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%.