查看更多>>摘要：While the global refrigeration capacity installed in ultra-low temperature applications is considerably lower than in standard refrigeration, the interest in such systems has risen during the last months due to their emerging demand for the preservation of mRNA vaccines. Several theoretical studies can be found in the literature for ultra-low temperature applications. However, there is still a lack of reliable experimental data that may be used for the models validation and, thereby, to perform more rigorous investigations. In light of this, the present manuscript proposes an ultra-low temperature refrigeration unit based on an indirect cascade system, developed by retrofitting a standard low-temperature R290 packaged unit. In this investigation, firstly, the suitability of the original components is assessed and, then, the unit is experimentally tested within a wide range of operating conditions. Thereby, the system's reliability is evaluated by assessing different parameters, such as discharge, condensing and evaporating temperatures and performance ratios. The main results showed a successful behaviour at the operating conditions tested, exhibiting a COP that ranged from 0.6 to 1.6 for cold room temperatures between -80 °C and -65 °C, respectively.
查看更多>>摘要：R1234yf and R1243zf have received extensive attention as environmentally refrigerants. However, the application of the pure R1234yf and R1243zf are limited due to their flammability and low volumetric refrigerating capacity. The mixed refrigerants are expected to achieve excellent thermophysical performance, safety, and environmental protection. Vapor-liquid equilibrium (VLE) is an extremely important thermophysical property of mixtures. By using a recirculation-analytical type VLE apparatus, the VLE data of the (R125 + R1234yf) and (R125 + R1243zf) binary systems were measured along six isotherms from 263.15 K to 313.15 K. The PR + vdW model is used to correlate the VLE data. The overall average relative deviation of pressure (ARD(p)) and the average absolute deviation of vapor phase mole fraction (AAD(y)) are 0.15% and 0.0011 for R125 + R1234yf system; and the ARD(p) and AAD(y) are 0.16% and 0.0011 for R125 + R1243zf system. The model has a good description of the two binary systems.
查看更多>>摘要：Air conditioners (AC) are responsible for the largest portion of energy use among all auxiliary components of vehicles. Most white-box models developed for dynamic modeling of vapor compression systems (VCS) are not adequate to model vehicle ACs due to stronger dynamics and disturbances caused by frequent door open/close, random numbers of passengers entering/leaving the vehicles as well as intermittent shading effect on roads. In this study, a novel physics-guided deep learning method is proposed for dynamic modeling of vehicle ACs based on both domain knowledge and historical operational data. To maximize the practical values of the model in control and diagnosis of ACs, this research aims at developing an integrated VCS model consisting of individual models of major AC components rather than a black-box model of the entire system. Domain knowledge guides the determination of model inputs and outputs, design of the model structure, and understanding of temporal relationship in developing dynamic heat exchanger models. A newly developed NARX-LSTM-MLP neural network is proposed for heat exchange process modelling. The compressor is modelled by a multiple-layer perception (MLP). Component models are integrated by referring to the physical system structure. Field AC operation data from a city bus collected by IoT sensors are used in this study. Validation results indicate good accordance between measurements and simulation results. The model developed is less expensive, more convenient, and more feasible for health monitoring of numerous vehicle ACs at city level in the context of IoT.
查看更多>>摘要：Pulse tube is one of the main parts of the pulse tube cryocooler. Ideally, the pulse tube works under adiabatic environment without heat transfer loss. But in real case, entropy is generated in the pulse tube itself from heat exchanged with the walls, turbulent mixing of the hot and cold gas segments, circulation of the gas within the tube, and end effect losses from the adiabatic to isothermal transition in the cycle. All these processes reduce the enthalpy flow in the pulse tube from the ideal acoustic power. However, no 1D model can sufficiently simulate these effects in the past. Also, most of the 1D models about the pulse tube part with a heat transfer coefficient is far from accurate, especially for micro pulse tube in which the heat transfer is more obvious. This work revealed that the basic heat transfer changes instantaneously and shows different tendencies in different time of one cycle, and no suitable law or explanation could fare well the phenomenon eventually. The phase angle changes from negative to positive that results in the relatively heat flux along the pulse tube becomes smaller, that could help the pulse tube to make a good performance.
