Critical heat flux (CHF) of convective boiling in a mini-tube due to power transient was measured. A platinum tube with an inner diameter of 1.0 mm was heated exponentially by a direct current power supply as Joule heating. The heated length of the platinum tube was 40.9 mm. The platinum tube was mounted vertically in the water-loop apparatus which consisted of a circulating pump, a pre-heater, a flow mater, a pressurizer, a cooler and a test section. The deionized water was pressurized by the pressurizer up to approximately 800 kPa to measure CHFs at the high subcooling. The upward flow velocity in the platinum tube was ranged from 5 to 11 m/s. The inlet subcooling was ranged from 92 to 117 K. The heat generation rate was controlled with exponential functions. The e-folding time of the heat generation rate was ranged from 30 ms to 18 s. As an experimental result, it was found that the CHFs increased with increasing the flow velocity and the inlet subcooling. The CHF also increased with decreasing the e-folding time of the heat generation rate. Since the heat generation rate of the platinum tube increased rapidly under the power transient condition, it was considered that the heat flux of the platinum tube increased until the vapor blanket covered the heated surface of the platinum tube.

The In Situ Adaptive Tabulation (ISAT) procedure, originally developed for the efficient computation of homogeneous reactions in chemically reacting flows, is adapted and demonstrated for reacting flow computations with complex heterogeneous (or surface) reactions. The treatment of heterogeneous reactions within a reacting flow calculation requires solution of a set of nonlinear differential algebraic equations at boundary faces/nodes, as opposed to the solution of an initial value problem for which the original ISAT procedure was developed. The modified ISAT algorithm, referred to as ISAT-S, is coupled to a three-dimensional unstructured reacting flow solver, and strategies for maximizing efficiency without hampering accuracy and convergence are developed. These include use of multiple binary tables, use of dynamic tolerance values to control errors, and periodic deletion and/or re-creation of the binary tables. The new procedure is demonstrated for steady-state catalytic combustion of a methane-air mixture on platinum using a 24-step reaction mechanism with 19 species, and for steady-state three-way catalytic conversion using a 61-step mechanism with 34 species. Both reaction mechanisms are first tested in simple 3D channel geometry with reacting walls, and the impact of various ISAT parameters is investigated. As a final step, the catalytic combustion mechanism is demonstrated in a laboratory-scale monolithic catalytic converter geometry with 57 channels discretized using 354,300 control volumes (4.6 million unknowns). For all of the cases considered, the reduction in the time taken to perform surface chemistry calculations alone was found to be a factor of 5–11.

Jet flow phenomenon is important in enhanced nucleate boiling heat transfer processes and applications. When heater sizes scale down, jet flow can be observed due to the Marangoni convection around bubbles staying on microscale heated surface. In this paper, two fluids were employed for comparing and demonstrating the effect of Marangoni convection on bubble behaviors on micro heating wire. One was ultrapure water and the other was aqueous n-butanol solution, a self-rewetting fluid. Bubble-top jet flow for water and multi-jet flows for n-butanol solution were observed around a platinum micro heating wire by high speed CCD camera. Corresponding numerical simulation proved that it is the Marangoni convection that attracts the sub-cooled water to flow from the super-heated microlayer at the bottom to the top of a stationary bubble. For n-butanol solution, however, the Marangoni convection can induce it to flow oppositely, which causes the subcooled solution to flow onto the heated surface. The simulation for the solution is in good agreement with the experiment where the subcooled liquid nears the bubble-top flow towards the bottom of bubble or the heated surface and hence the multi-jet flows occur. The multi-jet flows can sustain for a long period and cause transient chaos at the super-heated thin liquid layer near the heated surface. The temperature around the bubble presented sharp temperature gradient and the velocity in the near-wall region is almost vertical to the wall. The experimental and numerical studies on the effect of surface tension and thus Marangoni convection are crucial to the mechanisms of subcooled nucleate boiling of fluids.

