bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of pressure drop stabilization in arteries during variations in blood flow rate
Authors: KHAYUTIN V. M. , NIKOLSKY V. P. , ROGOZA A. N. , LUKOSHKOVA E. V.
Published: December 2008

The aim of this study was to clear up whether the flow‐induced dilation of arteries is sufficient to ensure invariability of pressure drop along these vessels under many‐fold increase in blood flow rate. In anaesthetized rats an arteriovenous shunt was constructed by connecting the saphenous artery and the femoral vein. Resistance of the shunt was changed by a device creating either rectangular flow pulses of different amplitude or slow linear ramps of flow rate. Pressure drop along the arterial cascade, from the aortic arch up to distal saphenous artery, was recorded and the steady‐state flow‐pressure drop relationship was determined. At low flow rates (< 0.25–0.3 ml min‐1) the pressure drop rose proportionally to flow, i.e. as in a system of ordinary tubes with non‐regulated diameter. In contrast, at higher flow rates (up to 1.5 ml min‐1) the pressure drop either remained unchanged under 3‐ to 5‐fold increases of flow rate or rose far less than the flow. Thus, the pressure drop stabilization effect has been evidenced. The threshold of this effect was about 0.3 ml min‐1, corresponding to the ‘resting’ flow in the saphenous artery, i.e. the narrowest of all the cascade vessels. This coincidence suggests that the pressure drop stabilization is determined by the flow‐induced dilation mainly of this artery.
Excitatory and depressant respiratory responses to chemical stimulation of the rostral ventrolateral medulla in the cat
Authors: BONGIANNI F. , CORDA M. , FONTANA G. A. , PANTALEO T.
Published: December 2008

The rostral ventrolateral medulla (rVLM) is known to play an important role in cardiorespiratory control. In the rVLM an ‘apnoea region’, in which unilateral focal blocks induce strong depressant effects on inspiratory activity up to complete apnoea, has been described. This study was designed to systematically investigate the effects provoked by unilateral micro‐injections (10–30 nl) ofd,l‐homocysteic acid 160 mm into this region on respiratory activity and arterial blood pressure in pentobarbitone anaesthetized, vagotomized, paralyzed and artificially ventilated cats. Micro‐injections into the rostral portion of this area caused depressant respiratory responses up to complete apnoea, while micro‐injections into more caudally located sites induced excitatory respiratory responses. Similar effects were observed in the activity of phrenic nerves and inspiration‐related medullary neurons of both the dorsal and ventral respiratory group. The respiratory responses could be accompanied by marked increases in blood pressure (≥30 mmHg), especially at locations ventral to the retrofacial and facial nucleus; however, they could also occur in the absence of appreciable changes or even in association with slight decreases in blood pressure. Similar respiratory and pressor effects were observed in carotid sinus denervated cats. The results indicate that two distinct rVLM neuronal populations, one located more rostrally and the other more caudally, may have an important role in the genesis and/or maintenance of respiratory rhythm by exerting respectively inhibitory and excitatory influences on inspiratory activity. Furthermore, they support the hypothesis that different neural substrates of the rVLM are involved in the regulation of respiratory and cardiovascular functions.
The effect of endothelin, neuropeptide Y, calcitonin gene‐related peptide and substance P on neutrophil functions
Authors: HAFSTRÖM I. , RINGERTZ B. , LUNDEBERG T. , PALMBLAD J.
Published: December 2008

Neuropeptides are putative mediators of inflammation. At physiological concentrations substance P has been shown to prime polymorphonuclear neutrophil granulocyte (PMN) chemiluminescence (CL). In the present study we show also that both endothelin and neuropeptide Y (NPY), but not calcitonin gene‐related peptide (CGRP) are able to prime PMN oxidative metabolism. At similar nanomolar concentrations SP and endothelin (but not NPY) also primed formyl‐methionyl‐leucyl‐phenylalanine (fMLP)‐induced rises of cytosolic calcium. On the other hand, NPY caused a direct and dose‐related increase of cytosolic calcium concentrations. None of the mentioned neuropeptides primed PMN aggregation or directly induced CL, aggregation or chemotaxis over a wide range of concentrations (1 fm‐1 μm).
Pulmonary function hysteresis during compression to, and decompression from 31.3 ATA
Authors: TAYLOR N. A. S. , CLARKE J. R.
Published: December 2008

Tracheal gas density breathing heliox at 31.3 atmospheres absolute (O2 at 0.42 ATA) is 6.287 g 1‐1, or approximately 5.5 times greater than air at 1 ATA. This constitutes a significant respiratory load, previously shown to induce respiratory adaptation. During a saturation dive to 31.3 ATA, five divers were exposed to this load for 16 days. This project aimed at investigating possible hysteresis in pulmonary function during dive compression, adaptation and decompression phases. Pulmonary function tests were performed at the surface in air, and at four pressure stops during compression and decompression, with divers breathing the helium‐oxygen gas mixture. Significant hysteresis patterns were observed for pooled maximal voluntary ventilation, forced expired volume at 1 s, peak expiratory flow, and maximum expiratory flows (P < 0.05), with post‐adaptation flows consistently exceeding those observed during compression. Two mechanisms may explain these observations. Differences may be attributable to positive effort‐dependence in the forced expiratory flow; or it is possible the subjects adapted to the respiratory load by modifying neural input to airway smooth muscle, thereby modifying airway resistance.
Thel‐arginine/nitric oxide pathway in the rabbit urethral lamina propria
Authors: ZYGMUNT P. K. E. , PERSSON K. , ALM P. , LARSSON B. , ANDERSSON K.‐E.
Published: December 2008

