Experimental Study Of The Heat Transfer Enhancement In A Downing-Channel Tube Fitted With Regularly Spaced Twisted Tape And Coiled Wire Inserts

Heat transfer enhancement techniques are widely used in chemical industry, power engineering, refrigeration, nuclear reactors and many other industries, especially in the applications of energy saving and waste heat recovery. Heat transfer enhancement can be classified into two categories of active and passive techniques. The passive techniques have been widely utilized since no additional external power is required in industries. Swirl flow devices form an important group of passive enhancement techniques, such as twisted tape and coiled wire inserts.The heat transfer and resistance characteristics of a circular tube fitted with18different regularly spaced twisted tape inserts,10different coiled wire inserts,14different combined regularly spaced twisted tapes and coiled wire respectively was experimentally studied with water as working fluid. In order to investigate the effect of geometric parameters on heat transfer and resistance characteristics, twist ratio (Y=4～6), width (D=17～23mm), fill ratio (S=0.4～0.6), segments (N=3～9), coil pitch (P=15～48mm) and wire diameter (e=0.711～2.032mm) were experimentally examined as variables. In the range of8000<Re<65000, the obtained results reveal that tube fitted with regularly spaced twisted tape can increase Nu number up to16.17%～22.53%and pressure drop is1.2～1.46times of the corresponding plain tube, tube fitted with coiled wire increase Nu number up to38.87%～92.15%and pressure drop is1.7～7.78times of the corresponding plain tube, tube fitted with compound element increase Nu number up to54.08%～89.53%and pressure drop is3.08～6.92times of the corresponding plain tube. The experimental results indicate that the combined regularly spaced twisted tape and coiled wire lead to better performance in heat transfer than the use of regularly spaced twisted tape or coiled wire alone. Geometric parameters have important influences on the heat transfer and flow resistance performance, for example, the wire diameter is stronger effect than the coil pitch, the coil pitch and the wire diameter of coiled wire are the governing parameters to signify the heat transfer and resistance characteristics in compound heat transfer enhancement. Through a multivariable regression, the correlation formulas for Nu number, flow resistance and pressure drop increment are fitted which can be the experimental basis for heat exchanger design and improved.The paper selects the best value of comprehensive heat transfer performance factor0as the goal. The experimental results indicate that the coiled wire inserts and compound heat transfer enhancement elements have better performance than regularly spaced twisted tape when the Reynolds number is less than40000.