A numerical investigation is performed to illustrate the mutual interaction between coolant jet issuing from shaped film cooling hole and cascade primary flow as well as the resulting film cooling performance under rotational condition. Four various film-hole geometries are utilized for comparison, including the conventional cylindrical hole, fan-shaped hole, converging slot-hole and diffused slot-hole. Results show that a strong radial flow is induced toward blade tip on the pressure side due to the rotational effect, thus affecting the interaction mechanism between the coolant jet and primary flow. In general, rotational effects on film cooling are behaved as two aspects. On one hand, it makes the coolant jet deflect toward blade tip, resulting in lateral film coverage improvement in the region adjacent to the film holes for the cylindrical hole and fan-shaped hole relative to the stationary condition. On the other hand, it weakens the flow momentum of coolant jet along the streamwise direction, causing degradation of local film cooling effectiveness far from the hole-exit except for the zone near blade tip. The shaped-hole performs favorable film cooling enhancement, especially under higher blowing ratio. Relative to the stationary case, film cooling improvement by the film-hole exit shaping is degraded a little under the rotational condition. Among the presented shaped-holes, the converging slot-hole achieves the highest film cooling effectiveness and the diffused slot-hole is the next under the same blowing ratio.
Three-dimensional thermosolutal natural convection and entropy generation within an inclined enclosure is investigated in the current study. A numerical method based on the finite volume method and a full multigrid technique is implemented to solve the governing equations. Effects of various parameters, namely, the aspect ratio ( Az ), buoyancy ratio (N) and inclination angle (γ) on the flow patterns, heat and mass transfer rates as well as entropy generation are predicted and discussed. A comparison of 2D and 3D models at normal situation γ=0° is conducted when N varied in the transition range -2≤N≤-0.6 demonstrating that the 2D assumption can be adopted for the 3D flows when -0.5≤N≤0. The numerical outcome of the present study shows that, the thermal and solutal isosurfaces exhibit a central stratification that significantly strengthens as Az is augmented. It is also found that decreasing the aspect ratio value Az leads to weakening the total entropy generation and reducing the 3D effects exhibited within the cavity. Especial attention is attributed to analyze the periodic flow pattern that appears for Ra=104, Az =2 and γ=75°. In terms of irreversibility criterion at the oscillatory regime, total entropy generation ( Stot ) and Bejan number (Be) are seen to oscillate with the same frequency but in opposing phases and with different amplitudes.
Ammonia absorption refrigeration has attracted attention due to its low refrigerating temperature and the absence of crystallization as well as good performance under vacuum conditions. However, its efficiency is still lower than the mechanical compression refrigeration system at present. The quality of heat and mass transfer in absorption process is vital for improving the performance of ammonia absorption refrigeration. The objectives of this work were to experimentally investigate the enhancing influence of nanoparticles on an ammonia/water falling film absorption process under pressure reducing conditions and to further explore the mechanism of absorption enhancement by the nanofluids. Our experimental results showed that the different kinds of nanoparticles used had different enhancing influences on the ammonia falling film absorption process, and the nanoparticles had different optimal enhancement concentrations. These concentrations were 0.2%, 0.1% and 0.1% in mass concentration for the Al2O3, ZnO and ZrO2 nanofluids, respectively. The effective absorption ratios increased with increasing initial ammonia concentration, indicating that the enhancing influence of the nanoparticle addition on the absorption was more obvious at a lower ammonia absorption potential. The absorption operating pressure is an important influencing factor. The enhancing effect of the nanoparticles was not represented without a sufficient absorption pressure (driving force). The enhanced absorption could be better explained by a combination of the two-film theory, the penetration theory and the surface update theory. The transportation effect and the vortex effect caused by the nanoparticles, as well as the Marangoni convection effect induced at the phase surface, could destroy the two-film stagnation layer assumed in the two-film theory. These effects could also enhance solute permeability and expedite the update of the surface, thus enhancing the absorption performance.
