**2. External structure**

The ability of a designer to minimize the thermal resistance between the source of heat dissipation and the thermal sink is essential in controlling maximum operating temperatures. While the convective heat transfer coefficient could potentially be enhanced

standard finite-element package, *FIDAP*) for a realistic value of the Reynolds number and for different geometric configurations (different fin external diameters and fin spacing). Then, he calculated the entropy generation rate from the flowfield, and examined both at a local level, to detect possible bad design spots (i.e., locations that corresponded to abnormally high entropy generation rates that could be cured by design improvements), and at an overall (integral) level, to assess the entropic performance of the heat exchanger. He gave optimal curves and determined the optimal spacing of fins using alternatively the entropy generation rate and the total heat transfer rate as objective functions: different

Tagliafico and Tanda (1996) presented a thermodynamic method for the comparison of plate fin heat exchanger performance. The researchers evaluated and scaled the entropy production of a given heat transfer surface geometry using that of corresponding reference configuration (a parallel-plate channel) with the same frontal area, volume, heat transfer duty, and mass flow rate to relate the relative merit of the surface geometry to corresponding irreversibility level. They applied their method to a number of plate-fin compact heat exchanger surfaces whose performance data were taken from Kays and London (1984). They examined six types of heat exchanger enhancements: the plain fin, louvered fin, strip fin, wavy fin, pin fin, and perforated fin. From this analysis, they found that the thermodynamic performance of the most suitable surfaces, among those considered in this study, turned out to be strongly related to the operating conditions (both heat transfer duty and mass flow rate). Also, they found that the strip fin was the

Muley and Maglik (1999) investigated performance optimization of plate heat exchangers with chevron plates. In this study, the researchers repeated Manglik and Fang's (1994) analysis but for corrugated rib surfaces used in plate heat exchangers. These devices also know as chevron ribs are widely used in process heat exchangers due there ease of construction and cleaning for fouling applications. They showed results for constant mass flow, constant pumping power, and constant pressure drop. They found that corrugated ribs at the fixed pumping power and fixed pressure drop constraints, led to a

Su et al. (1999) found a new way of fin design to minimize the irreversibilities due to heat transfer and fluid friction and maximize the available work of the working fluid. First, the researchers derived the general entropy generation formulas for fins according to the first and second law of thermodynamics. Then, they made a theoretical analysis on cylindrical pin fins and rectangular straight fins using the above formulas. They obtained the minimum entropy generation formulas for these two types of fins and proposed a principle for fin optimization, where the minimum entropy generation was chosen to be the objective function to be studied. They discussed in detail the influence of various parameters on fin

The ability of a designer to minimize the thermal resistance between the source of heat dissipation and the thermal sink is essential in controlling maximum operating temperatures. While the convective heat transfer coefficient could potentially be enhanced

optima arise, and the differences as well as the similarities were discussed in detail.

thermodynamically most efficient augmentation device.

thermodynamically more efficient system.

**2. External structure** 

entropy generation in forced convection heat transfer.

with an increase in the approach velocity, the dependence of heat transfer coefficient on the square root of the velocity in laminar flow results in diminished returns as velocity is increased. The second option for reducing film resistance is achieved by increasing the effective surface area for convective heat transfer. This is typically achieved through the use of heat sinks in single fluid heat exchangers and extended surfaces in two fluid heat exchangers. Heat sinks offer a low cost, convenient method for lowering the film resistance and in turn maintaining junction operating temperatures at a safe level in electronic components. Unfortunately, the selection of the most appropriate heat sink for a particular application can be very difficult given the many design options available. Thermal analysis tools, ranging from simple empirically derived correlations to powerful numerical simulation tools, can be used to analyze the thermal performance of heat sinks for a given set of design conditions. Regardless of which procedure is used, analysis tools only provide a performance assessment for a prescribed design where all design conditions are specified a priori. Following an exhaustive parametric analysis, design options can be assessed with respect to their influence on thermal performance, however, there is no guarantee that an "optimized" solution is obtained since the parametric analysis only provides a ranking of a limited set of test cases. The method of entropy generation minimization, pioneered by Bejan, provides a procedure for simultaneously assessing the parametric relevance of system
