An experimental investigation on heat transfer peculiarity of kerosene flowing in vertical upward high flux tubes at supercritical pressure is presented. Three inner-sintered steel powder coating tubes (high flux tubes) and one smooth tube are tested under the different super-critical pressure and different mass flux of kerosene in the experiment. Results are found that all three high flux tubes perform much better than smooth tube at the same parameters of the tube and same working conditions. It can be obviously deduced that the outer wall temperature is reduced by the disturbance in the flow field of the sintered metal coating at the inner tube-side, while the reduced mass flux can increase the wall temperature on the contrary. Heat transfer coefficient is found 2.5 times as the smooth tube, yet both too large and too small particle diameters of metal powder sintered on the tube surface can deteriorate heat transfer. Density and viscosity, thermal conductivity of kerosene at different temperatures and pressures under supercritical pressure can be evaluated by using the extended corresponding state principle, which shows good consistency with the experimental results.

A study was conducted to experimentally characterize the pool boiling performance of hydrofluorocarbon (HFC)-245fa. The motivation for this research is to characterize the performance of candidate refrigerants for potential use in automotive power electronics two-phase cooling systems. The HFC-245fa pool boiling experiments were conducted using horizontally oriented 1-cm 2 heated surfaces to quantify the effects of pressure and a microporous-enhanced coating on heat transfer coefficients and critical heat flux (CHF) values. Experiments were carried out at pressures ranging from 0.15 MPa to 1.1 MPa (reduced pressure range: 0.04–0.31). To enhance boiling heat transfer, a copper microporous coating was applied to the test surfaces. The coating was found to enhance heat transfer coefficients by as much as 430% and CHF by approximately 50%. Increasing pressure decreased the magnitude of the heat transfer coefficient enhancements but had minimal effect on CHF enhancements. The experimental data were then used to generate correlations for the boiling heat transfer coefficients and CHF values. Finally, the performance of HFC-245fa was compared to the performance of hydrofluoroolefin (HFO)-1234yf and HFC-134a at conditions of equivalent saturation temperatures and reduced pressures.

Heat pipes are used in many applications as an effective means for transferring heat from a source to a sink. The basic heat pipe typically consists of a solid metal casing within which a working fluid is sealed inside at a given pressure. The latent heat transfer via the heat pipe’s working fluid allows it to carry a larger amount of heat energy than would normally be possible with an identically dimensioned solid metal rod. Water is often used as a working fluid due to its high heat of vaporization and suitable operating range for electronics cooling. For many applications, especially space, aluminum is desired as a casing material for its high thermal conductivity, low weight, and low cost. However, water is incompatible for use with aluminum heat pipes because it forms a non-condensable gas (NCG), hydrogen, when they contact. In this work, an inorganic aqueous solution (IAS), which has thermophysical properties similar to water, has been used as the working fluid with an aluminum alloy 5052-H2 casing. The prepared thermosiphon underwent long-term lifetime testing and the results indicate no tube failure or significant NCG formation for the duration of the 9 week study. Furthermore, the data indicate that the IAS fluid not only inhibited NCG production but also led to a reduction in heat pipe thermal resistance over time. It is believed that the chemicals in IAS react with the aluminum surface to create a compact oxide layer and electrochemical reaction which prevents hydrogen generation. A secondary, hydrophilic surface coating is also generated by the fluid on top of the first oxide (passivation) layer. This hydrophilic layer is believed to be responsible for the heat transfer enhancement which was observed during testing and the reduction in ΔT (defined as T evap −T cond ) over time. Aluminum heat pipes used currently in practice utilize ammonia, or other non-water based working fluids, which have inferior latent heats of vaporization compared to water or an aqueous-based fluid such as IAS. The use of aluminum heat pipe casings in combination with a water-based fluid such as IAS has the potential to provide a significant increase in heat transport capability per device unit mass over traditional ammonia charged aluminum heat pipes.

Non-uniform heating during the austenization stage of hot-stamping can lead to inhomogeneous mechanical properties, coating thickness variations, and residual stresses in the stamped components. This paper presents a heat transfer model of an electrically-powered batch austenization furnace, with the objective of diagnosing and correcting factors that cause non-uniform blank heating. Radiation view factors are calculated with the Monte Carlo method. Convection with the furnace air and conduction through the blank and furnace door is also incorporated into a transient global domain energy balance. The model incorporates temperature- and phase-dependent radiative and thermophysical properties of the steel blanks and ceramic insulation of the furnace walls. The heat transfer model is validated with data obtained from instrumented blanks in an actual production cycle. The model is then used to optimize heater settings for the existing furnace using simulated annealing.

