Physical fundamentals of heat transfer

The driving force of the heat transfer process is the tendency of systems to equilibrate out temperature differences. The heat flux = Q/A is directly proportional to the temperature difference AT = Tb — T , the proportionality factor being the heat transfer coefficient h, which is also defined by the general heat transfer equation  

The indices b and iv denote the liquid bulk and the wall, through which heat transfer proceeds. 

In indirect heat transfer through a wall surface two heat transfer coefficients 

Only hi can be affected by stirring. This, in the case of viscous media, is usually determining (Uahi). For glass walls, enameled surfaces and thick-walled high pressure vessels the k/d term can be of decisive importance, ho can only be affected by flow conditions in pipes or the channels of jackets. 

Two alternatives are possible for investigating the heat transfer in the laminar flow range under steady-state conditions a division of the tank wall into two halves, whereby the upper half is electrically heated and the lower half is water-cooled [451], or, however, pumping the test liquid, which is cooled or heated, in a bypass. In the latter case, the tank bottom has to be designed as a distributor for the liquid throughput [311]. 

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