Isothermal Enclosure Research Articles

Roles and impacts of heat source/ sink and magnetic field on non-Darcy three-component Marangoni convection in a two-layer structure

In this study, the non-Darcy Three-Component Marangoni (NDTCM) convection issue is investigated in closed form using a non-Darcy model for the porous layer with constant heat source/ sink (HSS) and uniform vertical magnetic field in a two-layer system with a porous layer under a fluid layer. This two-layer construction has a rigid and adiabatic lower enclosure for the porous layer and a free adiabatic/ isothermal upper enclosure for the liquid layer. The thermal Marangoni numbers (TMNs) for lower rigid and upper free boundaries with surface tension, depending on both temperature and concentrations, are determined in closed form for two cases of temperature boundary conditions (TBCs), Case (i) Adiabatic–Adiabatic and Case (ii) Adiabatic–Isothermal. The ordinary differential equations are solved by an exact method of solution to attain an analytical expression for the Marangoni number. The impacts of applicable factors are discussed elaborately versus thermal ratio and shown graphically using MATHEMATICA. It is noticed that case (i) TBC is stable as the eigenvalue obtained is higher than that for case (ii) TBC for the fluid layer dominant (FLD) two-layer systems.

Molecular layer deposition of Nylon 2,6 polyamide polymer on flat and particle substrates in an isothermal enclosure containing a rotary reactor

Polyamide thin films, designated Nylon 2,6, were grown on flat and particle substrates using molecular layer deposition (MLD) in a custom-built isothermal enclosure containing a rotary reactor. The polyamide films were grown using sequential exposures of ethylene diamine and adipoyl chloride. The reactor and precursors were contained in a fiberglass oven to keep all reactor components at the same temperature. A growth rate of 4.0 Å/cycle at 67 °C was determined on flat substrates with ex situ x-ray reflectivity and spectroscopic ellipsometry. The temperature dependence of the Nylon 2,6 displayed a peak growth rate at 67 °C with decreasing growth rates above and below this temperature. X-ray photoelectron spectroscopy of the polyamide film on flat substrates also revealed an elemental composition consistent with the Nylon 2,6 polymer with a small amount of chlorine in the film. The isothermal reactor allowed MLD to be performed consistently on high surface area particles at low temperatures. Transmission electron microscopy (TEM) images showed growth of the Nylon 2,6 films on ZrO2, cellulose, and metformin particles that was consistent with the growth on witness wafers. The growth of the Nylon 2,6 films was also linear versus the number of MLD cycles. The TEM images displayed reproducible MLD growth on particles of varying size and composition. Fourier transform infrared spectroscopy and energy dispersive spectroscopy were consistent with the expected characteristics of the Nylon 2,6 polyamide film. Nylon 2,6 MLD should find application when low-temperature MLD is needed to coat thermally sensitive substrates such as organic films or pharmaceutical powders.

Enhancement of phase change materials melting performance in a rectangular enclosure under different inclination angle of fins

In the present investigation Melting rate enhancement of phase change materials in three sides adiabatic and one side isothermal rectangular enclosure by using inclinations of fins has been conducted. To enhance the melting rate performance of phase change materials inside a rectangular enclosure, the number of fins has been optimized under constant length and width of the enclosure and fixed fin thickness of 4 mm. To keep a constant mass of phase change material inside the enclosure length of fins was decreased as the fin number increased in the enclosure. The walled enclosure is maintained at a constant temperature of 333.5 K from the bottom side. Aluminum fins were selected for these numerical investigations. The optimum number of fins was found to be 2 fins for this scenario. Subsequently, investigations on an optimum number of fins in this scenario melting rate enhancement by three cases of fin inclination (60θ, 45θ, and 35θ inclination angle of the fin) have been simulated. Optimization of fins number density can be achieved from the present observation, since melting time effect of fins density was not showing only increasing function. Comparing to pure melting phase change material, the optimized fin number (2 fins) of this scenario can reduce the melting time by 43%. Additionally, 31% and 5.12% melting time decrease can be obtained by setting optimized fin of this scenario (2 fins) case in 45θ and 60θ inclinations of fins, respectively. Finally, the results of the melting time ratio were compared with those obtained through the use of fin inclination cases with no fin case. Approximately melting time of 45θ inclination of a fin is the 1800s shorter than that of 90θ fin inclination cases of the optimum fin number. These results show that, if fins can be optimized in number and setting inclination in a specific operative design the efficiency of the system can be augmented with a desirable reduction of the melting time of phase change materials.

