Surface tension measurement of aqueous polymer solutions

The surface tension of aqueous polymer solutions of polyacrylamide (PAM), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), and hydroxyethyl cellulose (HEC) was studied over a range of polymer concentrations by using the maximum bubble pressure method at temperatures ranging from 20 to 65°C. The surface tension of water was also measured by the maximum bubble pressure method as well as by the DuNoüy ring method over the same temperature range. The experimental water data are in excellent agreement with the well-established tabulated data in the literature.

For a fixed concentration, all of the polymer solutions exhibited a decrease in surface tension with increasing temperature level. When compared with water at a fixed temperature level, the PAM and CMC solutions showed slightly higher surface tension values, whereas the PAA solutions yielded values equal to those found for water. In the case of the HEC solutions, the measured surface tensions decreased with concentration at a fixed temperature level and were lower than the values found for water. For a concentration of 2000 wppm the surface tension values for the hydroxyethyl cellulose were of the order of 10% lower than those for water at a fixed temperature level.

A comparison of the new measurements with the relatively limited previously published studies showed good agreement.     

Rime- and glaze-ice accretion due to freezing rain falling vertically on a horizontal thermally insulated overhead line conductor

A theory of atmospheric icing due to freezing rain on an overhead line conductor (OHLC) is developed. The rain falls vertically on a horizontal OHLC that is thermally insulated. It is assumed that the collection efficiency of the accretion surface is unity and that this surface is in thermodynamic equilibrium with the environment.

For air temperature T_A 0°C and raindrop temperature T_D 0°C, the freezing rain accretes as rime ice, provided that the temperature of the ice surface T_l < 0°C. The evolution equation governing the mass transfer at the accretion surface is solved analytically, yielding the shape of the rime-ice surface. Equations governing the thermal state of the rime-ice deposit are also given. These determine the onset of wet growth or glaze accretion at the upper stagnation line during suitable environmental conditions.

For environmental conditions producing an ice surface at temperature T_l = 0°gC, the freezing accretes as glaze. Equations governing the heat and mass transfer at the surface determine the shape of the glaze surface and the downward viscous motion of the unfrozen water. For T_D < 0°C, glaze evolution equations are developed for T_A 0°C and T_A 0°C. Analytical solutions of these equations are obtained. In particular, when T_D < −T_A < 0°C, the evolution equation predicts a novel limiting growth that is triangular in shape. Further study of the mass and heat transfer conditions, in the neighborhood of this final stage of glaze accretion, shows that it is maintained in thermodynamic equilibrium with its warm air environment.     

Fluid flow and heat transfer around circular cylinder – flat and curved plates combinations

Experimental studies were carried out to investigate the fluid flow and heat transfer around a heated circular cylinder which was placed at various distances of a wall boundary with different geometries (flat or curved plate) with subcritical Reynolds number ranging from 3.5×10³ to 10⁴. The effects of plate geometry (aspect ratio: W|H=1.0,1.5 and 2.0, and rim angle, φ=0°,60°,90°, and 120°) and gap ratio, (G|D=0.0,0.86,2.0,7.0,10.0) on the fluid flow and heat transfer characteristics (static pressure around cylinder surface, wake width, base pressure, pressure drag coefficients, velocity distribution, and both local and mean Nusselt numbers) were presented. Also flow visualization was carried out to illustrate the flow patterns around the cylinder at various gap ratios (G|D). It was found that the heat transfer and fluid flow characteristics are dependent on the plate geometry at all tested gap ratios, except for G|D=7.0 and 10.0, they are independent of the plate geometry.     

Measurements of the contact angle between R134a and both aluminum and copper surfaces

Measurements of quasi-static advancing contact angles of refrigerant R134a on copper and aluminum surfaces are reported over a temperature range from 0 °C to 80 °C. The metal surfaces tested were aluminum (alloy 3003) and copper (alloy 101) plates. Measurements were done using a direct optical observation technique where the liquid meniscus at the surface of a vertical plate was captured using a high magnification camera system. The contact angle of solid–liquid interface was deduced by enhancing and manipulating the digital image using solid modeling software by drawing a tangent line to the meniscus at the intersection location of the solid, liquid and vapor. Values of the contact angle were found to vary between 8.3° and 5.6° for aluminum and between 5.1° and 6.5° for copper when the temperature rose from 0 °C to 80 °C. Maximum standard deviation amongst the measured values of contact angles was 1.3°.     

