Study of the thermocapillary effect on a wavy falling film using a fiber optical thickness probe

The wave flow of a water film down a vertical plate with a 150×150 mm heater has been experimentally studied. The effect of the heat flux on the film flow leads to the formation of periodically flowing rivulets and thin film between them due to the action of thermocapillary forces in spanwise direction. The local film thickness between rivulets is measured by means of a noncontact fiber optical probe. As the heat flux grows, the average film thickness continuously decreases but upon reaching about 50% of the initial thickness, the film spontaneously breaks down. It is found that the decrease of the wave amplitude between rivulets is caused by the reduction of the local Reynolds number and is in a qualitative agreement with the laws of the hydrodynamics for the isothermal case. That is, no appreciable effect of streamwise thermocapillary forces on the wave amplitudes is detected. The experimental results are in good agreement with recently published data obtained by the capacitance method.     

Temperature pulsations and thermocapillary effects on the surface of a wavy heated liquid film

The temperature pulsations and wave characteristics in water film flow along a vertical plate with a heater are investigated. Using an infrared scanner, the temperature field on the film surface is measured for various heat flux densities on the heater. Experimental data on the variation of the temperature with time on a local segment of the liquid film surface during wave transmission are obtained. In the absence of a heat flux the data obtained are in good agreement with the results of other researchers for an isothermal liquid film. When the down-flowing liquid is heated, the thermocapillary forces lead to the formation of rivulets and a thin film between them. It is shown that in the inter-rivulet zone the relative wave amplitude increases due to the action of the thermocapillary forces.     

Combined slit and plate–plate magnetorheometry of a magnetorheological fluid (MRF) and parameterization using the Casson model

We describe a magneto-slit die of 0.34 mm height and 4.25 mm width attached to a commercial piston capillary rheometer, enabling the measurement of apparent flow curves of a magnetorheological fluid (MRF) in the high shear rate regime (apparent shear rates 276 up to 20,700 s^???1, magnetic flux density up to 300 mT). The pressure gradient in the magnetized slit is measured via two pressure holes. While the flux density versus coil current without MRF could directly be measured by means of a Hall probe, the flux density with MRF was investigated by finite element simulations using Maxwell® 2D. The true shear stress versus shear rate is obtained by means of the Weissenberg–Rabinowitsch correction. The slit die results are compared to plate–plate measurements performed in a shear rate regime of 0.46 up to 210 s^???1. It is shown that the Casson model yields a pertinent fit of the true shear stress versus shear rate data from plate–plate geometry. Finally, a joint fit of the slit and plate–plate data covering a shear rate range of 1 up to 50,000 s^???1 is presented, again using the Casson model. The parameterization of the MRF behavior over the full shear rate regime investigated is of relevance for the design of MR devices, like, e.g., automotive dampers. In the Appendix, we demonstrate the drawbacks of the Bingham model in describing the same data. We also show the parameterization of the flow curves by applying the Herschel–Bulkley model.     

Intensification of Heat Transfer in a Downward Liquid-Film Flow

Heat transfer in a film flow of the FC-72 dielectric liquid down a vertical surface with an embedded 150×150 mm heater is experimentally examined in the range of Reynolds numbers Re = 5–375. A chart of liquid-film flow modes is constructed, and characteristic heat-transfer regions are identified. Data on the dependence of heater-wall temperature and local heat flux at the axis of symmetry of the heater on the longitudinal coordinate are obtained. Local and mean heat-transfer coefficients are calculated. It is shown that enhanced heat transfer is observed in the region where rivulets starts forming in the low-Reynolds-number liquid-film flow.     

