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October 11th, 2013, 04:35 AM   #1
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Heat transfer .. Could please help !

wanted90, on 11 Oct 2013 - 4:32 PM, said:

Hi all ..

Here I have two questions .. could please help how I can solve that

give me idea , and can please clear out the meaning of question ?


question number 1



question number 2

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October 11th, 2013, 08:02 AM   #2
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Re: Heat transfer .. Could please help !

Once you understand well the concept of heat transfer coefficient, you are able to solve examples like these.


where Q means heat, t means time, h is the heat transfer coefficient, A means area/surface, T means temperature.

This formula is for a hotter system, called the heater, and a continuos substance surround it. The heater gives heat to the surrounding, the obvious question is how much? It is not a well-defined question, you immediately ask back: during how much time? The good question is during one unit of time how much heat does it give? This is why the right side looks as it does, is the rate of the transfering heat. Bigger heat rate means faster heating. Usually we prefer the bigger heat rate, but for example I hate my toast-maker, just because it has so big heat reat that it burns the outside of the toast, while the inside is raw.
The formula says on what depends the heat rate. First of all, it is proportional to the surface (A) connected the heater with the substance. This is why the wall of a glazed tile stove is not smooth but full of fancy pattern, people increased its surface, so increased its performance (= heat rate). The formula also says that the heat rate is proportional to the difference of temperature. That is why I adjust my thermometer of my hot water boiler as low as possible, I mean as low that I can take a bath comfortably by only hot water. So the waste energy (the boiler heats my bathroom) is minimal. Finally we have reached the heat transfer coefficient (h), it binds together the heat rate with the two physical quantities. This coefficient (as coefficient in general) is the manifestation of linear proportionateness. The coifficent depends on the quality of the matter, both the heater and the substance, if you have a good microscopic model maybe you can calculate it from particle physics quantities, but in practice it is easier simply to measure it.
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October 14th, 2013, 03:03 AM   #3
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Re: Heat transfer .. Could please help !

thanks so much
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