Calculation method for the overall heat-transfer coefficient of a technical transport superstructure for perishables in traffic

High quality and safe transportation of perishable food cargo (PFC) by railway transport is impossible without knowledge of the heat engineering parameters of the technical transport (TT) used for transportation. Due to constant increase of requirements for PFC quality safety during transportation, the task of regular inspection and survey of TT is imperative. During transportation of (PFC by technical transport (TT) in the “thermos” mode, their quality is maintained for a limited period, that directly depends on the value of the overall heat transfer coefficient (coefficient K) of the TT superstructure. Concurrently, the use of coefficient K calculated based on the average values measured during the heat engineering tests (electric power, air temperature, average area of the TT superstructure) without considering significant fluctuations like conducting the tests without putting the TT into the specialized isothermal enclosure, enhances the risk of deterioration of the transported cargo quality transported during such transport.The proposed technique allows companies conducting heat engineering tests to calculate the coefficient K value measured using the method of internal heating without using a specialized isothermal enclosure. This is achieved by adding an expanded uncertainty on the measurement of coefficient K in its final value. The method was tested from 2016–2018 in the control process of the heat engineering parameters of the “thermos” cars. The results confirmed that all significant components of measurement uncertainties are considered in calculating the final value of coefficient K.Application of the proposed technique ensures safeguarding the quality and safety of transported goods during development of the PFC transportation conditions in the “thermos” mode connected with calculation of the cargo transportation time-limit.

Binary gas diffusivity estimates from transient, one-dimensional sublimation–diffusion experiments in a spherical enclosure

ABSTRACTBinary gas diffusivities DAB’s are extremely useful in the analysis/design of mass transfer systems and to develop correlations. This study used an unsteady experimental method to determine DAB’s in gas pairs starting with a sublimating solid (A) such as naphthalene or camphor and air (B). The cumulative fractional mass transferred from the surface of a solid A sphere placed concentrically within an isothermal spherical enclosure was followed gravimetrically with time. The experimental DAB,exp for the gas pair was determined by nonlinear regression using the solution to a transient, one-dimensional (radial) diffusion model. The model’s Case 1 option assumed impermeability (no flux of gas A) at the enclosure’s outer surface, while Case 2 assumed zero concentration of gas A at the same location. For naphthalene–air, DAB,exp overestimated the literature values, the errors ranging from −110 to −185% for Case 1 and −21 to −65% for Case 2. For camphor–air, the error in DAB,exp was −36% for Case 1 and −16% for Case 2. DAB,exp for camphor in atmospheric air is herein reported for the first time. Potential improvements to the experiments include automation of the sphere melt-casting process and tighter control of the enclosure’s environmental conditions. Likewise, the theoretical model could be extended to three dimensions with multicomponent diffusion to assess the effect of air humidity on the transport of gas A. This is the first attempt to determine DAB,exp for naphthalene–air and camphor–air from an unsteady sublimation–diffusion experiment and to model the results using rigorous mass transport theory.

Validated Simulations of Heat Transfer From a Vertical Heated-Rod Array to a Helium-Filled Isothermal Enclosure

Measurements of heat transfer from an array of vertical heater rods to the walls of a square, helium-filled enclosure are performed for a range of enclosure temperatures, helium pressures, and rod heat generation rates. This configuration is relevant to a used nuclear fuel assembly within a dry storage canister. The measurements are used to assess the accuracy of computational fluid dynamics (CFD)/radiation simulations in the same configuration. The simulations employ the measured enclosure temperatures as boundary conditions and predict the temperature difference between the rods and enclosure. These temperature differences are as large as 72 °C for some experiments. The measured temperature of rods near the periphery of the array is sensitive to small, uncontrolled variations in their location. As a result, those temperatures are not as useful for validating the simulations as measurements from rods near the array center. The simulated rod temperatures exhibit random differences from the measurements that are as large as 5.7 °C, but the systematic (average) error is 1 °C or less. The random difference between the simulated and measured maximum array temperature is 2.1 °C, which is less than 3% of the maximum rod-to-wall temperature difference.

