Analysis of the theoretical studies of the intensified heat exchange in pipes

Abstract

In the process of work on the basis of the analysis of the mathematical models, the disadvantages and advantages of different constructions of heat exchangers with spirals and their influence on the hydrodynamics and on the heat exchange of the spiral flows, are grounded. Most of the spiral heat exchangers have a complex shape. When using the screw twists of the flow, the increase of the heat exchange occurs due to the intensification of the heat exchange between the core of the flow and the boundary layer. This occurs during the turbulence of the swirling flow under the influence of the centrifugal forces. In this case, the effective speed is higher than during a normal flow turbulence. The process is more intensive at low Reynolds numbers. At laminar flow regimes the determining mechanism of heat transfer is the thermal conductivity across the flow along the normal to the wall. In this case, the heat transfer intensity is relatively low. To increase heat transfer, it is necessary to use some pipes with a helical heat exchange surface (with a single-start and multistart spiral knurling), in which there is a laminar spiral motion of the liquid. In contrast to the turbulent flow, in a laminar flow the thermal resistance in the channel is more evenly distributed over its entire cross section, so for the intensification of the heat transfer, some influence disturbing the flow within the wall zone of the flow, is needed.
The most promising are the heat exchangers made of pipes with a single-start or multistart spiral knurling. Unlike the tubular heat exchangers without any knurling, they have a larger heat exchange area and а lower material consumption. At the same time, in contrast to the tape inserts and twists, the pipes with knurling have a hydraulic resistance of the wall layer, which decreases faster than the pressure loss increases. The use of the pipes with a spiral knurling in energy piles with heat exchangers will reduce the weight and size characteristics not only of the heat exchanger, but also of the pile itself. In this case, the intensification of heat transfer is determined by the hydrodynamics of the flow in a viscous wall layer, i.e. the violation of the order of the liquid flow due to its twisting. The carried out analysis of the known mathematical models of heat exchange intensifiers allows to form the requirements for some perspective designs of heat exchangers. In the future this will make it possible to develop a new mathematical model of the hydrodynamics and heat transfer in a pile with an U-shaped heat exchanger, which takes into account all the disadvantages identified in the course of work. Based on the studies of the hydrodynamics and heat exchange processes, it is necessary to optimize the construction of the heat exchanger, namely the geometry of the cross section of the pipes, the shape of the pipe laying in the pile body, as well as the depth, angle and width of the spiral knurling deepening.