Dynamics of droplet breakup through a grid spacer in a rod bundle

Two-Phase Turbulence Statistics from High Fidelity Dispersed Droplet Flow Simulations in a Pressurized Water Reactor (PWR) Sub-Channel with Mixing Vanes

“…It is evident that a significant amount of energy is contained in the R xn component, especially towards the sub-channel center, and it increases with the upstream collision Weber number. As observed by the result presented in Figure 2 and the experiments by Cheung et al [17], the upstream collision Weber has a significant effect on the droplet SMD and distribution at the downstream location. Since the droplets provide significant feedback to the downstream turbulence, it is worthwhile to observe the effect of changing on the downstream turbulence behavior.…”

“…Since We a is significantly less than the critical Weber number [55], the droplets are not expected to undergo deformation or breakup due to aerodynamic effects. It must be emphasized that this is in harmony with the experimentally observed spherical topology of the droplets at the upstream location of the spacer-grid structures in the upper portions of the PWR channel [17,56]. More details on the droplet injection routine, the rationale for selecting the droplet parameters, accompanying assumptions and their relevance to the experiments can be found in [35,43] and are not reiterated herein for brevity.…”

“…For the We40 case, however, the droplets have the tendency to accumulate into larger blobs and also develop a liquid film over the leading edge of the spacer-grid surface, owing to a higher surface tension force coefficient. As noted by Cheung et al [17], a smaller downstream average droplet size (or Sauter mean diameter) correlates with a higher heat transfer coefficient, owing to a corresponding increase in surface area. Thus, although the current simulations are adiabatic, it can be conjectured that the heat transfer coefficient for the We80 case will be higher as compared to the We40 case.…”

“…where u d is the absolute droplet velocity and σ is the surface tension coefficient. The latter was adjusted in the simulation cases, as listed in Table 1, to ensure that the collision Weber number, Equation ( 14), is maintained at the user-specified value, herein chosen as We c = {40, 55, 80}, consistent with range of We c measured from experiments [17]. Based on these parameters, the resulting aerodynamics Weber number of the droplets was < 1 for all cases, defined as,…”

“…The latter phenomenon provides massive positive feedback to several heat transfer paths [16], viz., convective heat transfer enhancement from mass transfer due to rapid evaporation of smaller droplets, radiation heat transfer from fuel rods to droplet surface and interfacial heat transfer from steam to droplets. The heat transfer increase across spacers has been empirically correlated with the observed decrease in the Sauter mean diameter (SMD) of the droplets [17], which provides a statistical measure to characterize their morphology and is accounted for in STH codes [18,19]. It was noted by Hochreiter et al [16], however, that the aforementioned models for the DFFB regime have considerable implicit measurement uncertainty and model uncertainty related to their implementation in STH codes.…”