Heat Transfer Enhancement in a Double Sequential Impingement Channel

Sequential impingement channels can reduce the adverse effect of crossflow in narrow impingement channels, as well as increase the cooling efficiency. In this work, sequential impingement channels are experimentally investigated using the transient liquid crystal technique to assess their thermal performances. A low heat transfer region is identified in the downstream part of the first channel where the flow is discharged into the second plenum. Various means of increasing the heat transfer at this location are investigated. Ribs on the target plate allow for an increase of the average heat transfer coefficient with small losses in pressure. Reducing the channel cross-section increases the mean flow velocity and, combined with the ribs, allows for a further increase of the heat transfer. Additionally, the geometrical changes of the channel caused by the addition of a ramp with a rounded corner, allow to decrease the pressure losses associated with the discharge into the second plenum, which is not optimal in the baseline configuration due to the sharp corner of the purge hole. Further reducing the cross-section to increase the heat transfer, however, increases the pressure losses due to the small open area in the transition zone.

Effect of Varying Jet Diameter on the Heat Transfer Distributions of Narrow Impingement Channels

The development of integrally cast turbine airfoils allows the production of narrow impingement channels in a double-wall configuration, where the coolant is practically injected within the wall of the airfoil providing increased heat transfer capabilities. This study examines the cooling performance of narrow impingement channels with varying jet diameters using a single exit design in an attempt to regulate the generated crossflow. The channel consists of a single row of five inline jets tested at two different channel heights and over a range of engine representative Reynolds numbers. Detailed heat transfer coefficient distributions are evaluated over the complete interior surfaces of the channel using the transient liquid crystal technique. Additionally, local jet discharge coefficients are determined by probe traversing measurements for each individual jet. A 10%-increasing and a 10%-decreasing jet diameter pattern are compared with a baseline geometry of uniform jet size distribution, indicating a considerable effect of varying jet diameter on the heat transfer level and the development of the generated crossflow.

Detailed Heat Transfer Distributions of Narrow Impingement Channels With Varying Jet Diameter

The development of integrally cast turbine airfoils allows the production of narrow impingement channels in a double-wall configuration, where the coolant is practically injected within the wall of the airfoil providing increased heat transfer capabilities. This study examines the cooling performance of narrow impingement channels with varying jet diameters using a single exit design in an attempt to regulate the generated crossflow. The channel consist of a single row of five inline jets tested at two different channel heights and over a range of Reynolds numbers. Detailed heat transfer coefficient distributions are evaluated over the complete interior surfaces of the channel using the transient liquid crystal technique. Local jet discharge coefficients are determined by probe traversing measurements for each individual jet. A 10%-increasing and a 10%-decreasing jet diameter pattern is compared with a baseline geometry of uniform jet size distribution indicating a considerable effect of varying jet diameter on the heat transfer level and the development of the generated crossflow.

Detailed Heat Transfer Distributions of Narrow Impingement Channels for Cast-In Turbine Airfoils

The current capabilities of the foundry industry allow the production of integrally cast turbine airfoils. Impingement cooling effectiveness can be then further increased due to the manufacturing feasibility of narrow impingement cavities in a double-wall configuration. This study examines experimentally, using the transient liquid crystal technique, the cooling performance of narrow cavities consisting of a single row of five impingement holes. Heat transfer coefficient distributions are obtained for all channel interior surfaces over a range of engine realistic Reynolds numbers varying between 10,900 and 85,900. Effects of streamwise jet-to-jet spacing (X/D), channel width (Y/D), jet-to-target plate distance (Z/D), and jet offset position (Δy∕D) from the channel centerline are investigated composing a test matrix of 22 different geometries. Additionally, the target plate and sidewalls heat transfer rates are successfully correlated within the experimental uncertainties providing an empirical heat transfer model for narrow impingement channels. The results indicate similarities with multijet impingement configurations; however, the achievable heat transfer level is about 20% lower compared to periodic multijet impingement correlations found in open literature.

Hole Staggering Effect on the Cooling Performance of Narrow Impingement Channels Using the Transient Liquid Crystal Technique

This study examines experimentally the cooling performance of narrow impingement channels as could be cast-in in modern turbine airfoils. Full surface heat transfer coefficients are evaluated for the target plate and the sidewalls of the channels using the transient liquid crystal technique. Several narrow impingement channel geometries, consisting of a single row of five cooling holes, have been investigated composing a test matrix of nine different models. The experimental data are analyzed by means of various post-processing procedures aiming to clarify and quantify the effect of cooling hole offset position from the channel centerline on the local and average heat transfer coefficients and over a range of Reynolds numbers (11,100–86,000). The results indicated a noticeable effect of the jet pattern on the distribution of convection coefficients as well as similarities with conventional multi-jet impingement cooling systems.

Effect of Channel Orientation on the Heat Transfer Coefficient in the Smooth and Dimpled Rotating Rectangular Channels

The detailed distribution of the heat transfer coefficient on rotating smooth and dimpled rectangular channels were measured using the transient liquid crystal technique. The rotating speed of the channel was fixed at 500 rpm and the tested Reynolds number based on the channel hydraulic diameter was 10,000. A stationary surface and two different channel rotating orientations of 90 deg and 120 deg were tested in order to investigate the effects of channel orientation on the distribution of the heat transfer coefficient in smooth and dimpled rotating surfaces. Results show that the heat transfer coefficient on the trailing surface is higher than that on the leading surface. For the 120 deg channel orientation angle cases, a higher heat transfer coefficient was observed near the outer surface. In the dimpled channel, the effect of the Coriolis force induced secondary flow on the heat transfer coefficient was not as significant as that for the smooth channel case.