Internal Channel Flow Research Articles

Multi-Objective Optimization of Convective Heat Transfer for a Composite Internal and Film Cooling Structure

Abstract This paper conducted a multi-objective optimization work for a composite internal and film cooling structure. The pitch-to-height ratio of the ribs, the inclination angle of the ribs, and the inclination angle of the film hole are chosen as the three design variables to enhance the heat transfer performance, improve the film cooling effectiveness and reduce the pressure loss of the internal channel flow. During the optimization process, the Latin hypercube sampling method is adopted to select 26 sample points from the design space. The response values with higher fidelity at the sample points are calculated using computational fluid dynamics (CFD) simulations. Among the 26 sample points, 21 are used to construct a surrogate model of each objective function while the rest of them are adopted to validate the correctness of the established surrogate model. By combining the Kriging surrogate model with a nondominated sorting genetic algorithm, the Pareto optimal front is obtained after the optimization process. Finally, comparison and analysis are conducted with respect to the cooling performance and mechanisms between the reference model and the selected three representative optimized models. Results show that the optimized three models can not only improve the film cooling effectiveness but also reduce the pressure loss of the channel flow and enhance the heat transfer. In addition, it is found that the optimized model induces an anticlockwise rotating vortex, which entrains more coolant near the target surface. The inclined ribs of the optimized models induce a secondary flow along the inclined ribs, which enhances the flow mixing and augments the heat transfer performance.

Vortex dynamics based analysis of internal crossflow effect on film cooling performance

Film cooling is an important technology in protecting the turbine blade of modern aero-engine and gas turbine. Besides the widely studied parameters including the cooling hole geometry and blowing ratio, film cooling effectiveness is also affected by the crossflow in the internal cooling channel, which causes off-design inflow directions of the cooling hole. In-hole counter-rotating vortex pair is observed which accounts for the effect of internal crossflow on the film cooling. The current paper numerically studies the effect of internal channel flow on the film cooling performance by comparing the results of three configurations. A theoretical model about the boundary layer deformation is proposed to explain the formation of in-hole vortices based on the vortex dynamics, and two comparative cases with slip and no-slip channel wall respectively are used to highlight the importance of internal channel flow boundary conditions. By comparing adiabatic film cooling effectiveness under varying blowing ratios and internal crossflow velocities, it is shown that internal crossflow plays an important role in film cooling performance. Enhanced cooling effectiveness can be obtained at internal co-flow conditions where in-hole vortices has the opposite direction with downstream kidney vortices. Due to the competitive effects of distorted velocity profile and in-hole vortices, an optimal velocity of internal crossflow can be found at given blowing ratio and the optimal velocity magnitude slightly increases with increasing blowing ratio.

A parametric study on structural effects of hollow hybrid fin heat sinks in natural convection and radiation

This study investigates structural effects of hollow hybrid fin heat sinks (HHFHSs) under natural convection and radiation conducting parametric analysis. The HHFHS is a staggered array of hollow hybrid fins (HHFs) denoting hollow pin fins containing perforations near fin bases and integrated with radially-placed plate fins. CFD thermal models validated by measurements were utilized to explore structural effects of the HHFHS such as the fin spacing; number, width, and length of the radial plate fin; and the number of perforations. It is found that the surface area enhancement of the radial plate fin considerably improves the thermal performance of the HHFHS although it modestly deteriorates the mass-based performance. It is seen that the HHF spacing has stronger influence on the radiation heat transfer rate compared with other parameters. The result shows that the perforations enable internal channel flows resulting in an additional heat transfer enhancement.

Anisotropic minimum-dissipation (AMD) subgrid-scale model implemented in OpenFOAM: Verification and assessment in single-phase and multi-phase flows

Minimum-dissipation sub-grid models represent simple alternatives to the Smagorinsky-type approaches to the implementation of sub-grid scales’ (SGS) effects in the large-eddy simulation (LES) approach. Recently, the anisotropic minimum-dissipation (AMD) model has been introduced, which is a static type of eddy-viscosity SGS model and is a new addition to this family. This model is easy to implement; furthermore, not only can it consider the effect of various directions in computing sub-grid stress, it can also operate transitional flows from laminar to turbulent. For the first time, we implemented AMD in the open source package OpenFOAM. Foremost, we verified the OpenFOAM implementation of the AMD model for the prediction of isotropic decaying turbulence, a temporal mixing layer and an internal channel flow. To achieve this, decaying turbulence was considered for the isotropic turbulence produced by a grid with a mesh size of 5.08 × 10–2 m in a flow of mean velocity of 10 m/s. The temporal mixing layer was simulated at a Reynolds number based on half the initial vorticity thickness of 105. Channel flow was investigated at the three frictional Reynolds numbers of 180, 395 and 590. The verification results revealed that the implemented AMD model in OpenFOAM offered satisfactory accuracy to capture decaying turbulence and temporal mixing layer and calculate boundary layer velocity profiles and first and second-order turbulent parameters in the channel flow on anisotropic grids. Following this, for the first time, we evaluated AMD's performance in predicting external non-cavitating and cavitating flows over a 3D sphere. Non-cavitating and cavitating flow over the sphere were considered at Re = 2 × 104 and Re = 1.5 × 106, respectively. The AMD results were then compared with the direct numerical simulation (DNS) data and the numerical results obtained from the established SGS models such as dynamic Smagorinsky (DS), one equation eddy viscosity model (OEEVM), detached eddy simulation (DES), and delayed detached eddy simulation (DDES), where applicable. In treating sphere flow, in spite of the use of a coarse grid rather than the alternative SGS models, the AMD model predicted accurate results for pressure coefficient, shear stress, and separation point location over the sphere as well as the velocity profile in the wake region. It was detected that the simulation time of the AMD model was lower than that of DS in all the considered test cases by approximately 10–16%.

