Heat transfer distribution of swirling flame jet impinging on a flat plate using twisted tapes

Impinging flame jets have wide applications in industrial and domestic heating purpose. The aim of the present study is to study the effect of swirl on flame jet impingement heat transfer characteristics. Four twisted tapes of twist ratios 2, 3.2, 4.5 and 7.5 (corresponding swirl numbers of 0.79, 0.49, 0.35 and 0.21) are used to experimentally obtain the swirling flame jet. The effect of twist ratio at Reynolds number varying from 500 to 2500 and equivalence ratio varying from 0.7 to 1.5 for burner tip to impingement plate distances of 2 and 4 is studied. The effect of swirl is compared with that of no tape in the tube burner by estimating the average heat flux distribution and coefficient of variation of heat flux for two impingement regions - within two times the burner diameter from the stagnation point and four times diameter from the stagnation point. From the experimental study, it is found that swirl enhances the heat flux distribution by 40-140% at low Reynolds number. At higher Reynolds number the effect of swirl is negative and is found to decrease the average heat flux distribution by 10-40%. (C) 2015 Elsevier Ltd. All rights reserved

Heat transfer distribution for three interacting methane-air premixed impinging flame jets

Multiple impinging flame jets are widely used in industrial heating (Ex. ladle preheating) and melting applications (Ex. glass forming). In the present method, high resolution heat flux distribution is obtained by applying the inverse heat conduction technique using thermal infrared camera. The high resolution thermal imaging enables to capture the steep gradients of spatial heat flux distribution. Nusselt number and effectiveness distributions are obtained by analytical-numerical method of estimation of the adiabatic wall temperature. Three circular tube burners arranged in a staggered pattern with inter tube spacing (S/d) of 2 to 6 is considered. The ratio of distance from tube burner tip to the impingement plate (z/d) is varied from 2 to 6 while the Reynolds number is varied from 400 to 1000. For the smallest S/d = 2, the interaction amongst the flames is significant and leads to non-circular hot spot distribution for heat flux. The average Nusselt number and average effectiveness are higher for higher Re for all z/d unless the inner premixed cone touches the impingement plate. The average Nusselt number and average effectiveness are marginally higher for smaller S/d for the same z/d and Re. The coefficient of variance increases with the increase in S/d. (C) 2015 Elsevier Ltd. All rights reserved

Local heat transfer distribution on a flat plate impinged by a swirling jet generated by a twisted tape

An experimental investigation is conducted to study the local heat transfer distribution on a flat surface normally impinged by a swirling air jet. Twisted tapes of twist ratios equal to 2, 3.2, 4.5 and 7.5 (corresponding swirl numbers S = 0.79, 0.49, 0.35, 0.21) are inserted in a circular tube to generate swirling effect. Experiments are carried out for Reynolds number varying from 500 to 3000 for jet to plate spacing varying from 1 to 4. The local heat transfer characteristics are estimated using thermal images obtained by thermal infrared imaging technique. The jet flow profile on the target plate is evaluated by the flow visualisation carried out using lamp black technique. The heat transfer rate is found to initially increase with the increase in twist ratio (or decrease in swirl number) from 2 to 4.5 and thereafter it reduces with the increase in the twist ratio from 4.5 to 7.5. The heat transfer rate is maximum for a twist ratio of 4.5 and minimum for a twist ratio of 7.5. The jet to plate spacing also shows strong influence on the heat transfer rate. With the increase in jet to plate spacing, the heat transfer rate decreases. The maximum heat transfer rate is obtained at z/d = 1 for the different twist ratios and Reynolds number. (C) 2016 Elsevier Masson SAS. All rights reserved

A novel concept to estimate the steady state heat flux from impinging premixed flame jets in an enclosure by numerical IHCP technique

Most of the flame jet impingement in industrial and domestic applications is carried out in an enclosure. However, no data is available in the literature for impinging flame jets in an enclosure. The present experimental study proposes a novel concept for obtaining the heat flux distribution using numerical IHCP technique. The heat flux evaluated by the present method has high spatial resolution, requires short time and is found to be reasonably accurate. The heat transfer distribution of a single flame jet from tube burner of 7.5 mm diameter is studied at Reynolds number varying from 500 to 900 with and without enclosure. The heat transfer distribution of three circular tube burners of diameter 7.5 mm is studied at Reynolds number varying from 500 to 900 with and without enclosure. The ratio of distance from the burner tip to the impingement plate to the burner diameter is varied from 1.9 to 5.1. A comparison of average heat flux distribution of three tube burner pair (7.5 mm diameter) with two tube burner pair (11.25 mm diameter) is carried out for similar conditions. The presence of enclosure has negligible effect on the heat transfer distribution from flame jets for both single and multiple jets. It is found that two tube burner pair has higher (8% to 18%) average heat flux distribution and more uniformity as compared to three tube burner pair. (C) 2014 Elsevier Ltd. All rights reserved

Simultaneous estimation of heat transfer coefficient and reference temperature from impinging flame jets

