Akhilesh P. Rallabandi

Heat Transfer in Trailing Edge Wedge-Shaped Pin-Fin Channels With Slot Ejection Under High Rotation Numbers

The heat transfer characteristics of a rotating pin-fin roughened wedge shaped channel have been studied. The model incorporates ejection through slots machined on the narrower end of the wedge, simulating a rotor blade trailing edge. The copperplate regional average method is used to determine the heat transfer coefficient; pressure taps have been used to estimate the flow discharged through each slot. Tests have been conducted at high rotation (≈ 1 ) and buoyancy (≈ 2) numbers, in a pressurized rotating rig. Reynolds Numbers investigated range from 10,000 to 40,000 and rotational speeds range from 0–400rpm. Pin-fins studied are made of copper as well as non-conducting garolite. Results show high heat transfer coefficients in the proximity of the slot. A significant enhancement in heat transfer due to the pin-fins, compared with a smooth channel is observed. Even the non-conducting pin-fins, indicative of heat transfer on the end-wall show a significant enhancement in the heat transfer coefficient. Results also show a strong rotation effect, increasing significantly the heat transfer coefficient on the trailing surface — and reducing the heat transfer on the leading surface.Copyright

Heat Transfer and Pressure Drop Measurements for a Square Channel With 45 deg Round-Edged Ribs at High Reynolds Numbers

Experiments to determine heat transfer coefficients and friction factors are conducted on a stationary 45 deg parallel rib-roughened square channel, which simulates a turbine blade internal coolant passage. Copper plates fitted with silicone heaters and thermocouples are used to measure regionally averaged heat transfer coefficients. Reynolds numbers studied range from 30,000 to 400,000. The ribs studied have rounded (filleted) edges to account for manufacturing limitations of actual engine blades. The rib height (e) to hydraulic diameter (D) ratio (e/D) ranges from 0.1 to 0.2, while spacing (p) to height ratio (p/e) ranges from 5 to 10. Results indicate an increase in the heat transfer due to the ribs at the cost of a higher friction factor, especially at higher Reynolds numbers. Round-edged ribs experience a similar heat transfer coefficient and a lower friction factor compared with sharp-edged ribs, especially at higher values of the rib height. Correlations predicting Nu and f as a function of e/D, p/e, and Re are presented. Also presented are correlations for the heat transfer and friction roughness parameters (G and R, respectively).

Influence of Unsteady Wake With Trailing Edge Coolant Ejection on Turbine Blade Film Cooling

Detailed film cooling effectiveness distributions along a modeled turbine rotor blade under the combined effects of an upstream trailing edge unsteady wake with coolant ejection are presented using the pressure sensitive paint (PSP) mass transfer analogy method. The experiment is conducted in a low speed wind tunnel facility with a five blade linear cascade. The exit Reynolds number based on the axial chord is 370,000. Unsteady wakes and trailing edge coolant jets are produced by a spoked wheel-type wake generator with hollow rods equipped with several coolant ejections from holes. The coolant-to-mainstream density ratios for both the blade and trailing edge coolant ejection range from 1.5 to 2.0 for simulating realistic engine conditions. Blade blowing ratio studies are 0.5 and 1.0 on the suction surface and 1.0 and 2.0 on the pressure surface. The trailing edge jet blowing ratio and Strouhal numbers are 1.0 and 0.12, respectively. The results show that the unsteady wake reduces the overall film cooling effectiveness. However, the unsteady wake with trailing edge coolant ejection enhances the overall effectiveness. The results also show that the overall filming cooling effectiveness increases by using heavier coolant for trailing edge ejection and for blade surface film cooling.

