Unsteady Film Research Articles

Intensifying interfacial oscillations in falling film flows over rectangular corrugations

Unsteady film flows play an important role in intensifying heat and mass transfer processes, with applications, e.g., in falling film absorbers or reactors. In this context, the influence of surface structure modification on the wave dynamics of falling film flows is experimentally investigated based on localized film thickness time series data. Arrays of rectangular ridges oriented perpendicular to the main flow direction are considered, and an optimum ridge distance is identified, at which particularly strong interfacial oscillations are induced in the falling film. These potentially result from the interaction of the flow with a statically deformed base film under resonance-like conditions. The transient destabilization is amplified in the case of narrow ridge sizes, where inertia-driven flow features are particularly pronounced. With regard to mass transfer applications, the structure-induced increase in gas–liquid interfacial area may be of secondary importance compared to changes in internal flow conditions.

Investigation on the differences in unsteady film cooling behaviors of gas turbine blades between mainstream and cooling air pulsations for a cylindrical hole

In practice, film cooling on gas turbine blade inevitably works in an unsteady environment, which is introduced by periodical rotor/stator interaction and unsteady combustion. Previous studies have introduced two methods to simulate the realistic unsteady condition: 1) the pulsation of mainstream velocity; and 2) the pulsation of coolant injection. However, up to this point, the differences in instantaneous film cooling behaviors between these two methods remain unclear. This work presents a series of large eddy simulations to exhibit the unsteady flow and film cooling behaviors under steady and the two unsteady flow conditions. The numerical strategy is validated against our time-resolved experimental data. Time-averaged results show that the difference between the two pulsations is not significant if the averaged blowing ratio remains the same. However, the pulsation type plays a dominant role on the transient mode of film coverage. Under the steady condition, film coverage instability is induced by the unsteady trajectory of near-wall vortex structure; but with pulsed environments, the unsteadiness magnitude increases, and the area with high unsteadiness level enlarges. The pulsation of the mainstream velocity induces a more severe film coverage instability compared to the pulsation of the cooling air injection, because of the higher fluctuation energy of the mainstream bulk. Under mainstream pulsation, the probability distribution of instantaneous cooling effectiveness is the most scattered, and the corresponding fluctuation range is the largest.

Temporal evolution of demarcations between the unsteady and steady regions and between the film and meniscus during dip coating with vertical withdrawal of Newtonian fluids

Thus far studies on dip coating focus on the asymptotic layer thickness which is reached only in the lower part of the layer when the withdrawal time is sufficiently long. This work attempts to offer a complete description of the evolution of layer thickness profile with withdrawal time for vertical withdrawal of Newtonian fluids using simulation and the Buckingham π theorem. The results show that over time, the withdrawal process successively undergoes two regimes: (I) an unsteady film region and an unsteady meniscus region and (II) an unsteady film region connected to a steady meniscus region through an unsteady meniscus region. The controlling factors for the two regimes are identified at different values of capillary number and Reynolds number. Models are proposed to predict the heights of the demarcations between different regions. The models agree well with experimental results when the controlling forces are viscous force and gravity.

The Effect of Film Development on Primary Breakup in a Prefilming Airblast Atomizer

Abstract Liquid fueled gas turbines are likely to remain a dominant force in aviation propulsion for the foreseeable future, and therefore understanding the atomization process is key to meeting future emissions and performance legislation. To make experiments and simulations possible, simplified geometry and boundary conditions are often used, for example, simulations of primary atomization often use a fixed film height and velocity. This paper aims to quantify the effect of a fully developed unsteady film on the atomization process. A custom Coupled Level Set & Volume of Fluid (CLSVOF) solver with adaptive meshing built in OpenFOAM v9 is used. A simulation of the atomization process in the Karlsruhe Institute of Technology atomization experiment (Warncke et al., 2017, “Experimental and Numerical Investigation of the Primary Breakup of an Airblasted Liquid Sheet,” Int. J. Multiphase Flow, 91, pp. 208–224) is presented. A precursor simulation of the film development is used to provide accurate, temporally and spatially resolved inlet boundary conditions. These results are compared to previous CLSVOF simulations from Wetherell et al. (2020, “Coupled Level Set Volume of Fluid Simulations of Prefilming Airblast Atomization With Adaptive Meshing,” ASME Paper No. GT2020-14213)” using traditional boundary conditions. The unsteady film has doubled the modal ligament length and widened the distribution, and is now in better agreement with experimental measurements. A clear correlation in both time and space is observed between the film, atomization process, and spray. The sauter mean diameter (SMD) is significantly increased, again giving better agreement with the experiment. A discussion of extracting statistical descriptions of the spray is given, outlining the unfeasible computational cost required to converge droplet diameter distributions and other high order statistics for a case such as this.

