mechanism which was investigated from the qualitative point of view. Two heat transfer coefficient correlations from the literature have been evaluated through comparisons with experimental data. Deviations varied in the range from −25% to 42%.     

Flow boiling visualization in a vertical circular minichannel at high vapor quality

This paper reports on an experimental study of saturated flow boiling of R134a inside a circular vertical quartz tube coated with a transparent heater. The inner diameter of the tube was 1.33 mm and the heated length 235.5 mm. The flow pattern at high vapor qualities and the dryout of the liquid film were studied using a high speed CCD camera at the mass fluxes 47.4 and 124.4 kg/m² s in up flow at 6.425 bar. The heat fluxes ranged from 5 to 13.6 kW/m² for the lower mass flux and from 20 to 32.4 kW/m² for the higher mass flux.

The behavior of the flow close to dryout was found to be different at low and high mass flux. At low mass flux the location of the liquid front fluctuated with waves passing high up in the tube. In between the waves, a thin film was formed, slowly evaporating without breaking up.

At high mass flux the location of the liquid front was more stable. In this case the liquid film was seen to break up into liquid streams and dry zones on the tube wall.     

Thermal-hydraulic Phenomena Relevant to Global Dryout in a Hemispherical Narrow Gap

A series of experimental investigations on the cooling mechanism in hemispherical narrow gaps has been carried out. A visualization experiment, VISU-II, was done as the first step to get a visual observation of the flow behaviour inside a hemispherical gap and to understand the mechanism inducing global dryout. It was observed that the counter-current flow limitation (CCFL) phenomenon prevented water from wetting the heater surface and induced dryout. The CHFG test was performed to measure the critical power corresponding to global dryout and to investigate the inherent cooling mechanism in hemispherical narrow gaps. Temperature measurements over the heater surface show that the two-phase flow behaviour inside the gaps could be quite different from the other usual CHF experiments. The measured values of critical power are lower than the predictions by existing empirical CHF correlations based on the data measured with small-scale horizontal plates and vertical annulus. Received on 14 April 1998     

Numerical investigation of the boiling crisis for helical cruciform-shaped rods at high pressures

The limiting factor in heat removal in engineering systems involving liquid heat transfer is the boiling crisis. The boiling crisis is especially important in the field of nuclear engineering, as it is one of the main limits on the power extracted per unit volume for both Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR). The Helical Cruciform Fuel (HCF) bundle has been proposed to increase the power density in PWRs and BWRs through enhancement of the heat transfer area-to-volume ratio and mixing of the flow compared to the traditional cylindrical fuel rod bundle geometries. In order to explore the HCF improved performance limits, computational multiphase flow dynamics is pursued here, as the experimental testing of the conditions of interest in BWR and PWR is costly. The boiling crisis at low qualities is characterized by Departure from Nucleate Boiling (DNB) and at high qualities is understood to be the result of liquid film dryout. The two-phase Eulerian approach was used to develop a robust baseline framework for prediction of DNB under high pressures that predicts the trends in boiling crisis for engineering scale systems. Such numerical technique agreed with the trends observed in the collected experimental data related to HCF type geometry.     

Spectrum of density fluctuations in a particlefluid system—II. Polydisperse spheres

This paper presents an extension of the analysis shown in Part I to a polydisperse particle-fluid system. The density autocorrelation is shown to be a function of two quantities, a generalized Overlap function for which an analytical expression is derived, and the radial distribution function (RDF). In Fourier transform space, the density spectrum again appears to be a strong function of the mean particle size, and secondarily the mean particle separation distance. One unusual result is previously observed oscillations in the density spectrum of a monodisperse system of particles are severely dampened or even eliminated in the polydisperse case, depending on the width of the particle size distribution. Apparently contributions from different particle correlations interfere with each other, thereby reducing the coherent oscillations seen in the monodisperse particle-fluid system. Furthermore at large wavenumbers, the spectrum decays with a −2 power-law, independent of the shape of the particle size distribution. This behavior can be traced to the Overlap function which controls the behavior of the spectrum beyond the first peak. Remarkably the −2 power-law spectrum is determined by the shape of the particles (i.e. spheres) rather than their spatial distribution (RDF).

