A novel fluid for use as a working fluid in a heat pipe has been tested at UCLA. The fluid was discovered originally in use with a device consisting of a metal tube charged with the patented inorganic aqueous solution (IAS) which is evaporated when the tube is evacuated before use. According to the patent, this evaporation leaves a thin film which allows the tube to carry high heat flux loads with low temperature drop across the tube in a solid state mode. However, various experiments with these tubes have produced inconsistent results and there is some question as to whether the fluid is completely evaporated. The research on which this work is based, is focused on testing whether the charging fluid will operate as the working fluid in a heat pipe, in order to determine the nature of the IAS fluid. We charged a heat pipe apparatus with a biporous wick in order to investigate if the fluid plays a role in heat transfer. We have extensive data for this experiment using water as the working fluid which will use to compare the two sets of results. Testing has shown positive results in the reduction of the superheat required to drive heat fluxes through a wick compared to water. Some experiments have shown that the operating (temperature) range of the IAS is much larger than a standard heat pipe. It is theorized that the increase in performance of the IAS is due to an increased heat of vaporization. If this fluid is proven to be effective, it would lead to more effective and tunable heat transfer devices.

Heterogeneous vapor bubble nucleation was numerically simulated on surfaces containing nanometer-sized rectangular cavities of various sizes (ranging from 2.72–9.36 nm width & 4.09–8.34 nm depth). Non-equilibrium molecular dynamics simulations at constant wall temperature were carried out for the two-dimensional (2D) Lennard-Jones (LJ) fluid which was in thermal contact with LJ solid wall. Nucleation was induced by expanding the system volume to a metastable state while keeping the temperature of the solid wall constant. When the aspect ratio (depth-to-width) of the cavity was large (∼3-to-1), the nucleation rate was increased nearly three-fold compared to the flat surface for relatively large wettability; however, the nucleation rate was nearly the same for lower wettability surfaces. Nucleation was generally favored within the cavities due to the stabilization of the cavity side-walls. The value of cutoff radius, wall temperature, and solid-liquid wettability was varied to determine their effect on nucleation rate. The 2D model was validated by comparing thermodynamic averages of pressure and potential energy to predicted values from a 2D LJ equation of state.

The effects of rib-patterned surfaces and surface wettability on liquid flow in microchannels were experimentally investigated in this study. Microchannels were fabricated on single-crystal silicon wafers by photolithographic and wet-etching techniques. Rib structures were patterned in the silicon microchannel, and the surface was chemically treated by trichlorosilane to create the hydrophobic condition. Experiments with water as the working fluid were performed with these microchannels over a wide range of Reynolds number between 110 and 1914. The results for the rib-patterned microchannels showed that the friction factor with the hydraulic diameter based on the rib-to-upper-wall height was lower than that for the incompressible theory with the same height. The friction factor-Reynolds number products for the hydrophobic condition increased as Reynolds number increased in the laminar flow regime. The experimental results were also compared with the predictive expressions from the literature, and it was found that the experimental data for the small rib/cavity geometry was in good agreement with those in the literature.

A mechanistic model that to provide a quantitative understanding of the interplay of hydrodynamics, heat/mass transfer, and cracking reactions in the feed injection zone of a fluid catalytic cracking (FCC) riser reactor with a single nozzle spray. With the injection of an oil spray into a gas-solid flow, the collision between cold oil droplets and hot catalyst particles results in a strong momentum transfer that affects the spray hydrodynamics in terms of penetration and scattering. It also causes a significant heat transfer giving rise to rapid droplet vaporization and the attendant cooling of the catalyst. The presence of cracking reactions introduces volume expansion, changes in gas composition and volume fraction, and a cooling effect due to endothermicity. Accordingly, we in this study present an analysis of chemically-induced “entrance effects” in an FCC riser with a single nozzle spray. The cracking reaction network is described by a four-lump reaction model, while the ambient gas-solid transport is represented by a dense-phase riser flow. A Lagrangian modeling approach is adopted to track the spray trajectory as cracking reactions proceed. It is shown that cracking reactions play an important role in dictating the spray behavior, reaction and heat/mass transfer characteristics in the feed injection zone of an FCC riser.

