; AN EXPERIMENTAL STUDY OF FORCED CONVECTION GREEN HOUSE DRY-2
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AN EXPERIMENTAL STUDY OF FORCED CONVECTION GREEN HOUSE DRY-2

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									    INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME
               ENGINEERING AND TECHNOLOGY (IJARET)

ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
                                                                          IJARET
Volume 4, Issue 5, July – August 2013, pp. 10-16
© IAEME: www.iaeme.com/ijaret.asp                                         ©IAEME
Journal Impact Factor (2013): 5.8376 (Calculated by GISI)
www.jifactor.com




 AN EXPERIMENTAL STUDY OF FORCED CONVECTION GREEN HOUSE
                         DRYING

                  AjeetKumar Rai*, SarfarajAhamadIdrisi, Shahbaz Ahmad
                  Department of Mechanical Engg. SSET, SHIATS-DU Allahabad



ABSTRACT

        In the present study a greenhouse dryer is designed, fabricated and its performance is tested
in the force convection mode of heat transfer. A thermal model of the system is developed in the
forced convection greenhouse dryer and in the natural convection open sun drying mode.
Experiments were conducted in the premises of SHIATS-DU Allahabad at latitude of 25°N.
Measurements of solar intensity, relative humidity inside and outside the green house dryer, moisture
removal rate, air velocity and temperatures at different points were recorded. It is find that the
average convective heat transfer coefficient for the forced convection greenhouse drying mode is
higher than the open sun drying.

Key words: Forced convection green house dryer, convective mass transfer coefficient.

INTRODUCTION

        Now a day’s solar drying is a renewable and environmentally friendly technology. Solar
drying can be considered as an advancement of natural sun drying and it is a more efficient
technique of utilizing solar energy. For a better performance the solar drying systems must be
properly designed in order to meet particular drying requirements of specific products and to
give optimal performance. Designers should investigate the basic parameters such as
dimensions temperature, relative humidity, airflow rate and the characteristics of products to be
dried etc.
        The most common process of crop drying is known as open sun drying (OSD), during which
solar radiation falls directly on the crop surface and is absorbed up to certain limit of temperature.
The absorbed radiations heat up the crop and evaporate the moisture from the crop. Sodha et al.
modes.[1] 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.[2] Condori M, Luis S.
Greenhouse driers have the regular greenhouse structure (when not in use for crop production),

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME

where the product is placed in trays receiving solar radiation through the plastic cover, while
moisture is removed by n forced air flow. Mulet et al. [3] proposed a method of standardizing
open sun drying time by defining the equivalent time based on the average solar radiation input.
Hossain et al[4] redesigned, fabricated and installed a mixed mode type forced convection solar
tunnel dryer at the Department of Farm Power and Machinery, Bangladesh Agricultural University,
Bangladesh for drying of red and green chillies under the tropical weather condition. Shanmugam et
al. [5] designed and fabricated a desiccant integrated solar dryer to investigate its performance under
the hot and humid climatic conditions of Chennai, India during the month of June. Dincer and Dost
[6] presented a method to determine the moisture diffusion coefficient and moisture transfer
coefficient for a solid object by employing the drying coefficient and lag factor.
         Ratti and Crapiste [7] 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. Anwar and Tiwari [8]
evaluated the convective heat transfer coefficients for some crops under a simulated condition of
forced mode in indoor open and closed conditions. Manohar and Chandra [9] studied the drying
process in greenhouse type solar dryer using natural as well as forced ventilation and the drying
data were represented with the Page drying equation. Condon' and Saravia [10] presented an
analytical study of the evaporation rate in two types of forced convection greenhouse dryers using
single and double chamber systems. Kumar A and Tiwari [11] compared the convective mass
transfer coefficient of open sun drying, greenhouse dryer under natural and forced convections
for drying onion flakes. It was found 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 green house.
         The purpose of this work was to evaluate the heat transfer coefficient. The experiments were
conducted after the crop harvesting season May 2013. This study was limited to constant rate drying
from 7.5 to 8 hr of the day. The half hourly data for rate of moisture removal, crop temperature,
relative humidity inside and outside the greenhouse and ambient air temperature for the complete
drying period have been recorded. These data were used for determination of the convective heat
transfer coefficient at every half an hour of drying time for bitter melon with the following
conditions:(a) Open sun drying (OSD) under natural convection. (b) Greenhouse drying (FGHD)
under forced convection. A suitable empirical model is presented to regress the convective heat and
mass transfer coefficients as a function of drying time.

Materials and methods
       The Nusselt number is a function of Grash of number and Prandtlnumbes for natural
convection. The Nusselt number is a function of the Reynolds and Prandtl numbers for force
convection.

 Nu = [     ] =C(GrPr)n         (for natural convection),


Nu = [    ] =C (RePr)n        (for forced greenhouse convection)

Where C and n are the constants
Now the convective heat transfer coefficient under natural convection can be determined as -----

hc= [ ] C(GrPr)n



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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME

Moisture evaporation is given as

Qe= 0.0l6 hc [P (Tp) – γ P (Te)]

        The hc in the above expression with moisture evaporation is termed the convective mass
transfer coefficient in the case of crop drying.

