Direct contact heat transfer between two immiscible liquids flowing in a horizontal concentric annulus

Direct contact heat transfer between water and a heat transfer oil was investigated under non-boiling conditions in co-current turbulent flow through a horizontal concentric annulus. The ratio of the inner pipe diameter to the outer pipe diameter (aspect ratio) κ = 0.730−0.816; total liquid velocity (mixture velocity) V_T = 0.42−1.1 m/s; inlet oil temperature T_oi = 38−94°C; oil volume fraction in the flowing mixture φ_o = 0.25−0.75 were varied and their effects on the overall volumetric heat transfer coefficient U_v were determined at constant interfacial tension of 48 dynes/cm.

It was found that, in each concentric pipe set, the overall volumetric heat transfer coefficient increased with increasing dispersed phase volume fraction at each constant mixture velocity and reached a maximum at around φ_o = φ_w ≈ 0.5. The maximum U_v values increased with increasing total liquid velocity and decreasing aspect ratio of the annulus. The volumetric heat transfer coefficient was also found to increase with increasing inlet oil temperature and increasing total liquid velocity but to decrease with length along the test section keeping all other parameters constant. Empirical expressions for the volumetric heat transfer coefficient were obtained within the ranges of the experimental parameters.     

Ice accretion simulation on multi-element airfoils using extended Messinger model

In the current article, the problem of in-flight ice accumulation on multi-element airfoils is studied numerically. The analysis starts with flow field computation using the Hess-Smith panel method. The second step is the calculation of droplet trajectories and droplet collection efficiencies. In the next step, convective heat transfer coefficient distributions around the airfoil elements are calculated using the Integral Boundary-Layer Method. The formulation accounts for the surface roughness due to ice accretion. The fourth step consists of establishing the thermodynamic balance and computing ice accretion rates using the Extended Messinger Model. At low temperatures and low liquid water contents, rime ice occurs for which the ice shape is determined by a simple mass balance. At warmer temperatures and high liquid water contents, glaze ice forms for which the energy and mass conservation equations are combined to yield a single first order ordinary differential equation, solved numerically. Predicted ice shapes are compared with experimental shapes reported in the literature and good agreement is observed both for rime and glaze ice. Ice shapes and masses are also computed for realistic flight scenarios. The results indicate that the smaller elements in multielement configurations accumulate comparable and often greater amount of ice compared to larger elements. The results also indicate that the multi-layer approach yields more accurate results compared to the one-layer approach, especially for glaze ice conditions.     

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.     

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.     

Drag coefficient and upstream influence in three-dimensional stratified flow of finite depth

A numerical study of the three-dimensional stratified flow past a vertical square flat plate in a channel of finite depth is described. Particular attention is paid to the anomalous dependence of the drag coefficient C_D on parameter K( = ND/-πU), where N is the Brunt-Väisälä frequency, D is the half depth of the channel and U is the upstream velocity. It is shown that C_D generally increases with K, while it decreases locally at integral values of K. Time development of the upstream columnar disturbance and the corresponding variation of C_D reveals that the periodic variation of C_D with time for K > 1 comes from the successive upstream radiation of the columnar disturbances of the first internal wave mode. Although the propagation speed of the columnar disturbance is consistent with the prediction of linear theory, its time-dependent structure is different from the weakly nonlinear theory as has been shown by laboratory experiments.     

An ALE mesh movement scheme for long‐term in‐flight ice accretion

The rather irregular shapes that glaze ice may grow into while accreting over the surface of an aircraft represent a major difficulty in the numerical simulation of long periods of in‐flight icing. There is a constant need for remeshing: a wasteful procedure. In the framework of ALE formulations, a mesh movement scheme is presented, in which frame and elasticity analogies are loosely coupled. The resulting deformed mesh preserves the quality of elements, especially in the near‐wall region, where accurate prediction of heat flux and shear stresses is required. The proposed scheme handles mesh deformation in a computationally efficient manner by localizing the mesh deformation. The 2D problem of ice accretion over single and multi‐element airfoils is considered here as a numerical experiment. Experimentally measured glaze ice shapes were used to evaluate the performance of the present approach. Copyright © 2011 John Wiley & Sons, Ltd.     

Bottom bed regimes in a circulating fluidized bed boiler

This paper extends previous work on the fluidization regimes of the bottom bed of circulating flyidized bed (CFB) boilers. Pressure measurements were performed to obtain the time-average bottom bed voidage and to study the bed pressure fluctuations. The measurements were carried out in a 12 MW_th CFB boiler operated at 850°C and also under ambient conditions (40°C). Two bubbling regimes were identified: a “single bubble regime” with large single bubbles present at low fluidization velocities, and, at high fluidization velocities, an “exploding bubble regime” with bubbles often stretching all the way from the air distributor to the surface of the bottom bed. The exploding bubble regime results in a high through-flow of gas, indirectly seen from the low average voidage of the bottom bed, which is similar to that of a stationary fluidized bed boiler, despite the higher gas velocities in the CFB boiler. Methods to determine the fluidization velocity at the transition from the single to the exploding bubble regime are proposed and discussed. The transition velocity increases with an increase in particle size and bed height.     