A compact micro pulsating heat pipe was developed and tested to investigate thermal performance. Micro Flat Plate Pulsating Heat Pipe (FP-PHP) was fabricated using DRIE MEMS technique. A total of 10 parallel interconnected rectangular channels forming a meandering closed loop are engraved on the silicon wafer with a thickness of 1 mm. The top of the silicon wafer was covered by a transparent glass plate (#7740PyrexTM) with a thickness of 0.5 mm to allow visualization of the internal thermo-hydrodynamic behavior in the PHP. The overall FP-PHP has length of 50 mm, width of 15.5 mm, and thickness of 1.5 mm, respectively. The width and height of the engraved rectangular channel is 1 mm and 0.6 mm and the hydraulic diameter is 0.75 mm. The ethanol is used for working fluid. The results show that the FP-PHP without working fluid has thermal resistance of 17 °C/W and the FP-PHP with working fluid of filling ratio of 50% has thermal resistance of 4 °C/W. In other words, the FP-PHP has effective thermal conductivity of 650 W/mK which is about 1.6 times as much as of that of the Copper ( k eff = 400 W/mK). Therefore the developed FP-PHP can be used as compact high performance electronic cooling system.

The heat transfer and fluid flow characteristics of non-boiling two-phase flow in microchannels were experimentally investigated. The effects of channel diameter (140, 222, 334, and 506 μm) on the Nusselt number were considered. Air and water were used as the working fluids. Results were presented for the Nusselt number over a wide range of gas superficial velocity (1.24–40.1 m/s), liquid superficial velocity (0.57–2.13 m/s), and wall heat flux (0.34–0.95 MW/m 2 ). The results showed that the Nusselt number increased with increasing gas flow rate for the 506 μm and 334 μm channels, while the Nusselt number decreased with increasing gas flow for the 222 μm and 140 μm channels. Based on these experimental results, a transition channel diameter of about 235 μm to 260 μm, which distinguishes microchannels from minichannels, was suggested. By observing two-phase flow patterns within the microchannels, viscosity and surface tension were identified as the key factors that caused the heat transfer characteristics to change. In addition, new correlations for the forced convection Nusselt number were developed.

Oscillating flow is encountered frequently in many two-phase flow systems such as pulsating heat pipe, refrigerator with reciprocating compressor, etc. Thickness of liquid film formed between the tube wall and the vapor bubble is one of the crucial parameters to develop two-phase flow systems using micro tubes. However, liquid film formation and variation of oscillating flows are very complicated phenomena coupled with acceleration, deceleration, evaporation, condensation, etc. In the previous research, liquid film thickness in accelerating flow under adiabatic conditions was measured and compared with the correlation developed under steady condition [5]. In the present study, liquid film thickness in decelerating flow in a micro tube is investigated under adiabatic condition. Circular tubes with diameter, D = 1.0 mm, is used. Laser focus displacement meter is used to measure the liquid film thickness. Two-phase flow is obtained by introducing air from the open end of the test tube. Ethanol is used as a working fluid. At small capillary numbers, the effect of deceleration is negligible similar to the trend under accelerated condition. As capillary number increases, liquid film thicknesses in decelerated conditions become larger than the predictions of adiabatic steady correlation. However, liquid film thickness does not exceed the critical thickness at Re > Re crit . It is considered that liquid film thickness is affected by the altered velocity profiles in the liquid slug ahead of air-liquid interface according to accelerated or decelerated condition.

This study concerns the geometric design of a cylindrical micropin-fin heat sink with multiple row configurations. The objective is to maximize the rate of heat transfer from the solid to the fluid subject to total fin volume and manufacturing constraints. A heat sink with dimensions of 1 mm × 0.6 mm × 1 mm is used for the computational analysis. An automated gradient-based optimization algorithm, which effectively handles an objective function obtained from a computational fluid dynamics simulation is implemented. The optimal design is obtained as results of balance of conductive heat transfer along the pin-fins with laminar forced convection. In the first case, the fins are arranged in two rows of pin-fins with different geometric sizes (diameter, height, and spacing between the fins). The optimal configurations obtained as a function of thermal conductivity ratio and Reynolds number are found to be in good agreement with those obtained from theory and numerical optimization. In the second case, the fins are arranged in rows of three, the effect of thermal conductivity and Reynolds number on the optimal configuration and the maximized heat transfer rate from the arrays of cylinders is reported.

