Showing papers by "RV Ravikrishna published in 2012"

An experimental study on effervescent atomization of bio-oil fuels

Abstract: This paper presents the work on detailed characterization of effervescent spray of Jatropha and Pongamia pure plant oils. The spray characteristics of these biofuels are compared with those of diesel. Both macroscopic and microscopic spray characteristics at different injection pressures and gas-to-liquid ratio (GLR) have been studied. The particle/droplet imaging analysis (PDIA) technique along with direct imaging methods are used for the purpose of spray characterization. Due to their higher viscosity, pure plant oils showed poor atomization compared to diesel and a blend of diesel and pure plant oil at a given GLR. Pure plant oil sprays showed a lower spray cone angle when compared to diesel and blends at lower GLRs. However, the difference is not significant at higher GLRs. Droplet size measurements at 100 mm downstream of the exit orifice showed reduction in Sauter mean diameter (SMD) diameter with increase in GLR. A radial variation in the SMD is observed for the blend and pure plant oils. Pure oils showed a larger variation when compared to the blend. Spray unsteadiness has been characterized based on the image-to-image variation in the mean droplet diameter and fluctuations in the spray cone angle. Results showed that pure plant oil sprays are more unsteady at lower GLRs when compared to diesel and blend. A critical GLR is identified at which the spray becomes steady. The three regimes of spray operation, namely ``steady spray,'' ``pulsating spray,'' and ``spray and unbroken liquid jet'' are identified in the injection pressure-GLR parameter space for these pure plant oils. Two-phase flow imaging inside the exit orifice shows that for the pure plant oils, the flow is highly transient at low GLRs and the bubbly, slug, and annular two-phase flow regimes are all observed. However, at higher GLRs where the spray is steady, only the annular flow regime is observed.

Comparison of Jatropha Methyl Esther and diesel non-evaporating sprays

Abstract: The present experimental study compares various spray characteristics of diesel and jatropha methyl es ter (JME/ jatropha biodiesel) non-evaporating sprays. T hese sprays are studied in a specially fabricated h igh pressure chamber with optical access. The fuels were in jected at injection pressures of 500, 1000 and 1500 bar into a nitrogen environment maintained at 20, 25 and 35 ba r, respectively. First, the spray structure in term s of spray tip penetration and spray plume angle are compared. It is observed that the spray tip penetration is a round 2 to 5% higher and the spray plume angle is around 7 to 17% smaller for JME as compared to diesel. This ind icates a slower breakup for the jatropha biodiesel sprays. D roplet diameters are measured using the particle/dr oplet image analysis (PDIA) technique. For JME, around 5% higher droplet diameters are observed. Detailed prob ability distribution of droplets showed that jatropha biodi esel has more probability for larger droplet diamet ers (>18µm) and lesser probability for smaller droplet diameter s (<18µm) which explains the overall SMD trend. The main reason for larger droplet diameters is the higher v iscosity and surface tension of JME compared to die sel. The effect of fuel properties on the near nozzle struct ure is studied. A longer unbroken liquid length and narrower spray plume is observed for JME as compared to diesel indicating slower breakup.

Validation of a Modified Eddy Dissipation Concept Model for Stationary and Nonstationary Turbulent DiffusionFlames

Abstract: A transient flame simulation tool based on unsteady Reynolds average Navier Stokes (RANS) is characterized for stationary and nonstationary flame applications with a motivation of performing computationally affordable flame stability studies. Specifically, the KIVA-3V code is utilized with incorporation of a recently proposed modified eddy dissipation concept for simulating turbulence-chemistry interaction along with a model for radiation loss. Detailed comparison of velocities, turbulent kinetic energies, temperature, and species are made with the experimental data of the turbulent, non-premixed DLR_A CH4/H-2/N-2 jet flame. The comparison shows that the model is able to predict flame structure very well. The effect of some of the modeling assumptions is assessed, and strategies to model a stationary diffusion flame are recommended. Unsteady flame simulation capabilities of the numerical model are assessed by simulating an acoustically excited, experimental, oscillatory H-2-air diffusion flame. Comparisons are made with oscillatory velocity field and OH plots, and the numerical code is observed to predict transient flame structure well.

