Heat Transfer Distribution of Impinging Methane-Air Premixed Flame Jets

Abstract

Flame jets find importance in industrial and household applications like metal and glass melting/forming and cook stoves respectively. Heat transfer distribution of impinging flame jet was compared with that of the impinging air jet based on the experimental data reported in literature for methane-air flame jet and air jet impingement for Reynolds number, Re = 600 to 1400 and the non-dimensional nozzle tip to impingement plate distance, Z/d = 2 to 6. The comparative data based on mapping experimental data reported in literature suggested that there is a good agreement between the Nusselt numbers for higher Z/d near stagnation region. However, away from the stagnation region, the Nusselt number for flame jet is higher than that of air jet for similar operating conditions of Re and Z/d. A CFD simulation for impinging air jet and impinging flame jet was carried out using FLUENT software to explain the physics and reason for the deviations observed in experimental data. A scale analysis was carried out to identify the dominant forces and their influence on the heat transfer distribution on the impingement plate. Heat transfer from impinging flame jets to a flat plate has been assumed to be onedimensional in most of the investigations and without radiation loss treatment. In the present work, the exact nature of diffusion of heat in the plate is investigated via solution to multidimensional heat conduction problem. Two procedures have been employed – Duhamel theorem and three dimensional transient analytical inverse heat conduction problem (IHCP). The Duhamel theorem which is analytical model for transient one dimensional heat conduction was applied and its application failed the check of linearity requirement of the convection rate equation. From the solution by analytical IHCP for transient three dimensional heat conduction, the distribution of wall heat flux and the wall temperature was perfectly linear. This check confirmed that three dimensional approach has to be used. Experimental data is then analyzed by the three dimensional analytical IHCP for short and larger time intervals. It was found that for short time data, heat transfer coefficient and the reference temperature have oscillatory distribution along the radial direction on the impingement plate and for larger time data the oscillations die out. However, at larger time, radiation loss from the impingement plate becomes significant. The effect of variation in thermal conductivity of the impingement plate with the temperature on heat transfer coefficient and reference temperature is discussed. Anovel method was developed to correct the heat transfer coefficient and reference temperature to incorporate radiation losses. The deviation in heat transfer coefficient and reference temperature estimated without considering variable thermal conductivity and radiation loss for large time interval was upto 50%. The scope of the present technique is examined through its application to impinging jets with various configurations. The present study covers the applications of hot jet, cold jet and multiple jets with distinct Reynolds numbers and the nozzle-to-plate spacing and results confirms the validity of technique to impinging jets as well. Effect of plate thickness on the accuracy of the present technique is also studied. Upto 5 mm thick plates can be used in impinging jet applications without compromising much on accuracy. Use of present technique significantly reduces the experimental cost and time since it works on transient data of just few seconds Experiments were carried out on ribbed plates with three different geometrical shaped rib elements i.e. circular, rectangular and triangular. In addition, numerical simulations were performed to study flow field on and around ribs. During the experiments, Reynolds numbers varied from 600 to 1800 and burner tip to target plate distance from 2 to 4. Heat transfer coefficients were found lower whereas reference temperatures were observed higher on ribbed surfaces than smooth surfaces. Obstruction to the flow, flow separation and decrease in momentum are the reasons attributed for lower heat transfer rate to the ribbed surfaces.