High temperature flames, which can be produced via oxygen-enriched combustion, potentially have improved the combustion characteristics over air-only combustion flames due to their substantially higher flame temperatures. These conditions necessitate the use of non-intrusive optical measuring methods to measure the temperature and the chemical species in the flame. To develop an optical measurement calibration burner that can be used at high pressure and temperature conditions, a new calibration burner which employed water-cooled multi-hole nozzle was developed in this study. Premixed CH4/O2/N2 oxygen-enriched conditions were selected to investigate both the heat-resisting properties of the developed burner nozzle and the burner’s flame characteristics. OH-Planar Laser-Induced Fluorescence (OH-PLIF) measurements were conducted on the flames to observe the OH distributions. The flame temperature at 0.10 MPa was derived using a Boltzmann-plot for the OH fluorescence excitations. To verify the variation of molecular concentration with equivalence ratio for the experimental flames qualitatively, the experimentally acquired OH and CH chemiluminescence intensities were compared with the simulated partial pressure of OH* and CH*, respectively. Experimental results showed that the CH4/O2/N2 flames were stabilized on the burner nozzle in a wide range of oxygen-enrichment ratio, from 0.40 to 1.0 at atmospheric pressure. At an oxygen-enrichment ratio of 0.45, the flames were also stabilized in pressure conditions up to 0.49 MPa, while the inner nozzle temperature was lower than 400 K. The OH-PLIF images showed that the OH was distributed almost uniformly along the axial direction of the burner, and demonstrated similar characteristics to that of a flat flame. The derived maximum flame temperature at atmospheric pressure was approximately 2650 K at an oxygen-enrichment ratio of 0.80. The variation of the OH and CH chemiluminescence intensities with change of equivalence ratios corresponded roughly with the simulated partial pressures of OH* and CH* at each pressure condition.
The purpose of this paper is to investigate the effect of the wettability on MPCM suspensions thermal conductivity. The wettability of the capsules, characterized by contact angle between solid capsules and carrying fluid, was modified by mixing two selected surfactants into the suspensions, i.e.,cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate(SDS) and changing such surfactants’ additive amount. Meanwhile, the Hot Disk 2500s thermal analyzer was applied to test the static thermal conductivity of the MPCM suspensions in which the mass fraction of capsules is 10wt.% and obtained the relation between the thermal conductivity and the contact angle. The results indicated, when the mass fraction of surfactants falls into the range of 0 -0.05wt.%, the effect of the CTAB is more significant than the SDS. However, when the mass fraction of surfactants continue to increase from 0.05wt.%, the significance of the SDS exceeded the CTAB. It was also found that the decrease in the contact angle led to the growth in thermal conductivity for both Maxwell model’s theoretical value and experimental results. When the contact angle fell into the range of 45°-95°, the Maxwell model’s theoretical results of the thermal conductivity could predict the thermal conductivity of MPCM suspensions with a good accuracy, but inversely when the contact angles were smaller than 45°, a significant gap was found between the two results. To remove such gap a correction factor “A” which is associated with contact angles was proposed.
In order to study the thermal performance of a single U-tube VBHE in multiple-layer substrates, the similarity principle was used to build an indoor comprehensive miniature bench of single U-tube VBHE innovatively, and effects of inlet water temperature and hydrogeological geology on the thermal performance of VBHE were analyzed. In addition, numerical simulation was used to assist in the research. The results indicate that to build a laboratory-scale GSHP system model test apparatus based on the similarity principle will achieve long-term test of GSHP system with smaller human and material resources and a shorter experimental cycle. It's more efficient and convenient. The simplified use of uniform soil thermal and physical properties can cause great deviation. Heat flux per unit pipe of the VBHE increases linearly with the temperature difference's increasing between the inlet water and initial soil. The water temperature along the tube under thermal conductivity increasing and decreasing conditions does not decrease as linearly as the temperature's decreasing under the uniform thermal conductivity condition.
