EXPERIMENTAL AND CFD ANALYSIS OF WIRE COIL TURBULATORS IN BIOMASS BOILERS

To achieve as complete fuel burnout with as little excess air as possible, small wood log boilers (< 50 kW) use stage combustion. The first stage is often a pro-cess similar to downdraft gasification that consequently produces a flue gas lad-en with particulates. To prevent the build-up of solids and promote heat transfer in pipes of the convective part of these boilers, wire coils are used. The paper presents their in-situ examination together with CFD analysis. The analysis is carried out in a 460 mm long pipe, with a diameter of 82.5 mm, equipped with different wire coils for flue gas temperatures in the range between 300 degrees C and 150 degrees C. The analyzed coils are with and without a conical spring at their free end. The addition of this conical top is economical and should influence the rota-tion of the core flow. Proper pipe surface cleaning limited the analyzed wire coil designs to the dimensionless pitch, p/d, in the range between 0.36-0.61, dimen-sionless wire diameter e/d = 0.04-0.1, and pitch to wire diameter ratios p/e = 3.75-14.3, and three different angles (60 degrees, 90 degrees, and 120 degrees) of the conical top. The goals are to find the optimal flue gas velocity for the given operating condi-tions, pipe, and wire coil dimensions, and to investigate the addition of the coni-cal top on heat transfer enhancement. Several evaluation criteria are used to achieve the goals.