Research On Heat Transfer Enhancement For Natural Convection And Boiling In Converging-diverging Channels

The flow and heat transfer of natural convection and boiling in the converging anddiverging channel were researched in this paper. On the combination of experiment andnumerical simulation, the heat transfer of natural convection and boiling for water and60%sucrose solution in vertical converging-diverging (CD) tubes with regularly spaced twistedtapes (RSTT) were investigated. The effects of converging-diverging surfaces on naturalconvection and boiling were studied simultaneously.The flow and heat transfer of natural convection for water and60%sucrose solution invertical converging-diverging tube and smooth tube with regularly spaced twisted tapes werestudied experimentally. The effects of number of twisted tapes and twisted angle on flow andheat transfer were mainly presented. The variation of natural convection heat transfercoefficient with temperature difference in tube side was obtained under different twisted angleand number of twisted angle. The heat transfer coefficient for water and sucrose solution invertical smooth tube and CD tube with RSTT are much bigger than that in barely smooth andCD tube. Within the scope of the experimental conditions, the natural convection heat transfercoefficient increases with the number of twisted tapes, but the degree of increase will becomegradually smaller. The heat transfer coefficient for sucrose solution is different from that forwater under same number of twisted tapes. The twisted tapes with higher degree of twistedangle are more benefit for heat transfer performance.Based on the experimental research, the natural convection for water and sucrose solutionin vertical smooth and CD tubes with RSTT was investigated numerically. Under the analysisof temperature field and velocity field, it is found that the fluid in the central region of smoothtube is difficult to be heated because of the high heat resistance in near-wall region. When theRSTT were inserted into CD tube, the fluid in the central region was heated sufficiently. Thevelocity increases obviously when the fluid flows through the RSTT, and there is obviousaxial temperature gradient for water and sucrose solution in central region. Through theanalysis of flow resistance and heat transfer performance for CD tube with RSTT, thevariation of resistance coefficient and heat transfer coefficient with number of twisted tapeswas obtained, and the performance of comprehensive heat transfer enhancement wasevaluated. The heat transfer performance along axial direction was researched. The resultsindicated that the local heat transfer coefficient along axial direction in a CD tube with RSTTwas bigger than that in a smooth tube. It is found that the tangential velocity increasessignificantly for the compound effects of periodically converging-diverging surfaces and twisted tapes. The heat transfer boundary layer becomes thinner because of the highertangential velocity, and then the heat transfer is enhanced. The synergy of temperaturegradient field and velocity field for fluid in tube side was investigated. Because of thecompound effects, the synergy in a CD tube with RSTT is better than that in a smooth tube,especially in the region where the fluid flows through the twisted tapes.The boiling heat transfer of60%sucrose solution in a vertical tube with RSTT wasinvestigated experimentally. The curves of boiling heat transfer for evaporation of sucrosesolution were obtained under different number of twisted tapes and different twisted angle. Itis found that the twisted tapes is benefit for the boiling heat transfer, and the boiling heattransfer coefficient increases as the number of twisted tapes. Moreover, the empirical formulafor sucrose solution in a CD tube with RSTT was obtained by the fitting of the experimentaldata. Based on the experimental study, the optimization of energy conservation formultiple-effect evaporation system was conducted. The evaporator of CD tubes with RSTThas higher boiling heat transfer coefficient. Therefore, the consumption of fresh streamreduced under the same total heat transfer temperature and same total heat transfer area. Theparameters for each effect were obtained after optimization by the appropriate numericalmodel.The flow boiling in a converging-diverging channel was investigated numerically. Theeffects of physical thermal properties on heat and mass transfer of the two phases wereconsidered. It is found that heat transfer coefficient, flow pressure drop and gas-phase volumefraction in a CD channel is bigger than that in a smooth channel. The distribution of gas-phasevolume fraction along the heated time was obtained by unsteady simulation. The effects ofvelocity on flow boiling were studied. Because of the more vigorous exercise and biggerbubble departure frequency of the bubbles near the converging surfaces, the boiling heattransfer in a CD channel is enhanced significantly.The natural convection in a CD square enclosure and a CD channel was researchednumerically. The effects of rib height and pitch ratio for CD surfaces on the naturalconvection and heat transfer were particularly studied. The distribution of temperature filedand velocity field in the CD square enclosure was presented under different Rayleigh number.Within the scope of the experimental conditions, the Nusselt number increases as the Rayleighnumber, rib height and pitch ratio. The empirical correlation for Nusselt number in theenclosure was obtained based on simulated data. For the natural convection in a CD channel,the Nusselt number increases as rib height and then decrease gradually. The resistance coefficient increases as rib height and rarely decreases as pitch ratio. Because of theenlargement of pitch ratio, the Nusselt number increases and then remains unchanged. Thereasons of heat transfer enhancement for natural convection in converging-diverging surfaceswere revealed by analysis of temperature field and velocity field, axial heat transferperformance and filed synergy.