Numerical Investigation On Performances Of The Half Cylindrical Space Heat Exchangers With Orifice Baffles

Improving the performance of heat exchanger is beneficial to energy conservation and emission reduction.U-tube heat exchangers are widely utilized in the industrial production process for convenient manufacturing and installation as well as strong characteristics of withstanding high temperature and high pressure.Although the double tube-pass and single shell-pass structure is adopted for most occasions,double tube-pass and double shell-pass structure is needed for feed water heaters in the power plants and some situations with great fluid temperature difference needing counter-current U-tube heat exchanger for countercurrent heat transfer.This double tube-pass and double shell-pass structure with half cylindrical shell space is formed through installation of a longitudinal diaphragm in the shell space,which is obviously different from the common cylindrical space heat exchanger.To grasp the improvement direction of its performance,it is necessary to conduct in-depth research on the flow and heat transfer characteristics.The characteristics of heat exchangers are affected by the shell side support structure.The traditional segment baffle scheme has large dead zones,induced vibration,and easy fouling,while some effective heat transfer enhanced schemes such as the rod heat exchanger and helical heat exchanger are not applicable in half cylindrical shell space,the new unilateral ladder type helical baffle one has a complex structure.Therefore,the application of orifice baffles in the half cylindrical shell side is proposed in this paper,which has advantages of small vibration,no flow dead zone and simple manufacture.To comprehensively study the performances of half cylindrical heat exchangers with orifice baffles,the orifice schemes,the unilateral ladder spiral scheme and the segment scheme are compared in this paper.Besides,the influence of four shape schemes of orifice baffles（fillet triangular（F）,dual-foil square（D）,trefoil（T）and quatrefoil square（Q））with three baffle pitches（50 mm,40 mm,30 mm）are studied.Numerical investigation is adopted as the research method.The Design Modeler is utilized for geometric modeling,the unstructured mesh is applied,the RNG k-εturbulence model is selected,and the ANSYS Fluent software is used for the calculation.Their velocity field,temperature field,shell side pressure drop,heat transfer coefficient and comprehensive index are analyzed.The main research contents and achievements of this paper are as follows:The influence of four shapes of orifice baffle schemes of fillet triangular（F）,dual-foil square（D）,trefoil（T）and quatrefoil square（Q）on heat transfer performances is compared.The flow patterns of four orifice schemes are periodic longitudinal flow.The jet effect and eddy current effect in each periodic flow are beneficial to reduce scaling and to enhance heat transfer.The maximum velocity of the four orifice plate schemes is less than 0.65 m·s^-1,which meant a reduction of the induced vibration.The temperature of the fluid at center is higher than that at peripheral and the temperature distribution of the orifice scheme is greatly affected by the orifice baffle shape.The difference of the pressure dropΔp_oof four orifice schemes is small,and the largest is the scheme F and the smallest is the scheme Q among the four orifice scheme.The scheme F has the maximum K and h_oand the scheme Q has the minimum ones among the four orifice schemes.The scheme F has the maximum comprehensive index h_o·Δp_o^-1/3and the scheme D has the lowest one.It was concluded that the scheme F has the best heat transfer and comprehensive performances.The flow and heat transfer characteristics of the scheme F and the scheme T with the baffle pitches of 50 mm,40 mm and 30 mm are compared.The influence of baffle pitch on velocity and temperature distribution is not as great as the orifice baffle shape.The parameters ofΔp_o,K,h_oincrease gradually,but the comprehensive index h_o·Δp_o^-1/3decreases gradually with the decrease of baffle pitch at the same shell side flow rate.Therefore,proper baffle pitch should be selected in the heat exchanger design considering both pressure drop and heat transfer features comprehensively.Larger baffle pitch results in higher comprehensive performance,while smaller baffle pitch results in higher heat transfer performance.The flow and heat transfer characteristics are compared on different type heat exchangers of the fillet triangular orifice baffle scheme（F）,unilateral ladder spiral baffle scheme（U）and segmental baffle scheme（S）in the half cylindrical shell space.In the scheme F,the shell side fluid is of longitudinal flow pattern without dead zone or vibration.The jet flow is formed through the gaps,which is helpful for limiting the generation of dirt and enhancing heat transfer.In the scheme U,the spiral flow pattern was presented in the shell side space,and there is certain vibration but no dead zone.In the scheme S,the zigzag flow pattern is showed with large dead zone and induced vibration.The shell side heat transfer coefficient（h_o）and pressure drop（Δp_o）of the scheme F are smaller than those of the scheme U and the scheme S,while the comprehensive performance index(h_o·Δp_o^-1/3)of the scheme F is higher than that of the traditional scheme S,which means more energy saving of the scheme F.However,the heat exchange performance of the scheme F is relatively low and there is still room for improvement.