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Understanding heat transfer is important for developing high-performance internal combustion engines, but heat transfer mechanisms have not yet been elucidated. The objective of this study is to clarify the correlation between flow and heat transfer in an engine. Local instantaneous heat flux and advection speed adjacent to a wall were measured using a resistance-type three-point heat flux sensor to achieve this purpose. Furthermore, local instantaneous Nusselt and Reynolds numbers were calculated based on the measured heat flux and the advection speed using two characteristic lengths: (1) bore diameter and (2) penetration depth. Consequently, it was found that the ensemble-averaged Nusselt numbers varied exponentially as a function of the ensemble-averaged Reynolds numbers in both models, and good correlation equations could be derived. The penetration-depth model reproduced the experimental values more precisely. On the other hand, the Reynolds number index of the bore-diameter model corresponds with the heat transfer on the rear wall of a cylinder installed in a flow. This suggests an analogy between the engine and rear wall of a cylinder. Additionally, the impacts of each heat transfer factor were evaluated. As a result, it was found that a dominant factor until the heat flux peak is the temperature difference between the gas and wall. However, its effect becomes small after the heat flux peak, and it was found that the density and advection speed mainly affect the heat flux decrease.

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.

To develop higher efficiency and lower emission gasoline engines, ultra-lean burning under high Reynolds number conditions is desired. It is believed that enhancement of tumble flow in the engine cylinder is effective for increase of turbulent intensity, resulting in improvement of characteristics of flame propagation and ignition under a strong discharge, while the enhancement of tumble flow might cause heat loss from the wall. Investigations of characteristics of turbulence and distributions of wall and gas temperature in engine cylinders are still challenging due to transient and high pressure, and due to cycle-to-cycle variations. In the previous study (Jainski et al., 2013), a micro particle image velocimetry (micro PIV) measurements were conducted in an engine cylinder at up to 1100 rpm and the characteristics of velocity boundary layer around a cylinder head were investigated. The study has shown that the log-law does not properly present the measured velocity distributions near the wall. In our previous study (Shimura et al., 2018), a micro PIV was conducted in a motored engine cylinder to investigate velocity boundary layer characteristics near piston top before the top dead center (TDC) at a constant engine speed of 2000 rpm to deepen understanding characteristics of velocity boundary layer in engine cylinder with tumble flow. The velocity boundary layer was well fitted to the Blasius theory at 30 CAD before TDC and deviated from the theory at 15 CAD before TDC. However, the obtained data was two components of velocity in the measurement plane, which means that effects of magnitude of velocity were not clear in the previous measurement. In this study, stereoscopic micro PIV was conducted to elucidate the effects of magnitude and direction of velocity on the characteristics of velocity boundary layer near the piston top in the tumble enhanced SI IC optical engine. The tumble enhanced SI IC optical engine used in the previous study (Shimura et al., 2018; Matsuda et al., 2019) was used also in this study. The bore is 75 mm and the stroke is 112.5 mm. Length of the connecting rod is 250 mm. The engine has two intake valves of the diameter 29 mm and two exhaust valves of the diameter 25 mm. The compression ratio is 13.0. The optical access is achieved through the quartz glass cylinder. A tumble enhancing intake port is used for the sake of improvement of ignition and flame propagation. The engine speed can be set up to 2000 rpm at the maximum. The overall flow fields taken by a preliminary PIV experiment can be seen in the literatures (Shimura et al., 2018; Matsuda et al., 2019). The laser beams for PIV are from two Nd:YAG lasers (LOTIS, LS-2131, 150 mJ/pulse, 532 nm) are led to the same optical axis by a mirror and a polarizing beam splitter. The laser beam is formed into laser sheet of 180 µm thickness by laser sheet forming optics and led into the engine cylinder. The scattering light was collected by long distance microscope lenses (Quester, SZM100) and imaged onto CCD cameras (Princeton Technology, ES4020) in the stereoscopic alignment with 18 degrees. To compensate for the difference in the focal length caused by the quartz engine liner, a cylindrical lens of 1000 mm focal length was placed in front of each long distance microscope lens. SiO2 of 1 µm mean diameter was used for tracer particles. The micro PIV was operated at about 6.6 Hz to be synchronized with engine speed. The time separation of the successive particle images was 1.5 µs. The field of view of the micro PIV was 3.5 mm × 3.5 mm on the piston top including central axis of the cylinder. Here, x and y coordinates are set to the direction from the exhaust to the intake valves and the direction from the piston to the pent roof, respectively. z coordinates is perpendicular to x and y axes, and the orizin of the coordinates is set at the center of the piston top. The spatial resolution of PIV, which is defined by the size of interrogation region, is 108.8 µm × 54.4 µm. Vector spacing is 54.4 µm × 27.2 µm. The first vector position is about 27.2 µm away from the wall. The measurements were conducted at 345 CAD. The engine was motored at 2000 rpm and operated for three intake valve open timings of -30 CAD. The operation condition of the engine tested contain strong cycle-to-cycle variations, which results in the large root-mean-square values of velocity fluctuation near the center of the piston top (Shimura et al., 2018). To evaluate flow characteristics in the cycle-to-cycle variations, conditional averaging based on magnitude of fluid velocity is used in this study. Figure 1(a) shows a histogram of the magnitude of combined velocity of u and w. The magnitude of velocity can be considered as momentum of fluid because few fluctuation of density is considered and temperature boundary layer is enough thin compared to the velocity boundary layer. The large variations in the momentum can be observed in Fig. 1(a). The large variations are considered to be caused by the variations of tumble core locations. The fraction of the large momentum Here, the momentum are classified into C1 to C4 based on fractions (C1: 54.4%, C2: 19.5%, C3: 19.5%, C4: 6.6%). Figure 1(b) and (c) shows mean velocity distribution classified into C1 and C4 in Fig. 1(a). The distribution is fitted to the log-law velocity profile of developed wall turbulence. The mean velocity profile for C1 shows large discrepancy from that of general turbulent boundary layers, while that for C4 show relatively close to that of general turbulent boundary layer. C2 and C3, which are not shown here, have trend between the C1 and C4 profiles. These results show that the velocity profiles which can be assumed to be the developed turbulent boudary layer in the targeted condition is less than half of cycles, which means partial applicability of conventional CFD models for prediction of boundary layer of the engine condition.

