Abstract
Due to the extended geometry of internally rifled tubes with helical ribs, the rate of convective heat transfer within them is much higher compared to smooth tubes. Simultaneously, a rise in the contact surface area between the fluid and the solid body increases the friction factor. This paper presents the results of experimental testing performed to determine the friction factor in an internally rifled tube with helical ribs. The tests were carried out on a purpose-built test stand. The tested object was a rifled tube used in the evaporator of a once-through supercritical power boiler operating in a power plant in Poland. The friction factor results obtained from testing are compared to the results of calculations performed by means of correlations known from the literature. Finally, using experimental data, a new correlation is developed that enables the determination of the friction factor in internally rifled tubes with helical ribs.
Highlights
Heat exchangers with finned or ribbed tubes are common in thermal engineering
The heat transfer surface area is extended on the side of the fluid, which is characterized by a lower thermal capacity
It follows from the experiments that in the case of a two-phase flow of the used mediums, the friction-related pressure losses in tubes with internal helical ribs range from 1.6–2.7 times higher compared to those arising in smooth tubes
Summary
Heat exchangers with finned or ribbed tubes are common in thermal engineering. In most cases, the heat transfer surface area is extended on the side of the fluid, which is characterized by a lower thermal capacity. It follows from the experiments that in the case of a two-phase flow of the used mediums, the friction-related pressure losses in tubes with internal helical ribs range from 1.6–2.7 times higher compared to those arising in smooth tubes. This is an effect of the disturbance caused by the ribs in the flow of the boundary layer. Pressure-related losses arising in boiler evaporators made of rifled tubes have so far been determined using correlations developed for tubes installed in compact heat exchangers with different fluids and smaller diameters. The equation’s general form is selected so that a comparison with existing results of tests performed on rifled tubes can be made
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