Flow boiling in horizontal smooth tubes: New heat transfer results for R-134a at three saturation temperatures

The present study presents new flow boiling heat transfer results of R-134a flowing inside a 13.84 mm internal diameter, smooth horizontal copper tube. The heat transfer measurements were made over a wide range of test conditions: saturation temperatures of 5, 15 and 20 °C, (corresponding to reduced pressures of 0.08, 0.12 and 0.14), vapor qualities ranged from 0.01 to 0.99, mass velocities of 300 and 500 kg/m² s, and heat fluxes of 7.5 and 17.5 kW/m². The experimental results clearly show that a local minimum heat transfer coefficient systematically occurs within slug flow pattern or near the slug-to-intermittent flow pattern transition. The vapor quality x_min at which the local minimum occurs seems to be primarily sensitive to mass velocity and heat flux. Thus, it is influenced by the competition between nucleate and convective boiling mechanisms that control macroscale flow boiling. The experimental results were compared to four types of predictive methods: (a) strictly convective, (b) superposition, (c) strictly empirical and (d) flow pattern based. Generally, all the methods tend to underpredict the experimental data and the higher errors occur in two particular regions: low and high vapor qualities. These vapor qualities correspond to slug and annular patterns, respectively. For slug flow, methods that require the identification of nucleate boiling related regions tend to predict the heat transfer coefficient accurately. This emphasizes that for slug flows, heat transfer is not a simple juxtaposition of nucleate and convective boiling contributions, but that the integration of these two heat transfer mechanisms is also a function of flow parameters. The comparisons between experimental and predicted data show that the best overall results are obtained with superposition and flow pattern based methods.     

Void Fraction and Two-Phase Pressure Drops for Evaporating Flow over Horizontal Tube Bundles

A study has been performed to assess the accuracy of several existing void fraction and pressure drop correlations for diabatic, two-phase, shell-side flow over a horizontal tube bundle. The void fraction predictions have been applied in the computation of the static and momentum components of the two-phase pressure drop and have been compared to measured data for diabatic, vertical, up-flow through a twenty-tube bundle evaporator for refrigerants R-134a, R-410A, and R-507A. The tests were made with bundles comprised of plain tubes, low-finned tubes, and enhanced boiling tubes. The recent correlation of Feenstra et al. [5 Feenstra, P. A., Weaver, D. S. and Judd, R. L. 2000. An Improved Void Fraction Model for Two-Phase Cross-Flow in Horizontal Tube Bundles. Int. J. of Multiphase Flow, vol. 26: 1851–1873. [CSA][CROSSREF] [Google Scholar]] has been found to perform quite well. The void fraction predictions have also been compared to data for flow with oil in the refrigerant. The results show that the static pressure drop is much lower with oil due to the foaming of the refrigerant/oil mixture. Finally, by back calculating the expected frictional pressure drops from the measured data, the investigation has evaluated the leading method for predicting frictional losses for two-phase flow over tube bundles, and a new one has been proposed, applicable to low-mass velocities that are typical of flooded evaporators.     

Visualization of R-134a Flowing on Tube Arrays with Plain and Enhanced Surfaces under Adiabatic and Condensing Conditions

Experimental studies were performed to investigate the flow patterns of liquid films of R-134a on vertical arrays of horizontal tubes under adiabatic test conditions at room temperature and during condensation at a saturation temperature of 304 K. Three arrays with tube pitches of 25.5, 28.6, and 44.5 mm and up to ten tubes of four commercially available copper tubes with a nominal diameter of 19.05 mm and 544 mm in length were tested: a plain tube, a 26 fpi/1024 fpm low finned tube (Turbo-Chil), and two tubes with three-dimensional enhanced surface structures (Turbo-CSL and Gewa-C). Flow pattern observations and transitions were obtained and compared to existing flow pattern maps. The effect of the flow pattern on heat transfer was also investigated. The ideal flow modes (droplet, column, and sheet) could strictly be observed on the tubes with 3D enhanced surface structures. On the low-finned tube and the plain tube, the flow was very unstable, causing a part of liquid to leave the array sideways. For the 3D enhanced tubes, a difference in the formation of the sheets could be seen. The Gewa-C tube tended to form small sheets, while the Turbo-CSL formed large sheets at low film Reynolds numbers. A cyclic forward and backward motion of the sheet was observed that increased in amplitude with an increasing film Reynolds number. At high film Reynolds numbers, part of the liquid left the array of tubes sideways due to this oscillation, the effect of which on heat transfer was found to be more significant than the effect of the flow modes themselves.     

