Lesley M. Wright

Assessment of Steady State PSP, TSP, and IR Measurement Techniques for Flat Plate Film Cooling

Several steady state measurement techniques are used to measure the film cooling effectiveness on a flat plate. Pressure sensitive paint (PSP), temperature sensitive paint (TSP), and infrared (IR) thermography are used to measure the film cooling effectiveness. To compare these measurement techniques, a single row of cylindrical holes, with a compound angle, are used. Seven holes (D = 4 mm) are equally spaced 12 mm apart, and the hole length-to-diameter ratio is 9.92. The axial angle (θ) of the holes is 30°, and the compound angle (β) is 45°. In addition to evaluating the various measurement techniques the effect of the coolant blowing ratio is considered; effectiveness measurements are taken for blowing ratios, M, of 0.4, 0.6, 1.2, and 1.8. The effect of mainstream turbulence intensity is considered with the addition of a turbulence grid to the low speed wind tunnel. Of the three steady state measurement techniques considered in this study, PSP demonstrates the most promise for the measurement of the film cooling effectiveness. Because PSP is a mass transfer technique, film effectiveness measurements can be readily obtained near the film cooling holes. Although the heat transfer techniques of TSP and IR thermography are more desirable than traditional thermocouples or liquid crystal thermography, the applicability of measurements near the holes is questionable due to conduction problems associated with steady state heat transfer techniques.Copyright

Rib Spacing Effect on Heat Transfer in Rotating Two-Pass Ribbed Channel (AR 1:2)

Rib turbulators are commonly used to enhance the heat transfer within internal cooling passages of advanced gas turbine blades. Many factors affect the thermal performance of a cooling channel with ribs. This study experimentally investigates the effect of rib spacing on the heat transfer enhancement, pressure penalty, and thus the overall thermal performance in both rotating and nonrotating rectangular, cooling channels. In the 1:2 rectangular channels, 45 deg angled ribs are placed on the leading and trailing surfaces. The pitch of the ribs varies, so rib pitch-to-height (P/e) ratios of 10, 7.5, 5, and 3 are considered. Square ribs with a 1.59 mm x 1.59 mm cross section are used for all rib spacing, so the height-to-hydraulic diameter (e/D h ) ratio remains constant at 0.094. With a constant rotational speed of 550 rpm and the Reynolds number ranging from 5000 to 40,000, the rotation number in turn varies from 0.2 to 0.02. Because the skewed turbulators induce secondary flow along the length of the rib, the very close rib spacing of P/e = 3 has the best thermal performance in both rotating and nonrotating channels. This close spacing yields the greatest heat transfer enhancement, whereas the P/e = 5 spacing has the greatest pressure penalty. In addition, the effect of rotation is more pronounced in the channel with the rib spacing of 3. As more ribs are added, the channel is approaching a smooth channel, and the strength of the rotation induced vortices increases.

Effect of Rotation on Heat Transfer in Rectangular Channels with Pin-Fins

The effect of rotation on smooth narrow rectangular channels and narrow rectangular channels with pin fins is investigated in this study. Pin fins are commonly used in the narrow sections within the trailing edge of the turbine blade; the pin fins act as turbulators to enhance internal cooling while providing structural support in this narrow section of the blade. The rectangular channel is oriented at 150 deg with respect to the plane of rotation, and flow through the channel is radially outward. The focus of the study involves narrow channels with aspect ratios of 4:1 and 8:1. The enhancement due to both conducting (copper) pin fins and nonconducting (Plexiglas®) pins is investigated. Because of the varying aspect ratio of the channel, the height-to-diameter ratio (h p /D p ) of the pins varies from two, for an aspect ratio of 4:1, to unity, for an aspect ratio of 8:1. A staggered array of pins with uniform streamwise and spanwise spacing (x p /D p =s p /D p = 2.0) is studied. With this array, 42 pin fins are used, giving a projected surface density of 3.5 pins/in. 2 (0.543 pins/cm 2 ), for the leading or trailing surfaces

Heat transfer in rotating rectangular channels (AR=4:1) with V-shaped and angled rib turbulators with and without gaps

