Effects of impingement plate on thermal performance of premixed impinging flame jets

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Effects of impingement plate on thermal performance of premixed impinging flame jets

 

Author: Zhao, Zhen
Title: Effects of impingement plate on thermal performance of premixed impinging flame jets
Degree: Ph.D.
Year: 2006
Subject: Hong Kong Polytechnic University -- Dissertations
Heat -- Transmission
Jets -- Fluid dynamics
Department: Dept. of Mechanical Engineering
Pages: xix, 226 leaves : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2069720
URI: http://theses.lib.polyu.edu.hk/handle/200/2534
Abstract: Experiments have been carried out to investigate the effects of an impingement plate on the heat transfer characteristics of the impinging flame jet system. For the present study, the impingement plates were fabricated with three materials, viz. brass (k = 61W /mK), bronze (k = 26W/mK) and stainless steel (k = 14.9W/mK). It has been found that, under similar experimental conditions of the flame jet, the maximum local heat flux obtained with the brass plate(which has the highest thermal conductivity), is much higher than that obtained with the stainless steel plate(which has the lowest thermal conductivity). Such a phenomenon is quite obvious at the stagnation region but becomes less significant in the wall-jet region of the impingement plate. It is also found that the use of a low-conductivity impingement plate would lead to a larger suppression of heat transfer and a higher collection of unburned fuel at the stagnation point, as well as an outward shift of the location of maximum heat flux from there. More unburned fuel being consumed progressively beyond the stagnation region would lead to further shift of the location of the maximum heat flux. It has found that conduction, usually thought to be less significant in this kind of heat transfer, was actually playing a rather important role in the overall heat transfer from an impinging flame jet to the impingement plate. Experimental investigation has been conducted to study the thermal effect of the impingement plate surface emissivity, a parameter normally assumed to affect directly radiation heat transfer. Three brass plates of different surface emissivites (0.10, 0.38 and 0.98) were tested using the same premixed impinging flame jet. The phenomena of the occurrence of a cold region around the stagnation region and the outward shift of the location of maximum heat flux have also been observed. Although the surface emissivities are quite different, viz. 0.1, 0.38 and 0.98, the differences among maximum local heat fluxes of the impingement plates were found to be rather small, especially at the wall-jet region (where the maximum difference was only 8%). It is therefore concluded that the effect of surface emissivity, and hence radiation, on both heat flux and temperature distribution on an impingement plate is rather insignificant, especially at the wall-jet region. This implies that non-luminous radiation has insignificant influence on the thermal performance of the gas-fired premixed impingement flame jet system. To investigate the effect of surface roughness of the impingement plate on convective heat transfer of the flame jet system, three brass plates of significantly different surface roughness (1.035um, 100 um and 500um) have been tested using the same premixed impinging flame jet. A difference in the stagnation point heat flux of 40% was found between the smoothest and the roughest plates. It shows that a rough surface is able to act as a turbulence promoter. In view of the dominant role of convection in the flame jet system, a variation in plate surface roughness can generate quite a significant effect on the overall heat transfer from a premixed flame jet to the impingement plate. Multivariate regression models have been developed for the prediction of the area-averaged heat flux received in terms of each of three important plate characteristics, viz. thermal conductivity, surface emissivity and surface roughness. By means of a full factorial design, the integrated effect of each of these plate characteristics and the three test variables (i.e. Reynolds number, equivalence ratio, and nozzle-to-plate distance) on heat flux received can be estimated. At a confidence level of 95%, predictions given by all three area-averaged heat flux models have been found to agree closely with the experimental results A numerical study was conducted to simulate conduction heat transfer through the impingement plates. Assuming heat loss from the heated plate to the surrounding, thermo-chemical heat release and condensation are negligible, heat conduction through an impingement plate would equal to the convection and radiation transfer from the impinging flame jet to the impingement plate. The flame-side convective boundary conditions, a major difficulty in performing numerical study of the impinging flame jet heat transfer, can then be estimated by the polynomial fit with the experimental data. Predicted data obtained from the numerical simulation were then validated against experimental results and the model has been found to work satisfactorily. In addition, properties of the impingement plate vary with temperature and such variations can be easily accounted for by using the present simulation model. Moreover, by using the numerical simulation, the distribution of local heat flux and temperature within the solid plate can be determined, which can supply more information about the heat transfer of the impingement plate.

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