EXPERIMENTAL DETERMINATION OF CONVECTIVE HEAT TRANSFER COEFFICIENT DURING OPEN SUN AND GREENHOUSE DRYING OF APPLE SLICES

A small scale greenhouse type dryer was designed and tested. Convective heat transfer coefficient of apple slices was identified under open sun at natural convection mode and in greenhouse drying at natural and forced convection mode. The experiments were carried out in May-June, 2014 under the climatic conditions of Elazığ, Turkey (3914’ E 3841’ N). The rate of moisture removal, crop temperature, relative humidity inside and outside the greenhouse and ambient air temperature for complete drying was recorded at 30 min intervals. The values of constants (C and n) in Nusselt number expression were found by using linear regression analysis of the data gathered from apple slices. Finally, convective heat transfer coefficients were determined. The average convective heat transfer coefficient values for apple slices were calculated as 2.863 W/m C for open sun drying under natural convection mode, 2.065 W/m C for greenhouse drying under natural convection mode, 2.724 W/m C for greenhouse drying under forced convection mode, respectively. The convective heat transfer coefficient in open sun drying was slightly higher than forced convection in greenhouse drying. The convective heat transfer coefficient in greenhouse drying under natural mode was found as the lowest one.


INTRODUCTION
Food products can be produced in certain periods of the year and in a short time due to salient features.Generally, few intensive made products can be consumed as fresh in this period.Thus, a substantial portion of the product must be stored safely until they reach the consumer [1].
There are many techniques for a long storage of food products.These techniques are drying, cooling, treating with chemicals.The most common application among them is drying [2].Drying is defined as removal of the water or liquid in the material.As a result of this removal process, biochemical reactions and the growth of microorganisms which may take place in the products will be stopped down to a level, and then the products can withstand in a long time without impairing [1].
The term of drying, which is a heat and mass transfer process, under open sun drying is known as one of the oldest methods of food storage [2].But under open sun drying conditions, quality of the product seriously decreases due to dusts, impurities, insects and birds.Thus, marketing chances of low quality products decrease [3].To overcome this problem, drying process can be carried out by indoor drying systems.
Greenhouse type dryer is one of the indoor drying systems.In general, the greenhouse type dryers are evaluated as integral (direct) type dryers and sometimes they are evaluated as mixed (direct + indirect) mode type dryers [4].The greenhouse dryers have simple design and are easy to manufacture at low cost.Firstly, the greenhouse was used to cultivate agricultural products [5].In later years, it was used to heat ambient, soil solarization, poultry raising or aquaculture.After that, researchers discovered that the greenhouse could be used as dryer with low temperature.As a result, the greenhouse can be used both to cultivate agricultural products and to dry agricultural products.These favorable developments increased its economic value [5].According to the type of heat transfer, greenhouse type dryers are divided into two classes.These are greenhouse drying at natural convection and forced convection mode.Natural convection drying process works on the principle of thermosyphic effect.Humid air which formed in the forced convection of greenhouse dryer gets ventilated through the ventilator -provided at the roof or through the chimney of the dryer [5].Elkhadraoui et al. [6] performed an experimental analysis to examine the performance of a new mixed type solar greenhouse dryer at forced convection mode to dry red pepper and sultana grape.Experimental drying curves on the results of this study showed only the falling rate period.
One of the most important factors to be considered is the convective heat transfer coefficient.The convective heat transfer coefficient varies due to temperature difference between air and product.Thus, it is an important parameter in determining of drying rate and dryer design.Generally two methods can be used to evaluate convective heat transfer coefficients of products.These are dimensional analysis and direct measurement of heat transfer on a product bed by comparing the temperature curves with Shumann's exact solution.The dimensional analysis is mathematically simple and has a wide range of applications.This method is incomplete without sufficient experimental data, although it facilitates the interpretation and extends the range of application of experimental data by correlating them in terms of dimensional groups [3,7,8].
Although some theoretical and experimental studies were studied about drying, limited studies were carried out to evaluate convective heat transfer coefficient.Anwar and Tiwari [9] studied to find convective heat transfer coefficient in Open Sun Drying.They benefited from experimental data obtained from six various food products.Anwar and Tiwari [10] also worked on forced mode drying for six various food products.Akpinar [7,11] calculated the convective heat transfer coefficient in both natural and forced convection drying using experimental data from eight various products obtained.Kumar and Tiwari [12] studied natural and forced convection drying in greenhouse to investigate effect on convective mass transfer coefficient of mass and volume of the product.In this study, the researchers dried jaggery tray in three different sizes and in two different weights of 0.75 kg and 2.0 kg.Kumar and Tiwari [13] performed open sun and greenhouse drying for onion flakes in three different weights.In this study, the effect of mass on convective mass transfer coefficient during drying was found.Burmester and Eggers [14] studied heat and mass transfer to drying coffee.They were intended to understand heat and mass transfer during the coffee drying process.They also determined optimization of the industrial application transport coefficients and coffee properties.Barati and Esfahani [15] worked on simultaneous heat and mass transfer to investigate a new solution approach during the mango drying process.It was intended in this method to be useful for fast and accurate drying simulation in this method.Tzempelikos et all.[16] developed a numerical model for non-steady heat and mass transfer to drying cylindrical quince slices.It shows that this model is correct against experimental data for different air stream velocities (1 and 2 m/s) and temperatures (40, 50 and 60 °C).
In this study, a small scale greenhouse type dryer was designed and manufactured.Convective heat transfer coefficient of apple slices was determined under open sun at natural convection mode and in greenhouse drying at natural and forced convection mode.These values would be useful in designing a dryer for a given range of products and performance analysis of the drying process.

