Forced convective boiling heat transfer in microtubes at low mass and heat fluxes

Convective boiling of HCFC123 and FC72 in 0.19, 0.3 and 0.51 mm ID tubes is investigated. The experimental setup as well as the data reduction procedure has carefully been designed, so that the relative uncertainty interval of the measured heat transfer coefficient in microtubes is kept within ±10%. Up to 70 K liquid superheat over the saturation temperature is observed at low heat and mass fluxes. The onset of the superheat is found to be dependent on the mass flux and the boiling number of the refrigerant examined. In the saturated boiling regime, the heat transfer characteristics are much different from those in conventional-size tubes. The heat transfer coefficient is monotonically decreased with increasing the vapor quality, and becomes independent of the mass flux. Most empirical formulas are not in accordance with the present experimental data. Since the prediction using the nucleate boiling term of Kandlikar’s empirical correlations coincides with the present results, the convection effect should be minor in microtubes. On the other hand, the pressure loss characteristics are qualitatively in accordance with the conventional correlation formula while quantitatively much lower. These phenomena can be explained by the fact that the annular flow prevails in microtubes.     

Dynamics of aerospace vehicles at very high altitudes

Some basic questions of the flight mechanics of aerospace vehicles at very high altitudes are investigated and in some cases reviewed. In particular those of the spacecraft trajectories are considered along which the aerodynamic actions take place in a range of the Knudsen number between transition and free molecular flow. A recent physico-mathematical model is adopted in the evaluation of the aerodynamic and heat transfer coefficients. The stability characteristics are discussed of two reference vehicle configurations for which the time histories are also calculated relative to significant equilibrium conditions. In the final section of the paper the initial step of an aeroassisted orbit transfer is considered in order to evaluate the constraints due to the aerodynamic heating on this manoeuvre.Si studiano alcune questioni fondamentali della meccanica del volo di veicoli aerospaziali a quote molto elevate. Si considerano, in particolare, quei tratti delle traiettorie lungo i quali le azioni aerodinamiche si esplicano in un campo di numeri di Knudsen tra la transizione ed i flussi di molecole libere. Si adotta un recente modello fisico-matematico per calcolare i coefficienti aerodinamici e di scambio termico. Dopo aver discusso le caratteristiche di stabilit`a per due configurazioni di velivoli di riferimento, se ne calcolano le leggi orarie a partire da condizioni di equilibrio significative. Il lavoro termina con lo studio di un traferimento orbitale aeroassistito al fine di valutare i limiti a questa manovra dovuti al riscaldamento aerodinamico.     

Determination of stress-strain characteristics at very high strain rates

A modified version of the Kolsky thin-wafer technique is described. The method permits one to obtain the dynamic plastic properties of materials at strain rates as high as 10⁵ sec^?1. Data obtained from compression tests on high-purity aluminum are presented for strain rates ranging from 4000 to 120,000 sec^?1 at room temperature. Specimen-size effects and the effect of lateral inertia are taken into account in analyzing the data. The results plotted as stress vs. strain rate at constant strains (5 to 20 percent) show that, at the highest strain rates, the stress rises very rapidly with strain rate suggesting that a limiting strain rate is being reached. At the lower strain rates (10³ to 10⁴ sec^?1), the stress is linearly proportional to the strain rate indicating that the material is deforming in a viscous manner.     

Experimental verification of heat transfer coefficient for nucleate boiling at sub-atmospheric pressure and small heat fluxes

In this paper we study the influence of sub-atmospheric pressure on nucleate boiling. Sixteen correlations for pool boiling available in literature are gathered and evaluated. Analysis is performed in the pressure range 1–10 kPa and for heat flux densities 10–45 kW/m². Superheats are set between 6.2 and 28.7 K. The results of calculations were compared with experimental values for the same parameters. The experiments were conducted using isolated glass cylinder and water boiling above the copper plate. Results show that low pressure adjust the character of boiling curve—the curve flattened and the natural convection region of boiling is shifted towards higher wall temperature superheats due to the influence of low pressure on the bubble creation and process of its departure. In result, 8 of 16 analyzed correlations were determined as completely invalid in subatmospheric conditions and the remaining set of equations was compared to experimental results. Experimentally obtained values of heat transfer coefficients are between 1 and 2 kW/m²K. With mean absolute deviation (MAD) we have found that the most accurate approximation of heat transfer coefficient is obtained using Mostinski reduced pressure correlation (0.13–0.35 MAD) and Labuntsov correlation (0.12–0.89 MAD).     