Effect of pulsed coolant injection on unsteady film cooling characteristics with serrated trench design

To increase cooling effectiveness, film-hole designs with serrated trenches have been widely reported in the gas turbine industry. However, the unsteadiness level of the film coverage and the inevitable environmental pulsation are always neglected, which may lead to an unexpected failure of hot components by the instantaneous exposure to a high-temperature environment. In this work, a series of large eddy simulations are conducted to comprehensively evaluate the transient film cooling performance with serrated trench designs, and the effect of pulsed cooling air injection is discussed. Time-averaged results reveal that: 1) under steady condition, serrated trench design exhibits a higher film effectiveness in most regions compared to the traditional transverse trench, which supports the previously reported advantage. 2) Pulsed cooling air injection would cause a significant reduction of film effectiveness for the serrated trenches. Under the pulsed condition, film effectiveness with a serrated angle of 100° can be over 20% lower than that with a traditional transverse trench. Moreover, transient results indicate that under both steady and pulsed conditions, the serrated trenches always exhibit a lower steadiness level of film cooling, relative to the traditional transverse trench. Therefore, it should be prudent to suggest film cooling with serrated trenches in practical designs.

Self-Similar Solution to the Problem of Unsteady Film Condensation on a Vertical Semi-Infinite Plate

Self-Similar Solution to the Problem of Unsteady Film Condensation on a Vertical Semi-Infinite Plate

Numerical analysis of flow and heat transfer characteristics of unsteady film cooling of HPT shroud under high-speed rotation sweeping of the rotor blade

3D URANS simulations were performed to investigate the influence of high-speed blade rotation sweeping effect on the unsteady multi-hole film cooling performance of a high-pressure turbine (HPT) shroud. The blade rotation speed was 10,000 rpm and 15,000 rpm, the associated Strouhal numbers (St) were 2.35 and 3.52, and the blow ratio (M) was varied from 0.5 to 1.5. The instantaneous and time-averaged characteristics of film outflow mass rate, pressure distribution, film cooling effectiveness (FCE), and convective heat transfer coefficient (CHTC) of the HPT shroud with and without impinging jets were determined. The results show that the St and M have a substantial impact on the distribution of the FCE and CHTC on the shroud surface. At a high St and M, the FCE on the shroud surface is greater and more sensitive to the rise in M. As St increases, the suppression effect of the tip leaking vortex (TLV) on film and FCE on the shroud surface improved. The increased impingement jet effect has minimal impact on the FCE of the shroud. Under a high-speed blade rotation sweep, the net heat flux on the shroud wall with impingement/film cooling falls by approximately 22% when compared to the uncooled shroud surface.

Self-Similar Solution to the Problem of Unsteady Film Condensation on a Vertical Semi-Infinite Plate

Changes in the regimes of condensate film flow along the underlying surface in the presence of a body force are considered within the framework of a simplified physical and mathematical model of hydrodynamic and thermal processes occurring under unsteady film condensation on a vertical semi-infinite plate. The model is used to determine the applicability conditions of the zero-gravity approximation when considering film condensation. It is found that the validity of using the zero-gravity approximation to calculate the film thickness growth rate on any plate section depends not only on the magnitude of the body force, but also on time and distance between this section and the leading edge of the plate.

Thermocapillary thin film flow upon a porous heated stretchable cylinder

AbstractEarlier research work related to the unsteady flow of thin liquid film on a heated stretching cylinder has considered the boundary‐layer approximation under the assumption that the film thickness is thick. In this article, we have considered the full set of Navier–Stokes equations to study the unsteady film development over a porous heated stretching cylinder with the assumption that film height either coincides or lies within the boundary‐layer thickness. The effects of magnetic field, suction/injection, and cooling or heating of the cylinder have been considered for investigation. The governing set of coupled nonlinear partial differential equations is solved numerically by the implicit finite difference scheme (Crank–Nicolson). It was found that the film thinning rate diminishes with rising values of the porosity parameter and Hartmann number. It is also noted that the film thickness is enhanced for injection whereas opposite results are observed for suction through the porous cylinder. The thermocapillary effect showed that the thinning rate of the liquid film decreases or increases according to the surface of the cylinder being heated or cooled. It is observed that continuously increasing the stretching speed of the cylinder produces faster thinning of the liquid film. The temperature of the liquid film is the maximum at the surface of the cylinder and it decreases toward the free surface when the cylinder is heated. But the opposite phenomenon occurs when the cylinder is cooled.

