- Email: [email protected]
Study on Drying Uniformity of Static Small-sized Drying Box for Fruits and Vegetables
Available online at www.sciencedirect.com
ScienceDirect
Available online at www.sciencedirect.com Procedia Engineering00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 205 (2017) 2615–2622
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1922 October 2017, Jinan, China
Study on Drying Uniformity of Static Small-sized Drying Box for Fruits and Vegetables Ruichao Zhang, Jibo Long* College of Civil Engineering and Mechanics, Xiangtan University, Xiangtan, China
Abstract
In order to solve the problem of growing energy consumption in drying fruits and vegetables and advance the quality of fruits and vegetables processing, a kind of fruits and vegetables dryer was designed based on market demand for the dryer and drying characteristics of fruits and vegetables. The effects of air temperature and relative humidity on the evaporation rate of fruits and vegetables were analyzed, as well as the influence of design parameters on drying uniformity and energy consumption. A structure scheme of static small-sized dryer for fruits and vegetables with circulating airflow was created. Orthogonal test method was used to optimize the structure parameters of the dryer and the uniformity of drying air flow and drying uniformity were monitored by measuring temperature changes of fruits and vegetables. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Ltd. committee of the 10th International Symposium on Heating, Ventilation and Air Peer-review under responsibility of Elsevier the scientific Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Conditioning. Air Conditioning. Keywords:
Drying box for fruits and vegetables; Static; Drying uniformity; Air circulation; Penetration
1. Introduction The drying uniformity of Fruit and vegetable drying box is an important factor of affecting energy consumption of drying and baking quality. With the rapid development of agriculture in our country, fruit and vegetable drying is gradually from the traditional air drying transformed into drying equipment drying. Renewable energy sources such as solar energy and air energy have become an important research direction of drying heat source. solar energy[1,2]， * Corresponding author. Tel.: +86-0731-58298247; fax: +86-0731-58298247. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility ofthe scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. 10.1016/j.proeng.2017.10.201
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long /Procedia Engineering00 (2017) 000–000
2616 2
air source heat pump，as well as solar energy and air energy combination of multi energy drying heat source way, have gradually replaced the traditional drying heat source of non-renewable fossil fuels. While developing new energy sources, energy saving operation of drying equipment is another important measure to solve the problem of increasing energy consumption of fruits and vegetables drying. There are many affecting factors of saving energy performance of drying equipment. It was found that the connection between the various parts of the drying equipment reasonable，the structure of drying box airflow organization form in drying box and so on method can improve the efficiency of the use of drying heat[3-6]. Although the influence factors of energy-saving performance of drying equipment are many factors, however, the influence of various factors on energy-saving performance is reflected in the results of fruit and vegetable drying uniformity. This study designs a kind of static small-sized drying box for fruits and vegetables, and uses the method of orthogonal test to design the structural parameters of the drying box. 2. Drying principle The drying process of the material can be divided into two stages: the evaporation of the surface of the material and the migration of water vapor to the surface. Based on water vapor partial pressure difference, Dalton established the empirical formula for evaporation
E = C (Pqb − Pb )
(1)
The actual partial pressure of water vapor in the air can calculated as follows：
Pq = ψ ⋅ Pqb
(2)
By the above two formulas available：
E = C ⋅ Pqb (1 −ψ )
(3)
According to Goff-Gratch formula，when the temperature is higher than 0 ℃，the saturated vapor pressure in the wet air is a single-valued function of temperature, it is calculated as follows：
373.16 373.16 lg Pqb = −7.90298 − 1 + 5.028081 T T 11.344 1− 373T.16 −7 − 1.3816 ×10 10 − 1
(4)
−3.49149 373T.16 −1 + 8.1328 × 10 10 − 1 −3
+ lg(1013.246 )
Where E is evaporation rate, C is a coefficient related to wind speed , Pab is the saturated water vapor partial corresponding to pressure surface water temperature, Pq is water vapor pressure, ψ is the relative humidity of air, T is the absolute temperature of the air.
