Aerothermal testing of gas turbine components is usually performed under scaled laboratory conditions. The rig pressures are close to ambient pressure and applied temperatures are considerably lower. A geometric upscaling is performed to allow better access and resolution of applied measurement techniques.
In stationary application, the convective heat flux from the cooling film into the wall equals the heat flux through the wall. As proposed in [
29], setting the convective heat flux from Equation (
2) equal to the conductive wall heat flux with 1-D assumption results in
through a wall with the thickness
t and heat conductivity
k. For a correct thermal scaling, Equation (
3) shows the necessity of respecting the hot gas Biot number and the ratio of
. Coolant warming can be accounted for if needed [
30]. Depending on the operating parameters in the experiment, it can be reasonable and necessary to substitute the specimen materials to achieve Biot number similarity. For a cooled turbine blade, Albert et al. [
29] identified the Biot number to be
. The setup used for this article achieves this similarity and is described in the following.
2.1. Experimental Setup
The test rig consists of two rectangular channel sections that are connected together through a cavity in which a combustor double-wall is simulated. Measurements focus on impingement cooling in combination with effusion cooling and are performed under scaled aerothermal and aerodynamic conditions. Infrared measurement techniques will be used to derive cooling and heat flux quantities.
The infrastructure to drive the test section is shown in
Figure 2, left side. For the open loop main flow, the air is provided by a radial compressor, which is capable of providing a mass flow of 3 kg/s at a pressure ratio of 1.4. The air can be heated by electrical heaters with a maximum power of 450 kW. The power for each individual heater element (16 kW) is controlled through a Programmable-Logic-Controller-based proportional–integral–derivative (PID) controller by measuring the exit temperature of the air at each element, yielding a well mixed and uniform exit temperature. After heating, the air passes additional mixers, screens, and meshes to further improve temperature and velocity uniformity. Eventually the airflow is accelerated through a nozzle to achieve the target hot gas Reynolds number.
The coolant air is provided in a semi-closed loop. Compressed air is supplied by a rotary screw compressor and discharged through a regulated valve into the closed loop coolant system. The amount of introduced air into the closed loop is tracked by a mass flow meter and equals the ejected coolant in the test section. Thus, the given blowing ratios are always averaged values over the whole effusion specimen.
To circulate the air in the closed loop section and to achieve engine similar coolant cross flow Reynolds numbers, a blower is integrated. To maintain constant air temperature, the heat input of the blower is removed with a heat exchanger.
2.1.1. Test Section
The central test section is a scaled, planar double-wall combustor design, shown in
Figure 2, right side. Both the hot gas and the coolant channels are rectangular in cross section with parallel flow direction. They are connected via a cavity with an impingement plate on the coolant side and an effusion plate on the hot gas side.
The hot gas is introduced via the nozzle at the heater outlet. The exit width of the nozzle is
, and the height
250 mm. A turbulence grid generates an isotropic turbulence level of
9.8% (for design see [
31]). Further downstream, a boundary layer bleed is installed to obtain a reproducible boundary layer thickness in the hot gas channel. After the boundary layer bleed, the channel height reduces to
. The test specimen is integrated in the bottom wall of the channel where the connecting surfaces up- and downstream and at the sides are isolated with low thermal conductivity material (PEEK). The effusion specimen’s width and length are
and
.
The coolant channel has the same width and a height of 150 mm. The coolant flow channel inlet design (see ②) is adapted to increase coolant flow homogeneity (numerically designed and experimentally validated). The flow profile was further improved by introducing screens and meshes into the vertical portion of the coolant inlet. A turbulence grid with 5.0% (same design principle as above) was added in the coolant flow channel.
The connecting cavity is introduced through plastic frame made of PEEK and sealed with polytetrafluoroethylene (PTFE) against both channels. The height of this frame defines the distance between the specimen as specified for the different geometrical setups. A shift in longitudinal direction between impingement and effusion specimen can be set (an optional lateral shift can be realized with the test setup; the data, however, are not presented in this article) by moving the impingement specimen. The geometrical configurations shown in this article are summarized in
Table 1. This setup adds parallel cross flow conditions and thus simulates a combustor wall and engine realistic flow conditions, whereas most setups among different research papers (see
Section 1.1) use plenum driven coolant ejection.
In the hot gas channel as well as in the coolant channel, various accesses exist for flow measurements to set the operating point and to record measurement data, which will be addressed in the section for measurement techniques. The final test conditions are shown in
Table 2 for three of the six blowing ratios. By upscaling, a full similarity to engine conditions was achieved, most notably yielding high density ratios.