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 heat flux sensor was developed with micro-electro-mechanical systems (MEMS) technologies for investigating turbulent heat transfer characteristics in engines. The sensor has three thin-film resistance temperature detectors (RTDs) of a square 315 µm on a side on a 900 µm diameter circle in rotational symmetry. The performances of the MEMS systems sensor were tested in an open combustion chamber and a laboratory engine. In the open chamber tests, it was revealed that the MEMS sensor can measure the wall heat fluxes reflecting flow states of gas phase. In addition, the noise was evaluated as 3.8 kW/m2 with the standard deviation against the wall heat flux of a few hundred kW/m2. From these results, it was proved that the MEMS sensor has the potential to observe turbulent heat transfer on the order over 10 kW/m2 in the engine. In the laboratory engine test, the wall heat flux for continuous 200 cycles was measured with a good signal-to-noise ratio. The noise was evaluated as 13.4 kW/m2 with the standard deviation despite the noisy environment. Furthermore, it was proved that the MEMS sensor has the comparable scale with the turbulence in the engine because the three adjacent detectors measured similar but different phase oscillations in the local instantaneous heat fluxes. In addition, a heat flux vector reflecting the state of the local instantaneous heat transfer was visualized by the adjacent three-point measurement. It is expected that the three-point MEMS sensor will be a useful tool for the engine heat transfer research.

Investigation of microlayer formation characteristics is important for developing a reliable nucleate boiling heat transfer model based on accurate physical mechanisms. Although formation mechanisms of the thin liquid film in two-phase flow of confined spaces, such as micro-tubes and closely positioned parallel plates, have been thoroughly studied, microlayer formation mechanisms of pool boiling have been sparsely studied. In a previous study (Yabuki and Nakabeppu in Int J Heat Mass Transf 76:286–297, 2014; Int J Heat Mass Transf 100:851–860, 2016), the spatial distribution of initial microlayer thickness under pool boiling bubbles was calculated by transient heat conduction analysis using the local wall temperature measured with a MEMS sensor. In this study, the hydrodynamic characteristics of microlayer formation in pool boiling were investigated using the relationship between derived initial microlayer thickness and microlayer formation velocity determined by transient local heat flux data. The trend of microlayer thickness was found to change depending on the thickness of the velocity boundary layer outside the bubble foot. When the boundary layer thickness was thin, the initial microlayer thickness was determined by the boundary layer thickness, and the initial microlayer thickness proportionally increased with increasing boundary layer thickness. On the other hand, when the boundary layer was thick, the initial microlayer thickness decreased with increasing boundary layer thickness. In this thick boundary layer region, the momentum balance in the dynamic meniscus region became important, in addition to the boundary layer thickness, and the microlayer thickness, made dimensionless using boundary layer thickness, correlated with the Bond number.

