Formation Of Liquid Film Research Articles

Advanced microstructural characterisation of cast ATI 718Plus\xae\u2014effect of homogenisation heat treatments on secondary phases and repair welding behaviour

The influence of base metal conditions on the weld cracking behaviour of cast ATI 718Plus® is investigated by comparing 4 h and 24 h dwell time pseudo-hip homogenisation heat treatments at 1120, 1160 and 1190 °C with the as-cast microstructure. Scanning electron microscopy (SEM), X-ray diffraction (XRD) on electrolytically extracted powder and transmission electron microscopy (TEM) were used to identify Nb-rich secondary phases in interdendritic areas as the C14 Laves phase and Nb(Ti) MC-type carbides. All homogenisation heat treatments but the 1120 °C 4-h condition dissolve the Laves phase. A repair welding operation was simulated by linear groove multi-pass manual gas tungsten arc welding (GTAW). The Laves phase containing microstructures resulted in lower total crack length for heat affected zone cracking. Constitutional liquation of Nb(Ti) MC-type carbides is observed as a liquation mechanism in Laves-free microstructure, while thick liquid film formation due to the Laves eutectic melting could reduce the formation of weld cracks in microstructures containing the Laves phase.

Water washing of axial flow compressors: numerical study on the fate of injected droplets

In turbomachinery applications blade fouling represents a main cause of performance degradation. Among the different techniques currently available, online water washing is one of the most effective in removing deposit from the blades. Since this kind of washing is applied when the machine is close to design conditions, injected droplets are strongly accelerated when they reach the rotor blades and the understanding of their interaction with the blades is not straightforward. Moreover, undesirable phenomena like blades erosion or liquid film formation can occur. The present study aims at assessing droplets dragging from the injection system placed at the compressor inlet till the first stage rotor blades, with a focus on droplets impact locations, on the washing process and the associated risk of erosion. 3D numerical simulations of the whole compressor geometry (up to the first rotor stage) are performed by using Ansys Fluent to account for the asymmetric distribution of the sprays around of the machine struts, IGV and rotor blades. The simulations are carried out by adopting the k-ε realizable turbulence model with standard wall functions, coupled with the discretephase model to track injected droplets motion. Droplets-wall interaction is also accounted for by adopting the Stanton-Rutland model which define a droplet impact outcome depending on the impact conditions. The induced erosion is evaluated by adopting an erosion model previously developed by some of the authors and implemented in Fluent through the use of a User Defined Function (UDF). Two sets of simulations are performed, by considering the rotor still and rotating, representative of off-line and on-line water washing conditions, respectively. In the rotating simulation, the Multiple Reference Frame Model is used. The obtained results demonstrate that the washing process differs substantially between the fixed and the rotating case. Moreover, to quantify the water washing effectiveness and the erosion risk, new indices were introduced and computed for the main components of the machine. These indices can be considered as useful prescriptions in the optimization process of water washing systems.

Optimization of stator blade profile design for last stages of steam turbines based on the features of coarse droplets movement in inter-blade channels

Results of the investigation of coarse droplets movement in steam turbine blade passages are presented in the paper. The structure of generated streams of secondary droplets was considered. The analysis of the effect of these streams on the liquid film formation on the blade surfaces was performed. On the basis of obtained data, several recommendations for blade design operating in wet steam conditions were formulated. They were used to optimize the baseline profile geometry in order to control the coarse droplets streams and provide the increasing of liquid film suction effectiveness. The numerical and experimental comparison of the baseline and optimized geometries was performed.

Thin liquid film formation on hemispherical and conical substrate

AbstractThe deposition and coating of thin films onto curved rigid substrate, involving displacement of air by a liquid, has numerous applications within the technology sectors but faces two major challenges: (i) control of the local film thickness; (ii) ensuring that the coating remains stable. The work reported here investigates the full coverage of three‐dimensional curved geometries, of hemispherical and conical shape, by a continuously fed, gravity‐driven, thin liquid layer. The modelling approach adopted utilises a first integral formulation [1,2] of the Navier‐Stokes equations leading to a variational formulation in the case of steady flow and an advantageous re‐formulation of the dynamic boundary condition at the free surface [3]. Asymptotic analysis, underpinned by the long‐wave approximation, enables analytic solutions for the local film thickness to be obtained.

