Normal stress measurements on drag-reducing polymer solutions

Summary The elastic properties of very dilute solutions of a number of drag-reducing polymers differing either in chemical nature or molecular weight were investigated over a range of values of shear stress using the jet thrust method. Parallel drag reduction measurements were also made with the solutions. The results indicate a general relationship between the value of the first normal stress difference at the wall, (p ₁₁-p ₂₂, and the dragreducing ability.The data tends to confirm the generality of the correlation between the value of theWeissenberg number and the drag reduction reported byMetzner for a single polymer sample.     

Asymmetry in transitional pipe flow of drag-reducing polymer solutions

New experimental data are presented and discussed for fully developed pipe flow of shear-thinning, viscoelastic polymer solutions in the transitional regime between laminar and turbulent flow. The data confirm that such transitional flows exhibit significant departures from axisymmetry in contrast to the fully developed pipe flow of Newtonian fluids or both laminar and turbulent flows of such drag-reducing liquids. The azimuthal structure of the asymmetry is investigated together with its axial development and also the velocity fluctuation levels. These data do not lead to an explanation for the asymmetry but do suggest that the influence of the flow geometry both upstream and downstream can be ruled out.     

Steady and dynamic shear properties of non-aqueous drag-reducing polymer solutions

The steady and dynamic shear properties of two non-aqueous drag-reducers (a medium molecular weight polyisobutylene and a commercial organic drag-reducer) in kerosene solutions over a wide range of temperature and concentration were presented. The intrinsic and zero-shear viscosity results were used to identify the concentrate regimes of these solutions. A characteristic time constant λ₀, which was based on the spring-bead model for dilute solutions, was employed as the scaling parameter for both steady-shear and dynamic data over a wide range of concentration and temperature. The inadequacy of the Graessley reduced-variable method in the dilute region was illustrated. The shear-thinning behaviour of these polymer solutions could be described by the Carreau model. The dynamic data followed the Zimm and Rouse-like behaviour in the low and high frequency limits. The Cox-Merz rule was obeyed in the low shear rate and frequency regions. The Carreau and the zero-frequency Maxwell time constants appeared to be related to λ₀ by a constant factor over a wide range of polymer concentrations. The finding provides a method for extrapolating viscoelastic information into the drag reduction regime, and could be useful for interpretation of drag reduction results.     

Laminar,transitional and turbulent annular flow of drag-reducing polymer solutions

Mean and rms axial velocity-profile data obtained using laser Doppler anemometry are presented together with pressure-drop data for the flow through a concentric annulus (radius ratio κ = 0.506) of a Newtonian (a glycerine–water mixture) and non-Newtonian fluids—a semi-rigid shear-thinning polymer (a xanthan gum) and a polymer known to exhibit a yield stress (carbopol). A wider range of Reynolds numbers for the transitional flow regime is observed for the more shear-thinning fluids. In marked contrast to the Newtonian fluid, the higher shear stress on the inner wall compared to the outer wall does not lead to earlier transition for the non-Newtonian fluids where more turbulent activity is observed in the outer wall region. The mean axial velocity profiles show a slight shift (～5%) of the location of the maximum velocity towards the outer pipe wall within the transitional regime only for the Newtonian fluid.     

Pool boiling enhancement with surface treatments

Pool boiling heat transfer characteristics on treated surfaces were investigated experimentally. Surface treatments were performed with sandpapers, building micro structures by etching, and micro-porous coating. Copper blocks of 20 mm × 20 mm were used as test sections and PF5060 was used as a working fluid. The effect of wall superheat (0–35 K), surface orientation (0°, 45°, 90°), and subcooling (0, 5, 10 K) on treated surfaces were also investigated. Heat transfer performance on inclined surface was better than that of horizontal surface in all test sections. Because bubble generation was suppressed by subcooling, higher wall superheat was needed to initiate boiling in all surfaces. Micro-porous coated surface showed the highest heat transfer enhancement among tested surfaces.     

