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.

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.

scholarly journals The Application of Adiabatic Film Cooling Effectiveness to Non-Adiabatic Blade Cooling Design

1983 ◽  
Author(s):  
C. P. Lee ◽  
J. C. Han
Keyword(s):  
Heat Transfer ◽  
Film Cooling ◽  
Heat Transfer Analysis ◽  
Adiabatic Wall ◽  
Cooling Effectiveness ◽  
Film Cooling Effectiveness ◽  
Transfer Parameter ◽  
Adiabatic Effectiveness ◽  
C 32 ◽  
Heat Transfer Parameter

The effect of heat transfer on film cooling has been studied analytically. The proposed model shows that the non-adiabatic film cooling effectiveness will increase with increasing of the heat transfer parameter, Ū / (ρVCp)2, on the convex, the flat and the concave walls over the entire range of film cooling parameter, X/MS. On the convex wall with a blowing rate, M, of 0.51 and a heat transfer parameter of 10−3 at the typical engine conditions, the non-adiabatic effectiveness can be higher than the adiabatic effectiveness by 45% at a film cooling parameter of 103; while the film temperature can be lower than the adiabatic wall by 18°C (32°F) at a dimensionless distance of 500. The model can be extended and applied to the heat transfer analysis for any kind of turbine blade with film cooling.

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.

Paradoxes of Heat Transfer on a Permeable Wall

2010 ◽  
Author(s):  
A. I. Leontiev ◽  
V. G. Lushchik ◽  
A. E. Yakubenko
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Numerical Modeling ◽  
Turbulent Boundary Layer ◽  
Prandtl Number ◽  
Wall Temperature ◽  
Gas Injection ◽  
Gas Mixtures ◽  
Permeable Wall ◽  
Gas Temperature

Numerical modeling of a turbulent boundary layer on a permeable wall with gas injection is performed. New effects are discovered. It is shown in particular that the wall temperature in the region of the gas film may be lower than the injected gas temperature. This effect is especially essential for gas mixtures with low values of the Prandtl number.

A Novel Liquid Crystal Image Processing Technique Using Multiple Gas Temperature Steps to Determine Heat Transfer Coefficient Distribution and Adiabatic Wall Temperature

2004 ◽  
Vol 126 (4) ◽  
pp. 587-596 ◽  
Author(s):  
Abd Rahim Abu Talib ◽  
Andrew J. Neely ◽  
Peter T. Ireland ◽  
Andrew J. Mullender
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Liquid Crystal ◽  
Transfer Coefficient ◽  
Wall Temperature ◽  
Image Processing Technique ◽  
Transient Heat Transfer ◽  
Surface Measurement ◽  
Gas Temperature ◽  
Adiabatic Wall

This paper presents a novel experimental technique, which combines thermochromic liquid crystals with multiple steps in gas temperature, to determine heat transfer coefficient and adiabatic wall temperature distributions. The transient heat transfer experiments have been conducted on a flat plate using the low-temperature analogue of an ISO standard propane-air burner commonly used in aero-engine fire certification. The technique involves the measurement of the surface temperature response of an insulating model to a change in gas temperature. A coating comprising more than one thermochromic liquid crystal material is used to increase the range of the surface measurement and this is combined with multiple step changes in gas temperature. These measures induce several peaks in liquid crystal intensity throughout the transient experiment and these are shown to improve the accuracy. The current technique employs useful data from both the heating and cooling phases in the heat transfer test. To the authors’ knowledge, this has not been investigated before and it is likely to be very useful for other applications of the liquid crystal transient heat transfer experiment. The uncertainties in all measurements have been quantified and are presented in this paper.

scholarly journals METHOD FOR DETERMINING ADIABATIC FILM EFFECTIVENESS IN PRESENCE OF THERMAL BOUNDARY LAYER

2021 ◽  
pp. 1-20
Author(s):  
James Parker ◽  
Thomas Povey
Keyword(s):  
Boundary Layer ◽  
Film Cooling ◽  
Thermal Boundary Layer ◽  
Complex Function ◽  
Mixing Layer ◽  
Inlet Temperature ◽  
Gas Temperature ◽  
Point Of Interest ◽  
The Difference ◽  
Processing Techniques

Abstract In this paper we present a new method for determining adiabatic film effectiveness in film-cooling experiments with non-uniform inlet temperature distributions, in particular the situation of an inlet thermal boundary layer. This might arise in a quasi-steady experiment due to loss of heat from the mainstream flow to the inlet contraction walls, for example. In this situation the thermal boundary layer would be time varying. Adiabatic film effectiveness is generally normalised by the difference between mainstream and coolant gas temperatures. Most importantly these temperatures are generally assumed to be spatially—and, possibly temporally—uniform at the system inlet. In experiments with non-uniform inlet temperature, the relevant hot-gas temperature for a particular point of interest on a surface is not easily determined, being a complex function of both the inlet temperature profile and the flow-field between the inlet and the point of interest. In this situation, adiabatic film effectiveness cannot be uniquely defined using conventional processing techniques. We solve this problem by introducing the concept of equivalent mainstream effectiveness, a non-dimensional temperature for the mainstream that can be used to represent the thermal boundary layer profile at the inlet plane, or the effective temperature of the mainstream gas—which we refer to as the equivalent mainstream temperature—entrained into the mixing layer affecting the wall temperature at a particular point of interest.

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 Thermal-Boundary-Layer Response to Convected Far-Field Fluid Temperature Changes

2008 ◽  
Vol 130 (10) ◽  
Author(s):  
Hongwei Li ◽  
M. Razi Nalim
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Temperature Difference ◽  
Wall Temperature ◽  
Thermal Boundary Layer ◽  
Steady State Solution ◽  
Step Change ◽  
Far Field ◽  
Fluid Temperature ◽  
Constant Wall Temperature

Fluid flows of varying temperature occur in heat exchangers, nuclear reactors, nonsteady-flow devices, and combustion engines, among other applications with heat transfer processes that influence energy conversion efficiency. A general numerical method was developed with the capability to predict the transient laminar thermal-boundary-layer response for similar or nonsimilar flow and thermal behaviors. The method was tested for the step change in the far-field flow temperature of a two-dimensional semi-infinite flat plate with steady hydrodynamic boundary layer and constant wall temperature assumptions. Changes in the magnitude and sign of the fluid-wall temperature difference were considered, including flow with no initial temperature difference and built-up thermal boundary layer. The equations for momentum and energy were solved based on the Keller-box finite-difference method. The accuracy of the method was verified by comparing with related transient solutions, the steady-state solution, and by grid independence tests. The existence of a similarity solution is shown for a step change in the far-field temperature and is verified by the computed general solution. Transient heat transfer correlations are presented, which indicate that both magnitude and direction of heat transfer can be significantly different from predictions by quasisteady models commonly used. The deviation is greater and lasts longer for large Prandtl number fluids.

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