Interaction Mechanism and Loss Analysis of Mixing between Film Cooling Jet and Passage Vortex

The interaction between the film-cooling jet and vortex structures in the turbine passage plays an important role in the endwall cooling design. In this study, a simplified topology of a blunt body with a half-cylinder is introduced to simulate the formation of the leading-edge horseshoe vortex, where similarity compared with that in the turbine cascade is satisfied. The shaped cooling hole is located in the passage. With this specially designed model, the interaction mechanism between the cooling jet and the passage vortex can therefore be separated from the crossflow and the pressure gradient, which also affect the cooling jet. The loss-analysis method based on the entropy generation rate is introduced, which locates where losses of the cooling capacity occur and reveals the underlying mechanism during the mixing process. Results show that the cooling performance is sensitive to the hole location. The injection/passage vortex interaction can help enhance the coolant lateral coverage, thus improving the cooling performance when the hole is located at the downwash region. The coolant is able to conserve its structure in that, during the interaction process, the kidney vortex with the positive rotating direction can survive with the negative-rotating passage vortex, and the mixture is suppressed. However, the larger-scale passage vortex eats the negative leg of the kidney vortices when the cooling hole is at the upwash region. As a result, the coolant is fully entrained into the main flow. Changes in the blowing ratio alter the overall cooling effectiveness but have a negligible effect on the interaction mechanism. The optimum blowing ratio increases when the hole is located at the downwash region.

A novel cooling hole inspection method for turbine blade using 3D reconstruction of stereo vision

Cooling holes (number 100~200, diameter 0.3~1.2 mm) are important heat dissipation structures of a turbine blade in aero-engine. Due to the small sizes and similar contours of cooling holes, it is difficult to extract and match their features by traditional stereo reconstruction methods. This paper proposes a novel cooling hole inspection method for turbine blades utilizing 3D reconstruction technique of stereo vision, which combines the stereo vision principle with the invariance of cross-ratio. The feature points of contours are extracted and matched by calculating two intersection points of circular contours and a line through the center points of adjacent cooling holes. Additionally, the 3D points corresponding to the feature points are reconstructed, by which, the diameters of cooling holes can be calculated successfully. Moreover, measurement experiments verify the effectiveness and accuracy of the proposed method. The experimental results show that the average errors of the standard circle ruler and tiny hole sample are within 0.05 mm, which satisfies the inspection requirement of cooling holes.

Large Eddy Simulations on Film Cooling Flow Behaviors with Upstream Turbulent Boundary Layer Generated by Circular Cylinder

Large eddy simulations on film cooling hole array on a flat plate was carried out to investigate upstream turbulence effect. Circular cylinders were configured to create a turbulent boundary layer and its diameter has been adjusted to generate 13% upstream turbulence intensity in the main flow. Due to the small pitch to diameter configuration of the cylinder, two-dimensional LES analysis was carried out in advance and the results showed that LES was an essential method to resolve flow field around and downstream circular cylinder, which was not available in RANS simulations. The three-dimensional LES results showed reasonable agreement in turbulence intensity and normalized velocity distributions along the vertical with measured data. According to the blowing ratio, the cooling flow coverage on the surface along the stream-wise direction was varied and well agreed with measured data. Additionally, upstream boundary flows were partially ingested inside the cooling hole and discharged again near along the centerline of the cooling hole. This accounted for film cooling effectiveness distribution inside the cooling hole surface and along the centerline. The current study revealed that the LES for predicting turbulent boundary layer behaviors due to upstream turbulence generation source was an effective and feasible method. Moreover, the LES effectively resolved flow fields such as film cooling flow behaviors and corresponding film cooling effectiveness distributions.

EVALUATION OF THE RANS TURBULENCE MODEL PREDICTIVE CAPABILITY OF FLAT PLATE FILM COOLING

The two-equation turbulence models used for the present study are the commonly used standard k-ॉ model and k-ω model. In order to achieve this target, numerical simulation was initiated in Ansys Fluent to simulate a flow over a flat test surface with a diameter of 4mm straight, circular film cooling hole at angled injections of 25°, 30°, 35°and 40°. The comparison between the numerical calculations and the theoretical results showed the standard k-ω turbulence model gave better predictions against those with the standard k-ω turbulence models. The ability of k-ω model in closely predicting the cooling behavior is due to the precise modeling of the lateral spreading of the film. The isotropic two-equation turbulence models exhibited a huge dissent. The results also indicated that increasing the mass flow rates in the mainstream channels reduces the temperature distribution along the stream-wise direction.

