La experimentación y posterior modelización de los procesos de transporte y transferencia de calor y de masa en medios poroso saturados encuentra un gran número de dificultades que se derivan fundamentalmente de la heterogeneidad del propio medio, de la metodología de su parametrización estructural y física para asimilarlo a un medio continuo.Los objetivos planteados en la presente tesis se han orientado hacia un mejor conocimiento de la influencia de diversos parámetros estructurales del medio poroso, así como de las propiedades de las substancias que constituyen la matriz sólida y la fase fluida saturante, en las características de ebullición de un medio poroso inicialmente saturado, calentado por su frontera inferior y limitado por una capa superior del mismo líquido saturante.A tal fin, se ha estudiado la influencia de la estructura del medio poroso (granular o fibrilar) y de la naturaleza de la sustancia que constituye la matriz sólida sobre la permeabilidad del medio poroso al agua y a una solución acuosa de tensioactivo, de baja concentración. Así mismo se ha estudiado la influencia respecto a la conductividad y difusividad térmicas efectivas en régimen no estacionario. Por último, utilizando la misma variedad de medios porosos saturados, se estudia el proceso de ebullición hasta que se alcanzan condiciones de "dryout", y se analizan las consecuencias que resultan de la variación de la estructura física del medio poroso, de la naturaleza de la sustancia que constituye su matriz sólida y de las propiedades del fluido saturante.En lo referente a las características fluidodinámicas y térmicas de los medios porosos estudiados se ha podido concluir:- La adición de un tensioactivo al agua saturante del medio poroso produce un comportamiento diferente según la naturaleza del sólido: en caso de inorgánica (arena) ocasiona un aumento de la permeabilidad intrínseca, mientras que en caso de orgánica (fibras de algodón) produce una reducción tanto mayor cuanto menor es la porosidad del medio poroso. Las causas de este diferente comportamiento, son las notables diferencias de absorción del tensioactivo según el tipo de sólido (orgánico o inorgánico) y la mejora substancial de la humectación de la superficie del sólido inorgánico por el fluido lo que activa la eficacia de desplazamiento de toda fase no acuosa adsorbida o retenida entre partículas.- La difusividad térmica efectiva promediada espacialmente tiende al valor de la difusividad del componente del medio poroso de menor difusividad térmica a medida que transcurre el tiempo de calentamiento.- La difusividad térmica efectiva de los medios porosos saturados en los que &#61548;s / &#61548;l < 1 se aproxima a la de la fase líquida; en los medios para los que &#61548;s / &#61548;l >> 1, dicha difusividad térmica efectiva es un grado de orden superior a la de la fase líquida.- La adición de tensioactivo a la fase líquida saturante provoca la disminución de la conductividad térmica efectiva de medio poroso saturado en aquellos en que la fase sólida es granular e inorgánica.En lo referente al proceso de calentamiento de un medio poroso saturado hasta ebullición-"dryout" de su fase líquida se ha descrito un modelo físico de comportamiento de los diferentes medios porosos que comporta las siguientes fases:i) Calentamiento del medio hasta la temperatura de saturación de su fase líquida, con evidente aumento de volumen de las fases sólida y líquida por dilatación térmica.ii) Proceso de evaporación con formación de una capa bifásica cuya frontera superior se desplaza a la velocidad del frente de vapor. Simultáneamente se produce una disminución de la presión fluidoestática en la frontera de la capa bifásica, lo que se traduce en una reducción del reflujo de líquido hacia la placa calefactora.iii) Total desaturación de la entrefase medio poroso-placa calefactora al recibir por reflujo menos líquido del que es capaz de evaporar la placa calefactora. Aparición del "dryout" y elevación progresiva de la temperatura de la placa.iv) Aparición, en algún caso, de un fenómeno de basculamiento de la fase líquida desde la capa subenfriada a la zona desaturada del medio poroso. / A large number of difficulties are found in the experimentation and later modelization of transport and transfer heat and mass process in saturated porous media, which basically derive from the heterogeneity of the medium, the methodology of structural and physic parameterization to assimilate it to a continuous medium.The raised aims in this doctoral thesis have been directed towards a better knowledge of the influence of several structural parameters of the porous medium, as well as of the properties of the solid matrix and the saturating fluid phase, in the characteristics of boiling of an initially saturated porous medium, heated by its lower boundary and limited by an upper layer of the same saturating liquid.For this, the influence of the structure of the porous medium (granular or fibrous) and the nature of the solid matrix on the permeability to water and to a surfactant solution of lower concentration have been studied. The influence in relation to effective thermal conductivity and diffusivity in unstationary regime has also been studied. Finally, the boiling process until to achieve dryout conditions has been studied, and the consequences result from the variation of the physical structure of the porous medium, the nature of the solid matrix and the properties of the saturating fluid have been analyzed.About the fluid dynamic and thermal characteristics of the porous media studied, the thesis concludes that:- The addition of a surfactant to the saturating water of the porous medium produces a different behaviour depending on the nature of the solid: in inorganic matrix (sand) occasions an increase in the intrinsic permeability, whereas in organic matrix (cotton fibres) produces a decrease as greater as smaller is the porosity of the porous medium. The reasons of this different behaviour are the notable differences of absorption of the surfactant depending on the sort of solid matrix (organic or inorganic) and the important increase of the wetting of the inorganic solid's surface by the fluid activating the displacement of all adsorbed or retained not watery phase between particles.- The spatially averaged effective thermal diffusivity tends to the value of the diffusivity of the component of the porous medium with lower thermal diffusivity throughout the boiling process.- The effective thermal diffusivity of the saturated porous media which have &#61548;s / &#61548;l < 1 approaches to of the liquid phase; in the media with &#61548;s / &#61548;l >> 1, the effective thermal diffusivity is a grade of upper order to the of liquid phase.- The addition of surfactant to the saturating liquid phase gives rise to the decrease of the effective thermal conductivity of the saturated porous medium with granular and inorganic solid phase. A physical model of behaviour of the different saturated porous media concerning heating process until to achieve dryout conditions has been described considering the next phases:i) Heating of the medium until the saturation temperature of its liquid phase, with evident increase of volume of the solid and liquid phases by thermal dilatation.ii) Evaporation process with creation of a biphasic layer whose upper boundary displaces to the velocity of the vapour front. Simultaneously a decrease of the fluid static pressure in the boundary of the biphasic layer is produced, what result in a reduction of the reflux of liquid towards the heating plate.iii) Total unsaturation of the porous medium-heating plate interphase caused by to receive less liquid by reflux that the heating plate is capable of evaporating.iv) Appearance, in some case, of a fast phenomenon of turn upside down of the liquid phase from subcooled layer to the unsaturated zone of the porous medium.

