This paper investigates the CHF in saturated pool boiling of ethanol, R141b, and water on a 7 mm diameter vertical copper surface at high pressures. The pressures are from 0.1 to 3 MPa for ethanol, 0.1 to 1.5 MPa for R141b, and 0.1 to 0.8 MPa for water. The results show that the occurrence of CHF is accompanied by the formation of large vapor masses covering most of the heating surface with all three liquids over the whole range of pressures investigated here. The well-known Kutateladze-type CHF correlation explains the variations in the CHF with pressure well for ethanol and R141b, and underestimates the pressure dependence of the CHF for water. A correlation considering the effect of surface wettability on the CHF agrees fairly well with the CHF for water in the whole range of pressures here, when the temperature dependence of the contact angle determined from available data is incorporated into the correlation. This suggests that it is necessary to consider changes in surface wettability with pressure to be able to predict the CHF of water at high pressures.
The effect of ferric oxide concentration on particulate fouling in two-dimensional repeated rib tubes is investigated. Three repeated rib tubes with the range of roughness configuration 0.015≤e/Di≤0.030 and 10≤p/e≤20 are tested. The fouling curves show an asymptotic behavior. The fouling resistances of repeated rib tubes are higher than that of the plain tube. At low concentration of 750 ppm, however, they are approximately the same. The repeated rib tubes show stronger concentration dependencies compared with the plain tube. Within the test range, the repeated rib tube fouling resistance increases as p/e increases and e/Di decreases. The deposit inspection supports this trend. The effect of the deposit non-uniformity on fouling resistance and the effect of deposit on heat transfer performance are additionally examined.
In this study, experiments on a pulse detonation turbine engine (PDTE) were conducted. The final goal of this work is the self-sustained operation of a PDTE system in which all of the air used for its operation is supplied by a turbine-compressor driven by pulsed detonations in the system itself. Currently, air used for PDTE operation is supplied by using an external device because the air flow rate for PDTE operation is too high. That is, air is used not only for combustion but also for purge of the residual hot burned gas in a pulse-detonation combustor (PDC), and sometimes for the control of the turbine-inlet temperature by blowing secondary air into a PDTE system downstream of the PDC. In this work, for reducing the air flow rate for PDTE operation, the water-droplet-injection technique was introduced in two ways. First, water droplets were injected into the PDC for purging the residual hot burned gas. Second, water droplets were injected into the PDTE system downstream of the PDC for controlling the turbine-inlet temperature. In particular, the effect of the latter was analyzed by using simple model calculations. As a result, the turbine-inlet temperature was precisely controlled by the water-droplet injection, and the air flow rate for PDTE operation was drastically reduced.
The effect of expanded polystyrene (EPS) inclusions on thermal conductivity of lightweight concrete is studied. Various mixtures are produced by incorporating EPS aggregate at different volumes (0%, 10%, 20%, 30% and 40%) in concrete with three water/cement ratios (0.55, 0.50 and 0.45). The apparent density and thermal conductivity values of expanded polystyrene concrete decrease as the volume of EPS increases. The thermal conductivity increases with an exponential function of the apparent density whatever the water/cement ratios. The general thermal conductivity models can not well predict the present experimental results. Based on experimental results and composite approach, a new simplified model is proposed to evaluate the effective thermal conductivity with an equation of the plain concrete thermal conductivity, the EPS thermal conductivity, the density and the percentage of EPS particles. The model is applicable to the EPS lightweight concretes with different water and cement ratios.
This study was conducted to develop a meso-scale annular type combustor that uses two types of coaxial cylindrical flames, rich premixed flame and diffusion flame. The combustor consists of inner and outer porous tubes, and rich C3H8-air mixture and air issued, respectively, through the inner tube outwardly and through the outer tube inwardly, forms a cylindrical stagnation plane sandwiched by the inner rich premixed flame and the outer diffusion flame. A petal type flame was also observed in the downstream of the cylindrical flames. Maintaining a constant equivalence ratio φi and flow rate qi of the rich mixture, the overall equivalence ratio φall is controlled by varying the air flow rate qo of the outer tube. In order to investigate the combustion performance, the temperature was measured inside and at the outlet of the combustion chamber. The heat loss rate and exergy loss rate were evaluated from the obtained temperature data. The minimum values of the heat loss rate and exergy loss rate were 0.30 and 0.57, respectively. These loss rates were compared with the performance of other meso-scale combustors.
