With regard to the reduction of primary energy demand for heating and cooling the development of thermally driven absorption or adsorption chillers is pursued for several years. These machines use water as working fluid which evaporates in accordance with the operating conditions in a system pressure range between 7.6 and 20.7 mbar. As the development work show, the evaporator in ad-dition to the absorber or ad-sorber represents one of the key components for the cost-effective production, the reduction in the volume of construction and the optimization of the overall effi-ciency of these technologies. In this regard it is especially im-portant to improve the outer heat transfer between the evaporator wall and the working fluid due to the present low driving tempera-ture differences.

Since evaporator concepts are as-pired that do not require a cir-culation pump for the working fluid, the present study focuses on possibilities to improve the heat transfer as they are present in pool boiling. The range of wall superheat that needs to be improved is given in accordance with the operating process of the machine up to a maximum of 7 K. Object of investigation is the efficient heat transfer by means of evaporation from metallic short fiber structures. High heat transfer rates are expected as these structures can utilize ca-pillary action in combination with micro-zone evaporation. Fur-thermore, the regime of nucleate boiling is expected to be reached at the given wall superheats due to their porous structure. The potential of these short fiber structures is analyzed using three different boiling cases. Firstly, the case of completely flooding of the fiber structure, secondly, the capillary-feeding and thirdly, the partial flooding of the fiber structure. A test rig was adapted to the present system pressures and a suitable boiling vessel concept has been developed. Water as working fluid is considered in a system pres-sure range between 9 and 15 mbar.

Analyzing the influence of the various fiber structure parame-ters and the filling levels re-veals, that the range of nucleate boiling can be reached at the de-sired wall superheats with metal fiber structures. In doing so the initiation of nucleate boiling is significantly influenced both by the system pressure and the fill-ing level. In order to achieve maximum heat fluxes and the uti-lization of the optimal material it turns out that an evaporation process should be aimed for that exploits the capillary effect. The best results are achieved during the boiling case of capil-lary-feeding. Here, a considera-bly higher amount of heat is transferred in comparison to the complete flooding of the fiber structures especially at lower wall superheats. At the same time its potential is far from being exhausted.

Comparing the measuring results with known correlations to pre-dict the initiation of nucleate boiling and for the range of nu-cleate boiling, they do not show any agreement for almost any of the considered boiling cases. For the boiling case of capillary-feeding a good agreement is achieved in some cases in the boiling regime without any bubble formation. However, substantial variations occur if the formation of micro-zone areas changes. In this case the achievable heat transfer rate is highly underes-timated and reveals the signifi-cant influence of the micro-zone evaporation to the overall heat transfer.

The investigations further show that the appearance of nucleation hysteresis is not expected in the case of their potential use. Moreover, the results show only a slight change of the measurement result considering different de-grees of oxidation of the applied fiber structure material. Accord-ingly, for their potential use in the evaporator no change in heat transfer is expected even over a longer operation period.