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GR-1 Turbojet Project 3/8/04

Posted on March 8, 2004

       The GR-1 Turbojet Engine has been a great success for my first attempt at a jet engine. I have learned a lot from this project and I hope to learn a lot more as I get into the next phase of building the “Free-shaft” turbine PTO. I am very happy that my combustor does what it was designed to do; produce a relatively “low temperature” gas in quantity. For this reason I want to document the basic design of the combustor for other enthusiasts.
      
Note: I have not written this post as a how-to article because quite frankly there are just too many details that could be discussed in depth. This formula is just to convey the basic design of the GR-1 combustor....

The inducer diameter is the measurement taken from the compressor wheel at the intake side of the turbo as seen above. On this turbo it is 5.2 cm wide (value “I”).

       To document the dimensions of the GR-1 combustor, I have developed my GR-1 Combustor Formula. I based my formula on a metric value which is derived from the inducer diameter of the turbocharger being used making the formula adaptable to many different sized turbos. Using centimeters as my measurement value, I took the diameter of the inducer (Value “I” in cm) and based all my combustor measurements to this value. Below is a diagram of the GR-1 combustor cross section which explains the variables in the formula. Some values can be adjusted within reason to the sizes of material available.

                           The GR-1 Combustor Formula is as follows:

       The inside diameter (in cm) of the turbocharger’s inducer (inlet) shall be represented by the value “I”   *** Only use centimeters not inches!!! ***

The inside diameter (in cm) of the combustion liner:                   A = 1.3 x I

The length (in cm) of the combustion liner:                                    B = 3.85 x I

The inside diameter (in cm) of the combustion chamber:           C = 2.1 x I

The cross-section area (in square cm) of the bypass section:   D = 3.6 x I

The total area (in square cm) of the combustion liner holes:     E = 4 x I

The total number of holes (F) in the combustion liner:                 F = E ÷ 0.33

The individual size (G) of the liner holes (in square cm):             G = E ÷ F

       The overall length of the combustion chamber will vary with the adjustment of the bypass area. There is no set length for the combustion chamber itself, the bypass area in relation to the combustion liner must be measured before cutting the chamber to the proper length. See below ***

       The angle of the tapered combustor neck can be between 15 and 20 degrees in relation to the sides of the combustor itself. Try to avoid larger angles than 20 degrees (40 degrees total, side to side).

       The size if the combustor inlet and exhaust is determined by the size of the turbocharger used and may vary from model to model.

NOTE: This formula cannot be used with English Measurement (IE inches) Only use centimeters as your values. There are 2.54 cm in one inch.

       The Bypass Section of the combustor is the area between the end of the combustion liner and the tapered neck of the combustor. The bypass allows cool incoming air to envelope the hot gas rushing out of the combustor into the turbine. This helps to cool the engine greatly. The bypass area is represented by value “D” and can be adjusted by lengthening or shortening the depth of the combustion chamber itself. This will put the combustion liner closer or farther from the tapered end of the combustion chamber allowing adjustment of the bypass area.
      
*** There is no set value for the overall length of the combustion chamber as this value will need to be adjusted to achieve the proper clearance from the end of the flame tube creating the proper cross-section flow for the bypass section. It is a good idea to build the combustion chamber a little longer than needed so it can be trimmed to length once the combustion liner’s clearance has been obtained. Final alignment of the combustion liner should be performed before the combustor end plate assembly is welded to the combustion chamber tube. ***
       To properly adjust the bypass area in your combustor to the calculated bypass value for your turbo you can take the combustion liners outer diameter and divide it in half to get the radius of the combustion liner. In the case of the GR-1 the combustion liner’s outside diameter is 7.3 cm so:

                               7.3 ÷ 2 = 3.65 cm (combustion liner radius)

       You can now multiply the radius by it by itself and then multiply the answer by 3.14 (Pi) to get the area of the combustion liners outer diameter so:

     3.65 x 3.65 x 3.14 = 41.83 square cm (combustion liners diameter area)

       You can then take the required area of the bypass area which in this case is 18.72 square cm (D = 5.2 x 3.6) and add it to the 41.83 sqcm cm to get  60.55 sqcm. The combustion liner can now be mounted to a depth in the tapered neck of the combustor that has a area of 60.55 sqcm. That depth can be derived from the circumference of the tapered neck at a given point. We know that the value for this area is 60.55 sqcm but we do not have a circumference to help us figure out where on the combustor neck the combustion liner should be set.
       The easiest way to calculate this location is to figure out the circumference of the 60.55 sqcm value and compare it to the profile of the tapered neck. To get the circumference you will first need the radius of area in question. In this case the radius of a circle containing 60.55 sqcm is calculated by dividing the sqcm area by 3.14 (Pi) and then finding out the square root of the quotient (= sqcm ÷ 3.14). This will be the radius you are looking for so:

   60.55 ÷ 3.14 = 19.28      The square root of 19.28 = 4.39 cm (bypass radius)

       You can now take this radius and double it for a diameter of 8.78 cm and convert it to circumference by multiplying it by 3.14 (Pi):

                                   8.78 x 3.14 = 27.57 cm circumference      

       You can now cut out a paper ring template that is exactly 27.57 cm in circumference and slide it into the tapered neck of the combustor to see where it rests. This will be the depth that you want to make your combustion liner meet the tapered neck of the combustor. Of course your values may differ from this example as there are many different sized turbos.
       To calculate your combustion liner hole size take your value “G” and divide it by 3.14. You can then find the square root of the quotient (= G ÷ 3.14) which is equal to the radius of the hole size you need (which can be doubled for the diameter in cm). For example, a combustor needing a hole size if 0.3 square cm would end up needing a drill size of 0.618 cm or 6.18 mm (or the closest size):

       0.3 ÷ 3.14 = 0.0955      The square root of 0.0955 is = 0.309 cm radius

               Double the radius of 0.309 for a hole diameter of 0.618 in cm

       The holes in the combustion liner will sometimes vary in size from the injector to the exhaust side (seen above). The GR-1 combustion liner’s holes were all drilled at 1/4” and then widened or narrowed to allow for proper combustion. Keep in mind that the sum of the hole area should remain roughly the same so if you widen some holes you must narrow others. I had narrowed a few holes on the injector side by welding in the holes and then re-drilling them. This was done to even out the burn of fuel as in my combustor testing session. The burn characteristics may vary from combustor to combustor so proper testing is an important step.
       Note: This combustor design was originally intended for use with propane gas. See the
GR-7 project for modifications to this design for liquid fuel use...

Thanks for visiting my site!!!

Don Giandomenico

 

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