Heat Transfer Considerations Research Articles

Particle circulation loops in solar energy capture and storage: Gas–solid flow and heat transfer considerations

A novel application of powders relies on their use as heat transfer medium for heat capture, conveying and storage. The use of powders as heat transfer fluid in concentrated solar systems is discussed with respect to current technologies. The specific application reported upon is the use of powder loops in Solar Power Tower plants. In the proposed receiver technology, SiC powder is conveyed as a dense particle suspension through a multi-tube solar receiver in a bubbling fluidization mode, the upwards flow being established by pressurizing the powder feed. Tests were conducted with a single-tube receiver unit at the 1MW solar furnace of CNRS (Odeillo Font-Romeu, F). The measured wall-to-suspension heat transfer coefficient is a function of operating temperature, applied air velocity and imposed solid circulation flux: values increased with increasing solids flux from ∼430 to 1120W/m2K. Empirical approaches and a heat transfer model were applied to compare experimental and predicted values of the heat transfer coefficient, with a fair agreement obtained. The research moreover provides initial data concerning the overall economy of the system. The high temperature of the circulating powder leads to an increased power cycle efficiency, an increased storage density, reduced thermal power requirements, reduced heliostat field size, reduced parasitic power consumption and increased plant capacity factor.

Analytical considerations of slip flow and heat transfer through microfoams in mini/microchannels with asymmetric wall heat fluxes

Forced convection in a mini/microchannel filled with microfoam is analytically studied in the condition of uniform but asymmetrical heat fluxes. Velocity slip, thermal slip, local thermal non-equilibrium (LTNE) effect, and asymmetric heat fluxes imposed on the two parallel-plates sandwiching the microfoams are especially considered. For two walls with different heat fluxes, solid and fluid temperatures at the boundary are equal to each other based on self-coupling conditions. Explicit expressions for velocity and temperatures of solid and fluid are derived and the analytical solution agrees well with existing references. This solution can predict velocity, temperatures, friction loss and thermal performance of porous microchannel for wide ranges of Knudsen number and HF ratio. With the increased Knudsen number, the friction factor gradually decreases and the decreasing amplitude in slip flow region is very large. With the increased shape factor, the dimensionless pressure drop sharply decreases for small shape factor, while its value gradually approaches 1 for large shape factor. A maximum Nusselt number exists with the increase in Knudsen number. Heat transfer can be enhanced by increasing effective Biot number or decreasing thermal conductivity ratio. The analytical solution is symmetrical on the axis ξ = 1 in the region 0 < ξ < + ∞.

Modeling of friction along the tool-chip interface in Ti6Al4V alloy cutting

The aim of the paper is the modeling of chip formation in metal cutting in order to describe the thermomechanical interactions at the tool-chip interface (TCI). A particular attention is paid for the fully sticking case to complete previous modeling works which were more focused on the sliding regime or mixed sliding/sticking regime. The fully sticking contact is dominating due to the combined effects of high magnitudes of the normal stress, the average friction coefficient \( \left(\overline{\upmu}\to 1\right) \) and the temperature. This case is also well adapted for cutting of titanium alloy. In the present model, these local parameters are macroscopically expressed through the average friction coefficient \( \overline{\upmu} \), and the velocity field on the secondary shear zone is modeled using a new approach. The ratio between real area of contact A r and the apparent area A n is taken into consideration. The developed approach is also fully thermomechanically coupled with heat transfer consideration. In this way, it was possible to predict normal and shear stress according to the variation of cutting velocities, feed and rake angle. The model was supported by the experimental trends from Ti6Al4V alloy cutting tests and worn tools analysis. It was shown that the distribution of the ratio A r /A n may be considered as a good indicator to describe the spreading of the adhesion marks on the contact. The model also highlights how the influence of the apparent friction coefficient, the rake angle, the feed and the cutting speed acts on the tool-chip contact length.

