The prechamber combustion characteristics of a cogeneration system were studied using a rapid compression and expansion machine (RCEM) to improve the efficiency of cogeneration natural gas engines. The generated torch flames by the prechamber were used to investigate the effect of prechamber on the main combustion. In our previous study, we observed the correlation between the torch flame and the main flame, which is so-called “prechamber combustion.” In this study, the prechamber combustion characteristics of a gas engine were examined by visualization using a RCEM for methane and propane. In addition, the study mainly aimed to clarify the effects of the prechamber parameters, such as the equivalence ratio, initial pressure, and prechamber nozzle diameter, on the ignition and combustion in the main chamber for methane by comparing the combustion characteristics in the cases of methane and propane. Consequently, the ignitability of both fuels was improved by increasing the initial pressure and nozzle diameter. However, the effect of the equivalent ratio on the ignitability was different for the two fuels.

The water loss caused by evaporation heat dissipation is non-negligible for a natural draft counter-flow wet cooling tower (NDWCT), which is evidently influenced by many change factors. Based on Merkel method, the Merkel number is revised in consideration of the effect of water loss. Taking the NDWCTs equipped for 300MW and 600MW power plants as experimental objects, a mathematical model is established for predicting the evaporation loss of the NDWCT. The accuracy of the model is verified by experimental data, and the mathematical model for predicting evaporation loss is feasible. Then, the effect of change factors, including mass flow rate of circulating water and air parameters, on the tower evaporation loss is studied in the case of fixed the thermal load dissipated by the condenser. Results show that the evaporation loss varies little with the change of mass flow rate of circulating water. Therefore, Water conservation cannot be achieved by changing mass flow rate. With the increase of dry bulb temperature, the evaporation loss, as well as the rate of evaporation loss caused by unit temperature drop increases. As the relative air humidity increases, the NDWCT outlet water temperature increases, whereas the evaporation loss and the rate of evaporation loss caused by unit temperature difference decrease.

Moderate Intensity Low Oxygen Diluted (MILD) combustion has been investigated for a long time in order to minimize NO_X emission still enhancing thermal efficiencies in the combustion equipment. Many researches about MILD combustion have been recently performed, but studies on MILD combustion of renewable fuel such as ethanol has been very scarce and particularly, specific information on the NO_X production in ethanol-air MILD combustion has not been reported yet. In order to satisfy the condition for MILD combustion, internal recirculation is known to be essential in order to entrain the combustion products gas into air and fuel jets of combustion system. In this work, a series of numerical analysis with simplified opposed jet geometry have been done using the OPPDIF in Ansys program. Numerical analysis on how the recirculation ratio (K_V) affects NO_X emission in the ethanol-air combustion for MILD formation were carried out under the condition of various burnt gas dilution in reactant flow. The results show that the temperature was decreased by the increase of the recirculation ratio and the maximum heat release value became also low by the increase of the recirculation ratio. It was also found that the pyrolysis zone of the heat release was disappeared and the two heat release peaks are merged into one as combustion pattern is changed to MILD combustion mode.

For boiling heat transfer, it is important to improve both the critical heat flux and heat transfer coefficient. In general, the heat transfer coefficient is improved by promoting the nucleation of boiling bubbles on the heating surface. However, this decreases the critical heat flux. To improve the heat transfer coefficient without decreasing the critical heat flux, we previously developed a technique using a boiling bubble resonator, which consists of a material attached close to the heating surface that vibrates in response to the growth and collapse of boiling bubbles. In this study, we used spacer plates to vary the gap height between the heating surface and boiling bubble resonator to maximize the boiling heat transfer. By optimizing the gap height, the wall superheat decreased by 7 and 25 K at 0.8 and 5.6 MW/m², respectively. The maximum heat flux was 5.8 MW/m² with the optimized gap height. In addition, we observed sound signals when the boiling bubble resonator was optimally vibrating. Moreover, jet flow from the gap appeared with the vibration of the boiling bubble resonator.

