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Research Papers: Gas Turbines: Turbomachinery

# Effect of Variable Properties Within a Boundary Layer With Large Freestream-to-Wall Temperature Differences

[+] Author and Article Information
Nathan J. Greiner

Department of Aeronautics and Astronautics,
Air Force Institute of Technology,
Wright-Patterson AFB, OH 45433
e-mail: nathan.greiner@afit.edu

Marc D. Polanka, James L. Rutledge, Jacob R. Robertson

Department of Aeronautics and Astronautics,
Air Force Institute of Technology,
Wright-Patterson AFB, OH 45433

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 4, 2013; final manuscript received November 19, 2013; published online January 2, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(5), 052604 (Jan 02, 2014) (9 pages) Paper No: GTP-13-1400; doi: 10.1115/1.4026117 History: Received November 04, 2013; Revised November 19, 2013

## Abstract

Modern gas-turbine engines are characterized by high core-flow temperatures and significantly lower turbine-surface temperatures. This can lead to large property variations within the boundary layers on the turbine surfaces. However, cooling of turbines is generally studied near room temperature, where property variation within the boundary layer is negligible. The present study first employs computational fluid dynamics to validate two methods for quantifying the effect of variable properties in a boundary layer: the reference temperature method and the temperature ratio method. The computational results are then used to expand the generality of the temperature ratio method by proposing a slight modification. Next, these methods are used to quantify the effect of variable properties within a boundary layer on measurement techniques, which assume constant properties. Both low-temperature flows near ambient and high-temperature flows with a freestream temperature of $1600 K$ are considered under both laminar and turbulent conditions. The results show that variable properties have little effect on laminar flows at any temperature or turbulent flows at low temperatures such that constant property methods can be validly employed. However, variable properties are seen to have a profound effect on turbulent flows at high temperatures. For the high-temperature turbulent flow considered, the constant property methods are found to overpredict the convective heat transfer coefficient by up to $54.7%$ and underpredict the adiabatic wall temperature by up to $209 K$. Utilizing the variable property techniques, a new method for measuring the adiabatic wall temperature and variable property heat-transfer coefficient is proposed for variable property flows.

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## References

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## Figures

Fig. 1

Adiabatic wall temperature extrapolation of DeLallo [8] in a fuel-rich freestream (Φ = 1.3) with a normal trench-cooling plenum configuration supplied by N2 and air at various blowing ratios (M)

Fig. 2

Grid and boundary conditions for the computational domain

Fig. 3

Comparison of cf and Nux along a flat plate calculated via CFD to analytical correlations; T∞ = 1600 K, u∞ = 70 m/s

Fig. 4

Effect of temperature ratio on the ratio of variable property to constant property coefficients at constant Rex; u∞ = 70 m/s. (a) Coefficient of friction. (b) Convective heat-transfer coefficient.

Fig. 7

Temperature ratio method for large T∞-Tw: T∞ = 1600 K, u∞ = 70 m/s. (a) Convective heat transfer coefficients and heat fluxes calculated from turbulent and laminar correlations. (b) Apparent h (based on slope of q at Tw) and apparent Taw (abscissa-intercept of line tangent to q at Tw).

Fig. 6

Reference temperature method for large T∞-Tw: T∞ = 1600 K, u∞ = 70 m/s. (a) Convective heat-transfer coefficients and heat fluxes calculated from turbulent and laminar correlations. (b) Apparent h (based on slope of q at Tw) and apparent Taw (abscissa-intercept of line tangent to q at Tw).

Fig. 8

Effect of variable properties on heat flux over a large range of wall temperatures; T∞ = 1600 K, u∞ = 70 m/s

Fig. 5

Reference temperature method for small T∞-Tw: T∞ = 323 K, u∞ = 70 m/s. (a) Convective heat transfer coefficients and heat fluxes calculated from turbulent and laminar correlations. (b) Apparent h (based on slope of q at Tw) and apparent Taw (abscissa-intercept of line tangent to q at Tw).

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