Heat transfer investigations on impinging flame jets of a multi-port convex burner

Multi-port curved surface burners are used in industrial boilers, preheating of large diameter pipes, textile industries, preheating of cylindrical metal billets and other industrial applications. The present work reports the heat transfer characteristics of convex shaped burner with 49 inline ports arranged in an array of 7 each. The methane-air premixed flame jets are impinged on the concave surface of a curved plate placed with radial spacing of 12 mm. Two different burners with port diameters of 2 mm and 3 mm with pitch to diameter ratio of 2.33 are used. The flame jet is studied for Reynolds number (Re) ranging from 75 to 200 and equivalence ratio of 0.9-1.5. The characterisation is done by impinging the pre-characterised air jets of various flow rates on the rear surface and capturing the wall temperature. The wall temperature is analysed to get the distributions of Nusselt number and effectiveness. The variation of coefficient of variance of Nusselt number, average Nusselt number and thermal efficiency is reported in the present work. The Nusselt number and effectiveness distributions increase with the increase in mixture Reynolds number. For a fixed mixture Reynolds number, the optimum heat transfer is obtained for stoichiometric flame. The thermal efficiency and average heat transfer of 2 mm port diameter is observed to be 18-25% higher as compared with burner with port diameter of 3 mm. Correlations are developed to predict the average Nusselt number, average effectiveness, coefficient of variance (COV) of Nusselt number distribution and thermal efficiency

Effect of mixture composition on heat transfer characteristics of impinging methane-air flame jets of tube burners equipped with twisted tapes

Present study investigates the effect of the mixture equivalence ratio on the heat transfer characteristics of impinging premixed methane-air flame jets of tube burners equipped with twisted tape inserts. The induced swirl flow enhances the mixing process and improves the flame stability limits. The flame jet with mixture Reynolds number of 800 and burner to plate spacing of four times burner diameter is chosen based on the flame stability range. The heat transfer characteristics of non-swirling flame jet are compared with the swirling flame jets for various mixture compositions. Swirl flow is achieved with twisted tape inserts having twist ratios of 2, 3.2, 4.5 and 7.5. Twisted tape inserts resulted asymmetric flame. The flame shape is found to be highly influencing the heat transfer from the flame jet to the target surface. Better heat transfer distribution is achieved with stoichiometric mixture. However, the rich mixture produces more uniform heat flux distribution. Best thermal performance is observed for twisted tape with twist ratio of 7.5. The present study helps in better understanding and optimum design of the flame jet impingement heat transfer equipment. (C) 2016 Elsevier Masson SAS. All rights reserved

Air jet impingement technique for thermal characterisation of premixed methane-air impinging flame jets

A major part of the domestic and the industrial thermal energy requirement for the heating purpose is achieved by the combustion of hydrocarbon fuels using burners. The present study reveals that thermal characterisation of impinging flame jets can be performed with a steady state technique similar to thin metal foil technique used for impinging air jets. The target surface impinged by the premixed methane air flame jet is simultaneously cooled from the rear side by impinging air jets at different Reynolds number. One dimensional energy balance across the thickness of the plate is performed. The Nusselt number and the effectiveness distributions for a tube burner with the present technique matched reasonably well with the two-equation technique proposed in our previous work. Maximum deviations of 12% and 15% are observed for Nu and eta respectively. Correlations are developed for the local Nusselt number and effectiveness in terms of flame jet parameters. The burner is analysed for the thermal efficiency. For premixed cone flames, it is observed that the thermal efficiency increases with the Reynolds number and equivalence ratio and decreases with the burner tip to plate spacing. (C) 2016 Elsevier Ltd. All rights reserved

Heat transfer investigations on methane-air premixed flame jet exiting from a circular nozzle and impinging over semi-cylindrical surfaces

The flame jet impingement on curved surfaces fords applications in various direct flame heating situations. The design of heating equipment demands the estimation of the surface temperature and of the convective heat flux to the target surface at steady state for various impingement parameters. The characterisation of flame jets in terms of Nusselt number and effectiveness aids the designer to numerically estimate the steady state surface temperature and heat flux by specifying the convective boundary condition with spatial distribution of reference temperature. The present study reports the heat transfer characteristics of single flame jet impinging on convex and concave surfaces of a semi-cylindrical target plate. The effects of mixture Reynolds number, equivalence ratio, curvature ratio and burner to plate spacing on the heat transfer behavior are investigated. The uniformity of the heat flux distribution and the overall thermal efficiency are determined from the local heat flux distributions. The flow fields along the curvilinear axes influence the overall thermal performance. The concave surface produces higher thermal efficiency and better uniformity of heat flux over the impingement surface as compared with the convex surface

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

Separation of heat transfer components from impinging methane diffusion flames

The high heat transfer rates from impinging flame jets and plumes are extensively used in many industrial applications. Comprehensive literature review suggests that the characterization of flame impingement is done primarily by the measurement of heat flux onto the target surface. The use of in-situ probes limits the spatial resolution of the measurement. For a diffusion flame, radiation cannot be neglected and it is therefore necessary to determine the convective and radiative heat flux components to quantify the thermal boundary condition to the impingement surface. For determining the heat transfer characteristics of impinging diffusion flames, the target surface is impinged by a methane diffusion flame from the bottom and is simultaneously cooled from the top by air jets of different Reynolds number. At steady state, one dimensional energy balance across the impingement surface provides an equation with the three unknowns being the heat transfer coefficient of the flame jet, the reference temperature and the emissivity of the gas/flame. By keeping the flame jet impingement conditions same and varying the air jet impingement on the top surface, five different forms of the energy balance equation is obtained. A minimization technique, that makes use of the Nelder-Mead algorithm, is developed to solve for the over-determined system of equations. The obtained results are compared with the slope method that determines the effective heat transfer coefficient and the reference temperature. The impingement surface is modeled in FLUENT and the experimentally obtained heat transfer coefficient of the flame jet, the reference temperature and the emissivity of the gas/flame is provided as the boundary condition to numerically determine the surface temperature. For validation purpose, the impingement surface material and thickness is changed and the experimentally obtained and numerically determined wall temperatures are compared. It is demonstrated that the minimization technique is capable of separating the convective and radiative heat transfer components from impinging diffusion flames. (C) 2018 Elsevier Ltd. 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

Geotechnical and geoenvironmental engineering education during the pandemic

This paper reports the impact of coronavirus disease 2019 on the practice and delivery of geotechnical and geoenvironmental engineering (GGE) education modules, including lectures, lab sessions, student assessments and research activities, based on the feedback from faculty members in 14 countries/regions around the world. Faculty members have since adopted a series of contingent measures to enhance teaching and learning experience during the pandemic, which includes facilitating active learning, exploring new teaching content related to public health, expanding e-learning resources, implementing more engaged and student-centred assessment and delivering high-impact integrated education and research. The key challenges that faculty members are facing appear to be how to maximise the flexibility of learning and meet physical distancing requirements without compromising learning outcomes, education equity and interpersonal interactions in the traditional face-to-face teaching. Despite the challenges imposed by the pandemic, this could also be a good opportunity for faculty members obliged to lecture, to rethink and revise the existing contents and approaches of professing GGE education. Three future opportunities namely, smart learning, flipped learning and interdisciplinary education, are identified. The changes could potentially provide students with a more resilient, engaged, interactive and technology-based learning environment