Srinivasan K

This paper investigates temperature measurement using acoustic pyrometry in hot zones and flames. For experiments in flameless hot zones, a Hartmann whistle is used as the acoustic source. Time of flight of sound waves in the heated path is calculated by cross-correlating the signals received by two microphones placed at pipe ends. Acoustic pyrometry is also performed for estimation of flame temperature in kerosene wick and candle flames, using a speaker as the acoustic source. Since length scales of single flames are small, leading to short times-of-flight, appropriate temperature profiling strategies for flame temperature estimation are identified. © 2012 Elsevier Ltd. All rights reserved.

This paper investigates the effect of chamfer angles on the acoustic spectra and directivity of resonance tubes, kept axi-symmetrically in the flow field of a supersonic jet. Such tubes can be used for effective flow control, mixing, ignition etc. The jet impinges at the open end of the tube which is closed at the other end. The parameters being considered are the chamfer angle of the tube, nozzle pressure ratio and spacing (S) between nozzle exit and the tube inlet. The jet diameter and the tube inlet diameter are kept constant. Nozzle pressure ratio is varied from 4 to 6 in steps of 0.5. The chamfer angles considered are 15°, 30°, 45°. Acoustic pressure is measured in the far field region at emission angles varying from 37° to 135°, from the jet flow direction. The spectra clearly illustrates that the resonance tubes with chamfer has higher fundamental frequency than that of its absence. The fundamental frequency is observed to decrease with L/Dj for all chamfer angles. The frequencies obtained from experiments are compared with standard quarter wavelength theory. It is clear that the frequencies of the chamfered tubes are almost closer to the theory. At large tube lengths all the frequencies match well with theory but at small tube lengths only 30o chamfer is almost close to the theory. The fundamental frequency of 45o chamfer is found to be almost near to that of 0o chamfer. The minimum location of fundamental frequency as marked in Fig. 4 with S/Dj is found to be same for all L/Dj studied. The shadowgraph sequence (Fig. 5) shows that the low frequency components ∼2 kHz in the waterfall spectra (Fig. 6) are due to jet regurgitance. It is observed from Fig.7 that the directivity is seen to be higher for a tube with α = 30°. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

The current work experimentally investigates the effect of Hartmann cavity acoustics on the atomization of droplet sprays. Initially, the experiments are conducted on a single droplet to understand its behavior in the sound field of a Hartmann whistle. The atomization studies on single droplet reveal that the existence of sound field causes the droplet to undergo large deformation and become irregular in shape. The degree of droplet deformation is quantified based on smaller circularity and larger Feret's diameter. The increase in cone angle of spray to a higher value in the presence of acoustics in comparison to its absence shows that the acoustics enhances the atomization. The stroboscopic visualization of sprays in the presence of acoustics further reveals the breakup of ligaments, large scatter as well as the formation of more number of droplets, indicating atomization enhancement.

The burning characteristics of unsteady partially premixed flames have been investigated in this paper. Two reaction zones namely rich inner premixed zone and outer diffusion wing were observed for all the equivalence ratios considered. This kind of structure is called double flame structure. Flickering of flames were observed due to the formation of toroidal vortices around the flame surface. These toroidal vortices are convected up as a result of buoyant acceleration. The flames are observed to oscillate in two different modes namely, bulk flickering and tip flickering. In the bulk flickering mode, the flame tip is chopped off from the flame surface and in the tip flickering mode, the flame surface is observed to oscillate up and down without any breakage of flame tip. Flames with higher equivalence ratio (greater than 6) exhibit bulk flickering and flames with lower equivalence ratio (less than 6) experience tip flickering. As the equivalence ratio is lowered by partial premixing of the fuel with more air, the flame height is reduced and the strength of inner premixed flame is increased. The amplitude of oscillation of the flame height also decreases with reduction in the equivalence ratio due to the formation of smaller toroidal vortices but the frequency of oscillation is found to increase slightly. A generalised Strouhal number - Froude number correlation has been obtained in the form St ∝Fr-β, by varying the flow rates of air or fuel systematically, keeping the other parameters constant. The effect of increasing air flow rate is to increase the flickering frequency and increasing fuel flow rate is to decrease the flickering frequency.