Reliable plate–plate MRF magnetorheometry based on validated radial magnetic flux density profile simulations

The apparent shear stress from plate–plate magnetorheometry, using the commercial magnetocell MRD180/1T (Physica/Anton Paar) in standard configuration, is distinctly overestimated. The effect is due to a flux density maximum near the sample rim and radial migration of iron particles towards the rim. Radial magnetic flux density profiles were investigated by finite element simulations using the Maxwell^®2D code and by direct Hall probe measurements. The reliability of the finite element method results, both for the empty magnetocell and with magnetorheological fluid (MRF) in the measuring gap, allows conclusions on the true flux density within the MRF, which cannot be accessed by Hall probe measurements. If the MRF sits on top of the bottom yoke (standard configuration), the flux density maximum reaches twice the plateau value (0.74 T for 3 A coil current and 0.3 mm gap height of the investigated MRF). This yields a higher effective flux density and causes radial iron particle migration, resulting in an additional magnetic flux increase near the rim due to augmented MRF magnetisation. As a result, the rotor torque at constant rotary speed increases with time. Reliable results are achieved by a modification of the magnetocell, such that the MRF sits on a non-magnetic Hall disc of 1.5 mm thickness, allowing an online flux density measurement by a FW Bell Hall probe. In this configuration, the radial flux density profile near the rim remains sufficiently flat and no iron particle migration is detected.     

Experimental investigation of heat transfer and flow pattern from heated horizontal rectangular fin array under natural convection

This paper presents results of the experimental study conducted on heated horizontal rectangular fin array under natural convection. The temperature mapping and the prediction of the flow patterns over the fin array with variable fin spacing is carried out. Dimensionless fin spacing to height (S/H) ratio is varied from 0.05 to 0.3 and length to height ratio (L/H) = 5 is kept constant. The heater input to the fin array assembly is varied from 25 to 100 W. The single chimney flow pattern is observed from 8 to 12 mm fin spacing. The end flow is choked below 6 mm fin spacing. The single chimney flow pattern changes to sliding or end flow choking at 6 mm fin spacing. The average heat transfer coefficient (h_a) is very small (2.52–5.78 W/m² K) at 100 W for S = 5–12 mm. The h_a is very small (1.12–1.8 W/m² K) at 100 W for 2–4 mm fin spacing due to choked fin array end condition. The end flow is not sufficient to reach up to central portion of fin array and in the middle portion there is an unsteady down and up flow pattern resulting in sliding chimney. The central bottom portion of fin array channel does not contribute much in heat dissipation for S = 2–4 mm. The h_a has significantly improved at higher spacing as compared to lower spacing region. The single chimney flow pattern is preferred from heat transfer point of view. The optimum spacing is confirmed in the range of 8–10 mm. The average heat transfer results are compared with previous literature and showed similar trend and satisfactory agreement. An empirical equation has been proposed to correlate the average Nusselt number as a function of Grashof number and fin spacing to height ratio. The average error for this equation is ?0.32 %.     

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.     

Measurement of Dynamic Properties of Viscoelastic Materials

An improved method to measure the dynamic viscoelastic properties of elastomers is proposed. The method is based on the analysis of forced oscillation of a cylindrical sample loaded with an inertial mass. No special equipment or instrumentation other than the ordinary vibration measurement apparatus is required. Upper and lower surfaces of the viscoelastic material sample were bonded to a load disc and a rigid base plate, respectively. The rigid base plate was subject to forced oscillations driven by a vibration exciter. Two accelerometers were attached to monitor the displacement of the base plate and the load disc. The recorded magnitude ratio and the phase difference between the load disc and the base plate vibrations represent the axial, dynamic deformation of the sample. The data are sufficient to obtain the dynamic properties of the sample, oscillation properties of vibration exciter, whereas the sensitivity of gauges having no effect on the calculation results. For accurate calculation of the properties, a two-dimensional numerical model of cylindrical sample deformation was used. Therefore, a form factor, which takes into account the sample sizes in one-dimensional models, is not required in this method. Typical measurement of the viscoelastic properties of a silicone rubber Silastic® S2 were measured over the frequency range from 10 Hz to 3 kHz under deformations (ratio of vibration magnitude to sample thickness) from 10^?4% to 5%. It was shown that the modulus of elasticity and the loss tangent fall on a single curve when the ratio of load mass to sample mass changed from 1 to 20. When the sample diameter was varied from 8 to 40 mm, the modulus of elasticity fall on the same curve, but the loss tangent curves showed some degree of scatter. Studied temperature dependence and nonlinear behavior of viscoelastic properties is found not to be associated with this effect.     

