ASHRAE ST-16-013-2016 Experimental Setup and Methodology on Active Mechanisms for Enhancing Heat and Mass Transfer in Sorption Fluids.pdf
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1、122 2016 ASHRAEThis paper is based on findings resulting from ASHRAE Research Project RP-1462.ABSTRACTEnhancement of heat and mass transfer in sorption fluidscould improve the overall performance of absorption chillers.Activemechanismsareproposedasapotentialeffectivemeansto achieve this goal. A test
2、ing facility is needed to evaluate theimpactontheperformanceofthechillerafteraddinganactivemechanism. The challenges of this project include the fulfill-ment of mechanism motion to drive extra heat and mass trans-fer in an absorber, the measurement of related variables, andthe stability and repeatab
3、ility of findings. These challengescome from the fact that absorption chillers are closed-loopsystemswithlargeheatexchangers,havelowinsidepressures,andcansustainonlysmallpressuredropsalongtherefrigerantloop. Measures are needed to prevent the impact of vibrationon untargeted components in the system
4、. In this paper, weintroduce the details of the lab construction methodology,including the vibration table, the auxiliary water loop system,and the measuring instruments. Then, we present severalexamples to show the operation and testing procedure andstabilityofthetestsystem.Last,theexperimentplanma
5、trixandanalysis methodology are presented, which will be applied inthe next-phase experiment. The fulfillment of active mecha-nism and test methodology with a closed absorption chiller forperformance analysis can be references in similar thermalapplications.INTRODUCTIONAs a thermal-driven cooling de
6、vice, absorption chillersare considered great replacements to conventional electricity-driven vapor compression cycle (VCC) chillers. Despite theirrelatively low coefficient of performance (COP), absorptionchillers offer the unmatchable advantages of potentiallyutilizing a large portion of low-grade
7、 heat andenvironmentally benign working fluids. When integrated incombined cooling, heating, and power systems, absorptionchillers have the benefits of recovering waste heat, reducingpeak demand on electricity, and abating adverse effects on theenvironment.Numerous studies have shown the broad devel
8、opmentprospects and opportunities of using lithium bromide (LiBr)absorption chillers for thermal-comfort-oriented heating andcooling in buildings (Ali et al. 2008; Reay 2002; Shitara andNishiyama 1995). However, with respect to overallperformance and economical concerns, it is desired to ensurethat
9、absorption chillers have a more competitive COPcompared to their counterpart, electric VCC chillers. Mostcommercial absorption chiller COPs are from 0.6 to 1.1 whileVCC chiller COPs are about 3.0 to 6.0. Among the four maincomponents (a generator, a condenser, an evaporator, and anabsorber), the abs
10、orber is a key component that significantlyimpacts the overall chiller performance. By continuouslyremoving the boiled vapor out of the evaporator, the absorbercreates required low pressure in the closed loop and ensuresthe continuous operation of the system. Its structure andperformance determine t
11、he overall size, capacity andefficiency of the chiller. In addition, absorber performancealso affects the units cost and operating economics.Therefore, improving the performance in terms of heat andmass transfer efficiency in the absorber is an important issuefacing academia and industry.Experimenta
12、l Setup and Methodology onActive Mechanisms for Enhancing Heat andMass Transfer in Sorption FluidsZiqi Shen Yuebin Yu, PhD Josephine Lau, PhDStudent Member ASHRAE Associate Member ASHRAE Member ASHRAEZiqiShenisamastersstudentandYuebinYuandJosephineLauareassociateprofessorsattheDurhamSchoolofArchitec
13、turalEngineeringand Construction, University of Nebraska-Lincoln, Omaha, NE.ST-16-013 (RP-1462)Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 123A falling-film absorber is presently the most commonlyacceptedstructureforLiBrabsorptionchillers.Inanabsorber,the LiBr solution f
14、lows through the heat exchange tubebundles from the top of the exchanger to form the falling filmand maximize the solution exposure to the ambient watervapor coming from the evaporator. Depending on the flowrates, the mode of the falling film can be dripping, jet column,or fluid sheet. An aqueous so
15、lution of LiBr exhibits complexheat and mass transfer processes in a falling-film absorber.Many factors, from the structure to the operation, directly orindirectly impact absorber heat and mass transfer perfor-mance. These factors include the spray solutions concentra-tion, density, and temperature;
16、 cooling-water flow rate andtemperature; chamber pressure; tube diameter and arrange-ment; wall thickness; and surface conditions.Meanwhile, in falling-film absorption experiments,researchers discovered a noteworthy phenomenon caused byusing additives in the solution (Tomforde and Luke 2012).When an
17、 additive such as ethylhexyl is added as gaseoussurfactant of the falling-film absorption, a smooth falling filmwill quickly become turbulent. This phenomenon shows thatthe additive is primarily from the gas side, and gaseous addi-tivesarethedirectcauseofdisturbance(KulankaraandHerold2002), which en
18、hances heat and mass transfer. In the absorp-tion process, additives with the refrigerant vapor flow to theliquidfilmandadsorbonthefilmsurface.Thisoccursbecausethe two phases are not balanced. Additive in the liquid filmsurface is unequally distributed, resulting in an imbalance ofsurfacetensiongrad
19、ientandgeneratingmorepowerfulsurfaceflow, so heat and mass transfer is enhanced.MECHANISM MOTION ONABSORPTION ENHANCEMENTIn previous studies, researchers conducted various exper-iments to demonstrate that vibration could reduce the surfacetension and film equilibrium (Aoune and Ramshaw 1999;Cheng et
20、 al. 2009; Ellenberger and Krishna 2002; Liu et al.2004). They found films with less thickness were formed andthe surface of film presented some fluctuation after introduc-ing vibration. These two phenomena are beneficial to anabsorption process. A thinner film reduces the heat transferresistance th
21、rough the film and a fluctuating surface increasesmasstransferacrossthefilmliquid-vaporinterface.IftheLiBrsolution remains on the falling film too long after saturation,the system would have no future performance improvementduringtheabsorptionandsaturationperiod.Withagivenflowrate, a thinner film me
22、ans the droplets fall sooner from the topto the end of the absorber. Making the falling-film droplet fallsooner after it is saturated is one of the investigations inattempt to increase the chiller performance in our experiment.Enhancingheattransferwouldbenefitthemasstransfercorre-spondingly. Some re
23、searchers also introduced other kinds ofenhancement mechanism motions in absorption cooling andheating devices. For example, Reay (2002) got a higher trans-fer coefficient by using a plate-fin-type absorber with a rota-tion device and greatly reduced the absorber size. In anotherstudy, researchers d
24、emonstrated that significant enhancementof gas-liquid mass transfer in bubble columns can be achievedby subjecting the liquid phase to low-frequency vibrations(Ellenberger and Krishna 2002). In summary, it is expectedthat “thinning out” the falling film could enhance the heat andmass transfer from s
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