A non‐adrenergic, non‐cholinergic (NANC)‐mediated relaxation has been demonstrated in the female rabbit urethral lamina propria, both in response to electrical stimulation and after administration of acetylcholine. The present study was performed to investigate if these relaxations are mediated via thel‐arginine/nitric oxide (NO) pathway. In the lamina propria of the female rabbit urethra, numerous NADPH diaphorase‐positive, fine varicose nerve fibres were observed both around arteries, in and around smooth muscle bundles, and in arterial endothelium. Since NADPH diaphorase histochemistry may be a marker of NO synthase‐containing neurons, this finding suggests the occurrence of NO synthase in this tissue. Isolated preparations of the rabbit urethral lamina propria, contracted by noradrenaline, produced frequency‐dependent NANC relaxations in response to electrical field stimulation, and concentration‐dependent NANC relaxations in response to acetylcholine. Electrically induced relaxations were possible to evoke even in preparations where acetylcholine‐induced relaxation was poor or absent. Pretreatment with the NO synthase inhibitor, NG‐nitro‐l‐arginine (l‐NOARG) reduced the maximum relaxation obtained at 30 Hz to 12% of the control response. NG‐nitro‐d‐arginine (d‐NOARG) had no effect. The effects ofl‐NOARG were antagonized by the addition ofl‐arginine. Acetylcholine relaxed noradrenaline‐precontracted strips by 36%; vasoactive intestinal polypeptide (VIP) reduced the contraction by 95%.l‐NOARG, but notd‐NOARG, abolished or reversed acetylcholine‐induced relaxations, but failed to reduce the relaxations produced by VIP.

The results demonstrate the occurrence of NADPH diaphorase‐positive nerve fibres in the lamina propria of the female rabbit urethra. NANC‐mediated relaxations, sensitive to inhibition of NO synthase, were produced both by electrical stimulation and by acetylcholine. The fact that NANC nerve‐mediated relaxation was demonstrated also in preparations where no relaxant response to acetylcholine could be detected suggests that the NO which produces relaxation can be derived both from endothelium and from NANC nerves.
Effects of nitric oxide synthesis blockade and angiotensin II on blood flow and spontaneous vasomotion in the rat cerebral microcirculation
Authors: MORITA‐TSUZUKI Y. , BOUSKELA E. , HARDEBO J. E.
Published: December 2008

The effects ofNω‐nitro‐l‐arginine methyl ester (l‐NAME) an inhibitor of NO synthesis, or angiotensin II on the frequency and amplitude of rhythmic variations (vasomotion) in blood flow of the intact rat cerebral circulation were investigated using laser‐Doppler flowmetry (LDF). Experiments were performed on Sprague‐Dawley rats anaesthetized withα‐chloralose. The rat's head was fixed on a stereotaxic frame and the microvascular blood flow of the parietal cortex on the right or on both sides was measured via a small hole in the parietal bone, keeping the dura and a thin bone layer intact. Following the intravenous injection ofl‐NAME, the mean arterial blood pressure (MABP) increased to 123 ± 1 mmHg (1.25 mg kg‐1) or to 144 ± 3 mmHg (5.0 mg kg‐1) but no significant changes in cerebral blood flow (CBF) or vasomotion could be detected. The observed increase in MABP was sustained untill‐arginine was administered. In the presence ofl‐NAME, during stepwise reduction of MABP, CBF remained constant when MABP was kept between 60 and 130 mmHg, the vasomotion frequency was lower when MABP was above 80 mmHg but its amplitude was two times higher than in the control group. In another group of animals, angiotensin was infused to give comparable increments in blood pressure. In contrast tol‐NAME, angiotensin II had no effect on either frequency or amplitude of vasomotion, compared to the control group, within the whole range of MABP studied. Our study suggests that NO might be a significant modulator of the microvascular tonus in the brain and that it plays a significant role in controlling vasomotion frequency and amplitude.
Sharing sovereignty for global regulation: The cases of fuel economy and online gambling
Authors: Mikler John
Published: December 2008

Globalization is sometimes taken as a synonym for market liberalization, because it is claimed that power has flowed from states to markets. Whether happening as a result of undeniable “forces” or some hegemonic consensus, many on both the left and right of politics agree that this is a reality. However, this article argues that states which share sovereignty with market actors are able to influence outcomes beyond their borders. The cases of fuel economy and online gambling regulations are used to illustrate the point. In the former case, Japanese and European industry‐driven regulations are being “exported” in the attributes of the products of their car industries. In the latter, UK market‐friendly regulations are likely to be “exported” to the European region and beyond because of industry support, and market liberalization principles embodied in European Union institutions. Both cases indicate that sharing sovereignty in the process of making and implementing national regulations produces opportunities for global regulation.
Influence of Fermentation Conditions on Glucosinolates, Ascorbigen, and Ascorbic Acid Content in White Cabbage (Brassica oleracea var. capitata cv. Taler) Cultivated in Different Seasons
Authors: Martinez‐Villaluenga C. , Peñas E. , Frias J. , Ciska E. , Honke J. , Piskula M.K. , Kozlowska H. , Vidal‐Valverde C.
Published: December 2008

