Numerical Study of the High Speed Compressible Flow with Non-Equilibrium Condensation in a Supersonic Separator

Transcription

1 Journal of Clean Enery Technoloies, Vol. 3, No. 5, September 2015 Numerical Study of the Hih Speed Compressible Flow with Non-Equilibrium Condensation in a Supersonic Separator Liu Xinwei, Liu Zhonlian, and Li Yanxia dehydration technoloy, the main virtue of the supersonic separation technoloy is the small size and flexible structure. Therefore, supersonic separator can be used to remove condensable vapors such as water or natural as liquids (NGL) from a as stream in order to lower the (water or hydrocarbon) dew point of the as or strip the as with heavy hydrocarbons which can enerate additional investment. This new technoloy is suitable for unmanned operations, especially for the oil and as field in the offshore, desert and remote areas [6]. The first team known to carry out investments on supersonic separators was an enineerin roup from the Netherlands that is now affiliated with Twister BV [7]. At the same time, a roup of Russian specialists also work on the supersonic separator, and named them 3S technoloy [8]. Experimental testin system was constructed by Liu [9] to fully analyze the process of dew point depression in supersonic flows. A rane of field test investiations were conducted to predict the actual effect of supersonic dehydratin technoloy in SINOPEC Shenli Oilfield [10]. It has proved that this new supersonic dehydration technoloy for natural as can not only reduce the dew point of natural as but also recover liht hydrocarbon at the same time. However, the present test results still indicate that the separation efficiency of the supersonic separator is far from satisfactory effect. For the further research on the supersonic separator, the ability to explore the internal flow mechanism is sinificant. The flow in the supersonic separator is multi-component, stron swirlin and also with non-equilibrium phase condensation. In current experimental conditions, obtain the whole realizable flow field is difficult or even impossible. Fortunately, numerical simulations can reduce research costs and provide the convenience in side of discoverin new flow phenomena, reconizin flow details, extendin research topics, and so forth, and with the rapid development of computers, numerical methods become more and more promisin in understandin as-liquid two phase flow with hih speed phenomenon inside the supersonic separator. In the supersonic separation field, many numerical works have been done by researchers. For example, Malyshkina et al. [11], [12] presented their calculation results with two-dimensional Euler model of the natural as supersonic swirlin flow inside a supersonic separator. Jian et al. [13] developed a mathematical model to investiate the one-dimensional transonic flow of two-component as mixture with spontaneous condensation. But his researches only focus on Laval section of the supersonic separator. Bao et al. [14] established a mathematical model for phase equilibrium prediction of Abstract The present work is mainly focus on the internal flow in a supersonic separator. In order to explore the separation mechanism and heat transfer inside the separator, a fully three-dimensional numerical dynamic model with consideration of such effects as stron swirlin turbulent and non-equilibrium condensation process is established. The numerical model is conducted to determine the dominant factors on the separation performance. The mixture of methane and water liquid are chosen as workin fluid. The numerical model results are verified throuh comparison with experimental data obtained from the literature and the results demonstrated ood areement between the two ways. By usin the proposed numerical model, the distribution of pressure, temperature, velocity, droplet nucleation rate and Mach number are investiated. Based on the established numerical model, the swirlin intensity of the outlet of straiht tube versus the swirlin lenth ratio L5/L4 (L5 is the lenth of straiht tube, L4 is the lenth of the swirlin flow enerator, in this paper is 25mm) are also studied. The optimal value of L5/L4 should be 8 with the pressure loss ratio specified Furthermore, the simulation results can provide a foundation for the analysis of flow characteristic caused by the supersonic speed and nucleation phenomenon and also a basis for the desin and eometrical optimization of the supersonic separator. Index Terms Supersonic separator, as-liquid two phase model, condensation process, flow field. I. INTRODUCTION Considerin that conventional fossil fuel