On Realization of Ideal Multiflow Heat Exchange
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 4, July, 2020
GENERAL PROBLEMS OF TRANSPORT THEORY ON REALIZATION OF IDEAL MULTIFLOW HEAT EXCHANGE A. M. Tsirlin
UDC 66.01.011
It is shown that the conditions of ideal heat exchange, according to which at the given heat load and overall heat transfer coefficient the entropy production in the system attains its lower boundary, can be realized in a set of countercurrent heat exchangers on coordinated selection of temperatures and heat capacities of flows. The parameters of flows and the distribution of heat transfer coefficients and heat loads between two-flow heat exchangers for the Newtonian kinetics have been determined. The value of the minimal dissipation and the profile of the distribution of the overall heat transfer coefficient for the case where the temperature profile of one of the flows is fixed have been obtained. Keywords: ideal heat exchange, heat transfer coefficient, entropy, heat loads, Newtonian kinetics. Introduction. The thermodynamic approach to the analysis of technological systems ([1, 2, etc.]) allows one to isolate realizable systems and find estimates of their limiting possibilities (heat and mass transfer systems, heat engines and refrigerating machines, separation systems, chemical reactors, etc.). These estimates based on the relations of the thermodynamics of reversible processes (Carnot efficiency, reversible separation work) are very important but usually are highly overrated as compared with the possibilities of real systems. The reason is that they do not account for the flow intensity, contact surface magnitude, and for other factors associated with the given performance and final dimensions of apparatuses. Attempts at accounting for these factors and obtaining the characteristics of heat engines and refrigerating machines, which are optimal in the class of irreversible processes, led to an intense development of the methods of "optimization thermodynamics" or "thermodynamics at a finite time" [3]. Thus, along with the Carnot ideal reversible heat engine an ideal heat engine of maximum power [4, 5] or a heat engine of fixed power with a maximum efficiency appeared [6]. In all of these cases, the problem of an ideal heat engine was solved with natural limitations imposed on the coefficients of heat transfer of a working body with sources and on the engine power. It should be emphasized that it is possible to account also for other limitations, for example, on the value of friction of the heat engine with the cylinder surface and losses into the surrounding medium [7], but the limitations themselves are known with a great error, which means that in many cases their account is not important. For some processes the reversible estimates make no sense. This relates to stationary nonequilibrium systems, in which there are several reservoirs or substance and energy flows enter from outside. The most important example of such systems is furnished by heat transfer systems, the estimation of the thermodynamic perfection of which
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