ABSTRACT:  The content of glucosinolates (GLS), ascorbigen, and ascorbic acid in white cabbage (Brassica oleracea var. capitata cv. Taler) cultivated in different seasons (summer and winter) was determined, before and after spontaneous and starter‐induced fermentation. Different salt concentrations (0.5% NaCl or 1.5% NaCl) were used for sauerkraut production. Glucoiberin, sinigrin, and glucobrassicin were dominating in raw white cabbage cultivated either in winter or summer seasons. Ascorbigen precursor, glucobrassicin, was found higher in cabbage cultivated in winter (2.54 μmol/g dw) than those grown in summer (1.83 μmol/g dw). Cabbage fermented for 7 d was found to contain only traces of some GLS irrespective of the fermentation conditions used. Ascorbigen synthesis occurred during white cabbage fermentation. Brining cabbage at low salt concentration (0.5% NaCl) improved ascorbigen content in sauerkraut after 7 d of fermentation at 25 °C. The highest ascorbigen concentration was observed in low‐sodium (0.5% NaCl) sauerkraut produced from cabbage cultivated in winter submitted to either natural (109.0 μmol/100 g dw) or starter‐induced fermentation (108.3 and 104.6 μmol/100 g dw in cabbages fermented by L. plantarum and L. mesenteroides, respectively). Ascorbic acid content was found higher in cabbage cultivated in summer and fermentation process led to significant reductions. Therefore, the selection of cabbages with high glucobrassicin content and the production of low‐sodium sauerkrauts may provide enhanced health benefits towards prevention of chronic diseases.
Species‐Specific Effects of Sarcoplasmic Extracts on Lipid Oxidation in vitro
Authors: Ramanathan R. , Konda M.K.R. , Mancini R.A. , Faustman C.
Published: December 2008

ABSTRACT:  The degree to which lipid and myoglobin (Mb) oxidation processes interact in meat can be species‐specific. We investigated the effects of beef and pork sarcoplasmic extracts containing different Mb concentrations on lipid oxidation in a liposome system. Sarcoplasm was extracted from beef and pork longissimus dorsi and psoas major muscles. Beef sarcoplasm was diluted with 0.1 M phosphate buffer to obtain a Mb concentration equivalent to that in pork sarcoplasm. Conversely, equine heart Mb was added to pork sarcoplasm to match the myoglobin concentration of beef sarcoplasm. This resulted in beef and pork sarcoplasms, each with 2 different Mb concentrations for the longissimus (0.02 mM and 0.07 mM) and psoas (0.05 and 0.12 mM). Sarcoplasm (or phosphate buffer control) was incorporated within a phosphatidylcholine liposome preparation and incubated at 25°C. Thiobarbituric acid reactive substances (TBARS) were measured at 0, 30, 60, 90, and 120 min of incubation. Regardless of species, greater Mb concentration within the sarcoplasm increased lipid oxidation (P < 0.05). Across muscles, pork sarcoplasm had lower TBARS values than beef sarcoplasm (P < 0.05). Our results suggest that pork sarcoplasm has a lesser effect on lipid oxidation than beef sarcoplasm for a common Mb concentration. However, increased myoglobin concentration within sarcoplasm promotes lipid oxidation regardless of species.
Scientific Status Summary
Authors: Annous Bassam A. , Fratamico Pina M. , Smith James L.
Published: December 2008

The Institute of Food Technologists has issued this Scientific Status Summary to provide readers with a tutorial on biofilms, their purposeful mechanism of interaction (quorum sensing), and recent findings on how to inhibit their formation.
Immunohistochemistry is a more sensitive marker for the detection of myeloperoxidase in acute myeloid leukemia compared with flow cytometry and cytochemistry
Authors: SARAVANAN L. , JUNEJA S.
Published: December 2008

Myeloperoxidase (MPO) is an unequivocal marker of myeloid differentiation which is routinely detected using cytochemistry (CC), flow cytometry (FC) and immunohistochemistry (IH). Several studies have compared the use of two of these methods, but to our knowledge none has compared all three techniques. We compared the performance of these three modalities in the detection of MPO in 158 cases of acute myeloid leukaemia (AML). Discrepancies were noted in a total of 28 cases. Of 110 cases in which all three modalities were performed, 23 cases showed discrepancies. CC was the least sensitive marker, being negative in 11 of 23 cases in the presence of positive IH and/or FC. IH was the most sensitive marker with only one case being negative in the presence of a positive result by FC and/or CC. The results highlight the necessity of employing more than one method in determining the presence of MPO and confirm the important role of IH in the diagnosis of AML particularly in cases where MPO is not detected by CC and FC.
Influence of Inelastic Scattering at Metal-Dielectric Interfaces
Authors: Hopkins Patrick E. , Norris Pamela M. , Stevens Robert J.
Published: December 2008