Forced convection transient heat transfer coefficients have been measured for nitrogen gas flowing over a twisted heater due to exponentially increasing heat inputs ( Q 0 exp(t/τ) ). And then, the effect of heater configuration on transient heat transfer by a twisted heater has been investigated comparing to that of the plate heater. In the experiment, the platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as a test heater. For heat transfer enhancements in single-phase flow, it was twisted at the central part of the heater with an angle of 90 degrees with respect to the upper part of the heater. The heat generation rate was exponentially increased with a function of Q 0 exp(t/τ) . The gas flow velocity ranged from 1 to 4 m/s for the gas temperatures of 313K. The periods of heat generation rate ranged from 46 ms to 17 s. The surface temperature difference and heat flux increased exponentially as the heat generation rate increased with the exponential function. The heat transfer coefficients for twisted heater have been compared to those of the plate heater. They were 24 % higher than those of the plate one. The geometric effect (twisted effect) of heater in this study showed an enhancement on the heat transfer coefficient. It was considered that the heat transfer coefficients are affected by the change in the flow due to swirling flow on the twisted heater. Finally, the empirical correlations for quasi-steady-state heat transfer and transient one have been obtained based on the experimental data.

This paper proposes the use of electrochemical impedance spectroscopy (EIS) to measure characteristics of gold (Au)-coated membranes and their inherent limitations. In this work, the fabrication of a membrane permeate flow cell is described with the aim of subsequently studying the transport of ions through conductive polycarbonate track etched membrane (PCTE) by interrogating the system using EIS and CV measurements. In particular, we would like to ascertain the voltage range that can be applied to the Au-coated membrane without getting a considerable faradaic activity; the difference between platinum and Au electrode; the effects of different electrolyte concentrations and various applied DC potentials. We extend our previous work done [1] by studying the differences of using a hrydroxyl and methyl terminated self assembled monolayer (SAM). We also extend the quality of the monolayer with respect to the amount of time in which the monolayer is grown. Finally, finding the voltage in which a ‘defect free’ monolayer transforms from insulative to ‘leaky’ behavior extends a detailed analysis of the critical voltage of an alkane thiol.

A modified 3-ω method applied to a suspended platinum microwire was employed to measure the thermal conductivity and convective heat transfer coefficient of water-based single-walled carbon nanotubes (CNT) solution, and an expression for calculating the convective heat transfer coefficient in a free convective fluid was introduced. The measurement technique was validated for three model systems including vacuum, air, and deionized water. It is found that there is excellent agreement of these three model systems with theoretical predictions. In addition, the frequency dependence on the third harmonic response measured in deionized water reveals existence of a very low working frequency below 60 mHz. The thermal conductivity and convective heat transfer coefficient of a nanofluid (water-based single wall CNTs colloidal suspension) were determined to be 0.73±0.013 W/m·K and 14900±260 W/m 2 ·K respectively, which corresponds to enhancement of 19.4% in thermal conductivity and 18.9% in convective heat transfer as compared to water.

With the increase of power consumption in compact electronic devices, passive heat transfer cooling technologies with high heat flux characteristics are highly desired in microelectronics industries. Carbon nanotube (CNT) cluster/forest has high effective thermal conductivity, nano pore size and large porosity, which can be used as wick structure in a heat pipe heatspreader and provides high capillary force for high heat flux thermal management. In this research, investigations of high heat flux cooling of the CNT bi-wick structure are associated with the development of a reliable thermometer and high performance/interface free heater. A 100nm thick and 600μm wide Z-shaped platinum wire resistor is fabricated on the backside of a CNT sample substrate to heat a 2×2mm 2 wick area. As a heater, it provides direct heating effect without thermal interface and is capable of over 800°C high temperature operation. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation with temperature after the annealing process is applied. The CNT sample substrate is silicon. The backside of the silicon substrate is thermally oxidized to create a 2μm thick and pinhole-free SiO 2 layer so that the platinum heater and thermometer can survive from the server CNT growth environments and without any electrical leakage. For high heat flux cooling, the CNT bi-wick structure is composed of 250μm tall, 100μm wide stripe-like CNT clusters and 50μm empty space. Using 1×1cm 2 CNT bi-wick samples, experiments are completed in both the open and saturated environments. Testing results of CNT bi-wick structure demonstrate 600W/cm 2 heat transfer capacity and good thermal & mass transport characteristics in the nano level porous media.