State of the Art of High Heat Flux Cooling Technologies

The purpose of this literature review is to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips. Today, most development efforts are focused on three technologies: liquid cooling in copper or silicon micro-geometry heat dissipation elements, impingement of liquid jets directly on the silicon surface of the chip, and two-phase flow boiling in copper heat dissipation elements or plates with numerous microchannels. The principal challenge is to dissipate the high heat fluxes (current objective is 300 W/cm²) while maintaining the chip temperature below the targeted temperature of 85°C, while of second importance is how to predict the heat transfer coefficients and pressure drops of the cooling process. In this study, the state of the art of these three technologies from recent experimental articles (since 2003) is analyzed and a comparison of the respective merits and drawbacks of each technology is presented. The conclusion is that two-phase flow boiling in microchannels is the most promising approach; impingement cooling also has good prospects but single-phase liquid cooling is probably only a short-term solution. As an example of the state of the first technology, the Heat and Mass Transfer Laboratory at Ecole Polytechnique Fédérale de Lausanne has already achieved 200 W/cm² of cooling in a first prototype, with a low pumping power, good temperature uniformity, and at the required maximal operating temperature.     

The human homolog of Saccharomyces cerevisiae CDC45

In budding yeast Saccharomyces cerevisiae CDC45 is an essential gene required for initiation of DNA replication. A structurally related protein Tsd2 is necessary for DNA replication in Ustilago maydis. We have identified and cloned the gene for a human protein homologous to the fungal proteins. The human gene CDC45L is 30 kilobases long and contains 15 introns. The 16 exons encode a protein of 566 amino acids. The human protein is 52 and 49.5% similar to CDC45p and Tsd2p, respectively. The level of CDC45L mRNA peaks at G1-S transition, but total protein amount remains constant throughout the cell cycle. Consistent with a role of CDC45L protein in the initiation of DNA replication it co-immunoprecipitates from cell extracts with a putative replication initiator protein, human ORC2L. In addition, subcellular fractionation indicates that the association of the protein with the nuclear fraction becomes labile as S phase progresses. The CDC45L gene is located to chromosome 22q11.2 region by cytogenetics and by fluorescence in situ hybridization. This region, known as DiGeorge syndrome critical region, is a minimal area of 2 megabases, which is consistently deleted in DiGeorge syndrome and related disorders. The syndrome is marked by parathyroid hypoplasia, thymic aplasia, or hypoplasia and congenital cardiac abnormalities. CDC45L is the first gene mapped to the DiGeorge syndrome critical region interval whose loss may negatively affect cell proliferation.     

A model to predict embryonic development and hatching in lake whitefish (Coregonus clupeaformis) under variable incubation temperatures

We developed an incremental growth-at-temperature model to predict hatch timing for lake whitefish under varying incubation temperatures. The model extends earlier approaches by incorporating a temperature-dependence for development stage at hatching. Testing using experimentally reared embryos from Lakes Huron and Simcoe demonstrates improved predictive accuracy compared to previous models for asymmetrically varying thermal regimes that mimic aspects of those encountered under real-world conditions. The simple calibration of the model to different populations and its ability to predict observed phenomena such as thermally-induced mass hatching and hatching synchronization makes it a useful tool for the evaluation of thermal impacts from industrial discharges and climate change.     

