We focus on a formation process of a quite thin liquid film, known as ‘precursor film,’ ahead a droplet spreading on a smooth solid substrate. The spreading droplet on the solid substrate is accompanied with a movement of a visible boundary line so-called ‘macroscopic contact line.’ Existing studies have indicated there exist two major regions of the precursor film, that is, a region dominated by the fluid dynamics, and a region dominated by the molecular diffusion. Our group has dedicated our special effort to detect the formation process of the precursor film by applying a convectional laser interferometry and a high-speed camera, and to evaluate the spreading rate of the precursor film. In the present study, the existing length of the precursor film at a very early stage of the droplet spreading is evaluated by applying a Brewster-angle microscopy as well as the interferometer. We extend our attention to the advancing process of the precursor film on inclined substrate.

Low-Reynolds-number laminar channel flow is used in various heat/mass transfer applications, such as cooling and mixing. A low Reynolds number implies a low intensity of heat/mass transfer processes, since they rely only on the gradient diffusion. To enhance these processes, an active flow control by means of synthetic (zero-net-mass-flux) jets is proposed. This arrangement can be promising foremost in microscale. The present study is experimental in which a Reynolds number range of 200–500 is investigated. Measurement was performed mainly in air as the working fluid by means of hot-wire anemometry and the naphthalene sublimation technique. PIV experiments in water are also discussed. The experiments were performed in macroscale at the channel cross-section (20×100)mm and (40×200)mm in air and water, respectively. The results show that the low Reynolds number channel flow can be actuated by an array of synthetic jets, operating near the resonance frequency. The control effect of actuation and the heat transfer enhancement was quantified. The stagnation Nusselt number was enhanced by 10–30 times in comparison with the non-actuated channel flow. The results indicate that the present arrangement can be a useful tool for heat transfer enhancement in various applications, e.g., cooling and mixing.

In the present scenario of a global initiative toward a sustainable energy future, the polymer electrolyte fuel cell (PEFC) has emerged as one of the most promising alternative energy conversion devices for various applications. Despite tremendous progress in recent years, a pivotal performance limitation in the PEFC comes from liquid water transport and the resulting flooding phenomena. Liquid water blocks the open pore space in the electrode and the fibrous diffusion layer leading to hindered oxygen transport. The electrode is also the only component in the entire PEFC sandwich which produces waste heat from the electrochemical reaction. The cathode electrode, being the host to several competing transport mechanisms, plays a crucial role in the overall PEFC performance limitation. In this work, an electrode model is presented in order to elucidate the coupled heat and water transport mechanisms. Two scenarios are specifically considered: (1) conventional, Nafion ® impregnated, three-phase electrode with the hydrated polymeric membrane phase as the conveyer of protons where local electro-neutrality prevails; and (2) ultra-thin, two-phase, nano-structured electrode without the presence of ionomeric phase where charge accumulation due to electro-statics in the vicinity of the membrane-CL interface becomes important. The electrode model includes a physical description of heat and water balance along with electrochemical performance analysis in order to study the influence of electro-statics/electro-migration and phase change on the PEFC electrode performance.

A multi-dimensional mathematical model is formulated for simulating the transport and electrochemical reaction phenomena in a polymer electrolyte fuel cell (PEFC). The model describes the two-phase flows, electrochemical reaction kinetics, species transport, and heat transfer, as well as their intrinsic couplings within a PEFC. Two-dimensional model predictions are computed for the two typical operating temperatures at 40 and 80 °C. Computed results reveal that liquid water level may be lower at the higher temperature operation due to water vapor phase diffusion. Detailed water and temperature distributions are displayed to explain the water and heat transport and their interaction. The computed water-content profiles are compared with available experimental data obtained by neutron imaging.

The three-dimensional microstructure of a solid oxide fuel cell (SOFC) anode is directly observed using a focused ion beam and scanning electron microscope (FIB-SEM) technique. Microstructural parameters, which are closely related to transport phenomena in porous materials, are quantitatively evaluated by a random-walk-based diffusion simulation. Numerical simulation of the SOFC anode with the obtained microstructural parameters is also performed, and the result is in good agreement with the experimental counterparts. Combined with a sensitivity analysis for the SOFC performance, the relationships between the microstructural parameters and the power generation performance are discussed and guidelines for optimizing the anode microstructure are proposed.

