查看更多>>摘要：In ramjet propulsion systems, syelf-excited combustion instability is highly undesirable. It may lead to violent structural vibration and even mission failure. For this, self-excited combustion instabilities in such SFRJ are investigated numerically by 2D unsteady Reynolds-Average Navier-Stokes (URANS) model and varying the inlet mass flow rate mair. When combustion instability occurs, large-amplitude temperature, velocity, and reacting species wavy motions are observed along the axial flow direction. To shed light on the flow features and the energy distribution among the unstable eigenmodes, proper orthogonal (POD) and dynamic mode decomposition (DMD) analyses of nonlinear combustion instabilities in a solid-fuel Ramjet (SFRJ) combustor are conducted. The POD studies reveal that the fluctuation energy of the first six modes contributes to 99% of the total fluctuation energy of all modes. It means that the main features of the SFRJ combustors could be captured by using the first six modes. Unlike POD studies, DMD analysis captures the frequency information and spatial structures in the entire flow field. Examining the DMD growth rate reveals that all dominant DMD eigenmodes are marginally stable (growth rate = 0) or unstable (positive growth rate). And the frequency of each dominant mode could be enhanced by 3-10 Hz as themair is increased by 0.2 kg/s. The mode energy decreases with the increase of the mode order, with the decrease up to around 90%. The DMD analysis on velocity field indicated that the flow separation could be observed near the backward facing step. And the DMD mode structure of the temperature field can significantly affect the mode structures of Mach number and YCO2 field. In general, the present work provides detailed analyses of the SFRJ flow field in the presence of self-excited combustion instability by using POD and DMD approaches.
查看更多>>摘要：The use of two-phase nozzles for low-grade heat valorization by electricity production increases the energy recovery rate using Trilateral Flash or Wet to Dry cycles. A model benchmark for nozzle flow rate and efficiency estimation was conducted on experimental data from the literature. These data come from a series of tests made on geothermal energy production water two-phase nozzles. A new transitional bubble-to-droplet interfacial area density formulation (TA-BD model) is presented by the paper. It is compared to three models from literature. Two nozzles operating with different inlet and outlet conditions were modeled. The calibration flexibility and the robustness of the models are discussed in association with physical analysis. The paper shows how the models using single or redundant adaptation parameters fail to provide good results simultaneously on flow rate and efficiency. It appeared that the TA-BD model is the more flexible and robust. The Homogeneous Relaxation Model (HRM) model gives also good results. Furthermore, TA-BD model gives the lowest average discrepancies. Especially at the best efficiency point of the first test case, TA-BD model shows < 1% discrepancy where the HRM model has 18% discrepancy in efficiency. The TA-BD model appeared to be easier to calibrate than the HRM model. Finally, regarding the proposed TA-BD model, the sensitivity to the geometry and operating conditions shows that the interfacial area density formulation could be completed to include the effect of nozzle's section profile, the effect of the inlet temperature on bubbles number density, and the effect of outlet pressure on the droplets number density.
查看更多>>摘要：This study reports an experimental investigation of cooling power harvesting in LPG-fueled vehicles. The heat of vaporization for LPG in the vaporizer which is initially transferred from the engine coolant is modified by circulating low-temperature water by adding a chiller which placed on a three-passenger pick-up car. Low-temperature water is circulated from the chiller to the vaporizer and back to the chiller by a pump to transfer the cooling power. Meanwhile, the air from the cabin is flowed by an electric blower across the chiller and back to the cabin to transfer heat to the water loop in the chiller. The test was carried out on Daihatsu 1945 cm(3) at 1000, 2000, and 3000 rpm, where LPG consumption followed the engine load and obtained LPG mass flow rates of 0.022, 0.236, and 0.350 g.s(-1). With this vaporizer-chiller combination, the average cooling power of 41.51, 52.05, 110.29 W is obtained at 1000, 2000, and 3000 rpm, respectively. With an average cooling power of 124.5 W at 3000 rpm, it was proven to compensate for the thermal load in the cabin by 15.312, 11.67, and 80.31 kJ at 100, 2000, and 3000 rpm for 60 min of testing. This method can be applied as a secondary AC system to improve the performance of the main AC system in a vehicle. The actual cooling power from the LPG to the water loop can be increased by modifying the heat transfer contact area in the vaporizer cavity or by applying a high-efficiency vaporizer.
