Abstract: In this present study an open sun and greenhouse drying of onion flakes has been performed to study the effect of mass on convective heat transfer coefficient. Three sets of experiments with total quantity of onion as 300, 600 and 900 g were done. The onion was continuously dried for 33 h both in open sun and in the roof type even span greenhouse with floor area of 1.2 x 0.78 m2. Experiments were carried out during the months of October to December 2003 at IIT Delhi (28°35`N 72°12`E). Experiments were started at 8 am. The data obtained from experimentation under open sun and greenhouse conditions have been used to determine values of the constant `C` and exponent `n` by regression analysis and consequently, convective heat transfer coefficient. It is observed that there is a significant effect of mass on convective heat transfer coefficient for open as well as greenhouse drying. It is also observed that the rate of moisture evaporation in case of greenhouse drying is more than that in open sun drying during the off sunshine hours due to the stored energy inside the greenhouse. The experimental observations were analyzed in terms of percentage uncertainty also.
INTRODUCTION
Onion, Allium cepa, L., is considered as one of the most important crops in all countries. Onion ranks third highest in production in the world among seven major vegetables, namely onion, garlic, cauliflower, green peas, cabbage, tomato and green beans. The four major onion producing countries in the world are China with largest production of 3.93 million tones, followed by India with 3.35 million tones, USA 2.45 million tones and Turkey 1.55 million tones. In India, about 35-40% of onion is lost during post harvest, due to the lack of proper processing and storage facilities (Sarsavadia et al., 1999). In India, deterioration of considerable quantities of onion takes place during storage operation. Various preservative methods have been employed to minimize this loss. The most primitive method employed in preserving onion deterioration is that onion flakes are spread on the ground such as wheat, raisins, fig or apricot, exposed to the sun in order to be dried. This method is commonly known as Open Sun Drying (OSD). The dried crop can be stored for a considerable period without the fear of its deterioration. The rate of drying (moisture evaporation) depends on a number of external parameters (solar radiation, ambient temperature, wind velocity and relative humidity) and internal parameters (initial moisture contents, type of crops, crop absorptivity, mass of product per unit exposed area etc.). An advanced and alternative method to the traditional techniques is Greenhouse Drying (GHD), in which the product is placed in trays receiving solar radiation through the plastic cover, while moisture is removed by natural convection or forced air flow (Condorí and Luis, 1998). This double function, greenhouse and dryer, improves the rate of the initial investment (Condorí et al., 2001), thus maintaining the good quality, increasing the storage capacity and reducing the wastage of the crop simultaneously (Tiwari, 2003).
Mathematical modeling of crops drying under solar energy is a multifaceted problem relating simultaneous heat and mass transfer in a nonisotropic and nonhomogeneous nature of the agricultural products along with their irregular shape and changes in shape during drying of crop. Convective heat transfer coefficient is one of the most decisive parameters required for analysis and simulation of the drying process (Thompson et al., 1968.). Sodha et al. (1985) presented a simple analytical model based on simultaneous heat and mass transfer at the product surface and included the effect of wind speed, relative humidity, product thickness and heat conducted to the ground for open sun drying and for a cabinet dryer. Smith and Sokhansanj (1990) have developed a natural convection heat transfer model in which the density of air was assumed to be a function of temperature and absolute humidity. Ratti and Crapiste (1995) evaluated the heat transfer coefficient under forced convection from the data on crop drying and heat and mass balances. The experimental heat transfer coefficients were correlated by dimensionless expressions with Nusselt and Reynolds numbers. The experimental heat transfer coefficient values ranged from 25 to 90 Wm-2 K-1 for potatoes, apples and carrots. Goyal and Tiwari (1998) have studied heat and mass transfer in crop drying systems and have reported the values of convective heat transfer coefficient for wheat and gram as 12.68 and 9.62 Wm-2 °C-1, respectively, by using the simple regression and 9.67 and 10.85, respectively, for the same crops while using the multiple regression technique. Anwar and Tiwari (2001b) determined the convective heat transfer under open sun drying by using the linear regression technique. However, their study was limited to constant rate drying from 11 to 13.30 h of the day for the month of May and June for composite climate of New Delhi. Further they have reported that the value of convective heat transfer coefficient is of the order of 14.0 Wm-2 °C-1. Anwar and Tiwari (2001a) evaluated the convective heat transfer coefficients for some crops under a simulated condition of forced mode in indoor open and closed conditions. Jain and Tiwari (2003) evaluate the convective heat transfer coefficients for open sun drying for various crops and they developed a mathematical model to predict the crop temperature, moisture removal and solair temperature for a steady state condition.