Fundamental experiments were conducted on a cooling tube with an obstacle using liquid nitrogen as the coolant under a frost-formation condition. The background of this study is the development of a heat exchanger for an air-breathing engine, which cools the air using a cryogenic fuel. The obstacle is located in front of the cooling tube. The main purpose of the obstacle is to protect the cooling tube against damages due to foreign objects. The experimental results obtained using an obstacle with a V-shaped cross section show that the obstacle helps in reducing the pressure loss and improving the heat-transfer performance if the distance between the obstacle and the cooling tube is optimally selected. The experiments conducted using obstacles with several types of cross sections show that one of these obstacles with an arrowhead cross section is effective in suppressing the pressure loss, mitigating an adverse effect on the heat-transfer performance, and protecting the cooling tube against damages due to foreign objects.
In order to clarify the response characteristics of a lean methane-air premixed flame to equivalence ratio oscillation from actual measured values of the burning velocity, we developed a new burner with a wall-stagnating flow that allowed measurement of the flow field by using particle image velocimetry (PIV). To create fluctuations in the equivalence ratio only in the direction of flow without varying the velocity field, fuel and air flow rates were controlled by alternately vibrating two sets of loudspeakers. Burning velocity Su was calculated from the measured unburnt gas velocity ug and flame moving velocity uf. ug at the front edge of the flame was measured by PIV. uf was obtained using a high-speed video camera. For an oscillation with a mean equivalence ratio of 0.85, amplitude of 0.05 and frequency f ranging from 5 to 50 Hz, the following results were obtained: (1) The oscillation amplitude of the flame position changed quasi-steadily in the low frequency range, decreasing with increasing f in the frequency range greater than 40 Hz. (2) The oscillation amplitude of the burning velocity of dynamic flame ΔSud obtained by PIV measurement increased with respect to f, and reached maximum at 40 Hz. The maximum value became greater than that of the static flame ΔSus, over the same equivalence ratio range. This tendency was similar to the result obtained by approximating the flow field by a potential flow. (3) The frequency characteristics of the oscillation amplitude ratio of the burning velocity ΔS (= ΔSud/ΔSus) qualitatively correspond to the results ΔS(p) (= ΔSud(p)/ΔSus(p)) obtained by approximating the flow field by the potential flow. However, ΔS is quantitatively larger than ΔS(p), and the difference between ΔS and ΔS(p) is large in the high frequency range. This is because the approximation of the flow field by the potential flow underestimates the oscillation amplitude of the unburnt gas velocity, and the degree of the underestimation becomes noticeable in the high frequency range.
Saturated film boiling heat transfer around a vertical-finite-length cylinder has been investigated analytically to predict the local heat transfer coefficients at the bottom and lower vertical surfaces. Correlations of heat transfer for the vertical cylinder with top and bottom horizontal surface has been already analyzed by Momoki et al.(2007) and this correlations equations for heat transfer are in good agreement with the experimental data for the cylinders in saturated water. In the present study, this correlations are applied to estimate the local and average heat transfer rate through each surfaces of the cylinder. To predict the local heat transfer coefficient at the corner of the bottom surface and vertical lateral surface, Shigechi et al.(1999) analysis for film boiling heat transfer on horizontal bottom surface was modified in order to get finite vapor film thickness at the end of the bottom surface to predict finite value of local heat transfer coefficient at this end. In this modification, the vapor film thickness at the end of the bottom surface can be predicted and the prediction of the average heat transfer rate through all surfaces are in good agreement with the results of the Shigechi et al.'s method. Moreover, the local heat transfer coefficient at the end of the bottom surface can be predicted to discuss the local heat transfer characteristic of the bottom surface. The local heat transfer rate through the bottom surface and vertical lateral surface are described in terms of local Nusselt number with degree of superheat. The results on local heat transfer coefficient shows the highest value at the corner of the bottom surface and vertical lateral surface and it can be confirmed the experimental results of the vapor film collapse start from the corner of the bottom surface and vertical lateral surface of the cylinder in saturated film boiling.
