which is 0.3 % of the total mass-flow, appears to be well mixed with the leakage flow and makes little difference to the flow field downstream of the rotor.

Cooling is indispensable in rotors. It is also expensive, since cooling air is high pressure air that does no work. One would like to use cooling air for reasons other than heat transfer damage protection; for instance it could be used for the reduction of total pressure losses. The present study attempted to find whether coolant could be used for reducing the losses associated with tip leakage flow. The idea was to block the way of leakage flow entering into the tip gap, hence reducing the leakage mass flow. Results indicate that the concept might prove beneficial, but a relatively large amount of coolant is necessary to make significant improvements. Results from rotating frame measurements indicate that the leakage vortex is much stronger than the passage vortex, and is expected to be the most dominant vortical structure entering the downstream stage. Efforts are being made to increase the coolant mass flow rate as well as to measure heat transfer characteristics of the flow.

Detailed information and reduced data for this part of the study is presented in Appendix-

" Development of Tip Clearance Flow Downstream of a Rotor Blade with Coolant Injection," (Dey, D. and C. Camci), Proceedings of the 8th Int. Symposium on Transport Phenomena and Dynamics of Rotating Machinery ISROMAC-8, Vol. I, March 2000, pp: 572-579.

6.2.3 Heat transfer measurements on the tip surface using liquid crystal thermography

Convective heat transfer rates on the tip surface and on the endwall surface of the turbine rotor blades are presented in the form of convective heat transfer coefficients from steady state liquid crystal measurements. The Turbomachinery Heat Transfer Laboratory of the PSU has expertise in measuring heat transfer rates on surfaces with arbitrarily specified external boundaries as explained in Wiedner and Camci [1997] "Determination of Convective Heat Flux on State Heat Transfer Surfaces with Arbitrarily Specified Boundaries," the Transactions of the ASME, Journal of Heat Transfer, Vol.118, No.4, pp:l-8, November 1996. The approach described in this publication is extremely relevant to the success of the current heat transfer program because of the complicated external boundaries existing on the tip section of the rotor blade and on the endwall surface. The surface heater used in this study is formed from a 0.010 inch thick INCONEL 600 and attached to the tip section. The heat transfer surface is non-intrusive. The electric heater foil provides a prescribed wall heat flux boundary condition at the fluid solid interface. The liquid crystal coating on the heater surface provides the local wall temperature information. Local heat flux, wall temperature and free stream temperature information defines the heat transfer coefficient. The color information on the liquid crystal surface in the rotating frame is recorded from the stationary frame with the help of a stroboscope illumination system that was recently proven to be extremely useful in rotating frame applications. The Turbomachinery Heat Transfer Laboratory at PSU currently has all the components to complete liquid crystal thermography in the rotating frame. Although the project is currently terminated by the AGTSR , an additional heat transfer measurement effort is continuing with funds available from the Dept. of Aerospace Engineering at Penn State. The turbine blade conduction heat loss determination was completed in January 2002. We are expecting to continue our heat transfer efforts in Spring 2002 using limited funds available from the department. Some of our more current efforts in this area are summarized as follows:

A constant heat flux surface for turbine heat transfer research

Current Task and the Facility: A turbine blade mounted in the axial flow turbine facility of the Pennsylvania State University is shown in fig. 6.1. Twentynine rotor blades rotate at about 1330 rpm in air at ambient temperature (typically 20 — 30°C). One of the 29 blades needs to be instrumented for convective heat transfer coefficient measurements. The specific research interest is the tip platform surface. The electrical heater is powered by DC current transmitted through a slip-ring device. The current is limited to 5 Amps because of the slip-ring limitations.

Figure 6.1: Perspective view of blade in AFTRF

Inconel heater surface: Figure 6.2 shows our previous effort using a 0.001 inch thick Inconel sheet acting as a heater. Due to the specific tip geometry local heat flux is not constant. Currently we numerically calculate the local voltage and heat generation from the shape of the heater. The specific heat transfer measurement technique we use requires surface temperature measurements at the fluid-solid interface. We spray a thin black paint layer on top of the heater and then another thin layer of thermochromic liquid crystals. The red/yellow color band can be moved over the top surface by increasing the current passing through the heater.

Figure 6.3 shows a thermogram of the tip surface. The colors playing between black and blue/violet are calibrated for temperature. In figure 6.3 red/yellow color shows up at

Figure 6.2: Earlier work using Inconel heater about 42°C. all of the colors between the red/yellow and blue/violet represent a temperature bandwidth of about 1 °C . The liquid crystal color against temperature is precisely calibrated. The local temperature from liquid crystal coating and the local heat flux from the Inconel heater leads to the calculation of h (convective heat transfer coefficient) on the tip surface. Although this is an excellent method of measuring tip heat transfer, the surface wall heat flux distribution is not constant. This method requires matching a calculated wall heat flux with the measured wall temperature from the liquid crystal coating.

A constant heat flux heater could be used to perform convective heat transfer coefficient measurements on the tip surface shown in Figures 6.1- 6.3. The airfoil area in fig. 6.4 marked with dashed lines need to generate CONSTANT HEAT FLUX. The thickness of the heater system should be between 0.003 and 0.004 inches. The high aspect ratio rectangular strips near the leading edge and trailing edge are just electrical connectors with negligible resistance. The airfoil area is about 2.4 in2 = 1600 mm2. We are expecting to have an overall resistance of 10 OHMS in the airfoil area. This will require 10W , 40W and 160W at voltage settings of 10Y, 20Y and 40 Y. The corresponding heat flux will be 4.2 W/iri2, 16.7 W/in2 and 66.7 W/in2. The wedge shaped trailing edge zone of the airfoil is the most important region in this research. Therefore we need to make sure that we can generate "constant heat flux" in this almost triangular area. We are hoping that we can obtain a constant heat flux surface in the zone marked with dashed line.

Figure 6.6 shows an enlarged view of Figure 6.5. Note that the trailing edge region between y=50 mm and 100 mm is the most important measurement region. We need a heat flux distribution as uniform as possible in this region. We also need a good heat flux distribution near the leading edge (160 < y < 180 mm).

Figure 6.3: Thermogram of tip surface

An alternative strategy

Due to the extremely narrow geometry near the trailing edge (25 < y < 100), it may be worthwhile to investigate a heater shape as shown in fig. 6.7. The plan is to bend the heater surface around the trailing edge wedge (from the dotted lines). The heater surface is extended into the almost square regions added (starting from the dashed lines). High aspect ratio rectangular strips are electrical connectors (with negligible resistance). Power Density (AIRFOIL AREA =1600 MM2=2.4 SQ.INCH)

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