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long / Procedia Engineering 00 (2017) 000–000
2617 3
Migration of water vapor to the surface of the material is a complex process, according to the first law of Fick， the calculation formula of the amount of water vapor in the material under the concentration gradient can be expressed as follows：
W = −D ⋅ A ⋅
∂c ∂x
(5)
The diffusion coefficient of water vapor in the pores is as follows： 1.81
1 P T D = 0.0829 ⋅ ⋅ q ⋅ μ B T0
(6)
Substituting formula (6) into formula (5) and sorting it out can express as follows： 1.81
1 ψ ⋅ Pqb T ⋅ W = −0.0829 ⋅ μ B T0
⋅ A⋅
∂c ∂x
(7)
Where W is the amount of water vapor, D is the diffusion coefficient of water vapor in the pores, A is the cross-sectional area of the water vapor in the pores perpendicular to the direction of migration, c is water vapor concentration in porous media, μ is Water vapor diffusion resistance factor. From the above analysis can be obtained，for the same material, whether it is the evaporation of the surface of the material or the migration of water vapor into the surface, the temperature and relative humidity are important factors affecting the evaporation and migration of water vapor. The greater the drying air velocity, the higher the average air temperature flowing through the material, the smaller the average relative humidity, the shorter the drying time required, the more conducive to energy saving. However, changes in the flow rate of the drying air will have an effect on the change in airflow resistance, the relationship between airflow resistance and velocity can be expressed as follows：
p = ξ ⋅
ρv 2
(8)
2
The unit air volume of the fan power consumption is calculated as follows：
W=
P
η
=
1
η
ξ ⋅
ρv 2 2
(9)
Where P is the wind system resistance, ξ is the air duct resistance coefficient; v is the air velocity; ρ is the airflow density; W is the power consumption; ƞ is the comprehensive efficiency of the fan and the motor. It can be seen from the above formula, the greater the speed of drying air flow, the greater the fan air volume and unit air power consumption, the greater the required input power of the fan. And the greater the speed of drying air flow, the more detrimental to the drying area of the air evenly distributed，the more uneven the drying of fruits and vegetables，it will affect the energy consumption of drying box and the drying quality of fruit and vegetable.
2618 4
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long /Procedia Engineering00 (2017) 000–000
3. The model of drying box The fruit and vegetable drying box designed in this study is composed of drying area, air supply plenum chamber, air return plenum chamber, circulating duct and heating device. Drying area set up multi-layer material tray, set the windshield at the air inlet or outlet between the material trays, so that all the air through a material layer only once， the structure of drying box is show as follows. Thermal resistance
Drying box
Material tray
Inlet
Heat pump
x
a
Condenser
y
V
Air supply plenum chamber
Windshield Air return plenum chamber
Outlet
Figure. 1. The structure of the drying box
4. Simulation and analysis 4.1. Simulation method To reference the Fruit and vegetable drying box in effect，the influencing factors on drying air velocity and relative humidity are variable，except for drying airflow initial temperature and relative humidity, it also link with the airflow velocity that through the materials tray and the thickness of the material layer. The higher the airflow velocity that through the materials tray is, the lower temperature variation and relative humidity change per unit mass flow rate, the better effectiveness of drying box. However, the higher the air velocity that through the material tray is, the higher the air velocity between material tray is, that will exercises an influence on the energy consumption of drying airflow and the uniformity of material drying. In addition, in order to enhance the uniformity and reduce the energy consumption, there are two important design parameter: the area between material tray and the length of air convey in drying area. Thus, we studied drying airflow velocity at inlet of material tray, the thickness of material layer, the height of airflow passages between material tray, the length of air convey as the research object(in fig 1.) CFD was used to calculate the velocity that cross the material layer. We can estimate the uniformity of the fruit and vegetable drying box we designed based on the difference of air velocity at each point of material layer.
