tailieunhanh - ADVANCED THERMODYNAMICS ENGINEERING phần 3

Việc chuyển giao năng lượng thông qua kết quả nhiệt trong một gia tăng dữ liệu ngẫu nhiên trong khi truyền năng lượng thông qua công việc không. Trong Chương 3, chúng ta sẽ thấy rằng dS = δQrev / T (nhưng không phải δWrev / T hoặc PDV / T). | a b Figure 22 a. Carnot engine operating between hot gases and the ambient b. Carnot engine operating between water and the ambient. 100x 1 - 298 1000 70 kJ as illustrated in Figure 22a. Therefore the quality of energy at 1000 K is 70 . The entropy change of the hot gases is kJ K-1 -100 1000 while the entropy gain for the ambient is kJ K-1 . 30 kJ 300 K . Alternatively we can cool the hot gases using water to transfer the 100 kJ of energy. Assume that during this process the water temperature rises by 2 K from 399 K to 401 K with the average water temperature being 400 K as shown in Figure 22b . If a Carnot engine is placed between the water at 400 K and the ambient at 300 K then for the same 100 kJ of heat removed from radiator water we can extract only 100x 1 - 298 400 25 kJ and 75 kJ is rejected to the ambient. In this case the energy quality is only 25 of the extracted heat. Figure 23 illustrates the processes depicted in Figure 22a and b using a T-S diagram. The cycle A-B-C-D-A in Figure 23 represents the Carnot engine CE of Figure 22a while area ABJIA and EGKIE represent heat transfer from engine and to hot water respectively for Figure 22b while the area C-D-I-H represents the rejected heat of CE for the first case and the area D-C-H-K-J-I-D that for the latter case. Since more heat is rejected for the second case the work potential or the quality of the thermal energy is degraded to a smaller value at the lower temperature. This is due to the irreversible heat transfer or the temperature gradients between hot gases and radiator water as shown in Figure 22b . In general property gradients cause entropy generation. Now one might ask about the Maxwell-Boltzmann distribution of molecular velocities. Consider a monatomic gas within a container with rigid adiabatic walls. A pseudo temperature distribution exists for the monatomic gas. The question is whether with collision and transfer of energy there can be degradation of energy or generation of .