High Mass Velocity Research Articles

An experimental study on condensation of refrigerant R134a in a multi-port extruded tube

In the present study, the local characteristics of pressure drop and heat transfer are investigated experimentally for the condensation of pure refrigerant R134a in two kinds of 865 mm long multi-port extruded tubes having eight channels in 1.11 mm hydraulic diameter and 19 channels in 0.80 mm hydraulic diameter. The pressure drop is measured at an interval of 191 mm through small pressure measuring ports. The local heat transfer rate is measured in every subsection of 75 mm in effective cooling length using heat flux sensors. It is found that the experimental data of frictional pressure drop agree with the correlation of Mishima and Hibiki [Trans. JMSE (B) 61 (1995) 99], while the correlations of Chisholm and Laird [Trans. ASME 80 (1958) 227], Soliman et al. [Trans. ASME, Ser. C 90 (1998) 267], and Haraguchi et al. [Trans. JSME (B) 60 (1994) 239], overpredict. As a trial, the data of local heat transfer coefficient are also compared with correlations of Moser et al. [J. Heat Transfer 120 (1998) 410] and Haraguchi et al. [Trans. JSME (B) 60 (1994) 245]. The data of high mass velocity agree with the correlation of Moser et al., while those of low mass velocity show different trends. The correlation of Haraguchi et al. shows the trend similar to the data when the shear stress in their correlation is estimated using the correlation of Mishima and Hibiki.

Effects of fin shape on condensation in herringbone microfin tubes

Effects of fin height and helix angle on condensation inside a herringbone microfin tube have been experimentally investigated with five types of herringbone microfin tubes. Heat transfer coefficients are about 2–4 times higher than that of the helical microfin tube under high mass velocity conditions. In the low mass velocity, they are equal to that of the helical microfin tube. The heat transfer enhancement increases with fin height up to 0.18 mm; higher fin heights show enhancement values similar to the 0.18 mm results. Pressure drop increases with the fin height. Larger helix angle yields higher heat transfer and higher pressure drop. For the lowest fin and/or smallest helix angle, the pressure drop is comparable with that of the helical microfin tube, while the heat transfer enhancement is higher. The enhancement mechanism is discussed from flow pattern observations. Effect of mass transfer resistance for R410A is estimated and negligible effects have been proved.

Experimental Studies on Impact Behaviour of Woven Fabric Composites: Effect of Impact Parameters

Experimental studies have been presented on the impact behaviour of plain weave E-glass-epoxy laminates with different combinations of impactor mass and incident impact velocity with the same incident impact energy of 24.43 J. Four combinations of impactor mass-incident impact velocity have been considered. Impact tests have been carried out on an instrumented drop weight impact test apparatus. The unsupported area of the specimens during the impact loading was 127 mm 127 mm with clamped boundary condition. Thickness of the plate was 5 mm. Post-impact-compression testing of impacted specimens has been carried out using Compression-After-Impact test fixture as per NASA 1142 standard. It is observed that for the materials tested, damage tolerance is higher for low mass and high velocity combination as compared to high mass and low velocity combination.

Discovery of a very cool object with extraordinarily strong H$\mathsf{\alpha}$ emission

We report on the finding of the strongest Hα emission – pseudoequivalent width of 705 Å – known so far in a young, late type dwarf. This object, named as S Ori 71, is a substellar candidate member of the 1–8 Myr star cluster σ Orionis. Due to its overluminous location in color-magnitude diagrams, S Ori 71 might be younger than other cluster members, or a binary of similar components. Its mass is in the range 0.021–0.012 , depending on evolutionary models and possible binarity. The broad Hα line of S Ori 71 appears asymmetric, indicative of high velocity mass motions in the Hα forming region. The origin of this emission is unclear at the present time. We discuss three possible scenarios: accretion from a disk, mass exchange between the components of a binary system, and emission from a chromosphere.