Figure. 2. The diagram of material drying diagram
Zhang al. / /Procedia RuichaoRuichao Zhang and JiboetLong ProcediaEngineering Engineering205 00 (2017) (2017) 2615–2622 000–000
26195
Refer to existing structure design and operation parameters of the fruit and vegetable drying box，the values about the box are as follows：
Table 1 The value of each factor table factor level x(mm) y(mm) v(m/s) a(m)
1
2
3
80 20 1.2 1
100 40 1.4 1.5
120 60 1.6 2
The factors and the level shown in table above require computations and modeling about 81 times, in order to simplify the analysis，this study use Orthogonal test to select and analyze a few typical (factor level）from all influencing (factors level). various factors in Orthogonal test are shown below: Table 2 The group of Orthogonal test table Test number 1 2 3 4 5 6 7 8 9
x(mm) 80 80 80 100 100 100 120 120 120
y(mm) 20 40 60 20 40 60 20 40 60
v(m/s) 1.2 1.4 1.6 1.4 1.6 1.2 1.6 1.2 1.4
a(m) 1 1.5 2 2 1 1.5 1.5 2 1
4.2. Simulation results and analysis Table 3 The summary of orthogonal test results Test number
Vmax (m/s)
Vmin (m/s)
Range (m/s)
1 2 3 4 5 6 7 8 9
0.7989 1.0295 2.5115 1.8921 1.5230 0.8991 2.7381 1.8139 1.5081
0.0504 0.0032 0.0145 0.0412 0.0050 0.0271 0.0179 0.0034 0.1081
0.7485 1.0264 2.4970 1.8509 1.5180 0.8721 2.7202 1.8105 1.4001
V
(m/s)
0.2107 0.1628 0.4168 0.3729 0.3175 0.2401 0.3485 0.2586 0.3434
Variance 0.0258 0.0215 0.2790 0.0692 0.0544 0.0195 0.1106 0.0726 0.0405
The simulation results are indicated above. We can draw a conclusion from the table given above, the maximum air flow velocity pass through the materials tray in test 3 can reach to 2.5115m/s, the average air flow velocity pass through the materials tray is 0.4168m/s; The maximum air flow velocity pass through the materials tray in test 7 can reach to 2.7381m/s, the average air flow velocity pass through the materials tray is 0.3485m/s. However，the variance of both experiment are high，it shows that the two experiments can acquire a higher average rate of evaporation in drying initial stage，but the drying uniformity is not ideal. Average air flow velocity pass through the materials tray of test 1, test 2 and test 6 are fairly high, however, variance of the three tests are low. This shows that although average rate of evaporation at the beginning of the three tests are lower, the material drying uniformity are better. The minimum variance of experiment 6 is 0.0195, which shows that the combination of four parameters of experiment 6 during this period of simulation experiment has the best drying uniformity.
2620 6
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long /Procedia Engineering00 (2017) 000–000
5. Experimental test According to the above orthogonal test results，the uniformity of the sixth set of experiments was tested. Experimental parameters of the drying box and experimental conditions can reference to group 6 orthogonal test parameters. 5.1. Method for measuring uniformity of drying It can be obtained from the previous theoretical analysis，when the temperature in air supply plenum chamber uniformity，and the thickness and density of material layer distribute uniformity, during the drying process, the temperature of the material is related to the velocity of the air supply. The greater the penetration velocity of the material layer，the faster the temperature rise rate of the material. Therefore, this experimental study was conducted to study the distribution uniformity of the drying airflow by using the temperature rising rate of the Initial drying materials. The PT100 thermal resistors is used to measure the material temperature in Experimental，and recording thermal resistors data using XSR-70A inspection instrument. Heat resistance is installed in the middle of the thickness of the material layer when measured, to prevent the upper and lower air flow effect on the measuring temperature，wrap the material on the outer surface of thermal resistors，to prevent air flow heating thermal resistors directly. The installation position of thermal resistors is shown in Figure 1，and install PT100thermal resistors in the first, three, five layers, set 3 rows along the air flow direction in the material tray, 2 thermal resistors per row, 6 thermal resistors are installed in each tray. 5.2. Experimental results and analysis In the experiment, the dry material was replaced by wet broken paper, The experimental results are as follows：
Fig.3. The first layer resistance temperature change curve
The following conclusions from the above figure, in the initial stage of material temperature is low and the rate of temperature rise is large. Drying after 55 minutes, the average temperature of 6 heat resistance is 38.8 ℃.
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long / Procedia Engineering 00 (2017) 000–000
2621 7
Fig. 4. The third layer resistance temperature change curve
The following conclusions from the figure 4，the temperature variation of the third layers is the same as that of the first layer, drying after 55 minutes, the average temperature of 6 heat resistance is 37.1℃.. From the figure 5, the fifth layer of material after drying for 55 minutes, the average temperature of the thermal resistance is 38.7℃.
