CAPITA SELECTA IN CIVIL ENGINEERING

Master thesis proposal:

Kinetics of drying of compacted soils

Supervisor: Prof. Pierre Gerard

Students: Lê Ngoc Hà

29 May 2015

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Table of Contents

1. Why evaporation in soils is important? ....................................................................... 2

2. How to measure the evaporation ................................................................................ 3

2.1. Non-convective drying experiment ...................................................................... 3

2.2. Convective drying experiment ............................................................................. 4

2.3. Environmental chamber drying test ..................................................................... 5

3. Drying kinetics and process of evaporation ................................................................ 6

3.1. Mass Transfer model and drying curves ............................................................... 6

3.2. Issue on the wet bulb temperature ........................................................................ 6

4. Focus on temperature ................................................................................................. 9

4.1. Surface temperature measurement ....................................................................... 9

5. Objective of the master thesis ................................................................................... 10

References ...................................................................................................................... 11

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1. Why evaporation in soils is important?

Evaporation is the process that the water change from the liquid phase to the gas phase.

In this process, heat energy is necessary to supply to break the bonds of water molecules.

The heat energy is getting from the surrounding air or the body containing water itself.

Therefore, evaporation process might cause variety effects to the properties of the water

container such as soils, vegetation, food, etc. which are in need to study.

Specifically, in soil, evaporation process of soil water may have many affection to

different cases. The water evaporation may cause the changes of volume of the soil which

can give rise to ground movements then affect the building above (A.R.Estabragh et al,

2015). It is also the main component in water wetting-drying cycles which affect to the

properties and behaviour of soils: impact on the microstructure and mechanical properties

of lime-stabilized gypseous soils (A. Aldaood et al., 2014); soil biophysical controls over

rice straw decomposition and sequestration in soil (Shui-Hong Yao et al., 2011). Moreover,

evaporation changes the moisture content in soil which interacts with the soil properties (Y.

Le Bissonnais et al., 1995), change the properties of a clay-loam soil in resistance, soil

cohesion, adhesion, internal friction and metal friction angle, and aggregate size and

mechanical stability (G Rajaram, D.C Erbach, 1999). It is also one of main issues in

geotechnical engineering (Y.J. Cui et al., 2010)

Besides, evaporation also play an important role in food engineering. It is the drying

process which reduces moisture content in food to preserve the food and allow keeping

food for longer ( (M. N. A. Hawlader et al., 1991). That is an effective method of

preservation of grains, crops and variety kind of foods. (G.R. Askari, Z. Emam-Djomeh*

and S.M. Mousavi, 2006)

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2. How to measure the evaporation

2.1. Non-convective drying experiment

In non-convective drying test, saline solution is used to impose relative humidity

inside the chamber. The principle of this method is that saline solution with a precise

concentration and at a constant temperature would be in equilibrium with a set vapour

water pressure so that set a relative humidity (Young, 1967). Hence, by giving different

kind of saline, we get the corresponding relative humidity in the chamber as a constant

temperature. The following table is the different relative humidity vs saturated saline

solution at 20oC:

Table 1. Relative humidity vs. saturated saline solution at 20oC (S. Cariou et al., 2012)

Figure 1. Experimental device for drying test using saline solution

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According to a non-convective drying test to determine permeability of argillite (F.

Homand et al., 2004), the saline solution is used to keep the relative humidity constant. The

sample used in the test is cylindrical with the radius of 25mm and the length of 20mm. It is

introduced in the hermetic chamber of a specific experiment device (see Fig.1). The relative

humidity inside the chamber are controlled by the saline solution. By changing the saline

solution, the relative humidity are changed by drying step. When the sample is in

equilibrium with a given relative humidity, the next drying step is applied with a smaller

humidity. Each step, mass of sample is measured versus time until its body reach the

equilibrium. Under the hypothesis of study, the variation of mass is supposed equal to the

total liquid mass variation in the sample. In other words, that is the water evaporated from

the sample (S. Cariou et al., 2012), (F. Homand et al., 2004).

2.2. Convective drying experiment

In convective drying test, the sample is put in the dryer where the relative humidity

and wind velocity are imposed through the convection of a humid air around the material.

Each time, the experiment is performed at the ambient temperature in the chamber. The

water evaporation occurs at the surface of the sample so that the moisture is removed

from materials. Energy is needed to provide to this phenomenon. There are two coupled

processes occur at the boundary of materials at the same time: vapour and heat transfer.