Urea derived deposits in diesel exhaust gas after-treatment: Integration of urea decomposition kinetics into a CFD simulation

Formation of solid urea by-products in mixing sections of SCR systems is a well-known problem in development and design of exhaust gas after treatment systems. Spray impingement induces liquid film formation on mixer blades and tailpipe walls, from which urea by-products can be derived in dependence on temperature and residence time. This work presents a comprehensive modeling approach for prediction of by-product formation. An existing kinetic model for urea decomposition is integrated into CFD simulations by user coding. Implementation is evaluated by 3D simulations of thermogravimetric decomposition of urea and its by-products. Results are compared to experimental data and simulation results from a 0D model. Furthermore, the procedure is applied to simulations of a flow setup including urea injection for demonstration of the model capabilities.

MATHEMATICAL MODELING OF A NEW METHOD OF THERMAL PROTECTION BASED ON THE INJECTION OF SPECIAL COOLANTS

A closed mathematical model of heat and mass transfer under the non-isothermal filtration through the organized pores of coolants with a strong dependence of dynamic viscosity on temperature with further injection into a viscous gas-dynamic flotation with the formation of liquid film and evaporation under the influence of aerodynamic heat flows has been developed. The aim of the paper is to develop the physical and mathematical basis of thermal protection with the automatic supply of coolant having a strong dependence of dynamic viscosity on temperature (by 3–5 orders of magnitude when the temperature changes by 200–2500°C). Such thermal protection system allows for operating without the mass removal that preserves the geometry of the structural elements at the intensive heating, which is very relevant. To achieve this goal, a mathematical model of the automatic coolant supply with filtration through the organized pores, injection into a high-temperature gasdynamic boundary layer with the formation of the protective liquid film and evaporation is formulated. The results with respect to the mass flow rate of the coolant, the mass evaporation rate of the formed liquid film and the temperatures of the structure which in all cases remain below the evaporation temperature of the cooler have been obtained.

Effect of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC joint

Influence of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC superalloy was investigated systematically. The microstructure of as-cast base metal (BM) mainly consisted of “ogdoadically” diced cube γ′ particles, fanlike γ-γ′ eutectics, MC carbides, Cr–Mo borides and Ni–Zr intermetallics. Grain-boundary liquation cracking was observed in the HAZ of as-cast joint, resulting from the segregation of elemental boron. Pre-weld heat treatment had a significant influence on the size, volume fraction and distribution of γ′ particles. It also decreased the non-equilibrium segregation of boron during cooling, which inhibited the formation of grain-boundary liquid film and liquation cracking. The precipitation of MC carbides and nanoscaled γ′ particles in the fusion zone (FZ) improved the micro-hardness of the FZ. The micro-hardness of the BM and the HAZ were influenced by the volume fraction and the size of γ′ particles. All the joints fractured in the BM at RM and 900 °C. The as-cast and water quenched (WQ) joints failed with a quasi-cleavage fracture mode at RM and 900 °C while the tensile fracture of furnace cooled (FC) joint showed a ductile rupture at RM and 900 °C.

Characterization of Confined Liquid Jet Injected into Oscillating Air Crossflow

The aim of this work is to study the role of the liquid phase in the thermo-acoustic coupling that leads to combustion instabilities. In multipoint injection systems, the liquid fuel is injected through orifices as jets which are destabilized by the high-speed air flowing in a transverse direction. This paper is focused on the effect of an acoustic excitation imposed on the air flow on the various liquid features observed in actual fuel injection systems. This concerns the initial jet trajectory, its breakup, the formation and transport of the resulting spray, the liquid film formation on walls and its final atomization at the outlet of the injection system. Experimental investigations were performed on a simplified geometry designed to reproduce these main phenomena. Phase-averaged processing of the experimental data is used to analyze the relationship between the acoustic excitation and the liquid phase behavior. In particular, the phase delays imposed by the various processes involved are analyzed. The processing of high-speed video recordings reveals harmonic oscillation of the liquid column which periodically breaks into a spray structure or impacts the opposite wall, forming a liquid film. Using PDA measurements, it is shown that the resulting two-phase flow consists of alternating dense and diluted zones transported by the air flow. At the end of the channel, the liquid film flowing on the wall opposite the liquid jet is periodically atomized through the oscillating shear forces imposed by the air flow.