Fig. 5. The fifth layer resistance temperature change curve
After the inspection of the material drying, it was found that the above three layers of the temperature were better than the others. And found that the material density distribution of fifth layer, distribution density in the thermal resistance at high temperature of 40.7 ℃ is less than the low temperature thermal resistance at 36.8 ℃, the material density of the first layer and the third layer is uniform, therefore, the material distribution density uniformity on Drying Uniformity influence. From the drying box to run 55 minutes of dryness of the material point of view, compared with the traditional drying box, the drying time is shortened by more than 30%, and the drying uniformity is better, more energy efficient. 6. Conclusion To this end, this study designed a static small fruit and vegetable drying box, the results show that: (1) There are many factors that affect the uniformity of fruits and vegetables drying , not only the structural design parameters, the operating conditions parameters, but also the distribution of fruit and vegetable distribution in the material tray is also an important factor affecting the drying uniformity. (2) In this experiment, the static small-scale fruit and vegetable drying box was made in the form of through-type airflow in parallel with the material plate. The temperature rise rate of each material plate was similar, the average temperature was 38.8℃, 37.1℃, 38.7℃, 2.3℃,
2622 8
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long /Procedia Engineering00 (2017) 000–000
1.7℃, 3.9℃, the material drying evenly. (3) Compared with the traditional surface of the material tray flat-blowing drying box , the drying time of the material tray penetrating drying box in this experiment is less, and the drying uniformity is better, more conducive to energy-saving drying energy consumption. (4) Material tray penetrating fruit and vegetable drying, drying uniformity can be measured by the temperature of the fruits and vegetables during the drying process to monitor. References [1] H. Ma et al. Design of hybrid type mixed--mode solar dryer for fruits and vegetables, Transactions of the Chinese Society of Agricultural Engineering. 25(3)(2009) 50-54. [2] Chen et al. A study of the drying effect on lemon slices using a closed-type solar dryer, Solar Energy. 78(1)(2005) 97-103. [3] F. Lu et al. Structural Optimization Design of Peppers Dryer Box Based on CFD, Journal of Agricultural Mechanization Research. (9)(2015) 245-249. [4]J. Wang, Z. lv. Research of a Mixed Flow and Mobile Grain Dryer, Cereal and Feed Industry. (12)(2000) 15-16. [5]M. Aktas et al. Analysis of drying of melon in a solar-heat recovery assisted infrared dryer, Solar Energy. 137(2016) 500-515. [6]R. Daghigh, A Shafieian. An experimental study of a heat pipe evacuated tube solar dryer with heat recovery system, Renewable Energy. 96(2016) 872-880.
ScienceDirect
Available online at www.sciencedirect.com Procedia Engineering00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
Procedia Engineering 205 (2017) 2615–2622
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1922 October 2017, Jinan, China
Study on Drying Uniformity of Static Small-sized Drying Box for Fruits and Vegetables Ruichao Zhang, Jibo Long* College of Civil Engineering and Mechanics, Xiangtan University, Xiangtan, China
Abstract
In order to solve the problem of growing energy consumption in drying fruits and vegetables and advance the quality of fruits and vegetables processing, a kind of fruits and vegetables dryer was designed based on market demand for the dryer and drying characteristics of fruits and vegetables. The effects of air temperature and relative humidity on the evaporation rate of fruits and vegetables were analyzed, as well as the influence of design parameters on drying uniformity and energy consumption. A structure scheme of static small-sized dryer for fruits and vegetables with circulating airflow was created. Orthogonal test method was used to optimize the structure parameters of the dryer and the uniformity of drying air flow and drying uniformity were monitored by measuring temperature changes of fruits and vegetables. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Ltd. committee of the 10th International Symposium on Heating, Ventilation and Air Peer-review under responsibility of Elsevier the scientific Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Conditioning. Air Conditioning. Keywords:
Drying box for fruits and vegetables; Static; Drying uniformity; Air circulation; Penetration
1. Introduction The drying uniformity of Fruit and vegetable drying box is an important factor of affecting energy consumption of drying and baking quality. With the rapid development of agriculture in our country, fruit and vegetable drying is gradually from the traditional air drying transformed into drying equipment drying. Renewable energy sources such as solar energy and air energy have become an important research direction of drying heat source. solar energy[1,2]， * Corresponding author. Tel.: +86-0731-58298247; fax: +86-0731-58298247. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility ofthe scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. 10.1016/j.proeng.2017.10.201