In the thermal management of spacecraft, space thermal radiators play a vital role as heat sinks. A serial radiator with proven advantages in ground applications is proposed and analyzed for space applications. From the performance analysis, specific heat rejection (SHR) of serial radiator is found to be higher than parallel radiator by 80% for maximum diameter of the tube, 47% for maximum thickness of the fin, and 75% for maximum pitch of the tubes under consideration. Also, serial radiator requires four times higher pumping power than parallel radiator with geometric parameters and a maximum mass flowrate under consideration. In serial radiators, the cross conduction between the fins has a significant effect on its thermal performance. Thus, conjugate heat transfer simulations and optimization operations are to be performed iteratively to optimize the serial radiator, which is computationally costly. To reduce the computational time, artificial neural network (ANN) is trained using conjugate heat transfer simulations data and combined with the genetic algorithm (GA) to perform optimization. Taguchi's orthogonal arrays provided the partial fraction of conjugate heat transfer simulations set to train the ANN. Taguchi-Neuro-Genetic approach, a process that combines the features of three powerful techniques in different optimization phases, is used to optimize both parallel and serial radiators. The optimization aims to obtain a configuration that provides the lowest mass and lowest pumping power requirement for given heat rejection. Optimization results show that the conventional parallel radiator is about 20% heavier and requires about 35% more pumping power than the proposed serial radiator.

Experimental investigations are carried out on Hartmann whistles to explore the effect of external chamfer at the cavity mouth. The acoustic performance depends upon the cavity length, jet-to-cavity-spacing and external chamfer angle (15, 30, and 45). The modifications in spectral and directional characteristics of external chamfered Hartmann whistles are studied in detail and are compared with a regular cavity. The frequencies are observed to attain a minimum value at a chamfer angle of 30 along with modification in the acoustic spectra. In general, it is noticed that the external chamfered whistles are directive at an emission angle of around 45 where it is around 37 for a regular whistle. Numerical simulations portray the flow/shock oscillation features in external chamfered cavities. Simulations depict intense flow diversion at the mouth of chamfered cavities and elucidate the directivity shift as well as the enhancement of acoustic power observed experimentally. Thus, it is observed that Hartmann whistles with external chamfering could radiate acoustic power up to 2.3 times that of a non-chamfered whistle. © 2013 Elsevier Ltd. All rights reserved.

The burning characteristics of unsteady partially premixed flames have been investigated in this paper. Two reaction zones namely rich inner premixed zone and outer diffusion wing were observed for all the equivalence ratios considered. This kind of structure is called double flame structure. Flickering of flames were observed due to the formation of toroidal vortices around the flame surface. These toroidal vortices are convected up as a result of buoyant acceleration. The flames are observed to oscillate in two different modes namely, bulk flickering and tip flickering. In the bulk flickering mode, the flame tip is chopped off from the flame surface and in the tip flickering mode, the flame surface is observed to oscillate up and down without any breakage of flame tip. Flames with higher equivalence ratio (greater than 6) exhibit bulk flickering and flames with lower equivalence ratio (less than 6) experience tip flickering. As the equivalence ratio is lowered by partial premixing of the fuel with more air, the flame height is reduced and the strength of inner premixed flame is increased. The amplitude of oscillation of the flame height also decreases with reduction in the equivalence ratio due to the formation of smaller toroidal vortices but the frequency of oscillation is found to increase slightly. A generalised Strouhal number - Froude number correlation has been obtained in the form St αFr-β, by varying the flow rates of air or fuel systematically, keeping the other parameters constant. The effect of increasing air flow rate is to increase the flickering frequency and increasing fuel flow rate is to decrease the flickering frequency.