Experimental study of thermal performance in a closed loop pulsating heat pipe with alternating superhydrophobic channels

A pulsating heat pipe (PHP) with alternating hydrophilic/superhydrophobic channels was tested at vertical position, bottom heat mode and different heat power inputs. The device consists in a copper tube (internal/external diameters of 3.18/4.76 mm), bent into a planar serpentine of ten channels and five U-turns. The tube is partially functionalized with a superhydrophobic coating, in such a way to create an alternation of hydrophilic and superhydrophobic surfaces on the straight tubes along the loop, in the condenser zone. The aim is to investigate how the alternated wettability affects the start-up, the fluid motion along the tubes and the overall thermal performance of the device, which is compared to another PHP, having the same geometry and under the same working conditions, but completely hydrophilic. Distilled water, at 50% filling ratio is the working fluid. Power inputs varying from 20 to up to 350 W, in a stepwise increasing and decreasing fashion, are applied to the PHP. The condenser temperature is kept constant, at 20 °C. The device is monitored by sixteen thermocouples and one pressure transducer, mounted in contact to the fluid in the condenser region. Data analysis shows that the alternating wettability of tube sections strongly affects the flow motion, the start-up and the overall performance. In general, the alternating PHP presents a worse overall thermal performance: the thermal resistance is always higher and the start-up is achieved at higher heating power levels. Temperature at superhydrophobic surfaces exhibit a flat trend, suggesting that the flow was blocked in the functionalized area surface, while, for the hydrophilic inserts, more pronounced temperature fluctuations are observed. It is believed that the superhydrophobic coating hinders the liquid film formation, decreasing locally the flow motion. On the other hand, the enhancement of the inner wettability improves the flow motion, as the liquid film that covers the inner surface acts as lubricant.

CO2 absorption performance in a rotating disk reactor using DBU-glycerol as solvent

Abstract Gas–liquid mass transfer of rotating disk reactor was studied in CO2 absorption using 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU)-glycerol solution as solvent. Effects of the rotating disk structure and various operation parameters on the CO2 absorption rate and CO2 removal efficiency were investigated. The rotating disk with optimal holes is conducive to mass transfer of CO2 and the formation of thin liquid film at the opening increases the gas–liquid contact area. With the increase of rotating speed, the liquid flow pattern on the rotating disk surface changes from thin film flow to separated streams and creates extra liquid lines attached to the rim of the disk, which leads to a very complicated change on the CO2 absorption rate and CO2 removal efficiency. The overall gas-phase mass transfer coefficient increases 138% as the rotating speed increasing from 250 to 1400 r·min−1. Increasing temperature from 298 to 338 K can enhance the CO2 absorption rate due to lowering the viscosity of the solvent. The rate-determined step for the absorption is focused on the gas side. The rotating disk reactor can effectively enhance the absorption of CO2 with viscous DBU-glycerol solvents.

Simultaneous measurements of thin film thickness using total internal reflection fluorescence microscopy and disjoining pressure using Scheludko cell.

This work describes a method for measuring the thin film thickness using total internal reflection fluorescence microscopy, with the use of evanescent wave illumination. The thin liquid film was formed in a hole drilled at the center of a porous plate, which is used for measurement of the disjoining pressure by using the Scheludko cell method. The aim of simultaneous and in situ measurements of thin film thickness and disjoining pressure is to obtain the relationship between them, which is critical for explicitly depicting the thin film profile that determines the interfacial mass and heat fluxes in the thin film region near the triple line. This method can overcome the drawbacks of the optical methods that are insufficient for measuring the thickness of a thin film with curvature. The influence of structural forces formed by tracer nanoparticles seeded in the thin liquid film on the relationship was analyzed. The obtained expression for disjoining pressure vs thin film thickness provides a basis for analyzing the formation, evolution, and stability of the thin liquid film, which is the dominant mechanism of controlling the mesoscopic structure in many transport processes.

Numerical simulation of liquid film formation and its heat transfer through vapor bubble expansion in a microchannel

The evaporation of vapor bubbles inside a microchannel is important to realize a device with high cooling performance. The liquid film formed on the solid surface is essential for evaporative heat transfer from solid to fluid; its formation process and heat transfer characteristics need to be investigated. The expansion process of a single vapor bubble via evaporative heat transfer in microchannels was evaluated via a numerical simulation in this study. In the calculation model, the working fluid used was saturated FC-72 at 0.1013 MPa and the channel diameter was 200 µm. The superheat of the initial temperature field and wall were considered as parameters. To evaluate the heat transfer characteristics, the time variation of liquid film thickness was evaluated. The averaged liquid film thickness had a correlation with the capillary number. Additionally, the dominant heat transfer mode was estimated by decomposing the heat transfer rate into the heat-transfer rate through the liquid film, rear edge, and wake. When the superheat was low, the heat transfer mostly occurred via liquid film evaporation; the heat flux through the liquid film could be predicted using the liquid film thickness. On the other hand, in cases of higher superheat, owing to rapid expansion of the vapor bubble, no evaporative heat transfer occurred through the liquid film around the bubble head. It could be inferred from this study that the relationship between the thickness of the thermal boundary layer of the bubble and liquid film thickness is important for predicting the cooling effect of this phenomena. When the vapor bubble grows in the high superheat liquid, the rapid growth makes the liquid film thick, and the thick liquid film prevents the heat transfer between the liquid–vapor interface and heated wall.

