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Non-uniformity investigation in a combined thermal and microwave drying of silica gel
Applied Thermal Engineering 98 (2016) 872–879
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Non-uniformity investigation in a combined thermal and microwave drying of silica gel ChunFang Song a, Yan Wang a, Shuguang Wang b, ZhengWei Cui a,*, Yanfeng Xu a, Haiqing Zhu a a b
Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi, Jiangsu, 214063, China Agricultural College, Inner Mongolia Agricultural University, Huhhot, Inner Mongolia, 010019, China
H I G H L I G H T S
• • • • •
Uniformity in combined thermal and microwave (CTMW) dryer was analyzed. Local temperature and moisture of silica gel were determined to analyze uniformity. The effects of microwave power and temperature of the hot air on drying uniformity were investigated. The uniformity changes in the horizontal direction of the drying cavity were not obvious. The temperature gradient of silica gel between the layers can reach 5 °C.
A R T I C L E
I N F O
Article history: Received 23 April 2015 Accepted 22 December 2015 Available online 31 December 2015 Keywords: Combined thermal and microwave drying (CTMW) Silica gel Uniformity Microwave drying Temperature distribution
A B S T R A C T
Local temperature and moisture were determined to analyze the uniformity in vertical and horizontal planes in a combined thermal and microwave (CTMW) drying chamber. Furthermore, the effects of microwave power and temperature of hot air on drying uniformity were also investigated. It could be concluded that the dried silica gel in the center area of the drying chamber obtained the highest drying rate and temperature, followed by the silica in the outermost and the middle portions. The dehydration difference of the three layers decreased with the drying time, whereas the drying uniformity increased. Overall, the change in uniformity in the horizontal direction of the drying cavity was not obvious, and the temperature and moisture content gradients mainly occurred in the vertical direction. The temperature gradient between the layers reached 5 °C. As the hot air temperature and microwave power increased, the drying rate improved and the drying uniformity decreased. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Microwave (MW) heating offers several distinct benefits, including increasing throughput and higher energy efficiency, but its intensity and penetration depth depend on the physical and dielectric properties of the substrate and can vary with temperature, frequency, composition and shape. With bio-products characterized by low thermal conductivity, MW heating may exhibit a certain non-uniformity in the temperature distribution, leading to local overheating or even run-away loci [1–3], which not only damages the quality of the food due to hot spots, but also raises the issue of food safety as pathogenic microorganisms may not be destroyed in cold spots [4]. Heating uniformity in microwave processing can be improved in different ways. Currently, modification of relevant food or package
* Corresponding author. Tel./fax: (+86) 51085910390. E-mail address: [email protected] (Z. Cui). http://dx.doi.org/10.1016/j.applthermaleng.2015.12.089 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
parameters to optimize the heating uniformity are important in food product development [5–8]. Microwave oven designs such as rotating turntables in household microwave ovens, moving conveyor belts in industrial appliances and mode stirrers often result in uniformity [4]. However, due to their limited response in process performance, MW treatments have been renovated from MW irradiation alone to hybrid processing [9–12]. Combined thermal and microwave (CTMW) drying has been proposed for over a decade. Volumetric heating (caused by microwave power) drives moisture from the product’s interior toward the surface, where it is removed by the surrounding heated air currents. This coupled drying method has been shown to be a promising technology for reducing drying times and providing the desired temperature profiles needed for specific food processes [13–19]. To date, CTMW dryers have been successfully operated only on a small, experimental scale [20–22]. Attempts to make such dryers for industrial-scale use have been limited. In a laboratory-scale dryer, the volume of the cavity is small, and in such a small space, the nonuniformity of the microwave distribution is not notable. However,
C.F. Song et al./Applied Thermal Engineering 98 (2016) 872–879
the non-uniformity of the microwave distribution becomes a serious problem in an industrial-scale dryer. To overcome this difficulty, the main strategies available for use by most researchers are to move or float the drying materials around the cavity with convective air [21–23]. This strategy, however, leads to difficulties in managing the structure of the coupled dryer for large industrial-scale cavity. Moreover, most of the research on combination CTMW heating has been conducted in the context of microwave convection drying characteristics and simulations [24–26], in which the spatial uniformity of heating and drying was not deemed to be the key parameter and was therefore seldom reported. Most of these studies have treated the food to be dried as one lump sum so that spatial information from different points in the cavity was not recorded. Measurements of temperature and moisture distributions in the cavity during CTMW drying could capture the spatial and temporal heating patterns, and such information could provide valuable insights into the nature of combination heating. Such information could enable more effective designs for coupled CTMW dryers. However, no such study has been recorded in the literature. In this study, a novel CTMW dryer was designed and fabricated. In particular, a new strategy was tested that involved varying the distributions of microwaves and allowing the dried materials to remain motionless in the cavity. Using silica gel as the experimental material, changes in the temperature and moisture of samples in specified zones of the dryer plates were measured to evaluate the uniformity of heating and drying. The advantages of the combined CTMW method were apparent in the results, which demonstrated a more convenient means of managing temperature with greater uniformity in the microwave distribution. Such results raise the possibility of building CTMW dryers on a large or industrial scale. Therefore, the objectives of this study were (1) to determine local temperature and moisture in horizontal and vertical planes in the drying chamber using silica gel in an experimental test and (2) to explore the effect of MW power and the temperature of hot air on drying uniformity. 2. Materials and methods 2.1. Materials Silica gel (Model 10018360, National Medicine Group Chemical Reagent Co., Ltd., China) was used as the test material. Before the experiment, the gel was saturated with water vapor until the moisture content reached to 27.2% (w.b.). Silica gel has strong adsorption affinity for water vapor in air. Dry silica gel is blue, and wet silica gel shows different colors that depend on the quantity of crystallization water [29]. 2.2. Drying equipment The pilot-scale CTMW dryer was designed by the authors (Fig. 1a). The dimensions of length, width and height of the multimode rectangular cavity are 540 × 400 × 550 mm, and the volume is 115 L. There is a certain interval of 10 cm between the three trays. Each layer tray had an area of 0.17 m2 and was made of a glass fiber mesh grid. The heating feasibility of a material via MW irradiation depends on dielectric properties (or permittivity) [3]; trays with a low dielectric constant almost do not absorb MW energy and ensure it was absorbed by the working samples and allow air and water vapor to pass through them. The hot air production port was located in the back of the drying chamber (or cavity). This port consisted of a centrifugal fan, an electric heating tube and a circulating air duct. The hot air was cycled through the drying chamber, and the high humidity air was discharged through an air vent at the top of the dryer cavity. The size of the air vent could be adjusted to change the air velocity. The hot air temperature could be changed within
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a range of 40~120 °C, and the air velocity varied in the range of 0.3~1 m/s. The composition and structure of the microwave power feed port is shown in Fig. 1b. This structure consisted of a magnetron, a waveguide and an (electric and magnetic) EM field mode stirrer. The microwave power transmitted by the magnetron was transferred through the waveguide to a metal tray connected to the drying chamber. The metal mode stirrer was installed at the outlet of the waveguide and rotated by a motor at 25 rpm. This mode stirrer device mechanically changed the emission conditions and periodically disturbed the regularity of the EF (or electric field). The working frequency of the microwave was 2450 MHz, and a digital inverter solution for which the energy output could be adjusted from 100 to 1500 W by step-less high voltage regulation for the microwave oven (Model: WepeX 1600A, Shenzhen Mebmeet Electrical Technology Co., LTD, Shenzhen, China) was adopted. The nominal power is the actual power output of the microwave inverter indicated by its control panel (Model: WepeX-C1, Shenzhen Mebmeet Electrical Technology Co., LTD, Shenzhen, China). All times were measured using a PC-based stopwatch. Before the experiment, the hot air heater was opened for at least 30 min. 2.3. Determination and analysis methods 2.3.1. The absorbed power A calorimetric method was adopted to evaluate the nominal MW power P supplied to the sample. One liter of tap water was weighed in a beaker, and its initial temperature was read by a K-type thermocouple. Following a fulltime exposure to MW, the water was briefly mixed, and its temperature was read again to determine P. The absorbed power was determined by the following relationship [1]:
p = CPm
ΔT t
According to the calculation, nominal powers of 500 W, 600 W and 700 W in the experiments correspond to actual absorbed powers of 490 ± 5 W , 588 ± 10 W , and 685 ± 15 W , respectively. The humidity ratio:
MR =
M − Me M0 − Me
M——The mass of the samples after t minutes of drying time M 0——The initial mass of the sample M e ——The quality when the sample has moisture balance 2.3.2. Determination of drying uniformity The water uniformity K a was used to describe the drying uniformity of each dish: K a = ( x − Δx ) x × 100% x ——The average value of dehydration in all trays (g) Δx ——Variance of dehydration in all trays (reflecting water dispersion degree for dehydration in all dish) K a values closer to 100% indicate a greater degree of uniform drying. 2.3.3. Determination and analysis by Infrared thermal imaging An FLIR T440 infrared thermal imager (FLIR Systems company, USA) was preheated for 10 min before each test. Infrared emissivity was set at 0.95, and the detection distance was kept at a distance of 50 cm. Detected data were analyzed by FLIR Reporter software. This method of determining the temperature only allows for measurements of surface temperature.
