## 1 Introduction  
This paper addresses the issues of heat transfer in heat exchangers working under oscillatory flow conditions, which are typically found in thermoacoustic devices. Thermoacoustic engines are a new class of energy conversion devices (prime movers, refrigerators and heat pumps) whose operation relies on the interaction between heat and sound in close proximity of solid surfaces, a phenomenon identified as \"thermoacoustic effect\" [1,2]. Since in these devices the synchronization among the compression, expansion and heat transfer phases of the thermodynamic gas cycle is naturally accomplished by an acoustic wave, a number of technological benefits directly result. First of all, there is the complete absence of moving mechanical parts (pistons, sliding seals, etc.) which leads to engineering simplicity, reliability, longevity and low maintenance costs. Secondly, they are intrinsically low cost devices, being constituted basically by a small number of standard components made of inexpensive and common materials. Furthermore, they use environment friendly working fluids, can be employed in a large variety of applications (those involving heating, cooling or power generation [3, 4, 5]) and can be driven by different power sources (gas/biomass combustion, solar energy, waste heat, etc.). These characteristics make thermoacoustic technology a discipline of relevant interest for the energy industry giving it a primary position among emerging renewable energy technologies. A typical thermoacoustic device consists of (a) an acoustic network (acoustic resonator), (b) an electro-acoustic transducer, (c) a porous solid medium (namely a regenerator in travelling-wave systems [6] or a stack in standing-wave systems [7]) and (d) at least a pair of heat exchangers (HXs) [8]. The stack/regenerator is the component where the desired heat/sound energy conversion takes place. \"Hot\" and \"Cold\" HXs, placed in close proximity of both ends of the stack/regenerator, absorb or supply heat from its ends thus enabling heat communication with external heat sources and sinks. Heat exchangers constitute, in addition to the stack/regenerator, the highly dissipative components of thermoacoustic engines. Their porous structure entails, in fact, considerable flow resistance while steep thermal gradients are generally imposed on them to sustain the required heat fluxes. When designing efficient HXs aimed at transferring a target heat load the HX length along the direction of acoustic oscillation and the pore hydraulic radius should be simultaneously optimized in order to:  
* Provide the heat transfer surface area compatible with minimum acoustic power loss caused by thermal and viscous dissipation;
* Provide the temperature drop between the HX and the adjacent fluid compatible with minimum thermal irreversibility associated to heat transfer [9; 10]. An optimized HX should be able to achieve high transfer rates under small temperature differences in conjunction to low acoustic dissipation. Although thermoacoustic HXs constitute fundamental components of thermoacoustic devices their design constitute up to now a technical and engineering challenge. This is due to the fact that the flow is oscillatory and the existing knowledge for steady flow arrangements is of little practical value [11; 12; 13]. As a consequence, standard heat transfer correlations for steady flows cannot be directly applied to thermoacoustic HXs, for which reliable and unambiguous design criteria are still lacking. This work is concerned with the initial stage of a research aimed at studying the thermal performance of thermoacoustic HXs working under oscillatory flow conditions. The paper reports a detailed discussion of the design and fabrication of the experimental set-up, measurement methodology and test-HXs characterized by two different pore geometries, namely a circular pore geometry and a rectangular (i.e., straight fins) geometry. Some preliminary experimental results concerning the HX with straight fins are also shown.