Yanxing Wang

Comprehensive Study of Cryogenic Fluid Dynamics of Swirl Injectors at Supercritical Conditions

A comprehensive study is conducted to enhance the understanding of swirl injector flow dynamics at supercritical conditions. The formulation is based on full-conservation laws and accommodates real...

Geometric Effects on Liquid Oxygen/Kerosene Bi-Swirl Injector Flow Dynamics at Supercritical Conditions

A systematic study is carried out to investigate the flow dynamics and mixing of liquid oxygen/kerosene bi-swirl injectors at supercritical conditions. The theoretical framework is based on the ful...

A Three-Dimensional Analysis of Swirl Injector Flow Dynamics at Supercritical Conditions

1 Graduate Research Assistant, Georgia Institute of Technology, Email: xwang343@gatech.edu. 2 Mechanical Engineer, GE Global Research Center, Niskayuna, NY, 12309 3,4 Postdoctoral fellow, School of Aerospace Engineering 5 William R. T. Oakes Professor and Chair, School of Aerospace Engineering, Fellow AIAA. 1 American Institute of Aeronautics and Astronautics A Three-Dimensional Analysis of Swirl Injector Flow Dynamics at Supercritical Conditions

Liquid Monopropellant Combustion in Mesoscale Vortex Chamber

A WIDE variety of mesoand microscale combustion devices have been proposed and are under development to meet the power and energy density requirements of propulsion and powergeneration applications [1–4].Most of these devices employ gaseous fuels and, in particular, hydrogen, which has a fast reaction rate and high diffusivity, to demonstrate the feasibility of self-sustained combustion at millimeter scales or below. Liquid-phase storage and operations, however, are essential for taking full advantage of the high specific energy of a combustion-based power generator using hydrocarbon fuels. At macroscales, liquid fuels are typically sprayed into a combustor in droplet form to enhance the vaporization and ensuing burning rates. If the fuel was injected as a film on the chamber wall, the liquid surface area would not be large enough to sustain the needed vaporization rate. In combustors with dimensions in the subcentimeter-size range, however, because the specific area of the wall film increases as the combustor volume decreases, a liquid film can offer a surface area for vaporization that is as large as that of a vaporizing spray. The film on the combustor surface also cools the combustor wall to create a favorably distributed temperature profile that prevents heat losses for both endothermic liquid decomposition reactions and gasification. Heat transferred to the wall, which has been considered as heat loss in most miniaturized combustor applications, can be used for fuelfilm vaporization. Atomizers that are capable of producing submicron-scale droplets may, therefore, not be needed for liquid-fueled combustion in mesoand microscale combustors. Sirignano et al. [5,6] concluded that a 10-mm-diam combustor has a film surface area comparable to a droplet spray with a 10 m Sauter mean radius. The concept using wall film evaporation in small combustion chambers was demonstrated in a 3:14-cm-volume combustor [6] with gaseous air used as the oxidizer. Secondary air injection was recently introduced into the concept to enhance mixing and contain the reaction zone completely within the combustion chamber [7]. Furthermore, a portion of the flow residence time in the chamber was consumed by the mixing between the fuel and oxidizer. To circumvent the problem of short flow residence time in a smallscale combustion chamber and thereby avoid the difficulties associated with the mixing of fuel and oxidizer, we studied the usage of nitromethane (CH3NO2), a liquid monopropellant. A mesoscale vortex combustor developed in our previous work on gaseous combustion [8] was used because of its potential to implement the concept of wall film vaporization.

Central recirculation zones and instability waves in internal swirling flows with an annular entry

The characteristics of the central recirculation zone and the induced instability waves of a swirling flow in a cylindrical chamber with a slip head end have been numerically investigated using the Galerkin finite element method. The effects of Reynolds number as well as swirl level adjusted by the injection angle were examined systematically. The results indicate that at a high swirl level the flow is characterized by an axisymmetric central recirculation zone (CRZ). The fluid in the CRZ takes on a solid-body rotation driven by the outer main flow through a free shear layer. Both the solid-body rotating central flow and the free shear layer provide the potential for the development of instability waves. When the injection angle increases beyond a critical value, the basic axisymmetric flow loses stability, and instability waves develop. In the range of Reynolds numbers considered in this study, three kinds of instability were identified: inertial waves in the central flow, and azimuthal and longitudinal ...