Consequently, during the experiment, the relative humidity, the mass of material and the

temperature of the sample are changed. By using the sensors, those changes are recorded

at each time step. As a result, we will get corresponds of changes in mass of the sample

and the temperature inside the sample during the experiments. (P. Gerard et al., 2009)

Figure 2 Experimental set-up: (a) determination of drying kinetic curve of the sample,

(b) determination of temperature inside the sample

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According to an experiment convective drying of kaolin cylinder (Grzegorz,

Musielak and Jacek, Banaszak, 2007), the kaolin clay sample is cylindrically shaped with

the radius R=30mm and the height H=60mm. It is also absorbed an initial moisture content

of X0=0.45. Then the experiments are executed in the dryer chamber where the temperature

Ta and relative humidity a inside the chamber are recorded by the temperature probes and

hydrometer (see Figure 2 above). Moreover, the mass of the sample is also measured to

obtain the moisture removal vs time.

2.3. Environmental chamber drying test

The drying test is carried out using environmental chamber with large-scale sample

size. In the works of Song et al., 2013, the Fontainebleau sand sample is 1000 mm in

length, 800 mm in width, and 300 mm in height. In the chamber, various atmospheric

conditions including air relative humidity, temperature, and air-flow rate are controlled.

During the test, infrared thermometers and high-capacity tensiometers are installed at

different locations in the chamber and the sample o monitor soil surface temperature and

matric suction (see the figure 3).

Figure 3 Experimental set-up with the environmental chamber (Song et al., 2013)

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3. Drying kinetics and process of evaporation

3.1. Mass Transfer model and drying curves

From measuring the mass of the samples continuously, the kinetic of drying can be

analysed. Also, the surface temperature evolution of the sample are recorded. Those two

study bring relevant indications about the different processes and periods of drying. (P.

Gerard et al., 2009)

The first way to analyse experimental convective drying data is based on the sample

weight loss evolution with the drying time, for stable drying conditions .This classical

curve, mainly used for the study of the drying kinetic, emphasizes different drying periods

depending on the shape of the curve: a linear evolution during a first period and a non-

linear one for a second period of drying. (P. Gerard et al., 2009)

Figure 4. Theoretical drying curve

The drying process can also be analysed through the curve plotting the drying flow

(=−(1.0/A)·(dM/dt) with A the drying surface, M the weight of the sample and t the drying

time) as a function of the water content of the samples (Figure 4). This drying curve

provides a second and interesting way of analysis, because the mass derivative gives more

clear and complete indications about the drying kinetic of materials.

Indeed three periods of drying can be clearly observed: a preheating period, a

constant drying flow period and a falling flow period. The limit between the preheating and

the constant flow periods can be clearly distinguished on this curve even if these periods

are very short, because of the choice of plotting the water content instead of the drying time.

3.2. Issue on the wet bulb temperature

This drying curve is studied in parallel with the temporal evolution of the

temperature at the samples boundary, presented on Figure 5 which two cases: (a) where the

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air drying temperature Ta is higher than the initial temperature of the sample T0 (a) where

the air drying temperature Ta is equal to the initial temperature of the sample T0.

(a)

(b)

Figure 5. Theoretical temporal evolution of boundary temperature of a sample during drying test

(a) Ta > T0

(b) Ta=T0

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The preheating period corresponds to the increase of the drying rate (Figure 4).

Temperature at the outer boundary of the sample T0 increases up to the wet bulb

temperature Th of the surroundings (Figure 5). This period is very short. During the constant

drying flow period, the heat supplied by the surroundings is entirely used for the

vaporization of the liquid water. The temperature of the dried porous medium remains

constant and equal to the wet bulb temperature Th (Figure 5). The existence of a locally

uniform distribution of vapour concentration at the sample surface is assumed. The

evaporation occurs in a saturated boundary layer. The vapour and the heat transfers are only

influenced by the external conditions, i.e. the drying temperature or the air velocity.

This period continues as long as the moisture transport to the surface is sufficient to

maintain the drying rate. The falling rate period is characterized by a continuous increase

in the dried body temperature, beginning from the wet bulb temperature Th up to the

temperature of the drying medium Ta. (P. Gerard et al., 2009)

From the experiment of kaolin cylinder (Grzegorz, Musielak and Jacek, Banaszak,

2007) which has been mentioned at point 2.2, the three kaolin samples were dried to obtain

the drying curves and also measured the temperature at the surface of the samples

themselves. The experiments were performed at three temperatures of the drying medium:

(1) 41,5oC, (2) 50,8oC, (3) (1) 50,4oC .During the first period of drying under steady state

drying conditions the temperature inside the sample should be steady and equal to the wet

bulb temperature (Figure 6).