Heat transfer, pressure drop, and flow pattern recognition during condensation inside smooth, helical micro-fin, and herringbone tubes

This paper presents a study of flow regimes, pressure drops, and heat transfer coefficients during refrigerant condensation inside a smooth, an 18° helical micro-fin, and a herringbone tubes. Experimental work was conducted for condensing refrigerants R-22, R-407C, and R-134a at an average saturation temperature of 40 °C with mass fluxes ranging from 400 to 800 kg m⁻² s⁻¹, and with vapour qualities ranging from 0.85 to 0.95 at condenser inlet and from 0.05 to 0.15 at condenser outlet. These test conditions represent annular and intermittent (slug and plug) flow conditions. Results showed that transition from annular flow to intermittent flow, on average for the three refrigerants, occurred at a vapour quality of 0.49 for the smooth tube, 0.29 for the helical micro-fin tube, and 0.26 for the herringbone tube. These transition vapour qualities were also reflected in the pressure gradients, with the herringbone tube having the highest pressure gradient. The pressure gradients encountered in the herringbone tube were about 79% higher than that of the smooth tube and about 27% higher than that of the helical micro-fin tube. A widely used pressure drop correlation for condensation in helical micro-fin tubes was modified for the case of the herringbone tube. The modified correlation predicted the data within a 1% error with an absolute deviation of 7%. Heat transfer enhancement factors for the herringbone tube against the smooth tube were on average 70% higher while against the helical micro-fin tube it was 40% higher. A correlation for predicting heat transfer coefficients inside a helical micro-fin tube was modified for the herringbone tube. On average the correlation predicted the data to within 4% with an average standard deviation of 8%.     

Konsultative Assistenzsysteme für e-Government

Consultative Assistance Systems (CASS) enhance today’s transaction-oriented applications by case-driven, rule-based and interactive components geared towards individual needs of the end-user.

1. CASS supports in the resolution process with rare cases, which are typically complex in nature.
2. CASS guides the end-user in finding a solution that covers all legal and economical aspects of each case.
3. The example described in this article represents a CASS which supports the complex process of public procurement under the tendering by-laws. The application architecture used in this example can be transferred to many eGovernment scenarios.

     

Evaporation in microchannels: influence of the channel diameter on heat transfer

This paper discusses the effect of diameter on both flow boiling heat transfer and transition from macro to microchannel evaporation. A recently proposed three-zone flow boiling model based on evaporation of elongated bubbles in microchannels is briefly described and used for the present analysis. In the microscale range, the model predicts an increase in the two-phase heat transfer coefficient with a decrease of diameter for low values of vapor quality and a decrease of the heat transfer coefficient for larger values of vapor quality. This behavior is explained by the influence of the liquid film thickness, deposited periodically behind passing liquid slugs.     

"Hypnotist and Manner of Presentation Effects on a Standardized Hypnotic Susceptibility Test": Erratum.

Reports an error in the original article by D. Eugene Thome and Ernst G. Beier (Journal of Consulting and Clinical Psychology, 1968, 32, No. 5, pp. 610-612). There is a misspelling of J. V. Cabibi's name on page 611 and again in the reference list. The reference should read: Cabibi, J. V., Hughes, H. H., & Butler, J. R. Behavioral effects on the hypnotic process by a change in hypnotist. Journal of Clinical Psychology, 1966, 21, 334. (The following abstract of this article originally appeared in record 1969-00226-001). Administration of standardized hypnotic susceptibility tests requires, among others, 2 basic assumptions: administration of the test by different hypnotists has no significant effect on Ss' performances, and (2) administration of the test by way of different presentation methods has no significant effect on Ss' performances. 4 groups of 55 Ss each were administered the Harvard Group Scale of Hypnotic Susceptibility under 4 experimental conditions, i.e., 1 of 2 hypnotists, who either presented the scale in person or presented the scale by way of prerecorded tape. Results weakened the tenability of the basic assumptions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)