| Helical flow formed on the shell-side complex channels of the circumferential overlap trisection helical baffle electric heaters and vortex spiral flow formed on the shell-side complex channels of twisted elliptical tube heat exchangers are beneficial to enhance heat transfer.The trisection helical baffle electric heaters with“one-plus-two”U-tube units were proposed to improve the performance of traditional electric heaters with a multi-circle annular layout of U-tubes commonly used in industrial engineering.The helical baffle electric heaters possess many excellent advantages such as high heat transfer coefficients,low flow resistance,outstanding anti-vibration and anti-scaling,which is considered to be the most promising alternative to the segment baffle ones.The axial separation helical baffle scheme is a novel connect method of discontinuous helical baffles,which can create a larger helical pitch by separating the adjacent helical baffles with a small incline angle.Helical baffles are of small batches,large sorts and difficult to manufacture,the cheaper and popular 2D laser cutting machines can be used to manufacture helical baffles with small inclined angles with remediated abrasion of the sharp corner of the baffle holes to the tubes.Therefore,it is worth trying to apply the axial separation method to the trisection helical baffle electric heaters with“one-plus-two”units.There have been many studies on the flow and heat transfer characteristics of the twisted elliptical tube heat exchanger on the tube side and the shell side.However,the influence of the twisted elliptical tube rotation and tube layout are not noticed.Therefore,the heat transfer mechanism of helical flow and vortex spiral flow on the shell-side complex channels of the axial separation trisection helical baffle electric heaters and the coupling-vortex twisted elliptical tube heat exchangers were deeply investigated in this work.In order to study the flow and heat transfer characteristics and the temperature field uniformity of the axial separation trisection folded helical baffle electric heaters,the physical models of the axial separation and the end-to-end(abbreviated as normal)circumferential overlap folded trisection helical baffle electric heaters were established.The segment baffle scheme with a baffle span of 200mm was also studied for comparison.The one-third central symmetry“one-plus-two”unit tube layouts were adopted.The results show that the velocity fields of the axial separation helical baffle electric heaters are similar to those of the normal helical baffle ones.Main helical flow and the secondary flow are the main heat transfer enhancement mechanism of helical baffle on the shell side of electric heaters.As the V-notches become wider from the outmost peripheral to the central axis and the difference angle between the equivalent incline angle and the incline angle of helical baffles increases in axial separation schemes,the leakage flow gradually increases.Due to the increase of leakage flow between adjacent baffles,the axial separation helical baffle schemes are worse than the corresponding normal helical baffle schemes but are still much better than the segmental scheme.Among all helical baffle schemes,the larger the equivalent incline angle,the smaller the comprehensive index h·?p-1/3.To reduce the high-temperature areas at the end of U-tube bundles,an improved axial separation scheme was proposed by adding an end-to-end helical baffle at the end of U-tube bundles in axial separation schemes.The average tube wall temperatures of the improved axial separation schemes of FHM15(10)°,FHM20(10)°and FHM25(10)°are 49.0?134.0 K lower than that of segment scheme S200 and the heat transfer coefficient ratios of h·h-1S200 are higher than those of the corresponding original ones.To study the influence of different helical baffle configurations on the flow and heat transfer performance of electric heaters with a U-tube layout of15 one-plus-two units(45 U-tubes),four end-to-end helical baffle schemes,four axial separation helical baffle schemes and a segmental scheme S200 were constructed,all circumferential overlap trisection helical baffles are of plate or folded ears.The results show that the heat transfer coefficient,pressure drop and comprehensive index h·?p-1/3 of helical schemes show a stronger correlation with the equivalent inclined angle of helical baffles(or the number of baffles).Among the nine schemes,the normal scheme H16.6°has the highest h·?p-1/3 and the second-lowest pressure drop,the scheme FH20°has the relatively low heat transfer coefficient but the second-highest h·?p-1/3.Compared with S200,the averaged wall temperature of H16.6°/FH20°is decreased by 47.9 K/43.8 K.A variable-tube-length scheme was proposed to reduce the high-temperature zones at the end of U-tube bundles.For adapting to the position change of helical baffles of the last cycle in helical channels,three groups of U-tubes are consisted of four different lengths to form the variable-tube-length U-tube bundle.Each group of“one-plus-two”tubes corresponds to a group of trisection helical baffles.Besides,the influence of the outlet azimuth on temperature fields was also investigated for the helical baffle schemes and the segmental one.The simulation results show that the variable-tube-length scheme can effectively reduce the high-temperature areas at the end of U-tube bundles.Compared with the segment baffle scheme S200-90°with an outlet orientation of 90°,the averaged heat transfer coefficients and h·?p-1/3 of the variable-tube-length schemes HV15(10)°-180°/HV15(10)°-Ax are improved by 14.6%/11.8%and 16.1%/13.8%,and the averaged tube