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a
b
Fig. 1. Schematic of the CTMW dryer. 1. Multimode rectangular cavity; 2. Microwave input system; 3. Material tray; 4. Mode stirrer; 5. Drying oven door; 6. Observation window; 7. Vent; 8. Control panel; 9. Fan; 10. Heater. a. Structural schematic of the combined thermal and microwave oven. 1. Mode stirrer; 2. Metal tray; 3. Motor; 4. Wave guide; 5. Magnetron. b. Structural diagram of the microwave power feed port.
Data for the material temperature distribution were extracted, which included the temperature (Ti) and the points of sample temperature within the temperature scope (M i). The ratio of the points of sample temperature (M i ) within the temperature scope is K i ( K i = Mi ΣMi ) . The uniformity of drying is measured by the temperature standard deviation [28]. The average temperature T: T = k i * T i The standard deviation for temperature δ: δ = k i (T i−T) The uniformity of drying is measured by the value of the standard deviation for temperature [27].
2.4. Experimental methods 2.4.1. Determination of the bulk temperature distribution for hot air drying A hot-wire anemometer TSI9535-A (TSI Incorporated, Shoreview, MN, USA) combined with the data for the infrared thermal images was used to determine the bulk temperature distribution, and the air velocity was kept at 1 m/s for different hot air inlet
temperature treatments. Only the fan and heater were opened in the CTMW drier. 2.4.2. Determination of uniformity between different trays To obtain the same experimental conditions, 500 g of water was preheated for 10 min before each experiment under 600 W of MW power and a hot air temperature of 60 °C. A total of 500 g color silicone were accurately measured and evenly spread as a monolayer on the tray of the CTMW oven. The initial MW power density was 1.2 W/g. Samples were taken out to measure the weight, and the FLIR T440 infrared thermal imager was used to quickly take pictures every 5 min. Then, the samples were sent back to the CTMW oven as soon as possible; the time interval was less than 10 s until the moisture reached 5%. Each test was repeated 3 times. 2.4.3. Uniformity of the same tray The middle tray was chosen as the test plate. The infrared thermal imager was used to take pictures at intervals of 10 min, 20 min and 30 min. Material from 13 different parts of the tray was sampled, and 100 g silica gel particles were randomly sampled across the test
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Fig. 2. Distribution of the testing point of the material in the plate.
position. The distribution of 13 sample points is shown as Fig. 2. The Y-axis was from the front to the back of the tray. The X-axis was from the left to the right side of tray. The intersection point of the two axes was O (0, 0). 2.4.4. The effect of MW power on temperature distribution and drying uniformity A total of 500 g saturated silica gel particles was put into the oven and evenly spread on the fiberglass dry net with inlet air temperature of 60 °C and different MW powers of 500, 600, and 700 W. The drying process continued until the moisture reached 5% (w.b). 2.4.5. The effect of hot air temperature on the temperature distribution and drying uniformity The operation for testing the effect of hot air temperature on the temperature distribution and drying uniformity was the same as that of MW power. The silica gel was dried at a power of 600 W and different hot air temperatures of 50, 60, 70 °C until the moisture reached 3% (w.b.).
3. Results and discussion 3.1. Determination of the temperature distribution for hot air drying in the CTMW dryer The bulk temperature distribution for hot air drying in the CTMW dryer can be seen in Table 1. The bulk temperature became steady after 30 min. The bulk temperature of the upper layer is nearly equal to that of the middle layer, whereas the temperatures of these two layers are higher than that of the lower layer. The degree of uniformity for three layers is high (above 95%).
3.2. Assessment of average convective transmittance The correlations from Araszkiewicz et al. [30] for the heat transfer coefficient in the spout-fluidized-bed drier and Arballo et al [31] in the fixed bed dryer have been successfully used for carrots and soybean grains, and thus were incorporated in this study.
Table 1 Bulk temperature distribution for hot air drying in the CTMW dryer. Inlet hot-Air temperature (°C)
50 60 70
Bulk temperature (°C) Upper layer (min)
Middle layer (min)
The degree of uniformity K a (%)
Lower layer (min)
10
30
10
30
10
30
41.2 53.3 59.8
49.8 59.8 68.3
42.2 45.3 58.4
50.0 60.0 69.5
39.5 42.1 53.2
48.1 57.1 62.6
97.8% 96.3% 96.1%
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Correlation equation used for evaluation of the heat transfer coefficient: Nusselt number Nu = hTλdE = 2.0 + 0.6 ( Pr ) vdρ and Reynolds number Re = μairair
13
a
+ ( Re )
12
The air parameters are as follows:
ρair = −3.510101 × 10−8 T f3 + 1.583982684 × 10−5 T f2 − 4.6995202202 × 10−3 T f + 1.2913571428571 (10°C < T f < 80°C ) μair = −1.7676768 × 10−13 T f3 − 5.541125541 × 10−11T f2 + 4.983297258297 × 10−8 T f + 17.1964285714285 × 10−6 (10°C < T f < 80°C ) λair = 6.8181818 × 10−10 T f3 − 1.474025974 × 10−7 T f2 + 8.029112554113 × 10−5 T f + 0.0240835714285714 (10°C < T f < 80°C )
b
Pr = −2.272727 × −8 T f3 + 4.1991342 × 10−6 T f2 − 3.5335497835 × 10−4 T f + 0.719 (10°C < T f < 80°C ) where T f = TS +2Tair (Ts- Surface temperature of samples) and the sphere diameter equivalent to a particle (silica gel) of volume dE = 3 6 VπE Using the local Reynolds and Prandtl numbers, the average Nusselt numbers could be obtained as follows: 2.243 for a hot air temperature of 50 °C, 2.205 for 60 °C and 2.175 for 70 °C. Based on rig details (Fig. 1a). The working air is drawn in by the back blower. Then there is a positive pressure build-up in the cavity, and the air is vented via the little ceiling opening. In these operating conditions, the working air is strongly stirred in the cavity, and the obtained Nusselt numbers reflection convective heat transfer intensity in different hot air temperature, while the Nusselt relationships chosen can hardly be employed in this rig. 3.3. Uniformity between different trays
Fig. 3. Temperature (a) and standard deviation of temperature (b) as a function of drying time in each plate.