The present studies were undertaken to determine convective heat transfer coefficients for different mass at each stage of drying time for onion flakes with the following conditions:
| • | Open Sun Drying (OSD) under natural convection. |
| • | Greenhouse Drying (GHD) under natural convection. |
| • | Greenhouse Dying (GHD) under forced convection. |
The crop with high moisture content i.e., onion, is taken to see the effect of greenhouse on continuous drying of onion under natural and forced mode and the results have been compared with the open sun drying.
Materials and Methods
Experimental Set up
Different mass of onion flakes were kept in wire mesh tray having dimensions
of 0.4x0.24 m2. Three sets of experiment with total quantity of onion
flakes as 300, 600 and 900 g were done. The thickness of onion flakes of 3 mm
was spread as thin layers were used for each set of experimentation. A roof
type even span greenhouse with 1.20x0.78 m2 effective floor covering
area was made of PVC pipe and UV film covering.
| Fig. 1: | Onion drying under opens sun and natural convection mode
in greenhouse |
| Fig. 2: | Onion drying under force convection mode in greenhouse |
The central height and height of the walls were 0.60 and 0.40 m, respectively. An air vent with an effective opening of 0.0722 m2 was provided at the roof for natural convection. The experimental set up for open sun and greenhouse drying in the natural mode is shown in Fig. 1. For forced convection a fan of 120 mm sweep diameter with air velocity 5 m s-1 was provided on the sidewall of the greenhouse during the experiments Fig. 2. The orientation of the greenhouse was taken as east-west during the experiments.
Instrumentation
A six-channel digital temperature indicator with least count of 0.1°C
(accuracy ±0.1%) having 125°C range with calibrated copper-constantan
thermocouples was used to measure the onion and air temperature. To measure
the relative humidity a digital humidity meter (model Lutron HT-3003) was used.
It had a least count of 0.1% relative humidity with accuracy of ±3% on
the full-scale range of 5-99.9% of relative humidity. A top loading digital
balance of 6 kg weighing capacity, having a least count of 0.1 g with ±2%
on the full scale was used to weight the sample during drying.
Sample Preparation
Fresh onions were peeled and cut with the help of hand slicer in the form
of flakes of 3 mm thickness. The same sizes of samples were maintained simultaneously
for open sun drying and inside the greenhouse in all cases.
Experimentation
Experiments were carried out during the months of October to December 2003
at IIT Delhi (28°35'N 72°12'E). The prepared samples of onion were kept
in the wire mesh tray for the experiments. Observations were taken for open
sun and inside the greenhouse under natural as well as forced mode from 8 am
at every hour interval for the 33 h of continuous drying. Natural convection
under GHD was done with the air vent provided at the roof of the greenhouse.
Experiments in the forced mode under GHD were conducted by providing the ventilating
fan on the sidewall of the greenhouse.
The data of the experimental observations for the open sun and greenhouse drying for open as well as both modes natural and forced are presented in Table 1-9.
| Table 1: | Experimental data and results for convective heat transfer
coefficient for onion drying under open sun on Oct 3-4, 2003 (300 g) |
Numerical Computation
Determination of Convective Heat Transfer Coefficient
The convective heat transfer coefficient (hc) under natural convection
can be evaluated as (Anwar and Tiwari, 2001a; Jain and Tiwari, 2003):
| (1a) |
And under forced convection can be defined as:
| (1b) |
The rate of heat utilized to evaporate moisture is given as (Malik et al., 1982):
| (2) |
On substituting hc from Eq. 1 and Eq. 2 becomes:
| (3) |
Evaporated moisture can be determined by dividing Eq. 3 with the latent heat of vaporization (λ) and multiplying the area of Onion drying tray (At) and time interval (t):
| (4) |
Let
| (5) |
Taking logarithm on both sides Eq. 5 can be written:
| (6a) |
This is the form of a linear equation:
| (6b) |
Where:
| (7a) |
With
| (7b) |
Similarly in the case of forced convection mode:
| (8a) |
With
| (8b) |
By using the data from Table 1 to 9 for Tc, Te γ and mev, the values of y and x can be evaluated for different time interval and then the constant ‘C’ and exponent ‘n’ can be obtained from above Eq. 7 and 8 for natural and forced mode of drying. The constants ‘C’ and ‘n’ will be further used to evaluate convective heat transfer coefficient from Eq. 1 under natural and forced convection mode (Table 1-9).