Forced convection film boiling heat transfer from a horizontal cylinder in water and Freon-113 flowing upward perpendicular to the cylinder under subcooled conditions was measured for the flow velocities from 0 to 1 m/s at the system pressures ranging from 100 to 500 kPa: the platinum horizontal cylinders with diameters ranging from 0.7 to 5 mm were used as the test heaters. The film boiling heat transfer coefficients were obtained for the surface superheats from about 800 K for water and from about 400 K for Freon-113 down to minimum film boiling surface superheats. These heat transfer coefficients increase with the increase in flow velocity, liquid subcooling, system pressure, and with the decrease in cylinder diameter. A correlation for subcooled forced convection film boiling heat transfer was presented, which can describe the experimental data obtained within ±20% for the flow velocities below 0.7 m/s, and within −30% to +20% for the higher flow velocities. The correlation also predicted well the data by Shigechi (1983), Motte and Bromley (1957), and Sankaran and Witte (1990) obtained for the larger diameter cylinders and higher flow velocities in various liquids at the pressures of near atmospheric. The Shigechi’s data were in the range from about −20% to +15%, the data of Motte and Bromley were about ±30%, and the data of Sankaran and Witte were within +20% of the curves given by the corresponding predicted values.

In modeling catalytic combustion in a monolithic catalytic converter, it is generally assumed that the gas within the individual monolith channels does not interfere with thermal radiation. To date, no quantitative study has been undertaken to validate this assumption. Past studies for carbon monoxide combustion also appear to indicate that the emissivity of the washcoat has little effect on the thermal radiation field. In order to investigate these two issues, methane-air combustion on platinum is modeled inside a single channel of a monolith using a detailed surface reaction mechanism comprised of 24 reactions between 19 species. Radiation transport is modeled using the Discrete Ordinates Method and a gray formulation. Planck-mean absorption coefficients of the gases, calculated from the HITEMP and HITRAN databases, are used to investigate participating medium effects. All calculations were performed using the commercial CFD code, CFD-ACE+™, supplemented by user-subroutines for calculating the absorption coefficient of the gas mixture. Results show that the conversion percentages and temperature distributions are unaltered by the inclusion of participating medium radiation effects, verifying the commonly held belief, stated earlier. However, in strong contrast with carbon monoxide combustion, the emissivity of the washcoat was found to significantly affect flammability limits in the case of methane combustion—the flame being hotter and more stable for smaller values of emissivity.

The popular 3ω method of measuring thermophysical properties of solids is adapted for the simultaneous measurement of thermal conductivity and heat capacity in both liquids and gases. This technique is experimentally simple and has a lower susceptibility to random experimental noise, bulk fluid motion, radiation losses, and non-linear effects than other transient hot wire measurement methods. The compactness of the 3ω hotwire allows it to be used with different fluids in a variety of circumstances with very little specialized experimental equipment. Both the experimental setup and theoretical model are detailed. Experimental 3ω measurements were made in a variety of common fluids (air, water, and mineral oil) using commercially drawn 10μm platinum and 5μm tungsten hot wires which serve as both heating element and resistance thermometer. Measurements taken over a range of frequencies are numerically reduced to provide both thermal conductivity and heat capacity information. Experimental measurements and the corresponding analytical model are presented in terms of impedance or thermal resistance; a more physically meaningful and intuitive basis of comparison. Fluid properties are determined by curve-fitting an analytical model to experimental data using a least-squares approach. This technique allows both thermal conductivity and heat capacity (or thermal diffusivity) to be uniquely determined from a single measurement sequence.

This paper describes the heat transfer from a heated microcantilever to the substrate and the resultant temperature distributions. A four-point probe platinum resistive thermometer having a 140 nm lateral resolution has been fabricated on the SiO 2 -coated silicon substrate. The estimated temperature coefficient of resistance (TCR) of the thermometer is 0.0011 K −1 , approximately one third of the bulk value. When the heated cantilever scans over the thermometer, up to 70% of the cantilever power is transferred to the substrate through the air, heating up the substrate. The maximum substrate temperature rise measured with the thermometer is around 7 K. From the force-displacement experiment, the effective contact thermal conductance was estimated to be around 40 ± 20 nW/K. The obtained results will help further understanding of thermal behavior of the heated cantilever during the scanning and its effect on the substrate.

