Coupled Electromagnetic and Heat Transfer Simulations for RF Applicator Design for Efficient Heating of Materials

Transcription

1 Coupled Electomagnetic and Heat Tansfe Simulations fo RF Applicato Design fo Efficient Heating of Mateials Jeni Anto 1 and Raj C Thiagaajan 2 * 1 Reseache, Anna Univesity, Chennai, 2 ATOA Scientific Technologies Pivate Limited *Coesponding autho: ATOA Scientific Technologies Pvt Ltd, #204, Regent Pime Business Pak, #48 Whitefield Main Road, Whitefield, Bangaloe , India, Raj@atoastech.com Abstact: Conventional heating of mateial wastes enegy duing heating due to inheent adiation, conduction and convection based heating mechanism. Altenate efficient heating methods ae actively eseached fo impoved efficiency and quality. Radio fequency based electomagnetic heating is inceasingly used fo efficient heating in place of conventional adiation based heating.. The adio fequency based heating, which equies coupling of electomagnetics and heat tansfe fo pefomance evaluation is used fo design of an RF electomagnetic applicato. A dielectic disk is consideed fo heating pefomance evaluation. The methodology, mateial popeties used and simulation esults ae epoted. The unifomity of heat application o electomagnetic enegy distibution is used as metic to evaluate the efficiency of the RF heating applicato. The vitual design and heating esults and compaisons ae epoted. The multiphysics coupling and paametic modeling capability of COMSOL fo optimal design of applicato is highlighted. Keywods: Heating, RF heating, Electomagnetic heating, dielectic heating, coupled heat tansfe, unifom heating. 1.0 Intoduction RF based Electomagnetic heating method povides oppotunity fo efficient heating of mateials and impoved quality [1-5]. The enegy and cost efficient designs ae equied as a competitive diffeentiato fo new poduct and pocess development. Electomagnetic heating povides an altenative to conventional adiation based heating. This pape details about an RF applicato design fo electomagnetic heating of dielectic disk. The coupled electomagnetic heat tansfe fomulation methodology and implementation of the model in COMSOL is given. An analytical fomulation was also developed to pedict the oveall pefomance and is compaed with COMSOL simulations. The oveall design methodologies and simulation esults ae epoted. A special focus is given to the applicato design fo unifom field and hence heating. The inside out heating pefomance of RF heating compaed to conventional outside in heating is highlighted by compaing the powe unifomity index. The numeical Design of Expeiments (DoE) elated to applicato design is detailed. The paametes contibuting to the unifomity is highlighted and calibated against the physical fabication limitation. The optimal and finalized configuation and the esults ae also epoted. 2.0 Govening Equations The following two Maxwell equations goven the inteaction of electomagnetic field with mateial popeties and elate the time vaiations of one field to spatial vaiation of the othe [1-5]. E x H E H x E Whee, E, electicfield vecto, H, magneticfield vecto,, conductivity,, pemittivi ty,, pemeability. The applicato design in the RF fequency ange can be modeled using sinusoidal time vaying steady state govening equations, due the

2 electically small size of the applicato, as follows, x H E j E x E j H Whee,, is theangula fequency, j, is thecomplex opeato. The adio fequency inteaction with mateial fo heating is elated to the pemittivity of the mateial. The complex elative pemittivity compises a eal pat which is esponsible fo phase shift and the imaginay pat which is esponsible fo the enegy loss, as follows, Whee, is theelativepemittivi ty o dielecticconstant and j is theloss o dissipation facto. The govening equation elating the cuent density to the mateial popeties can be witten as follows, J (σ ε0 ε ω)e Whee, J, Cuent density j ε The dielectic loss component of inteest to RF heating can be witten as follows, J ε 0 ε ωe The powe dissipation facto pe unit volume (Pv) in W/m3 fo dielectic loss effect is, Pv ε0 ε ωe 2 This equation can be used to estimate the oveall volumetic behavio and limiting design paametes. 0 ε E The heating of mateials is govened by the Fouie heat tansfe equation, as follows, T C p ( k T ) Pv Whee, ρ, is thedensity, C,is thespecific heat, p k, is thethemalconductivi ty,and T,is thetempeatue. The volumetic tempeatues ise (dt) due to the powe deposited into the mateial can be calculated as follows, Pv t ΔT M C Whee, Ma,is themass, and t is a p thetime. The above equation assumes that the whole electical enegy is conveted into themal enegy and thee ae no heat and othe losses. The coupled electo themal numeical model will povide us the pedictive capability fo ealistic potential distibution, powe density and tempeatue. The analytical and numeical fomulation and esults ae used fo pefomance compaison Simulation methodology COMSOL Multiphysics was selected fo electical, themal and coupled electo themal simulation. The AC/DC module was used fo estimating the powe unifomity index and optimization of electode. The micowave heating module was used fo estimating the tempeatue distibution. The eades ae efeed to the COMSOL theoy manual fo futhe details elated to the theoy and implementation of coupled electomagnetic and themal simulation [1-5]. A paametic CAD model of the dielectic heating applicato and the heating enclosue was developed. This is used fo estimating the oveall pefomance. Fo electical field potential

