The functionality of silicon nanoparticles is strongly size-dependent, so there is a pressing need for laser diagnostics that can characterize aerosolized silicon nanoparticles. The present work is the first attempt to extend time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used for sizing soot, to size silicon nanoparticles. TiRe-LII measurements are made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon. Molecular dynamics (MD) is used to predict the accommodation coefficient between silicon nanoparticles and argon and helium, which is needed to interpret the TiRe-LII data. The MD-derived thermal accommodation coefficients will be validated by comparing them to experimentally-derived values found using transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis.

To fully model the physics present within the proposed supercritical water reactor (SCWR), the thermal hydraulics calculations (yielding temperatures and densities in each material as a function of space) must be coupled to the neutronic calculations (yielding reactivity and neutron flux shapes). To enable this full coupling, a 3D model of a supercritical water reactor is being implemented in the CFD software OpenFOAM with the same geometry as a 3D MCNP (neutronics) model. Coupling will performed through result exchange between the two codes — densities and temperatures from OpenFOAM to MCNP, with heat generation returned from the neutronics calculations. Use of a reduced-geometry model is advisable due to the high computational cost of each OpenFOAM/MCNP coupling iteration. In the present work, a 1.5D thermal model of a single fuel pin was coupled with a 3D MCNP model. The thermal model includes single channel analysis (cladding/coolant heat transfer) as well as heat transfer within the cladding, helium gas gap, and uranium dioxide fuel itself. These heat transfer zones provide specific data points of the radial temperature profile. Because no radial mesh is considered, full radial dependence is not possible. The model provides limited radial dependence unlike what a 1D code could provide; thus, 1.5D is used to indicate the incomplete radial dependence that is included in the model. Iteration between the two codes is performed until heat generation is converged to within 10% between successful MCNP results. A discussion of these 3D/1.5D coupled results and the path forward to fully 3D coupling is provided.

A helium filled orifice type pulse tube refrigerator (OPTR) was designed, built and operated to provide cryogenic cooling. The OTPR is a travelling wave thermoacoustic refrigerator that operates on a modified reverse Stirling cycle. The experimental studies are carried out at various values of the mean pressure of helium (0.35 MPa – 2.2 MPa), amplitudes of pressure oscillations, frequencies of operation and sizes of orifice opening. The experimental results are compared with the predictions from a detailed time-dependent numerical model. In the CFD model, the compressible forms of the continuity, momentum and energy equations are solved for both the refrigerant gas (helium) and the porous media regions (the regenerator and the three heat-exchangers) in the OPTR. An improved representation of heat transfer in the porous media is achieved by employing a thermal non-equilibrium model to couple the gas and solid (porous media) energy equations. The model predictions show better comparisons with the experimental results when the effects of wall thicknesses of the various components of the OPTR are included in the model.

Aerosolized metal nanoparticles have numerous important applications in materials science, but their functionality depends strongly on their size. Very recently, time-resolved laser-induced incandescence (TiRe-LII) has been investigated as a technique to size aerosolized metal nanoparticles, but this procedure requires an accurate model of the heat transfer through which laser-energized particles re-equilibrate with the bath gas. This paper presents a model for laser-energized molybdenum nanoparticles, which is then applied to analyze experimental TiRe-LII measurements made on molybdenum nanoparticles formed by photolysis of Mo(CO) 6 in helium, argon, nitrogen, and carbon dioxide. While it is possible to estimate the size distribution width, recovering nanoparticle sizes from TiRe-LII data requires independent knowledge of the thermal accommodation coefficient.

