Jamil A. Khan

Prediction of Temperature Distribution and Thermal History during Friction Stir Welding: Input Torque Based Model

AbstractA novel three-dimensional thermal model is proposed to study the transient temperature distributions during the friction stir welding of aluminium alloys. The moving heat source engendered by the rotation and linear traverse of the pin tool has been correlated with the actual machine power input. This power, obtained from experimental investigation, has been distributed to the different interfaces formed between the tool and the weldpiece based on the torques generated at different tool surfaces. Temperature dependent properties of the weld material have been used for the finite element based numerical modelling. Good agreement between the simulated temperature profiles and experimental data has been demonstrated. The effects of various heat transfer conditions at the bottom surface of the workpiece, thermal contact conductances at the interface between the workpiece and backing plate and different backing plate materials on the thermal profile in the weld material have also been investigated nume...

ENHANCEMENT OF HEAT TRANSFER BY INSERTING A METAL MATRIX INTO A PHASE CHANGE MATERIAL

Increase in the heat transfer rate during melting and freezing of a phase change material (PCM) with a low thermal conductivity is demonstrated by inserting a high-porosity metal matrix into the PCM. A vertical annulus space (r 1rr 0, 0zh)homogeneously fitted with water and an aluminum matrix, is selected for this study. The Navier-Stokes equations are modified to account for Darcys effect. For both the melting and freezing cases, the density inversion phenomenon of water is considered. The irregularity and time-varying nature of the solid and the liquid regions are accounted for by a geometric coordinate transformation. The numerical results are presented in the form of solid-liquid interface movements, isotherms, streamlines, and heat transfer rates for some representative cases. The heat transfer rates for enhanced cases show an order-of-magnitude increase over the base case, where no metal matrix is inserted.

Relationships between weld parameters, hardness distribution and temperature history in alloy 7050 friction stir welds

Abstract Aluminium alloy 7050 was friction stir welded using three different ratios of tool rotation rate to weld travel speed. Welds were made using travel speeds of between 0·85 and 5·1 mm s−1. Weld power and torque were recorded for each weld. An FEM simulation was used to calculate the time–temperature history for a subset of the welds. For each weld the hardness distribution with and without post-weld heat treatment was determined. The hardness distributions within the welds are rationalised based on the friction stir welding parameters and the resulting temperature histories. The analysis provides a basis for manipulation of weld parameters to achieve desired properties.

Prediction of nugget development during resistance spot welding using coupled thermal–electrical–mechanical model

AbstractAn axisymmetric finite element model employing coupled thermal–electrical–mechanical analysis of resistance spot welding is presented. The welding parameters considered include: heat generation at the faying surface and the workpiece–electrode surface; Joule heating at the workpiece and the electrode; and the thermal contact conductance between the electrode and the workpiece. The latent heat of phase change due to melting is accounted for. The effect of friction coefficient on the workpiece interface is also studied. The computed results agree well with the experimental data. Heat generation at the faying surface in the initial stages of welding dominates the nugget development, and Joule heating at long times governs the weld nugget growth. A parametric study is carried out for the nugget growth with specific consideration of resistance spot welding of Al alloys. Process control and modelling of resistance spot welding of Al alloys is more difficult than that for steel because of their high elec...

Numerical Simulation of Resistance Spot Welding Process

This paper develops a model to predict the nugget development during resistance spot welding (RSW) of Al-alloys. The model employs a coupled thermal-electrical-mechanical analysis and accounts for phase change and convective transport in the weld pool. The contact area and the pressure distribution are determined from a coupled thermal ? mechanical model. The model calculates time varying interface pressure. The knowledge of interface pressure allows for accurate prediction of interfacial heat generation. Temperature-dependent thermal-electrical-mechanical properties are used. The predicted nugget shape and size agree well with experimental data. The proposed model can be applied to predict the effects of the welding parameters and the electrode shapes on the nugget development.This paper develops a model to predict the nugget development during resistance spot welding (RSW) of Al-alloys. The model employs a coupled thermal-electrical-mechanical analysis and accounts for phase change and convective transport in the weld pool. The contact area and the pressure distribution are determined from a coupled thermal ? mechanical model. The model calculates time varying interface pressure. The knowledge of interface pressure allows for accurate prediction of interfacial heat generation. Temperature-dependent thermal-electrical-mechanical properties are used. The predicted nugget shape and size agree well with experimental data. The proposed model can be applied to predict the effects of the welding parameters and the electrode shapes on the nugget development.

