Swirl injectors

Swirl injectors are a very promising class of injectors. A swirl injector element can be seen in figure 2.6 [38]. A full injector will usually consist of multiple swirl ele-ments on the injector plate. These injectors have tangential inlets that go into a

Figure 2.6: Schematic of a swirl injector element [38]

swirl chamber, where the flow swirls around the walls with an air core in the center.

The rotational momentum of the flow results in a cone-shaped vortex sheet spray.

In figure 2.7 [38], pictures of the exiting flow pattern from a swirl injector element illustrate this more clearly.

(a) Swirl injector exit using N2O

(b) Swirl injector exit using Water with a pressure drop of 30 bar Figure 2.7: Swirl flow pictures taken by Bouziane et al. [38]

Many hot-fire tests show that swirl injectors can increase regression rates signif-icantly when compared to a traditional axial injection with the same mass flow rate of oxidizer. The increase varies greatly between different tests, with cases reporting

An Investigation into Hybrid Rocket Injectors

everything from a 16% to a 700% increase [13] [39]. The latter is an extreme case and should probably be taken with a grain of salt, but multiple cases report the regression rate increasing by factors of 2-3. Unfortunately, there does not seem to be much literature on how to design swirl injectors to achieve a specific regression rate increase, which is likely dependent on several factors. What is clear from the various experiments throughout the literature, though, is that introducing swirl will increase the regression rate. Part of the reason for this is that the centrifugal force of the swirling flow will drive the flame closer to the surface of the solid fuel grain, increasing heat transfer to the fuel grain [39]. An additional reason that has been suggested for the increased regression rates is that with a tangential velocity com-ponent, the effective velocity that governs the “apparent” oxidizer flux is increased.

This may somewhat alter the regression rate equations that were shown earlier, but the general point is that as the apparent oxidizer flux increases, the regression rate does the same as it is dependent on the oxidizer flux [13]. Another advantage swirl injectors have over the standard axial injectors is that the flow downstream of the injector may form a recirculation zone that protects it from heat transfer. This means that using a swirl injector could allow for a shorter pre-combustion chamber, as the high temperatures of the chamber are not going to affect the injector as much [39]. Furthermore, swirl injectors have been shown to improve combustion stability for some cases, which is also attributed to the recirculation zone. The recirculation could let the oxidizer be pre-heated and stabilize the flame sheet, preventing flame-holding instabilities [40].

While designing these injectors, there are a few things to keep in mind in terms of the geometry of the swirl element shown in figure 2.6. _D^Ls

s should be minimized to avoid friction losses, but needs to be bigger than 0.5 to stabilize the liquid flow and generate a uniform vortex sheet. For proper design, a recommended value of this ratio is 1. To minimize friction losses at the exit, _D^L0

0 should also be reduced. The

Lp

Dp ratio should also be larger than 1.3, as short inlet orifices may cause an unstable spray. As is clear, these swirl injectors have more “sources” of friction losses than simple orifices. Additionally, the existence of the air core makes the estimation of the discharge coefficient quite different for these injectors. The discharge coefficient (Cd) is a friction loss parameter that is very important for mass flow rate modeling and will be discussed in further detail in chapter 3. There are a few different empirical formulas for a swirl injector’s Cd. A convenient one is shown in equation 2.2, with the discharge coefficient being primarily influenced by 0.19 < _D^Ap

s∗D0 < 1.21 and While using swirl injectors can be an efficient way to increase the regression rate with some added benefits, the rocket designers must consider if this is necessary.

Depending on the optimal O/F ratio, higher regression rates may not always be desired. This can be particularly true whilst using high regression rate fuels such as the paraffin wax that Propulse intends to use. Too high regression rates could lead to decreased performance due to the change in the O/F ratio. Burning through the fuel grain too fast could even damage the chamber walls or the structural integrity of

the fuel grain itself. A highly fuel-rich mixture could also lead to significant amounts of unburnt fuel exit the nozzle. However, for traditional fuel materials such as HTPB that have historically had low regression rates, swirl injectors should be particularly useful. In any case, it could be wise to experiment with showerhead or impinging injectors first, to determine whether the regression rate needs further increases.

2.3.1 Vortex Injectors

Before moving forward, it should be mentioned that the swirl injectors from the previous section are sometimes referred to as vortex injectors in the literature. Al-though they are in many ways similar, a distinction is made here. See figure 2.8 [38]

for a schematic of a vortex injector.

Figure 2.8: Schematic of a vortex injector with 45^◦inclined orifices [38]

The vortex injectors also introduce a swirling or vortex flow into the combustion chamber, but instead of using a tangential inlet and a swirl chamber, they simply have inclined outlets. In the example from the figure above, the whole orifice is inclined as well. Thus, the flow gains a tangential velocity component and results in the flow pattern that can be seen in figure 2.9 [38]:

(a) Vortex injector exit usingN₂O

(b) Vortex injector exit using Water with a pressure drop of 30 bar Figure 2.9: Vortex flow pictures taken by Bouziane et al. [38]

An Investigation into Hybrid Rocket Injectors

Of note is that when comparing the water cold flow pictures of the swirl and vor-tex injectors, it seems as if the swirl variety shows a higher degree of atomization.

However, in 2.9a, the flow seems well atomized. If sufficient atomization can be obtained with this method, it could be an alternative that has many of the benefits of swirling flow while being easier to manufacture and design than the swirl injectors.

Additionally, the vortex injector is much more similar to the showerhead and impinging designs than the swirl injector is. As the work now will focus on mass flow rate modeling, it will be much easier to adapt the models to the vortex design than the swirl injector. This shows once again that the vortex injector might be a lot simpler to work with than swirl injectors, while still providing some of the same benefits. For an inexperienced group like Propulse NTNU, this could be particularly important.