2.1.2. Test Specimen
The scaling factor used for the engine geometry is
, resulting in a reference diameter of
. The specimen on the hot gas side with effusion holes has 159 laidback fan-shaped cooling holes distributed over 25 rows. In streamwise direction, the effusion hole pattern is shifted by
, i.e., the pattern repeats every 4th row as depicted in
Figure 3. In streamwise direction, every row of cooling hole has a distance of
to the next row. In a row, the pitch between each cooling hole is
. The effusion cooling holes are inclined by
to the surface and have a cylindrical entry diameter of
. The lateral opening angle and laidback opening angle are
(designed after the 7-7-7 configuration introduced by [
32]).
The impingement specimen has vertical cylindrical cooling holes with a diameter of
. Pitch and spacing of each cooling hole remain the same as for the effusion cooling holes, while their position is offset as described in
Figure 3, right side. The chosen placement maximizes the wetted area on the backside of the effusion specimen before the coolant is ejected through the effusion cooling holes. For the misaligned setup
MALO05, the impingement specimen is shifted by
in longitudinal direction (
y), placing the impingement exit opening on the rows of effusion entry holes. The origin for evaluation is placed at the start of the effusion specimen, in the lateral center of the channel.
The specimen with the effusion holes is made of TiAl6-V4 and has a thickness of . The impingement plate is made of stainless steel AISI 420 with a thickness of . The selection of material and thickness was derived from scaling the Biot number for a flat plate at jet engine conditions to experimental conditions.
The hydraulic porosity of the double-wall setup calculates to
with
being the total effective area of the impingement and effusion holes, and
representing the total area of the specimen. For the calculation of the total effective area
, an incompressible relationship between mass flow rate and pressure difference is assumed. Thus, assuming constant density and a pressure drop distributed over both specimen
(see also
Table 2), this yields
2.2. Measurement Methods and Derived Quantities
The main area of interest is the effusion-cooled specimen close to the hot gas side. For cooling and heat flux data, temperature field data are needed. The temperature data are acquired on both sides of the effusion specimen simultaneously using two infrared cameras. The access points are show in
Figure 2, right side.
The hot gas side of the specimen is recorded using a FLIR SC6000 MWIR camera (Teledyne FLIR LLC, USA) with a short pass filter (
2–4.1
m). The short pass filter increases the data quality with hot sapphire windows [
33]. Eight large sapphire windows give seamless access to the top surface. With the employed optical setup, a lateral width of 200 mm of the effusion specimen’s surface is recorded at a resolution of ≈
—thus, lateral averages consider approximately five effusion holes per row.
The cavity side of the effusion specimen is accessed via four round windows () in the impingement specimen. An InfraTec VarioCam hr head 720 researcher (Infratec GmbH, Germany) LWIR ( 7–14 m) is used. The windows are made of ZnSe with an anti-reflective coating to allow LW transmissivity of 90%. The windows are inset into and drilled with the impingement specimen pattern to avoid flow disturbance. Due to the anti-reflective coating and low angles of view, these holes only show a small temperature footprint in the final data. Temperature on the lower side of the effusion specimen is thus available at four streamwise locations, each in a circular shape. In each lateral direction, the hot gas recordings cover a wider area to mitigate boundary effects in the FE models presented below.
The data from the MWIR InSb detector are linearized ([
34]). Otherwise, both cameras share similar calibration procedures. The data are corrected using a two-point non-uniformity correction (NUC) and then calibrated using a camera specific pre-calibration and a final in situ calibration [
33]. With given temperatures, the residual non-uniformity is expected below
[
35]. The measurement surfaces are coated using
Nextel Velvet Coating, a high emissivity paint with
to reduce the effect of reflections (coating thickness
). Due to the very low thermal conductivity of the coating (
W/(m K), [
36]), the thermal resistance is considered in the model and the evaluation of the FE analysis (see below). The final in situ calibration was performed using a single thermocouple for each image, choosing the position with the lowest expected heat flux to reduce thermocouple measurement uncertainty. Finally, the data are mapped to 3D space for further processing using a camera position estimation technique [
37].