Nanoimprint Lithography (NIL) is becoming a powerful tool for nanolithography, nanofabrication and nanomanufacturing for nanotechnology applications. However, there is still a lack of systematic study of key processing parameters, which determine the imprinted pattern quality in terms of uniformity and replication fidelity. This research focuses on identifying the most important parameters in a nanoimprint process, in which microscale patterns were imprinted into polymethyl-methacrylate (PMMA) polymer with polydimethylsiloxane (PDMS) mold. The effects of several parameters such as pre-imprint temperature, pre-imprint pressure, imprint temperature, imprint pressure, imprint time, venting temperature and venting time, were varied in a certain range during the imprinting process. The imprinting results were analyzed with a three-level design of experiments (DOE) analysis. It was found that the pre-imprint pressure and imprint temperature are the key parameters. In addition, the DOE analysis is a powerful tool for NIL process optimization. As a practice, a vacuum assisted and selective coating (VASC) method based on a commerical nanoimprinting tool was developed to fabricate micro-hole arrays on a PbSe nanocrystal film to study its spectral response to IR radiation for applications such as IR detection and photovoltaic. The process optimization significantly improves imprinting quality.

Homogenous dispersing of nanoparticles in a base fluid is an excellent way to increase the thermal performance of heat transfer devices especially Heat Pipes (HPs). As a wickless, cheap and efficient heat pipe, Pulsating Heat Pipes (PHPs) are important candidates for thermal application considerations. In the present research an Open Loop Pulsating Heat Pipe (OLPHP) is fabricated and tested experimentally. The effects of working fluid namely, water, Silica Coated ferrofluid (SC ferrofluid), and ferrofluid without surface coating of nanoparticles (ferrofluid), charging ratio, heat input, and application of magnetic field on the overall thermal performance of the OLPHPs are investigated. Experimental results show that ferrofluid has better heat transport capability relative to SC ferrofluid. Furthermore, application of magnetic field improves the heat transfer performance of OLPHPs charged with both ferrofluids.

A quenching/diffusion analytical model has been developed for predicting the wall temperature and wall heat flux behind bubbles sliding in a confined narrow channel. The model is based on the concept of a well-mixed liquid region that enhances the heat transfer near the heated wall behind the bubble. Heat transfer in the liquid is treated as a one-dimensional transient conduction process until the flow field recovers back to its undisturbed level prior to bubble passage. The model is compared to experimental heat transfer results obtained in a high-aspect-ratio (1.2×23mm) rectangular, horizontal channel with one wide wall forming a uniform-heat-generation boundary and the other designed for optical access to the flow field. The working fluid was Novec™ 649. A thermochromic liquid crystal coating was applied to the outside of the uniform-heat-generation boundary, so that wall temperature variations could be obtained and heat transfer coefficients and Nusselt numbers could be obtained. The experiments were focused on high inlet subcooling, typically 15–50°C. The model is able to capture the elevated heat transfer rates measured in the channel without the need to consider nucleate boiling from the surface or microlayer evaporation from the sliding bubbles. Surface temperatures and wall heat fluxes were estimated for 17 different experimental conditions using the proposed model. Results agreed with the measured values within ±15% accuracy. The insight gathered from comparing the results of the proposed model to experimental results provides the basis for a better understanding of the physics of subcooled bubbly flow in narrow channels.

Nucleate pool boiling heat transfer has been experimentally studied at ambient temperature, on a surface with a novel nanofiber mat coating. The nano-textured surface was made of alumina ceramic substrate covered by an electrospun polymer nanofiber mat with a thickness of about 30 μm and immersed in saturated HCFC-123. The surfaces of the individual polymer nanofibers in the mat were copper-plated. Significant enhancements in nucleate boiling heat transfer as well as reduction of surface temperature have been achieved for the copper nanofiber-coated surface compared to a bare surface.

A Finite Difference Time Domain (FDTD) simulation is employed to calculate electromagnetic field and charge density distributions at the junction between a gold nanoparticle (NP) tip of a scanning ZnO nanowire and gold NP bio-markers. This three-dimensional simulation calculates the magnetic and electric field components in a large matrix of Yee cells by solving Maxwell’s curl equations. An absorbing boundary condition is included to eliminate reflection back into the simulation chamber. In the specific simulations considered here, a laser pulse of single wavelength is incident on the backside of a transparent silicon dioxide micro-cantilever, and coupled into a ZnO nanowire grown from an opening on a metal coating of the front side of the cantilever. The simulation results reveal local field enhancement between the gold NP tip of the nanowire and only one of three 20 nm gold NPs with a 28 nm empty spacing between two adjacent NPs. The charge density distributions in the gold tip and the gold NP are calculated and correlated with the local field enhancement, which makes the gold tip of the scanning nanowire waveguide attractive for use in imaging gold NP bio-labels on cell membranes.