Modelling of mixture preparation in a small engine with port fuel injection

Abstract: Computational Fluid Dynamics simulations are performed to study mixture preparation in the intake manifold of a small engine with Port Fuel Injection. Data from laser-based experiments on two injectors are used as inputs to the spray sub-model. The results reveal improved in-cylinder fuel distribution when the spray is directed onto the intake valve. The Sauter Mean Diameter of the spray is observed to have an insignificant effect during open valve injection, while the targeting is found to have a large influence. The methodology developed as part of this work can be used to further optimize injection parameters in such engines.

Experimental spray characterisation of air-assisted impinging jets

Abstract: Liquid atomization is a problem with applications in many fields. In the present study, an air-assisted impinging jet atomizer has been utilised and its spray characteristics have been studied at various conditions, using water as the liquid and nitrogen as the atomizing gas. Two configurations were studied: one having a central gas jet above two impinging liquid jets, and another having a central liquid jet targeted by two inclined gas jets. Backlit imaging and particle/droplet imaging and analysis techniques were utilised to characterise the sprays. It was observed that the configuration with a central gas jet and impinging liquid jets resulted in better atomization that the other case. It was observed that both the breakup length of the sheet and the resulting droplet sizes, reduced with an increase in the gas flow rate. The diameter of the gas jet orifice was also found to play an important role in changing the spread of the spray, the droplet sizes, and the radial distribution of droplet sizes. Overall, it is observed that by varying the gas-to-liquid ratio and nozzle orifices, it is possible to obtain a fine atomization of the spray using the air-assisted impinging jet configuration even at low injection pressures of below 0.4 MP a. Introduction Air-assisted atomization, in which kinetic energy of air is used to aid liquid breakup, has been used to atomize various kinds of liquids [ 1], [2], [3]. In most of these studies, either an annular/central liquid sheet is blasted with air jets, or gas is allowed to mix with the liquid inside the atomizer to form a spray. This method requires a large amount of gas to achieve droplet sizes of the order of 50 μm. Beck et al. [ 4] achieved a Sauter mean diameter (SMD) of 75 μm with water at a gas to liquid ratio (GLR) of the order of 1.0. On the other hand, impinging jet atomization has been used in rocket engines due to its good atomization and mixing characteristics [ 5], [6]. Impinging jet experiments with water by Shen et al. [ 7] showed that jet velocities of the order of 12.5 m/s was required to obtain an SMD in the range of 300 μm. In the present study, a liquid sheet is formed using two impinging liquid jets, and an air jet is directed on to this sheet to aid the atomization process. This combined effect of impinging jet and air assisted atomization helps in achieving smaller droplets at low liquid injection velocities. Experimental Setup and Methods The experimental setup consists of two parts, a liquid and gas supply system to the atomizer, and the optical setup to measure spray characteristics. In the supply system, high pressure nitrogen from a gas bottle is regulated to the gas reservoir tank and part of it is used to compress the liquid in the liquid reservoir. Liquid and gas flow rates are controlled using needle valves in their respective lines. The gas flow rate is measured using a thermal mass flow meter, while the liquid flow is measured using a gear flow meter. Digital pressure gages placed close to the atomizer measure the injection pressure. The spray is injected in a quiescent ambient condition. The spray is collected into a tank with a small exhaust fan at the bottom which creates sufficient suction to remove the mist without affecting the spray. The atomizer used in the present study consists of two liquid nozzles with an orifice of 0.76 mm in diameter, and a gas nozzle with an orifice diameter of 1.1 mm. The gas jet is placed over the impinging point of liquid jets at a distance of 7 mm, and making equal angles with them. The liquid jets impinge and form a liquid sheet in the plane perpendicular to that containing the axes of the injectors, and the gas supplied through the gas orifice is used to assist in the break-up of this sheet. The optical setup for spray characteristic measurements is based on the backlit direct imaging method. It uses a pulsed Nd:YAG laser along with a fluorescent diffuser as a light source to back-illuminate the spray, and a CCD camera with a resolution of 2048 X 2048 pixels to record the spray images. A pulse duration of around 10 ns is used. The schematic of the experimental setup is shown in Fig. 1. In the present study, two types of image-based measurements are performed, one to study the spray structure and the other for droplet size measurements. For the spray structure measurements, images are taken from the exit of the orifice to 80 mm downstream. Small portions of the spray are imaged using a long distance microscope for droplet size measurements. ∗Corresponding author: ammohan@mecheng.iisc.ernet.in