1. Introduction

The plate heat fin exchangers have a widely used application area. They are characterized by having secondary surfaces or fin structures. The function of the secondary (extended) surface is the enhancement of heat transfer performance of the heat exchanger in the allowable range of the pressure drop. The commonly used forms of the extended surface of the plate fin heat exchangers are the triangular or rectangular plain fin, offset strip fin, wavy fin, louvered fin and perforated fin as shown in Figure 1.

For extended surface application, fin geometries fall into two categories: continuous and interrupted surfaces. Continuous surfaces achieve heat transfer enhancement through the

distribution, and eproduction in any medium, provided the original work is properly cited.

Figure 1. Fin geometries for plate fin heat exchangers: (a) plain triangular fin; (b) plain rectangular fin; (c) wavy fin; (d) offset strip fin; (e) multi-louver fin; (f) perforated fin [1].

secondary flow patterns introduced by sudden velocity changes. On the other hand, interrupted surfaces achieve heat transfer enhancement by the continuous growth and destruction of laminar boundary layers on the interrupted portion of the geometry. One of the mostly used example of the interrupted surface is the louvered fin. The louvered fins were firstly investigated by Kays and London [2] in 1950s, and the popularities of the louvered fins have been maintained.

Today, the use of louvered fins has become popular in the fields of automotive, heating, cooling, air conditioning, power plants and food industry. Typical structure of the louvered fin is shown in Figure 2. The efforts of maximize the heat transfer and minimize the pressure drop in heat exchanger design are rapidly increasing due

Figure 2. Typical structure of a louver fin geometry [4].

to the restrictions of energy consumption applied by the governments. In this case, the importance of light, high surface density and energy efficient heat exchangers is increasing. The louvered fins are commonly used in heat exchanger field for reasons beyond simply increase the heat transfer surface area and decrease the volume, the amount of coolant and the costs [3].

The louvered fins enhance the heat transfer by providing multiple flat-plate leading edges with their associated high values of heat transfer coefficient. Although the louvered fins are similar in principle to the offset strip fin, they can enhance heat transfer by a factor of 2 or 3 compared with equivalent un-louvered surfaces. The louvers have the further advantage that the enhancement of heat transfer is gained without increase in flow resistance that results from the use of turbulators [5].

Louvers are generally formed by cutting the metal and pushing out the cut elements from the plane of the base metal. They can be manufactured by high-speed production techniques and as a result are less expensive than other interrupted flow geometries when produced in large quantities. Louvered fin geometries can be made in different dimensions of fin length, louver length or material thickness depending on fabrication techniques [2]. The physical principle of the louvered fin is based on the breaking the boundary layer of the flow that passes through the louvers. The structure of the flow phenomenon over the louvered fins is given in details at the following section.

Due to the extensive use of the louvered fins in the heat exchanger area, the researches have spent great efforts to improve the louvered fin geometry from 1950s. In this chapter, first the terminology and the fundamental concepts of flow phenomenon of the louvered fin are explained. In the following sections, the heat transfer and pressure drop characteristics of the louver fins are examined with respect to the geometrical variations of the louvered fin geometry adhering to the experimental and numerical studies in the literature. The empirical correlations are also summarized by a tabulated data.
