Transverse Trench Research Articles

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.

Drag reduction on micro-trench and micro-post superhydrophobic surfaces underneath a motorboat on the sea

Superhydrophobic (SHPo) surfaces can capture a thin layer of air called a plastron under water to reduce skin friction. Although a ~30 % drag reduction has been recently reported with longitudinal micro-trench SHPo surfaces under a boat and in a towing tank, the results lacked the consistency to establish a clear trend. Designed based on Yu et al. (J. Fluid Mech, vol. 962, 2023, A9), this work develops and tests a series of high-performance SHPo surface coupons that can sustain a pinned plastron underneath a passenger motorboat revamped to reach 14 knots. Importantly, plastrons in a pinned state, not just their existence, are confirmed during flow experiments for the first time. All the drag-reduction data measured on different longitudinal micro-trenches are found to collapse if plotted against slip length in wall units. In comparison, aligned posts and transverse trenches show less and little drag reduction, respectively, confirming the adverse effect of the spanwise slip in turbulent flows. This report not only verifies SHPo surfaces can provide a consistent drag reduction at high speeds in open sea but also shows that one may predict the amount of drag reduction in turbulent flows using the physical slip length obtained for Stokes flows.

Film Cooling of a Convex Surface with Coolant Supply Through Two Rows of Transverse Trenches

Film Cooling of a Convex Surface with Coolant Supply Through Two Rows of Transverse Trenches

Impact of external turbulence on film cooling designs using large eddy simulations

Large Eddy Simulations (LES) of film cooling flows were conducted to investigate the impact of high turbulence intensity in the main flow, representative of jet-engine combustor conditions. A comparison was made with LES simulations at zero turbulence at the main flow inlet boundary. The study investigated three different film cooling configurations, which involved cylindrical holes and two variations with holes embedded in trenches known as transverse and segmented trench. Different momentum ratios (I=3.5 and 8.3) relevant to combustor flows in jet-engines were applied. The LES simulations utilized OpenFOAM and solved the energy and passive scalar transport equations to determine film cooling efficiency (η), heat transfer coefficient (hf), and net heat flux reduction (NHFR). The numerical results were validated through comparison with experimental data. The mean and instantaneous results revealed that high turbulence intensity in the main flow significantly influenced film cooling performance. The film cooling efficiency was found to decrease with increased turbulence intensity, while for the ordinary film cooling design exhibiting jet lift-off, the increased mixing led to higher η. The heat transfer coefficient increased with turbulence intensity and momentum ratio, with turbulence intensity having a greater impact on ordinary designs and momentum ratio on trenched designs. Moreover, increasing turbulence intensity or momentum ratio resulted in lower NHFR for all designs. Hot spots were observed in the instantaneous results, with spatial variations depending on the film cooling design.

Study of thermal efficiency of a wall gas jet blown through the holes in the transverse trench

Study of thermal efficiency of a wall gas jet blown through the holes in the transverse trench

A combined cooling structure consisting of serrated trench and V-shaped surface for film holes: Flow structure and effectiveness improvement

Film cooling is a commonly used thermal protection technology for turbine airfoils. However, due to the counter-rotating vortex pair (CVP) generated by the interaction between the cooling jet and the mainstream, the coolant gradually breaks away from the wall, which is particularly evident at high blowing ratios. For the sake of improving the film cooling effectiveness (FCE) under high inlet temperature conditions, a novel film cooling structure (SerrTrench-VSS) that combines serrated trench film holes with shark skin-inspired V-shaped surface (VSS) is proposed. The RANS method and realizable k-ε model are used for the simulations. The flow characteristics, adiabatic FCE, and resistance loss are analyzed in detail when the blowing ratio (BR) is 0.5–1.5 and then compared with the traditional cylindrical film hole and transverse trench film hole. In addition, the impact of VSS height on the cooling performance is investigated and the best height of the structure is determined. The results show that the SerrTrench-VSS has better FCE. The serrated trench can destroy the CVP and improve the spanwise spreading ability of the coolant. Also, the VSS can converge the cooling jet downstream of the film hole toward the centerline, thereby improving its extension ability along the flow direction. In the studied range of blowing ratio, the SerrTrench-VSS has the best FCE, and the spanwise-averaged FCE increases by up to 10.50% when BR = 0.5. The empirical correlations between the globally-averaged FCE and the height ratio within the range of the present study are also given. For the serrated trench, the cooling jet has the best spanwise spreading and flow extension ability when the VSS height is 0.5D. The proposed novel film cooling structure (SerrTrench-VSS) can effectively improve the FCE and provide feasible means for the efficient cooling of a turbine blade under practical conditions.

