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To clarify the mechanisms of heat transfer on an engine wall is an important challenge because the heat loss on the wall is one of the dominant factors that limit the thermal efficiency. A thin-film resistance type heat flux sensor fabricated with Micro-Electro-Mechanical Systems (MEMS) technologies was applied to research on the heat transfer of an impinging diesel jet. The MEMS sensor had a high sensitivity and multiple measurement points with a scale comparable to the turbulence of in-cylinder flow for the investigation of local instantaneous heat transfer characteristics in engines. Measurements of wall temperature and heat flux were conducted in a constant volume chamber under the reference conditions of Engine Combustion Network (ENC) Spray A with variations in injection pressure. As a result, the maximum heat flux and heat transfer coefficient reached about 5 MW/m2 and 3 MW/(m2 K), respectively. The results were compared to those obtained with a commercially available thermocouple, and both sensors produced similar trends. In addition, fluid motion near the wall was estimated from the heat flux fluctuations using a cross-correlation analysis. The relationship between heat transfer and flow was investigated in a dimensionless number fashion, and it was compared to a semi-theoretical prediction that was based on a steady state laminar flow heat transfer model.

In this study, it was attempted to estimate the flow characteristics in the vicinity of an engine inner wall from the instantaneous local heat fluxes measured using a micro-electro-mechanical systems sensor. As the sensor has three resistance temperature detectors with a size of 315 µm fabricated on a circumference with a diameter of 900 µm in rotational symmetry, it can measure local heat flux on the equivalent scale of the turbulence of in-cylinder flow. The advective velocity and turbulent eddy scale were estimated from heat flux fluctuations using a cross-correlation analysis, and these were compared with results of particle image velocimetry performed under motored operation conditions. As a result, it was found that the micro-electro-mechanical systems sensor has the potential to detect the gas side information such as the wall parallel flow velocity. Although further verification of the physical meanings of the estimated characteristics is necessary, the micro-electro-mechanical systems sensor will become a powerful tool for engine diagnostics.

Wall heat transfer is one of the most important aspects of internal combustion engines. In this study, the local instantaneous heat transfer characteristics were investigated using an originally developed thin-film resistance-type sensor. The sensor, which was fabricated by micro-electro-mechanical systems (MEMS) technology, had advantages in the sensitivity, the spatial resolution, and the number of measurement points compared with conventional sensors. The MEMS sensor could clearly detect the cycle-by-cycle heat flux. Consequently, it was found that the local instantaneous heat flux had a strong cycle-to-cycle variability, the magnitude of which was in the order of 0.1–1 MW/m2, not only in fired operations but also a motored operation. The heat transfer coefficients also exhibited a strong cyclic variability with a magnitude of 0.1–1 kW/(m2⋅K). These results indicated that the heat transfer characteristics could not be properly evaluated only by the conventional analysis based on the ensemble-averaged data. Additionally, it was found that the heat fluxes at the adjacent points exhibited similar but different values, indicating that there were turbulent eddies of sub-millimeter scale and they affected the local heat transfer. Since the local instantaneous heat transfer had quite different characteristics from the average, advanced design and control based on the understandings of the local instantaneous characteristics will be need for the further improvement of the engine’s thermal efficiency.

A MEMS measurement technique was used to study the mechanisms of wall heat transfer and bubble growth during subcooled boiling of water in the isolated bubble region. The local wall temperature beneath a single bubble in subcooled boiling was measured at a sampling frequency of 50 kHz using a MEMS sensor. The wall and liquid phase heat transfers were quantitatively evaluated by the analysis with the measured local temperature and bubble size extracted from bubble images. The microlayer evaporation provided a significant contribution to the wall heat transfer and the bubble growth, but the rewetting heat transfer during the bubble departure process was insignificant in the wall heat transfer, as with our previous results for saturated boiling. The subcooling of the bulk liquid was found not to have significant influence to the microlayer evaporation. The condensation heat transfer from vapor bubble to bulk subcooled liquid was comparable to the microlayer evaporation heat transfer, and subcooled boiling single bubbles behaved like a heat pipe on the heated wall. The bubble oscillation on the surface was observed in one case. The evaporation of the microlayer which repeatedly reformed in the oscillation process transferred a high heat flux of around 1 MW/m 2 . Evaporation and formation characteristics of the microlayer were additionally examined.

