The relative impact of total and radiative heat flux in a backward-facing step combustor

Open Access
- Author:
- Colborn, Jennifer
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 15, 2024
- Committee Members:
- Robert Kunz, Professor in Charge/Director of Graduate Studies
Jacqueline O'Connor, Chair & Dissertation Advisor
Daniel Haworth, Major Field Member
David Hall, Outside Unit & Field Member
Karen Thole, Major Field Member
Xinyu Zhao, Special Member - Keywords:
- gas turbine combustion
radiative heat transfer
convective heat transfer
combustion
heat transfer
combustor heat transfer - Abstract:
- In an effort to increase overall engine efficiency, modern gas turbine engines have higher bypass ratios, leading to smaller engine cores. Pressure ratios and firing temperatures inside the combustors are increasing, creating a severe environment where advanced thermal management schemes are required to meet increasing heat transfer loads and durability standards. As a result, heat transfer in gas turbine combustors is quite complex, required advanced cooling schemes to handle the widely varying convective heat transfer to the liner. In combusting environments, radiation must also be considered; radiation is produced both luminously and non-luminously for combustion products as well as from soot. Variations in product and soot formation rates through the combustor will drive uneven radiative heat transfer, creating an environment where prediction of heat transfer is difficult. To better understand the sensitivity of both radiative and convective heat transfer to combustor operating parameters in combustor liner scenarios, a backward-facing step test section is used. Backward-facing steps are optically-accessible, non-proprietary, and nominally two-dimensional, allowing for simpler modeling and instrumentation. They also capture many of the critical flow features in gas turbine combustors that drive convective heat transfer to the wall and determine flame stability (recirculation, flow/wall impingement, and boundary layer recovery). A new experiment was designed with an upstream vitiator to preheat the air for observation of hot gaseous products effects on heat flux; a flame could also be stabilized in the shear layer separating from the backward-facing step for measurement of flame impacts on heat flux. Using a variety of visual diagnostics (particle image velocimetry, OH planar laser induced florescence, chemiluminescence, and mid-wave infrared imaging) and sensors (thermocouples, radiometers, and heat flux sensors) both the flow and heat flux to the wall through these flow features were characterized for a flame and for vitiated gases. Operating parameters including Reynolds number, gas and plate temperature, and gas concentration were varied to understand the sensitivity of wall heat flux to operating conditions. This work provides some of the first detailed measurements and comparisons to simulation of heat flux in reacting flows. For both vitiated and flame tests, the highest heat flux was seen at flow/flame-wall impingement and the lowest in the recirculation region. Vitiated tests show highest sensitivity to wall and gas temperature. The broadband spectral properties of combustion byproducts (particularly carbon dioxide and water) are shown to impact radiative heat transfer in the vitiated tests. A new method was developed to separate convective and radiative heat flux from a total and radiative heat flux measurements using simulations for insight into the spectral radiation properties of combustion byproducts. The sensors used for measuring heat flux only captured a portion of the incident radiation due to spectral limitations; not considering the full radiative load could lead to under-prediction of wall heat flux, decreasing durability of engine components. The importance of non-luminous radiation is additionally emphasized by the measured heat flux for both vitiated and flame testing being on the same order of magnitude. Flame tests highlighted the impacts of flame-wall interactions on heat flux, particularly the impacts of flame impingement. Higher temperature gradients in the test section floor are observed for flame tests than vitiated tests and flame wrinkling near the step is shown to increase with Reynolds number, corresponding to higher total heat flux measurements. Measurements in the impingement zone are shown to be most sensitive to Reynolds number, as increases in Reynolds number are driven by increased mass flow, increasing the thermal power of the flame and impacting the flame-wall behavior.