Convection heat transfer enhancement is an important issue since this problem is of particular interest in the field of energy and environment. Ag nano-solution is expected not only to enhance heat transfer but also to work for deodorization and antifungal effect. An experimental investigation on the convective heat transfer characteristics for NaHCO3-Ag nano-compound material solution in a long and straight heated pipe has been carried out. NaHCO3 compound materials with 400 ppm or 1000 ppm Ag nano-particle solved in pure water are considered to study the effect of Ag nano-particle on the heat transfer enhancement. The concentration of NaHCO3-Ag compound material in the water is varied 0.1 % to 1.0 %. The results indicate that the convective heat transfer coefficient is increased with an increase in the concentration of NaHCO3-Ag compound solution. At a given concentration, heat transfer coefficient is increased as the content of the Ag nano-particle is increased. Heat transfer enhancement ratio correlation using NaHCO3-Ag compound solution is also suggested.
In the present study, the flow characteristics and heat transfer of a thermo-sensitive magnetic fluid, which is a multiphase-flow material, were investigated experimentally. The mini-channels considered herein have a depth of 0.5 mm, with the nominal channel width being equal to five times the depth. The channel device was constructed from a Teflon tube. The operation of the device is based on the thermo-magnetic characteristics of the fluid, a suspension of Mn-Zn ferrite particles in kerosene, the magnetization of which is known to decrease with increasing temperature. The experimental parameters were magnetic force, the position of the magnet, and the temperature of the magnetic fluid. The experimental results indicate that force convection based on the magnetic characteristics of the fluid in the mini-channel exhibits excellent cooling performance. Furthermore, the flow characteristic of the thermo-sensitive magnetic fluid was found to be strongly dependent on the magnetic condition, such as the force and the position.
The present study numerically investigated the effect of tube arrangement on the thermal performance of the high temperature generator (HTG) of a double effect LiBr-water absorption system with the cooling capacity of 210 refrigeration tons. The HTG tubes were located behind the pre-mixed surface flame burner. The HTG was consisted with a set of circular tubes and flattened tubes in series. LiBr aqueous weak solution was flowed into the HTG tube, and then turned to strong solution at the exit of the HTG tube. FLUENT, as a commercial code, was used to estimate the thermal performance of the HTG. Standard k-ε model was applied for the turbulent flow of combustion gas around the HTG tubes. Key parameters were the diameter of the circular tube and tube pitch ratio of flattened and circular tubes in the HTG. The combustion gas temperature was increased and its heat transfer rate was decreased as the circular tube diameter was increased, and the pitch ratios of both circular and flattened were decreased.
The effect of inclination angle on the heat transfer and pressure drop characteristics of brazed aluminum heat exchangers having louvered fins is experimentally investigated. Three samples having different fin pitches (1.25, 1,5 and 2.0 mm) were tested. Results show that heat transfer coefficients are not affected by the inclination angle. Friction factors, however, increase as the inclination angle increases, with negligible difference between the forward and the backward inclination. Both the heat transfer coefficient and the friction factor are the smallest at Fp = 1.5 mm, followed by 2.0 mm and 1.25 mm. Possible explanation is provided considering the louver layout. Comparison of the present data with existing correlations is also made.
A cryogenic heat exchanger to remove carbon dioxide from landfill gas (LFG) is proposed and designed for applications to LNG production in distributed-scale. Since the major components of LFG are methane and carbon dioxide, CO2 removal is a significant pre-process in the liquefaction systems. A new and simple approach is proposed to directly remove carbon dioxide as frost on the surface wall along the cooling passage in a liquefying heat exchanger and to install two identical heat exchangers in parallel for alternative switching. As a first step of feasibility study, combined heat and mass transfer analysis is performed on the freeze-out process of CO2 in a counterflow heat exchanger, where CH4-CO2 mixture is cooled below its frost temperature in thermal contact with cold refrigerant. Engineering correlations for the analogy of heat and mass transfer are incorporated into numerical heat exchanger analysis with detailed fluid properties. The developed analytical model is used to estimate the distribution of CO2 accumulation and the required heat exchanger size with latent thermal load for the cryogenic CO2 removal in various operating conditions.
The cooling capability in compact electronic equipment of natural convective flow fields was investigated. A relationship between air passage width in the channel and natural cooling capability was obtained. Temperature and velocity measurements were carried out using a channel model of electronic equipment comprising a vertical duct with a rectangular cross-section. The channel model comprised two copper walls and two transparent acrylic walls. The clearance between the copper walls was used as a parameter in the channel model. Velocity profiles of natural cooling flow in the channel were quantitatively measured using particle image velocimetry (PIV). The temperature and velocity results demonstrated that changes in the velocity profiles closely depend on the wall clearances. Our studies indicate that a clearance of 8 to 10 mm is optimal for natural cooling in a channel of this type with the 56 mm channel depth.
