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The aim of this work was to determine the characteristic dimension governing transfers during convective dryin g . Parallelepipedic and cylindrical form of sweet potato was used. For the parallelepipedic form P_L-l-e, the thickness e is set to 1 cm while the length L and the width l are varying. The results show that the variation of the other dimensions other than the thickness e does not influence the transfers in a considerable way. The same observation is made for the cylindrical samples c_H-R by keeping the radius R constant. This present work therefore allows us to conclude that the thickness of the parallelepiped shaped samples and the radius of the cylindrical shapes, all being the smallest dimensions, characterize the transfers.

The drying of agrifood products is taking on a considerable scale [

This present work consists in determining this characteristic dimension for the samples of sweet potato of parallelepipedal and cubic shapes. We have fixed the characteristic dimension in the direction of the thickness of the samples of parallelepiped shape and of the radius for the samples of cylindrical shape.

A homogeneous macrostructured material sweet potato (Ipomoea batatas) is used to minimize the effects of macrostructure and component composition [

The tubers that served as samples for our experiments were purchased in a local market in the same heap in batches in order to reduce the differences in

material properties [

Washed and wiped with blotting paper, the samples are placed in the oven, the temperature of which is suitably adjusted for drying. We used an AIR concept, temperature varying from 40˚C to 250˚C, with PID regulation, with digital display. The samples are regularly withdrawn for the determination of the mass during the drying time and reintroduced into the oven. The mass is determined by a balance (SARTORIUS, 0.001 g precision, France). The measurement time is fast so as not to disturb the thermodynamic equilibrium. Given the regular withdrawal of samples, we consider that the transfers take place on all sides. The samples are placed directly on the racks. We calculate the initial areas as the total area of the sample from the dimensions measured with a Mitutoyo (Japan) precision 2.10 - 5 mm micrometer. Note that we were unable to measure air speed and relative humidity which are important parameters in the evaluation of drying. To do this, we compare the results of experiments which were carried out simultaneously and in the same oven to circumvent the influence of these two parameters.

The determination of the dry mass is made after a stay of 24 hours in an oven at 70˚C [

The initial water content of the product is the quotient of the total mass of water contained in the freshly cut product m_{e} divided by the mass of solid matter m_{s}.

X 0 = m e m s = m 0 − m s m s (1)

where m_{0} is the initial mass of the sample and ms is the dry mass of the sample.

The curves of the water contents as a function of the drying time were drawn from the experimental data. From the mass of the sample at time t, we deduce the water content according to:

X ( t ) = m ( t ) − m s m s (2)

where m(t) is the mass of the sample at time t of drying.

From the value of the mass m(t) and the dry mass m_{s} of the sample we calculate the water content X(t) according to the equation Eq-2. The relation X(t)-t gives us the kinetics of the variation of the water content of the product. To have the same basis for comparison, we normalize the water content at time t by the initial water content X_{0} of the product determined according to the Equation (1), which gives us the X/X_{0}-t curves.

= 2 − e = 1, P_L = 4 − l = 3 − e = 1 and P_L = 5 − l = 3 − e = 1. For each type of sample, three measurements are used to ensure the reliability of the experiments. The results show that the differences between the curves are minimal, going close to zero (0) kg_{e}/kg_{ms} for the most repetitive ones to 0.109 obtained, as the worst case, for samples 1 and 2 of the parallelepipedal samples P_L = 4 − l = 2 − e = 1 at the 161^{st} minute of drying. These small differences can be attributed to imperfect measurements, but also to the complex nature of agrifood products.

_{e}/kg_{ms}) for the most repetitive to 0.114 kg_{e}/kg_{ms} −0.079 kg_{e}/kg_{ms} = 0.035 kg_{e}/kg_{ms} obtained for samples 1 and 2 of the cylinders C_H = 2 − R = 1 at the 175^{th} minute of drying. These insignificant differences can be attributed to imperfect measurements, but also to the complex nature of agrifood products.

