bluuebunny/crossref_metadata_2025 huggingface dataset:
The Role of Adenosine in Regulation of Cerebral Blood Flow During Hypoxia in the Near‐Term Fetal Sheep
Authors: Blood Arlin B. , Hunter Christian J. , Power Gordon G.
Published: September 2002

The aim of this study was to determine in the near‐term ovine fetus the role of adenosine in the basal regulation of cerebral blood flow and in the increases in cerebral blood flow in response to acute hypoxic insult. We measured cerebral blood flow in chronically instrumented fetal sheep (127‐135 days gestation, term ≈145 days) using laser Doppler flowmetry probes implanted in the parietal cortices. Hypoxia was administered for 30 min by lowering the ewe's inspired oxygen to 10–12 % during an infusion of either saline or theophylline, a non‐specific adenosine receptor antagonist. The theophylline infusion was begun 30 min prior to and ended 30 min after the completion of the hypoxic insult. The administration of theophylline had no significant effect on cerebral blood flow during the baseline period. During control hypoxic periods, cerebral blood flow increased by ≈45 %. During theophylline experiments, however, there was no significant increase in cerebral blood flow during hypoxia. In the control experiments, cerebral blood flow returned to baseline levels during the recovery period, while in the theophylline experiments cerebral blood flow fell below baseline levels. We conclude that, in the near‐term ovine fetus, adenosine plays a minimal role in the regulation of basal cerebral blood flow. However, these data are strong evidence for the involvement of adenosine in increased fetal cerebral blood flow during an acute hypoxic insult. Finally, adenosine may also play an important role in the maintenance of fetal cerebral blood flow immediately following hypoxic insult.
c‐Fos Expression in Ouabain‐Treated Vascular Smooth Muscle Cells from rat Aorta: Evidence for an Intracellular‐Sodium‐Mediated, Calcium‐Independent Mechanism
Authors: Taurin Sebastien , Dulin Nickolai O. , Pchejetski Dimitri , Grygorczyk Ryszard , Tremblay Johanne , Hamet Pavel , Orlov Sergei N.
Published: September 2002

In this study, we examined the effect of Na+‐K+ pump inhibition on the expression of early response genes in vascular smooth muscle cells (VSMC) as possible intermediates of the massive RNA synthesis and protection against apoptosis seen in ouabain‐treated VSMC in our previous experiments. Incubation of VSMC with ouabain resulted in rapid induction of c‐Fos protein expression with an approximately sixfold elevation after 2 h of incubation. c‐Jun expression was increased by approximately fourfold after 12 h, whereas expression of activating transcription factor 2, cAMP/Ca2+ response element binding protein (CREB)‐1 and c‐Myc was not altered. Markedly augmented c‐Fos expression was also observed under Na+‐K+ pump inhibition in potassium‐depleted medium. Na+‐K+ pump inhibition triggered c‐Fos expression via elevation of the [Na+]i/[K+]i ratio. This conclusion follows from experiments showing the lack of effect of ouabain on c‐Fos expression in high‐potassium‐low‐sodium medium and from the comparison of dose responses of Na+‐K+ pump activity, [Na+]i and [K+]i content and c‐Fos expression to ouabain. A fourfold increment of c‐Fos mRNA was revealed 30 min following addition of ouabain to the incubation medium. At this time point, treatment with ouabain resulted in an approximately fourfold elevation of [Na+]i but did not affect [K+]i. Augmented c‐Fos expression was also observed under VSMC depolarization in high‐potassium medium. Increments in both c‐Fos expression and 45Ca uptake in depolarized VSMC were abolished under inhibition of L‐type Ca2+ channels with 0.1 μM nicardipine. Ouabain did not affect the free [Ca2+]i or the content of exchangeable [Ca2+]i. Ouabain‐induced c‐Fos expression was also insensitive to the presence of nicardipine and [Ca2+]o, as well as chelators of [Ca2+]o (EGTA) and [Ca2+]i (BAPTA). The effect of ouabain and serum on c‐Fos expression was additive. In contrast to serum, however, ouabain failed to activate the Elk‐1, serum response factor, CREB and activator protein‐1 transcription factors identified within the c‐Fos promoter. These results suggest that Na+‐K+ pump inhibition triggers c‐Fos expression via [Na+]i‐sensitive [Ca2+]i‐independent transcription factor(s) distinct from factors interacting with known response elements of this gene promoter.
Different Metabotropic Glutamate Receptors Play Opposite Roles in Synaptic Plasticity of the Rat Medial Vestibular Nuclei
Authors: Grassi Silvarosa , Frondaroli Adele , Pettorossi Vito Enrico
Published: September 2002