The Modular High Temperature Gas-Cooled Reactor (MHTGR) has been chosen as a reference design for the Next Generation Nuclear Plant (NGNP) project. This reactor consists of concentric stacks of graphite blocks containing embedded fuel elements. Helium will be used as the coolant and will flow through designed coolant channels interspaced axially within the graphite blocks as well as in the gaps separating the blocks (called the bypass flow). A key phenomenon that may lead to localized hot spots in the reactor is the degradation of heat transfer effects in the bypass flow due to geometry distortions. Geometry distortions are the result of the graphite blocks being irradiated with energetic neutrons as well as coefficient of thermal expansion effects due to temperature changes. Idaho State University is studying heat transfer within the bypass flow and is developing an experiment to study the deterioration of heat transfer in the bypass flow stemming from these geometry distortions. Experimental data gathered from this project will be used to benchmark numerical codes used in the design and safety analysis of the MHTGR. Baseline MHTGR operating conditions are for a system pressure of approximately 7 MPa and a helium exit temperature from the reactor of approximately 850 °C. In place of using helium at these extreme conditions, it is our desire to perform the experiments with air entering the experiment at atmospheric pressure and temperature. Additionally, it is desirable to have an open-air system as opposed to a closed helium system. In order to quantify the impacts on temperature increase as well as pressure drop, a scaling analysis will be performed to compare the respective values from both helium and air. Important non-dimensional parameters, such as Reynolds number, non-dimensional heat flux, the acceleration factor, and non-dimensional buoyancy, will be matched for the various conditions in order to provide a similitude between the helium and air. These factors cannot be matched all at once, except by using actual conditions. The range of Reynolds number will be chosen to ensure an operating regime from the purely laminar to completely turbulent. This paper presents the results of this scaling analysis.

Cryosurgery is also called as cryoablation or cryoleision. The third generation of cryo-machine use argon gas for cooling and helium for rewarming to destroy cancer cells. The probes may be put into the tumor during surgery or through the skin (percutaneously). After cryosurgery, the frozen tissue thaws and is either naturally absorbed by the body (for internal tumors), or it dissolves and forms a scab (for external tumors). The main purpose of this paper is to establish a preliminary computer assisted simulation in prostate tumor cryosurgery. A radiologist and an urologist in a medical center in addition to the engineering specialist from the university participated in this interdisciplinary research program. The first step of this simulation protocol is to trim hundreds of two-dimensional medical imaging photos from a patient through the imaging reconstructive software into building a three-dimensional solid modeling. The image data for each patient can be obtained from the x-ray computed tomography (CT), or magnetic resonance imaging (MRI) in the imaging department of hospital. It has successfully built up the related knowledge to overcome the complicacy between the medical imaging modalities and engineering graphic solid modeling with high resolution. It is worthy to mention here that the present solid modeling of prostate can demonstrate the variable diameters and courses of the prostate urethra in vivo. The second step focuses on thermal calculation. So far, there has been no existing commercial software for the specific purpose of the bioheat transfer problem. Hence, user subroutines must be added to the existing commercial software to simulate the clinical situation of cryosurgery. For example, the occurrence of phase change during some specified temperature range and the latent heat of fusion are also incorporated into bio-heat transfer model. It has successfully incorporated bioheat transfer model into the software program to fit the reality in thermal medicine. The third step supplies the data and knowledge concerned with the position of a tumor and the related mechanism of metabolism of living tissue and vessels. The number of probes, the position of each probe, and the operating time of each probe will be explored to ensure a complete killing of the tumor tissue while saving as much healthy surrounding tissue as possible. In this study, the three-dimensional transient temperature distributions based on cryosurgery for prostate tumors have been performed for several cases to find the optimal operating conditions. Different cryoprobes with different freezing time are considered to find the temperature distribution. The simulation results for cryosurgery of prostate tumors will be supplied for practicing physicians as reference to greatly improve the effectiveness of cryosurgery.

This paper studied the influences of shielding gas compositions on the transport phenomena in the metal domain during gas metal arc welding (GMAW). A comprehensive model was developed to simulate the time-dependent processes of the electrode melting; the droplet formation, detachment, transfer and impingement onto the workpiece; the weld pool dynamics and bead formation and their transient coupling with the arc plasma. The transient melt-flow velocity and temperature distributions in the metal shielded by pure argon and argon-helium mixtures with various mixing ratios are presented. It is predicted that the increase of helium content and the resulting arc contraction induce an upward electromagnetic force at the bottom of the droplet to sustain the droplet at the electrode tip. As a result, the more oblate droplet and the longer droplet formation time are produced. The behaviors of the predicted droplet shape and detachment frequency are in agreement with the published results. It is also found that, under the identical energy input, the weld bead has a shallower penetration depth and broader bead width when helium content increases.