resources are bein exhausted and the continuously increasin enery demand, explorin a sustainable alternate enery source is essential and urent. Natural as is one of the most important sources of enery in the world, which enerally accepted as a potential strateic enery form for sustainable development. Currently, sinificant challenes need to be overcome in the commercial exploitation and processin of natural as especially in China [1]. Thus, it is necessary to develop advanced technoloy to improve utilization efficiency of natural as. The primary innovation step should focus on the dehydration technoloy, which ures researchers to pay more attention on explorin advanced separation technoloy. The supersonic separation is a promisin new technoloy to remove water liquid and aseous components out of a mixed natural as [2]-[5]. Compared to the traditional Manuscript received July 1, 2014; revised Auust 28, The authors are with Beijin University of Technoloy, Collee of Environmental and Enery Enineerin,100124, Beijin, China ( Liuzhl@bjut.edu.cn). DOI: /JOCET.2015.V

2 Journal of Clean Enery Technoloies, Vol. 3, No. 5, September 2015 multi-component as separation process inside a supersonic separator. Jassim et al. [15], [16] adopted computational fluid dynamics technique to study behavior of hih-pressure natural as in supersonic nozzles. Rajaee Shooshtari, S. H [17] proposed a novel mass transfer approach to develop a reliable model for estimation of liquid droplet rowth rate for binary and multicomponent systems. Wen et al. [18] studied the separation efficiency usin Discrete Particle Method (DPM). It is convenient to execute the calculation of the as-droplets two-phase flow with discrete phase by usin the model of particle roup trajectory. Ma et al. [19] studied the performance of inner-core supersonic as separation device with droplet enlarement method. In his model, the as phase was treated as continuous medium and the particles as the discrete medium. As mentioned above, Most of the literature only concerned with the basic structure and preliminary test aspects of this device and not take into account the as dynamics mechanism and condensation aspects. Thus, establishin an accurate numerical model for hih speed process of two-phase flow with non-equilibrium condensation within the supersonic separator will provide a reat uidance for structure desin, furthermore, can provide a reliable method to predict the dehydration efficiency of the supersonic separator. Thus, in order to demonstrate the acceptable flow pattern and find the creditable explanation of separation mechanism in supersonic separator, this article established a 3D computational model based on the compressibility and stron swirlin characteristics coupled with the as-liquid two phase model and non-equilibrium condensation model which take into account the nucleation process and droplet rowth process. Usin this numerical model, we explore different parameters distribution within the supersonic separator. II. DESCRIPTION OF THE SUPERSONIC SEPARATOR The supersonic separator is mainly composed of three sections: a Laval nozzle (include converence part and diverence part), a straiht tube interated with a cyclone (include swirlin flow enerator), and diffuser part, as shown in Fi. 1. In the Laval nozzle part, the mixture as adiabatically expands, velocity increases and temperature drops, to promote the condensable as with formation of droplets. After that, the mixture of as and liquid or even solid phase is introduced into the straiht tube. In the straiht tube, centrifual effects enerated by the swirlin flow enerator composed of twist vanes drive the droplets towards the wall of the workin section. Condensed liquid droplets are separated from main as flow in the workin section. The remainin dry as stream flows into the diffuser in which about 50%-80% of the initial pressure is recovered. The wet as can be released or used for other purposes. 1-Saturated feed as inlet; 2-Laval Nozzle; 3-swirlin flow enerator; 4-straiht tube; 5-diffuser section; 6-dry as outlet; 7-wet as and slip-as outlet Fi. 1. Geometry structure of the supersonic separator. The velocity distribution from the orifices and the temperature distribution of the air jet were estimated by performin fluid analysis usin a three-dimensional (3D) finite element method (FEM) with FLUENT 14.0 software. The analyses were estimated based on varyin pressure. A. Numerical Methods The flow in the supersonic separator is assumed compressible and stron turbulent, the condensation process is also considered, relative overnin equations of the two phases are iven as follows: B. Gas Phase Control Equations Continuity equation v t Enery equation E t v v p y v v v m l rl v E p m h h t l k T v eff eff (2) (3) t Momentum equation v m (1) Gas state equation The Virial equation is applied, and its universal form is iven as follows: p RT 1 B C (4) 2 361