In the medial vestibular nuclei (MVN) of rat brainstem slices, the role of group II and III metabotropic glutamate receptors (mGluRs) and of the subtypes of group I mGluRs: mGluR1, mGluR5, was investigated in basal synaptic transmission and in the induction and maintenance of long‐term potentiation (LTP). We used selective antagonists and agonists for mGluRs and we analysed the field potentials evoked by vestibular afferent stimulation before and after high‐frequency stimulation (HFS) to induce LTP. The group II and III mGluR antagonist, (R,S)‐α‐2‐methyl‐4sulphonophenylglycine (MSPG), induced LTP per se and caused a reduction of the paired‐pulse facilitation (PPF) ratio indicating an enhancement of glutamate release. This suggests that group II and III mGluRs are activated under basal conditions to limit glutamate release. Both the group II and III mGluR selective antagonists, 2S‐2‐amino‐2‐(1S,2S‐2‐carboxycycloprop‐1‐yl)‐3‐(xanth‐9‐yl)propanoate (LY341495) and (R,S)‐α‐methylserine‐O‐phosphate (MSOP), induced LTP, and the selective agonists, (2R,4R)‐4‐aminopyrrolidine‐2,4‐dicarboxylate (APDC) and L(+)‐2‐amino‐4‐phosphonobutyric acid (L‐AP4) depressed the field potentials and prevented HFS‐LTP, with a prevailing contribution of group II mGluRs over that of group III mGluRs. The mGluR1 antagonist, 7‐(hydroxyimino)cyclopropa[b]chromen‐1a‐carboxylate ethyl ester (CPCCOEt) prevented the full development and maintenance of HFS‐LTP. By contrast, the mGluR5 antagonist, 2‐methyl‐6‐phenylethynylpyridine (MPEP) induced LTP per se, which was impeded by CPCCOEt, and it had no effect on LTP once induced by HFS. The PPF analysis showed an enhancement of glutamate release during MPEP potentiation. The group I mGluR agonist, (R,S)‐3,5‐dihydroxyphenylglycine (DHPG) induced LTP per se, which was blocked by CPCCOEt. By contrast the mGluR5 agonist, (R,S)‐2‐chloro‐5‐hydroxypheylglycine (CHPG) prevented LTP elicited by HFS and DHPG as well. In conclusion vestibular LTP is inhibited by group II and III mGluRs during the early induction phase while it is facilitated by mGluR1 for achieving its full expression and consolidation. An additional inhibitory control is exerted by mGluR5 at the level of this facilitatory phase.
Thermoregulatory Control of Sympathetic Fibres Supplying the Rat's Tail
Authors: Owens N. C. , Ootsuka Y. , Kanosue K. , McAllen R. M.
Published: September 2002

We investigated the thermoregulatory responses of sympathetic fibres supplying the tail in urethane‐anaesthetised rats. When skin and rectal temperatures were kept above 39 °C, tail sympathetic fibre activity was low or absent. When the trunk skin was cooled episodically by 2–7 °C by a water jacket, tail sympathetic activity increased in a graded fashion below a threshold skin temperature of 37.8 ± 0.6 °C, whether or not core (rectal) temperature changed. Repeated cooling episodes lowered body core temperature by 1.3–3.1 °C, and this independently activated tail sympathetic fibre activity, in a graded fashion, below a threshold rectal temperature of 38.4 ± 0.2 °C. Tail blood flow showed corresponding graded vasoconstrictor responses to skin and core cooling, albeit over a limited range. Tail sympathetic activity was more sensitive to core than to trunk skin cooling by a factor that varied widely (24‐fold) between animals. Combined skin and core cooling gave additive or facilitatory responses near threshold but occlusive interactions with stronger stimuli. Unilateral warming of the preoptic area reversibly inhibited tail sympathetic activity. This was true for activity generated by either skin or core cooling. Single tail sympathetic units behaved homogeneously. Their sensitivity to trunk skin cooling was 0.3 ± 0.08 spikes s−1°C−1 and to core cooling was 2.2 ± 0.5 spikes s−1°C−1. Their maximum sustained firing rate in the cold was 1.82 ± 0.35 spikes s−1.
Human Skeletal Muscle Fatty Acid and Glycerol Metabolism During Rest, Exercise and Recovery
Authors: van Hall G. , Sacchetti M. , Rådegran G. , Saltin B.
Published: September 2002

This study was conducted to investigate skeletal muscle fatty acid (FA) and glycerol kinetics and to determine the contribution of skeletal muscle to whole body FA and glycerol turnover during rest, 2 h of one‐leg knee‐extensor exercise at 65 % of maximal leg power output, and 3 h of recovery. To this aim, the leg femoral arterial‐venous difference technique was used in combination with a continuous infusion of [U‐13C]palmitate and [2H5]glycerol in five post‐absorptive healthy volunteers (22 ± 3 years). The influence of contamination from non‐skeletal muscle tissues, skin and subcutaneous adipose tissue, on FA and glycerol kinetics was studied by catheterization of the femoral vein in antegrade and retrograde directions. Substantially higher net leg FA and glycerol uptakes were observed with a retrograde compared to an antegrade catheter position, as a result of a much lower tracer‐calculated leg FA and glycerol release. The whole body FA rate of appearance (Ra) increased with exercise and decreased rapidly in recovery but stayed higher compared to pre‐exercise. The leg net FA uptake decreased immediately on cessation of exercise to near pre‐exercise level, but the tracer FA uptake and release decreased slowly and reached constant values after ≈1.5 h of recovery similar to pre‐exercise. Whole body FA reesterification (FA Rd ‐ FA oxidation; Rd, rate of disappearance) was ≈400 μmol min−1 at rest and during exercise, and increased during recovery to 495 μmol min−1. Leg FA reesterification was 17 μmol min−1 at rest and decreased to 9 μmol min−1 during recovery, due to a larger fraction of leg FA uptake being directed to oxidation. A net glycerol exchange across the leg could not be detected under all conditions, but a substantial leg glycerol uptake was observed, which was substantially higher during exercise. Total body skeletal muscle FA and glycerol uptake/release was estimated to account for 18–25 % of whole body Rd or Ra. In conclusion: (1) skeletal muscle FA and glycerol metabolism, using the leg arterial‐venous difference method, can only be studied if contamination from skin and subcutaneous adipose tissue is prevented; (2) whole body FA reesterification is unchanged when going from rest to exercise, but is increased during recovery; (3) in post‐absorptive man total body skeletal muscle contributes 17–24 % to whole body FA and glycerol turnover and FA reesterification at rest; (4) glycerol is taken up by skeletal muscle and the uptake increases many fold during exercise.
A Model of Atropine‐Resistant Theta Oscillations in Rat Hippocampal Area CA1
Authors: Gillies M. J. , Traub R. D. , LeBeau F. E. N. , Davies C. H. , Gloveli T. , Buhl E. H. , Whittington M. A.
Published: September 2002

