Microwave heating in a pressurized chamber

Transcription

1 Microwave heating in a ressurized chamber John F. Hunt, P.E., Research Mechanical Engineer Hongmei Gu, Post Doc. Research Associate Jerrold E. Winandy, Research Suervisory Engineer Phili Walsh, General Engineer USDA Forest Service, Forest Products Laboratory Madison, WI 59th FPS & SWST Conference, Quebec City, Canada June 19, 25

2 Presentation Overview Background Research Goal and Study Objective Why model heat transfer in wood 2D Finite Element Heat Transfer Models 2D heat transfer model comarison Current Research Why microwave heating Microwave Pressurized Chamber Conductive Heating Rates Microwave Heating Rates

3 Background RESEARCH GOAL: Maintain a healthy and sustainable forest through an understanding of rocessing conditions that can be used to maniulate the roerties of wood and wood comosites for secific erformance characteristics. STUDY OBJECTIVE: To understand and evaluate the effects of conventional and microwave heating on dimensional wood in a ressurized environment.

4 Background Why model heating in wood? Uncontrolled heating can cause serious defects in the roducts if not aroriately alied, either with conventional or microwave energy. Understanding the flow and generation of heat energy within wood is critical to wood heating control to obtain the desired final roduct material roerties.

5 Background 2D cellular model Wood is Anisotroic, axial- symmetric, orous material. Each growth ring consists of two significantly different structures earlywood and latewood. Radial Tangential

6 Background 2D cellular model Path-R Radial Path-T % MC Inut arameter:s Cell orosity Cell alignment Cellwall roerties Air roerties Path-R Line Line Radial Line Tangential Line Path-T Tangential

7 Background 2D cellular model Cellular Model With moisture content in wood at 3 MCs: Fiber Saturation Point (FSP); 5/5 water/vaor in the lumen; Fully saturated; FS Cellwal Free water in Cell lumen Water vaor in Cell lumen

8 1 Background 2D cellular model Effective Thermal conductivity (at four MC conditions).9 Effective thermal conductivity (W/m.K Model - % MC Model - Fiber Saturation Point Model - 5/5 water/vaor in lumen Model - Fully Saturated MacLean - % MC MacLean - Fiber Saturation Point MacLeans - 5/5 water/vaor in lumen MacLean - Fully Saturated Density (Kg/m3)

9 Current Research Microwave Heating of Wood Because microwaves aly heat energy to wood using a significantly different mechanism than conventional conduction heating, it has the otential of imarting imroved roerties not ossible with conventional heating and drying. Therefore with this research we hoe to gain an understanding of the heating rocess and be able to evaluate and redict the effects of microwave heating on dimensional wood in a ressurized environment and comare those with conventional heating. Defrost the article, Warm, heating/exansion moving the liquid out from the center of the article, soften the fiber, dee enetration.

10 Current Research Microwave Pressurized Chamber Currently: Two 1 Watt 2.45 GHz household microwave generators Right Side/End View Left Side View To Microwave Radiator Internal Press Mechanism

11 Current Research Temerature Measurement Temeratures inside the board are measured using either our own shielded thermocoules or commercial fiber otics.

12 Conductive Heating Rate Conductive Heating Rate two boards at ressurized heating two boards at non- ressurized heating Different moisture content; density thermal conductivity; secific heat; T dt = k ρ C T

13 Normalization Conductive T dt T ( dt T ( dt = ) ) k ρc NEW NEW T ρc ( ) k ( ρc ( ρc std ) ) old std T ( dt k k std old ρc ) OLD ( k T ( ) dt OLD ) old k thermal conductivity; ρ -- density; C secific heat; subscrit std for standardized board, which assumes a board having 7% orosity and 3% moisture content (density of 431g/cm 3 ) ;

14 Heat Caacity (ρc) ( ρ C ) = V% ( ρc ) + V% ( ρc ) + V% ( ρc ) + V% ( ρc Total cellwall cellwall boundwater boundwater vaor vaor freewater ) freewater Density Secific Heat (Kg/m3) (J/Kg K) Cell wall substance (%MC) Air in the lumen (%MC) Bound water in cell wall Saturated cell wall (FSP) Water vaor in cell lumen Free water in cell lumen Cell wall Swelling Cell wall Vaor Free water Density and secific heat for each comonent in wood is summarized from the literature; Volume fraction for each comonent is calculated based on the microscoic structure of wood as a function of orosity and moisture content; Porosity is calculated from the ovendry density measured for each board by the relationshi of: ρ ovendry = ρ Porosity + ρ air cellwall ( 1 Porosity) Total heat caacity is also affected by the temerature. The linear relationshi between temerature and secific heat from Siau s textbook was alied to the final total heat caacity

15 Thermal conductivity (k) 1.9 Model - % MC Thermal conductivity k is obtained from our revious cellular model -- a theoretical value as a function of density (or orosity) and moisture content; Effective thermal conductivity (W/m.K Model - Fiber Saturation Point Model - 5/5 water/vaor in lumen Model - Fully Saturated MacLean - % MC MacLean - Fiber Saturation Point MacLeans - 5/5 water/vaor in lumen MacLean - Fully Saturated Density (Kg/m3)

