Srinibas Karmakar

Puffing and Micro-Explosion Behavior in Combustion of Butanol/Jet A-1 and Acetone-Butanol-Ethanol (A-B-E)/Jet A-1 Fuel Droplets

ABSTRACT Butanol is mainly produced by acetone–butanol–ethanol (A-B-E) fermentation. The higher costs associated with the separation of butanol from the A-B-E mixture has prohibited the large-scale production of butanol. The direct use of the intermediate product (A-B-E mixture) could be an economical pathway if utilized for clean combustion. The present investigation deals with the puffing and micro-explosion characteristics in the combustion of a single droplet comprising butanol/Jet A-1, A-B-E/Jet A-1 blends, and A-B-E. The complete sequence of processes leading to micro-explosion, i.e., onset of nucleation, growth of vapor bubble, and subsequent breakup of droplet, for various fuel blends has been analyzed from the high-speed images. It is witnessed that puffing plays a crucial role in enhancing the micro-explosion especially in droplets with 50/50 composition. The probability of micro-explosion in droplets with A-B-E blends was found to be higher than that of butanol blends. Although the rate of bubble growth was almost similar for all butanol and A-B-E blends, the final bubble diameter before the droplet breakup was found to be higher for 50/50 blends than that of 30/70 blends. The occurrence of micro-explosion shortened the droplet lifetime, and this effect appeared to be stronger for droplets with 50/50 composition. Micro-explosion led to the ejection of both larger and smaller secondary droplets; however, puffing resulted in relatively smaller secondary droplets. Bubble growth and subsequent puffing/micro-explosion were also observed in the secondary droplets, which might play an important role in enhancing the atomization and, hence, combustion.

Atomization characteristics and instabilities in the combustion of multi-component fuel droplets with high volatility differential

We delineate and examine the successive stages of ligament-mediated atomization of burning multi-component fuel droplets. Time-resolved high-speed imaging experiments are performed with fuel blends (butanol/Jet A-1 and ethanol/Jet A-1) comprising wide volatility differential, which undergo distinct modes of secondary atomization. Upon the breakup of vapor bubble, depending on the aspect ratio, ligaments grow and break into well-defined (size) droplets for each mode of atomization. The breakup modes either induce mild/intense oscillations on the droplet or completely disintegrate the droplet (micro-explosion). For the blends with a relatively low volatility difference between the components, only bubble expansion contributes to the micro-explosion. In contrast, for blends with high volatility differential, both bubble growth as well as the instability at the interface contribute towards droplet breakup. The wrinkling pattern at the vapor-liquid interface suggests that a Rayleigh-Taylor type of instability triggered at the interface further expedites the droplet breakup.

Experimental investigations on nucleation, bubble growth, and micro-explosion characteristics during the combustion of ethanol/Jet A-1 fuel droplets

Abstract The combustion characteristics of ethanol/Jet A-1 fuel droplets having three different proportions of ethanol (10%, 30%, and 50% by vol.) are investigated in the present study. The large volatility differential between ethanol and Jet A-1 and the nominal immiscibility of the fuels seem to result in combustion characteristics that are rather different from our previous work on butanol/Jet A-1 droplets (miscible blends). Abrupt explosion was facilitated in fuel droplets comprising lower proportions of ethanol (10%), possibly due to insufficient nucleation sites inside the droplet and the partially unmixed fuel mixture. For the fuel droplets containing higher proportions of ethanol (30% and 50%), micro-explosion occurred through homogeneous nucleation, leading to the ejection of secondary droplets and subsequent significant reduction in the overall droplet lifetime. The rate of bubble growth is nearly similar in all the blends of ethanol; however, the evolution of ethanol vapor bubble is significantly faster than that of a vapor bubble in the blends of butanol. The probability of disruptive behavior is considerably higher in ethanol/Jet A-1 blends than that of butanol/Jet A-1 blends. The Sauter mean diameter of the secondary droplets produced from micro-explosion is larger for blends with a higher proportion of ethanol. Both abrupt explosion and micro-explosion create a large-scale distortion of the flame, which surrounds the parent droplet. The secondary droplets generated from abrupt explosion undergo rapid evaporation whereas the secondary droplets from micro-explosion carry their individual flame and evaporate slowly. The growth of vapor bubble was also witnessed in the secondary droplets, which leads to the further breakup of the droplet (puffing/micro-explosion).