The oxygen transfer characteristics of the gas diffusion layer are closely related to the cell performance of a polymer electrolyte fuel cell. In this study, a new hybrid gas diffusion layer is proposed in which two porous media with different wettabilities are arranged alternately for augmentation of the oxygen diffusivity in the gas diffusion layer. Since the movement of water from hydrophobic to hydrophilic media due to the difference in capillary pressure, the oxygen diffusion paths in the porous media can be maintained. The oxygen diffusion characteristics with respect to water saturation were measured using an experimental apparatus that uses a galvanic battery oxygen sensor as an oxygen absorber. The experimental results demonstrate that the hybrid structure has superior oxygen diffusion characteristics than a conventional gas diffusion layer with a single porous material with moisture. That is, the effective oxygen diffusivity of the hybrid configuration was almost five times larger than that of the single type at water saturation S = 0.2.

Graded refractive index media appear in numerous industrial applications such as non-isothermal flows, optics material processing, biological imaging. Refractive index gradient has been an early help for combusting flow visualisation. The numerical treatment of radiative transport is difficult in such media due to the curvature of rays, especially when the media are not optically thick. Computer-aided remote probing (inversion) is done today with the help of the diffusion approximation adapted to varying refractive index media but is unsuitable for thin media. Therefore, it is important to develop an approach allowing the use of the radiative transport equation which is the most complete formalism for radiative transfer to date and to couple it to reconstruction schemes. The aim of this study is to demonstrate the reconstruction of an arbitrary refractive index distribution from a least-squares gradient-based iterative inversion algorithm taking advantage of the full transient Radiative Transfer Equation (tRTE). The finite-difference discrete-ordinates method for the tRTE and its adjoint has been implemented, accounting for spatial changes in the distribution of the refractive index in a semi-transparent medium. A least-squares gradient-based iterative algorithm has been designed and elementary tests have been carried to demonstrate reconstruction possibilities.

The three dimension physico-mathematical model was established for the integrated planar solid oxide fuel cell (IP-SOFC) with the couples of multi components flow of reacting gas, heat transfer and electro-chemical process in order to reveal the inherent multi-scale effect of gas distributing duct and the porous support layer, and also, the microscale effect on the transport process in fuel cell. The mutual influences between heat transfer and chemical components transport were included in the model. In addition, the thermal effect of chemical reactions and its influences on polarizations of fuel cell were considered. And also, besides the Darcy diffusion, the Knudsen diffusion in the sub-microscale structure of the porous support is taken into consideration. Numerical simulation was employed to solve the model, by which, the output performance and polarization characteristics of a single cell were analyzed and compared for electrolyte-supported, anode-supported and cathode-supported SOFC, respectively. The present model was also validated comparing with the experimental data.

Thermal barrier coatings (TBCs) are applied to blades, vanes, combustion chamber walls, and exhaust nozzles in gas turbines not only to limit the heat transfer through the coatings but also to protect the metallic parts from the harsh oxidizing and corrosive thermal environment. There is a growing interest in operating these hot gas path (HGP) components at optimal conditions which has resulted in a continuous increase of the turbine inlet temperatures (TITs). This has resulted in the increase of heat load on the turbine components especially in the high pressure side of the turbine necessitating the need to protect the HGP components from the heat of the exhaust gases using novel TBC such as electron beam physical vapor deposition thermal barrier coatings (EBPVD TBCs) and Air Plasma Sprayed thermal barrier coatings (APS TBCs). This study focuses on the estimation of temperature distribution in the turbine metal substrate (IN738) and coating materials (EBPVD TBC and APS TBC) subjected to isothermal conditions (1573 K) around the turbine blade. The heat conduction in the turbine blade and TBC systems necessary for the evaluation of substrate thermal loads are assessed. The steady state 2D heat diffusion in the turbine blade is modeled using ANSYS FLUENT computational fluid dynamics (CFD) commercial package. Heat transfer by radiation is fully accounted for by solving the radiative transport equation (RTE) using the discrete ordinate method. The results show that APS TBCs are better heat flux suppressors than EBPVD TBCs due to differences in the morphology of the porosity present within the TBC layer. Increased temperature drops across the TBC leads to temperature reductions at the TGO/bond coat interface which slows the rate of the thermally induced failure mechanisms such as CTE mismatch strain in the TGO layer, growth rate of TGO, and impurity diffusion within the bond coat.