Large Eddy Simulation of trenched cylindrical film hole with backward compound angles

Many previous studies have demonstrated that both transverse trench and backward cooling injection are capable of improving jet lift-off problem of traditional cylindrical film hole at high blowing ratios. However, few studies have attempted to combine the above two cooling schemes to further alleviate the cooling deterioration phenomenon. This paper proposed two trenched film holes with different backward compound angles of β = 180° and 135°, and used highly-resolved Large Eddy Simulation method to explore in detail their unsteady flow fields including vortex structures, level of velocity fluctuation, turbulent shear stress, distributions of vorticity and adiabatic wall cooling effectiveness. The results are compared against those of the benchmark trenched film hole with β = 0°. The simulations were carried out under blowing ratio of 1.5, and the coolant to mainstream density ratio of 2.0.The detrimental symmetric counter-rotating vortex pair shown in the trench with β = 0° hole is replaced by an asymmetric vortex pair in the trench with β = 180° hole and only a single asymmetric vortex in the trench with β = 135° hole. Inside the trench, hairpin-like vortices are the dominant vortex structures for the trench with backward film holes. In the region downstream of the trench, the trench with β = 180° hole obtains the highest level of three velocity components fluctuations in the central region because of the direct intense collision between the coolant and mainstream, while the trench with β = 135° hole shows a highest level of lateral velocity component fluctuation in the lateral sides and a highest level of turbulent shear stress in the near wall region due to the significantly enhanced lateral motion of the coolant. The trenches with β = 180° and 135° holes significantly improve the lateral uniformity of the coolant, and respectively provide constantly high levels of 0.21 and 0.3 in laterally averaged cooling effectiveness on the wall downstream of the trench, compared to the trench with β = 0° hole with a low level of less than 0.2.

Evaluation of mist/air film cooling of C3X vane with serrate-type trenched holes

• Film cooling of C3X vane with serrate-type trenched holes using mist/air mixture under practical conditions is evaluated. • Both serrated transverse trench structure and addition of mist to air can significantly improve film cooling performance. • Replacing the trench edge by a double-curvature lip structure can further improve the film cooling effectiveness. • Film cooling effectiveness of the vane is reduced when the angle of the serrated trench is too large. • Vortex for the six trenched holes intensifies with the increase in the mass flow rate . Serrate-type trenched cooling holes have shown good film cooling performance for the cooling of turbine vanes using coolant air. Mist-assisted film cooling has also exhibited great potential for efficient cooling of a turbine vane. The combination of them may create a substantial improvement of cooling performance, which is explored in this simulation. This work studies the influence of serrate structure on the cooling performance of trenched holes for the C3X turbine vane under practical conditions by examining the flow and heat transfer characteristics of the mist/air mixture. The results prove the concept proposed for significantly improving the cooling performance. At the mass flow ratio R c = 0.25% ( M = 0.5) and R c = 0.50% ( M = 1.0), replacing the trench edge by a double-curvature lip structure can further improve the film cooling effectiveness. The film cooling effectiveness of the vane is reduced when the angle of the serrated trench is too large and the vortex for the six trenched holes intensifies with increasing mass flow ratio. The comparison of streamlines distribution and vortex structures at different blowing ratios by the Q criterion for the trenched holes in the flat plate reveals why the serrated trench structure can improve the cooling effectiveness: it can generate anti-counter-rotating vortices, making it difficult for the coolant film to separate from the wall. Although it is worth of further study on this novel design of serrated trench with mist/air film cooling under engine-representative conditions, this work provide heuristic guidance for the cooling of turbine vanes.

