Nozzle throat

On the expander side, the expander wheel is surrounded by the nozzle vanes. The nozzle vanes, in turn, reeeive gas from a toroidal spaee that is eonneeted to tlie expander inlet piping. Any non-uniformity in the torus spaee and/or in the nozzle vane design may result in a non-uniform pressure distribution around the expander wheel. Non-uniform gas pressure around the expander wheel will result in a non-uniform load and, henee, produee a gas dynamie radial load on the bearing. In the expander ease, however, the nozzle throat flow resistanee is mueh larger than the easing peripheral pressure nonuniformity. The latter aets as a buffer making the expander wheel eireumferential pressure variations smaller than those of the eompressor side. This smaller pressure variation produees mueh less radial load when eompared to that of the eompressor side. 

As normally designed, vapor flow through a typical high-lift safety reliefs valve is characterized by limiting sonic velocity and critical flow pressure conditions at the orifice (nozzle throat), and for a given orifice size and gas composition, mass flow is directly proportional to the absolute upstream pressure. 

In an effort to determine the processes responsible for this type of behavior, Akiba and Tanno (A3), Sehgal and Strand (S2), and Beckstead (B6) have studied the coupling between the dynamics of the combustion process and the dynamic ballistics of the combustion chamber as described by Eq. (7). Each of these investigators has postulated admittedly simplified but slightly different combustion models to couple with the transient ballistic equations. Each has examined the combined equations for regions of instability. The results of these studies suggest a correlation between the L of the motor (the ratio of combustion-chamber volume to nozzle throat area) and the frequency of the oscillations. 

A, Area of nozzle throat rit Mass flow rate... 

Critical (choked) flow will occur in the nozzle throat when the pressure ratio is... 

Equation (1.54) indicates that A/A becomes minimal at M = 1. The flow Mach number increases as A/A decreases when M < 1, and also increases as A/A increases when M > 1. When M = 1, the relationship A = A is obtained and is independent of Y- It is evident that A is the minimum cross-sectional area of the nozzle flow, the so-called nozzle throat", in which the flow velocity becomes the sonic velocity, furthermore, it is evident that the velocity increases in the subsonic flow of a convergent part and also increases in the supersonic flow of a divergent part. 

This maximum velocity depends on the molecular mass Mg, the specific heat y, and the stagnation temperature Tg. The velocity increases as y and Mg decrease, and as To increases. Based on Eq. (1.52), a simplified expression for mass flow rate in terms of the nozzle throat area Aj (= A ) and the chamber pressure (= pg) is given by... 

Equation (1.68) can be represented by a simplified expression for thrust in terms of nozzle throat area and chamber pressure ... 

Fig. 12.11 shows the structure of a rocket plume generated downstream of a rocket nozzle. The plume consists of a primary flame and a secondary flame.Fil The primary flame is generated by the exhaust combustion gas from the rocket motor without any effect of the ambient atmosphere. The primary flame is composed of oblique shock waves and expansion waves as a result of interaction with the ambient pressure. The structure is dependent on the expansion ratio of the nozzle, as described in Appendix C. Therefore, no diffusional mixing with ambient air occurs in the primary flame. The secondary flame is generated by mixing of the exhaust gas from the nozzle with the ambient air. The dimensions of the secondary flame are dependent not only on the combustion gas expelled from the exhaust nozzle, but also on the expansion ratio of the nozzle. A nitropolymer propellant composed of nc(0-466), ng(0-369), dep(0104), ec(0 029), and pbst(0.032) is used as a reference propellant to determine the effect of plume suppression. The burning rate characteristics of the propellants are shown in Fig. 6-31. Since the nitropolymer propellant is fuel-rich, the exhaust gas forms a combustible gaseous mixture with the ambient air. This gaseous mixture is ignited and afterburning occurs somewhat downstream of the nozzle exit. The major combustion products in the combustion chamber are CO, Hj, CO2, N2, and HjO. The fuel components are CO and H2, the mole fractions of which at the nozzle throat are co(0.47) and iH2(0.24).

Fig. 14.19 shows a typical set of pressure versus time curves obtained from tests on a rocket motor. When the I/D ratio defined in Fig. 14.19 is increased, the head-end chamber pressure is increased drastically immediately after the ignition stage. These grains are seven-pointed-star-shaped neutral-burning grains (diameter D = 114 mm), and are made of an AP-Al-CMDB propellant with the composition nc(0-25), ng(0-31), Ita(0-08), ap(0-27), and ai(0 09). The ratio of the initial burning surface area (Ayg) to the nozzle throat area (Aj), = AygjAp and the ratio of the... 

Since the initial port area of the propellant grain, A, is small, the flow velocity becomes large because the velocity at the nozzle throat is always at sonic level. Furthermore, when K is kept constant. Ay and Aj increase simultaneously with in-... 

Compute total mass flow rate at nozzle throat by mg = AAxVxpp... 

Fig. 15.4 A nozzle throat area controller based on insertion of a pintle into the throat.

Fig. 15.4 shows a schematic representation of a nozzle throat area controller used in a VFDR. The mass flow rate from the nozzle attached to the primary combustion chamber (gas generator) to the secondary combustion chamber (ramburner) is changed by inserting a pintle. The high-temperature gas produced in the gas generator flows into the ramburner through the pintled nozzle. The pintle inserted into the nozzle moves forward and backward in order to alter the nozzle throat area. As the nozzle throat area is made small, the mass flow rate increases according to the concept described above. The fuel-flow rate becomes throttable by the pintled nozzle. 

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