Additives Shear-Thinning and Turbulence Damping Influence on the Fitting Loss Coefficient of Bends

A large part of the auxiliary energy for cooling and heating systems in buildings is needed for the pumps that distribute the heat transfer medium in the thermal distribution network. Roughly half of the pressure losses in such systems are due to straight pipes and roughly 10 % are due to bends and T-pieces. Drag reducing additives have not so far been widely used for domestic thermal distribution networks. The BioNet-project at the Hermann-Rietschel-Institute in Berlin aims at adopting promising solutions for this specific application and evaluating potential energy savings. The specific typical technical conditions for this application, such as Reynolds numbers and pipe diameters are considered. In addition to measurements, numerical simulations (CFD) are used to adopt and evaluate the approaches to heating and cooling nets as well as to explain the physical principles of the developments. The flow of a drag-reducing surfactant solution in a bend with a curvature radius Rk/D of 1.5 is considered, where D=20 mm is the pipe inside diameter. It is known that the pressure drop of straight pipes can be reduced considerably with additives such as surfactants due to the damping of turbulent structures. Measurements were performed on two test setups to assess the influence of additives on the pressure drop in straight pipes as well as for a circuit containing ten bends. The experiments are further described in [1] and [5]. In the measurements of the circuit, a drag reduction of 40 % was found as compared to 60 % for the straight pipes. The investigations could so far not clearly explain why this was the case. Is it that the pressure drop of fittings may actually increase under specific circumstances if additives are used? The question already motivated other experimental studies, e.g. Gasljevic and Matthys[2]. This would mean that additives are then disadvantageous in thermohydraulic nets that contain many fittings such as bends. The physical properties of surfactant solutions depend on the actual chemical and physical system conditions such as water quality and the materials used for the pipes. In addition to the turbulence damping by the micelle structures which leads to a reduction of pressure losses, a non-Newtonian viscosity with a shear thinning behaviour is introduced that may actually increase the pressure losses. That is why a simple and straightforward model was needed to explain the basic influence of the shear-thinning behaviour and the turbulence damping in bends.