We experimentally investigated the promotion of deflagration-to-detonation transition (DDT) in hydrogen-air mixtures contained in a tube in which straight-shaped rods were installed as obstacles. In the experiments, the number of obstacle rods, their spacing, their blockage ratio, and the equivalence ratio of the hydrogen-air mixture were varied as the governing parameters. The obstacle rods had a spacing of 10 or 20 mm and a blockage ratio of 0.32, 0.41, or 0.51. As a result of an optimization of the obstacle-rod conditions, when fourteen rods, whose blockage ratio was 0.32, were installed in a tube with a spacing of 20 mm and with a hydrogen-air mixture with equivalence ratios from 0.8 to 1.2, the run-up distance to the DDT was shortened to approximately 20 times the tube diameter.

A three-dimensional numerical study is conducted to investigate the double-diffusive natural convection inside a trapezoidal solar pond filled with water and salt solution and subjected to discrete heat boundary conditions. It investigates the effect of stored thermal extraction on the non-convective zone (NCZ) of the solar pond. The heat extraction from the middle region on the lower convective zone (LCZ) is modeled as a discrete thermal boundary condition in the NCZ-LCZ. The governing equations are developed using the three-dimensional potential-vorticity formulation and solved using the finite volume method. The numerical computations are performed for the following parameter ranges: Rayleigh number 10³ ≤ Ra ≤ 10⁵ and buoyancy ratio－10≤ N ≤0. Two different cases related to the thermal boundary conditions are investigated. The most important results that are drawn from this study are: (i) the non-uniform heat extraction from the LCZ increased due to vortex formation which disturbed the structure of the flow, and (ii) the three-dimensional effects become more pronounced that caused a decrease in the efficiency of the solar pond. Therefore, when extracting the stored heat from the LCZ layer, it is imperative to maintain the surface temperature as constant as possible and to avoid getting discrete heat surfaces.

Conventional louver fins have an inherent problem of condensate drainage. In this study, a newly developed louver fin is introduced. The louver fin has an extension at leading edge. Dry and wet surface heat transfer and pressure drop characteristics of the heat exchanger made of the new fins were investigated, and results are compared with those having trailing edge extension fins or conventional fins. For conventional sample, significant differences between dry and wet j and f factors were observed. Dry j factors were 166% larger than wet j factors, and wet f factors were 68% larger than dry f factors. The discrepancies were significantly reduced for the samples having extensions, which suggests that extensions are effective in condensate control. The present sample having leading edge extension yielded higher dry j factors than the sample having trailing edge extension, probably due to proper allocation of louvers. Wet j factors of the two extended samples were approximately the same, which means that the condensate drainage is better for trailing edge extension than leading edge extension. Under dry condition, j/f^1/3 of leading edge sample were 132 % and 59% higher than those of trailing edge extension and conventional sample respectively. However, under wet condition, j/f^1/3 of leading edge extension sample were 87% and 205% higher than those of trailing edge extension and conventional sample. This confirms that the present leading edge extension sample shows better performance than trailing edge extension sample, both under dry and wet condition.

Effects of pressure and heat loss on the unstable motion of cellular-flame fronts in hydrogen-air lean premixed flames were numerically investigated. We adopted the reaction mechanism for hydrogen-oxygen combustion, modeled with seventeen reversible reactions of eight reactive species and a diluent. Two-dimensional unsteady reactive flow was treated, and the compressibility, viscosity, heat conduction, molecular diffusion and heat loss were taken into account. A sufficiently small disturbance was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation, and the linearly most unstable wavelength, i.e. the critical wavelength. As the pressure became higher, the maximum growth rate increased and the unstable range widened. These were due mainly to the decrease of flame thickness. As the heat loss became larger, the former decreased and the latter narrowed, which were due mainly to the decrease of burning velocity. To investigate the characteristics of cellular-flame fronts, a disturbance with the critical wavelength was superimposed. The superimposed disturbance developed owing to intrinsic instability, and then the cellular shape of flame fronts appeared. The burning velocity of a cellular flame normalized by that of a planar flame increased as the pressure became higher and the heat loss became larger. This indicated that the pressure and heat loss affected strongly the unstable motion of cellular-flame fronts. The burning velocity of a cellular flame increased monotonically with an increase in the space size. This was attributed to long-wavelength components of disturbances. Moreover, we estimated the fractal dimension of flame fronts through the box counting method. As the pressure and heat loss increased, the fractal dimension became larger, which denoted that the flame shape became more complicated.

