Effect of Rotation on Heat Transfer in AR = 2:1 and AR = 4:1 Channels Connected by a Series of Crossover Jets

Detailed heat transfer measurements using transient liquid crystal thermography were performed on a novel cooling design covering the mid-chord and trailing edge region of a typical gas turbine blade under rotation. The test section comprised of two channels with aspect ratio (AR) of 2:1 and 4:1, where the coolant was fed into the AR = 2:1 channel. Rib turbulators with a pitch-to-rib height ratio (p/e) of 10 and rib height-to-channel hydraulic diameter ratio (e/Dh) of 0.075 were placed in the AR = 2:1 channel at 60° relative to flow direction. The coolant after entering this section was routed to the AR = 4:1 section through a set of crossover jets. The 4:1 section had a realistic trapezoidal shape that mimics the trailing edge of an actual gas turbine blade. The pin fins were arranged in a staggered array with a center-to-center spacing of 2.5 times pin diameter. The trailing edge section consisted of radial and cutback exit holes for flow exit. Experiments were performed for Reynolds number of 20,000 at Rotation numbers (Ro) of 0, 0.1 and 0.14. The channel averaged heat transfer coefficient on trailing side was ~28% (AR = 2:1) and ~7.6% (AR = 4:1) higher than the leading side for Ro = 0.1. It is shown that the combination of crossover jets and pin-fins can be an effective method for cooling wedge shaped trailing edge channels over axial cooling flow designs.

Review: The flow and heat transfer investigation inside tapered and straight two pass channels with rib turbulators

This paper is a review of the number of experimental and CFD experiments performed with rib turbulators about the heat transfer and the at two pass channel. In respect to achieve higher thermal efficiency of gas turbines, efforts are made to raise the inlet temperature. The secondary fluxes resulting from the rib and U-shaped curvature play an important role enhancing heat transfer in the two pass ribbed channels. The ribs on the internal surface of cooling passage will increase the strength of heat transfer. This paper deals with the effect of tapered and straight two-pass channels with the influence of the variable rib cross-section on flow and heat transfer enhancement

Thermal Performance of V-Shaped and X-Shaped Ribs in Trapezoidal Cooling Channels

Convective heat transfer enhancement using rib turbulators is effective for turbine blade internal cooling. Detailed heat transfer measurement of X-shaped ribs in a trapezoidal cooling channel was experimentally conducted using infrared thermography. The novel X-shaped ribs were designed by combining two V-shaped ribs, and more secondary flows generated by the X rib delivered higher heat transfer enhancement. The Reynolds numbers in this study were 10,000, 20,000, and 30,000. These ribs were installed on two opposite walls of a trapezoidal channel in a staggered arrangement. The rib pitch-to-height ratios were 10 and 20, and the rib height-to-hydraulic diameter ratio was 0.128. Results indicated that higher heat transfer distribution was observed in the vicinity of the shorter base in the trapezoidal channel. The full X-shaped ribs and the V-shaped ribs demonstrated the highest Nusselt number ratios among all the cases. Although full X-shaped ribs contributed to higher heat transfer improvement due to intensified secondary flows, they also caused significant pressure loss. Therefore, the cutback X-shaped ribs were proposed by removing a segment in the rib at either upstream or downstream region. Consequently, the upstream cutback X-shaped rib and the V-shaped rib produced the highest thermal performance in this trapezoidal channel.

