Local total and radiative heat-transfer coefficients during the heat treatment of a workpiece in a fluidised bed

The heat-transfer coefficients around a workpiece immersed in an electrically heated heat treatment fluidised bed were studied. A suspension probe designed to simulate a workpiece of complex geometry was developed to measure local total and radiative heat-transfer coefficients at a high bed temperature. The probe consisted of an energy-storage region separated by insulation from the fluidised bed, except for the measuring surface, and a multi-thermocouple measurement system. Experiments in the fluidised bed were performed for a fluidising medium of 120-mesh alumina, a wide temperature range of 110–1050 °C and a fluidising number range of 1.18–4.24. It was found that the workpiece surface temperature has a more significant effect on heat transfer than the bed temperature. The total heat-transfer coefficient at the upper surface of the workpiece sharply decreased at the start of heating, and then steadily increased as heating progressed, while a sharp decrease became a rapid increase and then a slow increase for the radiative heat-transfer coefficient. A great difference in the heat-transfer coefficients around the workpiece was observed.

Radiative and total heat transfer measurements to a Titan Explorer model

Radiative and total heat transfer at the flow stagnation point of a 1:40.8 binary scaled model of the Titan Explorer vehicle were measured in the X3 expansion tube. Results from the current study illustrated that with the addition of CH4 into a N2 test gas radiative heat transfer could be detected. For a test gas of 5% CH4 and 95% N2, simulating an atmospheric model for Titanic aerocapture, approximately 4% of the experimentally measured total stagnation point heat transfer was found to be due to radiation. This was in comparison to < 1% measured for a test gas of pure nitrogen. When scaled to the flight vehicle, experimental results indicate a 64% contribution of radiation (test gas 5% CH4/95% N2). Previous numerical results however have predicted this contribution to be between 80-92%. Thus, experimental results from the current study suggest that numerical analyses are over-predicting the radiative heat transfer on the flight vehicle.

Heat transfer measurements on the first experimental layer of the FIRE II reentry vehicle in expansion tubes

The present study focused on simulating a trajectory point towards the end of the first experimental heatshield of the FIRE II vehicle, at a total flight time of 1639.53s. Scale replicas were sized according to binary scaling and instrumented with thermocouples for testing in the X1 expansion tube, located at The University of Queensland. Correlation of flight to experimental data was achieved through the separation, and independent treatment of the heat modes. Preliminary investigation indicates that the absolute value of radiant surface flux is conserved between two binary scaled models, whereas convective heat transfer increases with the length scale. This difference in the scaling techniques result in the overall contribution of radiative heat transfer diminishing to less than 1% in expansion tubes from a flight value of approximately 9-17%. From empirical correlation's it has been shown that the St √Re number decreases, under special circumstances, in expansion tubes by the percentage radiation present on the flight vehicle. Results obtained in this study give a strong indication that the relative radiative heat transfer contribution in the expansion tube tests is less than that in flight, supporting the analysis that the absolute value remains constant with binary scaling.

Heat transfer analysis of viscous incompressible fluid by combined natural convection and radiation in an open cavity

The effect of radiation on natural convection of Newtonian fluid contained in an open cavity is investigated in this study. The governing partial differential equations are solved numerically using the Alternate Direct Implicit method together with the Successive Over Relaxation method. The study is focused on studying the flow pattern and the convective and radiative heat transfer rates are studied for different values of radiation parameters namely, the optical thickness of the fluid, scattering albedo, and the Planck number. It was found that in the optically thin limit, an increase in the optical thickness of the fluid raises the temperature and radiation heat transfer of the fluid. However, a further increase in the optical thickness decreases the radiative heat transfer rate due to increase in the energy level of the fluid, which ultimately reduces the total heat transfer rate within the fluid.

Particle dynamics and heat transfer at workpiece surface in heat treatment fluidised beds

The particle behaviour is studied by the analysis of particle images taken with a high speed CCD digital video camera. The comparison of particle dynamics is performed for the fluidised beds without part, with single part and with multi-parts. The results show that there are significant differences in particle behaviours both in different beds and at different locations at part surfaces. The total and radiative heat transfer coefficients at different surfaces of a metallic component in a high temperature fluidised bed are measured by a heat transfer probe developed in the present work. The principle of the heat transfer probe is to measure the change in temperature of the heated metallic piece with time and, then, to extract the heat flux and heat transfer coefficients. The structure of the probe is optimized with numerical simulation of energy conservation for measuring the heat transfer coefficient of 150~600 W/m2 K. The relationship between the particle dynamics and the heat transfer is analysed to form the basis for future more rational designs of fluidised beds as well as for improved quality control.

Particle dynamics and heat transfer at workpiece surface in heat treatment fluidised beds

The particle behaviour in a heat treatment fluidised bed was studied by the analysis of particle images taken with a high speed CCD digital video camera. The comparison of particle dynamics was performed for the fluidised beds without part, with single part and with multi-parts. The results show that there are significant differences in particle behaviours both in different beds and at different locations of part surfaces. The total and radiative heat transfer coefficients at different surfaces of a metallic part in a fluidised bed were measured by a heat transfer probe developed in the present work. The structure of the probe was optimized with numerical simulation of energy conservation for measuring the heat transfer coefficient of 150-600 W/m2K. The relationship between the particle dynamics and the heat transfer was analysed to form the basis for future more rational designs of fluidised beds as well as for improved quality control.

