In the present work, liquid-liquid flow patterns positioned 40 mm downstream the inlet of microchannels were experimentally investigated, including the effect of hydraulic diameter ( D h ), liquid properties, aspect ratio of cross section ( a ) and inlet configuration. Deionized water, butanol, toluene and hexane were selected as probe fluids with water as the continuous phase. Cross-inlet microchannels of 200 μm * 200 μm ( D h = 200 μm), 400 μm * 400 μm ( D h = 400 μm), 600 μm * 600 μm ( D h = 600 μm) and 600 μm * 300 μm ( D h = 400 μm) as well as a T-inlet microchannel of 600 μm * 300 μm ( D h = 400 μm) were tested. For the tests in the microchannels of D h = 600 μm and 400 μm, the superficial velocities of the dispersed phase and continuous phase varied between 0.3 mm/s and 12 mm/s and between 0.2 mm/s and 50 mm/s, while in the microchannel of D h = 200 μm the superficial velocities of the dispersed phase and continuous phase ranged from 0.3 mm/s to 21 mm/s and from 0.2 mm/s to 150 mm/s. Annular flow, deformed interface flow, slug flow, intermittent flow, droplet and slug train flow and droplet flow were detected in the experiment. It shows that flow patterns depend on the hydraulic diameter, liquid properties, inlet configuration and aspect ratio significantly. Dimensionless analysis was employed to develop universal flow pattern maps regardless of the hydraulic diameter and liquid properties. It is indicated that an acceptable universal flow pattern map was derived based on the redefined dimensionless number Re i 0.2 * We i 0.4 , especially for the boundaries of the slug-droplet transitions, which are independent on the hydraulic diameter to some extent. The other dimensionless number We i * Oh i worked rather effectively to develop a universal flow pattern map independent on liquid properties. The boundaries of the flow pattern transitions in different liquid-liquid flow almost overlap with each other.

Heat transfer is a naturally occurring phenomenon which can be greatly enhanced by introducing longitudinal vortex generators (VGs). As the longitudinal vortices can potentially enhance heat transfer with small pressure loss penalty, VGs are widely used to enhance the heat transfer of flat-plate type heat exchangers. However, there are few researches which deal with its thermal optimization. Three dimensional numerical simulations are performed to study the effect of angle of attack and attach angle (angle between VG and wall) of vortex generator on the fluid flow and heat transfer characteristics of a flat-plate channel. The flow is assumed as steady state, incompressible and laminar within the range of studied Reynolds numbers ( Re = 380, 760, 1140). In the present work, the average and local Nusselt number and pressure drop are investigated for Rectangular vortex generator (RVG) with varying angle of attack and attach angle. The numerical results indicate that the heat transfer and pressure drop increases with increasing the angle of attack to a certain range and then decreases with increasing angle of attack. Moreover, the attach angle also plays an importance role; a 90° attach angle is not necessary for enhancing the heat transfer. Usually, heat transfer enhancement is achieved at the expense of pressure drop penalty. To find the optimal position of vortex generator to obtain maximum heat transfer and minimum pressure drop, the data obtained from numerical simulations are used to train a BRANN (Bayesian-regularized artificial neural network). This in turn is used to drive multi-objective genetic algorithm (MOGA) to find the optimal parameters of VGs in the form of Pareto front. The optimal values of these parameters are finally presented.

Most thermal engineers will model and analyze thermal engineering cases, using any of the numerous thermal analysis software, available in the market. These commercial software need years of continuous use to be fully mastered. Large companies can afford to acquire expensive software available in the market and train their engineers; but small companies do not have the financial means to acquire such expensive software. Thus for modeling and analysis, small companies or private practice need a different alternative. Excel is one of the programs that come with Microsoft Office suite of software, which is installed on any purchased computer. Most users of Microsoft office are proficient in using Word, and can use Excel as a spread sheet to speed up calculations. Technical personnel can easily use the charting capability of Excel, but very few engineers can use Excel for intensive Numerical Analysis. Engineers should be able to use the available inexpensive Excel software to perform numerical analysis at their work place. In this article three Heat Transfer Numerical cases using Microsoft Excel are discussed in detail. the first case is two dimensional steady state heat transfer with different isothermal boundary conditions. The second shows other boundary conditions: uniform heat flux, adiabatic, and convection. The third case is transient conditions. The results from the three cases are compared with results from Patran Thermal software.

