T.M Muruganandam

Performance of a nonuniform resonator based valve-less standing wave suction pump for gases is evaluated. Nonuniform resonator with a linear area variation was used to achieve high amplitude standing wave. High oscillating pressure at the small end of the resonator was rectified by fluidic diodes. Experiments were done with pairs of rectifiers, each of which consisted multiple diode elements in series. The performance of the valve-less standing wave pump is evaluated in terms of suction flow rate and suction pressure achieved. It was found that higher driving amplitudes gave higher flow rates and higher suction pressures. Increasing the number of diode elements also did the same. Smaller diode diameter cases produced higher suction pressures at similar flow rates. The measured maximum suction pressure was 4900 Pa and the maximum flow rate observed was 10.2 slpm of air respectively, while operating around 925 Hz.

This paper investigates the mixing of two supersonic streams in a supersonic exhaust diffuser using theoretical, numerical, and experimental approaches. It has focused attention on the startup and stationary operation of the two-stream supersonic diffusers. The diffuser geometry consists of a constant area throat which connects two ducts of convergent and divergent cross-sections. The effects of diffuser contraction ratio (CR) and convergence angle (θ) on the starting process, pressure recovery, and flow features. Experiments have been performed at five different secondary Mach number (Ms) values in the range of 1.8 < Ms < 2.6, at a fixed primary Mach number (Mp) value of 2. Synchronized pressure measurements are used to study the diffuser performance characteristics. Numerical simulations were also carried out to obtain a better insight into the flow field and computed results have been compared with the experimental results. It was found that the starting behavior has a hysteresis. As a result, the tunnel can be operated at lower pressure ratio than the starting pressure ratio. It is observed that the settling chamber pressure required to start a Constant-Area Diffuser (CAD) is decreased with increase in the diffuser length up to an L/D ratio of 9; subsequently, starting pressure remains almost constant with the further increase in the length due to frictional and pressure recovery losses. The present study provides valuable insights into the multi-stream supersonic mixing and diffusion problem.

Blowout process in premixed swirl dump combustors is known to have temporary partial extinction followed by re-ignition events as precursors. This re-ignition process is investigated using high-speed CH∗chemiluminescence and simultaneous TR-SPIV. It was found that during the extinction phase, the flame split into two zones, causing fresh mixture to enter the inner recirculation zone. The sudden loss of heat release causes the flow field to change such that the stagnation point moves further downstream, causing high negative velocity paths in the flow. The flame that was convected downstream, now uses these negative velocity paths to consume the fresh mixture that entered the IRZ. This is the re-ignition phase of the precursor event.

Based upon the results of a literature review, a set of physical parameters suitable for quantitative comparison in experimental and numerical studies is determined. The review showed the feasibility of modeling the process of the outflow of an immersed jet of inert gas into the surrounding space and the evolution of a free shear layer formed by hot and cold gases to analyze the process of convective and diffusion transport of various types of particles. In the model formulation we consider the problem of solid destruction by a melt of the same substance, as well as by a nitrogen stream at high temperature. The test simulation of the problems of two-phase interaction of a immersed jet of molten liquid and hot gas, on the whole, qualitatively showed the reproduction of the main characteristics of the flow: The generation of large-scale vortices around the jet in the induction zone, the oscillations of the jet when it collides with an obstacle, and the shape of the cavity formed.

An experimental study is conducted to investigate the performance of a non-uniform area resonator-based acoustic pump for air. Higher peak pressure amplitudes can be obtained by employing a non-uniform resonator. The pressure obtained has an oscillatory nature about the ambient pressure, with the frequency of operation equal to 933 Hz. The extraction of useful energy from the resonator is achieved by no-moving-part valves. In the present study, four different configurations of no-moving-part valves have been used for rectification. A series combination of three 15 mm length flow individual diode elements showed expected results with the diode having smaller diameter exhibiting better rectification. However, single diodes with length of 45 mm exhibited rectification in the opposite direction. The performance of the acoustic pump is evaluated based on its tank filling characteristics for a given driving power and a particular valve configuration. Maximum tank pressures of the order of 1000 Pa are obtained at maximum flow rates of the order of 10 slpm for the available range of driving powers.

