materials Article
Model Stirrer Based on a Multi-Material Turntable for Microwave Processing Materials Jinghua Ye 1 , Tao Hong 2 , Yuanyuan Wu 3 , Li Wu 1 , Yinhong Liao 1 , Huacheng Zhu 1, *, Yang Yang 1 and Kama Huang 1 1
2 3
*
College of Electronic and Information Engineering, Sichuan University, Chengdu 610065, China;
[email protected] (J.Y.);
[email protected] (L.W.);
[email protected] (Y.L.);
[email protected] (Y.Y.);
[email protected] (K.H.) College of Electronical Information Engineering, China West Normal University, Nanchong 637002, China;
[email protected] College of Information Science & Technology, Chengdu University of Technology, Chengdu 610000, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-28-8540-3882
Academic Editor: Ming Hu Received: 26 November 2016; Accepted: 19 January 2017; Published: 24 January 2017
Abstract: Microwaves have been widely used in the treatment of materials, such as heating, drying, and sterilization. However, the heating in the commonly used microwave applicators is usually uneven. In this paper, a novel multi-material turntable structure is creatively proposed to improve the temperature uniformity in microwave ovens. Three customized turntables consisting of polyethylene (PE) and alumina, PE and aluminum, and alumina and aluminum are, respectively, utilized in a domestic microwave oven in simulation. During the heating process, the processed material is placed on a fixed Teflon bracket which covers the constantly rotating turntable. Experiments are conducted to measure the surface and point temperatures using an infrared thermal imaging camera and optical fibers. Simulated results are compared qualitatively with the measured ones, which verifies the simulated models. Compared with the turntables consisting of a single material, a 26%–47% increase in temperature uniformity from adapting the multi-material turntable can be observed for the microwave-processed materials. Keywords: materials processing; microwave heating; multi-material turntable; heating uniformity
1. Introduction As a heating source, microwaves show many advantages over conventional heating methods, such as high heating efficiency, selective heating, easy controlling, and environmental friendliness. By virtue of these advantages, microwaves have been widely used to process materials in food engineering, chemical engineering, and many other fields [1–5]. For example, Liu et al. reported that good sterilization conditions can be observed under 2450 MHz microwave heating combined with 0.02 g·kg−1 ZnO nanoparticle addition [6]. Leadbeater et al. proposed that the preparation of biodiesel using microwave processing can result in a fast, easy route to this valuable biofuel with advantages of short reaction time, low oil/methanol ratio, and an ease of operation [7]. Lin et al. confirmed that the biodiesel yields from waste cooking oil can be improved by using microwave processing [8]. Roy et al. demonstrated that microwave processing is able to improve the modulus of Fe-Ni rupture by 60% than that of conventional samples [5]. However, there are still drawbacks limiting the large-scale industrial applications of microwave heating. Inhomogeneous heating is one of the major barriers [9]. When a high-power microwave is applied on the processed materials, the hot spots (huge temperature gradient at a given location) and
Materials 2017, 10, 95; doi:10.3390/ma10020095
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thermal runaway (the uncontrollable temperature rise due to strong dielectric loss and temperatureMaterials 2017, 10, 95 2 of 13 positive feedback of the material) caused by microwave uneven heating may take place, which possibly leads to the damageofofthe materials or even an by explosion. To overcome this problem, several positive feedback material) caused microwave uneven heating may take place,solutions which have been proposed, like changing the position of processed sample, changing the composition possibly leads to the damage of materials or even an explosion. To overcome this problem, severalof heated materials or adding a mode stirrer [10–15]. them, the last one is generally the most solutions have been proposed, like changing theAmong position of processed sample, changing the effective and employed composition of heatedwidely. materials or adding a mode stirrer [10–15]. Among them, the last one is Meanwhile, ovens are commonly used applicators for processing materials. generally the mostmicrowave effective and employed widely. The turntable, as one of the most intuitive common methods for increasing temperature uniformity, Meanwhile, microwave ovens are and commonly used applicators for processing materials. The onetoofcarry the most and common methods for increasing temperature isturntable, used as aastray andintuitive rotate the heated material in microwave ovens, so as to uniformity, improve the is used as auniformity tray to carry and rotate the heatedSome material in microwave ovens, so as tooven improve the temperature of microwave heating. studies related to the microwave turntable temperature of microwave heating. Someastudies to the microwave turntable have been putuniformity forward. Geedipalli et al. proposed discreterelated method to estimate theoven rotation of the ◦ each time have been forward. et with al. proposed a discrete estimate theThe rotation of the turntable byput rotating it 15Geedipalli the finite elementmethod methodto(FEM) [16]. temperature turntable by rotating itby 15°such eachrotations time withwas thestudied. finite element (FEM)a[16]. temperature uniformity contributed Liu et method al. proposed FEMThe model to predict uniformity contributed by such rotations was studied. Liu et al. proposed a FEM model to predict time-dependent temperature distributions of food samples with or without the rotation of turntable [17]. time-dependent temperature of food samples withand or without the rotation of