Steady state boiling crisis in a helium vertically heated natural circulation loop – Part 2: Friction pressure drop lessening

Experiments were conducted on a 2-m high two-phase helium natural circulation loop operating at 4.2K and 1atm. Two heated sections with different internal diameter (10 and 6mm) were tested. The power applied on the heated section wall was controlled in increasing and decreasing sequences, and temperature along the section, mass flow rate and pressure drop evolutions were registered. The post-CHF regime was studied watching simultaneously the evolution of boiling crisis onset along the test section and the evolution of pressure drop and mass flow rate. A significant lessening of friction was observed simultaneous to the development of the post-CHF regime, accompanied by a mass flow rate increase, which lets suppose that the vapor film in the film boiling regime acts as a lubricant. A model was created based on this idea and on heat transfer considerations. The predictions by this model are satisfactory for the low quality post-CHF regime.

A scale analysis model for film boiling heat transfer on a vertical flat plate with wide applicability

A model has been developed in present work, for a vertical flat plate under film boiling condition, based on scale analysis. The saturated and subcooled film boiling have been analyzed from heat and mass transfer consideration and appropriate vaporization criteria. The model takes care of natural convection and forced convection mode of heat transfer for subcooled case, while the saturated film boiling is treated with an assisting convection mode. The interfacial wave length is incorporated into the length scale and the effect of radiation heat transfer is accounted in the model. Expressions for Nusselt number have been developed for different situations. The present model recovers the Bromley scale for natural convection under saturated condition. The heat transfer characteristic is shown for relevant non-dimensional numbers pertaining to film boiling for various cases. A wide set of existing experimental data with ranges of wall superheat 260–1200°C, liquid subcooling 0–50°C and flow velocity 0–2.65m/s have been systematically utilized to arrive at the model closure.

Numerical study on the phase change heat transfer of LNG in glass wool based on the VOF method

In this study, the evaporation of a cryogenic gas in an insulating porous medium was investigated numerically and experimentally. The flow and heat transfer characteristics around the evaporating cryogenic gas in such a medium were analyzed based on the VOF (Volume of Fluid) method. The thermal leakage through the porous medium to the outer shell was then analyzed using heat transfer considerations. To support the numerical results, an experiment was then performed using LNG (Liquefied Natural Gas) as the fluid and glass wool as the porous medium. The numerical results for the outer shell temperature predict the experimental values well. Based on the validated numerical results, the transient behavior of temperature distribution in outer shell is analyzed. The effect of crack size on the reliability of insulation system is investigated. In addition, the behavior of the virtual evaporation velocity and the liquid film area was considered qualitatively. This work sheds light on technology related to cryogenic storage systems by suggesting a systematic method for predicting the temperature of the outer shell, the liquid film area, and virtual evaporation velocity.

Discrete unified gas kinetic scheme for all Knudsen number flows. II. Thermal compressible case

This paper is a continuation of our work on the development of multiscale numerical scheme from low-speed isothermal flow to compressible flows at high Mach numbers. In our earlier work [Z. L. Guo et al., Phys. Rev. E 88, 033305 (2013)], a discrete unified gas kinetic scheme (DUGKS) was developed for low-speed flows in which the Mach number is small so that the flow is nearly incompressible. In the current work, we extend the scheme to compressible flows with the inclusion of thermal effect and shock discontinuity based on the gas kinetic Shakhov model. This method is an explicit finite-volume scheme with the coupling of particle transport and collision in the flux evaluation at a cell interface. As a result, the time step of the method is not limited by the particle collision time. With the variation of the ratio between the time step and particle collision time, the scheme is an asymptotic preserving (AP) method, where both the Chapman-Enskog expansion for the Navier-Stokes solution in the continuum regime and the free transport mechanism in the rarefied limit can be precisely recovered with a second-order accuracy in both space and time. The DUGKS is an idealized multiscale method for all Knudsen number flow simulations. A number of numerical tests, including the shock structure problem, the Sod tube problem in a whole range of degree of rarefaction, and the two-dimensional Riemann problem in both continuum and rarefied regimes, are performed to validate the scheme. Comparisons with the results of direct simulation Monte Carlo (DSMC) and other benchmark data demonstrate that the DUGKS is a reliable and efficient method for multiscale flow problems.