Thermal boundary conductance is becoming increasingly important in microelectronic device design and thermal management. Although there has been much success in predicting and modeling thermal boundary conductance at low temperatures, the current models applied at temperatures more common in device operation are not adequate due to our current limited understanding of phonon transport channels. In this study, the scattering processes across Cr∕Si, Al∕Al2O3, Pt∕Al2O3, and Pt∕AlN interfaces were examined by transient thermoreflectance testing at high temperatures. At high temperatures, traditional models predict the thermal boundary conductance to be relatively constant in these systems due to assumptions about phonon elastic scattering. Experiments, however, show an increase in the conductance indicating inelastic phonon processes. Previous molecular dynamic simulations of simple interfaces indicate the presence of inelastic scattering, which increases interfacial transport linearly with temperature. The trends predicted computationally are similar to those found during experimental testing, exposing the role of multiple-phonon processes in thermal boundary conductance at high temperatures.
Simulation of Thermal Transport in Open-Cell Metal Foams: Effect of Periodic Unit-Cell Structure
Authors: Krishnan Shankar , Garimella Suresh V. , Murthy Jayathi Y.
Published: December 2008

Direct simulation of thermal transport in open-cell metal foams is conducted using different periodic unit-cell geometries. The periodic unit-cell structures are constructed by assuming the pore space to be spherical and subtracting the pore space from a unit cube of the metal. Different types of packing arrangement for spheres are considered—body centered cubic, face centered cubic, and the A15 lattice (similar to a Weaire-Phelan unit cell)—which give rise to different foam structures. Effective thermal conductivity, pressure drop, and Nusselt number are computed by imposing periodic boundary conditions for aluminum foams saturated with air or water. The computed values compare well with existing experimental measurements and semiempirical models for porosities greater than 80%. The effect of different foam packing arrangements on the computed thermal and fluid flow characteristics is discussed. The capabilities and limitations of the present approach are identified.
Heat Transfer Enhancement of Air Flowing Across Grooved Channels: Joint Effects of Channel Height and Groove Depth
Authors: Ridouane El Hassan , Campo Antonio
Published: December 2008

A numerical study was conducted to investigate convective heat transfer and laminar fluid flow in the developing region of two-dimensional parallel-plate channels with arrays of transverse hemicircular grooves cut into the plates. Air with uniform velocity and temperature enters the channel whose plates are at a uniform temperature. The finite-volume method is used to perform the computational analysis accounting for the traditional second-order-accurate QUICK and SIMPLE schemes. Steady-state results are presented for parallel-plate channels with and without hemicircular grooves for comparison purposes. The study revolves around four controlling parameters: (1) the height of the channel, (2) the relative groove depth, (3) the number of grooves, and (4) the Reynolds number. A prototypical 120‐cm-long channel contains two series of 3, 6, and 12 transverse grooves with four relative groove depths δ∕D of 0.125, 0.25, 0.375, and 0.5. Three ratios of channel height to groove print diameter H∕D of 0.5, 1, and 2 are employed. Computations are performed for Reynolds numbers based on the hydraulic diameter ranging from 1000 to 2500. It is found that the grooves enhance local heat transfer relative to a flat passage at locations near their downstream edge. The maximum heat transfer enhancement occurs at an optimal depth of the grooves. For purposes of engineering design, generalized correlation equations for the Nusselt number in terms of the pertinent Re, δ∕D, and the number of grooves N were constructed using nonlinear regression theory.
Experimental Study of the Flow Induced by a Sinusoidal Dielectric Barrier Discharge Actuator and Its Effects on a Flat Plate Natural Boundary Layer
Authors: Magnier Pierre , Boucinha Vincent , Dong BinJie , Weber Régine , Leroy-Chesneau Annie , Hong Dunpin
Published: December 2008

Since the mid-1990s, electrohydrodynamic actuators have been developed for modifying on subsonic airflows. The principle of plasma action is the use of the direct conversion of electrical energy into kinetic energy in order to act on the flow boundary layer. This paper presents our contribution to such an investigation concerning an electrohydrodynamic actuator consisting of several sinusoidal dielectric barrier discharges. First, the ionic wind induced by this actuator was measured with a pressure sensing probe. The induced flow velocity increased with the applied voltage and frequency. The particle image velocimetry system without external airflow showed the presence of induced swirls, generated by the ion movement in plasma. Then the action of this actuator on a flat plate boundary layer in parallel flow at zero incidence was studied in a subsonic wind tunnel. Experiments were performed for 15 m/s and 22 m/s. They showed that electric discharges (±8 kV, 1 kHz) acting on a laminar flow tripped the laminar-to-turbulent transition. Moreover, higher applied voltages (up to ±12 kV, 1 kHz) were necessary for modifying turbulent boundary layers.
DSMC Simulation: Validation and Application to Low Speed Gas Flows in Microchannels
Authors: Ewart T. , Firpo J. L. , Graur I. A. , Perrier P. , Méolans J. G.
Published: December 2008

A direct simulation Monte Carlo method (DSMC) solver, adapted to the subsonic microflow, is developed under the object-conception language (C++). Some technical details critical in these DSMC computations are provided. The numerical simulations of gas flow in a microchannel are carried out using the developed DSMC solver. Streamwise velocity distributions in the slip flow regime are compared with the analytical solution based on the Navier–Stokes equations with the velocity slip boundary condition. Satisfactory agreements have been achieved. Furthermore, the domain of the validity of this continuum approach is discussed. Simulations are then extended to the transitional flow regime. Streamwise velocity distributions are also compared with the results of the numerical solutions of the linearized Boltzmann equation. We emphasize the influence of the accommodation coefficient on the velocity profiles and on the mass flow rate. The simulation results on the mass flow rate are compared with the experimental data, which allow us to validate the “experimental” technique of the determination of the accommodation coefficient.
Liquid Crystal Imaging of Flow Boiling in Minichannels
Authors: Hollingsworth D. Keith
Published: December 2008