turntable Pitchai et al. proposed a FEM distributions model coupling electromagnetic heat transfer equations to obtain [17]. Pitchai et al. proposed aofFEM modelmulti-component coupling electromagnetic heat transfer equations to the temperature distribution a rotating meal. Aand microbial inactivation kinetics obtain the temperature distribution of a rotating multi-component meal. A microbial inactivation model for Salmonella Heidelberg was also studied to assess the food safety risk [18]. However, most of kinetics for Salmonella Heidelberg was of also studied to assess the food safety risk [18]. the studiesmodel mentioned above focused on the effect position changes on heating uniformity. Actually, However, of the studies mentioned above on thestirrers effect of position changes on the turntablemost in microwave ovens can also play thefocused role of mode when it is composed ofheating different uniformity. Actually, the turntable in microwave ovens can also play the role of mode stirrers when materials. It is supposed to be an economical and convenient way to access uniform temperature it is composed of different materials. It is supposed to be an economical and convenient way to access distribution in the process of material treatment. uniform temperature distribution in the process of material treatment. In this paper, a novel and simple multi-material turntable structure is first proposed to improve In this paper, a novel and simple multi-material turntable structure is first proposed to improve the heating uniformity of microwave processing materials effectively. Polyethylene (PE), alumina, the heating uniformity of microwave processing materials effectively. Polyethylene (PE), alumina, and aluminum are selected as the material composition of turntable, as shown in Figure 1. The selection and aluminum are selected as the material composition of turntable, as shown in Figure 1. The of these materials are random, because these materials are commonly used. Additionally, as long as selection of these materials are random, because these materials are commonly used. Additionally, the turntable is made of heterogeneous materials, the rotation can stir the electromagnetic field of as long as the turntable is made of heterogeneous materials, the rotation can stir the electromagnetic microwave ovens. Multi-physics models are built and simulated to obtain the temperature distributions field of microwave ovens. Multi-physics models are built and simulated to obtain the temperature ofdistributions processed samples with a constantly rotating multi-material turntable. The results are experimentally of processed samples with a constantly rotating multi-material turntable. The results verified using an infrared thermal camera andimaging optical fibers. of all the are experimentally verified using imaging an infrared thermal cameraThe andheating optical uniformity fibers. The heating models are next analyzed by various criterions. The conclusion is drawn that multi-material turntables uniformity of all the models are next analyzed by various criterions. The conclusion is drawn that have great potential to improve microwave heating uniformity. multi-material turntables have great potential to improve microwave heating uniformity.
(a)
(b)
(c)
Figure 1.1. The Figure The proposed proposed turntable turntable structure structure and and the the corresponding corresponding experimental experimentalturntable. turntable. (a) PE-aluminum turntable; (b) PE-alumina turntable; and (c) alumina-aluminum turntable. (a) PE-aluminum turntable; (b) PE-alumina turntable; and (c) alumina-aluminum turntable.
2. Methodology 2. Methodology 2.1.Model ModelDescription Description 2.1. Thegeometric geometric model model isis developed developed based based on The on aa domestic domestic microwave microwave oven oven(P70D20TL-D4, (P70D20TL-D4, Guangdong Galanz Enterprises Co., Ltd., Shunde, China) model with a rotating turntable composed Guangdong Galanz Enterprises Co., Ltd., Shunde, China) model with a rotating turntable composed of of different materials. The geometric model consists of six components, including a cavity, a different materials. The geometric model consists of six components, including a cavity, a waveguide, waveguide, a rotating turntable, a PE tray, a Teflon bracket and a heated sample, as shown in Figure 2. The waveguide and the cavity are filled with air. The turntable is formed by multiple materials, as shown in Figure 1, or a single material in different simulations, and it is placed on a PE tray. In order
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a rotating turntable, a PE tray, a Teflon bracket and a heated sample, as shown in Figure 2. The waveguide and the cavity are filled with air. The turntable is formed by multiple materials, as shown in Figure 1, or a single material in different simulations, and it is placed on a PE tray. In orderMaterials to observe the effect of the multi-material turntables on the heating uniformity, the heated 2017, 10, 95 3 of 13 potato slice is placed on the Teflon bracket, which is located above the center of the turntable. to observe the effect of the multi-material turntables the heating the heated As shown in Figure 2d, the coordinate origin of theon model is theuniformity, lower right cornerpotato of the cavity. slice is placed on the Teflon bracket, which is located above the center of the turntable. As shown in The rotation of the turntable in this applicator is continuous, covering the entire 360 degrees of rotation Figure 2d, the coordinate origin of the model is the lower right corner of the cavity. The rotation of in 6 s. The waveguide is fed by a TE10 modecovering with an effective power of 300 W at a frequency of the turntable in this applicator is continuous, the entire 360 degrees of rotation in 6 s. The 2.45 GHz. waveguide is fed by a TE10 mode with an effective power of 300 W at a frequency of 2.45 GHz.