3 Journal of Clean Enery Technoloies, Vol. 3, No. 5, September 2015 where B and C are second and third Virial coefficient, respectively. C. Liquid Gas Control Equation Continuity equation l y l yvl m Momentum equation (6) l 1 y k 1 y U l k 1 y Gk, t, (13) 1 y k k 1 y 1 y k Droplet species equation l y l yvl l J (7) Due to the existence of the water vapor, in the supersonic separator two sinificant processes will take place, one is non-equilibrium condensation nucleation and the other one is the droplet rowth. It is extremely important to predict the two processes of nucleation and droplet rowth accurately for the numerical simulation on the supersonic two phase flow. In this study, the description of the mass eneration rate ṁ provided by Yan [20] is chosen for the nucleation model, which indicate the sum of mass increase due to condensation and due to rowth/demise of the formed droplet. The equations to express the condensation phenomenon can be written as follows. Droplet mass equation m 1 y J l 4 rc3 r 4 r 2 l N 3 1 y 1 y U l 1 y (14) 1 y t, k 1 y (9) The droplet nucleation rate J and droplet rowth rate [21] 4 rc3 2 exp M m3 3KbT 1 C T T r p p l l hl 2 RT 2 k C 1 Gk, C2 D. Numerical Solver All calculations are run with help of the CFD code FLUENT The as and liquid phase conservation equations are discretized usin finite-volume method. The non-equilibrium condensation model has been implemented into FLUENT Solver by UDF (user defined function). Droplet species equation solved by UDS (user defined Scalar) module include in the FLUENT Solver. Quick scheme is used to discretize convective terms, and coupled procedure is used to solve the momentum and enery equations simultaneously. First-order upwind schemes are set for density term. Second-order upwind schemes are selected for the necessary terms, such as turbulent kinetic enery, and turbulent dissipation rate. r can be written as follows: qc 1 l liquid phases on the continuous as phase, U is the as phase-weihted velocity, and Gk, is the production of turbulent kinetic enery. 3C 2 f1 ln, f 2 2 B s, f3 s2 s 2 J Here k and epresent the influence of the dispersed (8) 2 l R T f1 f 2 f 3 ps S 1 and where N is the number density for droplets, and the rc denotes the critical droplet radius [13], which is based on the classical nucleation theory and can be written as: rc (12) Here hl is the latent heat of evaporation at pressure p and γ is the ratio of specific heat capacities.ps is the saturation pressure of water vapor at the local temperature T. It is universally that for the hih speed flow within the supersonic separator the turbulence intensity plays an important role in the flow pattern. Due to the concentrations of the liquid phase are dilute, the dispersed turbulence model is appropriate model for current calculation. The k and ε equations describin the Dispersed Turbulence Mode are as follows: (5) l yvl l yvl l yi l y l v vl v vl m 2 1 hl hl RT RT (10) E. Calculation Domain and Boundary Conditions In order to ive a successful demonstration of the flow field within the supersonic separator, the whole structure was chosen for the calculation domain, Table I listed the main eometrical data of the separator. Fi. 2 shows the three-dimensional calculation domain. The rid is intensified near the throat and swirlin flow reions. The three-dimensional rids of the eometry were meshin with GAMBIT Such meshin method is based on the check (11) In the above equations, qc is the condensation coefficient, Kb is the Boltzmann constant, Mm is sinle water molecule mass, σ is the droplet surface tension, and θ is a nonisothermal correction factor that is iven by the followin relation: 362

