An experimental investigation of heat transfer from the tubes to the two-phase foam system was performed. Statically stable gas-liquid foam flow was used as a coolant. An investigation was performed on the experimental laboratory set-up consisting of the foam generator, an experimental channel and tube bundles. Two different geometries of in-line tube bundles were used for the experiments. Regularities of heat transfer of the tube bundles to the foam flow under the 180˚ degree turn were analysed in the work. The results of the investigation showed that heat transfer intensity is much higher than that for the one-phase airflow under the same conditions. The heat transfer character of frontal and further tubes to downward foam flow is different in comparison with the one-phase coolant flow. After the turn, local void fraction of the foam is less on the inner side of the foam flow. Therefore heat transfer intensity of the inner side-line tubes is higher than for other tubes of the bundle. The results of the investigation were generalized by criterion equations, which can be used for the calculation and design of the statically stable gas-liquid foam heat exchangers with the in-line tube bundles.
The paper analyses low grade heat recovery problem for modern woodfuel cogeneration plant. The woodfuel flue gas, behind the condensing economizer, still contains a considerable amount of heat, main part of which is the latent one. To recover this low grade heat, the heat pump technology can be used, which is related with additional consumption of energy (electric, mechanical or heat). Another technique that could be applied is a heat regeneration when flue gas heat, mostly latent, is transmitted to air blown towards burning chamber. Therefore, the analysed heat recovery system operates mainly like mass regenerator which contains only blowers that use some electric energy. The regenerator consists of two cyclically operating columns with packing material. Energetic analysis demonstrates that 13% of additional heat can be produced utilizing this low grade heat. The economic valuation shows that investment in a heat recovery system is quite effective; the payback time is about four years.
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