The Mechanical Engineering Department at California Polytechnic State University in San Luis Obispo currently maintains a lab-scale hybrid rocket motor for which nitrous oxide is utilized as the oxidizer in the combustion system. Because of its availability, the same two-phase (gas and liquid) nitrous oxide that is used in the combustion system is also routed around the throat of the hybrid rocket’s converging-diverging nozzle as a coolant. While this coolant system has proven effective empirically in previous tests, the physics behind the flow of the two-phase mixture is largely unexplained. This thesis provides a method for predicting some of its behavior by modeling it using the classic gas dynamics scenarios of Rayleigh and Fanno flows which refer to one-dimensional, compressible, inviscid flow in a constant area duct with heat addition and friction. The two-phase model produced utilizes a separated phase with interface exchange model for predicting whether or not dryout occurs. The Shah correlation is used to predict heat transfer coefficients in the nucleate boiling regime. The homogeneous flow model is utilized to predict pressure drop. It is proposed that a Dittus-Boelter based correlation much like that of Groeneveld be developed for modeling heat transfer coefficients upon the collection of sufficient data. Data was collected from a series of tests on the hybrid rocket nozzle to validate this model. The tests were first run for the simplified case of an ideal gas (helium) coolant to verify the experimental setup and promote confidence in subsequent two-phase experimental results. The results of these tests showed good agreement with a combined Rayleigh-Fanno model with a few exceptions including: (1) reduced experimental gas pressure and temperature in the annulus entrance and exit regions compared to the model and (2) reduced experimentally measured copper temperatures uniformly through the annulus. These discrepancies are likely explained by the geometry of the flowpath and location of the copper thermocouples respectively. Next, a series of two-phase cooled experiments were run. Similar trends were seen to the helium experiment with regards to entrance and exit regions. The two-phase Rayleigh homogeneous flow model underpredicted pressure drop presumably due to the inviscid assumption. Ambiguity was observed in the fluid temperature measurements but the trend seemed to suggest that mild thermal non-equilibrium existed. In both cases, the dryout model predicted that mist flow (a post-CHF regime) occurred over most of the annulus. Several modifications should be implemented in future endeavors. These include: (1) collecting more data to produce a heat transfer coefficient correlation specific to the nitrous oxide system of interest, (2) accounting for thermal non-equilibrium, (3) accounting for entrance and exit effects, and (4) developing a two-phase Fanno model.