Flow of drag-reducing surfactant solutions in rough pipes

The paper reports the results of experimental study of the flow of hexadecyltrimethylammonium chloride (CTAC) solutions with addition of sodium salicylate (NaSal) in the rough pipes. Measurements were performed in the range of the surfactant concentration from 200 to 400 ppm at a constant molar ratio CTAC/NaSal of 1:2. Five pipes of the relative roughness k/D varying from 1.2 × 10^?2 to 5.6 × 10^?2, obtained by the covering of inner surface of the pipes with glued silicon carbide particles of different size, were studied. The roughness was observed to increase the drag of flow of CTAC/NaSal solutions already at Reynolds numbers higher than 800. With increasing relative roughness k/D, the critical value of Reynolds number, at which the drag reduction disappears, was found to decrease. However, no influence of the roughness on the critical shear stress was noted. The ratio of the critical Reynolds number for rough pipes to that of hydraulically smooth pipes was independent of the surfactant concentration. The degree of drag reduction by the flow of surfactants was greater in rough pipes than in smooth pipes.     

Tests of drag-reducing polymer coated on a riblet surface

Experiments have been carried out at BMT where the drag reduction due to Hoechst U-groove riblets, a polymer coating, and the two combined were measured in a towing tank on a one-third scale model of the America's Cup winning yacht, Australia II. The results indicated that the riblet/polymer combination offered an overall improvement in drag reduction characteristics over either riblets or polymer coating alone, with a maximum reduction of 3.5% observed for a non-dimensional S ⁺=8. The qualitative behaviour of the drag reduction was similar to that recorded in earlier pipe flow experiments, employing an injection of polymer additive and 3M V-groove riblets, but contrary to that recorded in studies of an axisymmetric body, also coated with 3M riblets, in a drop tank filled with a polymer solution.     

Pool boiling heat transfer performance of Newtonian nanofluids

Experimental measurements were carried out on the boiling heat transfer characteristics of γ-Al₂O₃/water and SnO₂/water Newtonian nanofluids. Nanofluids are liquid suspensions containing nanoparticles with sizes smaller than 100 nm. In this research, suspensions with different concentrations of γ-Al₂O₃ and SnO₂ nanoparticles in water were studied under nucleate pool boiling heat transfer conditions. Results show that nanofluids possess noticeably higher boiling heat transfer coefficients than the base fluid. The boiling heat transfer coefficients depend on the type and concentration of nanoparticles.     

Pool boiling performance of finned surfaces in R-113

Performance of horizontal copper heaters with a transverse fin structure was investigated for pool boiling heat transfer and critical heat flux limits. Data were obtained for 5.1 and 7.6 cm diameter structured cooper and brass heaters in saturated R-113 boiling at pressures ranging between 0.037 and 1 atm. The fin structure consisted of 0.16 cm×0.16 cm×0.32 cm high square fins with an interfin spacing of 0.16 cm. Following a similar methodology to Haley and Westwater¹, a numerical analysis of the heat transfer phenomenon was performed by solving the one-dimensional fin conduction equation with a non-linear heat transfer boundary condition obtained from the previously reported data for R-113 boiling on plain surfaces. The predictions agreed with the data at the 1 atm pressure levels but showed deviations at the low pressure levels. The results showed that, compared with plain surface heaters of the same diameters the finned structured surfaces investigated: (a) decreased the wall temperature differences for a given heat flux and saturated pool boiling conditions, thus improving the nucleate boiling heat transfer coefficients, and (b) increased the critical heat flux limits, calculated as the power input divided by the heater projected area, by a factor of 2–2.5.     

Pool boiling — And the effect of pool geometry

Simple correlationsh=C·qⁿ for experimental results in nucleate pool boiling often exhibit a wide scatter in the constant and the exponent. Among the many parameters responsible, an effect from confining boundaries causing flow recirculation, cannot be excluded. Heat transfer experiments were performed in a cylindrical and rectangular vessel on finned tubes in a single and twin arrangement. The cylindrical vessel had a smaller fluid cross section area than the rectangular vessel. Although the tubes were identical in geometry (except for length) and surface roughness, they yielded clearly different results. The heat transfer coefficients in the cylindrical vessel were always larger than in the rectangular one.     

