Abstract
In recent years, a number of authors have studied entropy generation in Wells turbines. This is potentially a very interesting topic, as it can provide important insights into the irreversibilities of the system, as well as a methodology for identifying, and possibly minimizing, the main sources of loss. Unfortunately, the approach used in these studies contains some crude simplifications that lead to a severe underestimation of entropy generation and, more importantly, to misleading conclusions. This paper contains a re-examination of the mechanisms for entropy generation in fluid flow, with a particular emphasis on RANS equations. An appropriate methodology for estimating entropy generation in isolated airfoils and Wells turbines is presented. Results are verified for different flow conditions, and a comparison with theoretical values is presented.
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06 May 2019
In the original publication of the article, Eqs. (26)���(28) are incorrect due to a missing �� symbol, inadvertently omitted when reporting the equations in the article.
06 May 2019
In the original publication of the article, Eqs. (26)���(28) are incorrect due to a missing �� symbol, inadvertently omitted when reporting the equations in the article.
Abbreviations
- CFD:
-
Computational fluid dynamics
- DNS:
-
Direct numerical simulation
- LES:
-
Large eddy simulation
- RANS:
-
Reynolds Averaged Navier Stokes
- \({\mathbf{a}}\) :
-
Acceleration (m s−2)
- A :
-
Area (m2)
- \(c_p\) :
-
Constant pressure specific heat (m2 s−2 K−1)
- \(c_v\) :
-
Constant volume specific heat (m2 s−2 K−1)
- dS :
-
Differential surface (m2)
- D :
-
Drag force (kg m s−2)
- f :
-
Frequency (s−1)
- \({\mathbf{F}}\) :
-
Force (kg m s−2)
- h :
-
Specific enthalpy (m2 s−2)
- H :
-
Total specific enthalpy (m2 s−2)
- k :
-
Specific turbulent kinetic energy (m2 s−2)
- p :
-
Pressure (kg m−1 s−2)
- \({\mathbf{q}}\) :
-
Thermal flux (kg s−3)
- \(r_{tip}\) :
-
Turbine tip radius (m)
- R :
-
Gas constant (m2 s−2 K−1)
- s :
-
Specific entropy (m2 s−2 K−1)
- \(S_1\) :
-
Near-field (surface) (m2)
- \(S_2\) :
-
Far-field (surface) (m2)
- \(\dot{S}_G\) :
-
Entropy generation rate (kg m2 s−3 s−1)
- t :
-
Time (s)
- T :
-
Temperature (K), torque (kg m2 s−2)
- \(u,{\mathbf {u}}\) :
-
Velocity (m s−1)
- \(V_a\) :
-
Turbine axial flow velocity (m s−1)
- \(\epsilon\) :
-
Turbulent dissipation rate (m2 s−3)
- \(\Phi\) :
-
Dissipation function (s−2)
- \(\lambda\) :
-
Thermal conductivity (kg m s−3 K−1)
- \(\lambda _t\) :
-
Turbulent conductivity (kg m s−3 K−1)
- \(\mu\) :
-
Dynamic viscosity (kg m−1 s−1)
- \(\mu _t\) :
-
Turbulent dynamic viscosity (kg m−1 s−1)
- \(\omega\) :
-
Specific turbulent dissipation rate (s−1), rotational speed (s−1)
- \(\Omega\) :
-
Mid-field (volume) (m\(^{3}\))
- \({\varvec{\varPi }}\) :
-
Viscous stress tensor (kg m−1 s−2)
- \({\varvec{\varPi }_R}\) :
-
Reynolds stress tensor (kg m−1 s−2)
- \(\rho\) :
-
Density (kg m−3)
- \(\sigma _T\) :
-
Entropy generation rate per unit mass due to heat transfer (m2 s−3 K−1)
- \(\sigma _V\) :
-
Entropy generation rate per unit mass due to fluid flow (m2 s−3 K−1)
- \(c_d\) :
-
Drag coefficient
- e :
-
Nepero constant
- k :
-
Non-dimensional frequency
- \(K_{\dot{S}_G}\) :
-
Non-dimensional entropy generation rate
- M :
-
Mach number
- \({\mathbf {n}}\) :
-
Surface normal unit vector
- Re :
-
Reynolds number
- \(\alpha\) :
-
Incidence angle
- \(\gamma\) :
-
Ratio of specific heats
- \(\phi\) :
-
Turbine flow coefficient
- \(\cdot\) :
-
Dot product
- \(\varDelta\) :
-
Difference
- \(\frac{\partial ()}{\partial t}\) :
-
Partial time derivative (s−1)
- \(\frac{D()}{Dt}\) :
-
Total time derivative (s−1)
- \(\nabla\) :
-
Nabla operator (m−1)
- \(\nabla ^2\) :
-
Laplacian operator (m−2)
- \(\nabla ^s\) :
-
Sum of gradient and gradient transposed (m−1)
- \(\otimes\) :
-
Cross product
- \(\overline{()}\) :
-
Time average
- \('\) :
-
Fluctuating part
- eff :
-
Effective
- \(\infty\) :
-
At infinity (upstream)
- mf :
-
Mean flow
- t :
-
Turbulent, due to turbulence
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This work has been funded by the Regione Autonoma Sardegna under Grant F72F16002880002 (L.R. 7/2007 n. 7 - year 2015).
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Ghisu, T., Cambuli, F., Puddu, P. et al. Numerical evaluation of entropy generation in isolated airfoils and Wells turbines. Meccanica 53, 3437–3456 (2018). https://doi.org/10.1007/s11012-018-0896-1
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DOI: https://doi.org/10.1007/s11012-018-0896-1