ENHANCEMENT OF HEAT TRANSFER USING TURBULENCE PROMOTERS

          Three cases of heat transfer enhancement by turbulence promoters were adopted in order to increase the thermal performance of a double pipe heat exchanger of 1245 mm effective length, 28 mm outer diameter and changeable  inner diameter (11 or 14 mm). Wire coils of 1 mm diameter and 10, 20, 30 and 40 mm coiling pitches were used as turbulence promoters to augment heart transfer inside the inner tube of heat exchanger at a Reynolds number range of 5000 to 40000 based on smooth tube diameter. Two new types of turbulence promoters are used to enhance heat transfer in the annulus of the same double pipe heat exchanger for a Reynolds number range of 3000 to 10000 based on smooth annulus equivalent diameter. The first was by wire coils of 1 and 2.2 mm diameters and 10, 20, 30 and 40 mm coiling pitches set up on the outer surface of the inner tube. The second was by circular ribs of 2.2 mm diameter and the same pitches and position. Water was used as the working fluid in the two sides. Variation in the experimental conditions was attained by changing the mass flowrates of unenhanced side and changing the inlet temperature of hot fluid. These conditions were followed in order to increase the data points in addition to observe the effect of these conditions. Heat transfer is increased inside the inner tube by 2.43 folds compared to smooth tube at the same Reynolds number accompanied by friction factor increase of 4.75 folds. For the annulus-side enhancement, heat transfer is increased by 3.25 folds, compared to smooth annulus with an increase of friction factor of 2.63 folds. New correlations of Nusselt number and friction factor for the tube and annulus sides were proposed as functions of Reynolds number, Prandtl number and geometrical characteristics of inserts and sizes of tubes and annuli. In addition, performance evaluation criteria (PEC) were applied to the results, in order to determine the most beneficial method.