Flow boiling heat transfer of R134a and R404A in a microfin tube at low mass fluxes and low heat fluxes

An experimental investigation of flow boiling heat transfer in a commercially available microfin tube with 9.52 mm outer diameter has been carried out. The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15°. The fin height is 0.24 mm and the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long and is electrically heated. The experiments have been performed at saturation temperatures between 0 and −20°C. The mass flux was varied between 25 and 150 kg/m²s, the heat flux from 15,000 W/m² down to 1,000 W/m². All measurements have been performed at constant inlet vapour quality ranging from 0.1 to 0.7. The measured heat transfer coefficients range from 1,300 to 15,700 W/m²K for R134a and from 912 to 11,451 W/m²K for R404A. The mean heat transfer coefficient of R134a is in average 1.5 times higher than for R404A. The mean heat transfer coefficient has been compared with the correlations by Koyama et al. and by Kandlikar. The deviations are within ±30% and ±15%, respectively. The influence of the mass flux on the heat transfer is most significant between 25 and 62.5 kg/m²s, where the flow pattern changes from stratified wavy flow to almost annular flow. This flow pattern transition is shifted to lower mass fluxes for the microfin tube compared to the smooth tube.     

New method for testing composites at very high strain rates

A method was developed for testing and characterizing composite materials at strain rates in the 100 to 500 s^?1 regime. The method utilizes a thin ring specimen, 10.16 cm (4 in.) in diameter, 2.54 cm (1 in.) wide and 6–8 plies thick. This specimen is loaded by an internal pressure pulse applied explosively through a liquid. Pressure is measured by means of a calibrated steel ring instrumented with strain gages. Strains in the composite specimen are measured with strain gages. Strains in the calibration and specimen rings are recorded with a digital processing oscilloscope. The equation of motion is solved numerically and the data processed by the mini-computer attached to the oscilloscope. Results are obtained, and plotted by an X-Y plotter in the form of a dynamic stress-strain curve. Unidirectional 0-deg, 90-deg and 10-deg off-axis graphite/epoxy rings were tested at strain rates up to 690 s^?1. Times to failure ranged between 30 and 60 μs. The 0-deg properties which are governed by the fibers do not vary much from the static ones with only small increases in modulus. The 90-deg properties show much higher than static modulus and strength. The dynamic in-plane shear properties, obtained from the 10-deg off-axis specimens, are noticeably higher than static ones. In all cases the dynamic ultimate strains do not vary much from the static values.     

Rheology and birefringence of Fomblin YR at very high shear rates

The rheological and structural properties of perfluoropolyether (PFPE) lubricant films including viscosity, shear stress, and birefringence were measured at relatively low to extremely high shear rates using a rotational optical rheometer. The viscosity of various films with different thicknesses exhibit Newtonian behavior up to a shear rate 1 × 10⁴ s⁻¹, with a transition to shear-thinning behavior obvious at higher shear rates. Birefringence of these films was also measured for the first time, and these results indicate chain alignment with shear in the shear-thinning regime. The shear rate at which alignment occurs is similar to that of the onset of shear thinning. This correlation between chain alignment and shear thinning provides direct evidence that the ability of PFPEs to lubricate hard drives at high shear rates is a direct consequence of the ability of the applied shear field to align the molecules on a molecular level.     

Interferometric testing at very high temperatures by TV holography (ESPI)

This paper describes how TV-holography or electronic-speckle-pattern interferometry (ESP) can be used to monitor and measure with interferometric sensitivity the surface behavior, vibrations and deformations of objects heated to temperatures up to +3000°C. If problems due to rapid microstructure changes and the object actually melting are disregarded, the temperature limits are determined by the accessible laser illumination due to the increasing background radiantion.     

Experimental observations of a flexible slider crank mechanism at very high speeds

A slider crank mechanism has been constructed and operated for the purpose of investigating steady state rod bending vibration induced by a very high speed crank. Features include a combination flywheel and adjustable length crank, a thin aluminum connecting rod, and a piston sliding on steel rod slide axes. A strain gage on the rod and magnetic pickup on the crank sensed rod strain and crank speed, respectively.For this system configuration, experimental results are categorized as small, intermediate and large crank length response. Small and intermediate cranks response was amplified due to a large superharmonic component of twice the crank speed frequency and at crank speeds near 1/2 the first natural frequency of the rod. Beyond that speed, period doubling occurred over a range of speeds for intermediate length cranks. The occurrence of period doubling was experimentally sudden and audibly noticeable, and characterized by the onset of frequency components of 1/2, 3/2, 5/2, and 7/2 times the crank speed. For large crank sizes of 0.5, 1, and 2 inches an amplified response also appeared in each at a certain speed, but at speeds lower than in the small and intermediate crank cases. Larger cranks required more frequency components to describe the response than smaller cranks. Experimental responses were correlated with computer simulations of a one mode nonlinear ordinary differential equation model, and over a wide range of speeds and for a representative of a small, intermediate, and large crank length.     

On the direct measurement of very large strain at high strain rates

When dynamic plastic strain exceeds 4-percent deformation in completely annealed polycrystalline aluminum, difficulties in the optical measurment of strain occur because of changes in the diffuse-ambient-background light arising from the growth of a mottled surface, or “organe peel.” This paper describes how the diffraction-grating technique may be modified to measure dynamic plastic strain for very large strain at high strain rates in the presence of changing light intensity. The experimental results obtained show that the strain-rate-independent finite-amplitude wave theory, governed by the present writer's generalized, linearly temperature-dependent parabolic stress-strain law, still applies.     