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long /Procedia Engineering00 (2017) 000–000
2616 2
air source heat pump，as well as solar energy and air energy combination of multi energy drying heat source way, have gradually replaced the traditional drying heat source of non-renewable fossil fuels. While developing new energy sources, energy saving operation of drying equipment is another important measure to solve the problem of increasing energy consumption of fruits and vegetables drying. There are many affecting factors of saving energy performance of drying equipment. It was found that the connection between the various parts of the drying equipment reasonable，the structure of drying box airflow organization form in drying box and so on method can improve the efficiency of the use of drying heat[3-6]. Although the influence factors of energy-saving performance of drying equipment are many factors, however, the influence of various factors on energy-saving performance is reflected in the results of fruit and vegetable drying uniformity. This study designs a kind of static small-sized drying box for fruits and vegetables, and uses the method of orthogonal test to design the structural parameters of the drying box. 2. Drying principle The drying process of the material can be divided into two stages: the evaporation of the surface of the material and the migration of water vapor to the surface. Based on water vapor partial pressure difference, Dalton established the empirical formula for evaporation
E = C (Pqb − Pb )
(1)
The actual partial pressure of water vapor in the air can calculated as follows：
Pq = ψ ⋅ Pqb
(2)
By the above two formulas available：
E = C ⋅ Pqb (1 −ψ )
(3)
According to Goff-Gratch formula，when the temperature is higher than 0 ℃，the saturated vapor pressure in the wet air is a single-valued function of temperature, it is calculated as follows：
373.16 373.16 lg Pqb = −7.90298 − 1 + 5.028081 T T 11.344 1− 373T.16 −7 − 1.3816 ×10 10 − 1
(4)
−3.49149 373T.16 −1 + 8.1328 × 10 10 − 1 −3
+ lg(1013.246 )
Where E is evaporation rate, C is a coefficient related to wind speed , Pab is the saturated water vapor partial corresponding to pressure surface water temperature, Pq is water vapor pressure, ψ is the relative humidity of air, T is the absolute temperature of the air.
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long / Procedia Engineering 00 (2017) 000–000
2617 3
Migration of water vapor to the surface of the material is a complex process, according to the first law of Fick， the calculation formula of the amount of water vapor in the material under the concentration gradient can be expressed as follows：
W = −D ⋅ A ⋅
∂c ∂x
(5)
The diffusion coefficient of water vapor in the pores is as follows： 1.81
1 P T D = 0.0829 ⋅ ⋅ q ⋅ μ B T0
(6)
Substituting formula (6) into formula (5) and sorting it out can express as follows： 1.81
1 ψ ⋅ Pqb T ⋅ W = −0.0829 ⋅ μ B T0
⋅ A⋅
∂c ∂x
(7)
Where W is the amount of water vapor, D is the diffusion coefficient of water vapor in the pores, A is the cross-sectional area of the water vapor in the pores perpendicular to the direction of migration, c is water vapor concentration in porous media, μ is Water vapor diffusion resistance factor. From the above analysis can be obtained，for the same material, whether it is the evaporation of the surface of the material or the migration of water vapor into the surface, the temperature and relative humidity are important factors affecting the evaporation and migration of water vapor. The greater the drying air velocity, the higher the average air temperature flowing through the material, the smaller the average relative humidity, the shorter the drying time required, the more conducive to energy saving. However, changes in the flow rate of the drying air will have an effect on the change in airflow resistance, the relationship between airflow resistance and velocity can be expressed as follows：
p = ξ ⋅
ρv 2
(8)
2
The unit air volume of the fan power consumption is calculated as follows：
W=
P
η
=
1
η
ξ ⋅
ρv 2 2
(9)
Where P is the wind system resistance, ξ is the air duct resistance coefficient; v is the air velocity; ρ is the airflow density; W is the power consumption; ƞ is the comprehensive efficiency of the fan and the motor. It can be seen from the above formula, the greater the speed of drying air flow, the greater the fan air volume and unit air power consumption, the greater the required input power of the fan. And the greater the speed of drying air flow, the more detrimental to the drying area of the air evenly distributed，the more uneven the drying of fruits and vegetables，it will affect the energy consumption of drying box and the drying quality of fruit and vegetable.
