As phase change processes play an important role in the metallurgical and thermal manufacturing industries, the ability to predict its phenomena is considerable engineering interest. The term of phase change (or phase transition) means the transformation from one thermodynamic state to another, such as the transitions between solid, liquid, and gaseous states of matter. During a solid-liquid phase transition of matter, it has different thermodynamic properties along with the thermal behavior of matter due to the absorption and release of latent heat. Also, these processes involve complex heat and mass transfer phenomena and occur as an intermediate state, i.e. mushy region, where the material can be mixed liquid and solid. Usually a solid-liquid phase interface and its front motion are irregular in shape in that process. Therefore, it needs a mathematical description on a fixed grid method in order to attack and predict the phase change characteristics.
Meanwhile, the UWS(Urea-Water Solution) used as a reducing agent in the Urea-SCR(Selective Catalytic Reduction) system is an eutectic mixture, which has a concentration of 32.5wt%. Therefore, the UWS of the liquid phase has the same composition as that of the original solid phase and it has a freezing point of ?11°C lower than any mixture of the components. Owing to the fact, the UWS is frozen in cold environment where the ambient temperature drops below the freezing point. In case of low ambient temperature levels, thus, the urea tank module must be separately heated to ensure a quantity of the liquid UWS during cold start conditions to reduce the initial NOx emissions from exhausted gases. For this reason, it is necessary to adopt a heating apparatus for thawing the frozen UWS in the cold start conditions.
In this study, therefore, numerical analysis was conducted to investigate the thawing process including a complex solid-liquid phase change with electric heating module in urea tank by using the commercial CFD software Fluent 17.0. After introducing the numerical methodologies related to melting, the validation work was conducted by analyzing the phase front motion in literature and comparing with an experimental test of the UWS thawing. In order to check whether the number of grids is in proper range, the grid convergence test was also performed.
The results show that the melting and thermal characteristics, such as a heat transfer of conduction and convection, phase transition, phase interface motion, and natural convection etc., in the thawing process of the frozen UWS with model 2 and 3 of the heating module. It can be found that conduction, convection and combined heat transfer can be classified according to the thawing step, and the average velocity of the phase interface motion is faster in the upper region than in the lower region. Also, the natural convection becomes more actively as time goes on in the domain. When comparing the thawing performance according to the shape of the electric heating module, it is concluded that not only the heat transfer area but also the volume of the heating module is an important factor affecting its performance.