查看更多>>摘要：Gas engine heat pump (GEHP) is gradually developing due to the high energy efficiency. However, the gas engine is easily disturbed by load fluctuation, which affects the stability of GEHP operation and causes higher nitrogen oxides (NOx) emission concentrations. Therefore, an efficient control method is necessary to ensure system stability, improve energy efficiency and reduce NOx emissions. In this paper, a coupling control strategy of superheat and NOx is proposed. The results show that the superheat can maintain stability with the small overshoot and adjustment time of less than 400 s when the engine speed changes. When the superheat setting value changed continuously, the superheat overshoot and error are less than 0.5 °C and the adjustment time is about 100 s. Meanwhile, the system outlet temperature is stable, the overshoot is less than 1 °C. The amplitude variation of the refrigerating capacity is similar to the amplitude variation of the superheat. The variation range of total heat capacity is greater than that of refrigerating capacity. In addition, the performance and emission of the GEHP are respectively studied when the superheat is 4 °C and 7 °C, at the optimum excess air coefficient. The NOx emissions are always below 100 ppm, but the primary energy ratio (PER) is higher under the superheat of 4 °C. Therefore, the proposed coupling control strategy can achieve good stability, excellent performance and emission characteristics.
查看更多>>摘要：A new correlation is presented which was verified with a very wide range of data for plain mini and macro channels. The data include 42 fluids (water, halocarbon refrigerants, hydrocarbons, carbon dioxide, ammonia, cryogens, chemicals) in single and multi-channels of diameter 0.38 mm to 41 mm, of various shapes (round, rectangular, triangular), partly or fully heated, in horizontal and vertical orientations. Reduced pressures varied from 0.0046 to 0.787 and mass flux from 15 to 2437 kgm?2s?1. The 10387 data points from 111 sources were predicted with mean absolute deviation (MAD) of 18.8%. The same data were also compared to 17 other correlations. The present correlation was found significantly more accurate than others. The results of this research are presented and discussed.
查看更多>>摘要：This paper proposes a modified auto-cascade refrigeration cycle (MARC) with a self-recuperator. The introduced self-recuperator and associated expansion valve effectively increases the refrigerant enriched with more low-boiling component in the evaporator. This case could improve the cycle performance by further choosing appropriate design-dependent cycle parameters. The energy and exergy analysis methods are used to compare and evaluate the performance of MARC with conventional auto-cascade refrigeration cycle (CARC). The simulation results show that under all given working conditions, the COP and exergy efficiency of MARC are superior to those of CARC. Among R290/R170 and R600a/R1150, R600a/R1150 is much better refrigerant mixture. Under a typical working condition, the COP of MARC using R600a/R1150 is 68.17% higher than that using R290/R170. The performance improvement of MARC is more obvious when R600a/R1150 is used. Under the typical working condition, the COP of MARC is increased by 6.24% and 24.17% using R290/R170 and R600a/R1150, respectively. When initial mass fraction of R1150 is about 0.6, the two cycles have maximum COP and exergy efficiency, and the maximum COP of MARC is 24.26% higher than that of CARC. In addition, the increase in the two-phase flow vapor quality at the condenser outlet can improve the COP, but reduce the refrigeration capacity. The COP of MARC increases by 37.56% when the quality increases from 0.4 to 0.6. It is also found for MARC that COP and refrigeration capacity are positively correlated with intermediate pressure.
查看更多>>摘要：Methane and ethane are the main components of liquefied natural gas (LNG) and mixed-refrigerant liquefaction cycles. The pool boiling characteristics and bubble dynamic parameters of methane/ethane mixtures are important for the design and optimization of LNG systems. In this study, visual experiments were carried out to investigate the pool boiling of methane, ethane and their mixtures on a horizontal flat surface. Heat transfer characteristics and bubble departure diameters were acquired at 0.3 MPa for different concentrations of the mixtures. The bubble departure diameter increases with the increase in superheat temperature and dimensionless Jacob number for all concentrations of the mixture. The non-linear variation with concentrations of the bubble departure diameter can be observed due to the effects of mixtures. The bubble departure diameters for mixtures were predicted by twelve existing correlations, with Cole's correlation having the lowest mean absolute relative deviation (MARD) of 46.28%, with only 38.78% points within ±30% deviation. Therefore, a new model for mixtures was developed based on force equilibrium analysis in this study. Marangoni force is taken into account in the proposed model, as well as the effects of mass transfer on the bubble growth rate in mixtures. The MARD of the new model is 18.78%, which is better than all the twelve existing correlations.
