Experimental and numerical analysis of transient heat transfer in the heater of a Stirling Engine immersed in a Fluidized Bed Combustor

A system for micro-CHP, having good flexibility on the fuel source by adopting a Fluidized Bed Combustor (FBC) and a Stirling Engine (SE) with the heater immersed in the fluidized bed, is under development. The proposed configuration opens for several improvements on the overall efficiency, but new challenging issues have also emerged. There is experimental evidence of the high efficiency of heat transfer between the outer side of a tube bundle heater of the SE and the FBC. The performance of the SE, designed for exhausts of a gas-fired burner, has been matched with just a fraction of the heater immersed in the hot bed of a FBC. Heat transfer coefficients on the outer side of the heater resulted to be one order of magnitude higher than those achieved when the heater is placed in a hot gas stream. This permits a downsizing of the heater, hence a significant reduction of the dead volume. However, accurate design under these conditions requires accurate prediction of the inner side heat transfer process involving short tubes, for which reliable correlations are not available. Therefore, an investigation of the effective heat transfer achievable in short tubes has been carried out with a CFD model. Transient fluid motion and compressibility effects are taken into account. Results of the experiments and details of the model and relevant simulations are presented and discussed in the paper.