Determination of tensile flow stress beyond necking at very high strain rate

The objective of this effort was to extend the Bridgman analysis of tensile necking to obtain stress-strain data beyond the point of onset of necking from a split Hopkinson bar. For this purpose, combined analytical and experimental techniques were considered. The analytical efforts were focused on validating the use of Bridgman solutions for high rate of deformation through a finite-element analysis of a tapered tensile specimen. The experimental technique involved the development of a photographic system using a light-emitting diode and a 35-mm rotating drum camera for the observation of necking during dynamic tensile tests conducted with a split Hopkinson tension bar. The developed new technique was successfully used to measure neck profiles of 6061-T6 aluminum, HY100 and 1020 steel tensile specimens. The measured profiles were used with the Bridgman analysis and stress-strain data were obtained to over 70-percent strain.     

Dynamic self-similar debonding of interface at very high velocity: Anti-plane case

This paper analyzes the anti-plane problem of dynamic self-similar debonding of interface at very high velocity. The debonding is modeled as an interface crack propagating self-similarly from zero-length. The extending speed is assumed to be transonic or supersonic. We first consider the dynamic debonding under moving concentrated loads. The moving dislocation model of self-similar propagation of an interface crack is used to formulate the problem to a singular integral equation which is solved analytically. The singularity of stresses near the crack tip is discussed and the dynamic stress intensity factors are presented. Finally the solution of dynamic debonding underx ²-type loads is obtained by using the superposition method.     

Critical heat fluxes in the boiling of ethanol at flow speeds of 50 to 115 m/sec

Experimental results are given on the critical heat fluxes for flows of ethanol between 40 and 65 t/m²-sec and temperature differences corresponding to the conditions for normal boiling crises. The results agree with theoretical calculations.     

Nucleate boiling heat transfer in aqueous solutions with carbon nanotubes up to critical heat fluxes

In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHFs) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60 °C. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001%, 0.001%, 0.01%, and 0.05%. For the dispersion of CNTs, polyvinyl pyrrolidone polymer is used in distilled water. Pool boiling HTCs are taken from 10 kW/m² to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the aqueous solutions with CNTs are lower than those of pure water in the entire nucleate boiling regime. On the other hand, critical heat flux of the aqueous solution is enhanced greatly showing up to 200% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle on the test surface and extends the nucleate boiling regime to very high heat fluxes and reduces the formation of large vapor canopy at near CHF. Thus, a significant increase in CHF results in.     

A theoretical and experimental study on subcooled flow boiling at high heat flux

In a subcooled flow boiling system at high heat flux, the major heat transfer mechanism places emphasis on a very thin liquid layer, known as the sublayer which is trapped between the heated surface and the vapor blankets. Base on the convective boiling heat transfer dominated by the heat conduction through the liquid sublayer, a theoretical model for subcooled flow boiling heat transfer has been developed. To provide useful data in the simulation of Light Water Reactors (LWRs) conditions, heat transfer experiments for up-flow boiling water through a vertical tube at the pressure ranging from 6.9 to 15.5 MPa have been conducted. The experimental results are compared with the predictions of the present model and other five famous correlations. For the LWRs subcooled flow boiling, the comparison reveals that the present model show the best agreement with the measured data.

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Eine theoretische und experimentelle Studie über unterkühltes Sieden bei hoher Wärmestromdichte

Zusammenfassung In einem Fluidsystem, das bei hohem Wärmefluß den Effekt des unterkühlten Siedens zeigt, findet der wesentliche Wärmetransportmechanismus in einer sehr dünnen Schicht statt, die zwischen der beheizten Oberfläche und den Dampfpolstern liegt und als Unterschicht bekannt ist. Basierend auf den Gesetzmäßigkeiten des konvektiven Siedens unter dominierendem Einfluß der Wärmeleitung durch die Flüssigkeits-Unterschicht wurde ein theoretisches Modell zu Beschreibung des Wärmeübergangs bei unterkühltem Sieden entwickelt. Um nützliche Daten für die Simulation der in Leichtwasserreaktoren (LWR) herrschenden Bedingungen zu gewinnen, erfolgten die Experimente bei Aufwärtsströmung siedenden Wassers in einem senkrechten Rohr im Druckbereich 6,9 bis 15,5 MPa. Diese experimentellen Ergebnisse werden mit Vorausberechnungen nach dem erstellten Theoriemodell, sowie jenen nach fünf der bekanntesten Korrelationen verglichen. Für unterkühltes Sieden in Leichtwasserreaktoren zeigte sich hierbei, daß die Experimente am besten durch das neuentwickelte Modell wiedergegeben werden.