Structural and Mechanical Properties of Gastropod Connective and Smooth Muscle Tissue

The pedal integument of terrestrial gastropod Arion rufus is composed mainly of smooth muscle cells (SMCs, 45 %), haemocoelic cavities (36 %), and collagen connective tissue. Using stereological methods, SMC two-dimensional length density (0.12 μm^?1), numerical density (426,000 mm^?3), and mean distance (31 μm) in the cluster were assessed. The average SMC could be approximated by an ellipsoid 72 μm in length with semi-axes of 3 μm. Three-dimensional reconstructions of SMCs and haemocoelic cavities of gastropod tissue were created using serial thick and semi-thin sections. These reconstructions showed the spatial arrangement of individual SMCs within the tissue: longitudinally, perpendicularly, and obliquely oriented to the main axis of the gastropod body. Using uniaxial mechanical loading with linearly increasing load or elongation at various loading rates (2, 10, and 20 mN/min; 2 and 3 mm/min) in transverse and longitudinal orientations to the main gastropod body axis, the Young’s modulus of elasticity for small (23–27 kPa) and large deformations (49–132 kPa) as well as ultimate stress (105–250 kPa) and strain (300–400 %) were determined. There was a trend toward stiffer integument tissue in the longitudinal direction compared to the transversal direction and toward increasing stiffness with loading velocity.     

Numerical investigation of the coupled interaction between an unsteady aerodynamic flow field and a water film coating on a circular cylinder

Rain–wind induced vibration is an aeroelastic phenomenon that occurs on the inclined cables of cable-stayed bridges and arises due to the interaction between the unsteady wind loading and the formation of water rivulets on the cable surface. A new numerical method has been developed at the University of Strathclyde to simulate the influence of the external flow field on the rivulet dynamics and vice versa. The approach is to couple a Discrete Vortex Method solver to determine the external flow field and unsteady aerodynamic loading, and a pseudo-spectral solver based on lubrication theory to model the evolution and growth of the water rivulets on the cable surface under external loading. Results of this coupled model are presented, to provide detailed information on the development of water rivulets and their interaction with the aerodynamic field. In particular, the effect of the initial water film thickness and the angle of attack in plane on the resulting rivulets are investigated. The results are consistent with previous full scale and experimental observations with rivulets forming on the upper surface of the cable only in configurations where rain–wind induced vibration has been observed. Additionally, the thickness of the lower rivulet is found to be self-limiting in all configurations. The results demonstrate that the model can be used to enhance the understanding of the underlying physical mechanisms of rain–wind-induced vibration.     

Experiments on the flow of a thin liquid film over a horizontal stationary and rotating disk surface

Experiments on characterization of thin liquid films flowing over stationary and rotating disk surfaces are described. The thin liquid film was created by introducing deionized water from a flow collar at the center of an aluminum disk with a known initial film thickness and uniform radial velocity. Radial film thickness distribution was measured using a non-intrusive laser light interface reflection technique that enabled the measurement of the instantaneous film thickness over a finite segment of the disk. Experiments were performed for a range of flow rates between 3.0 lpm and 15.0 lpm, corresponding to Reynolds numbers based on the liquid inlet gap height and velocity between 238 and 1,188. The angular speed of the disk was varied from 0 rpm to 300 rpm. When the disk was stationary, a circular hydraulic jump was present in the liquid film. The liquid-film thickness in the subcritical region (downstream of the hydraulic jump) was an order of magnitude greater than that in the supercritical region (upstream of the hydraulic jump) which was of the order of 0.3 mm. As the Reynolds number increased, the hydraulic jump migrated toward the edge of the disk. In the case of rotation, the liquid-film thickness exhibited a maximum on the disk surface. The liquid-film inertia and friction influenced the inner region where the film thickness progressively increased. The outer region where the film thickness decreased was primarily affected by the centrifugal forces. A flow visualization study of the thin film was also performed to determine the characteristics of the waves on the free surface. At high rotational speeds, spiral waves were observed on the liquid film. It was also determined that the angle of the waves which form on the liquid surface was a function of the ratio of local radial to tangential velocity.     