Numerical and experimental studies were conducted to investigate the effects of hydrogen and helium addition to fuel on soot formation in atmospheric axisymmetric coflow laminar methane-air diffusion flame. Soot temperature and volume fraction distributions were measured using a two-dimensional two-color technique. Numerically the conservation equations of mass, momentum, energy, and species in the limit of low-Mach number were solved. Detailed gas-phase chemistry and thermal and transport properties were accounted for. Radiative heat transfer by CO, CO 2 , H 2 O, and soot was calculated using the discrete-ordinates method with the radiative properties of the mixture obtained from a wide-band model. Soot was modeled using a two-equation semi-empirical model in which the mechanisms for inception and surface growth are assumed to be PAH coagulation and H-abstraction acetylene addition. Both experimental and numerical results show that helium addition is more efficient than hydrogen addition in reducing soot formation in the methane flame. These results are different from the previous investigations in ethylene flames where the hydrogen addition was found to be more effective in reducing soot formation than helium addition due to the additional chemical suppression of hydrogen on soot. It is suggested here that hydrogen chemically enhances soot formation when added to methane.

Forced convection transient heat transfer for helium gas at various periods of exponentially increase of heat input to a horizontal plate (ribbon) was experimentally and theoretically studied. In the experimental studies, the authors measured heat flux, surface temperature, and transient heat transfer coefficients for forced convection flow of helium gas over a horizontal plate under wide experimental conditions. The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate, τ, ranged from 46 ms to 17 s. The pressures were from 400 to 800 kPa. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlations for quasi-steady-state heat transfer and transient heat transfer were obtained based on the experimental data. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. It was obtained that the surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. The values of numerical solutions for surface temperature and heat flux at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities.

Thermoacoustic refrigeration is an environmentally safe refrigeration technology that has evolved over the past three decades [1–5]. The influence of working fluid on the performance of the thermoacoustic refrigerator (TAR) expressed in terms of the cooling load and the coefficient of performance is discussed in the paper. The calculations rely on the short stack boundary layer approximation. A simple model of a one-dimensional half wavelength resonance tube equipped with stack plates and a pair of heat exchangers was used as the physical model. It is known that a TAR with noble gases and their mixtures as working fluids can achieve high values of the coefficient of performance (COP) because of small Prandtl number values. The present study revealed and quantified that cooling load behavior is quite different from the performance in terms of the COP: the highest cooling load is achieved with pure Helium as the working fluid. A reason is the very high sound speed in Helium. TARs with Helium as the working fluids deliver the highest cooling load of all gases and their mixtures examined here, therefore it has been suggested as a limiting case of the most powerful TAR. The influence of geometry and thermophysical parameters of the device on TAR performance was studied systematically and a performance sensitiviy analysis was presented with particular emphasis on TAR cooling load.

Forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a horizontal cylinder and a plate (ribbon) was experimentally and theoretically studied. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. The dependence of transient heat transfer on the gas flowing velocity becomes weaker when the period becomes very shorter. However, the gas temperature in this study shows little influence on the heat transfer coefficient. Empirical correlations for quasi-steady-state heat transfer and transient heat transfer were obtained based on the experimental data. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. The values of numerical solution for surface temperature and heat flux were compared and discussed with authors’ experimental data. It was clarified that the surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. The temperature distribution near the heater becomes larger as the surface temperature increases. The values of numerical solution for surface temperature and heat flux agree well with the experimental data for the cylinder diameter of 1 mm. However, the heat fluxes show some differences from the experimental values for the cylinder diameters of 0.7 mm and 2.0 mm. And for the numerical solution for a plate, the values of numerical solutions for surface temperature and heat flux at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities.

