CFD MODELLING OF DIRECT CONTACT STEAM INJECTION

Transcription

1 Fifth Internationa Conference on CFD in the Process Industries CSIRO, Meourne, Austraia Decemer 006 CFD MODELLING OF DIRECT CONTACT STEAM INJECTION Curtis MARSH 1 and Denis WITHERS 1 CFD Design & Engineering Ltd, PO Box 30003, Christchurch 846, New Zeaand cmarsh@engineeringcfd.com Aughinish Aumina Ltd, Askeaton, Ireand ABSTRACT The direct injection of steam into a process stream is a method of heating used in many process industries. The amount of research in this area however is imited to the nucear industry, with appications reating to reactor cooing systems. There are no genera CFD modes avaiae for designs reating to the process industry. A a-scae experiment was conducted to investigate the direct injection of steam into water. The height of the steam pume was measured against different water temperatures providing a means to vaidate the suroutine used in the CFD modes. A suroutine was deveoped to descrie the heat and mass transfer etween the vapour and iquid phases, ased on previous work y the author on fashing fows. The suroutine was compied into FLUENT (ver 6..16) using the Euerian mutiphase mode. The mode was then appied to an industria scae proem giving unique insights into the operation of the equipment and the ehaviour of the condensing steam. Key words: Steam; two phase fow; condensing; injection; heat and mass transfer NOMENCLATURE A 1/m interfacia area a - voume fraction C p J/kg-K specific heat h W/m -K heat transfer coefficient i at J/kg atent heat Ja - Jaco numer J 1/m 3 -s source term k W/m-K therma conductivity N 1/m 3 ue numer density Nu - Nusset numer ρ kg/m 3 density R m radius T K temperature Suscripts ue E energy iquid M mass sat saturation v vapour INTRODUCTION The injection of steam into a iquid is a direct heating method used in many process operations and has severa advantages over other means of heat transfer, such as not eing affected y the degree of superheat, heat transfer does not deteriorate with scaing and arge increases in temperature can e achieved over very short time scaes. The overa energy aance for this process is readiy cacuated with the properties of steam we defined over a arge temperature range. The ehaviour of steam as it condenses whie in direct contact with a su cooed iquid is not so we defined, with research imited to the nucear industry. Lee and No (1998a) present resuts and theories reating to nucear reactor cooing systems using the RELAP code, deveoped for cacuating therma- hydrauic transients in water-cooed nucear reactors. Chun Kim and Park (1996) present experimenta resuts of steam injected into sucooed water and characterise pume shapes for different steam mass fux. Lee and No (1998) aso present experiment data of steam experiments and have puished condensation regime maps descriing three regimes, steam cavity, chugging and su-sonic jetting. Whie commercia CFD codes can readiy mode iquid vapour systems there are currenty no suroutines or puished theories that aow this particuar heat and mass transfer process to e accuratey modeed. The performance of a high pressure steam injection system at Aughinish Aumina Ltd was giving variae resuts with very sma changes in process conditions. A project invoving oth experimenta work and numerica simuations was undertaken to etter understand the dynamics of the direct contact steam condensing process, with the aim of creating a practica approach of modeing this process with CFD and designing an improved steam injection system. The key aspect identified for this work was that the mixing of the fuid is principay uoyancy driven and the rate of steam coapse is a key variae. MODEL DESCRIPTION Condensing Steam Mode The interface etween the iquid and vapour is rapidy changing and contains oth arge and sma surface features and ues, over which the heat and mass transfer takes pace. It is not practica to mode the sma fow features with CFD as the required grid size and the time steps woud e prohiitivey sma and not practica for industria appications. To cacuate the rate of heat and mass transfer the theory from previous work modeing fashing fows (Marsh, 004) was used as a starting point. The rate of energy transfer is ased on three key parameters; interfacia area, heat transfer coefficient and the driving force?t. 1

