查看更多>>摘要：This paper proposes a novel ejector-enhanced auto-cascade refrigeration cycle (NEARC). In the novel cycle, the ejector not only replaces an expansion valve to recover partial expansion work, but also greatly reduces the throttling loss of the other expansion valve connected to the evaporator. The energy and exergy analysis methods are used to evaluate and compare the performance of NEARC using R290/R170 with conventional auto-cascade refrigeration cycle (CARC) and previously proposed ejector-enhanced auto-cascade refrigeration cycle (EARC). The simulation results show that under all given working conditions, the COP and exergy efficiency of NEARC are superior to those of CARC, but not always superior to those of EARC. With the change of initial mass fraction of R290, the COP and exergy efficiency of the three cycles all have maximum values. The maximum COP and exergy efficiency of NEARC are 42.85% and 42.71% higher than those of CARC, and 18.10% and 17.99% higher than those of EARC, respectively. When initial mass fraction of R290 is about 0.5, CARC and EARC have the best performance, and NEARC performs best when initial mass fraction of R290 is about 0.7. The comparison results demonstrate that the novel cycle has great energy-saving potential.
查看更多>>摘要：In summer air-conditioning, conventional vapor compression systems cool the process air to its dew point temperature for dehumidification and further heat it to required supply temperature. This involves extra cooling and subsequent heating. Liquid desiccant systems avert this issue by separately handling latent and sensible loads, while running primarily on low grade thermal energy. Falling film type liquid desiccant dehumidifiers are advantageous over other types, mainly, as they reduce desiccant carryover into supply air stream. The current work aims towards the analysis and the performance improvement of a falling film type dehumidifier with the help of wavy profile of the working surface. Detailed comparison of the two commonly used desiccants, i.e., LiCl and CaCl2 are carried out to analyse the performance of the dehumidifier. The present work also includes the characterisation of the wave profile and the analysis in terms of various important parameters, such as, the dehumidification effectiveness, the moisture removal rate and the change in specific humidity, missing in earlier studies. In order to study the performance, a simplified 2-D transient finite volume model is taken into consideration to simulate a multiphase, multi-component conjugate heat and mass transfer system using ANSYS Fluent 19.2. The volume of fluid method is used to trace out the liquid-gas interface. With back-end modification of the solver, the thermophysical properties of the liquid film are evaluated using empirical relationships, as a function of temperature and concentration. The penetration model of mass transfer provides the local mass transfer coefficients. The moisture removing the potential of the aqueous solutions of LiCl and CaCl2 are eval-uated at quasi steady-state conditions. The numerical solver is validated with the already published experimental results with a maximum deviation of 10.31%. A sinusoidal wavy profile of the wall introduced in the system enhances the turbulence intensity by 3.8% and the dehumidification performance by 33.18% and 18% for LiCl and CaCl2, respectively. It is due to the enhanced liquid-gas interface area. The mean film thickness is found to improve by 15.25% and 13.38% for LiCl and CaCl2, respectively. The effect of process parameters on the per-formance of the dehumidifier is also studied. It has been observed that the dehumidification effectiveness de-creases from 86.14% to 31.1% with increase in inlet air velocity from 0.2 m/s to 2.0 m/s. It has also been found that a lower concentration of the desiccant solution and lower velocity of air yield higher dehumidification effectiveness. However, the moisture removal rate of the system will be compromised. Compared to the previ-ously studied cases, the current configuration of the sinusoidal profile with amplitude to wavelength ratio of 1:15, yields superior dehumidification performance.
查看更多>>摘要：Water scarcity is among the major crucial issues confronting humanity, caused by the world's growth in population, the socio-economic development and the global climate change. Several desalination technologies are available to tackle the increased freshwater water demand. Therefore, the solar energy-driven distillation is among the most conceivable alternatives for supplying safe water with reducing the energy cost. The conventional solar distiller is a sustainable and economical process. However, the overall productivity is generally low. In this study, conceptual improvements on the design and distillation process of a wick type solar still are presented for the purpose of overcoming the low efficiency of the classic system. The novelty of this study is presented in combining three ideas to improve the still efficiency through creating an alternative mode of condensation based on forced convection, achieving a substantial gain in the specific weight and automating the distillation process. Hence, reducing the heat losses and enhancing the still productivity are achieved while maintaining the simplicity of the distiller and without calling for any complicated or cost-intensive technology. Furthermore, this investigation offers an innovative analytical process based on mathematical modelling tools as well as, an experimental approach. The results inferred that the advanced system produces significantly high distillate outputs of about 4.03 L.m(-2).d(-1) for an average of 380 W.m(-2) of mean solar radiation. Thereafter, the numerical results were experimentally validated, and it was proven that the efficiency of the advanced solar still was enhanced by 32% in comparison to the conventional solar still. The physic-chemical and bacterial analysis of the produced water revealed that the improved solar still can supply good quality of drinking.
