Experimental and numerical study of NOx formation in a domestic H2/air coaxial burner at low Reynolds number

Thermal NOx formation in H2 /air jet flames from a coaxial burner is studied experimentally and numerically. The aim is to study possible NOx reduction strategies for domestic gas boiler burners. Following a flame splitting method strategy, a single burner is studied at different inlet powers (from 0.2 to 1.0 kW). The effect of three different fuel-air ratios (or equivalence ratio φ) is considered by varying the coaxial air stream, with fuel-air ratios corresponding to values of φ < 1, relevant for domestic boiler applications (here φ = 0.77, φ = 0.83 and φ = 0.91). NOx concentrations increase with increasing inlet power between 0.2 and 0.6 kW and numerical results are in good correspondence with available experimental data. The opposite trend is observed above 0.6 kW and no numerical results are obtained, indicating a transition from laminar to turbulent flames. On the other hand, in contrast to the observations made in turbulent non-premixed flames, reducing the equivalence ratio implies higher NOx concentrations in the low Reynolds number flames considered. The numerical results in the laminar regime are used to highlight and quantify three competing main factors concerning NOx production in order to interpret the experimental observations: the volume of the region where NOx is produced, and within this region, the competition between residence time and NOx reaction rate. Based on this analysis, different design strategies for low NOx hydrogen diffusion burners are finally discussed.The authors are grateful to the Basque Government for funding this research through projects IT781-13 and IT1314-19. Part of this work is also supported at Ciemat by the project #PID2019-108592RB- C42/AEI/10.13039/50110 0 011033

Numerical Study of a Laminar Hydrogen Diffusion Flame Based on the Non-Premixed Finite Rate Chemistry Model; Thermal NOx Assessment

The present study examines thermal NOx formation on a laminar hydrogen diffusion flame based on a patented multi-flame diffusion burner installed in a domestic boiler. The analysis is performed through CFD modeling by means of ANSYS-FLUENT 19.0. Detailed chemical, thermal, and transport parameters are implemented, in order to obtain accurate results using finite-rate chemistry together with laminar and low-Reynolds turbulence models. The Damköhler number is calculated using a Matlab code developed in-house. The chemical kinetics of the thermal route are analyzed for several inlet flame power levels, so as to demonstrate the effectiveness of the flame-splitting method at reducing the formation of thermal NOx. The numerical results are contrasted through measurements from literature for validation purposes. In the present work, the flame power is varied from 0.05 kW up to 0.8 kW. In contrast to turbulent non-premixed hydrogen flames, the numerical results provide a quasi-constant trend in thermal formation at higher power levels (0.4–0.8 kW). The heat exchange rate between the flame and the combustion chamber and its influence on thermal NOx formation are all carefully analyzed.Authors are grateful to the Basque Government for funding this research through projects IT781-13 and IT1314-19 and to all those involved in the different stages for their guidance and invaluable help

Predicting the induction hardened case in 42CrMo4 cylinders

Induction hardening has the potential to produce favorable surface integrity that can improve fatigue performance and extend the lifetime of a component. The localized superficial heating provided by induction is the main advantage of this process, as it allows the core to remain intact and, therefore, ductile, while the surface is hardened. Achieving favorable characteristics in the hardened case is of great importance, as this process is usually applied to load bearing and wear-susceptible metallic components. The simulation of the hardening process by induction heating is a complex and challenging task at which many efforts have been directed in the last years. Due to the numerous interactions of the many physics that take part in the process (electromagnetic, thermal, microstructural and mechanical), a highly coupled finite element model is required for its numerical simulation. In this work, a semi-analytical induction heating model is used to compute the induction hardening process, predicting the size and shape of the hardened layer and the distribution of the hardness. Using the semi-analytical model allows the computational time to be much faster compared to a fully coupled model using a commercial software, where the time consumption for the presented 2D case is reduced by 20 %. Experimental validation is presented for cylindrical 42CrMo4 billets heated by a short solenoidal inductor, which shows good agreement with the predicted results, reaching an average error of 3.2 % in temperature estimations

Lifetime prediction of bonded structural patch repairs for wind turbine pitch bearing strengthening

&lt;p&gt;There is a growing trend in the use of adhesively bonded patches to repair structures that are subjected to fatigue loads in an attempt to reduce the propagation of an existing crack. The fatigue lifetime prediction of the adhesive used in bonded patch repair is a state-of-the-art challenge, especially when in- service conditions must also be considered. Different approaches have been identified in the literature to analyse structural failure; the methods based on fracture mechanics seem more promising for adhesives because the presence of defects is also considered in the analysis. A considerable amount of work has been undertaken to analyse the delamina-tion of layered structures using cohesive zone modelling (CZM). However, research analysing fatigue delamination in bonded thick-walled structures (&gt; 30 mm) is lacking. The present work proposes a methodology to predict the lifetime of an adhesively bonded patch repair applied to a thick-walled structure, speci-fically looking at the bearing ring structural fatigue (RSF) life extension of a pitch bearing commonly used in wind turbines&lt;/p&gt

A semi-analytical coupled simulation approach for induction heating

The numerical simulation of the induction heating process can be computationally expensive, especially if ferromagnetic materials are studied. There are several analytical models that describe the electromagnetic phenomena. However, these are very limited by the geometry of the coil and the workpiece. Thus, the usual method for computing more complex systems is to use the finite element method to solve the set of equations in the multiphysical system, but this easily becomes very time consuming. This paper deals with the problem of solving a coupled electromagnetic - thermal problem with higher computational efficiency. For this purpose, a semi-analytical modeling strategy is proposed, that is based on an initial finite element computation, followed by the use of analytical electromagnetic equations to solve the coupled electromagnetic-thermal problem. The usage of the simplified model is restricted to simple geometrical features such as flat or curved surfaces with great curvature to skin depth ratio. Numerical and experimental validation of the model show an average error between 0.9% and 4.1% in the prediction of the temperature evolution, reaching a greater accuracy than other analyzed commercial softwares. A 3D case of a double-row large size ball bearing is also presented, fully validating the proposed approach in terms of computational time and accuracy for complex industrial cases

Spindle and cutting tools for internal suction of chip and dust in CFRP trimming

Chip and dust removal during the trimming of carbon-fibre reinforced polymer (CFRP) parts is still an important challenge to overcome by the manufacturing industry to protect operators from the health hazards associated to CFRP dust and chips. Therefore, novel PCD cutting tools and a special spindle with internal suction capability have been designed and developed, providing an effective chip and dust removal from the working area during robotic trimming. Nevertheless, to guarantee a totally clean machining process not only conventional process aspects such as part surface quality, tool wear and productivity must be considered, but also the effectiveness of the chip and dust suction must be studied. Thereby, experimental process optimization has been carried out, analysing the effect of different process conditions on suction rate and surface integrity. Finally, optimum cutting conditions are identified obtaining a 98.4% suction rate resulting in a safe CFRP machining process