Quantitative liquid crystal thermography was used to investigate boiling incipience and nucleate flow boiling in rectangular mini-channels with channel heights of 2 mm to 500 μm. Distributions of surface temperature along the heated surface were measured from the liquid crystal images, and streamwise profiles of heat transfer coefficient on the heated surface were calculated. The working fluid was the refrigerant R-11. Observations of the boiling incipience superheat excursion, the hysteresis phenomenon, and saturated flow boiling are presented. Comparisons to established two-phase heat transfer correlations are performed to investigate the existence of “thin channel” effects.
Minichannels in Polymer Electrolyte Membrane Fuel Cells
Authors: Trabold Thomas A.
Published: December 2008

This paper provides an overview of the application of minichannels, typically on the order of 1 mm hydraulic diameter, in the design of polymer electrolyte membrane (PEM) fuel cells. In these electrochemical devices, minichannels deliver reactant hydrogen and oxygen to the anode and cathode electrodes, respectively, while transporting product water out of the cell. The channels must be designed for low pressure drop, to avoid excessive parasitic power losses from gas handling equipment. However, the channels also need to operate in a flow regime in which the overall water balance in the fuel cell can be maintained. The various aspects of minichannel design, including size and cross-sectional shape, are discussed, with particular emphasis on fuel cell water management. In addition to reviewing these fundamental aspects of minichannel design, examples are given of new experimental tools currently under development which are applied to relate channel water transport and accumulation to fuel cell performance.
Wide Variety of Possible Applications of Micro-Thermofluid Control
Authors: Yoshida Hideo
Published: December 2008

The possible applications of micro-thermofluid control are overviewed, mainly on the basis of the total of seventeen proceedings of IEEE International Workshops or Conferences on Micro Electro Mechanical Systems held since 1987. The contents consist of three aspects. The first is thermofluid control in microsystems; particularly, the attention is focused on the various types of micropumps. The second is thermofluid control in miniaturized thermofluid machines such as a microturbine and a microcooler. The third is micro-thermofluid control for conventional-sized flow phenomena such as turbulent shear flows and a flapping-wing micro-aerial vehicle. The author stresses that, in this field, considerable advances have been achieved by relatively young researchers with various backgrounds other than the classical theromofluid dynamics, and many challenging works are in progress which will lead to new possibilities in the field of thermofluid dynamics. Also, the author has gained the impression that the researchers’ byword has recently changed from “what can MEMS research do?” to “what should MEMS research do?”
Single-Phase Heat Transfer Enhancement Techniques in Microchannel and Minichannel Flows
Authors: Steinke Mark E. , Kandlikar Satish G.
Published: December 2008

The single-phase heat transfer enhancement techniques are well established for conventional channels and compact heat exchangers. The major techniques include flow transition, breakup of boundary layer, entrance region, vibration, electric fields, swirl flow, secondary flow and mixers. In the present paper, the applicability of these techniques for single-phase flows in microchannels and minichannels is evaluated. The microchannel and minichannel single-phase heat transfer enhancement devices will extend the applicability of single-phase cooling for critical applications, such as chip cooling, before more aggressive cooling techniques, such as flow boiling, are considered.
Direct Comparison Between a Variety of Microchannels: Part 2 — Experimental Description and Flow Friction Measurement
Authors: Eason Cormac , Dalton Tara , O’Mathu´na Cian , Davies Mark , Slattery Orla
Published: December 2008

Part 1 of this paper (Eason et al 2004) investigates the manufacturing of a variety of microchannels, produced by wet and dry etching in silicon, as well as precision mechanical sawing in silicon and thermoset plastic. This paper describes the experimental equipment and methods used to measure the pressure flow characteristics of the manufactured channels. A custom designed test system has been built in order to test each sample using the same inlet and outlet manifolds, pressure tappings, pumping system and instrumentation. The pressure drop across each set of channels was measured using an inductive pressure transducer. The mass flow rate through the system is measured by weighing the flow from the system in a given time. The measured pressure flow behaviour was compared with theoretical values as calculated from macro scale theory. Channel dimensions used for this calculation are as measured in part 1 of this paper. Error analysis was then carried out in order to determine the overall accuracy of the experimental work and determine whether any deviation from theoretical values is of experimental significance. This step is essential in any attempt to determine whether microchannel flows are indeed different to macro scale flows in a fundamental way. The deep reactive ion etched (DRIE) channels show the most significant lack of correlation with theoretical predictions. Compensation must be introduced to deal with the difference in cross section between the perfectly rectangular channels used for the theoretical prediction and the actual cross section of the channels.
Flow and Heat Transfer in a Simple Minichannel System Involving Two Straight Sections Separated by a 180° Bend
Authors: Oosthuizen Patrick H.
Published: December 2008