Specimen and Grain Size Effects of Al1100 on Strain and Strain Rate Hardening at Various Strain Rates for Al1100

This paper is concerned with comparison of the tensile properties of Al1100 thin film in a micro-scale to that of Al1100 sheet in a macro-scale. The material properties of Al1100 film and sheet with a thickness of 96 μm and 1 mm respectively have been investigated at strain rates ranging from 0.001 to 100 s^?1. The experiments were conducted with Static Micro-Material Testing Machine (SMMTM) and High Speed Micro-Material Testing Machine (HSMMTM) for micro-specimens and with Instron 5583 and high speed material testing machine (HSMTM) for macro-specimens. A reliable jig system for SMMTM and HSMMTM has been newly developed for easy installation of a specimen and accurate alignment between a specimen and the jig system to enhance the reproducibility of tests. The digital image correlation (DIC) method is employed to measure the axial strain of the specimens. In order to obtain a fine speckle pattern for the DIC method, a novel technique is employed to print the speckle pattern with fine particles by blowing sprayed particles before printing. The grain sizes of two Al1100 specimens have been compared and the number of grains in the gauge cross-section has been calculated to obtain the grain number which is related to the specimen size effect. Electron Back Scattered Diffraction (EBSD) images were obtained for both micro-specimens and macro-specimens and analyzed to measure the grain size. The Al1100 film with a smaller average grain size shows larger strain hardening than the Al1100 sheet with a larger average grain size.     

Experimental study of the effect of spray inclination on ultrafast cooling of a hot steel plate

The ultrafast cooling that occurs during high mass flux air-atomized spray impingement on a hot 6 mm thick stainless steel plate has been studied experimentally in terms of the nozzle inclination between 0° and 60°. The average mass flux of water used in the study accounts to 510 kg/m² s. The coolants used in the study are pure water and surfactant water of 600 ppm concentration. The initial temperature of the plate has been maintained at 900 °C, which is the temperature of a hot strip on run-out table in steel industry. The transient surface heat flux and temperature histories have been estimated by an inverse heat solver using measured temperature input data. Heat transfer results demonstrates that optimum cooling efficiency (~2.76 MW/m², 194 °C/s) for pure water has been achieved at 30° nozzle orientation. The inclined nozzle has not been found beneficial when surfactant water is used as the coolant.     

Mechanics of shaft-loaded blister test for thin film suspended on compliant substrate

Based on the von Kármán plate theory, the mechanics of a shaft-loaded blister test for thin film/substrate systems is studied by considering elastic substrate deformations and residual stresses in these films. In testing, films are attached to a substrate provided with a circular hole, through which loading is applied to the film by a flat-ended shaft of circular cross-section. The effect of substrate deformation on the deflection of the loaded film is taken into account by using a line spring model. For small deflections, an analytical solution is derived, while for large deflections a numerical solution is obtained using the shooting method. The resulting load-shaft displacement relation, which is essential in blister tests, compares favorably with finite element analysis.     

Fiber Bragg Grating Based Stress Measurement for Films Deposited onto Cylindrical Surfaces

Shape of a substrate directly influences the residual stress in thin film coatings. In this study, a method involving Fiber Bragg Grating (FBG) was used to measure residual stress in a film deposited on a cylindrical surface. An FBG has a cylindrical surface and its Bragg wavelength shifts continuously when a film is being deposited on the sensor’s surface. Herein, we calculated the residual strain in the film from the wavelength shift of the Bragg grating by studying the transfer of the residual strain of the cylindrical film to the core of the optical fiber substrate during deposition. By employing the energy method, we derived expressions that related the strain in the core of fiber to the residual strain in single layer films, bilayer films, and multilayer cylindrical films. As an example, we demonstrated a detailed process for testing the stress and the strain distribution across a nickel (Ni) film electrodeposited on the surface of a nickel-phosphorus (Ni-P) alloy-coated optical fiber. The results indicated that the measured strain repeatability was less than 500 μ? and the strain sensitivity was more than ?2 × 10^?3 pm/μ?, when the thickness of the film was less than 5 μm. The negative sign on the strain sensitivity indicated that the tensile strain in the film produced compressive strain in the core of the optical fiber. The FBG sensor system has high test speed, and integrates measurement and signal transmission. This method provides an effective and convenient approach to measure stress in a film deposited on a cylindrical surface.     

