Study of turbulent single-phase heat transfer and onset of nucleate boiling in high aspect ratio mini-channels to support the MITR LEU conversion/

TL;DR: In this paper, the single and two-phase heat transfer in high aspect ratio mini-channels has not been well-characterized, especially in regard to the onset of nucleate boiling.

Abstract: Heat transfer in high aspect ratio mini-channels has important applications for materials test reactors using plate-type fuel. These fuel plates typically possess coolant channels with hydraulic diameters on the order of 4 mm or less. The single and two-phase heat transfer in such channels has not been well-characterized, especially in regard to the onset of nucleate boiling. While surface effects are known to dramatically influence the incipience of boiling, they have not been widely considered under forced convection. Since the limiting safety system setting for the MITR is the onset of nucleate boiling, there is considerable interest in better characterizing the phenomenon in such channels. This study presents a first-of-a-kind, two-phase flow facility designed to measure the singlephase heat transfer coefficient and onset of nucleate boiling in a high aspect ratio mini-channel over a wide range of flow conditions while also permitting high speed visualization of the entire surface. The single-phase heat transfer coefficient is measured for mass fluxes ranging from 750 kg/m2-sec up to 6000 kg/m2-sec and for subcoolings ranging from 20 °C to 70 °C. The onset of nucleate boiling superheat and heat flux are measured for mass fluxes ranging from 750 kg/m2sec to 3000 kg/m2-sec and for subcoolings ranging from 10 °C to 45 °C. Measurements are supported with high speed videography to visualize bubble incipience when conditions permit. The influence of surface wettability on the incipience point is also investigated by performing tests on a surface oxidized at high temperature in air. Using a boundary layer analysis along with experimental data obtained in the study, a semianalytical correlation is developed to predict the single-phase heat transfer coefficient in high aspect ratio rectangular channels. The correlation accounts for effects from secondary flows and heating asymmetry, and is suitable for both the transition and fully turbulent flow regimes. The new correlation predicts the Nusselt number with a mean absolute error of 4.9% in the range of 2.2

TL;DR: The introduction of statistical methods into the analysis of aeronautical experimental data, whether for quality control in production, for the interpretation of the results of structural and aerodynamic laboratory experiments, or for airline operation, has been brought about only in recent years, it may by now be fair to assert that their advantages are no longer in dispute.

Abstract: WHILE the introduction of statistical methods into the analysis of aeronautical experimental data, whether for quality control in production, for the interpretation of the results of structural and aerodynamic laboratory experiments, or for airline operation, has been brought about only in recent years, it may by now be fair to assert that their advantages and even their indispensability are no longer in dispute. Hitherto, investigations on these lines have usually involved, explicitly or implicitly, only the ‘normal curve of error’ and allied considerations; owing, it may be thought, to the controllability of the various manufacturing or laboratory techniques, but also perhaps to the scarcity of data hitherto available. It may well be, however, that with the accumulation of information arising out of investigations planned with particular reference to the statistical analysis of their results the whole range of the apparatus for statistical analysis, usually confined to such fields as those of biology or economics, will be called into full play.

TL;DR: In this paper, the effect of asymmetric heating on the Nusselt number was analyzed using a boundary layer analysis with a two-region wall layer model, similar to that originally proposed by Prandtl.

Abstract: Experimental results are presented for single-phase heat transfer in a narrow rectangular minichannel heated on one side. The aspect ratio and gap thickness of the test channel were 29:1 and 1.96 mm, respectively. Friction pressure drop and Nusselt numbers are reported for the transition and fully turbulent flow regimes, with Prandtl numbers ranging from 2.2 to 5.4. Turbulent friction pressure drop for the high aspect ratio channel is well-correlated by the Blasius solution when a modified Reynolds number, based upon a laminar equivalent diameter, is utilized. The critical Reynolds number for the channel falls between 3500 and 4000, with Nusselt numbers in the transition regime being reasonably predicted by Gnielinski's correlation. The dependence of the heat transfer coefficient on the Prandtl number is larger than that predicted by circular tube correlations, and is likely a result of the asymmetric heating. The problem of asymmetric heating condition is approached theoretically using a boundary layer analysis with a two-region wall layer model, similar to that originally proposed by Prandtl. The analysis clarifies the influence of asymmetric heating on the Nusselt number and correctly predicts the experimentally observed trend with Prandtl number. Furthermore, a semi-analytic correlation is derived from the analysis that accountsmore » for the effect of aspect ratio and asymmetric heating, and is shown to predict the experimental results of this study with a mean absolute error (MAE) of less than 5% for 4000 < Re < 70,000.« less

TL;DR: In order to increase the fission yield per burst, efforts have been directed toward the development of cores which maintain dimensional stability when subjected to more extreme temperature cycles than may be tolerated in normal uranium metal.