2618 4
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long /Procedia Engineering00 (2017) 000–000
3. The model of drying box The fruit and vegetable drying box designed in this study is composed of drying area, air supply plenum chamber, air return plenum chamber, circulating duct and heating device. Drying area set up multi-layer material tray, set the windshield at the air inlet or outlet between the material trays, so that all the air through a material layer only once， the structure of drying box is show as follows. Thermal resistance
Drying box
Material tray
Inlet
Heat pump
x
a
Condenser
y
V
Air supply plenum chamber
Windshield Air return plenum chamber
Outlet
Figure. 1. The structure of the drying box
4. Simulation and analysis 4.1. Simulation method To reference the Fruit and vegetable drying box in effect，the influencing factors on drying air velocity and relative humidity are variable，except for drying airflow initial temperature and relative humidity, it also link with the airflow velocity that through the materials tray and the thickness of the material layer. The higher the airflow velocity that through the materials tray is, the lower temperature variation and relative humidity change per unit mass flow rate, the better effectiveness of drying box. However, the higher the air velocity that through the material tray is, the higher the air velocity between material tray is, that will exercises an influence on the energy consumption of drying airflow and the uniformity of material drying. In addition, in order to enhance the uniformity and reduce the energy consumption, there are two important design parameter: the area between material tray and the length of air convey in drying area. Thus, we studied drying airflow velocity at inlet of material tray, the thickness of material layer, the height of airflow passages between material tray, the length of air convey as the research object(in fig 1.) CFD was used to calculate the velocity that cross the material layer. We can estimate the uniformity of the fruit and vegetable drying box we designed based on the difference of air velocity at each point of material layer.
Figure. 2. The diagram of material drying diagram
Zhang al. / /Procedia RuichaoRuichao Zhang and JiboetLong ProcediaEngineering Engineering205 00 (2017) (2017) 2615–2622 000–000
26195
Refer to existing structure design and operation parameters of the fruit and vegetable drying box，the values about the box are as follows：
Table 1 The value of each factor table factor level x(mm) y(mm) v(m/s) a(m)
1
2
3
80 20 1.2 1
100 40 1.4 1.5
120 60 1.6 2
The factors and the level shown in table above require computations and modeling about 81 times, in order to simplify the analysis，this study use Orthogonal test to select and analyze a few typical (factor level）from all influencing (factors level). various factors in Orthogonal test are shown below: Table 2 The group of Orthogonal test table Test number 1 2 3 4 5 6 7 8 9
x(mm) 80 80 80 100 100 100 120 120 120
y(mm) 20 40 60 20 40 60 20 40 60
v(m/s) 1.2 1.4 1.6 1.4 1.6 1.2 1.6 1.2 1.4
a(m) 1 1.5 2 2 1 1.5 1.5 2 1
4.2. Simulation results and analysis Table 3 The summary of orthogonal test results Test number
Vmax (m/s)
Vmin (m/s)
Range (m/s)
1 2 3 4 5 6 7 8 9
0.7989 1.0295 2.5115 1.8921 1.5230 0.8991 2.7381 1.8139 1.5081
0.0504 0.0032 0.0145 0.0412 0.0050 0.0271 0.0179 0.0034 0.1081
0.7485 1.0264 2.4970 1.8509 1.5180 0.8721 2.7202 1.8105 1.4001
V
(m/s)
0.2107 0.1628 0.4168 0.3729 0.3175 0.2401 0.3485 0.2586 0.3434
Variance 0.0258 0.0215 0.2790 0.0692 0.0544 0.0195 0.1106 0.0726 0.0405
The simulation results are indicated above. We can draw a conclusion from the table given above, the maximum air flow velocity pass through the materials tray in test 3 can reach to 2.5115m/s, the average air flow velocity pass through the materials tray is 0.4168m/s; The maximum air flow velocity pass through the materials tray in test 7 can reach to 2.7381m/s, the average air flow velocity pass through the materials tray is 0.3485m/s. However，the variance of both experiment are high，it shows that the two experiments can acquire a higher average rate of evaporation in drying initial stage，but the drying uniformity is not ideal. Average air flow velocity pass through the materials tray of test 1, test 2 and test 6 are fairly high, however, variance of the three tests are low. This shows that although average rate of evaporation at the beginning of the three tests are lower, the material drying uniformity are better. The minimum variance of experiment 6 is 0.0195, which shows that the combination of four parameters of experiment 6 during this period of simulation experiment has the best drying uniformity.