查看更多>>摘要：As an effective method for heat management of hypersonic vehicles, regenerative cooling faces a severe problem of insufficient cooling capacity under high-speed conditions. Aiming to increase the cooling capacity of a given fuel, we conducted an optimization study by considering the influence of working conditions, chemical kinetics, and chemical routes. Via establishing a framework of multi-physical simulation by coupling the catalytic reactions with complex heat transfer process from subcritical to supercritical status, we conducted a parametric study of the effects of working conditions (i.e., inlet temperature and inlet velocity) to reveal the influence of physical heat sink, and different chemical kinetics and chemical routes to optimize the chemical heat sink. As a limiting case study, the surface coking process was also investigated. With the consideration of both physical and chemical heat sinks, the regenerative cooling capacity of a hydrocarbon fuel can be effectively increased via proper optimization. Using n-Decane as an example, a total heat sink of 2.5 MJ/kg is obtained under typical working conditions. A maximum heat sink of 5.3 MJ/kg could be obtained by engineering chemical routes with ethylene and hydrogen as the final cracking products, under inlet conditions of 473 K and 0.042 m/s. Results also reveal that it is essential to reduce the temperature of the wall to minimize carbon deposition. For practical applications, careful consideration of the synergies among the inlet conditions, reaction kinetics and routes, and coking should be performed to maximize the cooling capacity of a hydrocarbon fuel.
查看更多>>摘要：The performance of the ejector relies on the geometrical parameters and the fluid phase state. Although the optimization of geometries of the ejector has been conducted in previous studies, the influence of different optimization sequence of geometric parameters was ignored in previous literatures. To close the knowledge gap, the goal of this paper is to investigate whether different optimization sequences affect the ejector performance and whether the ejector performance is the same under different optimization sequences after multiple rounds of optimization. Therefore, three geometrical parameters, namely the constant-pressure mixing chamber length (L-pm), the constant-area mixing chamber length (L-am) and diameter (D-am), are selected for the optimization study; besides, multi-round optimization with six optimization sequences of these three parameters is conducted by CFD simulations under four different combinations of primary flow liquid volume fraction (LVF1) and sec-ondary flow liquid volume fraction (LVF2) (LVF1 = 0 plus LVF2 = 0, LVF1 = 0 plus LVF2 = 0.06, LVF1 = 0.06 plus LVF2 = 0, LVF1 = 0.06 plus LVF2 = 0.06) for the first time. The results showed that: (1) for each LVF combi-nation, the three optimal parameters and corresponding maximum ER produced by one round optimization of six different optimization sequences are evidently different; (2) after multiple rounds of optimization, for LVF1 = 0 plus LVF2 = 0 or LVF1 = 0.06 plus LVF2 = 0.06, ultimate optimal geometrical parameters and maximum ER are the same with each other for six optimization sequences, however, for LVF1 = 0 plus LVF2 = 0.06 or LVF1 = 0.06 plus LVF2 = 0, each of them still has two different ultimate maximum ER; (4) for those four combinations, different sequence takes different optimization rounds, recommended sequences are D-am -> L-am -> L-pm (S6), L-pm -> L-am -> D-am (S1) or L-pm -> D-am -> L-am (S2), and L-am -> L-pm -> D-am (S3) and D-am -> L-am -> L-pm, respectively; and (5) the ultimate optimal parameters and maximum ER in four combinations differ significantly because they are largely dependent on the inlet fluid states.
Mondal, Md Hasan TarekAkhtaruzzaman, MdIslsm, Md AzadulSarker, Md Sazzat Hossain...
15页
查看更多>>摘要：Investigation of exergy transfer is an important aspect for all grain drying industry. Exergy transfer of rice husk fired cyclonic furnace assisted recirculating mixed flow dryer (MFD) equipped with a heat exchanger, was sought in this study. A cyclonic furnace (CF) having capacity 0.5 MW and recirculating MFD were designed and developed to investigate exergy transfer during grain drying. Efficiency of furnace was determined to quantify performance of CF for industrial grain drying system. Exergetic efficiency was also thoroughfare performed for the developed CF, air mixing chamber (heat exchanger) and MFD. Thermal efficiency of the CF varied from 68.96 to 87.01% during drying of high moisture maize grain. Exergy efficiency of the CF, heat exchanger and drying chamber marked between 58.8 and 63.5%, 74.6-93.29%, and 3.03-21.9%, respectively. Exploration of exergy inflow persist between 53.32 and 60.51 kJ/s and 7.58 to 9.81 kJ/s while exergy outflow ranged between 7.58 and 9.81 kJ/s and 1.52 to 3.10 kJ/s, respectively for heat exchanger and drying chamber. Results based on exergy losses indicates that 44.01-51.54 kJ/s and 5.87-7.50 kJ/s of exergy is just loss through heat exchanger and drying chamber, respectively. Exergy improvement potential and sustainability index varied between 1.02 and 1.80 kJ/s and 1.17-1.40 kJ/s, respectively. Quality of final product is in acceptable range as color difference of dehydrated maize kernel varied from 9.19 +/- 0.46 to 17.05 +/- 0.13. Exergy results indicate that CF can aptly be applied for grain drying industry in commercial scale.
