Post by finiteparts on Jul 19, 2020 13:44:48 GMT -5
Hi Patty,
Interesting engine and hopefully you can get it running better soon. I thought that I might share my initial thoughts on what may have been happening on your first startup and ask a few questions as well.
So when I read about your first startup, my initial thoughts jumped straight to insufficient fired torque...i.e. a hung start. If we talk about the start up of a single shaft gas turbine, there are two really big issues that have to be addressed.
The first issue is that during startup, in order to get the rotor to accelerate, we have to have more torque being produced in the turbine than what is being consumed in the compressor and bearings. The problem here is at low rotor speeds, the turbine (and compressor for that matter) is operating really far away from the design point. This means that the flows entering the turbine are highly off-incidence and thus the turbine efficiency is total crap. The fix for this is to either crank up the wick and push more fuel, or increase the torque provided by the starter.
Now we jump to the second big problem. At the low pressure ratios that you see during starting, the airflow through the combustor is so slow that you are not getting any real mixing energy within the combustor. This leads to larger fuel droplets and very slow reaction rates. This is a contributing factor on why you see flame shooting out the tailpipes of older engines. This means that the combustion efficiency is also total crap and pushing in more fuel does nothing to increase the fired torque. So you can see that the only really effective means of alleviating a hung start is to increase the starter torque. On full size aircraft engines, a hung start is often an indicator that your batteries are weak or not holding enough charge.
I have to admit that I really hate to see people just start drilling holes in combustors when things don't go right. Especially when they have no idea what the problem is.
So let's talk about drilling in more holes. When we size the through-flow area of the liner holes, what we are trying to control is the amount of mixing that occurs in the combustion chamber. Smaller through flow area means higher flow speeds through the liner and thus higher mixing of the fuel and air within the combustor. So the last thing that you would want to do with a engine that seems to have a hung start is to increase the through flow area across the liner, since it would give you even worse mixing and exacerbate the low combustion efficiency. I see that you were trying to increase the airflow to the primary zone, but I would suggest trying to understand what the primary zone fuel to air ratio was before taking a drill to it.
Here is another problem with just drilling more holes. With more through flow area in the liner, the airflow velocity in the vaporizer tube will reduce, thus reducing it's ability to process fuel. Most people, understandably, think that the primary mechanism of fuel preparation in a "vaporizer" style combustor is vaporization, when in fact it is the airblast atomization of the fuel from the vaporizer discharge lip. If you disagree, then go back and read Sotheran's paper on the Rolls Royce experience with vaporizers.
asmedigitalcollection.asme.org/gasturbinespower/article-abstract/106/1/88/407339/The-Rolls-Royce-Annular-Vaporizer-Combustor?redirectedFrom=fulltext
I would venture to say that they have more experience with vaporizers than any other manufacturer out there, and when they tell you that the name "vaporizer" is really a misnomer, you probably should believe them. If you look at the size progression of the vaporizers from the Sapphire and Viper walking sticks to the very small t-vaporizer that are used in the Olympus or the RB199, you can visually see how little the area has to do with the system. You cannot get enough area to vaporize all the fuel and in fact, you wouldn't want to, because the fuel is the only thing keeping your vaporizer from burning away. That is why they often try to introduce the fuel tangentially, so that it films the entire inner surface and keeps the surface temps of the vaporizer below either an oxidation limit or melting.
The fact that you say that the change from pure diesel to diesel+gasoline mix did not change much, also tells me that your vaporizers are not the problem.
So my suggestion is to find a way to increase the spool rpms before bringing on the liquid fuel, because it seems to me that you have a hung start. When you do get onto liquid fuel, move the throttle slowly, because slow accelerations require smaller turbine inlet temperature increases than do rapid accelerations.
So here are my questions:
1. What are you using to control the liquid fuel flow? What I am getting at here is are you overfueling in the transient and thus getting high EGTs?
2. What RPM are you switching to the liquid fuel? Are you reaching a steady state with the gaseous fuel and then slowly bringing on the diesel?