Computational Fluid Dynamic Simulation of Single and Two-Phase Vortex Flow—A Comparison of Flow Field and Energy Separation

In the present work, a computational fluid dynamic (CFD) simulation has been performed to investigate single and two-phase vortex tube. Air in compressed form and partially condensed phase are used as working fluid, respectively. Simulation has been carried out using commercial CFD software package fluent 6.3.26. A detailed study has been performed to generate the profiles of velocity, pressure, and pathlines. These profiles provide an insight on how the process of energy separation as well as the flow field in the vortex tube gets affected on introduction of a liquid phase. The result shows that in case of cryogenic vortex tube, the flow reversal takes place closer to wall due to presence of a very thin wall adhering liquid film, while, in single-phase flow vortex tube, flow reversal is observed at the central portion. The model also predicts that presence of recirculation zone near warm end diminishes the refrigeration effect of vortex tube for two-phase flow.

Bubble dynamics and atomization mechanisms in burning multi-component droplets

We examine the complete sequence of events associated with the transition in the topology of a single droplet into multiple fragments of secondary droplets in the context of burning multi-component miscible mixtures. The multi-component blends consist of tetradecane as a lower volatile component, while butanol and acetone-butanol-ethanol (A-B-E) are used as the higher volatile constituents. In addition to the widely recognized theory of bubble growth via micro-bubble coalescence, we reveal that the vapor bubble growth also occurs through the merging of large bubbles during the combustion of droplets. The initial bubble growth (Regime I) and collapse cycles were found to increase the rate of bubble nucleation in the droplet, which in turn leads to the growth and merging of two or more vapor bubbles into a single larger bubble (Regime II). The final stage of bubble growth (Regime III) is associated with the Rayleigh-Taylor (RT) instability at the vapor-liquid interface. After the inception of the RT instability, capillary wave propagation is also witnessed on the droplet surface. The breakup of a vapor bubble results in the creation of a ligament that subsequently undergoes pinch-off into one or more secondary droplets. The ligament pinch-off mechanisms are categorized into two types, i.e., tip breakup and tip-base breakup, which govern the diameter and velocity of secondary droplets along with succeeding volumetric shape oscillations in the parent droplet. In particular, the ligament tip-base pinch-off mechanism results in a bimodal distribution of secondary droplets. After the initial breakup event, a vapor bubble may grow either in the secondary droplet or inside the developing ligament, leading to a sequential cascade of breakup events.We examine the complete sequence of events associated with the transition in the topology of a single droplet into multiple fragments of secondary droplets in the context of burning multi-component miscible mixtures. The multi-component blends consist of tetradecane as a lower volatile component, while butanol and acetone-butanol-ethanol (A-B-E) are used as the higher volatile constituents. In addition to the widely recognized theory of bubble growth via micro-bubble coalescence, we reveal that the vapor bubble growth also occurs through the merging of large bubbles during the combustion of droplets. The initial bubble growth (Regime I) and collapse cycles were found to increase the rate of bubble nucleation in the droplet, which in turn leads to the growth and merging of two or more vapor bubbles into a single larger bubble (Regime II). The final stage of bubble growth (Regime III) is associated with the Rayleigh-Taylor (RT) instability at the vapor-liquid interface. After the inception of the RT instabi...

Puffing and Micro-explosion behavior of Ethanol/Jet A-1 Fuel Droplets

The secondary atomization caused by puffing and micro-explosion phenomena play a significant role in enhancing atomization in the combustion chamber. The physical processes such as bubble nucleation and dynamics of bubble growth leading to puffing/microexplosion are not well understood. Puffing and Micro-explosion phenomena in the ethanol/Jet A-1 blends (E10, E30, and E50) have been studied for relatively larger diameter droplets. Micro-explosions were found to reduce the total burning time of the droplet. The abrupt explosion phenomenon was noticed only in E10 blend, which was a dominant phenomenon in this particular blend. Micro-explosions were categorized as undisturbed micro-explosion and puffing induced micro-explosion. Undisturbed micro-explosion is considered as a special case where the bubble growth leading to micro-explosion occurs without prior puffing. Puffing was observed to increase the growth of the vapor bubble as well as the intensity of breakup. Puffing/micro-explosion phenomenon was characterized by the normalized squared onset diameter (NOD). The onset of bubble growth was found to be the prerequisite of puffing and only in a few cases the onset of bubble nucleation lead directly to micro-explosion. NOD was observed to decrease with the increase in the proportion of ethanol component. Temporal variation of bubble radius has been shown for all the ethanol/Jet A-1 blends. The lower degree of superheating of the ethanol component leads to the puffing phenomenon whereas the higher degree of superheating leads to the micro-explosion phenomenon. Puffing was observed to be the dominant source of droplet ejections. The velocity of ejected droplets was found to increase with the increase in the ethanol proportion.