Experimental investigation of hole-geometry effect on unsteady characteristics of film cooling at turbine vane leading edge

In present work, the hole-geometry effect on the unsteady characteristics of film cooling at leading-edge of a turbine vane was experimentally studied under blowing ratios (BRs) from 0.5 to 3.0. The typical double-row staggered layout was applied in the stagnation zone. Selected hole-geometries include the cylindrical-hole and fan-shaped-hole, as well as the cylindrical-holes in a transverse trench and an individual crater. Through employing the infrared thermography and planar quantitative light sheet techniques, the unsteadiness of surface film effectiveness and in-plane jet-concentration were analyzed. The level of cooling unsteadiness was estimated by the standard deviations of these scalar parameters. A comparison of both techniques revealed that the flow unsteadiness at the interface of mainstream and cooling air jet is the essence of surface film effectiveness fluctuation. Under small BRs, the fan-shaped-hole displays the best film coverages; however, adding the transverse trench can generate the highest film effectiveness under large BRs. Relative to the cylindrical-hole, the maximum increments in area-averaged effectiveness can be above 10% for both the new designs. The transverse trench design can generate the lowest-level cooling unsteadiness, and improve evidently the uniformity of film effectiveness over entire leading-edge. However, two challenges for trench design should be concerned. The first was the faster decay in film effectiveness, in comparison with the fan-shaped-hole jets. The second was the high-level cooling unsteadiness on the sidewalls of trench.

Film cooling performance enhancement of serrate-type trenched cooling holes by injecting mist into the cooling air

Mist-assisted film cooling has exhibited great potential for efficient cooling of a turbine vane due to the vaporization of water droplets flowing with the air. To further improve the film coverage and cooling effectiveness, it is necessary to find a structure that allows the coolant flow to expand laterally and inhibits the separation of the coolant jet from the wall. Therefore, we propose in this work to study the influence of serrate structure on the cooling performance of trenched holes by examining the flow and heat transfer characteristics of the mist/air mixture. As water evaporation is involved in this process, the dry-bulb temperature may not be suitable for evaluating the film cooling performance. Therefore, a wet-bulb-temperature-based film cooling effectiveness (WFCE) is proposed. The results show that the included angle of serrate plays a significant role in WFCE. In addition, the area-average WFCE of the trenched holes increases first and then decreases as the blowing ratio increases. The concentration of mist and the diameter of droplets are found to be crucial for significantly affecting the WFCE. The relationship between the film cooling effectiveness and the contribution of mist is explicated by empirical correlation with the mist concentration and droplet diameter. The empirical correlation, especially the exponential increasing trend with the square root of mist concentration at a constant droplet diameter, provides the basis for further theoretical analysis of the mechanism of cooling effectiveness improvement. Under the practical condition, it is also proved that the serrated transverse trench structure and the addition of mist to the coolant air can improve significantly the film cooling performance for the turbine vane.

