Transient Heat Transfer Behavior Research Articles

Heat transfer of gas–solid two-phase mixtures flowing through a packed bed

This paper reports an experimental study on both transient and steady-state heat transfer behavior of a gas–solid two-phase mixture flowing through a packed bed under constant wall temperature conditions. A logarithmic mean temperature difference (LMTD) method is used to process the temperature data to obtain the overall heat transfer coefficient. The influences of particle loading and gas flow Reynolds number are investigated. The results show that the introduction of suspended particles greatly enhances heat transfer between the flowing gas–solid two-phase mixture and the packed bed, and the enhancement increases approximately linearly with increasing particle loading. The heat transfer coefficient data are processed to give the Nusselt number, which is found to correlate well to the Reynolds number, the Archimedes number and the suspended particle loading ratio. A comparison of the data of this work with the published data reveals large discrepancy. Possible reasons for the discrepancy are discussed.

Transient Analysis of Heat and Moisture Transfer with Sorption/Desorption and Phase Change in Fibrous Clothing Insulation

This article reports on a numerical investigation of the transient heat and moisture transfer behavior in clothing insulation made of fibrous battings and covering fabrics. The finite-volume method (FVM) is applied in the mathematical formation and numerical calculation. The method provides more accurate representation of the element properties and their grid locations. Combined with the upstream scheme in dealing with the moisture bulk flow and considering the effect of condensate on the permeability of the fibrous batting, the present model improves the accuracy of the numerical solution. The calculated results of water content and temperature distributions in two kinds of clothing assemblies show improved agreement with the experimental ones. Based on the new method, the effects of various clothing parameters on the condensation and moisture absorption within clothing are investigated numerically and discussed.

Heat Transfer Behavior for a Stationary or Rotating MCM Disk With an Unconfined Round Jet Impingement

A series of experimental investigations on the studies related to fluid flow and transient mixed convection from a horizontally unconfined stationary or rotating ceramic-based multichip module (MCM) disk with unconfined jet impingement have been successfully conducted. The fluid flow and heat transfer behavior explored includes the streamwise velocity and turbulence intensity distributions, transient dimensionless temperature distribution on the MCM disk surface, transient heat flux distribution of input power, and transient chip and average heat transfer characteristics on the MCM disk surface. Besides, two new correlations of transient stagnation and average Nusselt numbers in terms of jet Reynolds number, ratio of jet separation distance to nozzle diameter and time elapsed during the transient period, are presented for the cases of stationary MCM disk. For the cases of rotating MCM disk, a complete composite correlation of steady-state average Nusselt number for mixed convection due to jet impingement, disk rotation and buoyancy is proposed. As compared with the steady-state results, if the transient chip and average heat transfer behaviors may be considered as a superposition of a series of quasisteady states, the transient chip and average Nusselt numbers in all the present transient experiments can be properly predicted by the existing steady-state correlations when t>6min in the power-on transient period.

A numerical and experimental investigation of three-dimensional ground heat transfer

A three-dimensional numerical simulation of transient heat transfer behaviour measured during a full-scale experiment is presented. The experimental data, provided by others, was measured at a purpose built, full-scale test-house, over a 5-year period. The paper addresses some of the practical problems associated with undertaking multi-dimensional simulations, even when the problem is reasonably well defined. Comparisons are made between numerical results and measured data over an annual cycle. Good correlation of results has been achieved. Phase lag of ground temperature variations was also accurately represented in the simulation. The need to pre-condition the simulation to arrive at realistic initial conditions is shown to be necessary in this class of problem. Practical application: At the design stage a predictive assessment of building energy consumption and dissipation is clearly of value. However, the accuracy of any simulation fundamentally depends on the precision with which relevant factors are included. Above-ground insulation of buildings has steadily improved and as a consequence thermal losses due to earth-contact have become proportionally more important. Within this context, this paper aims to provide a case study for validation of models against full-scale field-measured data. The work will also contribute to innovative design that may utilize the thermal mass of foundation materials to achieve the required thermal performance.