查看更多>>摘要：This paper is the last leg of the previous three papers of the same series with experimental study carried on a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes. Three different tube configurations were tested. (1) bare tube (2) with full length solid PVC nonconductive round rod insert (12.7 mm diameter) and (3) 33% cut insert. For the same flow, tubes with rods developed annular flow within the annular portion between the tube and the rod. Source of heat was hot ethylene glycol/water solution flowing on the shell side. Saturation temperature of ammonia was -20 °C ≤ Tsat ≤ -2 °C, heat flux range 5 kW m?2 ≤ q˙ ≤ 45 kW m?2 and mass flow rate of 0.008 and 0.015 kg s?1. Tests were conducted for each mass flow rate at four saturation temperature. Two-phase heat transfer coefficient increased with saturation temperature and heat flux for all cases with initial increase and then a drop at the onset of partial dry out. The tube with 33% cut rod exhibited better overall performance. Results were also compared with plate type evaporator data.
查看更多>>摘要：R1234ze(E) has emerged in the recent years as low Global Warming Potential substitute for R134a in refrigeration and air-conditioning systems. As a drawback, R1234ze(E) is classified as a mildly flammable fluid (A2L class) and, in the search for non-flammable alternatives to R134a, hydrofluorocarbon/hydrofluoroolefin binary mixtures can be considered. In the present work, condensation tests are performed with R1234ze(E) and non-flammable binary mixtures R450A (R1234ze(E)/R134a at 58.0/42.0% by mass) and R515B (R1234ze(E)/R227ea at 91.1/8.9% by mass) inside two channels with inner diameter equal to 3.38 mm and 0.96 mm. R515B is an azeotropic mixture whereas R450A is a near-azeotropic blend (temperature glide 0.6 K at 40 °C). Heat transfer coefficients are measured at 40 °C saturation temperature and mass flux from 40 kg m?2 s?1 to 600 kg m?2 s?1. Flow pattern visualizations are recorded by a high-speed camera in the 3.38 mm inner diameter tube. Two-phase pressure gradients are measured in the 0.96 mm test section at mass flux equal to 200 and 400 kg m?2 s?1. The prediction accuracy of condensation heat transfer and two-phase pressure drop models is assessed against the experimental results. A comparative study between the tested fluids and R134a, accounting for both the heat transfer coefficients and the two-phase pressure drops, is performed.
查看更多>>摘要：High temperature condensation is relevant for the heat pump system used for the district heating and the organic Rankine cycle system used for combined heat and power generation. By using a zeotropic mixture as working fluid in such systems, it is possible to achieve a higher system efficiency compared with those of pure fluids. Only two previous studies addressing zeotropic mixture high temperature condensation in plate heat exchangers are available in the open literature, and these studies considered ethanol/water mixtures. The current works present an experimental analysis of high temperature condensation of zeotropic mixtures in a plate heat exchanger. We analyzed the condensation of the hydrofluoroolefin mixture R1234ze(E)/R1233zd(E) and the hydrofluorocarbon mixture R134a/R245fa with different mass compositions at various condensation temperatures and mass fluxes. Based on the experimental results, we analyzed the temperature effects on the condensation heat transfer and identified a convective-dominated condensation heat transfer process. Furthermore, the results suggest that the mixture R1234ze(E)/R1233zd(E) has the up to 8.5% and 4.4% higher heat transfer coefficient and frictional pressure drop, respectively, than the R134a/R245fa mixture. A modified Silver-Bell-Ghaly model predicted the heat transfer data with a 7.1% mean absolute percentage deviation between the predicted and experimental results, and a new pressure drop correlation developed using the mixture test data predicted the pressure drop results with a 6.2% mean absolute percentage deviation.
