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The classical performance indicators for extended surfaces, ef ciency and effectiveness, cannot be used for the proper design of nned systems subject to time-dependent processes, such as heat exchangers or electric devices. Based on the network simulation method, a network model of a n-wall assembly, whose admittance is identical to the thermal admittance of the system, has been designed for the whole system. A new n performance indicator, output admittance, is proposed, and frequency analysis of the system is carried out. The simulated numerical response is rapidly obtained by running the network in the appropriate circuit resolution software. This method is especially useful for studying complex thermal transmission functions such as admittance, evaluating modulus, phase, and real and imaginary components of the thermal signal.Extended surfaces are a widely used solution for the augmentation of heat transfer in applications such as heat exchangers, air-cooled heat engines, electric and electronic devices, etc. Many of these applications are subject to variable operating conditions in noncyclic (start-up of machines) or cyclic (on-off cycles or rotating machines) ways. Nevertheless, the complexity of the calculations needed has led to the performance of ns under time-dependent excitation not receiving much attention to date. Furthermore, the isolated n is merely an idealization, and one of the main aspects which must be considered in applications involving a set of ns is the inuence of the wall surface. The in uence of the wall to which the n is attached is quite important for assembly performance. The study of the n-wall system as a set is a far more realistic approach to considering n arrays. However, another dif culty now arises: the inadequacy of the classical extended surface performance indicators, ef ciency and effectiveness, for suitably representing nned wall performance and so in the proper design of such systems.Various articles discuss the transient heat transfer in individual straight ns under harmonic boundary 31

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The heat transfer from cylinder to air of a two-stroke internal combustion finned engine has been simulated. For this purpose, a 2D model of the block and head has been done. Starting from the geometry of a real engine, annular cylindrical and spherical symmetric walls to fins has been used to obtain an equivalent simplified geometry, where the heat transfer rate is the same as that in the real engine. The cylinder body, cylinder head (both provided with fins), and piston have been numerically analyzed and optimized in order to minimize engine dimensions. The maximum temperature admissible at the hottest point of the engine has been adopted as the limiting condition. An engine temperature map has been presented as additional information. Cyclically variable inner cylinder conditions (thermodynamic, thermal, and chemical) proceed from a previous work; convective and radiative inner boundary conditions and convective external conditions have also been taken into account in this work. A network simulation model has been used as the numerical tool in the analysis presented.

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