IMPROVING THE PERFORMANCE OF A THERMAL COMPRESSOR IN A STEAM EVAPORATOR VIA CFD ANALYSIS

Proceedngs of 29 ASME Internatonal Mechancal Engneerng Congress & Exposton IMECE29 November 13-19, Lae Buena Vsta, Florda IMECE29-12651 IMPROVING THE PERFORMANCE OF A THERMAL COMPRESSOR IN A STEAM EVAPORATOR VIA CFD ANALYSIS Xanchang L Department of Mechancal Engneerng Lamar Unversty Beaumont, TX 7771 Tng Wang and Benamn Day Energy Converson and Conservaton Center Unversty of New Orleans New Orleans, LA 7148 ABSTRACT Eectors have been wdely used n many applcatons such as water desalnaton, steam turbne power generaton, refrgeraton systems, and chemcal plants. The advantage of an eector system les n ts extremely relable and stable operaton due to the complete absence of movng parts. However, the performance depends on a number of factors, among whch the flow channel confguraton/arrangement s very crtcal. A comprehensve study manly focusng on the senstvty of performance on the geometrc arrangement was conducted n ths paper to mprove an exstng thermal compressor performance n a steam evaporator. The performance s measured by the sucton (secondary) flow rate of the prmary steam et from a low-pressure vapor plenum. Numercal smulaton s employed to nvestgate the thermalflow behavor. It s observed that any downstream resstance wll serously mpede the sucton flow rate. In addton, the sucton rate s found to be senstve to the locaton of the et ext, and there s an optmum locaton where the et should be ssued. A well-contoured dffuser can ncrease the sucton rate sgnfcantly. However, the sze of sucton openng to the plenum s less mportant, and a contoured annular passage to gude the entraned flow shows lttle effect on the overall performance. Based on the numercal results the steam sucton rate of the best case n the confnement of the current study s approxmately 43% the et flow rate, whle some cases wth medocre desgn can only produce an entranment of 24% the et flow. Keywords: Eector, thermal compressor, performance smulaton NOMENCLATURE c p specfc heat (J/g-K) D eector ppe dameter (m) d dameter of steam et or contracton cones (m) turbulence netc energy (m 2 /s 2 ) h convectve heat transfer coeffcent (W/m 2 -K) L, l length (m) m& mass flow rate (g/s) P pressure (N/m 2 ) Pr Prandtl number, ν/α Re Reynolds number, ud/ν T temperature (K, o F) u streamwse velocty component (m/s) u', T' turbulence fluctuaton terms v radal velocty component (m/s) x, y coordnates Gree α thermal dffusvty (m 2 /s) ε turbulence dsspaton rate (m 2 /s 3 ) η sucton rate, m & s / m& λ heat conductvty (W/m-K) μ dynamc vscosty (g/m-s) ν nematc vscosty (m 2 /s) ρ densty (g/m 3 ) τ stress tensor (g/m-s 2 ) Subscrpt et s secondary flow t turbulent

1. INTRODUCTION An eector generally utlzes the momentum of a hghvelocty prmary et of vapor to entran and accelerate a medum n stll or movng n a low speed. The functons of an eector nclude mantanng vacuum n evaporaton, removng ar from condensers as a vacuum pump, augmentng thrust, or ncreasng vapor pressure as a thermal compressor. The thermal compressor s an eector, but t further utlzes the thermal energy to augment the performance by reducng the sze of a conventonal mult-stage evaporator. Eectors have been wdely appled n water desalnaton plants, steam turbnes, geothermal power plants, refrgeraton systems, and chemcal plants. The applcaton of eectors n these felds has a long hstory. For example, for many years, steam et eectors have been the drvng force to produce vacuum n the chemcal process ndustres. Power [1] systematcally documented the worng mechansm of eector stages and dscussed the engneerng calculatons. A revew was also gven n [1] to steam-et refrgeraton, steam-et and gas-et compressors, lqud et eductors, and desuperheaters. As the pressure for hgh energy effcency ncreases, a number of studes have been conducted to mprove the performance of an eector or a system wth eectors. The aerothermal performance of an eector can be defned as the vacuum mantaned or the amount of medum removed through a certan