On the Vapor Film Destabilization Mechanism during Unsteady Film Boiling

On the Vapor Film Destabilization Mechanism during Unsteady Film Boiling

Numerical investigation of unsteady film cooling on turbine blade squealer tip with pressure side coolant

Unsteady film cooling characteristic from pressure side coolant in squealer tip in turbine stage was investigated by numerical simulation. Due to unsteady flow in turbine stage, flow characteristics and blowing ratio of coolant from various holes fluctuate and change cooling effectiveness on squealer rim, cavity floor and side wall. In unsteady simulation, deterioration of coolant coverage causes unsatisfied cooling performance on squealer tip, which results in the maximum reduction of 18.3% and 20% of cooling effectiveness on squealer rim and cavity floor at specific instants compared to steady simulation. In different moments, pressure side coolant increases average heat transfer coefficient on pressure side squealer rim by 15% ∼ 47% and forms discrete distribution of high heat transfer coefficient there. Unsteady heat transfer on squealer rim is much intense compared to steady condition. Raising blowing ratio and decreasing cavity depth promote cooling effectiveness in tip cavity. Too small blowing ratio might lead to the risk of high temperature mainstream invasion into some pressure side film holes. Cooling effectiveness in tip cavity increases with cavity depth decreasing. Due to relative motion between blade tip and shroud, scraping vortex pushed rolling vortex more closely to pressure side and thus facilitates coolant to touch cavity floor.

Effects of Blowing and Density Ratios on Unsteady Film Cooling Effectiveness Using Fast-Response Pressure-Sensitive Paint

Abstract The transient effect of unsteady wakes on film cooling effectiveness using a fast response pressure sensitive paint (PSP) was studied. Testing was performed in a five-passage, low-speed wind tunnel using a scaled-up high pressure, blade profile. The blowing ratios studied include 0.4, 0.8, and 1.2 with a density ratio of 1.0 and 1.5 at each blowing ratio. An upstream wake generator produced Strouhal numbers of 0.1 and 0.3 while the wind tunnel inlet mainstream Reynolds number was held at a constant value of 300,000 for every test case. The results in this paper confirm the overall time averaged reduction in film cooling effectiveness with the addition of an unsteady wake while providing more detailed information on the transient response of the film cooling effectiveness, presented in the form of film cooling effectiveness RMS values. Results show that film cooling effectiveness RMS values increase on the leading-edge region of the blade and show little change on the pressure side of the blade with an increase in the Strouhal number. On the suction side, the area-averaged effectiveness RMS values decrease for the 0.8 and 1.2 blowing ratios and remain unchanged for the 0.4 blowing ratio. A change in the density ratio of the coolant from 1.0 to 1.5 has a minimal effect on area-averaged effectiveness RMS values for all areas of the blade.

Unsteady film cooling characteristics of the high-pressure turbine shroud with blade rotation in an aero-engine

Abstract Three-dimensional unsteady numerical simulations were conducted to investigate the detailed film cooling mechanism of the high-pressure turbine shroud with the first-stage turbine blade and guide vane for an aero-engine under the high-speed rotation of blades and rotor-stator interaction. The slip mesh was used to realize the relative motion between the rotating blade and the stationary turbine shroud. It is found that the coolant jet is alternately influenced by the hot mainstream, tip clearance leakage flow and leakage vortex due to the high rotational speed of blades. The film cooling characteristics of the turbine shroud significantly present an unsteady and periodic flow and heat transfer phenomenon. The insufficient cooling margin for film holes at the upstream of the blade leading edge can occur not only under high blowing ratios due to the coolant jet liftoff, but also at low blowing ratios due to the insufficient coolant flow rate as a result of the high exit pressure. A novel shroud cooling structure with coolant supply by the added throttle chamber is put forward, and expected to provide better thermal protection for the high-pressure turbine shroud near the leading edge of blades with no extra increase in the total mass flow rate of coolant.

Detached Eddy Simulation of the Unsteady Flow and Film Cooling Characteristics of an Endwall With an Interrupted Slot

Abstract The flows close to a high-pressure turbine endwall are highly three-dimensional and unsteady. These flows increase the aerodynamic loss and augment the heat transfer. In this paper, the unsteady flows and film cooling characteristics of an endwall were studied by using the Detached eddy simulation (DES). First, the unsteady flows and film cooling characteristics of a slot on a flat plate were studied at three different coolant mass flow ratios. A comparison of the experimental and numerical results showed that the DES offered an improved ability to capture the distribution of film cooling effectiveness. The aerothermal characteristics of the interrupted slot on a vane endwall were then studied. By analyzing the unsteady flow as well as the mixing process, it was found that the shedding vortices produced by the step lip played a dominant role in the film cooling distribution and the formation of the horseshoe vortex. The coupling of the shedding vortices with the horseshoe vortex had an important influence on the film effectiveness on the endwall. Finally, the effect of the axial distance between the interrupted slot and the vane leading edge on the formation of the horseshoe vortex was further studied. The instantaneous vorticity contours showed that varying the axial position of the interrupted slot affected the development of the horseshoe vortex. As the axial distance increased, the horseshoe vortex disappeared periodically and then reappeared. As the axial distance decreased, the horseshoe vortex retained a steady pattern upstream of the vane leading edge.