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.

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) of the minute samples in the range of microgram to nanogram were studied using original cantilever-type calorimeters. The micro-fabricated calorimeter with a heater and thermal sensors was able to perform a fast temperature scan at above 1000 K/s and a high-resolution heat measurement. The DTA of minuscule metal samples demonstrated some advances such as the thermal analysis of a 20 ng level indium and observation of a strange phase transition of a binary alloy. The power compensation type DSC using a thermal feedback system was also performed. Thermal information of a microgram level sample was observed as splitting into the DSC and DTA signals because of a mismatch between the sample and the calorimeter. Although there remains some room for improvement in terms of the heat flow detection, the behavior of the compensation system in the DSC was theoretically understood through a lumped model. Those experiments also produced some findings, such as a fin effect with sample loading, a measurable weight range, a calibration of the calorimeter and a product design concept. The development of the nano-DTA and nano-DSC will enable breakthroughs for the fast calorimetry of the microscopic size samples.

Subcooled flow boiling experiments have been conducted using water at atmospheric pressure in a rectangular flow channel. These experiments are conducted in support of a US DOE Nuclear HUB project, the Consortium for Advanced Simulation of Light Water Reactors (CASL). CASL aims at the development of an environment for predictive simulation of light water reactors, and subcooled flow boiling is investigated experimentally in order to develop and validate 3-D Interface Tracking Models (ITMs). In the present experiments, a MEMS sensor has been used to obtain wall temperature response during nucleation, growth and departure of a vapor bubble in subcooled water flowing upward through a vertical rectangular test section. High-speed video images and MEMS sensor data collected at 5000 fps and 50 kHz, respectively, revealed rapid microlayer evaporation, dryout and rewetting phenomena under a single vapor bubble following nucleation accompanied by rapid wall temperature changes of up to ∼10 K.

Bio-chemical calorimeters with a MEMS (Micro-Electro-Mechanical Systems) thermopile sensor have been studied for monitoring detailed processes of the biochemical reactions of a minute sample with a high temporal resolution. The bio-calorimeters are generally divided into a batch-type and a flow-type. We developed a highly sensitive batch-type calorimeter which can detect a 100 nW level thermal reaction. However it shows a long settling time of 2 hours because of the heat capacity of a whole calorimeter. Thus, the flow-type calorimeters in passive and active mode have been studied for measuring the thermal reactions in an early stage after starting an analysis. The flow-type calorimeter consists of the MEMS differential thermopile sensor, a pair of micro channel reactor in a PDMS (polydimethylsiloxane) sheet in a three-fold thermostat chamber. The calorimeter in the passive mode was tested with dilution reactions of ethanol to water and NaCl aqueous solution to water. It was shown that the calorimeter detects exo- and endothermic reaction over 250 nW at solution flow rate of 0.05 ~ 1 µl/min with a settling time of about 4 minutes. In the active mode, a response test was conducted by using heat removal by water flow from the reactor channel. The active calorimetry enhances the response time about three to four times faster.

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.

Gas absorption and pressure drop characteristics of a slug flow in microchannel were experimentally studied with CO2 and water system in expectation of high absorption performance by large specific interface area, toroidal flow in slug and high heat transfer capability of microchannel. Absorption rate was measured by detecting volume change of gas bubble along the channel with photo-interrupters, and then absorption time constant was formulated as a function of inner diameter, bubble velocity, and liquid slug length. Pressure drop was also formulated by fitting experimental data to an equation modeled as a sum of Poiseuille flow part and toroidal flow part. Based on the formulas, high mass transfer performance and compactness of the proposed system was demonstrated by comparison with typical gas absorption systems such as falling liquid film and dispersion bubbles. The microchannel gas absorber, of which channel length was set to keep outlet liquid concentration a specified value, was estimated to be more compact by reducing a channel diameter with a slight increase of pressure drop.