Theta frequency oscillations are a predominant feature of rhythmic activity in the hippocampus. We demonstrate that hippocampal area CA1 generates atropine‐resistant theta population oscillations in response to metabotropic glutamate receptor activation under conditions of reduced AMPA receptor activation. This activity occurred in the absence of inputs from area CA3 and extra‐ammonic areas. Field theta oscillations were co‐expressed with pyramidal distal apical dendritic burst spiking and were temporally related to trains of IPSPs with slow kinetics. Pyramidal somatic responses showed theta oscillations consisted of compound inhibitory synaptic potentials with initial IPSPs with slow kinetics followed by trains of smaller, faster IPSPs. Pharmacological modulation of IPSPs altered the theta oscillation suggesting an inhibitory network origin. Somatic IPSPs, dendritic burst firing and stratum pyramidale interneuron activity were all temporally correlated with spiking in stratum oriens interneurons demonstrating intrinsic theta‐frequency oscillations. Disruption of spiking in these interneurons was accompanied by a loss of both field theta and theta frequency IPSP trains. We suggest that population theta oscillations can be generated as a consequence of intrinsic theta frequency spiking activity in a subset of stratum oriens interneurons controlling electrogenesis in pyramidal cell apical dendrites.
Hibernating without Oxygen: Physiological Adaptations of the Painted Turtle
Authors: Jackson Donald C.
Published: September 2002

Many freshwater turtles in temperate climates may experience winter periods trapped under ice unable to breathe, in anoxic mud, or in water depleted of O2. To survive, these animals must not only retain function while anoxic, but they must do so for extended periods of time. Two general physiological adaptive responses appear to underlie this capacity for long‐term survival. The first is a coordinated depression of metabolic processes within the cells, both the glycolytic pathway that produces ATP and the cellular processes, such as ion pumping, that consume ATP. As a result, both the rate of substrate depletion and the rate of lactic acid production are slowed greatly. The second is an exploitation of the extensive buffering capacity of the turtle's shell and skeleton to neutralize the large amount of lactic acid that eventually accumulates. Two separate shell mechanisms are involved: release of carbonate buffers from the shell and uptake of lactic acid into the shell where it is buffered and sequestered. Together, the metabolic and buffering mechanisms permit animals to survive for 3–4 months at 3 °C with no O2 and with circulating lactate levels of 150 mmol l−1 or more.
Effects of Thyroxine on Myosin Isoform Expression and Mechanical Properties in Guinea‐Pig Smooth Muscle
Authors: Löfgren Mia , Fagher Katarina , Woodard Geoffrey , Arner Anders
Published: September 2002

Information on the effects of thyroid hormone on smooth muscle contractile protein expression and mechanical properties is sparse. We have addressed the following questions. (1) Can thyroxine hormone alter myosin isoform composition in smooth muscle? (2) Can a change in myosin isoform composition lead to altered mechanical properties in smooth muscle? (3) Are alterations, if occurring, equal in fast and slow smooth muscle types? Guinea‐pigs were treated with thyroxine (T4) for 12 days. Control animals were given physiological saline solution. Maximal unloaded shortening velocity (Vmax) was measured in chemically skinned, maximally activated muscle preparations from the aorta and the taenia coli. Vmax increased following thyroxine treatment, by approximately 20 % in the taenia coli. In the aorta, no significant increase in Vmax could be detected. The sensitivity of isometric force to inorganic phosphate (Pi) was increased in the taenia coli following thyroxine treatment. The expression of mRNA (determined with RT‐PCR) for the myosin heavy chain with the seven amino acid insert increased by approximately 70 % in the aorta and about 25 % in the taenia coli following thyroxine treatment. Western blot analysis showed an increase in the inserted myosin heavy chain form in the taenia coli. Expression of mRNA for the myosin essential light chains and the corresponding proteins did not change significantly in either muscle type. No alterations in non‐muscle myosin heavy chain isoforms could be detected after thyroxine treatment. In conclusion, thyroxine treatment alters the isoform composition of myosin in fast and slow smooth muscles in vivo. This change is sufficient to increase shortening velocity and sensitivity of isometric force to Pi in the fast, but not in the slow, smooth muscle type.
Mechanisms Underlying the Frequency Dependence of Contraction and [Ca2+]i Transients in Mouse Ventricular Myocytes
Authors: Antoons Gudrun , Mubagwa Kanigula , Nevelsteen Ines , Sipido Karin R.
Published: September 2002