Substituting the value of hc in above, Qe = 0.016[ ] C(GrPr)n [P(Tp)-γP(Te)]
The moisture evaporated found by dividing the latent heat of vaporization (k) and multiplying by the
area of the tray (At) and time interval (t).

Mev = tAt = 0.016 [P(Tp) – γP(Te)] tAt C (Gr Pr)n ,
          Z=0.016 [P(Tp) – γP(Te)] tAt, Mev=ZC(Gr Pr)n
         Taking the logarithm of both side, ln [    ] = ln C+ ln (GrPr)n

This is the form of a linear equation Y = mX0 + C0, where
       Y =ln[ ],        X0= ln[Gr Pr], m = n; and C0= ln C;         thus C = eC0
                                              (for natural convection)
Similarly, for forced green house convection
      Y =ln[ ],        X0= ln[Re Pr], m = n; and C0= ln C;          thus C = eC0

Experimental set up
        Two stainless steel wires mesh trays of 0.50×0.50m2 were used to accommodate 0.400 kg
samples of bitter melon as thin layers, respectively. A roof type even span greenhouse with an
effective floor covering 1.0 × 1.0 m2 has been made of aluminum plate (of L-shape c/s) and plastic
film covering of 1.5mm. thikness. The central height and height of the walls were 1.285 and 1.0 m,
respectively. An air vent with legs 0.1m andan effective opening provided at the roofof 0.15×0.15
m2and a fan of 150 mm sweep diameter with air velocity 5 m/s was provided on the sidewallof dryer
for forced convection. A continuous supply of A/C current 220V was supplied. The experimental set
up for open sun drying and forced convection drying mode is shown in Fig. 1 and 2.The greenhouse
had an east-west orientation during the experiments.




          Fig. 1 Open sun drying (OSD)                  Fig.2 Forced green house drying (GHD)


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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME




                            Fig.3 Instruments used in the experiments

Instrumentation builds up
       A digital humidity/temperature meter was used to measure the relative humidity and
temperature of air in the greenhouse, of ambient and above the crop surface.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
and 1°C temperature with accuracy of ± 1 % on the full scale range of 10-80°C. A non-contact
thermometer, having a least count of 1°C and full scale range of 18 to 260°C was used for
measurement of the crop temperature. A top loading digital balance (gold line) of 500 g weighing
capacity, having a least count of 0.01gm was used to weigh the sample during drying. The difference
in wave length calibrated bysolarimeter (Central Electronics Ltd., India).It measures solar radiation
in m W/cm2, having a least count of 2 m W/cm2 with ±2% accuracy of the full scale range of 0-120
mW/cm2. The air velocity across the greenhouse section was measured with an electronic digital
anemometer, a fan to produce forced convection of Zigma Pvt. Ltd. fitted on side wall of the green
house dryer

Sample preparation
       The same sizes of samples were maintainedof fresh bitter melon (karalla) cut into small slices
5 mm thickness with the help of chips cutter. The slices were soaked in water for 4 h and than
conditioned in a shed for 1/2 h after removing the excess water. The same sizes of samples were
same for open sun drying and inside the forcedconvectiongreenhouse in all the cases.

Experimentation
        The experiments on OSD were always under natural convection and forced convection under
FGHD was done with the help of air vent provided at the roof and an A/C fan on the side of the
greenhouse dryer.The experiments were revised type in nature. Experiments were conducted in the
months of May 2013 for open and forced convection green house dryer by using a fan on the side of
dryer, in the Climatic conditions of SHIATS Allahabad. The 0.400 kg samples were kept in the wire
mesh tray for the experiments. Observations were taken under open sun and inside the forced
greenhouse simultaneously. The observations were recorded from 9 am at every½ hour interval for
the 17 times continuous drying. All the experiments of forced greenhouse drying (FGHD) have been
conducted simultaneously with the open sun drying (OSD) for experimental study.




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME

RESULTS AND DISCUSSION

        Variation of relative humidity with respect to the time of the dayareshown in fig.4.At 1.30
‘O’clock relative humidity inside the green house and of the environment becomes equal.Variation
of solar intensity with respect to the time of the dayshows in the fig 5. It is highest at 1.00 ‘O’ clock.
Solar intensity and falls down in the afternoon time than the rate of rise in the morning. In the fig 6
the variation of air velocity is also shown which plays an important role in open sun drying. This has
promoted the faster rate of moisture removal in open sun drying than in the FGHD in the initial stage
of drying. The values of constants ‘C’ and ‘n’ are obtained by simple linear regression analysis, and
thus the values of hc were determined for both open sun drying and forced convection greenhouse
drying mode. Fig.8 shows the variation of convective heat transfer coefficients Vs time for open sun
drying and FGHD drying modes. It is observed that the maximum rate of moisture removal took
place in the beginning of the drying time. The mass removal rate becomes nearly constant after 240
minutes of drying time in FGHD. The convective heat transfer coefficient inside forced convection
greenhouse drying mode is more than open sun drying.Similar results have been observed by D. Jain
et al.[12].and S K Shukla et al [13].Whereas Ajeet Kumar Rai et.al.[14] have reported that the
convective heat transfer coefficient is higher in open sun drying when it is compared with the green
house drying in natural convection mode.
Result Data forOpen sun and forced convection Green house drying modes
                                   C       n       hc (W/m2°C) hcav (W/m2°C)
Open sun drying mode                                         0.87          0.31                      1.862-3.265         2.545
Forced convection green house                                 1.46      0.3824                       1.693-3.673         2.832
Drying mode

