Helical Roughness Research Articles

Numerical study of turbulent heat transfer and particle deposition in enhanced pipes with helical roughness

Large-eddy simulations of turbulent heat transfer and solid particle deposition in helically rib-roughened pipe flows have been performed for different Reynolds numbers Re and various particle diameters Dp. An Euler–Lagrange approach, using cyclic boundary conditions for the continuous and the dispersed phase, have been applied to achieve a fully developed turbulent flow. An adhesion and removal model have been added to the multiphase large-eddy simulations to take into account the physical effect of particle re-entrainment. The complex interactions between particle-laden turbulent flow and the structured pipe wall in multiple-started helically ribbed pipes are numerically investigated with regard to heat transfer, pressure loss, and particulate deposition. The results of the Nusselt numbers Nu, friction factors fd, and particle deposition rates Ṅd are presented for each geometry variant. For same Reynolds numbers Re, significant differences of those values have been observed for the differently structured pipes.

Analysis of Reverse Currents Parameters and Gas Content in Liquid behind the Roughness Ridge

Based on experimental studies, the effect of gas content in the liquid behind the ridges of artificial helical roughness on film thickness and the value of the heat transfer coefficient is shown. The numerical results for the reverse currents velocity distribution and pressure difference along the ridge height and the cavity length, as well as experimental values of gas content in the liquid separation zone, arising from the flow over a regular roughness, are presented. The scale of reverse currents, their location behind the roughness ridge, and gas content depend on the height and shape of the ridge, the irrigation density, and the physical properties of the liquid. It was concluded that the gas bubbles in the liquid form in reverse current zones in the cavities of the artificial roughness due to reduced pressure compared to gas pressure at the interface and, therefore, gas suction from the interface zone. The dependence for calculating gas content via the thickness of the liquid layer in the roughness cavity is proposed.

HEAT TRANSFER TO A TANGENTIALLY INJECTED SWIRL FLOW THROUGH AN ARTIFICIALLY ROUGHENED ANNULUS

This paper provides heat transfer enhancement and flow friction data for swirl flows enter tangentially to an annulus roughened with helical-ribs. The effects of the roughness intensity, tangential to total momentum flux ratio and Reynolds number were studied. The tested annulus has a radius ratio of 0.25 with the inner surface insulated and the outer one was heated with a uniform heat flux. The inner surface of the outer tube was roughened with helically coiled wires which act as turbulence promoters breaking up the viscous sublayer formed in the fluid region nearest to the wall. Roughness was made by using helical-wires at two different diameter to pitch ratios (d/p) of 0.025 and 0.050 within a range of Reynolds number from 5x103 to 25x103, and at values of the tangential to total momentum flux ratios of 1.73, 2.94, 3.46 and 5.88. It was found that there is a significant enhancement in the heat transfer due to inlet injection swirl compared with that gained by helical roughness. Also, the efficiency index is critically dependent on the roughness Reynolds number, momentum flux ratio and the roughness intensity and it is almost less than that for the rough plain annulus. New correlations for Nusselt number enhancement ratio and the Fanning friction factor ratio were obtained as functions of Reynolds number, momentum flux and d/p ratios. Also, a maximum Nusselt number enhancement ratio of about 2.0 was found corresponding to 8.7 fold increase in the flow friction factor for a single entry injector with Mt/MT of 5.88 at relatively dense roughness with d/p of 0.05.