The present study investigates the behaviour of the shock train in a typical Ramjet engine under the influence of shock and expansion waves at the entry of a low aspect ratio (1:0.75) rectangular duct/isolator at supersonic Mach number (M = 1.7). The start/unstart characteristics are investigated through steady/unsteady pressure measurements under different back and dynamic pressures while the shock train dynamics are captured through instantaneous Schlieren flow visualisation. Two parameters, namely pressure recovery and the pressure gradient, is derived to assess the duct/isolator performance. For a given back pressure, with maximum blockage (9% above nominal), the duct/isolator flow is established when the dynamic pressure is increased by 23.5%. The unsteady pressure measurements indicate different scales of eddies above 80 Hz (with and without flap deflection). Under the no flap deflection (no back pressure) condition, the maximum fluctuating pressure component is 0.01% and 0.1% of the stagnation pressure at X/L = 0.03 (close to the entry of the duct) and X/L = 0.53 (middle of the duct), respectively. Once the flap is deflected (δ = 8°), decay in eddies by one order is noticed. Further increase in back pressure (δ ≥ 11°) leads the flow to unstart where eddies are observed to be disappeared.

The total temperature achieved by vitiation of air using hydrogen–oxygen combustion is predicted after heat transfer losses. The adiabatic temperature, combustion products, and its properties are acquired using GASEQ. The heat transfer losses include losses due to convection and radiation of the mixture. The hot jet from individual burner enters the preheater with a velocity, which in turn causes convection to occur from the gas to the surface of the preheater. The convection loss is predicted using basic convection heat transfer equation used for heat transfer of hot gases flowing over a plate. The radiation loss was predicted using the emittance of the combustion products at different temperatures using existing analytical data. The emittance of the mixture is a function of adiabatic temperature of the combustion, chamber pressure, and partial pressure of water vapour in the combustion product. The prediction is compared with experimental total temperature, and calculated using the chamber pressure PO. The predicted total temperature is also compared with thermocouple value in one of the tests. The prediction helps in the design of supersonic combustion experiments which are largely dependent on the total temperature of the preheater. The prediction was used get the total temperatures up to 1100 K in the preheater.

Feasibility of multistaging of a nonuniform area resonator based fluidic pump, both in series as well as in parallel, is discussed in comparison with single stage pump. Multistaging up to two stages was carried out in this study. Multistaging was successful in achieving higher flow rates and higher back pressures than a single stage pump. Series pump delivered 1.8 times the pressure for zero flow rate, whereas parallel pump delivered 2.3 times the flow rate at zero back pressure. Of all three configurations, single stage pump was found to have the best efficiency based on the input power requirements. It was also found that the acoustic interactions between the stages can affect the performance of the pump.

Tomography technique was used to recover the three-dimensional density distribution of a flow field generated by a rectangular nozzle. The flow field was investigated through background oriented schlieren (BOS) setup, where the flow induced density gradients were captured as a 2D projected image. These 2D information were post-processed to retrieve back the phase of the interrogating beam, which later used in tomography. The research was an attempt to use iterative tomography technique with BOS captured data for quantifying the three-dimensional flow field. The ability of the proposed technique, which required relatively lesser projections to recover the three-dimensional domain was verified through iterative phase numerical simulations. Even CFD simulations showed good agreement with experimental data.

Flow around the open strut cavity has been investigated numerically. Open strut cavity is the cavity between two struts placed in tandem, along the flow. The simulation is done for the inlet Mach number of 2, with stagnation conditions of 3 bar and 300 K. Three different aspect ratios 4, 6 and 8 are simulated. Various flow features around the cavity are found to be similar to that of the wall mounted cavity. Pressure oscillations, inside the cavity, with a dominant mode at one of the Rossiter mode frequencies has been confirmed. The dominant mode is found to vary with aspect ratio of the cavity. Open strut cavity with aspect ratio of 8 behaved as closed cavity with a full bow shock in front of the trailing strut. Study of flow near the leading wall reveals the asymmetric nature of the flow in the symmetric configuration. The traveling compression wave is found to interact differently with the two shear layers, thus introducing asymmetry near the leading wall.