Sakthi Kumar | Indian Institute of Science (original) (raw)

Papers by Sakthi Kumar

Search for potential alternative jet fuels is intensified in recent years to meet stringent envir... more Search for potential alternative jet fuels is intensified in recent years to meet stringent environmental regulations imposed to tackle degraded air quality caused by fossil fuel combustion. The present study describes atomization characteristics of blends of jatropha-derived biofuel with conventional aviation kerosene (Jet A-1) discharging into ambient atmospheric air from a dual-orifice atomizer used in aircraft engines. The biofuel blends are characterized in detail and meet current ASTM D7566 specifications. The experiments are conducted by discharging fuel spray into quiescent atmospheric air in a fuel spray booth to measure spray characteristics such as fuel discharge behavior, spray cone angle, drop size distribution and spray patternation at six different flow conditions. The characteristics of spray cone angle are obtained by capturing images of spray and the measurements of spray drop size distribution are obtained using laser diffraction particle analyzer (LDPA). A mechanical patternator system comprising 144 measurement cells is used to deduce spray patternation at different location from the injector exit. A systematic comparison on the atomization characteristics between the sprays of biofuel blends and the 100% Jet A-1 is presented. The measured spray characteristics of jatrophaderived alternative jet fuels follow the trends obtained for Jet A-1 sprays satisfactorily both in qualitative and quantitative terms.

Atomization characteristics of alternative jet fuel from aircraft engine atomizers are presented.... more Atomization characteristics of alternative jet fuel from aircraft engine atomizers are presented. The alternative jet fuel is a blend of 75% camelina-and 15% jatropha-derived biofuels and 10% aromatics (Van-SOL 53). Experiments are conducted to measure spray characteristics such as fuel discharge behavior, spray cone angle, drop size distribution and spray patternation at different flow conditions. The measured characteristics of the alternative jet fuel spray are compared with that of conventional jet fuel (Jet A-1).

The atomization characteristics of aviation biofuel discharging from a simplex swirl atomizer int... more The atomization characteristics of aviation biofuel discharging from a simplex swirl atomizer into quiescent atmospheric air are studied. The aviation biofuel is a mixture of 90% commercially available camelina-derived biofuel and 10% VonSol-53 (aromatics). The experiments are conducted in a spray test facility at varying fuel flow rate conditions. The measured characteristics include atomizer flow number, spray cone angle, breakup length of liquid sheet, wavelength of undulations on liquid sheet, and spray droplet size. The characteristics of biofuel sheet breakup are deduced from the captured images of biofuel spray. The measurements of spray droplet size distribution are obtained using Spraytec. The experimentally measured characteristics of the biofuel sheet breakup are compared with the predictions obtained from the liquid film breakup model proposed by . The measurements of wavelength and breakup length of the biofuel sheet discharging from the simplex swirl atomizer agree well with the model predictions. The model-predicted droplet size for the biofuel spray is significantly higher than the experimentally measured Sauter mean diameter (SMD). The spray droplets formed from the liquid sheet breakup undergo secondary atomization until 35-45 mm from the atomizer exit and thereafter the SMD increases downstream due to the combined effect of fuel evaporation and droplet coalescence. A good comparison is observed between the experimentally measured SMD of the biofuel spray and the predictions obtained using the empirical correlation reported in literature for sprays discharging from simplex swirl atomizers.

