Fluid Flow And Compound Heat Transfer Enhancement For Shell Side Of Double-pipe Heat Exchanger Enhanced By Helical Fins

Installation of helical fins on the outer wall of the inner pipe can improve heat transfer performance of the shell side of a double-pipe heat exchanger. The shell side can be regarded as a helical channel with rectangular cross section when the helical fins span the full width of the annular. Therefore, in order to improve heat transfer performance of the shell side enhanced by helical fins further, fluid flow characteristic and heat transfer performance of helical channel with rectangular cross section were investigated numerically at first. The numerical results show that, heat transfer coefficients increase with curvature, Reynolds number and Prandtl number increasing and decrease with aspect ratio increasing. At low Re, torsion has little effect on heat transfer coefficient. At high Re, heat transfer coefficient has a maximum value when torsion is equal to half of curvature. The synergy between velocity field and temperature field is poor at the center of the rectangular cross section. On the basis of the results obtained from calculation we knew that, the way to enhance heat transfer performance for the shell side only enhanced by helical fins is that some measurements should be taken to improve the secondary flow and the synergy between velocity field and temperature field at the center of the cross section. Thus, some vortex generators were installed along the centerline of the helical channel.Fluid flow characteristics of the helically rectangular channel with pins were investigated numerically and experimentally. The results were compared with those of a helical channel without pins. The results show that, by pin fins being installed, the secondary flow in the cross section is improved significantly. Correspondingly, the absolute values of the vorticity increase in the cross section. The pin can increase the turbulence intensity and the fluctuation frequency of the instantaneous velocity.Based on the results obtained from the investigation on the fluid flow of the helical channel with and without pins, some vortex generators (pin, wing and winglet) were constructed with helical fins to enhance heat transfer. Heat transfer performance of the compound heat transfer enhancement was investigated experimentally. For comparison convenient, heat transfer performances of a heat exchanger without any enhancement and those of the heat exchangers only enhanced by helical fins were also examined experimentally. The comprehensive performances of the heat exchanger enhanced by helical fins and by the compound enhancement were evaluated under three conditions, i.e., identical mass flow (IMF), identical pressure drop (IPD) and identical pumping power (IPP).The results show that, for the heat exchanger only enhanced by helical fins, the smaller pitch angle is, the higher heat transfer coeffient can be obtained. The pressure drop however increases with pitch angle decreasing, especially at small pitch angle. The compound heat transfer enhancement with helical fins and vortex generators has advantages over the heat transfer enhancement only by helical fins at short pitch. The delta winglet pairs have superior performance than other vortex generators when the heat transfer area and the number of the vortex generators are same.