The Effects of Outgassing Through Cylindrical Walls on the Transport Coefficients

Luma Hikmat Yaseen
Professor Dr. Nada B. Nakkash
Dr. Mohammed N. Latif

 Abundant data on the momentum, heat and mass transfer is available in the literature. Most of this data is confined to impermeable surfaces. It is known from studies concerning flat plates that blowing into the boundary layer can severely reduce the transport coefficients (friction factor,mass and heat transfer coefficients). This work studies the influence of blowing on the friction factor and mass transfer 
coefficient for the flow normal to a tube bank with outgassing porous cylinders. The friction factor and the mass transfer coefficients are measured for the investigated range of Reynolds number and blowing rate, and expressed in terms of various dimensionless groups.A test rig was built for the present work purposes consists of a rectangular duct (3.113.20 × cm). Five rows of three tubes of porous tube in staggered arrangements fixed across the duct with 1.5 pitches (both longitudinal and transversal).The outside and the inside diameters of the tubes were 25.4 and 21.4 mm respectively.Measurements for the friction factor was done for 3000 < Re < 17,000 and blowing rate up to 4×10 . It was found that the blowing into boundary layer for cylinders increase the overall friction factor from 10% up to 50%depending on the blowing rate.The mass transfer coefficient was measured for 5000 <Re<16,000 and blowing rate •−53". A new definition for the blowing rate was used and expressed by the ∞<<10G/m10 mass transfer driving force. Two successful formulas were modified to account the outgassing effect on the mass transfer coefficient. The mass transfer coefficient for the outgassing boundary layer can be obtained by first determining the non-blowing coefficient and multiplying it with the correction factor ϕ. The first one modified to account the density variation associated with the boundary layer change and the final model obtained given as: