Optimization of twin shafted concrete mixer

Abstract

Mixing is an integrated part of modern technology. There are numerous applications of this process in the fields of pharmacology, chemistry, food industry, and construction industry. Static mixers, co-axial mixers, paravisc mixers, helical ribbon blade impeller mixers, and twin sigma blade mixers can be given as some examples for general mixing machines. When it comes to concrete mixing, industry mostly prefers drum mixers, pan or planet mixers, continuous concrete mixers or twin shafted concrete mixers. Mixers are specialized according to its purpose to use. Although different kinds of mixers are used for various applications, main target is increasing efficiency and decreasing operating costs. Geometrical optimizations may lead to a better rheology for mixing procedure helping increase efficiency. Additionally, there are some researches aiming to investigate wearing behavior of blades under certain mixing conditions. Obviously, the goal of these investigations is to decrease operating costs of mixer in terms of material or energy costs. Computational Fluid Dynamics (CFD) can be considered as a beneficial method to understand flow characteristics of mixers. Especially, well constructed such sort of numerical studies yield substantial force or moment data to investigators without the need of expensive experimental setup. Main parameters determining the efficiency of a twin shafted mixing machine can be considered as shape of blades, angle of blades, size of mixing tank, filling level, and distance between shafts. Results obtained from CFD analysis is evaluated accordingly and design is modified to improve mixer efficiency. Needless to say that, after numerical optimizations, final geometry should be prepared for experiment in order to acquire detailed analysis. In spite of the fact that CFD calculations provide us with beneficial moment and power data, obtaining segregation information which exhibits the mixing performance from the outputs of CFD calculations can be quite hard. Instead, using Discrete Element Method (DEM) gives us detailed segregation data in a short computing time. Besides of segregation data, DEM is quick and easy to attain moment and power values by employing DEM. Fundamentally concrete mixing process can be split into three substages, first, dry phase defining the mixing while aggregates like sand and gravel is introduced into tank, second, wet phase including water addition period, and finally, final mixing phase decreasing segregations within the mixture until a certain limit (Kozic et al,2016). In this study, optimization of a twin shafted concrete mixer was aimed by focusing on third stage (final mixing phase). In order to gather enough knowledge about the effect of different blade angles over the mixing process Fluent was employed for CFD modeling, and EDEM for DEM modelling.