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

2013 ◽  
Author(s):  
Nathan J. Greiner ◽  
Marc D. Polanka ◽  
Jacob R. Robertson ◽  
James L. Rutledge
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Wall Temperature ◽  
Film Cooling ◽  
Linear Trend ◽  
Linear Method ◽  
Ratio Method ◽  
Variable Properties ◽  
Adiabatic Wall ◽  
Constant Property

Modern aviation combustors run at high fuel-air ratios to achieve high turbine inlet temperatures and higher turbine efficiencies. To maximize turbine durability in such extreme temperatures, the blades are fitted with film cooling schemes to form a layer of cool air between the blade and the hot core flow. Two terms that are utilized to evaluate a cooling scheme are the heat transfer coefficient (h) and the local driving temperature, namely, the adiabatic wall temperature (Taw). The literature presents a method for calculating these two parameters by assuming the heat flux (q) is proportional to the difference in freestream and wall temperatures (T∞ − Tw). Several researchers have shown the viability of this approach by altering the wall temperature over a finite range in low temperature environment. A linear trend ensues where the slope is h and the q = 0 intercept is adiabatic wall temperature. This technique has proven valuable since constant h is known to be a valid assumption for constant property flow. The current study explores the validity of this assumption by analytically predicting and experimentally measuring the h and q at high T∞ and low Tw characteristic of a modern combustor. Both a reference temperature method and temperature ratio method were applied to model the effects of variable properties within the boundary layer. To explore the linearity of the heat transfer with driving temperature, the analysis determined the apparent h and Taw which would be measured over small ranges of Tw by the linear method discussed in the literature. This study shows that, over large Tw ranges, property variations play a significant role. It is also shown that the linear trend technique is valid even at high temperature conditions but only when used in small temperature ranges. Finally, this investigation shows that the apparent Taw used in the linear convective heat transfer assumption is a valid driving temperature over small ranges of Tw but cannot always be interpreted literally as the temperature where q(Taw) = 0.

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

2014 ◽  
Vol 136 (5) ◽  
Author(s):  
Nathan J. Greiner ◽  
Marc D. Polanka ◽  
James L. Rutledge ◽  
Jacob R. Robertson
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Turbulent Flows ◽  
Wall Temperature ◽  
Ratio Method ◽  
Variable Properties ◽  
Temperature Ratio ◽  
Adiabatic Wall ◽  
Constant Property ◽  
Variable Property

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.

The Effects of Conjugate Heat Transfer on the Thermal Field Above a Film Cooled Wall

2011 ◽  
Author(s):  
Jason E. Dees ◽  
David G. Bogard ◽  
Gustavo A. Ledezma ◽  
Gregory M. Laskowski
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Wall Temperature ◽  
Film Cooling ◽  
Thermal Boundary Layer ◽  
Heat Transfer Analysis ◽  
Gas Temperature ◽  
Adiabatic Wall ◽  
Thermal Fields ◽  
Conducting Wall

Common gas turbine heat transfer analysis methods rely on the assumption that the driving temperature for heat transfer to a film cooled wall can be approximated by the adiabatic wall temperature. This assumption implies that the gas temperature above a film cooled adiabatic wall is representative of the overlying gas temperature on a film cooled conducting wall. This assumption has never been evaluated experimentally. In order for the adiabatic wall temperature as driving temperature for heat transfer assumption to be valid, the developing thermal boundary layer that exists above a conducting wall must not significantly affect the overriding gas temperature. In this paper, thermal fields above conducting and adiabatic walls of identical geometry and at the same experimental conditions were measured. These measurements allow for a direct comparison of the thermal fields above each wall in order to determine the validity of the adiabatic wall temperature as driving temperature for heat transfer assumption. In cases where the film cooling jet was detached, a very clear effect of the developing thermal boundary layer on the gas temperature above the wall was measured. In this case, the temperatures above the wall were clearly not well represented by the adiabatic wall temperature. For cases where the film cooling jet remained attached, differences in the thermal fields above the adiabatic and conducting wall were small, indicating a very thin thermal boundary layer existed beneath the coolant jet.