In most mammalian species force of contraction of cardiac muscle increases with increasing rate of stimulation, i.e. a positive force‐frequency relationship. In single mouse ventricular cells, both positive and negative relationships have been described and little is known about the underlying mechanisms. We studied enzymatically isolated single ventricular mouse myocytes, at 30 °C. During field stimulation, amplitude of unloaded cell shortening increased with increasing frequency of stimulation (0.04 ± 0.01 ΔL/L0 at 1 Hz to 0.07 ± 0.01 ΔL/L0 at 4 Hz, n= 12, P < 0.05). During whole cell voltage clamp with 50 μM [K5‐fluo‐3]pip, both peak and baseline [Ca2+]i increased at higher stimulation frequencies, but the net Δ[Ca2+]i increased only modestly from 1.59 ± 0.08 ΔF/F0 at 1 Hz, to 1.71 ± 0.11 ΔF/F0 at 4 Hz (n= 17, P < 0.05). When a 1 s pause was interposed during stimulation at 2 and 4 Hz, [Ca2+]i transients were significantly larger (at 4 Hz, peak F/F0 increased by 78 ± 2 %, n= 5). SR Ca2+ content assessed during caffeine application, significantly increased from 91 ± 24 μmol l−1 at 1 Hz to 173 ± 20 μmol l−1 at 4 Hz (n= 5, P < 0.05). Peak ICa,L decreased at higher frequencies (by 28 ± 6 % at 2 Hz, and 45 ± 8 % at 4 Hz), due to slow recovery from inactivation. This loss of ICa,L resulted in reduced fractional release. Thus, in mouse ventricular myocytes the [Ca2+]i‐frequency response depends on a balance between the increase in SR content and the loss of trigger ICa,L. Small changes in this balance may contribute to variability in frequency‐dependent behaviour. In addition, there may be a regulation of the contractile response downstream of [Ca2+]i.
Role for cAMP and Protein Phosphatase in the Presynaptic Expression of Mouse Hippocampal Mossy Fibre Depotentiation
Authors: Huang Chiung‐Chun , Chen Yea‐Lin , Liang Ying‐Ching , Hsu Kuei‐Sen
Published: September 2002

Long‐term potentiation (LTP) at the hippocampal mossy fibre‐CA3 synapses can be reversed (depotentiated) by long trains of low‐frequency stimulation (LFS). In the present study, we showed that this depotentiation is triggered by a presynaptic group II metabotropic glutamate receptor (mGluR), which reduces cytosolic cAMP level, leading to a reversal of cellular processes responsible for mossy fibre LTP expression. Furthermore, we found that both the presynaptic activity‐induced elevation of Ca2+ and the activation of protein phosphatase (PP) activity are required for the induction of depotentiation. Thus, we conclude that mossy fibre depotentiation is expressed presynaptically through the activation of both presynaptic mGluR‐ and PP‐coupled signalling cascades, and that the bidirectional long‐term plasticity at the mossy fibre‐CA3 synapses is likely to be regulated by presynaptic Ca2+‐dependent processes.
Film Cooling Effectiveness and Heat Transfer Coefficient Distributions Around Diffusion Shaped Holes
Authors: Yu Y. , Yen C.-H. , Shih T. I.-P. , Chyu M. K. , Gogineni S.
Published: September 2002

Presented in this paper is an experimental study focusing on the effects of diffusion hole-geometry on overall film cooling performance. The study consists of three different but closely related hole shapes: (1) Shape A: straight circular hole with a 30 deg inclined injection, (2) Shape B: same as Shape A but with a 10 deg forward diffusion, and (3) Shape C: same as Shape B with an additional 10 deg lateral diffusion. The blowing ratios tested are 0.5 and 1.0. The density ratio is nominally equal to one. Measurements of the experiment use a transient liquid crystal technique that reveals local distributions of both film effectiveness (η) and heat transfer coefficient (h). The data obtained indicate that Shape C with a combined forward and lateral diffusion produces a significant increase in η and decrease in h as compared to Shape A, the baseline case. These improvements combined yield an about 20 percent to 30 percent reduction in heat transfer or thermal load on the film protected surface. Shape B, with forward diffusion only, shows a much less significant change in both film effectiveness and overall heat transfer reduction than Shape C. However, it has the lowest heat transfer coefficient in the vicinity of the injection hole among all the three hole-shapes studied. A flow visualization study using pulsed laser sheet-light reveals that Shape A and Shape B inherit quite similar flow structures. The coolant injected out of Shape C flows much closer to the protected wall than that of Shape A and Shape B.
Directional Spectral Emittance of a Packed Bed: Influence of the Temperature Gradient in the Medium
Authors: Baillis Dominique , Sacadura Jean-Franc¸ois
Published: September 2002

Recently a new experimental set-up for measuring the directional spectral emittance has been developed. Both sides of packed opaque spheres are simultaneously heated with identical power laser beams (CO2 10.6 μm) and the isothermal condition in the medium is assumed. In this paper we consider the coupled conduction-radiation equations to investigate the effect of the temperature nonuniformity on the calculated value of the emittance and to verify if the isothermal assumption is valid. Moreover, the sensitivity of the emittance and the temperature profile to the values of parameters (such as the porosity, the face temperatures, and the particle hemispherical reflectivity, etc.) is studied.
Effectiveness Correlations for Heat and Moisture Transfer Processes in an Enthalpy Exchanger With Membrane Cores
Authors: Zhang L. Z. , Niu J. L.
Published: September 2002