2 Interfacia Area The interfacia area is the surface area etween the vapour phase and the iquid phase over which the energy and mass transfer takes pace. This is in effect the tota surface area of the vapour ues and is expressed in terms of area per unit voume. The approach used y Binkov et a (1993) determined the numer of ues and then cacuates the radius of the ue ased on the oca vapour fraction. For spherica ues, the interfacia area (A i, ), and the oca vapour fraction (a) are shown in equations 1 and respectivey. Ai, = 4πR N B (1) α = 4 π R3N 3 B Comining these equations gives an equation (3) for the interfacia area. A i, = 3α When considering fashing fows the ue numer density is determined from ue nuceation theory. However with the process of coapsing steam the ues are formed y the reak-up of arger ues and sugs. As there is no information avaiae for this variae for condensing fows, a constant ue density per unit voume was assumed and the vaue ased on the experimenta work that foows. Heat Transfer Coefficient The heat transfer coefficient etween the iquid and vapour phases was cacuated using the foowing equations; c Ja = p, 1Ja Nu = 1 + π ρ T ρ i v at 1 k Nu h = R R π 6Ja 3 π + 6Ja The formuation for the heat transfer coefficient h is ased on the Nusset numer Nu and the Jaco numer Ja. Driving Force The driving force for the condensation of steam is the difference etween the oca iquid temperature and the iquid saturation temperature. sat Energy and Mass Source Terms () (3) (4) (5) (6) Τ = Τ, Τ (7) The energy transfer was assumed to act ony across the interfacia area and thus the tota energy transfer was cacuated from the product of the driving force?t, interfacia area and the heat transfer coefficient. The tota energy transferred was then used as the source term (J E ) in the energy transport equation for each phase within the mutiphase mode in the foowing form. J E = hai T (8) As the iquid cannot exceed the oca saturation temperature without changing ack to steam this source term was ony considered to act when the iquor temperature was eow the saturation temperature. The phase change from iquid to vapour was not considered to e significant. As the superheated steam reduces in temperature and reaches the oca saturation temperature, the mass transfer process egins. The mass source term (J M ) eing equa to the energy source term divided y the atent heat of condensation, where the steam temperature is equa to oca saturation temperature. EXPERIMENT J ha T i M = (9) iat The theory presented aove aows the direct contact condensing process to e defined, however it eaves one variae undefined, the density of ues used to cacuate the interfacia area etween the phases. To determine this parameter and to visuaise the actua condensing process the foowing experiment was devised. Autocave Test Tank Throtte Vave Figure 1: Schematic of Experiment Figure : Experimenta Layout. T P P 3-Way Vave Condenser Baance Vave

3 An autocave with a 1 gaon capacity was used as the steam source aowing a cosey controed set point of steam temperature to e achieved. The steam fow was controed y a fine neede vave and high accuracy pressure transducer. To cacuate the mass fow of steam a three-way vave was used to direct the steam to a condensing coi, aowing the voume of condensate to e directy measured over time. A second neede vave (aance vave) was used to ensure that the pressure drop through the condensing coi was equivaent to the test vesse, confirmed y verifying a consistent pressure drop across the first vave. When steady state conditions were estaished the steam was introduced to the test vesse through a 6mm stainess stee tue aowing the condensing process to e oserved. An exampe of the steam pume is shown in figure 3. correct interfacia area etween the phases to e determined, competing the mathematica mode of the condensing process. IMPLEMENTATION OF THE NUMERICAL MODEL The presented theory was impemented within Fuent (version 6..16) through the use of a user defined suroutine and soved as an unsteady time dependant proem. There are severa mutiphase modes that coud e empoyed to simuate the two phase system of a sucooed iquid and steam, such as the Voume of Fuid (VOF) mode, mixture mode or the Euerian mode. The VOF mode is not suitae as the interface etween the two phases is rapidy changing and not aways we defined. The Euerian mode was chosen over the mixture mode as each phase is treated separatey, aowing the energy of each phase to e considered and aows for the proposed suroutine to e easiy inked in to the sover parameters. Vaidation Figure 3: Steam Pume The temperature of the steam and water were varied to gain an understanding of the different vapour coapsing regimes and to etter understand the reationship etween the steam pume and the degree of tank mixing. Whie hoding the steam temperature and fow rate constant and varying the iquid temperature the height of the steam pume coud e measured. This reationship is shown in figure 4 and foows an exponentia reationship. Pume Height (mm) Steam Pume Height vs Water Temp Water Temperature (oc) Figure 4: Reationship etween pume height and steam temperature. This reationship was used as a means to determine a representative vaue for the ue density aowing the A -D axi-symmetric simuation was created, equivaent to the experimenta test tank, with steam fow at 1 kg/hr and a temperature of 105 C. By impementing the theory presented aove and assuming an aritrary ue density a transient soution was used and the mean ue height determined. The ue density was varied and the simuation repeated to determine the most appropriate empirica vaue for ue density. With this empirica vaue determined, simuations were repeated for a range of temperatures. The resuting ue height profie can e compared to the experimenta data as shown in figure 5. Pume Height (mm) Comparison with Experiment Experiement CFD Differentia Temperature (oc) Figure 5: Cacuated pume height versus experimenta data. Considering the random variation in the ue height in oth the experimenta and numerica resuts the proposed mode was considered suitae for the proposed appication. As a the key parameters for the suroutine, with the exception of the ue density, are cacuated from known materia properties, it is assumed that the appication to a much arger scae proem wi have acceptae accuracy. APPLICATION TO PROCESS PLANT This mode was used to mode the injection of steam into a digestion vesse at Aughinish Aumina. The Digester is 4m in diameter and over 30m ta. The steam is injected into 3

