EXPERIMENTAL INVESTIGATION ON HEAT TRANSFER AND FLUID FLOW CHARACTERISTICS IN THREE FLUID HEAT EXCHANGER USING MULTI PASSIVE TECHNIQUES

Enhancing heat transfer and reducing pressure drops is the main aim of the present experimental study. This can be achieved in a three-fluid heat exchanger (TFHE) by using a combination of two enhancement techniques. This experimental work shows that the two enhancement techniques provide more heat transfer (HT) under controlled pressure drops compared with a smooth circular tube for industrial applications in water and space heating simultaneously. First, change the cross-section of an innermost tube from circular to hexagonal; second, insert 1-mm diameter spring wire with pitches of 5, 10, and 15 mm in the hexagonal tube through which atmospheric airflows. Waste hot fluid (WHF) and normal water (NW) circulated through the inner and outer annuli of the helical coil (HC). The experiments were carried out under turbulent conditions, where the Reynolds number for WHF, NW, and air varies in the ranges of 7500 to 25,000, 2000 to 4000, and 3000 to 11,000, respectively. The experimentally obtained Nusselt number for the HC and air side is compared and validated with the Nusselt number obtained from correlations reported in literature. The parameters that have the greatest impact on the results are the pitch of the spring, the inlet temperature of the waste hot fluid (WHF), and the mass flow rate of all fluids. It is achieved from the experiments that the Nusselt number rises as the Reynolds number rises. Due to the hexagonal tube, the percentage increment in Nusselt number for the HC and air side is found to be 5.46% to 7.60% and 12.15% to 26.35%, respectively, compared with a smooth circular tube with minor changes in pressure drops. The use of spring inserts in a hexagonal tube causes the maximum increment in Nusselt number and friction factor for the HC side, it is found to be 7.87% to 21.45% and 15.94% to 23.20%, and for the air side, 33.03% to 67.13%, and 15.68% to 24.83%, respectively, compared with a smooth circular tube at lowest pitch.