Browsing by Author "Park, Suhyeon"

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    Experimental Investigation of Flow and Wall Heat Transfer in an Optical Combustor for Reacting Swirl Flows
    Park, Suhyeon (Virginia Tech, 2018-02-23)
    The study of flow fields and heat transfer characteristics inside a gas turbine combustor provides one of the most serious challenges for gas turbine researchers because of the harsh environment at high temperatures. Design improvements of gas turbine combustors for higher efficiency, reduced pollutant emissions, safety and durability require better understanding of combustion in swirl flows and thermal energy transfer from the turbulent reacting flows to solid surfaces. Therefore, accurate measurement and prediction of the flows and heat loads are indispensable. This dissertation presents flow details and wall heat flux measurements for reacting flow conditions in a model gas turbine combustor. The objective is to experimentally investigate the effects of combustor operating conditions on the reacting swirl flows and heat transfer on the liner wall. The results shows the behavior of swirling flows inside a combustor generated by an industrial lean pre-mixed, axial swirl fuel nozzle and associated heat loads. Planar particle image velocimetry (PIV) data were analyzed to understand the characteristics of the flow field. Experiments were conducted with various air flow rates, equivalence ratios, pilot fuel split ratios, and inlet air temperatures. Methane and propane were used as fuel. Characterizing the impingement location on the liner, and the turbulent kinetic energy (TKE) distribution were a main part of the investigation. Proper orthogonal decomposition (POD) further analyzed the data to compare coherent structures in the reacting and non-reacting flows. Comparison between reacting and non-reacting flows yielded very striking differences. Self-similarity of the flow were observed at different operating conditions. Flow temperature measurements with a thermocouple scanning probe setup revealed the temperature distribution and flow structure. Features of premixed swirl flame were observed in the measurement. Non-uniformity of flow temperature near liner wall was observed ranging from 1000 K to 1400 K. The results provide insights on the driving mechanism of convection heat transfer. As a novel non-intrusive measurement technique for reacting flows, flame infrared radiation was measured with a thermographic camera. Features of the flame and swirl flow were observed from reconstructed map of measured IR radiation projection using Abel transformation. Flow structures in the infrared measurement agreed with observations of flame luminosity images and the temperature map. The effect of equivalence ratio on the IR radiation was observed. Liner wall temperature and heat transfer were measured with infrared thermographic camera. The combustor was operated under reacting condition to test realistic heat load inside the industrial combustors. Using quartz glass liner and KG2 filter glass, the IR camera could measure inner wall surface temperature through the glass at high temperature. Time resolved axial distributions of inner/outer wall temperature were obtained, and hot side heat flux distribution was also calculated from time accurate solution of finite difference method. The information about flows and wall heat transfer found in this work are beneficial for numerical simulations for optimized combustor cooling design. Measurement data of flow temperature, velocity field, infrared radiation, and heat transfer can be used as validation purpose or for direct inputs as boundary conditions. Time-independent location of peak location of liner wall temperature was found from time resolved wall temperature measurements and PIV flow measurements. This indicates the location where the cooling design should be able to compensate for the temperature increase in lean premixed swirl combustors. The characteristics on the swirl flows found in this study points out that the reacting changes the flow structure significantly, while the operating conditions has minor effect on the structure. The limitation of non-reacting testing must be well considered for experimental combustor studies. However, reacting testing can be performed cost-effectively for reduced number of conditions, utilizing self-similar characteristics of the flows found in this study.
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    Non-intrusive sensing of air velocity, humidity, and temperature using tunable diode laser absorption spectroscopy
    Park, Suhyeon (Virginia Tech, 2015-06)
    This work will report the non-intrusive sensing of air velocity, humidity, and temperature using tunable diode laser absorption spectroscopy (TDLAS), and discuss the potential applications of such sensors for in situ monitoring and active control for wind energy. The sensing technique utilizes the absorption features of water vapor in ambient air to monitor multiple flow parameters including velocity, humidity, and temperature simultaneously and non-intrusively [1-3]. The TDLAS technique does not require pre-calibration or seeding and extensively employs fiber-optics technologies to facilitate its implementation. As a result, the sensor enjoys advantages such as low maintenance cost and scalability, which are especially attractive characteristics for practical deployment in power generation systems such as wind turbines. In this work, we will discuss the fundamentals of TDLAS technologies and the results obtained in a series of laboratory demonstrations. Figure 1 shows a schematic to illustrate the concept of TDLAS. The output of a laser diode (both in terms of it power and wavelength) was changed by modulating its driving current (for example, a 2 kHz ramp signal in this work). As a result such modulation, the wavelength of the output scans across a certain spectral range (near the 1392 nm for example in this work) to detect an absorption line of water vapor. Typically, the output of the laser were split into three parts using fiber couplers. The first part, a small portion (10% in this work) of the output, was fed into a Mach-Zehnder interferometer (MZI) to monitor the wavelength scan during the modulation as shown. The rest of 90% of the output was then split into two equal parts to be used as the probe beams. The probe beams are pitched at an angle and directed into the target flow as shown. The transmitted laser beams are detected by two photodiodes at the opposite side of the flow, whose signals are then collected by a data acquisition systems for further analysis. In the signal analysis, the magnitude of the absorption peak was used to determine the concentration of the water vapor in the measurement region, the relative strengths of two absorption peaks to determine the temperature, and the Doppler shift between the two beams to determine flow velocity [2]. A set of example measurements is shown in Figure 2 below to illustrate the data analysis mentioned above. The results show an absorption peak near 1392 nm, measured by two probe laser beams, one crossed the flow at -45 degrees (slide blue line) and the other at 45 degrees (dashed red line). As mentioned above, the absolute magnitude of such absorption signals (defined as absorbance as shown in Figure 2) is used to determine the concentration of water vapor. As shown in Figure 2, the absorption signals peak at different wavelengths because the Doppler shift caused by the flow. This Doppler shift (Δν) was calculated to be 4 x 10^(-4) cm-1 for this measurement as shown in the inset of Figure 2, based on which the flow velocity was determined to be 12.4 m/s. In this measurement, the target flow (which was generated by a small open jet tunnel) was also characterized by hotwire for comparison purposes. Figure 3 shows the flow velocity measured at multiple locations by the hotwire. We can see the velocity followed a distribution in the measurement region. The velocity distribution peaked at 20 m/s at the center of the flow, and the arithmetic mean of distribution was 11.7 m/s. In comparison, the TDLAS technique determines a line-of-sight integrated measurement of 12.4 m/s. In summary, the above example demonstrated the advantages of the TDLAS technique for monitoring multiple flow properties simultaneously and non-intrusively. Due to the advancement and maturity in diode lasers and fiber technologies, such technique is especially attractive for in situ application in practical systems.