Analysis of the spray wall impingement of urea-water solution for automotive SCR De-NOx systems

Selective catalytic reduction (SCR) is the most popular automotive exhaust after-treatment solution for nitrogen oxides (NOx) emission reduction. The mitigation of solid deposit formation is a big challenge for SCR system operation. Urea-water spray impingement is an important parameter which effects on solid deposit formation, should be investigated duly. This study presents an experimental investigation of urea-water spray impingement on the heated wall of automotive SCR systems for diesel engine exhaust gases. The investigation focused on the impingement conditions and distribution of the spray droplets, which are important parameters for system performance. This work was conducted with a commercial pressure driven SCR injector into an optically accessible test chamber at different wall temperatures and injection height. The spray impingement phenomena captured by means of Z-type shadowgraph high-speed Imaging technique. The injection quantity of the injector was measured to determine the injection rate. The high-speed imaging reveals in detail spray distributions, liquid film formation and urea crystallization. Relevant dimensions of spray distribution after the impingement have been estimated with digital image processing. The investigation reveals that high wall temperature produces swirl and bouncing that entrain rebounded droplets of the impinging spray on the wall, which leads to improved mixing. High temperature also causes longer spray front projection length which is an important factor for the mitigation of urea deposits.

Thermographic characterization of thin liquid film formation and evaporation in microchannels.

The science of transport in microchannels has greatly benefited applications ranging from micro-mixing, chemical synthesis and biological analysis to compact and efficient energy devices. One of the most critical and intricate phenomena in this field of science is the dynamics of thin liquid film formation during the flow of liquid and gas/vapor mixtures. These films can form in microseconds and be less than a micrometer thick, while dominating thermal transport in phase-change processes. Here, we report the captured details of these phenomena using a new measurement technique with unprecedented spatial and temporal resolutions of 20 μm and 100 μs, respectively. Thin films with thicknesses ranging from 1 to 20 μm forming around elongated bubbles over a capillary number range of 0.025 to 0.1 are characterized. The measurements suggest that these films thermally develop and evaporate at timescales in the order of 1-10 ms, two orders of magnitude longer than their formation timescale. The formation, reflow and evaporation of the liquid film constitute a complex dynamic involving variations of the film thickness over the periphery of a rectangular channel, leading to a thicker liquid film feeding (through lateral capillary wicking) a much thinner rapidly evaporating film. As a result, the thinner film dictates the rate of surface heat transfer while the thicker film determines the duration of thin film evaporation. A modified Bretherton model provides the best fit to the experimental results.

Heat Transfer and Friction Measurement in Pin-Fin Arrays Under Mist Flow Condition

Abstract Effects of air/water mist flow on endwall heat transfer in a square channel were experimentally investigated using infrared thermography. The purpose was to study the detailed heat transfer contour variation caused by the generation of the liquid films. The surface was roughened with staggered partial pin-fin arrays to enhance flow mixing and liquid entrainment. Two streamwise spacings (Xp/d = 3 and 6) of the fin array were investigated. The gas Reynolds number ranged from 7900 to 24,000. The calculated droplet deposition velocity was comparable to the literature results and was not substantially affected by the gas Reynolds number or fin spacing. For the pin-fin array, heat transfer was dominated by the water accumulation and liquid film formation, which was dependent on the carrier gas flow rate and fin spacing. Furthermore, thick liquid fragments entrained between the fins substantially enhanced local convective heat transfer coefficients. The average heat transfer enhancement on the finned surfaces using mist flow was four times as high as the air flow. The pressure drop from the mist flow was two times as high as the air flow.

Effect of liquid viscosity and surface tension on mass separation of shear-driven liquid film at a sharp corner

Formation of a thin liquid film along a wall that is driven by an adjacent high velocity gas has many applications such as liquid atomizers, fuel film transport in internal combustion engines, and refrigerant systems. At a geometric singularity like a sharp corner, the liquid film may remain attached to the wall or become separated depending on gas-liquid flow conditions. Mean film layer inertia and instabilities, which form large amplitude waves at liquid film interface are two mechanisms for the separation of shear-driven films from a sharp corner. Inertial force due to the mean film layer and the interface layer which includes large amplitude waves both influence the liquid mass separation at the corner. In this study, the effect of liquid film properties such as viscosity and surface tension on these processes and ultimately the liquid mass separation of the shear-driven liquid film from a sharp corner was investigated. Experimental results revealed that as liquid film viscosity decreased, more mass became separated from the sharp corner due to an increase in both large amplitude wave amplitudes and mean film layer inertial force. This study also showed that although liquids with smaller surface tension developed a thiner mean film layer and less large amplitude waves at the interface, the resultant high force imbalance between the destabilizing inertial force and surface tension restoring force led to a higher liquid mass separation at the sharp corner.

