Effect of Wall Impingement on Heterogeneous Structure in Diesel Sprays

A 2-D phase doppler technique was used for the measurements of the velocity, size, and flight direction of droplets in diesel sprays. The data acquisition rate of the phase doppler system was 250 kHz. Diesel fuel sprays injected intermittently into the atmosphere were investigated. The injector orifice was 0.113 mm in diameter. The rail pressure was set at 40 MPa by using a common rail system. The injection period was 3.0 ms and the time interval between injections was 330 ms. Measurement position was located at 40 mm from the nozzle exit for free sprays. In order to evaluate velocity vectors of each droplet, velocity components with angles plus and minus 45 degrees to the spray axis were measured. The data measured at each position was 10,000 and was accumulated over about 1,000 injections. It was found that most droplets near the spray center had velocity vectors along the spray axis. Droplets in the spray periphery region had the velocity component from the spray center to the spray periphery at a certain time from the start of injection. And droplets had the velocity component from the spray periphery to the spray center. It is understood that a heterogeneous structure exists in the spray periphery region. The arithmetic mean size of the droplets which had the velocity component to the spray center was smaller than that of the droplet which had the velocity component to the spray periphery. A flat wall was located at 20 mm from the nozzle exit at an angle of 45 degrees from the spray axis. The velocity, size, and flight direction of droplets after impingement were measured. It was made clear that the droplet size is affected by wall impingement. Droplets fly downstream and some droplets are entrained by vortex in spray periphery region in impinged spray. A strong vortex is observed when the spray impinged on a wall. The time variation of droplet size of impinged spray was different from that of free spray. The velocity of impinged spray droplets flying inward was lower than these flying outward.