For the purpose of evaluation of debris transport on containment floor of nuclear power plants, a model to calculate the transient flow field on the floor is developed. Two-dimensional shallow water equation (SWE) is solved by finite volume method (FVM) with unstructured triangular grid. The predictor-corrector method with an approximate Riemann solver of the Harten-Lax-van Leer (HLL) scheme is used to reserve second order accuracy in time and to capture water spreading on dry bed. An experiment of a dam-break is simulated. The present model is also applied in the calculation of the flow field of actual containment and the prediction result is discussed in terms of the transient behavior including the water spreading and the transient flow rates entering the Holdup Volume Tank (HVT).
The purpose of this paper is to investigate the effect of the ambient pressure on the overall spray characteristics of dimethyl ether (DME) and biodiesel fuel spray in a high-pressure combustion chamber. In order to investigate the spray performance under the various ambient pressure conditions, the high-pressure combustion chamber, which can be controlled by nitrogen gas to obtain the ambient pressure conditions, was applied. To determine the macroscopic structure of the spray, such as the spray development process and the axial/radial distance, frozen images of the spray can be obtained directly using a high-speed camera with a light source according to the elapsed time after energizing begins. At the same time, the results from the DME and biodiesel spray under atmospheric conditions were compared with the results from the spray under various ambient pressure conditions in the chamber. According to the spray characteristics, the axial and maximum radial distances of the biodiesel and DME spray decreased with an increase in the ambient pressure. Also, it was shown that the SMD (Sauter mean diameter) of droplets becomes smaller than that of the spray under the atmosphere condition as the ambient pressure in the high pressure chamber increases.
Power generation systems based on the oxy-coal combustion with carbon dioxide capture and storage (CCS) capability are being proposed and discussed lately. The proposed systems are evolving and various alternatives are to be comparatively evaluated. This paper presents a proposed approach for performance evaluation of a commercial scale power plant, which is currently being considered for ‘retrofitting’ for the demonstration of the concept. System components to be included in the discussion are listed. Evaluation criteria in terms of performance and economics are summarized based on the system heat and mass balance and simple performance parameters such as the fuel to power efficiency and brief introduction of the 2nd law analysis. Cases are selected for comparative evaluation, based on the site-specific requirements. With limited information available, preliminary evaluation is attempted for the cases.
Oxy-fuel boilers have been developed to capture CO2 from the exhaust gas. A 50 kW class model burner has been developed and tested in a furnace type boiler. The burner has been scaled up to 0.5 and 3 MW class for fire-tube type boilers. The burners are commonly laid out in a coaxial type to effectively heat the combustion chamber of boilers. Burners are devised to support air and oxy-fuel combustion modes for the retrofitting scenario. FGR (flue gas recirculation) has been tried during the scale-up procedure. Oxy-fuel combustion yields stretched flame to uniformly heat the combustion chamber. It also provides the high CO2 concentration, which is over 90% in dry base. However, pure oxy-fuel combustion increases NO concentration, because of the reduced flow rate. The FGR can suppress the thermal NOx induced by the infiltration of the air.
Direct methanol fuel cell (DMFC) is one of promising candidates for power sources of mobile devices. Efficient operation of fuel cell system is very important for long-sustained power supply due to limited tank size. It is necessary to investigate performance characteristics of the fuel cell stack for efficient control of the DMFC system. We have developed an electrochemical model of DMFC using experimental data. The generated voltage is modeled according to various operating conditions; methanol concentration, cell temperature, and load current. The crossover phenomenon is also included in cell model. The model fits the experimental data well within 10 percent error. It is found that the efficiency of the fuel cell is mainly dependent on concentration of the methanol solution. Maximum efficiency line exists at specific concentrations of methanol solution with respect to the amount of generated power. Also efficient control method of the stack is presented.
Water management is important for the development of polymer electrolyte fuel cells (PEFCs) having high performance and reliability. Experimental and numerical studies were carried out on PEFCs with serpentine channels. Two channel widths, 2 mm and 8 mm, were examined. It was found that the rib of each separator played the role of a cover that reduced the rate of water vapor removal from the gas diffusion layer.
A new anode with a proton conductor (Barium-Cerium/Yttrium oxide (BCY): BaCe0.8Y0.2O3-δ) was proposed for a high-power, solid-oxide fuel cell. In the new anode, the proton-conducting material was included in a conventional anode made of nickel (Ni) and Gd-doped Ceria (GDC) in the ratio of GDC:BCY=0:100, 50:50, 90:10, 100:0 vol.%. With 3% humidity hydrogen fuel, the overpotential of the BCY anode became smaller compared with that of the conventional Ni/GDC anode. The most striking feature is that the anode with the 10%-BCY has the lowest overpotential among all anodes due to the high oxide-ion conductivity by the 90%-GDC and the reaction enhancement and/or the high hydrogen-adsorption ratio by the 10%-BCY.