Numerical modeling of the mechanism of coarse droplets deposition on surfaces of a steam turbine nozzle blade cascade

The numerical model describing the interaction of coarse droplets with the surfaces of inter-blade channels is presented. This method uses a statistical approach to simulate the splashing process of primary droplet after its collision with the wall. This technique was verified by experimental investigation of wet steam flow in the studied nozzle blade cascade using the flow laser diagnostics system and PTV method. Numerical studies of wet steam were performed in order to investigate the structure of liquid particles streams and liquid film formation conditions in the nozzle blade passage. The influence of primary droplets inlet angle and steam density on the liquid phase parameters has been considered.

Anomalous Friction between Agar Gels under Accelerated Motion.

Understanding the friction phenomena on a gel surface under accelerated conditions is important for the designing of functional materials. However, there are few reports on friction under such conditions. In the present study, the effects of velocity, normal force, and gel hardness on the friction force were evaluated between two agar gels under sinusoidal motion. We found a friction phenomenon with an extremely low friction coefficient on the gel surfaces: the friction coefficient became less than 0.02 when sliding velocity increased. In addition, the profile of the friction coefficient was different between outward and homeward processes in the reciprocating sliding motion. In the outward direction, the low friction coefficient was maintained even if the sliding velocity decreased. On the other hand, the friction coefficient increased with sliding velocity in the homeward direction. This characteristic friction profile is caused by a long relaxation time on the gel surfaces. When the gel substrate is rubbed for a shorter time than the relaxation time, the morphology of the gel surface becomes unstable. Under such conditions, the formation and extinction of a thick liquid film can induce a super lubrication state and the asymmetric friction phenomena. These findings are useful not only for developing functional materials but also for understanding nonequilibrium phenomena in soft biological systems.

Mechanical Properties and Brittleness of Shale with Different Degrees of Fracturing-Fluid Saturation

The mechanical characteristics of Longmaxi Formation shale with different degrees of fracturing-fluid saturation were characterized by applying triaxial compression tests at a confining pressure of 50 MPa. The test samples were collected from fresh outcrop shale in Dayou, Chongqing, China and the shale brittleness was evaluated based on brittleness-drop coefficient, stress decrease coefficient and softening modulus. The weakening of related rock parameters of shale specimens being immersed in fracturing-fluid for different time periods was studied and discussed. The degree of deterioration of the peak strengths, elastic and softening moduli and brittleness were significant and varied exponentially when the samples were soaked in fracturing-fluid. The samples were found to fail by shear on the whole accompanied by varying degrees of bedding plane cracking. With increase of sample immersion time, the number of shear failure surfaces changes from one to two and finally to more than three. The length and number of cracks parallel to bedding planes increased gradually, however, no cracks were formed perpendicular to the bedding plane even when the shale was soaked for a long time. The weakening of the brittleness and mechanical parameters with sample fracturing-fluid saturation are mainly related to change of stress state at the crack tips caused by hydration swelling, the dissolution caused by alkaline fracturing-fluid and the formation of liquid film on the surface of shale particles, all of which are the results of mechanical–physical–chemical coupling.

A preliminary assessment on a two-phase steam condensation model in nuclear containment applications

Steam condensation in the presence of air is a vital heat transfer process in postulated loss of coolant accidents. To numerically evaluate local and global thermal hydraulic properties, several single-phase models have been developed in previous studies. However, potential mutual interactions between the liquid and gas phase (e.g. evaporation, radioactive material retention, etc.) impose requirements on two-phase models. Accordingly, the present work performed CFD evaluations on a two-phase boundary layer model (TPBL) which concerned the gas phase, liquid phase and their mutual interactions. Assessments were performed by comparing calculated results with the COPAIN and TOSQAN experimental data. The COPAIN cases show that predicted local and average heat fluxes are generally within 25% and 15% deviation, respectively. Detailed properties were obtained on the formation, distribution, and thermal resistance of liquid film as well as the model computational costs. In the TOSQAN cases, two steady states, including steam-air mixtures and steam-air-helium mixtures were considered. Via these cases, the applicability of the TPBL model in predicting local field profiles like velocity, temperature and concentration distribution was discussed. Results demonstrate that the calculated results overall agree well with the experimental ones. In general, the TPBL model is feasible in postulated accident analysis.