Calculation of critical heat transfer in horizontal evaporator pipes in cooling systems of high-rise buildings

Andrey Aksenov,Anna Malysheva,D Safarik,Y Tabunschikov,V Murgul

doi:10.1051/e3sconf/20183302037

Andrey Aksenov, Anna Malysheva + Show 3 more

Open Access

https://doi.org/10.1051/e3sconf/20183302037

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Abstract

An exact calculation of the heat exchange of evaporative surfaces is possible only if the physical processes of hydrodynamics of two-phase flows are considered in detail. Especially this task is relevant for the design of refrigeration supply systems for high-rise buildings, where powerful refrigeration equipment and branched networks of refrigerants are used. On the basis of experimental studies and developed mathematical model of asymmetric dispersed-annular flow of steam-water flow in horizontal steam-generating pipes, a calculation formula has been obtained for determining the boundaries of the zone of improved heat transfer and the critical value of the heat flux density. A new theoretical approach to the solution of the problem of the flow structure of a two-phase flow is proposed. The applied method of dissipative characteristics of a two-phase flow in pipes and the principle of a minimum rate of entropy increase in stabilized flows made it possible to obtain formulas that directly reflect the influence of the viscous characteristics of the gas and liquid media on their distribution in the flow. The study showed a significant effect of gravitational forces on the nature of the phase distribution in the cross section of the evaporative tubes. At a mass velocity of a two-phase flow less than 700 kg / m2s, the volume content of the liquid phase near the upper outer generating lines of the tube is almost an order of magnitude lower than the lower one. The calculation of the heat transfer crisis in horizontal evaporative tubes is obtained. The calculated dependence is in good agreement with the experimental data of the author and a number of foreign researchers. The formula generalizes the experimental data for pipes with the diameter of 6-40 mm in the pressure of 2-7 MPa.

Highlights

Design and calculation of heat exchange surfaces with their different orientations in space, including horizontal ones, is of wide use in steam generators of thermal installations in various branches of industrial heat power engineering
From the thermodynamic point of view, in the flow of a viscous liquid degradation occurs, as in all other real processes taking place in nature, the characteristic feature of which is the increase in the entropy of the system
Internal nonequilibrium processes always act in a direction that causes a decrease in the rate of entropy increase

Summary

Introduction

Design and calculation of heat exchange surfaces with their different orientations in space, including horizontal ones, is of wide use in steam generators of thermal installations in various branches of industrial heat power engineering. From the thermodynamic point of view, in the flow of a viscous liquid degradation occurs (energy depreciation), as in all other real processes taking place in nature, the characteristic feature of which is the increase in the entropy of the system. In the thermodynamics of irreversible processes, this law is formulated as follows: as the system goes over to the stationary-nonequilibrium state, the value of the incremental increase in entropy decreases, and when the stationary-nonequilibrium state is reached, it is of the smallest value compatible with external constraints [1, 2]

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