Heat Transfer Components Research Articles

Calculation of Heating Patterns in Microwave Sintering using a 3D Finite-Difference Code

ABSTRACTAnalysis of heating patterns in microwave sintering experiments provide information on the contributions of the various heat transfer components to the overall temperature pattern. Measured temperature patterns provide limited information on overall effects. Numerical simulations provide a cost effective way from which the effect of geometry, material properties and the presence of stimulus such as SiC rods or sheets on the heating pattern can be studied separately. Parametric studies allow us to identify the most significant properties and provide guidelines for the routine successful utilization of microwave sintering experiments. These guidelines may also facilitate the scale up and commercialization of microwave sintering.In this paper we describe a thermal model that calculates the temperature distribution in ceramic samples and insulation under realistic microwave sintering conditions. The calculation process involves a two-step procedure. The first step is to calculate the microwave power deposition in the sample and surrounding insulation. 3D FDTD calculations, described in a companion paper[1,2], are used for this purpose. The other step involves calculation of the temperature distribution using a 3D finite-difference heat-transfer program developed in our Departments[3]. Results illustrating the effect of thickness of insulation and the placement of SiC rod susceptors in picket-fence arrangement are presented.

An investigation on instantaneous local heat transfer coefficients in high-temperature fluidized beds—II. Statistical analysis

Statistical analysis is employed to make a further insight into the nature of instantaneous local heat transfer coefficients obtained in Part 1 of this paper. The computed functions from these coefficients are the variance, autocorrelation function and power spectral density function. These functions and parameters have been employed to characterize the dynamic behaviors around the immersed horizontal tube and thus to investigate the mechanism of bed-to-tube heat transfer in high temperature fluidized beds. The bubble phase heat transfer component is separated from instantaneous local heat transfer coefficients, and is found to be increased approximately exponentially with increasing bed temperature.

A study of the particle convection contribution to heat transfer in gas fluidized beds

A model for the particle convection heat transfer component of the overall heat transfer between a submerged surface and a bubbling fluidized bed is presented. A convective boundary condition combined with surface coverage concepts are introduced to describe the instantaneous wall-to-bed heat transfer. Both thermally lumped and distributed formulations with constant and variable properties for the model are presented and studied. As formulated, the model is not restricted to uniform bed particle size distributions. Experimental measurements obtained from instantaneous local heat flux probes are compared to the model predictions. The thermally distributed, variable property model formulation provides very good agreement with the experimental data over a wide range of particle sizes and fluidization velocities. For particle size beds ranging from 256 to 4000 μm, the model displayed a very accurate trend prediction relative to available data, with magnitude predictions of 2–28% error, depending upon particle size over a wide range of fluidization velocity.

Combined natural convection and radiation heat transfer from a shrouded vertical fin array

Experiments were performed to determine the combined-mode natural convection/radiation heat transfer characteristics of a shrouded array of rectangular, vertical fins. The investigated parameters included the height and longitudinal length of the fins, the clearance gap between the shroud and the fin tips and the fin-to-ambient temperature difference. Shroud walls having different thermal characteristics were used (highly conducting and insulated). For each configuration, the Rayleigh number ranged from 40 to 700. It was found that the positioning of a shrouding surface close to the fins decreased the rate of combined-mode heat transfer when compared to the unshrouded case. Greater heat transfer rates were found for the conducting wall shroud in comparison with the insulated one. Calculations performed showed that, when the clearence gap between the shroud and the fin tips was zero, the convective component of the total heat transfer was dominant. For other clearence gaps, the contribution of radiation was of the same order of magnitude as the convective one. The contribution of the radiative component was greater for small values of the Rayleigh number and for clearence gaps different from zero.

