Abstract. External heat transfer coefficient changing from corridor tube bundle to the reverse cross fluid flow with low-frequency asymmetrical pulsations was analyzed experimentally. The experiments were performed for Reynolds numbers in the tube bundle in the range of 100 ≤ Re ≤ 500 and changing of flow impulses frequency was limited with values 0.125 ≤ f ≤ 0.5 Hz, fluid pulsations amplitude in the tube bundle corresponded to 0.5 d ≤ A ≤ 1.2 d mm. A comparison of steady and non-steady conditions was made. The most efficient regime of pulsations in the studied range was found.

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Open-cell foams are attractive for heat transfer enhancement in many engineering applications. Forced pulsations can lead to additional heat transfer enhancement in porous media. Studies of heat transfer in open-cell foams under forced pulsation conditions are limited. Therefore, in this work, the possibility of heat transfer enhancement in porous media with flow pulsations is studied by a numerical simulation. To generate the 3D open-cell foams, the Laguerre–Voronoi tessellation method was used. The foam porosity was 0.743, 0.864, and 0.954. The Reynolds numbers ranged from 10 to 55, and the products of the relative amplitude and the Strouhal numbers ranged from 0.114 to 0.344. Heat transfer was studied under the conditions of symmetric and asymmetric pulsations. The results of numerical simulation showed that an increase in the amplitude of pulsations led to an augmentation of heat transfer for all studied porosities. The maximum intensification of heat transfer was 43%. Symmetric pulsations were more efficient than asymmetric pulsations, with Reynolds numbers less than 25. The Thermal Performance Factor was always higher for asymmetric pulsations, due to the friction factor for symmetrical pulsations being much higher than for asymmetric pulsations. Based on the results of a numerical simulation, empirical correlations were obtained to predict the heat transfer intensification in porous media for a steady and pulsating flow.

Scale buildup on the tube surface in the intertubular space of the shell-and-tube heat exchangers reduce their efficiency. The topical issue is the search for clean-in-place methods. The tube bundle cleaning by low-frequency nonsymmetrical pulsations is understudied. The aim of the paper is numerical analysis of the influence of pulsations on the key cleaning factors (wall shear stress, erosion rate). For the numerical experiment the symmetrical element of a staggered tube bundle with a crossflow of turbine oil (T22) (Re = 100; Pr = 273) and the quartz sand as a cleaning agent is used. The model of incompressible fluid flow comprises the system of Navier-Stokes and continuity equations, the turbulent model Spallart-Allmaras. The motion of solid particles is calculated by the discrete element method, and the erosion rate is calculated by the Campos-Amezcua method. In unsteady conditions with time step 0,001 sec, numerical simulations are performed in Ansys Fluent. Pulsations are generated on entry boundary condition. To estimate the flow pulsation efficiency, the wall shear stresses on the central tube of bundle and erosion rates are compared under the same average rate in steady and nonsteady flow. It is found that asymmetrical flow pulsations (duty cycle 0,25) increase of wall shear stress in all the modes under consideration (amplitude 25 ≤ A/d ≤ 35, frequency 0,3125 ≤ f ≤ 0,5 Гц), but an increase in erosion rate takes place only at maximal frequency. The amplitude variation displaces the localization of the reinforcing effect of flow pulsations on the tube surface. However, it is found that flow pulsations increase the wall shear stress and erosion rate in the front and rear sides of the tubes that are most susceptible to scale buildup. The conducted analysis confirms the significant influence of asymmetrical pulsations on cleaning factors and the perspective of their application for intensification of tube bundle cleaning. The detected effects can be the base to develop new technologies of cleaning intertubular space of heat exchangers.