Numerical investigation of nozzle shape effect on the flash boiling phenomenon

Abstract

Small nozzles may be used as an element of novel steam traps in steam heating systems or shell-coil (JAD) heat exchangers. Phenomena occurring in these nozzles should be analyzed to find their optimal operating points and avoid steam losses. Therefore, the paper presents a numerical investigation of the effect of the nozzle shape on the flash boiling phenomena. Particularly, the study aimed to check the relation between the shape of the decompression part of the nozzle and the mass flow rate of the mixture of water and vapor flowing through it for different pressures and undercooling of water at the nozzle inlet. Two geometries of the divergent part of the nozzle, i.e. the stepped and conical, were considered. In the analysis, two-phase flow in the decompression part of the nozzle was accounted for by applying the mixture model. The dynamics of the phase change process during the flash boiling were described with the Zwart-Gerber-Belamri model, additionally supplied with the polynomial relationship linking the saturation pressure and temperature of the water. In the first step, a credibility analysis of the model was conducted. The simulated results were compared to the literature’s reference results, and a good agreement was achieved. Moreover, the independence of results on the grid size and turbulence modeling in the bulk flow and near-wall region was shown. In the second step, a series of simulations were carried out for both nozzle geometries (stepped and conical), three nozzle neck diameters, three pressures, and eleven undercooling of water at the inlet to the nozzle. The shape of the divergent part of the nozzle was found to influence the characteristics of flash boiling flow significantly. However, it did not affect the mass flow rate of water flashing through the nozzle. For both geometries, the predicted mass flow rates were the same.