Enhancement of Near Wall Mixing of an Impinging Jet by Means of a Bump on the Wall

It is found that a thin and rich mixture layer on a wall is formed after impingement of a gas jet of fuel on the wall. The measured thickness of the mixture layer is about 2 mm. and its dispersion rate after the end of injection is much lower, compared to that of a space gas jet. This phenomenon in a small D.I. diesel engine is known as “wall wetting” or “wall fuel accumulation” which has an important influence on engine fuel consumption and emissions.

This paper presents a technique for enhancing the near wall mixing of an impinging jet by means of a bump on the wall. The development of a wall jet formed after an impingement of a gas jet has been investigated by simultaneously measuring the near wall velocity and concentration distribution. It has been found that a wall jet is stripped off the wall and ejected as a secondary jet when the wall jet encounters a bump of the wall. Effects of the bump height, the location of the bump and the impingement angle are studied at a number of typical conditions. The distributions of velocity and concentration of a secondary jet are also investigated in detail and the non-dimensional distributions for velocity and concentration of a secondary jet are derived based on the data statistics. In addition the concept of concentration mean dispersion rate is employed in data processing in order to understand the effects of the bump geometry on mixing rate. It has experimentally confirmed that a controllable mixing process of “space-wall-space” for an impinging jet may be realized by means of a bump setting on the wall.