This chapter consists of six sections in which distinctive experimental works to explore the mechanisms of nucleate boiling are described. Sections 2.1 and 2.2 discuss the detailed heat transfer mechanisms during the bubble nucleation process. A sophisticated MEMS sensor is used to measure the surface temperature in Section 2.1 and the microlayer structure is investigated using laser techniques in Section 2.2. Section 2.3 discusses the heat transfer characteristics of mini/microchannel boiling systems that receive increasing attention in various applications. Sections 2.4 and 2.5 deal with nucleate boiling heat transfer of mixtures. Section 2.4 reports the surface tension and the critical heat flux of high-carbon alcohol solutions. In Section 2.5, mixture effect is investigated from various standpoints; the heat transfer characteristics of immiscible mixtures are also discussed. In the last section (2.6), to elucidate the heat transfer mechanisms and the void development in subcooled flow boiling, bubble dynamics is explored based on the visualization results using a high-speed camera.

Heat transfer characteristics of isolated bubble saturated pool boiling of water were investigated by local wall temperature measurement using an original micro-electro-mechanical systems (MEMS) sensor and a wall heat transfer evaluation. The measured temperature clearly indicated dynamic heat transfer phenomena below an isolated bubble including microlayer evaporation, dry-out of the microlayer, and rewetting of the dry-out region. Local heat removal due to microlayer evaporation exceeded 1 MW/m2. Microlayer evaporation was found to have a central role in the wall heat transfer. The contribution of microlayer evaporation to bubble growth was about 50%, and did not vary significantly with wall superheat. The remaining 50% of the latent heat in the bubble was transferred from the superheated liquid surrounding the bubble. Finally, the spatial distribution of the initial microlayer thickness was calculated from the local heat flux of the wall.

Heat transfer and formation characteristics of the microlayer are investigated by the surface temperature measurement with a MEMS thermal sensor and the wall heat transfer evaluation using an experimental data. The temperature data clearly indicated transition of heat transfer phenomenon on the heated surface from microlayer formation via dry-out of the microlayer to rewetting of the dry-out region. Microlayer evaporation transferred extremely high heat flux over 1 MW/m2 and was found to be the dominant heat transfer mechanism in the wall heat transfer during isolated bubble boiling of water. The contribution of microlayer evaporation to bubble growth was found to be 50% and not vary with wall superheat. Finally the spatial distribution of initial thickness of microlayer was calculated from the wall heat transfer.Copyright © 2013 by ASME

A new technique, which uses the Atomic Force Microscope (AFM) coupled with a micro-thermocouple cantilever, for measurement of microscale temperature profile has been developed. Topological and thermal images of any solid sample can be obtained simultaneously by this technique. Using a handmade thin-wire thermocouple and a thin-film thermocouple deposited on a commercially available Si3N4 cantilever, some working small electric devices were measured in both air and vacuum. Since heat conduction through the air between the sample and the probe was dominant in air, clear but distorted thermal images were obtained. In vacuum, heat transfer through the contact point between them was so small that a signal to noise ration was low and the conventional signal amplifying technique was not helpful. The Lock-in technique was employed to detect such small signals and finally, a true temperature image of sample surface was acquired. Also, the thermal environment surrounding the cantilever in the AFM is discussed in this paper.

The mechanism of isolated bubble pool nucleate boiling of water is studied by a novel approach method using the developed MEMS thermal sensor. The local temperature variation beneath isolated bubble was measured using the MEMS sensor at different six wall superheats. Evaporation and dry-out of the microlayer and the rewetting of the dry-out area were obviously observed in the measured temperature variation. Wall heat transfer was numerically calculated by transient heat conduction simulation with the measured temperature as a surface boundary condition. The results showed that the microlayer evaporation transfers high heat flux of a few MW/m2 , and dominantly contributes to the heat transport from the heating wall during the bubble growth phase. The ratio of the heat transferred from the wall to the latent heat in the bubble at the departure decreased with increasing wall superheat. In other words, the contribution of the heat transfer from the superheated liquid layer surrounding the bubble becomes important with increasing wall superheat. Moreover, the microlayer thickness was calculated by integrating the local heat flux. The derived initial thickness of the microlayer was independent from the wall superheat and became thick as distance from the nucleation site increases.Copyright © 2011 by ASME

Metallic point contacts have been extensively studied from the viewpoint of their interesting, and often nonlinear, electrical properties. Their thermal characteristics, however, have largely been ignored, even though they show great potential as microscale temperature sensors. It has been previously demonstrated that when a temperature drop exists across a point contact consisting of two identical metals, a thermoelectric voltage can be generated, provided the mean contact radius is comparable in size to the electron mean free path. In the present experimental study, a point contact is formed by pressing a sharply etched Ag whisker against either an Ag or a Cu flat plate. In addition to confirming the previous results, the feasibility of using such a point contact is demonstrated by calibrating the voltage output against the nondimensionalized plate temperature. Furthermore, the thermoelectric voltage at an Ag–Cu point contact is also presented, showing that a point contact made from dissimiliar metals is even more promising than one made from identical materials. Finally, the point-contact thermal resistance is shown to depend nonlinearly on the electrical resistance, or contact area, and on the temperature drop.