The degree of uniformity of each dehydration plate can be seen in Table 2. The dehydration rate was highest in the beginning stages of drying. The dehydrations for the upper, middle and lower layer were 29.94 g, 30.91 g and 36.28 g, respectively, and the variance of dehydration for all trays was 7.78 after 10 min of drying. The dehydrations for the upper, middle and lower layer were 9.89 g, 9.13 g and 8.12 g, respectively, after 40 min of drying, and the variance of dehydration for all trays decreased considerably to 0.73. The results showed that the degree of non-uniformity decreased gradually. In addition, the dehydration of the lower (bottom) tray differed substantially from the other layers, which showed that the degree of non-uniform drying in the vertical direction is greater than in the horizontal direction. The degree of uniformity of all trays was 75.97% after 10 min of drying and 94.14% after 40 min of drying, which indicates that the dehydration difference of the three layers decreased with drying time, whereas the degree of drying uniformity increased.
Silica gel particles were dried in the CTMW under the conditions of 600 W of MW power and an air temperature of 60 °C. The temperature change of the sample layers could be seen from Fig. 3a. The surface temperature of the samples increased as the drying time increased. There was a certain temperature gradient from the bottom to the top of the material in which the variation during the 10 min of drying was more obvious; this temperature gradient was approximately 5 °C (Fig. 3a). Because MW heat as a whole mass, the whole materials are heated up at a fairly uniform rate (as long as the materials are not larger than half of the wavelength). However, the material surface always lost heat faster due to cooler ambient conditions compared with the inner part. Hot air is, by itself, relatively efficient at removing free water at or near the surface, whereas the unique pumping action of dielectric heating provides an efficient way of removing internal free water
Table 2 Degree of uniformity of each dehydration plate. Drying time (min)
10 20 30 40
Dehydration (g) Upper layer
Middle layer
Lower layer
29.94 26.11 17.56 9.89
30.91 27.35 16.24 9.13
36.28 29.46 14.53 8.12
Average amount of dehydration X
Variance of dehydration Δx
The degree of uniformity K a (%)
32.38 27.64 16.11 9.02
7.78 1.98 1.54 0.73
75.97% 92.84% 90.04% 94.14%
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as well as bound water [2]. With more hot air, the influence of the pressure gradient is greater on the total mass removal. The ambient condition of the lower layer is colder than the upper and middle layers, which results in lower temperatures of the samples in the lower layer. With increasing drying time, the temperature gradient became smaller (Fig. 3a). Furthermore, with increasing drying time, the standard deviation (Fig. 3b) between the material layers tended to decline. The different layers of samples could continue uniformly absorbing MW energy during the drying process. Moisture and temperature changes during this process were consistent with the material. There was also a certain moisture gradient in the vertical direction. When moist bio-substrates are placed in the MW drier, the MW power absorbed by the substrate is converted into heat, thus generating volumetric heating within the sample. In turn, the electromagnetic field displacement can be a function of both local temperature and moisture due to the substrate’s dielectric properties. Commonly for bio-substrates, in one or more points, saturation is eventually reached, and water vapor forms from liquid water. The vapor outwardly diffuses, accelerating the heat transport and lowering the absorbed MW power from the zones where the moisture is depleted [25]. Then, it can be seen that as local moisture decreases, the local temperature increases correspondingly, and the absorbed MW energy decreases due to dielectric properties, which can be affected by local moisture and temperature. Therefore, the temperature distribution is again altered by the moisture distribution.
3.4. Uniformity of the same tray From Fig. 4, it can be observed that velocity in the central portion of each layer of the trays was the highest, followed by the outermost and middle portions. The central portion (point 7) had the lowest moisture content after 10 min (4.83%) and 30 min (2.32%). The middle portion (points 4, 6, 8, 10) of silica gel has the highest moisture content after drying for 10 min (point 8, 8.78%) and 30 min (point 8, 3.06%). The moisture of the outermost portion quickly attracts hot air, which results in the lower moisture content. The heat of the central portion is not easily sent out, which produces a hot spot in the MW chamber, and the moisture content of the central portion becomes the lowest.
a
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3.5. Single factor uniform of CTMW drying 3.5.1. Effects of MW power on the temperature distribution and drying uniformity Fig. 5 shows that with increasing MW power, the average temperature of the sample surface increased. This increase in temperature may be due to more MW power being absorbed and transformed into heat energy, which accelerated the evaporation of moisture, thereby increasing the final temperature of the material. During the initial drying period, the material showed high moisture content and strong MW absorption capacity, and the velocity of temperature rose rapidly. At the final stage, the moisture content was greatly reduced with drying time, and the MW energy absorbed by the samples also decreased, which resulted in the sample temperature increasing slowly and maintaining the steady state. Under the different drying conditions, MW power changes do not affect the field mode but do influence the field intensity. The different curve magnitude is affected by field intensity. Additionally, it seems that the change in field mode due to load conditions (dry or wet) is minimal; therefore, the curve changing can maintain its consistency. During the drying process, the standard deviation for temperature first rose, then fell and finally stabilized. With the MW power increased, the standard deviation for temperature increased. After 40 min of drying, the standard deviations for temperature were 1.3, 1.5 and 2.0, indicating that the drying uniformity decreased with increasing MW power.
3.5.2. Effects of hot air temperature on the temperature distribution and drying uniformity The effect of hot air temperature on drying uniformity is shown in Fig. 6. The effect of changing the inlet air temperature was the same as the trend of MW power. In the initial drying stage, different ventilation temperatures affect the material surface temperature, but no notable difference in the conditions of different hot air temperature was observed, which was due to the high moisture content of the material and the internal material rapidly heating to evaporate moisture at this stage. The water vapor was forced to move outward. The main role of the hot air was to accelerate evaporation of the surface moisture of the material, thereby reducing the temperature of the material surface. At the accelerated drying stage,
b
Fig. 4. Distribution of material moisture content after 10 min (a) and 30 min (b).