Analytical and numerical solutions of inward spherical solidification of a superheated melt with radiative-convective heat transfer and density jump at freezing front

Analytical and numerical solutions of a general problem related to the radially symmetric inward spherical solidification of a superheated melt have been studied in this paper. In the radiation-convection type boundary conditions, the heat transfer coefficient has been taken as time dependent which could be infinite, at time,t=0. This is necessary, for the initiation of instantaneous solidification of superheated melt, over its surface. The analytical solution consists of employing suitable fictitious initial temperatures and fictitious extensions of the original region occupied by the melt. The numerical solution consists of finite difference scheme in which the grid points move with the freezing front. The numerical scheme can handle with ease the density changes in the solid and liquid states and the shrinkage or expansions of volumes due to density changes. In the numerical results, obtained for the moving boundary and temperatures, the effects of several parameters such as latent heat, Boltzmann constant, density ratios, heat transfer coefficients, etc. have been shown. The correctness of numerical results has also been checked by satisfying the integral heat balance at every timestep.

Effect of wall emissivities on radiation heat transfer in glass tank forehearths

In an earlier work, we had proposed a two-band, non-grey radiative transfer model for heat transfer in forehearths with simultaneous optically thick and thin approximations for molten glass interiors and at boundaries. Here using the same model, the radiative interaction of the top-crown and bottom-refractory walls with interior layers of shallow molten glass is studied by varying the wall emissivities. The forehearth exit temperature profiles for higher wall emissivities (0.9) show better conditioning of the glass for white flint glasses (optically thin).

Numerical simulation of heat and mass transfer in fluidised bed heat treatment furnaces

In this paper, a novel combined theoretical and computational model is developed to simulate the heat and mass transfer between a fluidised bed and a workpiece surface, and within the workpiece by considering the fluidised bed as a medium consisting of a double-particle layer and an even porous layer. The heat and mass-transfer flux from the fluidised bed to the workpiece surface is contributed by dense and bubble phases, respectively. The convective heat and mass transfer is simulated by analysing the gas dynamics in the fluidised bed, while radiative heat transfer is modelled by simulating photon emission in a three-dimensional particle array. The simulation shows that convection is approximately constant, while radiation contributes significantly to the heat transfer. The heat-transfer coefficient on an immersed surface near particles is about 6–10 times that on other areas. The transient heat and mass-transfer coefficient, heat and mass-transfer flux on any surface of the workpiece, transient temperature and carbon distributions at any position of the workpiece during the metal carburising process are studied with the simulation.

Modelling of transient heat transfer in annealing furnaces

This is a study of heat transfer in a lift-off furnace which is employed in the batch annealing of a stack of coils of steel strip. The objective of the project is to investigate the various factors which govern the furnace design and the heat transfer resistances, so as to reduce the time of the annealing cycle, and hence minimize the operating costs. The work involved mathematical modelling of patterns of gas flow and modes of heat transfer. These models are: Heat conduction and its conjectures in the steel coils;Convective heat transfer in the plates separating the coils in the stack and in other parts of the furnace; and Radiative and convective heat transfer in the furnace by using the long furnace model. An important part of the project is the development of numerical methods and computations to solve the transient models. A limited number of temperature measurements was available from experiments on a test coil in an industrial furnace. The mathematical model agreed well with these data. The model has been used to show the following characteristics of annealing furnaces, and to suggest further developments which would lead to significant savings: - The location of the limiting temperature in a coil is nearer to the hollow core than to the outer periphery. - Thermal expansion of the steel tends to open the coils, reduces their thermal conductivity in the radial direction, and hence prolongs the annealing cycle. Increasing the tension in the coils and/or heating from the core would overcome this heat transfer resistance. - The shape and dimensions of the convective channels in the plates have significant effect on heat convection in the stack. An optimal design of a channel is shown to be of a width-to-height ratio equal to 9. - Increasing the cooling rate, by using a fluidized bed instead of the normal shell and tube exchanger, would shorten the cooling time by about 15%, but increase the temperature differential in the stack. - For a specific charge weight, a stack of different-sized coils will have a shorter annealing cycle than one of equally-sized coils, provided that production constraints allow the stacking order to be optimal. - Recycle of hot flue gases to the firing zone of the furnace would produce a. decrease in the thermal efficiency up to 30% but decreases the heating time by about 26%.

Analytical thermal study on nonlinear fundamental heat transfer cases using a novel computational technique

In this paper a novel computational technique called Parameterized Perturbation Method (PPM) is used to obtain the solutions of nonlinear fundamental heat conduction equations. Three well known problems in the area of heat transfer are addressed to be solved. An analytical investigation is carried out for: (a) the temperature distribution in a fin with a temperature-dependent thermal conductivity, (b) the cooling of the lumped system with variable specific heat, and (c) the temperature distribution of a convective-radiative fin. The validity of the results of PPM solution was verified via comparison with numerical results obtained using a fourth order Runge-Kutta method. These comparisons revealed that PPM is a powerful approach for solving these problems. Also, the results showed that the main attributions of this method are very straightforward calculations and low computational burden compared to previous analytical and numerical approaches.