传热专题小论文.doc

Weapons to Fight Against Cold Winter It becomes more and more annoying to get through such cold winter. Girls are born to be crazy about beauty, they could not stand wearing like a zongzi. In another words, we students dont have an electric stove, not to mention an air conditioner, to warm ourselves. According to such conditions, some new kind of heating tools appear. When it comes to warming ourselves, hot-water bags come first, nearly 80% girls around me are using them, and even some boys are fans of them. Its convenience make the popularity , as you just need charge it and a few minutes later, you will get a heated bag. It is actually an energy changing process as it coverts electricity into heat by boiling the water in the closed bag. it applies a simple forced circulation system with a series path just as convection through the liquid boundary layer and in the form of boiling, conduction across the PVC cover of the bag ,conduction between the bag and our hands. Firstly the electricity changed into heat via an electric resistance, the larger value the resistance is, the more heat can be converted. And then the heat flux flows to the water by convection through the liquid boundary layer adjacent to the metal surface. It is followed that the density of those heated water decreases, they need more room but as volume is limited they will mix with other cold water ,at last, the water would be boiled as a result of the local temperature difference and bubbles formed at the heat transfer surface. it is a kind of phase change heat convection. After that, heat conducts from the inside wall to the outside wall of the bag. More often than not, the bag is made of PVC material with some properties such as impermeability to water and antioxidation. Taking consumers safety into consideration, the conductivity of the PVC bag should be neither too good or too bad. If it conducts heat very well, people may be scald or uncomfortable. Conversely, bad conductivity could not meet peoples requirement. Whats more, the water in the bag is not the normal water, some ions are added to improve its specific heat capacity and electrical conductivity. In addition, heat pitch, in Chinese we called “ nuanbaobao”, is an another kind of the latest fashionable heat tool. Just stick it on our inner clothes and it can generate heat over 20 hours at most. Many girls, especially those who like wearing skirts during winter are big fans of it. In fact, it actually makes use of the chemistry reaction. As we all know, there exist two types of chemistry reaction , exothermic reaction and endothermal reaction. And the heat pitch is a tiny bag with some chemistry material in it. On the surface of bag there is a plastic cover to prevent the oxygen from getting in. If you want to use it, just tear the cover off and stick it to your inner clothes. The reason why it can give out heat is that as long as you get rid of the plastic cover the oxygen gas will get in and it will react with the iron powder and some other solid catalysts. As this is an exothermic reaction you can get the heat. The reaction heating rate is under control on account of the structure and material of the bag and the proportion of those reagents. The remains of the reaction can be used as a good flower soil. In order to protect you from possible danger, dont get in direct touch with the pitch as it may arise hypothermia scald. And if you expose the pitch in the air, it will accelerate the reaction thus shorten the heating time . All though winter becomes more and more unbearable, we can always think of useful ways to warm ourselves. And heat transfer is of great importance to