Molecular tagging thermometry with adjustable temperature sensitivity

We report an extension to the technique of molecular tagging thermometry which allows for adjustable temperature sensitivity. The temperature dependence of laser-induced phosphorescence of the water-soluble phosphorescent triplex (1-BrNp•Mβ-CD•ROH) is used to conduct temperature measurements in aqueous flows. It is shown that the temperature sensitivity of phosphorescence intensity can be adjusted by changing the time delay between the laser excitation pulse and the start of the phosphorescence emission acquisition. For example, for a phosphorescence integration period of 1 ms, the temperature sensitivity of the measured phosphorescence intensity varies in the range 8.15–18.2% per °C at 25°C as the time delay changes from 1 to7 ms. This temperature sensitivity is much higher than that of most fluorescent dyes used for temperature measurements (e.g. less than about 2% per °C for Rhodamine B). The implementation and application of this new approach are demonstrated by conducting temperature measurements in the wake of a heated cylinder.     

2D temperature measurements in the wake of a heated cylinder using LIF

A technique is described to measure the instantaneous 2D temperature distribution in the wake of a heated cylinder using `laser-induced fluorescence'. Rhodamine B, a fluorescent dye, is used as a temperature indicator. The relation between fluorescence intensity and temperature is determined by means of calibration experiments in the temperature range of 20–35 °C with an accuracy of ±0.1 °C. The temperature distribution behind the heated cylinder is well visible and can be measured with a high spatial resolution. Corrections for variation in laser energy and intensity distribution in the laser sheet have to be made to further improve the accuracy of the measuring method. Received: 3 January 2001/Accepted: 18 May 2001     

Synthesis and characterization of BaMgAl10O17：Eu2＋ phosphor prepared by homogeneous precipitation

A homogeneous precipitation process based on urea hydrolysis reaction was exploited to synthesize BaMgAl10O17：Eu2＋ phosphor. The process parameters, such as the dosage of urea, the calcination tem- peratures and the concentration of Eu2＋, were refined in light of the characterization of the products. The experimental results revealed that pure and well-crystallized BaMgAl10O17：Eu2＋ phosphor could be obtained at 1250℃, a much lower temperature than that for traditional solid-state reaction. The as-prepared phosphor particles were small in grain size, regular in morphology, and uniform in size distribution. Because of the high homogeneity of the process, the as-prepared phosphor exhibited stronger emission intensity and higher thermal stability than the sample prepared by solid-state reaction at 1600℃.     

Turbulent mixed convection flow over a backward-facing step––the effect of the step heights

Effect of the backward-facing step heights on turbulent mixed convection flow along a vertical flat plate is examined experimentally. The step geometry consists of an adiabatic backward-facing step, an upstream wall and a downstream wall. Both the upstream and downstream walls are heated to a uniform and constant temperature. Laser–Doppler velocimeter and cold wire anemometer were used, respectively, to measure simultaneously the time-mean velocity and temperature distributions and their turbulent fluctuations. The experiment was carried out for step heights of 0, 11, and 22 mm, at a free stream air velocity, u_∞, of 0.41 m/s, and a temperature difference, ΔT, of 30 °C between the heated walls and the free stream air. The present results reveal that the turbulence intensity of the streamwise and transverse velocity fluctuations and the intensity of temperature fluctuations downstream of the step increase as the step height increases. Also, it was found that both the reattachment length and the heat transfer rate from the downstream heated wall increase with increasing step height.     

Thermal characteristics of a high heat flux micro-evaporator

Our purpose is to design a high heat flux micro-evaporator that can remove more than 100 W/cm². For this purpose a thin liquid film is evaporized. The liquid film is stabilized in micro-channels by capillary forces. The micro-channels are fabricated by chemical etching on silicon to reduce thermal resistance. For the experiments, the channel plate is heated by an ITO thin film heater deposited on the opposite side of the channel plate. Influence of heat flux, coolant flow rate, and inlet temperature on the temperature of the heater element are investigated. Water is used as working fluid. A maximal heat flux of 125 W/cm² could be achieved for water inlet temperature of 90 °C and flow rate of 1.0 mL/min. The temperature of the heater element is kept constant at about 120 °C with fluctuations within 8 °C. The measured pressure drop is less than 1000 Pa.     

Convective boiling of halocarbon refrigerants flowing in a horizontal copper tube – an experimental study

An experimental study of convective boiling of refrigerants R-22, R-134a and R-404A in a 12.7 mm internal diameter, 2 m long, horizontal copper tube has been performed. Experiments involved a relatively wide range of operational conditions. Experiments were performed at the evaporating temperatures of 8°C and 15°C. Quality, mass velocity and heat flux varied in the following ranges: 5% to saturated vapor, 50–500 kg/(s m²); and 5–20 kW/m². Effects of these physical parameters over the heat transfer coefficient have been investigated. High quality experiments were also performed up to the point of the tube surface dryout, a mechanism which was investigated from the qualitative point of view. Two heat transfer coefficient correlations from the literature have been evaluated through comparisons with experimental data. Deviations varied in the range from −25% to 42%.     

