与环境交换的热和功,Р离开体系的能量:Р体系能量的积累= 进入体系的能量-离开体系的能量РNote: Δ= 2-1(Exit - Entrance)Р离开体系的物质帯走的能量,Р7Р精品课件Р其中, 是流体的质量流速,kg/s; 是热交换速率,J/s; 是功率。对于界面是刚性的体系,不存在体积功, 只包括两部分,一是流体进入和离开体系所做的流动功率;另一种是轴功率(shaft work rate), ,它是通过机械装置,如泵,热机,等做的功。因此Р8Р精品课件Р这就是敞开体系热力学第一定律的通用形式。Р因为 H = U + PV,得到Р9Р精品课件РAn insulated, electrically heated tank for hot water contains 190 kg of liquid water at 60 0C when a power outage occurs. If water is withdrawn from the tank at a steady rate of 0.2 kg/s, how long will it take for the temperature of the water in the tank to drop from 60 to 35 0C? Assume that cold water enters the tank at 10 0C, and Cv=Cp=C, independent of T and P.?(一个保温很好、容量为190kg 的电热水器将水加热到60 0C时,突然断电。如果此时电热水器以0.2 kg/s的质量流速放出热水,问需要多长时间电热水器里面的水由60 0C 降到 35 0C?假设:电热水器进口的冷水温度为10 0C,水的CV = CP = C,与温度无关。)РProblemР10Р精品课件
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