181,095 research outputs found
Free-convection condensation on single horizontal pin-fin tubes
No full textPhDNew experimental data are reported for free-convection condensation of ethylene
glycol and R-113 on three-dimensional pin-fin tubes. Effects of pin geometry and tube
thermal conductivity (for copper, brass and bronze giving a mean range of 400, 120 and
80 W/m K over the range of temperature of interest) were investigated. All tests were
performed at near atmospheric pressure with downward flowing vapour at low velocity.
Heat-transfer enhancement was found to be approximately twice the corresponding
active surface area of the tubes, i.e. the surface area of the parts of the tube and pin
surface not covered by condensate retained by surface tension. For ethylene glycol, the
best performing pin-fin tube gave a heat-transfer enhancement of 5.8, about 24 %
higher than the ‘equivalent’ two-dimensional integral-fin tube (i.e. with the same finroot
diameter, longitudinal fin spacing and thickness and fin height). For R-113, the
best enhancement was 5.9, about 10 % higher than the equivalent integral-fin tube.
For both fluids tested, vapour-side, heat-transfer enhancement was found to increase
with decreasing circumferential pin spacing and increasing pin height. Circumferential
pin thickness had little effect on heat-transfer enhancement. Effects of tube thermal
conductivity were found to be more significant for ethylene glycol than R-113.
Retention angle measurements were made under static conditions (without
condensation) and were found to be larger than for equivalent integral-fin tubes. An
expression for condensate retention angle on pin-fin tubes was proposed and found to
agree with the measured retention angles to ±15%.
A semi-empirical model for condensation heat transfer on horizontal pin-fin tubes has
been developed which accounts for the combined effect of gravity and surface tension.
The model predicts the majority of available data to ±20 %
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Forced-convection condensation heat-transfer on horizontal integral-fin tubes including effects of liquid retention
No full textPhDAccurate and repeatable heat-transfer data are reported for forced-convection filmwise
condensation of steam and ethylene glycol flowing vertically downward over two
single, horizontal instrumented integral-fin tubes and one plain tube. Vapour-side,
heat-transfer coefficients were obtained by direct measurement of the tube wall
temperature using specially manufactured, instrumented tubes with thermocouples
embedded in the tube walls. Both tubes had fin height of 1.6 mm and fin root diameter
of 12.7 mm, with fin thickness and spacing of 0.3 mm and 0.6 mm, respectively for
the first tube and 0.5 mm and 1.0 mm respectively for the second. Tests were
performed at atmospheric pressure for steam with nominal vapour velocities from
2.4 m/s to 10.5 m/s and at three pressures below atmospheric with nominal vapour
velocities from 8.4 m/s to 57 m/s for steam and 13 m/s to 82 m/s for ethylene glycol.
The data show that both the finned tubes provide an increase in heat flux at the same
vapour-side temperature difference with increasing vapour velocity. Visual
observations were made and photographs obtained of the condensate retention angle
at each combination of vapour velocity and pressure. It was observed that the
curvature of the meniscus was distorted by the increase in vapour velocity and in
many cases, the extent of condensate flooding changed compared to its value in the
quiescent vapour case.