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a
b
Fig. 5. Effect of MW power on drying uniformity: a: Average temperature; b: Standard deviation of temperature.
the standard deviation of temperature increased through the drying process. In the falling drying stage, the standard deviation of temperature decreased gradually, and the drying uniformity improved. For different drying conditions, the lower air temperature could obtain better drying uniformity, perhaps because the rising hot air can hinder the delivery of water inside the particles; thus, the hot air temperature in the CTMW should not be too high. 4. Conclusions For a single-port CTMW drier, the temperature and moisture changes at different times and spatial positions were studied using silica gel as samples. In the horizontal direction, it could be concluded that the dried silica gel in the central part dried the fastest, followed by the gel in the outermost and middle portions in the same tray. In the vertical direction, the fastest drying rate for the samples was obtained in the upper layer, followed by the middle and lower layers. The temperature in the horizontal direction was almost consistent, and the center area was slightly higher than the surrounding areas. The temperature gradient in the vertical direction between each layer was 5 °C. In the CTMW drying process, the heating uniformity was improved. The moisture distribution was strongly consistent with the temperature distribution. As hot air temperature
a
b
Fig. 6. Effect of hot air temperature on drying uniformity: a: Average temperature; b: Standard deviation of temperature.
and MW power increased, the drying rate improved, and the drying uniformity decreased. Acknowledgements This work is funded by the Program of the National Natural Science Foundation of China under contract No 21206051, Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology under contract No.FM-201503, Fundamental Research Funds for the Central Universities under contract No JUSRP5151. References [1] M. Pace, M.V. De Bonis, F. Marra, G. Ruocco, Characterization of a combination oven prototype: effects of microwave exposure and enhanced convection to local temperature rise in moist substrate, Int. Commun. Heat Mass Transf. 38 (2011) 557–564. [2] A.S. Mujumdar, Handbook of Industrial Drying, 3rd ed., CRC Press, Boca Raton, FL, 2007, pp. 286–304. [3] S. Chandrasekaran, S. Ramanathan, T. Basak, Microwave food processing – a review, Food Res. Int. 52 (1) (2013) 243–261. [4] R. Vadivambal, D.S. Jayas, Non-uniform temperature distribution during microwave heating of food materials – a review, Food Bioprocess Technol. 3 (2) (2010) 161–171. [5] M. Zhang, H. Jiang, R.X. Lim, Recent developments in microwave-assisted drying of vegetables, fruits and aquatic products- drying kinetics and quality considerations, Drying Technol. 28 (11) (2010) 1307–1316.
C.F. Song et al./Applied Thermal Engineering 98 (2016) 872–879
[6] C.F. Song, Z.W. Cui, G.Y. Jin, A.S. Mujumdar, J.F. Yu, Effects of four different drying methods on the quality characteristics of peeled litchis (litchi chinensis Sonn.), Drying Technol. 33 (2015) 583–590. [7] Z.Y. Li, R.F. Wang, T. Kudra, Uniformity issue in microwave drying, Drying Technol. 29 (6) (2011) 652–660. [8] M. Zhang, J. Tang, A.S. Mujumdar, S. Wang, Trends in microwave-related drying of fruits and vegetables, Trends Food Sci. Technol. 17 (10) (2006) 524–534. [9] Y.C. Wang, M. Zhang, A.S. Mujumdar, K.J. Mothibe, S.M. Roknul Azam, Study of drying uniformity in pulsed spouted microwave-vacuum drying of stem lettuce slices with regard to product quality, Drying Technol. 31 (1) (2013) 91–101. [10] L.L. Huang, M. Zhang, Trends in development of dried vegetable products as snacks, Drying Technol. 30 (5) (2012) 448–461. [11] F. Marra, M.V. De Bonis, G. Ruocco, Combined microwaves and convection heating: a conjugate approach, J. Food Eng. 97 (1) (2010) 31–39. [12] I. Alibas, Microwave, air and combined microwave–air-drying parameters of pumpkin slices, LWT-Food Sci. Technol. 40 (8) (2007) 1445–1451. [13] A.A. Barba, A. Dalmoro, M. D’amore, Microwave assisted drying of cellulose derivative (HPMC) granular solids, Powder Technol. 237 (2013) 581–585. [14] J. Varith, P. Dijkanarukkul, A. Achariyaviriya, Combined microwave-hot air drying of peeled longan, J. Food Eng. 81 (2) (2007) 459–468. [15] G.S.V. Beaudry, C.R. Raghavan, T.J. Rennie, Effect of four drying methods on the quality of osmotically dehydrated cranberries, Drying Technol. 22 (3) (2004) 521–539. [16] C.B. Koskiniemi, V.D. Truong, J. Simunovic, R.F. McFeeters, Improvement of heating uniformity in packaged acidified vegetables pasteurized with a 915 MHz continuous microwave system, J. Food Eng. 105 (2011) 149– 160. [17] P. Sharmag, Drying of garlic (A lliums atiuvm) cloves by microwave hot air combination, J. Food Eng. 50 (9) (2001) 9–105. [18] A. Manickavasagan, D.S. Jayas, N.D.G. White, Non-uniformity of surface temperatures of grain after microwave treatment in an Industrial microwave dryer, Drying Technol. 24 (12) (2006) 1559–1567.
879
[19] R.F. Wang, H.H. Huo, R.B. Dou, Z.Y. Li, A.S. Mujumdar, Effect of the inside placement of electrically conductive beads on electric field uniformity in a microwave applicator, Drying Technol. 32 (16) (2014) 1997–2004. [20] S.M. Wang, Z.C. Hu, Y.B. Han, Z.X. Gu, Effects of magnetron arrangement and power combination of microwave on drying uniformity of carrot, Drying Technol. 31 (11) (2013) 1206–1211. [21] A. Ilknur, Microwave, air and combined microwave-air-drying parameters of pumpkin slices, LWT-Food Sci. Technol. 40 (8) (2007) 1445–1451. [22] L. Malafronte, G. Lamberti, A.A. Barba, Combined convective and microwave assisted drying. Experiments and modeling, J. Food Eng. 112 (2012) 304–312. [23] W.Q. Yan, M. Zhang, L.L. Huang, J. Tang, A.S. Mujumdar, J.C. Sun, Study on optimization of puffing characteristics of potato cubes by spouted bed drying enhanced with microwave, J. Sci. Food Agric. 90 (8) (2010) 1300–1307. [24] W.A.M. McMinn, M.A.M. Khraisheh, T.R.A. Magee, Modelling the mass transfer during convective, microwave and combined microwave-convective drying of solid slabs and cylinders, Food Res. Int. 36 (9–10) (2003) 977–983. [25] M. De Bonis V, P. Caccavale, G. Ruocco, Convective control to microwave exposure of moist substrates. Part 1: model methodology, Int. J. Heat Mass Transf. 86 (2015) 943–949. [26] M.V. De Bonis , P. Caccavale, G. Ruocco, Convective control to microwave exposure of moist substrates. Part II: model validation and application, Int. J. Heat Mass Transf. 86 (2015) 950–956. [27] D.G. Prabhanjan, H.S. Ramaswamy, Microwave-assisted convective air drying of thin layer carrots, J. Food Eng. 25 (1995) 283–293. [28] P. Vajda, G. Guiochon, Surface excess isotherms of organic solvent mixtures in a system made of liquid carbon dioxide and a silica gel surface, J. Chromatogr. A. 1308 (2013) 139–143. [29] V. Sebera, A. Nasswettrová, K. Nikl, Finite element analysis of mode stirrer impact on electric field uniformity in a microwave applicator, Drying Technol. 30 (13) (2012) 1388–1396. [30] M. Araszkiewicz, A. Koziol, A. Oskwarek, M. Lupinski, Microwave drying of porous materials, Drying Technol. 22 (10) (2004) 2331–2341. [31] J.R. Arballo, L.A. Campañone, R.H. Mascheroni, Modeling of microwave drying of fruits, Drying Technol. 28 (10) (2010) 1178–1184.