Experimental investigation of heat transfer and pressure drop characteristics of flow through spirally fluted tubes

Spirally fluted tubes are used extensively in the design of tubular heat exchangers. In previous investigations, results for tubes with flute depths e/D_vi < 0.2 were reported, with most correlations applicable for Re ≥ 5000. This paper presents the results of an experimental investigation of the heat transfer and pressure drop characteristics of spirally fluted tubes with the following tube and flow parameter ranges: flute depth e/D_vi = 0.1−0.4, flute pitch p/D_vi = 0.4−7.3, helix angle θ/90° = 0.3−0.65, Re = 500−80,000, and Pr = 2−7. The heat transfer coefficients inside the fluted tube were obtained from measured values of the overall heat transfer coefficient using a nonlinear regression scheme. The friction factor data obtained consisted of 507 data points. The proposed correlation for the friction factor predicts 96% of the database within ±20%. The heat transfer correlation for the range 500 ≤ Re ≤ 5000 predicts 76% of the database (178 data points) within ±20%, and the correlation for the higher Re range predicts 97% of the 342 data points within ±20%. Comparison of heat transfer and friction data show that these tubes are most effective in the laminar and transition flow regimes. The present results show that the increase of flute depth in the range considered does not improve heat transfer.     

Convective heat transfer for cold tube bundles with ice formations in a stream of water at steady state

Experiments were conducted on cold-tube banks subjected to a cross-flow of water. The tubes were internally cooled below the freezing temperature and became enveloped in ice. The resulting ice shapes, which formed on the outside surfaces of the tubes, were allowed to stabilize, and their impact on the total steady-state rate of energy exchange between the tubes and the flowing water was investigated.

Both in-line and staggered tube-bank geometries were considered, with tests conducted in the low to moderate Reynolds number range (Re_d = 100−2,000) and for cooling-temperature ratio variations of 0.5 < Θ < 8. The ice formations were directly observed and photographed, and the total heat transfer rate for the tube bank was inferred from a simple energy balance on the system.

The ice shapes that formed around the tubes were described by one of three distinct categories: ice formutions with no linkage occurring between any adjacent tubes; ice formations with partial linkage of some adjacent tubes; and, for the staggered tube bank, a complete linkage of a majority of the tubes.

The experiments showed that the ice formations dramatically affected the convective heat transfer rate of the tube banks (when compared to nonicing tube banks at the same Re_d) and that the change in heat transfer rate is dependent on the tube-bank geometry. In the no-link category, the ice formations were found to either increase or decrease the tube-bank heat transfer rate depending on the amount of ice-build accumulation, the staggered configuration showing a greater overall rise with Θ than the in-line geometry. Ice linkage between adjacent tubes was found to be detrimental to the heat transfer rate of the staggered bank; however, the same phenomenon on the in-line tube bank did not seriously impede its heat transfer rate. Correlations expressing the heat transfer behavior of both in-line and staggered tube banks with ice formations at steady state have been developed.     

Temperature measurement of a curved surface using thermographic phosphors

An optical technique for surface temperature measurement based on the fluorescent emission of rare earth ion-doped phosphors was demonstrated in an experiment with a heated cylinder in crossflow. In this experiment, a uniform heat flux was imposed by applying a constant voltage across the thin stainless steel cylinder surface to produce surface temperatures between 24°C and 55°C. The fluorescent emission of a thermographic phosphor, lanthanum oxysulfide doped with europium (La₂0₂S:Eu³⁺) deposited on the surface, was recorded to determine the temperature distribution at the curved surface. When excited by ultraviolet radiation, the phosphor emits a spectrum containing certain emission lines, the intensities of which vary with temperature. For a single temperature-sensitive line, ratios of the intensity at a reference temperature to the intensity at different temperatures were correlated as a function of surface temperature. The use of intensity ratio correlations avoids complications due to geometric (viewing angle) effects. Digitized images of the cylinder permitted calculation of surface temperatures and local Nusselt numbers. Differences between surface temperatures measured by calibrated thermocouples and temperatures determined from the phosphor technique were at most 1.2°C.     

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.     

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%.     

Automatic control system of a rear-wheel drive vehicle moving on a sloped weak sandy terrain

The general mechanism of tractive performance of a four-wheel vehicle with rear-wheel drive moving up and down a sloped sandy soil has been considered theoretically. For the given vehicle dimensions and terrain-wheel system constants, the relationships among the effective tractive or braking effort of the vehicle, the amount of sinkage of the front and rear wheels, and the slip ratio were analysed by simulation. The optimum eccentricity of the vehicle’s center of gravity and the optimum application height of the drawbar-pull for obtaining the largest value of maximum effective tractive or braking effort could be calculated by means of the analytical simulation program. For a 5.88 kN weight vehicle, it was found that the optimum eccentricity of the center of gravity e_opt was 1/6 for the range of slope angle—0βπ/24 rad during driving action of the rear wheel and e_opt was also 1/6 for the range of slope angle—π/24β0 rad during braking action of the rear wheel. The optimum application height H_opt was found to be 35 cm for the range of slope angle 0βπ/24 rad during driving action of the rear wheel and H_opt was 0 cm for the range of slope angle—π/24β0 rad during braking action of the rear wheel.     

