Sean C. Jenkins

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor—Part I: Sensor and Benchmarks

A novel heat flux sensor was tested that allows for time-resolved heat flux measurements in internal ribbed channels related to the study of passages in gas turbine blades. The working principle of the atomic layer thermopile (ALTP) sensor is based on a thermoelectric field created by a temperature gradient over an yttrium-barium-copper-oxide (YBCO) crystal (the transverse Seebeck effect). The sensors very fast frequency response allows for highly time-resolved heat flux measurements up to the 1 MHz range. This paper explains the design and working principle of the sensor, as well as the benchmark-ing of the sensor for several flow conditions. For internal cooling passages, this novel sensor allows for highly accurate, time-resolved measurements of heat transfer coefficients, leading to a greater understanding of the influence of fluctuations in temperature fields.

The Effects of High Mainstream Turbulence and Turbine Vane Film Cooling on the Dispersion of a Simulated Hot Streak

This paper presents the combined effects of high turbulence and film cooling on the dispersion of a simulated hot streak as it passes over a scaled-up nozzle guide vane. Experimental data demonstrates a considerable decay in the strength of a hot streak due to turbulence effects alone. Film cooling further reduces the peak temperature values resulting in a reduction of the peak temperature in the hot streak on the order of 75% relative to the upstream peak temperature in the hot streak. Comparisons are made between high turbulence (Tu = 20%) and moderate turbulence (Ta = 3.5%) as well as between different blowing conditions for the suction side, showerhead, and pressure side film cooling holes on a simulated nozzle guide vane.

The Effects of the Vane and Mainstream Turbulence Level on Hot Streak Attenuation

This paper discusses the effects of varying pitch position on the attenuation of a simulated hot streak in a vane cascade under conditions of low and high turbulence. Measurements describe the effect of both small and large changes in pitch away from the stagnation line as evidenced by hot streak variations in the wake and at the trailing edge. Under both turbulence conditions, the vane serves to increase or decrease the attenuation rate depending on the location of the hot streak relative to the vane due to isolating of the hot streak core by the vane wall. Additional attenuation resulted due to the action of the wake on the hot streak.

The Influence of Including a Partially Smooth Section in the 2nd Leg of an Internally Ribbed Two Pass Cooling Channel

Internal cooling schemes for blades in a gas turbine engine often are subject to compromises between increased pressure losses in return for greater levels of heat transfer required to maintain durability levels in the engine’s harsh environment. Rib configurations have been the subject of much study in past years, however these configurations are normally presumed to be used in “full-coverage” mode, meaning that the ribs are placed in the channel in a continuous and uniform manner. This study investigates the interaction between the bend effects downstream of a 180° bend, which cause higher local heat transfer, and the effect of ribs. Some of the ribs directly downstream of the 180° bend in the 2nd leg of a two pass high aspect ratio (4:1) channel were removed and the effect on heat transfer was assessed. Experimental results showed that the heat transfer level recovered quickly once ribs were encountered. As expected, some decrease in heat transfer was observed in the region where ribs were removed; however total pressure losses in the channel were also much lower. Results include detailed two-dimensional heat transfer distributions determined by the transient liquid crystal method as well as an analysis of the balance between pressure recovery and local heat transfer levels. Generally, for the accuracy of the transient liquid crystal technique in complex three-dimensional flows, strongly varying fluid temperatures present in and downstream of the bend region must be taken into account. For this study, time and position dependent fluid temperature distributions were measured to account for these effects, making it possible to obtain high quality heat transfer results in those regions.Copyright

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor—Part II: Heat Transfer Results

Measurements using a novel heat flux sensor were performed in an internal ribbed channel representing the internal cooling passages of a gas turbine blade. These measurements allowed for the characterization of heat transfer turbulence levels and unsteadiness not previously available for internal cooling channels. In the study of heat transfer, often the fluctuations can be equally as important as the mean values for understanding the heat loads in a system. In this study, comparisons are made between the time-averaged values obtained using this sensor and detailed surface measurements using the transient thermal liquid crystal technique. The time-averaged heat flux sensor and transient TLC results showed very good agreement, validating both methods. Time-resolved measurements were also corroborated with hot film measurements at the wall at the location of the sensor to better clarify the influence of unsteadiness in the velocity field at the wall on fluctuations in the heat flux. These measurements resulted in turbulence intensities of the velocity and heat flux of 20%. The velocity and heat flux integral length scales were about 60% and 35% of the channel width, respectively, resulting in a turbulent Prandtl number of 1.7 at the wall.

