Turbine Blade Entropy Generation Rate: Part I — The Boundary Layer Defined

2000 ◽  
Author(s):  
M. R. D. Davies ◽  
F. K. O’Donnell ◽  
A. J. Niven
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Entropy Generation ◽  
Leading Edge ◽  
Stress Generation ◽  
Entropy Generation Rate ◽  
Suction Surface ◽  
Viscous Shear ◽  
Layer Flow ◽  
Hot Film

The profile loss of a gas turbine blade is normally associated with the entropy increase due to the boundary layer phenomena of viscous shear, Reynolds stress generation and heat transfer. To establish the relative contributions of laminar, transitional and turbulent adiabatic boundary layer flow, to the overall entropy generation (as described in part two of this paper), detailed hot film and hot wire measurements have been made over the suction surface of a turbine blade mounted within a subsonic linear cascade. At a Reynolds number of 185 × 103, a natural transition region was found between 53 and 70% suction surface length, followed by a slowly relaxing turbulent boundary layer. The wall shear stress distribution indicated a peak in the leading edge region, dropping to a minimum value prior to transition, with a sharp rise over the transitional length before decreasing within the turbulent portion. The measurements were compared with predictions obtained from a commercial computational fluid dynamics code, which utilised the renormalisation group theory (RNG) turbulence model.

Turbine Blade Entropy Generation Rate: Part II — The Measured Loss

2000 ◽  
Author(s):  
F. K. O’Donnell ◽  
M. R. D. Davies
Keyword(s):  
Boundary Layer ◽  
Turbine Blade ◽  
Entropy Generation ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Local Entropy ◽  
Suction Surface ◽  
Layer Velocity ◽  
Local Entropy Generation ◽  
Profile Loss

Using detailed boundary layer velocity measurements the profile loss, expressed in terms of local entropy generation rate, is evaluated at discrete locations along the suction surface of a turbine blade in a subsonic linear cascade at a chord Reynolds number of 1.8 × 103 under adiabatic test conditions. The distribution of loss through the entire boundary layer is thus established with particular attention given to the comparison of the relative contributions from the laminar, transitional and turbulent regions. It is found that 75% of the entropy generation occurs in the laminar region of the blade, with transition being one of the key features of the overall loss distribution. Traditional correlation methods are considered and shown to give accurate results when compared to the experimental measurements within both the laminar and turbulent regions, the application of such correlations is however dependant upon knowledge of the onset and extent of transition. Finally it is demonstrated that an existing method for the evaluation of local entropy generation rate from measurements of wall shear stress in laminar flow, may be adapted for use in turbulent flow and hence the possibility is presented for the measurement of loss from surface mounted sensors.

Surface Flow Patterns as Visualised by Dust Deposits on the Blades of a Fan

1963 ◽  
Vol 67 (633) ◽  
pp. 589-594 ◽  
Author(s):  
E. T. Hignett ◽  
M. M. Gibson
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Leading Edge ◽  
Surface Flow ◽  
Scale Model ◽  
Flow Visualisation ◽  
Pressure Gradients ◽  
Suction Surface ◽  
Layer Flow ◽  
Dust Deposits

Investigations by one of the authors in connection with the design of a fan for a blower type of wind tunnel showed that regular and repeatable dust patterns occurred on the blades of a one-quarter scale model fan of 18 inches diameter. Dust was deposited on the fan blades along the leading-edge and on the suction surface over an area thought to be the turbulent region of the boundary layer. The introduction of isolated protuberances on the dust free area of a blade gave rise to turbulence wedges in which dust was also deposited and this was interpreted as confirmation of the coincidence of the dust deposits with regions of turbulent boundary-layer flow. These deposits showed the existence of a considerable extent of laminar flow on the suction surface of each blade close to the root, a region where high lift coefficients would be expected with associated adverse pressure gradients. Two-dimensional wind tunnel experiments were made to confirm the interpretation of the observed dust patterns by comparison with the smoke filament and volatile liquid methods of flow visualisation and these are reported in Reference 2.