4 the vesse at 310 C at a rate of 350 t/hr at a point eow the iquid surface. Coder iquor is added in this region at 00 C and is heated to around 50 C. The steam is superheated y around 50 C and comes into contact with the su-cooed caustic iquor. The steam quicky ooses the superheat and rapidy condenses. An unstructured grid was deveoped for the geometry, incuding the interna nozzes and affe pates. A surface oundary ayer grid was used, competing the mesh with tetrahedra ces in the interior. The tota ce count was 650,000 ces. The soution was strongy transient in nature and required time steps of seconds, taking 1 week to sove 10 seconds of soution time. Figure 8: Steam Pume after 4 seconds. An iso-surface of 50% steam voume fraction is shown in figures 6, 7 and 8, cooured y steam temperature. These resuts give an idea of how transient the soution is, with the steam pume strongy infuenced y the fow of iquor and the reative temperatures of the phases. Appication of the new steam condensing mode was successfuy appied to an industria proem and aowed severa design concepts to e evauated. These resuts aso gave new insights into the operation of the equipment and improved understanding of how process factors affect the rate of steam condensation CONCLUSION Figure 6: Steam Pume after 0 seconds. Existing heat and mass transfer theories were reviewed and adapted for the appication of condensing steam. The new direct contact steam condensing mode was incorporated into a user defined suroutine for incusion into a commercia CFD software package. The theory required one empirica constant to e defined to compete the mathematica mode and this was otained through experimenta work. The resuts of the experimenta work provided insight into the condensing process, y iustrating the different condensing regimes. The experimenta resuts aso aowed the numerica mode to e vaidated, with the resuts in reasonae agreement over the temperature range considered. Figure 7: Steam Pume after seconds. Appication of the new steam condensing mode was successfuy appied to an industria proem and aowed severa design concepts to e evauated. These resuts aso gave new insights into the operation of the equipment and improved understanding of how process factors affect the rate of steam condensation. ACKNOWLEDGEMENTS The author woud ike to thank the Aughinish Aumina for permission to puish this work and their aoratory staff for their kind assistance with the experimenta work. 4

5 REFERENCES BLINKOV, V.N., JONES, O.C., NIGMATULIN, B.I., (1993), Nuceation and fashing in nozzes-. Comparison with experiments using five-equation mode for vapour void deveopment. Journa of Mutiphase Fow 19(6): CHUN M.H., KIM Y.S., PARK J.W., (1996) An investigation of direct condensation of steam jet in sucooed water Int Comm Heat Mass Transfer 3(7) LEE, S.I., NO H.C., (1998) Improvement of direct contact condensation mode of RELAP5/MOD3.1 for passive high-pressure injection system Annas of Nucear Energy 5(9): LEE, S.I., NO H.C., (1998a) Gravity driven injection experiments and direct contact condensation regime map for passive high-pressure injection system Nucear Engineering and Design MARSH, C., (004). Numerica simuation of fashing fow. MEng Thesis, University of Limerick, Ireand. 5