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Non-uniformity investigation in a combined thermal and microwave drying of silica gel ChunFang Song a, Yan Wang a, Shuguang Wang b, ZhengWei Cui a,*, Yanfeng Xu a, Haiqing Zhu a a b
Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi, Jiangsu, 214063, China Agricultural College, Inner Mongolia Agricultural University, Huhhot, Inner Mongolia, 010019, China
H I G H L I G H T S
• • • • •
Uniformity in combined thermal and microwave (CTMW) dryer was analyzed. Local temperature and moisture of silica gel were determined to analyze uniformity. The effects of microwave power and temperature of the hot air on drying uniformity were investigated. The uniformity changes in the horizontal direction of the drying cavity were not obvious. The temperature gradient of silica gel between the layers can reach 5 °C.
A R T I C L E
I N F O
Article history: Received 23 April 2015 Accepted 22 December 2015 Available online 31 December 2015 Keywords: Combined thermal and microwave drying (CTMW) Silica gel Uniformity Microwave drying Temperature distribution
A B S T R A C T
Local temperature and moisture were determined to analyze the uniformity in vertical and horizontal planes in a combined thermal and microwave (CTMW) drying chamber. Furthermore, the effects of microwave power and temperature of hot air on drying uniformity were also investigated. It could be concluded that the dried silica gel in the center area of the drying chamber obtained the highest drying rate and temperature, followed by the silica in the outermost and the middle portions. The dehydration difference of the three layers decreased with the drying time, whereas the drying uniformity increased. Overall, the change in uniformity in the horizontal direction of the drying cavity was not obvious, and the temperature and moisture content gradients mainly occurred in the vertical direction. The temperature gradient between the layers reached 5 °C. As the hot air temperature and microwave power increased, the drying rate improved and the drying uniformity decreased. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Microwave (MW) heating offers several distinct benefits, including increasing throughput and higher energy efficiency, but its intensity and penetration depth depend on the physical and dielectric properties of the substrate and can vary with temperature, frequency, composition and shape. With bio-products characterized by low thermal conductivity, MW heating may exhibit a certain non-uniformity in the temperature distribution, leading to local overheating or even run-away loci [1–3], which not only damages the quality of the food due to hot spots, but also raises the issue of food safety as pathogenic microorganisms may not be destroyed in cold spots [4]. Heating uniformity in microwave processing can be improved in different ways. Currently, modification of relevant food or package
* Corresponding author. Tel./fax: (+86) 51085910390. E-mail address: [email protected] (Z. Cui). http://dx.doi.org/10.1016/j.applthermaleng.2015.12.089 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
parameters to optimize the heating uniformity are important in food product development [5–8]. Microwave oven designs such as rotating turntables in household microwave ovens, moving conveyor belts in industrial appliances and mode stirrers often result in uniformity [4]. However, due to their limited response in process performance, MW treatments have been renovated from MW irradiation alone to hybrid processing [9–12]. Combined thermal and microwave (CTMW) drying has been proposed for over a decade. Volumetric heating (caused by microwave power) drives moisture from the product’s interior toward the surface, where it is removed by the surrounding heated air currents. This coupled drying method has been shown to be a promising technology for reducing drying times and providing the desired temperature profiles needed for specific food processes [13–19]. To date, CTMW dryers have been successfully operated only on a small, experimental scale [20–22]. Attempts to make such dryers for industrial-scale use have been limited. In a laboratory-scale dryer, the volume of the cavity is small, and in such a small space, the nonuniformity of the microwave distribution is not notable. However,
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the non-uniformity of the microwave distribution becomes a serious problem in an industrial-scale dryer. To overcome this difficulty, the main strategies available for use by most researchers are to move or float the drying materials around the cavity with convective air [21–23]. This strategy, however, leads to difficulties in managing the structure of the coupled dryer for large industrial-scale cavity. Moreover, most of the research on combination CTMW heating has been conducted in the context of microwave convection drying characteristics and simulations [24–26], in which the spatial uniformity of heating and drying was not deemed to be the key parameter and was therefore seldom reported. Most of these studies have treated the food to be dried as one lump sum so that spatial information from different points in the cavity was not recorded. Measurements of temperature and moisture distributions in the cavity during CTMW drying could capture the spatial and temporal heating patterns, and such information could provide valuable insights into the nature of combination heating. Such information could enable more effective designs for coupled CTMW dryers. However, no such study has been recorded in the literature. In this study, a novel CTMW dryer was designed and fabricated. In particular, a new strategy was tested that involved varying the distributions of microwaves and allowing the dried materials to remain motionless in the cavity. Using silica gel as the experimental material, changes in the temperature and moisture of samples in specified zones of the dryer plates were measured to evaluate the uniformity of heating and drying. The advantages of the combined CTMW method were apparent in the results, which demonstrated a more convenient means of managing temperature with greater uniformity in the microwave distribution. Such results raise the possibility of building CTMW dryers on a large or industrial scale. Therefore, the objectives of this study were (1) to determine local temperature and moisture in horizontal and vertical planes in the drying chamber using silica gel in an experimental test and (2) to explore the effect of MW power and the temperature of hot air on drying uniformity. 2. Materials and methods 2.1. Materials Silica gel (Model 10018360, National Medicine Group Chemical Reagent Co., Ltd., China) was used as the test material. Before the experiment, the gel was saturated with water vapor until the moisture content reached to 27.2% (w.b.). Silica gel has strong adsorption affinity for water vapor in air. Dry silica gel is blue, and wet silica gel shows different colors that depend on the quantity of crystallization water [29]. 2.2. Drying equipment The pilot-scale CTMW dryer was designed by the authors (Fig. 1a). The dimensions of length, width and height of the multimode rectangular cavity are 540 × 400 × 550 mm, and the volume is 115 L. There is a certain interval of 10 cm between the three trays. Each layer tray had an area of 0.17 m2 and was made of a glass fiber mesh grid. The heating feasibility of a material via MW irradiation depends on dielectric properties (or permittivity) [3]; trays with a low dielectric constant almost do not absorb MW energy and ensure it was absorbed by the working samples and allow air and water vapor to pass through them. The hot air production port was located in the back of the drying chamber (or cavity). This port consisted of a centrifugal fan, an electric heating tube and a circulating air duct. The hot air was cycled through the drying chamber, and the high humidity air was discharged through an air vent at the top of the dryer cavity. The size of the air vent could be adjusted to change the air velocity. The hot air temperature could be changed within
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a range of 40~120 °C, and the air velocity varied in the range of 0.3~1 m/s. The composition and structure of the microwave power feed port is shown in Fig. 1b. This structure consisted of a magnetron, a waveguide and an (electric and magnetic) EM field mode stirrer. The microwave power transmitted by the magnetron was transferred through the waveguide to a metal tray connected to the drying chamber. The metal mode stirrer was installed at the outlet of the waveguide and rotated by a motor at 25 rpm. This mode stirrer device mechanically changed the emission conditions and periodically disturbed the regularity of the EF (or electric field). The working frequency of the microwave was 2450 MHz, and a digital inverter solution for which the energy output could be adjusted from 100 to 1500 W by step-less high voltage regulation for the microwave oven (Model: WepeX 1600A, Shenzhen Mebmeet Electrical Technology Co., LTD, Shenzhen, China) was adopted. The nominal power is the actual power output of the microwave inverter indicated by its control panel (Model: WepeX-C1, Shenzhen Mebmeet Electrical Technology Co., LTD, Shenzhen, China). All times were measured using a PC-based stopwatch. Before the experiment, the hot air heater was opened for at least 30 min. 2.3. Determination and analysis methods 2.3.1. The absorbed power A calorimetric method was adopted to evaluate the nominal MW power P supplied to the sample. One liter of tap water was weighed in a beaker, and its initial temperature was read by a K-type thermocouple. Following a fulltime exposure to MW, the water was briefly mixed, and its temperature was read again to determine P. The absorbed power was determined by the following relationship [1]:
p = CPm
ΔT t
According to the calculation, nominal powers of 500 W, 600 W and 700 W in the experiments correspond to actual absorbed powers of 490 ± 5 W , 588 ± 10 W , and 685 ± 15 W , respectively. The humidity ratio:
MR =
M − Me M0 − Me
M——The mass of the samples after t minutes of drying time M 0——The initial mass of the sample M e ——The quality when the sample has moisture balance 2.3.2. Determination of drying uniformity The water uniformity K a was used to describe the drying uniformity of each dish: K a = ( x − Δx ) x × 100% x ——The average value of dehydration in all trays (g) Δx ——Variance of dehydration in all trays (reflecting water dispersion degree for dehydration in all dish) K a values closer to 100% indicate a greater degree of uniform drying. 2.3.3. Determination and analysis by Infrared thermal imaging An FLIR T440 infrared thermal imager (FLIR Systems company, USA) was preheated for 10 min before each test. Infrared emissivity was set at 0.95, and the detection distance was kept at a distance of 50 cm. Detected data were analyzed by FLIR Reporter software. This method of determining the temperature only allows for measurements of surface temperature.
