Work is positive when the surroundings do work on the system (i.e., the system contracts). Thus, work can be done by the system on the surroundings or vice versa. The boundaries of the system can expand or contract. The only types of work that are present are expansion/compression of the system and flow work.This is often a good assumption when the fluid is not moving near the speed of sound, the change in height over the system is not large, or the system temperature variations are not large. We neglect kinetic and potential energy carried by the mass. Mass can carry internal energy into or out of the system.The mass flow rate into the system is positive, whereas flow rates out of the system to the surroundings are negative. Mass flows into or out of the system along one boundary of the system.We make the following assumptions and definitions: 1, an open system allows mass and energy to flow into or out of the system. We can then use the tables of the results to solve problems.We begin with the first law of thermodynamics applied to an open thermodynamic system. This final equation is used to determine values of specific enthalpy for a given temperature for a given gas. The specific heat capacity (cp) is called the specific heat at constant pressure and is related to the universal gas constant of the equation of state. There is also a “specific” form of the enthalpy equation, which is just the derived form divided by the mass of the gas. Enthalpy is a very useful state variable when solving gas dynamics problems. The enthalpy is also used in our evaluation of the change of entropy as required by the second law of thermodynamics. We can apply the conservation of energy equation to determine the work performed during the various strokes of a four-stroke engine. The enthalpy is used in our derivation of the conservation of energy for a gas. Using the enthalpy for a gas lets us easily solve problems involving the first law of thermodynamics by measuring the temperature. The internal energy of a gas is hard to measure, but the temperature of a gas is easy to measure. We have enclosed the terms of the equation in parentheses with a “p” subscript to remind ourselves that this equation is only true for a constant pressure process. Now substitute the value for “Q” which we talked about earlier: (H2 – H1)p = Cp (T2 – T1)p The (E + p * V) can be replaced by the enthalpy (H) H2 – H1 = Q Let’s group the conditions at state 2 and the conditions at state 1 together: (E2 + p * V2) – (E1 + p * V1) = Q Substitute the expression for the work: E2 – E1 = Q – p * The first law of thermodynamics tells us: E2 – E1 = Q – W Then the work is given by: W = p * (V2 – V1) Let us select a constant pressure process. And second, that the work will be of the form pressure times the change of volume. First, the amount of work depends on the process used to change the state. From our slide on heat transfer, we know that we can represent the amount of heat transfer by a constant (C), called the heat capacity, times the difference in temperature (T): Q = C * (T2 – T1)įrom our slide on the work done by a gas, we know two things. How does one use this new variable called enthalpy? Let’s consider the first law of thermodynamics applied to a gas system with both heat transfer (Q) and work (W) done by the system in going from state 1 to state 2. For a gas, a useful additional state variable is the enthalpy (H) which is defined to be the sum of the internal energy (E) plus the product of the pressure (p) and volume (V), H = E + p * V These new variables will often make the analysis of a system much simpler. We are free to define additional state variables which are combinations of existing state variables. From our study of the first law of thermodynamics, we have found that the internal energy of a gas is also a state variable, that is, a variable which depends only on the state of the gas and not on any process that produced that state. The state of a gas is defined by several properties including the temperature, pressure, and volume which the gas occupies. Like the Wright brothers, we are most interested in thermodynamics for the role it plays in engine design. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. Thermodynamics is a branch of physics which deals with the energy and work of a system. Home > Beginners Guide to Aeronautics Enthalpy
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