3 distibution, the AC/DC module was used. Obtaining unifom powe distibution, i.e., the powe unifomity is a citical challenge and hence a method to estimate the powe unifomity index (PUI) is implemented in COMSOL. The impotance is given to the PUI estimation. The Equation fo PUI is given as PUI= ( (( ) )) Whee the Volume of the sample is, is the local powe dissipation density. It is given as, whee f is the fequency of input signal, is the vacuum pemittivity, is the imaginay pat of the elative pemittivity of the sample (Dissipation facto), and E is the electic field stength. The unit of this tem is W/m 3. is the Aveage Powe Dissipation. It is calculated fom the fomula We used paametic model, pobe vaiable and deived values of COMSOL Multiphysics fo pedicting the PUI as function of optimization paametes. Fom the above equations, it is clea that PUI efes to the unifomity of electic field powe distibution. It is undestood that PUI should be smalle fo bette RF powe unifomity in the dielectic mateial. The objective function of optimisation was to minimise the PUI as function of design vaiables such as electode size, distance, electode shape. This DOE helped us to locate the best design paametes of the applicato fo lowest PUI. The paametic model enabled to pefom sweeps to exploe the oveall tend. A detailed analysis was then pefomed fo the final selected esults. The effects of electode distance, size, shape, ai volume, sample distance fom the electode ae paameteized fo the design of expeiments. The dielectic sample size was fixed at constant size of 50 mm diamete and 4 mm thickness. The simulation Methodology and steps ae summaized as follows, a. Pepocessing of the CAD model of the sample, electodes and ai volume. b. Define Mateials and find capacitance of the stuctue using COMSOL. Then numeically benchmaking the COMSOL pediction with analytical. c. DoE of electode configuation, distance, dielectic disk size and shape. d. Optimization of stuctue fo lowest powe unifomity. The dielectic popeties (Dielectic constant and loss facto) of the disk ae shown in table 1. Futhe, specific heat capacity of 2059 J/Kg C, and density of 1541 kg/m3 is consideed. In addition a ange of popeties ae also used fo estimation of bounds fo DoE. The tempeatue dependent mateial popety models ae also used as appopiate to facto the themal ise. Table 1. Typical mateial popeties used fo Simulation. Mateial Ai Wate (20oC) Wate (100oC) Dielectic Disk Results and Discussion This section povides the citical obsevation and esults elated to the applicato design fo unifom heating of dielectic disk. The DoE esults, pedictive pefomance of the applicato electode, optimized electode geomety ae detailed. The PUI and heating esults ae shown in Figue 1-4. Figue 1 and 2 shows the Powe unifomity optimization esults. Figue 3 and 4 shows the RF heating pefomance as function of dielectic popeties of the disk. The size of electode was changed fom 30 mm to 100 mm diamete and esults ae as shown in figue 1, fo an electode gap distance of 10 mm. The PUI was lowe aound the diamete of the dielectic disk and is lowest at aound 48 mm. This is compaable to the size of the sample. An electode size of 46 to 50 mm will povide us PUI of less than 0.1. The electical field and field stength distibution is at the lowest PUI is

4 shown in figue 1. Futhe the sample distance between the electodes was investigated and the sample at the cente povided the lowest PUI. Figue 1. Powe unifomity index as a function of the diamete of the applicato. An investigation pefomed to study the effect of mateial popeties on the heat up pefomance using the micowave-heating module. The sample dielectic constant and loss facto vaied fom 6.5 to 15 and 1 to 10, espectively. The incease in dielectic constant and loss facto inceases the heat buildup. The loss facto plays a significant ole in the heat buildup. This simulation is study the effect of mateial popety on the pefomance. The tempeatue contou plots ae shown in figue 3 and 4. Figue 3. Typical coupled electo themal esults and tempeatue distibution fo Dielectic constant of 6.5 and loss facto 1 Anothe DoE was pefomed on a lage electode with an angle between top and bottom suface. An electode diamete of 100 mm is consideed with lowe intenal diamete as function of angle. The esults ae shown in figue 2. The lowest PUI was obseved at at aound 80 degee. Figue 4. Typical coupled electo themal esults and tempeatue distibution fo Dielectic constant of 15 and loss facto 10 Figue 2 Powe unifomity index as a function of the edge angle of the applicato. Based on the applicato design and heating pefomance optimization, a RF heating setup fo dielectic disk was fabicated and expeimentally evaluated fo unifom heating. The esults wee in good ageement with pediction and futhe helped fo expeimental optimization.

5 5.0 Conclusions Electomagnetic RF heating povides an altenate and enegy efficient heating method fo pocessing of mateials. A bief intoduction about heating of mateials was given. The govening equation elated to RF heating was given. A metic to measue the unifomity fo RF heating was developed and implemented in COMSOL. A coupled electo themal simulations fo pedicting the unifomity of RF heating was pefomed. Optimal distance and configuation of the applicato electode was identified fo unifom and efficient heating. These investigations wee used fo expeimental design and development of an RF based heate. Technologies fo the 100:20 eseach fund fo this wok and appoval fo publication. 6.0 Refeences 1. G. Tiwai et.al., Analysis of adio fequency (RF) powe distibution in dy food mateials, Jounal of Food Engineeing 104, , V. Komaov et.al, Pemittivity and Measuements, Encyclopedia of RF and Micowave Engineeing, Edited by Kai Chang, John Wiley & Sons, Inc, D Stuega, Micowave Mateial Inteactions and Dielectic Popeties, Key Ingedients fo Mastey of Chemical Micowave Pocesses, Micowaves in Oganic Synthesis, Second edition. Edited by A. Loupy, WILEY-VCH Velag GmbH & Co, Bila, S.L., Wang, S., & Tang, J., Compute simulation of adio fequency heating of model fuit immesed in wate. Jounal of Food Engineeing, 84(2), G. Bodie, Simultaneous heat and moistue diffusion duing micowave heating of moist wood, Applied Engineeing in Agicultue, Vol. 23(2): , Acknowledgements The authos would like to thank the Reseach and Innovation division of ATOA scientific