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a
b
Fig. 1. Schematic of the CTMW dryer. 1. Multimode rectangular cavity; 2. Microwave input system; 3. Material tray; 4. Mode stirrer; 5. Drying oven door; 6. Observation window; 7. Vent; 8. Control panel; 9. Fan; 10. Heater. a. Structural schematic of the combined thermal and microwave oven. 1. Mode stirrer; 2. Metal tray; 3. Motor; 4. Wave guide; 5. Magnetron. b. Structural diagram of the microwave power feed port.
Data for the material temperature distribution were extracted, which included the temperature (Ti) and the points of sample temperature within the temperature scope (M i). The ratio of the points of sample temperature (M i ) within the temperature scope is K i ( K i = Mi ΣMi ) . The uniformity of drying is measured by the temperature standard deviation [28]. The average temperature T: T = k i * T i The standard deviation for temperature δ: δ = k i (T i−T) The uniformity of drying is measured by the value of the standard deviation for temperature [27].
2.4. Experimental methods 2.4.1. Determination of the bulk temperature distribution for hot air drying A hot-wire anemometer TSI9535-A (TSI Incorporated, Shoreview, MN, USA) combined with the data for the infrared thermal images was used to determine the bulk temperature distribution, and the air velocity was kept at 1 m/s for different hot air inlet
temperature treatments. Only the fan and heater were opened in the CTMW drier. 2.4.2. Determination of uniformity between different trays To obtain the same experimental conditions, 500 g of water was preheated for 10 min before each experiment under 600 W of MW power and a hot air temperature of 60 °C. A total of 500 g color silicone were accurately measured and evenly spread as a monolayer on the tray of the CTMW oven. The initial MW power density was 1.2 W/g. Samples were taken out to measure the weight, and the FLIR T440 infrared thermal imager was used to quickly take pictures every 5 min. Then, the samples were sent back to the CTMW oven as soon as possible; the time interval was less than 10 s until the moisture reached 5%. Each test was repeated 3 times. 2.4.3. Uniformity of the same tray The middle tray was chosen as the test plate. The infrared thermal imager was used to take pictures at intervals of 10 min, 20 min and 30 min. Material from 13 different parts of the tray was sampled, and 100 g silica gel particles were randomly sampled across the test
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Fig. 2. Distribution of the testing point of the material in the plate.
position. The distribution of 13 sample points is shown as Fig. 2. The Y-axis was from the front to the back of the tray. The X-axis was from the left to the right side of tray. The intersection point of the two axes was O (0, 0). 2.4.4. The effect of MW power on temperature distribution and drying uniformity A total of 500 g saturated silica gel particles was put into the oven and evenly spread on the fiberglass dry net with inlet air temperature of 60 °C and different MW powers of 500, 600, and 700 W. The drying process continued until the moisture reached 5% (w.b). 2.4.5. The effect of hot air temperature on the temperature distribution and drying uniformity The operation for testing the effect of hot air temperature on the temperature distribution and drying uniformity was the same as that of MW power. The silica gel was dried at a power of 600 W and different hot air temperatures of 50, 60, 70 °C until the moisture reached 3% (w.b.).
3. Results and discussion 3.1. Determination of the temperature distribution for hot air drying in the CTMW dryer The bulk temperature distribution for hot air drying in the CTMW dryer can be seen in Table 1. The bulk temperature became steady after 30 min. The bulk temperature of the upper layer is nearly equal to that of the middle layer, whereas the temperatures of these two layers are higher than that of the lower layer. The degree of uniformity for three layers is high (above 95%).
3.2. Assessment of average convective transmittance The correlations from Araszkiewicz et al. [30] for the heat transfer coefficient in the spout-fluidized-bed drier and Arballo et al [31] in the fixed bed dryer have been successfully used for carrots and soybean grains, and thus were incorporated in this study.
Table 1 Bulk temperature distribution for hot air drying in the CTMW dryer. Inlet hot-Air temperature (°C)
50 60 70
Bulk temperature (°C) Upper layer (min)
Middle layer (min)
The degree of uniformity K a (%)
Lower layer (min)
10
30
10
30
10
30
41.2 53.3 59.8
49.8 59.8 68.3
42.2 45.3 58.4
50.0 60.0 69.5
39.5 42.1 53.2
48.1 57.1 62.6
97.8% 96.3% 96.1%
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Correlation equation used for evaluation of the heat transfer coefficient: Nusselt number Nu = hTλdE = 2.0 + 0.6 ( Pr ) vdρ and Reynolds number Re = μairair
13
a
+ ( Re )
12
The air parameters are as follows:
ρair = −3.510101 × 10−8 T f3 + 1.583982684 × 10−5 T f2 − 4.6995202202 × 10−3 T f + 1.2913571428571 (10°C < T f < 80°C ) μair = −1.7676768 × 10−13 T f3 − 5.541125541 × 10−11T f2 + 4.983297258297 × 10−8 T f + 17.1964285714285 × 10−6 (10°C < T f < 80°C ) λair = 6.8181818 × 10−10 T f3 − 1.474025974 × 10−7 T f2 + 8.029112554113 × 10−5 T f + 0.0240835714285714 (10°C < T f < 80°C )
b
Pr = −2.272727 × −8 T f3 + 4.1991342 × 10−6 T f2 − 3.5335497835 × 10−4 T f + 0.719 (10°C < T f < 80°C ) where T f = TS +2Tair (Ts- Surface temperature of samples) and the sphere diameter equivalent to a particle (silica gel) of volume dE = 3 6 VπE Using the local Reynolds and Prandtl numbers, the average Nusselt numbers could be obtained as follows: 2.243 for a hot air temperature of 50 °C, 2.205 for 60 °C and 2.175 for 70 °C. Based on rig details (Fig. 1a). The working air is drawn in by the back blower. Then there is a positive pressure build-up in the cavity, and the air is vented via the little ceiling opening. In these operating conditions, the working air is strongly stirred in the cavity, and the obtained Nusselt numbers reflection convective heat transfer intensity in different hot air temperature, while the Nusselt relationships chosen can hardly be employed in this rig. 3.3. Uniformity between different trays
Fig. 3. Temperature (a) and standard deviation of temperature (b) as a function of drying time in each plate.