4 Temperature(K) Journal of Clean Enery Technoloies, Vol. 3, No. 5, September 2015 of rid independence. Preliminary test about rid independence of the computational domain showed that it is unnecessary to increase the number of the cells beyond the To retain the calculation speed advantae comin with the use of a reular block-structure element, the multi-block technique was applied to make the concentration of rid density focused on the areas where sinificant phenomena were expected. Fi. 2. Mesh partitionin stratey of the supersonic separator. Boundary conditions in the supersonic separator are defined as follows: 1) At the separator entrance, compressed air with known pressure and total temperature are specified, 2) At the dry and wet outlet, pressure is assumed, 3) The wall of the supersonic separator is assumed adiabatic and no slip conditions are used for flow velocity components. F. Grid Independence Analysis 1) Model verification Althouh the detailed numerical solution methods are introduced in the above pararaphs, it is necessary to validate whether these methods could be used to predict the complex swirlin flow in a supersonic separator. However, as far as the present authors could know up to now, only a few sample results about the experimental test have been published in the open literature. So, in this paper, another supersonic separator that used in the previous experiment by Liu [9] was also simulated to verify the accuracy of the numerical model. For the verification case, the workin fluid is mixture of air and water to keep the same material properties as the experiment conditions. Fi. 3 shows the comparison of the temperature distribution alon the separator centerline between the simulations results and experiment. The comparison curve indicates that althouh some calculation data is slihtly deviation compare to the experiment ones, there is still a reat areement between the two in the whole trend. TABLE I: THE BASIC DIMENSIONS OF THE STRUCTURE d i d t d s d h L 1 L 2 L 3 L 4 L 5 L 6 Computational model Experimentalresluts[13] Simulation results separator used for verification axial distance Fi. 3. Temperature comparison of the simulation results versus the experimental results. III. RESULTS AND DISCUSSIONS Bearin in mind that the workin fluid is the mixture of the methane and water in the next section of this paper, the whole calculation structure has been iven (Fi. 2). The calculation conditions are as follows: the inlet pressure is 0.45MPa, the temperature is 300K, and the outlet static pressure is 0.25MPa. A. Analysis of the Flow Field For the supersonic separator, the mainly drivin force is pressure difference between the inlet and outlet (in this paper dry outlet and wet outlet keep the same pressure value). Fi. 4 shows the pressure contour on a cuttin plane passin throuh the axial of the supersonic separator. As is apparent in Fi. 4 that alon the whole supersonic separator at first the pressure decrease radually when at the shock wave location the minimum pressure is only 0.2MPa then it jump abruptly to a bier value, after the shock wave [22] location the pressure will not appear obviously chane except the bifurcation point where the condensate droplets were removed. In the supersonic separator, pressurized as expands to low pressure will lead to low temperature and hih speed. To complete the analysis of the mechanism of flow characteristic, Fi. 5-Fi. 7 depict the contour profile of velocity, temperature and Mach number on the plan passin throuh the axis, respectively. It is observed from Fi. 6 that the minimum temperature is 190K appears in the diverent part of the Laval nozzle. Such a low temperature provides ood condition for the water vapor condensation. Care must be taken with the formation of hiher temperature at diffuser section that nearby the trailin section of the diffuser the temperature increase to 310K which is hiher than the inlet temperature. This is because the stron vortex effect enerated by the centrifual force will cause fiercely kinetic enery transfer [23]. It can be seen from the Fi. 5 and Fi. 7 that the maximum velocity will be 450m/s correspondinly the Mach number is 1.7. These conditions can maintain the as flow in the supersonic state at the diverin part of the Laval nozzle and keep as residence time in this section 363

5 Journal of Clean Enery Technoloies, Vol. 3, No. 5, September 2015 below two milliseconds to avoid extra hydrate inhibitors. Besides, at the trailin section of the swirlin flow enerator flow parameters include the pressure, temperature, velocity and Mach number will appear slihtly obvious fluctuation. In current operational condition, after the swirlin flow enerator temperature will increase while pressure, velocity and Mach number will decrease. Fi. 4. Pressure distribution on the plane passin throuh the axis (Pa). Fi. 5. Gas phase velocity distribution on the plane passin throuh the axis (m/s). Fi. 6. Gas phase temperature distribution on the plane passin throuh the axis (K). Fi. 7. Gas phase Mach number distribution on the plane passin throuh the axis. Fi. 8. Nucleation rate distribution on the plane passin throuh the axis (lo J, J (k-1s-1)). Fi. 9. Liquid fraction distribution on the plane passin throuh the axis. Within the supersonic separator, the