The influences of the endwall corner jet (ECJ) with different locations, yaw angles and jet-to-inflow total pressure ratios on the aerodynamic performance of a high-speed compressor cascade are parametrically investigated by numerical simulation. The results show that the ECJ could weaken the boundary layer separation, reduce the loss and increase the pressure rise effectively by inputting momentum to the low energy corner region. The optimal ECJ location for the loss reduction is slightly downstream of the separation line. With the increase of the yaw angle, more loss reduction is obtained in the near endwall region, whereas the loss near the midspan is enhanced due to the enlarged separation along the blade height. A higher jet-to-inflow total pressure ratio could enhance the pressure rise of the cascade, whereas the mixing losses between the jet and the low energy fluid are also strengthened. The benefit of loss reduction degrades when the jet-to-inflow total pressure ratio is higher than 1.1. Moreover, the ECJ could obtain considerable loss reduction for the incidence ranging from -4° to +4°. A maximum loss reduction up to 15.0% is obtained at the incidence of +2° by the ECJ located at 60% chord with a yaw angle of 30°, whereas the jet-to-inflow mass flow ratio is only 0.57%.
The interactions between Transient Rayleigh-Bénard convection and volumetric radiation are investigated by means of the lattice Boltzmann method (LBM) performed for a two dimensional participating Rayleigh-Bénard cell. Given that, the analysis of the transient convection-radiation finds applications in combustion chambers, rocket propulsion systems, the design of reactors, heat pipes, etc. in this paper, we extended the mesoscopic Lattice Boltzmann model for analyzing the coupled engineering problem Rayleigh-Bénard Convection with thermal radiation. In order to highlight and assess the aim and the computational advantage of computing the radiative information too using the LBM and to demonstrate the workability of the LBM to a such coupled problem in two dimensional media, first, transient Rayleigh-Bénard convection is solved using the lattice Boltzmann method (LBM) and then are compared with those available in the literature. The coupled transient case, Rayleigh-Bénard convection-radiation in participating media is extended, where LBM, is used, both to calculate the volumetric radiative information needed for the energy equation, which is solved using the LBM. Results of this recent approach LBM-LBM work are compared with those available in the literature. In all cases, good agreement has been obtained. Indeed, the recent numerical approach is found to be efficient, accurate, and numerically stable for the simulation of fluid flows with heat and mass transfer in presence of volumetric radiation in participating medium. The steady state stream-functions, isotherms and pressure distribution were compared with results available in the literature. It is found that the recent approach provides accurate results and it is computationally more efficient than others CFD numerical methods which approve the workability of this recent approach and this make it a new potential computational tool for solving a large class of engineering problems.
A novel Hamiltonian-based method is introduced to the two-dimensional (2-D) transient heat conduction in a rectangular domain with partial temperature and partial heat flux density on one boundary. This boundary condition is very difficult to deal with in the classical Lagrangian solving system. Because of this, a total unknown vector consisting of both temperature and heat flux density is regarded as the primary unknown so that the problem is converted to the Hamiltonian form. By using the Laplace transform and method of separation of variables, the total unknown vector is solved and expressed in terms of symplectic eigensolutions in the complex frequency domain (s-domain). The undetermined coefficients of the symplectic series are obtained according to a generalized adjoint symplectic orthogonality. In this manner, analytical expressions for the rectangular domain with specific mixed boundary conditions are achieved in the s-domain. Highly accurate numerical results in the time domain (t-domain) are then obtained by using inverse Laplace transform. Numerical examples are given to demonstrate the efficiency and accuracy of the proposed method.
This study investigated the heat generation behavior of normally-on GaN FET consisting of multi-chip AlGaN/GaN high electron mobility transistors (HEMTs) cascoded with a low-voltage MOSFET (LVMOS) and a SiC Schottky barrier diode (SBD) in a new design package to enable high power applications. The electric field intensity distribution and the hot spot position of the devices were analyzed by electrothermal simulation and the infrared temperature measurement. The transient thermal characteristics are probed by temperature sensitive parameters (TSPs). The changes in on-resistance (RON), maximum drain current (IDMAX), and transconductance (gm) with temperature from 25 °C to 150 °C are measured, and the correlations are investigated. Two paralleled GaN-HEMT, LVMOS, and SiC SBD were then integrated on a directly bonded copper (DBC) substrate in the four-pin metal case TO-257 and a newly designed REC-2015 package to evaluate steady thermal performance improvement of packaging. The temperature distribution of parallel-connected GaN HEMTs were analyzed in numerical thermal simulations and infrared thermography measurements. The analytical results of thermal analysis were confirmed by comparing with the infrared thermographic measurements and numerical results obtained from simulations using Ansys Icepak. According to the thermal measurement at power dissipation of less than 24 W, the peak temperatures of the GaN HEMTs are 144.7 °C and 132.6 °C with TO-257 and REC-2015 package.
