Award Date

5-1-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Committee Member

Yitung Chen

Second Committee Member

Robert Boehm

Third Committee Member

Kwang Kim

Fourth Committee Member

Hui Zhao

Fifth Committee Member

Jacimaria Batista

Number of Pages

161

Abstract

The scope of this project is to numerically and experimentally dry cooling process in air-cooled condensers (ACCs) designed for concentrated solar power (CSP) applications. This effort is driven by the growing economic and political pressure to reduce water consumption during power generation due to limited water resources in the arid geographic climate of the southwestern United States. A computational approach is used in conjunction with experimental validation to gain a more complete understanding of these systems.

Traditionally research into ACCs have been largely limited to air-side heat transfer modelling as it accounts for a large portion of the total thermal resistance. Recent studies, however, indicate that impact of steam-side thermal resistances is often more significant than previously reported. Additionally, as the thermal performance of the air-side heat transfer is improved, the heat and mass transfer characteristics of the steam-side must be considered in greater detail. To improve the mathematical model of the steam-side phenomena, the impact of both rotation and divergence of the nonconservative Nusselt velocity vector field on film growth and mass conservation are investigated and a new film thickness equation is introduced. Nusselt thin film assumptions are implemented to derive the governing differential equations for mass, and energy and momentum. A transformation for effective peripheral angle during stratification is derived based on the assumed linear interface between liquid and vapor of Chato at the lower part of the tube. An expression for entrance length, L_e^* is introduced as a function of inclination angle. The results for heat transfer ratio have been compared with experimental data and an empirical correlation from literature and a good agreement was obtained and shown between them.

An analysis of filmwise condensation of a pure vapor in inclined non-circular tubes using cubic Bezier curves is presented. Nusselt thin film theory is modified to derive the governing differential equations for momentum, mass, and energy for a generalized surface contour. Elliptical and ovoid tube profiles are investigated and the film thickness and Nusselt number are compared to that of a circular tube on the basis of equivalent surface area. It was found that for a given surface area, the pressure drop in a circular tube is the lowest, however, entropy generation due to heat transfer can reduced by increasing |Y_2 |/X_2. Ovoid tubes can achieve superior heat transfer performance over elliptical tubes on the basis of equivalent aspect ratio, |Y2|/X2 however, this enhancement diminishes with increased inclination angle. The pareto dominant solutions are correlated to inclination angle and heat transfer enhancement and a new relation is proposed.

To relax the standard isothermal condition applied to the wall, the air-side heat transfer coefficient is coupled with the film thickness equation to investigate the impact on the condensate film growth and the HTC of the condenser. The air-side fluid flow is modeled using ANSYS Fluent and the Nusselt thin film theory was used to describe the film thickness in the upper part of the tube interior and a pool condensation model is used to define the axial flow in the lower part. The analysis was successful in showing that air-side transport phenomena can have a measurable impact on the condensate film distribution.

Finally, A full-sized ACC test apparatus was constructed to investigate dry cooling performance and potential modifications. 608 data points were experimentally obtained over a wide range of ambient temperature and compared with 8 commonly used correlations from the available literatures. The ambient temperature changes from 3° to 45°C, the steam mass flux varies from 3 to 18 kg/(m2·s), vapor quality ranges from 0.51 to 0.86. An improved two-phase frictional pressure drop correlation based on the Wallis correlation is proposed. The new correlation agrees well with the experimental database and outperforms all other tested correlations with a MAPE of 16.84% and a NRMSE of 20.45% while being able to predict 91.41% of the experimental data within 30%.

Disciplines

Aerodynamics and Fluid Mechanics | Mechanical Engineering | Thermodynamics

File Format

pdf

File Size

7.9 MB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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