Lean premixed combustion can be expected to reduce the high-temperature area and NOx emission. However, it has a risk of flashback due to flame propagation. It can be thought to modulate the gas swirl intensity to prevent flashback and to stabilize combustion. In this research, we developed time-variable-angle swirl vanes which had 56 mm of the inner diameter, 70 mm of the outer diameter and consisted of 36 vanes, each vane was directly connected to a stepping motor, and the swirl intensity could be changed periodically by keeping the constant air ratio and flow rate. We confirmed the periodical movement of the swirl vane angle using a high-speed camera. The flame position could be made reciprocating move upstream and downstream by the periodic change of the swirl vane angle. The upstream direction flow at the tip of the flame appeared when the flame moved in the upstream direction with increasing vane angle and the downstream direction flow appeared when that moved in the downstream direction with decreasing the vane angle. When the flame propagation was changed from upstream direction to downstream, the upstream direction flow near the flame tip was weakened. In advance of changing the flame propagation from upstream direction to downstream, the axial velocity near the flame tip changed from decreasing to increasing.

Targeting the evaluation of the universal droplet breakup process, measurements were made on diesel fuel sprays injected from two solenoid type injectors with different specifications. The velocity and size of the spray droplets were measured using a laser 2-focus velocimeter (L2F). The velocity of small droplets that follow the flow was taken as the air velocity. The Weber number was evaluated using the velocity of the droplets relative to air as the representative velocity. Since the spray expands in a conical shape with the nozzle hole at its apex, the measurement points were placed on a straight line from the apex, which is the estimated flight direction of droplets. The change in droplet size in the flight direction was considered to be due to secondary breakup, and the rate of decrease in droplet size during this process was evaluated. It was confirmed that the velocity and size of the droplets inside the spray injected from the two injectors varied over time, and the spatial distribution of the Weber number and the rate of droplet size decrease in the middle of the injection period was non-uniform and different. It was found that there is a positive correlation between the Weber number and the rate of droplet size decrease for both sprays, and that the relationship is nearly identical despite the fact that the characteristics are different between the two sprays. The secondary breakup process was shown to be independent of the injection conditions such as the diameter and number of nozzle holes.

The concept of heat transfer enhancement in a plate fin heat sink (PFHS) using twisted tape is presented. The airflow behavior in the flow channel and heat sink performance are investigated at the Reynolds numbers between 2000 and 5000. The twisted tape and perforated twisted tape with twist ratio between 2.5 and 3.5 are equipped between the fins of the PFHS. For perforated twisted tape, the holes are drilled along the twisted tape length with the ratio of perforation diameter to twisted taped width (d/W) between 0.2 and 0.6. The heat transfer coefficient and pressure drop are enhanced by decreasing the twist ratio and increasing the Reynolds number and the d/W ratio between 0.2 and 0.4. However, they are dropped when the d/W ratio higher than 0.4. The highest thermal performance factors of the plate fin heat sink equipped with twisted tape (PFHSTT) and the plate fin heat sink equipped with perforated twisted tape (PFHSPTT) are 1.28 and 1.33, respectively. The correlations of the Nusselt number and friction factor related to twist ratio and d/W ratio are generated and proposed for designing and selecting in the future.

This research article intends to discuss on the role and effects of dispersing solution combustion derived magnesia nanoflakes (~17 nm) within the biodiesel-diesel blends and pure diesel termed as nanofuels, in order to investigate the functional and pollutant emissions of a single-cylinder, electrically loaded, water-cooled diesel engine. The fuels focussed in this study are a blend of palm oil biodiesel and regular diesel dispersed with 50 ppm magnesia nanoflakes, and a pure diesel dispersed with 50 ppm magnesia nanoflakes. These fuels are compared with regular diesel which is considered as the base reference fuel, as well as with the biodiesel-diesel blend. From the experimental measurements, we inferred that the fuel density, viscous nature, and calorific value enhanced with the addition of nanoflakes. As for the engine performance attributes, the brake specific fuel consumption (BSFC) is lessened by 3.08% and 2.88% for particle dispersed biodiesel-diesel blend and particle dispersed diesel, respectively, whereas the brake thermal efficiency (BTE) enhances by 5.04% for particle dispersed biodiesel-diesel blend and 2.74% for particle dispersed diesel. With reference to emission, the unburnt hydrocarbon (UHC), white damp (CO), particulate exhaust or smoke, and the nitrogen oxides (NOx) are reduced by 9.51%, 18.71%, 13.64%, and 5.63%, respectively for particle dispersed biodiesel-diesel blend and 10.35%, 16.54%, 13.64%, 19.47%, and 4.70%, respectively for particle dispersed diesel.