condensation phenomenon is another important process need to be demonstrated clearly. Thus, Fi. 8 presents the loarithmic nucleation rate contour under current operational condition, from this fiure it can be known that nucleation mainly occurs at the diverin reion of the Laval nozzle, especially at upstream of the swirlin flow enerator the nucleation rate reaches the maximum value k-1s-1. Throuh the above qualitative description of the flow field within the supersonic separator, it is reasonable to conclude that the thermodynamic phenomenon include pressure expansion, cryoenic process, supersonic reion, shock wave location and spontaneous condensation have been reproduced. These further provide evidence that the numerical model established in this study is suitable for the full three-dimensional simulation on the flow characteristic of a supersonic separator. Accordin to the workin principle of the supersonic separator one can conclude that swirlin intensity in the straiht section also play a sinificant role in satisfyin its performance, so the swirlin intensity can reard as one important factor used for evaluatin centrifual force which will cause the variation of the separation efficiency. Generally, hiher swirlin intensity will lead to more condensate separated to the walls form the main stream, and weaker swirlin intensity will lower the separation efficiency. However, in current separator structure the swirlin enerator is located in the straiht tube; the lenth of straiht tube must have a reat effect on the swirlin intensity. When the lenth of straiht tube is too lon, stron swirlin intensity will cause much more enery losses, too short lenth cannot separate the liquid thorouhly. Therefore, there may be an optimal value of swirl section for the best separation performance. 0.9 swirlin intensity case1 case2 case3 case4 0.5 γ=0.71 γ=0.51 γ=0.35 γ= L5/L4 Fi. 10. Swirlin intensity of the straiht tube exit as a function of dimensionless parameter L5/L4 with different pressure loss ratio. 364

6 Journal of Clean Enery Technoloies, Vol. 3, No. 5, September 2015 The principle aim of the next section in this paper is investiate the relationship between the straiht tube and swirlin intensity. At first we defined two dimensionless parameters, swirlin lenth ratio L5/L4 (L5 is the lenth of straiht tube, L4 is the lenth of the swirlin flow enerator, in this paper is 25mm), swirlin intensity S which is the ratio of the tanential velocity to the axial velocity, respectively. The detailed form of the S is defined as follows. S ut uw listed as follows: 1) The numerical method developed in this paper has a hiher credibility for solvin supersonic non-equilibrium condensation flow, and can predict the location of maximum droplet nucleation. 2) For the condensation flow within the supersonic separator, the obvious nucleation rate will occur at diverin section of the Laval nozzle, especially upstream the swirlin flow enerator, the maximum nucleation rate will k-1s-1. 3) In the current study, the pressure loss ratio and the swirlin lenth ratio L5/L4 have proved to play a reat role in improvin the swirlin separation effect. It was obtained that when the pressure loss ratio is 071 the optimal L5/L4 value should be 8, with the decrease of the pressure loss ratio the optimal L5/L4 should be decreased correspondinly. (15) where ut is the area-weihted averae tanential speed of the flow at the exit of the straiht tube and uw denotes the area-weihted averae axial speed at the exit of the straiht tube. Besides, there need another factor to demonstrate the performance of supersonic separator, pressure loss ratio γ, which s defined as the ratio of the pressure loss p of the supersonic separator to its inlet pressure pinlet, that is, p poutlet p inlet pinlet pinlet ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No ) and the Research Fund for the Doctoral Proram of Hiher Education (Grant No ). (16) Poutlet is pressure at as outlet (both dry-outlet and wet-outlet are identical). Drawin the important relationship between these dimensionless parameters help us to better understand the physics process of the problem within the supersonic separator. Fi. 10 shows the variation of swirlin intensity obtained at the exit location of the straiht tube versus the dimensionless parameter L5 /L4 with different pressure loss ratio cases. It is observed that at the certain case1 with the increase of the L5 /L4 at the value ranes from 1 to 8 the swirlin intensity decrease slihtly, but when the L5 /L4 exceed the value 8 the swirlin intensity decrease fiercely. Under different pressure loss ratio