查看更多>>摘要：Earth-to-air heat exchanger (EAHE) system is a geothermal environmental technique able to reduce the energy consumption and their costs for heating/cooling, by using the undisturbed temperature of the ground for conditioning of air. This study aims to use a tubular heat exchanger to heat an agricultural greenhouse in the Baniyas area in a simple, more economical, and environmentally friendly way. In addition, to assess the length, diameter, and flow velocity parameters that affect the efficiency of the heat exchanger. Tests were carried out in a Mediterranean climate at Tartous Governorate in the coastal area of west Syria. The exchanger pipes of 20 m length were buried at 1 m deep. The diameters used are 10.16 and 15.24 cm for each studied exchanger with two velocities 1.5 and 2 m/s. Two exchangers were set up along a plastic greenhouse. Tests were performed through a month starting from 1 December 2020. The results showed a significant effect of the parameters on both exchangers heating performance. During the heating period, soil temperature was between 18 and 19 degrees C and inlet air temperature average was between 11 and 12 degrees C. An increase in the length of the tube to 20 m caused an increase in the heating performance to 56% and 36.28% for 10.16 cm and 15.24 cm diameter, respectively. Therefore, the smaller diameter tube was better, in addition, decreasing airflow velocity during the pipe from 2 to 1.5 m/s caused an increase in temperature from 5.5 to 6.99 degrees C.
查看更多>>摘要：The Humidification Dehumidification (HDH) desalination system is a promising method for production of potable water in isolated locations. Among different component constituting the HDH desalination system, humidifier and its packing are paramount factors in determining system productivity. Several experimental investigations were conducted to study the influence of packing on humidifier performance. However, numerical analysis dealing with performance of humidifier packing is still not available. Such an investigation will be useful to select the optimized packing for a humidifier. In the present work, performance analysis of 15 different types of random packing have been carried out using Scilab software. From this study, it is concluded that metal packing with highest surface area to volume ratio, lowest packing factor and smallest packing diameter functions as the best humidifier packing. Experimental study was also conducted in a pilot plant of open air open water HDH cycle using pall ring 25 mm diameter metal packing, which has shown the highest performance index in the numerical study. Experimental results obtained were found to be in agreement with the numerical results. Present work is a useful tool in designing new packing material of desired properties since the packing performance is a strong function of its geometrical properties.
查看更多>>摘要：In this paper we report the results of a numerical investigation focused on the evaluation of the most suitable conditions to make it work efficiently, with respect to the crucial parameters like the frequency of the cycle and fluid velocity vs variable lengths of the wires of a single bunch of elastocaloric elements to be employed in SUSSTAIN-EL. SUSSTAIN-EL will be the first Italian elastocaloric device for cooling and it is being developed by University of Naples Federico II, as a goal of the SUSSTAINEBLE project. The prototype is designed to be rotary and formed by bunches of elastocaloric wires crossed by air as heat transfer fluid. This paper introduces a two-dimensional model carrying the following originalities with respect to the other tools already published in literature: the 2-D design and possibility of reproducing step by step the velocity and the pressure field of fluid, to predict more accurately the solid-to-fluid heat exchange. The revealed trade-off between the maximization of temperature span and cooling power suggests the choice of 0.119 Hz and 7 m s(-1) as cycle frequency and air flow velocity. The effect of the length is remarkable from 100 mm to 200 mm but not so noticeable for a further increment of 100 mm.
Menendez Perez, AlbertoAltamirano, Carlos Fernandez-AballiBorrajo Perez, Ruben
11页
查看更多>>摘要：This paper studies the performance of vortex generators (VG) in compact louvered fins and tubes heat exchangers. CFD simulations and genetic algorithm optimization were used to create surrogate models for the Colburn and friction factor of the heat exchanger as a function of VG positioning, size, and angle of attack. The models were used to study how the design parameters affects heat transfer enhancement and pressure losses. The study was conducted with a Reynolds number of 208 based in the hydraulic diameter of the flow channel. It was found that the performance of the VG in the first row of tubes is coupled to the aspect ratio of the VG of the second row of tubes. Further that vortex generators with large angles of attack produce recirculation areas behind them diminishing the heat transfer. Also, that best performing vortex generators correspond to small aspect ratios. Yet, these produce a low impact on the heat transfer enhancement when their angles of attack are greater than 50 degrees. Vortex generators with large aspect ratios produce no significant effects on the heat exchanger performance. Larger vortex generators produce stronger and more intense vortices, greater drag, and frictional losses. This work shows how the different parameters of the VG affect heat transfer and pressure losses. The optimal model presents a PEC value of 1.055, in correspondence with a j/j(0) ratio of 10.43 and a f/f(0) ratio of 14,38.