In parallel, experiments involving simulated condensation on finned tubes were
conducted using horizontal finned tubes in a vertical wind tunnel. Condensate was
simulated by liquid (water, ethylene glycol and R-113) supplied to the tube via small
holes between the fins along the top of the tube. Downward air velocities up to 24 m/s
were used and retention angles were determined from still photograph. Eight tubes
with a diameter at the fin root of 12.7 mm were tested. Five tubes of which had fin
height of 0.8 mm and spacing between fins of 0.5 mm, 0.75 mm, 1.0 mm, 1.25 mm
and 1.5 mm and three tubes had fin height 1.6 mm with fin spacings 0.6 mm, 1.0 mm
and 1.5 mm. The results were repeatable on different days and suggested, for all tubes
and fluids, that the retention angle asymptotically approached a value around 80o to
85o (from either lower or higher values at zero vapour velocity) with increase in air
velocity. Good agreement was found with observations taken during the condensation
experiments
Sound production and associated behavior of tagged fin whales (Balaenoptera physalus) in the Southern California Bight
No full textBackground: For marine animals, acoustic communication is critical for many life functions, yet individual calling behavior is poorly understood for most large whale species. These topics are important for understanding whale social behavior and can also serve as a baseline for behavioral studies assessing whale response to disturbance. Using a new technique for identifying the calling individual, we measured body orientation, dive behavior, and surface social behavior in relation to call production for tagged fin whales in Southern California. Results: Behavioral metrics associated with elevated call rates included shallow maximum dive depths (10–15 m), little body movement, negative pitch in body orientation, and moderate body roll. Calling whales were also more likely to be traveling than milling, in groups rather than solitary, and without change in group size compared to non-calling whales. Conclusions: These are the first descriptions of body posture and depths at which fin whales are most likely to call, and some possible sound propagation and/or anatomical reasons for these results are considered. The call behavior characterizations presented here will help in predicting calling behavior from surface behavior, informing interpretation of passive acoustic data, and determining the effects of anthropogenic sound on whales in Southern California.Peer reviewe
Aspects of tissue morphogenesis and organisation in the zebra fish, Brachydanio rerio
No full textThis thesis is mainly concerned with the impact of cellular
mechanisms which influence a particular instance of tissue
morphogenesis, namely development of the caudal fin of the
zebra fish, Brachydanio rerio.
Ultrastructural analysis of fin fold morphogenesis in situ
reveals that specific changes in epidermal cell shaping occur
as a transient ectodermal ridge is generated. The ridge is
converted to a fin fold by further changes in cell shaping
which are spatio-temporally associated with the deposition
of extracellular matrix material.
Microsurgical excisions of small portions of early folds
support the suggestion that cell shape modulation and extracellular matrix organisation interact reciprocally during early
fold formation. Studies using cytochalisin B show that actinoid
microfilaments play an important role in generating the changes
in epidermal cell shaping associated with fin fold morphogenesis.
They also eJiminate the possibility that overlying peridermis
and e(Xiermis which flank putative fin folds exert any great
influence on the morphology of fold epidermis during apical
ectodermal ridge generation. Furthermore, experiments employing tunicamycin indicate that cell adhesion and extracellular
matrix deployment are partic.llarly important in converting
the apical ectodermal ridge into a fin fold and in subsequent
stabilisation of the early fin fold. Microtubules do not appear
to influence early fin fold morphogenesis although they are
important during a later phase of fin fold expansion.
The spatial relationships between extracellular matrix
orientation and cytoskeletal alignment in cell layers associated with the scales of certain teleosts have also been assessed.
These studies have involved electron and immunufluorescence
microscopy. They show that fibroblastic cells found at varying
locations on the surfaces of scales display three types of micro-
tubule arrays. Two of these arrays show intercellular alignment
of microtubules which are spatially correlated with patterns
of extracellular matrix deposition
Frosting Performance of Fin-and-Tube Evaporators with Small Copper Tubes Diameter