(a)
(b)
(c)
(d)
Figure 2. Geometry of the 3-D microwave oven: (a) y-z plane view; (b) x-z plane view; (c) x-y plane
Figure 2. Geometry of the 3-D microwave oven: (a) y-z plane view; (b) x-z plane view; (c) x-y plane top top view; and (d) 3D view. view; and (d) 3D view. The initial temperature of the air, turntable and food inside the oven is 20 °C. The input parameters of the model are listed in Table 1. The thermal and dielectric properties potatoes are The initial temperature of the air, turntable and food inside the oven is 20 ◦ C. of The input parameters obtained from the literature [4,19]. Considering the narrow temperature range, the thermal and of the model are listed in Table 1. The thermal and dielectric properties of potatoes are obtained from electromagnetic parameters of the potatoes are treated as constant in the calculation.
the literature [4,19]. Considering the narrow temperature range, the thermal and electromagnetic 1. Summary material properties applied in the model. parameters of the potatoesTable are treated asofconstant in the calculation. Parameter
Domains
Value
Table 1. Summary of material properties applied Air in the model. 1 Parameter Dielectric constant (ε′)
Dielectric constant (ε0 ) Dielectric loss factor (ε′′) Specific heat capacity (Cp, J/(kg·K)) 3) (ε00 ) Dielectric loss factor Density (ρ, kg/m Thermal conductivity (k, W/(m3·K)) Specific heat capacity (C , J/(kg ·K)) 2·K)) Heat transfer coefficient (h, pW/(m Density (ρ, kg/m3 ) Thermal conductivity (k, (S/m) W/(m3 ·K)) Electrical conductivity Heat transfer coefficient (h, W/(m2 ·K))
Electrical conductivity (S/m)
Alumina 9 Domains Value PE 2.3 Aluminum Air 1 1 Teflon Alumina 9 2.1 Potato PE 2.357 Aluminum 1 17 Potato Teflon 2.1 0 Others Potato 57 Potato 3640 Potato 17 Potato Others 01050 Potato 0.648 Potato 3640 Potato–Air 10 Potato 1050 Aluminum 3.774 × 107 Potato 0.648 0 Others Potato–Air 10 Aluminum 3.774 × 107 Others 0
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2.2. Multi-Physics Calculation Microwave-assisted processing of materials involves multiple physics, electromagnetic in the cavity and heated samples, as well as mass and heat transport. 2.2.1. Governing Equation The electromagnetic energy distribution inside an oven cavity is governed by the Maxwell’s waveform equation [20]:
∇ × µr−1 × (∇ × E) − k20 (ε0 εr − jσ/ω0 ) E = 0
(1)
where E is the electric field, ε0 is the permittivity of vacuum, and εr is the relative permittivity of the medium. µr denotes the permeability, while ω0 represents the angular frequency. k0 is the wave number in a vacuum, and σ is the electrical conductivity. Then, the power dissipated inside the potato can be obtained by the following equation [21,22]: 1 (2) Pd = ω0 ε0 ε”| E|2 2 where ε” is the imaginary part of relative permittivity of potato. The dissipated power term is the heat source in transient heat transfer of potato domain [23,24]. ∂T = k∇2 T + Pd (3) ∂t where ρ, Cp and k are the density, heat capacity and thermal conductivity of potatoes, respectively. ρCp
2.2.2. Boundary Condition Electromagnetic boundary condition The metallic waveguide and cavity walls are considered as perfect electric conductors, where the following boundary condition applies [25]: Etangential = 0
(4)
Heat transfer boundary condition In this model, the upper surface of the potato slice exchanges heat with air by convection, expressed as [26]: − k·∂T/∂n = h · ( T − Tair ) (5) where ∂T/∂n is the gradient of the temperature perpendicular to the temperature field interface, Tair is the temperature of the air inside the oven, and h is the heat transfer coefficient with the value of 10 W/(m2 ·K), which is a typical value used for natural convective heat transfer in air [18]. The thermal boundaries between potato slices and the Teflon bracket are set as insulators. 2.3. Continuous Algorithm for Turntable Rotation To simulate the heating process, we adopt a continuous algorithm, which can avoid re-meshing during the calculation process, so as to obtain accurate results. In this algorithm, the permittivity of the rotating region is set as a function of space and time to represent the rotation of the multi-material turntable. As for this model, the rotating region is turntable region. Since the turntable is rotating on the x-y plane, it can be treated as a two-dimensional cylindrical coordinate system. The components of the multi-material turntable are expressed as Component A and Component B for short, as shown in Figure 3a. It can be noted that all the mesh dots inside the turntable region satisfy the following constraint condition: r < Rc
(6)
where Rc is the radius of the turntable, and r is the distance from the mesh dots to the origin (0,0). At the initial moment, mesh dots inside Component A also satisfy the constraint condition as: 0