In Hong Kong, the number of parallel traders has increased quickly within these years. Passengers bring not only handbags as planned for the subway system four decades ago. There are heavy luggage put in trolleys carried by parallel traders. Goods include milk powder, electronic devices, food and drinks. The increase in fire load will enhance the fire risk. Although the subway management limits the maximum allowable luggage to 23 kg, the combustible amount is still large in train cars. Heat release rate (HRR) is the most important parameter in fire hazard assessment. A 1:15 scale modeling test was carried out to find the HRR in a train car under limited ventilation with parallel goods in this paper. The goods were simulated by a propanol pool fire. HRR for six scenarios with all closed door to all opened door were studied. Mass loss rate of fuel, oxygen consumption, air velocity and fire duration were measured. Ventilation was provided by opening different numbers of doors of the train car. The train car fire can then be ventilation-controlled or fuel-controlled. The burning phenomena of fire in the train under limited ventilation including steady burning, oscillating fire and ghosting flame were studied. It was found that the fire size and duration depended mainly on ventilation when oxygen was very limited. When the amount of oxygen supply reached a critical value (the fire at the decay period with continuous supply of oxygen), the fire tended to be fuel-controlled. Burning characteristics of train fire under limited ventilation were also observed. They were different from fires burning in open area. Oscillating flame, ghosting flame and self-extinction were observed. Results are useful to assess fire hazards associated with parallel trading activities. Three repeated experiments on the captioned subject have been done with average results of all experiments presented.

The effects of droplet train impingement on spreading-splashing transition and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of HFE-7100 droplet train was generated using a piezo-electric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet impingement velocity. A thin layer of Indium Tin Oxide (ITO) was coated on a translucent sapphire substrate, which was used as heating element. High-speed and infrared imaging techniques were employed to characterize the hydrodynamics and heat transfer of droplet train impingement. Numerically, the high frequency droplet train impingement process was simulated using ANSYS-Fluent with the Volume of Fluid (VOF) method [1]. The heat transfer process was simulated by applying constant heat flux conditions on the droplet receiving surface. Droplet-induced spreading-splashing transition behavior was investigated by increasing the droplet Weber number while holding flow rate constant. High speed crown propagation images showed that at low-Weber number ( We < 400), droplet impingements resulted in smooth spreading of the droplet-induced crown. However, within the transitional droplet Weber number range ( We = 400–500), fingering and splashing (i.e. emergence of secondary droplets) could be observed at the crown’s rim. At high droplet Weber number ( We > 800), breakup of the crown was observed during the crown propagation process in which the liquid film behaved chaotically. Droplet-induced spreading-splashing transition phenomena were also investigated numerically. Reasonable agreement was reached between the experimental and numerical results in terms of crown morphology at different droplet Weber number values. The effects of spreading-splashing transition on surface heat transfer were also investigated at fixed flow rate conditions. Time-averaged Infrared (IR) temperature measurements indicate that heat flux-surface temperature curves are linear at low surface temperatures and before the onset of dry-out, which indicate that single phase forced convection is the primary heat transfer mechanism under those conditions. Numerical heat transfer simulations were performed within the single phase forced convection regime only. Instantaneous numerical results reveal that droplet-induced crown propagation effectively convect heat radially outward within the droplet impingement zone. Under high heat flux conditions, a sharp increase in surface temperature was observed experimentally when dry-out appeared on the heater surface. It was also found that strong splashing ( We > 800) is unfavorable for heat transfer at high surface temperature due to the onset of instabilities seen in the liquid film, which leads to dry-out conditions. In summary, the results indicate that droplet Weber number is a significant factor in the spreading-splashing transition and surface heat transfer.