Flow instability and transient flow patterns inside intercrossed silicon microchannel array in a micro-timescale

The objective of this study is to visualize the transient flow patterns and heat transfer behaviors at low mass fluxes and high heat fluxes. The silicon chip consists of the intercrossed microchannel array with 10 triangular microchannels with the hydraulic diameter of 155.4 μm, and five transverse trapezoid microchannels, separating the triangular microchannels into six independent zones. The chip is horizontally positioned. Liquid acetone is used as the working fluid. Tests were performed in the range of mass flux 40–80 kg/m 2 s and heat flux 107–216 kW/m 2. It is found that all the microchannels repeat the flow patterns in the timescale of milliseconds, with three substages: liquid refilling stage, transient stratified flow stage and partial/fully dry-out stage. Stratified flow is the dominant flow due to the low liquid Froude numbers. The axial liquid film thickness for each separated microchannel zone is increased along the flow direction, caused by the large momentum force due to evaporation on the interface of the vapor and the settled liquid film, and the high vapor shear stress applied on the vapor/liquid interface. The non-uniform axial liquid film distributions cause the earlier dry-out of the triangular microchannel upstream. However, dry-out always takes place earlier in the downstream zone than that in the upstream zone. The vapor slug near the triangular microchannel exit may be entrained in the long liquid plug during the liquid refilling stage. Besides, the steep jumped liquid film thickness may take place near the channel exit for the stratified flow. Both indicate the strong geometry effects. Due to the compact size and high thermal conductivity of the silicon chip, the chip surface temperature variations versus time are identified by the infrared radiator image system, especially in the ending area of the thin film heater.

Heat transfer of gas flow through a packed bed

This paper reports an experimental study of both the transient and steady-state heat transfer behaviour of a gas flowing through a packed bed under the constant wall temperature conditions. Effective thermal conductivities and convective heat transfer coefficient are derived based on the steady-state measurements and the two-dimensional axial dispersion plug flow (2DADPF) model. The results reveal a large temperature drop at the wall region and the temperature drop depends on the axial distance from the inlet. The 2DADPF model predicts the axial temperature distribution fairly well, but the prediction is poor for the radial temperature distribution. Length-dependent behaviour of the effective heat transfer parameters and non-uniform flow behaviour are proposed to be responsible. A comparison with previously published correlations and data in the literature shows that the relationships proposed by Bunnell et al. and Demirel et al. agree well with the measured effective radial thermal conductivity, whereas the wall–fluid heat transfer coefficient is better represented by the Li–Finlayson correlation.

Transient Radiative Heat Transfer from a Plasma Produced by a Capillary Discharge

The objective of this work is to develop a better understanding of the transient behavior of radiative heat transfer from a plasma. The plasma generation occurred within a 3.2-mm-diam and 26-mm-long polyethylene capillary. Because of its high temperature and high pressure, the plasma evolved from the capillary into an ambient air environment as an underexpanded supersonic jet that interacted with a stagnation plate. Various diagnostic techniques were used. They include heat flux and pressure gauges mounted on the stagnation plate, heat flux gauges and silicon photodiodes mounted below the plasma jet, as well as current transducers interfaced with the electrical circuit. The heat flux gauges were manufactured via sputtering and calibrated using a standard convection oven. A fused-silica window, placed about 1 mm above the gauges, ensured that only the radiative heat flux transmitted by the window was deduced. The row of heat flux gauges mounted below the plasma provided an assessment of the fraction of the radiative heat flux transmitted by the fused-silica window. The results show that the peak of emitted radiant flux occurs immediately after the peak of the discharge of electrical energy, which usually occurred a relatively long time prior to arrival of the precursor shock on the stagnation plate.

Cooling Performance of Using a Confined Slot Jet Impinging onto Heated Heat Sinks

A series of experimental investigations on the studies related to transient- and steady-state cooling performance from the horizontally heated heat sinks with a confined slot jet impingement have been conducted. The relevant parameters influencing the transient convective cooling performance include the steady-state Grashof number, ratio of jet separation distance to nozzle width, ratio of heat sink height to nozzle width, and jet Reynolds number. The transient heat transfer behaviors such as the temperature distribution, local and average Nusselt numbers on the heated heat sinks have been systematically explored. Two empirical correlations of steady-state local and average Nusselt numbers are presented. Furthermore, a complete composite correlation of steady-state average Nusselt number for mixed convection due to jet impingement and buoyancy is proposed. This empirical correlation obtained by data regression is in good agreement with the experimental data. The maximum and average regression errors are 7.46% and 2.87%, respectively.