Absorption refrigeration cycles, the developed as an alternative to vapour-compression refrigeration cycles they are not effective at low temperatures. When the absorption and vapour compression cycles are combined as cascade the consumed compressor work can be reduced considerably, but it requires the use of heat energy at low temperature (solar energy, geothermal energy, waste heat). In this study, the absorption part has been designed to improve the performance of absorption-vapour compression cascade cycle as serial flow double effect. The detailed thermodynamic analysis has been made of the double effect absorption-vapour compression cascade refrigeration cycle. For the novel cycle working fluid used R-134a for vapour compression section and LiBr-H2O for absorption section. This cycle has been compared with single effect absorption-vapour compression cascade cycle and one stage vapour compression refrigeration cycle. The results indicate that the electrical energy consumption in the novel cycle is 73% lower than the one stage vapour compression refrigeration cycle. Also, the thermal energy consumption in the cascade cycle is 38% lower than the single effect absorption-vapour compression cascade refrigeration cycle. It is found that the the minimum and maximum exergy efficiency occurs in the cooling set and the low pressure generator (LPG) as 21.85% and 99.58%, respectively.
Computational results of flow structure, pressure loss and heat transfer characteristics in triple-start corrugated tubes are reported. The influences of the depth ratio (DR = 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14 and 0.16) and pitch ratio (PR = 0.5, 0.65, 0.75, 1.0, 1.5 and 2.0) were investigated in turbulent flow regime, Re = 5000 to 20,000. The computational results indicated that the triple-start corrugated tubes generate main swirl flow and helical swirl flow which helps to reduce the thermal boundary layer thickness and enhance the heat transfer rate. The flow and heat transfer become under fully developed periodic condition around x/D = 6.0. The friction factor monotonically increases with the rise of DR values and decrease PR values while maximum heat transfer rate is found at DR = 0.08 and PR = 0.75. Nusselt numbers and friction factors of triple-start corrugated tubes in the investigated range are found to be 0.8 to 2.31 and 1.0 to 17.14 times over those of the straight circular smooth tube, respectively. For the range studied, the triple-start corrugated tube with DR = 0.06 and PR = 0.75 offers the maximum thermal enhancement factor of 1.21 at Re = 5000.
In this study, we have examined the flow field and the flame surface density in premixed combustion by simultaneous particle image velocimetry (PIV) and OH-planar laser induced fluorescence (OH-PIIF) measurements. Turbulent flames are formed in a cyclone-jet combustor under weak to highly turbulence. The flame surface density is evaluated by the orthogonal OH-PLIF for vertical and horizontal imaging. In terms of the flame front detected by OH fluorescence, the flow field of turbulent flames including the strain rate has been discussed. It is found that the axial velocity becomes the maximum around the center axis, whereas the rms axial or radial velocity takes its maximum at the position far from the center axis. The larger velocity fluctuation is observed by increasing the mean exit velocity of Um. The radial position of peak OH fluorescence signal is not in the region of large rms velocity, except for the condition where the local extinction is observed. Hence, the large velocity fluctuation induces the local extinction through the high strain rate. The flame surface density is increased with an increase in rms velocity. The range of three-dimensional flame surface density (3D FSD) is from 0.2 to 0.8 mm-1, which corresponds similarly to that of the Bunsen flame. Moreover, based on two-dimensional (2D) and 3D FSDs, the increasing factor of 3D effects is not the same value of homogeneous turbulence. Recognizing a linear relationship between the rms axial velocity and the integrated FSD, the increase of FSD is simply caused by the flame wrinkling due to the turbulence, even at the condition where the local extinction occurs.