The atomization characteristics of blends of bioderived camelina hydrogenated renewable jet (HRJ)... more The atomization characteristics of blends of bioderived camelina hydrogenated renewable jet (HRJ) alternative fuel with conventional aviation kerosene (Jet A-1) discharging into ambient atmospheric air from a dual-orifice atomizer used in aircraft engines are described. The spray tests are conducted in a spray test facility at six different test flow conditions to compare the atomization of alternative fuels with that of Jet A-1. The fuel sprays are characterized in terms of fuel discharge, spray cone angle, drop size distribution, and spray patternation. The measurements of spray drop size distribution are obtained using laser diffraction based Spraytec equipment. The characteristics of fuel discharge and cone angle of alternative fuel sprays do not show any changes from that of Jet A-1 sprays. The characteristics of spray drop size, evaluated in terms of the variation of mean drop size along the spray axis, for the alternative fuel sprays remain unaffected by the variation in fuel properties between the alternative fuels and Jet A-1. The measurements on spray patternation, obtained using a mechanical patternator at a distance 5.1 cm from the atomizer exit, show an enhanced fuel concentration in the vicinity of spray axis region for the alternative fuel sprays discharging from the dual-orifice atomizer.

Pressure swirl atomizers discharging hollow cone liquid spray are commonly employed in aircraft g... more Pressure swirl atomizers discharging hollow cone liquid spray are commonly employed in aircraft gas turbine engines as fuel injectors. Studies on the characteristics of fuel spray discharging from such atomizers assume significance from the view of combustion considerations. The present study deals with an experimental study analyzing the role of liquid properties on the characteristics of swirling spray from an aircraft engine simplex atomizer. The experiments are carried out by discharging spray into quiescent atmospheric air at varying flow rate conditions with different liquids (pure water, water-glycerol mixture, and Jet A-1 fuel). Experimental measurements to describe the breakup process of conical liquid film and the characteristics of spray drop size are deduced with the aid of backlighting photographic technique and laser diffraction instrument Spraytec. A systematic analysis is carried out to elucidate the role of liquid properties on the sheet breakup and spray drop size characteristics. The observed trends on the variation of spray characteristics with varying flow conditions are explained by adopting already existing theoretical model on liquid sheet breakup and empirical correlations on mean drop size developed for sprays from pressure swirl atomizers.

Studies on the spray formation from orifices play a pivotal role in the development of atomizers ... more Studies on the spray formation from orifices play a pivotal role in the development of atomizers employed in aerospace and automobile engines. Significant academic interest is shown in recent years to understand the breakup and spray formation of liquid jets discharging from non-circular orifices. The manuscript presents preliminary results on the features of spray formation of water jets discharging from elliptical, triangular, and square orifices of equivalent diameter in the range 0.96 mm to 1.06 mm. The experiments are carried out in a spray test facility with pressure drop across the orifice as high as 42 bar.

This template illustrates the formatting of full length papers for ILASS 2016 proceedings. Please... more This template illustrates the formatting of full length papers for ILASS 2016 proceedings. Please do not change any formatting, such as margins. The abstract should be no longer than 200words.

Understanding the interfacial oscillation characteristics of liquid jets discharging from orifice... more Understanding the interfacial oscillation characteristics of liquid jets discharging from orifices is essential to describe their breakup mechanism and subsequent spray formation. The present work deals with an experimental study on the interfacial oscillation of elliptical liquid jets. The experiments are conducted with an elliptical orifice of equivalent diameter 4.93 mm with major and minor axis lengths 7.12 mm and 3.45 mm respectively. The results are obtained for three different liquids: pure water, water-glycerol mixture, and Jet A-1 fuel. The liquid jets are visualized using backlight shadow-graphy technique involving a high intensity strobe lamp and a digital camera. The interfacial oscillation of liquid jets is characterized by measuring the wavelength and amplitude of jet oscillation from recorded digital images. The wavelength of jet oscillation (λ) of elliptical liquid jet increases linearly with increase in liquid jet velocity (U o ). The jets of Jet A-1, owing to its significantly lower surface tension compared to the other two liquids, display higher value of wavelength for a given U o . The experimental measurements of λ recorded at different flow conditions exhibit good agreement with the theoretical predictions obtained using mathematical model proposed by Rayleigh (1879) to describe capillary phenomena of liquid jet exhibiting undulations in its cross section. For a given liquid jet Weber number (We), the elliptical jets of Jet A-1 undergo a larger increase in λ compared to the jets of other two liquids. Further the variation of maximum and minimum amplitudes (D max and D min ) of jet oscillation of elliptical liquid jets with We is discussed.

