Liquid Potential Temperature Research Articles

CFD water spray model development and physical parameter study on the evaporative cooling

Polydisperse evaporating spray study is complex due to the influence of a large number of physical parameters. Several studies have performed CFD simulations to investigate the cooling performance of water spray systems, but a few have investigated their impact upon heat exchangers. For industrial applications, developing a new and simple approach to simulate polydisperse evaporating sprays upon complex 3D geometries is of great interest. Thus this paper is the first contribution to a CFD numerical tool development to study water spray impact on heat exchangers and presents a CFD water spray model. The spray model is divided into two steps: the spray formation and its dispersion in air flow. The spray development step describes the moment from droplet injection to the position where droplet velocity equals air velocity. This position and the spray dimension are accessed through the droplet trajectory analysis, while the amount of liquid water evaporated is obtained by integrating the droplet size decrease equation. This first part provides boundary conditions for the second step used in a 3D CFD software: Code_Saturne. This CFD code solves the Navier-Stokes equations for the spray with the k-ε turbulence model. Three transport variables are introduced: the liquid potential temperature, θL, the total water specific humidity, qw, which are conservative variables for the evaporation processes; and the total number, of droplets, Nc. The droplet evaporation is added to the Nc equation through a source term approach. A lognormal law is also used to represent and follow the evolution of the droplet spectra. The model results are compared with experimental results from droplets injected in counter-flow configurations in a wind tunnel. Temperature fields show good agreements with the experimental data. Finally, this paper provides a parametric analysis of water evaporation and air cooling upon a specified surface. The impacts of the relative humidity, spray angle, water mass flow rate and droplet size distribution are investigated. Our approach is an alternative to classical Lagrangian approaches used in spray applications. It provides accurate and consistent results with low computational time in comparison with the literature.

Modelling atmospheric effects on performance and plume dispersal from natural draft wet cooling towers

Abstract The study of cooling towers in realistic atmospheric conditions is important to assess their performance and plume dispersal as a function of wind speed and hygrometric conditions for their economical and public health consequences. In the present work the plume formation and its dispersal are modeled using a finite volume method on grid including details of the towers. The air mass flow and heat transfer are simulated inside and outside the tower. The equations for the transport of momentum, mass and liquid potential temperature are solved using the CFD software Code Saturne. In contrast to other studies dealing with the performance of cooling towers, the exchange processes inside the tower are represented with a source terms approach. The adiabatic expansion of the plume at the exit of the tower in the atmosphere is accounted for using the thermodynamical laws. The results of the simulation are compared to the measurement at Bugey nuclear power plant. The results of the model are shown to be in good agreement with the field measurements from point of view of the air flow structures, plume patterns and thermodynamical variables. Based on this reference simulation we study the variation of the cooling tower performance against the wind speed and quantify the effect of the ambient wind speed increase on the reduction of the natural draft. We also discuss the sheltering effect that the upstream tower can have on the other.

Buoyancy Flux Modeling for Cloudy Boundary Layers

The modeling of the buoyancy flux in a partly cloudy atmospheric boundary layer is complicated by its dependence on the correlation between clouds and vertical velocity elements, which can vary significantly with the underlying layer dynamics (e.g., stratocumulus versus shallow cumulus). The buoyancy flux is sometimes modeled in terms of higher-order statistics, such as vertical velocity skewness or saturation variance, to try to capture some of these dynamical effects. In this work an approximate expression for the buoyancy flux is formulated solely in terms of the liquid potential temperature and total water profiles and their respective flux profiles. The predictions compare favorably with the results of an extensive set of large-eddy simulations (LES), including simulations of stratocumulus, shallow cumulus, and transitional behavior in between. This formulation is combined with previous results on the relation between cloud-top entrainment rate and circulation structure to predict the behavior of quasi-steady cumulus-coupled boundary layers as a function of a basic set of physical input parameters. These predictions also compare favorably with LES results.

Binormal Model of Ensemble Partial Cloudiness

Abstract A new formulation of partial cloudiness parameterization has been introduced that agrees with that for a random model in one limit and approaches the simple updraft/downdraft model of larger-scale models in the limit of very highly skewed flow. Each of the conserved variables, liquid potential temperature and total humidity, along with the vertical velocity, are assumed to have probability distributions that may be parameterized as combinations of two multivariate normal distributions. This allows the skewness of the variables to be controlled by the bias between the means of the two normals and their relative fractions. It also provides a smooth transition between the normal distribution and the two limiting delta function distribution of the updraft/downdraft model. Comparisons with large-eddy-simulation data show this new model to be valid over a much wider range of conditions than the single normal distribution. When a simple cloud-top entrainment instability (CTEI) analysis is made using the...