Experimental investigation of the effects of simultaneously using plate fins and a helical screw rod with a core rod on a double-tube heat exchanger's thermal performance

Fossil fuel consumption is increasing, causing pollution, global warming, and heating system expenses, among other negative environmental repercussions. It highlights the need to increase the efficiencies of heating/air conditioning components. For this purpose, the heat exchanger's efficiency is pivotal, so experiments were conducted to increase the efficiency of a concentric double-tube heat exchanger (DTHX). The effects of simultaneously using a helical screw rod, a core rod, and plate fins were studied. Specifically, the thermal performance factor, tube-side heat transfer coefficient, Nusselt number, and friction factor were experimentally examined. Plate fins (PF) were inserted into the annulus, and a helical screw rod and a core rod (HSR-CR) were inserted in an inner tube. For this research, a 23-mm-diameter helical screw rod with a core rod (HSR-CR) was inserted freely into an inner tube with a hot water stream. It had a twist ratio 4.826 and a 1.5 mm clearance to the tube wall. In addition, the aluminium plate fins (PFs) were inserted loosely into the outer tube (annulus) in the stream of cold air with an 8 mm clearance to the annulus wall. They were distributed as 30 rings along the entire outer perimeter of the inner tube so that they formed 90 degrees. Each ring contains 14 plate fins with heights of 30 mm. The experiments were conducted applying two different temperatures (313.15 and 323.15 K), three different hot water flow rates (0.2, 1.0, and 2 lpm) in the inner tube with Reynold's range (236 <= Re < 3000), and a constant airflow rate of 1250 lpm was maintained in a counterflow setup. The analysis revealed that incorporating HSR-CR and PF enhanced thermal properties compared to a smooth DTHX and individual use of HSR-CR or PF. Consequently, the overall heat transfer coefficient, the Nusselt number, and the friction factor increased by 204.54%-293%, 85.9%-275%, and 1.323-1.817, respectively. The double-tube heat exchanger's performance maximum values (TPF) at 313.15 K were 1.69, 2.614, and 3.07, respectively, at Reynold's numbers associated with the three flow rates (0.2, 1, and 2 lpm). At 323.15 K, at the same flow rates, the thermal performance factor (TPF) values were 1.775, 2.65, and 2.974, respectively.