The influence of radiative heat transfer on the limit pull rate in Czochralski crystal growth of silicon

In this paper the results of an analytical study of the influence of the radiative heat transfer on the limit pull rate in Czochralski process are reported. An analysis of the contribution of the radiative heat transfer components shows that the limit pull rate depends non-monotonically on the effective temperature for radiation and, hence, at some temperature the limit pull rate has a maximum. For Czochralski crystal growth of silicon, the limit pull rate is shown to have the maximum when the effective temperature for radiation is within the 500–650 K range — depending on the convective heat transfer on the crystal surface. For commercially used apparatus, this temperature is evaluated to be about 1300 K. The most considerable contributions to this temperature are made by the radiative flux from the melt and crucible surfaces. The increase of the limit pull rate is possible (up to 50%) if the surface of the growing crystal can be protected from these surfaces, for example, by means of a special cooler installed around the crystal and kept at 500–650 K.

The porosity in a fluidized bed heat transfer model

A mathematical model of heat transfer between a fluidized bed and an immersed surface and a model of gas flow and porosity, both recently published, were combined and further modified in the area of low velocities where the particle convective component of heat transfer is low or neglectable. Experimental work consisted of measurements of both porosity and heat transfer in a cold fluidized bed of sand. Five samples of sand, each having a narrow size range, were investigated; the mean particle diameter varied from 0.43 to 1.55 mm. Also two mixtures of these samples, which yielded a wide particle size distribution, were tested. Experimental results showed good agreement with model predictions, for both porosity and heat transfer.

Determination of particle and gas convective heat transfer components in a circulating fluidized bed

In order to study the role of particles in augmenting heat transfer from the wall of a circulating fluidized bed (CFB), simultaneous heat and mass transfer experiments were carried out. For heat transfer, particles are important hydrodynamically, augmenting gas convection, and are a source of internal energy, i.e. particle convection. For mass transfer, only the former occurs. Simultaneous heat and mass transfer experiments using naphthalene as a sublimation material were performed in a 20 cm diameter circulating bed operating at atmospheric conditions. The presence of particles in the circulating bed causes an order of magnitude increase in the bed to wall heat transfer in comparison to single-phase turbulent gas flow. In contrast, the mass transfer is increased by 50% over single-phase gas flow. The gas convection component of the total heat transfer, found from the mass transfer experiments, varied from 10 to 20% of the total heat transfer. In this range of solids concentration between 12 and 80 kg/m 3, particle convection dominates. The superficial gas velocity has little influence on the particle convection or on the gas convection component. The particle convection varies with the density of particles in the core, probably due to variations in the wall fraction covered by particle clusters. Gas convection is insensitive to the density of particles in the core.

Investigations into the thermal performance of multilayer insulation (300-77 K) Part 1: Calorimetric studies

The thermal performance of multilayer insulation (MLI), consisting of double aluminized Mylar radiation shields and nylon net thermal spacers, was evaluated using a double guarded cylindrical calorimeter and a tank calorimeter over the temperature range 300-77 K. The degradation in the effective thermal conductivity of MLI was evaluated to be 1.68 using the calorimeters. The optimum number of layers for the MLI was 40 to 50 at a layer density of 25 layers cm −1. The temperature profile and heat flux through the MLI were obtained as a function of vacuum level for different numbers of insulation layers. The temperature profile of the MLI indicates the relative predominance of the conduction and radiation components of heat transfer through the insulation. It is observed that for a given number of layers, the temperature of a specific layer between the cold and warm boundaries decreases with an increase in chamber pressure and vice versa.

Heat transfer of a spray droplet in a PWR pressurizer

Heat transfer rates to spray droplets under conditions corresponding to those of spray transients in a pressurizer of pressurized water reactor (PWR) have been predicted by a simple droplet model with internal thermal resistance and partial internal mixing. In those processes, the temperature distributions in the droplet have been obtained using the integral method, and the physical properties of the saturated steam-hydrogen gas mixture surrounding the droplets are estimated applying the concept of compressibility factor and using appropriate correlations. Results have been provided for the temporal variations of total heat flux with its convection and condensation heat transfer components, dimensionless droplet bulk temperature and droplet flight distance. The effects of ambient pressure, initial droplet size, concentration of hydrogen gas in the mixture, initial injection velocity, and spray angle on the heat transfer of spray droplets have been discussed.