The degree of uniformity of each dehydration plate can be seen in Table 2. The dehydration rate was highest in the beginning stages of drying. The dehydrations for the upper, middle and lower layer were 29.94 g, 30.91 g and 36.28 g, respectively, and the variance of dehydration for all trays was 7.78 after 10 min of drying. The dehydrations for the upper, middle and lower layer were 9.89 g, 9.13 g and 8.12 g, respectively, after 40 min of drying, and the variance of dehydration for all trays decreased considerably to 0.73. The results showed that the degree of non-uniformity decreased gradually. In addition, the dehydration of the lower (bottom) tray differed substantially from the other layers, which showed that the degree of non-uniform drying in the vertical direction is greater than in the horizontal direction. The degree of uniformity of all trays was 75.97% after 10 min of drying and 94.14% after 40 min of drying, which indicates that the dehydration difference of the three layers decreased with drying time, whereas the degree of drying uniformity increased.
Silica gel particles were dried in the CTMW under the conditions of 600 W of MW power and an air temperature of 60 °C. The temperature change of the sample layers could be seen from Fig. 3a. The surface temperature of the samples increased as the drying time increased. There was a certain temperature gradient from the bottom to the top of the material in which the variation during the 10 min of drying was more obvious; this temperature gradient was approximately 5 °C (Fig. 3a). Because MW heat as a whole mass, the whole materials are heated up at a fairly uniform rate (as long as the materials are not larger than half of the wavelength). However, the material surface always lost heat faster due to cooler ambient conditions compared with the inner part. Hot air is, by itself, relatively efficient at removing free water at or near the surface, whereas the unique pumping action of dielectric heating provides an efficient way of removing internal free water
Table 2 Degree of uniformity of each dehydration plate. Drying time (min)
10 20 30 40
Dehydration (g) Upper layer
Middle layer
Lower layer
29.94 26.11 17.56 9.89
30.91 27.35 16.24 9.13
36.28 29.46 14.53 8.12
Average amount of dehydration X
Variance of dehydration Δx
The degree of uniformity K a (%)
32.38 27.64 16.11 9.02
7.78 1.98 1.54 0.73
75.97% 92.84% 90.04% 94.14%
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as well as bound water [2]. With more hot air, the influence of the pressure gradient is greater on the total mass removal. The ambient condition of the lower layer is colder than the upper and middle layers, which results in lower temperatures of the samples in the lower layer. With increasing drying time, the temperature gradient became smaller (Fig. 3a). Furthermore, with increasing drying time, the standard deviation (Fig. 3b) between the material layers tended to decline. The different layers of samples could continue uniformly absorbing MW energy during the drying process. Moisture and temperature changes during this process were consistent with the material. There was also a certain moisture gradient in the vertical direction. When moist bio-substrates are placed in the MW drier, the MW power absorbed by the substrate is converted into heat, thus generating volumetric heating within the sample. In turn, the electromagnetic field displacement can be a function of both local temperature and moisture due to the substrate’s dielectric properties. Commonly for bio-substrates, in one or more points, saturation is eventually reached, and water vapor forms from liquid water. The vapor outwardly diffuses, accelerating the heat transport and lowering the absorbed MW power from the zones where the moisture is depleted [25]. Then, it can be seen that as local moisture decreases, the local temperature increases correspondingly, and the absorbed MW energy decreases due to dielectric properties, which can be affected by local moisture and temperature. Therefore, the temperature distribution is again altered by the moisture distribution.
3.4. Uniformity of the same tray From Fig. 4, it can be observed that velocity in the central portion of each layer of the trays was the highest, followed by the outermost and middle portions. The central portion (point 7) had the lowest moisture content after 10 min (4.83%) and 30 min (2.32%). The middle portion (points 4, 6, 8, 10) of silica gel has the highest moisture content after drying for 10 min (point 8, 8.78%) and 30 min (point 8, 3.06%). The moisture of the outermost portion quickly attracts hot air, which results in the lower moisture content. The heat of the central portion is not easily sent out, which produces a hot spot in the MW chamber, and the moisture content of the central portion becomes the lowest.
a
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3.5. Single factor uniform of CTMW drying 3.5.1. Effects of MW power on the temperature distribution and drying uniformity Fig. 5 shows that with increasing MW power, the average temperature of the sample surface increased. This increase in temperature may be due to more MW power being absorbed and transformed into heat energy, which accelerated the evaporation of moisture, thereby increasing the final temperature of the material. During the initial drying period, the material showed high moisture content and strong MW absorption capacity, and the velocity of temperature rose rapidly. At the final stage, the moisture content was greatly reduced with drying time, and the MW energy absorbed by the samples also decreased, which resulted in the sample temperature increasing slowly and maintaining the steady state. Under the different drying conditions, MW power changes do not affect the field mode but do influence the field intensity. The different curve magnitude is affected by field intensity. Additionally, it seems that the change in field mode due to load conditions (dry or wet) is minimal; therefore, the curve changing can maintain its consistency. During the drying process, the standard deviation for temperature first rose, then fell and finally stabilized. With the MW power increased, the standard deviation for temperature increased. After 40 min of drying, the standard deviations for temperature were 1.3, 1.5 and 2.0, indicating that the drying uniformity decreased with increasing MW power.
3.5.2. Effects of hot air temperature on the temperature distribution and drying uniformity The effect of hot air temperature on drying uniformity is shown in Fig. 6. The effect of changing the inlet air temperature was the same as the trend of MW power. In the initial drying stage, different ventilation temperatures affect the material surface temperature, but no notable difference in the conditions of different hot air temperature was observed, which was due to the high moisture content of the material and the internal material rapidly heating to evaporate moisture at this stage. The water vapor was forced to move outward. The main role of the hot air was to accelerate evaporation of the surface moisture of the material, thereby reducing the temperature of the material surface. At the accelerated drying stage,
b
Fig. 4. Distribution of material moisture content after 10 min (a) and 30 min (b).
