Transcritical injection

Transcritical injection, particularly Type II, is also becoming increasingly important in research related to rocket combustors. It has been studied more than any other type, with cryogenic liquids as the fuel. In addition, the higher chamber pressures being used for gas turbine and diesel engines to increase efficiency and performance often increase the pressure above the fuel s critical value. This happens if the fuel is injected into an environment where combustion raises the temperature above critical conditions for the fuel. Thus, a subcritical temperature, supercritical pressure fuel is injected into a supercritical chamber. 

Of the many studies on super- or transcritical injection, Newman and Brzustowski [2] were the first to observe how an injected fluid behaves near the critical point. They noted the decreased effect of surface tension. 

Mayer and Telaar [13] also compared supercritical injection to gas-gas mixing. They noted that transcritical injection forms a regime that lies between subcritical spray breakup and supercritical jet breakup. 

Transcritical injection is mainly dependent on thermodynamic conditions... 

Surface tension can play a role in transcritical injection, depending on mixture... 

Injections of types II, III, and IV are typically labeled transcritical, since they cross the injectant critical conditions at some point during the injection process. 

Mayer et al. [3] described Type II injection as a mixing between two fluids rather than a spray entering a gas. They also observed that surface tension vanished at transcritical (subcritical temperature, but supercritical pressure) conditions. 

Much of the available research agrees that supercritical jets typically behave similar to turbulent gas jets, specifically variable-density gas injection. Chehroudi et al. [6] first found favorable comparison between measured transcritical (Type II or IV) jet spreading angles and those of variable-density gas jets. Barata et al. [7] used numerical variable-density gas jet models to predict experimental results, and found good agreement. 

Seebald and Sojka [23] reported transcritical (Type HI injection) concentration profiles with a maximum concentration that was offset from the centerline. They attributed this to a temperature gradient across their jet, which affected density calculations. 

Seebald, P. Sojka, P. E. (2008). An experimental study of transcritical C02 injection, ILASS-Europe 2008 Conference, Italy. ILASS08-A055. 

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