hgh-pressure flow. The eector performance depends on a number of factors, and the flow channel desgn/arrangement s very crtcal. For example, Balamurugan et al. [2] performed detaled experments as well as numercal smulaton to understand the hydrodynamc characterstcs of the eector geometry. It was shown that there s an optmum rato of nozzle area to throat area (area rato) at whch the lqud entranment rate s the hghest. For a wde varety of eector geometres and operatng condtons, the lqud entranment rate correlates wth the pressure dfference between the water surface n the sucton chamber and the throat ext. For a steam-eector refrgeraton system, Chen and Sun [3] expermentally nvestgated the controllng parameters of a steam eector, ncludng operatng condtons and the ext Mach number of the prmary nozzle. Operaton maps were constructed to help the desgn of steam-eectors and from the expermental results the emprcal equatons were correspondngly derved. It was reported that an excessvely hgh Mach number at the prmary nozzle ext s not necessary, and a moderate value should be 4.35. Rusly et al. [4] modeled several eector desgns usng fnte volume CFD technques to resolve the flow dynamcs n the eectors. Flow condtons resultng from eector geometry varatons were dscussed. It was found that the maxmum entranment rato happens n the eector ust before a shoc occurs and the poston of the nozzle s an mportant eector desgn parameter. Panthong et al. [5] predcted numercally the flow phenomena and performance of two types of steam eectors used n a refrgeraton system. They concluded that the computatonal flud dynamcs (CFD) can predct eector performance very well and revealed the effect of operatng condtons on an effectve area s drectly related to ts performance. In addton, t was found that the flow pattern does not depend much on the sucton zone. Rffat and Evertt [6] desgned a steam et eector for ar condtonng n a small motor vehcle. The coolant and/or exhaust heat from the engne was used to drve the eector cycle. Expermental tests and numercal smulaton were used to valdate the desgn and determne the performance. Research has also been conducted for applcaton of eectors n desalnaton. J et al. [7] establshed a smulaton model of steam eectors used n a thermal vapor compresson desalnaton system. The performance under dfferent operatng condtons was then calculated. The results ndcated that entranment rato decreased wth ncreasng compresson pressure when the compresson pressure exceeded desgn value, and the entranment rato mantaned at a constant value when compresson vapor pressure was lower than the desgn value. A motve steam pressure ether lower or hgher than the desgn pont could result n bad performance. Elady et al. [8] presented an expermental nvestgaton of the performance of the eector used n desalnaton applcatons wth ar as the worng flud. The effect of the operatng condtons, eector geometry, and the relatve poston of the prmary nozzle ext wthn the mxng chamber on the eector performance was studed. The results showed that usng a convergent-dvergent nozzle enhances the performance of the eector. The entranment rato ncreases wth ncreasng prmary pressure, and the entraned flow reaches a maxmum at a certan prmary pressure. Recently, Negeed [9] nvestgated the performance of the eector by usng numercal smulaton (CFD). The eector s for desalnaton applcatons and the worng flud s ar. The effect of operatng condtons and eector geometry on eector performance was examned. The results showed that eector performance ncreases by ncreasng the prmary nozzle throat dameter, and the entraned flow reaches a maxmum at a certan dameter. The optmum poston wth respect to the eector's mxng chamber and the mxng length were determned. Dscusson on fundamental flow feld and control of an eector s also seen n the publc doman. Km et al. [1] conducted a numercal study on the flow characterstcs nsde a varable eector. The varable eector s used to obtan a specfc recrculaton rato under a gven operatng pressure rato by varyng the eector throat area rato. The