The effect of an asymptotically large pressure gradient on the correlation of several important higher-order moments is revisited for the polydisperse system. The relatively simple relationships developed for the monodisperse system are lost in the polydisperse case because particles of different sizes will be influenced differently by an applied pressure gradient. The result is moments that are of different order in velocity can no longer be related to each other (as they were in the monodisperse system), even in this idealized flow. A more comprehensive understanding of this phenomenon can only be achieved through direct numerical simulation or experiment.     

Two-phase flow measurements with sharp-edged orifices

This paper contains the results of a set of two-phase flow measurements of 4 different ratios of vapor to liquid density (up to 0.328) across a sharp-edged circular orifice. Test fluid was R-113. Tests were carried out upon 3 orifices whose diameter ratios were 0.312, 0.439 and 0.625. The test quality ranged from 0–100%, while the mass velocity from 917–1477 kg/m².s. On the basis of a modified separated flow model, a relationship is developed for the flow rate and quality and is compared with experimental data and 5 proposed correlations. Comparison shows this method can be used to calculate the flow rate or the quality of vapor liquid (or steam water) mixture in the range 0.00455 to 0.328 of the density ratio, and in pipe size ranging from 8 to 75 mm (β = 0.25–0.75).

The RMS error of this method is about 12% when the quality, x, ranges from 2% to 100%.     

Minimum fuel trajectory transfer in a general gravity field using a finite number of engine firings to cause velocity discontinuities

A method is presented to compute the firing logic for a rocket to transfer from one arbitrary trajectory to another such trajectory in a fixed maximum number of firings of the engine in a manner to minimize the amount of fuel consumed while satisfying various orbital constraints.

The iterative procedure presented may be used to solve other constrained non-linear parameter minimization problems. The method involves iteratively minimizing the first differential of the “cost” function subject to improving the first differentials of the constraints by using linear programming. Both equality and inequality constraints are amenable to solution by this method.     

Experimental study of CHF in vertical and horizontal tubes cooled by freon-12

The influence of test section orientation and diameter on flow boiling crisis occurring in tubes has been studied experimentally using Freon-12 as a coolant. At low mass flux the critical heat flux (CHF) was lower in horizontal flow than in vertical. As either the liquid or vapour velocity, or both, were increased the vertical and horizontal CHF results converged. Above a mass flux of 4Mg · m⁻² · s⁻¹ the results were essentially identical.

The effect of tube diameter on boiling crisis in general depends crucially on the parameters which are maintained constant when the comparison is made.     

Large-scale turbulence phenomena in compound rectangular channels

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.     

On the radiation properties of an asymmetric cylinder

The radiation properties of an asymmetric cylinder, formed by slicing a circular cylinder with a plane making an angle π/2 − Λ with the cylinder axis, are investigated as a model problem of relevance to noise emission by novel aeroengine intakes. We consider the scattering of incident duct modes and use asymptotic analysis in the limit Λ 1; the unsteady flow then comprises an inner incompressible region around the cylinder rim and an outer region comprising the rest of space, and the radiation in the outer region is determined using the method of matched asymptotic expansions. From this we are able to conclude that the correction to the radiation directivity is O(Λ), whilst the correction to the total integrated sound power reaching infinity is much weaker, and is in fact only O(Λ²).     

Effect of velocity and pipeline configuration on dispersion in turbulent hydrocarbon-water flow using laser image processing

Drop size distribution and concentration profile data for hydrocarbon-water mixtures are obtained in a 8.2 cm dia pipe at a range of velocities for a straight horizontal pipe, horizontal and vertical flow after one bend and vertical flow after three bends. The laser image processing technique employed in this project is proven reliable.

The maximum drop size (d₉₉), is more dependent on the number of upstream interactive bends than on the velocity. The drop size distributions follow a Rosin-Rammler power law. The values of Rosin-Rammler exponents, based on this work, are on average 2.1 for all the configurations studied.

