Determination of the flow field downstream of an annular gap steam injector

Abstract: Background Tetra Pak manufactures and distributes so called VTIS facilities for sterilization and pasteurization of for example milk and ice-cream. These VTIS facilities uses a common sterilization process called ultra high temperature (UHT) treatment. UHT treatment involves heating the product to 130-150ºC for one to six seconds depending on type of product. In the VTIS facility the temperature raise in the UHT treatment is obtained with an annular gap steam injector. The annular gap steam injector injects high pressure steam directly in to the product which is going to be sterilized, this raises the temperature of the product almost instantaneously. When quickly raising the temperature of a product the product can burn to the walls of the facility and cause so called fouling (product burn-on due to high temperatures). The fouling has been thoroughly investigated but it is still not totally eliminated. The fouling changes character depending on flow, pressure and the settings of the annular gap steam injector in the VTIS facility. On account of this it is interesting to investigate if the flow field after the annular gap steam injector changes when the flow, pressure and the settings of the annular gap steam injector changes and if this can be connected to the fouling that occurs. Methods The flow field investigations has been made with the optical flow velocity measuring technique Particle Image Velocimetry (PIV) which takes pictures of the flow field using a camera, a high powered laser and particles in the flow. The PIV measurements took place in a pilot plant build to mimic the annular gap injector part of the VTIS facility. Measurements were done after the annular gap steam injector in a part of the VTIS facility called the holding cell, 750 mm of approximately 4 000 mm of the holding cell, at three different heights of the holding cell, was investigated. The pilot plant was run under the same conditions as the VTIS facility in order to mimic the flow field in the VTIS facility. Results When injecting high pressure steam in to the product a jet emerges from the annular gap steam injector. Close to the annular gap steam injector the jet consists of steam but short after the steam has condensed and the jet only consists of liquid. The diameter of the jet is narrow close to the annular gap steam injector and then grows until it covers the whole diameter of the holding cell. Before the jet has reached its full diameter a back flow is present close to the walls of the holding cell. When the jet has reached its full diameter and covers the whole diameter of the holding cell no more back flow can be detected