Condensation in a horizontal tube initially of pure vapor which completely condenses at the outlet, is theoretically analyzed. The tube interior is divided into three regions : the first is entirely vapor, the second is a thin film region where Nusselt-type condensation occurs, and the last is the tube bottom region where condensate accumulates and flows to the outlet. It is important to estimate the height of the accumulated condensate region to predict the total heat transfer. Therefore, the inlet level of the condensate as variable parameters, as well as two opposite factors, shear stress and gravity, is considered in this third region. Compared with a previous analysis which neglected the gravitational force, the result of this analysis nearly matches the experimental data.

This paper describes an experimental investigation of heat transfer mechanism beneath isolated bubble during nucleate boiling with MEMS sensors having high temporal and spatial resolution in temperature measurement. The MEMS sensor fabricated for the boiling research includes eight thin film thermocouples and an electrolysis trigger on the topside of 20 × 20 mm2 silicon substrate and thin film heater on the backside. The electrolysis trigger initiates bubble growth by supplying hydrogen gasses as bubble nuclei with the electrolysis of the water by two electrodes. In the experiment, temperature fluctuation beneath an isolated bubble during saturated nucleate boiling of water was measured with the sensor. The measurement data presented strong evaporation and dry-out of the microlayer in the bubble growth phase and rewetting of the dry-out area in the bubble departure phase. Moreover, heat transfer induced by the boiling bubble was evaluated by computing local heat flux through a transient heat conduction simulation in the sensor substrate using the measured data as boundary condition. The heat transfer analysis shows that the local heat flux in the microlayer evaporation area has high value of the order of MW/m2 , and the maximum value of about 2 MW/m2 is indicated near the center in an early phase of the bubble growth. On the other hand, the heat flux is very low of around zero at the dry-out area, where microlayer had disappeared completely, and slight increase was observed at the rewetting area. Total heat transferred from the surface reached to about half of latent heat in the bubble until the bubble departure. Finally, initial thickness of the microlayer under the bubble was estimated by integrating the derived local heat flux. As the result, it was distributed in a few μm within the measurement area.Copyright © 2010 by ASME

Temperature variation beneath isolated bubble during saturated boiling of water was measured with a MEMS (Micro-Electro-Mechanical Systems) sensor having high temporal and spatial resolution. Then, local heat transfer from the heated surface was evaluated by a transient heat conduction analysis of the wall with measured temperature data as a boundary condition. The MEMS sensor on a 20 × 20 mm2 silicon substrate includes an electrolysis trigger and eight thin film thermocouples on the top side, and two thin film heaters on the back side. The thin film thermocouple was calibrated with a thermal scan method using two alloy samples with different melting point. The condition of the sensor was smoothly controlled with the heater. The bubble is initiated with electrolysis at a gap of the trigger electrode, where slight hydrogen gasses are supplied as bubble nuclei. Then, local and fast temperature variations in wide region are measured with the thermocouples with cutoff frequency of 100 kHz arranged in a line at 40 – 2000 μm far from the trigger gap. Measured temperature data presents formation of microlayer and expansion of dryout area in bubble growth process and rewetting in bubble departure process. The numerical analysis showed that average heat flux beneath the bubble indicated the maximum value of 19 W/cm2 during the microlayer evaporation, and then after hitting a bottom slightly lower than a heat flux at the bubble nucleation, recovers to the nucleation level. The contribution of the heat transfer from the heated wall was evaluated to approximately one-fourth of latent heat in the bubble at departure.Copyright © 2009 by ASME

Condensation heat transfer of a nonazeotropic binary R113-R114 vapor mixture in a vertical tube is investigated experimentally and theoretically, for cases of both constant and varying wall temperature along a flow direction. Experimental results show that a minimum temperature difference is required for complete condensation at an outlet of the condenser tube. This temperature difference and the heat transfer coefficients can be accurately predicted by a theory which assumes a well-mixed condition in the condensate, that is, a uniform concentration profile in the condensed liquid. When the fin height is 0.2 mm, augmentation of heat transfer by an inner finned surface occurrs in the liquid film but not in a vapor phase. For both cases of the constant and varying wall temperature, the heat transfer coefficients correlate well when an averaged temperature difference between the wall and the bulk flow is used as a characteristic temperature difference.