Modelling and Optimizing Operating Conditions of Heat Exchanger with Finned Elliptical Tubes

Abstract

High performance heat exchangers are widely used in many industries such as petrochemical industry, automotive industry, energetic industry and many others. The name of this kind of heat exchangers is in consistency with their structure. Large heat transfer area leads to high thermal efficiency of the device. Its working principle is to cool rapidly large amount of gaseous or liquid medium. Because of its compact size, it is possible to use it for easy installation in various systems, like the heating, drying, air conditioning and the other systems. High thermal performance of this type of heat exchangers (Nagatha, 2004; Ibrahim & Gomaa, 2009) is achieved due to large packed rectangular fins on tube surface, as well as the shape of the tubes. The elliptical shape ensures better distribution of velocity along the tube circumference during the flow, as well as lower pressure drop in comparison to the circular shape (Matos et al, 2004; Jang & Yang, 1998; Li et al, 2006; Nishiyama et al, 1988; Khan et al, 2004; Saboya & Saboya, 1981; Saboya & Sparrow, 1974; Bordallo & Saboya, 1999). Consequently, the coefficient of heat transfer in the elliptical shape is larger when compared to the circular shape (Idem et al, 1990; Rocha et al, 1997; Taler, 2009). It brings about that the heat exchange process is more efficient in elliptical than in circular tubes. To use all the aforementioned advantages, it is necessary to ensure uniform distribution of velocity in all the tubes. Failing that, it leads to large differences in mean temperature in the tubes. Consequently, excessive thermal stress occurs, that may cause the heat exchanger to break down. Small volume of collectors of the heat exchangers implicates possibility of improper flow condition inside the tubes, causing unsuitable inner distribution of thermal and mechanical loads. Damages to a tube, Fig. 1, as well as a tube bundle prone to buckling, Fig. 2, prove, that such a failure is possible (Łopata & Oclon, 2010; Oclon & Łopata, 2011). Compressible stresses are undesirable (Łopata & Oclon, 2010; Łopata & Oclon, 2011) in this type of construction, because the allowable compressible load (allowable buckling stress) for a slender tube is significantly lower than the safe tensile load. The simplified method to assess failure of a high performance heat exchanger with finned elliptical tubes is presented in the paper. The diagram, shown on Fig. 3, presents the method. The geometrical representation of construction – a CAD model is prepared at first. Next, on the basis of prepared geometry, numerical representation of the model is created. Afterwards, the flow computation CFD (Ferziger & Peric 1999; Anderson, 1995; Gresho &