Piston-cylinder example

When a polymer is extruded through an orifice such as a capillary die, a phenomenon called die swell is often observed. In this case, as the polymer exits the cylindrical die, the diameter of the extrudate increases to a diameter larger than the diameter of the capillary die, as shown in Fig. 3.9. That is, it increases in diameter as a function of the time after the polymer exits the die. Newtonian materials or pure power law materials would not exhibit this strong of a time-dependent response. Instead they may exhibit an instantaneous small increase in diameter, but no substantial time-dependent effect will be observed. The time-dependent die swell is an example of the polymer s viscoelastic response. From a simplified viewpoint the undisturbed polymer molecules are forced to change shape as they move from the large area of the upstream piston cylinder into the capillary. For short times in the capillary, the molecules remember their previous molecular shape and structure and try to return to that structure after they exit the die. If the time is substantially longer than the relaxation time of the polymer, then the molecules assume a new configuration in the capillary and there will be less die swell. 

Let ub take a cylinder filled with gas and a piston holding the gas in the cylinder, and let us consider the action of some continuous field of force on this Bystem. As long ob the piston is not moved and the external field of force is unchanged, the above condition is fulfilled. However, if we move the piston, for example (considering the action of the piston as a field of elastic forces), the condition is violated in this case we act on the gas, inasmuch as we change in time the existing external field of force. Cf. Section 23b. 

The nature of reversible processes is illustrated by the example of a simple expansion of gas in a piston/cylinder arrangement. The apparatus shown in Fig. 2.2 is imagined to exist in an evacuated space. The gas trapped inside the cylinder is chosen as the system all else is the surroundings. Expansion processes result when mass is removed from tlie piston. F or simplicity, assume that the piston slides within the cylinder without friction and that the piston and cylinder neither absorb nor transmit heat. Moreover, because the density of the gas in the cylinder is low and because the mass of gas is small, we ignore the effects of gravity on the contents of the... 

A thermodynamic state is a macroscopic condition of a system in which the properties of the system are held at selected fixed values independent of time. The properties of the system are held constant by its boundaries and the surroundings. For example, a system comprising 2 mol helium (He) gas can be held in a piston-cylinder apparatus that maintains the system pressure at 1.5 atm, and the apparatus may be immersed in a heat bath that maintains the system temperature at 298 K. The properties of pressure (P) and temperature (T) are then said to be constrained to the values 1 atm and 298 K, respectively. The piston-cylinder and the heat bath are the constraints that maintain the selected values of the properties P and T. 

IRREVERSIBLE EXPANSION OF AN IDEAL GAS Consider a gas confined within a piston-cylinder arrangement and held at constant temperature in a heat bath. Suppose the external pressure is abruptly reduced and held constant at the new lower value. The gas immediately expands against the piston until its internal pressure declines to match the new external pressure. The total entropy of system plus surroundings will increase during this expansion. In preparation for a quantitative example, a general comparison of irreversible and reversible processes connecting the same initial and final states provides insight into why the total entropy increases in a spontaneous process. 

A small pilot valve controlling admission to and exhaust from this cylinder is mechanically operated by the main steam piston. An example of such a valve motion is the Cameron pump, illustrated in Fig. 

Fig. 1 An example of a piston-cylinder high pressure apparatus.

The trials should deliberately investigate the mode for variations and failure route (care must be taken that trial-induced variations in the product are not released for consumer retail unless extensively checked). For example, failure/variation in a vacuum filling line can be caused by loss of vacuum, the result of seal damage from abrasion with the bottle rims. In a piston/ cylinder operation, density variations in the product (requiring strict analytical control) are a principal source of fill weight variation. 

Advantages of oxide calorimetry are (l) ease of dissolution of compounds rich in AI2O3, MgO, and other refractories, (2) relatively small enthalpies of solution which are independent of variations in the amount of solute, the presence of other solutes, or small changes in melt composition, and (3) the ability to work with small amounts of sample (200-ii00 mg total for several duplicate runs) which enables one to study, for example, phases synthesized at high pressure in a piston-cylinder apparatus. 