Boiling water in small channels that are formed along turbine blades has been examined since the 1970s as a means to dissipating large amounts of heat. Later, similar geometries could be found in cooling systems for computers, fusion reactors, rocket nozzles, avionics, hybrid vehicle power electronics, and space systems. This paper addresses (a) the implementation of two-phase micro-channel heat sinks in these applications, (b) the fluid physics and limitations of boiling in small passages, and effective tools for predicting the thermal performance of heat sinks, and (c) means to enhance this performance. It is shown that despite many hundreds of publications attempting to predict the performance of two-phase micro-channel heat sinks, there are only a handful of predictive tools that can tackle broad ranges of geometrical and operating parameters or different fluids. Development of these tools is complicated by a lack of reliable databases and the drastic differences in boiling behavior of different fluids in small passages. For example, flow boiling of certain fluids in very small diameter channels may be no different than in macro-channels. Conversely, other fluids may exhibit considerable ‘confinement’ even in seemingly large diameter channels. It is shown that cutting-edge heat transfer enhancement techniques, such as the use of nano-fluids and carbon nanotube coatings, with proven merits to single-phase macro systems, may not offer similar advantages to microchannel heat sinks. Better performance may be achieved by careful optimization of the heat sink’s geometrical parameters and by adapting a new class of hybrid cooling schemes that combine the benefits of micro-channel flow with those of jet impingement.

The horizontal frame members that often protrude from the inner surface of a window can significantly effect the convective heat transfer rate from this inner surface to the room. The purpose of the present numerical study was to determine how the size of a pair of horizontal frame members effect this heat transfer rate. The flow has been assumed to be steady and conditions under which laminar, transitional, and turbulent flows occur are considered. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being dealt with using the Boussinesq approach. The governing equations have been solved using the FLUENT commercial CFD code. The k-epsilon turbulence model with standard wall functions and with buoyancy force effects fully accounted for has been used. The solution has the following parameters: the Rayleigh number, the Prandtl number, the dimensionless window recess depth, and the dimensionless width and depth of the frame members. Results have been obtained for a Prandtl number of 0.74.

Thermal distortion is a critical design consideration for the Haystack Ultrawide-band Satellite Imaging Radar (HUSIR) with respect to its performance at W-band. This design consideration is needed due to the thermal distortion effects on the surface accuracy of a parabolic reflector. For example, a tight surface tolerance of ∼100 microns root-mean-squared is required to obtain 85 percent antenna performance efficiency for the 37 meter (120 foot) diameter reflector. An understanding of the temperature and velocity fields aids compensation of these losses. Computational fluid dynamics models (CFD) are too computationally expensive to implement in a control algorithm. Therefore, this work applies proper orthogonal decomposition (POD) to simulated CFD data and creates a reduced order model of the fluid system that characterizes the dominant features of both the temperature and velocity fields. A case study of the HUSIR’s convective flow inside a dome is illustrated.

In the present study, slip phenomena are investigated in two different sets of experiments conducted in gases and one Newtonian liquid. Overall, differences in near-surface slip behavior are illustrated for these two different fluid mediums, where the slip is induced surface roughness and rarefaction in the gases, and by surface roughness and intermolecular interactions in the liquid. Within both sets of experiments, flows are induced within micro-fluidic passages by rotation within C-shaped fluid chambers formed between a rotating disk and a stationary surface. When gases are employed, accommodation coefficients are determined in a unique manner from experimental results and analysis based on the Navier-Stokes equations. In all cases, roughness size is large compared to molecular mean free path. When channel height is defined at the tops of the roughness elements, slip is believed to be a result of rarefaction as well as fluid shear. With this arrangement, tangential accommodation coefficients decrease and slip velocity magnitudes increase, at a particular value of Knudsen number, as the level of surface roughness increases. With Newtonian water as the working fluid, hydrophobic roughness is used to induce near-wall slip in the fluid chamber. The magnitudes of slip length and slip velocities increase as the average size of the surface roughness becomes larger. The resulting slip length data show a high degree of organization when normalized using the fluid chamber height, such that experimental data obtained using different chamber heights and different disk roughness magnitudes collapse along a single line, illustrating strong linear dependence of the slip length on the normalized radial-line-averaged shear stress.