Experimental study of turbulence in isothermal and stably stratified homogeneous shear flows: Mean flow measurements

The evolution of freestream turbulence under the combined action of linear shear and stable linear temperature profile is investigated. The experiment is carried out in a small, open circuit, low-speed test cell that uses air as working fluid. The temperature gradient formed at the entrance to the test section by means of an array of 24 horizontal, differentially heated elements is varied to get a maximum Brunt-Vaisala frequency N_o[=({g/T_m}{∂T/∂y})^1/2] of 3.1⁻¹. Linear velocity profiles are produced using screens of variable mesh size. The Reynolds number Re_M based on centre-line velocity and mesh size is varied from 80 to 175. Isothermal studies are carried out in four different experiments with varying velocity gradients. The effect of inlet turbulence level on growth of turbulence is studied in these flows by keeping the shear parameter Sh (=(x/u)(∂u/∂y)) constant. The range of shear parameters considered is 2.5–7.0. Shear and stratification combined produce a maximum gradient Richardson number Ri_g (= N_o²/(∂u/∂y)²) of 0.0145. Results have been presented in terms of evolution of variance of velocity fluctuations, Reynolds shear stress and temperature fluctuations. Measurements show the following: In isothermal flows the growth rate of turbulence quantities depends on both shear parameter and inlet turbulence level. There are distinct stages in the evolution of the flow and that can be identified by the power-law exponent of growth of turbulence. Shear is seen to promote the growth of turbulence and accelerate it towards a fully developed equilibrium state. Stratification initially suppresses the growth of turbulence and, hence, enhances the degree of underdevelopment. Under these conditions shear becomes active and subsequently enhances the growth rate of turbulence quantities.     

Heat and mass transfer in unsaturated porous media with solid–liquid change

 A model on heat and mass transfer in unsaturated porous media with solid/liquid phase change was developed with extending the three-variable model previously proposed. The movement of air phase and its effect on the motion of water is considered. The model has been checked with comparison of the experimental results of the temperature distribution for two dimensional freezing process. The evolution of air pressure, water and ice saturation were predicted by solving the governing equations. The ice segregation and moisture movement toward the front of freezing were numerically simulated. Received on 8 December 1999     

A unified constitutive model for superalloys

A physically based unified constitutive model is presented for an aircraft engine nickelbase superalloy. The model accounts for deformation modes that can be activated under different stress, time, and temperature combinations. Two internal state variables and a flow function have been utilized to prdict strain rate sensitivity, stress hold creep, strain hold relaxation, monotonic loading, cyclic loading, and thermal mechanical cycling. In the model flow function, creep deformation and plasticity deformation modes have been incorporated over a wide range of temperatures (0.4 < T/T_melt < 0.75). The model is checked with independent isothermal and thermal mechanical experiments. Different temperature ranges are explored to assess model capabilities.     

Correlations predicting frictional pressure drop and liquid holdup during horizontal gas-liquid pipe flow with a small liquid holdup

Experimental data and correlations available in the literature for the liquid holdup ε_L and the pressure gradient ΔP_TP/L for gas-liquid pipe flow, generally, do not cover the domain 0 < ε_L < 0.06. Reliable pressure-drop correlations for this holdup range are important for calculating flow rates of natural gas, containing traces of condensate. In the present paper attention is focused on reliable measurements of ε_L and ΔP_TPIL values and on the development of a phenomenological model for the liquid-holdup range 0 < ε_L < 0.06. This model is called the “apparent rough surface” model and is referred to as the ARS model. The experimental results presented in this paper refer to air-water and air-water + ethyleneglycol systems with varying transport properties in horizontal straight smooth glass tubes under steady-state conditions. The holdup and pressure gradient values predicted with the ARS model agree satisfactorily with both our experimental results and data obtained from the literature referring to small liquid-holdup values 0 < ε_L < 0.06. Further, it has been shown that in the domain 38 < < 72 mPa m the interfacial tension of the gas-liquid system has no significant effect on the liquid holdup. The pressure gradient, however, increases slightly with decreasing surface tension values.     

Experimental and numerical determinations of the initial surface of phase transformation under biaxial loading in some polycrystalline shape-memory alloys

Biaxial proportional loading such as tension (compression)–internal pressure and bi-compression tests are performed on a Cu-Zn-Al and Cu-Al-Be shape memory polycrystals. These tests lead to the experimental determination of the initial surface of phase transformation (austenite→martensite) in the principal stress space (σ₁,σ₂). A first “micro–macro” modeling is performed as follows. Lattice measurements of the cubic austenite and the monoclinic martensite cells are used to determine the “nature” of the phase transformation, i.e. an exact interface between the parent phase and an untwinned martensite variant. The yield surface is obtained by a simple (Sachs constant stress) averaging procedure assuming random texture. A second modeling, performed in the context of the thermodynamics of irreversible processes, consists of a phenomenological approach at the scale of the polycrystal. These two models fit the experimental phase transformation surface well.     

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.