A convergent-divergent nozzle, also called De Laval nozzle, CD nozzle or con di nozzle, is a tube that is somewhat pinched in the middle so as to accelerate the flow of gas, which is passing through it. Since the gas flows symmetrically along the nozzle’s axis starting from the inlet to the exhaust, a constriction in the middle will produce a change in pressure and as well as in the velocity in order to satisfy the law of conservation of Energy. As the gas, which emanated from the combustion chamber, enters the nozzle at subsonic velocities (velocity below the speed of sound) with high pressure. As it reaches the nozzle throat it will be contracted down until it reaches sonic velocities. At this point the pressure will be reduced and as the gas leaves the nozzle throat, it will continue to expand and will reach supersonic velocity (velocities above the speed of sound) since the cross sectional area of the nozzle increases. Important considerations in the analysis of gas flow in the CD nozzle includes that the gas is assumed to be ideal, the gas is at constant entropy, there is little amount of heat gained or lost or none at all, the gas flow is constant, the behaviour of the gas flow is compressible, and the mass flow in the nozzle should be sufficient to produce supersonic flow.
The CD nozzle is commonly located at the end of the combustion chamber and it controls the gas expansion at the exhaust to efficiently convert the energy at the combustion into thrust. By reaching sonic velocities at the nozzle throat, thrust is being produced and carried throughout the rest of the expansion cone. The two major configurations in the CD nozzle are submerged and external. In the submerged type the nozzle entry, the throat, and the exhaust parts are inside the combustion chamber. In the external type, all parts of the CD nozzle are entirely separated from the combustion chamber (see figure 1).
The CD nozzle consists of three major sections: the convergent section where the subsonic gas flow will be accelerated thus reducing the pressure and increasing its kinetic energy; the throat where it supports the transonic flow; and the divergent section that furthers accelerate the supersonic flow and eventually matching the exit pressure to the outside pressure. If the two pressures do not matched, under a lower outside pressure the CD nozzle will create expansion waves and under a higher outside pressure the CD nozzle will create oblique shock wave.
A carburettor is a mechanical device that combines or blends fuel and air for an internal combustion engine. The working principle of a carburettor is based on Bernoulli’s theorem that the pressure will be lower if the air moves faster. This principle is similar to CD nozzle wherein a constriction also exists. The carburettor mechanism is actuated by the throttle linkages which controls the flow of air intake to the engine. The speed of which, therefore the pressure, determines the fuel being drawn into the air stream. A venturi type carburettor can either be fixed or variable. In a fixed venturi carburettor fuel flow is being altered by varying air velocity. In the variable venturi carburettor the slide, wherein it simultaneously alters the flow of air, varies the fuel jet opening. A carburettor consists of a “throat” or an open pipe or barrel wherein the air flows through the air inlet of an engine. A venturi form of a pipe is used, a narrowing of the cross sectional area at the middle or a constriction just like in a CD nozzle, to increase the velocity of air coming through it. A throttle valve exists below the venturi to control the airflow, either to restrict or block the air intake. This enables the control of air/fuel mixture inside the system thus regulating the power and speed of the engine. The control of the throttle comes from a mechanical linkage of joints and rods to the accelerator, usually a pedal or an equivalent control of a vehicle or equipment. The fuel is introduced in the system in response to a specific pressure drop in the venturi by means of orifices.
The nozzle’s throat area refers to the minimum cross sectional area of the constriction in a CD nozzle or in a verturi carburettor. It is in this area where the gas flow gains kinetic energy thus producing supersonic speed after passing through it (see figure 1).
The Vena contracta is an effect that occurs in air flow through an available opening. To effectively reduce the size of the opening the flowing air “sticks” at the edges of the opening thus reducing the inlet’s capacity to a specific percentage of its size. Though the vena contracta reduces the air inlet capacity it increases the air velocity flowing through.
Choked flow occurs if the gas velocity, which is travelling through the restriction, tends to increase towards the speed of sound. The choke plane is the physical point where the choking occurs. The downstream pressure will not be a factor with respect to the gas velocity at this point.
The pressure loss, considering the CD nozzle, occurs at the restriction and eventually causing a higher gas flow velocity. This enabled to increase the kinetic energy of the gas to develop supersonic speed at the throat thus producing thrust at the exhaust section. This thrust is most commonly applied in modern rocket engines and supersonic jet engines.
Isentropic process is a process wherein the entropy of a working fluid is considered constant. Entropy is a measure on how far the differences in pressure, density, chemical potential, and temperature might exist in the system.
Isentropic velocity is a condition that is considered reversible and adiabatic. No energy is being added or loss during the flow due to the dissipative effects or friction. In a CD nozzle the ideal gas is at constant entropy and considered adiabatic.
The flow coefficient is a measure of how efficient a fluid flows. It has a relationship to the pressure drop at the restriction or at the orifice and its equivalent flow rate.
References
Heldstab, Wayne. The Secret of Super Mileage Carburettors: How they Work and How to Build Them.
Pavlovich, Ivan (1964). Thermodynamics. Western Hemisphere by Mcmillan New York.
Risacher. How to Design, Build, and Test Small Liquid Fuel-Rocket Engines. Retrieved January 29, 2007, from the risacher.org website: http://www.risacher.org/rocket/intro.html
Sutton, George P. (1992). Rocket Propulsion Elements: An Introduction to the Engineering of Rockets, 6th Edition, Wiley-Interscience.
ThinkQuest. Bernoulli’s Principle. Retrieved January 28, 2007, from the ThinkQuest website:
http://library.thinkquest.org/27948/bernoulli.html