scholarly journals An Investigation of Treating Adiabatic Wall Temperature as the Driving Temperature in Film Cooling Studies

2011 ◽  
Author(s):  
Lei Zhao ◽  
Ting Wang
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Wall Temperature ◽  
Film Cooling ◽  
Injection Hole ◽  
Adiabatic Wall ◽  
Temperature Potential ◽  
Driving Potential

In film cooling heat transfer analysis, one of the core concepts is to deem film cooled adiabatic wall temperature (Taw) as the driving potential for the actual heat flux over the film-cooled surface. Theoretically, the concept of treating Taw as the driving temperature potential is drawn from compressible flow theory when viscous dissipation becomes the heat source near the wall and creates higher wall temperature than in the flowing gas. But in conditions where viscous dissipation is negligible, which is common in experiments under laboratory conditions, the heat source is not from near the wall but from the main hot gas stream; therefore, the concept of treating the adiabatic wall temperature as the driving potential is subjected to examination. To help investigate the role that Taw plays, a series of computational simulations are conducted under typical film cooling conditions over a conjugate wall with internal flow cooling. The result and analysis support the validity of this concept to be used in the film cooling by showing that Taw is indeed the driving temperature potential on the hypothetical zero wall thickness condition, ie. Taw is always higher than Tw with underneath (or internal) cooling and the adiabatic film heat transfer coefficient (haf) is always positive. However, in the conjugate wall cases, Taw is not always higher than wall temperature (Tw), and therefore, Taw does not always play the role as the driving potential. Reversed heat transfer through the airfoil wall from downstream to upstream is possible, and this reversed heat flow will make Tw > Taw in the near injection hole region. Yet evidence supports that Taw can be used to correctly predict the heat flux direction and always result in a positive adiabatic heat transfer coefficient (haf). The results further suggest that two different test walls are recommended for conducting film cooling experiments: a low thermal conductivity material should be used for obtaining accurate Taw and a relative high thermal conductivity material be used for conjugate cooling experiment. Insulating a high-conductivity wall will result in Taw distribution that will not provide correct heat flux or haf values near the injection hole.

A Transient Infrared Thermography Method for Simultaneous Film Cooling Effectiveness and Heat Transfer Coefficient Measurements From a Single Test

2004 ◽  
Vol 126 (4) ◽  
pp. 597-603 ◽  
Author(s):  
Srinath V. Ekkad ◽  
Shichuan Ou ◽  
Richard B. Rivir
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Crystal ◽  
Transfer Coefficient ◽  
Infrared Thermography ◽  
Wall Temperature ◽  
Film Cooling ◽  
Initial Model ◽  
Adiabatic Wall ◽  
The Heat Transfer Coefficient

In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (nondimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to determine the two unknowns (the heat transfer coefficient and film effectiveness) at every point on the test surface. The advantage of this technique over existing techniques is the ability to obtain the information using a single transient test. Transient liquid crystal techniques have been one of the standard techniques for determining h and η for turbine film cooling for several years. Liquid crystal techniques do not account for nonuniform initial model temperatures while the transient IR technique measures the entire initial model distribution. The transient liquid crystal technique is very sensitive to the angle of illumination and view while the IR technique is not. The IR technique is more robust in being able to take measurements over a wider temperature range which improves the accuracy of h and η. The IR requires less intensive calibration than liquid crystal techniques. Results are presented for film cooling downstream of a single hole on a turbine blade leading edge model.