Utilizing efficiently and securely hydrogen as clean energy source, it is required not only to analyze the experimental data under a certain condition but also to create the mathematical model for the prediction of flame propagation velocity under various conditions. Thus, it is significant to understand the characteristics of dynamic behavior of hydrogen/air premixed flames and to elucidate the effects of addition of inert gas, i.e. carbon dioxide CO₂ and water vapor H₂O. We performed the experiments of hydrogen explosion in two types of closed chambers to observe spherically expanding flames using Schlieren photography. Wrinkles on the flame surface were clearly observed in low equivalence ratios. Analyzing the Schlieren images, the flame propagation velocity depending on the flame radius was obtained. Increasing the addition of inert gas, the propagation velocity decreased, especially in the case of CO₂ addition. The propagation velocity increased monotonically as the flame radius became larger. The appearance of flame acceleration was found, which was caused by the evolution of wrinkles on the flame surface. Moreover, the Markstein length decreased as the concentration of inert gas became higher, indicating that the addition of inert gas promoted the instability of hydrogen flames. Furthermore, the wrinkling factor, closely related with the increment in propagation velocity, decreased as the inert-gas concentration became higher. The wrinkling factor normalized by the propagation velocity of flat flame increased, on the other hand, under the conditions of high inert-gas concentration, except for near the quenching conditions. This indicated that the addition of CO₂ or H₂O promoted the unstable motion of hydrogen flames, which could be due to the enhancement of the diffusive-thermal effect. Based on the characteristics of dynamic behavior of hydrogen flames, the parameters used in the mathematical model on propagation velocity including flame acceleration was obtained, and then the flame propagation velocity under various conditions was predicted.

To describe permeable roughness effects on turbulence statistics, our previously reported PIV (particle image velocimetry) experimental data (Shimizu et al., 2019; Okazaki et al., 2020) of turbulent channel flows over two different permeable roughness types are assessed. The considered roughness elements were porous transverse square ribs and rectangular sectioned short flat plates which were mounted on porous walls with constant spacings. The spacings were controlled to reproduce the so-called k- and d-type roughness geometries. The values of the pore-per-inch of the materials of the roughness elements and the porous wall were the same in each case. Three different foamed porous materials were considered while their porosities were approximately 0.8. By the assessment of the logarithmic mean velocity profiles of the two roughness cases, it is confirmed that a linear relationship between the zero-plane displacement and the roughness scale exists. The data of the flat porous walls without roughness elements (Suga et al., 2010; 2017) are also confirmed to have the same trend regardless of the porous media. We found an effective displacement parameter which linearly increases as the pore diameter. Using the pore-scale Reynolds number, we then propose a correlation that describes the value of the variable von Kàrmàn constant and a simple formula for estimating the equivalent sand grain roughness height for all cases presently assessed.

In this study, a constant fill ratio vapor chamber using water and ferrofluid as the coolants on the thermal performance and thermal optimization to provide the cooling range are presented. The liquid flow rate of the condenser side ranges from 0.029−0.099 kg/s, the input power of 100−300 W, heat source size of 30 mm × 30 mm, and the water and ferrofluid with φ=0 %, φ=0.005 % and φ=0.05 % as coolants are investigated. The results indicate that the minimum thermal resistance of a traditional vapor chamber of 0.126 ℃/W at m° = 0.075 kg/s and vapor chamber with a mini channel of 0.077 ℃/W at m° = 0.049 kg/s at φ=0.005 % are obtained. The vapor chamber with and without mini channel at a certain fill ratio of 26 % and 33 %, the heat source size of 30 mm × 30 mm, and the mass flow rate of ≥ 0.042 kg/s are withstanding the heat load range from 100–200 W. However, to achieve the high heat load range from 100–300 W, the vapor chamber with mini channel using the Fe₃O₄ ferrofluid φ=0.005 % is covering the entire cooling range and is recommended in electronic cooling applications.