Evolution and transition mechanisms of internal swirling flows with tangential entry

The characteristics and transition mechanisms of different states of swirling flow in a cylindrical chamber have been numerically investigated using the Galerkin finite element method. The effects of the Reynolds number and swirl level were examined, and a unified theory connecting different flow states was established. The development of each flow state is considered as a result of the interaction and competition between basic mechanisms: (1) the centrifugal effect, which drives an axisymmetric central recirculation zone (CRZ); (2) flow instabilities, which develop at the free shear layer and the central solid-body rotating flow; (3) the bouncing and restoring effects of the injected flow, which facilitate the convergence of flow on the centerline and the formation of bubble-type vortex breakdown; and (4) the damping effect of the end-induced flow, which suppresses the development of the instability waves. The results show that the CRZ, together with the free shear layer on its surface, composes the basi...

An Experimental Study of Monopropellant Combustion in Small Volume

and diameter of 5 mm was experimentally investigated. The meso-scale combustor utilized a vortex combustion concept, which has been demonstrated to be useful for stabilizing combustion of gaseous hydrocarbon air mixtures in small volumes. The monopropellants were injected tangentially from the backend of the cylindrical combustor and the combustion products exited the chamber tangentially at the other end. Stable combustion of nitromethane was not found achievable at atmospheric pressure, although combustion can be self-sustained by enhancing the kinetic rates of the reaction with the addition of a small amount of oxygen (air). Pressurization of the combustor has an equivalent effect as oxygen enrichment on flame stabilization due to the high sensitivity of nitromethane kinetic rates to pressure. Stable combustion in the vortex combustor was achieved at pressures higher than 260 psi, although various hydrocarbon compounds such as CH4, C2H2, and C2H4 are found in the FT-IR spectra of the combustion products at low pressures suggesting incomplete combustion due to insufficient flow residence time. Complete combustion was achieved at chamber pressures above 350 psi. The chemical power input was ~230W for all the cases investigated.

Modeling and simulation of liquid monopropellant combustion in microthrusters

A comprehensive numerical analysis is developed to study the combustion of liquid monopropellant in a small-volume vortex chamber. The instantaneous burning rate of the liquid propellant is obtained from well-caliberated experiments as a function of local pressure and temperature. The basis of the present work is a two-phase flow analysis based on the level-set approach. The model allows for a detailed investigation of the liquid-film motion and gas-phase flow development. As a specific example, the case with liquid nitromethane in a volume of 108 mm 3 is examined systematically. I. Introduction ith the fast development of Micro Electro Mechanical Systems (MEMS) in the past 10 years, the miniaturization of small-size power supply devices with high energy density has been receiving considerable attention. The use of combustion for power generation provides enormous advantages over currently available batteries in terms of specific power generation, even when the conversion efficiency from thermal to electrical energy is taken into account. The challenges associated with micro-scale combustion modeling are considerably different from those of macro-scale devices. The chamber internal flow is predominantly laminar, and the surface phenomena play an important role in dictating the system characteristics. The effects of miniaturization on the fluid mechanics, heat transfer, and combustion characteristics involved in micro power devices have recently been analyzed by Fernandez-Pello 1 and Ketsdever et al. 2 . As first described by Waitz et al. 3 , a micro-scale combustor is more constrained by an inadequate residence time for complete combustion and a high rate of heat loss from the combustor. Both features can lead to incomplete reaction or even quenching of chemical reaction. The commonly used methods to acquire stable combustion in a micro-scale combustor are (1) balancing the flow residence time and chemical reaction times and (2) optimizing the thermal conditions, such as the “Swiss roll” burner developed by Weinberg et al. 4 . For liquid propellants, one major challenge for all miniature combustion concepts is the increasing surface-tovolume (S/V) ratio with decreasing size. At small scales, a high S/V ratio often leads to flame quenching, particularly for premixed flames. One alternative for high S/V combustors is to deliver the liquid fuel as a film on the combustor surface. Such a propellant delivery technique simultaneously cools the combustor wall and increases the liquid surface for vaporization. In current larger combustion systems, to keep the ratio of liquid surface area to volume large enough to sustain high fuel vaporization rates, the fuel is injected as a spray. The intention is to vaporize the liquid as a spray before the liquid deposits on the combustor wall. If the fuel was filmed in these larger devices, the liquid surface area would not be large enough to sustain the needed vaporization rate. On the other hand, because the specific area of the wall film increases as the combustor volume decreases, the liquid film can offer a surface area for vaporization as large as a vaporizing spray in the subcentimeter-size range. At one atmosphere, a 10 mm diameter combustor has a film surface area greater than would occur for a droplet spray with 10 µm mean radius 5 . The film has more surface area per volume of liquid than the spray if the combustor diameter is sufficiently small or the pressure is sufficiently low. Furthermore, the liquid film offers protection from heat losses and quenching. With the liquid film on the solid surface, the wall temperature will not exceed the boiling point of the liquid. Although the film thickness is on the order of tens of microns, the Reynolds number is larger than unity, indicating that viscous forces do not prevent the movement of liquid along the solid surface. Generally the liquid film is always in the laminar range.