| Table 2: | Experimental data and results for convective heat transfer
coefficient for onion drying under natural mode on Oct 3-4, 2003 (300 g) |
| Fig. 3a: | Rate of moisture evaporation in onion drying under open sun for different mass |
| Fig. 3b: | Rate of moisture evaporation in onion drying under natural
mode for different mass using greenhouse |
| Fig. 3c: | Rate of moisture evaporation in onion drying under force mode
for different mass using greenhouse |
Computation Technique
The rate of moisture evaporated corresponding to the onion temperature (Tc)
and temperature above the crop surface (Te) were calculated at each
hour interval for each case and its values are given in Table
1 to 9 and also shown in Fig. 3.
| Appendix A |
| Appendix B |
The physical properties of humid air were evaluated for the mean temperature of Tc and Te by using the expressions given in the Appendix A. The values of C and n were obtained from Eq. 7b and 8b for natural and forced mode by using the linear regression analysis at increments of every hour of observation and thus, the values of convective heat transfer coefficient (hc) were computed from Eq. 1 at the corresponding hour of drying. The computer program was prepared in the MATLAB software to evaluate convective heat transfer coefficient (Chapman, 2003).
The experimental error has been determined in terms of percent uncertainty (internal+external) for the most sensitive parameter, i.e., the rate of moisture evaporation (Appendix B).
| Table 3: | Experimental data and results for convective heat transfer
coefficient for onion drying under forced mode on Oct 3-4, 2003 (300 g) |
Results and Discussion
The results of convective heat transfer coefficient by using the values of ‘C’ and ‘n’ are also given in Table 1 to 9 for open sun drying and greenhouse drying.
The values of C and n as obtained by Anwar and Tiwari (2001b) for open sun drying for 602.9 g onion flakes are 1.00 and 0.31, respectively. While those obtained by Jain and Tiwari (2003) for open sun drying for 600 g onion flakes are 1.0064 and 0.2579, respectively. However, the values of C and n as obtained by present work are 0.472 and 0.17 respectively for open sun drying for 600 g onion flakes. The variation in the values may be due to the different drying hours of the experiment. The present work is based on continuous drying for 33 h and the observations are taken after one hour interval. While the drying hours for the experiment done by Anwar and Tiwari (2001b) and Jain and Tiwari (2003) were for 2 h and 30 min with observations taken at 15 min interval.
In the case of indoor open simulation under forced mode, Anwar and Tiwari (2001a) found the values of C and n as 0.99 and 0.75, respectively for 625.3 g of onion flakes. However, in case of indoor closed simulation under forced mode, they found these values as 0.99 and 0.59, respectively. In our present work for 600 g of onion flakes for greenhouse drying under forced mode, the values of C and n are calculated as 0.936 and 0.298, respectively. This variation in the values is due to different environmental conditions of both the experiments performed.
| Fig. 4: | Variation in convective heat transfer coefficient for different
modes of drying (300 g) |
| Fig. 5: | Variation in convective heat transfer coefficient for different
modes of drying (600 g) |
| Fig. 6: | Variation in convective heat transfer coefficient for different
modes of drying (900 g) |
Table 1 to 3 demonstrates the values of convective heat transfer coefficient for 300 g onion under open sun as well as greenhouse drying. It has been observed that the values of hc are constant for forced mode of operation. However convective heat transfer coefficient varies for open sun and greenhouse drying under natural mode. Further it is important to note that the convective heat transfer coefficient is more in natural mode as compared to forced mode greenhouse as well as open sun drying due to more operating temperature inside the greenhouse.
| Table 5: | Experimental data and results for convective heat transfer
coefficient for onion drying under open sun on Oct 18-19, 2003 (600 g) |
| Fig. 7a: | Variation in convective heat transfer coefficient in onion
drying under open sun for different mass |
| Table 5: | Experimental data and results for convective heat transfer
coefficient for onion drying under natural mode on Oct 18-19, 2003 (600
g) |
| Fig. 7b: | Variation in convective heat transfer coefficient in onion
drying under natural mode for different mass using greenhouse |
| Table 6: | Experimental data and results for convective heat transfer
coefficient for onion drying under forced mode on Oct 23-24, 2003 (600 g) |
| Fig. 7c: | Variation in convective heat transfer coefficient in onion
drying under force mode for different mass using greenhouse |
| Table 7: | Experimental data and results for convective heat transfer
coefficient for onion drying under open sun on Nov 21-22, 2003 (900 g) |
While in open sun drying, more convective heat transfer coefficient occurs than forced mode due to high wind velocity. These results have also been depicted in Fig. 4.
