![]() The degree of sustainable superheat prior to void formation depends upon the availability of nucleation sites on the walls and in the bulk of the fluid. Bubble nucleation and growth rely on heat transfer at the vapor/liquid interface, which introduces a time delay (typically ~ 1ms) in the development of voids, similar in many respects to the case of a condensation shock described above. Generalizing to two-phase critical flow-where it is now assumed that reservoir A contains liquid at or near saturation conditions-it can be seen that for a sufficiently long flow path, the static pressure of the fluid accelerating through the connector will eventually fall to a level where flashing to vapor begins. For example, in an isentropic flow in a De Laval nozzle, the critical pressure ratio is given by:įigure 2. Axial pressure profiles in a De Laval nozzle. For a converging/diverging nozzle, the choked plane forms at the minimum flow area. The geometry of the flow path has a direct bearing on the flow. The ratio of the critical pressure P c at the choked plane to the inlet pressure P 0 is known as the critical pressure ratio (P c/ P 0). ![]() However, further reductions in P 1 will increase pressure drop across the choked plane-where the pressure gradient is now mathematically indeterminate-although physically the pressure drop occurs over a finite distance, resulting in a large pressure gradient. Reducing P 1 promotes the flow of fluid from A to B at a rate which increases with pressure drop until the velocity at some point in the connector achieves the local sonic velocity.Ī choked plane forms at this location, and further reductions in downstream pressure have no effect on conditions upstream as the rarefaction waves travel at the local sound speed and are stalled at the choked plane. To illustrate this, consider Figure 1, where a large reservoir is connected by a flow path of arbitrary geometry to another reservoir whose thermodynamic state can be controlled precisely.įigure 1. Flow between two arbitrarily large reservoirs. The occurrence of choking in a system is conventionally defined as the maximum mass flow rate as a function of downstream pressure. Single-phase choking is well understood but, with the introduction of a phase change, the behavior of fluid becomes far more complex. Experimental data may be available for a specific application but reliance often has to be placed upon theoretical models, which, in turn, must be validated against well-qualified data. Critical, or choked, flow is not only an interesting academic problem but is also important in many practical applications, such as in power generation and in chemical process industries where, without a precise knowledge of critical flow behavior, safety or performance of a system may be compromised.
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