Transient Convective Heat Transfer of Air Jet Impinging Onto a Confined Ceramic-Based MCM Disk

Transient heat transfer behavior from a horizontally confined ceramic-based MCM disk with jet impingement has been systematically explored. The relevant parameters influencing heat transfer performance are the steady-state Grashof number, jet Reynolds number, and ratio of jet separation distance to nozzle diameter. In addition, an effective time, ton, representing a certain transient time when the mixed convection effect due to jet impingement and buoyancy becomes significant relative to heat conduction, is introduced. Both the transient chip and average Nusselt numbers on the MCM disk surface decrease with time in a very beginning period of 0⩽t<ton, whereas it gradually increases or keeps constant with time and finally approaches the steady-state value in the period of ton⩽t<ts. As compared with the steady-state results, if the transient chip and average heat transfer behaviors may be considered as a superposition of a series of quasi-steady states, the transient chip and average Nusselt numbers in all the present transient experiments can be properly predicted by the existing steady-state correlations when t⩾4min in the power-on transient period.

Transient heat transfer behavior of one-dimensional symmetric thermoelectric SMA actuators

The main purpose of this paper is to study the transient heat transfer behavior of one-dimensional symmetric thermoelectric Shape Memory Alloy (SMA) actuators, without assuming a small ratio between the layer thicknesses of SMA and semiconductor. For the transient heat transfer with a constant current density, it is proved that there is an upper bound for the current density so that the temperature distribution along the SMA layer is stable. For the transient cooling problem with a constant current density, it is proved that there is a lower bound for the current density so that the temperature at the interface between SMA and semiconductor layers is always decreasing to its stable state, and it may not be always decreasing if the current density is below the lower bound. Estimates for these bounds are derived. The physical implications of main results are also discussed.

TRANSIENT NATURAL CONVECTION OF WATER NEAR ITS DENSITY EXTREMUM IN A RECTANGULAR CAVITY FILLED WITH POROUS MEDIUM

A numerical study is performed to analyze the transient flow field and heat transfer behavior of cold water in a rectangular enclosure filed with a porous medium. The numerical method was employed by the SIMPLE-C algorithm. The accurate nonlinear relation between density and temperature developed by Gebhart and Mollendorf was used. The numerical results show that, for R l, only one counterclockwise vortex exists in the space the entire time. For τ > 10−3, the heat transfer rate increases with increase in Darcy number for the entire time. However, as time increases, the hea...

The transient thermal response of a glass-fiber insulation slab with hygroscopic effects

The Brunauer-Emmett-Teller (BET) equation, representing adsorption isotherms, is used in a one-dimensional, transient vapor-diffusion model for heat and moisture transport in a typical, mediumdry density glass-fiber insulation slab to account for the hygroscopic effects (water vapor adsorption, desorption, and capillary condensation). The correction to the latent enthalpy of phase change, used in the energy equation, is derived from empirical desorption isotherms. The results obtained, for two types of boundary conditions (a closed system and an open system), show that the effects of hygroscopicity on the transient temperature distribution are significant for a slab with one boundary open to moist air. The sensitivity of hygroscopicity on the transient heat and mass transfer behavior can be depicted by a transition Fourier number, which is proportional to the adsorption capacity of the glass-fiber slab and increases as the cold side temperature and ambient relative humidity increase. Given the uncertainty in the adsorption properties of fiber-glass and experimental data, the agreement between the model and measured data is reasonable.

Design criteria for stability in cable-in-conduit conductors

Stability is a key point in the design of large conductors for application in fusion machines such as NET or ITER. In the electromagnetically noisy environment characterizing the surroundings of a plasma, the superconductor should be stable against all expected heat deposition, including a.c. losses driven by plasma disruptions. Since all sources of thermal perturbations cannot be adequately identified, predicted or calculated, the design must allow for suitable margins. To ensure this, stability must be considered as a design criterion which is checked by means of analytical and numerical methods and later calibrated against experimental measurements of transient heat transfer and stability behaviour. In this the procedure developed at NET is described for the design and analysis of the stability margin of conductors for the toroidal and poloidal field coils. These methods are applied to the generation of stability curves and maps which are used to set operating limits for the 40 kA NET conductor. The design criteria and analytical methods can be appropriately modified for other conductors and coil applications.