A Prediction Method for the Local Entropy Generation Rate in a Transitional Boundary Layer With a Free Stream Pressure Gradient

2002 ◽  
Author(s):  
Ed Walsh ◽  
Roy Myose ◽  
Mark Davies
Keyword(s):  
Boundary Layer ◽  
Entropy Generation ◽  
Prediction Method ◽  
Accurate Method ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Suction Surface ◽  
Turbulent Region ◽  
Percentage Difference ◽  
Moderate Reynolds Number

To design an aerodynamically efficient blade the distribution of entropy generation on the blade surface should be known. Having only knowledge of the integrated loss, makes the task of improving the efficiency of a blade extremely difficult. A method to predict the entropy generation rate in steady, two-dimensional, incompressible, adiabatic boundary layer flows is presented, which gives both the distribution and magnitude of the entropy generation rate. This prediction method is based upon five correlations which are used to determine the: 1. entropy generated in the laminar region; 2. entropy generated in the turbulent region; 3. location of transition; 4. length of transition; 5. entropy generated in the transition region. These are then used to predict the entropy generation rate on the suction surface of a turbine rotor blade at a moderate Reynolds number; comparisons are then drawn with past measurements. The aim is to develop a quick, simple and relatively accurate method for the prediction of entropy in the boundary layers of turbomachines, although the method is not confined to this application. The only information required to implement this prediction method is the boundary layer edge velocity distribution and the turbulence intensity. A benefit of this method is that it does not rely upon dissipative CFD predictions, which are both slow to use in a design process and not yet sufficiently trustworthy. The dissipation coefficient and entropy generation rate predicted for this test case compare well to experimental measurements, with the percentage difference between the integrated entropy measured and predicted being approximately 13%. However, the difference in the turbulent region is found to be as high as 30%.

An Entropic Minimization Technique for Turbine Blade Profiles

2002 ◽  
Author(s):  
Ed Walsh ◽  
Mark Davies ◽  
Roy Myose
Keyword(s):  
Boundary Layer ◽  
Velocity Distribution ◽  
Boundary Layers ◽  
Turbine Blade ◽  
Entropy Generation ◽  
Incompressible Flows ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Minimization Technique ◽  
Boundary Layer Edge

The optimization of the boundary layer edge velocity distribution may hold the key to the minimization of entropy generation in the boundary layers of turbomachinery blades. A preliminary optimization analysis in the laminar region of a non film cooled turbine blade is presented, which demonstrates the concept of how the entropy generation rate may be reduced by varying the boundary layer edge velocity distribution along the suction surface, whilst holding the work done by the blade constant. In the laminar region the analytical technique developed by Pohlhausen and others to predict the boundary layer momentum thickness in the presence of pressure gradients has been adopted to predict the entropy generated as described in other papers by the same authors. The result gives an expression for the entropy generation rate in terms of the boundary layer edge velocity distribution for incompressible flows. The boundary layer edge velocity distribution may then be represented as a polynomial with undefined variables. This allows a minimization technique to be used to minimize the entropy generation rate on these variables. Constraints are included to keep the work output constant and the diffusion low to avoid separation. In this analysis it is only the laminar region that is considered for minimization, thus it is necessary to ensure that the modified boundary layer edge velocity distribution does not undergo transition earlier than the baseline boundary layer edge velocity distribution. This is accomplished by considering transition and separation criteria available in the literature. The result of this analysis indicates that the entropy generation rate may be reduced in the laminar boundary layers by using this technique.

scholarly journals Inherent irreversibility impacts on thermal boundary layer flow over a mobile plate with convective surface boundary conditions

2019 ◽  
Vol 5 (2) ◽  
pp. 45
Author(s):  
Sajjad Haider ◽  
Adnan Saeed Butt ◽  
Asif Ali ◽  
Yun-Zhang Li ◽  
Tufail Hussain
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Temperature Profile ◽  
Entropy Generation ◽  
Free Stream ◽  
Entropy Generation Rate ◽  
Surface Boundary ◽  
Entropy Generation Number ◽  
Surface Boundary Conditions ◽  
Layer Flow