Scanning thermal microscopy (SThM) can measure thermal image with a nano-scale spatial resolution. However, there remains an issue in quantitative temperature measurement. We proposed an active temperature measurement method that provides a real temperature image by compensating a variation in contact thermal conductance. Performance of the active method was examined by a multi-function cantilever made with micro-fabrication process. Response test of the cantilever showed about 50 Hz cut off frequency for both passive and active method. Temperature measurement test indicated that sensitivity of heat flow detection was not enough to measure real temperature regardless of the thermal contact conductance. Imaging test demonstrated that the active method takes temperature image closer to real temperature distribution than the passive method.

We are studying the removal of the SPM by a humidity swing method, in which sample air is consecutively humidified with heating and dehumidified with cooling. First, the removal effect of this method was experimentally demonstrated. The sample air was humidified to saturated moist air up to 66°C and then cooled to 13°C. The results showed the decrease of the SPM concentration. The SPM survival ratio is plotted against the drop of absolute humidity through the dehumidification; and the survival ratio exponentially decreased to ∼0.2. Also, the dependence of removal characteristic on SPM concentration was not shown for 10 ∼ 10000 particles/cc. Also, growth of SPM due to nuclear condensation was observed in the experiments. However, the number of SPM grown to several micrometers where SPM significantly decreases with gravity was very small. Therefore gravitational sedimentation would not be enhanced. Next, SPM transfer in the dehumidifier was calculated. The heat and mass transfer through the dehumidifier was calculated; and then, variation of SPM concentration by suction flow and themophpresis were obtained. The analysis of SPM transfer showed that the dominant phenomenon for SPM transport in the dehumidifier is suction flow. However, the calculation considering thermophoresis and suction flow predicted the removal effects several times lower than the experiments. Thus, the removal mechanism of the humidity swing method should be explained by unidentified phenomena.Copyright © 2013 by ASME

Solidification of an aqueous solution was studied using a laser interferometry technique combined with an optical microscope. In order to measure the concentration distribution in an aqueous NaCl solution near an ice crystal and the three-dimensional shape of the ice crystal in a known temperature field, a directional solidification stage was used. This was composed of low- and high-temperature blocks and a moving bed. In our experiment, ice crystals showed four shapes: flat, treelike, swordlike, and needlelike. As a result of the interferometry experiment, it was observed that the concentration increases in the bulk solution for thick samples, swordlike ice crystals have an asymmetric triangular cross section, and treelike crystals have a flat and low top. We also studied ice formation response to injection of a high-concentration solution.

Solidification of an aqueous solution is studied using a laser interferometry technique combined with an optical microscope. In order to measure the concentration distribution in an aqueous NaCl solution near an ice crystal and three-dimensional shape of the ice crystal under a known temperature field, a directional solidification stage is used, which is composed of low-and high-temperature blocks and a moving bed. In this experiment ice crystals showed four shapes : flat, treelike, swordlike and needlelike. As a result of the interferometry experiment, it was observed that concentration increases in bulk solution for thick samples, swordlike ice crystal has an asymmetric triangular cross section, and that treelike crystal has a flat and low top. Also, ice formation response to injection of a high concenttration solution was studied.

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

When a water–ethanol binary mixture condenses on a flat plate, one observes that the liquid film condensate rises locally and eventually forms many droplets on the film. Usually, filmwise condensation is expected because both substances are completely soluble in each other and they wet a copper plate well. This paper presents the droplet growth mechanism during so-called pseudo-dropwise condensation. Instability analysis is used to determine the transition from filmwise condensation to pseudo-dropwise condensation theoretically. In a stress balance at the vapor–liquid interface, the analysis considers not only the surface tension itself, but also the surface tension variation due to changes in temperature and concentration, assuming saturation conditions at the interface. Numerical results indicate that the Marangoni effect plays a more important role than the absolute value of the surface tension in pseudo-dropwise condensation. The change in surface tension with temperature is not always negative; it becomes positive for certain mixtures due to the dependence on concentration. Pseudo-dropwise condensation is only realized when surface tension increases with temperature. This analysis qualitatively predicts the critical Marangoni number experimentally observed during water–ethanol mixture condensation.

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.