Transient convection in saturated porous layers with internal heat sources

The present investigation is focused on the two-dimensional, transient behavior of convective heat transfer in porous, rectangular ducts saturated with a fluid and in which there is uniform internal heat generation. In earlier works the steady-state multiplicity features of this flow have been studied. In the present work the evolutionary path to such steady states is examined. In several cases, a sustained oscillatory behavior has been observed. The solution structure is governed by two parameters, namely the aspect ratio of the duct, γ = b/ a and the Rayleigh number, R = KβgaA′ Q g αvk. For a duct with an aspect ratio of unity, a complicated solution structure is observed upon increasing the dynamical parameter. A steady, symmetric two-cell pattern observed for R of up to 4400 gives way to a periodic regime for R of up to 5400, then to a chaotic regime over a narrow range of R and a return to a steady-state solution at R = 5800. Upon increasing γ to 8, several multiple steady-state solutions are observed. The transition to oscillatory convection occurs at an earlier value of R with increasing y. None of the oscillatory solutions are symmetric about the center line.

Basic researches on thermo-hydraulic non-equilibrium phenomena related to nuclear reactor safety.

A review was made of recent developments of fundamental researches on thermo-hydraulic non-equilibrium phenomena related to light water reactor safety, in relation to problems to be solved for the improvement of safety analysis codes. As for the problems related to flow conditions, fundamental researches on basic conservation equations and constitutive equations for transient two-phase flow were reviewed. Regarding to the problems related to thermal non- equilibrium phenomena, fundamental researches on film boiling in pool and forced convection, transient boiling heat transfer and flow behavior caused by pressure transients were reviewed.

Transient behavior of heat removal from a cylindrical heat storage vessel packed with spherical porous particles

The transient heat transfer behavior in the case of heat removal from a cylindrical heat storage vessel packed with spherical particles was investigated experimentally for various factors (flow rate, diameter of spherical particles packed, temperature difference between flowing cold air and spherical particles accumulating heat, and physical properties of spherical particles). The experiments were covered in ranges of Reynolds number based on the mean diameter of spherical particles packed Red= 10.3–2200, porosityɛ=0.310 to 0.475, ratio of spherical particle diameter to cylinder diameterd/D = 0.0075–0.177 and ratio of length of the cylinder to cylinder diameterL/D=2.5–10. It was found that especially the flow rate and the dimension of spherical particles played an important role in estimating the transient local heat transfer characteristics near the wall of the cylindrical vessel in the present heat storage system. As flow rate and diameter of spherical particles were increased under a given diameter of the cylinder heat storage vessel, the mean heat transfer coefficient between the flow cold air and the hot spherical particles increased and the time period to finish removing heat from the vessel reduced. In addition, the useful experimental correlation equations of mean heat transfer coefficient between both phases and the time period to finish removing heat from the vessel were derived with the functional relationship of Nusselt numberNud=f [modified Prandtl numberPr* (d/D), Red) and Fourier numberFo = f(d/D, L/D, Pr*, Red).

Transient heat transfer behaviors in cylindrical porous beds at relatively large Reynolds numbers

The transient behavior of heat transfer in a cylindrical porous bed was examined experimentally under various factors (flow rate, diameter of spherical solid particle, temperature of flowing fluid and physical properties of porous bed). In these factors, it was understood that especially flow rate and the diameter of the particled have important role in evaluating the transient behavior of heat transfer in the porous bed. That is, as the flow rate and the diameter of the particle under a constant diameterD of the cylindrical bed are increased, mean heat transfer coefficient between flowing fluid and the solid particles is increased and the time period to reach a thermally steady state is decreased. The useful experimental correlation equations of mean heat transfer coefficient and the time period to reach the steady state were derived with the functional relationships of Nusselt numberNud=f(d/D, Reynolds numberRed) and Fourier numberFo =f (modified Prandtl numberPr*, d/D, Red).

Transient heat transfer and recovery behavior of superconductors

Transient heat transfer from bare and coated copper surfaces to saturated liquid helium was measured with a time constant of ∼ 5 μsec. In another experiment recovery of a superconducting wire after a temperature excursion was studied. The results show a large difference between bare and coated surfaces, latter recover at higher surface heat fluxes. While the recovery time of the coated surface is a smooth function of the temperature excursion, that of the bare surface exhibits a discontinuity, being higher for a small disturbance.