Circulation characteristics in short evaporation elements

Abstract

The development and creation of new equipment designs with flame heating and an intermediate heat-transfer medium with compact convective surfaces and improved thermotechnical parameters represent an important scientific-technical problem, Research results on vessels for which gas tubes serve as convective surfaces are described in [1][2][3].Combustion products move longitudinally within the pipes. It is known that during the longitudinal flow of a heating heat-exchange medium past heat-exchange surfaces, the coefficient of convective heat transfer is 1.5-2 times lower than that for a transverse flow. In this connection, the TsKBN has developed a vessel design with flame heating in which the convective surfaces are evaporative elements built in accordance with a type of Field tubes. The evaporative elements take up the thermal energy from a transversally impinging flow of combustion products.Building of a vessel with Field tubes made it possible to improve considerably the thermotechnical characteristics as a result of intensification of heat exchange and improvement in the specific density of the energy transfer to the volume of the intermediate heat-exchange medium.The evaporating time of the evaporative element depends heavily on the temperature regime of the channel walls being heated, i.e., on the conditions under which the walls of the tubes are cooled by the circulating flow. The TsKBN has fabricated and assembled a special bench to investigate the circulation characteristics in short Field tubes.An experimental channel was built in the form of a circuit with a natural circulation (Fig. 1).Intermediate heat-exchange medium 2, which is located in upper tank 1 flows into downcomer 3, proceeds to the lower end, turns through 180 ~ and is directed into an overhead slot. Thermal energy from electric coil 6 is supplied to external tube 4.Water-steam mixture 5 is fed through the circular slot from bottom to top. The experimental channel is covered by layer of insulation 7 over the heated length.An experimental channel with a heated length Lhe = 600 and 400 mm was investigated under heat demands Q of from 0.79 to 5 kW. The diameter of the external 40 • 2-mm heated tube was held constant in all regimes. To vary the geometric parameterfsl/fd e of the evaporative channel (the ratio of the area of the flow section of the overhead slotfs l to the area of the flow section of the downcomer fde), the diameter of the downcomer was altered by a factor of eight (by a factor of three for Lhe = 400 ram). Variation in the parameterfsl/Jd c was associated with experimental determination of the geometric parameters resulting in maximum circulation possibilities for a circuit with natural circulation.The heat demand on the tube was determined from the formulawhere N = IU is the electric power of the coils, I is the strength of the current, U is the voltage, Qp = t~lFl (twl -ts) + ~2F2(tw2 -ts) is the thermal energy lost by the channel to the surrounding space, ct 1 = (c~ c + c~r) is the coefficient of convective heat exchange f...