What are fluids?
Matter exists in several states. These states are known as physical states or states of matter. The first three states of matter are well known because we can experience them in everyday life. They are solid, liquid and gaseous (Figure 3-3). “Other” states are not considered at this time because they occur only in severe physical conditions.
Matter, whether in its liquid or gaseous state, is a fluid, and although liquids and gases react under different conditions, they have the basic properties of liquids. That is, when a shear force is applied to it, a liquid is constantly deformed. Unlike solids, which do not change much in response to a force, liquids deform and deform until a force is applied (Figure 2-3). When closed in a closed container, the liquid takes the form of a container. An enclosed liquid occupies only the bottom of the container, not all available space such as gas.
Liquids, like solids, are molecules that interact with each other. Tracing the velocity and position of each of these molecules is almost impossible, so a hypothesis is made to simplify models and calculations in fluid mechanics. This hypothesis is a continuous hypothesis, it is assumed that the effect of individual properties of molecules is negligible compared to the properties of the whole liquid. Thus, according to the continuous hypothesis, a low-volume element (∙ ∙) defines properties such as pressure, temperature, density, and velocity (Figure 3-4). The continuous hypothesis also requires that these properties constantly change between two adjacent volume elements. Hence, the continuous hypothesis requires that the fluid be continuous over its entire volume instead of being made up of individual molecules.
What are the main characteristics of liquids?
The four main characteristics of liquids are density, specific gravity, dynamic viscosity and vapor pressure. This section details these four features.
The density of a fluid is its mass ratio per unit volume, ie
The density of a substance varies with pressure. This change is usually small for liquids and solids, but very important for gases because they are very compact. Increasing the pressure applied to the gas greatly affects its density. In contrast, liquids are relatively incompressible, and increasing the pressure does not significantly change their density. It has a density of kilograms per cubic meter (kilograms per cubic meter) in SI units and pounds per cubic foot (lb / ft3) in normal US units. The density of solids and liquids also changes with temperature. If you need to compress a sub-position at a certain temperature, refer to density as a function of the temperature table.
Specific gravity, or relative density, is the ratio of the density of a substance to the density of an equal volume of water:
An ideal liquid cannot withstand shear force. Therefore, the forces in a small volume element are only equal to the normal forces, as Figure 3-5 shows. At rest, the pressure that a liquid exerts on a small volume element depends only on the weight of the liquid above that element. The pressure due to the weight of the liquid is proportional to the depth of the pressure measurement and only to this depth. The shape of the ship has no effect on hydrostatic pressure. For example, if two tanks are full of water but have different diameters, the hydrostatic pressure is the same at the bottom of both tanks due to the weight of the water (Figure 3-6). The absolute pressure at the bottom of the liquid column is the amount of atmospheric pressure and the hydrostatic pressure due to the weight of the liquid. Is a mathematical expression used to calculate the pressure at the bottom of a ship
Blaise Pascal (1662-1623) The French mathematician, physicist, and philosopher deduced from the fact that a fluid column withstands a pressure that differs only in height from the fluid column, the external pressure exerted on an enclosed fluid, to any Part of the fluid fades.
Manufacturers of U-tube barometers must carefully select the manometer fluid to ensure optimal measurement accuracy. The use of water manometers is limited to measuring pressure close to atmospheric pressure because a low pressure change causes a relatively large displacement of water. For example, to measure a 7 kPa (1 psig) barometer with a water barometer, the manometer column must be more than 71 cm (28 inches) high. The manufacturer can use mercury instead of water to significantly increase the measurement of a manometer. The density of mercury is 13.6 times the density of water. Thus, for a given pressure, the displacement of mercury is 13.6 times less than the displacement of water. Liquid pressure measurement is accurate enough to serve as a standard for calibrating other pressure gauges.
Bourdon pipe pressure
The Bourdon sphygmomanometer provides a direct pressure reading tube. They use an element of initial measurement called a Bourdon tube to sense pressure. Figure 3-12 shows a typical Bourdon tube barometer. The pressure gauge consists of a needle indicator connected to a Bourdon tube via a gear connection, which is a flexible C-shaped coil tube. The Bourdon tube is hollow and connects directly to the process fluid line. As the pressure increases, the Bourdon tube flattens, which moves the gear link and causes the needle pointer to dial. Bourdon pipe is made of materials that have elastic properties so that it deforms under pressure and returns to its original shape when it is no longer under pressure .
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