Film flow measurements at several axial positions in round pipes with variousaxial power distributions are presented for conditions corresponding to normaloperation of a BWR. It is confirmed that the film flow rate approaches zero atthe onset of dryout. Selected phenomenological models of annular two-phaseflow are shown to reasonably reproduce the measurements. It is concluded thatmodels are in better agreement with measurements if terms corresponding topossible boiling induced entrainment are excluded. A method to perform film flow analysis in subchannels as a post-processto a standard two-field subchannel code is suggested. It is shown that thisapproach may yield accurate prediction of dryout power in rod bundles to alow computational cost and that the influence of the power distribution is wellpredicted by the model. / QC 20100618

<p>Measurements of film mass flow rate for annular, diabatic steam-water flow in tubes are presented. The measurements were carried out with four axial power distributions and at several axial positions at conditions typical for boiling water reactors, i.e. 7 MPa pressure and total mass flux in a range from 750 to 1750 kg/m2s. The results show that the influence of the axial power distribution on the dryout power corresponds to a consistent tendency in the film flow rate and that the film tends to zero when dryout is approached. Furthermore it is demonstrated that two selected phenomenological models of annular flow well predict the present data. A model for additional entrainment due to boiling is shown to degrade the predictions.</p>

Two-phase heat transfer in mini/micro-channels is capable of meeting the high cooling demands of modern high heat flux applications. The phase change process ensures better temperature uniformity and control for local hot spots. Furthermore, these compact channels could be helpful in reducing the required charge and material inventories.Environmental concerns—mainly ozone depletion and global warming—have instigated a search for new alternatives in refrigeration industry. While new compounds are being developed to address stringent legislative demands, natural alternatives are also coming into prominence. A limited number of investigators have reported on thermal performance of such alternatives. The current study is therefore focused on saturated flow boiling heat transfer, pressure drop and dryout characteristics for three low global warming potential (GWP) refrigerants (R152a, R600a and R1234yf) in a vertical mini-channel.In this study experiments were carried out by uniformly heating a test section (stainless steel tube with 1.60 mm inside diameter and 245 mm heated length) at 27 and 32 oC saturation temperature with 50-500 kg/m2s mass velocities. The effects of various parameters of interest (like heat flux, mass flux, system pressure, vapor quality, operating media) on flow boiling heat transfer, frictional pressure drop and dryout characteristics were recorded. R134a, which has been widely used in several applications, is utilized as a reference case for comparison of thermal performance in this study.Experimental results for saturated boiling heat transfer showed strong influence of heat flux and system pressure with insignificant contributions from mass flux and vapor quality. Two phase frictional pressure drop increased with mass flux, vapor quality and with reduced operating pressure. The dryout heat flux remained unaffected with variation in saturation temperature, critical vapor quality in most cases was about 85%. The experimental results (boiling heat transfer, two-phase pressure drop and dryout heat flux) were compared with well-known macro and micro-scale correlations from the literature. / <p>QC 20141124</p>