Analysis of Film Cooling and Full-Coverage Film Cooling of Gas Turbine Blades

1984 ◽  
Vol 106 (1) ◽  
pp. 206-213 ◽  
Author(s):  
E. R. G. Eckert
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Solid Surface ◽  
Wall Temperature ◽  
Film Cooling ◽  
Turbine Blades ◽  
Adiabatic Wall ◽  
Gas Turbine Blades ◽  
Full Coverage ◽  
Cooling Air

Film cooling has become a standard method for the protection of the skin of gas turbine blades against the influence of the hot gas stream. The cooling air is usually injected into the boundary layer covering the skin through one or two rows of holes. A calculation method to predict heat transfer to the skin of a film cooled wall based on two parameters—the film effectiveness and a heat transfer coefficient defined with the adiabatic wall temperature—has been widely accepted. More recently, those sections of a turbine blade skin requiring intensive cooling are covered over its entire area with holes through which cooling air is ejected. A different method to predict the temperature of this section by this “full coverage film cooling” has been proposed which is based on two different parameters θ and K. The air used for the cooling of the perforated section of the skin also provides protection to a solid section located downstream in the normal film cooling process. The two methods are reviewed, and it is discussed under what conditions and in which way results obtained with one method can be transformed to the parameters used in the other one. Published data [8, 9] are used to calculate film cooling effectiveness values and Stanton numbers based on the adiabatic wall temperature for a perforated wall and a solid surface downstream of 11 rows of holes with coolant injection. The results demonstrate the advantage of this method which has been shown in previous experiments with ejection through one or two rows of holes, for film cooling of a solid surface. For full-coverage film cooling, there is still the advantage that a heat transfer coefficient defined with the adiabatic wall temperature is independent of temperature difference within the restrictions imposed by the superposition model.

Adiabatic Wall Temperature and Heat Transfer Downstream of Injection Through Two Rows of Holes

1978 ◽  
Vol 100 (2) ◽  
pp. 303-307 ◽  
Author(s):  
M. Y. Jabbari ◽  
R. J. Goldstein
Keyword(s):  
Heat Transfer ◽  
Experimental Investigation ◽  
Heat Transfer Coefficient ◽  
Wall Temperature ◽  
Film Cooling ◽  
Adiabatic Wall ◽  
Cooling Effectiveness ◽  
Film Cooling Effectiveness ◽  
Single Row ◽  
The Heat Transfer Coefficient

Results of an experimental investigation of film cooling and heat transfer following injection through two staggered rows of holes are reported. The two staggered rows are considerably more effective in protecting the wall than a single row. The film cooling effectiveness at locations beyond about 30-hole dia downstream of injection is laterally uniform. The heat transfer coefficient is within a few percent of that without injection at low blowing rates, but it increases rapidly as the blowing rate increases above unity.

scholarly journals An Investigation of Treating Adiabatic Wall Temperature as the Driving Temperature in Film Cooling Studies

2012 ◽  
Vol 134 (6) ◽  
Author(s):  
Lei Zhao ◽  
Ting Wang
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Wall Temperature ◽  
Film Cooling ◽  
Injection Hole ◽  
Adiabatic Wall ◽  
Temperature Potential ◽  
Driving Potential

In film cooling heat transfer analysis, one of the core concepts is to deem film cooled adiabatic wall temperature (Taw) as the driving potential for the actual heat flux over the film-cooled surface. Theoretically, the concept of treating Taw as the driving temperature potential is drawn from compressible flow theory when viscous dissipation becomes the heat source near the wall and creates higher wall temperature than in the flowing gas. But in conditions where viscous dissipation is negligible, which is common in experiments under laboratory conditions, the heat source is not from near the wall but from the main hot gas stream; therefore, the concept of treating the adiabatic wall temperature as the driving potential is subjected to examination. To help investigate the role that Taw plays, a series of computational simulations are conducted under typical film cooling conditions over a conjugate wall with internal flow cooling. The result and analysis support the validity of this concept to be used in the film cooling by showing that Taw is indeed the driving temperature potential on the hypothetical zero wall thickness condition, i.e., Taw is always higher than Tw with underneath (or internal) cooling and the adiabatic film heat transfer coefficient (haf) is always positive. However, in the conjugate wall cases, Taw is not always higher than wall temperature (Tw), and therefore, Taw does not always play the role as the driving potential. Reversed heat transfer through the airfoil wall from downstream to upstream is possible, and this reversed heat flow will make Tw > Taw in the near injection hole region. Yet evidence supports that Taw can be used to correctly predict the heat flux direction and always result in a positive adiabatic heat transfer coefficient (haf). The results further suggest that two different test walls are recommended for conducting film cooling experiments: a low thermal conductivity material should be used for obtaining accurate Taw and a relative high thermal conductivity material be used for conjugate cooling experiment. Insulating a high-conductivity wall will result in Taw distribution that will not provide correct heat flux or haf values near the injection hole.