Study of airflow in a cold-region tunnel using a standard k − ε turbulence model and air-rock heat transfer characteristics: validation of the CFD results

An efficient computational fluid dynamics (CFD) method for simulating the flow and convective heat transfer process of airflow in a tunnel is required to analyze the freezing and thawing of surrounding rock and to apply the results to the design of the insulation layer for a tunnel located in a cold region. Comparisons of experimental data and CFD results using a standard k ? ε turbulence model, a wall function, a thermal function and an adaptive finite element method are presented. Comparison of the results indicated that the proposed model and simulation method are efficient at determining the solid–air interface heat coefficient in a thin and infinitely wide horizontal plate and the hydrodynamic and thermal fields in a 3-D cavity. After demonstrating that the necessary validations are satisfied, this paper presents an analysis of the characteristics of airflow and air–rock heat transfer in a cold-region tunnel.     

An experimental investigation on the surface water transport process over an airfoil by using a digital image projection technique

In the present study, an experimental investigation was conducted to characterize the transient behavior of the surface water film and rivulet flows driven by boundary layer airflows over a NACA0012 airfoil in order to elucidate underlying physics of the important micro-physical processes pertinent to aircraft icing phenomena. A digital image projection (DIP) technique was developed to quantitatively measure the film thickness distribution of the surface water film/rivulet flows over the airfoil at different test conditions. The time-resolved DIP measurements reveal that micro-sized water droplets carried by the oncoming airflow impinged onto the airfoil surface, mainly in the region near the airfoil leading edge. After impingement, the water droplets formed thin water film that runs back over the airfoil surface, driven by the boundary layer airflow. As the water film advanced downstream, the contact line was found to bugle locally and developed into isolated water rivulets further downstream. The front lobes of the rivulets quickly advanced along the airfoil and then shed from the airfoil trailing edge, resulting in isolated water transport channels over the airfoil surface. The water channels were responsible for transporting the water mass impinging at the airfoil leading edge. Additionally, the transition location of the surface water transport process from film flows to rivulet flows was found to occur further upstream with increasing velocity of the oncoming airflow. The thickness of the water film/rivulet flows was found to increase monotonically with the increasing distance away from the airfoil leading edge. The runback velocity of the water rivulets was found to increase rapidly with the increasing airflow velocity, while the rivulet width and the gap between the neighboring rivulets decreased as the airflow velocity increased.     

Periodic boiling in parallel micro-channels at low vapor quality

Based on experimental investigations the present study evaluates instability and heat transfer phenomenon under condition of periodic flow boiling of water and ethanol in parallel triangular micro-channels. Tests were performed in the range of hydraulic diameter 100–220 μm, mass flux 32–200 kg/m² s, heat flux 120–270 kW/m², vapor quality x = 0.01–0.08. The period between successive events depends on the boiling number and decreases with an increase in the boiling number. The initial film thickness decreases with increasing heat flux. When the liquid film reached the minimum initial film thickness CHF regime occurred. Temporal variations of pressure drop, fluid and heater temperatures were periodic. Oscillation frequency is the same for the pressure drop, for the fluid temperature at the outlet manifold, and for the mean and maximum heater temperature fluctuations. All these fluctuations are in phase. The CHF phenomenon is different from that observed in a single channel of conventional size. A key difference between micro-channel heat sink and single conventional channel is amplification of parallel-channel instability prior to CHF. The dimensionless experimental values of the heat transfer coefficient are presented as the Nusselt number dependence on the Eotvos number and the boiling number.     

Thermoviscoelastic deformations of functionally graded thin plates

By considering the influence of the mid-plane strains due to the inhomogeneous property of FGMs across the thickness, a constitutive equation of thermoviscoelastic functionally graded thin plates is reduced on the basis of the Kirchhoff's hypothesis for the classical plate theory. The corresponding simplified Gurtin's type variational principle of functionally graded thin plates is presented by means of the modern convolution bilinear forms. By using the Navier analytic method or combining the Ritz method in the spatial domain and the Legendre interpolation method in the temporal domain, the static thermoelastic deformations or quasi-static thermoviscoelastic deformations of functionally graded thin plates under mechanical or thermal loads are studied.     

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).