sonc and supersonc nozzles were adopted as prmary drvng nozzles, and a movable cone cylnder, nserted nto a conventonal eector-dffuser system, was used to change the eector throat area rato. Km et al. [11] nvestgated the nfluence of varous geometrc parameters and pressure ratos on the Coanda eector performance. In Coanda eectors, the secondary flow s dragged nto the eector, followng a curved contour wthout separaton, due to the prmary flow momentum. The effect of varous factors, such as the pressure rato, prmary nozzle and eector confguratons on the system performance was examned. It was found that the performance of the Coanda eector strongly depends on the prmary nozzle confguraton and the pressure rato. The mxng layer growth plays a maor role n optmzng the performance of the Coanda eector as t 2 Copyrght 29 by ASME

decdes the rato of secondary mass flow rate to prmary mass flow rate and the mxng length. Srveeraul et al. [12] studed the flow mxng process of a steam eector used n a et refrgeraton cycle by usng the software pacage (FLUENT). The flow structure of the modeled eectors was analyzed when ts operatng condtons and geometres were vared. A comprehensve study was conducted n ths study to mprove the performance of an exstng thermal compressor n a steam evaporator. The system s desgned to dstll brne water effectvely wth a 2-effect evaporator. Tradtonally, the multple-stage thermal compressors that apply the vacuum sucton prncple (Bernoull's prncple) can be used to ncrease the steam flow exchange rate va a seres of steam ets. The 2-effect evaporator s to reduce the conventonal system s sze from eght stages to two stages wth a smlar captal cost. The desgned output s 16 gallons/mn (.1 m 3 /s) by usng a steam flow of 1, lbm/hr at 45 psg. However, after nstallaton the actual output was only 47-6 gallon/mn, whch s sgnfcantly lower than the desgned value. Tths study s to employ CFD smulaton to nvestgate the thermal-flow behavor and the reason of falure and to provde an mproved desgn to acheve the best performance under the current space constrant. The performance s measured by the sucton (secondary) flow rate of the prmary steam et from a low-pressure vapor plenum. More specfcally, the effects of the et ext locaton, dffuser contour, sucton openng sze, and downstream resstance on the eector sucton flow rate are explored. 2. GOVERNING EQUATIONS AND NUMERICAL METHOD The tme-averaged steady-state Naver-Stoes equatons as well as the equatons for mass and energy for compressble flows need to be solved, and the equatons are gven as follows. ( ρu ) = (1) P ( ρu u ) = ρg + ( τ - ρu' u' ) v (2) T ( ρc u T) = λ - ρc u' T' + μφ p p x x x (3) where τ s the symmetrc stress tensor defned as u u 2 u τ = + μ δ. (4) 3 μφ s the vscous dsspaton and λ s the heat conductvty. The terms of ρ u' and ρc pu' T' represent the Reynolds stresses u' and turbulent heat fluxes, respectvely. These two terms need be modeled properly for a turbulent flow. Turbulence Model A wdely appled group of turbulence models are the -ε model, whch, based on the Boussnesq hypothess, relates the Reynolds stresses to the mean velocty as u u 2 u 2 = δ ρδ (5) 3 - ρu' u' μt + 3 where s the turbulent netc energy and μ t s the turbulent vscosty. Correspondngly, the turbulent heat fluxes can be modeled wth the turbulent heat conductvty (λ t ) as gven by ρc u' T' p T μ T t = λt = cp, (6) Prt where Pr t s the turbulence Prandtl number. The standard -ε model s manly vald for fully-turbulent flows wth hgh Reynolds number. To smulate flows wth both hghly turbulent and low velocty lamnar zones, the RNG-based -ε model s a better opton [13]. The RNG -ε model was derved usng a renormalzaton group theory, a statstcal technque, although the equatons are n the smlar form to the standard -ε model. However, the RNG theory provdes an analytcally-derved dfferental formula for the effectve vscosty that accounts for low-reynolds-number effects, and an analytcal formula for turbulent Prandtl numbers. The equatons for the turbulent netc energy () and the dsspaton rate (ε) are: ( ρu) = αμeff + G ρε + Y. (7) M 2 ε ε ε ( ρu ε) = αεμ eff + C1εG C2ερ R. (8) ε The term G s the generaton of turbulence netc energy due to the mean velocty gradents, and Y M represents the contrbuton of the fluctuatng dlataton n compressble turbulence to the overall dsspaton rate. α and α ε are the nverse effectve Prandtl numbers for and ε, respectvely. R ε n the ε equaton s an addtonal term to the standard -ε model. Ths term s desgned to sgnfcantly mprove the accuracy for rapdly straned flows. Note the and ε equatons n a general format may nclude more terms to tae nto account the effect of buoyant effect as well as extra sources. For hgh-reynolds-number flows, the formula for turbulent vscosty s reduced to 2 μ t = ρc μ / ε (9) where the constant C μ s.845, whle ths constant s.9 for the standard -ε model. The constants C 1ε and C 2ε n the RNG model are 1.42 and 1.68, respectvely. Enhanced Wall Functon Effectve use of the RNG model depends on an approprate treatment of the near-wall regon. Specal treatment s needed n the regon close to the wall. The enhanced wall functon s one of the methods that model the near-wall flow. In the enhanced wall treatment, the two-layer model s combned wth the wall functons. The whole doman s separated nto a vscosty-affected regon and a fully turbulent regon by defnng a turbulent Reynolds number, Re y, 1/2 Rey = y / ν (1) 3 Copyrght 29 by ASME

where s the turbulence netc energy and y s the dstance from the wall. The two-equaton -ε model s used n the fully turbulent regon where Re y > 2, and the one-equaton model of Wolfsten [14] s used n the vscosty-affected regon wth Re y < 2. The turbulent vscostes calculated from these two regons are blended wth a blendng functon (θ) to smoothen the transton. μ t, enhanced θμ t + (1 θ)μ t,l = (11) where μ t s the vscosty from the -ε model, and μ t,l s the vscosty from the near-wall one-equaton model. The blendng functon s defned so that t s equal to at the wall and 1 n the fully turbulent regon. The lnear (lamnar) and logarthmc (turbulent) laws of the wall are also blended to mae the wall functons applcable throughout the entre nearwall regon. The smlar approach to the near-wall temperature dstrbuton s also employed. More detals can be seen n [15]. Numercal Method The commercal software pacage Fluent (verson 6.3.26) s used n ths study. The governng equatons are dscretzed by the fnte volume method, and the second order upwnd scheme s adopted for spatal dscretzaton of the convecton terms [15]. The segregated solver s used n the smulaton, and the SIMPLE algorthm (Patanar [16]) s employed to couple the pressure and velocty. The convergence crtera of teratve soluton have been nsured when the resdual of all varables are less than specfc values. The value s 1-4 for contnuty and momentum, and 1-6 for energy. 3. GEOMETRY AND BOUNDARY CONDITIONS As documented n the later part of ths paper, the exstng desgn s frst analyzed, followed by dfferent modfcatons on the geometrc confguraton to reveal the senstvty of the performance to dfferent flow channel arrangements. Based on analyss of the feld data va estmated energy and mass balances, the sucton flow rate s very low -- only about 24% of the desgned value, whch should be about 3.5 tmes of the et flow rate. The smplfed geometry of the exstng desgn s shown n Fgure 1(a). It was assumed the flow s axsymmetrc. The total length of the ppe s L=6.3 m (248 nches) wth a dameter of D=.58 m (2 nches). The dameter of the et s d=.58 m (2 nches), nectng from the same locaton as the contracton cone entrance n the manstream drecton. The frst contracton cone s close to the steam et and has a length of l 1 =.559 m (22 nches). The other two downstream contracton cones have a length of.533 m (21 nches) each (l 2 =l 3 ). The ext dameters of the three contracton cones (d 1, d 2, and d 3 ) are.127,.112, and.12 m (or 5., 4.4 and 4. nches), respectvely. The left cone s located at L 1 =.711 m (28 nches) from the left end of the ppe. The dstance between the other two cones s.58m (2 nches) (L 2 ). Startng