Steady, incompressible flow through a basic minichannel system has been considered. Slip at the wall has been assumed to be negligible. The channel has a rectangular cross-section. The flow enters the channel with a uniform velocity and temperature and passes through a straight channel section that has a length that is 30 times the width of the channel. The flow then passes around a 180° bend. Following the bend, the flow passes down another straight channel which also has a length of 30 times the size of the channel. A uniform heat flux is applied over the entire surface of the duct. The flow geometry does not represent any that is likely to occur in minichannel system applications but is adequate for the evaluation of the assumptions often adopted regarding pressure losses, flow development lengths and heat transfer rates in the preliminary design real such systems. The governing equations have been written in dimensionless form and solutions to these equations have obtained using a commercial finite-element software package, FIDAP. The solution has the following parameters: the Reynolds number, Re, the Prandtl number, Pr, the dimensionless height-to-width ratio of the channel H = h / w, and the dimensionless radius of the bend Ri = ri / w. Here w is the width of the channel, h is the height of the channel and ri is the radius of the inside surface of the bend. Results are only presented for P r = 0.7 and Ri = 1. Re values between 10 and 500 and H values between 0.5 and 2 have been considered.
Numerical Modelling of Heat Transfer in Rectangular Microchannels
Authors: Gamrat Gabriel , Favre-Marinet Michel , Asendrych Dariusz
Published: December 2008

The paper presents both three and two-dimensional numerical analysis of convective heat transfer in microchannels. The three-dimensional geometry of the microchannel heat sink followed the details of the experimental facility used during a previous research step. The heat sink consisted of a very high aspect ratio rectangular microchannel. Two channel heights, namely 1mm and 0.3mm (0.1mm), were used for 3D (2D) numerical model respectively. Water was employed as the cooling liquid. The Reynolds number ranged from 200 to 3000. In the paper, the thermal entrance effect is analyzed in terms of heat transfer efficiency. Finally, the comparison between measured and computed heat flux and temperature fields is presented. Contrary to the experimental work, the numerical analysis did not reveal any significant scale effect in heat transfer in microchannel heat sink up to the smallest size considered (0.1 mm).
A Review of Single-Phase Liquid Flow and Heat Transfer in Microchannels
Authors: Shen Peiqing , Aliabadi Shahrouz K. , Abedi Jalal
Published: December 2008

Fluid flow and heat transfer in microchannels have been important research area during the past decade. The understanding and explanation of the fundamental mechanisms of flow and heat transfer are critical to the application of microchannel systems to many important industrial and research projects. We present a review of the literatures on fluid flow and heat transfer of single-phase liquid in microchannels. Recent experimental and theoretical studies are both covered. The emphasis has been on studies on single-phase liquid flows. As a conclusion, although further work needs to be done, carefully designed experiments have obtained data that agree well with the conventional theory developed for larger channels. The theoretical explanation of some experimental results, which deviate the conventional theory for larger channels, is still under development.
Conjugated Heat Transfer in 2-Dimensional Mini and Micro Channels: Influence of Axial Conduction in the Walls
Authors: Maranzana G. , Perry I. , Maillet D.
Published: December 2008

For small Reynolds numbers, conductive heat transfer in the wall of mini-micro channels can become quite multidimensional: the wall heat flux density does not stay uniform and heat transfer mainly occurs at the entrance of the channels. The use of a ID model to invert measurements designed for estimating the convective heat transfer coefficient can lead to misinterpretations such as a variation of the Nusselt number with the Reynolds number. Three analytical models of conjugated heat transfer in channels are proposed, and the potential inversion of measurements is considered. A non-dimensional number M quantifying the relative part of conductive axial heat transfer in walls is introduced.
Validation of a Second-Order Slip Model for Transition-Regime, Gaseous Flows
Authors: Hadjiconstantinou Nicolas G.
Published: December 2008

We discuss and validate a recently proposed second-order slip model for dilute gas flows. Our discussion focuses on the importance of quantitatively accounting for the effect of Knudsen layers close to the walls. This is important, not only for obtaining an accurate slip model but also for interpreting the results of the latter since in transition-regime flows the Knudsen layers penetrate large parts of the flow. Our extensive validation illustrates the above points by comparing direct Monte Carlo solutions to the slip model predictions for an unsteady flow. Excellent agreement is found between simulation and the slip model predictions up to Kn = 0.4, for both the velocity profile and stress at the wall. This demonstrates that the proposed second-order slip model reliably describes arbitrary flowfields (and related stress fields) in a predictive manner at least up to Kn = 0.4 for both steady and transient problems.
Simulation of Low Knudsen Number Isothermal Flow Past a Confined Spherical Particle in a Micro-Pipe
Authors: Barber Robert W. , Gu X. J. , Emerson David R.
Published: December 2008

Low Knudsen number isothermal slip flow past a confined spherical particle has been investigated using a specially adapted Navier-Stokes solver. Knudsen numbers covering the continuum and slip-flow regimes (Kn ≤ 10−1) are considered while the Reynolds number is varied between 10−3 ≤ Re ≤ 0.5. In addition, blockage effects are studied by varying the ratio between the diameter of the pipe (H) and the diameter of the particle (D). A particularly important aspect of the present study is the proper formulation of the slip-velocity boundary condition over the curved surface of the particle. This is achieved by recasting Maxwell’s conventional velocity-slip equation as a function of the wall shear stress in order to account correctly for the curvature. The results show that blockage effects are extremely important in the continuum regime and cause amplification in the hydrodynamic drag on the particle. However, blockage phenomena are shown to be less important as the Knudsen number is increased. At the upper limit of the slip-flow regime, Kn ≈ 10−1, blockage amplification effects are reduced by almost 50% for a pipesphere geometry of H/D = 2.
A Criterion for the Experimental Validation of the Slip-Flow Models for Incompressible Rarefied Gases Through Microchannels
Authors: Morini G. L. , Lorenzini M. , Spiga M.
Published: December 2008

