THE HEAT AND FLUID FLOW ANALYSIS FOR WATER HEATER

Transcription

1 Ln, C.-N., et al.: The Heat and Flud Flow Analyss for Water Heater S81 THE HEAT AND FLUID FLOW ANALYSIS FOR WATER HEATER by Chen-Nan LIN a*, Cheng-Ch WANG a, and Y-Pn KUO b a Department of Mechancal Engneerng, Far East Unversty, Hsn-Shh, Tanan, Tawan b Department of Electronc Engneerng, Far East Unversty, Hsn-Shh, Tanan, Tawan Orgnal scentfc paper UDC:66.046:662.75: DOI: /TSCI11S1081L In ths paper, the heat transfer and flud flow are studed for the water heater of RV cars, n whch the hot water s heated by the combuston energy of lquefed petroleum gases. Three types of combuston tubes are performed n ths nvestgaton, whch are crcular tube, ellptc tube and ellptc tube wth screwed wre nserted. The heat transfer performances of numercal smulaton results are compared wth those of the expermental works; they are n good trend agreement. The ellptc combuston tube performs better than the crcular one, whch ndcates the average 7% energy savng for the ellptc combuston tube and 12% energy savng for the ellptc combuston tube wth screwed wre under statc heatng. Key words: numercal smulaton, water heater, combuston Introducton Water heaters are wdely used n varous ndustral and domestc applcatons for hot water. The heatng energy may come from dfferent knds of resources, such as petroleum fuel, natural gas, electrcal power or solar energy. And combuston s stll the convenent way to produce thermal energy for outdoor uses. The recreaton vehcle s one example, n whch the water heater adopts lquefed petroleum gases or lquefed natural gas as fuel. The combuston reacton occurs n the combuston chamber, whch s enclosed by the water tank fllng wth cold water. The thermal energy s then transferred from the produced gases through the chamber walls to the cold water wthn the tank. The structure of the water heater s shown n fg. 1. The fuel s njected from a nozzle and flows through a Ventura tube to nduce the frst nlet fresh ar. The mxed gases are led to the combuston chamber through the gas tube, and the second nlet ar s also supplemented near the outlet of the mxed gas tube to make up the proper ar to fuel rato. It s dffcult to measure the temperatures and gases flow dstrbutons n the combuston chamber by expermental work due to the hgh temperatures, whch are normally above 2000 K. Therefore, the numerous numercal smulatons were wdely used to analyze the heat and flud flow n these years [1-4]. Saade and Koznsk s research [5] presented and *ncorrespondng author; e-mal: lncn@cc.feu.edu.tw

2 S82 Ln, C.-N., et al.: The Heat and Flud Flow Analyss for Water Heater valdated n a seres of combuston of fbrous sludge experments. Roux et al. [6] studed the numercal smulatons n a premxed swrled combustor, and used the LES model combned wth acoustc analyss; the numercal results qute matched the expermental data. Consderng the energy consumptons savng, Yutaka et al. [7] used the honeycomb regenerator to exchange the heat between the exhausted gas and the preheated ar and fuel to ncrease the heat effcent; the temperature of the preheated ar was up to 1600 K, and they also reduced the NO x producton n ther experment. Zhang and Neh [8] studed a novel vortex Fgure 1. The constructon of water heater combustor, whch s characterzed by a strong swrl and low temperature combuston envronment. The results show that the swrl provdes an effectve control of the gas-partcle slp velocty n the vortex combustor. Ths study ams to ncrease the heat flux by mprovng the combuston chamber n the water heater. The heat flux s descrbed by the equaton of Q = UADT. The temperature dfference, DT, s between the combusted produced gases and the cold water. The overall heat transfer coeffcent, U, s determned by the heat convecton coeffcent at both sdes of the combuston tube, whch depends on the heat transfer coeffcents and the thermal conductvty of the combuston tube. The heat transfer coeffcent, h, and the contacted area, A, are further consdered to enhance the heat transfer rate n ths study. However, a screwed wre s nserted n the second combusted chamber to ncrease the gas mxture, whch s a passve devce to ncrease the heat transfer coeffcent. Also, the larger contact area can be adopted by squeezng the combuston chamber from crcular shape to be ellptc shape. Under the same volume of combuston chamber, the ellptc tube performs larger surface area to volume rato (A/V) than crcular tube. Theory analyss Three types of combuston tubes are analyzed n ths nvestgaton ncludng crcular tube, ellptc tube and ellptc tube wth screwed wre nserted. The combuston tubes are separated nto two chambers by a baffle plate, whch enlarges the length of the flow channels to ncrease the wall heat flux between the gases and cold water. The geometrc dagrams of the three combuston tubes are shown n fg. 2. The combuston tube s placed nsde the water tank to heat the cold water. Consderng the water tank capacty, the volume of the ellptc tube (pabl) s equal to the volume of the crcular tube (pr 2 L). The dameter of the combuston tube s 4 nches for the crcular ppe, and the ellptc tubes are made of fve-nch crcular ppes, and squeezed to be 1:2:7 of aspect rato. The contnuty equaton, momentum equaton, energy equaton and speces dffusve equatons are as follows: x ( u ) p u u j 2 ul ( uu j ) j ( uu j ) x x x x x 3 x x j l j 0 (1) (2)

