05-05-2011, 10:11 AM
ABSTRACT
A specially designed producer gas carburetor is comprehensively analyzed for its mixing performance and response with a CFD modeling. The model is made up of a mixer chamber that has the essential orifices for air and fuel (producer gas) inlets to generate stable stoichiometric mixture at near to ambient conditions using the induction of the engine as the driving pressure differential for the flow, and tested for a case of engine of 25 kWe capacity.The CFD simulations are carried out followed with experimental studies to validate the analysis. The results show a consistency in the experimental data and the modeling has provided a good insight into the flow details and has paved way in optimization in the geometrical design to get a good mixing efficiency.
Keywords: computational fluid dynamics, producer gas, carburetor, air/fuel ratio, turbulence
NOMENCLATURE
μ molecular viscosity,
μT turbulent viscosity
Cε1 constant =1.45
Cε2 constant 1.9.
σk turbulent model constant for kinetic energy =1
σε constant for k-ε model =1.3.
INTRODUCTION
In the current state of technological advances, it is recognized that Biomass is one of the viable and sustainable renewable resources and new technologies emerging out of biomass based gasification systems find a significant role in bridging the energy crisis. The advanced biomass gasification systems are known to generate producer gas as the combustible fuel that is clean enough to be used in Direct Injection gas engines. However in order to adapt standard gas engines few of its components need modifications before they are used in the biomass power plants. Since this area is an emerging one and the technology has not been disseminated to the scale of driving market, it is essential that specialized components that require modification need be studied. Carburetor is one of the important components in such Category and it is identified that additional research work is to be carried out in establishing a design procedure for this application. [3]. The work presented here is an effort in this regard.
International Conference on Computational Fluid Dynamics, Acoustics, Heat Transfer and Electromagnetics Air/fuel ratio characteristic exert a large influence on exhaust emission and fuel economy in Internal Combustion engine.[5]. With increasing demand for high fuel efficiency and low emission, the need to supply the engine cylinders with a well defined mixture under all circumstances has become more essential for better engine performance. Carburetors are in general defined as devices where a flow induced pressure drop forces a fuel flow into the air stream [1]. An ideal carburetor would provide a mixture of appropriate air-fuel (A/F) ratio to the engine over its entire range of operation from no load to full load condition. To ensure proper performance, Carburetors should be reproducible and have unequivocal adjustment procedures.
CFD software used for cold flow analysis is CFX 10.The k-ε turbulence model is most commonly used and is considered to be the best model between computational time and precision. [2]. The geometric model is built using Ansys workbench.
PRODUCER GAS CARBURETOR
Mixing devices for gases used in gas engines generally referred to as carburetor, for mixing air and gaseous fuels are commonly attached to the intake manifold of an internal combustion engine. In gas carburetor the mixing of air and gaseous fuels needs to be in a proper ratio for a particular engine load and speed. In designing the producer gas carburetor, simplicity and ruggedness have always been considered as a basic requirement to achieve easy adjustment and reproducible performance. The effective area reduction of gas and air entry holes is considered by taking a suitable coefficient of discharge. The air and fuel flow is through orifices into the mixing chamber of the carburetor which has baffle plates that enables proper mixing of air and fuel. The producer gas carburetor is being designed to have air and fuel flow at ambient conditions to be stoichiometry. The producer gas carburetor is as shown in the Figure 1 has orifices placed at air and gas inlets such that the A/F ratio at ambient flow condition should be stoichiometry for a engine suction pressure of a 25 KWe engine. The amount fuel flow inside the carburetor is controlled by a butterfly valve which is located prior to the fuel inlet orifice. The pressure balancing electronic controller drives suitably the butterfly valve with the help of a motor that brings the valve for a null pressure differential across the manifolds for the fuel and air attached upstream to the main engine manifold and works in suction pressures. If the differential pressure at both the carburetor manifolds are maintained at zero, with the manifolds tuned for their effective flow areas to match the ideal mixture condition, then the mixture flow what we get at engine intake manifold will be stoichiometry. The Figure 1 shown below is the geometric model of the producer gas carburetor designed and analyzed for optimal pressure drop with good mixing ability
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