Enhancement of air entrainment in ejector-diffuser using plate guidance at slots to reduce infrared emission

Ejector-diffuser reduces infrared emissions and are installed in combat aircraft to counter the threat of heat-seeking missile. The specific role of an ejector-diffuser is to reduce the heat emissions without substantially affecting the engine performance. The present study investigates a new design of ejector-diffuser wherein straight-plates and hybrid-straight-plates are installed at each slot for improving the ejector-diffuser performance. The evaluation criteria of an ejector-diffuser is specified in terms of air entrainment through the slots, thermal characteristics, and recovery of pressure. This work is carried out in two stages. In the first part, the orientation of the plate at the slot is investigated by varying the angle between the slot and diffuser axis over the range [Formula: see text]. The overall mass entrainment increases from 2.88 to 4.04 with the increase in plate angle. Further, the thermal characteristics also improves with increase in plate angle, but the pressure recovery decreases from 0.701 to 0.155. In the second part, the straight-plate at the slots are partially/fully replaced by hybrid-plate. Two configurations are proposed by first introducing a hybrid-plate at the first slot and straight-plate at the other slots, and subsequently by introducing hybrid-plate at all the slots. It is found that the pressure recovery in both the cases shows a significant improvement compared to the straight-plate case, the value being close to 0.75 for both the cases. However, the cumulative mass entrained by the first configuration of the hybrid-plate is better than the second configuration and is similar to the straight-plate guidance of 28°. Thus, the current study proposes an IRSS device having the hybrid-plate at the first slot and the straight-plate guidance at the remaining slots which reduces infrared emissions with minimum loading on the engine.

Instability of supersonic cold streams feeding galaxies – IV. Survival of radiatively cooling streams

ABSTRACT We study the effects of Kelvin–Helmholtz Instability (KHI) on the cold streams that feed massive haloes at high redshift, generalizing our earlier results to include the effects of radiative cooling and heating from a UV background, using analytic models and high resolution idealized simulations. We currently do not consider self-shielding, thermal conduction, or gravity. A key parameter in determining the fate of the streams is the ratio of the cooling time in the turbulent mixing layer which forms between the stream and the background following the onset of the instability, $t_{\rm cool,\, mix}$, to the time in which the mixing layer expands to the width of the stream in the non-radiative case, tshear. This can be converted into a critical stream radius, Rs, crit, such that $R_{\rm s}/R_{\rm s,crit}=t_{\rm shear}/t_{\rm cool,\, mix}$. If Rs < Rs, crit, the non-linear evolution proceeds similarly to the non-radiative case studied by Mandelker et al. If Rs > Rs,crit, which we find to almost always be the case for astrophysical cold streams, the stream is not disrupted by KHI. Rather, background mass cools and condenses on to the stream, and can increase the mass of cold gas by a factor of ∼3 within 10 stream sound crossing times. The mass entrainment induces thermal energy losses from the background and kinetic energy losses from the stream, which we model analytically. Roughly half of the dissipated energy is radiated away from gas with $T \lt 5\times 10^4\, {\rm K}$, suggesting much of it will be emitted in Ly α.

A Review of Russian Snow Avalanche Models—From Analytical Solutions to Novel 3D Models

The article is a review of mathematical models of snow avalanches that have been proposed since the middle of the 20th century and are still in use. The main attention is paid to the work of researchers from the Soviet Union and Russia, since many of their works were published only in Russian and are not widely available. Mathematical models of various levels of complexity for avalanches of various types—from dense to powder-snow avalanches—are discussed. Analytical solutions including formulas for the avalanche front speed are described. The results of simulations of the movement of avalanches are given that were used to create avalanche hazard maps. The last part of the article is devoted to constructing models of a new type, in which avalanches are considered as laminar or turbulent flows of non-Newtonian fluids, using the full (not depth-averaged) equations of continuum mechanics. The results of a numerical study of the effect of non-Newtonian rheology and mass entrainment on the avalanche dynamics are presented.

On the influence of hypersonic flow at the speed of a reflow heat-proof surface in terms of destruction

The results on the distribution of heat flows coming to the refractory plate of a hypersonic aircraft moving at different distances from the Earth's surface with cosmic velocities are obtained. The results of studies related to the study of phase transitions in the wall boundary layer occurring during the flow of hypersonic flow ablating surface are presented. The influence of the catalytic wall on the heat flow is considered. The main attention is paid to the analysis of the surface entrainment of high-speed aircraft, based on a detailed account of the mechanism of heterogeneous catalytic reactions in the conditions of surface mass transfer. The temperature distribution over the thickness of the boundary layer at the critical point of a blunted body with a refractory coating for a particular section of the flight path is given. Mass entrainment from the surface of crystalline refractory bodies is determined. Ill. 7. Ref. 16.