Evaporation from a packed bed of porous particles into superheated vapor

Abstract An experimental and computational study of evaporation into superheated steam from porous particles, which form a packed bed, is performed. The goal of the study is to develop a predictive model of the drying process in packed beds with steam as the drying medium. The direct experimental measurement using load cells of the weight of alumina particles of various diameters in a packed bed during the drying process produced a complete continuous drying curve. A range of experimental conditions are examined, and a correlating equation for the overall heat transfer coefficient developed. Evaporation from a single spherical porous particle is modeled as occurring at a receding liquid/vapor interface within the particle. The drying process in a single particle is assumed to be thermally controlled. The single particle model is incorporated into an overall model for drying in a packed bed. The model predicted overall and local drying rates, as well as transient temperature distributions in the packing. The results of this study indicate that drying in a packed bed of significant depth occurs in three phases. The three phases of drying correspond to periods where the entire bed is in the constant drying rate regime, portions of the bed have entered the falling rate regime, and the entire bed is in the falling rate regime, respectively. The existence of these phases of drying are confirmed by both measured temperature distributions and measured drying rates. Predictions of the overall drying rate from the model show favorable agreement with measured drying curves.

Thermal load deviation model for superheater and reheater of a utility boiler

The extreme steam temperature deviation experienced in the superheater and reheater of a utility boiler can seriously affect its economic and safe operation. This temperature deviation is one of the root causes of boiler tube failures (BTF), which causes about 40% of the forced power station outages. The steam temperature deviation is mainly due to the thermal load deviation in the lateral direction of the superheater and reheater. This variation is very difficult to measure in situ using direct experimental techniques. In this paper, we propose a thermal load model that is based on the power plant thermodynamic parameters, thermal deviation theory, and flow rate deviation theory. It is found that the calculated results from our model agree well with the in situ experimental results. The predicted BTF positions are the same as that in the reheater of a 300 MW utility boiler at Wujing Power Plant. The proposed model has been used to improve the design of utility boiler in Boiler Works, predict the possible BTF in the design stage, and assess the existing designs. This model can also be applied to utility boilers of different manufactures, and has been successfully applied to the BTF prediction and prevention in the Power Station.

Electrolytic stimulation of denitrification in sand columns

Denitrification in two sand columns inoculated with a mixed culture of denitrifying microorganisms was stimulated when an electric current was passed using electrodes inserted within the columns. An average nitrogen removal efficiency of 49% was achieved at a current of 20 mA. Abiotic column experiments showed an average nitrogen removal efficiency of 20% at a current of 20 mA, indicating that the direct electrochemical reduction of nitrate on the cathode was occurring. Statistical analysis showed that at a 95% confidence level, the performance of the two columns with respect to the nitrogen removal efficiency was similar, the increase in nitrogen removal efficiency with an increase in passed current was significant and the difference in the performance between the abiotic and biologically active columns was significant. At a current of 20 mA, the 95% confidence interval for the contribution of biological activity to denitrification in the column is 18 to 69%.

Effective heat transfer in a metal-hydride-based hydrogen separation process

Abstract This paper presents the results of experimental and analytical study of the thermal cycling absorption process (TCAP); which is a metal-hydride-based hydrogen separation system configured as a helical shell-and-tube heat exchanger. The column (tube side) is packed with Palladium deposited on kieselguhr (Pd/k). This packed column is thermally cycled by a hot and cold nitrogen gas on its exterior surface (shell side), while a stream of hydrogen mixed with other inert gases are passed through the packed column. Hydrogen gas is absorbed and desorbed from the Palladium, causing a separation from the gas stream. The rate at which the hydrogen is separated depends only on how quickly the Pd/k can be thermally cycled. In this paper we present a transient heat transfer analysis to model the heat transfer in the Pd/k packed column. To improve the efficiency of the TPAC, metallic foam was added in the Pd/k packed column. It was observed that adding metallic foam significantly improved the separation rate of hydrogen. Thermal cycling times for varying packed column diameters, materials, and compositions are also determined. Comparison of performance is made between an existing 1.25 in (31.75 mm) column versus a 2 in (50.8 mm) column tube. A parametric argument is presented to optimize the material selection and geometric design of a TCAP heat exchanger.

Numerical Thermal Model of Resistance Spot Welding in Aluminum

A three-dimensional thermal model for resistance spot welding in aluminum is presented. The numerical model, validated with experimental findings, considered phase change and the associated weld pool convection. A parametric study was performed to determine the influence of welding features such as welding current, faying surface (workpiece contact surface) electrical contact resistance, and electrode-workpiece (E/W) thermal contact conductance. These parameters have significant effects on the nugget and heat-affected-zone geometry. The phase change morphology, including melting and solidification rates and weld pool dynamics, was also significantly influenced by the parameters studied. The strongest convection was observed at the center of the molten pool in a vertical plane, aligned with gravity. Although two prominent convection cells developed, the phase change morphology was not significantly affected by convection due to the short welding time (less than 0.1 s) and low fluid velocity (smaller than 0.01 mm/s). The nugget grew nonlinearly with increasing current and faying surface electrical contact resistance, whereas it diminished with increasing E/W thermal contact conductance. The influence of electrical contact resistance at the faying surface on nugget size was less pronounced than that of the other parameters. The length of time that the weld pool existed was directly proportional to current and indirectly proportional to E/W thermal contact conductance.