Spatiotemporal distributions of sweeping jet film cooling with a compact geometry

The spatiotemporal distributions of the coolant coverage behind the 777-shaped hole, sweeping jet (SJ), and compact SJ were quantified comprehensively. Nitrogen gas was selected as the coolant, and its blowing ratio (M) was set from M = 1.0 to 3.0. The fast pressure-sensitive paint technique was applied to measure the instantaneous, mean, and unsteady film cooling effectiveness of the three configurations, and these data are compared side-by-side with the 777-shaped hole. The measured velocity spectra demonstrated a close level of Strouhal number for the SJ (St = 1.6–1.7) and compact SJ (St = 1.5–1.6). Due to the dynamic nature, both the SJ and compact SJ exhibited a highly unstable cooling effectiveness over the surface. Their effectiveness values were found to be lower than the 777 hole at relatively low M, but the compact SJ surpassed it and showed the highest effectiveness (i.e., the best cooling performance) as M ≥ 2.0 due to the widest coolant spreading. Compared with the SJ, the effectiveness of the compact SJ was consistently higher, but its coherence of flow structure was reduced, as revealed by a proper orthogonal decomposition analysis. Further simulations vividly describe the flow structures and oscillating processes inside the sweeping actuators. The SJ with compact geometry exhibited a lower exit momentum and more uniform coolant coverage than the SJ, leading to augmented adiabatic effectiveness.

Nonlinear dynamic response and bifurcation of asymmetric rotor system supported in axial-grooved gas-lubricated bearings

Dynamic characteristics of the asymmetric rotor system supported in axial-grooved gas-lubricated bearings are studied. In order to solve nonlinear dynamic response of rotor system effectively, a hybrid numerical model is established by coupling the motion equation of rotor with the rational function model of the gas film forces. The rational function model of the gas film forces of gas-lubricated bearing is established based on vector fitting theory. By using the hybrid numerical model, the repeated calculations of the unsteady Reynolds equation and gas film forces are avoided; the continuous rotor trajectory and the dynamic gas film forces can be calculated simultaneously; and for the rotor system supported in the same bearings, the computing cost can be saved effectively. The nonlinear dynamic responses of asymmetric rotor system supported in axial-grooved gas-lubricated bearings are investigated by trajectory diagrams, frequency spectrum, Poincaré maps, and time series. The bifurcations are analyzed by the bifurcation diagrams with different rotating speeds and mass eccentricities. The dynamic behaviors of the asymmetric rotor system appear complex nonlinear dynamic phenomenon and specific bifurcation characteristics.

The intelligent networks for double-diffusion and MHD analysis of thin film flow over a stretched surface

This study presents a novel application of soft-computing through intelligent, neural networks backpropagated by Levenberg–Marquardt scheme (NNs-BLMS) to solve the mathematical model of unsteady thin film flow of magnetized Maxwell fluid with thermo-diffusion effects and chemical reaction (TFFMFTDECR) over a horizontal rotating disk. The expression for thermophoretic velocity is accounted. Energy expression is deliberated with the addition of non-uniform heat source. The PDEs of mathematical model of TFFMFTDECR are transformed to ODEs by the application of similarity transformations. A dataset is generated through Adams method for the proposed NNs-BLMS in case of various scenarios of TFFMFTDECR model by variation of rotation parameter, magnetic parameter, space dependent heat sink/source parameter, temperature dependent heat sink/source parameter and chemical reaction controlling parameter. The designed computational solver NNs-BLMS is implemented by performing training, testing and validation for the solution of TFFMFTDECR system for different variants. Variation of various physical parameters are designed via plots and explain in details. It is depicted that thin film thickness increases for higher values of disk rotation parameter, while it diminishes for higher magnetic parameter. Furthermore, higher values of Dufour number and the corresponding diminishing values of Soret number causes enhancement in fluid temperature profile. Further the effectiveness of NNs-BLMS is validated by comparing the results of the proposed solver and the standard solution of TFFMFTDECR model through error analyses, histogram representations and regression analyses.

