BUBBLES DYNAMICS

Purpose-The purpose of this paper is to numerically simulate the nanofluid boiling inside a tube in turbulent flow regime and to investigate the effect of adding volume faction of CuO nanoparticles on the boiling process. Design/methodology/approach-To make sure the accuracy of the obtained numerical results, the results of this paper have been compared with the experimental results and an acceptable coincidence has been achieved. In the current paper, by Euler-Euler method, the phase change of boiling phenomenon has been modeled. The presented results are the local Nusselt number distribution, temperature distribution of wall, the distribution of volume fraction of vapor phase and fluid temperature at the center of the tube. Findings-The obtained results indicate that using nanofluid is very effective in the postponement of the boiling process. Hence, by change the amount of volume fraction of nanoparticles in base fluid, the location of phase change and bubble creation are changed. Also, at the Reynolds numbers of 50,000, 100,000 and 150,000 with the volume fraction of 2 per cent, the beginning locations of phase change process are, respectively, 2D, 10D and 13D, and for the volume fraction of 4 per cent, the beginning locations of phase change are 4D, 18D and 19D, respectively. These results indicate that, as the volume fraction of nanoparticles increases, the location of the start of the phase change process is postponed that this issue causes the increment of heat transfer from wall to fluid and the reduction of wall temperature. In general, it can be stated that, in boiling flows, using nanofluid because of the delay in boiling phenomenon has a good effect on heat transfer enhancement of heated walls. Also, the obtained results show that, by increasing Reynolds number, the created vapor phase reduces that leads to increase of the Nusselt number. Originality/value-The paper investigates the effect of using nanofluid in phase change process of cooling fluid. Nomenclature A = Area, m 2 ; d = Solid particle molecule diameter, mm; d 1 = Input diameter, mm; d 2 = Middle diameter, mm; d 3 = Output diameter, mm; F = Friction factor; h = Convection heat transfer coefficient, W/m 2 K; k = Heat conductivity coefficient, W/m K; L 1 = Input branch length, mm; L 2 = Middle Branch Length, mm; L 3 = Output branch length, mm; Nu = Nusselt number; P = Perimeter, m; P = Pressure, Pa; Pr = Prandtl number; q" = Heat flux, W/m 2 ; Re = Reynolds number; T = Temperature, K; U = Velocity component along x; and V = Velocity component along y. Greek symbols m = Viscosity, Pa.s; a = Input branch angle, deg; b = Volumetric expansion coefficient, K À1 ; b = Middle Branch angle, deg; g = Output branch angle, deg; w = Volume fraction; and r = Density, kg/m 3. Super-and Sub-scripts f = Fluid; P = Solid articles; nf = Nanofluid; and ave = Average.

Methane (CH4) is the most abundant hydrocarbon and one of the most important greenhouse gases in the atmosphere. CH4 bubble growth and migration within muddy aquatic sediments are closely associated with sediment fracturing. In this paper we present the modeling of buoyancy-driven CH4 bubble growth in fine-grained muddy aquatic sediment prior to the beginning of its rise. We designed a coupled mechanical/reaction-transport numerical model that enables a differential fracturing over the bubble front (as it occurs in nature), when the fracturing increment stays constant at the bubble head and subsides towards bubble tail during bubble growth. We show that this differential fracturing over the bubble front controls the bubble shape and size temporal evolution, and is significantly affected by the critical stress intensity factor of the muddy sediment. The intercalated stages of elastic expansion and fracturing during the bubble growth shorten with time as the bubble approaches its terminal size (prior to its ascent). Our simulations reveal a high asymmetry in the bubble shape growing with time, with respect to its initial symmetric penny-shaped configuration. It was found that the bubble grows allometrically, while the importance of the bubble surface area growth with time. We also confirmed the earlier predictions about the ”inverted tear-drop” bubble cross-section just prior to the beginning of its rise. Modeling of the terminal bubble characteristics will permit prediction of the delivery of gaseous methane from the sediment to the atmosphere via the water column.