Predicting and optimizing multirow film cooling with trenches using gated recurrent unit neural network

The film cooling of cylindrical holes embedded in transverse trenches under superposition has shown promise for protecting the critical components of a high-pressure turbine from thermal damage. To optimize the relevant parameters and provide a suitable film cooling strategy, it is important to predict the effectiveness of lateral-averaged adiabatic film cooling with the trench effect on the surface of a blade. However, high-fidelity semi-empirical correlations for film cooling under superposition conditions with a trench have rarely been examined. This study establishes a gated recurrent unit (GRU) neural network model to predict the effectiveness of lateral-averaged film cooling under multiple-row superposition conditions with a trench. In general, a GRU neural network model is built with a large sequence of one-dimensional parameters, including the depth and width of the trench, compound angle, location of the hole, and blowing ratio. The computational fluid dynamics (CFD) method is used to provide a training dataset for the model. After careful testing and validation, the results predicted by the GRU agreed well with the CFD results. Moreover, the performance and robustness of the GRU were better than those of other recurrent neural network models, such as the long short-term memory model. Integrated with the GRU model, the sparrow search algorithm was adopted to optimize the parameters of the trench. The film cooling effectiveness of the optimized case improved by 1.6% compared with the best case, 28.5% compared with the worst case in dataset, and 23.5% compared with the no-trench case.

Effect of length-to-diameter ratio on film cooling and heat transfer performances of simple and compound cylindrical-holes in transverse trenches with various depths

Advances in trenched-hole film cooling result in a fact that short-holes exposing in a transverse trench have a significantly large potential for application in designs of future double-wall cooled blade. In present work, combined influences of hole-length, trench-depth, compound angle and cooling air flowrate on the film effectiveness and heat transfer features of trenched-holes were discussed deeply. Film effectiveness and heat transfer coefficient were measured by an infrared thermal imaging system. Numerical simulation as an auxiliary tool was conducted to provide necessary flow details. Two typical length-to-diameter ratios of L/D = 2 and 5, three trench-depths of H = 0.5D, 0.75D and 1.0D and two compound-angles of film-hole of 0° and 45° were designed. Cooling air flowrate was controlled in a relatively large range from 0.5 to 4.0 in averaged blowing ratio (BR). The results revealed the various jet-mechanisms caused by L/D while relatively regular variations of turbulent intensity of jets. Thus, the complicated trends in film effectiveness while regular trends in heat transfer coefficient can be clearly captured. The combined influences of trends above can bring to the more undesirable thermal protections for the short-hole jet. The L/D-induced decrement in area-averaged net heat flux reduction (NHFR) can reach 60% at most. To effectively improve film coverage and NHFR, the short-holes were properly embedded in the deeper trench. And the sensitivity analysis of trench-depth to discharge coefficient showed the negligible variation below 5%. The NHFR of the shallowest trench can be improved through application of compound angle, inducing an enlarged aerodynamic loss below 10%.

Study on Film Cooling Performance of Round Hole Embedded in Different Shaped Craters and Trenches

Film cooling effectiveness can be improved significantly by embedding a round hole in trenches or craters. In this study, film cooling performances of a transverse trench, W-shaped trench and elliptic trench were compared and analyzed in detail. The CFD models for trench film cooling were established and validated via the experimental results. Inside the transverse trench, a pair of recirculating vortices is formed, which promotes the coolant spreading in a lateral direction. The decrease of trench width and increase of trench depth both improve the film cooling effectiveness of the transverse trench. For the W-shaped trench, the guide effect of the corner angle further improves the lateral spreading capability of coolant and generates higher cooling effectiveness than a transverse trench with the same depth and width. The flow characteristics of the elliptic trench are similar to that of the round hole, and the kidney vortex pair takes a dominant role in the flow fields downstream of the coolant exit. Accordingly, the elliptic trench generates the worst cooling performance in these shaped trenches. The increase of trench depth and decrease of trench width both result in an increase of the discharge coefficient for trench film cooling. For the W-shaped trench, the increase of the corner angle causes a decrease of the discharge coefficient. For the elliptic trench, the discharge coefficient increases with the decrease of the elliptic aspect ratio (major axis/minor axis).