The performance correlations for the effectiveness of heat and moisture transfer processes in an enthalpy exchanger with membrane cores are presented. The physical phenomena relevant to the heat and moisture transfer in these devices have been used to develop a novel set of correlations based on the relevant dimensionless parameters. The total enthalpy effectiveness can be calculated from sensible effectiveness, latent effectiveness, and the ratio of latent to sensible energy differences across the unit. Studies show that the sensible effectiveness is a function of NTU, the number of transfer units for heat; while the latent effectiveness is a function of NTUL, the number of transfer units for moisture. The relations between NTUL and NTU are derived and studied with the proper separation of moisture resistance for membranes. This newly developed dimensionless parameter, NTUL, is to summarize the sorption characteristics of membrane material, the exchanger configurations, as well as the operating conditions. A number of experimental results on an enthalpy exchanger with novel hydrophilic membrane cores has been used to valid these correlations.
A Cavity Activation and Bubble Growth Model of the Leidenfrost Point
Authors: Bernardin John D. , Mudawar Issam
Published: September 2002

This study presents a new mechanistic model of the Leidenfrost point (LFP); the minimum liquid/solid interface temperature required to support film boiling on a smooth surface. The model is structured around bubble nucleation, growth, and merging criteria, as well as surface cavity size characterization. It is postulated that for liquid/solid interface temperatures at and above the LFP, a sufficient number of cavities (about 20 percent) are activated and the bubble growth rates are sufficiently fast that a continuous vapor layer is established nearly instantaneously between the liquid and the solid. The model is applicable to both pools of liquid and sessile droplets. The effect of surface cavity distribution on the LFP predicted by the model is verified for boiling on aluminum, nickel and silver surfaces, as well as on a liquid gallium surface. The model exhibits good agreement with experimental sessile droplet data for water, FC-72, and acetone. While the model was developed for smooth surfaces on which the roughness asperities are of the same magnitude as the cavity radii (0.1–1.0 μm), it is capable of predicting the boundary or limiting Leidenfrost temperature for rougher surfaces with good accuracy.
Experimental Study on Forced Convection Heat Transfer Inside Horizontal Tubes in an Absorption/Compression Heat Pump
Authors: Yong and Li , Sumathy K.
Published: September 2002

Quasi-local absorption heat transfer coefficients and pressure drop inside a horizontal tube absorber have been investigated experimentally, with R-22/DMA as the working pair. The absorber is a counterflow coaxial tube-in-tube heat-exchanger with the working fluid flowing in the inner tube while the water moves through the annulus. A large temperature gliding has been experienced during the absorption process. Experimental results show that the heat transfer coefficient of the forced convective vapor absorption process is higher compared to the vertical falling film absorption. A qualitative study is made to analyze the effect of mass flux, vapor quality and solution concentration on pressure drop and heat transfer coefficients. On the basis of the experimental results, a new correlation is proposed whereby the two-phase heat transfer is taken as a product of the forced convection of the absorption and the combined effect of heat and mass transfer at the interface. The correlation is found to predict the experimental data almost within 30 percent.
Thermal Aspects in the Continuous Chemical Vapor Deposition of Silicon
Authors: Yoo Hoseon , Jaluria Yogesh
Published: September 2002

This paper deals with the continuous chemical vapor deposition of silicon in a horizontal cold wall reactor, paying special attention to a moving susceptor. A two-dimensional numerical model, which accounts for variable properties, thermal diffusion, radiative heat exchange among surfaces, and conjugate heat transfer between the gas and susceptor, is developed and validated. Scale analysis for the susceptor energy balance enables the identification of the characteristic parameters and the prediction of their qualitative effects before carrying out a detailed analysis. The results from the scale analysis are found to be consistent with the numerical predictions. The results show that the present continuous system is characterized by two newly defined parameters: conductance ratio and susceptor parameter. A pair of performance curves that relate the deposition efficiency to each parameter successfully provide the susceptor-related design conditions. It is also revealed that there exists an optimum length of heating zone that maximizes the deposition efficiency.
Direct Chill Casting of Aluminum Alloys: Modeling and Experiments on Industrial Scale Ingots
Authors: Vreeman Christopher J. , Schloz J. David , Krane Matthew John M.
Published: September 2002

A continuum mixture model of the direct chill casting process is compared to experimental results from industrial scale aluminum billets. The model, which includes the transport of free-floating solid particles, can simulate the effect of a grain refiner on macrosegregation and fluid flow. It is applied to an Al-6 wt% Cu alloy and the effect of grain refiner on macrosegregation, sump profile, and temperature fields are presented. Two 45 cm diameter billets were cast under production conditions with and without grain refiner. Temperature and composition measurements and sump profiles are compared to the numerical results. The comparison shows some agreement for the grain refined case. It is believed that an incorrect assumption about the actual grain structure prevents good agreement in the non-grain refined billet.
On the Accuracy of Beam-Averaged Interferometric Heat Transfer Measurements
Authors: Naylor D.
Published: September 2002