Effect of Upstream Step on Flat Plate Film Cooling Effectiveness Using PSP

The effect of a step positioned upstream of a row of film cooling holes on the film cooling effectiveness is studied systematically using the steady state Pressure Sensitive Paint (PSP) technique. The upstream step effect is studied on four separate hole geometries: simple angled (axial angle 30°) and compound angled (axial angle 30°, compound angle 45°) cylindrical and fan-shaped film cooling holes. Each plate considered has 7 holes, each hole 4mm in diameter. The plates with cylindrical holes have a spacing of 3 diameters (12mm) between the centers of two consecutive holes, while the fan-shaped holes have a spacing of 3.75d (15mm). Three different step heights (12.5%d, 25%d and 37.5%d) are studied. Also studied is the effect of the width of the step; the distance of the step upstream of the hole and the positioning of the step downstream of the film-cooling hole. Four separate blowing ratios are reported for all tests: M = 0.3, M = 0.6, M = 1.0 and M = 1.5. All studies have been conducted with a mainstream of 25m/s velocity at an ambient temperature of 22C. Results indicate an increase in film-cooling effectiveness in the region near the hole due to the upstream step for all the plates considered. This increase due to the step is found to be most significant in the case of compound angled cylindrical holes and least significant in the case of simple angled fan-shaped holes.Copyright

Unsteady Wake and Coolant Density Effects on Turbine Blade Film Cooling Using PSP Technique

The effect of an unsteady stator wake (simulated by wake rods mounted on a spoke wheel wake generator) on the modeled rotor blade is studied using the Pressure Sensitive Paint (PSP) mass transfer analogy method. Emphasis of the current study is on the mid-span region of the blade. The flow is in the low Mach number (incompressible) regime. The suction (convex) side has simple angled cylindrical film-cooling holes; the pressure (concave) side has compound angled cylindrical film cooling holes. The blade also has radial shower-head leading edge film cooling holes. Strouhal numbers studied range from 0 to 0.36; the exit Reynolds Number based on the axial chord is 530,000. Blowing ratios range from 0.5 to 2.0 on the suction side; 0.5 to 4.0 on the pressure side. Density ratios studied range from 1.0 to 2.5, to simulate actual engine conditions. The convex suction surface experiences film-cooling jet lift-off at higher blowing ratios, resulting in low effectiveness values. The film coolant is found to reattach downstream on the concave pressure surface, increasing effectiveness at higher blowing ratios. Results show deterioration in film cooling effectiveness due to increased local turbulence caused by the unsteady wake, especially on the suction side. Results also show a monotonic increase in film-cooling effectiveness on increasing the coolant to mainstream density ratio.© 2010 ASME

Heat Transfer and Pressure Drop Measurements for a Square Channel With 45Deg Round Edged Ribs at High Reynolds Numbers

Experiments to determine heat transfer coefficients and friction factors are conducted on a stationary 45 deg parallel rib roughened square channel which simulates a turbine blade internal coolant passage. Copper plates fitted with silicone heaters and thermocouples are used to measure regionally averaged heat transfer coefficients. Reynolds numbers studied range from 30,000 to 400,000. The ribs studied have rounded (filleted) edges to account for manufacturing limitations of actual engine blades. The rib height (e) to hydraulic diameter (D) ratio (e/D) ranges from 0.1 to 0.2; spacing (p) to height ratio (p/e) ranges from 5 to 10. Results indicate an increase in heat transfer due to ribs at the cost of a higher friction factor, especially at higher Reynolds Numbers. Round edged ribs experience a similar heat transfer coefficient and a lower friction factor compared to sharp edged ribs, especially at higher values of rib height. Correlations predicting Nu and f as a function of e/D, p/e and Re are presented. Also presented are correlations for heat transfer and friction roughness parameters (G and R).© 2009 ASME

Heat Transfer Measurements in Rotating Blade-Shape Serpentine Coolant Passage With Ribbed Walls at High Reynolds Numbers

Flow in the internal three-pass serpentine rib turbulated passages of an advanced high pressure rotor blade is simulated on a 1:1 scale in the laboratory. Tests to measure the effect of rotation on the Nusselt number are conducted at rotation numbers up to 0.4 and Reynolds numbers from 75,000 to 165,000. To achieve this similitude, pressurized Freon R134a vapor is utilized as the working fluid. Experimental heat transfer coefficient measurements are made using the copperplate regional average method. Regional heat transfer coefficients are correlated with rotation numbers. An increase in heat transfer rates due to rotation is observed in radially outward passes; a reduction in heat transfer rate is observed in the radially inward pass. Strikingly, a significant deterioration in heat transfer is noticed in the “hub” region — between the radially inward second pass and the radially outward third pass. This heat transfer reduction is critical for turbine cooling designs.© 2013 ASME