Figure 6. a) Change of mass of the sample b) Change of temperature inside the sample

The theoretical values of wet bulb temperatures are 26.2, 27.5, and 31.4 ◦C for three

drying conditions considered (40.5, 50.8, and 60.4◦C). In our experiments, we noticed

higher values of temperature inside the samples: about 30, 35, and 40◦C. The main reason

for this is that moisture evaporation from the sample was rendered difficult due to the small

volume of the laboratory chamber and small exchange of the drying medium inside the

chamber with outside air. Such specific drying conditions generate a lower energy flux

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evaporated with steam from the sample surface than the theoretical value calculated for

drying in free air, which leads to higher sample temperature than the theoretical wet bulb

temperature, as expected and measured.

4. Focus on temperature

4.1. Surface temperature measurement

There is lack of method and experiment to measure the surface temperature of the

soil sample in drying test. As the experiment introduced in previous point, the results

obtained from the test and the ones obtained from the numerical model are not matched.

4.2. Method to measure surface temperature

a. Thermo couples

A thermocouple is a temperature-measuring device consisting of two dissimilar conductors

that contact each other at one or more spots, where a temperature differential is experienced

by the different conductors (or semiconductors). It produces a voltage when the temperature

of one of the spots differs from the reference temperature at other parts of the circuit.

Thermocouples are a widely used type of temperature sensor for measurement and

control,[1] and can also convert a temperature gradient into electricity. Commercial

thermocouples are inexpensive,[2] interchangeable, are supplied with standard connectors,

and can measure a wide range of temperatures. In contrast to most other methods of

temperature measurement, thermocouples are self powered and require no external form of

excitation. The main limitation with thermocouples is accuracy; system errors of less than

one degree Celsius (°C) can be difficult to achieve.[3] (Wikipedia).

Thermocouple could be used to measure the surface temperature of soil in drying

test.

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b. Therma camera

Therma camera is a device used to take the infrared picture of the cross sections

that show the temperature of the samples.

(Stefan J. Kowalski, Grzegorz Musielak, and Jacek Banaszak, 2010)Therma CAM B2

camera made by the FLIR SYSTEMS firm

Imaging Performance

Field of view/min focus

distance 34° x 25° / 0.1m

Thermal sensitivity < 0.10° C at 25° C

Detector type Focal plane array (FPA) uncooled microbolometer; 160 x 120 pixels

Spectral range 7.5 to 13 μm

5. Objective of the master thesis

The objective of the master thesis is to assess how the temperature evolves at the surface of

specimen and how influences the drying kinetics.

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References

A. Aldaood et al. (2014). Impact of wetting–drying cycles on the microstructure and

mechanical properties of lime-stabilized gypseous soils. Engineering Geology 174.

A.R.Estabragh et al. (2015). Laboratory investigation of the effect of cyclic wetting and

drying. Soils and Foundations.

F. Homand et al. (2004). Permeability determination of a deep argillite in

saturatedPermeability determination of a deep argillite in saturated. International

Journal of Heat and Mass Transfer 47.

G Rajaram, D.C Erbach. (1999). Effect of wetting and drying on soil physical

properties1Effect of wetting and drying on soil physical properties. Journal of

Terramechanics.

G.R. Askari, Z. Emam-Djomeh* and S.M. Mousavi. (2006). Effects of Combined Coating

and Microwave Assisted Hot-airDrying on the Texture, Microstructure and

RehydrationCharacteristics of Apple Slices. Food Science and Technology

International.

Grzegorz, Musielak and Jacek, Banaszak. (2007). Non-linear heat and mass transfer

during convective drying of kaolin cylinder under non-steady conditions.

Transport in porous media, 121--134.

M. N. A. Hawlader et al. (1991). Drying Characteristics of Tomatoes . Journal of Food

Engineering 14 .

P. Gerard et al. (2009). Study of the soil–atmosphere moisture exchanges through

convective drying tests in non-isothermal conditions. INTERNATIONAL

JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN

GEOMECHANICS.

S. Cariou et al. (2012). Experimental measurements and water transfer models for the

drying of argillite. International Journal of Rock Mechanics & Mining Sciences

54.

Shui-Hong Yao et al. (2011). Soil biophysical controls over rice straw decomposition and

sequestration in soil: The effects of drying intensity and frequency of drying and

wetting cycles. Soil Biology and Biochemistry.

Song et al. (2013). Experimental study on water evaporation from sand using

environmental chamber. Canadian Geotechnical Journal, 115--128.

Song, W. (2014). Experimental investigation of water evaporation from sand and clay

using an environmental chamber. Paris.

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Y. Le Bissonnais et al. (1995). Interactions between soil properties and moisture content

in crust formation, runoff and interrill erosion from tilled loess soils . Catena 25.

Y.J. Cui et al. (2010). Simulating the water content and temperature changes in an

experimental embankment using meteorological data. Engineering Geology 114.

Young, J. F. (1967). Humidity control in the laboratory using salt solutions-a review . J.

appl. Chem., vol 17.