Irreversible electroporation (IRE) has been studied as a less invasive method for tumor treatment. Since the mechanism of the treatment is based on the fatal perforation of the cell membrane caused by an external electric field, a tumor can be ablated non-thermally if an appropriate electric field is selected. However, an electric field more than a few kV/cm is required to accomplish ablation. In this study, we aim to examine the feasibility of a comb-shaped miniature electrode for reducing the required voltage for IRE. The reduction of the applied voltage while maintaining the potential difference was realized by narrowing the gap between the electrodes. A 150-μm-wide miniature electrode with a 100-μm gap between its teeth was fabricated using photolithography. In the experiment, the electrode was contacted onto a tissue phantom consisting of fibroblasts cultured in agarose gel three-dimensionally. After the application of electric pulses, cell ablation depth was examined using fluorescent staining. The miniature electrode successfully ablated the cells at the surface of the tissue phantom by the application of 90 electric pulses at 100 V. The maximum and average ablation depth were 72.7 μm and 61.0 ± 11 μm, respectively, which was approximately 40 % of that estimated by the numerical analysis. Our study showed that the contact-IRE using a miniature electrode in the order of sub-millimeter does ablate the superficial cells of targeted tissues upon the application of electric pulses of less than 100 V; however, further studies are required to maximize the ablation depth under the constraint of limited applied voltage.
A novel technology to reduce soot is developed utilizing a high-frequency standing wave of 20 kHz applied to a methane-air lifted jet flame base. The amount of soot was measured using the transmitted light attenuation method. A mixing profile of the fuel jet was visualized using acetone planar-laser-induced fluorescence in order to measure the mixing status when the ultrasonic wave was applied. The blow-off and reattached limit were increased by the standing wave. Soot was also clearly decreased under some conditions. This was detected both through a disappearance of the luminous flame and a qualitative measurement using the transmitted light attenuation method. The luminous intensity of the acetone fluorescence at the middle of the fuel jet decreased with height because the fuel jet diffused more efficiently with ultrasonic waves.
A comparative investigation of leakage flow and film-cooling characteristics on different squealer tips is conducted under the conditions of stationary and moving casing. Firstly, the numerical method used in the calculations has been verified. Then, the influences of film-hole arrangement, squealer height and blowing ratio are studied in detail. Three different film-hole configurations are taken into consideration. For type-A and type-B, the film holes are arranged in one row, but located on the middle-camber line and near the suction side squealer, respectively. Type-C places two rows of 7 film holes at the leading edge and one row of 6 film holes at the middle chord region. The results show that the relative motion of the casing leads to the formation of a scraping vortex near the casing, which not only compresses the cavity vortex, but also acts as a pair of labyrinths with the cavity vortex to the leakage flow. The effect of the moving casing on the film-cooling effectiveness of squealer tip varies with the film-hole arrangements. When the casing is moving, the tip leakage loss increases with the decrease of the rim height because the blockage effect of the vortexes in the cavity decreases. Nevertheless, type-C obtains the best tip cooling performance in both cases of stationary and moving casing.
The influences of thermal radiation and non -uniform heat source/sink in Falkner-Skan flow and heat transfer characteristics of nanofluids have been investigated. Three different types of nanoparticles, namely Copper, Cu, Alumina Al2O3, and Titania TiO2 are considered by using water-based fluid with Prandtl number Pr = 6.2. The partial differential equations are transformed into the system of ordinary differential equations by applying similarity transformations which is then solved numerically. The result revealed that utilizing Cu-water nanofluid enhanced the heat transfer rate in comparison with Al2O3 -water and TiO2 -water nanofluids. The heat transfer rate is significantly boosted with an increase in the radiation parameter. Further, the opposite trend is observed with an increase in source/sink parameters and Eckert number.
This paper reports a microelectromechanical systems (MEMS) mirror with electrothermal polymer actuators for the diffusion sensor. A compact and high-speed diffusion sensor is desirable for point-of-care testing because of its real-time monitorability and portability, and because diffusion coefficient reflects the abnormality of biological samples such as proteins. Herein, a fringe-tunable electrothermal Fresnel mirror (FEFM) is analyzed to maximize the mirror's angular shift while maintaining the repeatability of the actuator drive. The thermal-response-speed and temperature-distribution characteristics were examined. The proposed fabrication process contributed toward improving the yield and quality of the device. The diffusion coefficient was successfully measured using the fabricated FEFM. Moreover, by making the fringe spacing 7.1 times narrower than its initial value, the decay time of diffracted light became 50 times faster than that of the wider fringe, thereby showing reasonable agreement with theory. The results validated the development of a compact, high-speed diffusion sensor that realizes control of the decay time of the mass diffusion in a transient grating using an FEFM.