An experimental study on post dryout heat transfer regime in annuli with flow obstacles was conducted in the High-pressure Water Test (HWAT) loop at the Royal Institute of Technology in Stockholm, Sweden. An annulus consisting of two concentric heated pipes (12.7x24.3) mm, with total heated length equal to 3650 mm was employed as a test section. Three kinds of flow obstacles were used: pin-spacers, cylindrical obstacles and grid obstacles. The experiments performed in the test section with pin-spacers only were considered as the reference case. In two consecutive sets of runs, additional obstacles were placed inside the flow channel while keeping the pin spacers in the same positions. In that way the net effect of obstacles on heat transfer was measured. The experimental investigations were performed in a wide range of the operational conditions: mass flux (500-1750) kg/(m2s), inlet subcooling (10-40) K and system pressure (5-7) MPa. The wall superheat was measured at 88 different axial positions (40 on the inner tube and 48 on the outer tube) for the conditions mentioned above. A local heat transfer coefficient was calculated based on the measured annulus wall temperatures and the saturated fluid (water) properties. The results show an enhancement of the heat transfer coefficient downstream of flow obstacles. The most significant influence has been observed in case of pin spacers. This result is consistent with blockage area of various obstacles, which was the highest in case of pin spacers. The data obtained in more than 200 runs were compared with two pre-dryout and post-dryout correlations. The correlations show a slight over-prediction of the heat transfer coefficient in both pre-dryout and post-dryout regions. The thesis contains a detailed description of experimental procedures as well as an analysis of the results of measurements. / QC 20111024 / SKC: Post Dryout Regime

Measurements of film mass flow rate for annular, diabatic steam-water flow in tubes are presented. The measurements were carried out with four axial power distributions and at several axial positions at conditions typical for boiling water reactors, i.e. 7 MPa pressure and total mass flux in a range from 750 to 1750 kg/m2s. The results show that the influence of the axial power distribution on the dryout power corresponds to a consistent tendency in the film flow rate and that the film tends to zero when dryout is approached. Furthermore it is demonstrated that two selected phenomenological models of annular flow well predict the present data. A model for additional entrainment due to boiling is shown to degrade the predictions. / QC 20101108

<p>The effect of pressure transients on the onset of intermittent dryout in upward annular flow was experimentally investigated in order to resolve the conflict between the observations drawn from two major data sets in the literature. A delay in time to the onset of dryout at the test section exit relative to the time predicted based on steady-state data was observed in the R-12 experiments of Celata et al (1988; 1991). Steady-state prediction methods were sufficient to predict the upstream progression of a pre-existing dryout front in the water experiments of Lyons and Swinnerton (1983). Steady state and pressure transient dryout experiments were performed using water with outlet pressures of 2 to 6 MPa and mass fluxes of 1000 to 2500 kg/m2/s in an electrically heated 1.32 m long 4.6 mm ID vertical Inconel 600 tube with depressurisation rates of up to 1.0 MPa/s. Transient experiments were performed with a small margin to dryout and with post-dryout initial conditions in order to test the hypothesis that these initial conditions influenced the onset of dryout during transients. The results of a comparison between the steady dryout data and two dryout prediction methods---the Biasi et al (1967) correlation and the 2006 CHF look-up table (Groeneveld et al, 2007)---were used to develop correction factor correlations to reduce systematic error when these methods were used to predict the transient time to dryout. These modified methods yielded mean predicted dryout delays of -0.1 and 1.5 s respectively with standard deviations of approximately 3 s. There was no statistically significant variation between the pre- and post-dryout initial conditions. Based on this result it was concluded that the initial conditions did not affect the observed time to dryout. The mean wall temperature exhibited a discontinuous decrease as the heat flux approached 92 to 95% of the dryout value. It was postulated that this was caused by a heat transfer regime change from liquid film evaporation to droplet evaporation based on the observations of Hewitt (1970), Doroschuk et al (1970) and Groeneveld (2011). For the range of conditions of the present work the onset of intermittent dryout (Groeneveld, 1986) was caused by deterioration of droplet evaporation heat transfer. Celata et al (1988) noted that in their pressure transient experiments the decrease in saturation temperature drove a rapid increase in the heat flux to the fluid. This was caused by the release of stored thermal energy as the test section wall cooled. Celata et al (1991) stated that the systematic dryout delay was observed for depressurisation rates greater than 0.2 MPa/s. Using Celata et al's (1988) pressure transient data it was concluded that the stored thermal energy transient did not influence the onset of intermittent dryout when rho_w c_pw L_w *(dT_sat/dt)<0.3*q''_a.</p> / Doctor of Philosophy (PhD)