In this paper a genetic algorithm-polynomial neural network approach is used in order to model the effect of important parameters on heat transfer as well as fluid flow characteristics for a double-pipe helical heat exchanger by using numerical-certified results. In this way, overall heat transfer coefficient ( U o ), inner and annular pressure drop ( ΔP in , ΔP an ) are modeled with respect to the variation of inner and annular dean number, inner and annular Prandtl number, and pitch of coil which are defined as input (design) variables. The numerical-certified data was randomly divided into test and train sections which the former is used for benchmark. The GA-PNN structure was instructed by 75 percent of the numerical-validated data. 25 percent of the primary data which had been considered for testing procedure were entered into GA-PNN proposed models and results were compared by statistical criteria.

In this paper, piston wind effect on smoke diffusion characteristic in subway tunnel is studied by using three-dimensional transient computational fluid dynamics (CFD) method. In the first simulation case, fire disaster is simulated with homogeneous resting initial field condition. In the second simulation case, the train’s decelerating process till stopping in the tunnel is simulated for getting three-dimensional tunnel air velocity field distribution. Then the final heterogeneous air velocity field when the train stops in the tunnel is taken as initial field condition and the same fire scenario as the first case is simulated again. The data obtained under both initial conditions are compared by detecting people evacuation safety and the influence of initial air velocity field is analyzed. The results show that the inertial air velocity field caused by train’s movement has significant influence on smoke diffusion at the first few minutes of fire disaster, which is the key time for people’s evacuation. The adopted method in this paper and the simulation result could be used in establishing more effective subway fire evacuation plan.

Thermal analysis of a cylindrical tissue subject to a train of ultrashort pulse irradiations was made by developing a combined time-dependent radiation and conduction bio-heat transfer model. Ultrashort pulsed radiation transport in the cylindrical tissue is simulated using the transient discrete ordinate method. Treatment of focused beam is introduced. The model skin tissue is stratified as three layers with different optical, thermal and physiological properties. Comparisons between the collimated irradiation and focused beam are conducted. The effect of pulse train is investigated.

In this paper, three dimensional air flow distributions in typical island platform both with and without PSD (platform screen door) system were simulated for the whole process of the train entering, staying in and departing from the platform. Based on the data obtained from the simulation with and without PSD system, the effects of PSD system on platform air environment were analyzed. At last, both the advantages and disadvantages of PSD system were concluded in view of improving passengers’ comfort and air quality.

Turbulent forced convection correlations are available in the literature for gases (Pr ∼ 0.7), but the test data leave a gap in the range of Prandtl (Pr) number between 0.1 and 0.7 occupied by binary gas mixtures. In this paper we develop a turbulent forced convection correlation for the Nusselt (Nu) number of in-tube binary gas mixtures for the ranges of Reynolds (Re) number between 10 4 and 10 6 and Prandtl (Pr) number between 0.1 and 0.7. A fully connected back-propagation Artificial Neural Network (ANN) is used to learn the pattern of Nu as a function of Re and Pr. Available test data in the range of 0.001 < Pr < 0.1 and 0.7 < Pr < 1000 are provided to the ANN. The test data are separated in two sets to train and test the neural network. A training set with 80% of the data is used to predict a testing set with the remaining 20% of the data. After the network is trained, we make use of the excellent nonlinear interpolation capabilities of ANNs, to predict values of Nu for the sought range 0.1 < Pr < 0.7. These predictions are later used to generate a correlation that aptly covers the complete range of Prandtl numbers.

The three dimensional air temperature distributions on subway platform under both natural ventilation mode and over-platform supply/ under-platform exhaust (OSUE) air conditioning system were simulated during a single train entering, staying and departing processes by using Computational Fluid Dynamics (CFD) method. On basis of the simulation, the comprehensive influences of both the train’s piston effect and the air conditioning mode on the air environment in different part of the platform were analyzed.

