**2. Boundary layer over a flat plate surface**

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

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‐

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

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

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

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

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

contradiction was needed to be considered.

166 Wind Tunnel Designs and Their Diverse Engineering Applications

ed ones by the model showed a satisfactory agreement.

resistance to heat and mass transfer processes.

to the droplet by the nozzle was another problem.

applicable for cases at high air temperatures.

diameter droplet of 200 µm.

a single droplet shape.

A boundary layer developed over a flat-plate surface plays a great role in the mechanisms of convective drying. Very little work has been done on the conjugated problem of heat and mass transfer during a flow over a drying bed. In this paper, through an experimental and mathematical approach, we attempt to understand the role of the thermal boundary layer on the interface behavior and the drying mechanisms for porous mediums. Beds of desert sand, beach sand and glass beads were subjected to forced convective drying in a scaled-up wind tunnel.

A laboratory-scale dryer designed for this work is shown in Figure 1. The apparatus consists of wind tunnel, molecular-sieve air dryer, 3 KW air heater and a fan. Through the wind tun‐ nel a controlled flow of hot, dry air, with an average velocity of 1 m s-1, was passed over a sample mounted flush with the tunnel floor. The last section of the wind tunnel (converging section) was designed to be opened easily for installation of the test bed.

**Figure 1.** Experimental apparatus composed of: 1. fan; 2. molecular-sieve air dryer; 3. voltage regulator; 4. air heater; 5. wind tunnel; 6. observation port; 7. joint; 8. thermocouple socket; 9. thermocouple selector.

A glass tray (100 cm x 36 cm x 2.5 cm) attached to a flat metal plate was especially designed for this study. The top surface of the glass tray was at the same level of the metal plate that formed the floor of the wind tunnel. The sides and the bottom of the tray are insulated with neoprene rubber to minimize heat transfer *via* the glass wall. Figure 2 shows both the wind tunnel and the glass tray.

Ten thermocouples were inserted from one side of the tray to facilitate measurement of sur‐ face temperature distribution along the bed. At the same side of the tray, another ten ther‐ mocouples were inserted but at lower depth, 2.2 cm from the surface, to measure the bottom temperature distribution.

**Figure 2.** Details of (a) Wind tunnel: 1. smoothing grid; 2. observation port; 3.thermocouple socket; 4. joint, (b) Glass tray (clear) and flat plate (gray): 1. the leading edge; 2. thermocouple socket; 3. thermocouples.

The experiment was initiated by switching on the centrifugal fan and then the electric air heater. The voltage regulator was adjusted to provide the desired air temperature. The air temperature was monitored until it reached a steady state. This state was normally required 1 - 2 hours to be achieved.

When the apparatus achieved a constant air temperature, the drying process was com‐ menced. The metal plate and the glass tray, containing the sample, were placed carefully in‐ side the tunnel, allowing the hot air to pass over the surface of the bed. The initial readings of time and temperature were then registered immediately. During the experiment, temper‐ ature distributions were measured at intervals of approximately 20 minutes. The tempera‐ ture at each distance was measured by using the thermocouple selector and registered with an accuracy ± 0.05ºC.

Two types of sand were subjected to the drying process. The first type was a fine sand of average grain size 220 µm diameter, with a moisture content of 0.17 kg kg-1, taken from the desert of Kuwait, Burgan ( N 28o 44 00 E 047o 42 00). The nature of the desert sand is usually fine and dry. The other type of sand was a beach sand of average grain size 300 µm diame‐ ter and 0.24 kg kg-1 moisture content, taken from the north coast of Kuwait, Bobiyan Island ( N 29o 46 00 E 048o 22 00). The north coast is a place where the Shatt Al-Arab River (Iraq) falls into the Arabian Gulf. This makes the coast more muddy and less saline than the south coast. In addition, the beach sand contains a lot of small shells of various shapes. Glass beads of 400 µm diameter and 0.2 kg kg-1 initial moisture content were also selected, for test as a porous medium for comparison. The bed of glass beads is considered a typical sample for drying experiments, since the sizes of the beads are almost identical. The two types of sand and the glass beads beds are good examples for testing drying processes.
