Numerical and Experimental Study of Thermal-Hydraulic Processes in the Convergent-Divergent Channels of Plate Heat Exchangers with Etched Channels

The cost and efficiency of power installations depend to a great extent on the design characteristics of heat transfer equipment. These characteristics are especially important for gas turbine units with regenerators operating on low boiling working fluids with low temperature differences and high pressure differences. For such highly efficient energy systems with regeneration, the use of plate heat exchangers containing microchannels obtained by chemical etching of their surface holds promise. For improving their thermal-hydraulic efficiency, versions of extended heating surfaces with convergent-divergent channels are proposed in the article. A set of calculated and experimental studies of the newly developed channels for plate heat exchangers was carried out. Convective heat transfer was numerically simulated, the results of which made it possible to determine the influence of key geometrical parameters, namely, the relative segment length L/H and expansion amplitude A/H on the thermal-hydraulic characteristics in a wide range of Reynolds number values (2500 ≤ Re ≤ 15 000). It is shown that with a periodically narrowed and widened channel cross section, flow acceleration and deceleration zones are produced; directed action on the boundary layers takes place, and turbulent mixing is enhanced. This helps eliminate stagnant zones and obtain more uniform distribution of heat removal over the surface. Based on mathematical modeling, correlation dependences for the Nusselt number and the friction coefficient as functions of the Reynolds number have been obtained, which are suitable for engineering assessments of the efficiency of heat exchangers with similar geometrical parameters of the channels. To verify the numerical study results, an experimental setup was developed, and tests of channel models with recording the heat transfer and pressure losses were carried out, which have confirmed the correctness of the theoretical approach and repeatability of the revealed regularities. The obtained results can serve for confirming the rational choice of geometrical parameters and practical applicability of modified plate heat exchangers with chemically etched channels as part of compact regenerators of advanced power facilities.