The paper reports first steps in an analysis, using Computational Fluid Dynamics (CFD), of a leak of hot (>250°C) lead bismuth eutectic (LBE) from the MEGAPIE spallation source target into the helium-filled gap between the target hull and a close-fitting, double-walled safety container. Issues addressed are: (1) determining the initial impact pressure of the LBE on the inner shell of the safety container, which could result in the gap between the shells being closed, and its cooling capacity compromised; (2) LBE freezing on the cold shell surface could delay action of the leak detection system, and hence the beam trip control; and (3) heat transfer from the hot LBE to the D 2 O circulating between the safety container walls could induce boiling and subsequent pressurization of the circuit. Steps in the model development needed for the simulation are described, which include, in particular, the implementation of a liquid/gas interface-tracking model based on the Level Set approach. Results confirm that the safety container will be adequately cooled by the D 2 O during normal target operation, but first indications are that boiling might occur within the D 2 O circuit if substantial amounts of LBE leak from the target vessel unless its outer surface is coated with an insulating layer. Pressures generated by the impact of a slug of LBE on the safety window do not threaten its structural integrity, and no delay of the leak detection equipment as a consequence of LBE freezing is anticipated.

Mercury flow is used in chemical and nuclear systems. The sound speed of mercury is similar to that of water, but the density is over an order of magnitude greater, making the consequences of fluid transients like fluid column separation followed by fluid-hammer much more severe in mercury systems than in water systems. Fluid transients in a mercury flow loop are considered and fluid transients in a loop filled with mercury and 0.5% by volume helium bubbles are evaluated. The helium bubbles add compressibility, which helps mitigate consequences of some transients of concern to nuclear applications.

A computational model to study the stability characteristics of an evolving buoyant helium gas jet in ambient air environment is presented. Numerical formulation incorporates a segregated approach to solve for the transport equations of helium mass fraction coupled with the conservation equations of mixture mass and momentum using a staggered grid method. The operating parameters correspond to the Reynolds number varying from 30 to 300 to demarcate the flow dynamics in oscillating and non-oscillating regimes. Computed velocity and concentration fields were used to analyze the flow structure in the evolving jet. For Re = 300 case, results showed that an instability mode that sets in during the evolution process in Earth gravity is absent in zero gravity, signifying the importance of buoyancy. Though buoyancy initiates the instability, below a certain jet exit velocity, diffusion dominates the entrainment process to make the jet non-oscillatory as observed for the Re = 30 case. Initiation of the instability was found to be dependent on the interaction of buoyancy and momentum forces along the jet shear layer.

The transition from laminar to turbulent flow in helium jets discharged into air was studied using Rainbow Schlieren Deflectometry technique. In particular, the effects of buoyancy on jet oscillations and flow transition length were considered. Experiments to simulate microgravity were conducted in the 2.2s drop tower at NASA Glenn Research Center. The jet Reynolds numbers varied from 800 to 1200 and the jet Richardson numbers ranged between 0.01 and 0.004. Schlieren images revealed substantial variations in the flow structure during the drop. Fast Fourier Transform (FFT) analysis of the data obtained in Earth gravity experiments revealed the existence of a discrete oscillating frequency in the transition region, which matched the frequency in the upstream laminar regime. In microgravity, the transition occurred farther downstream indicating laminarization of the jet in the absence of buoyancy. The amplitude of jet oscillations was reduced by up to an order of magnitude in microgravity. Results suggest that jet oscillations were buoyancy induced and that the brief microgravity period may not be sufficient for the oscillations to completely subside.

A three-dimensional computational analysis is employed to investigate the transient flow and heat transport within an annular-core High-Temperature Reactor pebble bed, the blind core, and the surrounding graphite structures including 36 embedded coolant feeding channels. Well established models of pressure loss in the pebble bed, heat transfer between helium and the pebbles, and the heat transport within the porous medium by conduction and radiation (using an effective conductivity) have been implemented into an extended version of CFX-4. The graphite structures of the reflector are simulated closely coupled to the flow simulation as heat conducting solids. The model has been verified for axisymmetric power distributions by comparison with known results from 2D analysis. Various 3D-power distributions are assumed to represent a possible loss of forced cooling incident after unusual plant operation such as asymmetric positioning of absorber rods. It is confirmed, that 2D analysis is not adequate, because the predicted maximum temperature of the fuel elements after fission shutdown may exceed the maximum temperature predicted by 2D analysis.