CTE DB SI 1: Propagación interior

As stated in Section 2.6.5 studies were conducted by Mcalley in 2006 into the development of a liquid fuel system for pulsejet engines, with much work conducted on valveless engines. While his testing showed limited success, mostly due to poor injector placement, he developed numerous injector styles which showed produced useful results. The initial injector studies revolved around the design of multi-holed pin injectors, created from 1/8” stainless steel. Flow visualisation and engine tests were performed on varying size holes and hole positions. The initial tests produced 6 and 12 hole injectors with 1.7mm holes. Flow visulisation of the injectors can be seen in Figure 57 and Figure 58. It can be seen that the flow pattern from the 12 hole swirl injector aims to promote better mixing in the engine, however it was found that this injector design was unable to sustain thrust within the engine. The 6 hole opposed spray injector was successful in sustaining thrust, but only with forced air. It can be seen in the flow visualisation that flow is only exiting through the bottom injector. This

Section 3.12 Fuel Injector Design

concluded that the injector hole size was too large, as the pressure drop was not great enough to force fuel out of the other injector holes.

Figure 57- 12 hole swirl injector

Figure 58- 6 hole opposed spray injector

Continued development with decreasing hole sizes progressively showed that a single or double hole injector, with hole size 0.51mm was the most effective injector design, as they provided high pressure injection, which promoted better air/fuel mixing.

After looking at the work undertaken by Mcalley, we undertook an investigation into the performance of a simple pin hole injector, to allow estimates to be made regarding required injector sizing and performance. Initial calculations based of work from Williams (1990), suggested that an orifice diameter of 0.1mm would be large enough to supply 300ml/min of petrol for a supply pressure of 3bar. We were unable to test these calculations as such small orifice sizes were not able to be manufactured. Also,

the calculations did not account for the dynamic behaviour of the engine, or the high operating temperatures. The high temperatures during operation would reduce the diameter of the orifice, causing a significant reduction in flow rate.

The operating characteristics of swirl injectors were also analysed, but in depth numerical calculations were not undertaken. Swirl injectors produce greater spray angles than pin hole injectors, due to the added tangential component of velocity.

However, the performance of these injectors varies dramatically depending on the internal structure. Therefore, as we were not designing our own injectors, this exercise was done to obtain knowledge regarding general injector performance, to allow decisions to be made during engine testing, regarding the performance of the system.

Following this, a review of commercially available injectors was conducted. It was found that very few injectors met the specific requirements of both size and fuel flow rate, whilst being able to handle the required temperatures. Two main injectors were found, the first manufactured by BETE, who are a custom nozzle manufacture for a wide range of industries in the United States.

The BETE PJ series offered flow rates between 0.043 to 5.34 L/min from a supply pressure of between 3 and 10Bar. The injector offered was a cone spray type injector, as shown in Figure 59.

Figure 59-BETE PJ Cone Spray Injector

Cone spray injectors produce highly atomised fuel spray, in a 90 degree arc. This increases mixing compared with a single pinhole injector, producing droplet sizes of

Section 3.12 Fuel Injector Design

under 50 microns (BETE 2008). From the data provided a maximum flow rate of 0.34l/min could be achieved at 5 Bar through a 0.51mm orifice. However despite this the size of the injector was still larger than desired, as the intake of the pulsejet only has a diameter of 29mm.

The other injector which was found was a similar design spray injector available through a local Australian dealer. The designs were constructed from 5mm stainless steel tube, with the injector nozzle fitting inside the tube diameter, as seen in Figure 60. The manufacturer also quoted droplet sizes of 25 microns.

Figure 60-5mm stainless steel injectors

From the data available it was seen that a seen that a single injector could only supply a maximum of 170m/min though a 1mm orifice. However due to the size of the injectors it was determined that 3 injectors could be placed down the length of the intake to achieve the required fuel flow rates. Due to the availability, small droplet size and significantly smaller physical size, these injectors were purchased.

Six injectors were purchased as the fuel flow rate was unknown. Two 1mm, two 0.8mm and two 0.6mm injectors as the flow rate was expected to be between 200 and 350ml/min. The different sizes would ensure that once the required fuel flow rate was

found, that the optimum injector configuration could be determined. A table of fuel flow rates for each size injector can be found in Table 11.

Table 11

Orifice size

Fuel Flow rate (5 bar)

0.6mm 0.101

0.8mm 0.138

1mm 0.178