A Transient Infrared Thermography Method for Simultaneous Film Cooling Effectiveness and Heat Transfer Coefficient Measurements From a Single Test

2004 ◽  
Author(s):  
Srinath V. Ekkad ◽  
Shichuan Ou ◽  
Richard B. Rivir
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Crystal ◽  
Transfer Coefficient ◽  
Infrared Thermography ◽  
Wall Temperature ◽  
Film Cooling ◽  
Single Test ◽  
Initial Model ◽  
Adiabatic Wall

In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (non-dimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to determine the two unknowns (heat transfer coefficient and film effectiveness) at every point on the test surface. The advantage of this technique over existing techniques is the ability to obtain the information using a single transient test. Transient liquid crystal techniques have been one of the standard techniques for determining h and η for turbine film cooling for several years. Liquid crystal techniques do not account for non uniform initial model temperatures while the transient IR technique measures the entire initial model distribution. The transient liquid crystal technique is very sensitive to the angle of illumination and view while the IR technique is not. The IR technique is more robust in being able to take measurements over a wider temperature range which improves the accuracy of h and η. The IR requires less intensive calibration than liquid crystal techniques. Results are presented for film cooling downstream of a single hole on a turbine blade leading edge model.

Methods for Measuring and Computing the Adiabatic-Wall Temperature

2020 ◽  
Author(s):  
James Peck ◽  
Jason Liu ◽  
Kenneth M. Bryden ◽  
Tom I-P. Shih
Keyword(s):  
Heat Transfer ◽  
Wall Temperature ◽  
Film Cooling ◽  
Heated Surface ◽  
Experimental Studies ◽  
New Method ◽  
Multiple Sources ◽  
Adiabatic Wall ◽  
Different Temperatures ◽  
Computational Fluid Dynamics Cfd

Abstract For convective heat transfer involving multiple sources of different temperatures in the flow field such as in film cooling, the adiabatic-wall temperature, Tad, is used as the reference temperature to define the heat-transfer coefficient (HTC). Studies based on computational Fluid Dynamics (CFD) have always obtained Tad by requiring the cooled or heated surface to be adiabatic. Similarly, most experimental studies that measured Tad have sought to mimic adiabatic wall by using solids with very low thermal conductivity. Other experimental studies have obtained Tad by making two assumptions: (1) Tad at any given point on the cooled or heated surface is independent of the surface temperature, Ts, and the surface heat flux, qs″, at that point and (2) the HTC, had, which equals qs″/(Ts – Tad), depends only on the ratio of qs″ to Ts – Tad. With these two assumptions, measuring qs″ at two different Ts or vice versa at a point yields Tad and had at that point. In this study, CFD simulations, based on steady RANS, were performed to assess the assumptions invoked by CFD and experimental studies that seek to obtain Tad and had. The assessment was made by studying film cooling of a flat plate with an adiabatic wall and with isothermal walls, where the temperature of the isothermal wall, Ts, ranged from the lowest to the highest temperatures in the flow. Results from this study show Tad obtained by enforcing an adiabatic wall does not satisfy the requirement: where and when Ts – Tad = 0, qs″ = 0. Therefore, had approaches infinity where Ts – Tad is either zero or nearly zero, but qs″ ≠ 0. Also, obtaining Tad and had by measuring two sets of (Ts, qs″) was found to yield non-unique values that depended strongly upon the pair of (Ts, qs″) chosen. To overcome the shortcomings of existing methods, a new method was developed in this study to obtain Tad that does satisfy the requirement: Ts – Tad = 0 where and when qs″ = 0. Also, the method developed yields an had that is continuous across Ts – Tad = 0. By using the new method developed, errors in Tad and had obtained by existing methods were assessed.

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