A cratered film-cooling hole was investigated to determine the effect of shape parameters on film-cooling performance, and shape of the hole was optimized to improve film-cooling effectiveness. Numerical analyses were performed using three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations. The numerical results were validated by comparison with experimental data for film-cooling effectiveness for cratered film cooling. The ratio of the major axis length of the crater to the diameter of the hole, the ratio of the minor axis length of the crater to the diameter of the hole, and the ratio of the depth of the crater to the diameter of the hole were chosen as the design variables. From the results of a parametric study, the effects of these three design variables on film-cooling effectiveness were evaluated. The spatially-averaged film-cooling effectiveness was defined as the objective function. The values of the objective function were numerically evaluated through a RANS analysis at the design points determined by Latin hypercube sampling to construct surrogate models, i.e., RSA, RBNN, and Kriging models. Sequential quadratic programming was applied to search for the optimal point from the constructed surrogate models. The result showed that the Kriging model gave best optimization result and the film-cooling effectiveness for a cratered hole was improved by about 20% through the optimization in comparison with the reference geometry.
In this study, an injection rate meter that can be used to measure marine engine nozzle injection rates is developed and the results obtained using this equipment are presented. This main focus is on differences resulting from changing the rack stroke using marine diesel oil. In addition, marine engine injection system design variables are shown to be related to by the shape of the cam profile to nozzle geometry and to the control parameters influencing the injection quantity (fuel rack position) and timing. Changing the load and fuel rack position, we calculated the injection rate by using the Zeuch method, simultaneously measuring the amount of fuel injected from the marine nozzle for 10 minutes. The difference between calculated and measured quantities is less than 1%. Comparison demonstrated the reliability of the developed injection meter. In addition, injection rate characteristics of a marine nozzle under various conditions were investigated. This study shows that it is possible to measure the injection rate and quantity of the complete marine engine using the Zeuch method
Contributions of the advection, turbulent transport and diffusion terms to the Nusselt number and the torque coefficient in the Taylor-Couette flow were evaluated. Integrating the energy equation in the Taylor-Couette flow twice gave an equation in which the Nusselt number was decomposed into the three terms which were the advection, turbulent transport and diffusion terms. Integrating twice the momentum equation with respect to the azimuthal velocity in the Taylor-Couette flow gave an equation in which the torque coefficient was decomposed into the three terms. The two equations obtained here made it possible to evaluate the contribution of the three terms to the Nusselt number and the torque coefficient. Large eddy simulation was performed to obtain the velocity and temperature fields in two cases of Taylor number, Ta=4000 and 8000. Using the above equations to this field data showed that the contribution of the advection term was the highest, more than 70% for the Nusselt number and more than 55% for the torque coefficient due to the Taylor vortex which retained the structure in the axis-normal plane. The contribution of the turbulent transport term increased with the increase of the Taylor number while the advection term decreased. The contribution of diffusion term was small and did not change so much.
The structure of a counterflow-type cylindrical diffusion flame was studied experimentally to determine the effect of stretch and curvature. The cylindrical flame used in this study had a concave curvature with respect to the fuel stream. The fuels used were propane and methane, diluted with nitrogen, argon or helium. Diluent gases were used to change the Lewis number, Lef, of the fuel. Our results indicated that (1) the fuel was preheated to a high temperature by the concentric inward heat flow from the flame because the flame shape is concave curvature with respect to the fuel flow; (2) at constant flame stretch, flame temperature increased with a decrease in flame radius when Lef > 1, but decreased when Lef < 1; and (3) at constant flame radius, flame temperature decreased with increased flame stretch rate when Lef > 1, but increased when Lef < 1.
This paper shows the performance evaluation of the variable refrigerant flow (VRF) air-conditioning system at different outdoor air temperatures during cooling and heating operations. The purpose of this study is to determine the real performance of the system when operating at different outdoor temperatures. The one outdoor unit's and two indoor units' VRF system was used as a test specimen in the controlled testing chambers in which temperature and humidity were controlled. Several test cases were done covering from partial to full thermal loadings for the different outdoor air temperatures. The results showed that the outdoor air temperature much affected the performance of the VRF air-conditioning system. It showed that during the cooling operation, as the outdoor air dry bulb temperature increases, the coefficient of the performance ratio decreases. In the heating operation, as the outdoor air dry bulb with corresponding wet bulb temperature decreases, the coefficient of the performance ratio decreases. The defrosting mode, which occurs during the heating operation, affects the indoor chamber's temperature as the air temperature becomes very low due to the indoor units having become an evaporator. The results of the study showed the importance of the VRF air-conditioning system performances at different outdoor air dry bulb and wet bulb temperatures. Hence, it is important to consider the outdoor air temperatures when calculating the energy consumption of the VRF air-conditioning system to be installed in new and/or retrofitted buildings as they affect the electric energy consumption.