This paper investigates the effect of outer chamfer on the frequency and amplitude characteristics of resonance cavities, placed axi-symmetrically in the flow field of a supersonic jet. Such cavities can be gainfully used for flow control, atomization, mixing, ignition etc. The parameters being considered are the nozzle pressure ratio, chamfer angle at the outer surface of the cavity, cavity length and stand-off distance between nozzle exit and the cavity inlet. The cavity inlet diameter and jet diameter are kept constant. The chamfer angles adopted for the present study includes 15°, 30°, 45°. The acoustic pressure is measured in the far field region at a fixed radius of around 64Dj in order to avoid flow effects. The emission angles considered for the present study varied from 37° to 135° in steps of 2°, measured from the jet flow direction. The frequencies of non-chamfered cavity and 15° chamfered cavity are almost same thereafter it follows decreasing and increasing trend with chamfer angles. It is also noticed that all the frequencies approach a minimum value at a chamfer angle of 30°. The fundamental frequencies of all outlet chamfered cavities at a nozzle pressure ratio (NPR) of 5 are observed to decrease with increase of the non-dimensional cavity length (L/Dj). The frequencies obtained from experiments are compared with those obtained using Quarter wavelength formula (QWL) for an open - closed cavity. The decreasing trend of the frequency with L/Dj is found same for both experiments and theory, but the theoretical values are slightly higher at small L/Dj's. The spectral variation of frequency components at NPR = 4 show that there is no modification of the frequency components at L/Dj = 4.28 but the increase of L/Dj to 5.71 causes the increase of broad part between two successive peaks at 15o and 45o chamfer. It is seen that the minimum location of frequency index (frequency with chamfer/frequency without chamfer) with S/Dj at NPR = 4 is same for all chamfer angles. Shadowgraph sequence clearly illustrates the flow oscillation in front of the cavity mouth for all stand-off distances normalized with jet diameter. It is observed that the overall sound pressure level attains maximum at small L/Dj almost for all chamfer angles. It is seen that for all chamfer angles the overall sound pressure level follows a decreasing trend with S/Dj. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

Symmetric to asymmetric transitions of laminar-premixed methane-air flames anchoring in symmetric sudden expansion channels are explored. Numerical and experimental studies on flame anchoring phenomena are carried out. Simulations predict attachment of cold flow to one of the side walls (Coanda effect), above a critical Reynolds number. However, with symmetric ignition near the sudden expansion, initially asymmetric cold flow becomes symmetric and a symmetric flame anchors at the sudden expansion. Below a critical value of Reynolds number (ReC,R), both steady flow and flame are symmetric. When fuel-air mixture is ignited asymmetrically close to the sudden expansion for Re > ReC,R, both the flow and the flame attach to one of the side walls asymmetrically if the transverse distance of ignition is greater than a critical value. The flow attachment length is always smaller than flame attachment length. Experimentally visualized flame anchoring phenomena for ignition near the exit are in conformity with corresponding numerical predictions.

This paper presents numerical results on the near-field interactions of particle-laden twin supersonic jets operating in underexpanded mode, with special emphasis on the effects of particles on jet flow expansion. Although plane jets have been considered primarily, a few cases of twin round jets have also been simulated for the sake of comparison. Three turbulence models (Spalart-Allmaras, k-ε (realizable), and k-ω (sst)) have been considered for incorporating turbulent interaction effects in two-phase supersonic jets. In general, the k-ω (sst) model gives better predictions in the reverse flow as well as in the shock regions. The recirculatory flow is enhanced at high underexpansion (UE) ratios, and the size and strength of the recirculation region are much smaller for twin round jets than those for plane jets. In particle-laden jets, small particles (corresponding to Stokes number less than 0.05) are entrained in the recirculation zone, whereas particles of larger size are not trapped. With large particles, a particle-free zone occurs close to the nozzle wall in the divergent section. Small particles follow the gas phase with negligible non-equilibrium effects, but they also cause higher two-phase drag, resulting in a higher degree of jet UE.