The symbiotic star CH Cygni - IV. Basic kinematics of the circumstellar matter during active phases

We give a brief summary of common basic characteristics of active phases during the whole (1963‐2000) observed symbiotic life of CH Cygni. We identify two types of outbursts: (1) those with pronounced signatures of a high-velocity mass outflow, and (2) those displaying signatures of only a mass inflow to the central star. On the basis of the observed Hα emission, we investigate mass loss from the active star during the period from when radio jets appeared (1984.6) to the most recent activity (1998‐2000). A simple expression, which relates the observed Hα luminosity to the mass-loss rate, is derived. We obtained mass-loss rates of ˙

Condensation heat transfer of herringbone micro fin tubes

Experiments of in-tube condensation of R410A were carried out for a smooth tube, a helical micro fin tube and three types of herringbone micro fin tubes, which have different fin height and helix angle. In the herringbone micro fin tube, the micro fins work to remove liquid at fin-diverging parts and collect liquid at fin-converging parts. In the high mass velocity region the heat transfer coefficients of all the herringbone tubes are higher than those of the helical micro fin tube. In the low mass velocity region, however, the heat transfer coefficients of the herringbone micro fin tubes are slightly lower than those of the helical micro fin tube. The heat transfer coefficient is lower for smaller helix angle tube and effect of fin height on the heat transfer is small. Pressure drop of the herringbone tube of which helix angle is small is lower than that of the helical micro fin tube, while that of other tubes is higher. Flow configurations of the herringbone tubes were observed with R123 as working fluid and the heat transfer enhancement mechanism is discussed.

1913 サブチャンネル解析コード COBRA-TF による稠密炉心限界熱流束実験の解析

It is important to evaluate the thermal margin of the Reduced-Moderation Water Reactor which consists of tight lattice fuel assemblies with gap clearance around 1.0mm. Subchannel analyses may provide valuable information to supplement thermal hydraulic experiments. To assess the applicability of subchannel analysis for tight lattice cores, critical heat flux experiments for tight lattice cores were analyzed with COBRA-TF code. The test section was a 7-rod bundle with rod diameter of 12.3 mm, rod gap of 1.0 mm and heated length of 1.8 m. It was found that COBRA-TF gives good prediction of critical power for mass velocity of 400-500kg/(m^2s), while it underestimates the critical power for lower mass velocity and overestimates for higher mass velocity. With modification of interfacial heat transfer model, the difference between measured and predicted powers in the high mass velocity region was reduced.

Condensation of downward-flowing HFC134a in a staggered bundle of horizontal finned tubes: effect of fin geometry

Experimental results are presented that show the effect of fin geometry on condensation of refrigerant HFC134a in a staggered bundle of horizontal finned tubes. Two types of conventional low-fin tubes and three types of three-dimensional fin tubes were tested. The refrigerant mass velocity ranged from 8 to 23 kg/m 2s and the condensation temperature difference from 1.5 to 12 K. The effect of condensate inundation was more significant for the three-dimensional fin tubes than for the low-fin tubes. In most cases, the highest performance was obtained by the tube with a three-dimensional structure at the tip of low fins. In the case of high mass velocity and high condensate inundation rate, however, the highest performance was obtained by one of the low-fin tubes. The results were compared with previous results for bundles of smooth tubes and low-fin tubes.

A theoretical model of critical heat flux in flow boiling at low qualities

A new theoretical critical heat flux (CHF) model was developed for the forced convective flow boiling at high pressure, high mass velocity, and low quality. The present model for an intermittent vapor blanket was basically derived from the sublayer dryout theory without including any empirical constant. The vapor blanket velocity was estimated by an axial force balance, and the thickness of vapor blanket was determined by a radial force balance for the Marangoni force and lift force. Based on the comparison of the predicted CHF with the experimental data taken from previous studies, the present CHF model showed satisfactory results with reasonable accuracy.