The simulated triple vacuum glazing consists of three, 4 mm thick glass panes with two vacuum gaps with each internal glass surface coated with a low emittance coating. The two vacuum gaps are sealed by an indium based sealant and separated by a stainless steel pillar array with a height of 0.12 mm and a pillar diameter of 0.3 mm spaced at 25 mm. Both solder glass and indium based sealants have been successfully applied in vacuum glazing previously. The thermal performance of the triple vacuum glazing was simulated using a finite volume model. The simulation results show that although the thermal conductivity of solder glass (1 W.m −1 .K −1 ) and indium (83.7 W.m −1 .K −1 ) are very different, the increase in heat transmission of triple vacuum glazing with a 10mm frame rebate resulting from the use of an indium edge seal compared to a solder glass edge seal was 0.48%. Increasing both edge seal widths from 3 mm to 10 mm led to a 24.7% increase in heat transmission of the triple vacuum glazing without a frame and an 18.3% increase for a glazing with a 10 mm frame rebate depth. Increasing the rebate depth in a solid wood frame from 0 to 15 mm decreased the heat transmission of the triple vacuum glazing by 32.9%. The heat transmission of a simulated 0.5 m by 0.5 m triple vacuum glazing was 32.2% greater than that of 1 m by 1 m triple vacuum glazing.

In this paper the interfacial characteristics of a liquid flowing over a 1cm 2 array of hydrophobic cylindrical micropillars located within a microchannel are investigated. The microchannel was 12mm wide and 32mm long with an average channel height of approximately 83μm. Hydrophobic coating of the channel was achieved via a controlled flow of a trichlorosilane and ethanol solution. A method to remove lodged gas bubbles from a microchannel was successfully demonstrated, while maintaining the favorable Cassie-Baxter wetting state (gas/vapor layer present) of the micropillar structures. This was achieved using degassed water to dissolve low-curvature gas bubbles, while ohmically heating the silicon substrate to reassert and maintain the Cassie-Baxter wetting state of the hydrophobic micropillars. During this experimentation it was discovered that the part wetting and dewetting of a superhydrophobic (SH) surface within a microchannel could be achieved using similar methods. The onset of surface wetting (Wenzel wetting state) was achieved by pumping degassed water through the microchannel. Surface dewetting was then accomplished through substrate heating by the increase in the trapped gas layer pressure, the water vapor pressure and outgassing from the lightly degassed fluid. These reactions force the gas/vapor layer to expand laterally throughout the micropillar array, thus restoring the Cassie-Baxter wetting state. The reported results demonstrate a low-power method for effectively reversing the Wenzel wetting state of a SH surface under microchannel flow conditions and may prove to be a useful technique for manipulating fluid flow within microfluidic devices.

Automotive industry frequently needs to test new products, according to different production parameters, in order to determine the actual thermal behavior of bodies before mass production is implemented. Numerical simulation of these processes can reduce the very expensive and time consuming experimental procedures. For the drying and hardening process of the top paint applied in the coating process, the body temperature must be raised according to the paint manufacturer regulations. Consequently, prediction of temperature distribution of the car body during various zones of ovens is very vital in the design and performance analysis of the paint dryers. In this research, a novel semi-analytical approach has been used to predict the body temperature variation during the curing process. Considering the energy balance for the body, a set of differential equation has been extracted, depending on the oven zone. These equations can be solved numerically to find the transient temperature profile of the car body. Some parameters in these equations have been achieved by experimental procedure. The results show that the present model predictions are in a good agreement with the experimental data. Therefore, the developed model has a reasonable accuracy and can be used as an efficient robust approach to distinguish overall thermal behavior of the body. These techniques can be used to optimize the design of curing paint oven.

The present work reports on flow boiling visualization of refrigerant R-134a in a vertical circular channel with internal diameter of 1.33 mm and 235 mm in heated length. Quartz tube with a homogeneous ITO-coating is used allowing heating and simultaneous visualization. Flow patterns have been observed along the heated length with the aid of a digital camera with close-up lenses. From the flow boiling visualization, seven distinct two-phase flow patterns have been observed: Isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow , and mist flow . Two-phase flow pattern observations are presented in the form of flow pattern maps. Finally, the experimental flow pattern map is compared to models developed for conventional sizes as well as to a microscale map for air-water mixtures available in the literature, showing a large discrepancy.

The methanol steam reforming plays an important role for hydrogen supply to the proton membrane exchange fuel cell in the portable power applications. The catalyst coating on the walls of channels is often used in the fabrication of the reactors in the reformer to minimize the pressure loss. In this study, the temperature and concentration fields in the reactors for the methanol steam reforming were investigated numerically. The methanol conversion is usually used to evaluate the performance of the reformer. The effects of the inlet gas temperature in the heat supply channel and inlet velocity in the reforming channel on the performance of the methanol steam reforming are presented.