Nickel plating is widely used for a corrosion- and wear-resistant finish. Typical applications, with a thin top coat of electrodeposited chromium, are decorative trim for automotive and consumer products and office furniture. Nickel deposits are also used for nondecorative purposes for improved wear resistance, for example, on pistons, cylinder walls, ball studs, and so forth. 

Apart from the individual component, the components in a diesel engine are constructed by a 3-dimensional solid body. For example, the piston, cylinder head, body, crankshaft, and so on are wholly constructed by a solid body, so the 3-dimensional solid modeling software must be provided. 

For example, say we wish to study the piston-cylinder assembly in Figure 1.1. The usual choice of system, surroundings, and boundary are labeled. The boundary is depicted by the dashed line just inside the walls of the cylinder and below the piston. The system contains the gas within the piston-cylinder assembly but not the physical housing. The surroundings are on the other side of the boundary and comprise the rest of the universe. Likewise the system, surroundings, and boundary of an open system are labeled in Figure 1.2. In this case, the inlet and outlet flow streams, labeled in and out, respectively, allow mass to flow into and out of the system, across the system boundary. 

Figure 1.8 provides a schematic representation of each of these quantities. The two piston-cylinder assemblies depicted on the left represent cases for which the saturation pressure is defined. In these systems, pure species a is in vapor-liquid equihbiium at temperatures Ty and Tg, respectively, where T2 is greater than Ti. In each case, there is a unique pressure at which the two phases can be in equilibrium—defined as the saturation pressure, Pf. For example, pure water at 293 K (20°C) has a saturation pressure of 2.34 kPa. Said another way, for pure water to boil at 293 K, the pressure of the system must be 2.34 kPa. If the pressure is higher, water will exist only as a liquid. Conversely,... 

Equation (2.7) is often encountered in thermodynamics the work described by this equation will be referred to as Pv work. On a molecular scale, the energy transfer by Pv work can be understood in terms of momentum transfer of the molecules in the system when they bounce off the moving boundary, as discussed in Section 1.3. A piston-cylinder assembly is a common system that is used to obtain work (e.g., in your automobile). Example 2.3 illustrates how work is calculated for such a system. 

Figure E23 Example of a process in which energy is transferred from the system to the surroundings by Pv work expansion of a gas in a piston—cylinder assembly. The surroundings are maintained at 1 bar.

To help solidify these abstract ideas, a concrete example is illustrative. We will compare the value of work for six processes. We will label these cases process A through process F. Three processes (A, C, and E) entail isothermal expansion of a piston-cylinder assembly between the same states state 1 and state 2. The other three (B, D, and F) consist of the opposite process, isothermal compression between state 2 and state 1. An isothermal process results in the limit of fast heat transfer with the surroundings. We could perform a similar analysis on adiabatic processes where there is no energy transfer via heat between the system and the surroundings. 

Figure E2.10A Piston-cylinder assembly with a spring attached to the piston. The initial state of the system for Example 2.10 is shown.

Figure 2.16 An ideal gas in a piston-cylinder assembly undergoing a reversible, adiabatic expansion. In this example, is constant. See if you can predict the signs of At/, (), and W for this process in the table.

EXAMPLES. Calculation of Entropy Change for an Irreversible, Isothermal Compression A piston-cylinder device initially contains 0.50 of an ideal gas at 150 kPa and 20°C. The gas is subjected to a constant external pressure of 400 kPa and compressed in an isothermal process. Assume the surroundings are at 20 C. Take Cp = 25R and assume the ideal gas model holds. (a) Determine the heat transfer (in kj) during the process. (b) What is the entropy change of the system, surroundings, and universe (c) Is the process reversible, irreversible, or impossible ... 

Eunctional or hard chromium plating (169,175) is a successfljl way of protecting a variety of industrial devices from wear and friction. The most important examples are cylinder liners and piston rings for internal combustion engines. Eunctional chromium deposits must be appHed to hard substrates, such as steel, and are appHed in a wide variety of thicknesses ranging from 2.5 to 500 ]Am. 

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