Laser interferometry can be used in a three dimensional temperature field to measure the average fluid temperature and heat transfer rate, integrated over the length of an experimental model. However, such measurements are inherently approximate when the surface temperature varies in the direction of the test/object beam. In this study an analysis is performed to determine the accuracy of beam-averaged heat transfer rate measurements made in ideal gases. Two analysis methods are considered. The first method is based on extrapolation of the near-wall temperature field to obtain the surface gradient. In the second method, the temperature gradient at the surface is obtained directly from the gradient of the fringe field. The results show that the intrinsic error in the measurements depends strongly upon the form and magnitude of the temperature variation in the light beam direction. Although the error in the measured heat transfer rate is shown to be small for many commonly encountered conditions, it can be greater than ten percent in extreme cases.
Bulk Temperature Development in Transient Heat Transfer Measurements Using Heater Foils
Authors: von Wolfersdorf Jens
Published: September 2002

The time and space development of the fluid bulk temperature in a transient heat transfer experiment for internal channel cooling investigations using heater foils is addressed. An analytical solution for uniform heat transfer coefficients is derived which shows the effect of wall heating on the bulk temperature during a transient test run. A simplified model is proposed for characterizing the bulk temperature development by introducing an upstream heat transfer parameter. With this, analytical solutions for the local wall temperature history can be derived. The presented solution can be used for data reduction of transient tests of this kind.
A Natural Circulation Model of the Closed Loop, Two-Phase Thermosyphon for Electronics Cooling
Authors: Haider S. I. , Joshi Yogendra K. , Nakayama Wataru
Published: September 2002

This study presents a model for the two-phase flow and heat transfer in the closed loop, two-phase thermosyphon (CLTPT) involving co-current natural circulation. The focus is on CLTPTs for electronics cooling that exhibit complex two-phase flow patterns due to the closed loop geometry and small tube size. The present model is based on mass, momentum, and energy balances in the evaporator, rising tube, condenser, and the falling tube. The homogeneous two-phase flow model is used to evaluate the friction pressure drop of the two-phase flow imposed by the available gravitational head through the loop. The saturation temperature dictates both the heat source (chip) temperature and the condenser heat rejection capacity. Thermodynamic constraints are applied to model the saturation temperature, which also depends upon the local heat transfer coefficient and the two-phase flow patterns inside the condenser. The boiling characteristics of the enhanced structure are used to predict the chip temperature. The model is compared with experimental data for dielectric working fluid PF-5060 and is in general agreement with the observed trends. The degradation of condensation heat transfer coefficient due to diminished vapor convective effects, and the presence of subcooled liquid in the condenser are expected to cause higher thermal resistance at low heat fluxes. The local condensation heat transfer coefficient is a major area of uncertainty.
Recent research advances in the dynamic behavior of shells: 1989–2000, Part 2: Homogeneous shells
Authors: Qatu Mohamad S
Published: September 2002

Shell-like structures are used in various engineering applications including civil, aerospace, mechanical, marine, and automotive engineering. This article reviews most of the recent research done in the field of dynamic response of homogeneous shells with special attention to free vibrations. Literature on dynamics of laminated composite shells was reviewed in Part 1 published in the July 2002 issue of AMR. Emphasis is given to the theory being applied (thin, thick, 3D, nonlinear,…), the shell geometries that were subject to dynamics research (cylindrical, conical, spherical,…), the analysis method (exact, Ritz, finite elements,…), and the various complicating effects (initial stress, imperfection, added masses and springs, elastic supports, rotating shells, interaction with fluids, and others). This review article contains 606 references.
Nonstaggered APPLE Algorithm for Incompressible Viscous Flow in Curvilinear Coordinates
Authors: Lee S. L. , Chen Y. F.
Published: September 2002

The NAPPLE algorithm for incompressible viscous flow on Cartesian grid system is extended to nonorthogonal curvilinear grid system in this paper. A pressure-linked equation is obtained by substituting the discretized momentum equations into the discretized continuity equation. Instead of employing a velocity interpolation such as pressure-weighted interpolation method (PWIM), a particular approximation is adopted to circumvent the checkerboard error such that the solution does not depend on the under-relaxation factor. This is a distinctive feature of the present method. Furthermore, the pressure is directly solved from the pressure-linked equation without recourse to a pressure-correction equation. In the use of the NAPPLE algorithm, solving the pressure-linked equation is as simple as solving a heat conduction equation. Through two well-documented examples, performance of the NAPPLE algorithm is validated for both buoyancy-driven and pressure-driven flows.
Investigation of Circumferential Variation of Heat Transfer Coefficients During In-Tube Evaporation for R-22 and R-407C Using Liquid Crystal
Authors: Yoon Seok Ho , Kim Min Soo
Published: September 2002

Heat transfer coefficients during evaporation in a horizontal smooth tube for R-22 and R-407C (R-32/125/134a, 23/25/52 wt.%) have been measured using thermochromic liquid crystal. Focus has been put on the circumferential variation of heat transfer coefficients at several cross-sections of the test tube with inner diameter of 11.3 mm for several vapor qualities of refrigerant. The inner wall temperatures were calculated by one dimensional heat conduction equation from the measured outer wall temperatures, which were obtained using an image processing technique with thermochromic liquid crystal (TLC). The relation between measured temperature and color information (Red-Green-Blue values) of thermochromic liquid crystal was calibrated by a neural network method. Results show that circumferential variation of heat transfer coefficients for R-22 is quite large with the highest heat transfer coefficient at the top of the tube. For zeotropic mixture of R-407C, similar trend has been observed with less difference between the heat transfer coefficients at the top and bottom than that of R-22.
Rewetting of an Infinite Slab With Uniform Heating Under Quasi-Steady Conditions
Authors: Satapathy A. K. , Sahoo R. K.
Published: September 2002