3 Ln, C.-N., et al.: The Heat and Flud Flow Analyss for Water Heater S83 [ ( E p)] k S eff T h j j j ( )( eff ) j h (3) ( Y ) J R (4) where uu j s Reynolds stress, u j T s Reynolds heat flux, E s total nternal energy, Y s mass fracton of speces O 2,N 2 and C 3 H 8, J s dffuson flux, R s mass source term due to combuston reacton. Propane gas was used as the fuel n ths combuston mode, and the fnte rate volume reacton combuston s consdered n ths study. The chemcal reacton equaton s: C H 5(O 3.76N ) 3CO 4H O 18.8 Q (5) N2 Fgure 2. (a) the crcular combuston tube (b) the ellptc combuston tube (c) the ellptc combuston tube nserted screw wre The standard two equaton k-e model was adopted n the smulaton; they are: ( u ) Gk Gb x x t k (6) ( u ) c1 ( Gk c3gb ) c2 (7) x x x k k t j where G k s the producton of turbulence knetc energy, G b the generaton of turbulence The fuel nlet pressure was set as Pa n gauge pressure and the temperature s set to be 300 K. The nlet and outlet pressure are consdered as pressure boundary condton of atmosphere and the temperature s set to be 300 K.The conjugated heat transfer occurs n the chamber wall, and the heat conducton equaton s solved. The exteror wall of combuston chamber s set to be the temperature of water. Numercal scheme The CFD Fluent 6.2 code s appled to analyze the combuston heat transfer. Grd ndependence s tested for dfferent combuston chamber; the errors are controlled wthn 0.5 K of average temperature dfferences exhausted gas at outlet boundary. The SIMPLE scheme s used to solve the pressure-velocty couplng equatons; frst order upwnd scheme s used to treat the convectve terms. The total grds of combuston chamber are 600,000 after the grds test for three sets of grds numbers; they are about 400,000, 600,000 and 800,000. The errors of wall heat flux are less than 1% between 600,000 and 800,000, and the estmated numercal error s 2.6% by numercal convergence analyss n ths numercal smulaton. Expermental analyss The outlne of expermental confgure s shown n fg. 1; the lquefed petroleum gas were suppled from the storage tank, where the flow rate s throttled by the regulator, and the 2

4 S84 Ln, C.-N., et al.: The Heat and Flud Flow Analyss for Water Heater gnton are controlled by the electrc sparker to produce the ntal combuston. The mass flow meter s used to measure the flow rates of the petroleum gases; the accuracy s wthn 0.1% of measured range. The accuracy of the pressure transmtter s 1% of span range. The varable area type s adopted n the flow meter for the dynamc test, and the accuracy s controlled wthn 2%. The K-type thermocouples are placed at the nlet and outlet tube of ar and water sdes. The total expermental uncertanty s 4% for the statc heatng and 5% for the floodng heatng n ths measurement. Results and dscussons The temperature dstrbutons and contours at dfferent sectons locaton are shown n fg. 3. The mxture of fuel gas and ar are njected nto the left chamber sde, and then combusted reacton occurs at the gnted regon. The produced gases flow through the head regon along the rght sde chamber and are exhausted to the outsde. fg. 3(a) dsplays that the temperature ncreases rapdly and then gradually decays along the gases flowng paths due to heat transfer to the chamber wall. By comparng the temperature profles of the three cases, t s found that the average temperatures at dfferent secton planes of the crcular type are hgher than those of the other two types, whch shows that the heat flux s the lowest n the crcular type. Also, the ellptc chamber wth screwed wre nserted demonstrates lower temperature profles than the types wthout wre nserted. The temperature dstrbutons at the second chamber are unform at every secton plane for case 1. Contrarly, the temperatures adjacent to the cold water are hgher than those of baffle sde for ellptc chamber, whch s due to the flowng of most gas along the wall of cold water sde. Fgure 3. The temperature contours at dfferent y-plane secton; (a) crcular combuston tube, (b) ellptc combuston tube; (c) ellptc combuston tube nserted screw wre The streamlnes released from the fuel nlet and velocty dstrbutons are dsplayed as fg. 4. The gases flow straghtly to the chamber head and change the drecton through the secondary chamber to the outlet. Crculatng flow appears at the secondary chamber of case 3;