Negatively Buoyant Plume Flow in a Baffled Heat Exchanger

A numerical simulation of transient two-dimensional negatively buoyant flow into a straight baffle situated below an isothermal circular cylinder in an initially isothermal enclosure is presented for both an adiabatic and a highly conducting baffle for Rayleigh numbers from 106 to 107. Results show the effects of baffle offset, width, and length on the point where viscous flow develops and on velocity profiles within the baffle. Results are interpreted to guide the design of straight baffles to reduce destruction of stratification in thermal stores using an immersed heat exchanger. The preferred geometry is a low-conductivity baffle of width equal to the effective width of the heat exchanger and 15 or more cylinder diameters in length to ensure nearly fully developed flow at the baffle outlet.

Analysis of the Comparison of TPW Realizations in Europe in Light of CCT Recommendation 2 (CI-2005)

Three comparisons of different triple-point-of-water (TPW) realizations in Europe have been organized under the auspices of EUROMET (EUROMET Projects 278, 549, and 714). Thirty European national metrology institutes were involved in these three comparisons that took place from 1994 to 2005. The aim of these successive projects was to assess the uncertainties associated with the practical realization of the triple point of water in Europe. Fifty-four TPW local cells were compared to a traveling standard cell (ref 679) circulated with an isothermal enclosure. The same equipment was used for the three projects, and LNE-INM regularly checked the stability of the TPW standard cell. Recently, LNE-INM has devoted efforts to bring the French standard at the triple point of water into close agreement with CIPM Recommendation 2 (CI-2005). The isotopic fractionation between water and ice when the cell is in use was experimentally studied. Several new TPW cells delivered by the manufacturer with water samples were added to our batch of reference cells. A French laboratory analyzed the isotopic compositions of these samples. These actions allow the French national definition of temperature at the triple point of water to be changed. A new temperature was associated with TPW cell 679 in agreement with the CIPM recommendation. In this presentation, the latest TPW cell measurements carried out by LNE-INM are presented. The results from EUROMET Projects 278, 549, and 714 are investigated in light of these changes.

Prediction of airflow patterns in a ventilated enclosure with zonal methods

A zonal model with two different approaches for expressing mass flow rates (the conventional method based on relations for flow in conduits and the shear stress QSNA-Newton’s approximation) is used to predict airflow patterns in a ventilated isothermal enclosure. A new proposed 3D wall jet model gives much better agreement with experimental results than the previously available 2D wall jet model. The outlet boundary condition ( m ˙ out = m ˙ in or P out = P atm) has no influence on the mass flow rates. However, the pressure condition should be chosen when there is more than one outlet. Based on the calculated results, it is recommended that the shear stress approximation should be privileged over the conventional method since the latter predicts completely unrealistic average cell pressures with both outlet boundary conditions.

Two-dimensional simulations of natural convection/radiation heat transfer for BWR assembly within isothermal enclosure

AbstractThe current scoping study identifies the significant heat transfer effects for a 7 × 7 boiling water reactor (BWR) assembly within an isothermal basket opening inside a transport cask. A two-dimensional finite volume mesh is constructed that models the assembly components and cover gas. Computational fluid dynamics (CFD) simulations calculate the buoyancy induced gas motion, conduction and radiation within the components. Simulations use different basket surface temperatures, fuel heat generation rates and cladding surface emissivities, for both nitrogen and helium cover gases at atmospheric pressure. An analytical conduction/radiation model is developed for the thermal resistance between the channel and basket. Results using buoyancy induced gas motion compared to stagnant gas simulations show that natural convection is significant only at low basket temperatures, with nitrogen gas. Helium and high basket temperature simulations exhibit no significant temperature reduction from natural convection...

Modeling Transient Conduction in Enclosed Regions Between Isothermal Boundaries of Arbitrary Shape

Analytical models are developed to predict dimensionless heat flow rate for transient conduction in the doubly connected region formed between an arbitrarily shaped, isothermal inner body and its surrounding isothermal enclosure. This modeling procedure is based on limiting cases and trends identified in the exact solution for the concentric spheres. Asymptotic solutions for the limiting cases of small and large relative domain size, as well as for short and long times, are combined to provide comprehensive models applicable to the full range of the independent parameters. Validation of the models by the spherical enclosure solution and numerical data for the concentric cubes has shown excellent agreement between the model and the data, within less than a 3% rms difference for most cases.