from the left end, the sucton openng s.61 m (24 nches) (L 3 ). Fgure 1 (b) shows the geometrc scheme of the best case by combnng all the favorable features, a contoured contracton cone wth a long dffuser added downstream (66 nches or 1.68 m) and a transton pece (4 nches or.1 m) between the contracton and the dffuser. The dffuser has a dffuson angle of 6.5 o. There s no contracton cone downstream. Sucton Openng L 1 Sucton Openng Steam Jet (d) Cone 2 φ=d 31 φ=d 2 D Cone 1 l 1 l 2 L Steam Jet Outlet Contoured contracton cone Transton secton (.1 m) Dffuser l=1.68m (a) Outlet (d 3 ) Cone 3 (b) Fgure 1: Geometrc scheme of (a) the exstng desgn and (b) the case wth best performance For numercal analyss, the sucton openng and the outlet are at a constant pressure (atmosphere). The et velocty s 1 m/s wth a total flow rate of.99 g/s. The steam et s assumed to be at 45 K (35 o F), and the sucton flow has a temperature of K (212 o F). These parameters gve a et Reynolds number (Re) of 135,556, whch means the flow s turbulent. Heat transfer between the et and sucton flow s calculated, and the compressblty effect s also consdered. Notce that n the real stuaton the et mass flow rate s hgher due to ts hgher pressure. It s beleved that the mechansm of sucton presented n ths study can be appled to the real system wth hgher pressures. Because the crtcal factor s the pressure dfference, the actual pressure plays a secondary role. 4. MESHING AND GRID INDEPENDENCE STUDY The characterstcs of baselne case meshng are shown n Fg. 2. The non-unform unstructured mesh s employed n the whole doman. There are more cells close to the regons wth large velocty or pressure gradents. The total number of cells n ths study s about 11,184. Grd ndependence s examned for all the cases, and one of the examples s shown n Table 1. It s observed that when the cell number ncreases to 44,736, the ext mass flow rate and the maxmum pressure change only by less than 1.%. Therefore, t can be proven that the overall performance s not affected by the grd and the results are ndependent of the employed mesh numbers. 6.5 o 4 Copyrght 29 by ASME

Grd close to the et of the ntal desgn Overall grds for the optmzed desgn Grd close to the et of the optmzed desgn Fgure 2: Computatonal meshes for dfferent cases Table 1: Overall performance wth dfferent grd systems Cases Coarse Grd Fne Grd Number of cells 11184 44736 Flow rate at ext (g/s).123.122 Maxmum statc pressure (Pa) 375 5. RESULTS AND DISCUSSION 5.1 Baselne case: Exstng Desgn The computed pressure and temperature felds of the exstng desgn are shown n Fg. 3, and the velocty feld and stream functon dstrbuton are shown n Fg. 4. -79.2 Statc Pressure Temperature Exstng desgn Jet flow rate:.99 g/s Sucton flow rate:.24 g/s Pa 45 K Fgure 3: Pressure and temperature felds of the exstng desgn Velocty Vector The fgures show the statc pressure s hgh between the frst and second contracton cones. The hgh temperature et mxes wth the cool entraned (suced) steam to become a moderate temperature flow. Strong recrculaton occurs nsde the contracton cones and n the suddenly opened secton mmedately downstream of the contracton cones. The recrculaton sgnfes neffcent aerodynamc performance and ncreased pressure losses as well as entropy producton. The smulaton result ndcates that the sucton flow rate s only about 24% of the steam et flow rate, whch s sgnfcantly lower than the desgned value of 35% of the et flow rate. A sucton rate can be defned as η = m & / m& (12) s It s essentally the rato of mass flow rate of secondary (sucton) flow and the et flow. The low sucton rate s consdered as the man culprt of the low output of dstlled water. Therefore, t s necessary to explore parameters that can potentally affect the sucton flow rate. Almost more than 2 cases have been smulated, and only the cases wth favorable results are presented next. 5.2 Effect of Downstream Contracton Cones The effect of downstream contracton cones s conducted by removng one of the downstream cones each tme. Fgure 5 shows that the sucton rate s ncreased