10987

10118

كارشناسي ارشد

تبديل انرژي

اصفهان: دانشگاه صنعتي اصفهان، دانشكده مهندسي مكانيك

1394

چهارده، 82ص.: مصور

ابراهيم شيراني، محمدرضا سليم پور

محمدرضا توكلي

94/11/18

احمد صابونچي، علي اكبر عالم رجبي

مهندسي مكانيك

ID10118

چكيده يكي از راهكارهاي افزايش انتقال حرارت در مبدلهاي حرارتي اسكككتفاده از افزونههايي مانند محيط متخلخل اسككت هدف در اين پروژه دسككتيابي به چيدمان بهبوديافته و توزيع طولي بهينه ماده متخلخل در لوله جهت باال بردن انتقال حرارت و بصكورت همزمان كنترل سكطح افت فشكار در ميزان توجيهپذير اسكت رژيم جريان آرام و اجباري با ك ك ك ك سكرعت ورودي 5 1 متر بر ثانيه در نظر گرفته شده است نسبت حجم ماده متخلخل به حجم لوله در تمامي هندسهها برابر مقدار ثابت 84 1 در نظر گرفته شكد قيد در اين مسئله ميزان ماده متخلخل و درجات آزادي شامل تعداد بلوك هاي ماده متخلخل فاصله بلوكها و نسبت اندازه اين بلوكها است شرايط مرزي ديواره لوله در دو حالت متفاوت شككار حرارتي ديواره ثابت و دماي ثابت ديواره در نظر گرفتهشككده اسككت با افزايش تعداد تكههاي ماده متخلخل در لوله ميزان انتقال حرارت هم افزايش يافت همچنين همواره براي داشكككتن بيشكككترين ميزان انتقال حرارت بزر ترين به كوچك بعد از اين تكه جاگذاري ميشككدند بعد از تكه از ماده متخلخل در ابتداي لوله و بقيه تكهها هم از بزر تعيين هندسككه بهينه در مسككئله تأثير سككاير پارامترهاي ماده متخلخل از جمله تخلخل عدد دارسككي و نسككبت ضككريب هدايت حرارتي مؤثر محيط متخلخل به سككيال موردمطالعه قرار گرفت براي بررسككي عملكرد ماده متخلخل از معيار ارزيابي عملكرد PEC اسكتفاده شكد با افزايش تخلخل عدد دارسكي و نسكبت ضريب هدايت حرارتي مؤثر محيط متخلخل به سككيال عدد PEC افزايش مييابد در حالت سككه تكه ماده متخلخل و در بهترين حالت براي شككرط مرزي شككار ثابت عدد 22 PEC و عدد ناسككلت 53 افزايش يافت براي شككرط مرزي دماي ثابت ديواره اين دو پارامتر به ترتيب 3 12 و 43 افزايش پيدا كرد درنهايت تمامي اين شككبيهسككازيها كه با فرض تعادل گرمايي موضككعي انجام شد با فرض عدم تعادل گرمايي موضعي مورد مقايسه قرار گرفت و اين نتيجه حاصل شد كه در عدد14 tkr اين دو فرض تطكابق خوبي دارند اما به ازاي 19 tkr اختالف زيادي در نتايج به وجود خواهد آمد بطوريكه عدد ناسكككلت متوسط در دو حالت 19 درصد اختالف پيدا ميكند كلمات كليدي انتقال حرارت محيط متخلخل طراحي ساختاري مدل عدم تعادل گرمايي موضعي مدل تعادل گرمايي موضعي

83Porous Media Longitudinal Arrangement Design for Improvement of the Internal Convective Heat Seyed Navid Roohani Isfahani n roohani@me iut ac ir Date of Submission 2016 01 16 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor M R Salimpour E Shirani salimpour@cc iut ac ir eshirani@cc iut ac irAbstractDue to the lack of energy sources improving energy conversion efficiency in heat exchangers is an importantsubject Also increasing the heat transfer in systems that are related to the energy consumption could reducethe environment pollution Different ways were suggested for enhancing the heat transfer in heat exchangers among them convective heat transfer in systems including porous plug ins attract many researchers due tothe wide applications of these systems in engineering processes and equipment like geothermal energyrecovery air heaters drying processes electronics cooling solid matrix heat exchangers decreasing sizeof heat exchangers for aerial industries etc By using porous media the space and material needed forheat exchanger would be reduced effectively The aim of this thesis is to achieve improved alignment andoptimal longitudinal distribution of porous material segments in the tube to enhance heat transfer withappropriate and controllable pressure drop Ratio between heat transfer and pressure for tube with porousmedia should be higher than tube without porous material Laminar and forced flow regime with the inputspeed of 5 0 meters per second is considered Volume ratio of porous material fixed to the tube at all times isconsidered as much as 18 of total volume of tube which is a critical quantity for porous material in tube Indicating degrees of freedom are the number of blocks of porous material distance between the blocks andsize of them Simulation was done using FLUENT software Wall boundary conditions in two different modesof constant heat flux and a constant temperature of the wall are taken into account A performance evaluationcriteria PEC which is the ratio of Nu number to pressure loss in tube with porous material in compare totube without porous media is defined to find best mode of porous media arrangement As the number ofpieces of porous material increased the heat transfer in tube increased too Moreover order of segments isfrom biggest to smallest in arrangement with highest PEC It is determined that three blocks of porous mediacould effectively increase heat transfer in compare with one and two segments geometry After determiningthe optimal geometry of the problem the impact of other porous material parameters such as porosity Darcynumber and the effective thermal conductivity of porous media was studied in fluid and porous region Inoptimized circumstances PEC increases 66 for constant heat flux boundary condition and Nu numberincrease 95 For constant temperature at the wall PEC is determined 62 9 higher than tube without porousmedia Moreover Nu number increases 95 for constant wall temperature boundary condition in tube Finally all the simulations that assume local thermal equilibrium as a model to simulate heat transfer inporous media were compared with the Local thermal non equilibrium assumption and it was concluded thatthermal coefficient ratio less than 10 these two models match very good However for thermal coefficientratio higher than these two models shows huge difference For TKR number higher than 30 it is determinedthat average Nu number difference is higher than 32 Keywords Heat transfer Porous media Convective heat transfer Performance evaluationcriteria Local thermal equilibrium Local thermal non equilibrium

ابراهيم شيراني، محمدرضا سليم پور

محمدرضا توكلي

احمد صابونچي، علي اكبر عالم رجبي