This paper reports experimental investigation of the scaling effect on thermal transport in platinum nanoheaters deposited on a single crystalline silicon substrate. The size dependency of the thermal resistances of nanoheaters with dimensions from 5 μm down to 100 nm is experimentally obtained. Additional Joule heating and electrical resistance thermometry in heater/sensor pairs of various dimensions and spacing (100 nm to 400nm) are carried out in order to study the temperature distribution in the vicinity of these nanoheaters. The results of these two sets of measurements confirm that the thermal resistances of smaller nanoheaters are dominated by the interface resistance between the platinum and silicon substrate. Furthermore, analytical solutions of the BTE around a hot spot are used to evaluate and examine the significance of the localized heating or sub-continuum transport around a hotspot.

Pressure sensitive paint (PSP) technique is based on the interaction of oxygen molecules with luminescent molecules, and it seems suitable for analyses of high Knudsen number flows which require diagnostic tools in the molecular level. However, application of the PSP technique to micro-devices is still very difficult because of the thickness of PSP layers of the order of microns and the aggregation of luminescent molecules caused by the “painting” methods. To resolve the problems of ordinary PSPs mentioned above, we adopt the Langmuir-Blodgett (LB) method to fabricate pressure sensitive molecular films (PSMFs) having nanometer order thickness. The fundamental properties of the PSMFs such as the pressure sensitivity and the surface roughness are examined, to evaluate the feasibility of the PSMF technique for pressure measurement around micro-devices. A PSMF based on palladium (II) mesoporphyrin IX shows high pressure sensitivity in low pressure regime, while a PSMF based on platinum (II) mesoporphyrin IX is optimum for atmospheric pressure. It is also clarified that the PSMFs do not degrade the roughness of solid surfaces. The results indicate the feasibility of the PSMF technique for pressure measurement in high Knudsen number flows such as micro flows.

The dependence of molecular motion on the dissociative adsorption mechanism of hydrogen molecule (H 2 ) on platinum (Pt) surface was studied by Molecular Dynamics (MD) method. An interaction between atoms was considered by the Embedded Atom Method (EAM). A potential between an H atom and a Pt atom was determined from results of Density Functional Theory (DFT). Dissociation probabilities of three surface conditions, that is, (1) when the surface temperature is 300 K, (2) when the surface temperature is 0 K with allowing motion of the surface atoms and (3) when the surface temperature is 0 K with prohibiting motion of the surface atoms, were obtained. From results of the simulations, the effect of surface motion on dissociation probability was analyzed as a function of initial energy of the dissociating molecule or the surface conditions. First, it was concluded that the increase in the dissociation probability of the case (3) by the increase in the initial translational energy of H 2 molecule is gentle compared with those of the other cases. Additionally, the minimum initial translational energy of H 2 molecule of case (3) at which the H 2 molecule can dissociate is the smallest among all of three cases. It was found that this is because the range of the dissociation barrier distribution for the case (3) is wider than those for the other cases due to the thermal motion of surface atoms. Moreover, the effect of translational and rotational motion of molecule on the dissociation probability was analyzed. It was concluded that the dissociation probability increases with the increase in the translational energy while it decreases with the increase in the rotational energy when the rotational energy is small.