3. Where are you EGT probes located? Got a picture?
Good luck!
Chris
Interesting engine and hopefully you can get it running better soon. I thought that I might share my initial thoughts on what may have been happening on your first startup and ask a few questions as well.
So when I read about your first startup, my initial thoughts jumped straight to insufficient fired torque...i.e. a hung start. If we talk about the start up of a single shaft gas turbine, there are two really big issues that have to be addressed.
The first issue is that during startup, in order to get the rotor to accelerate, we have to have more torque being produced in the turbine than what is being consumed in the compressor and bearings. The problem here is at low rotor speeds, the turbine (and compressor for that matter) is operating really far away from the design point. This means that the flows entering the turbine are highly off-incidence and thus the turbine efficiency is total crap. The fix for this is to either crank up the wick and push more fuel, or increase the torque provided by the starter.
Now we jump to the second big problem. At the low pressure ratios that you see during starting, the airflow through the combustor is so slow that you are not getting any real mixing energy within the combustor. This leads to larger fuel droplets and very slow reaction rates. This is a contributing factor on why you see flame shooting out the tailpipes of older engines. This means that the combustion efficiency is also total crap and pushing in more fuel does nothing to increase the fired torque. So you can see that the only really effective means of alleviating a hung start is to increase the starter torque. On full size aircraft engines, a hung start is often an indicator that your batteries are weak or not holding enough charge.
I have to admit that I really hate to see people just start drilling holes in combustors when things don't go right. Especially when they have no idea what the problem is.
So let's talk about drilling in more holes. When we size the through-flow area of the liner holes, what we are trying to control is the amount of mixing that occurs in the combustion chamber. Smaller through flow area means higher flow speeds through the liner and thus higher mixing of the fuel and air within the combustor. So the last thing that you would want to do with a engine that seems to have a hung start is to increase the through flow area across the liner, since it would give you even worse mixing and exacerbate the low combustion efficiency. I see that you were trying to increase the airflow to the primary zone, but I would suggest trying to understand what the primary zone fuel to air ratio was before taking a drill to it.
Here is another problem with just drilling more holes. With more through flow area in the liner, the airflow velocity in the vaporizer tube will reduce, thus reducing it's ability to process fuel. Most people, understandably, think that the primary mechanism of fuel preparation in a "vaporizer" style combustor is vaporization, when in fact it is the airblast atomization of the fuel from the vaporizer discharge lip. If you disagree, then go back and read Sotheran's paper on the Rolls Royce experience with vaporizers.
asmedigitalcollection.asme.org/gasturbinespower/article-abstract/106/1/88/407339/The-Rolls-Royce-Annular-Vaporizer-Combustor?redirectedFrom=fulltext
I would venture to say that they have more experience with vaporizers than any other manufacturer out there, and when they tell you that the name "vaporizer" is really a misnomer, you probably should believe them. If you look at the size progression of the vaporizers from the Sapphire and Viper walking sticks to the very small t-vaporizer that are used in the Olympus or the RB199, you can visually see how little the area has to do with the system. You cannot get enough area to vaporize all the fuel and in fact, you wouldn't want to, because the fuel is the only thing keeping your vaporizer from burning away. That is why they often try to introduce the fuel tangentially, so that it films the entire inner surface and keeps the surface temps of the vaporizer below either an oxidation limit or melting.
The fact that you say that the change from pure diesel to diesel+gasoline mix did not change much, also tells me that your vaporizers are not the problem.
So my suggestion is to find a way to increase the spool rpms before bringing on the liquid fuel, because it seems to me that you have a hung start. When you do get onto liquid fuel, move the throttle slowly, because slow accelerations require smaller turbine inlet temperature increases than do rapid accelerations.
So here are my questions:
1. What are you using to control the liquid fuel flow? What I am getting at here is are you overfueling in the transient and thus getting high EGTs?
2. What RPM are you switching to the liquid fuel? Are you reaching a steady state with the gaseous fuel and then slowly bringing on the diesel?
3. Where are you EGT probes located? Got a picture?
Good luck!
Chris