Effect of thermal barrier coating on thermal load of the rotor of the small aviation wankel engine

Due to its high-altitude, low-temperature, high-load, and air-cooled working environment, small aviation rotary engines have problems such as large component load and low heat dissipation efficiency. As the main moving part of the engine, the rotor is continuously exposed to the complex temperature field of the engine. As an effective high-temperature protective coating, the thermal barrier coating can isolate the heat load generated by the work of the combustion chamber and effectively improve the complex work condition of the triangular rotor. This paper takes the triangular rotor of a small aviation Wankel engine as the research object, and establishes the finite element model of the rotor and the coating. The engine thermodynamic simulation model is established by Simulink, and the combustion chamber temperature and heat transfer coefficient are calculated. The heat transfer coefficients of the other surfaces of the rotor were calculated by series thermal resistance, which were used as boundary conditions for finite element analysis of the rotor and the coating. The temperature field, stress field and deformation of the rotor before and after processing the thermal barrier coating are compared. The results showed that after the thermal barrier coating, the temperature of the rotor will drop by about 50K on average. The temperature of the pit and cooling hole of the rotor will drop by 17K and 16K respectively, and the temperature of the inner edge and side end surface of the sealing groove will drop by about 10K. The stress values at the inner side of the rotor seal groove, the inner cavity cooling hole, and the inner hole of the rotor are reduced by about 35.4MPa, 29.4MPa, 33.4MPa, respectively, and the stress value at the bonding layer is 150MPa, which is significantly higher than the stress value at the corresponding position of the original rotor, indicating that there is stress Concentration phenomenon. At the same time, the deformation at both ends of the rotor seal groove is reduced from 61.92μm to 52.55μm, and the difference in the axial deformation of each position is less than 3mm. It can be obtained that the thermal barrier coating can effectively reduce the radial deformation of the rotor and has little effect on the axial deformation of the rotor.

The Assessment of the Maximum Heat Production and Cooling Effectiveness of Three Different Drill Types (Conical vs. Cylindrical vs. Horizontal) during Implant Bed Preparation—An In Vitro Study

The aim of this experimental study was to verify thermal diffusion differences, by measuring the maximum temperature achieved with different drill shapes. Synthetic bone blocks of type I density made from solid rigid polyurethane (PUR) foam were used to perform the drilling procedures. The experiment was conducted at three different rotation speeds: 800, 3000 and 5000 rpm. Conical drills (with and without an internal cooling hole) were compared with horizontal drills and disc drills. The temperature during drilling for implant bed preparation was estimated with the use of thermocouples and an infrared (IR) camera. The temperature during drilling with disc cutters for lateral basal implants did not exceed 33 ∘C and the temperature decreased in proportion to higher drill speed. The results indicate that the tested design is safe and will not cause bone overheating.

THE DUFOUR EFFECT IN FILM COOLING EXPERIMENTS WITH FOREIGN GASES

In typical film cooling experiments, the adiabatic wall temperature may be determined from surface temperature measurements on a low thermal conductivity model in a low temperature wind tunnel. In such experiments, it is generally accepted that the adiabatic wall temperature must be bounded between the coolant temperature and the freestream recovery temperature as they represent the lowest and highest temperature introduced into the experiment. Many studies have utilized foreign gas coolants to alter the coolant properties such as density and specific heat to more appropriately simulate engine representative flows. In this paper, we show that the often ignored Dufour effect can alter the thermal physics in such an experiment from those relevant to the engine environment that we generally wish to simulate. The Dufour effect is an off-diagonal coupling of heat and mass transfer that can induce temperature gradients even in what would otherwise be isothermal experiments. These temperature gradients can result in significant errors in calibration of various experimental techniques, as well as lead to results that at first glance may appear non-physical such as adiabatic effectiveness values not bounded by zero and one. This work explores Dufour effect induced temperature separation on two common cooling flow schemes, a leading edge with compound injection through a cylindrical cooling hole, and a flat plate with axial injection through a 7-7-7 shaped cooling hole. Air, argon, carbon dioxide, helium, and nitrogen coolant were utilized due to their usage in recent film cooling studies.