(106g) The Effects of Out Gassing Through Cylindrical Walls on the Transport Coefficients

    This work concerns with studying the influence of blowing on friction factor and mass transfer coefficient when the flow is normal to a tube bank with outgassing porous cylinders. The friction factor and the mass transfer coefficients are measured for a range of Reynold’s number and blowing rate and expressed in terms of various dimensionless groups.

    The test rig consists of a rectangular duct (20.3 × 11.3 cm), five rows of three porous tubes in staggered arrangements fixed across the duct with 1.5 pitches (both longitudinal and transversal). The outside and the inside diameters of tubes were 25.4 and 21.4 mm, respectively. 

    The friction factor measurements was studied for Reynold’s number range of

3000 < Re <17,000 and blowing rate up to 4×10^-3. The results show that the blowing into the boundary layer for the cylinders increases the overall friction factor from 10% to 50% depending on the blowing rate. The mass transfer coefficient was measured for Reynold’s number range of 5000 <Re <16,000 and blowing rate of 10^-5<m/G_∞ <10^-3.

    A new definition of the blowing rate was considered and expressed by the mass transfer driving force. Two successful formulas were obtained to account the outgassing effect on the mass transfer coefficient. The mass transfer coefficient for the outgassing boundary layer was obtained first by determining the non-blowing coefficient and multiplying it with the correction factor ϕ.

    The first model was modified to account the density variation associated with the boundary layer change:

                             0.51 Re ^-0.5       ρ_s

           St_m Sc ^2/3 = -------------- (------) ^0.4

                           (1 + β_m) ^4.7    ρ_∞

 With the coefficient of determination R2 = 0.93.

    The second model depends on the flat plate correction factor:

                                               Ln(1+ β_m)

           St_m Sc ^2/3 = 0.5 Re^-0.5 (--------------- ) ^9.94 

                                                  β_m

With the coefficient of determination R2 = 0.92.

These two models are found to be in agreement with the previous literature and a reduction of about 80% in the mass transfer coefficient has been obtained due to the outgassing.