Numerical study on unsteady film cooling performance of turbine rotor considering influences of inlet non-uniformities and upstream coolant

High-pressure turbine (HPT) rotor blades work in a harsh environment with high temperature and rotational speed. Affected by blade row interaction and inlet non-uniformities, the rotor flow field behaves with significant unsteady characteristics. In order to study the unsteady film cooling performance of the rotor blade, unsteady numerical simulations were conducted on the film-cooled first stage vane and blade of GE-E3 engine HPT. Time-resolved results of cases with hot streak (HS) and combined HS and swirl were compared to the case with uniform inlet total temperature and velocity to investigate the effect of inlet non-uniformities. A simulation with non-film-cooled stator vane (NFCS) was also studied to reveal the impact of stator film coolant. The results showed that unsteady film cooling effectiveness fluctuations at the leading edge (LE) region are the most significant, with the mean-to-peak fluctuation amplitude reaching over ±100%. LE cooling air unsteady excursion induced by the stagnation line oscillation is the main cause of this behavior. Vane coolant and inlet non-uniformities have shown a distinguished influence on the coolant attachment and migration at LE, thus affecting the film cooling effectiveness. At blade pressure side (PS) and suction side (SS), vane coolant and inlet non-uniformities affect the fluctuation amplitude of film cooling effectiveness by changing the static pressure perturbation behavior. Simultaneously, the existence of HS will decrease the SS time-averaged film effectiveness by a factor of 10%. In comparison, swirl has shown a negative effect on PS film effectiveness by a factor of 6%.

Heat transfer under quenching of cylindrical bodies in subcooled liquids

s.Quenching of high-temperature bodies in liquids is inherent for important technological processes. Despite a large number of experimental studies, there is yet no common understanding of an existence of the two distinct modes of the process. The physical nature of the intense cooling regime occurred at the surface temperatures thermodynamically excluding direct liquid/solid contact, requires new studies. This problem is very complex, as the process depends on plenty of factors (liquid subcooling, properties of a coolant and a cooled body, the surface characteristics). However, its solution is essential, since incipience of the intensive heat transfer regime of cooling means a sharp decrease of the process duration. This is actual in connection with the development of so-called the tolerant fuel for NPP, where there is a need to justify the behavior of new materials under the conditions of re-flooding of the core in the event of a loss of coolant accident. On this way, new purposeful experimental researches are necessary. The aim of this work is an experimental study of the effect of coating properties on unsteady film boiling. For this, cylindrical copper specimens coated with gold and Cr18Ni25 alloy were made. Cr18Ni25 coatings had different thickness and roughness. To analyze the effect of liquid properties on cooling processes, water and ethanol were used as a coolant. The experiments were carried out in a wide range of subcooling, both at atmospheric and at elevated pressures, the maximum subcooling for ethanol being 150 K. The experimental results revealed a strong influence on the quenching rate of the coolant physical properties and the thermal effusivity of a cooled body that is in agreement with the model developed by the authors earlier. The experimental results showed a strong influence on the process of both the physical properties of the coolant and the set of coating properties (thermal activity, thickness and roughness), which qualitatively agrees with the model developed by the authors earlier. An essential conjugated effect of the thickness and roughness of the low conductance coating is a principally new result, especially for ethanol as a coolant. This allows determining a direction of further theoretical and experimental researches, which have to account for the separate influence of the coating properties, thickness, roughness height and a liquid subcooling on incipience of the intense heat transfer regime.

Numerical Study on the Unsteady Film Cooling Characteristics of Turbine

In view of the main flow pulsation phenomenon and the new pulsating film cooling mode in the aeroturbine engine, the numerical calculation method is used to study the film cooling characteristics under the condition of the main flow velocity pulsation and cooling gas velocity pulsation with the classical circular film hole as the research object. In order to eliminate the influence of other factors on the film cooling characteristics of turbine blades as much as possible, a circular film hole of flat plate model similar to the small curvature section of turbine blades was established, and the influence of different cooling air pulse frequency, blowing ratio and mainstream pulse frequency on the film cooling characteristics was analyzed in depth. The results show that: with the increase of cooling air pulse frequency, the film cooling efficiency of cooling gas pulsation scheme can be achieved approximately the steadystate condition with blowing ratio of 1.0 while only half of the cooling air flow is consumed; at high blowing ratio, the cooling effect will be better when using the cooling air pulsating film cooling scheme compared with the steady-state scheme; in the main pulsating frequency range of the study, the frequency of main flow pulsation has little effect on film cooling efficiency.