This study is to investigate an effect of natural convection or natural circulation on a transport process by molecular diffusion in a stratified fluid layer consisting of two component gases. There are many experiments and analysis regarding natural convection in a vertical slot or natural circulation in a circular tube. However, there are few studies on natural convection or circulation with molecular diffusion in the stratified fluid layer consisting of two component gases. It is confirmed that these phenomena appear when the depressurization accident occurs in the very high temperature reactor (VHTR). Therefore it is important to evaluate the transport and mixing processes of two or more component gases during the depressurization accident of the VHTR. The experiment has been performed regarding the combined phenomena of molecular diffusion and natural convection or natural circulation in a parallel vertical slot filled with two component gases. The one vertical slot consists of the heated wall and the other side cooled wall. The other one consists of both the cooled wall. The dimension of heated wall is 500mm×200mm and thickness is 3mm. The width of the slot is 20mm and the aspect ratio of the slot is 25. Combination of nitrogen/argon is used as the two component gas system. The density change of the gas mixture and the gas temperature distribution in the slots were obtained. The mixing process of the heavier gas from the bottom side of the slot filled with the lighter gas was discussed in this paper.

Microtextured superhydrophobic surfaces have become ubiquitous in a myriad of engineering applications. These surfaces have shown potential in friction reduction applications and could be poised to make a big impact in thermal management applications. For instance higher heat transfer rate with less pumping power might be achievable through the aid of superhydrophobic surfaces. However, past and current research on superhydrophobic surface has focused mainly on modifying either the chemical component or the roughness factors of such surfaces. The purpose of this paper is to account for the thermal effects of the heated fluid flowing in superhydrophobic microfluidic channels. Herein we characterize the wetting behavior as a function of temperature of microtextured superhydrophobic surfaces, for both active and passive thermal management applications. A series of PDMS microtextured samples were fabricated using micromachining and soft lithography techniques. Flow measurements were performed using the superhydrophobic microfluidic channel. The channel surface roughness was large enough to induce the Cassie-Baxter state, a phenomenon in which a liquid rests on top of a textured surface with a gas layer trapped underneath the liquid layer. This gas layer induces a two-phase flow, and friction reduction can be achieved for the liquid channel flow. With this channel, flow rates were measured by varying the equilibrium temperature of the substrate. The temperature in the constant pressure source was controlled by circulating the water through a water-bath. As the heating reached a certain threshold the curvature of the liquid-gas interface was reversed and dewetting of the penetrated liquid layer was observed. This result suggests that the Cassie state in fluid flow can be prolonged even under increased pressure drops by increasing the temperature in the gas layer.

The recent rapid development of computational fluid dynamics (CFD) technology provides a powerful tool to obtain accurate temperature distribution of prismatic fuel blocks. In general, however, a CFD analysis requires tremendous computational efforts to analyze whole prismatic fuel blocks due to complex geometry and near wall treatment to resolve the boundary layer of the fluid flow. Such efforts might be a huge burden for a designer who wants a large number of calculations with various design options. Recently, a practical method to analyze a thermal behavior of prismatic fuel blocks has been developed by the present authors to overcome the demerits of CFD and system code calculations. The developed method solves three-dimensional heat conduction in prismatic fuel blocks like a CFD code. For the fluid, however, the present method adopts one-dimensional conservation equations like a system code. Such a combination enables significantly reduced computational effort with reasonable computational accuracy. This paper presents an intensive thermo-fluid analysis for prismatic fuel assemblies using the new method. In order to examine the validity and the accuracy of the developed method, at first, single standard and control fuel assemblies have been analyzed and compared with the CFD results. Then the developed method has been applied to a whole core thermal analysis of a prismatic reactor.

The transient natural convection of variable and constant viscosity fluids in a side cooled cubical cavity is studied experimentally. The convective flow in the cavity is visualized by the shadowgraph technique. The fluid was initially preheated and enough time is allowed to assure uniform temperature distribution. Temperature difference is introduced by a sudden cooling of the two side walls. The results indicate that the transient flow development is characterized by the following distinct processes: the initial growth of the vertical boundary layer and horizontal intrusions, the interaction of the horizontal intrusions and the stratification. New scaling relations are introduced to include the effect of the viscosity variation with temperature. The new suggested scalings showed good agreement with the measurements based on the shadowgraph images.