A modified turbulent mixing model with the consideration of heat transfer between hot buoyant plume and sidewalls in a closed stairwell

Previous studies on the turbulent mixing process in closed shafts did not take into account the heat transfer from the hot buoyant plume to the boundaries such as walls. In this paper, a modified theoretical model predicting the one-dimensional turbulent mixing process in vertical shafts is proposed with the heat transfer from the hot buoyant plume to the boundaries being involved. A set of small scale experiments were conducted to validate this model. A propane gas burner was used as the heat source to provide steady heat release rate. The temperature rise in the stairwell exponentially decreases with the height. The comparison of the model predicted and experimental results show that the Cooper’s turbulent mixing model without the boundary effect gives higher predictions to the temperatures in the stairwell compared to the experimental data whereas the current modified model leads to more comparable results. Therefore the heat transfer process between the plume and the boundaries should be included in any modeling for the case of buoyant plume rising in closed shafts.

Solids Deposition from Wax–Solvent–Water “Waxy” Mixtures Using a Cold Finger Apparatus

The solids deposition from one-phase and two-phase waxy mixtures (comprising a multicomponent paraffinic wax dissolved in a multicomponent solvent, and water) was studied using a cold finger experimental apparatus. The deposition experiments were performed with a 10 mass % wax solution, containing 0, 10, 20, and 30 vol % water, with two different rates of agitation, and for nine different deposition times ranging from 30 s to 24 h. The water content of the deposit was found to be not related to the water content of the waxy mixture. The short-duration experiments showed the deposition process to be very fast, with more than half of the deposition process completed in 30 s. Following a rapid rate of deposition initially, the deposit mass was observed to increase slowly to reach steady state at about 12 h. The deposit mass decreased with an increase in the agitation speed. The deposition data were modeled satisfactorily with a steady-state heat-transfer model and an unsteady-state model based on the moving boundary problem formulation. The results confirmed the liquid–deposit interface temperature to be equal to the wax appearance temperature (WAT) of the wax solution throughout the deposition process, i.e., for all deposition times. The results of this study provide further confirmation that the solids deposition process can be described adequately with an approach based solely on heat-transfer considerations.

Numerical Study and Structural Optimization of a Top Combustion Hot Blast Stove

The hot blast stove is one of the most important equipment devices in the blast furnace iron making process. The temperature and duration of hot air are the crucial parameters to assess the performance of the hot blast stove. In order to sustain the desired high temperature air, it requires rapid completed combustion reaction, stable flue gas flow structure, and uniform temperature distribution throughout the regenerator. In the present work, a 3D numerical model with all essential turbulence, heat transfer, and combustion considerations has been developed to assess the performance of a typical hot air stove. The flow field of the whole domain and temperature distribution within the regenerator were simulated using the model. The predicted results show that the velocity at each nozzle varies substantially due to the uneven pressure distribution in cavity of the traditional hot blast stove, generating the eccentric vortex that leads to nonuniform temperature distribution in the regenerator. In order to solve this problem, a new structure design of top combustion regenerative hot blast stove is proposed. Numerical simulations were then carried out to compare based on the performance of the new hot blast stove design against the traditional hot blast stove.

Moisture Buffering Effect of Gypsum Board in a Marine Climate: A Field Experimental Study

Durability, acceptable indoor air quality, energy efficiency and aesthetics are all pillars of good design in healthy buildings. A new approach for optimizing all four of these pillars is whole-building performance design. This approach involves the consideration of heat, air and moisture (HAM) transfer and control of a building, specifically, how the coupled relations between different transient systems (mechanical system, building envelope, indoor environment, outdoor environment, and occupants) affect the building performance and operation. Ventilation is one of the means of controlling indoor humidity in buildings. Its effectiveness depends on the supply air moisture level and the ventilation rate. The drier the supply air is, the higher its capacity to remove indoor humidity. In a marine climate where the outdoor air is relatively moist, higher ventilation rate is required to achieve the same level of indoor humidity in a cold and dry climate. In this study, the potential benefit of interior gypsum finishing in lowering indoor humidity peaks, through the moisture buffering process, and thereby reducing ventilation rates are investigated. A field experimental study is conducted using two identical test facilities at the Whole Building Performance Research Laboratory in Burnaby, British Columbia to test this hypothesis in a marine climate. Initial benchmarking of the recently commissioned test buildings was undertaken to ensure they behaved similarly under identical conditions. Each building was outfitted with an occupant simulator unit, which provided the humidification that would be produced by occupants. The occupants simulators were programmed based on moisture production data analysis from a real high-occupancy apartment suite, to provide two different moisture generation profiles representing typical and high intensities, scaled down to the size of the test buildings. Following benchmarking, three tests were conducted to evaluate the effect of ventilation rate, moisture generation intensity, and moisture buffering ability of finishing surfaces on indoor moisture levels. Preliminary experimental test results are presented. Future tests will be undertaken to consider other factors such as indoor air quality based on carbon dioxide concentration, heating and ventilation energy consumption, and alternative finishing materials.