On the Thermal Interaction Between an Isothermal Cylinder and Its Isothermal Enclosure for Cylinder Rayleigh Numbers of Order 104

An experimental investigation is made of the thermal interaction between a horizontal isothermal cylinder centrally located in a water-cooled isothermal cubical enclosure. The study is restricted to laminar flow and cylinder Rayleigh numbers of order 104. The application of interest is the cooling of electronic systems. This field is currently lacking in techniques that can measure the complex fluid phenomena encountered in real systems. The paper therefore begins with an experimental review of interferometry to assess its applicability as a potential solution to this need. Based on this review, a real time Digital Moire´ Subtraction interferometer is used to measure temperature profiles, and local Nusselt number distributions in two regions of interest: the plume impingement on the ceiling of the enclosure, and the upper corner region of the enclosure. A Mach-Zehnder interferometer is used for the cylinder Nusselt number distribution. Results are compared both qualitatively and quantitatively with a numerical simulation run on a commercial CFD package widely used for electronic system temperature predictions. The paper gives considerable insight into the nature of the enclosure heat transfer and an indication of the accuracy of a widely used predictive code.

Interlaboratory comparison of realizations of the triple point of water

An interlaboratory comparison of the different realizations of the triple point of water (TPW) in Europe has been organized by EUROMET (Project No. 278). This project was based on the circulation of one TPW cell and an isothermal enclosure. The national metrology institutes of thirteen countries and the Bureau International des Poids et Mesures were involved in the work. Twenty-seven TPW cells were compared. Half of the results lie between −0.05 mK and +0.05 mK of the mean value, and all but two of the results are within 0.10 mK of the mean.

On Gaseous Free-Convection Heat Transfer With Well-Defined Boundary Conditions

The scaling of free convection heat transfer is investigated. The nondimensional groups for Boussinesq and fully compressible variable property free convection, driven by isothermal surfaces, are derived using a previously published novel method of dimensional analysis. Both flows are described by a different set of groups. The applicability of each flow description is experimentally investigated for the case of the isothermal horizontal cylinder in an air-filled isothermal enclosure. The approach taken to the boundary conditions differs from that of previous investigations. Here, it is argued that the best definition of the boundary conditions is achieved for heat exchange between the cylinder and the enclosure rather than the cylinder and an arbitrarily chosen fluid region. The enclosure temperature is shown both analytically and experimentally to affect the Nusselt number. The previously published view that the Boussinesq approximation has only a limited range of application is confirmed, and the groups derived for variable property compressible free convection are demonstrated to be correct experimentally. A new correlation for horizontal cylinder Nusselt number prediction is presented. [S0022-1481(00)01604-2]

A Comparison of Natural Convection Heat Transfer for a Staggered Versus an Aligned Array of Horizontal Spent Nuclear Fuel Rods Within a Rectangular Enclosure

Natural convection heat transfer was experimentally investigated in a staggered array of heated cylinders, oriented horizontally within a rectangular isothermal enclosure. The test conditions were characteristic of a spent-fuel assembly during transport or horizontal dry storage. The assembly was configured with a pitch-to-diameter ratio of 1.33 and backfilled with pressurized helium or nitrogen. The backfill pressure was varied between 1 and 5 atm, while the assembly power was varied between 1 and 5 W per heater rod. The resulting data are presented in the form of Nusselt-Rayleigh number correlations, where the Nusselt number has been corrected for thermal radiation using a numerical technique. The staggered-array data are compared to previous data for a similar-pitch aligned rod array (a simulated boiling water reactor fuel assembly) to determine if convective heat transfer is enhanced or hindered in a staggered configuration. For the overall array, both the staggered and aligned configurations yield Nusselt-Rayleigh curves with a three-regime trend, which suggests distinct conduction and convection regimes separated by a transition regime. For lower Rayleigh numbers (<106), representative of the conduction regime, the aligned-array Nusselt number is 10 to 12% higher than the corresponding staggered-array value. However, in the convection regime at higher Rayleigh numbers, the staggered-array Nusselt number slightly exceeds the aligned- array Nusselt number. This is attributed to the fact that the staggered array begins to transition into the convection regime at lower Rayleigh number than the aligned array. For both configurations, the slope of the Nusselt-Rayleigh curve in the convection regime suggests turbulent flow conditions.