from 24% to 5% by removng one cone, and removng both the cones downstream can lft the sucton rate to 14% -- a sx-fold augmentaton! The reverse flow nsde the contracton s apparently weaened; however, the flow recrculaton downstream of the contracton stll occurs, and the area of recrculaton becomes even larger after the cone s removed. Wth both downstream contracton cones beng removed, the sucton flow rate ncreases, and the temperature of the mxed flow becomes lower, as shown n Fg. 5(b). Sucton rate: 24% Sucton rate: 5% Sucton rate: 14% (a) Stream functon Sucton rate: 24% Stream Functon 15 m/s Sucton rate: 5% Jet flow rate: Sucton flow rate:.99 g/s.24 g/s Fgure 4: Velocty feld and stream functon of the exstng desgn Sucton rate: 14% (b) Temperature 45 K Fgure 5: Effect of downstream resstance on the eector flow pattern and sucton rate 5 Copyrght 29 by ASME

From these results, t can be clearly seen that the downstream contracton cones do not provde addtonal momentum transfer or sucton power as orgnally desgned. Instead, they adversely create hgh flow resstance and sgnfcantly mpede the sucton performance of the frst-stage steam et. 5.3 Effect of Addng a Downstream Dffuser To reduce the flow recrculaton downstream the contracton cone, a dffuser s added nto the ppe followng the desgn concept of the standard Ventur nozzle. The length of the dffuser s 1.68 m (66 nches), whch gves a dffusng angle of 6.5 o to avod flow separaton near the wall. Fgure 6 shows the comparson between the cases wth and wthout the downstream dffuser. The flow recrculaton area s oblvous n the downstream of the dffuser. The flow separaton s neglgble nsde the dffuser. As a result, the sucton rate s ncreased to 365%, a 2.6-fold ncrease from the case wthout the dffuser and 15.2 tmes more than the exstng desgn. The temperature of mxed flow becomes even lower due to the hgh sucton flow rate. Based on these results, t can be concluded that employng a downstream dffuser to reduce aerodynamc losses s extremely mportant. It s not dffcult to completely remove the flow separaton by reducng the dffuser s ncluded angle to below 6 o, whch wll mae the dffuser even longer. (a) Stream functon Reverse flow Sucton rate: 14% Sucton rate: 365% Sucton rate: 14% Sucton rate: 365% 45 K 15 m/s (b) Temperature and velocty vector Fgure 6: Effect of downstream dffuser on the eector flow pattern and sucton rate progressvely. Fgure 7 shows the results of the two cases: one s wth the steam et located n the plane of cone entrance, and the other case s wth the steam et beng moved half way nto the contracton cone. Both cases nclude one downstream contracton cone. It s seen that the reverse flow becomes stronger n the second case, whch results n a reducton of sucton flow rate from 5% to 18%. After comparng ths wth many other cases, t s concluded that the best result occurs by placng the steam et necton rght at the centerlne at the contracton cone s entrance. A slght dsplacement of the et wll not result n any sgnfcant change n the sucton flow rate. (a) Stream functon Sucton rate: 5% Sucton rate: 5% Sucton rate: 18% Sucton rate: 18% 15 m/s (b) Velocty vector Fgure 7: Effect of et locaton on the flow feld and eector performance 5.5 Effect of the Contracton Cone Wall Contour Whle the smple straght-wall contracton cone and dffuser are easer to manufacture than the contoured ones, the potental enhancement of the sucton flow rate s nvestgated by addng contoured wall to the contracton cone. Fgure 8 shows the results wth a modfed cone and dffuser geometry. The contracton cone has a contoured wall, and a small secton of straght transton (4 nches) s added between the cone and dffuser to mae the transton from the convergent cone to the dvergent cone more smoothly. The result ndcates the contoured contracton cone and the added transton pece ndeed ncrease the sucton rate about 2% from 365% to 43%. Note that due to the hgh sucton flow, the maxmum velocty s hgher than the et velocty n ths case. Sucton rate: 365% 5.4 Effect of the Locaton of the Steam Jet To search for a more effectve sucton, the effect of the locaton of the steam et ext s examned. Several cases are studed by movng the steam nectng locaton from at the contracton cone entrance away and nto the contracton (a) Stream functon Sucton rate: 43% 6 Copyrght 29 by ASME

