Kyubok Ahn

Fuel-Rich Combustion Characteristics of Biswirl Coaxial Injectors

An experimental study was performed to investigate the combustion characteristics of liquid–liquid swirl coaxial injectors in fuel-rich conditions. Liquid oxygen and kerosene (Jet A-1) were burned in a range of mixture ratios (0.29–0.41) and chamber pressures (46–65 bar) in a gas generator for a liquid rocket engine. An injector head was connected to a water-cooled chamber and a short nozzle with or without an extension pipe between the chamber and the nozzle. The extension pipe acoustically simulated a turbine inlet manifold. The injector head had 37 identical swirl coaxial injectors. It is found that the characteristic velocity and combustion gas temperature are seldom influencedby the extension pipe, but are only functions of themixture ratio. The dynamic pressure data show that the combustion instability in the fuel-rich gas generator equipped with biswirl coaxial injectors can be significantly affected by themixture ratio and also by the extension pipe, which influences the resonant frequency in the chamber.

Combustion Dynamics of Swirl Coaxial Injectors in Fuel-Rich Combustion

An experimental study was carried out to investigate the combustion characteristics of liquid–liquid swirl coaxial injectors in a fuel-rich gas generator. Propellants of liquid oxygen and kerosene (jet A-1) were burned in a range of mixture ratios (0:29 0:38) and chamber pressures (53:6 57:9 bar) in several combustion chambers with differentdiametersandlengths.Aninjectorheadhad37identicalswirlcoaxialinjectorswithlargerecesslengthsand internal-mixingcharacteristics.Theinjectorheadwasconnectedtocombustionchambersandachokednozzleeither with or without an extension pipe. It was found that the combustion characteristic velocity, combustion gas temperature, and combustion dynamics were seldom influenced by the increase of injector recess length in the present fuel-rich conditions. The results of dynamic pressure data and swirl-injector dynamics suggest that the longitudinal-modecombustioninstabilityinthefuel-richgasgeneratorequippedwithbiswirlcoaxialinjectorscould besignificantlyaffected bytherelationship betweenthe resonant frequencyin thecombustion chamberandthe flow dynamics of the swirl injector.

Effects of Orifice Internal Flow on Liquid Jets in Subsonic Crossflows

The effects of orifice internal flow on the spray plume characteristics of the liquid jet injected perpendicularly into subsonic crossflows were investigated experimentally. The internal flows are classified into threemodes: steady flow, cavitationflow, andhydraulic flipflow.Thesemodes aremainly determined by the ratio of the injector lengthL to the injector diameter d and by the shape of the orifice internal edge. To study the spray plume characteristics corresponding to each mode, three measurement techniques were applied: mass-flow-rate measurement, direct photography of internal injector orifices, and planar liquid laser-induced fluorescence. From the results of themassflow-rate measurement and direct photography, each mode of orifice internal flow without air crossflow was classified through the discharge coefficient patterns and internal–external flow shape. Steady flow shows no significant change on the flow pattern, and the discharge coefficient increases as the pressure differential increases. However, unsteady flows, including cavitation and hydraulic flip, have strong bubble envelopes due to a sudden reduction of flow passage, and these phenomena affect the discharge coefficient patterns. The penetrations and the width of the liquid spray plume were analyzed using the images obtained from the planar liquid laser-induced fluorescence method, and they were compared with the previous results. The penetrations and width were formulated with the liquid–air momentum flux ratio q and the ratio of the distance from the injector exit to the injector diameter x=d. It is found that the spray plume trajectory is determined by the liquid column diameter of the orifice exit and the liquid–air momentum flux ratio using the jet velocity at the orifice exit. It is also found that it is better to use the nominal parameters in the case of cavitationflow, and it is better to use the effective parameters in the case of hydraulic flip flow.

Application of PIV to Over-expanded Supersonic Flows: Possibilities and Limits

A bstractTwo-dimensional velocity distributions outside a Mach 2.0 supersonic nozzle have been investigated using a digital particle im age velocimetry (PIV). Mean velocities , vor ticity field and volume dilatation field were obtained from PIV images using 0 .33 μm titanium dioxide (TiO2) particle. The seeding particle of larger size , 1.4 μrn Ti02, was also used for the experimental comparison of velocity lag downstream of shock waves. The results have been compared and analyzed with schlieren photographs for the locations of shock waves and over-expanded shock structure to inspect possibilities and limits of a PIV technique to over-expanded supersonic flows. It is found that although the quantitative velocity measurement using PIV on over-expanded supersonic flows with large velocity and pressure gradients is limited, the locations of normal shock and oblique shock waves can be resolved by the axial/radial velocity fields, and over-expanded shock structure can be predicted by vorticity field and volume dilatation field which are acquired from the spatial differential of the velocity field.

Flow Control Characteristics of Cavitating Venturi in a Liquid Rocket Engine Test Facility

The flow rate control of a cavitating venturi has been investigated with downstream pressure variation. A set of cavitating venturies for a liquid rocket engine thrust chamber firing test facility have been designed and manufactured. The flow characteristics of the cavitating venturies have been analyzed by experimental and computational methods. Results showed that constant mass flow rate condition was established by the cavitation inside the venturi. However, upstream pressure less than the actual design pressure of the cavitating venturi could not supply a constant flow rate.

Characterization of Side-Dump Combustor Flowfield Using Particle Image Velocimetry

An experimental study was carried out to investigate the instantaneous flowfield in a side-dump combustor using particle image velocimetry and examine the systematic effects of geometric variations (inlet angle and dome height) on the mean flowfield and the recirculation zones. The instantaneous velocity fields showed that the recirculation zones in the combustor consisted of large-scale and small-scale vortices, and the flow pattern might change periodically due to the oscillation of two inlet jets. The mean velocity fields indicated that as the inlet angle increased, the height of the secondary recirculation zone and the maximum reverse mass flow rate at transport into the primary recirculation zone increased almost linearly. The results also indicated that there was an optimum dome height for each inlet angle even though the dome height did not significantly affect the flowfield downstream of the combustor inlet. The values of the maximum reverse mass flow rate in the secondary recirculation zone were equivalent to those in the primary recirculation zone, so the secondary recirculation zone was believed to be as important as the primary recirculation zone for flame stabilization.

An Experimental Study on Pressure Loss in Straight Cooling Channels

A regeneratively-cooled channel in a liquid rocket engine is used to effectively cool a combustion chamber inner wall from hot combustion gas, and the heat transfer/pressure loss characteristics should be predicted in advance to design cooling channels. In the present research, five cooling channels with different geometric dimensions were designed and the channels were respectively manufactured using cutter and endmill. By changing coolant velocity and downstream pressure, the effects of manufacturing method, channel shape, and flow condition on pressure losses were experimentally investigated and the results were compared with the analytical results. At same channel shape and flow condition, the pressure loss in the channel machined by the cutter was lower than that by the endmill. It was also found that the pressure loss ratio between the experimental result and the analytical data changed with the channel shape and flow condition.

Spray Plume Characteristics of Liquid Jets in Subsonic Crossflows

The effect of internal liquid flow on spray plume characteristics was performed experimentally in subsonic cross flows. The injector internal flow was classified as three modes such as a normal, cavitation, and hydraulic flip. The objectives of the research are to investigate the effect of internal liquid flow on the spray plume characteristics and compare the trajectory of spray plume with previous works. The results suggest that the trajectory and width of spray plume can be correlated as a function of liquid/air momentum flux ratio(q), injector diameter and normalized distance from the injector exit(x/d). Its also found that the injector internal turbulence influences the spray plume characteristics significantly.