surface temperature is decreased by 40.7 K/32.3 K,respectively.The averaged heat transfer coefficients and h·?p-1/3 of the normal axial separation schemes H15(10)°-180°/H15(10)°-Ax are increased by13.8%/10.0%and 15.5%/12.3%,and the averaged Tw is decreased by 37.7 K/31.4 K.The outlet azimuth of both helical baffle schemes and segment scheme imposes significant effects on the temperature fields at the end of U-tube bundles.Within the studied scope,both helical baffle and segment baffle electric heaters with radial outlets are superior to those with axial outlets,the best relative positions between the U-tube bundle and the outlet allow the fluid to smoothly sweep the U-tube elbow behind the last baffle.The heat transfer coefficients of two helical baffle schemes HV15(10)°-180°/H15(10)°-180°are 0.7%/3.5%higher than the corresponding axial outlet helical schemes and the averaged wall temperature of U-tubes is decreased by 3.1 K/9.3 K.Aiming at the problems of manufacturing difficulty of helical baffles and the serious leakage between adjacent baffles,a modified axial separation helical baffle scheme with two bending straight edges and a small incline angle was proposed.The manufacture can be achieved by two procedures:firstly,the cheap 2D laser cutting method is adopted to process the helical baffles with a small incline angle;secondly,two straight edges are folded mechanically.Simulations were conducted on nine axial helical baffle electric heaters and a segment baffle scheme S200.The results show that,compared with the corresponding conventional axial separation helical baffle schemes,the novel bent helical baffle electric heaters possess excellent heat transfer performances and more uniform temperature fields,the leakage flow can be significantly reduced,especially in the vicinity of axially central regions.Compared with the segmental scheme S200,the averaged heat transfer coefficients and the comprehensive index h·?p-1/3 of the bent helical baffle schemes of BH15(10)°,BH20(10)°and BH25(10)°schemes are increased by 35.7%/18.5%,17.1%/27.3%and 5.3%/35.5%,the average wall temperature decreased by 72.3 K,45.7 K and 24.4 K,respectively.Within the studied scope,the larger the equivalent incline angle,the more obvious the advantages of the new axial separation bent helical baffles.The averaged heat transfer coefficients and the comprehensive index h·?p-1/3 of BH15(10)°,BH20(10)°,BH25(10)°are increased by 6.5%/2.3%,12.2%/7.3%and 20.4%/13.0%,respectively.The averaged wall temperature is decreased by 12.5 K,25.4 K and 54.7 K,respectively.To explore the effects of heat transfer enhancement of vortex flow on the twisted elliptical tube heat exchangers,a coupling-vortex checkerboard square tube layout and a 1/3 axisymmetric coupling-vortex alternating V-shaped triangular tube layout twisted elliptical tube heat exchangers were proposed.Heat exchangers with the coupling-vortex twisted elliptical tube layout,the parallel-vortex twisted elliptical tube layout,the smooth oval tubes and the smooth round tubes were simulated to investigate the effects of tube layout,the twisted direction of elliptical tubes,the ratio Amax/Bmin of the main axis to the minor axis of twisted tubes and the twisted spiral pitch Hs.The fluid on shell-side channels flowing along two adjacent tubes with opposite rotation directions that forms concurrent flow can enhance heat transfer.Merely concurrents exist in the coupling-vortex checkerboard square tube layout schemes but both concurrents and countercurrents exist in the triangular tube layout coupling-vortex schemes.Because only the twisted elliptical tubes need to be reversely twisted during the manufacturing process,the tube layout method will not produce any extra manufacturing and installation costs.Numerical simulation results demonstrate that all twisted elliptical tube schemes can form good spiral flows and secondary flows,which benefit from irregular channels of twisted elliptical tube heat exchangers,a larger ratio of Amax/Bmin and a smaller twisting pitch.Due to the concurrents and strong secondary flow,the coupling-vortex schemes have a higher heat transfer coefficient,a slightly higher pressure drop and more uniform temperature fields than the relative parallel-vortex schemes.The Nusselt number Nu and the friction coefficient f of all twisted elliptical tube schemes are higher than the smooth elliptical tube scheme and the smooth round tube scheme.The temperature fields of the coupling-vortex schemes are more uniform than those of the corresponding parallel-vortex schemes.As the Reynolds number Re increases,Nu and Nu·f-1/3 are increased but f is decreased.As the twist pitch increases,Nu and f are decreased while Nu·f-1/3 is increased.As the ratio Amax/Bmin increases,Nu,f and Nu·f-1/3 are increased.Compared with the square tube layout smooth round tube scheme?R10.8,the averaged Nu/Nu·f-1/3 of?C12.3S50,?C12.3S100and?C14.2S50 are increased by 39.7%/18.1%,26.7%/24.7%and 81.5%/25.6%.Compared with the corresponding parallel-vortex schemes,the coupling-vortex schemes have greater advantages under a shorter twisting pitch,a smaller Amax/Bmin(larger than 1.0)and a lower Reynolds number.The averaged Nu,f and Nu·f-1/3 of the coupling-vortex scheme?C12.3S50/?C12.3S100 are increased by12.8%/9.9%,15.2%/10.5%and 7.6%/6.3%,respectively.Compared with triangular tube layout scheme?R10.8,the Nu/Nu·f-1/3 of?C14.2S50 and?C12.3S50 are increased by 132.8%/47.1%and67.1%/38.0%,respectively.The maximum and average Nu/Nu·f-1/3 of the coupling-vortex scheme?C12.3S50 are increased by 9.5%/6.4%and 7.8%/4.9%than the parallel-vortex scheme?P12.3S50. |
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