Journal of Thermal Science and Technology Vol. 12(2017) No. 2

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 ( A_z ), 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 A_z is augmented. It is also found that decreasing the aspect ratio value A_z 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, A_z =2 and γ=75°. In terms of irreversibility criterion at the oscillatory regime, total entropy generation ( S_tot ) 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 (R_ON), maximum drain current (I_DMAX), and transconductance (g_m) 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.