Achintya Mukhopadhyay

Heat Transfer Enhancement and Entropy Generation in a Square Enclosure in the Presence of Adiabatic and Isothermal Blocks

In the present work, heat transfer and entropy generation characteristics are numerically investigated in presence of single and double obstructive blocks within a square enclosure. It is found that the adiabatic block(s) enhance(s) the heat transfer marginally up to a critical size in a convection-dominated regime. On the other hand, the enhancement parameter is observed to be more with an increase in block size in a lower range of Rayleigh numbers for an isothermal block. The entropy generation for thermal irreversibility is observed to be several orders higher than that due to viscous dissipation in all cases.

Trends in Comprehensive Modeling of Spray Formation

Comprehensive modeling of spray formation in liquid fuel injectors involves modeling of (i) internal hydrodynamics of fuel injector (ii) break up of liquid sheet leading to primary and secondary atomization and (iii) prediction of size and velocity distributions of droplets in the spray. Comprehensive models addressing all the three aspects are rare though some work has been reported that incorporate two of the three aspects. However, significant volume of literature exists on the individual modules. In the present work, progress and current trends in the individual modules have been extensively reviewed and their implications on development of comprehensive models have been discussed. The unresolved issues and future research directions are also indicated.

Instability analysis of a plane liquid sheet sandwiched between two gas streams of nonzero unequal velocities.

Due to interaction with the surrounding fluid, waves grow on fluid sheet and disintegration of the liquid sheet takes place when amplitude of those waves reaches a critical value. This break up leads to the formation of ligaments of fluid sheet. These ligaments are further break up into drops due to surface tension effect. This instability of thin liquid sheet followed by break up into drops is commonly known as atomization. So the nature of growing disturbances on the fluid interfaces due to interaction with the surrounding gas plays a significant role in the break up mechanism of thin liquid sheet. In past Squire [1], Hagerty and Shea [2] have investigated the instability of thin liquid sheet using a linear theory. Li [3] and Ibrahim [4] studied a temporal instability analysis of the liquid sheet moving in a stagnant gas, indicated that there are only two types of waves that were amplified at a given frequency, namely sinuous (in-phase) and varicose (out of phase). Li [5] performed a temporal stability analysis of liquid sheet in gas streams of unequal velocities where one gas stream was static and it was found that there exist two more mode of instability, which are para-sinuous (when phase difference between two interfaces is close to zero) and para-varicose (when phase difference between two interfaces is close to π).Also for large Weber number the para-sinuous mode was found to be more unstable than para-varicose mode. Surface tension of the liquid has a stabilizing effect whereas liquid viscosity had dual effect depending on the conditions. The spatial instability analysis of Chuech [6], Li [3] and Iibrahim [4] considered a liquid sheet moving in a quiescent gas medium; however in practical application such as air blast atomizer, liquid sheet is subjected to gas velocities on both sides. As explained by Witherspoon and Parthasarathi [7] results of Li [5] and Ibrahim [4] can’t be readily extrapolated for the case of liquid sheet subjected to gas motion. So Present study deals with the break up behaviour of the liquid sheet subjected to non-zero unequal gas flow on both side of the liquid sheet. Effects of different properties such as density ratio, surface tension or Weber number on the instability of the plane fluid sheet have also been studied. As SMD (mean droplet diameter) depends on the dominant wave length and growth rate, so how this growth rate behaves with different parametric situation that also been tested. So motivation of the preset work isa) Investigation of the stability of a thin liquid sheet subjected to non-zero unequal velocities on both sides. 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 2 5 August 2009, Denver, Colorado AIAA 2009-5157

Characterization of Turbulent Combustion Systems Using Dynamical Systems Theory

Turbulent combustion which is ubiquitous in all real engines in power and propulsion industries has inspired the combustion community to a great extent in recent years. Turbulence being the most significant unresolved problem gets more complicated by the interaction with combustion as combustion involves a large number of chemical reactions occurring at different time scales. A researcher often focuses on some specialized problems of turbulent combustion as it has many different aspects to investigate. One such challenging aspect of turbulent combustion is combustion dynamics. Many such facets of combustion dynamics have been understood through modelling, simulation and experiments. The present chapter proposes a survey of combustion dynamics which has been addressed under the parlance of dynamical systems theory. More recently, combustion instability in turbulent combustors such as modern low-(NO_x) gas turbine has gained a lot of attention. The stable state is generally characterized by combustion noise which is generated by turbulent reactive flow. A transition occurs from combustion noise to combustion instability through a dynamical regime called intermittency. Combustion instability is, in general, detrimental for all combustion systems except pulse combustors where combustion instability is deliberately maintained for better performance. The dynamical transition in pulse combustor has also been analyzed both theoretically and experimentally. The analysis of a nonlinear analytical model using dynamical systems theory reveals the regime of limit cycle oscillations, Hopf bifurcation, period-doubling bifurcations and so on. A case study of numerical continuation in pulse combustor model will be explained in detail at the end of this chapter.

Experimental Investigation of a Hollow Cone Spray Using Laser Diagnostics

Atomization of fuel is a key integral part for efficient combustion in gas turbines. This demands a thorough investigation of the spray characteristics using innovative and useful spray diagnostics techniques. In this work, an experimental study is carried out on commercial hollow cone nozzle (Lechler) using laser diagnostics techniques. A hollow cone spray is useful in many applications because of its ability to produce fine droplets. But apart from the droplet diameter, the velocity field in the spray is also an important parameter to monitor and has been addressed in this work. Kerosene is used as the test fuel which is recycled using a plunger pump providing a variation in the injection pressure from 100psi to 300psi. An innovative diagnostic technique used in this study is through illumination of the spray with a continuous laser sheet and capturing the same with a high speed camera. A ray of laser beam is converted to a planer sheet using a lens combination which is used to illuminate a cross section of the hollow cone spray. This provides a continuous planar light source which allows capturing high speed images at 285 fps. The high speed images, thus obtained are processed to understand the non-linearity associated with disintegration of the spray into fine droplets. The images are shown to follow a fractal representation and the fractal dimension is found to increase with rise in injection pressure. Also, using PDPA, the droplet diameter distribution is calculated at different spatial and radial locations at wide range of pressure.Copyright