An Instrument for Local Radiative Heat Transfer Measurement Around a Horizontal Tube Immersed in a Fluidized Bed

An instrument for the measurement of the radiative component of total heat transfer in a high-temperature gas fluidized bed is described. The main objective of this paper is to emphasize the design, instrumentation, and calibration of this device. The results are presented and discussed elsewhere (Alavizadeh, 1985; Alavizadeh et al., 1985). The design makes use of a silicon window to transmit the radiative heat flux to a thermopile-type heat flow detector located at the base of a cavity. The window material thermal conductivity is sufficiently large to prevent conduction errors due to the convective component of total heat transfer. Also, its transmission and mechanical hardness are well suited for the fluid bed environment. The device has been calibrated using a blackbody source both before and after exposure to a fluidized bed, indicating the effect of the abrasive bed environment on performance. The instrument has been used to measure local radiative heat transfer around a horizontal tube. Typical results for a particle size of 2.14 mm and a bed temperature of 1050 K are presented and discussed to illustrate instrument performance.

Review on Heat Transfer in Gas Solid Fluidized Beds

This review covers those aspects of heat transfer in fluidized beds related to bed/immersed tubes and bed/vessel walls heat transfer coefficients correlations. These correlations generally incorporated the effects of operating conditions, bed and gas thermophysical properties and bed size and geometry. Experimental as well as empirical dimensionless models are presented. The general tendency in the literature is towards building up more realistic hydrodynamic models which incorporate particle and gas convective heat transfer components, solids residence time and local bubble behaviour at immersed surfaces. It is also generally recognized in the literature that heat transfer cannot be satisfactorily explained on the basis of stationery models but rather on the basis of unsteady state heat conduction models of solids movement induced by rising bubbles. The aspects of research mostly overlooked in the surveyed investigations were those related to interphase heat transfer and the effect of operation under elevated temperatures and pressures.

Heat transfer performance of a toluene loaded two-phase thermosyphon

Large dimension thermosyphons are efficient heat transfer components in heat recovery systems. Their performance limits depend on the following parameters: geometrical (length, diameter, inclination angle), physical (fluid, fill charge), thermal (temperature, heat flux). An experimental investigation was carried out with a large dimension, closed, two-phase thermosyphon which correspond to a device used in industrial recuperators. A vertical or inclinded steel thermosyphon, 3 m long and 27 mm inner diameter, was tested at temperatures varying from 100°C to 300°C with toluene as the working fluid. The lower part of the pipe was electrically heated along a variable length and the upper zone was cooled with an air stream whose flow rate and temperature were controlled. The maximum heat flux was measured as a function of temperature for different liquid fill charges and inclination angles. From these experimental data, boiling and condensation heat transfer coefficients were deduced. It was observed that the critical heat flux depends little on the fill ratio unless the charge is less than 20% for which a local dry-out occurs. The optimal fill charge was found to be between 20% and 50%. Experimental data have been compared with existing theories. The inclination effects have been taken into account with an empirical formula.

Effects of Radiation on Enhanced Electronic Cooling

The radiation component of heat transfer has been shown to provide a significant portion of the power dissipation capability for certain applications of enhanced surfaces in electronic cooling. Experimental work using two surface coatings has shown that the radiation interchange can be increased in natural-convection cooled environments for both horizontally and vertically oriented heat sinks. It was also shown that, to a lesser degree, the radiation component of heat transfer is effective in a forced convection environment. Based on experimental data from an oxidized aluminium heat sink, the radiation component was evaluated using the geometry constraints of a heat sink enclosed within a system environment consisting of gray reradiation walls. The equations used were then modified to simulate a change in heat-sink surface finish. The results of this investigation indicate that, for an aluminium heat sink, in the test configuration evaluated, the radiation component in a natural convection environment dissipated approximately 5% of the total heat flow.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Maximal values of the conductive component of the external heat transfer in miscible dispersed media

On the basis of a two-zone heat-transfer model, a universal form of the dependence of the maximal magnitude of the conductive component of the external heat transfer on governing factors is set up.