2620 6
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long /Procedia Engineering00 (2017) 000–000
5. Experimental test According to the above orthogonal test results，the uniformity of the sixth set of experiments was tested. Experimental parameters of the drying box and experimental conditions can reference to group 6 orthogonal test parameters. 5.1. Method for measuring uniformity of drying It can be obtained from the previous theoretical analysis，when the temperature in air supply plenum chamber uniformity，and the thickness and density of material layer distribute uniformity, during the drying process, the temperature of the material is related to the velocity of the air supply. The greater the penetration velocity of the material layer，the faster the temperature rise rate of the material. Therefore, this experimental study was conducted to study the distribution uniformity of the drying airflow by using the temperature rising rate of the Initial drying materials. The PT100 thermal resistors is used to measure the material temperature in Experimental，and recording thermal resistors data using XSR-70A inspection instrument. Heat resistance is installed in the middle of the thickness of the material layer when measured, to prevent the upper and lower air flow effect on the measuring temperature，wrap the material on the outer surface of thermal resistors，to prevent air flow heating thermal resistors directly. The installation position of thermal resistors is shown in Figure 1，and install PT100thermal resistors in the first, three, five layers, set 3 rows along the air flow direction in the material tray, 2 thermal resistors per row, 6 thermal resistors are installed in each tray. 5.2. Experimental results and analysis In the experiment, the dry material was replaced by wet broken paper, The experimental results are as follows：
Fig.3. The first layer resistance temperature change curve
The following conclusions from the above figure, in the initial stage of material temperature is low and the rate of temperature rise is large. Drying after 55 minutes, the average temperature of 6 heat resistance is 38.8 ℃.
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long / Procedia Engineering 00 (2017) 000–000
2621 7
Fig. 4. The third layer resistance temperature change curve
The following conclusions from the figure 4，the temperature variation of the third layers is the same as that of the first layer, drying after 55 minutes, the average temperature of 6 heat resistance is 37.1℃.. From the figure 5, the fifth layer of material after drying for 55 minutes, the average temperature of the thermal resistance is 38.7℃.
Fig. 5. The fifth layer resistance temperature change curve
After the inspection of the material drying, it was found that the above three layers of the temperature were better than the others. And found that the material density distribution of fifth layer, distribution density in the thermal resistance at high temperature of 40.7 ℃ is less than the low temperature thermal resistance at 36.8 ℃, the material density of the first layer and the third layer is uniform, therefore, the material distribution density uniformity on Drying Uniformity influence. From the drying box to run 55 minutes of dryness of the material point of view, compared with the traditional drying box, the drying time is shortened by more than 30%, and the drying uniformity is better, more energy efficient. 6. Conclusion To this end, this study designed a static small fruit and vegetable drying box, the results show that: (1) There are many factors that affect the uniformity of fruits and vegetables drying , not only the structural design parameters, the operating conditions parameters, but also the distribution of fruit and vegetable distribution in the material tray is also an important factor affecting the drying uniformity. (2) In this experiment, the static small-scale fruit and vegetable drying box was made in the form of through-type airflow in parallel with the material plate. The temperature rise rate of each material plate was similar, the average temperature was 38.8℃, 37.1℃, 38.7℃, 2.3℃,
2622 8
Ruichao Zhang et al. / Procedia Engineering 205 (2017) 2615–2622 Ruichao Zhang and Jibo Long /Procedia Engineering00 (2017) 000–000
1.7℃, 3.9℃, the material drying evenly. (3) Compared with the traditional surface of the material tray flat-blowing drying box , the drying time of the material tray penetrating drying box in this experiment is less, and the drying uniformity is better, more conducive to energy-saving drying energy consumption. (4) Material tray penetrating fruit and vegetable drying, drying uniformity can be measured by the temperature of the fruits and vegetables during the drying process to monitor. References [1] H. Ma et al. Design of hybrid type mixed--mode solar dryer for fruits and vegetables, Transactions of the Chinese Society of Agricultural Engineering. 25(3)(2009) 50-54. [2] Chen et al. A study of the drying effect on lemon slices using a closed-type solar dryer, Solar Energy. 78(1)(2005) 97-103. [3] F. Lu et al. Structural Optimization Design of Peppers Dryer Box Based on CFD, Journal of Agricultural Mechanization Research. (9)(2015) 245-249. [4]J. Wang, Z. lv. Research of a Mixed Flow and Mobile Grain Dryer, Cereal and Feed Industry. (12)(2000) 15-16. [5]M. Aktas et al. Analysis of drying of melon in a solar-heat recovery assisted infrared dryer, Solar Energy. 137(2016) 500-515. [6]R. Daghigh, A Shafieian. An experimental study of a heat pipe evacuated tube solar dryer with heat recovery system, Renewable Energy. 96(2016) 872-880.