Heat and mass transfer during condensation of water steam from the products of combustion on the surface of profilled finning tubes
Full-scale development in Ukraine of a huge potential for energy saving due to the utilization of the heat of exhaust gases of industrial power and technological units that use hydrocarbon fuels is impossible without deep cooling of these gases in condensing heat exchangers.
The essence of the development is the use of condensed heat exchangers as heat exchange surfaces of finned flat-oval tubes, which are characterized by low aerodynamic drag and a reduced feeding zone of accumulation of the condensate film. In addition, the presence of a system of effective rectangular fins welded to a flat-oval tube in comparison with conventional
round fins leads to an increase of the efficiency of the fin and increases the contact surface area of phases when it flowing of a vapor-gas mixture under conditions of deep utilization.
For finned flat-oval surfaces that work in the conditions of vapor condensation from a gas-vapor mixture, there are no calculated ratios that can be used as the basis for the development of modern methods for heat and aerodynamic calculations and optimization of designs of surfacing condensation heat exchengers, which are based on highly developed
surfaces with low aerodynamic drag.
Within the framework of the research work, experimental models of steel flat-oval tubes with incomplete transverse fins are manufactured whose geometric characteristics lie in the following ranges: transverse and longitudinal dimensions of the flat-oval tube, respectively d1 = 15 mm and d2 = (30-42) mm; dimensions of the fins: height h = (20-25) mm, length l = (45-50)
mm, thickness δ = (0.8-1.0) mm, pitch between fins t = (3.5-4.0 ) mm.
Studies of heat and mass transfer and aerodynamic drag of surfaces from finned flat-oval tubes under conditions of condensation of water vapor from combustion products are carried out with a wide variation of their geometric characteristics and flow velocity. The influence on heat and mass exchange and drag of geometrical characteristics of surfaces and regime factors is
determined. The quantitative relationship and influence of the water vapor concentration on the heat transfer intensity is determined, the optimum range of thermal and aerodynamic characteristics of the finned surfaces is established, which operate under conditions of drop - film condensation of water vapor on the surface, the mechanism of heat exchange intensification and the hydrodynamic flow of the gas - vapor mixture flow interacting with the highly developed finned surface. On the basis of complex studies, new generalizing dependencies for the calculation of heat and mass transfer and aerodynamic drag of finned surfaces from flat-oval tubes have been proposed, which expands the class of problems devoted to the condensation of water vapor from combustion products on the heat exchange surface, and their results are the basis for the development of modern methods for calculating surface condensation apparatus.
Based on the results of experimental studies, engineering methods for the heat and aerodynamic calculations of surfaces from finned flat-oval tubes have been obtained for the first time. These methods allowing the development of new types of condensation surface heat recovery on a new element base in the range of flow velocity changes from 2 m/s to 20 m/s, the flow temperature of the combined vapor-gas mixture (60-130)°С, the surface temperature of the
tube is (20-80)°С and the concentration of water vapor in the coolant flow (7-200) g/kg. Technical, constructive and schematic solutions for creating a surface condensation heat exchanger with reduced material and operating costs have been developed. The results of the thermoaerodynamic calculations showed that the condensation heat exchanger made from finned flat-oval pipes has a mass-dimensional characteristics of (20-25%) less than that of the heat exchanger made of a traditional round-shaped finned bimetallic tubes
with the same heat power of the apparatus.