The heat flux data are derived using an FE model to compute heat fluxes from boundary temperatures. The model geometry is chosen so that the geometry represents a repeatable pattern, the top side is fully covered with measured temperatures, and the bottom side is mostly covered, leaving a small uncovered region outside of the final evaluation areas, the latter being the main constraint. The geometry is shown in
Figure 4, left side view, for the hot gas side, and right side view for the cavity side. The colors show exemplary temperature data mapped from the infrared images for the access at position two. The perimeter walls are modeled as adiabatic due to the symmetry approach. The walls inside the cooling holes are modeled specifying a heat transfer coefficient and a constant sink temperature of 300 K. The heat transfer coefficient was derived from a Nusselt correlation for pipe flow evaluated at each operating point separately using the Reynolds numbers computed for the effusion holes. The resulting heat transfer coefficient ranges from
W/(m
2 K) for low blowing ratios up to ≈
W/(m
2 K) for high blowing ratios.
The surfaces with measured temperatures have to consider the coating due to its thermal resistance behavior. This is included using a thin layer model with no lateral but high wall normal thermal resistance. The temperature measured with the thermography systems is subsequently applied to the outer side of the thin layer. Any calculation considering thermal conductivity uses surrogate values derived from the multiple layer model. The materials used in the model are considered with temperature-dependent thermal conductivity.
Using this approach, a discrete model can be built for each of the four access windows from below. The derived data in
Section 3 are thus shown for each of the windows 1 to 4 and are averaged in three important areas of the model. These areas are shown in
Figure 4 in red: one area on the top side which connects four effusion exit holes with a parallelogram shape, and two different areas on the bottom. The first area on the bottom side (subscript
imp) is defined as the wetted area (
Figure 3), and the second area (subscript
eff) is defined as the
shifted area from the top side.
Additionally, the hot gas channel was modeled and a radiation analysis was performed for each test case using the full coverage temperature data on the top side of the effusion side to derive the calorimetric irradiance heat flux. This radiation heat flux is then added as an additional boundary condition to the described FE models, thus yielding the true convective heat flux .
Thus, the heat transfer coefficient is defined as
with
being either the hot gas recovery temperature or the coolant recovery temperature. The reference temperatures are evaluated for each simulation separately from the data acquisition log files. This heat transfer coefficient is the used to derive Nusselt numbers and Biot numbers. The former are computed with the temperature-dependent heat conductivity of either coolant or hot gas, and the latter are computed using the effective thermal conductivity from the FE model, including effects of the coating and temperature dependence of material properties included in the FE model. This final post processing is performed using [
38]—including automated data export and computation of derived quantities for each simulation.
Measurement uncertainty is derived by adding up several different sources. As described above, non-uniformity is expected below K. The calibration thermocouple uncertainty is given with K for a type K class I thermocouple, which is, however, shared between all measurements alike. Additional uncertainties are added when cross checking the final calibrated temperatures to other thermocouples not used for the calibration ( K) and a positional uncertainty considering the low-conductivity paint ( K). Thus, the non-bias temperature error adds up to = 3.5%, referenced on the temperature span between hot gas and coolant. For the FE simulations, this uncertainty has to be considered on both boundary sides alike, doubling its effective value. Assuming linear error propagation (which is not fully correct due to temperature-dependent material properties), this error propagates to heat fluxes and thus Nusselt and Biot numbers. Adding general modeling errors in the FE (e.g., uncertainty for in-hole boundary conditions, adiabatic modeling of perimeter), the final uncertainty is expected to be below 15%.
A validation of measurement data can only be performed checking plausibility by considering other data available. Comparing, e.g., impingement jet Nusselt numbers (quantity and shape, Figure 8) or total cooling effectiveness on the hot gas side, a good agreement with the available literature is found.
Additionally, several flow measurements are performed. In the hot gas channel, a pitot tube including a thermocouple and a static tap in the wall is used to measure the bulk velocity and temperature, and thus Reynolds number. The probe is placed in the center of the hot gas channel (, , ). Additional probes at the channel side record static pressure in streamwise direction.
In the coolant channel, a pitot-static tube was used for bulk velocity measurements. Previously expressed concerns about unequal mass distribution in the coolant channel indicated that the usage of a pitot-static probe delivers more reliable results of bulk velocity measurements than a simpler pitot tube with a static port at the wall. The coolant temperature is monitored with a thermocouple probe. Additional pressure taps in the cavity (four along the streamwise direction) enable the measurement of pressure losses due to impingement cooling and effusion cooling separately. All parameters are constantly monitored and recorded, and the time used is resolved for the computation of relevant quantities.