Phase change and free surface phenomena of materials are very difficult to resolve due to the change in properties of the material from one phase to another based on temperature distribution with heat transfer type. Therefore, most of the thawing-related problems have been researched with limited problem set by assumption. Besides, the numerical analysis considering both phase change and free surface flow has been conducted for the melting and melting of PCM (Phase Change Material).
On the other hand, Urea-SCR system has a complication in freezing on low temperature environment below -20℃ during winter season in Northern Europe and North America. It is due to the fact that the urea water used as reducing agent has a freezing point of -11.7℃. In particular, urea water tanks, urea flow paths, urea water inside the gear pump, or urea water remaining in the form of ice pellets, make the gear pump inoperable and as a result, in such cold environment the supply of urea water to the injector becomes impossible. Therefore, in order to supply urea water in a stable manner, it is required to apply the thawing system in the reducing agent supply module and as a matter of fact, it is highly necessary to study the efficient heat transfer from the heat source to the flow path and the gear pump.
In this study, the numerical analysis of the thawing and heat transfer phenomena for urea water is carried out in the integrated urea water supply module of the Urea-SCR system, which will be implemented in 3L class small diesel engine vehicles. Computational numerical analysis was carried out using ANSYS Fluent 17.0, a commercial CFD program, and the numerical model used in the analysis was compared with the surface boundary shift results of pure gallium melting experiments conducted by Gau and Viskanta, and the free surface shifts performed by Martin and Moyce. Experimental and comparative verification was performed. After verifying the numerical model, the thawing phenomenon of the urea water was analyzed by the electric heating module in the urea tank having a volume of about 3.7 ℓ mounted on the Urea-SCR device of a 3L diesel engine vehicle. In particular, the thawing phenomenon of frozen urea water in a urea tank, flow path, gear pump was investigated, and heat transfer characteristics were analyzed through temperature change and liquid fraction distribution according to material change and pump operating condition.
As a result, it was found that convective heat transfer was predominant in the upper urea water tank, where the heat was transferred by the initial heating and conduction. In addition, in the lower pump module, Case 3, which applied Al6061 to the inlet and outlet pipes, confirmed that thawing was completed as soon as possible. As the urea water was discharged by the pump operation, the upper region of the urea water tank was thawed more, and the lower region had little difference in phase boundary movement. This is because convective heat transfer is introduced into the upper portion by the discharge of urea water, and the mass flow rate of the urea water discharged in the lower portion is very small.