Unsteady waves in boiling solutions

The evolution of shock waves and pulses in vapor-liquid binary media with a bubble structure is considered. A number of binary systems are calculated. It is established that in some cases the wave profiles in binary systems do not lie between the corresponding limiting profiles calculated for the boiling single-component systems. It is found that this effect is associated with the diffusional retardation of the phase transitions in binary media.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 97–101, July–August, 1990.     

Boundary layer of a body of revolution in a drag-reducing polymer solution

A possible method is presented for calculating an axisymmetric boundary layer when a body moves in weak polymer solutions with constant concentration. The method is based on the use of a velocity profile and system of integral equations which take into account most fully the effects of the transverse curvature of the body's surface. The computational scheme makes it possible to take into account the change in the flow regimes in the boundary layer.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 40–48, May–June, 1981.     

Pool boiling from GEWA surfaces in water and R-113

Pool boiling heat transfer measurements are reported for water and R-113 with several horizontal cylindrical test sections: plain, T-finned GEWA-T (with various gaps) and standard low fin GEWA-K. With R-113 the enhancement is a well-behaved function ofS _T , reaching a maximum of 2 atS _T = 0.25 mm. With distilled water, the performance depends in a more complex way on gap width. The maximum enhancement of 1.6 is obtained atS _T =0.35 mm. Visual observations of the actual test sections and models indicated a complex liquid-vapor exchange mechanism. Bubble generation with adjacent liquid feed was observed at random locations around the periphery of the test section. A semi-theoretical equation was developed which correlates the experimental data quite well.

---

Sieden bei freier Konvektion von GEWA-Oberflächen in Wasser und R-113

Zusammenfassung Es wird über Wärmeübergangsmessungen beim Sieden unter freier Konvektion mit Wasser und R-113 an verschiedenen zylindrischen, horizontal angeordneten Versuchsobjekten berichtet, die folgende Oberflächenbeschaffenheiten hatten: glatt, T-berippt GEWA-T (mit unterschiedlichen Abständen) und serienmäßig mit niedrigen Rippen versehen GEWA-K. Bei R-113 ist die Verbesserung im Wärmeübergang eine erwartungsgemäße Funktion vonS _T , die einen Maximalwert von 2 beiS _T =0,25 mm erreicht. Bei destilliertem Wasser hängt das Verhalten in komplizierter For von der Lückenweite ab. Die maximale Verbesserung liegt bei 1,6 und wird fürS _T =0,35 mm erreicht. Optische Beobachtungen an den Versuchsobjekten und Modellen zeigten einen komplizierten Flüssigkeits-Dampf-Austauschmechanismus. Es wurde Blasenbildung mit benachbartem Flüssigkeitszustrom an willkürlich wechselnden Stellen um die Peripherie des Versuchsobjektes beobachtet. Eine halbtheoretische Gleichung wurde entwickelt, welche die experimentellen Daten gut wiedergibt.

---

Nomenclature A surface area - A _t tunnel/channel wall area - A liquid film area - C _q constant in natural convection correlation - C _T constant in GEWA-T Model - D outside diameter - D _b base diameter (fin root diameter) - D _tc diameter at thermocouple location - h heat transfer coefficient - h _fg latent heat - h _t heat transfer coefficient at the tunnel wall - k _l thermal conductivity of liquid - L length of test section - N _A number of active nucleation sites - ql latent heat transfer rate - q heat flux - q_ex natural convection heat flux - q latent heat flux - S _t GEWA-T gap width - T wall-minus-saturation temperature difference - T _gn vapor temperature - T _s saturation temperature - T _w wall temperature - x, y constants in natural convection correlation - liquid film thickness - surface tension Dedicated to Prof. Dr.-Ing. U. Grigull's 75th birthday     