Multi-scale heat transfer in fluidized bed reactors by Eulerian CFD modeling

In the previous work, the multi-scale chemical reaction model was developed by Wang et al. (2014) to consider the impact of the meso-scale structure on reaction performance. This paper incorporates multi-scale heat transfer to further improve the previous multi-scale chemical reaction model. The temperature field in the cluster phase and the dispersed phase is predicted. A cluster structure-dependent (CSD) drag model is used to obtain local structural parameters. By comparison with experimental data, there is a slight improvement on predicted results with consideration of the multi-scale heat transfer. Meanwhile, the inter-phase heat transfer between the cluster phase and the dispersed phase is evaluated. The results indicate that the effect of the inter-phase heat transfer on the temperature field is less significant than the effect of the inter-phase mass transfer on gas components.

Understanding Slag Freeze Linings

Slag freeze linings, the formation of protective deposit layers on the inner walls of furnaces and reactors, are increasingly used in industrial pyrometallurgical processes to ensure that furnace integrity is maintained in these aggressive, high-temperature environments. Most previous studies of freeze-linings have analyzed the formation of slag deposits based solely on heat transfer considerations. These thermal models have assumed that the interface between the stationary frozen layer and the agitated molten bath at steady-state deposit thickness consists of the primary phase, which stays in contact with the bulk liquid at the liquidus temperature. Recent experimental studies, however, have clearly demonstrated that the temperature of the deposit/liquid bath interface can be lower than the liquidus temperature of the bulk liquid. A conceptual framework has been proposed to explain the observations and the factors influencing the microstructure and the temperature of the interface at steady-state conditions. The observations are consistent with a dynamic steady state that is a balance between (I) the rate of nucleation and growth of solids on detached crystals in a subliquidus layer as this fluid material moves toward the stagnant deposit interface and (II) the dissolution of these detached crystals as they are transported away from the interface by turbulent eddies. It is argued that the assumption that the interface temperature is the liquidus of the bulk material represents only a limiting condition, and that the interface temperature can be between T liquidus and T solidus depending on the process conditions and bath chemistry. These findings have implications for the modeling approach and boundary conditions required to accurately describe these systems. They also indicate the opportunity to integrate considerations of heat and mass flows with the selection of melt chemistries in the design of future high temperature industrial reactors.

Thermal performance analysis of porous medium solar receiver with quartz window to minimize heat flux gradient

Exposure under concentrated solar radiation increases the temperature of volumetric receiver which can cause high thermal stress and damage the receiver. The Plano-convex quartz window is introduced with the aim to minimize heat flux gradient of porous medium receiver. Thermal performance of porous medium receiver with quartz window is numerically studied while the fluid inlet is located at the side wall which would be more practicable. The Monte Carlo ray tracing (MCRT) method is used to calculate the radiative heat transfer in the solar collector system with quartz window, and the local thermal non-equilibrium (LTNE) model with the consideration of radiative heat transfer in the porous medium receiver is used to calculate the fluid phase and solid phase temperature distribution of the porous medium receiver. The numerical results indicated that the pressure distribution and temperature distribution for the condition of fluid inlet located at the side wall is different from that for the condition of fluid inlet located at the front surface.

Universality of tip singularity formation in freezing water drops.

