Heat transfer characteristics of impinging premixed flame jet with induced swirl

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Heat transfer characteristics of impinging premixed flame jet with induced swirl

 

Author: Huang, Xiaoqun
Title: Heat transfer characteristics of impinging premixed flame jet with induced swirl
Degree: M.Phil.
Year: 2006
Subject: Hong Kong Polytechnic University -- Dissertations.
Heat -- Transmission.
Jets -- Fluid dynamics.
Department: Dept. of Mechanical Engineering
Pages: xxi, 150 leaves : ill. ; 31 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2093857
URI: http://theses.lib.polyu.edu.hk/handle/200/187
Abstract: The premixed gas-fired impinging flame jets are widely used for domestic and industrial applications because of their advantages in offering high heat transfer rate, rapid combustion process and lower emission of air pollutants. In addition, they are of simple construction, convenient and safe to use, and very suitable for low Reynolds number and low pressure applications. The heat transfer characteristics of impinging flame jets have been investigated by many researchers. There have been several methods applied successfully to enhance their flammable limits for the production of stable and reliable flame, and hence enhance their heat transfer performance. For example, swirl has been successfully adopted in industrial combustors utilizing turbulent diffusion flame jets to extend the flammable limits as well as to improve the combustion efficiency, to enhance the heating performance and to reduce the pollution emissions. The use of swirl appears to be a rather promising method, but the application of swirl to the small-scale domestic and industrial appliances utilizing premixed gas-fired impinging flame jets at low Reynolds numbers has not yet been investigated. The present study had been conducted to fill this gap by studying the heat transfer from a small-scale premixed butane/air circular impinging flame jet with induced swirl to a horizontally placed copper impingement plate. It was essentially an experimental study, and the major parameters affecting the impingement heat transfer including: Reynolds number of the air/fuel jet, Swirl number of the air/fuel jet, nozzle-to-plate distance and equivalence ratio of the air/fuel mixture, had been fully investigated. Butane gas was used as the gaseous fuel for the present investigation, as it is one of the major gaseous fuels used in Hong Kong. The first part of the present study was to confirm the feasibility of introducing swirl to a gas-fired premixed circular flame jet operating at low Reynolds numbers and low pressure. After reviewing the relevant literature, there were several approaches considered. The ability of each of these approaches to produce the swirling flame was examined. It was decided that the swirl is better induced by mixing and balancing the axial and tangential flows of working fluids into the conical chamber of the burner, where the tangential flow is admitted into the burner through two symmetrical tangential inlets. It had been confirmed from this part of study that it is feasible to apply the laminar premixed gas-fired swirling impinging flame jets under quite a wide range of operation conditions. The second part of the present study was to investigate the thermal characteristics of the swirling impinging flame jets. Two experimental setups, namely I and II, had been used to produce the swirling flame jets, in which different methods were applied to mix and distribute the air and fuel streams into the burner. For the entire heat transfer characteristics study, the local heat flux distributions on the impingement plate under different operating conditions were measured and compared. The total heat flux on the impingement surface was obtained by integrating the local heat flux along the whole impingement area, from which the area-averaged-heat-flux was then determined. In the experimental setup I, the air and fuel streams were admitted into the burner via the tangential and axial inlets, respectively. Thus, ratio between the tangential flow and axial flow which determines the Swirl number is directly linked to the equivalence ratio (i.e. air/fuel ratio). This approach enables a rather high swirl intensity to be obtained, but effects of the Swirl number and equivalence ratio on the heat transfer performance of the swirling impinging flame can not be separately identified. The method used to calculate the Swirl number is referred as the "Geometric Swirl Number Calculation Method", where the Swirl number is calculated directly from the tangential and axial flow rates. In the experimental setup II, the air and fuel were premixed and the air/fuel mixture was divided into two separate streams, which were then supplied to the burner via the axial and tangential inlets. Each of these two streams can be adjusted and measured by a flow meter. In this approach, the Swirl number can be determined with the aid of the photographs obtained by applying the "Smoke Flow Visualization Technique". This approach enables the production of a very stable swirling impinging flame having rather good thermal performance. In addition, effects of Swirl number and equivalence ratio can be individually identified, but a rather low swirl intensity can only be generated with this approach. Experimental results obtained from both experimental setups I and II were discussed and compared with each other, as well as with the findings obtained from the previous work on non-swirling flame jets. In the final part of the present study, semi-analytical non-dimensional equations had been developed with the aid of the present experimental results and the multiple regression method in Matlab 7.0. These equations provide quick but accurate predictions of the convective heat transfer coefficient, both local and area-averaged, between the swirling impinging flame jet and the impingement plate. The predictions obtained from these equations had been compared with the findings obtained from similar impinging flame jets without swirl induced, which were reported in literature.

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