5 Ln, C.-N., et al.: The Heat and Flud Flow Analyss for Water Heater S85 the produced gas flows around the screwed wre and the flow path s longer than others, whch enhances the perturbaton of exhausted gas and ncreases the heat transfer rate. The mxture of fuel and ar leaves the outlet of the mxed tube, whch causes the maxmum velocty to appear. And then the mxture flows nto the enlarged chamber, nducng the secondary ar nto the chamber. The maxmum velocty slghtly decays and appears n the central regons of the frst chamber due to the boundary layer effect. When the produced gases flow around the top of the combuston chambers, the maxmum veloctes appear along the water sde wall and flow faster than the neghbor flud near the baffle plane due to the nertal forces. Fgure 4. The streamlnes and veloctes contours for the three combuston chambers The dagram of the heat flux n the combuston tube walls vared wth tank water temperature for the three types of chambers are shown n fg 5. The heat transfer rate decreases wth the temperature of the surroundng cold water due to the lower temperature dfference between produced gases and the cold water. And the heat flux of the crcular combuston tube s averagely 7% lower than that of the ellptc tube wthout screw wre, and 12% lower than that of the ellptc tube wth nserted screw wre. The outlet temperatures of the exhausted gas at the ext of the combuston chamber also are shown n fg. 5. Ths reveals that the gas outlet temperature wll ncrease wth the water temperature for both the expermental and numercal results. Whle the cold water receves thermal energy to ncrease the temperature, t wll cause that the temperature dfferences between these two fluds decreases and the heat flux s reduced. The gas outlet temperature, therefore, ncreases to the degree as the temperature of the cold water. The expermental data are also shown n fg. 5; the temperatures are averagely about C hgher than the numercal results, whch yelds 10-14% dfferences. The dfferences caused by the thermocouples receve the radaton heat from the produced gases nsde the combuston chamber. If we compare case 3 wth case 2, t s obvous that the temperature gradent feld s steeper for case 3, ths s because the nserted screwed wre dsturbs the flow streams; more heat s transferred to the wall, nducng the lower ext temperature for case 3. Conclusons The nvestgaton of the three types of combuston tubes crcular tube, ellptc tube and ellptc tube wth screwed wre are performed n ths study. The ellptc combuston tubes perform better than the crcular one. For example, the

6 S86 Ln, C.-N., et al.: The Heat and Flud Flow Analyss for Water Heater ellptc combuston tubes wth and wthout the screwed wre are able to save 12% and 7% of energy under statc heatng. The crculatng flow appears at the secondary chamber of ellptc chamber wth screwed wre nserted, whch enhances the perturbaton of exhausted gas and ncreases the heat transfer rate. The results of the numercal smulaton are n consderably good agreement wth the expermental work; the average devatons are about 10-14%. References Fgure 5. The wall heat flux and outlet gas temperature for dfferent water temperatures [1] Furuhata, T., et al., Performance of Numercal Spray Combuston Smulaton, Energy Converson and Management, 38 (1997), 10-13, pp [2] Koh, P. T. L., Nguyen, T. V., Jorgensen, F. R. A., Numercal Modelng of Combuston n a Znc Flash Smelter, Appled Mathematcal Modelng, 22 (1998), 11, pp [3] Mtsuru, Y., et al., Modelng of Eddy Characterstc Tme n LES for Calculatng Turbulent Dffuson Flame, Internatonal Journal of Heat and Mass Transfer, 45 (2002), 11, pp [4] Yn, C., et al., Investgaton of the Flow, Combuston, Heat-transfer and Emssons from a 609 MW Utlty Tangentally Fred Pulverzed-Coal Boler, Fuel, 81 (2002), 8, pp [5] Raafat, G. S., Janusz, A. K., Numercal Modelng and TGA/FTIR/GCMS Investgaton of Fbrous Resdue Combuston, Journal of Bomass & Boenergy, 18 (2000), 5, pp [6] Roux, S., et al., Studes of Mean and Unsteady Flow n a Swrled Combustor Usng Experments, Acoustc Analyss, and Large Eddy Smulatons, Combuston and Flame, 141 (2005), 1-2, pp [7] Yutaka, S., et al., Heat Transfer Improvement and NO x Reducton by Hghly Preheated Ar Combuston, Energy Converson and Management, 38 (1997), 10-13, pp [8] Zhang, J., Neh, S., Swrlng, Reactng, Turbulent Gas-Partcle Flow n a Vortex Combustor, Powder Technology, 112 (2000), 1-2, pp Paper submtted: July 12, 2010 Paper revsed: September 16, 2010 Paper accepted: November 11, 2010