Sucton rate: 365% Sucton rate: 43% 124m/s (b) Velocty vector Fgure 8: Effect of the contracton cone contour on the eector performance 5.6 Effect of the Sucton Openng Sze The sucton openng s the secton connectng the evaporatng volume to the thermal eector. The openng s ntally desgned nto a cut-through secton on the chamber wall housng the steam nector and the contracton cone. The openng s.61-m (24 nch) long and cut-through about one half of the ppe surface. As an approxmaton, the sucton openng s treated as an axsymmetrc openng slot durng the smulaton. Snce t s not clear f the openng sze would affect the sucton flow rate, the effect of the sucton openng sze s examned. Two cases are compared: Case (a) has a baselne openng sze, and Case (b) has a smaller sucton openng (1/3 of the baselne openng). The results n Fg. 9 ndcate that the stream functon as well as the velocty vector does not change much n these two cases. The correspondng sucton flow rates are almost the same at 365%. Bg sucton nlet Small sucton nlet (a) Stream functon Bg sucton nlet Stream functon Stream functon the exstng contracton wall and the steam et necton tube. Two cases are compared: one wthout and the other wth the flow gude. Both cases have the downstream dffuser and a downstream contracton cone. The results n Fg. 1 show that these two cases have smlar sucton rate: 14% (.13 g/s) n the frst case versus 99% (.982 g/s) n the second. It can be seen that the beneft of addng the flow gude s neglgble and s not worth the trouble of beng added. (K) 45 Sucton rate: 14% (m/s) 14 (a) Temperature and velocty wthout sucton flow gude (K) 45 Sucton rate: 99% (b) Temperature and velocty vector wth sucton flow gude Fgure 1: Comparson of the eector performance wth and wthout sucton flow gude 5.8 The Optmal Case By combnng all the favorable features together, an optmal case s smulated wth a contoured contracton cone, a long dffuser downstream, a transton pece added between the contracton and the dffuser, and all downstream contracton cones removed. The result n Fg. 11 shows the smooth flow feld wth a mnmzed flow recrculaton zone. The sucton flow rate s.4233 g/s or 43% the et flow rate. Fgure 12 compares the dstrbutons of pressure and temperature along the eector centerlne between the optmal case and exstng desgn. A sgnfcant deference can be clearly seen. Further optmzaton can be conducted. Crculaton (m/s) 15 Stream functon Small sucton nlet Statc pressure Sucton rate: 365% 124 m/s (b) Velocty vector Fgure 9: Effect of the sucton openng sze 5.7 Effect of Addng a Sucton Flow Gude Consderng the large flow recrculaton near the contracton entrance, a contoured flow gude s added between -144 Temperature 455 Pa 45 K Fgure 11: Flow pattern and temperature dstrbuton for the optmzed eector case 7 Copyrght 29 by ASME

Statc pressure (pa) Exstng desgn Optmal case Temperature (K) 6. Addng a contoured annular passage to gude the entraned flow shows lttle effect on the sucton rate. 7. The optmal desgn s found by modfyng the contracton cone wall profle to become aerodynamcally contoured, addng a long dffuser downstream the contracton cone, removng all the contracton cones downstream, and locatng the steam et necton at the center of the contracton cone entrance. The sucton rate of the optmal case s.423 g/s, whch s 4.3 tmes the flow rate of the steam et and 18 tmes hgher than the exstng desgn. Dstance from the et (m) Fgure 12: Temperature and pressure dstrbutons along the eector centerlne n dfferent cases Ths study shows dfferent ways to enhance the sucton flow rate of the thermal compressor n a new steam evaporator. The actual sucton flow rate s subect to the numercal uncertanty when appled to real stuatons; however, the concepts obtaned from the numercal smulaton are extremely useful and nstrumental n provdng the soluton to the problems encountered by the exstng system. 