The concentration profiles as a function of velocity for straight horizontal flow are obtained and show the transition from stratified to adequately dispersed flow at about 2.3 m/s velocity. The concentration profiles for horizontal or vertical flow after one bend show dispersed flow in some cases; however, in other cases swirling makes representative sampling more difficult.

Vertical downflow after three interactive bends breaks the droplets to a finer size, and concentration profiles obtained in this location are more uniform than the other configurations studied. Representative sampling can be accomplished in this location even at 0.7–1.0 m/s velocity, in a 8.2 cm pipe.     

RANS simulations for transition and turbulent flow regimes in wire-wrapped rod bundles

Internal flows through rod assemblies are commonly found in heat exchangers, steam generators, and nuclear reactors. One of the fuel assembly designs considered for liquid metal-cooled reactors utilizes wires helically wrapped around each fuel rod as spacers. The wires keep the fuel pins separated, enhancing the turbulent mixing, and heat transfer, but also affecting the pressure drop. It is of interest the understanding of the fluid flow phenomena in the sodium-cooled fast reactor as it is one of the Generation IV advanced reactor designs and it has been a motivating topic of research for the last decade. A wire-wrapped fuel assembly replica with 61-pins has been in operation at the Thermal-Hydraulic Research Laboratory of Texas A&M University. This facility produced high-fidelity velocity and pressure drop data for validation of computational fluid dynamics codes. This study investigates the effects of geometrical features and operating conditions on the flow behavior of the 61-pin wire wrapped bundle using Reynolds-Averaged Navier-Stokes (RANS) models to predict the axial and transverse pressure drops for a range of Reynolds numbers from 1,270 to 100,000. The friction factor predictions were in satisfactory agreement with the experimental data and the Upgraded Chen and Todreas correlation. The internal subchannel velocity results were compared with experimental data and Large Eddy Simulations (LES) and found in reasonable agreement. This study demonstrates that RANS is a suitable approach in predicting velocity and pressure fields in wire-wrapped rod bundles, with a relatively low computational effort.     

Steady laminar flow through thin curved pipes

Let R, τ denote, respectively, the radius of curvature and radius of torsion of the pipe (centre-line) and let a be a typical cross-sectional diameter.

The major part of the present paper addresses the case of flows through pipes of constant cross-section; (Re)²(a/R), Re(a/τ), (a/R) and (a/τ) all being small. Re is the Reynolds number for the flow. It is found that, even without further specifications of the details of the pipe, many important results can be obtained about the secondary flow which occurs and the pressure losses resulting from it. For example, it is shown that an important feature of such flows is valid for any corss-sectional shape; this was not obvious from previous works which treated only special cases having significant symmetries. Also, a new method for calculating the modified axial pressure gradient is presented which reduces dramatically the amount of work required therefor.

The remainder of the paper presents some results for similar flows through pipes of varying cross-section.     

Thermal and hydraulic performance of a three-phase fluidized bed cooling tower

An experimental study was made of the thermal and hydraulic characteristics of a three-phase fluidized bed cooling tower. The experiments were carried out in a packed tower of 200 mm diameter and 2.5 m height. The packing used was spongy rubber balls 12.7 mm in diameter and with a density of 375 kg/m³. The tower characteristic was evaluated. The air-side pressure drop and the minimum fluidization velocity were measured as a function of water/air mass flux ratio (0.4–2), static bed height (300–500 mm), and hot water inlet temperature (301–334 K).

The experimental results indicate that the tower characteristics KaV/L increases with increases in the bed static height and hot water inlet temperature and with decreases in the water/air mass flux ratio. It is also shown that the air-side pressure drop increases very slowly with increases in air velocity. The minimum, fluidization velocity was found to be independent of the static bed height.

The data obtained were used to develop a correlation between the tower characteristics, hot water inlet temperature, static bed height, and the water/air mass flux ratio. The mass transfer coefficient of the three-phase fluidized bed cooling tower is much higher than that of packed-bed cooling towers with higher packing height.     

Two-phase stratified flow splitting at a T-jun

The objective of this study is to investigate, experimentally and theoretically, two-phase splitting under stratified wavy flow conditions at a regular horizontal T -junction with an inclined branch arm.