In this paragraph we seek to verify if the thickness e of the samples in parallelepiped

shape is the characteristic dimension of drying. Indeed, according to Ouoba, 2013 [

To do this, we have set a thickness e = 1 cm facing lengths of 3 cm, 4 cm, 5 cm and widths of 2 cm, 3 cm. the sample is denoted here P_L-l-e according to the dimensions length L, width l, thickness e used for these parallelepipedic shapes P.

_{0} of the three tests by type of sample is plotted as a function of the drying time. This average makes it possible to reduce the differences in handling errors and difficulties linked to the nature of the product. As we can see, the results are very superposable.

The maximum standard water content deviation is 0.223 kg_{e}/kg_{ms} − 0.144 kg_{e}/kg_{ms} = 0.079 kg_{e}/kg_{ms} between P_L = 3 − l = 2 − e = 1 and P_L = 5 − l = 4 − e = 1 at the 140^{th} minute of drying. this difference is negligible and occurs when the material undergoes visible deformations of its structure which can also interfere with the transfer path.

The search for the characteristic dimension continues here with the cylindrical samples. We look for it in the radial direction, fixing the radius R = 1 cm as the smallest dimension. This corroborates the idea of Ouoba, 2013 [

path for the migration of water from the inside to the outside of the product. This is the smallest dimension compared to the height of the samples.

To do this, we have set a radius R = 1 cm facing heights of 2 cm, 3 cm, 4 cm and 5 cm. The sample is denoted here c_H-R according to the dimensions height H, radius R used for these cylindrical shapes c.

_{0} of the three tests by type of sample is plotted as a function of the drying time. The goal is to reduce the effects of handling and difficulties linked to the nature of the product.

Here also, it can be seen, according to

The maximum standard water content deviation is 0.302 kg_{e}/kg_{ms} − 0.217 kg_{e}/kg_{ms} = 0.085 kg_{e}/kg_{ms} between c_H = 2 − R = 1 and c_H = 5 − R = 1 at the 119^{th} minute of drying. This difference is also negligible and occurs when the material undergoes visible deformations of its structure as well.

From the foregoing, the thickness of the parallelepiped samples turns out to be the dimension which best governs the transfers versus its length and its width. Likewise, with regard to cylindrical samples, the fixed radius smaller than the height is the characteristic dimension of the transfers in the product during its convective drying.

In any case, the thickness and the radius in this present work are the smallest

dimensions. According to, Ouoba, 2013 [

For these samples of parallelepipedal and cylindrical shape, the notion of characteristic dimension, as being the shortest distance between the outside and the point most unfavorable for drying is verified.

This work highlights the importance and the existence of a characteristic dimension that dictated the transfers during the convective drying of organic products such as sweet potato.

So, for two forms, parallelepipedal and spherical forms, cut in sweet potato material are considered to be homogenous. The most important is the characteristic dimension described by Ouoba, 2013 [

In the case of samples with parallelepipedic shape, this characteristic dimension is the smallest dimension, i.e. the thickness. Indeed, for different lengths and widths Lxl ranging from 2 cm × 2 cm, 3 cm × 2 cm, 4 cm × 2 cm, 5 cm × 2 cm, all having the same thickness e = 1 cm dry in a similar way.

In the case of samples of cylindrical shape, this characteristic dimension is, for these present works, the radius which is the smallest dimension. Indeed, for different heights H ranging from 2 cm, 3 cm, 4 cm and 5 cm, all having the same radius = 1 cm dry similarly.

This work corroborates the notion of characteristic dimension described by Ouoba, 2013 [

It would therefore be recommended that dryers consider this characteristic dimension for uniform drying of the organic products to be dried.

Also, to deepen the importance of the notion of the characteristic dimension, wider experiments, varying the shapes and sizes are to be carried for further studies.

The authors declare no conflicts of interest regarding the publication of this paper.

Ouoba, K.H., Ganame, A.-S. and Zougmore, F. (2021) Research of the Characteristic Dimension of the Transfers during the Convective Drying of the Sweet Potato for the Parallelepipedic and Spherical Shapes. Advances in Materials Physics and Chemistry, 11, 267-276. https://doi.org/10.4236/ampc.2021.1112022