Study On Single-and Two-phase Flow And Heat Transfer Characteristics In Microchannels With Periodic Expansion-constriction Cross Sections

With the gradual perfection of micromachining technology in the new century,the area of international heat transfer has begun to meet new challenge, that is,developing the second-generation microchannel heat transfer technology, whichstructure is more complex. The microchannel with periodic expansion-constrictioncross sections is one of them. On one hand, as the cross section of the microchannelchanging periodically, the boundary layer will be intermitted periodically, which canenhance heat transfer; On the other hand, the periodic variation of transverse crosssection will influence the gas-liquid two-phase flow pattern, and then exert importanteffects on the flow and heat transfer characteristics. Although a great deal of results,made by traditional microchannel with constant transverse cross section, provide a lotof valuable information for microchannel with periodic expansion-constriction crosssections, we are still unclear whether the results of the flow and heat transfercharacteristics from traditional microchannel can be suitable for the microchannelwith periodic expansion-constriction cross sections. A lot of questions should beclarified through further research. Therefore, this paper has further studied on thecharacteristics of the single-phase and gas-liquid two-phase flow and heat transfer inthe microchannel with periodic expansion-constriction cross sections.Firstly, based on the microscale heat transfer enhancement mechanism, a varietyof microchannels with periodic expansion-constriction cross sections have beendesigned and processed. By combining the rectangular straight microchannel andadopting the experimental and numerical calculation methods, the fluid flow and heattransfer characteristics of water in the microchannels with periodicexpansion-constriction cross sections have been studied in detail and in depth. Themajor conclusions can be drawn as follows. The transition from laminar to turbulentflow occurs at a transition Reynolds number of1000-1100for the microchannels, andthe reentrant cavities just play a marginal role on the transition process. With anincrease of the Reynolds number, the apparent friction factor for the microchannelswith expansion-constriction cross sections increase rapidly and are obviously higherthan those of the straight microchannel. In the lower Reynolds number range, the heattransfer of the microchannals with expansion-constriction cross sections are not asgood as that of the straight microchannel. In the higher Reynolds number range, theheat transfer enhancement of the microchannals with expansion-constriction cross sections have an advantage over that of the straight microchannel. The heat transferenhancement mechanism can be contributed to: The second flow in the reentrantcavity brings about chaotic advection and enhances the convective fluid mixing; Thethermal boundary layer is interrupted periodically and redevelops along the constantcross-section segment; due to the periodic expansion-constriction cross section, theheat transfer surface area has been increased.Secondly, three–dimensional CFD simulations of laminar flow and heat transferin microchannels with different operation and geometry parameters have beenperformed under constant wall heat flux at the substrate for the microchannel withperiodic expansion-constriction cross sections. The paper has studied the effects of aseries of characteristics on the properties of the flow and heat transfer, such as theshape and size of microchannel, the variation of transverse cross section area, thechange of flow field and fluid velocity, etc. Based on the thermal enhancement factorperformances, the optimal geometric parameters are obtained in principle for themicrochannels with periodic expansion-constriction cross sections. And the resultsshow that: When the Reynolds number is lower, the water in the reentrant cavityflows so slow that the main flow slip over the reentrant cavities which reducespressure drop but drastically decreases heat transfer mainly attributing to thesignificantly lower conduction levels. When the Reynolds number is larger, thecontribution of the developing thermal boundary layer and the fluid mixingenhancement effect enlarge heat transfer significantly. There are three most notableeffects of the interrupted microchannel with rectangular ribs in the transversemicrochambers on the mainstream flow, including mainstream flow separation,recirculation or vortex, interrupted boundary layer. The rectangular rib not only canpromote the efficient mixing of the cold mainstream cold fluid and the hot secondary,but also interrupt the thermal boundary layer to improve the convective heat transfercoefficient. When the Reynolds number is lower, the rectangular rib can significantlyimprove the heat transfer characteristics. With the Reynolds number is larger, the heattransfer enhancement due the rectangular rib gradually reduces.Furthermore, based on the rectangular straight microchannel, the gas-liquidtwo-phase flow pattern and pressure drop characteristics in the microchannel withperiodic expansion-constriction cross sections are investigated experimentally bymeans of the IDT high-speed camera mounted together with a Nikon microscope. Thegas-liquid two-phase pressure drop characteristics were analyzed by the homogeneous flow model and the separated flow model. The results show that, due to the periodicexpansion-constriction cross section, the intermittent sub flow pattern is very differentfrom that of the straight microchannel. With the change of the gas-liquid two-phasesuperficial velocity, the annular/single-phase alternating flow pattern, theslug/annular/single-phase alternating flow pattern, the slug/single-phase alternatingflow pattern, mist/slug alternating flow pattern and mist/single-phase alternating flowpattern presents in turn. The two-phase flow regime maps for the four microchannelsare obtained. The homogeneous flow model can not predict the the two-phase frictionmultiplier data. Despite the separated flow model introducing the gas-liquidtwo-phase interaction to a certain extent, the gas-liquid two-phase movement andspace distribution in the microchannel with periodic expansion-constriction crosssections can not be predicted accurately.Finally, in virtue of the experimental research and numerical simulation, thegas-liquid two-phase flow patterns and changes along the flow direction in the PDMSsingle microchannels have been studied. The observed flow patterns are intermittentflow and separated flow. For intermittent flow, the gas scatters in the liquid phase orthe liquid scatters in the gas phase, and the two forms appears in turn. For separatedflow, the gas flows mainly along the wall side of gas inlet, and the liquid flows mainlyalong the liquid side of water inlet. There is a obvious interface between the twophases and the interface fluctuates along the flow direction.