A Multiscale Study of Freeze-Volume Expansion in Urea-Water Solutions Within SCR Systems under Low-Temperature Conditions: from Molecular Disorder to Ordered Packing

The anomalous volume expansion of supercooled aqueous solutions during freezing raises fundamental questions about the underlying microstructural evolution, yet its atomic-scale origin remains unclear. A 32.5 wt% urea aqueous solution (UE32) is the standard working fluid in selective catalytic reduction (SCR) systems for diesel vehicles. At 233 K, ten experiments gave a mean freezing expansion of 7.44% with a sample standard deviation of 0.47 percentage points. Such expansion may cause structural failure in cold-region applications. This study combines molecular dynamics (MD), sequential thermo-mechanical finite element analysis, and macroscopic experiments. MD reveals a ‘loose-packing’ mechanism in which urea rejected by the growing ice lattice obstructs complete ice-network coalescence and creates a lower-density heterogeneous structure. This non-ideal behaviour is represented by the signed density deviation Δρ(T) = ρMD(T)−ρideal(T), with R2 = 0.991; at 262 K the signed relative density deficit is −2.75% (magnitude 2.75%). The continuum model predicts a volumetric expansion of 7.17%. Its absolute difference from the experimental mean is 0.27 percentage points, corresponding to a relative deviation of 3.63%; the prediction lies within one experimental standard deviation. Pressure tests show that the integrated expansion chamber limits the equilibrium pressure to approximately 0.505 MPa, while finite element analysis predicts a 99.6% reduction in the maximum PA66 wall stress relative to the rigid reference model. The combined MD, thermo-mechanical finite-element, and experimental results support the proposed loose-packing mechanism and validate the multiscale framework under the investigated cooling and single-freezing conditions