This research conducted microgravity experiments to investigate the flame-spread characteristics of the fuel-droplet-cloud element with uneven droplet spacing, which is a basic element of a randomly distributed droplet cloud at the critical condition for group-combustion occurrence. Flame spread to a droplet followed by burning with two-droplet interaction was observed in microgravity to investigate the effect of flame-spread direction and local interactive effect. The results show that the flame-spread rate to a droplet in a perpendicular direction to the axis of two interacting droplets was greater than that to the droplet in the same direction as the axis of two interacting droplets. The temperature distribution around burning droplets was measured by the Thin Filament Pyrometry (TFP) method based on radiation from 14-micron SiC fibers suspending droplets at their intersections. The flame-spread-limit distance increased with two-droplet interaction in both flame-spread directions. This also shows the dependence of the flame-spread direction. The flame spreading after two-droplet interaction in different directions is discussed considering the temperature distribution development. An approximation of the flame-spread-limit distance is also presented.
This study focused on the dependency of the flame location of a premixed propane-air flame on turbulence intensity and length scale. The flame location was investigated using a diffuser-type combustor to show the response of the flame location to varying turbulence intensities and length scales without changing the mixture velocity, i.e., the thermal power. Combustion simulations were conducted using a coherent flame model within the framework of Reynolds-averaged Navier-Stokes equations under unsteady state conditions. The flame generally moved toward the combustor inlet with increases in turbulence intensity and length scale. The combustion and inlet turbulence caused a flow separation mainly downstream of the flame front. Consequently, the secondary flow structures influenced the flame topology and location.
As fiberoptics industries have been attempting to use a silica preform of larger diameter in order to reduce facility down time and increase optical fiber manufacturing productivity, this computational study investigates how the use of large sized preform in sized up draw furnace affects high speed glass fiber drawing process. Present computational model for glass fiber drawing simulations employs iterative scheme between one-dimensional momentum balance model for neck-down profile of heated and softened preform and two-dimensional axisymmetric thermo-fluid computations of convective and radiative heat transport for preform heating and purge gas flow inside muffle tube. Prediction of preform neck-down shape for 10 cm diameter preform has been verified with measured profile from actual optical fiber drawing test. The effects of larger preform diameter up to 20 cm are appreciated and discussed on temperature distribution of preform and drawn glass fiber and fiber draw tension. This study also shows that fine control of heating condition is necessary to maintain proper amount of draw tension in high precision glass fiber drawing.
Operational flexibility, such as fast start-up time, ability to adapt to the load change as well as high efficiency has made the gas turbines as one of the most important energy devices among the thermal power systems. It is well-known that gas turbines efficiency decreases with an increase in the ambient atmospheric temperature. For that purpose, recently fogging of water droplets have been utilized to increase the power output of these industrial gas turbines. Based on fogging principle, Advanced Humid Air Turbines (AHAT) are developed which utilizes the humid air to increase the thermal efficiency of the gas turbine systems. In this paper, the characteristics of the humid air system are investigated experimentally. Extensive high-speed images were taken using a high-speed camera. Analytical models are proposed based on the mass and energy conservation principle to understand the effects of the thickness of the trailing edge of the droplets size distribution after the trailing edge of the cascade blades. From the proposed model and experimental data, it is found that the primary droplets formed are inversely proportional to the Weber number (based on the blade thickness). It is also concluded that if the Weber number of the two profiles were kept constant, then the one with greater trailing edge thickness would result in larger primary droplet size and vice versa.
Transient three-dimensional numerical computations are implemented to clarify the natural convective heat transfer characteristics of the Rayleigh-Benard convection of Al2O3-water nanofluids induced in a shallow vertical cylindrical enclosure. The thermophysical properties of nanofluids, assumed to be a single-phase fluid in the numerical computations, are estimated using the experimental correlation equations reported by Khanafer and Vafai (2011). When the average Nusselt numbers of nanofluids are plotted against the Rayleigh number defined by the thermophysical properties of water, computations for four different volume fractions of nanoparticles were below the average Nusselt number curve experimentally reported by Silveston (1958). The diagram also reveals that the increase of nanoparticles in the base fluid delays the generation of Rayleigh-Benard convection. However, the average Nusselt numbers of nanofluids almost agreed with the average Nusselt numbers of Silveston without depending on the volume fraction of nanoparticles when plotted against the Rayleigh number defined by the thermophysical properties of nanofluids. Thus, the natural convection heat transfer rates of Al2O3-water nanofluids can be considered to be similar to the general fluids reported by Silveston as long as experimental thermophysical properties are employed.
