Volume 8, Issue 1 (January 2011)
Heat Transfer Stages Recognition by Boiling Acoustic During Quenching
In order to control the quenching process and provide uniform quenching, wetting of the specimen with the quenching medium is very important since this will provide equable heat transfer and therefore uniform quenching conditions for the specimen. During the quenching of the specimens in liquids, in the boundary layer between the specimen and the quenching medium, three characteristic phenomena occur in the following sequence: Film boiling, bubble boiling, and convection. The phenomena concerned proceed with different rates of intensity at different locations at the specimen surface or at its entire surface. The quenching process is always accompanied by evaporation of the quenching medium at the specimen surface, where the characteristic boundary layer will form. Vaporization and boiling is always involved with sound generated by formation, oscillation, and decay of the bubbles, predominantly in the range of hearing, and which slightly round this threshold. During the experiment of dip quenching, the acoustic emission generated was captured with the hydrophone. The sound pressure changes are indicated in a digital sound record. A spectrum analysis of sound pressure is shown in the diagrams indicating variations of the sound pressure level as a function of frequency and time, in three-dimensional diagrams of sound pressure changes, as a time-variation of pressure amplitude, as a signal power spectrum, etc. The variations of sound pressure in dependence of the acoustic signal captured confirm the phenomena in the boundary layer between the specimen and the quenching medium. From the acoustic signals captured in the course of the quenching process, it can be inferred (1) the occurrence and continuing of film boiling, in which intense emission will occur due to the evaporation of a quenching medium followed by generation characteristic sound signals with high amplitude; (2) this is followed by bubble boiling with a characteristic calm emission of the vapor bubbles. Sound signals have constant amplitude values; and (3) at the conclusion of the quenching process, the bubble emission will reduce and eventually stop, which is reflected in the amplitude value of the sound signal. The aim of the experimental study of quenching by measuring the characteristics of the sound signal is to monitor the quenching process and obtain optimal results.