Hole Blockage Research Articles

Experimental/Numerical Investigation on the Effects of Trailing-Edge Cooling Hole Blockage on Heat Transfer in a Trailing-Edge Cooling Channel

Hot and harsh environments, sometimes experienced by gas turbine airfoils, can create undesirable effects such as clogging of the cooling holes. Clogging of the cooling holes along the trailing edge of an airfoil on the tip side and its effects on the heat transfer coefficients in the cooling cavity around the clogged holes is the main focus of this investigation. Local and average heat transfer coefficients were measured in a test section simulating a rib-roughened trailing edge cooling cavity of a turbine airfoil. The rig was made up of two adjacent channels, each with a trapezoidal cross sectional area. The first channel supplied the cooling air to the trailing-edge channel through a row of racetrack-shaped slots on the partition wall between the two channels. Eleven crossover jets, issued from these slots entered the trailing-edge channel, impinged on eleven radial ribs and exited from a second row of race-track shaped slots on the opposite wall that simulated the cooling holes along the trailing edge of the airfoil. Tests were run for the baseline case with all exit holes open and for cases in which 2, 3, and 4 exit holes on the airfoil tip side were clogged. All tests were run for two crossover jet angles. The first set of tests were run for zero angle between the jet axis and the trailing-edge channel centerline. The jets were then tilted towards the ribs by five degrees. Results of the two set of tests for a range of jet Reynolds number from 10,000 to 35,000 were compared. The numerical models contained the entire trailing-edge and supply channels with all slots and ribs to simulate exactly the tested geometries. They were meshed with all-hexa structured mesh of high near-wall concentration. A pressure-correction based, multiblock, multigrid, unstructured/adaptive commercial software was used in this investigation. The realizable k-ε turbulence model in combination with enhanced wall treatment approach for the near wall regions were used for turbulence closure. Boundary conditions identical to those of the experiments were applied and several turbulence model results were compared. The numerical analyses also provided the share of each crossover and each exit hole from the total flow for different geometries. The major conclusions of this study were: (a) clogging of the exit holes near the airfoil tip alters the distribution of the coolant mass flow rate through the crossover holes and changes the flow structure. Depending on the number of clogged exit holes (from 3 to 6, out of 12), the tip-end crossover hole experienced from 35% to 49% reductions in its mass flow rate while the root-end crossover hole, under the same conditions, experienced an increase of the same magnitude in its mass flow rate. (b) Up to 64% reduction in heat transfer coefficients on the tip-end surface areas around the clogged holes were observed which might have devastating effects on the airfoil life. At the same time, a gain in heat transfer coefficient of up 40% was observed around the root-end due to increased crossover flows. (c) Numerical heat transfer results with the use of the realizable k-ε turbulence model in combination with enhanced wall treatment approach for the near wall regions were generally in a reasonable agreement with the test results. The overall difference between the CFD and test results was about 10%.

Sand transport in a two pass internal cooling duct with rib turbulators

Abstract Jet engines often operate under dirty conditions where large amounts of particulate matter can be ingested, especially, sand, ash and dirt. Particulate matter in different engine components can lead to degradation in performance. The focus of this study is to investigate the sand transport and deposition in the internal cooling passages of turbine blades. A two pass stationary square duct with rib turbulators subjected to sand ingestion is studied using Large Eddy Simulations (LES). Each pass has ribs on two opposite walls and aligned normal to the main flow direction. The rib pitch to rib height ( P / e ) is 9.28, the rib height to channel hydraulic diameter ( e / D h ) is 0.0625 and calculations have been carried out for a bulk Reynolds number of 25,000. Particle sizes in the range 0.5–25 μm are considered, with the same size distribution as found in Arizona Road Dust (medium). Large Eddy Simulation (LES) with a wall-model is used to model the flow and sand particles are modeled using a discrete Lagrangian framework. Results quantify the distribution of particle impingement density on all surfaces. Highest particle impingement density is found in the first quarter section of the second pass after the 180° turn, where the recorded impingement is more than twice that of any other region. It is also found that the average particle impingement per pitch is 28% higher in the second pass than the first pass. Results show lower particle tendency to impact the region immediately behind the rib in the first pass compared to the second pass where particle impingement is more uniform in the region between two ribs. The rib face facing the flow is by far is the most susceptible to impingement and hence deposition and erosion. The results of this simulation are compared to experiments conducted on an identical two pass geometry with Arizona Road Dust particles. The numerical predictions showed good qualitative agreement with experimental measurements. These results identify the damage prone areas in the internal cooling passages of a turbine blade under the influence of sand ingestion. This information can help modify the geometry of the blade or location of film cooling holes to avoid hole blockage and degradation of heat transfer at the walls.