The supercharged boiler drum is partly surrounded by the furnace flue gas and hot air, and it's difficult to precisely determine the temperature field from the direct method. This paper presents the coupling method of direct and inverse heat conduction problem to calculate the transient temperature field of the drum. To reduce the influence of boundary condition, the drum is divided into two regions and the temperature fields are respectively determined from the direct and inverse method according to heating condition of the outer wall. For the junction of the two regions, the temperature determined from the inverse method is assigned to the direct method. Then the coupling method is realized, the whole temperature field of drum is obtained. The proposed method is validated through experimental data and the result simulated by ANSYS, during a cold startup process. The comparison of results shows high accuracy of the coupling method and strong adaptability of complex boundary condition.
Flow in a concentric annular passage with a rotating inner cylinder, which is called Taylor-Couette flow, is important in industrial applications, such as electric motor which requires not only effective cooling of rotating shaft but also saving power required for the axis rotation. When the through flow is superposed, which is called Taylor-Couette-Poiseuille flow, it affects the cooling efficiency and the torque required for the axis rotation. To the authors' knowledge, previous studies have been focused on either the Nusselt number or the torque coefficient in the Taylor-Couette-Poiseuille flow. Therefore, it is difficult to estimate the through-flow effects on both of them under the same geometry and flow conditions. In this study, the through-flow effects on both the Nusselt number and the torque coefficient in the Taylor-Couette-Poiseuille flow under the same geometry and flow conditions were investigated by performing large eddy simulation. The through-flow Reynolds number, Re, was varied from 500 to 8000 under constant Taylor and Prandtl numbers of Ta=4000 and Pr=0.71, respectively. The Nusselt number and the torque coefficient had similar trend to each other with the increase of Re. They decreased by 25% for the change of Re from 0 to 1000 and were nearly constant for the change of Re from 4000 to 8000. Contribution of the advection, turbulent transport and diffusion terms to the Nusselt number and the torque coefficient were evaluated by using the equations proposed by the authors. The contribution of the advection term was nearly zero for Re from 500 to 8000, which was contrary to the case without through-flow (Re=0). As Re increased, the contribution of the turbulent transport term decreased but that of the diffusion term did not change so much. The friction factor in the axial direction varied as Re-0.75 of which power was between laminar (Re-1) and turbulent (Re-0.25) correlations in a smooth stationary pipe flow.
Recently, there have been many studies on dehumidifiers with a low flow rate of liquid desiccant, in which a plate-type heat exchanger with internal cooling is adopted. If the low flow rate of a liquid desiccant maintains a thin liquid film on the heat exchanger surface, heat and mass transfer performance would improve, the stability of the liquid film would increase, and there would be no entrainment of desiccant droplets. In this study, we built an experimental apparatus to measure the regeneration performance of a liquid desiccant and investigated the wettability of the liquid desiccant along the vertical surface of a plate-type heat exchanger. Liquid desiccant was applied to the top of plate heat exchangers having different groove gaps, on which we had applied a hydrophilic coating to enhance the wettability of the liquid desiccant. In a visualization test, we found that a plate heat exchanger with a 1.0-mm groove gap resulted in the greatest wettability. As the mass flow rate of the supplied liquid desiccant increases, the wettability of the liquid desiccant is enhanced, and the regeneration rate and mass transfer coefficient can also be enhanced. We developed experimental correlations for the heat and mass transfer performances of the heat exchanger.
A stabilized composite phase change material for heat storage was synthesized by adding expanded graphite and disodium hydrogen phosphate to sodium acetate trihydrate with the method of vacuum adsorption. The effects of expanded graphite and disodium hydrogen phosphate were experimentally explored and the thermal properties of the composite material were characterized. The experimental results indicated that disodium hydrogen phosphate was an excellent nucleating agent for sodium acetate trihydrate. With addition of 1% disodium hydrogen phosphate, the supercooling degree of sodium acetate trihydrate decreased significantly from over 38°C to about 0.5°C. The addition of expanded graphite was also helpful to ameliorate supercooling of sodium acetate trihydrate. Furthermore, phase separation of sodium acetate trihydrate could be effectively eliminated by adding expanded graphite. Compared with the disodium hydrogen phosphate/sodium acetate trihydrate composite material (without addition of expanded graphite), the heat storage/release time of the expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate composite material was shorten by 75.3%. With the optimal ingredient proportion of 8% expanded graphite, 1% disodium hydrogen phosphate and 91% sodium acetate trihydrate, the composite material became a stabilized 'solid-solid' phase change energy storage material with excellent thermal performance. Its thermal conductivity was greatly improved, and the phase change latent heat reached 233.5kJ/kg. The supercooling and phase separation phenomenon were no longer observed.
Edited and published by : The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Produced and listed by : Showa Joho Process Co., Ltd.(Vol.8 No.3-) Sanbi Printing Co., Ltd.(Vol.1 No.1-Vol.8 No.2)