This paper is devoted to analyzing the friction factor of incompressible rarefied gas flow through microchannels. A theoretical investigation is conducted in order to underline the conditions for experimentally evidencing rarefaction effects on the pressure drop. It is demonstrated that for a fixed geometry of the microchannel cross section it is possible to calculate the minimum value of the Knudsen number for which the rarefaction effects can be observed experimentally, taking into account the experimental uncertainties on the evaluation of the friction factor.
Flow Boiling in Mini and Microchannels
Authors: Di´az M. Cortina , Boye H. , Hapke I. , Schmidt J. , Staate Y. , Zhekov Z.
Published: December 2008

Flow boiling heat transfer characteristics of water and hydrocarbons in mini and microchannels are experimentally studied. Two different test section geometries are employed; a circular channel with a hydraulic diameter of 1500 μm, and rectangular channels with height values of 300–700 μm and a width of 10mm. In both facilities the fluid flows upwards and the test sections, made of the nickel alloy Inconel 600, are directly electrically heated. Thus the evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 25 and 350 kg/(m2s) and heat fluxes from 20 to 350 kW/m2 at an inlet pressure of 0.3 MPa are examined. Infrared thermography is applied to scan the outer wall temperatures. These allow the identification of different boiling regions, boiling mechanisms and the determination of the local heat transfer coefficients. Measurements are carried out in initial, saturated and post-dryout boiling regions. The experimental results in the region of saturated boiling are compared with available correlations and with a physically founded model developed for convective boiling.
Modeling and Design of Micro Groove Falling Film Evaporators
Authors: Hasebe Shohei , Shikazono Naoki , Kasagi Nobuhide
Published: December 2008

In the present study, heat transfer in a falling film micro groove evaporator has been simulated by an analytical model. The flow and thermal fields were divided in two regions, i.e. macroscopic flow inside the groove and the microscopic flow where intensive evaporation takes place at the thin film interline region. For the micro region model, pressure in the liquid film was expressed as a sum of surface tension and disjoining pressure effects. The film thickness profile was obtained by solving the 4th order differential equation by Runge-Kutta method. Then, this micro region model was combined with the macro region model. Macro region model solves one dimensional bulk flow inside the groove with gravitational effect taken into account. Constant curvature of the liquid vapor surface was assumed for the macro flow. It is shown that the gravitational force is essential for providing the liquid to wide range of heat transfer area. Thus, diverging branch evaporator is investigated. It is demonstrated that this concept has large potentiality for improving the performance of the micro groove falling film evaporator.
Visualization and Flow Pattern Map for Flow Boiling of Water in a Vertical Micro Channel
Authors: Sobierska E. , Shuai J. , Mertz R. , Kulenovic R. , Groll M.
Published: December 2008

Experiments on flow boiling of water are carried out for a rectangular vertical channel with dimension (width × depth) 0.86 × 2.0 mm2 (hydraulic diameter 1.2 mm). The Confinement number, Co = [σ/(g(ρL − ρG)dh2)]0.5, for the investigated range of operating parameters is about 2.2. So, applying the criterion Co > 0.5 for micro channels, the investigated channel has to be classified as “micro”. Water as working fluid is used with different mass and heat fluxes. Investigations of heat transfer and pressure drop were carried out. Results of flow visualisation are presented. On the basis of observations, a flow pattern map is obtained and the results are compared with existing flow pattern transition boundaries proposed by Taitel et al. (1980) and Mishima and Ishii (1984). The paper contains recent experimental results of an ongoing EU-project. Previous results have been presented in Shuai et al. (2002, 2003).
Influence of Selected Parameters on Boiling Heat Transfer in Minichannels
Authors: Piasecka Magdalena , Poniewski Mieczyslaw E.
Published: December 2008

The experimental investigations cover heat transfer of refrigerants R 123 and R 11 flowing through vertical minichannels of 40 mm wide rectangular section and depths of 1 mm, 1.5 mm and 2 mm. The heating foil, supplied with controlled direct current, constitutes one of the surfaces of the minichannel. The liquid crystal thermography technique is applied in order to measure the two-dimensional temperature field of the heating surface. The investigations focus on the transition from single-phase forced convection to nucleate boiling, i.e. in the zone of boiling incipience. The present work aims to examine and analyze how the selected parameters (inlet pressure, inlet liquid subcooling, liquid flow velocity) affect nucleate boiling incipience for various geometry (changeable depth) of the minichannel. Furthermore, the investigations are intended to develop a correlation for the calculations of the Nusselt number under the conditions of boiling incipience in the minichannel. The equations are derived as modifications of the already developed ones [Piasecka, 2002; Piasecka and Poniewski, 2003b,c; Piasecka et al., 2004] and as a function of changeable parameters in the experimental investigations.
Modeling Bubble Behavior on a Heated Surface With Surface Pores and Sub-Surface Channels
Authors: Chen Yuming , Groll Manfred , Mertz Rainer
Published: December 2008

