Download - Capita Selecta Report _ Le Ngoc Ha
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.