Simulation of a Low-Density Nozzle Jet Exhausting into Different Backpressures

2003

This paper describes the numerical studies of a rarefied plume flow expanding through a nozzle into a vacuum, especially focusing on investigating the nozzle performance, the angular distributions of molecular flux in the nozzle plume and the influence of the backflow contamination for the variation of nozzle geometries and gas/surface interaction models. The direct simulation Monte Carlo (DSMC) method is employed for determining inside the nozzle and in the nozzle plume. The simulation results indicate that the half-angle of the diverging section in the highest thrust coefficient is 25 • − 30 • and this value varies with the expansion ratio of the nozzle. The descent of the half-angle brings about the increase of the molecules that are scattered in the backflow region.

AIP Conference Proceedings

An important problem for space vehicles is contamination of the spacecraft surface by combustion products and unburned fuel exhausting from control thruster nozzles. The objective of the present work is a detailed simulation of the backflow in the case of gas exhaustion from the nozzle into vacuum. A specific feature of this problem is the drastic expansion of the initially dense flow at the nozzle exit, which makes necessary simulations of the near-continuum flow inside the nozzle, the transitional flow near the nozzle exit, and the free-molecular flow outside the nozzle. The simulations are performed by a combined Navier-Stokes/DSMC approach.

37th AIAA Thermophysics Conference, 2004

This work is prompted by recent experiments on a multiphase (gas/droplets/cooling film) flow expanding from a supersonic nozzle into vacuum. A reverse motion of droplets (in the direction opposite to the flow in the plume core) has been experimentally observed near the nozzle lip. To understand this phenomenon, we have performed a numerical investigation of backflow formation. A hybrid Navier-Stokes/Direct Simulation Monte Carlo approach has been used to simulate the flow in different regimes -from a dense flow inside the nozzle, through very fast expansion near the nozzle lip, to a rarefied, freemolecular flow in the backflow region. A Lagrangian particle algorithm has been employed to trace the droplet motion in the gas flow. It has been shown that the gas backflow constitutes only a small part of the total mass flow rate. As a result, aerodynamic forces are insufficient to turn the droplets around the nozzle lip, and it seems that none of the droplets from the nozzle cannot reach the backflow region. Thus, it can be assumed that all droplets in the backflow originate from the cooling film being destroyed on the nozzle lip. Further, to investigate the viscous expansion flow near the nozzle lip in more detail, a model problem -the flow over a plane wall turning by a large angle (an expansion corner), has been studied using both continuum and kinetic modeling. It has been shown that, due to viscous effects, the flow deviates drastically from the classical Prandtl-Meyer solution. For large deflection angles, the decrease in the flow Mach number and the growth of the flow temperature are observed instead of their increase and fall, respectively. Reasons for such behavior are discussed, and the limits of applicability of the Navier-Stokes solution are analyzed.

2007

Jet flows at the exit of an under-expanded nozzle are investigated numerically for nitrogen gas and for different reservoir thermodynamic states. The simple polytropic, i.e., constant isochoric specific heat, van der Waals model has been used to account for gas non-idealities due to the vicinity of the liquid-vapor saturation curve. Differently from the ideal gas behavior, in which the flow features depend only on the reservoir to ambient pressure ratio and on the polytropic exponent, under-expanded jets of van der Waals gases are found to depend also on the thermodynamic state in the reservoir. In particular, for reservoir states along the critical isochor, the Mach disk radius decreases and its distance from the exit of the nozzle increases as the reservoir pressure is increased, for constant reservoir to ambient pressure ratios. These features are originated from the variations of the pressure at the exit of the nozzle, which decreases as the reservoir pressure is increased.

International Journal of Low-Carbon Technologies

Nozzles are widely used to control the rate of flow, speed, direction, mass, shape and pressure of the stream in connection with many different engineering applications. This paper presents the performance predicted by a computational fluid dynamic (CFD) model, which are 3D models that utilize parametric analysis, realizable k-epsilon turbulence models and experimental measurement for a jet. Jet flows are ejected from three different slot nozzles: round-shaped nozzle, rectangular-shaped nozzle and 2D-contoured nozzle. In this numerical study, velocities of free jets have been predicted for different axial distances from the nozzle exit in the range of $0.2\le z/B\le 12$ when center velocity at the nozzle exit. CFD simulation results are compared to experimental results from literature. These results are consistent with the existing experiments.

Journal of Propulsion and Power, 1993

Pitot pressures and flow angles are measured in the plume of a nozzle flowing nitrogen and exhausting to a vacuum. Total pressures are measured with Pitot tubes sized for specific regions of the plume and flow angles measured with a conical probe. The measurement area for total pressure extends 480 mm (16 exit diameters) downstream of the nozzle exit plane and radially to 60 mm (1.9 exit diameters} off the plume axis. The measurement area for flow angle extends to 160 mm (5 exit diameters) downstream and radially to 60 ram. The measurements are compared to results from a numerical simulation of the flow that is based on kinetic theory and uses the direct-simulation Monte Carlo (DSMC) method. Comparisons of computed results from the DSMC method with measurements of flow angle display good agreement in the Far-field of the plume and improve with increasing distance from the exit plane. Pitot pressures computed from the DSMC method are in reasonably good agreement with experimental results over the entire measurement area.

The objective of this dissertation was to develop a methodology for performing simulations of a complete nozzle plume system. The combined approach involves four steps. First, a continuum simulation of the nozzle plume system is performed.

27th Joint Propulsion Conference, 1991

Measurements of Pitot pressure were made in the exit plane and plume of a lowdensity, nitrogen nozzle flow. Two numerical computer codes were used to analyze the flow, including one based on continuum theory using the explicit MacCormack method, and the other on kinetic theory using the method of direct-simulation Monte Carlo (DSMC). The continuum analysis was carried to the nozzle exit plane and the results were compared to the measurements. The DSMC analysis was extended into the plume of the nozzle flow and the results were compared with measurements at the exit plane and axial stations 12, 24 and 36 mm into the near-field plume. Two experimental apparatus were used that differed in design and gave slightly different profiles of pressure measurements. The DSMC method compared well with the measurements from each apparatus at all axial stations and provided a more accurate prediction of the flow than the continuum method, verifying the validity of DSMC for such calculations.

Aerospace

Detailed knowledge of jet plume development in the near-field (the first 10-15 nozzle exit diameters for a round jet) is important in aero-engine propulsion system design, e.g., for jet noise and plume infrared (IR) signature assessment. Nozzle exit Mach numbers are often high subsonic but improperly expanded (e.g., shock-containing) plumes also occur; high Reynolds numbers (O (10 6)) are typical. The near-field is obviously influenced by nozzle exit conditions (velocity/turbulence profiles) so knowledge of exit boundary layer characteristics is desirable. Therefore, an experimental study was carried out to provide detailed data on nozzle inlet and exit conditions and near-field development for convergent round nozzles operated at Nozzle Pressure Ratios (NPRs) corresponding to high subsonic and supersonic (underexpanded) jet plumes. Both pneumatic probe and Laser Doppler Anemometry (LDA) measurements were made. The data revealed that internal nozzle acceleration led to a dramatic reduction in wall boundary layer thickness and a more laminar-like profile shape. The addition of a parallel wall extension to the end of the nozzle allowed the boundary layer to return to a turbulent state, increasing its thickness, and removing vena contracta effects. Differences in nozzle exit boundary layers exerted a noticeable influence but only in the first few diameters of plume development. The addition of the exit extension removed the vena contracta effects of the convergence only design. At underexpanded NPRs, this change to nozzle geometry modified the shock cell pattern and shortened the potential core length of the jet.