Catalytic combustion of hydrogen-air boundary layers involves the adsorption of hydrogen and oxygen into a platinum coated surface, chemical reactions of the adsorbed species and the desorption of the resulting products. Re-adsorption of some produced gases is also possible. The catalytic reactions can be beneficial in porous burners and catalytic reactors that use low equivalence ratios. In this case the porous burner flame can be stabilized at low temperatures to prevent any substantial gas emissions, such as nitrogen oxides. The present paper is concerned with the numerical computation of heat transfer and chemical reactions in hydrogen-air mixture boundary layers that flow over platinum coated hot plates. Chemical reactions are included in the gas phase as well as on the solid platinum surface. In the gas phase, eight species are involved in 26 elementary reactions. On the platinum hot surface, additional surface species are included that are involved in 14 additional surface chemical reactions. The platinum surface temperature is fixed, while the properties of the reacting flow are computed. The flow configuration investigated in the present paper is that of a parallel boundary layer. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Hybrid differencing is used to ensure that the finite-difference coefficients are always positive or equal to zero to reflect the real effect of neighboring nodes on a typical central node. The finite-volume equations are solved, iteratively, for the reacting gas flow properties. On the platinum surface, surface species balance equations, under steady-state conditions, are solved numerically. A non-uniform computational grid is used, concentrating most of the nodes in the boundary sub-layer adjoining the catalytic surface. The computed OH concentration is compared with experimental and numerical data of similar geometry. The obtained agreement is fairly good, with differences observed for the location of the peak value of OH. Surface temperature of 1170 K caused fast reactions on the catalytic surface in a very small part at the leading edge of the catalytic flat plate. The computational results for heat and mass transfer and chemical surface reactions at the gas-surface interface are correlated by non-dimensional relations.

Polymerase Chain Reaction (PCR) is a molecular biological method for the in vitro amplification of nucleic acid molecules which has wide applications in the area of genetics, medicine and biochemistry. The typical three step PCR cycle consists of heating the sample to 90–94 °C to denature double-stranded DNA, cooling down to 50–54 °C to anneal the specific primers to the single stranded DNA and finally increasing the temperature to 70–75 °C for extension of the primers with thermostable DNA polymerase. The temperature sensitivity of the reaction requires precise temperature control and proper thermal isolation of these three zones. In this paper we present the design of a continuous flow PCR microfluidic device with the channels fabricated in (poly) dimethylsiloxane (PDMS) and thin film Platinum Resistance Temperature Detector (RTD) elements fabricated on glass substrate to define the three different temperature zones. The fluidic arrangement has a water jacket layer to minimize evaporation from the porous PDMS walls. A detailed thermo fluidic model of the device is presented to predict the performance and efficacy of the proposed design. Numerical simulations are carried out to find the temperature distribution and temperature gradients in the device and a parametric study is done by varying flow rate, heat flux and channel dimensions in order to optimize the design for achieving temperature isolation and sharp temperature gradients between different zones.

The effect of the cathode catalyst layer’s structure and composition on the overall performance of a polymer electrolyte fuel cell (PEMFC) is investigated numerically. The starting point of the sub-grid scale catalyst layer model is the well-known flooded agglomerate concept. The proposed model addresses the effects of Nafion loading, platinum loading, platinum/carbon ratio, agglomerate size and cathode layer thickness. The sub-grid scale model is first validated against experimental data and previously published results, and then embedded within a two-dimensional validated computational fluid dynamics code that can predict the overall performance of the fuel cell. The integrated model is used to explore a wide range of the compositional and structural parameter space, mentioned earlier. In each case, the model is able to correctly predict the trends observed by past experimental studies. The studies show that the presence of an optimal performance with varying Nafion content in the cathode is more due to the local agglomerate-level mass transport and conductivity losses in the polymer coating around the agglomerates than due to the amount of Nafion within the agglomerate. It was also found that platinum mass loading needs to be at a moderate level in order to optimize fuel cell performance. Cathode tortuosity has a significant effect on fuel cell performance at low values of agglomerate radius.

