Hi
I simulated an axial turbine. Now I have faced with strange problem.
Performance of any turbine can be improved if the losses are identified and minimized. Several losses can be defined by flow separation, small or large vortices creation or flow circulation, etc.
In fact, when the turbine have lower efficiency than others turbines, it means that this turbine have more losses than others. In other words, the mentioned turbine have more flow separation, vortices or flow circulation than others. This again means that mentioned turbine have more entropy generation than others because entropy generation include velocity gradient terms and when separation and circulation(losses) increase, entropy generation also increases. Is my statement right?am i right?
Now, i designed an improved blade and a thicker blade(constant cord) than to base blade. . My results show that the Improved blade have better efficiency and lower entropy generation than to base blade. These results are logical . than to base blade. Is it possible that entropy generation decreases with decreasing efficiency?
I am grateful that guide me about this case.
One of the problems that we have is the problem of regular or irregular thermodynamics.
Hi,
when you are using thinner blade, the contact area of blade with the surrounding fluid is small than the thicker blade. hence, the entropy generation is lesser in thinner blade than the thicker blade.
are you calculating the entropy generation due to both heat and fluid dissipation? if no then the equations, which you are using for the calculation of entropy generation, gave you only the entropy generation due to fluid viscous dissipation. you have to also consider the entropy generation due to heat generation because the speed of turbine is very high (2000 rpm).
about efficiency, you cant consider just one factor entropy generation to calculate efficiency.
Ali,
Entropy is a thermodynamic state. It is not the definition that you have used. You are discussing something that it is not entropy.
Hi,
First, get to the basics! Evaluate the stagnation pressure loss between the inlet and the the outlet surfaces. Use the mean values. Let us know the results. If the mean stagnation pressure increases you are in trouble...
Hi professor,
I used total pressure for calculating efficiency. (please see the following figure). And also, total pressure loss(between the inlet and the the outlet surfaces) increase with increasing axial velocity that it is logical. And also published papers confirm this case.(please see published paper-fig 4, 6 and 7).
At a stagnation point, the velocity of the fluid is zero. If the gravity head of the fluid at a particular point in a fluid flow is zero, then the stagnation pressure at that particular point is equal to total pressure. However, in general total pressure differs from stagnation pressure in that total pressure equals the sum of stagnation pressure and gravity head.(see the following link-Wikipedia)
P total = 0 + P stagnation
I am grateful that guide me more.
Thanks.
Let W be the work actually performed, Wmax be the maximum possible work allowed by the Second Law of Thermodynamics, Qh be the heat input from the hot reservoir, Tc be the temperature of the cold reservoir, E be the actual efficiency, and Ecarnot be the Carnot (maximum possible) efficiency. Then the change in total entropy can be written as delta S = (Wmax - W)/Tc = Qh(Ecarnot - E)/Tc. Since Tc is fixed (the ambient temperature), it seems that delta S can decrease with decreasing E only if Qh decreases faster with decreasing E than (Ecarnot - E) increases with decreasing E. Might thicker blades impede heat flow Qh from the hot reservoir sufficiently that Qh decreases faster than (Ecarnot - E) increases?
Hi Ali,
The rule of thumb for the Wells turbine is simple:
- Thicker blades separate at higher incidence angle at a cost of increasing the drag at low incidences.
- Larger leading edge radius, for the same thickness, postpone separation and increase the drag at low incidences.
So, for a first design you need a compromise between these effects along the span. Keep in mind that the Wells turbine operate under reciprocating flow conditions. The flow is never steady nor periodic due to the random characteristics of the ocean waves. Additionally, speed control is mandatory for the Wells turbine.
In my opinion, before you get too deep in the CFD analysis, you should check the book chapter Falcão AFO, Gato LMC. Air turbines. In: Sayigh A, editor. Comprehensive Renewable Energy. Ocean Energy, vol. 8. Oxford: Elsevier; 2012. p. 111e49. http://dx.doi.org/10.1016/B978-0-08-087872-0.00805-2 (this is almost mandatory), Ralph Starzmann PhD thesis (https://www.researchgate.net/profile/Ralf_Starzmann), and the classical book about Turbomachines written by Dixon.
Entropy: A concept that is not a physical quantity
https://www.researchgate.net/publication/230554936_Entropy_A_concept_that_is_not_a_physical_quantity
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