<p class="abstract"><strong>Background:</strong> The irreversibility impacts on flow and heat transfer processes can be quantified through entropy analysis. It is a significant tool which can be utilized to deduce about the energy losses. The current study investigates the inherent irreversibility impacts during a flow of boundary layer and heat transfer on a mobile plate.</p><p class="abstract"><strong>Methods:</strong> The flow is examined under thermal radiation and convective heat conditions. The fundamental governing equations of flow and heat phenomenon are transmuted into ordinary differential equations by employing similarity transmutations and shooting technique is utilized in order to solve the resultant equations. The temperature and velocity profiles are acquired to reckon Bejan and entropy generation number. Pertinent results are elucidated graphically for the movement of plate and flow in same and opposite directions.  </p><p class="abstract"><strong>Results:</strong> A decline in temperature profile is noted with rise in values of <em>Pr</em> in both cases when the movement of surface and free stream is in similar and converse directions. A decrease in temperature is observed for both cases with increase in <em>N<sub>R</sub></em> while with the rise in Biot number <em>a</em>, the temperature profile also increases. Entropy generation rate near the surface is high in case when surface and free stream are moving in opposite directions as compared to case when they move in same directions.</p><p class="abstract"><strong>Conclusions:</strong> It is observed that irreversibility impacts are more remarkable when the movement of fluid and plate is in opposite direction. Moreover, irreversibility impacts of heat transfer are prominent in free stream region.</p><p class="abstract"> </p><br /><em></em>

Investigation of Hot Film Calibration for Entropy Generation Rate Calculations

2002 ◽  
Author(s):  
R. Mahon ◽  
P. Frawley ◽  
M. R. D. Davies
Keyword(s):  
Boundary Layer ◽  
Numerical Methods ◽  
Circular Cylinder ◽  
Constant Temperature ◽  
Entropy Generation ◽  
Cross Flow ◽  
Entropy Generation Rate ◽  
Hot Wire ◽  
Hot Film ◽  
The Relationship

The objective of this paper is to investigate in detail the relationship between results obtained from flow over a circular cylinder in cross flow using Hot Film and Hot Wire Constant Temperature Anemometry (C.T.A.). The experimental results are compared with those obtained using numerical methods. The results obtained from Hot Wire Anemometry are used to attempt to calibrate the Hot Film Sensors for the purpose of evaluating entropy generation rates in the boundary layer of the cylinder.

The Effect of Reynolds Number, Compressibility and Free Stream Turbulence on Profile Entropy Generation Rate

2002 ◽  
Author(s):  
Philip C. Griffin ◽  
Mark R. D. Davies ◽  
Francis K. O’Donnell ◽  
Ed Walsh
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Laminar Boundary Layer ◽  
Entropy Generation ◽  
Free Stream ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Suction Surface ◽  
Free Stream Turbulence ◽  
Layer Velocity

Detailed aerodynamic data from the suction surface boundary layer on a turbine blade arranged in a linear subsonic cascade was acquired under high free stream turbulence conditions (∼ 5.2%) generated using a perforated plate placed upstream of the cascade. In addition, data was also obtained from a transonic turbine cascade utilizing the same blade profile but of much smaller chord at free stream turbulence levels of 3.5%. Velocity profiles from the laminar, transitional and turbulent boundary layers were measured at various locations along the airfoil suction surface for the incompressible regime at ReC of 76,000. For the compressible test cases, boundary layer velocity profiles were measured at two locations towards the aft section of the blade at ReC of 163,000 and MEx of 0.37 respectively. For both cases the boundary layer velocity profiles were acquired by traversing a single normal hot wire probe normal to the blade surface. In addition the extent of the transition region over the blade surface was determined for both compressible and incompressible regimes by the use of an array of heated thin film sensors over a range of Reynolds and exit Mach numbers. It was observed that an earlier transition ensued at high free stream turbulence conditions in comparison to a previous investigation at comparable ReC and lower turbulence level (0.8% Tu). In addition comparisons were made to existing incompressible data at ReC = 185,000 and 0.8% free stream turbulence intensity. One of the primary observations resulting from an earlier transition was a thicker turbulent boundary layer, but in addition it was also noted that shear strain rates in the laminar boundary layer were significantly higher than those obtained at the 0.8% turbulence intensity. Further analyses also elucidated the presence of fluctuating components of velocity in the laminar boundary layer and were attributed to the effects of the free stream turbulence. This leads to the notion of a hybrid boundary layer, possessing both laminar and turbulent characteristics. These findings have implications regarding the profile loss of the blade, that is the loss generated in blade boundary layers and wakes normally associated with phenomena such as viscous shear, Reynolds stress production, shock wave formation and heat transfer across temperature differences and can be quantified in terms of the amount of entropy generated. For the purposes of this study entropy creation is solely restricted to that arising due to fluid dynamic phenomena, thereby assuming an adiabatic and quasi-isothermal flow. The entropy generation rate per unit volume is obtained directly from the boundary layer velocity profile; further integration gives rise to the entropy generation rate over the boundary layer at a point or over the entire suction surface length. Even though the number of quantitative measurement points on the transonic cascade was limited due to the very thin boundary layer present, no effects attributable to compressibility were observed on the entropy generation rate at the Mach number in question. Increased free stream turbulence had a greater effect on the generated entropy due to increased viscous shear in the laminar boundary layer and increased Reynolds stress production. In contrast, free stream turbulence did not have any significant effect on the turbulent boundary layer in the context of this study, as it was observed that the amount of entropy generated in the turbulent boundary layer was approximately equivalent for both turbulence levels at comparable Reynolds number.