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a
b
Fig. 5. Effect of MW power on drying uniformity: a: Average temperature; b: Standard deviation of temperature.
the standard deviation of temperature increased through the drying process. In the falling drying stage, the standard deviation of temperature decreased gradually, and the drying uniformity improved. For different drying conditions, the lower air temperature could obtain better drying uniformity, perhaps because the rising hot air can hinder the delivery of water inside the particles; thus, the hot air temperature in the CTMW should not be too high. 4. Conclusions For a single-port CTMW drier, the temperature and moisture changes at different times and spatial positions were studied using silica gel as samples. In the horizontal direction, it could be concluded that the dried silica gel in the central part dried the fastest, followed by the gel in the outermost and middle portions in the same tray. In the vertical direction, the fastest drying rate for the samples was obtained in the upper layer, followed by the middle and lower layers. The temperature in the horizontal direction was almost consistent, and the center area was slightly higher than the surrounding areas. The temperature gradient in the vertical direction between each layer was 5 °C. In the CTMW drying process, the heating uniformity was improved. The moisture distribution was strongly consistent with the temperature distribution. As hot air temperature
a
b
Fig. 6. Effect of hot air temperature on drying uniformity: a: Average temperature; b: Standard deviation of temperature.
and MW power increased, the drying rate improved, and the drying uniformity decreased. Acknowledgements This work is funded by the Program of the National Natural Science Foundation of China under contract No 21206051, Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology under contract No.FM-201503, Fundamental Research Funds for the Central Universities under contract No JUSRP5151. References [1] M. Pace, M.V. De Bonis, F. Marra, G. Ruocco, Characterization of a combination oven prototype: effects of microwave exposure and enhanced convection to local temperature rise in moist substrate, Int. Commun. Heat Mass Transf. 38 (2011) 557–564. [2] A.S. Mujumdar, Handbook of Industrial Drying, 3rd ed., CRC Press, Boca Raton, FL, 2007, pp. 286–304. [3] S. Chandrasekaran, S. Ramanathan, T. Basak, Microwave food processing – a review, Food Res. Int. 52 (1) (2013) 243–261. [4] R. Vadivambal, D.S. Jayas, Non-uniform temperature distribution during microwave heating of food materials – a review, Food Bioprocess Technol. 3 (2) (2010) 161–171. [5] M. Zhang, H. Jiang, R.X. Lim, Recent developments in microwave-assisted drying of vegetables, fruits and aquatic products- drying kinetics and quality considerations, Drying Technol. 28 (11) (2010) 1307–1316.
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[6] C.F. Song, Z.W. Cui, G.Y. Jin, A.S. Mujumdar, J.F. Yu, Effects of four different drying methods on the quality characteristics of peeled litchis (litchi chinensis Sonn.), Drying Technol. 33 (2015) 583–590. [7] Z.Y. Li, R.F. Wang, T. Kudra, Uniformity issue in microwave drying, Drying Technol. 29 (6) (2011) 652–660. [8] M. Zhang, J. Tang, A.S. Mujumdar, S. Wang, Trends in microwave-related drying of fruits and vegetables, Trends Food Sci. Technol. 17 (10) (2006) 524–534. [9] Y.C. Wang, M. Zhang, A.S. Mujumdar, K.J. Mothibe, S.M. Roknul Azam, Study of drying uniformity in pulsed spouted microwave-vacuum drying of stem lettuce slices with regard to product quality, Drying Technol. 31 (1) (2013) 91–101. [10] L.L. Huang, M. Zhang, Trends in development of dried vegetable products as snacks, Drying Technol. 30 (5) (2012) 448–461. [11] F. Marra, M.V. De Bonis, G. Ruocco, Combined microwaves and convection heating: a conjugate approach, J. Food Eng. 97 (1) (2010) 31–39. [12] I. Alibas, Microwave, air and combined microwave–air-drying parameters of pumpkin slices, LWT-Food Sci. Technol. 40 (8) (2007) 1445–1451. [13] A.A. Barba, A. Dalmoro, M. D’amore, Microwave assisted drying of cellulose derivative (HPMC) granular solids, Powder Technol. 237 (2013) 581–585. [14] J. Varith, P. Dijkanarukkul, A. Achariyaviriya, Combined microwave-hot air drying of peeled longan, J. Food Eng. 81 (2) (2007) 459–468. [15] G.S.V. Beaudry, C.R. Raghavan, T.J. Rennie, Effect of four drying methods on the quality of osmotically dehydrated cranberries, Drying Technol. 22 (3) (2004) 521–539. [16] C.B. Koskiniemi, V.D. Truong, J. Simunovic, R.F. McFeeters, Improvement of heating uniformity in packaged acidified vegetables pasteurized with a 915 MHz continuous microwave system, J. Food Eng. 105 (2011) 149– 160. [17] P. Sharmag, Drying of garlic (A lliums atiuvm) cloves by microwave hot air combination, J. Food Eng. 50 (9) (2001) 9–105. [18] A. Manickavasagan, D.S. Jayas, N.D.G. White, Non-uniformity of surface temperatures of grain after microwave treatment in an Industrial microwave dryer, Drying Technol. 24 (12) (2006) 1559–1567.
879
[19] R.F. Wang, H.H. Huo, R.B. Dou, Z.Y. Li, A.S. Mujumdar, Effect of the inside placement of electrically conductive beads on electric field uniformity in a microwave applicator, Drying Technol. 32 (16) (2014) 1997–2004. [20] S.M. Wang, Z.C. Hu, Y.B. Han, Z.X. Gu, Effects of magnetron arrangement and power combination of microwave on drying uniformity of carrot, Drying Technol. 31 (11) (2013) 1206–1211. [21] A. Ilknur, Microwave, air and combined microwave-air-drying parameters of pumpkin slices, LWT-Food Sci. Technol. 40 (8) (2007) 1445–1451. [22] L. Malafronte, G. Lamberti, A.A. Barba, Combined convective and microwave assisted drying. Experiments and modeling, J. Food Eng. 112 (2012) 304–312. [23] W.Q. Yan, M. Zhang, L.L. Huang, J. Tang, A.S. Mujumdar, J.C. Sun, Study on optimization of puffing characteristics of potato cubes by spouted bed drying enhanced with microwave, J. Sci. Food Agric. 90 (8) (2010) 1300–1307. [24] W.A.M. McMinn, M.A.M. Khraisheh, T.R.A. Magee, Modelling the mass transfer during convective, microwave and combined microwave-convective drying of solid slabs and cylinders, Food Res. Int. 36 (9–10) (2003) 977–983. [25] M. De Bonis V, P. Caccavale, G. Ruocco, Convective control to microwave exposure of moist substrates. Part 1: model methodology, Int. J. Heat Mass Transf. 86 (2015) 943–949. [26] M.V. De Bonis , P. Caccavale, G. Ruocco, Convective control to microwave exposure of moist substrates. Part II: model validation and application, Int. J. Heat Mass Transf. 86 (2015) 950–956. [27] D.G. Prabhanjan, H.S. Ramaswamy, Microwave-assisted convective air drying of thin layer carrots, J. Food Eng. 25 (1995) 283–293. [28] P. Vajda, G. Guiochon, Surface excess isotherms of organic solvent mixtures in a system made of liquid carbon dioxide and a silica gel surface, J. Chromatogr. A. 1308 (2013) 139–143. [29] V. Sebera, A. Nasswettrová, K. Nikl, Finite element analysis of mode stirrer impact on electric field uniformity in a microwave applicator, Drying Technol. 30 (13) (2012) 1388–1396. [30] M. Araszkiewicz, A. Koziol, A. Oskwarek, M. Lupinski, Microwave drying of porous materials, Drying Technol. 22 (10) (2004) 2331–2341. [31] J.R. Arballo, L.A. Campañone, R.H. Mascheroni, Modeling of microwave drying of fruits, Drying Technol. 28 (10) (2010) 1178–1184.