Preswirl Nozzle

A preswirl nozzle is a special kind of orifice (Figure 78), used to impart a tangential velocity to gas before it enters a rotating device. That way, the loss due to the difference in circumferential velocity between the air entering the rotating device and the rotating device itself can be decreased. In the Figure $\boldsymbol{v}_{rot}$ is the rotational velocity of the orifice the preswirl nozzle is serving, $\boldsymbol{v}_{abs}$ is the absolute velocity of the air leaving the preswirl nozzle and $\boldsymbol{v}_{rel}$ is its velocity as seen by an observer rotating with the orifice (the so-called relative velocity). The velocity entering the calculation of the discharge coefficient of the rotating orifice is the tangential component $v$ of the velocity of the rotating device as seen by the air leaving the preswirl nozzle (which is $-\boldsymbol{v}_{rel}$). This velocity can be modified by a multiplicative factor $k_\phi$.

The geometry of a preswirl nozzle can be quite complicated and the discharge coefficient should be ideally determined by experiments on the actual device.

Figure 78: Geometry of the preswirl nozzle fluid section and the orifice it serves

The discharge coefficients provided by CalculiX are merely a rough estimate and are based on [35]. For this purpose the preswirl nozzle must be described by the following constants (to be specified in that order on the line beneath the *FLUID SECTION, TYPE=PRESWIRL NOZZLE card):

• the cross section $A$.
• the angle with the engine axis at the exit of the nozzle.
• $k_\phi$.
• the number of a curve.

The angle at the exit of the nozzle is used to determine the circumferential velocity of the gas leaving the nozzle. This is stored for use in the (rotating) device following the nozzle. The curve number can be used to distinguish between several measured curves. Right now, only one curve is coded. More specific curves can be implemented by the user, the appropriate routine to do so is cd_preswirlnozzle.f. Alternatively, the usr can enter an own curve in the input deck listing $Y=C_d$ versus $X=p_{s_2}/p_{t_1}$. In that case the input reads

• the cross section $A$.
• the angle with the engine axis at the exit of the nozzle.
• $k_\phi$.
• not used.
• not used.
• $X_1$.
• $Y_1$.
• $X_2$.
• $Y_2$.
• ..

Example files: moehring, vortex1, vortex2, vortex3.