The influence of the hole blocking layers on the electroluminescence stability of phosphorescent organic light emitting devices

Delayed electroluminescence (EL) measurements are used to investigate electroluminescence stability in phosphorescent organic light emitting devices (PHOLEDs) containing typical hole blocking layers (HBLs). The results show a strong correlation between the extent of hole blockage capacity of the HBL and the rate of deterioration in device EL efficiency, pointing to the major role that the build-up of hole space charges in the emitting layer (EML) plays in EL degradation. In this regard, the use of a strongly blocking material significantly increases the build-up of holes in the EML, and accelerates EL degradation.

New scenarios of charge transport in PEDT:PSS conducting polymer: From hole resonant tunneling to cationic motion and relaxation

This work reports a series of distinct current density∼field ( JE) characteristics of the poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonic acid) (or PDET:PSS) conducting polymer system. Symmetrical and asymmetrical negative differential resistance (NDR) behaviors are observed for metal/PEDT:PSS/metal sandwich devices, and these novel behaviors are related to hole injection and resonant tunneling, hole blockage and extraction, and ionic motion in the PEDT:PSS films. Both the PEDT:PSS/metal interface configuration and film thickness have strong effect on the manifestation of the NDR characteristic. It is proposed that phase segregation in μm-thick PEDT:PSS films could lead to the conversion of hole-dominated transport to ionic motion-dominated conduction, thus causing a drastic change in JE characteristics of films with different thickness. Our results highlight the important effect of PEDT:PSS/metal interface and PEDT:PSS film thickness on charge conduction, and broaden our current understanding of this model organic system with both electronic and ionic conducting components.

Gamma ray tomography—An experimental analysis of fractional gas hold-up in bubble columns

Abstract Gas hold-up is one of the most important hydrodynamic parameters involved in the design, development, scale-up and troubleshooting of multiphase systems. Gamma ray tomography (GRT) is an extensively used non-destructive and non-invasive technique for the measurement of the gas hold-up profiles in multiphase system such as bubble columns and stirred vessels. The tomographic measurements can be subsequently used for the qualitative as well as quantitative estimation of the performance of multiphase system under given set of operating conditions. The non-destructive technique of grid scanning which forms the basis of GRT was used to analyze the performance of the laboratory scale bubble column as well as industrial scale sectionalized bubble column. The later had a total height of 8.5 m divided into 10 equal sections. The hydrodynamic performance obtained in the form of chordal gas hold-up was shown to be highly influenced as a result of the sparger design altercations. The non-performance of spargers due to the hole blockage was studied in a bubble column of 0.2 m diameter by analyzing the reconstructed gas hold-up profile at two different axial locations. An industrial sectionalized bubble column was also studied to obtain the desired information on the change in gas hold-up in the axial direction as a result of increased e ¯ G due to intrinsic reaction kinetics giving rise to the formation of additional gas at every stage.

Effects of Surface Deposition, Hole Blockage, and Thermal Barrier Coating Spallation on Vane Endwall Film Cooling

With the increase in usage of gas turbines for power generation and given that natural gas resources continue to be depleted, it has become increasingly important to search for alternate fuels. One source of alternate fuels is coal derived synthetic fuels. Coal derived fuels, however, contain traces of ash and other contaminants that can deposit on vane and turbine surfaces affecting their heat transfer through reduced film cooling. The endwall of a first stage vane is one such region that can be susceptible to depositions from these contaminants. This study uses a large-scale turbine vane cascade in which the following effects on film cooling adiabatic effectiveness were investigated in the endwall region: the effect of near-hole deposition, the effect of partial film cooling hole blockage, and the effect of spallation of a thermal barrier coating. The results indicated that deposits near the hole exit can sometimes improve the cooling effectiveness at the leading edge, but with increased deposition heights the cooling deteriorates. Partial hole blockage studies revealed that the cooling effectiveness deteriorates with increases in the number of blocked holes. Spallation studies showed that for a spalled endwall surface downstream of the leading edge cooling row, cooling effectiveness worsened with an increase in blowing ratio.

The use of suction to suppress disturbances in laminar flow caused by insect and other surface debris

Aircraft that employ extensive laminar flow for drag reduction need measures to counter the potential breakdown to turbulence caused by surface debris picked up near the ground. Decontamination methods are being developed but may not be suitable where suction through microperforated skins is used to control the flow, owing to the likelihood of hole blockage. This paper examines an alternative, where the existing suction system is used to draw away the developing turbulent wedges behind individual particles, thus providing a degree of tolerance to surface contamination. A wind tunnel investigation was conducted, using a flat plate model with a configurable suction region, to simulate the essential features of the boundary layer on a laminar flow engine nacelle. This enabled the influence of the various flow and geometric parameters to be assessed. The results, taken at Reynolds numbers/m in the range 1.5-4.5 × 106, showed that turbulent wedges could be stabilized and removed, but that this required high levels of suction. The attendant increases in drag and suction system power make the method unattractive economically, although this situation would change if the increased suction could be applied only where required. The condition of self-induced transition at high suction flows was also investigated and was shown to limit the speed at which particle-induced turbulence could be suppressed.