cases the variation trend of these curves are slihtly different. Accordin to the different pressure loss ratio, by decreasin it, the critical value of L5 /L4 will become smaller and the decrease trend of the swirlin intensity is more obvious. These results leads to the conclusion that by increasin the L5 /L4 in a certain rane, the performance of the supersonic separator should be improved because of the increasin wall surface area for particles collection and extend of the resident time for as/liquid separation. However, too hih of the L5 /L4 will cause more friction loss, impairin swirlin separation effect. In this study, the optimal lenth of the straiht tube should be 8 times the lenth of swirlin flow enerator at the pressure loss ratio case1 and 6 times at the other pressure loss ratio cases. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] IV. CONCLUSIONS A numerical analysis of supersonic separator has been established to analyze the flow pattern within the separator. The accuracy of the CFD simulations has been validated throuh comparison with experimental data obtained throuh literature. Based on the numerical model for the supersonic separator, the main conclusions are obtained and [14] [15] 365 Y. Son et al., The status of natural as hydrate research in China: A review, Renewable and Sustainable Enery Reviews, vol. 31, pp , M. Bettin et al., Supersonic separator apparatus and method, US Patent b2, J. M. Brouwer, G. Bakker, H-J. Verschoof, and D. H. Epsom, Twister supersonic as conditionin-first commercial offshore experience, GPA paper, P. B. Machado, J. G. M. 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7 Journal of Clean Enery Technoloies, Vol. 3, No. 5, September 2015 [16] E. Jassim, M. A. Abdi, and Y. Muzychka, Computational fluid dynamics study for flow of natural as throuh hih-pressure supersonic nozzles: part 1, real as effects and shockwave, Petroleum Science and Technoloy, vol. 26, pp , [17] S. H. R. Shooshtari and A. Shahsavand, Reliable prediction of condensation rates for purification of natural as via supersonic separators, Separation and Purification Technoloy, vol. 116, pp , [18] C. Wen, X. W. Cao, Y. Yan, and J. Zhan, Evaluation of natural as dehydration in supersonic swirlin separators applyin the discrete particle method, Advanced Powder Technoloy, vol. 23, pp , [19] Q. Ma et al., Performance of inner-core supersonic as separation device with droplet enlarement method, Chinese Journal of Chemical Enineerin, vol. 17, pp , [20] Y. Yan and S. Shen, Numerical simulation on non-equilibrium spontaneous condensation in supersonic steam flow, International Communications in Heat and Mass Transfer, vol. 36, pp , [21] Y. Yan, S. Shen, T. Kon, and K. Zhan, Numerical investiation of homoeneous nucleation and shock effect in hih-speed transonic steam flow, Heat Transfer Enineerin, vol. 31, pp , [22] C. Wen, X. W. Cao, Y. Yan, and W. L. Li, Numerical simulation of natural as flows in diffusers for supersonic separators, Enery, vol. 37, pp , [23] M. Farzaneh-Gord and M. Kararan, Recoverin enery at entry of natural as into customer premises by employin a counter-flow vortex tube, Oil Gas Sci. Technol. Rev IFP Eneries Nouvelles, vol. 65, pp , Liu Xinwei was born in Hebei Province in Presently he is study in Beijin University of Technoloy, he is a candidate for doctor deree of thermal enineerin; he received bachelor deree of buildin environment and equipment enineerin in Tanshan collee. Currently, he is specialized in numerical simulation technoloy and optimization of the flow and heat transfer and supersonic separation technoloy of natural as. Liu Zhonlian was born in Hebei Province in He is a professor in thermal fluids & enery enineerin, at Collee of Environmental & Enery Enineerin, Principal of Beijin-Dublin International Collee at BJUT, Beijin University of Technoloy, Pinleyuan 100, Chaoyan District, Beijin , China. His research direction include strenthen the heat transfer and its application in hih and new technoloy, environmental enery technoloy research and development, numerical simulation technoloy and optimization of the flow and heat transfer et al. Li Yanxia was born in Shandon Province. She received master deree in Beijin University of Technoloy in July Then she received doctor deree in Beijin University of Technoloy in July Since June 2003, she has been a lecturer and teachin assistance at Beijin University of Technoloy, Collee of Environmental and Enery Enineerin. Her main research direction is environmental enery research and development of hih and new technoloy. 366

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