Heat transfer in microsphere insulation

The results of an investigation of heat transfer in a new type of insulation (microsphere insulation) are presented. The effects of the microsphere diameter, the concentration of metallized microspheres and the residual gas pressure on the thermal conductivity of the insulation were investigated. Measurements were made of the thermal conductivity at 77 to 300 K of microspheres with differing diameters (e.g. 95, 130 and 270 μm) and of samples with silver metallized microsphere concentrations of 7 and 32%. Measurements of average thermal conductivity (77–296 K) were made at residual gas pressuresk(p) in the range from 10−3 Pa to 105 Pa for pure nitrogen. The component of heat transfer by gas,k gc (p), was estimated.

The thermodynamic design of heat and mass transfer processes and devices

This review article places in perspective the new work devoted both to the analysis of the thermodynamic irreversibility of heat and mass transfer components and systems and to the design of these devices on the basis of entropy generation minimization. The review focuses on the fundamental mechanisms responsible for the generation of entropy in heat and fluid flow and on the design tradeoff of balancing the heat transfer irreversibility against the fluid flow irreversibility. Applications are selected from the fields of heat exchanger design, thermal energy storage, and mass exchanger design. This article provides a comprehensive, up-to-date review of second-daw analyses published in the heat and mass transfer literature during the last decade.

Measurement of Thermal Conductivity of Nonwovens Using a Dynamic Method

A nonsteady-state cooling method has been used to measure the thermal diffusivity (α) of nonwovens. The method is relatively simple and requires a minimum of spe cialized equipment. The calculations required to obtain α from cooling curves are somewhat involved but can be easily performed on a small computer. The apparatus allows α to be measured at various degrees of sample compression. Thermal conduc tivity can be obtained from α if heat capacity and bulk density are known. The tech nique was used to study the effect of fiber structural parameters on the conductivity of nonwovens. The results indicate that in agreement with established theory, con ductivity decreases if the nonwoven is compressed or is made with finer fabrics or has a reflective coating on the fiber surface. This occurs because of a reduction in the radiative component of heat transfer. Fiber shape has no effect on thermal conductivity. Nonwovens made with experimental hollow fibers have lower thermal conductivity than nonwovens containing solid fibers of the same size; no explanation is currently available for this phenomenon.

Effect of the percolation heat transfer component on the temperature field and thawing dynamics of soils

The effect of the convective heat transfer component on the temperature field and thawing front dynamics of soils is investigated for high fluid percolation velocities in the thawed zone. The steady state interchange and approximate self-similarity methods are used to obtain upper and lower bounds of the solution of the Stefan problem with a convective heat transfer component in a porous medium. From the results of the calculations conclusions are drawn concerning the accuracy and limits of applicability of the solutions obtained.

Trial Production and Examination of the Device Measuring Heat Transfer Components at the Sunlit Window with Shading

Trial Production and Examination of the Device Measuring Heat Transfer Components at the Sunlit Window with Shading

Heat flow and ground water movement in the Bohemian cretaceous basin (Czechoslovakia)

Sub-surface temperature fields may be considerably affected by active ground water systems, thereby seriously hampering the interpretation of heat flow data. Quantitative evaluation of the convective component of heat transfer is thus very important in cases such as large sedimentary basins with vast underground water circulation. We propose in this study a simple model of horizontal aquifer. This model was used to examine the effect of the lateral convection on the surface heat flow near the recharge zone of basinal margins. The perturbation of the heat flow field above the aquifer was calculated for various aquifer geometry and various flow velocities and the regional scale dependence of the perturbation on the hydraulic properties of the aquifer was demonstrated. The model was applied to the Bohemian Cretaceous Basin and it was shown that within a few kilometres from the recharge zones the observed surface heat flow may be underestimated by up to several tens of percent. The procedure was tested in two locations in this area, in an attempt to make hydrogeological corrections to the measured heat flow values in several boreholes.