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor: Part I — Sensor and Benchmarks

A novel heat flux sensor was tested which allows for time-resolved heat flux measurements in internal ribbed channels related to the study of passages in gas turbine blades. The working principle of the Atomic Layer Thermopile (ALTP) sensor is based on a thermoelectric field created by a temperature gradient over an YBCO crystal (the transverse Seebeck effect). The sensors very fast frequency response allows for highly time-resolved heat flux measurements up to the 1 MHz range. This paper explains the design and working principle of the sensor, as well as the benchmarking of the sensor for several flow conditions. For internal cooling passages, this novel sensor allows for highly accurate, time-resolved measurements of heat transfer coefficients, leading to a greater understanding of the influence of fluctuations in temperature fields.Copyright

Validation and Analysis of Numerical Results for a Varying Aspect Ratio Two-Pass Internal Cooling Channel

Numerical results for an internal ribbed cooling channel including a 180° bend with a 2:1 inlet and 1:1 aspect ratio outlet channel were validated against experimental results in terms of spatially resolved heat transfer distributions, pressure losses, and velocity distributions. The numerical domain consisted of one rib segment in the inlet channel and three ribs segments in the outlet channel to reduce the overall numerical effort and allow for an extensive parametric study. The results showed good agreement for both heat transfer magnitudes and spatial distributions and the numerical results captured the predominate flow physics resulting from the 180° bend. The production of Dean vortices and acceleration of the flow in the bend produced strongly increased heat transfer on both the ribbed and unribbed walls in the outlet channel in addition to increases due to the ribs. Numerical simulations were performed for a wide range of divider wall-to-tip wall distances, which influenced the position of the highest heat transfer levels on the outlet walls and changed the shape of the heat transfer distribution on the tip wall. Analysis of section averages of heat transfer in the bend and outlet channel showed a strong influence of the tip wall distance while no effect was seen upstream of the bend. A similarly large effect on pressure losses in the bend was observed with varying tip wall position. Trends in averaged heat transfer varied linearly with tip wall distance while pressure losses followed a non-linear trend, resulting in an optimum tip wall distance with respect to heat transfer efficiency.Copyright

The Effect of Ribs and Tip Wall Distance on Heat Transfer for a Varying Aspect Ratio Two-Pass Ribbed Internal Cooling Channel

Internal cooling channels with differing aspect ratios are typically found in gas turbine blades due to the varying thickness of the blade from the leading to trailing edge. These serpentine passages often contain several 180° bends, which are sharp edged in the region of the blade tip. The 180° bend has a pronounced effect on the heat transfer characteristics in the outlet channel and tip wall, where a strong influence is seen due to the divider wall-to-tip wall distance in the bend. The present study investigates the effect of the divider wall-to-tip wall distance for a ribbed two-pass cooling channel with a 2:1 inlet and 1:1 outlet channel. Spatially resolved heat transfer measurements were made using the transient thermochromic liquid crystal technique for a smooth and a ribbed configuration using parallel 45° ribs. Effects of the 180° bend on heat transfer and rib-induced enhancements were identified separately and bend effects were found to dominate the heat transfer increase in the outlet channel near the bend. Pressure losses due to the bend and ribs were also independently evaluated for a range of tip wall distances. Results show that the smaller tip wall distances increase heat transfer on the tip wall and outlet channel, but at the cost of an increased pressure loss. An optimum tip wall position is suggested, forming a compromise between heat transfer improvement and increased pressure losses.Copyright

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor: Part II — Heat Transfer Results

Measurements using a novel heat flux sensor were performed in an internal ribbed channel representing the internal cooling passages of a gas turbine blade. These measurements allowed for the characterization of heat transfer turbulence levels and unsteadiness not previously available for internal cooling channels. In the study of heat transfer, often the fluctuations can be equally as important as the mean values for understanding the heat loads in a system. In this study comparisons are made between the time-averaged values obtained using this sensor and detailed surface measurements using the transient thermal liquid crystal technique. The time-averaged heat flux sensor and transient TLC results showed very good agreement, validating both methods. Time-resolved measurements were also corroborated with hot film measurements at the wall at the location of the sensor to better clarify the influence of unsteadiness in the velocity field at the wall on fluctuations in the heat flux. These measurements resulted in turbulence intensities of the velocity and heat flux of about 20%. The velocity and heat flux integral length scales were about 60% and 35% of the channel width respectively, resulting in a turbulent Prandtl number of about 1.7 at the wall.Copyright