Momentum accommodation factor

In Eq. (1), Fm is the momentum accommodation factor and has a value close to unity for the gas-solid couples used most commonly in engineering, and is also taken so in this work. In Eq. (2), Ts is the temperature of the fluid molecules at the wall, T is the wall temperature, y is the ratio of the specific heats of the fluid, and Ft is the thermal accommodation factor. Ft may take a value in the range O.O-I.O, depending on the gas and solid surface, the gas temperature and pressure, the temperature difference between the gas and the surface, and is determined experimentally. 

As mentioned earlier, the factor a is the thermal accommodation coefficient and am the momentum accommodation or reflection coefficient. From the data of Rosenblatt and LaMer (1946), Schmitt (1959), and Keng and Orr (1966), as a first approximation a value of 1.25 seems reasonable for Cm, whereas for Ct a value of 2 is a good approximation (Brock, 1962b). These numbers then imply values of 0.89 for am and 0.97 for at. 

WSRC has developed a special code, FLOWTRAN-TF, based on the conservation of mass, energy, and momentum to account for two-phase flow, heat transfer effects, and cross-rib gap flows in assembly subchannels. The heat conduction models developed for FLOWTRAN-FI have been incorporated in FLOWTRAN-TF. Each subchannel coolant node has radially adjacent fuel surface temperature nodes to accommodate the heat transfer in the cell.. Rib fin effects are also handled in the same manner as they are in FLOWTRAN-FI. In order to initiate the computation, an air void fraction must be assigned to the computational cell above the fuel. This is done by assuming an air void volume in the first (top) axial cell as adjusted by an experimentally determined partitioning factor. Results from FLOWTRAN-TF have been shown to be relatively insensitive to the value assigned. Two-phase flow across the ribs is modeled by the application of an assumed partition factor based on values given in the literature. 

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