Post by Johansson on Sept 2, 2014 13:49:14 GMT -5
Thanks a lot guys for voting, I appreciate it a lot!
(for those of you who won´t read the previous page, I´ve entered my bike build in Boca Bearings inventor competition. 5000USD in first prize and I promise a burnout video if I win, so I´d be glad if you guys would vote on me!)
www.bocabearings.com/innovation-contest/ContestantDetails.aspx?ProjectID=137
Good progress! Hope you sort the small issues soon, can't wait to see a videos of you, blasting down the road again!
How is the outboard turbine build going? Very interesting too!
Cheers
Erik
Thank you Erik! I am actually working on the outboard engine right now while sorting out what to do about the bike, I´ll post a progress report after I´ve been at it again tomorrow.
I think a wind speed gauge would work great...unless there is a high amount of velocity at the inlet that might lead to a high suction...we would be sad to see the engine FOD out after you decided to complete a windspeed meter ingestion test...Ha!
No, really I think it is a good idea because you don't need to calculate the flow velocity. Actually, you could prepare a chart showing the mass flow at standard atmospheric conditions based on the airbox inlet area and the inlet flow velocity, that way during the test, you could glance at the chart and see where your mass flow range is.
John, I think the potential for residual swirl in the exhaust is high. Usually there is only a small operating range where the exhaust from the turbine exits in a fully axial direction. The potential for flow issues in a curved tube with a swirling flow seem like it would be quite "loss-y" and thus there would be a very high static pressure component. Curved passages set up secondary flows due to the forces needed to "turn" the flow and there is usually a spec given for how soon you can turn the flow after a turbine. The "visual test" of Ander's pipe didn't seem like it would be an issue, but the data he took made it seem like there are flow issues. I am not sure how the residual swirl from the turbine would effect the secondary flows in the pipe, but usually, swirling flows tend to highlight areas of flow separation. The use of deswirl vanes would be great at one flow condition, but then as the residual swirl changes the deswirl vanes might be severely off-incidence and cause flow separation and blockage, leading to even higher static pressures at the turbine exit.
Turboshaft and power generation turbines are quite sensitive to their exhaust systems. Since they are not trying to use the residual kinetic energy in the exhaust stream for any purpose (thrust, a subsequent turbine stage, etc.) they try to diffuse the flow carefully in order to recover the kinetic energy into static pressure. It is common to hear that the exhaust side of the turbine is actually operating at a static pressure below atmospheric! This took me a little bit of time to come to grips with back when I first heard it. The beauty of this for a large power generation turbine (or any for that reason, just the big ones spend more money on natural gas and thus it is a bigger cost driver!) is that the pressure ratio across the turbine is increased, so the required firing temperature is lower, thus lower fuel burn, etc.. Conversely, this also points the other side of this coin, increased losses in the exhaust lead to higher static pressures at the back side of the turbine, lowering the turbine pressure ratios and thus causing higher firing temps to make the required power for the system.
I struggled with a good way to describe this pressure effect, but I will give it a try and hopefully it will be clear. At the turbine exit plane, you have a certain total pressure available to "drive" the exhaust out through the exhaust plane farther downstream, made up of the static pressure component (potential energy component) and the dynamic pressure component that is due to the momentum of the flow, i.e. kinetic energy. If you generate flow losses, you are using some of that dynamic portion of the pressure to make the eddies, viscous shear, heat and other flow losses, thus using up some of that kinetic energy.
At the exhaust plane where the flow meets the atmosphere, the exhaust jet has to have the same static pressure as the atmosphere (known as the "free jet condition", unless the flow is sonic or above, then this doesn't apply). So if you have lost a portion of the dynamic pressure (the momentum driving the flow out), then you will need more of the static portion of the pressure to push the flow out. So with the static pressure fixed at the exhaust plane, due to the losses, nature readjusts the pressure balance so that you have a higher total/static pressure at the turbine than the case with no losses. But if you can recover a large portion the dynamic pressure into static pressure (kinetic energy into potential energy), then you don't need as much total/static pressure at the turbine. So in the ideal case, you can see that the static pressure at the turbine can be below the ambient pressure by as much as the dynamic component of the pressure at the turbine.
In fact, when you are assuming that the exhaust pressure equals ambient in your calculations, this is what you are assuming...perfect exhaust expansion (no losses) to ambient static pressure, which is not realistic, but commonly done.
Thus, even the exhaust system needs to be thought out quite well for a really good engine.
Good luck! ~ Chris
I´ll try not to drop the wind speed gauge into the engine.
Very interesting info about the exhaust duct downstream the freepower turbine! Much food for thought.
I´ve ordered the wind speed gauge, a P2 wire probe and a couple of high temp probes and gauges today, so when they arrive I have some work to do!