Although rapid pyrolysis affords higher volatile yield than slow pyrolysis, the change in the yield of components at different heating rates have not been reported in detail. Moreover, few studies have assessed the changes in the tar component yield with respect to biomass type. Therefore, from a practical point of view, this study quantitatively determined the effects of the heating rate and biomass type on the tar component yield through gas chromatography-mass spectrometry. In addition, the mechanism of tar formation was investigated. The main results of the study are as follows: (1) Changes in the yield and component of biomass tar due to increase in the heating rate by a factor of 100 were quantitatively determined. An increased heating rate resulted in a higher yield of aromatic compounds and induced the formation of benzene, toluene, and other compounds. At slow heating rates, the yield of odorous components such as vanillin, furfural, acids, and aldehydes increased. At the middle heating rate (1.0 K/s), a significant increase in the amount of phenols containing OH and O groups was observed. (2) For woody biomass, acetic acid, cellulose-derived glucose, catechol, phenols, and furfural were identified as the major tar components. (3) The tar components volatilized from the wood trunk, bark, and grass are affected by the primary content of the biomass constituents.

Theoretical examinations based on absorption line databases were carried out to investigate the influence of turbulence-radiation interaction on the radiative heat transfer arriving at the wall of large-scale industrial furnaces including hydrocarbon flame, where the re-absorption of radiative energy by combustion gas on its path toward objects to be heated cannot be neglected. In this study, we combined an improved version of our previous method for reducing the calculation load required for tracing turbulent fluctuation in temperature in great detail and an efficient method proposed in our previous papers to reduce the enormous calculation load contingent on detailed non-gray analysis. When we combined these methods with a governing equation solver for obtaining the spatial distribution of time-averaged values of temperature, concentration, velocity, and so on, we could evaluate the heat transfer including radiation in large-scale industrial furnaces enveloping turbulent hydrocarbon flame with sufficient accuracy equivalent to Line-by-Line analysis and with a feasible calculation load. Our application of this calculation method to large-scale furnaces enveloping hydrocarbon flame revealed that neglecting the turbulence-radiation interaction in numerical simulation gave rise to an obvious change in the heat flux distribution on the side wall and in the spatial distribution of the time-averaged temperature. In addition, change in the total amount of radiative energy arriving at the side wall caused by neglecting the turbulence-radiation interaction was fairly small compared with the change observed in our previous report on a model optical path imaging the typical course of radiative energy in large-scale industrial furnaces fueled by propane.

For a cooling or heating coil of a building air conditioning system, large diameter tubes (over 12.7 mm) with wave fins are frequently used. Furthermore, there have been attempts to apply oval tubes to the heating or cooling coils. In this study, nine samples - three herringbone wave fin/round tube, three smooth wave fin/round tube and three smooth wave fin/oval tube samples - were tested under dry condition. All of the samples had the same original tube diameter (12.7 mm). For the smooth wave fin samples, fins were identical, and for the round tube samples, fin corrugation angles were almost the same. Tests were conducted varying the frontal air velocity from 1.0 to 4.0 m/s. Results showed that the round tube samples yielded higher conductance per volume (η_o h_o A_o / V_o ) than the oval tube sample. Similarly, the smooth wave fin samples yielded a higher conductance per volume than the herringbone wave fin samples. At one row configuration, however, a significant difference among samples existed between the heat transfer conductance per volume and the heat transfer coefficient, and the reason was explained by the difference in fin efficiency between the round and the oval geometry. The pressure drops of the round tube samples were larger than those of the oval tube samples. Similarly, the smooth wave fin samples yielded larger pressure drops than the herringbone wave fin samples. A performance evaluation revealed that the smooth wave fin samples yielded larger heat transfer conductances per volume than the herringbone wave fin samples at the same pumping power per volume.