The paper involves developing practical aspects of the using enhanced surfaces in heat transfer research when two types of devices are applied. The first one is the heat exchanger with the minichannel furnished with enhanced heating surface. The second type includes prototype solar collectors with the developed surface of absorber’s pipes or smooth pipes covered by the absorber plate, with an external developed surface. The author’s Polish patent No. A1 396579 "A structure for boiling heat transfer enhancement" was used in described heat transfer devices construction.

    Heat transfer in small channels has been studied intensively over the last few years, especially as regards the application in cooling electronic components. Mini heat exchangers are used to provide higher cooling capability for new technologies. It means reducing their sizes and costs, while the consumed power is identical. Owing to the change of the state which accompanies flow boiling in small channels, it is feasible to meet contradictory needs simultaneously, i.e. obtain a heat flux as large as possible at small temperature difference between the heating surface and the saturated liquid while retaining small dimensions of heat transfer systems. The use of microstructured surfaces allows additional intensification of the process. The series of studies pursued at the Kielce University of Technology includes research on flow boiling heat transfer in a cooling fluid flow along the minichannel with plain or microstructured heating wall and various orientations. The results were described in numerous publications.

The essential part of the experimental stand is the test section with a rectangular minichannel 1 mm deep, 40 mm wide and 360 mm long. The heating element for FC-72 flowing in the minichannel was the thin alloy foil designated as Haynes-230. There is a microstructure on the side of the foil which comes into contact with fluid in the channel. Two types of microstructured heating surfaces: one with micro-recesses distributed evenly, and another with mini-recesses distributed unevenly, were used. The micro-recesses were performed by laser drilling. The diameter of the single micro-recess is usually 10 mm, its depth is 3 mm. 5÷7 mm high layers of melted metal deposit annularly around the recesses, forming structures that can be named as “craters”. Micro-recesses are evenly distributed every 100 mm in both axes. The mini-recesses were obtained by spark erosion. The melted metal foil and electrode material, a few mm high, reaching locally 5 mm, accumulate around the recesses. The depth of the cavity craters is usually below 1 mm.  It is possible to observe both surfaces of the channel through glass panes. One pane allows observing changes in the temperature distribution on the plain side of the foil thanks to the liquid crystal thermography. The latter one allows observing the two-phase flow patterns on the microstructured  foil side. The final results are presented as local heat transfer coefficients.

The observations have confirmed experimentally that boiling incipience occurs in lower heat flux supplied to the enhanced foil which constitutes a heating surface of the minichannel in comparison to results from the studies on similar minichannels employing the plain foil. Thus, the heating surfaces with the proposed arrangement of recesses make it possible to provide a large number of nucleation sites. It was found that prototype solar collectors with recesses formed on the surface of the absorber's pipes have higher energy efficiency in comparison with the collectors with plain absorbers. To sum up, the analysis of all results of the discussed studies indicates that the application of enhanced surfaces allows achieving effective heat transfer.

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