Catalytic combustion of hydrogen-air mixtures involves the adsorption of the fuel and oxidant into a platinum surface, chemical reactions of the adsorbed species and the desorption of the resulting products. Re-adsorption of some produced gases is also possible. The catalytic reactions can be beneficial in porous burners that use low equivalence ratios. In this case the porous burner flame can be stabilized at low temperatures to prevent any substantial gas emissions, such as nitric oxide. The present paper is concerned with the numerical computation of heat transfer and chemical reactions in flowing hydrogen-air mixtures axisymmetrically around a platinum-coated thin cylinder. Chemical reactions are included in the gas phase and in the solid platinum surface. In the gas phase 8 species are involved in 24 elementary reactions. On the platinum hot surface, additional surface species are included that are involved in 14 additional surface chemical reactions. The platinum surface temperature is fixed, while the properties of the reacting flow are computed. The flow configuration investigated here is the parallel boundary layer reacting flow over a cylinder. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to ensure that the influence coefficients are always positive to reflect the physical effect of neighboring nodes on a typical central node. The finite-volume equations are solved iteratively for the reacting gas flow properties. On the platinum surface, surface species balance equations, under steady-state conditions, are solved numerically by an under-relaxed linear algorithm. A non-uniform computational grid is used, concentrating most of the nodes near the catalytic surface. Surface temperatures, 1150 K and 1300 K, caused fast reactions on the catalytic surface, with very slow chemical reactions in the flowing gas. These slow reactions produce mainly intermediate hydrocarbons and unstable species. The computational results for the chemical reaction boundary layer thickness and mass transfer at the gas-surface interface are correlated by non-dimensional relations, taking the Reynolds number as the independent variable. Chemical kinetic relations for the reaction rate are obtained which are dependant on reactants concentrations and surface temperature.

Forced convection film boiling heat transfer on a horizontal cylinder in saturated water and Freon-113 flowing upward perpendicular to the cylinder was measured for the flow velocities ranging from zero to 1 m/s at the system pressures ranging from 100 to 500 kPa: the platinum cylinders with the diameters ranging from 0.7 to 5 mm were used as the test cylinder heaters. The existing correlation for forced convection film boiling heat transfer given by Bromley et al. could not well describe the experimental data obtained, especially those for the higher pressures. The forced convection film boiling heat transfer correlation including the radiation contribution from the cylinders with various diameters for saturation conditions was developed based on forced convection two-phase laminar boundary layer film boiling model and the experimental data obtained. The experimental data agreed with the corresponding values derived from the correlation within ±15% for the flow velocities below 0.7 m/s, and within −30% to +15% for higher flow velocities. It was confirmed that the experimental data obtained by Bromley et al. for the horizontal carbon cylinders with the diameters ranging from 9.83 to 16.2 mm and with the significant radiation effect from the cylinder surfaces in various liquids for the various flow velocities up to 4.4 m/s at an atmospheric pressure agreed with the corresponding values derived from the new correlation within ±20%.

As analysis of thermal-technical parameters of the existing polymer exchange membrane fuel cells (PEMFC) indicated, their electrical efficiency is not above 32–37% which is due to relatively high (10–15%) electric power consumption to cover own needs. Its major percentage falls on pump and compressor drive (air, fuel, water supply) using the AC electric engines connected via converter to FC. A real alternative allowing the efficiency increase up to 45% and higher is to apply hybrid plants when an AC turboelectric generator (TEG) is built into the thermodynamic cycle, the latter operating at the expense of utilization of the FC reagent stream power. TEG is a micro turbocompressor with incorporated turbo generator manufactured on the basis of the micro electric-mechanical system (MEMS) technology. The numerical study over the hybrid cycle effectiveness revealed that the optimum efficiency is ensured provided the TEG power does not exceed 12–15% of the total hybrid power plant power. Considering a value of the PEMFC mean power of around 8, a need arises in TEG of 500 to 1500W power to operate as a part of hybrid engine. Among various auxiliaries involved into the PEMFC, there should be devices that ensure getting target values in terms of hydrogen purity, a dominant synthetic gas fraction, at the FC inlet. A need to get and maintain hydrogen purity on the level of 99.99% is related with that if CO content in mixture is higher 0.01%, then a normal operation of FC fails due to “poisoning” effect of the platinum-containing catalysts of the FC electrodes. To forestall this, the palladium molecular membranes are now typically applied which causes the filtered flow temperature limitations within values not exceeding 500–550°C. At the same time, PEMFC in most cases must operate on air and syngas produced by reforming, with their temperature amounting to 800–900°C. To provide this, we need to have ceramic molecular membranes (CMM) made of a ceramic support in which macropores nano pores are formed (characteristic size of less 0.3nm), the latter achieved through application of a complex of various chemical and physical processes. In the paper presented, findings of numerical studies over a hybrid engine plant are provided, this hybrid plant (PEMFC + MEMS) as compared with alternatives to existing and offered schemes, and, also, test data on the zeolite-based support CMM are demonstrated.