An experimental study was performed to measure the regionally averaged heat transfer distributions in a rotating ribbed channel with an aspect ratio of 4:1. The Reynolds number, based on a hydraulic diameter, varies from 5,000 to 40,000. The rotation number ranges from 0 to 0.3, and the inlet coolant-to-wall density ratio (Δρ/ρ) is maintained around 0.122. Six different configurations of ribs, oriented at an angle of 45° to the direction of flow, are placed on both the leading and trailing surfaces: parallel and staggered V-shaped ribs without gaps, parallel and staggered angled ribs without gaps, parallel V-shaped ribs with gaps, and parallel angled ribs with gaps are considered. The rib height-to-hydraulic diameter ratio (e/Dh ) is 0.078, and the rib pitch-to-height ratio (P/e) is 10. The channel orientation with respect to the plane of rotation is 135°. The results show that V-shaped rib configuration produces more heat transfer enhancement than the angled rib configurations. It is also shown that there is only negligible difference between the heat transfer enhancement due to the staggered V-shaped ribs without gaps and the enhancement due to the parallel V-shaped ribs without gaps. The same is true for the staggered and parallel angled ribs without gaps. Also, the parallel V-shaped ribs without gaps produce more heat transfer enhancement than the V-shaped ribs with gaps, while the parallel angled ribs with gaps experience overall greater heat transfer enhancement than the angled ribs without gaps. Finally, all surfaces undergo heat transfer enhancement by rotating the channel compared to the stationary channel for all cases.Copyright

Effectiveness Distributions on Turbine Blade Cascade Platforms Through Simulated Stator-Rotor Seals

A five-blade linear cascade is used to experimentally investigate turbine-blade platform cooling. Three slot configurations placed upstream of the blades are used to model advanced seals between the stator and rotor. The seal configurations include vertical injection of the coolant onto the platform, a redirection of the coolant onto the platform, and a labyrinthlike configuration between the stator endwall and rotor platform. The coolant flow rate through the seals varies from 0.5 to 2.0% of the mainstream flow. The film-cooling effectiveness is measured on the platform using pressure-sensitive paint. The upstream slots cover 1.5 passages with the coolant exiting the slot 3.87 cm upstream of the leading edge of the blades. The mainstream Reynolds number is 3.1 x 105 based on the inlet velocity and the chord length of the scaled high-pressure turbine blade. With the pressure-sensitive-paint measurement technique, the effect of the passage-induced secondary flow on the film-cooling effectiveness is easily captured, as the distribution of the film effectiveness is very nonuniform through the passage. In addition, the more advanced seal configurations, considered in the present study, yield reduced film-cooling effectiveness compared with the more fundamental inclined slot configurations.

Heat transfer in rotating rectangular channels with V-shaped and angled ribs

An experimental study was performed to measure the regionally averaged heat-transfer distributions in a rotating ribbed channel with an aspect ratio of 4:1. The Reynolds number, based on hydraulic diameter, varies from 5 × 10 3 to 40 × × 10 3 . The rotation number ranges from 0 to 0.3, and the inlet coolant-to-wall density ratio (∆ρ/ρ )i smaintained around 0.122. Six different configurations of ribs, oriented at an angle of 45 deg to the direction of flow, are placed on both the leading and trailing surfaces: 1) parallel V-shaped ribs without gaps, 2) staggered V-shaped ribs without gaps, 3) parallel V-shaped ribs with gaps, 4) parallel angled ribs without gaps, 5) staggered angled ribs without gaps, and 6) parallel angled ribs with gaps. The rib-height-to-hydraulic-diameter ratio (e/Dh )i s0.078, and the rib-pitch-to-height ratio (P/e )i s10. The channel orientation with respect to the plane of rotation is 135 deg. The results show that the V-shaped rib configuration produces more heat-transfer enhancement than the angled rib configurations. It is also shown that there is only a negligible difference between the heat-transfer enhancement due to the staggered V-shaped ribs without gaps and the enhancement due to the parallel V-shaped ribs without gaps. The same is true for the staggered and parallel angled ribs without gaps. Also, the parallel V-shaped ribs without gaps produce more heat-transfer enhancement than the V-shaped ribs with gaps, whereas the parallel angled ribs with gaps produce more heat-transfer enhancement than the angled ribs without gaps. Finally, rotation further increases the heat transfer from all surfaces above that of the stationary channels.

Buoyancy Effects on Heat Transfer in Five Different Aspect-Ratio Rectangular Channels With Smooth Walls and 45-Degree Ribbed Walls

This paper experimentally studies the effects of the buoyancy force and channel aspect ratio on heat transfer in two-pass rotating rectangular channels with smooth walls and 45° ribbed walls. The channel aspect ratios include 4:1, 2:1, 1:1, 1:2 and 1:4. Four Reynolds numbers are studied: 5000, 10000, 25000 and 40000. The rotation speed is fixed at 550 rpm for all tests, and for each channel, two channel orientations are studied: 90° and 45° or 135°, with respect to the plane of rotation. Rib turbulators are placed on the leading and trailing walls of the channels at an angle of 45° to the flow direction. The ribs have a 1.59 by 1.59 mm square cross section, and the rib pitch-to-height ratio (P/e) is 10 for all tests. The effects of the local buoyancy parameter and channel aspect ratio on the regional Nusselt number ratio are presented. The results show that increasing the local buoyancy parameter increases the Nusselt number ratio on the trailing surface and decreases the Nusselt number ratio on the leading surface in the first pass for all channels. However, the trend of the Nusselt number ratio in the second pass is more complicated due to the strong effect of the 180° turn. Results are also presented for this critical turn region of the two-pass channels. In addition to these regions, the channel averaged heat transfer, friction factor, and thermal performance are determined for each channel. With the channels having comparable Nusselt number ratios, the 1:4 channel has the superior thermal performance because it incurs the least pressure penalty.Copyright