The near wake structure of a square cylinder

The near wake structure of a square cross section cylinder in flow perpendicular to its length was investigated experimentally over a Reynolds number (based on cylinder width) range of 6700–43,000. The wake structure and the characteristics of the instability wave, scaling on θ at separation, were strongly dependent on the incidence angle () of the freestream velocity. The nondimensional frequency (St_θ) of the instability wave varied within the range predicted for laminar instability frequencies for flat plate wakes, jets and shear layers. For = 22.5°, the freestream velocity was accelerated over the side walls and the deflection of the streamlines (from both sides of the cylinder) towards the center line was higher compared to the streamlines for = 0°. This caused the vortices from both sides of the cylinder to merge by x/d 2, giving the mean velocity distribution typical of a wake profile. For = 0°, the vortices shed from both sides of the cylinder did not merge until x/d 4.5. The separation boundary layer for all cases was either transitional or turbulent, yet the results showed good qualitative, and for some cases even quantitative, agreement with linearized stability results for small amplitude disturbances waves in laminar separation layers.     

Effect of mass transfer on flow across a staggered tube bundle

Over a range of Reynolds numbers from 0·6 × 10⁵ to 1·75 × 10⁵ tests were made on a seven rows deep tube bank. These tests were made using a specially instrumented porous cylinder which could be located in any position within the bank. Mass transfer through the porous surface simulating the condensation process in a surface condenser, was applied, and its effect on local parameters investigated. The distribution of static pressure and skin friction was determined around tubes in different rows in the bank. From these measurements, the pressure drag and friction drag were estimated. The total pressure drop across the bank was also measured.

Results showed that, for typical steam condenser loadings, the contribution of the pressure drag to the total drag does not change appreciably with suction. However, the skin friction contribution does change considerably with suction.     

Total hemispherical emissivity of oxidized Inconel 718 in the temperature range 300–1000°C

Total hemispherical emissivities were measured for Inconel 718 as a function of sample temperature. Measurements were made for both unoxidized and oxidized samples. The oxidation temperatures were 1000°C, 1100°C and 1142°C and the oxidation times were 15, 30 and 60 min, respectively. The oxidized samples showed a significant increase in emissivity over the unoxidized one which was in an as-received condition. No apparent pattern was observed in the change of emissivity as a function of oxidation time at a given oxidation temperature. In some cases, emissivity measurements made with increasing temperature were greater than those made with descending temperature. One possible explanation for this is the spalling of the oxide layer as the sample area contracted with descending sample temperature.     

Hydrodynamics and boiling phenomena of water droplets impinging on hot solid

The collision of single water droplets with a hot Inconel 625 alloy surface was investigated by a two-directional flash photography technique using two digital still cameras and three flash units. The experiments were conducted under the following conditions: the pre-impact diameters of the droplets ranged from 0.53 to 0.60 mm, the impact velocities ranged from 1.7 m/s to 4.1 m/s, and the solid surface temperatures ranged from 170 °C to 500 °C. When a droplet impacted onto the solid at a temperature of 170 °C, weak boiling was observed at the liquid/solid interface. At temperatures of 200 or 300 °C, numerous vapor bubbles were formed. Numerous secondary droplets then jetted upward from the deforming droplet due to the blowout of the vapor bubbles into the atmosphere. No secondary droplets were observed for a surface temperature of 500 °C at the low-impact Weber numbers (∼30) associated with the impact inertia of the droplets. Experiments using 2.5-mm-diameter droplets were also conducted. The dimensionless collision behaviors of large and small droplets were compared under the same Weber number conditions. At temperatures of less than or equal to 300 °C, the blowout of vapor bubbles occurred at early stages for a large droplet. At a surface temperature of 500 °C, the two dimensionless deformation behaviors of the droplets were very similar to each other.     

New printed circuit heat exchanger with S-shaped fins for hot water supplier

A new PCHE with an S-shaped fin configuration was applied to a hot water supplier in which cold water of 7 °C is warmed to 90 °C through heat-exchange with supercritical CO₂ of 118 °C and 11.5 MPa pressure. The fin and plate configurations were determined using 3D CFD simulations for the CO₂ side and H₂O side and the thermal–hydraulic performance of hot water supplier was evaluated. Compared with a hot water supplier that is currently used in a residential heat pump, the new PCHE provides about 3.3 times less volume; and lower pressure drop by 37% in the CO₂ side and by 10 times in H₂O side.     

