Search Results

A commercial CFD package was used to develop a three-dimensional, fully turbulent model of the compressible flow across a complex-geometry venturi, such as those typically found in small engine carburetors. The results of the CFD simulations were used to understand the effect of the different obstacles in the flow on the overall discharge coefficient and the static pressure at the tip of the fuel tube. It was found that the obstacles located at the converging nozzle of the venturi do not cause significant pressure losses, while those obstacles that create wakes in the flow, such as the fuel tube and throttle plate, are responsible for most of the pressure losses. This result indicated that an overall discharge coefficient can be used to correct the mass flow rate, while a localized correction factor can be determined from three-dimensional CFD simulations in order to calculate the static pressure at locations of interest within the venturi.

This paper presents a theoretical analysis of the fuel and air flows within the idle circuit found in simple carburetors. The idle circuit is modeled numerically using a dynamic model that considers the resistances of the flow paths as well as the inertia of the fuel. The modeling methodology is flexible, in that the organization and techniques can be applied to any configuration and geometry. The numerical model calculates the fuel flow response of carburetor idle/transition circuits to pressure variations associated with air flow through the venturi and around the throttle plate. The model is implemented for a typical small engine carburetor and the nominal results are presented for this specific design.