**1. Introduction**

Drying of solids provides a technical challenge due to the presence of complex interactions between the simultaneous processes of heat and mass transfer, both on the surface and with‐ in the structure of the materials being dried. Internal moisture flow can occur by a complex mechanism depending on the structure of the solid body, moisture content, temperature and pressure in capillaries and pores. External conditions such as temperature, humidity, pressure, the flow velocity of the drying medium and the area of exposed surface also have a great effect on the mechanisms of drying.

Theoretical and experimental studies [1-6] reported the forced convection heat and mass transfer across different shapes. Most of these studies have considered a heated solid surface with a uniform surface temperature. However, this situation is not the same as in the drying process, where heat and mass transfer occurs simultaneously and the interfacial tempera‐ ture and moisture content vary during the drying.

Evaporation from a flat plate surface to a laminar boundary layer was theoretically analyzed [7]. It was calculated distributions of the interfacial temperature and local Nusselt and Sher‐ wood numbers for a parallel flow where both Prandtl and Schmidt numbers are unity. A conclusion stated that the characteristics of heat and mass transfer are highly conjugated and significantly influenced by the temperature dependency of vapor-liquid equilibrium, the magnitude of the latent heat of the phase change, and the thermal conductance of the flat plate. The work needs to be extended for the case of unsteady state conditions and to be re‐ peated for a drying bed.

The variation in the surface temperature for a flat bed of a capillary porous material was dis‐ cussed in [8]. The authors presented graphs that show an increase in surface temperature

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from the leading edge of the flat plate during the periods of drying. They also showed graphs in which the surface temperature decreased. In general, a physical meaning for that contradiction was needed to be considered.

As in [9], Jomaa *et al.* presented a simulation of the high-temperature drying of a paste in a scaled-up wind tunnel. They attempted to study the influence of the local air flow and ther‐ mal radiation on the drying behaviour of the product. A rapid air velocity (8 m s-1) was used in the experiment, and an empirical model was derived to predict temperature and solvent content along the conveyer belt. Comparison of the experimental results with those predict‐ ed ones by the model showed a satisfactory agreement.

Evaporation of a pure liquid droplet has been widely studied both theoretically and experi‐ mentally [10-14]. However, in spray dryers and droplet drying applications, the droplets al‐ ways consisted of multi-component mixtures of liquids and sometimes dissolved solids, forming a complicated multiphase composition. This makes analyzing heat and mass trans‐ fer processes more difficult. This effect is attributed to the presence of various components that vaporize at different rates, giving rise to a gradient in concentration in the liquid and vapor phase. In addition, a solid crust forms at the outer surface of droplet which acts as a resistance to heat and mass transfer processes.

Various experimental techniques have been used [15-19] to study the mechanisms of drying for a single droplet containing dissolved solids. The droplet was suspended freely from the end of a stable nozzle fixed in a wind tunnel. Air flow was hitting the droplet from one side causing a significant disturbance to the shape of droplet. Therefore, there was some difficul‐ ty in recording the weight and temperature of the droplet. The transferred heat conduction to the droplet by the nozzle was another problem.

Cheong *et al.* [20] proposed a mathematical model to predict core temperature for drying a free suspended droplet against time. Reasonable agreement was obtained between the pre‐ dicted and the experimental results at an air temperature of 20ºC. However, the predicted temperature was less accurate at higher temperatures (50ºC and 70ºC); the model was not applicable for cases at high air temperatures.

A mathematical model was modified in [21], taking into account the droplet shrinkage. The droplet was assumed first to undergo sensible heating with no mass change. The model showed that temperature distribution within the droplet cannot be ignored even for a small diameter droplet of 200 µm.

Wind tunnel definitely is considered one of the best tools to investigate and to study the role of boundary layer and the mechanisms of drying process. The most important variables in any drying process such as air flow, temperature and humidity are usually easy to be con‐ trolled inside the wind tunnel. In the current study, through an experimental work and mathematical approach, we attempt to understand the role of the boundary layer on the in‐ terface behavior and the drying mechanisms for various materials of a flat plate surface and a single droplet shape.