Numerical evaluation of film cooling performance of transverse trenched holes with shaped lips

In this work, the influence of several lip structures on the effectiveness of film cooling and the flow fields of transverse trenched holes at different blowing ratios (BRs) ranging from 0.5 to 2.0 is numerically investigated, using the Reynolds averaged Navier Stokes method with the realizable k-ε turbulence model. The trenches include the traditional transverse trench, the bevel trenches, and the fillet trenches. It shows that the trenched holes produce weaker counter-rotating vortex pairs, which can enhance the effectiveness of film cooling compared with the standard cylindrical film hole. It demonstrates that the lip structures can improve the effectiveness of film cooling of the trenches at low BRs, but it has a negative effect at high BRs. It is found that the lower lip structures have less influence on the effectiveness of film cooling at different BRs than the upper lip structures. It is also found that the upper part of the fillet structure employs the Coanda effect to make the coolant flow close to the wall, thereby improving the effectiveness of film cooling; while the lower part of the fillet structure is conducive to converting the coolant flow into a vertical jet, thereby improving the effectiveness of film cooling at low BRs.

Effect of transverse trench on film cooling performances of typical fan-shaped film-holes at concave and convex walls

By means of a time-resolved infrared thermal imaging system, the effect of transverse trench on the film cooling performances of a fan-shaped film-hole at typical concave and convex walls was discussed deeply, from a combined view of the time-averaged parameter evaluation and cooling unsteadiness level analysis. Harmful cooling unsteadiness with high level can be estimated by large standard deviation (SD) of transient film effectiveness, which is generated by the intensified temporal evolutions of vortex-footprints at wall. Comparisons of the time-averaged experimental results revealed the design of trench at curved walls can effectively improve the film effectiveness, completely different from the phenomenon of previous flat-plate studies. The increments in area-averaged effectiveness are nearly non-sensitive to cooling air flowrate, which can reach 27% and 16% at the concave and convex walls, respectively. Meanwhile, the trench can cause the relatively small aerodynamic loss at curved walls. Analysis of cooling unsteadiness can provide the new horizon to estimate the benefits of film cooling design. Thus, the transverse trench structure is strongly suggested at convex wall, due to the reduced level of cooling unsteadiness; however, the improved level of cooling unsteadiness can impede the direct application of transverse trench at concave wall. Steady numerical simulations were also conducted to provide the detailed knowledge of flow mechanisms. The variations of intensities of vortex-system result in the different jet reattachments downstream of trench caused by wall curvature. The difference of cooling unsteadiness levels is mainly determined by the turbulence quality of impingement to trench-edge. An additional information from the present study was introduction of trench can obviously change the trends in film effectiveness and SD-contour with wall curvature. The variations of turbulence dissipation cause a break of the wall curvature effect on film effectiveness at blowing ratio (BR) of 1.5, indicting the highest effectiveness is acquired by the convex model under small BRs whereas by the concave model under large BRs.

Structure of the flow in the near-wall gas jet injected through circular holes in a transverse trench

Structure of the flow in the near-wall gas jet injected through circular holes in a transverse trench

On the effect of geometry of w-wave trenches on film cooling performance of gas turbine blades

In the present study, three types of w-wave trenches with different amplitude configurations are compared with transverse trench (TT), and the use of variable radius fillet (VRF) on downstream lips at different blowing ratio is numerically investigated to measure heat transfer coefficient, and cooling effectiveness. The numerical results are obtained by three-dimensional Reynolds average Navier–Stokes equations (RANS) while employing shear stress transport turbulence models, which are validated by comparing with experimental data. The trench width is kept constant in all cases, yet the three different amplitudes and variable fillet radiuses offered a variety of designs in trench film cooling. The results showed that w-wave trenches impressively improved film cooling effectiveness over the transverse trench, and utilizing fillets at downstream lips of the trench caused significant enhancement on both lateral averaged and centerline cooling performance. Due to the w-wave trench configuration, anti-counter-rotating vortices responsible for pushing coolant film toward the near-wall were formed throughout of downstream wall of the trench, and the cooling flow thus had a more uniform structure. The heat transfer coefficient distributions of filleted w-wave trenches are observed to be more uniform than simple w-wave and transverse trench under all blowing ratio conditions. Moreover, enlargement of the fillet radius in Cases 2 and 3 yielded to the growth of centerline coolant flow, which in turn resulted in the improvement of film cooling effectiveness at all blowing ratios.

