An analytical and experimental analysis of a very fast thermal transient

According to some international standards, some products, developed for use under heavy thermal conditions, have to be tested by subjecting them for a short time to a particular heating and cooling thermal stress to allow the m an acceptable future operative life. It is possible to obtain these fast thermal gradients in confined environments, called climatic chambers where the air is heated by an electrical resistance and is cooled with a finned e vaporator which is linked to a vapour compression system subjected to a par ticular control system of the refrigerating power. In particular, in this p aper the air and object tested thermal transients are studied from an analy tical and experimental point of view. The study of the mathematical model i s realized assuming simplified hypotheses about the air, the object and the air cooled evaporator temperature. The most complex circumstances are rela ted to a very fast temperature decrease because under this working conditio n the mathematical model is characterized by a nonlinear differential syste m. The nonlinear term is represented by the refrigerating power that varies in a definite range with the evaporator temperature according to a sinusoi d trend. For this power a suitable analytical expression, derived by the co ntrol system performance and by the compressor characteristic, has been fou nd. The analytical-experimental comparison during a cooling thermal stress of typical products subjected to international standard tests as the electr onic boards, has been carried out showing acceptable results. The model pre sented is useful to foresee the climatic chamber performances in the presen ce of a specific refrigerating power trend; this is the start-point for the design of the vapour compression plant and its control system. Copyright ( C) 2001 John Wiley & Sons, Ltd.