A drop of water deposited on a cold plate freezes into an ice drop with a pointy tip. While this phenomenon clearly finds its origin in the expansion of water upon freezing, a quantitative description of the tip singularity has remained elusive. Here we demonstrate how the geometry of the freezing front, determined by heat transfer considerations, is crucial for the tip formation. We perform systematic measurements of the angles of the conical tip, and reveal the dynamics of the solidification front in a Hele-Shaw geometry. It is found that the cone angle is independent of substrate temperature and wetting angle, suggesting a universal, self-similar mechanism that does not depend on the rate of solidification. We propose a model for the freezing front and derive resulting tip angles analytically, in good agreement with the experiments.

Numerical investigation on practicability of reducing MCST by using grid spacer in a tight rod bundle

The numerical investigation was carried out to reveal the practicability of reducing the maximum cladding surface temperature (MCST) within the inner sub-channel of a tight, hexagon rod bundle using commercial CFD code STAR CCM+ 6.04. The special heat transfer and pressure drop characteristics caused by four existing grid spacer designs were discussed in detail by analyzing the effects of grid strap length, different flow enhancing features and different Reynolds numbers. It was found that the local heat transfer within the grid strap is greatly enhanced due to the raised flow velocity. Both the standard grid spacer and the grid spacer with split-vanes cause decreased heat transfer in the downstream region. The friction drag is very influential in the tight rod bundle and can eliminate the positive effect of flow blockage on the heat transfer performance. The grid spacer with flow blockage discs induces relatively good heat transfer performance and higher pressure drop within sub-channels, indicating a tradeoff between the heat transfer augmentation and the pressure drop. The combination of multiple existing grid spacers can reduce the MCST to a certain level, but the corresponding disadvantages cannot be ignored. The improved grid spacer design was proposed based on the overall considerations of heat transfer and pressure drop characteristics and has been proved more suitable to widely reduce MCST for SCWR than any other grid spacer designs involved in present study.

The effect of moisture transportation on energy efficiency and IAQ in residential buildings

Abstract The building envelope is normally subject to thermal and moisture gradients in practice. Due to the interrelationship, thermal and moisture transfer should be simultaneously calculated for an accurate building performance evaluation. The aim of this study is to investigate the effect of moisture transportation on energy efficiency, thermal comfort and mould growth risks in residential buildings based on the hygrothermal simulation. First, hygrothermal simulation is conducted to evaluate the energy efficiency and IAQ in the selected residential building. This paper also provides characteristics of the hygrothermal properties of domestic building materials which are used for the selected residential building. Differences between experimental data for the domestic materials and properties from embedded data in the hygrothermal simulation program are discussed. Second, a hygrothermal calculation model is calibrated with the measurement data for indoor air temperature and relative humidity in the selected residential building. Lastly, we illustrate the effect of moisture transfer on the overall building performance by comparing the hygrothermal simulation results with thermal-only simulation (only consideration of heat transfer through the building envelope) results. The results suggest that heating and cooling energy can be underestimated without consideration of moisture transportation mechanisms in the building energy simulation. Moreover, results show that the moisture buffering effect could significantly reduce the amplitude of relative humidity fluctuations during all seasons.

Effects of Magnetic Field on the MHD Flow of a Third Grade Fluid through Inclined Channel with Ohmic Heating

In this paper, we investigate the combine effects of magnetic field on the MHD flow of a third grade fluid through inclined channel in the presence of a uniform magnetic field with the consideration of heat transfer. Three different problems, Couette flow, Poiseuille flow and Couette-Poiseuille flow have been analysed. The non-linear differential equations governing the flow and heat transfer are solved for the velocity and temperature profile by employing the regular perturbation technique and the results are presented graphically.

Heat Transfer Considerations in a Tubular Carbon Fuel Cell

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Modeling the burnout of solid polydisperse fuel under the conditions of external heat transfer

A self-similar burnout mode of solid polydisperse fuel is considered taking into consideration heat transfer between fuel particles, gases, and combustion chamber walls. A polydisperse composition of fuel is taken into account by introducing particle distribution functions by radiuses obtained for the kinetic and diffusion combustion modes. Equations for calculating the temperatures of particles and gases are presented, which are written for particles average with respect to their distribution functions by radiuses taking into account the fuel burnout ratio. The proposed equations take into consideration the influence of fuel composition, air excess factor, and gas recirculation ratio. Calculated graphs depicting the variation of particle and gas temperatures, and the fuel burnout ratio are presented for an anthracite-fired boiler.