This research experimentally investigates the exhaust gas characteristics of a methane-air lifted-jet flame with elevated stream temperatures up to 450 K. Emission indices of NOx and CO were measured by direct gas sampling using a gas detector tube. The index of NOx decreased or did not change with increasing stream temperature up to 350 K, and then increased with further temperature increases. The index of CO changed in an opposite manner to that of NOx with increasing stream temperature. These tendencies demonstrated that the generation of emission gases did not monotonically increase with stream temperature. These non-monotonic properties of the emission indices were caused by particular variations in the lift-off height of flame and flame length. In order to examine these flame characteristics, we measured the flame curvature with OH-PLIF and related it to the lift-off height of triple flame. These flame properties could explain the unique emission characteristics of a lifted-jet flame with stream heating.
The effects of the surface structures and the surface structural clearances at the nanometer scale on the thermal resistance at a Lennard-Jones liquid-solid interface, as well as the self-diffusion behaviors of liquid molecules, were investigated directly by the non-equilibrium classical molecular dynamics simulations. When the potential parameter between liquid molecules and nanostructure atoms is equal to that between liquid molecules and solid wall atoms, in other words, in the case of nano-engineered surface, the geometric surface area change depending on the nanostructures as well as their clearances and the self-diffusion coefficient change of the liquid molecules at the interface depending on the nanostructural clearances cause the thermal resistance change depending on the nanostructures at the liquid-solid interface. When the potential parameter between liquid molecules and nanostructure atoms is different from that between liquid molecules and solid wall atoms, in other words, in the case of a modified surface at the nanometer precision, the interfacial thermal resistance is much dependent on the potential parameter between liquid molecules and nanostructure atoms itself rather than the geometric surface area at the molecular scale.
Magnetic refrigeration is a new environmentally benign technology and a promising alternative to conventional vapor-cycle refrigeration. Among these technologies the household refrigerator without a freezing compartment shows very good prospects for a successful application. This article starts with the general principle of magnetic refrigeration and then also describes the maximum specific cooling capacity of magneto caloric materials. The specific cooling power of a magnetocaloric material is found to be large even for medium magnetic field changes, especially if the frequency is not too small. For a domestic magnetic refrigerator, a comparison with a standard compressor refrigerator is presented. The modeling of a rotary magnetic refrigerator is described and its dynamic behavior is investigated. The physical model is based on a mapping of the magneto-thermodynamic problem from a cylinder onto two rectangles. In this model, in a basic centre cell, two coupled linear partial differential equations are solved, which have been programmed in the Modelica language. Steady-state solutions are envisaged to determine the coefficient of performance, COP, for these conditions. In future work the developed model shall be applied for an optimization of the magnetic refrigerator and to determine the related best parameters.
Ethanol oxidation rate in high pressure steam at the pressure of 10 MPa in the temperature ranging from 430 to 490 °C was studied with the fuel equivalence ratio of 0.4. On the assumption that the reaction order is first order, the reaction rate parameters obtained in the experiments are 1011.6 ± 0.4 s-1 for the pre-exponential factor and 166.5 ± 6.1 kJ·mol-1 for the activation energy. This ethanol oxidation rate in high-pressure steam was equivalent to that in supercritical water oxidation of ethanol. Liquid products were acetaldehyde and acetic acid and gas phase products were carbon monoxide, carbon dioxide, methane and ethane. A parallel reaction network of first order model well described the characteristics of the ethanol decomposition to acetaldehyde and acetic acid. Utilisation of the high-pressure steam oxidation process can overcome the problem of pressure tightness of the reactor in supercritical water oxidation, and the process can be employed in practical facilities.
Melting of an ice by a steam is sometimes employed, for example, to remove a frost, an ice, and a snow that adhered to the structure surface. It is usually regarded that the melting performance by steam is naturally higher than that by high-temperature dry air due to the effect of large latent heat of condensation. In the present study, the effect of latent heat of condensation on the melting performance of ice by steam has been investigated experimentally. Both in the flow of a saturated steam and a dry air at 100°C, the melting performance of horizontal ice cylinder were investigated under a variety of Reynolds number. A horizontal ice cylinder of 120mm diameter was adopted as the test piece. The melting rate in a flow of both saturated steam and dry air at 100°C was measured. The Reynolds number of the ambient flow was set at 550, 910, and 1270. The melting heat transfer characteristic was evaluated from the melting rate that was assessed from the shape of the ice cylinder. The experimental results show that the merits of the melting of an ice by a steam are not only the effect of latent heat of condensation, but also the existence of thick liquid film with temperature difference of 100°C.
The freezing behavior of liposomes such as internal freezing, contraction by dehydration and disruption of membrane after thawing was examined by microscopic observation. Liposome suspensions were prepared by gentle hydration method using phosphatidylcholine (egg) and distilled water. The observed liposomes ranged in diameter from 5 to 150 µm. The sample of liposome suspensions was cooled from -1 °C to -50 °C at cooling rates of 1, 2 and 5 °C/min and heated to 0 °C at heating rate of 10 °C/min. As a result, two different patterns for freezing were observed: internal freezing and contraction without internal freezing. When the internal freezing was observed, the membrane was unexceptionally disrupted after thawing. When the internal freezing was not observed, two different cases were observed after thawing: disruptive and contractive conditions. These different freezing patterns were primarily dependent on the liposome size. In addition, the cooling rate became a key factor determining the freezing patterns in small liposomes.