No full textIn modern heat pump systems, the heat exchangers use enhanced heat transfer surfaces for both air and refrigerant sides. In air conditioning applications, conventional 9.5 mm (3/8 inch) tube diameter fin-and-tube coils are slowly being replaced by microchannel heat exchangers. However, during heating mode the energy performance of heat pump systems with microchannel outdoor coils are generally lower than those of fin-and-tube direct-expansion evaporators due to a higher frequency of defrost cycles. A different approach might be to utilize fin-and-tube technology, which has proven records of excellent water drainage characteristics and good performance in frosting operating conditions, and enhance the air side heat transfer rates by introducing a larger number of small diameter copper tubes. In this paper, six fin-and-tube coils with copper tube diameter ranging from 5 mm (1/5 inch) to 7mm (8/29 inch) were experimentally investigated in frosting operating conditions. The laboratory experiments were conducted in an air flow wind tunnel at Oklahoma State University. Experimental data on heat transfer rate and air-side pressure drop across the coils were measured and the fin density and the tube diameter were varied in a parametric fashion during the experimental campaign. The performance of the fin-and-tube coils were also compared to those of a conventional 9.5 mm copper tube diameter fin-and-tube heat exchanger and of a microchannel heat exchanger that had similar air-side frontal area and at similar operating conditions of outdoor direct-expansion evaporators in heat pump systems for residential applications. The trends of the data during frosting operation suggested that reducing the tube diameter was beneficial for frosting performance at low fin density while was harmful at high fin density. The data showed that increasing the fin density increased the capacity but significantly reduced the time for heating service for the evaporator. Small copper tube diameter resulted in about 11% higher initial capacity at dry start conditions and about 4% higher average integrated capacity when considering the entire frost period. The data discussed in this paper serve as basis for future research on direct-expansion evaporators for air-source heat pump applications, in which the frosting of the outdoor heat exchangers is still one of the major concerns
Enhancement of Round Tube and Flat Tube-Louver Fin Heat Exchanger Performance Using Deluge Water Cooling
No full textAn experimental study has been conducted to evaluate the performance of a compact round-tube louver-fin condenser, each with frontal areas of 0.25 m2 in both dry and wet conditions. Deluge water cooling is achieved by incorporating a perforated bottom plate-type water distributor on top of the round tube heat exchanger. Water is used as a refrigerant, and enters the heat exchanger tubes at 35°C temperature. Ambient air and deluge cooling water are both maintained at 22°C temperature. Heat exchanger capacity and air-side pressure drop are measured with the heat exchanger angle set at 0° and 21° from vertical, with a frontal air velocity of 1.4 m/s and 3.5 m/s without deluge water cooling, and a frontal air velocity of 1.4 m/s with deluge water cooling. For both heat exchangers, the capacity was significantly enhanced with the use of deluge water cooling and with the heat exchanger angle set at 21° from vertical
Frost Formation on Fan-Supplied Tube-Fin Evaporators: A Visual and Numerical Analysis
No full textThis paper presents an experimental and numerical study to evaluate the thermal-hydraulic performance of light commercial capacity evaporators operating under frosting conditions. A tube-fin heat exchanger was tested on a closed-loop wind-tunnel considering the fan performance. A visual analysis of the frost formation processes was carried out to compare the frost layer morphology in different conditions. A mathematical model was developed and the numerical results were compared with the experimental accumulated mass of frost, air flow rate and cooling capacity, with all the predictions falling within the experimental band of uncertainties. Based on this comparison, a frost density correlation was proposed for a typical range of light commercial refrigeration applications. The results show evaporator cooling capacity reductions up to 40% due to frost formation process. Moreover, the effects of air flow reduction and low conductivity frost layer on the overall thermal resistance were analyzed, when it was found that the former is the main cause of the cooling capacity reduction under frosting conditions
Air-Side Heat Transfer Performance of Louver Fin and Multi-Tube Heat Exchanger for Fuel-Cell Cooling Application
No full textThe present work is performed to evaluate the heat transfer performance of a heat exchanger used in a fuel cell. Because of material constraints and performance requirements, a louver fin heat exchanger is modified for use with conventional micro-channel tubes and with multiple small-diameter tubes (a so-called multi-tube). Prototype heat exchangers are tested, and the air-side heat transfer, pressure drop, and fan power are measured in a wind tunnel and simulated using a commercial code. The air-side pressure drop and heat transfer coefficient of the multi-tubes show similar trends to those of the flat-tube heat exchanger if the contact resistance is negligible. The tube spacing of the prototype multi-tube heat exchangers has a small effect on the pressure drop and heat transfer, but it has a profound effect on air-side heat transfer performance because of the contact resistance between the tubes and louver fins. The air-side pressure drop agrees well with an empirical correlation for flat tubes
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