ASHRAE ST-16-013-2016 Experimental Setup and Methodology on Active Mechanisms for Enhancing Heat and Mass Transfer in Sorption Fluids.pdf

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

25、everal aspects.There are several kinds of mechanism motions as optionsfor enhancing the chiller performance: rotation, agitation, andvibration. In these mechanism types, rotation is not preferredbecause it can introduce a centrifugal force on the film andthen break the surface balance of falling fil

26、m. It is also impos-sible to introduce agitation into a film-style absorption chiller.Compared to other mechanism motions, vibration is mucheasier to realize in industrial products. In our last paper(Behfar et al. 2014), we completed a literature review on theeffects of the vibration mechanism on ab

27、sorption chillers. Wefoundpositiveexperimentaldatatosupportthehypothesisthatmechanism motion enhances absorption performance. Mostof the literature introduced the vibration motion, while somepapers also discussed the rotation mechanism (Aoune andRamshaw 1999). The two most important elements for vib

28、ra-tion are frequency and amplitude. In the reviewed literature, itwas shown that absorption performance increased with ahigher frequency; however, the quantitative relationship wasnot established (Ellenberger and Krishna 2002). Meanwhile,wehavenotyetfoundanystudythatdemonstratedwhetheranexcessive f

29、requency will reduce heat and mass transfer.Besides, the absorption performance does not have a linearlyproportional relationship with amplitude. Liu et al. (2004)found that the performance actually decreased when theamplitude was higher than an optimum value. In their study,there was also a floor t

30、hreshold value for effective amplitude.At the beginning of the vibration below the floor threshold,there was no obvious enhancement caused by a vibration.After continually adding a vibration above the floor thresholdvalue, the enhancement from vibration was observed.Although the literature shows the

31、 effect of mechanism motionon heat and mass transfer, a comprehensive understanding onhow the frequency and amplitude, individually and jointly,impact the performance is very limited.The situation becomes more complicated when an additiveis present. Although the additives effect in LiBr solution is

32、stilldebated,themostacceptedtheoryisthattheadditivecouldreducesurface tension and arouse Marangoni convection (Frances andOjer 2004). Introducing enhancement mechanism motion mayalso increase the additive performance by increasing the turbu-lence on the surface (Marangoni convection) and correspond-

33、ingly increase the performance of absorption. Enhancementmechanism motion could further break the surface tension, thusmaking the falling film separate from the tubes more evenly.Currently, there is no literature discussing how enhancementmechanisms may affect the absorption under the condition with

34、andwithoutadditive.Becauseadditiveshavedifferentcharacter-istics, different kinds of enhancement mechanism motion mayPublished in ASHRAE Transactions, Volume 122, Part 2 124 ASHRAE Transactionsalsoaffecttheperformanceindifferentdegrees.Thus,continuousresearch is very needed to further explore the un

35、known cause/effect relationships with the aforementioned means for heat andmass transfer enhancement in sorption fluids.OBJECTIVES AND EXPERIMENTAL CHALLENGESUpon review of the findings, two main objectives are set forthis project:1. Develop mechanisms of motions that might enhance thecoupledheatand

36、masstransferprocessofabsorptionchillersand construct a test rig to evaluate them2. Identifytherelationshipsbetweentheheatandmasstransferand vibration amplitude and vibration frequency and otherinfluencing factorsItisexpectedthat,withthedesignedtestrigandexperiments,we can provide solid data with a w

37、ide range of vibration ampli-tude and frequency and operation parameters to serve Objective2. The comprehensive data could be used to develop modelingtools to quantify the vibration effect on absorption chillers. Thisanalysis will fill in the blank of our knowledge on heat and masstransfer enhanceme

38、nt of horizontal tube falling-film sorptionsystems.The challenges we are facing to accomplish Objective 1 ofconstructing the test rig are as follows:Mechanism motion and its fulfillment. Mechanism motionbasically includes vibration and rotation and it can be localon the absorber or expanded to the w

39、hole chiller. Mean-while, implementation of such a mechanism requires con-sideration of cost and feasibility. Potential negative impactto the chillers vacuum environment should be avoided.Measurements of the variables. The absorber is an enclosedcomponent of the closed-loop system and cannot be easi

40、lyseparated from other parts. Because the system is closedand in a vacuum, it is very difficult to measure the heat andmass transfer on the refrigerant side or obtain all the relatedvariables directly. In addition to the limited measurements,is the reliability and quality of the measurement and anal

41、y-sis results. Accuracy of the sensors should be checked toreduce the propagated error associated with the compo-nent-level and system-level heat and mass transfer analysis.Stable and repetitive environment. As a closed-loop sys-tem, any change on the four thermal components of thechiller could impa

42、ct the system operation. To serve Objec-tive 2, a controllable steady-state condition needs to bemaintained with and without mechanism motion during theoperation. A set of artificial thermal source and sink shouldbe constructed and controlled for this purpose.In the following section, we describe th

43、e solutions corre-sponding to the aforementioned three challenges.TEST RIG CONSTRUCTION METHODOLOGYTechnical Analysis and Mechanism MotionAsweanalyzedbefore,mechanismmotionmayreducethesolution film thickness and then reduce heat and mass transferresistance. At the same time, it may increase the drop

44、let drop-ping rate by reducing surface tension. On the other hand, mech-anism motion may also promote the additive effect andconsequently enhance the absorption performance. Becausemost absorption chillers use horizontal tubes, thinning out thefilm should let the motion happen in a vertical directio

45、n ratherthan a horizontal direction. A motion in the horizontal directiononly makes the film pendulate on the tubes but does not essen-tially change the film thickness.To introduce vibration into a system, there are manyoptions. One is to vibrate just the tubes in the absorber.However, this kind of

46、design will cause an issue to the vacuum,which is critical to the normal operation of absorption chillers.Asmallpressurelossintheabsorberwillresultinalargereduc-tion of the cooling performance (Xie et al. 2008). The seconddesign option is using solution to pulse the horizontal tubebundle for vibrati

47、on. However, it is very hard to control thevibrationamplitudeandfrequency.Consideringthecomplexityinvolved in absorption chillers, such as dimension variationvacuum requirement, different container arrangements, andtube connections, the best scheme at present is using commer-cial equipment to conduc

48、t the mechanism experiments andintroduce vibration to the entire chiller. Another reason forvibrating the whole chiller is that most commercial productscombine the absorber and evaporator, as well as the condenserand generator, into two containers, separately. Having aspecially designed absorption s

49、ystem and vibrating only theabsorbernotonlyincreasesthecost,butalsopreventsthegener-alization of the findings to commercial products. Meanwhile,separating the absorber from the evaporator into two containersincreases the pressure drop between them, which is not desiredin an absorption system.Absorption ChillerBased on the analysis, we chose RXZ (95/85)-1.2ZS typehot-water LiBr absorption chiller from a manufacturer. Thischiller con