查看更多>>摘要：The regenerative evaporative cooler is experimentally investigated in this study and the predictive machine learning model is developed based on the test data to predict the energy and exergy performances of the device. This statistical machine learning framework is used to study the effect of six input operating variables (air inlet temperature, inlet air flow rate, air inlet specific humidity, water flow rate, inlet water temperature and extraction ratio) on the output variables (supply air temperature, cooling capacity, dew point effectiveness, coefficient of performance and exergy efficiency). The linear and polynomial regression models are followed to see the exactness of the model and check the train score against the test score. The effect of all the input variables on the output variables are discussed as well. In the case study, the developed machine learning model is used to predict the performances of fabricated regenerative evaporative cooler based on the field weather data of different days of 2020 in Varanasi. Model train score is 0.9974; the test score is 0.9912, the mean squared error is 0.1427, and the root mean squared error is 0.3778. This model can be used to predict the cooler performance under varied operating conditions.
Hinmers S.Atkinson G.H.Critoph R.E.van der Pal M....
24页
查看更多>>摘要：This work has focussed on the development of an accurate method for testing and modelling the reaction kinetics involved in ammonia-salt adsorption reactions, something not achieved consistently to date. A Large Temperature Jump (LTJ) test cell has been developed for testing ammonia-salt reactions in real machine conditions. A Large Temperature Jump (LTJ) test cell has been developed for testing ammonia-salt reactions in real machine conditions. This was used to validate a new approach to modelling the behaviour and simulate the performance of chemisorption machines. A derivation of the heat transfer and thermodynamic equations are presented, and a finite difference model described which has been validated for the adsorption and desorption reactions of ammonia into halide salts within a porous matrix. The model is implemented in a MATLAB? program. Large Temperature Jump (LTJ) tests have been conducted on manganese chloride and barium chloride to validate the model and to identify the physical parameters which characterise the dynamic performance of the sorbent. The manganese chloride and barium chloride were impregnated in expanded natural graphite (ENG) (SGL SIGRATHERM?) board. The ENG board gave rise to practicable samples (31.5 mm OD ? over ?” tube) undergoing a desorption reaction in under 250 seconds with the fluid temperature 15°C above the equilibrium temperature, an order of magnitude faster than observed elsewhere. A new test method has been developed enabling an accurate single heat of reaction to be identified due to reduced hysteresis, which is reported for barium and manganese chloride. The model has been validated using experimental data from LTJ tests of two geometric configurations in radial heat transfer: discs heated/cooled from the outside radius (‘tube-side’) and annuli heated from the inner radius (‘shell-side’). The empirical data obtained is a milestone towards designed and optimised salt generators.
查看更多>>摘要：To reduce freezing-induced damage, a novel freezing method based on pressurized CO2 pre-injection combined with ultrasound-assistance was developed for quick freezing of pieces of honeydew melon. The samples were first exposed to CO2 at different pressures (0.2 MPa, 0.5 MPa and 0.8 MPa) and then placed in an ultrasound-assisted freezing device. The ultrasonic intensity was 0.15 W/cm2 at 20 kHz. The results indicate that pre-injection of CO2 enhanced ultrasonic cavitation and significantly (p < 0.05) increased the nucleation temperature, thus shortening the freezing time. CO2 combined with ultrasonic assisted freezing can reduce the variation in the state of water in the sample after freezing and thawing. The results of SEM, cryo-microscopy and inverted fluorescence microscopic observations show that the cellular structure of the sample was better protected and the tissue was less damaged by ice crystals. This suggests that CO2 pre-injection combined with ultrasound assist during freezing can effectively control ice nucleation, ice crystal growth and reduce freezing-induced damage.
NSTL
Elsevier