Experimental data have been acquired with the side arm at horizontal conditions, downward inclination angles of −5, −10, −25, −40 and −60°, and upward inclinations of 1, 5, 10, 20, and 35° from the horizontal. The data reveal that gravity forces have a significant effect on the flow splitting. For downward inclination of the side arm more liquid is diverted into the branch arm, as compared to the case in which the side arm was horizontal. All the liquid was found to be diverted into the branch arm when the branch arm inclination was increased to −60°. For upward inclination angles a significant amount of the inlet gas has to be diverted into the side arm in order to get any liquid to flow into that arm. However, once liquid has started flowing, not much more additional gas has to be diverted into the side arm to get all of the liquid to flow into the branch. At 35° almost all the gas has to be diverted for any liquid flow into the branch.

A mechanistic model has been developed for the prediction of the splitting phenomenon for both the horizontal and the downward orientations of the side arm. The model is based on the momentum equations applied for the separation streamlines of the gas phase and the liquid phase. Very good agreement is observed between the prediction of the model and the data acquired for all the cases.     

Modelling Pressurized Water Reactor cores in terms of porous media

The aim of this study is to develop a tractable model of a nuclear reactor core taking the complexity of the structure (including its nonlinear behaviour) and fluid flow coupling into account. The mechanical behaviour modelling includes the dynamics of both the fuel assemblies and the fluid. Each rod bundle is modelled in the form of a deformable porous medium; then, the velocity field of the fluid and the displacement field of the structure are defined over the whole domain. The fluid and the structure are first modelled separately, before being linked together. The equations of motion for the structure are obtained using a Lagrangian approach and, to be able to link up the fluid and the structure, the equations of motion for the fluid are obtained using an arbitrary Lagragian Eulerian approach. The finite element method is applied to spatially discretize the equations. Simulations are performed to analyse the effects of the characteristics of the fluid and of the structure. Finally, the model is validated with a test involving two fuel assemblies, showing good agreement with the experimental data.     

Inertial instability of the Stewartson -layer

Inertial stability of a vertical shear layer (Stewartson E^1/4-layer) on the sidewall of a cylindrical tank with respect to stationary axisymmetric perturbations is inverstigated by means of a linear theory. The stability is determined by two non-dimensional parameters, the Rossby number Ro = U/2ΩL and Ekman number E = v/ΩH², where U and L = (E/4)1/4H are the characteristic velocity and width of the shear layer, respectively, Ω the angular velocity of the basic rotation, v the kinematic viscosity and H the depth of the tank.

For a given Ekman number, the flow is more unstable for larger values of the Rossby number. For E = 10⁻⁴, which is a typical value of the Ekman number realized in rotating tank experiments, the critical Rossby number Ro_c for instability and the critical axial wavenumber m_c non-dimensionalized by L⁻¹ are found to be 1.3670 and 8.97, respectively. The value of Ro_c increases and that of m_c decreases with increasing E.     

Evaluating the results of the single-blow transient heat exchanger test

Single-blow test data are often used as the basis for fundamental convective heat transfer correlations. However, if the heat transfer model used to relate the measured temperature history to an avergge heat transfer coefficient does not accurately describe the experiment, then the accuracy of the data may be low.

The results of simulations are presented that quantify the error in the estimated heat transfer coefficient due to specific common mismatches between the model and the experiment. The areas of mismatches include inlett fluid temper history, longitudinal conduction in the solid, variable local convective coefficient, and effective core mass.

One symptom of a mismatch between the model and the experiment is that different criteria for comparing the measured and predicted temperature histories may yield different estimates for the performance of the same heat exchanger. The results obtained using direct temperature history and derivative and integral evaluation criteria are compared. It is shown that even if two different crit yield similar estimates for the average heat transfer coefficient, these estimates may be far from correct.

The results presented in this paper lead to the recommendation that singl-blow facilities be calibrated with tests on a core of known performance to demonstrate the appropriateness of the model as well as the accuracy of the measurements and thus establish a minimum level of confidence in new data.