Experimental and CFD analysis of wire coil turbulators in biomass boilers

Abstract

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 process similar to downdraft gasification that consequently produces a flue gas laden 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 °C and 150 °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 rotation 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, dimensionless wire diameter e/d = 0.04-0.1, and pitch to wire diameter ratios p/e = 3.75-14.3, and three different angles (60⁰, 90⁰, and 120⁰) of the conical top. The goals are to find the optimal flue gas velocity for the given operating conditions, pipe, and wire coil dimensions, and to investigate the addition of the conical top on heat transfer enhancement. Several evaluation criteria are used to achieve the goals.