The increasing importance of improving efficiency and reducing capital costs has lead to significant work studying advanced Brayton cycles for high temperature energy conversion. Using compact, highly efficient, diffusion-bonded heat exchangers for the recuperators, has been a noteworthy improvement in the design of advanced carbon dioxide Brayton Cycles. These heat exchangers will operate near the pseudocritical point of carbon dioxide, making use of the drastic variation of the thermo-physical properties. This paper focuses on the experimental measurements of heat transfer under cooling conditions, as well as pressure drop characteristics within a prototypic printed circuit heat exchanger. Studies utilize type-316 stainless steel, nine channel, semi-circular test section, and supercritical carbon dioxide serves as the working fluid throughout all experiments. The test section channels have a hydraulic diameter of 1.16mm and a length of 0.5m. The mini-channels are fabricated using current chemical etching technology, emulating techniques used in current diffusion bonded printed circuit heat exchanger manufacturing. Local heat transfer values were determined using measured wall temperatures and heat fluxes over a large set of experimental parameters that varied system pressure, inlet temperature, and mass flux. Experimentally determined heat transfer coefficients and pressure drop data are compared to correlations and earlier data available in literature. Modeling predictions using the CFD package FLUENT are included to supplement experimental data. All nine channels were modeled using known inlet conditions and measured wall temperatures as boundary conditions. The FLUENT results show excellent agreement in total power removal for the near pseudocritical region, as well as regions where carbon dioxide is a high or low density fluid.

Wax deposition is a critical operational problem in crude oil transportation through pipelines in cold environments. Accurate prediction of the wax deposition is crucial for the efficient design of subsea lines. Wax deposition is a complex process for which the basic mechanisms are still not fully understood. Although Fick’s molecular diffusion model is considered by several authors as the leading deposition mechanism, it is shown that it does not represent well the wax deposition thickness, measured during the transient regime, in a simple experiment, in a rectangular channel, with a laboratory oil-wax mixture. Another important wax deposition mechanism identified is associated with the rheological properties of the fluid, since oil-paraffin mixtures shows a non-Newtonian behavior at temperatures below the fluid Wax Appearance Temperature. The mixture can be modeled as a Bingham fluid, with a dependence of the yield stress on wax concentration, temperature and rate of cooling. The present paper presents a numerical model for predicting wax deposition in channel flows considering the influence of rheological properties combined with a diffusion-based deposition mechanism. To determine the amount of deposit, the conservation equations of mass, momentum, energy and wax concentration in the mixture were numerically solved with the finite volume method. A nonorthogonal moving coordinate system that adapts to the wax interface deposit geometry was employed. The results demonstrated that additional deposition is obtained as a result of the non Newtonian behavior of the fluid. This trend is in agreement with experimental observation conducted in previous studies.

Hot-wire and hot-film anemometry are widely used in steady flows for instantaneous velocity measurements, and their use has been extended to velocity and wall shear stress measurements in unsteady flows. The technique of hot-film anemometry relies on the Reynolds analogy which relates the diffusion of heat to the momentum exchange. The paper investigates the applicability of the analogy in linearly varying flows. The investigation is a combination of CFD analyses using the Transition SST model and experimental measurements. Results show that, in a linearly accelerating flow, while wall shear stress increases immediately upon the onset of acceleration, heat transfer indicates a relative lag in response. A quantitative analysis of the effects of flow parameters shows that the deviant behaviour is especially pronounced with increasing acceleration and/or reduced initial flow Reynolds number. The initial deviation can be predicted using a non-dimensional parameter based on turbulence timescales and acceleration rate, thereby providing a possible solution to correcting wall shear stress measurements using hot-film anemometry in fast accelerating flows.

We consider regularizations of the convective term that preserve symmetry and conservation properties exactly. This yields a novel class of regularizations that restrain the convective production of small scales in an unconditionally stable manner Numerically, one of the most critical issues is the discrete filtering; properties required are, in general, not preserved by classical LES filters. Alternatively, here we propose to construct filters with the general form F = I + Σ m = 1 M d m D˜ m where D˜ is the discrete diffusive operator. Then, the coefficients, d m , follow from the requirement that, at the smallest grid scale k c , the damping effect to the wavevector-triple (k c , p, k c − p) interactions must be virtually independent of the p-th Fourier-mode. This allows an optimal control of the subtle balance between convection and diffusion to stop the vortex-stretching. Finally, the proposed method is tested for an air-filled differentially heated cavity of aspect ratio 4 by direct comparison with DNS reference results.

