LAMINAR FLUID-FLOW AND HEAT-TRANSFER IN AN ANNULUS WITH AN EXTERNALLY ENHANCED INNER TUBE

Laminar forced convection in a double-pipe heat exchanger is studied numerically. In this study, an isothermal tube with periodic enhancements (promoters) is placed concentrically inside an insulated circular tube. Pressure drop and heat transfer characteristics of promoters are obtained at various geometric and flow conditions. These are compared to an unenhanced (circular) tube annulus of identical length and heat transfer surface area while the mass flow rate and the Reynolds number are kept the same. A promoter with Gaussian shape establishes superiority over a cosine shape and other normal distribution shapes, since it provides high heat transfer enhancement with a small increase in the pressure drop. Effects of promoter length and spacing on the pressure drop and heat transfer are small. The pressure drop is influenced significantly by the promoter height and the annular gap, while the promoter height is the only significant geometric parameter affecting the heat transfer. At Reynolds numbers of less than 500, the pressure drop in the enhanced tube annulus is 50 percent more than that in the unenhanced tube annulus. Heat transfer enhancement, though small (about 20 percent) at a Peclet number (Re(D) Pr) less than 200, increases with the Reynolds number and the Prandtl number. At the highest Re(D) (= 1,000) and Pr (= 5) investigated, heat transfer from the enhanced tube annulus is about eight times more than that from the unenhanced tube annulus. However, for this case the pressure drop increases by only a factor of two.