Effect of Impingement Surface Velocity on Slot Jet and Slot Jet Reattachment Nozzles’ Flow Field

Slot jet reattachment (SJR) nozzle is developed in an attempt to enhance heat and mass transfer characteristics while effectively controlling the impingement surface force exerted by the jet flow. In the SJR nozzle, the jet is directed outward from the nozzle exit and it then reattaches on an adjacent surface in its vicinity. The turbulent mixing occurs at the boundaries of the free stream induces secondary flow by mass entrainment and causes the flow to reattach the surface in the form of an oval reattachment at close nozzle to surface spacing [1]. All the previous studies had considered a stationary reattachment surface. This paper, for the first time, investigates the impact of reattachment surface movement on the flow structure of SJR nozzle with three different exit angles of +45°, +20°, and +10°. Specifically, this numerical study is carried out by varying the surface-to-jet velocity ratio (u* = up/ue) from 0 to 1.5 and comparing of flow reattachment flow fields to those of a regular slot jet (SJ) nozzle, where up is the speed of reattachment surface (moving plate) and ue is the jet exit velocity. In this study, jet exit temperature is kept constant at the room temperature of 20°C and all comparisons were performed at the same Reynolds number of 7,900. Additionally, the effect of SJR air exit angle on the peak surface pressure is investigated.

Inline-Slot Ejector Diffuser for a Warship to Suppress IR Signatures

The infrared (IR) signatures emitted by warships can substantially reduce the survivability chances while operating in the enemy area, as these signatures can be tracked and locked-on by the heat seeking missiles. The primary source for the IR signatures in a warship is the gases emanating from the exhaust of the gas turbine engine. These signatures can be suppressed by installing the passive infrared suppressors such as ejector diffusers, downstream of the turbine exhaust. The prime objective of an ejector diffuser is to reduce the exhaust gases temperature with minimal back pressure. In the present investigation, inline-slot conical ejector diffuser is numerically studied. From the open literature it is found that only step-slot ejector diffuser has been explored and no work on the inline-slot ejector diffuser is reported. Preliminary investigations on the inline-slot ejector diffuser show the performance to be better in terms of mass entrainment and static pressure recovery than the step-slot ejector diffuser, which is commonly used with warship power plants. The focus of the present study is to establish the effect of nozzle exit conditions (i) inlet swirl (S), and (ii) inlet turbulence intensity (TI) on the performance of inline-slot ejector diffuser. In the first part, inlet swirl is varied in the range 0≤S≤0.3 in step of 0.05, and in the second part TI is varied in the range of 1%≤TI≤15% in step of 3%. The performance is evaluated in terms of local and cumulative mass entrainment ratios, non-dimensional temperature distribution, and static pressure recovery. For the first part, it is seen that there is 3% drop in cumulative mass entrainment with the increase in swirl number from 0 to 0.3 and this can be attributed to the drop in potential core region. Higher wall temperatures in the mixing tube are observed for all the configurations with swirl cases. Static pressure recovery increases with the increase in the swirl number. For the second part, the effect of turbulence intensity on the performance of inline slot ejector diffuser is carried out. It is seen that the mass entrainment decreases by ~5% when turbulence intensity is increased from 1% to 15%. No significant effect of turbulence intensity is seen on the temperature distribution and pressure recovery.

Comparison between a Conventional and a New IRS Device in Terms of Air Entrainment: An Experimental and Numerical Analysis

An experimental study was conducted on a laboratory-scale new infrared suppression (IRS) device to assess the mass entrainment of ambient air into it under various operating conditions. A numerical analysis was also undertaken to assess the mass entrainment rate against pertinent input parameters independently. The numerical results were validated against the experimental data to ensure the reliability of the numerical analysis of the new IRS device at real scales. The numerical method solves the three-dimensional, incompressible Navier–Stokes equations; the mass continuity equation; and the two-equation–based eddy viscosity model for the turbulent k-epsilon equations in the flow field. Numerical assessment of the air entrainment was performed for the conventional and the newly proposed IRS devices. A number of experiments on the new IRS device were carried out under various operating conditions. From the numerical study, it was observed that the conventional IRS device performs better than the new IRS device up to a geometric ratio of 1.4 (which is the ratio of diameters of the successive funnels used in an IRS device). Beyond the geometric ratio of 1.4, the newly proposed IRS device outperforms the conventional one significantly. For the new IRS device, the maximum mass entrainment was found to occur when four funnels were used and the nozzle was kept in flush condition with the lower opening of the bottom-most funnel. Mass entrainment increases with the nozzle-exit Reynolds number for the range of values considered in the study.