A Procedure to Predict Subcooled-Water-Flow-Boiling CHF in Uniformly Heated Tubes for High-Heat-Flux Applications

A model to predict critical heat flux (CHF) for high-heat-flux subcooled flow boiling in uniformly heated tubes is proposed. The present CHF model is based on the mechanism of wall-attached bubble coalescence. To take into account the enhanced condensation due to high subcooling and high mass velocity in small-diameter tubes, a mechanistic approach is adopted to evaluate the nonequilibrium flow quality and void fraction in the subcooled water flow boiling. Comparison of the model predictions against ~3100 subcooled water CHF data shows relatively good agreement over a wide range of parameters that covers the operating conditions of fusion reactor components. The operating ranges of the present database cover 0.33 ≤ D ≤ 37.5 mm, 0.002 ≤ L ≤ 4 m, 0.1 ≤ P ≤ 20 MPa, 0.37 ≤ G ≤ 134 Mg/m2·s, 49 ≤ Δhsub,in ≤ 1659 kJ/kg, –1.25 ≤ xem < 0, and 1.05 ≤ qCHF ≤ 276 MW/m2.

Condensation heat transfer and flow pattern inside a herringbone-type micro-fin tube

Condensation heat transfer and pressure drop of R410A and R22 in a newly proposed herringbone-type micro-fin tube are measured and compared to those of a helical micro-fin tube and a smooth tube. The heat transfer coefficient of the herringbone micro-fin tube is higher than that of the helical micro-fin tube in the high mass velocity region, while it has slightly lower value in the low mass velocity region. Pressure drop of the herringbone micro-fin tube is, however, higher than that of the helical micro-fin tube. Flow patterns of the herringbone micro-fin tube are observed and the heat transfer enhancement mechanism is discussed. The heat transfer coefficient and pressure drop of the helical micro-fin tube is predicted well with previously proposed correlations, while those of the herringbone-type micro-fin tube has higher value than the predicted values. Preliminary correlations for the pressure drop and the heat transfer coefficient are proposed for the herringbone micro-fin tube.

Ultra-high critical heat flux (CHF) for subcooled water flow boiling—I: CHF data and parametric effects for small diameter tubes

Ultra-high critical heat flux (CHF) data, with many values exceeding 100 MW m −2, were obtained using high mass velocity, subcooled water flow through short, small diameter tubes. These tests produced the highest CHF of 276 MW m −2 reported in the literature for a uniformly heated tube which surpassed the prior record of 228 MW m −2. The data include broad ranges of tube diameter (0.406–2.54 mm) , heated length-to-diameter ratio (2.4–34.1) , mass velocity (5000–134 000 kg m −2 s −1) , inlet temperature (18–70°C) , and outlet pressure (2.5–172.4 bars) . The parametric trends of CHF were ascertained relative to all important flow and geometrical parameters. CHF increased with increasing mass velocity, increasing subcooling, decreasing tube diameter, and decreasing heated length-to-diameter ratio. For a constant inlet temperature, CHF increased with increasing pressure for pressures up to 30 bars, remained fairly constant between 30 and 150 bars, and decreased afterwards as the critical pressure was approached. CHF was accompanied by physical burnout of the tube wall near the exit and tube material had little effect on the magnitude of CHF. The pressure drop for most conditions was fairly constant, albeit as high as 153.4 bars, over the entire range of heat fluxes, from the single-phase flow condition corresponding to zero heat flux up to CHF, proving that CHF was triggered even with negligible net vapor production. These high pressure drops indicate special attention should be exercised when employing the high mass velocity flows necessary to attaining ultra-high CHF. These high pressure drops also render the practice of referencing CHF data reSlative to a single measured pressure value very misleading.

Ultra-high critical heat flux (CHF) for subcooled water flow boiling—II: high-CHF database and design equations

A high-CHF (critical heat flux) database for subcooled flow boiling of water in tubes was compiled from the world literature. The bulk of the database is for high mass velocity flow ( G ⩾5000 kg m −2 s −1) in small diameter tubes ( D ⩽ 3 mm) . Over half of the 1596 valid data points in the database had CHF values in excess of 25 MW m −2. The database included the highest CHF for uniform heating ever reported in the literature, 276 MW m −2, which was obtained in Part I of the present study. A pressure drop model applicable to subcooled flow boiling in tubes with inlet and outlet plenums was developed in order to determine the pressure at the end of the heated length where burnout (CHF) was observed. The pressure at this location is often significantly different from that measured downstream in the outlet plenum. A subcooled high-CHF correlation superior to previously published correlations was systematically developed using data only from the authors institution and Kiev Polytechnic Institute, Kiev, Ukraine. The correlation was validated with data covering a broad range of pressures (from atmospheric to near the critical point) and mass velocities (1500–134 000 kg m −2 s −1) which are required in the cooling of high-heat-flux components, such as those in fusion reactors. Although it was developed by examining the parametric trends exhibited by a small fraction of the database, the correlation had a root-mean-square error of 19.5% for the entire database, lowest among the correlations tested. In sharp contrast to other correlations, this new, dimensionless CHF correlation consisted of a single equation having only five adjustable constants.