Entropy Generation for Bypass Transitional Boundary Layers

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Richard S. Skifton ◽  
Ralph S. Budwig ◽  
John C. Crepeau ◽  
Tao Xing
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Flat Plate ◽  
Entropy Generation ◽  
Boundary Layer Flow ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Transitional Region ◽  
Bypass Transition ◽  
Layer Flow

The principal purpose of this study is to understand the entropy generation rate in bypass, transitional, boundary-layer flow better. The experimental work utilized particle image velocimetry (PIV) and particle tracking velocimetry (PTV) to measure flow along a flat plate. The flow past the flat plate was under the influence of a negligible “zero” pressure gradient, followed by the installation of an adverse pressure gradient. Further, the boundary layer flow was artificially tripped to turbulence (called “bypass” transition) by means of elevated freestream turbulence. The entropy generation rate was seen to behave similar to that of published computational fluid dynamics (CFD) and direct numerical simulation (DNS) results. The observations from this work show the relative decrease of viscous contributions to entropy generation rate through the transition process, while the turbulent contributions of entropy generation rate greatly increase through the same transitional flow. A basic understanding of entropy generation rate over a flat plate is that a large majority of the contributions come within a wall coordinate less than 30. However, within the transitional region of the boundary layer, a tradeoff between viscous and turbulent dissipation begins to take place where a significant amount of the entropy generation rate is seen out toward the boundary layer edge.

Second Law Analysis of Boundary Layer Flow With Variable Fluid Properties

2017 ◽  
Vol 139 (10) ◽  
Author(s):  
M. I. Afridi ◽  
M. Qasim ◽  
O. D. Makinde
Keyword(s):  
Boundary Layer ◽  
Differential Equations ◽  
Entropy Generation ◽  
Boundary Layer Flow ◽  
Numerical Solutions ◽  
Nonlinear Partial Differential Equations ◽  
Entropy Generation Rate ◽  
Bejan Number ◽  
Entropy Generation Number ◽  
Layer Flow

An entropy generation analysis of steady boundary layer flow of viscous fluid with variable properties over an exponentially stretching sheet is presented. The basic nonlinear partial differential equations that govern the flow are reduced to ordinary differential equations by using appropriate transformations. Numerical solutions are obtained by using shooting technique along with Runge–Kutta method. Expressions for the dimensionless volumetric entropy generation rate (NG) and Bejan number are also obtained. The effects of different dimensionless emerging parameters on entropy generation number (NG) and Bejan number (Be) are investigated graphically in detail.

scholarly journals Entropy Generation Measurement in a Laminar Turbine Blade Boundary-Layer

1997 ◽  
Author(s):  
John D. Wallace ◽  
Mark R. D. Davies
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Heat Transfer Rate ◽  
Turbine Blade ◽  
Entropy Generation ◽  
Transfer Rate ◽  
Surface Heat ◽  
Entropy Generation Rate ◽  
Blade Surface ◽  
Surface Heat Transfer

This paper demonstrates a method of calculating the entropy generation rate in an incompressible laminar turbine blade boundary-layer from measurements of surface heat transfer rate. It is shown that the entropy generated by fluid friction in an incompressible blade boundary-layer is significantly less than that generated by heat transfer at engine representative temperature ratios. The centre blade in a low-speed linear cascade is electrically heated and isolated from the airflow with a bypass valve. Upon opening the valve the blade is transiently cooled and thin film heat transfer gauges, painted on machinable glass ceramic inserts mounted into the surface of the blade, are used to record blade surface temperature and surface heat transfer rate signals; local Nusselt numbers are then calculated. Non-dimensional temperature distributions are derived across the boundary-layer using the blade surface heat transfer rate and a similarity condition. The equation describing the local entropy generation per unit volume is then integrated through the boundary-layer at each chordwise measurement point on the blade surface.

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