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Truyền khối, công nghệ hóa học, máy bị, truyền khối Trịnh Văn Dũng, các quá trình hóa học, ĐH BK HCM, hấp phụ, chưng cất......Truyền khối, công nghệ hóa học, máy bị, truyền khối Trịnh Văn Dũng, các quá trình hóa học, ĐH BK HCM, hấp phụ, chưng cất
019:006 Exercises on heat transfer by thermal radiation --Solutions 1. In outer space heat transfer by convection and conduction cannot occur. How is thermal energy removed from a spacecraft? The only mechanism for heat removal on a spacecraft is by radiation. 2. You remove a roasted chicken from the oven but will not be ready to serve it for 15 minutes. What can you to keep it from getting cold too fast? The chicken should be covered immediately with aluminum foil. The rate of heat loss by radiation is proportional to T4. When it is hottest it radiates at its highest rate, so this heat loss must be reduced as soon as possible. 3. Often when cooking a pie, the edges of the pie crust will get brown before the rest of the crust. What can be done to prevent the edges of the pie crust from burning? You should wrap the edges of the pie crust with Al foil which will reflect the heat. 4. If you want to keep an object cool, is it better to paint it with silver paint or black paint? You want to allow it to radiate as much heat as possible and dark (black) objects radiate more than light objects 5. Two objects have the same temperature and the same area but object A has an emissivity eA = 0.2 and object B has an emissivity eB = 0.8. How much radiation is emitted per second from object B compared to object A? The rate of radiation emission is proportional to the emissivity, e. So QA ~ eA, and QB ~ eB. Therefore QB/QA = 0.8/0.2 = 4. 6. Two objects have the same temperature and the same emissivities but object A has an area of m2 and object B has an area of 10 m2. How much radiation is emitted per second from object A compared to object B? The rate of radiation emission is ~ Area of the object. Therefore, QA/QB = AA/AB = 10/2 = 5. 7. Two objects have the same areas and the same emissivities but object A has a temperature of 150 K and object B has a temperature of 450 K. How much radiation is emitted per second from object B compared to object A? Q ~ T4, so that QB/QA = (TB)4/(TA)4 = (450/150)4 = (3)4 = 81.