Study On Flow And Heat Transfer Performances Of Trisection Helical Baffled Electric Heaters

Trisection helical baffled electric heaters with one-plus-two U-tube units are a novel type of electric heaters to optimize flow and heat transfer performances in replaceing the conventionally-used segment baffled electric heaters with multi-ring-annular U-tube layouts. One-plus-two U-tube units layout scheme can eliminate the adverse impact on flow and heat transfer caused by non-uniform tube pitches of multi-ring-annular U-tube layouts, and trisection helical baffle scheme can apparently enhance the flow and heat transfer performance and improve surface temperature uniformity of electric heaters due to the high efficiency and low resistance characteristics as well as overcome the defects of conventional segment baffled electric heaters such as dead-zone flow, low heat transfer efficiency, high flow resistances, and easily induced vibrations. Based on the trisection helical scheme, folded baffles are designed to create two "ears" perpendicular to the axis of the cylinder to fix the helical baffles with three sets of rod and spacing sleeve assembles by folding each inclined sector baffle at both corners of the curved edge and straight edges. The leakage at triangle zoons between two adjacent plain baffles is minimized and the ring tube plate at the free end of U-tubes could be omitted thus the corresponding flow resistance is eliminated in the folded baffled scheme compared with the plain one. The folded baffled structure can optimize the fluid flow and temperature distribution and extend service life of electric heaters and it is more compact and convenient for procession and installation, and of lower cost with less rod and sleeve tube assembles since each one can fix two sets of adjacent baffles. This scheme has better effect on energy saving optimization design of electric heaters.Firstly, numerical simulation models of segment baffled, trisection helical baffled and folded trisection helical baffled schemes with one-plus-two U-tube units layout were established based on the flow and heat transfer properties of electric heaters in this paper. FLUENT CFD and visualization software TECPLOT were adopted to analyze the flow and heat transfer properties of the electric heaters.Secondly, on the basis of the numerical comparative investigation of three electric heater schemes on the flow and heat transfer performance parameters, visualization study combined with data analysis were conducted to investigate the velocity, pressure and temperature fields of fluid flow and performances of heat transfer coefficient as well as temperature distribution of U-tubes surface. The heat transfer coefficient, comprehensive index, average and highest surface temperature of the U-tubes were analyzed detailedly to reveal the effects of baffle structure on heating performance of circumferential overlapped trisection helical baffled electric heaters with one-plus-two U-tube unit layout. A comprehensive study was conducted on the impacts of the inclined angle of baffle plate on flow and heat transfer performances of the folded helical baffled electric heaters. Electric heaters with incline angles of 20°,25°,30° single helical thread and 30° double helical thread schemes and segment schemes with baffle pitches of 200 and 250 mm were designed and simulated. The results show that the optimum incline angle is 25° in terms of flow and heat transfer comprehensive performances and surface temperature uniformity indexes at the designed condition.Thirdly, numerical simulation and visual study were conducted on flow and heat performances of helical baffled electric heaters with different baffle structures and assembled configurations. Fluid flow and pressure fields of trisection helical baffled schemes and folded trisection helical baffled schemes with the same baffle incline angle or the same spiral pitch were investigated to reveal the flow characters of the folded schemes. Surface temperature uniformity of trisection helical baffled schemes with different tube length was studied to explore heat transfer potential of the trisection scheme. Flow and heat transfer performances of single and double helical thread schemes were compared with heating performance at relatively big incline angles. The impact on heating performance of azimuth angle of tube bundle which relative to the inlet pipe was studied. The scheme with the highest heat transfer coefficient was explored at an allowable pressure drop to meet the aim of flow and heat transfer enhancement Numerical results show that the folded circumferential overlapped trisection helical baffled schemes have high heat transfer coefficient and lower flow resistance, which not only inherited the excellent flow and heat transfer performance of trisection helical schemes, but also simplified the structure of the electric heaters, and they also greatly improved the surface temperature uniformity of U-tubes. At the same working condition, compared with the segment scheme, the heat transfer coefficient and comprehensive index of folded helical schemes are enhanced by 7%-20% and 24%-32% respectively, while the average and maximum surface temperatures of U-tubes are dropped by 15K-50K and 130K-151K, respectively. The folded helical baffled scheme can enhance the flow and heat transfer performance as trisection helical scheme does, and optimize fluid velocity and temperature distributions as well as fasilitate the manufacture process and installation of the baffles. The further investigation on schemes with different tube length shows that the trisection helical baffled scheme can save heating area by 30%at least. Comparative study on optimum scheme under the allowable pressure drop shows that the trisection helical baffles can significantly enhance the heat performance of electric heaters and reduce the maximum surface temperature of U-tubes and the area of hop spots at the allowable pressure drop, thus the operation safety can be guaranteed.Fourthly, the visualization study combined with data analysis was conducted to investigate heating performance under design and off-design conditions of the heating oil electric heaters. The results show that the heat transfer coefficient and comprehensive index of the folded helical oil heat electric heater with incline angle of 15° are enhanced by 18% and 17% respectively, while the average and the maximum surface temperatures of U-tubes are dropped by 14K and 94K compared with those of the segment baffled one with baffle pitch of 100 mm, and the hot spots, which are common in segment baffled scheme, are almost vanished in the folded helical schemes.