Abstract: In order to increase the fission yield per burst, efforts have been directed toward the development of cores which maintain dimensional stability when subjected to more extreme temperature cycles than may be tolerated in normal uranium metal There also have been attempts to eliminate inertial difficulties related to the quenching delay associated with the finite time for translating fission energy into surface or volume expansion This delay leads to an effective broadening of the bursts at a given yield The second series of burst reactors as referred to here are those which employ an alloy of uranium, specifically 10 weight percent molybdenum (U-10 w/o Mo), in which extensive metallurgical tests have indicated relatively small crystal growth and excellent phase stability during or following repeated large temperature cycles (~500°C) Research Reactor) at Oak Ridge National Laboratory, Molly-G or FBR (Fast Burst Reactor) at White Sands Missile Range, and Super Kukla at Lawrence Radiation Laboratory; three additional models similar to HPRR and Molly-G are in the final planning stages, one at Sandia Corporation, one at Aberdeen Proving Grounds, and one at LASL None of these reactors has produced more than a total of ~300 bursts to be compared with many thousands formore » a typical reactor of the first series Accordingly, such devices may be considered in a relatively early stage of development« less

TL;DR: The first edition of this book as mentioned in this paper was published in 1992 and was used for the first year of a physics course at the University of Sheffield. But it was not intended to be a statistics text, nor was it intended to serve as a statistic text, but an introdution to the mathematics required for the analysis of measurements at the level of a first year laboratory course.

Abstract: Students in a science or engineering curriculum ought to be introduced early to the requirement that a meaningful measurement result should always be accompanied by a statement of its uncertainty. This book has been written specifically with this objective in mind. That the first edition has been successful in doing this is attested to by its popularity with both faculty and students, and its translation into six languages. This book is not a statistics text - nor was it intended to be - but an introdution to the mathematics required for the analysis of measurements at the level of a first-year laboratory course. Part 1 begins with uncertainty as a qualitative concept and builds slowly, using many numerical examples and exercises for the student, to develop methods for quantifying uncertainty, and ultimately relating it to the standard deviation of a statistical distribution. Along the way, Taylor develops the rules for expressing and combining (`propagating') uncertainties, and introduces the student to the gaussian (normal) distribution and some of its properties. Part 2 covers, with somewhat more mathematical rigor, specific topics such as data rejection criteria, the binomial and Poisson distributions, covariance and correlation, least-squares fitting, and the chi-squared test. I was not familiar with the first edition, and from a quick scan of the Preface I looked forward to reading this book and learning something about the state of statistical analysis in first-year university texts today. I was disappointed (in part with what the level of the book implies about the sad state of preparation of today's students). Although there are now two ISO publications ( International Vocabulary of Basic and General Terms in Metrology (VIM) and Guide to the Expression of Uncertainty in Measurement (GUM), Geneva, 1993), Taylor makes no mention of either, and never gives a formal definition of `uncertainty' (although he ultimately associates `random uncertainty' with the standard deviation of a gaussian distribution). The book also does not clearly define `error', or the distinction between error and uncertainty. The important point, that the `propagation of uncertainty' is additive in terms of variances is valid for any distributions with finite variance, is not emphasized; instead Taylor restricts the discussion solely to the normal distribution and or those that can be approximated by it. I also find it unfortunate that the book does not clearly distinguish between the variance of a sample , the variance of a distribution , and the sample estimate of the variance of the distribution ( or ). Instead, he accepts the fact that formulas for the variance with either N or N - 1 dividing the sum of the squares of the deviations from the mean exist in the literature and concludes simply: `Nevertheless, you need to be aware of both definitions. In the physics laboratory, using the more conservative... def- inition... is almost always best.' In spite of these shortcomings, the book is a significant contribution to a student laboratory reading list, and it is written at a level that facilitates a self-study program. It has an important message to deliver and it appears to be delivering it well.