(for those of you who won´t read the previous page, I´ve entered my bike build in Boca Bearings inventor competition. 5000USD in first prize and I promise a burnout video if I win, so I´d be glad if you guys would vote on me!)
www.bocabearings.com/innovation-contest/ContestantDetails.aspx?ProjectID=137
Hi Anders
Good progress! Hope you sort the small issues soon, can't wait to see a videos of you, blasting down the road again!
How is the outboard turbine build going? Very interesting too!
Cheers
Erik
Thank you Erik! I am actually working on the outboard engine right now while sorting out what to do about the bike, I´ll post a progress report after I´ve been at it again tomorrow.
Hi Anders,
I think a wind speed gauge would work great...unless there is a high amount of velocity at the inlet that might lead to a high suction...we would be sad to see the engine FOD out after you decided to complete a windspeed meter ingestion test...Ha!
No, really I think it is a good idea because you don't need to calculate the flow velocity. Actually, you could prepare a chart showing the mass flow at standard atmospheric conditions based on the airbox inlet area and the inlet flow velocity, that way during the test, you could glance at the chart and see where your mass flow range is.
John, I think the potential for residual swirl in the exhaust is high. Usually there is only a small operating range where the exhaust from the turbine exits in a fully axial direction. The potential for flow issues in a curved tube with a swirling flow seem like it would be quite "loss-y" and thus there would be a very high static pressure component. Curved passages set up secondary flows due to the forces needed to "turn" the flow and there is usually a spec given for how soon you can turn the flow after a turbine. The "visual test" of Ander's pipe didn't seem like it would be an issue, but the data he took made it seem like there are flow issues. I am not sure how the residual swirl from the turbine would effect the secondary flows in the pipe, but usually, swirling flows tend to highlight areas of flow separation. The use of deswirl vanes would be great at one flow condition, but then as the residual swirl changes the deswirl vanes might be severely off-incidence and cause flow separation and blockage, leading to even higher static pressures at the turbine exit.
Turboshaft and power generation turbines are quite sensitive to their exhaust systems. Since they are not trying to use the residual kinetic energy in the exhaust stream for any purpose (thrust, a subsequent turbine stage, etc.) they try to diffuse the flow carefully in order to recover the kinetic energy into static pressure. It is common to hear that the exhaust side of the turbine is actually operating at a static pressure below atmospheric! This took me a little bit of time to come to grips with back when I first heard it. The beauty of this for a large power generation turbine (or any for that reason, just the big ones spend more money on natural gas and thus it is a bigger cost driver!) is that the pressure ratio across the turbine is increased, so the required firing temperature is lower, thus lower fuel burn, etc.. Conversely, this also points the other side of this coin, increased losses in the exhaust lead to higher static pressures at the back side of the turbine, lowering the turbine pressure ratios and thus causing higher firing temps to make the required power for the system.
I struggled with a good way to describe this pressure effect, but I will give it a try and hopefully it will be clear. At the turbine exit plane, you have a certain total pressure available to "drive" the exhaust out through the exhaust plane farther downstream, made up of the static pressure component (potential energy component) and the dynamic pressure component that is due to the momentum of the flow, i.e. kinetic energy. If you generate flow losses, you are using some of that dynamic portion of the pressure to make the eddies, viscous shear, heat and other flow losses, thus using up some of that kinetic energy.
At the exhaust plane where the flow meets the atmosphere, the exhaust jet has to have the same static pressure as the atmosphere (known as the "free jet condition", unless the flow is sonic or above, then this doesn't apply). So if you have lost a portion of the dynamic pressure (the momentum driving the flow out), then you will need more of the static portion of the pressure to push the flow out. So with the static pressure fixed at the exhaust plane, due to the losses, nature readjusts the pressure balance so that you have a higher total/static pressure at the turbine than the case with no losses. But if you can recover a large portion the dynamic pressure into static pressure (kinetic energy into potential energy), then you don't need as much total/static pressure at the turbine. So in the ideal case, you can see that the static pressure at the turbine can be below the ambient pressure by as much as the dynamic component of the pressure at the turbine.
In fact, when you are assuming that the exhaust pressure equals ambient in your calculations, this is what you are assuming...perfect exhaust expansion (no losses) to ambient static pressure, which is not realistic, but commonly done.
Thus, even the exhaust system needs to be thought out quite well for a really good engine.
Good luck! ~ Chris
I´ll try not to drop the wind speed gauge into the engine.
Very interesting info about the exhaust duct downstream the freepower turbine! Much food for thought.
I´ve ordered the wind speed gauge, a P2 wire probe and a couple of high temp probes and gauges today, so when they arrive I have some work to do!