In this paper weld pool shapes of type 304 stainless steel obtained through computational fluid dynamics simulations (CFD) will be used to train artificial neural networks. The neural network is then used to determine the experimental parameters of the ND:YAG laser system on 304 steel by feeding it with the parametrized, experimentally obtained weld pool parameters. The output of the neural network are the process parameters like laser spot diameter and laser power on the surface of the weld pool. These parameters are very difficult to determine accurately through experiments but are necessary inputs for CFD simulations of the weld pool. Accurate CFD simulation is required for simulation and modeling of the solidified micro structure of the material.

Recently, a new laser micromachining technique using multiple ultrashort pulses with a picosecond-to-nanosecond separation as a train group has demonstrated some promising application potential, which also has high scientific merits in micro-/nano-scale ultrafast energy transport. However, the underlying physics of this technique remain poorly understood. In this study, the improved two-temperature model recently developed by the authors is used to analyze the pulse train technology, in which the full-run quantum treatments are used to calculate the significantly varying properties, including the electron heat capacity, electron relaxation time, electron conductivity, reflectivity, and absorption coefficient. This paper reports the effects of the pulse number per train, pulse separation, and repetition rate. Results from the present study have demonstrated that 1) it can increase the photon efficiency and further reduces the heat-affected zone; 2) the number of bursts required for melting decreases with the repetition rate, but at tens of MHz or higher, the repetition rate has a negligible impact; 3) the lattice temperature right after the thermolization time first increases to a peak and then decreases as the pulse separation increases.

This paper makes use of a new methodology for heat transfer increase through flow structures modifications. Intended to help railway designers in handling cooling issues, it is applied to improve the roof-mounted equipment design of a modern railway coach, namely the CORADIA TER 2N NG produced by the ALSTOM Transport company. The brake resistor, a key equipment in charge of dissipating the train kinetic energy as heat into the surrounding air during braking phases, has been particularly considered. To do so, a simple model including a heated obstacle inside a three-sided lead-driven cavity is used, and simple geometry variations are suggested. Their impact on heat transfer is then estimated through numerical simulations while experimental tests validate the results obtained.

In the HTGR (High Temperature Gas Cooled Reactor), the Reactor Cavity Cooling System (RCCS) is equipped to remove the heat transferred from the reactor vessel to the structure of the containment. The function of the RCCS is to dissipate the heat from the reactor cavity during normal operation including shutdown. The system also removes the decay heat during the loss of forced convection (LOFC) accident. A new concept of the water pool type RCCS was proposed at Seoul National University. The system mainly consists of two parts, water pool located between the containment and reactor vessel and five trains of air cooling system installed in the water pool. In normal operations, the heat loss from the reactor vessel is transferred into the water pool via cavity and it is removed by the forced convection of air flowing through the cooling pipes. During the LOFC accident, the after heat is passively removed by the water tank without the forced convection of air and the RCCS water pool is designed to provide sufficient passive cooling capacity of the after heat removal for three days. In the present study, experiments and numerical calculations using CFX5.7 for the water pool and cooling pipe were performed to investigate the heat transfer characteristics and evaluate the heat transfer coefficient model of the MARS-GCR (Multi-dimensional Analysis of Reactor Safety for Gas Cooled Reactor Analysis) which was developed for the safety analysis of the gas cooled reactor. From the results of the experiments and CFX calculations, heat transfer coefficients inside the cooling pipe were calculated and those were used for the assessment for the heat transfer coefficient model of the MARS-GCR.

Noise reduction concepts applicable to small underwater vehicles and in the nascent stages of research, are considered. These questions are asked. How to lower the propulsor rotational rate while maintaining forward speed? How to reduce blade tonal? How to supplant the propulsor drive train, which is the main source of hull vibration and radiated noise? The highlights of progress in passive and active approaches are reviewed. The approaches are inspired by unsteady high lift principles employed in flying and swimming animals and by emerging artificial muscle technology.