Pool boiling heat transfer of ultra-light copper foam with open cells

High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, with the heating surface area of 12.0 mm by 12.0 mm, using acetone as the working fluid. The foam covers have ppi (pores per inch) from 30 to 90, cover thickness from 2.0 to 5.0 mm, and porosity of 0.88 and 0.95. The surface superheats are from −20 to 190 K, and the heat fluxes reach 140 W/cm². The 30 and 60 ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. Cage bubbles were observed to be those with liquid filled inside and vented to the pool liquid. For the 90 ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid.     

Pool boiling heat transfer of tandem tubes provided with the novel microstructures

The paper presents experimental data on pool boiling heat transfer of tandem tubes, arranged one above the other in the same vertical plane. The outer surface of the tubes is provided with the novel microstructures. The structure elements are micropins created by electrolytic deposition of copper upon the tube, using a specially treated polycarbonate foil. By this technique the pins diameter can be varied from 0.1 μm up to 25 μm, the pins height goes up to 100 μm at densities up to 1 × 10⁹ pins/cm² and pins inclination almost up to 180° regarding the base surface. Micropins with several different inclinations can be created simultaneously on the same surface.Experiments were conducted with two different microstructures using the refrigerant R134a and the highly wetting Fluorinert liquid FC-3284 at pressures of 5-9 bar and 0.5-1.5 bar, respectively. The advantages of the novel microstructure regarding the boiling heat transfer for tandem tubes turned out to be practically the same as for a single tube arrangement. Microstructured tubes have the superheat independent on the heat flux, they show a very low boiling inception superheats (below 2 K), are highly effective in comparison with a technically smooth tubes, and operate stable over the long periods of time.     

Diffusion of slot injected drag-reducing polymer solution in a LEBU modified turbulent boundary layer

The effects of a tandem set of large eddy breakup (LEBU) devices on the diffusion of drag-reducing polymer solution and of water injected into a turbulent boundary layer flow have been studied. Laser Doppler velocimeter measurements were taken in the LEBU modified boundary layer with and without polymer injection. A laser-induced fluorescence technique was used to examine the development of concentration profiles of the injected fluids with increasing distance from the injection slot for a range of injection rates. The diffusion rate of water, a passive contaminant, was diminished by the LEBU devices over a distance of only 10 to 15 boundary layer thicknesses before returning to the case of an unmodified flow; whereas the devices did have a major effect on the diffusion of polymer over the entire streamwise distance studied compared to the case of an unmodified flow. Large reductions of turbulent normal and shear stresses were observed downstream of the devices, especially with polymer injection.A version of this paper was presented at the 12^th Symposium on Turbulence, University of Missouri-Rolla, 24–26 September, 1990     

Viscoelastic properties of polymer solutions

Summary Measurements were taken of the thrust of liquid jets ejecting from a long capillary into air for aqueous solutions of polyacrylamide (ET 597) of various concentrations. The measurements were then used to determine axial normal stresses for two capillary diameters: 1.52 mm (L/D = 201.5) and 2.44 mm (L/D = 207.2). The results show that the calculated values of the axial normal stress are higher for the larger capillary diameter than for the smaller, as recently reported byPowell andMiddleman. It has been further found that this diameter effect becomes more pronounced as the concentration of solute is increased. For the same materials, we also measured the primary normal stress difference by means of aWeissenberg rheogoniometer. Comparison between the two kinds of measurements shows that the magnitude of axial normal stresses is much smaller than that of normal stress differences over the range of shear rates studied (200 20,000 sec^–1) for the materials investigated. This result seems to point out the necessity of measuring the wall normal stresses, which are believed to depend not only on the capillary diameter, but also on the concentration. The authors therefore contend that, in general, the measurement of axial normal stresses alone is not sufficient to completely determine the elastic properties of viscoelastic solutions.