Experimental Setup
The experiments were conducted in the months of May-June 2014 in the climatic conditions of Elazig-Turkey (39°14' E 38°41' N).In this study, apples are used as material.When the raws were selected, it was sure that the materials have same size and also non-crushed and non-decayed structure.The apples were given some treatments such as reduction peeling, size reduction, slicing.Finally they were put on a wire mesh tray and dried there.The measured quantity of product was kept in a wire mesh tray of 0.17 x 0.30 m 2 size directly over the electronic balance of 3500 g capacity having a least count of 0.01 g.For the dried process of the apple, a greenhouse type dryer was designed and manufactured.The frame of the greenhouse made of wood and covered with a plastic sheet.The greenhouse type dryer was established on concrete floors.The product temperatures and the temperature of air leaving the product surface at different locations were measured by iron-constantan thermocouples connected to a twelve channel-automatic digital thermometer (ELIMKO 6400), with a least count 0.1 °C.Precautions were taken so that placement of the thermocouple would not affect the reading being shown by the balance.An EXTECH 444731 model, thermo hygrometer humidity measuring instrument with a least count 0.1 RH was used for relative humidity measurement just above the product surface.The humidity-measuring instrument was kept on the product surface with its sensing ports facing downwards towards the product surface.Every time, it was kept 2 min before recording observations.For all the observations the data for the rate of moisture removal, crop temperature, relative humidity inside and outside the greenhouse and ambient air temperature for complete drying were recorded at 30 min time intervals.In Figure 1. was seen the schematic picture of the prepared set of experiments can be seen and in Figure 2 the photo of this system can be seen.

Computation Procedure
The values of observations for apple slices are shown in Table 1 under open sun at natural convection mode, in Table 2 in greenhouse drying at natural convection mode and in Table 3 in greenhouse drying at forced convection mode.
The convective heat transfer coefficient (hc) can be determined using the expression for Nusselt number as [5,10,17,18]: or natural convection: forced convection: The rate of heat utilized for moisture evaporation is given as [5,11,12,14] On substituting hc from Eq. ( 1), Eq. ( 4) becomes Q e ̇= 0.016 The moisture evaporated is determined by dividing Eq. ( 5) and ( 6) by the latent heat of vaporization (λ) and multiplying with the area of tray (At) and time interval (t) Eq. ( 12) and ( 13) is the analogy of an equation of a straight line, The different physical properties of humid air, i.e. density (ρv), thermal conductivity (Kv), specific heat (Cv) and viscosity (μv), used in the computation of Reynolds number (Re), Grashof number (Gr) and Prandtl number (Pr) have been determined using the following polynomial expressions [5,12,13,18].For obtaining physical properties of humid air, Ti is taken as average of product temperature (Tc) and exit air temperature (Te): The values of the constants C and n have been determined by linear regression analysis by using measured data of the product and exit air temperatures, exit air relative humidity and moisture evaporated during a certain time period.

RESULTS AND DISCUSSION
The average product temperature (T ̅ c), exit air temperature (T ̅ e) and exit air relative humidity (γ ̅) have been used for determining the physical properties of humid air which, in turn, used for calculating the values of Reynolds number, Grashof number and Prandtl number.The constants, C and n, were determined by linear regression analysis and considered further for obtaining the values of the convective heat transfer coefficients by Eq.1.
Measured and calculated values of the apple slices which were dried in open sun drying, in greenhouse at natural convection and forced convection drying are showed in Table 1, Table 2 and Table 3, respectively.According to the time change, the convective heat transfer coefficient values which were obtained from experiments are showed in Figure 3.The convective heat transfer coefficients were found between 1.36 and 3.60 W/m²°C in open sun drying.The convective heat transfer coefficients were also found between 1.75 and 2.23 W/m²°C in greenhouse drying at natural convection.In addition, the convective heat transfer coefficients were found between 2.70 and 2.74 W/m²°C in greenhouse drying at forced convection.According to Figure 3, especially in the beginning, the convective heat transfer coefficients were found to be at the highest value in open sun drying because of wind effect outdoor and temperature difference.At first, the convective heat transfer coefficient in open sun drying was slightly higher than in greenhouse drying at forced convection.However, towards the end of the drying process, the highest value belongs to the greenhouse drying at forced convection because of external conditions.The convective heat transfer coefficient variation versus time was almost constant in the greenhouse under the forced convection case, since it was not affected by external conditions as musch as the other systems did.Although the temperature difference was more than others, the convective heat transfer coefficient in greenhouse drying under natural mode was found to be the lowest one due to the high moisture content.
The average values of C, n and hc for apple slices have been showed at Table 4 in open sun drying, in greenhouse drying at natural and forced convection.The average convective heat transfer coefficient values that were obtained in studies in the literature are showed in Table 5.
According to Table 5, the range of the convective heat transfer coefficient (hc) is partly consistent with experimental studies in the literature.The differences of this study from the ones in literature are that: the climate conditions and the applied drying process.
Dried apple slices are showed in Figure 4. Shrinkage and loose of coloration were not observed in those products.

Figure 3 .
Figure 3. Variation of the convective heat transfer coefficient with time for different drying conditions and modes

Table 4 .
C, n constants and average convective heat transfer coefficient (hc) values of apples slices that dried in open sun drying, in greenhouse drying at natural and forced convection.

Table 1 .
The measured and calculated values for apple slicess in open sun drying

Table 2 .
The measured and calculated values for apple slicess in greenhouse with natural convection drying

Table 3 .
The measured and calculated values for apple slicess in greenhouse with forced convection drying

Table 5 .
The average convective heat transfer coefficient from the cited references