Experimental study of the effect of a transverse trench depth on film cooling effectiveness

The experimental results of film cooling effectiveness of a wall gas jet injected through inclined cylindrical holes into a transverse trench are presented. Measurements were carried out using IR thermography. The results of film cooling effectiveness through holes embedded into a trench were compared with the traditional well-known method of cooling with injecting without a trench. The thermal efficiency of the wall jet is much higher when injecting through the holes into the trench than the corresponding value when the wall jet is fed through inclined holes without a trench (the maximal value is more than 3 times higher). The particular advantage of this method of coolant supply is achieved at high injection parameters. The results of our measurements are compared with the experimental and numerical data of other authors, and a reasonable agreement is obtained.

Heat Transfer in Highly Turbulent Separated Flows: A Review

The study of flows with a high degree of turbulence in boundary layers, near-wall jets, gas curtains, separated flows behind various obstacles, as well as during combustion is of great importance for increasing energy efficiency of the flow around various elements in the ducts of gas-dynamic installations. This paper gives some general characteristics of experimental work on the study of friction and heat transfer on a smooth surface, in near-wall jets, and gas curtains under conditions of increased free-stream turbulence. Taking into account the significant effect of high external turbulence on dynamics and heat transfer of separated flows, a similar effect on the flow behind various obstacles is analyzed. First of all, the classical cases of flow separation behind a single backward-facing step and a rib are considered. Then, more complex cases of the flow around a rib oriented at different angles to the flow are analyzed, as well as a system of ribs and a transverse trench with straight and inclined walls in a turbulent flow around them. The features of separated flow in a turbulized stream around a cylinder, leading to an increase in the width of the vortex wake, frequency of vortex separation, and increase in the average heat transfer coefficient are analyzed. The experimental results of the author are compared with data of other researchers. The structure of separated flow at high turbulence—characteristic dimensions of the separation region, parameters of the mixing layer, and pressure distribution—are compared with the conditions of low-turbulent flow. Much attention is paid to thermal characteristics: temperature profiles across the shear layer, temperature distributions over the surface, and local and average heat transfer coefficients. It is shown that external turbulence has a much stronger effect on the separated flow than on the boundary layer on a flat surface. For separated flows, its intensifying effect on heat transfer is more pronounced behind a rib than behind a step. The factor of heat transfer intensification by external turbulence is most pronounced in the transverse cavity and in the system of ribs.

Study on leading-edge trenched holes arrangement under real turbine flow conditions

Turbine guide vane faces extremely high thermal load, especially on the leading edge region. In order to enhance cooling performance, the present study investigates the effects of leading-edge film cooling arrangements by adiabatic and aero-thermal coupled simulation. The three-row cooling design of the 25° inclined angle with the trenched holes is used to replace five-row showerhead holes. The model is employed after conducting the numerical validation. The results show that at coolant mass flow of 0.00171 kg/s, three-row showerhead film cooling with trenched holes can provide higher laterally averaged adiabatic cooling effectiveness than three-row round holes. It can also improve cooling performance in most of the leading-edge regions, compared with five-row showerhead cooling arrangements. The three-row cooling design with trenched holes can still obtain the highest area-averaged effectiveness on either the suction side or the pressure side of the leading edge. Besides, the trenched holes are also beneficial to the reduction of the aerodynamic losses. In the aero-thermal coupled simulation, the improvement of the three-row cooling design with trenched holes decreases because of the internal cooling effect. It is also proved that the transverse trench would not lead to a local high temperature region on the metal surface. The trenched configuration above can be used on the first-stage turbine vane to increase turbine inlet temperature, improve engine efficiency, and prolong engine life.