Bubble behavior and pressure fluctuation during boiling are modeled for enhanced surfaces with surface pores and sub-surface channels. The hydraulic diameter of the channels is about 0.42 mm. It is assumed that the latent heat flux is solely generated by thin-film evaporation on the channel surface. The evaporation rate is given by a semi-empirical correlation including effects of surface tension and liquid viscosity. The activation pressure and the bubble departure size are different for different pores. The vapor pressure inside the bubble is related to the channel pressure by the orifice equation. The bubble growth rate is determined by a modified Rayleigh equation. The vapor mass in a given channel volume is given by the mass conservation equation. Thus the instantaneous channel pressure and vapor volume can be determined which, in turn, govern the dynamic processes of bubble initiation and growth on various pores. This model demonstrates for the first time the dynamic picture of boiling phenomena on enhanced surfaces, e.g. the instantaneous activation, growth and detachment of bubbles on various pores, the fluctuation of channel pressure and vapor mass etc.. The model has been tested for boiling of propane and isobutane on an enhanced tube. The predictions agree reasonably well with the experiments.
Experimental Study on Flow Boiling of HFC134a in a Multi-Port Extruded Tube
Authors: Kuwahara Ken , Koyama Shigeru , Kazari Kengo
Published: December 2008

In the present study, the local heat transfer and pressure drop characteristics are investigated experimentally for the flow boiling of refrigerant HFC134a in a multi-port extruded tube of 1.06mm in hydraulic diameter. The test tube is 865mm in total length made of aluminum. The pressure drop is measured at an interval of 191 mm, and the local heat transfer coefficient is measured in every subsection of 75mm in effective heating length. Experimental ranges are as follows: the mass velocity of G = 100–700 kg/m2s, the inlet temperature of Tin = 5.9–11.4 °C and inlet pressure of about 0.5 MPa. The data of pressure drop are compared with a few previous correlations for small diameter tubes, and the correlations can predict the data relatively good agreement. The data of heat transfer coefficient is compared with the correlations of Yu et al. proposed for relatively large diameter tubes. It is found that there are some differences about two phase multiplier factor of convective heat transfer between the circular channel and rectangular channel.
Evaporation of Gravity- and Gas Flow-Driven Thin Liquid Films in Micro- and Minigrooves
Authors: Gambaryan-Roisman T. , Stephan P.
Published: December 2008

Using microstructured wall surfaces may improve the heat transfer performance of falling film or shear-driven film cooling devices enormously. The advantages of the structured surface include the prevention of the formation of dry patches on hot surfaces, the promotion of ultra-thin film evaporation, and a wavy motion of the film that enhances mixing of the liquid. We develop a model describing the hydrodynamics and heat transfer by evaporation of gravity- and gas flow-driven liquid films on grooved surfaces. For low Reynolds numbers or low liquid mass fluxes the heat transfer is governed by the evaporation of the ultra-thin film at a micro region, in the vicinity of the three-phase contact line. We investigate the hydrodynamic stability of the film flow using the long-wave theory. In addition to the films completely covering the wall structure, we study the stability characteristics of a thin liquid film partly covering the grooved wall, so that the flow region is bounded by contact lines. Two cases are analyzed: fully wetting liquids and liquids which form a small but finite contact angle with the wall material.
Two-Phase Flow Phenomena for Boiling in Two Parallel Microchannels
Authors: Li H. Y. , Lee P. C. , Tseng F. G. , Pan Chin
Published: December 2008

Boiling heat transfer and corresponding two-phase flow phenomena are of significant interest for the design of a compact evaporator. The present work investigates experimentally, using a high-speed digital CCD camera, the two-phase flow phenomena for boiling in a silicon-based, two parallel trapezoid microchannels, which were prepared by the combination of silicon bulk micro machining and Pyrex-silicon wafer bonding. Onset of nucleate boiling, bubbly flow, slug flow, and partial dry out slug flow are typically observed along the flow direction. The appearance of the partial dryout slug flow may degrade the nucleate boiling heat transfer in the microchannel. At a low flow rate, reversed vapor flow is observed. In such a flow pattern, liquid droplets are formed intermittently on the inner wall of top Pyrex glass due to vapor condensation. Moreover, the reversed vapor flow usually accompanies with large magnitude two-phase flow oscillations.
Characteristics of Earlier Nucleation for Boiling in Microchannels
Authors: Tian Y. , Wang X. D. , Peng X. F.
Published: December 2008

In this study, the nucleus formation and bubble growth at beginning of boiling process in microchannels was investigated. Canonical molecular distribution was introduced to analyze characteristics of nucleate boiling when liquid molecular number is very small. The minimum bulk phase volume in which phase change was enable to occur was determined from thermodynamic theory of bubble formation in a superheated liquid and from the energy distribution of the molecules in the bulk phase at a given temperature and pressure. The comparison of the free energy decrease during nucleus formation on a flat wall and at a corner of walls indicates that nuclei more easily form at a corner than on a flat wall for wetting liquids. The experimental results demonstrated that the superheat temperature increased as diameter of microchannels decreased. The experimental measurements are in a quite agreement with theoretical predictions.