查看更多>>摘要：In this paper, the 2D model of the elliptical tube is established to simulate the film hydrodynamics and heat transfer performance. The volume of fluid (VOF) is used and the model is verified by the experimental data in the previous literature. The effects of the heating condition, fluid medium, inlet temperature (Tin) on the film thickness and heat transfer coefficients are explored. The studied 11 refrigerants include ethane, propane, R123, R1234yf, R1234ze(E), R125, R134a, R143a, R152a, R227ea, R245fa at Tin = -30, -10 and 10 degrees C. The results show at Re = 2000, the local heat transfer coefficients of the propane at Tin = 10 degrees C under fixed heat flux are 19% greater than those under fixed wall temperature. Both the local film thickness and local heat transfer coefficients increase with Reynolds number (Re). The overall external heat transfer coefficients increase with Re, but the growth rate slows down at Re >= 1500. Fluids have different results of film thickness and heat transfer coefficients even they have a similar Kapitza number (Ka). Different from the water and seawater, the heat transfer coefficients reduce as the inlet temperature increases for the refrigerants. A correlation is proposed to predict local film thickness on both circular and elliptical tubes. A correlation is acquired by the fitting method to predict the overall external heat transfer coefficients under the heating condition of fixed wall temperature, which can capture 89% of 132 data within the deviations of +/- 10%.
查看更多>>摘要：To improve the utilization rate of geothermal resource using organic Rankine cycle (ORC), an organic Rankine flash cycle (ORFC) combined with insufficient evaporation and flash process is proposed to reduce the irreversibility of the power generation cycle. The thermodynamic performance and techno-economic performance of ORFC and two-stage series organic Rankine cycle (TSORC) are optimized, respectively. The effects of evaporation temperature, flash temperature and outlet dryness on the performance of two systems are discussed. The results show that with the increase of heat source temperature, the optimal operating condition value of the system increases, which leads to improving the thermodynamic and techno-economic performance significantly. R601a and R601 are the most suitable working fluids for TSORC and ORFC, respectively. The heat exchanger is the main component that causes the exergy destruction of the system, and its improvement potential is the highest in all components. The thermodynamic performance and techno-economic performances of ORFC are better than those of TSORC. The irreversible loss of the system can be reduced by the insufficient evaporation combined with flash separator.
查看更多>>摘要：In this work, we studied the wall heat flux partitioning during the pool boiling of water on thin metallic surfaces. We conducted boiling experiments on surfaces where we engineered nucleation sites by nanosecond-fiber-laser texturing. These nucleation sites form triangular lattice patterns with different pitches. We measured the time-dependent temperature and heat flux distributions on the boiling surface using an infrared camera. We devel-oped post-processing algorithms to measure, based on these distributions, all the fundamental boiling parameters used in heat flux partitioning models (e.g., nucleation site density, bubble wait and growth time, and bubble footprint radius) and the actual partitioning of the heat flux, i.e., how much heat is transferred by evaporation of the microlayer, rewetting of the surface, and convective effects. This work reveals that the mechanisms of heat transfer on substrates of small thermal capacity are very different compared to substrates of large thermal capacity. With water, the bubble microlayer typically does not dry out and the surface temperature at rewetting is practically the same as the rewetting fluid temperature. These effects limit the efficiency of microlayer evaporation and rewetting heat transfer. Instead, convective effects generated by the bubble growth process remove most of the energy from the heated surface. This behavior is captured by a heat flux partitioning model that we re-derived from first principles to describe the heat transfer mechanisms on substrate of small thermal capacity.
查看更多>>摘要：The helical micro fin tubes (HFT) are commonly used in various double pipe heat exchangers because of the excellent processing and anti-fouling performance. It is of great significance to further improve the overall efficiency of the HFT so as to diminish energy consumption. In this work, the heat transfer and flow characteristics of the HFT are studied by numerical simulation. The results show that the heat transfer enhancement factors of the HFT are the secondary flow generated near the wall and the increase of the heat exchange area. In addition, the effects of the geometrical parameters on thermal-hydraulic performance are studied at Re = 36,636. It is found that the micro fin height (e), the helical angle (phi), and the number of starts (Ns) have a significant impact on the overall performance, and there is a strong mutual coupling between them. According to the parametric analysis, the HFT with a low micro fin height and a large number of starts is considered to be a better geometrical type. Finally, in order to select (or design) the HFT quickly under the specific working conditions, based on the exergy destruction minimization principle, the geometrical parameters are optimized by using the artificial neural network and genetic algorithm. An optimal solution (e = 0.23 mm, phi = 36.1 degrees, and Ns = 66) is selected from the Pareto front by the TOPSIS method. The results indicate that the optimal solution has a sensible balance between the exergy destruction caused by heat transfer and fluid flow. Besides, it has a better thermal-hydraulic performance as well (PEC = 1.73). This work fills the gap of heat transfer and the geometrical optimization study of HFT based on the second law of thermodynamics and provides strong evidence that the exergy destruction minimization principle is still applicable in the case of the periodic model and fully developed turbulence. We hope that it will be contributed to the structural design of the HFT.
NSTL
Elsevier