IMPORTANCIA Y USOS

To obtain high temperatures with minimum power and minimum thermal gradients across the sample, heat convection and heat conduction had to be reduced as much as possible. To reduce convective processes, a compact furnace design was employed. To reduce heat conduction the sample was surrounded by insulating material as any direct contact with conductive material would allow heat to leak away from the sample. Good insulation was essential in the radial and in the axial directions for safety reasons and to achieve temperature uniformity along the sample. The lower and upper blocks used to hold the furnace together and insulate the top and bottom of the furnace do not undergo any stresses and could therefore be made up of Pyrophilite. All insulating materials and components were manufactured and ordered following specific design requirements by Ceramic Substrates and Components, UK. Properties of the insulating materials used are listed in Table 4.3.

4.4.2.1. Requirements and Design Choice

The inner walls of the pressure vessel have a maximum continuous service temperature of 300° C. This meant that a maximum temperature drop of 700°C was to be achieved between the coils and the vessel wall. Having chosen the size of the external heating element, there were 15 mm of radial space between the outer coil and the vessel wall in which temperatures had to fall drastically. To fill up this space, the use of many materials was investigated, including Zirconia powder, alumina power, and Nitrasil. A pure

C h u p t e i 4 - D c s i p n o f t h e I l i p l i T c i n p c r a t n r c T v i a x i a l D i f o n n a t i o n . I p p o r o / i i s

alumina blanket 40mm thick, but compressible to 15mm, was finally chosen. It was compacted and wrapped around the outer coil with a glass fibre adhesive tape.

Thermal insulation in the axial direction was controlled by two alumina pistons and discs between the rock and the steel pistons that reduced the temperature from approximately 1000°C at the bottom of the metal piston to 200°C at the bottom o f the alumina piston. This was essential to control temperature gradients across the length of the sample and to protect 0 -rings at the bottom o f the set-up and the stain gauges at the top of the furnace. The success of the axial insulation can be seen in Figure 4.10, where the change in colour of the metal jacket evidences where most o f the heating occurred. Two discs were placed at the top and bottom of the sample between the spacers and the specimen (Figure 4.8, Figure 5.16) in order to protect them from potential rock melt. Alumina was chosen for this purpose due to its mechanical and physical characteristics, in particular, its high compressive strength and its low thermal conductivity. The spacers in fact, suffer compressive stresses as well as direct contact with the high temperature specimen.

Property Unit V alue

Composition % 99.5 - 99.7 AIO2

Density kg/m^ 3700-3950

Porosity % NIL

Compressive Strength GPa 2.6

Modulus of Rupture MPa 343

Safe Continues Temperature °C 1650

Melting Point °C 2030

Thermal Conductivity @100 °C W/mK 25-30

Thermal Conductivity @ 1000 °C W/mK 5

Specific Heat J/kgK 850

Coefficient of Linear Expansion 0-1000°C 8.1x 10

20°C Ohm/cm >10"^

500°C Ohm/cm 1.5 xlO "

Resistivity 1000°C Ohm/cm 5.0x10*

1500°C Ohm/cm LOxl O*

C l u i p r e r 4 - D e s i g n o f t h e Tciiip cru fii ri ' T r i o x i o l D e f o n n a t i o n A p p a r a t u s

A hollow alumina cylinder 3mm wall thickness was used as a support and separator between the two sets o f coils. At the base between the furnace and the bottom plug and at the top, a less-expensive Pyrophilite block and disc were used for insulation. These two discs functioned as lids to the furnace and were screwed to the outer copper sleeve. Due to all the inlet holes for thermocouples, the heating elements and sample set-up, they had to be made from an insulating material yet easily machinable; and pyrophillite met these requirements.

Across the sample itself, convection of any confining fluids (gas or liquid) becomes the dominant heat- transfer mechanism at elevated temperatures. Colder gas will sink and hotter gas will rise, inducing an upward thermal gradient along the vessel and sample. Filling all empty spaces within the vessel with insulating material, gas circulation was sufficiently inhibited to restrict convective temperature differences along the sample to 20°C or less. In addition, a solid steel cylinder filled the void between the actuator and the top of the furnace. By putting the top of the furnace in direct contact with a large steel mass such as the actuator, which acted as a heat sink, all residual heat at the top of the furnace and close to the load cell is conducted away. Further cooling was achieved by running cold water through five rings of copper tubing coiled around the outside of the vessel. The tubing absorbs excess heat from the apparatus and also protects the sealing rings at the bottom of the pressure vessel. However, the load cell (at the top of the vessel) reached limiting temperatures during tests performed at the highest pressures and temperatures (see Chapter 5). This restricted the use of the apparatus at temperatures above 900°C and confining pressures above lOMPa.

To calculate the heat flu x from the rock sample at 1000 ° C t o the Alumina spacers and the steel pistons, the general Fourier Law fo r I-dimensional heat flow is used,

q = A k ( T , - T 2)/L, [ 1 ]

where q is the heat flu x in Watts; k is the coefficient o f thermal diffusivity in W/m K; L is the element length in meters; A is the area through which heat is conducted in meters square; T/ is the high temperature element from which heat is diffused; 73 is the temperature o f the element to which heat is being diffused in degrees K. Substituting the corresponding values, the heat flux through the Alumina spacers results in 394 Watts. If to this, we add, the radial heat flux and the radiation from rock a total of 7885W flows through the spacers. Using equation 2, this translated to a temperature of 152°C.

AT = Thigh —T room ~ AQ / m C, [ 2 ] where C is the specific heat in Jkg’’ K’‘; m is the mass in kg; and AQ is the heat flow in Watts.

This result refers to the worst case in which the coils are being used at full power and the rock is at maximum temperature.

The same combination of equation 1 and equation 2 were used to calculate the temperature drop between the outer coil and the inner vessel wall. A maximum temperature of 96°C was obtained at the walls of the pressure vessel.

All these calculations ignore convective effects for which calculations would be complicated and the results have too big approximations. These calculations were intended only to give a rough idea of the temperatures reached at key point of the vessel, for material choice decisions.

( luipfi'r 4- P i' s i p n o f rlic f / ip l i I o n i p r r a i i i r f I r i a x i a l D o fo r n u ir ia ii A p p o r a t i t s

MAINS

TC TC TC TO INNER C O IL S ► T O O U T E R CO ILS TO VESSEL WALL T R A N S F O R M E R T R A N S F O R M E R O U T E R C O I L RELA'' M AS TE R T C SLAVE T C O V E R . T E M P E R A T U R E . C O N T R O L L E R C O N T R O L L E R

3-PHASE

C h a p t i ' r 4- D c s i p n o f t h e I f i p h T e m p e r a t u r e T r i a x i a l D e f o r m a t i o n A p p a r a t u s

-1

Figure 4.10- Metal jacket and specimen. The axial insulation ensures that only the central part of the sample set-up, where the specimen is placed, is heated. This is evidenced by the change in colour suffered by the metal as its heated.

In document Estudio de conexiones entre elementos estructurales de caña guadua sometidos a carga axial (página 41-51)