The single-point compression principle, based on the collision of pulsed supermulti-jets, which is proposed in our previous reports, has the potential of obtaining both a high compression ratio and relatively low combustion noise, leading to a lower exhaust gas temperature, i.e., high thermal efficiency for the next generation of engines. The supermulti-jets also enclose high-temperature combustion gas around the chamber center, which means less heat loss to the chamber wall, i.e., higher thermal efficiency due to the air-insulation effect. Here, experimental and computational visualizations around the compression point should be examined in order to confirm the occurrence of single-point compression. Thus, in the present paper, we present experimental Schlieren photographs of flows formed by the collision of supermulti-jets without combustion and the results of unsteady three-dimensional computations conducted with the compressible Navier-Stokes equations, while the Cubic Interpolated pseudo-Particle (CIP) and Combined Unified Procedure (CUP) method is employed as numerical algorithm. Comparison of the experimental and computational results show fairly good agreement in time and space. Schlieren photographs and computational visualizations obtained for various conditions of four-, eight-, and sixteen- nozzles of jets are axial symmetrical, which will indicate the single-point compression based on the collision of supermulti-jets. Computations for asymmetrical distribution of seven nozzles also bring results showing nearly symmetric flow.
Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling is widely used in application of thermal engineering and considerably more efficient than single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circular spots (0.5 - 2 mm in diameter and 1.5 - 3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and low incipience temperature overshoot with water as working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests under heat loads 30 W to 260 W reveal the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation-site density. The surface with hydrophobic spots with diameter 1 mm and pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. A comparison of three evaporator surfaces with identical wettability patterns but with different surface topographies and coating thicknesses is carried out experimentally. The results show superior heat transfer rates and wear resistance on the surface coated with HNTs spots thanks to the large contact angle, great thickness, and durability of the coating layer.
In this work, the existing theoretical heat conductive models such as: Cattaneo-Vernotte model, simplified thermomass model, and single-phase-lag two-step model are summarized, and then a general model of hyperbolic heat conduction (HHC) is presented with boundary conditions prescribed as: (1) temperature at the boundary; (2) heat flux (not the temperature gradient) at the boundary. The convective boundary condition is not considered because it is impossible to produce a fluid motion at such time scale, e.g. picoseconds. In the context of HHC, the reciprocity relation of Green's function is systematically proven, based on which Green's function solution equation of the general model is completely derived. Meanwhile, Green's functions are deduced under several different sets of boundary conditions. Thus, solution to HHC based on Green's function is obtained, and it consists of three parts, i.e. boundary conditions, initial conditions and heat generations. The priority of the present solution is that the time-dependent boundary condition and heat generation may be dealt with, and that it is easy to extend the solution to multi-dimensional case. The accuracy of the solution is verified through numerical examples, and Laplace transform method is adopted to avoid the dispersions around the front of temperature wave for jump-type boundary conditions, thus the solution is further perfected.
Based on the continuum assumption and Navier-Stokes equation, three-dimensional fluid flow and conjugate heat transfer in heat sinks with rectangle microchannels have been studied by numerical simulation. The validation of this approach has been demonstrated by comparisons with analytical solutions. Results were obtained for the detailed description of the local and average heat transfer characteristics including wall and fluid temperatures, heat flux, Nusselt number (Nu). Effects of the Peclet number (Pe), the relative conductivity (kw/kf) and the relative thickness (W/D) of solid substrate were presented and discussed. At low Pe flow, axial conduction dominates the heat transfer, which results in a large upstream region which is pre-warmed by axial conduction, so axial conduction cannot be neglected at small Pe. The increase of Pe induces an enhancement of convection, the heat exchange mainly happens on heat region by convection under this condition. A more uniform streamwise temperature distribution at fluid-solid interface indicates the enhancement of the pre-warmed effect along with the increase of kw/kf. The study also reveals that the Nux of low kw/kf material is higher than the Nux of high kw/kf material. The increase of wall thickness has an effect on axial distribution of the Nux, the trend is similar to that of an increase of the wall thermal conductivity kw/kf. It was concluded that the average Nusselt number is proportional to thermal conductivity of the wall. For low Pe, the fluid is heated by the wall by means of convection on heat region, when Pe ≤ 1, a slight increase of the Nu on heat region is shown as the increase of Pe. When Pe > 10, the convection is the dominant factor in heat transfer, an obvious increase of Nu is shown with increase of Pe. For high Pe flow, the heat transfer enhancement as increase of Pe is weak.