A pilot plant for continuous flow microwave-assisted chemical reaction combined with microreactors was developed and water heating tests were conducted for evaluation of the developed plant. We first designed a continuous-flow microwave-assisted chemical reactor in which a reaction solution absorbs microwave energy efficiently and can be used in chemical processes using microreactors. We designed the reactor using electromagnetic simulation and found that the energy absorption rate by the water was more than 80% by conducting water heating tests. Next, we developed a microwave apparatus having a single microwave generator that can heat reaction solutions in four reaction fields simultaneously in order to increase throughput. We also designed a four-branch waveguide using electromagnetic simulation, and found that the transmission efficiency at 99%. Finally, we developed the pilot plant using the developed microwave apparatus and conducted water heating tests. The temperatures in the respective reaction fields were controlled within ±1.1 K at 353.2 K. Moreover, the energy absorption rates by the water were about 90% in the respective reaction fields, whereas the energy absorption rate was about 40% when 100 cm3 of water was heated by a commercially available multimode microwave chemical reactor.
In this work we develop a new spectrophotometer system for measuring thermal radiation characteristics of real surfaces of thermal engineering. This system measures transition of spectra of normal incidence hemispherical reflectance RNH, normal incidence specular reflectance RNN, normal incidence diffuse reflectance RND, normal incidence absorptance AN and normal emittance εN of real surfaces in a near-ultraviolet through infrared region of wavelength 0.30∼11 µm simultaneously and repeatedly with a cycle time of 6 s. The system is applied to measure the spectrum transition of the reflectances, absorptance and emittance of a nickel surface which is prepared as a clean optically smooth surface and is oxidized in high-temperature air to be changed to an oxidized rough real surface. Microscopic mechanisms of the spectrum transition are discussed, to illustrate the performance of the developed spectrophotometer system for thermal engineering applications.
A confined, round, laminar impinging jet, fed upward against the gravity, is studied, both experimentally and numerically, under the influence of an impingement-surface heating. It is a submerged water jet that steeply separates from the surface at a small distance from the stagnation point, due to a downward natural convection. The region after the separation point turns into a dead zone where the heat transfer deteriorates. It severely undermines the overall heat-transfer efficiency of the jet. Moreover, the separated jet is found to be highly transient, with the separation location oscillating in a cyclic fashion. Its mechanism is also discussed in detail.
The ignition behavior of a newly developed biomass briquette, Bio-coke (BIC), is investigated. The fuel has unique features such as economical advantages for its versatility of biomass resources, high volumetric calorific value because of its high density (1300 kg/m3; twice or more than that of ordinary wood pellets) and high mechanical strength. The ignition characteristics of cylindrical BIC blocks (48 mm in diameter and 85 mm in length), important when using the fuel in actual combustion furnaces, are investigated in high temperature air flows (473-873 K, 550-750 NL/min.). In the experiments, preheated air is blown onto the bottom surface of BIC cylinders and the ignition behavior of the bottom surface is observed monitoring the surface temperature as well as the time dependent mass loss rates. The results show two ignition modes; (1) solid surface ignition preceding gas-phase ignition in high air temperature conditions (T≥598K), and (2) gas-phase ignition accompanied by simultaneous surface ignition occurring at relatively low air temperature conditions. The appearance of each mode depends on the preheated air supply condition in terms of the air temperature, flow velocity, and moisture content of the fuel. The rate of evolution of volatile gases is closely correlated with the temperature distribution inside the BIC briquette which depends on the heating rate, implying that variations in the temperature distribution inside the fuel could be one reason for the appearance of the observed ignition modes. It is suggested that the temperature distribution inside the fuel has to be taken into account in the control of the ignition behavior of BIC briquettes.
The effect of antifreeze solution liquid film on the frost prevention is experimentally investigated. It is desirable that the antifreeze solution spreads widely on the heat exchanger surface forming thin liquid film to prevent frost nucleation while having small thermal resistance across the film. A porous layer coating technique is adopted to improve the wettability of the antifreeze solution on heat exchanger surface. The antifreeze solution spreads widely on the heat exchanger surface with 100 µm thickness by the capillary force resulted from the porous structure. It is observed that the antifreeze solution liquid film prevents a heat exchanger from frosting. The reductions of heat and mass transfer rate caused by the thin liquid film are only 1-2% compared with those for non-liquid film surface.
In this study pyrolysis kinetics of jute stick (white jute: Corchorus capsularis; tossa jute: Corchorus olitorius) and tamarind (Tamarindus indica) seed available in Bangladesh have been investigated thermogravimetrically in a nitrogen atmosphere at heating rates of 10 and 60°C/min over a temperature range of 30 to 800°C. The two biomass solid wastes exhibited similar behaviors in that, the weight loss region is shifted to a higher temperature range and the weight loss rate is increased with increasing heating rate. The percentage of total weight loss is higher for jute stick and is lower for tamarind seed. The overall rate equation for the two biomass wastes has been modeled satisfactorily by one simplified equation from which the kinetic parameters of unreacted materials based on the Arrhenius form can be determined. The predicted rate equation compares fairly well with the measured TG and DTG data.