The two-dimensional quasi-steady conduction equation governing conduction controlled rewetting of an infinite slab, with one side flooded and the other side subjected to a constant heat flux, has been solved by Wiener-Hopf technique. The solution yields the quench front temperature as a function of various model parameters such as Peclet number, Biot number and dimensionless heat flux. Also, the critical (dryout) heat flux is obtained by setting the Peclet number equal to zero, which gives the minimum heat flux required to prevent the hot surface being rewetted.
Energy and Exergy Balance in the Process of Spray Combustion in a Gas Turbine Combustor
Authors: Som S. K. , Sharma N. Y.
Published: September 2002

A theoretical model of exergy balance based on availability transfer and flow availability in the process of spray combustion in a gas turbine combustor has been developed to evaluate the total thermodynamic irreversibility and second law efficiency of the process at various operating conditions, for fuels with different volatilities. The velocity, temperature and concentration fields in the combustor, required for the evaluation of the flow availabilities and process irreversibilities, have been computed numerically from a two phase separated flow model of spray combustion. The total thermodynamic irreversibility in the process of spray combustion has been determined from the difference in the flow availability at inlet and outlet of the combustor. The irreversibility caused by the gas phase processes in the combustor has been obtained from the entropy transport equation, while that due to the inter-phase transport processes has been obtained as a difference of gas phase irreversibilities from the total irreversibility. A comparative picture of the variations of combustion efficiency and second law efficiency at different operating conditions for fuels with different volatilities has been made to throw light on the trade off between the effectiveness of combustion and the lost work in the process of spray combustion in a gas turbine combustor.
R-22 and Zeotropic R-22/R-142b Mixture Condensation in Microfin, High-Fin, and Twisted Tape Insert Tubes
Authors: Smit F. J. , Meyer J. P.
Published: September 2002

Using mixtures of the zeotropic refrigerant mixture R-22/R-142b, a series of experiments was performed to determine the sectional and average heat transfer coefficients. Experiments were also conducted to compare three different heat transfer enhancement methods to that of smooth tubes. They were microfins, twisted tapes, and high fins. Measurements at different mass fluxes were obtained at six refrigerant mass fractions from 100 percent R-22 up to a 50 percent/50 percent mixture of R-22/R-142b. All condensation measurements were conducted at an isobaric inlet pressure of 2.43 MPa. This pressure corresponds to a saturation temperature of 60°C for R-22. The measurements were taken in 9.53 mm outer diameter smooth tubes and microfin tubes with lengths of 1603 mm. The heat transfer coefficients were determined with the Log Mean Temperature Difference equations. It was found that microfins were more suitable as an enhancement method than twisted tubes or high fins. Also, that the heat transfer coefficients and pressure drops decrease as the mass fraction of R-142b increases.
Contact mechanics of multilayered rough surfaces
Authors: Bhushan Bharat , Peng Wei
Published: September 2002

The deposition of layers is an effective way to improve the tribological performance of rough surfaces. The contact mechanics of layered rough surfaces needs to be studied to optimize layer parameters. Since 1995 a lot of progress has been made in the development of numerical contact models, which analyze the contact behavior of layered rough surfaces with no assumption concerning the roughness distribution as well as the effect of interfacial liquid film on the contact statistics. These models predict the contact pressure profile on the interface and contact statistics, namely fractional contact area, the maximum value of contact pressure, von Mises and principal tensile stresses, and relative meniscus force. The results allow the specification of layer properties to reduce friction, stiction, and wear of layered rough surfaces. A comprehensive review of these numerical contact models is presented here. Based on the formulation of contact problems, these models are classified into three categories: direct formulation, weighted residual formulation, and minimum total potential energy formulation. The numerical methods applied in these models include Finite Difference Method (FDM), Finite Element Method (FEM), and Boundary Element Method (BEM). Typical examples of layered rough surfaces of contact simulated by those models are presented. The examples contain data for various surface topographies, elastic and elastic-plastic material properties, normal and tangential loading conditions, and dry and wet interfaces. A 3D BEM model based on a variational principle is described in detail for its capability to analyze the layered rough surfaces of contact involving a large number of contact points. Applications of the model to magnetic storage devices and MicroElectroMechanical Systems (MEMS) are presented. This review article contains 92 references.
On the Potential for Homogeneous Nucleation of Salt From Aqueous Solution in a Natural Convection Boundary Layer
Authors: Smith Kenneth A. , Hodes Marc , Griffith Peter
Published: September 2002

