Experimental Investigation On Heat Transfer Performance Of Trisection Helical Baffle Electric Heaters With One-plus-two U-tube Units

As a new technology,the trisection helical baffle electric heater is designed with one-plus-two U-tube units and is supported by both axial separation and circumferential overlap trisection helical baffles,to overcome the deficiencies of nonuniform tube spacing,low heat transfer coefficient and high flow resistance with traditional segmental baffle electric heaters with multi-ring-annular U-tube schemes.The new electric heater is composed of one-plus-two U-tube units better fitting the equilateral triangular tube layout scheme of different tubes with step of 9 U-tubes in a cylindrical space.The structure with circumferential overlap trisection helical baffles has advantages of not only higher heat transfer coefficient,low flow resistances,no dead-zone flow,no induced vibrations and fouling,but also limited leakage problem at the triangle zones between adjacent baffles.The axial separation approach,which adopts small inclined angle circumferential overlap helical baffles,and adds spacing sleeves between adjacent baffles so as to realize greater helix pitch with alleviated abrasion of sharp-corners of inclined baffle holes on the tubes manufactured by 2D laser beam cutting machine.Thus,the helical baffle technique can be adopted to electric heaters with feasible manufacture approach of 2D raser beam cutting for multiple types and small batches features,to improve the processing efficiency and reduce the cost of the helical baffles.To verify the heat transfer strengthening effect of circumferential overlap trisection helical baffle electric heaters and compare the performances of the new structure with axial separation helical baffle scheme and the normal end-to-end one,experimental study were conducted on heat transfer performances of both 3 normal and 2 axial separation circumferential overlap trisection helical baffle electric heaters and a segment one with 9U-tubes arranged with one-plus-two U-tube unit and with water as working medium.Meanwhile,through the specific analysis of relevant indicators such as heat transfer coefficient h,pressure drop△p,comprehensive index h·△p^-1/3and the average tube wall temperatures T_w,the influences of different baffle structures on the performance of electric heaters are discussed.The experimental results suggest that the heat transfer coefficient and comprehensive index of the five circumferential overlap trisection helical baffle electric heater schemes are generally higher,and their average wall temperatures of the electric heating tube are lower than those of the segmental baffle electric heater scheme.The specific data are as follows:compared with the segmental scheme,the heat transfer coefficients of the helical baffle electric heater schemes increase by 6.9%to 48.2%,the comprehensive indexes increase by6.9%to 47%and the average wall temperatures of the electric heating tube reduce by 1.42 K to 8.0 K.By comparing the normal schemes with the axial separation schemes with the same equivalent inclined angle,it can be found that the latter has a slightly lower heat transfer coefficient and comprehensive index.Still it is much better than the segmental baffle electric heater scheme.The heat transfer coefficients of the two axial separation schemes of H8（6）°and H10（6）°are 22.24%and 11.91%higher and the comprehensive indexes are 20.26%and11.49%higher,and the average wall temperature of the electric heating tube is 5.36 K and3.11 K lower respectively than those of the segmental baffle scheme.The universal calculation model of mass center equivalent rectangle was adopted for calculation of the trisection helical baffle electric heaters featured three different helical angles（H6°,H8°,H10°）under the same experimental conditions.The comparison of the correlated and experimental results is satisfactory which further verified the universal calculation model of mass center equivalent rectangle.By comparing the correlated results using the model of mass center equivalent rectangle with the experimental results under the same conditions,the uncertainty is only in the range of-3.5%to 17%.Based on HTRI software,a quadrant helical baffle heat exchanger model was also established to study the influences of different inclined angles and helix pitches on the performances of the quadrant helical baffle heat exchangers.The calculation results show that the heat transfer coefficient curves of the heat exchangers with the same inclined angle but different pitches（including three baffle connection types of normal end-to-end,axial overlap and axial separation）tend to coincide and gradually increase with the increasing flow.The results indicate that model established by HTRI software for the heat transfer coefficient of the quadrant helical baffle heat exchanger is only affected by the baffle inclined angle rather than the helix pitch.