                             20                                                      1500                                   Solar Intensity
   Humidity %




                             15
                                                                                   Solar Intensity




                                                                                     1000
                             10
                                                                                              500
                             5
                             0                                                                       0
                                  0   100   200 300 400      500     600                                 0   100 200 300 400 500 600
                                             Time (minute)                                                       Time(minute)

       Fig.4. Variation of relative humidity Vs time                                  Fig.5. Variation of solar intensity Vs time


                                                         Air Velocity                                                             FGHD
                                                                                            160
                             2                                                                                                    Mev (gm)
                                                                                            140
        Air velocity (m/s)




                                                                                            120
                     1.5                                                                    100
                                                                                   Mev (gm)




                                                                                             80
                             1
                                                                                             60
                     0.5                                                                     40
                                                                                             20
                             0                                                                0
                                  0   100 200 300 400 500 600                               -20 0            100 200     300 400 500 600
                                          Time(minute)                                                                 Time (minute)


                             Fig. 6-variation of air velocity Vs time            Fig.7-variation of Moisture removal rate Vs time

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME

CONCLUSION

        A greenhouse dryer is designed and fabricated to work in force convection mode. A thermal
model of the system is developed. The convective heat transfer coefficients for bitter melon under
open sun and forced convection mode were determined by using the values of the constants ‘C’and
‘n’ in the expression of Nusselt number. C and n in the open sun drying were found to be 0.87 and
0.31, whereas for the forced convection greenhouse drying the corresponding values are found to be
1.46 and 0.3824 respectively.The maximum values of convective heat transfer coefficients under
open sun drying and forced convection greenhouse drying were found to be 3.26513 W/m2 °C, and
3.67323 W/m2°C.Where the average convective heat transfer coefficient for theforced convection
greenhouse drying mode is higher than the open sun drying. It is concluded that-



                              4                                                     FGHD hc Curve
                             3.5                                                    OSD hc Curve

                              3
                             2.5
              hc (W/m2 °C)




                              2
                             1.5
                              1
                             0.5
                              0
                                   0   30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510

                                                              Time(minute)


             Fig. 8Variation of Convective heat transfer coefficient (W/m2°C) Vs time

1. The maximum rate of moisture evaporation took place in the beginning of the drying time (1-4 h).
The mass transfer rate became approximately constant after 6 h of drying time.
2. The convective mass transfer coefficient in the beginning of drying behaves like a wetted surface
and at the end of the drying like as a dry surface.
3. The convective mass transfer coefficient as a function of drying time has been established withthe
help of a two term exponential curve model.

REFERENCES

  [1]   Sodha M. S., Dang A., Bansal P.K., and Sharma S.B. (1985). An analytical and
        experimental study of open sun drying and a cabinet type dryer. Energy conversion and
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  [2]   Condori M, Luis S. The performance of forced convection greenhouse driers. Renewable
        Energy 1998;13(4):453-69.
  [3]   MuletA, Berna A, Rossello, CaiiellasJ. Analysis of open sun drying experiments
        .Drying Technol 1993;11(6): 1385-400.
  [4]   Hossain M. A., Bala B. K. “Drying of hot chili using solar tunnel dryer” Solar Energy
        81(2007): 85-92.

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                              nced
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 5, July – August (2013), © IAEME

  [5]    Shanmugam V.,Nataranjan E “Experimental Investigation of forced convection and
                      tegrated
         desiccant integrated solar dryer” Renewable Energy 31 (2006): 1239-121239 12
  [6]    Dincer I, Dost S. A modelling study for moisture diffusivities and moisture transfer
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APPENDIX

 TP(T)=exp[ 25.317―{353.44/ (Ti+273.15)}]
     Gr = gL3βρ2∆T/µ2          µv = 1.718×10-5 +4.620×10-8Ti
     Re = ρ v L/µ              ρv = 353.44/ (Ti+273.15)
 Pr = µ Cp/K            =0.0244+0.7673×10
                     Kv=0.0244+0.7673×10-4Ti
 Where, Ti=(Tp+Te)/2  Cv= 999.2+0.1434 Ti+1.101×10-4Ti2-6.7581×10-8Ti3




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