Shahzad, Muhammad WakilBurhan, MuhammadChen, QianJamil, Muhammad Ahmad...
8页
查看更多>>摘要：The energy efficiency of seawater desalination processes is usually expressed in terms of kWh electricity or lowgrade heat per cubic meter of water produced. This energy efficiency evaluation criteria unfortunately omitted the embedded quality of derived energy input. To have fair comparison of assorted desalination processes, it is important to consider quantity as well as quality of derived energy input based on their generation mechanisms. The numerator (m3 of distillate produced) and denominator (kWh_derived energy consumption) terms in energy efficiency evaluation are to be benchmark onto a common platform for fair evaluation and comparison. An inadequate comparison may result in an inferior adaptation of desalination methods that can lead to high economical destruction. In this article, a detailed thermodynamic framework has been developed to convert cogeneration-based electricity and heat into standard primary energy input. The proposed standard primary energy platform will help to demystify the quality and quantity aspects of derived energy supply. The thermodynamic based rigorous calculations show that 1.813 units of primary energy are required to produce one unit of electricity due to conversion efficiencies and loses involved in the power plant. On the other hand, one unit lowpressure steam to operate thermally driven desalination cycles need only 0.0944 units of primary energy. This stark difference clearly shows that omitting the grade of energy in performance evaluation can lead to an inefficient installation decision. This proposed framework will provide a basic ground for future efficient processes selection and assorted processes evaluation at common platform.
查看更多>>摘要：Accurate estimation of specific heat capacity (C-p) is of essential importance for characterization of the heat transfer performance of nanofluids in many applications. In this study, the particle size, interface, and temperature effects on the C-p & nbsp;of Cu-water nanofluid and Cu nanoparticle are systematically studied. Also, the TIP4P rigid and the SPC/Fw flexible water models are compared to demonstrate their influence on the estimation of C-p. The investigation is performed by using the molecular dynamics simulation method, at the mean temperatures of 300 K, 350 K, and 400 K. The results show that the C-p & nbsp;increases with increasing nanoparticles size, but increases with decreasing temperature. The increase is attributed to the interface effect demonstrated by the vibrational density of state (VDOS). The VDOS mismatch is close to zero with the increase in the C-p & nbsp;of the Cu-water nanofluids. In contrast, there is a substantial divergence of the C-p & nbsp;of the nanoparticles from the theoretical values. The reason can be owing to the effect of particle size and the interaction with the surrounding water molecules. Compared to the TIP4P model, the SPC/Fw model shows an increase in the Cp. It is attributed to the intramolecular degrees of freedom that existed in the model, since they provide a small, but probably significant, contribution to intermolecular interactions. This work is helpful for understanding the enhancement mechanisms of specific heat capacity for nanofluids and the suspended nanoparticles used in, e.g., solar thermal applications.
查看更多>>摘要：Aiming at flow boiling in a micro circular tube with the inner diameter of 0.5 mm, experimental investigation on the flow and heat transfer characteristics of the thin liquid film has been carried out. Water was used as working fluid, and it was heated by a transparent ITO heater. The thickness of the thin liquid film was measured by a laser focus displacement meter, and the flow boiling visualization was performed simultaneously by using a highspeed camera. The result of synchronized flow visualization and flow boiling measurement showed that the temperature and pressure fluctuation was strictly consistent with the bubble cluster generation. The mechanism of flow boiling instability was analyzed. The void fraction based characteristics such as the bubble and liquid slug duration, and pair generation period were obtained from the dynamic distribution diagram of liquid film thickness. The experimental data of void fraction showed a good agreement with the homogeneous model. The temporary thin liquid film variation provides an opportunity to probing the physics of bubble dynamics and instability during flow boiling.
NSTL
Elsevier