16 Normalized Conductive Heating Rate Correlation between heating rate and moisture content for conductive heating is -.825; Correlation with (ρc) total is about Higher internal temeratures are ossible with ressurized heating; Moisture loss is less with ressurized heating;

17 Microwave Heating Rate Raw MW heating rate 66 boards ressurized heating 66 boards non- ressurized heating Different moisture content; density; secific heat; ower value; T dt = N ρ C T

18 Normalization Microwave T dt = T ( ) dt T ( ) dt N f N ρc NEW NEW N Microwave = T ρc ( ) k ( ρc ( ρc 11 tanδ loss tagent; std T ( ) dt ) old kstd ) N std ower value; 2 fε' tanδ E ; E electric old field ( * ρc ( N T ( ) dt OLD see frequency; ε' relative electric const.; strength note) OLD ) old N is a difficult arameter to obtain exerimentally. But N might be able to be determined by studying the ratio of N/k (MW heating arameter over thermal conductivity). This ratio can be obtained by testing the same board (having the same orosity and relatively same MC and density conditions), comaring the two heating rates to obtain N/k ratio. This factor will be used in the MW heating rate normalization * from Gregory I. Torgovnikov. (Dielectric Proerties of Wood and Wood-based Materials

19 Microwave Heating rate Correlation between heating rate and moisture content for MW heating is about -.778; Correlation with (ρc) total is about Pressurization increase the microwave heating rate; Higher internal temeratures are ossible with ressurized heating;

20 Absorbed Energy by Microwave vs. Conductive Heating Microwave heat u wood instantly; Energy absorbed into wood by Microwave heating is about 2-42 times more than the energy absorbed by conductive heating during the early 25 minutes; Moisture in wood has little effect on energy absorbed by MW heating, but significant effect on energy absorbed by conductive heating.

21 Absorbed Energy by Microwave vs. Conductive Heating Absorbed energy = ρc T; C is function of temerature, too Energy absorbed into wood by Microwave heating is about times more than the energy absorbed by conductive heating during the early 2 minutes; High moisture board absorbed more energy than low moisture board by conductive heating in a non-ressurized chamber.

22 Pressurized vs. Non-ressurized Heating Energy Pressurized environment has a ositive effect on conductive heating energy absortion, but has a little negative effect on microwave heating energy absortion.

23 ( ρ C ) = V% ( ρc ) + V% ( ρc ) + V% ( ρc ) + V% ( ρc Heat transfer in wood where does the energy go into wood? Total cellwall cellwall boundwater boundwater vaor vaor freewater ) freewater Cell wall Swelling Cell wall Bound water Vaor Free water Volume fraction for each comonent is calculated based on the microscoic structure of wood as a function of orosity and moisture content; Assume a unit cell with orosity (% - 95%) swells to (1+ l) (1+2 l); Moisture content ranges from % to the maximum moisture content at each orosity;

24 Heat transfer in wood where does the energy go into wood?.8 Porosity Energy total Energy Cellwall %Total.1 % Energy vaor Porosity Porosity Moisture Content Moisture Content Moisture Content Total stored energy Percent of total energy stored in cell wall Percent stored in lumen vaor.8 Porosity % Energy free water Energy bound water %Total Porosity Moisture Content 1.5 Moisture Content Percent of total energy stored in bound water Percent of total energy stored in free water

25 Heat transfer in wood where does the energy go into wood? 12%.8 Porosity.6.4 1%.2 Percent in Total Store Energy 8% 6% 4% 2% cell wall free water total bound water vaor.8.6 Energy Cellwall %Total Moisture Content Percent of total energy stored in cell wall % % 2% 4% 6% 8% 1% 12% 14% 16% 18% 2% -2% Moisture content (%).8 Porosity.6.4 % Energy free water Porosity.4.3 Energy bound water %Total Moisture Content Moisture Content Percent of total energy stored in bound water Percent of total energy stored in free water

26 Summary The heating rate is correlated to moisture content and heat caacity acity of each board by both conductive and microwave heating; Higher internal temeratures are ossible with ressurized heating; Energy absorbed into wood by Microwave heating is instant and 2-4 times for ressurized and times for non-ressurized more than the energy absorbed by conductive heating during the early 2 minutes; Moisture in wood has little effect on energy absorbed by MW heating, but significant effect on energy absorbed by conductive heating. High h moisture board absorbed more energy than low moisture board by conductive heating in both ressurized and non-ressurized chamber. Pressurized environment has a ositive effect on conductive heating energy absortion, but has a little negative effect on microwave heating energy absortion.

27 Acknowledgement Jane O Dell, General Engineer, Forest Products Laboratory; The Performance Engineered Comosite grou, USDA Forest Products Laboratory. 59th FPS & SWST Conference, Quebec City, Canada June 19, 25

28 Questions and Comments? Thank you! 59th FPS & SWST Conference, Quebec City, Canada June 19, 25