The Influence Of Return Bend On The Liquid Nitrogen Flow And Heat Transfer In U-tubes

So far, the research on the cryogenic two-phase flow pattern, pressure drop and heattransfer has mainly concentrated in the straight tube. With the emergence and use of highefficiency heat exchanger, such as compact heat exchanger, the pipeline system isdominated by the U-tubes, which is made of straight pipes and return bends. However,there is very little research on the effect of return bends on the cryogenic two-phase flow.When single phase flow passes return bends, the secondary flow is developed along withthe main stream, as the results of the centrifugal force and pressure difference on thecross-section. When two-phase flow passes return bends, due to the density differencebetween two phases, the secondary flow can change the flow behavior and phasedistribution in return bends, resulting in the changing of heat transfer model and thepressure drop calculation method in U-tubes. In addition, the safety accidents happenedmany times in the cryogenic industrial processes because of the U-tubes cracking. Theroot reason is a lack of understanding of the influence of return bends on the flow and heattransfer. Obviously, the influence of return bends on the cryogenic two-phase flowbehavior, heat transfer and pressure drop is a typical and urgent problem to be solved.Firstly, the high-speed cameras is used for carrying out the visualization studies ongas/vapor-liquid two-phase flow in adiabatic and non-adiabatic U-tubes and invertedU-tubes. The mixture of nitrogen gas and water is employed in adiabatic U-tube andinverted U-tube. The boiling liquid nitrogen is in the non-adiabatic U-tube and inverted U-tube. The visualization studies found that the return bends change the flow pattern inadiabatic U-tube and inverted U-tube by aggregating the bubbles or separating the gascolumn. The flow pattern and its transition diagram of the nitrogen-water mixture inU-tube and inverted U-tube is played and the influence of curvature ratio of return bend onflow pattern and its transition is discussed. The liquid nitrogen boiling in the non-adiabaticU-tube and inverted U-tube only shows the film falling annular flow and the stirring flow.Due to the existence of return bend, the film falling annular flow and the stirring flow istransformed to each other in upstream and downstream section. By comparing the flowbehavior of nitrogen gas-water mixture and boiling liquid nitrogen, the commoncharacteristic in U-tubes is found that the inner side wall of return bend is often exposed tothe gas/vapor.Secondly, the numerical simulation is conducted for analyzing the single-phase andboiling two-phase liquid nitrogen flow and heat transfer in U-tubes. It is found that thefluid almost hasn’t crosswise movement in the upstream straight section, develops theDean Vortex in the return bend and the Dean Vortex decays into a single vortex in thedownstream straight pipe as single-phase liquid nitrogen flows in the U-tube. The PlaneVortices Intensity is proposed for quantifying the process of Dean Vortex decaying in thedownstream pipe. Three advantages of Plane Vortices Intensity as quantifying and judgingthe affected area in the downstream section by the return bend are described, which areexcluding the impact of heat flux; synergy with the Reynolds number; and reactingdifferently caused by the curvature ratio differences. A correlation for the affected length isproposed based on the numerical simulation data.It is also found that the classical parabolic temperature distribution in the upstreamstraight pipe is reconstructed by the secondary flow in the return bend. The cold coreflows to the outer side, while the fluid with higher temperature to the inner side. Theclassic parabolic temperature distribution is gradually restored in the downstream sectionagain. The wall temperature in return bend has significantly decreased, and thecircumferential distribution of the wall temperature presents differently. The heat transfercoefficient achieves to the maximum near the exit of the return bend. The contribution ofthe return bend to the entire U-tube heat transfer is significant.As for the numerical analysis of boiling two-phase liquid nitrogen flow and heattransfer in U-tubes, two fluid model and a suitable closure equations is chosen. As an example, a detailed numerical analysis and results discussed on the tube of WD6R20iscarried out. It is found that the temperature field and local gas volume fraction are affectedby the velocity field. In the return bend, the inner wall is prone to dry-out. This is becauseof there forming a vapor accumulation near the inner wall. The secondary flow makes theboiling bubbles generated in the inner the wall stay inner side and transport the boilingbubbles generated elsewhere except the inner wall to the inner side. When the fluid flowsinto the downstream straight section, the vapor accumulation is replaced by the “vapordistributing uniform near the wall” with the secondary flow disappearance.Thirdly, the experimental measurements of liquid nitrogen flows in six differentU-tubes (UD6R20, UD8R32, UD8R60, WD4R20, WD6R20and WD8R40) are conducted.The purpose is to study the pressure drop and heat transfer characteristics of boiling liquidnitrogen in U-tubes. The experimental results show that the influence of return bend on thepressure drop in the downstream straight pipe can not be neglected. A new calculationalgorithm, namely ΔPsplus ΔPcin this paper, for total pressure drop in U-tube is proposedon condition that single-phase flows in U-tube. The newΔPsplus ΔPcalgorithm includesthe impact of return bend. When liquid nitrogen flow is boiling in U-tube, the proposedΔPsplus ΔPcalgorithm is extended to the two-phase flow total pressure drop in U-tubes. Itis found that the total pressure drop in U-tube calculated by the combination of ΔPsplusΔPcand Chisholm B coefficient model is in good agreement with the experimentalmeasurements.When liquid nitrogen is boiling in the U-tubes, the local heat transfer characteristicsare affected obviously by the heat flux, the inlet pressure and curvature ratio. The localheat transfer characteristics in U-tube are different from that in the straight one. One of thespecific performance is that a local dry out is easy to develop near the inner wall in thereturn bend, in consequence of the inner wall temperature rising rapidly.A statistics on1581points of liquid nitrogen boiling flow is conducted. It is foundthat it is reasonable to use vapor quality x=0.25as the boundary of boiling flow heattransfer dominant mechanism. It is suuposed that the nucleate boiling takes change of thechannel as x <0.25; the forced convection evaporation takes change of the channel as x>0.25. This paper proposed a new correlation for liquid nitrogen boiling heat transfer inU-tubes. The new correlation contains five dimensionless numbers: Bo, We, X, KP, and Dn.The different dominant mechanism of flow boiling heat transfer and the effect of various parameters on the liquid nitrogen boiling in the U-tube can be described accurately by thenew correlation. The heat transfer coefficient calculated by the new correlation can be ingood agreement with experiments.