In this paper the concentration dependency of mass diffusion coefficients in binary system was investigated. We have developed a novel and accurate visualization system using a small area of transient diffusion fields by adopting a phase shifting technique. Through accurate visualization of the transient diffusion field, it is possible to determine the mass diffusion coefficient. Unlike a conventional interferometer, the proposed system provides high spatial resolution profiles of concentration even though the target area is less than 1.0 mm. This allows the measurement of local transient diffusion field with a high accuracy. The determination of mass diffusion coefficient of each component in multi-component system was also conducted. For the accurate and reliable measurement of mass diffusion coefficient, the experimental error should be taken into account. The experimental data usually contains unexpected accidental error and inherent errors of the measurement system. In this study, an optimization technique using conjugate gradient method is developed for the precise determination of the mass diffusion coefficients. The difference between the experimental and numerical concentration distribution is set as the objective function for the optimization method. The conjugate gradient method searches the optimal value by minimizing the objective function. For the concentration dependency evaluation, sodium chloride (NaCl) in pure water was selected as solute. For determination of each mass diffusion coefficient in multi-component system, NaCl and lysozyme in buffer solution was selected. The experiments were performed under isothermal conditions. The proposed measurement method was validated by comparing the measured data with those available in the literature. The results indicated that the concentration dependency was successfully investigated from the experimental data. The mass diffusion coefficient of each component also could be determined from the experimental data as evidenced by good agreement with the published data. The difference between the reference and determined value of mass diffusion coefficient was less than 10%. It can be said that the diffusion of each solute inside the cell progresses independently within the dilute concentration ranges and the superposition principle of concentration of NaCl and lysozyme was satisfied. The influence of concentration of solution on the diffusion process and allowable concentration range of the superposition principle are determined and discussed.

The present work summarizes the theory and describes the algorithm related to the construction of an open source mixed symbolic-numerical computational code named UNIT — Un ified I ntegral T ransforms, that provides a development platform for finding solutions of linear and nonlinear partial differential equations via integral transforms. The reported research was performed by making use of the symbolic computational system Mathematica v.7.0 and the hybrid numerical-analytical methodology Generalized Integral Transform Technique — GITT. The aim here is to illustrate the robust and precision controlled simulation of multidimensional nonlinear transient convection-diffusion problems, while providing a brief introduction of this open source code. Test cases are selected based on nonlinear multi-dimensional formulations of the Burgers equations, with the establishment of reference results for specific numerical values of the governing parameters. Special aspects and computational behaviors of the algorithm are then discussed, demonstrating the implemented possibilities within the present version of the UNIT code.

In this study, the importance of drainage on dropwise condensation of a flowing air-steam mixture is investigated. The initial phase of drop growth, when diffusion is not limiting, is artificially made more important to separate the diffusion resistance to heat transfer. An apparatus with controlled removal of condensate droplets from the condenser plates is designed and tested. The dropwise condensation process is frequently interrupted upon which nucleation restarts each time. Non-artificial drainage occurs at low frequencies of typically 4 large drops per second per dm 2 . Condensate removal at such a frequency that does not allow formation of large drops artificially makes the initial phase of drop growth more important. The results are believed to be important for the explaining of differences between filmwise and dropwise condensation heat transfer. It is found that the total heat transfer resistance decreases with increasing droplet removal frequency, f. When f is increased, the ratio of condensate mass flow rate to gas mass flow rate increased as well, by 11% for a droplet removal frequency of 0.8 Hz. With increasing f, the relative importance of convective heat transfer decreases.