Frost deposition and its growth occur on the cold surface of the heat exchangers by being exposed to humid air, which cause serious problems on the heat transfer performance due to the thermal resistance between the cooling surface and the ambient air. The structure of frost is porous consisting of ice crystal and air, which makes the thermal conductivity of frost remarkably low. In this study, the microstructure of the frost layer is measured by using the X-ray μCT at SPring-8 BL20B2 beam-line. Frost is formed on the cooling surface of 6 mm in diameter. 600 frost-layer projection images are taken by the X-ray radiography with rotation of the cooling surface through 180 degrees in 45 seconds. The reconstructed three-dimensional image clearly showed the microstructure of the frost layer, which consisted of three regions; the ice droplets region, the columnar crystals region and the dendrite crystals region for the 10 min frosting duration. The ice droplets were formed on the cooling surface with about 80 - 150 μm in height. The columnar crystals grew linearly from the surface of the ice droplets up to 400 μm height. At higher than 400 μm, the frost crystals branched and changed its structure to the dendrite crystals. For the 20 min frosting duration, the upper layer of the frost is covered by the laterally growing crystals. The frost density profile was directly estimated from the three-dimensional frost structure. The frost density showed the constant about 260 kg/m3 at the columnar crystal region and it gradually decreased in the dendrite crystal region.
We have developed an experimental apparatus for the measurement of diffusion coefficient of aqueous methanol solutions in a polymer electrolyte membrane (PEM) for direct methanol fuel cells (DMFCs), based on the infrared Soret forced Rayleigh scattering (IR-SFRS) method. Because mass transport phenomena in PEMs play significant roles, evaluation and understanding of the mass diffusion in PEMs is important. In the present study, we developed a new sample cell using a single crystal diamond window, which transmits both the heating laser with the wavelength of 9.714 μm and the probing laser with the wavelength of 639 nm. The heating wavelength was selected from the emission range of a valuable-wavelength carbon dioxide laser to utilize the absorption of methanol due to CO stretching. By using a single crystal diamond window, the scattered light of the probing laser generated at the window material which causes unpreferable effects on detection signals was reduced to about 1/150, compared to a polycrystalline diamond window. To evaluate the validity of the measurement system, we carried out experiments on the aqueous methanol solutions at a temperature of T = 298.2 K. The mass diffusion coefficient of the aqueous methanol solution with the mole fraction of 0.7 measured in the present study agreed with the measured values by the diaphragm cell technique within the relative standard uncertainty of 8.9%. After the validation, we performed experiments on three concentrations of aqueous methanol solutions in a PEM of Nafion 117 at T = 298.2 K. The mass diffusion in Nafion was observed in several hundreds of milliseconds. We obtained the mass diffusion coefficient with the relative standard uncertainties of 2.5-5.3%, depending on the methanol concentration. The mass diffusion coefficient in Nafion 117 was smaller than that of bulk aqueous methanol solutions, and increased with the increase in the methanol mole fraction.
In order to understand heat transfer mechanisms in the combustion chamber with multi-elements injector and provide benchmark data of wall heat fluxes for the CFD code validation, experiments were carried out in a GH2/GO2 heat-sink combustion chamber. To obtain the transient heat flux in the experiments, the Levenberg-Marquardt method was modified and applied to the rocket combustion chamber based on the temperature measured by single coaxial thermocouple (Named “single-point method”). The comparison between the single-point method and the two-point method with temperatures measured in two points shows that the single-point method could be used as heat flux measurements of the heat-sink combustion chamber in engineering. Heat flux distribution was obtained in experimental conditions of different work time and chamber pressure by the modified method, feasibly and efficiently. For the transient variations of the heat flux, an inverse variation trend of heat flux to time in the cylindrical segment and the nozzle segment was observed. A typical variation of the averaged heat flux was obtained under the conditions with different chamber pressure and work time. The results of wall heat flux scaled with pressure to the power 0.8 can be uniformized for all pressure levels. The heat fluxes obtained in the cylindrical chamber and nozzle section may be applied for the life cycle prediction of rocket engines.