Heat transfer from impinging flame jets to a flat plate has been assumed to be one-dimensional 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 IHCP. The Duhamel theorem which is analytical model for transient one dimensional heat conduction is applied but 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 is 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 is 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 variations in thermal conductivity of the impingement plate with the temperature on heat transfer coefficient and reference temperature is discussed. A novel method is 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 is upto 50%

Thermocouple error correction for measuring the flame temperature with determination of emissivity and heat transfer coefficient

Temperature measurement by thermocouples is prone to errors due to conduction and radiation losses and therefore has to be corrected for precise measurement. The temperature dependent emissivity of the thermocouple wires is measured by the use of thermal infrared camera. The measured emissivities are found to be 20%-40% lower than the theoretical values predicted from theory of electromagnetism. A transient technique is employed for finding the heat transfer coefficients for the lead wire and the bead of the thermocouple. This method does not require the data of thermal properties and velocity of the burnt gases. The heat transfer coefficients obtained from the present method have an average deviation of 20% from the available heat transfer correlations in literature for non-reacting convective flow over cylinders and spheres. The parametric study of thermocouple error using the numerical code confirmed the existence of a minimum wire length beyond which the conduction loss is a constant minimal. Temperature of premixed methane-air flames stabilised on 16 mm diameter tube burner is measured by three B-type thermocouples of wire diameters: 0.15 mm, 0.30 mm, and 0.60 mm. The measurements are made at three distances from the burner tip (thermocouple tip to burner tip/burner diameter = 2, 4, and 6) at an equivalence ratio of 1 for the tube Reynolds number varying from 1000 to 2200. These measured flame temperatures are corrected by the present numerical procedure, the multi-element method, and the extrapolation method. The flame temperatures estimated by the two-element method and extrapolation method deviate from numerical results within 2.5% and 4%, respectively. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790471

An experimental and numerical investigation of heat transfer distribution of perforated plate burner flames impinging on a flat plate

Heat transfer from perforated plate burner flame impinging on a flat plate finds importance in industrial (gas fired boiler) and domestic heating (household gas burner and commercial hotel gas burners) applications. A limited study on heat transfer distribution of this kind of burner plate is found in the literature. in the present work, high resolution heat flux is estimated by the inverse heat conduction (IHCP) technique based on use of analytical solution for semi-infinite medium for the impingement plate. Inline, star and staggered holes patterns with three different inter-hole distances (pitch) are considered in the present study. Methane air premixed flame of Reynolds number varying from 50 to 600 and an equivalence ratio varying from 0.6 to 1.2 is considered. The hole to impingement plate distance is varied from 3 to 7. From the experimental results, it is found that the inline and staggered patterns have the same heat flux averaged over an area of 50 mm x 50 mm for different Reynolds number. The intermediate pitch of 7 mm is the optimal pitch over the entire mixture flow range considered in the present study. The specific fuel consumption for the star pattern is less by 40-60% as compared with the inline pattern for p/d = 1.67, Re = 50-300 and z/d = 3-7. A numerical simulation is carried out using CFD software to explain the shift in the peak heat flux away from the geometric intended location. (C) 2015 Elsevier Masson SAS. All rights reserved

Local heat transfer distribution of an impinging air jet through a crossflow

An experimental investigation is conducted to study the effect of crossflow on the local heat transfer distribution of a flat surface normally impinged by round air jet. The influence of jet-to-plate distances (4 d of 4, 6 and 12), the crossflow velocity to the jet velocity (M varying from 1/6 to 1/12) and the Reynolds number (Re varying from 6000 to 12,000) on the heat transfer distribution was studied. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. The center-line distribution of the Nusselt number along the length of the plate is provided and a correlation for the stagnation point Nusselt number varying with the experimental parameters was proposed. It is observed that the distance of the stagnation point from the geometric impingement point and its magnitude increase with the increase in M. This distance is seen to be higher for higher values of z/d. (C) 2014 Elsevier Masson SAS. All rights reserved

Effect of preheated mixture on heat transfer characteristics of impinging methane-air premixed flame jet

Energy from spent flame or other low grade energy can be used to increase the temperature of the air before mixing with fuel. This would improve the heat transfer characteristics of the impinging flame jet. The studies on impinging flame jets reported in the literature are based on the fuel-air mixture at ambient temperature. In the present work, the inlet air for mixture is heated by an electrical heater. The heat flux distribution is estimated using an inverse heat conduction (IHCP) technique. The Nusselt number (Nu) and effectiveness (eta) distributions are obtained by estimating the adiabatic wall temperature (T-aw) by the analytical-numerical method. A circular burner of 13.5 mm is used for impingement on quartz plate of 3 mm thickness. Reynolds number (Re) varying from 500 to 2000 for the non-dimensional burner tip to impingement plate spacing (Z/d) of 2-6 and stoichiometric condition (phi = 1.0) is considered for varying preheated condition. The effect of equivalence ratio is studied for phi = 0.75 to 1.5 for Re = 1000 and Z/d = 4. By increase in preheat temperature, the stagnation point heat flux increases from 20% to 50% unless the inner premixed zone touches the impingement plate. CFD simulations are carried out in FLUENT software to explain the distribution of heat flux. (C) 2015 Elsevier Ltd. All rights reserved