Thursday, 9 June 2011

The Practical Chamber

As previously mentioned, I decided to use nominal bore hydraulic pipe to form the tubular section of the chamber. Nominal bore hydraulic pipe is readily available. It is manufactured to verifiable, laid down standards governing pressure rating and dimensional tolerance. 

Confusingly it is specified by a non dimensional figure (in Imperial units) for the diameter. A schedule number is used for the wall thickness.

The practical chamber had to include the volume required to deliver an L* of 1.52m (60 inch). However, as the nozzle and convergence were to be separately fabricated, the volume of the tubular portion had to be reduced accordingly.

 In addition I needed to include the external jacket with a coolant gap of 3.2 mm (0.125 inch). The task reduced to finding a combination of nominal bore pipe diameters that represented a good compromise in the light of these requirements. 

I finally decided on a combination of 1.5 inch, schedule 10 nominal bore pipe for the internal tube and 2 inch, schedule 10 for the external tube. This was the best compromise I could achieve, the coolant gap becoming the final arbiter. Below are the key dimensions of both pipe sizes:-

1.5 inch Schedule 10 

External diameter = 48.26 mm (1.9 inch)
Internal diameter = 42.72 mm 1.68 inch)
Wall thickness = 2.77 mm (0.109 inch)

2 inch Schedule 10

External diameter = 60.32 mm (2.37 inch)
Internal diameter = 54.79 mm (2.16 inch)
Wall thickness = 2.77 mm (0.109 inch)

For a small chamber, the volume of the convergent section is generally assumed to be about 1/10 that of the tubular section. So the volume of the tubular section needs to be reduced by 1/10 compared to the theoretical value. It will be recalled that the proposed value for Vc was 0.121 x 10^-3 cubic metres. The volume of the tubular section, less 1/10, is thus 0.109 x 10^-3 cubic metres. The volume of the convergent portion is then 0.012 x 10^-3 cubic metres.

Examination of the internal diameter of the 2" tube and external diameter of the 1.5" shows that the coolant gap will be 3.265 mm (0.128 inch). The theoretical chamber diameter and length are 50.3 mm (1.98 inch) and 61 mm (2.4 inch) respectively. The external diameter of the 1.5 inch tube is 48.26 mm (1.9 inch) and the internal is 42.72 mm (1.68 inch).

 I decided that this internal dimension would be acceptable for the chamber. The external diameter of 48.26 mm (1.9 inch) still gives sufficient area for the injector design I plan to use. The length of the chamber was extended to 75 mm (2.95 inch) to compensate for the slightly smaller diameter. This gave a chamber volume of 0.107 x 10^-3 cubic metres. I decided that lengthening the chamber to 75 mm was as far as I was prepared to extend it. I had to bear in mind the fact that I was increasing the heated area and hence exacerbating the cooling problem. The volume of the convergence will be adjusted to 0.014 x 10^-3 cubic metres to ensure an L* of 1.52 m (60 inch). Finally, the contraction ratio Ec comes out at 4.25 as opposed to the theoretical 5. This will still be more than sufficient.

The exact chamber dimensions are far less critical than the nozzle dimensions in terms of performance. It is felt that the practical chamber represents a good compromise between the theoretical dimensions and the limitations of the constructional method chosen.

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