Heat Transfer in a Rib Turbulated Pin Fin Array for Trailing Edge Cooling

Ribs were added into a pin fin array for a uniquely new cooling concept enabled through additive manufacturing. Both heat transfer mechanisms are highly non-linear; thus, cannot be superimposed. Heat transfer measurements are obtained using the thermochromic liquid crystal technique in a trapezoidal duct with pin fins and rib turbulators. Three pin blockage ratios and four rib heights at Reynolds numbers between 40,000 and 106,000 were tested. The Nusselt number augmentation is generally higher at the longer base of the trapezoidal duct. The same high heat transfer trend is seen at the columns closer to the longer base of the trapezoidal duct than on the shorter base. Through the length of the duct, the flow shifts from the nose region to the larger opening on the opposite wall. Also, it is observed that increasing the blockage ratio as well as increasing the rib height, has a positive impact on heat transfer as ribs act as additional extended surfaces and alter the near-wall flow field. The heat transfer augmentation of pins and ribs is found to not be equal to the sum of both. The observed heat transfer augmentation of the combined cases exceeded over the rib and pin only cases by up to 100%, but the weighted friction factor also doubled. The combination of ribs and pins is an excellent concept to achieve more uniform cooling over an array at higher levels when pressure drop is not of concern.

Effect of Rotation on Heat Transfer in AR = 2:1 and AR = 4:1 Channels Connected by a Series of Crossover Jets

Detailed heat transfer measurements using transient liquid crystal thermography were performed on a novel cooling design covering the mid-chord and trailing edge region of a typical gas turbine blade under stationary and rotating conditions. The test section comprised of two channels with aspect ratio (AR) of 2:1 (mid-chord) and 4:1 (trailing edge), where the coolant was fed into the AR = 2:1 channel from the root. Rib turbulators with a pitch-to-rib height ratio (p/e) of 10 and rib height-to-channel hydraulic diameter ratio (e/Dh) of 0.075 were placed in the AR = 2:1 channel at an angle of 60° relative to the direction of flow. The coolant after entering this section was routed to the AR = 4:1 section through a set of crossover jets. The purpose of the crossover jets was to induce sideways impingement onto the pin fins that were placed in the 4:1 section to enhance heat transfer. The 4:1 section had a realistic trapezoidal shape that mimics the trailing edge of an actual gas turbine blade. The pin fins were arranged in a staggered array with a center-to-center spacing of 2.5 times the pin diameter in both spanwise and streamwise directions. The trailing edge section consisted of both radial and cutback exit holes for flow exit. Experiments were performed for a Reynolds number (ReDh(AR = 2:1)) of 20,000 at Rotation numbers (RoDh(AR = 2:1)) of 0, 0.1 and 0.14. The channel averaged heat transfer coefficient on trailing side was ∼28% (AR = 2:1) and ∼7.6% (AR = 4:1) higher than the leading side for Rotation number (Ro) of 0.1. It is shown that the combination of crossover jets and pin-fins can be an effective method for cooling wedge shaped trailing edge channels over axial cooling flow designs.

HEAT TRANSFER IN A ROTATING, TWO-PASS, VARIABLE ASPECT RATIO COOLING CHANNEL WITH PROFILED V-SHAPED RIBS

The thermal performance of two V-type rib configurations is measured in a rotating, two-pass cooling channel. The coolant travels radially outward in the rectangular first pass (AR = 4:1), and travels radially inward in the second pass (AR = 2:1). Both the passages are oriented 90° to the direction of rotation. The LS and TS of the channel are roughened with V-type ribs. The first V-shaped configuration has a narrow gap at the apex of the V. The configuration is modified by off-setting one leg of the V to create a staggered discrete, V-shaped configuration. The ribs are oriented 45° relative to the streamwise coolant direction. The heat transfer enhancement and frictional losses are measured with varying Reynolds and rotation numbers. The Reynolds number varies from 10,000 to 45,000 in the AR = 4:1 first pass; this corresponds to 16,000 to 73,500 in the AR = 2:1 second pass. The maximum rotation numbers are 0.39 and 0.16 in the first and second passes, respectively. The heat transfer enhancement on both the leading and trailing surfaces of the first pass of the 45° V-shaped channel is slightly reduced with rotation. In the second pass, the heat transfer increases on the leading surface while it decreases on the trailing surface. The 45° staggered, discrete V-shaped ribs provide increased heat transfer and thermal performance compared to the traditional V-shaped and standard, 45° angled rib turbulators.