Blade Element Momentum Theory

[stokesy34]

Hi there, im currently an undergraduate looking at trying out different airfoils for analysis to do with tidal turbine blades. I'm currently researching previous papers about blades that have been designed and they talk about using BEMT. They give all parameters required but I don't know how to input these or get results anywhere. The graphs/ answers they recieved were chord length(m)vs(r/R) and blade twist angle(degrees) vs (r/R).

The whole process is somewhat confusing at the moment and is causing me to stress a lot!

Any help on the matter would be much appreciated!

Thank you

 

[rb1957]

sorry, but "no student posting" ... ask a prof/TA.

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

The NREL has many pages of resources for you on their website, even a forum for their own analysis codes.

STF

 

[SAITAETGrad]

The reason for r/R is to make the analysis unitless. It is a common simplification for rotor analysis. First you need to understand that BEMT (Blade Element Moementum Theory) involves the combination of blade element and momentum analysis. First learn how to do these first. Gordon Leishman's book is a good primer on the subject. Sure, the book says helicopters but autorotation is what you need to know and the first few chapters work up this subject. Now don't come back until you graduate...

 

[KENAT]

thread1-363479 was a recent thread touching on similar territory.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pilotmm]

Since the turbine generate a pressure field upstream the blades of that turbine do not see the undisturbed flow field, they see a flow field that has already been modified by the turbine. So this created a circular problem: to estimate the modified flow field I would use 1D momentum theory and Bernoulli to estimate the flow speed at the rotor plane, but to know that I need to know the rotor thrust. To know the rotor thrust I need to estimate the modified flow field (i.e. the flow as seen by the blade) to apply the blade element theory. Like all circular problems one applies numerical methods, so one would iterate and voila you have an answer. This whole process is BEMT. Furthermore the theory with some additional assumptions gives an upper limit on power (the Betz limit) and can be used to estimate optimal blade geometry.

I took a quick glance at the wiki article

http://en.wikipedia.org/wiki/Blade_element_momentum_theory

and it seems accurate enough. Go ahead and read to indulge your curiosity but if this is an undergraduate project I would not implement your own BEMT code. To do it properly for wind turbines (which are basically the same as tidal turbines) there are several corrections that you need to apply. Furthermore like any numerical method there are all these different potential pit fall's you need to be aware and correct for to make a robust code required for general use. In other words implementing BEMT is too much work to complete within a typical under project time frame.

Instead I would use free code by NREL they use to have a stripped down easy to use version called wt_perf but I couldn't find it instead use

https://nwtc.nrel.gov/AeroDyn.

This should give you what you need to get the different forces.