A Predictive Model for Condensation in Small Hydraulic Diameter Tubes Having Axial Micro-Fins

A semiempirical model is proposed to predict the condensation coefficient inside small hydraulic diameter extruded aluminum tubes having microgrooves. The model accounts for the effects of vapor shear and surface tension forces. Surface tension force is effective in enhancing the condensation coefficient as long as the fin tips are not flooded by condensate. This enhancement increases as mass velocity is reduced. At high mass velocity the flow is vapor shear controlled and the surface tension contribution is very small. The surface tension effect is strongly affected by the fin geometry. A smaller fin tip radius provides a higher surface tension drainage force. A large cross sectional area in the interfin region will allow the surface tension enhancement to occur at lower vapor quality. Separate models are developed for the surface tension and vapor shear controlled regimes and the models are combined in the form of an asymptotic equation. The vapor shear model is based on use of an equivalent mass velocity and the heat-momentum transfer analogy. The surface tension model is analytically based. The model is validated by predicting the authors data for two tube geometries using R-12 and R-134a, and the model predicts 95 percent of the condensation data within ±16 percent.

Evaluation of Subcooled Critical Heat Flux Correlations Using the PU-BTPFL CHF Database for Vertical Upflow of Water in a Uniformly Heated Round Tube

A simple methodology for assessing the predictive ability of critical heat flux (CHF) correlations applicable to subcooled flow boiling in a uniformly heated vertical tube is developed. Popular correlations published in handbooks and review articles as well as the most recent correlations are analyzed with the PU-BTPFL CHF database, which contains 29 718 CHF data points. This database is the largest collection of CHF data (vertical upflow of water in a uniformly heated round tube) ever cited in the world literature. The parametric ranges of the CHF database are diameters from 0.3 to 45 mm, length-to-diameter ratios from 2 to 2484, mass velocities from 0.01 × 103 to 138 × 103 kg/m2·s, pressures from 1 to 223 bars, inlet subcoolings from 0 to 347°C, inlet qualities from −2.63 to 0.00, outlet subcoolings from 0 to 305°C, outlet qualities from −2.13 to 1.00, and CHFs from 0.05 × 106 to 276 × 106 W/m2. The database contains 4357 data points having a subcooled outlet condition at CHF. A correlation published elsewhere is the most accurate in both low- and high-mass velocity regions, having been developed with a larger database than most correlations. In general, CHF correlations developed from data covering a limited range of flow conditions cannot be extended to other flow conditions without much uncertainty.

The mass and angular momentum loss from the edge of the accretion disk and the evolution of cataclysmic binaries

We give the mass and angular momentum loss from the edge of the accretion disk of a cataclysmic binary and model theoretically the effect of these losses on the evolution of the system. The results of the calculation show that the high-velocity mass outflows observed by IUE come from the edge of the disk and the angular momentum loss there could be the mechanism that drives the evolution of cataclysmic variables with periods longer than 3 hours.

