Fluid flow sensor for industrial quench baths

G.E. Totten, H.M. Tensi, G.M. Webster:

Proceedings of the 8th Seminar of the International Federation for Heat Treatment and Surface Engineering , IFHTSE 2001, Dubrovnik- Cavtat, Croatia, 12 – 14 September 2001.

Quenching severity is agitation dependent. Therefore, magnitude and turbulence of fluid flow around a part in the quench zone is critically important relative to the uniformity of heat transfer throughout the quenching process [1-5]. One of the greatest contributors to non-uniform hardness, increased thermal stresses, cracking and distortion is non-uniform fluid flow throughout the quench zone in production quench tanks. [6] The impact of non-uniform flow on distortion and cracking has been discussed previously. [7,8] These, and other, references have clearly shown the necessity of optimizing the uniformity of fluid flow in the quench zone to provide optimum control of distortion and to minimize cracking. Some of the classic methods of measuring fluid flow on both a laboratory and commercial scale include: turbine velocimeters [9], streak photography [10,11], pitot-static tube [12], electromagnetic current meter [13], hot-film anemometer [9], and laser Doppler velocimetry [8,9] Although none of these methods are generally unsuitable for continuously monitoring fluid flow in quench tanks during heat treat processing, they have provided invaluable insight into the fluid mechanics of the quenching process.  For example, streak photography was conducted on a model of a quench tank for an integral quench furnace.  Computational fluid dynamics (CFD) modeling is increasingly used to examine the uniformity of fluid flow in a quench tank. Totten and Lally reported one of the first examples of the application of this methodology to illustrate the non-uniformity of  quench tank fluid flow [9]. This work was followed by a studies reported by Garwood, et.al.[13,14].   Bogh used CFD analysis to examine the impact on quench non-uniformity of the placement of submerged spray eductors at various locations around a rack of aluminum panels. [15]. More recently, Halva and Volný [16] have used CFD analysis to examine the homogeneity of fluid flow as a function of agitator placement.  An example of the use of CFD modeling to design quench system with improved flow uniformity has recently been reported by IIT Flygt. [17].  A study was sponsored by SAE – AMEC Committee to evaluate the concentration limitations to meet Mil. Handbook 5 design minimums for Type I quenchants for aluminum heat treating standard development. Unfortunately, the results were too scattered to achieve the desired goal. CFD analysis was performed which illustrated the variance in physical property data was likely due to flow velocity variation in the quench tank [18]. The most recently reported example of CFD modeling was conducted on a classic laboratory apparatus used for cooling curve analysis. The results of this work showed that even this system was susceptible to significant flow variation in the quench zone. These CFD studies have clearly shown that in most cases it is not possible to achieve perfectly uniform fluid flow in the quench zone. In addition, experimental work reported by Titus showed substantial variation of fluid flow in the quench zone of a batch integral quench furnace [20].  Therefore, from these and other studies, it is clear that it is important that fluid flow velocity be measured during quench processing in the workshop.  Various approaches that have been reported to date measure quench severity and fluid flow will be reviewed here.

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