The results of convective heat transfer coefficient for 600 g onion for different drying modes reported in Table 4 to 6 have been shown in Fig. 5. It is important to note that the convective heat transfer coefficient is constant under forced mode unlike natural mode in greenhouse and open sun drying. Further it is observed that convective heat transfer coefficient for forced mode is more for 600 g onion in comparison with other modes due to decrease in relative humidity inside the greenhouse resulting in increased partial pressure. The convective heat transfer coefficient for greenhouse drying in natural mode is higher than open sun drying due to high operating temperature (Fig. 5) as also concluded earlier for 300 g (Fig. 4).
The results of Table 7 to 9 for 900 g onion have been shown in Fig. 6 for all cases. The trends of variation of convective heat transfer coefficient are similar to the case of 600 g except for few hours at the beginning. The value of convective heat transfer coefficient is reduced from 3 to 2.5 Wm-2 °C-1 for greenhouse drying under forced mode. It may be due to non exposure of some onion flakes due to higher layer thickness in case of 900 g.
| Table 8: | Experimental data and results for convective heat transfer
coefficient for onion drying under natural mode on Nov 21-22, 2003 (900
g) |
It is also important to observe that the convective heat transfer coefficient in open sun drying is slightly more than greenhouse drying under natural mode due to wind effect outside greenhouse.
In order to see the effect of mass of onion on convective heat transfer coefficient, the results of Fig. 4 to 6 have been further shown in Fig. 7. One can easily conclude from Fig. 7 that the convective heat transfer coefficient strongly depends on the mass and thickness of layer of onion flakes. The convective heat transfer coefficient increases by 90 and 135%, respectively as the mass of onion flakes is increased from 300 to 900 g in case of open sun drying and greenhouse drying under forced mode due to faster rate of moisture removal. However this increase is reduced to 30% in case of greenhouse drying under natural mode. It may be due to lower coefficient of diffusion of the greenhouse resulting in the increased relative humidity inside greenhouse.
The above results are found to be within the percent uncertainty of 21.71, 19.94 and 20.40 for open sun drying, greenhouse drying under natural mode and greenhouse drying under forced mode, respectively as given in Table 10.
| Table 9: | Experimental data and results for convective heat transfer
coefficient for onion drying under forced mode on Dec 4-5, 2003 (900 g) |
| Table 10: | Experimental percent uncertainties under different modes
of drying |
Conclusions
On the basis of the present studies, the following conclusions were made:
The value of convective heat transfer coefficient
| • | Depends significantly on the mass of the onion to be dried and the mode of drying. |
| • | There is 30-135% increase in the convective heat transfer as the mass of the onion flakes is increased from 300 to 900 g for different modes of drying. |
Nomenclature
| At | Area of onion flakes tray (m2) |
| C | Experimental constant |
| c | Intersection in straight line equation |
| cp | Specific heat of humid air (Jkg-1 °C-1) |
| g | Acceleration due to gravity (m s-2) |
| Gr | Grashof number = |
| hc | Convective heat transfer coefficient (Wm-2 °C-1) |
| k | Thermal conductivity of humid air (Wm-1 °C-1) |
| m | Slope in straight line equation |
| mev | Moisture evaporation (kg) |
| n | Exponent |
| Nu | Nusselt number = hcX0/k |
| Pr | Prandtl number of humid air = μcp/k |
| P (T) | Vapor pressure at temperature T (Nm-2) |
| Q | The rate of heat utilized to evaporate moisture (Jm-2 s-1) |
| Re | Reynolds number = ρvX0/μ |
| t | Time (s) |
| Te | Temperature above onion flakes (°C) |
| Tc | Temperature of onion (°C) |
| Ti | Mean temperature of the Tc and Te (°C) |
| ΔT | Effective temperature difference (°C) |
| X0 | Characteristic dimension (m) |
Greek Letters
| β | Coefficient of volumetric expansion (°C-1) |
| γ | Relative humidity (%) |
| λ | Latent heat of vaporization (J kg-1) |
| μ | Dynamic viscosity (N s m-2) |
| ρ | Density (kg m-3) |