Condensation of R-12 in small hydraulic diameter extruded aluminum tubes with and without micro-fins

This manuscript provides heat transfer data for R-12 condensation and subcooled liquid in small hydraulic diameter, flat extruded aluminum tubes. The tube outside dimensions are 16 mm × 3 mm (high) × 0.5 rrm (wall thickness). The tubes contain three internal membranes, which separate the flow into four parallel channels. Two internal geometries were tested: one had a plain inner surface and the other had micro-fins, 0.2 mm high. Data are presented for the following range of variables: vapor qualities (12–97%), mass velocity (400–1400 kg s−1 m−2), and heat flux (4–12 kW m−2). The overall heat transfer coefficient was measured for water-to-refrigerant heat transfer, and the modified Wilson plot method used to determine the heat transfer coefficient for water-side flow in the annulus. Then, the tube-side condensation coefficient was extracted from the measured UA-value. The data show that the condensation coefficient increases with heat flux to the 0.20 power. The subcooled heat transfer coefficient for both geometries is well predicted using the Petukhov equation with hydraulic diameter. At low mass velocity, the Akers correlation agrees well with the plain tube data, and overpredicts the data 10–20% at high mass velocity. The micro-fin tube shows significantly higher performance than predicted by the Akers correlation (based on hydraulic diameter) for vapor qualities greater than 0.5. The authors propose that surface tension force is effective in enhancing the condensation coefficient for vapor quality greater than 0.5. The proposed surface tension enhancement is particularly strong at the lower mass velocities.

Flow boiling heat transfer to saturated pure components and non-azeotropic mixtures in a horizontal tube Wärmeübergang beim Strömungssieden von gesättigten reinen Komponenten und binären nicht azeotropen Gemischen im horizontalen Rohr

In horizontal flow boiling conditions the heat transfer is affected by the saturation pressure, the composition of the mixture, the vapour fraction, the heat flux and the mass velocity. Furthermore, the heat transfer coefficients are different at different axial and circumferential positions on the heat transfer surface. In this work the calculation of averaged heat transfer coefficients is based on local (point) values. For the prediction of local heat transfer coefficients of pure substances a new correlation is proposed. Local heat transfer coefficients for two pure substances (SF 6 and CCl 2F 2) and mixtures were measured for the condition of uniform wall temperature over the whole composition range and various values of the affecting parameters. In the nucleate flow boiling region a pronounced reduction of the local heat transfer coefficients occurs, which is dependent on the liquid composition, the heat flux and the saturation pressure. This reduction of nucleate flow boiling heat transfer coefficients of mixtures is predicted by the model of Schlünder. However, from the experimental results it can be concluded, that the reduction in vaporization heat transfer is smaller for high mass velocities. With a simple model for the evaluation of the wetting boundary circumferential averaged heat transfer coefficients can be computed.

Growth and form of two-dimensional rotating aggregates.

We propose a two-dimensional particle-cluster aggregation model which considers explicitly linear and angular momentum conservation. Each aggregate is grown from a seed of given mass and given initial angular velocity, to which particles of unitary mass and ballistic trajectories are added. The fractal dimensions of the resulting aggregates are calculated and Fourier transform analysis is performed. Very different features are found, as initial conditions are varied. Particularly, spiral structures are found for a seed of high mass and high initial angular velocity.

Local Heat Transfer for Subcooled Flow Boiling with Water

In this paper, local heat transfer coefficients are predicted for turbulent water subcooled flow boiling through uniformly heated circular tubes. Correlations by Petukhov and by Shah are modified slightly. however, the correlation suggested by Kandlikar is improved significantly by requiring that it approach more accurate limits near the onset of fully developed boiling and the onset of nucleate boiling for subcooled flow. Excellent agreement is obtained with data corresponding to conditions of high inlet subcooling (183[degrees]C), high mass velocity (4.4 to 31.5 Mg/m[sup 2][center dot]s), and a large ratio of the axial coordinate to the diameter (95.5). The exit subcooling varies from 53.0 to 81.5[degrees]C. For smaller ratios ([lt]50.0), the accuracy decreases. In all cases, the local film temperature is the characteristic temperature. When the associated critical heat flux (CHF) data are examined in a Stanton number-Peclet number space, St [lt] 0.0065 and Pe [gt] 10[sup 5] in all cases. Comparisons with the Saha-Zuber criterion for bubble detachment show that moderately subcooled and high-velocity flows re characterized by a multiboundary layer phenomenon that includes an attached bubble layer. These results show that the bubble layer's existence can now be documented for a wide variety of fluids and conditions without flowmore » visualizations.« less