Effective thermal conductivity is an important parameter for the design of porous ceramic coatings that are exposed to high temperatures. Pore shapes which change in relation with the coating parameters and coating method affect the effective thermal conductivity significantly. In this study the effective thermal conductivity of 8 wt % yttria stabilized ZrO 2 coating deposited by air plasma spraying have been calculated by finite element method using digital images. The effects of pore shape on effective thermal conductivity have been discussed.

The thin film evaporation process through use of thin micro-scale sintered copper mesh screen was proven to be a very effective heat transfer mechanism with high critical heat flux (CHF). This efficient heat transfer mechanism is widely used in designing heat pipe, Capillary Pumped Loops (CPL), and drying process, however, the nucleation process and meniscus dynamics at the liquid-vapor-solid interface are not directly observed and systematically studied. Very few visual investigation in thin film evaporation has been conducted. In the existing two visual studies, the interface thermal resistance between coating and the heated wall was not seriously considered, and the heat flux was limited below 35 W/cm 2 . In this visualization investigation, the nucleation process and meniscus dynamics from initial condition to drying out were observed and well documented. To minimize the interface thermal resistance, the micro scale wicking was sintered to heated wall directly. High quality images were acquired through a well-designed visualization system. The majority of nucleate bubbles, whose diameters are at a magnitude of 10 μm, were found to form on the top wire surfaces instead of inside the porous media at moderate heat flux. Few large size bubbles were observed to grow inside capillary wicks, however, their presence did not seem to stop the evaporation process as reported before. The menisci receding process was visually captured for the first time. The minimum menisci radius was found to form at the smallest corners and pores. It is also illustrated the thin liquid area increases when the menisci recede and the thin liquid film evaporation is the dominant heat transfer mode at high heat flux. The present work visually confirms the heat transfer regimes of evaporation on micro porous media, which was proposed by Li and Peterson [2], and further improves the understanding to the nucleate boiling and thin liquid film evaporation on the surfaces of micro sintered copper mesh screen.

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.

Vertically oriented carbon nanotube (CNT) arrays can increase real contact in thermal interfaces and efficiently transfer heat between the mating solids. In this study, a paraffin wax that changes phase at approximately 50°C was applied to interfaces with CNT arrays directly synthesized on one side of the interface, and to foil/CNT interfaces with CNT arrays directly synthesized on one side and both sides of the foil. The bulk thermal resistances of single-sided CNT array/wax interfaces were measured using a transient photoacoustic (PA) technique to range from approximately 2 to 3 mm 2 ·K/W under moderate pressures. The bulk thermal resistances of foil/CNT/wax interfaces were measured with the PA technique to range from approximately 10 to 20 mm 2 ·K/W under moderate pressures. For each sample structure, the addition of paraffin wax to the CNT arrays produced significant reductions in thermal resistance. We surmise that this improved thermal performance could be a function of the wettability of paraffin wax to CNTs. A hydrophilic wetting angle of approximately 47° was observed at the interface of liquid wax and the free tips of a CNT array at 60°C, and field-emission scanning electron microscope images taken after PA testing revealed individual CNTs that were blanketed with a wax coating.

The design of a two phase heat exchanger requires a fundamental knowledge of the parameters affecting film condensation. The interaction of roughened surfaces and change of surface energy by the addition of a surfactant to the system for quiescent steam was examined in this study. Three types of roughened surfaces were generated by roughening a polished surface with sandpaper, electroplating to obtain a thin porous coating of nodules onto the condenser surface, and soldering of porous aluminum foam to the condenser face. The addition of a nonionic surfactant, alcohol alkoxylate, to the system was evaluated for the roughened surfaces. For a Reynolds number less than 50, the roughened surfaces examined all degraded the condensation heat transfer. The addition of a surfactant to the system enhanced condensation heat transfer on polished surfaces almost two fold.

Four types of steady-state boiling experiments were conducted to investigate the efficacy of two distinctly different heat transfer enhancement methods for external reactor vessel cooling under severe accident conditions. One method involved the use of a thin vessel coating and the other involved the use of an enhanced insulation structure. By comparing the results obtained in the four types of experiments, the separate and integral effect of vessel coating and insulation structure were determined. Correlation equations were obtained for the nucleate boiling heat transfer and the critical heat flux. It was found that both enhancement methods were quite effective. Depending on the angular location, the local critical heat flux could be enhanced by 1.4 to 2.5 times using vessel coating alone whereas it could be enhanced by 1.8 to 3.0 times using an enhanced insulation structure alone. When both vessel coating and insulation structure were used simultaneously, the integral effect on the enhancement was found much less than the product of the two separate effects, indicating possible competing mechanisms (i.e., interference) between the two enhancement methods.