Recent studies have examined the rate of salt deposition by natural convection on a cylinder heated above the solubility temperature corresponding to the concentration of salt in the surrounding solution at conditions typical of the Supercritical Water Oxidation (SCWO) process (Hodes et al. [1,2], Hodes [3]). The total deposition rate of salt on the cylinder is the sum of the rate of deposition at the salt layer-solution interface (SLSI) formed on the cylinder and that within the porous salt layer. The rate of deposition at the SLSI cannot be computed without determining whether or not salt nucleates homogeneously in the adjacent (natural convection) boundary layer. A methodology to determine whether or not homogeneous nucleation in the boundary layer is possible is presented here. Temperature and concentration profiles in the boundary layer are computed under the assumption that homogeneous nucleation does not occur. If, under this assumption, supersaturation does not occur, homogeneous nucleation is impossible. If supersaturation is present, homogeneous nucleation may or may not occur depending on the amount of metastability the solution can tolerate. It is shown that the Lewis number is the critical solution property in determining whether or not homogeneous nucleation is possible and a simple formula is developed to predict the Lewis number below which homogeneous nucleation is impossible for a given solubility boundary and set of operating conditions. Finally, the theory is shown to be consistent with experimental observations for which homogeneous nucleation is absent or present.
Heat Transfer Characteristics of Melting Ice Spheres Under Forced and Mixed Convection
Authors: Hao Y. L. , Tao Y.-X.
Published: September 2002

An experimental investigation is conducted to characterize melting and heat transfer during convective melting of an ice sphere in horizontally flowing water. The shape changes of melting ice spheres with time at different velocities and supply temperatures of water and at different initial ice temperatures are recorded using a digital video camcorder. From the recorded images, the time variations of local melting rate, local heat transfer coefficient and local Nusselt number at various angular positions are obtained. The effects of water temperature and velocity on these local parameters are analyzed and the total melting rate and average heat transfer coefficient are determined. Guided by a shape factor characterizing the particle shape variation, an empirical correlation for average Nusselt number in convective melting of a solid particle is obtained. The experimental results provide important particle-level information needed to improve the accuracy of numerical models for convective melting of a single particle or packed particles.
Thermal and Electrical Energy Transport and Conversion in Nanoscale Electron Field Emission Processes
Authors: Fisher T. S. , Walker D. G.
Published: September 2002

This paper considers the theory of electron field emission from nanoscale emitters with particular focus on thermal and electrical energy transport. The foundational theory of field emission is explored, and a model is presented that accounts explicitly for the energy band curvature produced by nanoscale tip emitters. The results indicate that the inclusion of band curvature strongly influences the energetic distribution of electrons for emitter radii less than 50 nm. The energy exchange process between emitted and replacement electrons is shown to allow high local energy transfer rates that can be exploited in direct thermal-to-electrical energy conversion processes. The dependence of energy conversion rates on material and operational parameters is demonstrated. Throughout the paper, opportunities for further research involving nanoscale heat transfer, materials development, and modeling are highlighted.
Evaporation of Water Droplets Placed on a Heated Horizontal Surface
Authors: Ruiz Orlando E. , Black William Z.
Published: September 2002

A numerical analysis of the evaporation process of small water droplets with diameters of 1 mm or less that are gently deposited on a hot isothermal solid surface has been performed. This study considers the internal fluid motion that occurs as a result of the thermocapillary convection in the droplet and it determines the effect of fluid motion on the heat transfer between the drop and the solid surface. This study is particularly relevant because the internal fluid motion has not been considered in previous numerical and analytical models presented in the literature. To assess the effects of internal fluid motion, the model results are compared to numerical results provided by a heat conduction model that neglects the fluid motion. The Navier-Stokes and Thermal Energy equations are solved using the Artificial Compressibility Method with Dual Time Stepping. Boundary-fitted grids are used to track the changes in the droplet surface shape during the evaporation process. The numerical simulations have demonstrated that the internal fluid motion provides vastly different temperature distributions in the drop compared to the results from the heat conduction model that neglects fluid motion. The evolution of the droplet geometry was simulated from an initial spherical-shaped cap until the contact angle was close to the receding contact angle.
Nontraditional
theories of shells and plates
Authors: Ambartsumian SA
Published: September 2002

In this review, I shall discuss some of the aspects of the Mechanics of Solid Deformable Media, which have been for more than half a century among my main research priorities.

It is somewhat difficult to write about research to which one has devoted his life. First of all, because you have to write about yourself, describing personal feelings—successes and failures, triumphs and defeats. It is hardly going to be of major interest for the readers. Thus, I am just going to note down, with corresponding comments some results, obtained by my colleagues, followers and myself. I think this can be rather interesting for junior researchers, who have not tasted yet the joy of success and the bitterness of the failure. This might be interesting as well for the mature scholars with relatively high self-esteem, willing to present each and every obtained result as a discovery—classical theory to be followed. The weight of a scientific result is evaluated by time. Time and coming generations of researchers will show the place in science of all results, even those which are widely advertised.
Interface and Strain Effects on the Thermal Conductivity of Heterostructures: A Molecular Dynamics Study
Authors: Abramson Alexis R. , Tien Chang-Lin , Majumdar Arun
Published: September 2002

Molecular dynamics simulations are used to examine how thermal transport is affected by the presence of one or more interfaces. Parameters such as film thickness, the ratio of respective material composition, the number of interfaces per unit length, and lattice strain are considered. Results indicate that for simple nanoscale strained heterostructures containing a single interface, the effective thermal conductivity may be less than half the value of an average of the thermal conductivities of the respective unstrained thin films. Increasing the number of interfaces per unit length, however, does not necessarily result in a corresponding decrease in the effective thermal conductivity of the superlattice.