Heat Transfer in Two-Pass Rotating Rectangular Channels (AR=2:1) with Discrete Ribs

This paper reports the heat transfer coefficients and friction factors in a two-pass rotating rectangular channel with ribs, applicable to an internally cooled turbine blade. The channel aspect ratio is 2:1. Five different turbulators are studied: 45-deg angled ribs, V-shaped ribs, discrete 45-deg angled ribs, discrete V-shaped ribs, and crossed V-shaped ribs. The ribs are placed on the leading and trailing surfaces. The Reynolds number ranges from 5000 to 40,000. The corresponding rotation numbers vary from 0.206 to 0.026 for a fixed rotation speed of 550 rpm. The rib-height-to-hydraulic-diameter ratio (elD) is 0.094, the rib-pitch-to-height ratio (Pie) is 10, and the inlet-coolant-to-wall-density ratio (Ap/p) is maintained around 0.115. For each case, two channel orientations with respect to the plane of rotation are studied, 90 and 135 deg. The results show that the V-shaped ribs and discrete V-shaped ribs have higher heat transfer enhancement than the 45-deg angled ribs and discrete 45-deg angled ribs for both rotating and nonrotating cases. The pressure measurements show the 45-deg angled ribs incurred the highest frictional losses. Based on the present study, the discrete V-shaped ribs have the best overall thermal performance in both rotating and nonrotating channels.

Rib Spacing Effect on Heat Transfer and Pressure Loss in a Rotating Two-Pass Rectangular Channel (AR=1:2) With 45-Degree Angled Ribs

Rib turbulators are commonly used to enhance the heat transfer within internal cooling passages of advanced gas turbine blades. Many factors affect the thermal performance of a cooling channel with ribs. This study experimentally investigates the effect of rib spacing on the heat transfer enhancement, pressure penalty, and thus the overall thermal performance in both rotating and non-rotating rectangular, cooling channels. In the 1:2 rectangular channels, 45° angled ribs are placed on the leading and trailing surfaces. The pitch of the ribs varies, so rib pitch-to-height (P/e) ratios of 10, 7.5, 5, and 3 are considered. Square ribs with a 1.59 mm × 1.59 mm cross-section are used for all spacings, so the height-to-hydraulic diameter (e/Dh ) ratio remains constant at 0.094. With a constant rotational speed of 550 rpm and the Reynolds number ranging from 5000 to 40000, the rotation number in turn varies from 0.2 to 0.02. Because the skewed turbulators induce secondary flow along the length of the rib, the very close rib spacing of P/e = 3, has the best thermal performance in both rotating and non-rotating channels. This close spacing yields the greatest heat transfer enhancement, while the P/e = 5 spacing has the greatest pressure penalty. In addition, the effect of rotation is more pronounced in the channel with the rib spacing of 3. As more ribs are added, the channel is approaching a smooth channel, and the strength of the rotation induced vortices increases.Copyright

Assessment of Steady State PSP and Transient Ir Measurement Techniques for Leading Edge Film Cooling

Film cooling is commonly used on the leading edge of turbine blades to protect the blade surface from hot mainstream gases in the turbine. Obtaining detailed film cooling effectiveness distributions on the leading edge can be challenging. This paper considers two measurement techniques which can be applied to the leading edge (modeled by a cylinder) to obtain detailed distributions of the film effectiveness. A steady state pressure sensitive paint (PSP) technique and a transient infrared (IR) thermography technique are used to obtain detailed film cooling effectiveness distributions on the cylinder. The cylinder, 7.62 cm in diameter, is placed in a low speed wind tunnel, with the mainstream flow having a Reynolds number of 100,900 (based on the cylinder diameter). The cylinder has two rows of film cooling holes located at ±15° from the cylinder’s stagnation line. The pitch-to-diameter ratio of the film holes is 4, and holes are inclined 30° in spanwise direction. PSP continues to show promise for film cooling effectiveness measurements. Detailed distributions can be obtained near the film cooling holes because this technique relies on mass transfer rather than heat transfer. In order to reduce the error caused by conduction in heat transfer experiments, transient measurement techniques are favorable. Transient IR measurements are taken, and film cooling effectiveness is determined on the cylinder’s surface. Although the effect of conduction is reduced with the transient IR technique (compared to a steady state heat transfer experiment), heat conduction through the cylinder has not been eliminated (or even minimized). Without correction, the results obtained from transient heat transfer experiments must be used cautiously. For this reason, PSP is developing a niche within the gas turbine community for detailed film cooling effectiveness measurements.Copyright