A multi-dimensional discrete-ordinates method for polarized radiative transfer. Part I: Validation for randomly oriented axisymmetric particles

A polarized multi-dimensional radiative transfer model based on the discreteordinates method is presented. The model solves the monochromatic vector radiative transfer equation (VRTE) that considers polarization using the four Stokes parameters. For the VRTE, the intensity of the scalar radiative transfer equation is replaced by the Stokes intensity vector; the position-dependent scalar extinction coefficient is replaced by a direction- and position-dependent 4 × 4 extinction matrix; the position-dependent scalar absorption coefficient is replaced by a direction- and position-dependent emission (absorption) vector; and the scalar phase function is replaced by a scattering phase matrix. The model can solve the VRTE for anisotropically scattering one-, two-, or three-dimensional Cartesian geometries. Validation for one-dimensional polarized radiative transfer compares model results with benchmark cases available in the literature. For two- and three-dimensional geometries, the model is tested by using a one-dimensional system as input and running in three-dimensional mode. A validation for a three-dimensional geometry based on Kirchoff s law for an isothermal enclosure is also presented. The model uses a parallel computing paradigm where each Stokes parameter is assigned to a computer processing unit. In Part II, the model will be validated for systems containing oriented particles, and benchmark results for three-dimensional geometries will be given.

A Continuously Weighed Pycnometer for Measuring Fluid Properties

This paper documents the construction details and experimental procedures for the continuously weighed pycnometer used in our laboratory to measure fluid properties such as densities, saturation pressures, phase boundaries, and isothermal compressibilities. This apparatus consists of a cell suspended from an electronic balance at all times and a variable volume bellows cell which is used to adjust the confined sample density within an isothermal enclosure. The principal advantages of this apparatus are rapid measurement and relatively high accuracy. Some experimental results are presented to illustrate capability. The accessible experimental ranges are 100−450 K up to 200 MPa; measurement accuracy is 0.1% or better.

Natural Convection Heat Transfer in Horizontal Rod-Bundle Enclosures

Natural convection heat transfer in enclosed horizontal N × N arrays (N = 3, 5, and 7) of electrically heated rods with a pitch-to-diameter ratio (P/d) of 1.35 has been experimentally investigated. Each array was positioned in an isothermal square enclosure with a width-to-diameter ratio (W/d) of 20.6. Pressurized air or helium was used as the working fluid. It was observed that the bottom-tow rods were relatively insensitive to increases in the array size, as they exhibit only slight temperature variations, but the top-row rods demonstrated substantial temperature increases. Natural convection correlations in the form of Nusselt number (Nud) as a function of modified Rayleigh number (Rad*) were obtained for each rod in each array. The correlations cover three flow regimes of conduction, transition, and convection in the range of 6.45 &lt; Rad* &lt; 3.08 × 105. A generalized enclosure Nusselt number was correlated as a function of enclosure modified Rayleigh number and the array size (N). Comparison of the data with previous numerical prediction showed that this correlation may be readily used to obtain a conservative estimate of the maximum temperature in the arrays with N = 3, 5, 7, and 9.

NATURAL CONVECTION OVER A ROTATING CYLINDRICAL HEAT SOURCE IN A RECTANGULAR ENCLOSURE

A numerical study of laminar two-dimensional natural convection heat transfer from a uniformly heated horizontal cylinder rotating about its center, and placed in an isothermal rectangular enclosure, is performed using a spectral element method. The physical aspects of the flow and its thermal behavior are studied for a wide range of pure natural convection to mixed convection at low and high rotational speeds of the cylinder. The computer program has been validated against experimental correlations available on pure natural convection of heated bodies in enclosures. The rotation of the cylinder has been found to enhance the heat transfer. At low ratios of Rayleigh number to the square of the rotational Reynolds number, Ra / Reω 2, the maximum temperature on the cylinder surface is decreased by as much as 25–35% from similar cases with fixed cylinders. At moderate values of Ra/ Reω 2, the thermal plume rising above the cylinder is shifted in the rotation direction and the angular shift decreases as Ra / R...

Natural convection heat transfer around two heated cylinders in an isothermal enclosure including the effect of wall conductance

A numerical study has been conducted for natural convection heat transfer for air around two vertically separated horizontal heated cylinders placed inside an isothermal rectangular enclosure having finite wall conductances. The interaction between convection in the fluid filled cavity and conduction in the walls surrounding the cavity is investigated. Results have been obtained for Rayleigh numbers (Ra) between 103 and 106, dimensionless wall thickness (W) between 0.5 and 1.375 and dimensionless wall‐fluid thermal conductivity ratio (α) between 0.01 and 5.0. The results indicate that wall heat conduction reduces the average temperature differences across the cavity, partially stabilizes the flow, and decreases natural convection heat transfer. The overall heat transfer coefficient for both cylinders is correlated with CRan for different W and α.