An air-ingress accident followed by a pipe break is considered as a critical event for a very high temperature gas-cooled reactor (VHTR) safety. Following helium depressurization, it is anticipated that unless countermeasures are taken, air will enter the core through the break leading to oxidation of the in-core graphite structure. Thus, without mitigation features, this accident might lead to severe exothermic chemical reactions of graphite and oxygen depending on the accident scenario and the design. Under extreme circumstances, a loss of core structural integrity may occur along with excessive release of radiological inventory. Idaho National Laboratory under the auspices of the U.S. Department of Energy is performing research and development (R&D) that focuses on key phenomena important during challenging scenarios that may occur in the VHTR. Phenomena Identification and Ranking Table (PIRT) studies to date have identified the air ingress event, following on the heels of a VHTR depressurization, as very important (Oh et al. 2006, Schultz et al. 2006). Consequently, the development of advanced air ingress-related models and verification and validation (V&V) requirements are part of the experimental validation plan. This paper discusses about various air-ingress mitigation concepts applicable for the VHTRs. The study begins with identifying important factors (or phenomena) associated with the air-ingress accident using a root-cause analysis. By preventing main causes of the important events identified in the root-cause diagram, the basic air-ingress mitigation ideas can be conceptually derived. The main concepts include (1) preventing structural degradation of graphite supporters; (2) preventing local stress concentration in the supporter; (3) preventing graphite oxidation; (4) preventing air ingress; (5) preventing density gradient driven flow; (6) preventing fluid density gradient; (7) preventing fluid temperature gradient; (7) preventing high temperature. Based on the basic concepts listed above, various air-ingress mitigation methods are proposed in this study. Among them, the following one mitigation idea was extensively investigated using computational fluid dynamic codes (CFD) in terms of helium injection in the lower plenum. The main idea of the helium injection method is to replace air in the core and the lower plenum upper part by buoyancy force. This method reduces graphite oxidation damage in the severe locations of the reactor inside. To validate this method, CFD simulations are addressed here. A simple 2-D CFD model was developed based on the GT-MHR 600MWt as a reference design. The simulation results showed that the helium replaces the air flow into the core and significantly reduces the air concentration in the core and bottom reflector potentially protecting oxidation damage. According to the simulation results, even small helium flow was sufficient to remove air in the core, mitigating the air-ingress successfully.

A slug flow with phase change in a vertical mini-tube is numerically simulated on the basis of a scheme of continuum mechanics. To formulate two-phase flow, the volume of fluid, VOF, method is employed; the advection of the gas-liquid surface is expressed by the piecewise linear interface calculation, PLIC, scheme, while the effect of surface tension is evaluated by the continuum surface force, CSF, model. Since the treatment of liquid film between a bubble and tube wall is crucially important to properly predict both heat transfer and resulting fluid flow in a mini-tube, a semi-empirical approach based on subsidiary knowledge estimated from a preliminary experiment is newly proposed. Further, an additional numerical procedure is introduced to obtain allowable mass conservation even in the thin-film region with intense evaporation. Consequently, by introducing only one parameter, the physical meaning of which is clear, the bubble behavior is reasonably predicted, and its detailed mechanism is clarified.

An experimental study of the free convection heat transfer from horizontally held wires to carbon dioxide at near-critical pressures has been performed. In the experiments, platinum wires of 25.4 μm, 76.2 μm and 100 μm, as well as a 101.6 μm diameter nichrome 60/20 wire were used. The pressure (P) and bulk temperature (T b ) of the fluid were varied in the range: 6.34 MPa ≤ P ≤ 9.60 MPa and 10 °C ≤ T b ≤ 33.3 °C, respectively. Visual observation of the flow was realized using a high speed camera. The bulk fluid temperature is introduced as a new reference temperature for the calculation of fluid properties. Correlations were developed to predict the natural convection heat transfer rate at both subcritical and supercritical pressures. The correlations developed predict almost all the experimental data from the current study and those reported in the literature to within ± 15%. No bubble-like flow pattern was observed for both platinum and nichrome wires.

A two-dimensional numerical model of an integrated microscale hydrogen-air combustor, recuperator, and oil heat exchanger is presented. The recuperator is used to preheat the incoming reactant gases entering the combustor. Heterogeneous catalytic combustion occurs on the walls of the microchannel in the presence of platinum catalyst. The maximum oil temperature was constrained based on the properties of the fluid used. The recuperator gas outlet temperature was constrained to be larger than the saturation temperature for water to avoid condensation in the exhaust gas stream. The performance of the combustor is documented for variations in geometric and fluidic parameters. The geometric parameters include the length and location of the catalyst bed, and the length of the device. Varied fluidic parameters include the oil inlet temperature, flow rates of oil and reactants, and the equivalence ratio of the reactants. For the range of parameters studied, results indicate that increasing the catalytic surface length as well as the channel length increases the efficiency.