Abstract
ABSTRACT To enhance the air-cooling process in geothermal power plants for economical utilization of the exhaust steam from expansion, a natural-draft thermal chimney design was proposed and studied here in this paper. In view of the necessity of accurate velocity field measurements which would provide further insight into the physics behind the evolving plumes above heated horizontal cylinders, Particle Image Velocimetry (PIV) was employed to experimentally investigate the buoyant flow in the thermal chimney system. Two configurations have been tested to understand the flow induced by the horizontally heated cylinders inside the thermal chimney. Firstly, flow field above a single row of cylinders was tested while they were isothermally heated to simulate of an air-cooled condenser. After that, a second row of cylindrical heaters (air-heater) was added above the first row to enhance the buoyant flow, aiming at enhancing the air side flow of the air-cooled condenser. Flow characteristics and velocity enhancement were studied for both configurations. The results show that significant flow unsteadiness occur near the cylindrical heaters because of the non-steady crossing flows between adjacent cylinders, and the unsteadiness attenuates in the downstream. The effects of cylinder row distance, surface temperature as well as downstream distance on the flow field were then analyzed. Flow velocity is increased by the air-heater as the buoyancy force is enhanced, proving the idea of flow enhancement of the thermal chimney configuration. It is also observed that the velocity fluctuation, turbulent kinetic energy and vorticity change significantly after adding the second row of heaters. The present study provides further insight into natural convection flow theory of heated cylinders for a Rayleigh number range of 1.3E4 to 2.2E4, which is fundamental for the flow enhancement designing of the proposed natural-convection-driven cooling system.
Highlights
To utilize renewable energies to aid the air flow and enhance the natural draft air-cooling, the thermal chimney concept is verified to be an economic and environment-friendly cooling solution in many fields (Sun, Zhiqiang, and Kamel 2017; Zhai, Song, and Wang 2011; Zou et al 2012)
The present study provides further insight into natural convection flow theory of heated cylinders for a Rayleigh number range of 1.3E4 to 2.2E4, which is fundamental for the flow enhancement designing of the proposed natural-convection-driven cooling system
To validate the velocity profile, 3D CFD research was conducted and it was confirmed that the errors in the mean velocity between Particle Image Velocimetry (PIV) and CFD at targeted nominal temperatures are in a range of (−15-+20)% (Li et al 2020)
Summary
To utilize renewable energies to aid the air flow and enhance the natural draft air-cooling, the thermal chimney concept is verified to be an economic and environment-friendly cooling solution in many fields (Sun, Zhiqiang, and Kamel 2017; Zhai, Song, and Wang 2011; Zou et al 2012). Since Nusselt in 1915 pro posed the Nusselt number concerning natural convection heat transfer from a single horizontal cylinder, numerous heat transfer studies of horizontal cylinders have been conducted experimentally, numerically and analytically It can be con cluded from the large amount of literature that in addition to the local heat transfer of the cylinders, researchers have recog nized the necessity of accurate velocity field measurements which would provide further insight into the physics behind the evolving plumes above heated horizontal cylinders (Churchill and Humbert 1975; Morgan 1975). To evaluate the thermal chimney performance, investigations of flow field above cylindrical heating elements and in particular about how the buoyancy plumes develop above cylinders should be initially conducted. By developing the lab-scaled model and conducting experimental analysis in this work, the designing feasibility, flow physics as well as the trade-off between the thermal chimney effect and the extra flow resistance caused by the upper row heating elements is expected to be implicated
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