As the first step of simulation, a temperature field for solidifying cast steel and cast iron roll was created. The convection in the liquid is not comprised since in the first approximation, the convection does not influence the analysed occurrence of the C → E (columnar to equiaxed grains) transition in the roll. The obtained temperature field allows to study the dynamics of its behavior observed in the middle of the mould thickness. This midpoint of the mould thickness was treated as an operating point for the C → E transition. A full accumulation of the heat in the mould was postulated for the C → E transition. Thus, a plateau at the T ( t ) curve was observed at the midpoint. The range of the plateau existence t C ↔ t E corresponded to the incubation period, t C R ↔ t E R that appeared before fully equiaxed grains formation. At the second step of simulation, the thermal gradients field was studied. Three ranges were distinguished: a/ for the formation of the columnar structure (the C–zone): ( T ˙ ≫ 0 and ( G | t < t C R − G | t = t C R ) ≫ 0 ) , b/ for the C → E transition (from columnar to fully equiaxed structure): ( T ˙ ≈ 0 and ( G | t = t C R − G | t = t E R ) ≈ 0 ) , c/ for the formation of the fully equiaxed structure (the E–zone): ( T ˙ < 0 and ( G | t = t E R − G | t > t E R ) ≈ 0 ) . The columnar structure formation was significantly slowed down during incubation period. It resulted from a competition between columnar growth and equiaxed growth expected at that period of time. The ( G | t = t C R − G | t = t E R ) ≈ 0) relationship was postulated to correspond well with the critical thermal gradient, G crit. . A simulation was performed for the cast steel and cast iron rolls solidifying as if in industrial condition. Since the incubation divides the roll into two zones (columnar and equiaxed) some experiments dealing with solidification were made on semi-industrial scale. A macrosegregation equation for both mentioned zones was formulated. It was based on a recent equation for redistribution after back-diffusion. The role of the back-diffusion parameter was emphasized as a factor responsible for the redistribution in columnar structure and equiaxed structure.

Characteristics of turbulent mass transfer around a rotating circular cylinder have been investigated by Direct Numerical Simulation. The concentration field was computed for three different cases of Schmidt number, Sc = 1, 10 and 100 at Re * = 336. Our results confirm that the thickness of the Nernst diffusion layer decreases as Sc increases. Wall-limiting behavior within the Nernst diffusion layer was examined and compared with those of channel flow. Concentration fluctuation was found to be time-scaled with ( r + ) 2 while the time scale ratio equals the Schmidt number throughout the Nernst diffusion layer. Scalar modeling closure constants were determined, and turned out to vary considerably within the diffusion layer.

We suggest a model of rain scavenging of soluble gaseous pollutants in the atmosphere. It is shown that below-cloud gas scavenging is determined by non-stationary convective diffusion equation with the effective Peclet number. The obtained equation was analyzed numerically in the case of log-normal droplet size distribution. Calculations of scavenging coefficient and the rates of precipitation scavenging are performed for wet removal of ammonia (NH 3 ) and sulfur dioxide (SO 2 ) from the atmosphere. It is shown that scavenging coefficient is non-stationary and height-dependent. It is found also that the scavenging coefficient strongly depends on initial concentration distribution of soluble gaseous pollutants in the atmosphere. It is shown that in the case of linear distribution of the initial concentration of gaseous pollutants whereby the initial concentration of gaseous pollutants decreases with altitude, the scavenging coefficient increases with height in the beginning of rainfall. At the later stage of the rain scavenging coefficient decreases with height in the upper below-cloud layers of the atmosphere.

We analyze non-isothermal absorption of trace gases by the rain droplets with internal circulation which is caused by interfacial shear stresses. It is assumed that the temperature and concentration of soluble trace gases in the atmosphere varies in a vertical direction. The rate of scavenging of soluble trace gases by falling rain droplets is determined by solving heat and mass transfer equations. In the analysis we accounted for the accumulation of the absorbate in the bulk of the falling rain droplet. The problem is solved in the approximation of a thin concentration and temperature boundary layers in the droplet and in the surrounding air. We assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and concentration of the absorbate and temperature are homogeneous and time-dependent in the bulk. By combining the generalized similarity transformation method with Duhamel’s theorem, the system of transient conjugate equations of convective diffusion and energy conservation for absorbate transport in liquid and gaseous phases with time-dependent boundary conditions is reduced to a system of linear convolution Volterra integral equations of the second kind which is solved numerically. Calculations are performed using available experimental data on nocturnal temperature profiles in the atmosphere. It is shown than if concentration of a trace gas in the atmosphere is homogeneous and temperature in the atmosphere increases with altitude, droplet absorbs gas during all the period of its fall. Neglecting temperature inhomogenity in the atmosphere described by nocturnal temperature inversion leads to essential underestimation of the trace gas concentration in a droplet on the ground.