This work presents an experimental study on the oscillation behavior of liquid jets discharging f... more This work presents an experimental study on the oscillation behavior of liquid jets discharging from elliptical, triangular, and square shaped non-circular orifices. The measurements of wavelength and amplitude of jet oscillation are extracted from digital images of the liquid jets captured via still photography technique at different flow conditions using three different liquids (pure water, water-glycerol mixture, and Jet A-1 fuel). The measured wavelength of jet oscillation ( λ), irrespective of the shape geometry of the tested non-circular orifices, increases linearly with increase in liquid jet velocity ( U o ) and scales with the liquid jet Weber number ( We ) as λ/ D eq ∝ We 0.5 . The experimental measurements of λ recorded at different flow conditions exhibit good agreement with the theoretical predictions obtained using the mathematical model proposed by Rayleigh (1879) for liquid jet undulating in its cross section. For a given U o , the liquid jet of Jet A-1 displays higher value of λ and undergoes a larger increase in λ along the axis of the jet compared to the jets of other two liquids. Further the trends on the variation of maximum and minimum amplitudes of the first wave segment of liquid jets discharging from the elliptical and square orifices with U o are discussed.

Interaction of liquid droplets with hot surfaces is commonly encountered in practical application... more Interaction of liquid droplets with hot surfaces is commonly encountered in practical applications involving spray cooling. The physical roughness characteristics of the target hot surface could affect the interaction process drastically. One of the commonly encountered morphologies of surface is that of a groove-texture as seen in micro-finned heat exchangers. In this work, we study the impact of water droplets on a heated stainless steel surface textured with parallel linear grooves. The maximum spread, D m attained by the impacting droplet is one of the crucial factors governing the heat transfer from the surface to the impacting droplet. The focus of the present study is to understand the trend of D m with surface temperature, T s and droplet impact velocity, U o during water droplet impact on a superheated groove-textured surface. T s is varied from 120 o C to 305 o C and U o is varied from 0.5 m/s to 1.8 m/s corresponding to a variation in impact Weber number, We = ρU o 2 D o / from 10 to 110 (here ρ and  are the density and surface tension of water at ambient temperature, and D o is the droplet diameter prior to impact which is kept constant at around 2.59 mm). A smooth stainless steel surface is used as reference. The morphological details and maximum spread measurements of impacting droplets are obtained from front-view images taken using a digital high speed video camera. At any given T s , maximum spread factor (D m /D o ) measured perpendicular to grooves, β m, as well as on reference smooth surface, β m increases with We with β m, less than β m at all the impact conditions studied here. At a given We, as T s increases β m, increases and this effect diminishes at higher We. In contrast, β m decreases as T s increases. The difference between β m and β m, is seen to decrease with T s and increase with We. This indicates that the effect of contact line pinning, which causes β m, to be less than β m for droplet impact at ambient temperature conditions, diminishes with increase in T s and decrease in We.

High speed imaging and analysis of water drop impact on a superheated groove-textured surface, TG... more High speed imaging and analysis of water drop impact on a superheated groove-textured surface, TGS3 were conducted for surface temperatures, T s ranging from 120 0 C to 305 0 C and for drop Weber number, We ranging from 10 to 110. From 120 0 C to 180 0 C a transition from static to contact boiling is observed on both TGS3 and smooth surface, RS1 for all We. In static boiling, the drop morphological dynamics and boiling initiation time, t i which decreases with We, are similar on both the surfaces; in contact boiling the drop morphology corresponding to t i on the surfaces differ, with t i being higher on TGS3 than on RS1. At higher T s qualitative differences are seen in drop morphological dynamics on RS1 (rim fragmentation, central mass ejection, and Leidenfrost drop) and TGS3 (parachute-like structure formation, lift-off, and rupture).