In this paper we propose a new level set approach to describe not only a flame's surface but also to the flame's spatial distribution. First, we derived the mathematical formulation for a one-dimensional laminar premixed flame, where the steady flame has a finite thickness depending on the diffusion flux whose physical quantity such as temperature has a relation to index function G . Further, to investigate the relationship between the present model and Inage's model, we extracted the physical meaning from the energy equation. We also validated another important parameter, the heat reaction release rate using the modified G-equation. The analysis of the heat release term leads to the definition of local flame speed, describing the distribution in the flame thickness. We evaluated the distribution of local flame speed with scalar G based on the one-dimensional solutions of premixed flames obtained by the detailed chemical reaction GRI-Mech3.0 using CHEMKIN. For CH4/Air premixed fuel, we carried out a series of calculations with different fuel rates and inlet temperatures. Based on the linear distribution of local flame speed, the modified G-equation can again be certified as the hyperbolic tangent profile.
The structure of cathode catalyst layer (CCL) has strong relationship with the performance of polymer electrolyte fuel cells (PEFCs). We investigated the relationship between the catalyst layer structure and the cell performance experimentally. Multi-layered CCL is used to investigate the effect of the layer design on the cell performance. Membrane side of CCL works, as a reaction area, more actively than the gas diffusion layer (GDL) side at low relative humidity (RH) due to the lower proton conductivity. On the other hand, when the cathode gas has less oxygen partial pressure at high RH, GDL side is more active than membrane side owing to low diffusivity of oxygen. We suggest that the volumetric catalyst concentration of the CCL membrane side should be higher at low RH, however at high RH with lower oxygen partial pressure in cathode gas, the GDL side should have higher concentration. Simple theoretical model is employed to see the behavior of the reaction distribution in the catalyst layer.
This paper describes the results of experimental and analytical work on a phase-changematerial(PCM)-based transient cooling module. The module is made of low-cost materials, yet it is designed to achieve a reasonably high level of heat transfer performance. Paraffin is used as PCM, and it fills the space studded with metallic pin fins. We measured transient temperature rises at several spots of the module. Also, the effects of the dimensional parameters of the pin fins on the heat dissipation performance were investigated. The measured temperatures explicitly reflect the thermal absorption effect of PCM. Analytical work is conducted using a thermal network model where equivalent thermal capacitance are attached to the nodes of the network. The model is validated by the experimental observations, and the simulation code is expected to serve as an efficient analytical tool in the design of PCM-based cooling modules.
This paper presents numerical simulations of thermoelectric materials for local Seebeck coefficient measurements by a heated microprobe. The Galerkin finite element method is used to solve the governing equations with boundary conditions to obtain temperature and induced thermoelectric voltage distributions. The measured local Seebeck coefficients obtained from the potential Seebeck microprobe (PSM) model and the simplified PSM model are compared; the influence of probe tip size and heating time is investigated. A method is proposed for obtaining the spatial resolution of the PSM apparatus by increasing inhomogeneity size gradually. The simulation results indicate that the simplified PSM model can be efficiently used to simulate the local Seebeck coefficient measurements by the PSM apparatus. The temperature distribution is similar to the voltage distribution for homogeneous thermoelectric material but is different from the distorted voltage distribution for thermoelectric material with inhomogeneity. The spatial resolution of the PSM apparatus increases with decreasing probe tip size, heating time, and thermal conductivity of the thermoelectric material.
The performance of a compact cryogenic heat exchanger with metal woven wire screen matrix units was studied. Using different structural parameters, the heat transfer and hydrodynamic characteristics of the units were analyzed. Comparative analysis was also performed on the impact of different unit geometries and on the operational parameters affecting heat transfer performance and resistance characteristics. Data were interpreted to derive relevant performance characteristics such as mass flow, heat load, temperature difference in heat transfer, Reynolds number, heat transfer coefficient, inlet and outlet temperature, and resistance losses, among others. This study also introduced the structural parameters of optimal hot and cold section proportions and the operational range of thermal parameters. The cryogenic heat exchanger of interest had a hot and cold cross-sectional area ratio of 0.44; when optimized, the unit can work within the temperature range of 80—295K with a mass flow between 0 and 1 g/s and a heat load from 35 to 400 W. This condition produced maximum heat exchanger efficiency at 94% with both hot and cold fluid flow resistances less than 3.0 kPa. The units can constitute a low-weight ratio with a larger specific surface area. Based on the principles of characteristic evaluation, the assessment results were presented based on an integrated feature of heat transfer units, which includes their thermal properties, resistance characteristics, and structural parameters, among others. The criterion for the improvement and performance optimization of the space applications of cryogenic woven wire screen matrix heat exchanger units was also proposed.