مدل سازي و شبيه سازي ديناميك سيال ﴿CFD﴾ يك مبدل حرارتي تحت تاثير الكتروهيدروديناميك در جهت تقويت انتقال گرما

چكيده انگليسي :

Computational Fluid Dynamics CFD Modeling of a Heat Exchanger under EHD Effect for Enhancing Heat Transfer Milad Nahavandi m nahavandi@ce iut ac ir Date of Submission 2 7 2012 Department of Chemical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor Arjomand Mehrabani Zeinabad Arjomand@cc iut ac irAbstract Enhancement of heat transfer is very important in saving energy and protection of environment Onthe other hand heat transfer duty of heat exchangers can be improved by heat transfer enhancementtechniques In general these techniques can be categorized into two groups of active and passive Theactive techniques require external forces e g electric field acoustic or surface vibration The passivetechniques require fluid additives or special surface geometries Electrohydrodynamic EHD has been usedwidely as an active method for enhancing heat transfer It protects environmental issues and energy sources The electrohydrodynamic EHD enhancement of heat transfers refers to the coupling of an electric fieldwith the fluid field in a dielectric fluid medium In this technique either a DC or an AC high voltage low current electric field is applied in the dielectric field medium flowing between a charged and a receiving grounded electrode Previous researches in the field of EHD augmented two phase flow have shown thatthe electric field induces an additional electrical body force on the flowing fluids One of its applications isin condensation of vapor In this investigation condensation of R 134a through interior pipe of a verticaldouble pipe condenser in presence of electric field simulated based on computational fluid dynamics CFD software Comsol Multiphysics The double pipe condenser with length of 1 meter is composed of aninternal pipe with internal and external diameter of 28 mm and 31 mm and external pipe with correspondingdiameter of 31 mm and 34 mm The condenser was equipped with a concentric copper electrode at variousdiameters of 4 6 8 12 mm Variation of phase field fluid temperature condensation heat flux electricfield dielectric permitivity and imposed electrohydrodynamic force on passing fluid through internal pipewere collected and analyzed After validation of simulation data with experimental results variation of R 134a condensation heat transfer coefficient versus applied voltages saturated vapour qualities electrodediameters and temperature differences between cold surface and the vapour were evaluated Simulationresults show an increasing on thermal conductivity of liquid R 134a influence of electric field at differenttemperature This coefficient increases by increasing the electric field strength and it decreases byincreasing temperature Also condensation coefficient of vapour R 134a decreases by increasing operatingpressure so for the vapour with quality of 75 at pressure up to 1200 kPa decreases to 0 04 and for thevapour with quality of 100 at pressure up to 2000 kPa drops to 0 18 Simulation results showscondensation heat transfer coefficient of R 134a saturated vapor under applied voltages 2 4 and 6 kV ishigher than similar value in the absence of electric field and it increases for higher electrode diameter Based on this result the maximum increase of R 134a condensation heat transfer coefficient is 37 7 byusing of the 12 mm electrode diameter applying of 6 kV applied voltages 20 Kelvin temperaturedifference 1948 kPa operating pressure and flowing of 0 015 kg s R 134a vapour Furthermore simulationresults show switching of applied electric polarity to pipe and electrode has no effect on condensation heattransfer coefficient of R 134a Keywords condensation heat transfer coefficient condensation coefficient phase field electrohydrodynamic force

نهاوندي، ميلاد

مدل سازي و شبيه سازي ديناميك سيال ﴿CFD﴾ يك مبدل حرارتي تحت تاثير الكتروهيدروديناميك در جهت تقويت انتقال گرما

ضريب انتقال گرماي چگالش , ضريب چگالش , ميدان فازي