6. CONCLUSIONS Based on numercal smulaton, the followng conclusons can be made: 1. The downstream contracton cones create large flow resstance and mpede the overall sucton performance. Nether addtonal momentum nor addtonal sucton flow rate s provded as clamed n the exstng desgn. Removng the downstream cones sgnfcantly ncreases the sucton capacty. For example, the sucton rate s doubled by removng the frst contracton cone and ncreases 5.8 tmes by removng the second one. 2. Addng a dffuser downstream the contracton cone also provdes a sgnfcant ncrease of the sucton flow rate. A smple straght dffuser wth an ncluded angle of 6.5 o ncreases sucton flow rate by 2.6 tmes. 3. The locaton of the steam et ext affects the flow sucton rate. The best locaton s at the center of the contracton cone entrance. The sucton capacty reduces when the steam et necton locaton s moved nto or away from the entrance plane. When the necton locaton s placed half way between the entrance and the contracton ext, the sucton capacty loses 67%. 4. Employng an aerodynamcally contoured contracton cone provdes a 2% augmentaton of sucton flow rate, whch s defntely a favorable effect although not sgnfcant. 5. The sucton openng connected to the vapor plenum s not mportant. Shrnng the sucton openng to one or twothrds of the orgnal sze does not change the sucton flow rate. REFERENCES [1] Power, R.B., 1994, Steam et eectors for the process ndustres, McGraw-Hll, New Yor, NY, USA. [2] Balamurugan, S., Gaar, V.G., and Patwardhan, A.W., 28, Effect of eector confguraton on hydrodynamc characterstcs of gas-lqud eectors, Chemcal Engneerng Scence, v. 63, pp. 721-731. [3] Chen, Y.M., Sun, C. Y., 1997, Expermental study of the performance characterstcs of a steam-eector refrgeraton system, Expermental Thermal and Flud Scence, v.15, pp. 384-394. [4] Rusly, E., Aye, Lu, Charters, W.W.S., and Oo, A., 25, CFD analyss of eector n a combned eector coolng system, Internatonal Journal of Refrgeraton, v. 28, pp. 192-111. [5] Panthong, K., Seehanam, W., Behna, M., Srveeraul, T., and Aphornratana, S., 27, Investgaton and mprovement of eector refrgeraton system usng computatonal flud dynamcs technque, Energy Converson and Management, v. 48, pp. 2556-2564. [6] Rffat, S.B. and Evertt, P., 1999, Expermental and CFD modellng of an eector system for vehcle ar condtonng, Journal of the Insttute of Energy, v. 72, pp. 41-47. [7] J, J., N, H., L, L., Wang, R., 28, Performance smulaton and analyss of steam eector under dfferent operatng condton, Journal of Chemcal Industry and Engneerng (Chna), v. 59, pp. 557-561. [8] Elady, M., Karameldn, A., Negeed, E.S., and El- Bayoumy, R, 28, Expermental nvestgaton of the effect of eector geometry on ts performance, Internatonal Journal of Nuclear Desalnaton, v. 3, pp. 215-229. [9] Negeed, E-S. R., 29, Enhancement of eector performance for a desalnaton system, Internatonal Journal of Nuclear Desalnaton, v. 3, pp.247-259. [1] Km, H.D., Lee, J.H., Setoguch, T., and Matsuo, S., 26, Computatonal analyss of a varable eector flow, Journal of Thermal Scence, v. 15, pp. 14-144. [11] Km, H.D., Lee, J.H., Setoguch, T., and Matsuo, S., 26, Optmzaton study of a Coanda eector, Journal of Thermal Scence, v. 15, pp. 331-336. [12] Srveeraul, T., Aphornratana, S., Chunnanond, K., 27, Performance predcton of steam eector usng computatonal flud dynamcs: Part 2. Flow structure of a steam eector nfluenced by operatng pressures and 8 Copyrght 29 by ASME

geometres, Int. J. of Thermal Scences, v. 46, pp. 823-833. [13] Choudhury, D., 1993, Introducton to the Renormalzaton Group Method and Turbulence Modelng, Fluent Inc. Techncal Memorandum TM-17. [14] Wolfsten, M., 1969, The Velocty and Temperature Dstrbuton of One-Dmensonal Flow wth Turbulence Augmentaton and Pressure Gradent, Int. J. Heat Mass Transfer, v. 12, pp. 31-318. [15] Fluent Manual, Verson 6.3.23, Fluent Inc, 26. [16] Patanar, S.V., Numercal Heat Transfer and Flud Flow, McGraw-Hll, New Yor, USA, 198. 9 Copyrght 29 by ASME