In this paper, both first and second laws of thermodynamics are employed to examine the combined effects of nonlinear thermal radiation, buoyancy forces, thermophoresis and Brownian motion on entropy generation rate in hydromagnetic couple stress nanofluid flow through a vertical channel with permeable walls. The model equations of momentum, energy balance and nanoparticle concentration are obtained and tackled numerically using a shooting technique coupled with Runge-Kutta-Fehlberg integration scheme. The numerical results for velocity, temperature and nanoparticles concentration profiles are utilised to determine the skin friction, Nusselt number, Sherwood number, entropy generation rate and Bejan number. It is found that the entropy production in the flow system can be effectively minimized by regulating the values of the thermophysical parameters for efficient operation. Some other interesting results are displayed graphically and discussed quantitatively.
Estimation of temperature distribution in tissues and organs is critically important for treatments such as hyperthermia, radiofrequency ablation and cryosurgery which expose malignant tissue to extreme temperatures that are different from the physiological temperature. Commonly, the bioheat equation, instead of heat conduction equation, is used for estimation to incorporate the effect of blood perfusion, because the heat transfer in tissues is significantly affected by blood perfusion in addition to thermophysical properties of tissues. Nevertheless, in many cases, the rate of blood perfusion is not available for human tissues and organs. This study therefore aims to examine if we can use the normal heat conduction equation with apparent thermophysical properties to take the effect of blood perfusion into account. Feasibility was checked by comparing the results obtained from the heat conduction equation and the bioheat equation. The result indicated that the simulation with the apparent thermal conductivity or specific heat capacity does not agree well with the temperature distribution inside a tissue with blood perfusion. However, the apparent thermal conductivity was useful to estimate the size of growing ice ball produced during cryosurgery.
This paper shows the performance evaluation of the variable refrigerant flow (VRF) air-conditioning system subjected to very low outdoor air temperature at which the heating-defrosting cyclic operation occurred. The objectives of the study are to know the effect of the heating-defrosting cyclic operation in the VRF air-conditioning performances and of the indoor environment thermal conditions. The one outdoor unit's and two indoor units' VRF air-conditioning 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 constant dry bulb temperature with different wet bulb temperature and for both dry bulb temperature and wet bulb temperature different. The results of the study show that the heating-defrosting cyclic operation affected the performance of the VRF air-conditioning system. It shows that the heating time changes depend on both dry bulb and wet bulb temperatures. It shows that the defrosting time is almost the same for different dry bulb and wet bulb temperatures for different heating times. The cyclic heating-defrosting operation affected the indoor air temperature by lowering the indoor air temperature during defrosting. Based on the results, necessary measures are needed to address the effect of the defrosting in the VRF air-conditioning system performance and of the indoor thermal environment conditions. The heating-defrosting cyclic operation is expected to occur when the VRF air-conditioning system is installed in an actual building in a temperate climate, and in the performance evaluation of the different VRF air-conditioning systems under these conditions, it is important to know how it affects the system's operation behavior, energy consumption and support of the indoor environment thermal conditions.
This study examined the melting behavior and heat transfer characteristics of a water-insoluble material immersed in water. n-Hexadecane, popularly used as a phase change material, was selected as the water-insoluble material. A rectangular n-hexadecane solid was immersed in water. The n-hexadecane block was then vertically fixed onto the copper plate of the cooling wall. The flow structure of the free convection of water was visualized by mixing tracer particles with the water, and a laser sheet entered from the opposite side of the cooling wall as a light source. Its melting behavior and melting rate were then observed under various water temperatures in a test vessel. The local heat transfer coefficients of n-hexadecane were calculated from its melting rate and latent heat. In addition, the thickness of the melting liquid of n-hexadecane was calculated using a simple analysis model. As the experimental results, it flows upward along the n-hexadecane block, accumulates at the top of the solid as a droplet, and then flows upward vertically because the melting liquid of n-hexadecane does not diffuse in water. The calculated thickness of the melting liquid of n-hexadecane increases drastically near the lower end and increases almost monotonically and at a mild rate in other parts of the block. The analytical and experimental local heat transfer coefficients showed good agreement at low water temperatures. The calculated local heat transfer coefficients changed slightly, except for those at the top and low parts of the block. The local heat transfer coefficients increased in most locations as time elapsed in the experiment.