The viscoelastic behavior of melts of virgin and recycled polycarbonate reinforced with short glass fibers

Three series of shear oscillatory tests are performed on polycarbonate melts reinforced with short glass fibers at the temperatures T₁=250 and T₂=290 °C. The content of glass fibers ranges from 0 to 20 wt.%. In the first series, virgin polycarbonate is used, in the other series, dynamic tests are performed on recycled polymer, whereas in the third series, a mixture of virgin with recycled polycarbonates is employed. Constitutive equations are derived for the viscoelastic behavior of a polymer melt at isothermal deformations with small strains. A polymer is treated as an equivalent transient network of strands that rearrange at random times as they are agitated by thermal fluctuations. The time-dependent response of a network is determined by four adjustable parameters that are found by fitting the experimental data. Excellent agreement is demonstrated between the observations and the results of numerical simulation. The study focuses on the effects of temperature and filler content on the material constants in the stress–strain relations.     

Temperature spectra from a turbulent thermal plume by ultrasound scattering

This paper reports the results of a study on temperature inhomogeneities conducted on a thermal plume by using ultrasound scattering as a non-intrusive measurement technique. The plume rises from a metallic disk which can be heated up to 800 °C. The working fluid is air at atmospheric pressure. In the measurement technique, an incoming ultrasound wave is emitted towards the thermal plume. The incident wave is scattered because of non-linear couplings with the flow instabilities present in the measurement region. The scattered wave carries information about those flow instabilities. The technique allows for the retrieving of this information. The shape of the obtained spectrum of temperature fluctuations as a function of wave vector modulus is consistent with previous theoretical analysis. Three qualitatively different regions were identified: first, a production region characterized by a q² law; secondly, a region with behavior as per q⁻³ associated with a buoyancy region and; finally, a dissipation region associated with a q⁻⁷ law. These spectral regions characterize the energy transfers mechanisms among the length scales of flow investigated here. A coefficient of anisotropy γ was defined to analyze anisotropic features of the flow.     

Vortex shedding and surface pressures on a square cylinder at incidence to a uniform air stream

This article describes results of experiments on vortex-shedding frequencies and surface pressures of a square cylinder at non-zero angle of incidence. The range of Reynolds numbers was 2000–21 000, but the lower range was emphasized. For Reynolds numbers greater than 5300, the Strouhal number shows a similar trend with changing angle of incidence; that is, a rapid rise in Strouhal number occurs at an angle of around 13°. The occurrence of such a jump in Strouhal number was found to be associated with onset of the flow reattachment, bringing in a strong pressure recovery on the lower side face of the cylinder. For lower Reynolds numbers Re=2000–3300, the maximum Strouhal number occurs at a relatively higher angle of 17°. Around this angle, the pressure measurements exhibit a rather weak pressure recovery, suggesting a less firm shear-layer reattachment to the side face of the cylinder. The nature of the reattaching flow was further examined by spectral analysis of the fluctuating pressure coefficients measured on the lower side face of the cylinder.     

Influence of Density Inversion on Thawing Around a Cylinder in a Frozen Saturated Porous Medium

An analytical study is made of the convective flow field produced when a warm cylinder maintained at a fixed temperature above freezing is buried in saturated frozen porous medium. The flow field is shown to have a double cell pattern due to the density inversion of water at ~ 4°C, with downward convection of heat dominating at cylinder temperatures of below ~ 10°C and upward heat convection dominating at temperatures greater than this. The analysis uses a perturbation technique to determine the first-order convective correction to the flow and temperature fields around the cylinder for a quasi-static case. It demonstrates that the porous medium permeability and the cylinder temperature are the dominant factors in determining the point at which convection heat transfer becomes significant, with convection expected to be insignificant for Darcy permeabilies lower than 10⁻⁵ m/s. The analysis also gives an indication of the rates of thawing occurring in different directions without resorting to numerical methods. The practical implications of a thawing pattern significantly different to that predicted by conduction theory only are discussed briefly with respect to the problem of differential thaw settlement of arctic pipelines.     

Thermal behavior of spiral fin-and-tube heat exchanger having fly ash deposit

This research investigates the effect of fly-ash deposit on thermal performance of a cross-flow heat exchanger having a set of spiral finned-tubes as a heat transfer surface. A stream of warm air having high content of fly-ash is exchanging heat with a cool water stream in the tubes. In this study, the temperature of the heat exchanger surface is lower than the dew point temperature of air, thus there is condensation of moisture in the air stream on the heat exchanger surface. The affecting parameters such as the fin spacing, the air mass flow rate, the fly-ash mass flow rate and the inlet temperature of warm air are varied while the volume flow rate and the inlet temperature of the cold water stream are kept constant at 10 l/min and 5 °C, respectively.

From the experiment, it is found that as the testing period is shorter than 8 h the thermal resistance due to the fouling increases with time. Moreover, the deposit of fly-ash on the heat transfer surface is directly proportional to the dust–air ratio and the amount of condensate on heat exchange surface. However, the deposit of fly-ash is inversely proportional to the fin spacing. The empirical model for evaluating the thermal resistance is also developed in this work and the simulated results agree well with those of the measured data.