The Effect of Rib Turbulators on Film Cooling Effectiveness of Round Compound Angle Holes Fed by an Internal Cross-Flow

Early stage gas turbine blades feature complicated internal geometries in order to enhance internal heat transfer and to supply coolant for film cooling. Most film cooling experiments decouple the effect of internal coolant feed from external film cooling effectiveness, even though engine parts are commonly fed by cross-flow and feature internal rib turbulators which can affect film cooling. Experiments measuring adiabatic effectiveness were conducted to investigate the effects of turbulated perpendicular cross-flow on a row of 45 deg compound angle cylindrical film cooling holes for a total of eight internal rib configurations. The ribs were angled to the direction of prevailing internal cross-flow at two different angles: 45 deg or 135 deg. The ribs were also positioned at two different spanwise locations relative to the cooling holes: in the middle of the cooling hole pitch and slightly intersecting the holes. Experiments were conducted at a density ratio of DR = 1.5 for a range of blowing ratios including M = 0.5, 0.75, 1.0, 1.5, and 2.0. This study demonstrates that peak effectiveness can be attained through the optimization of cross-flow direction relative to the compound angle direction and rib configuration, verifying the importance of hole inlet conditions in film cooling experiments. It was found that ribs tend to reduce adiabatic effectiveness relative to a baseline, smooth-walled configuration. Rib configurations that directed the internal coolant forward in the direction of the mainstream resulted in higher peak adiabatic effectiveness. However, no other parameters could consistently be identified correlating to increased film cooling performance. It is likely that a combination of factors is responsible for influencing performance, including internal local pressure caused by the ribs, the internal channel flow field, in-hole vortices, and jet exit velocity profiles. This study also attempted to replicate the possibility that film cooling holes may intersect ribs and found that a hole which partially intersects a rib still maintains moderate levels of effectiveness.

CONJUGATED HEAT TRANSFER ANALYSIS OF A FILM COOLING PASSAGE WITH TURBULATOR RIBS

In this study, the adiabatic film cooling effectiveness and conjugate heat transfer characteristics for a circular hole are investigated numerically. Seven turbulator ribs are located on the internal wall, and a film cooling hole between two adjacent ribs has an inclination angle of 30°. The secondary flow rate is affected by the internal channel flow velocity, and the external film cooling is combined with the internal channel flow conditions. In the conjugate heat transfer research, two different plate materials are considered to analyze the heat conduction characteristics. The results show that the turbulator ribs enhance the downstream heat transfer and provide a more extensive coverage area and effective cooling protection. More blade-wall heat is transferred by heat conduction, which makes the area-average wall temperature for the high thermal conductivity case to decrease along the secondary flow direction. The wall temperature distribution and film cooling effectiveness are affected significantly by heat conduction. (Less)

A Methodology to Measure Aerodynamic Forces on Cylinders in Channel Flow

While the measurement of drag and lift forces on a body in external flow is common practice, the same cannot be said for aerodynamic forces on bodies in internal flows. The inherent difficulty in making force measurements on a body in an internal channel flow is decoupling the body from the bounding walls. The methodology presented in this paper uses a technique to overcome this constraint to accurately measure two components of force on a single cylinder within a single row array, with an aspect ratio (height-to-diameter ratio) of 1. Experiments were conducted with air over a range of Reynolds numbers between 7500 and 35,000 and for three different spanwise pin spacings. Experimental results indicated an increase in cylinder drag with a reduction in spanwise pin spacing. The gas turbine and electronics industries use cylinders or pin fins in internal flow channels to increase heat transfer augmentation through high turbulence and increased surface area. The flow fields in these obstructed channels are difficult to predict, so these measurements can be used to directly compare with predicted drag and lift forces.

Heat transfer and friction behaviors in rectangular channels with varying number of ribbed walls

An experimental study of surface heat transfer and friction characteristics of a fully developed turbulent air flow in a square channel with transverse ribs on one, two, three, and four walls is reported. Tests were performed for Reynolds numbers ranging from 10,000 to 80,000. The pitch-to-rib height ratio, P/ e, was kept at 8 and rib-height-to-channel hydraulic diameter ratio, e/ D h was kept at 0.0625. The channel length-to-hydraulic diameter ratio, L/ D h, was 20. The heat transfer coefficient and friction factor results were enhanced with the increase in the number of ribbed walls. The friction roughness function, R( e +), was almost constant over the entire range of tests performed and was within comparable limits of the previously published data. The heat transfer roughness function, G( e +), increased with roughness Reynolds number and compared well with previous work in this area. Both correlations could be used to predict the friction factor and heat transfer coefficient in a rectangular channel with varying number of ribbed walls. The results of this investigation could be used in various applications of turbulent internal channel flows involving different number of rib roughened walls.