Thermo-fluid dynamics of woody biomass flue gas in the heat accumulation stoves.

The research aims to clarify some aspects of the thermo-fluid dynamics of woody biomass flue gas within the refractory twisted conduit inside heat accumulation stoves. These are traditional heating elements in the European Alpine regions, whose history began in the fifteenth century. The high temperature flue gas flows in a twisted conduit, releasing heat along its path to the refractory. The heat stored in the refractory is then released slowly into the environment mainly as a radiant component. The physical phenomena that occur in an accumulation stove, once the activation energy in the combustion chamber has been provided, continues, initially increasing temperature and velocity of flue gases and then decreasing the two variables until the end of the reaction. The decreasing temperatures of flue gases, flowing from the combustion chamber in the twisted conduit, tends to cause the transition of the flow regime from laminar to turbulent conditions. Moreover the continuous changes of direction imposed by the curves cause local contractions and expansions of the flux and, consequently, energy losses which are difficult to evaluate. The way the heat transfer occurs appears to depend mainly on gas flow conditions (laminar or turbulent) and on the radiative properties of the hot particle cloud inside the flue gas. The physical properties of the conduit (roughness of the internal surface), as well as the thermodynamic properties of the refractory material (its mass and its geometric arrangement), play a fundamental role in the functionality of the stove. This paper describes some analysis performed on heat transport and exchange processes inside the flue gases and between them and the refractory. The importance of the radiation properties of the flue gasses has been highlighted. The numerical results, obtained with COMSOL® Non Isothermal Flow (k-epsilon turbulent model), have been compared with laboratory measures.