Numerical study on the effect of using spiral tube with lobed cross section in double-pipe heat exchangers

turbulent flow characteristics and heat transfer applications of a twisted double-pipe heat exchanger (dphe) with four different lobed cross sections are numerically investigated. geometrical modifications are made for both inner and outer tubes of double-pipe heat exchangers. the numerical analyses are done based on the performance evaluation criterion (pec), which is the relation between heat transfer rates and pressure losses. it is found out that pec increase is more considerable regarding the outer tubes of the dphes. upon simulations, it is observed that heat transfer and pressure drop decrease with the increase of lobe number in a tube, while the other tube was held smooth. at this point, maximum of respectively 240 and 85% increase in nusselt number and pressure drop are observed. moreover, numerical results show that 3-lobed cross section for both inner and outer tubes presents the best results in double-pipe heat exchangers, in which an enhancement of more than 200% in pec is observed. on the other hand, in some cases, using lobed tubes may lead to inappropriate results; in the worst scenario, considering 6 as the lobe number of the inner tube, the pec eventuates to 0.8.