Microchannel heat exchangers present many advantages, such as reduced size, high thermal efficiency and low fluid inventory; and are increasingly being used for heat transfer in a wide variety of applications including heat pumps, automotive air conditioners and for cooling of electronics.However, the fundamentals of fluid flow and heat transfer in microscalegeometries are not yet fully understood. The aim of this thesis is to contribute to a better understanding of the underlying physical phenomena in single-phase and specially flow boiling heat transfer of refrigerants in small channels. For this purpose, well-characterized heat transfer experiments have been performed in uniformly heated, single, circular, vertical channels ranging from 0.64 to 1.70 mm in diameter and using R-134a, R-22 and R-245fa as working fluids. Furthermore, flow visualization tests have been carried out to clarify the relation between the two-phase flow behavior and the boiling heat transfer characteristics. Single-phase flow experiments with subcooled liquid refrigerant have confirmed that conventional macroscale theory on single-phase flow and heat transfer is valid for circular channels as small as 640μm in diameter. Through high-speed flow boiling visualization of R-134a under non adiabatic conditions seven flow patterns have been observed: isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. Annular-type flow patterns are dominant for vapor qualities above 0.2. Onset of nucleate boiling and subcooled flow boiling heat transfer of R-134a has been investigated. The wall superheat needed to initiate boiling was found as large as 18 ºC. The experimental heat transfer coefficients have been compared to predictions from subcooled flow boiling correlationsav ailable in the literature showing poor agreement. Saturated flow boiling heat transfer experiments have been performed with the 640 μm diameter test section. The heat transfer coefficient has been found to increase with heat flux and system pressure and not to change with vapor quality or mass flux when the quality is less than ∼0.5. For vapor qualities above this value, the heat transfer coefficient decreases with vapor quality. This deterioration of the heat transfer coefficient is believed to be caused by the occurrence of intermittent dryout in this vapor quality range. The experimental database, consisting of 1027 data points, has been compared against predictions from correlations available in the literature. The best results are obtained with the correlations by Liu and Winterton (1991) and by Bertsch et al. (2009). However, better design tools to correctly predict the flow boiling heat transfer coefficient in small geometries need to be developed. Dryout incipience and critical heat flux (CHF) have been investigated in detail. CHF data is compared to existing macro and microscale correlations. The comparison shows best agreement with the classical Katto and Ohno (1984) correlation, developed for conventional large tubes. / QC 20101101

This work has been aimed at developing a mechanistic, transient, 3-D numerical model to predict the behavior of an evaporating thin liquid film on a non-uniformly heated cylindrical rod with simultaneous parallel and cross flow of vapor. Interest in this problem has been motivated by the fact that the liquid film on a full-length boiling water reactor fuel rod may experience significant axial and azimuthal heat flux gradients and cross flow due to variations in the thermal-hydraulic conditions in surrounding subchannels caused by proximity to inserted control blade tip and/or the top of part-length fuel rods. Such heat flux gradients coupled with localized cross flow may cause the liquid film on the fuel rod surface to rupture, thereby forming a dry hot spot. These localized dryout phenomena can not be accurately predicted by traditional subchannel analysis methods in conjunction with empirical dryout correlations. To this end, a numerical model based on the Level Contour Reconstruction Method was developed. The Standard k- turbulence model is included. A cylindrical coordinate system has been used to enhance the resolution of the Level Contour Reconstruction Model. Satisfactory agreement has been achieved between the model predictions and experimental data. A model of this type is necessary to supplement current state-of-the-art BWR core thermal-hydraulic design methods based on subchannel analysis techniques coupled with empirical dry out correlations. In essence, such a model would provide the core designer with a "magnifying glass" by which the behavior of the liquid film at specific locations within the core (specific axial node on specific location within a specific bundle in the subchannel analysis model) can be closely examined. A tool of this type would allow the designer to examine the effectiveness of possible design changes and/or modified control strategies to prevent conditions leading to localized film instability and possible fuel failure.