Articles
An Unusual Approach To Study Convection, Radiation, And Size Dependent Heating And Cooling
March 23, 2009
•White Paper: An Unusual Approach To Study Convection, Radiation, And Size Dependent Heating And Cooling
By FLIR Systems, Inc
For any kind of quantitative analysis it is most convenient for thermographers to treat thermal images as representing situations with thermal equilibrium conditions. Unfortunately, this is nearly never the case which may pose problems even in qualitative analysis of images. Temperature differences in any situation result from energy flows into a system (heating by electrical power, contact to thermal bath, absorption of radiation, e.g., microwaves, sun radiation etc.) and energy flows from a system to the surrounding. The former leads to a heating whereas the latter results in cooling of an object. The cooling of objects is usually considered to be due to three fundamental modes, first conduction of heat, second convection and third, radiative transfer of energy . Although these three modes of energy flow are quite different from each other, one often finds a very simple law for their combined action to describe the cooling curves of hot objects. This law, mostly referred to as Newton´s law of cooling can be expressed as the statement, that the temperature difference between an object and its surrounding decreases exponentially, if there is no additional heating involved. The respective time constant t is characteristic for the object under study, i.e. it depends on its properties (heat capacity, geometry etc.).
In the following, heating and cooling of objects of different geometries and size will be studied. It will be shown, that for certain objects, characterized by so called Biot numbers below certain values, it is possible to estimate characteristic time constants of objects and thus judge whether thermal equilibrium - which may be static or dynamic- is indeed established after certain times. Furthermore, it will be demonstrated that, although radiative heat loss follows a nonlinear power law with temperature, Newton´s law of cooling can indeed often be used as an approximation for the cooling mechanisms. The experiments use cheese cubes, metal cubes and spheres as well as light bulbs and soft drink cans and bottles.
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