查看更多>>摘要：Modeling study for predictions of oil retention and critical refrigerant mass flux in compressor suction line was presented in the current work. Three approaches were developed for prediction of oil retention amount in suction line, namely, annular flow model, flow-pattern based method and empirical void fraction model. The annular flow model assumed that annular flow exists as the flow pattern, while the flow-pattern based method described the flow behavior of refrigerant/oil mixture by the flow pattern classifications and double circle model. Based on the proposed models, new correlations of interfacial friction factor and void fraction were established. The experimental data for R32 with coexistence oil PVE68 inside horizontal, vertical and inclined suction lines were used as the foundation and validation of the models. The results showed that all the models display satisfying prediction for oil retention. The flow-pattern based method exhibited the best results, which yielded the mean absolute deviation of 9.35% and captured 87.83% of the data within ±20% error bands. Moreover, two models for prediction of minimum refrigerant mass flux were recommended in order to ensure successful oil return. These models could serve as tools for operation optimization incorporating the lubricant oil effect.
查看更多>>摘要：To overcome the limitations of absorption refrigeration system brought by commercial working pairs of NH3/H2O and H2O/LiBr for the utilization of low-grade heat, a new class of working pairs consisting of dimethyl ether (DME) and ionic liquids (ILs) are developed. The solubilities of DME in three ILs [BMIM][BF4], [BMIM][PF6] and [BMIM][Tf2N] were firstly measured and correlated. Then, COPs of the single-effect absorption refrigeration system (ARS) and absorption-compression hybrid refrigeration system (ACRS) using DME/IL were calculated under different conditions. The maximum COP of ARS using the three DME/IL working pairs can be up to 32.0% larger than that using NH3/H2O, while the maximum COP of ACRS using the three DME/IL working pairs can be up to 38.3% larger than that using NH3/H2O under studied conditions. The three DME/IL working pairs also show much better cooling performance than HFC/IL and HFO/IL working pairs which are R152a/[HMIM][Tf2N] and R1234ze(E)/[HMIM][Tf2N]. In ACRS, DME/ILs can drop the optimal generation temperature in ARS by more than 10 K. DME/ILs are very promising alternative absorption refrigeration working pairs due to the high COP, wide applicability and environmental friendliness.
查看更多>>摘要：The effects of thawing frozen meats using physical field-based methods and traditional thawing methods on frozen meat were examined is a critical analysis of relevant literature. Based on published data it can be concluded that physical field thawing methods reduce thawing cooking losses and retain color and texture of the thawed meat. Compared with meat samples thawed at room temperature, the loss due to ultrasonic thawing of pork, beef and mutton was lower by about 43%, 45% and 43%, respectively. The corresponding cooking loss decreased by 8.1%, 7.5% and 10.10%, respectively, under the test conditions. The results of MMb content and TBARS indicates that physical field thawing reduces protein and lipid oxidation of meat. Compared with the room temperature thawing group, the MMB content of the ultrasonic thawing group decreased by 10.13%, 15.70% and 12.69%, while the TBARS value of the ultrasonic thawing group was also the lowest, which decreased by 14.58%, 15.20% and 15.87% compared with the room temperature thawing group. LF-NMR results show that the ultrasonic thawing has the highest content of bound water and immobilized water. These results indicate that the physical field thawing methods can better maintain the quality of meat samples. Among different thawing methods tested ultrasonic thawing showed the best effects.
查看更多>>摘要：Air source heat pump (ASHP) has advantages of high efficiency, eco-friendly operation, and convenient installation, but frost accumulation on the outdoor heat exchanger is one of the basic concerns about its operation. Reverse cycle defrosting (RCD) is widely used for defrosting, however, when RCD begins and ends, the four-way reversing valve quickly switches and the system pressure needs to be rebalanced which will cause the incomplete evaporation of refrigerant into the compressor. Thus, it leads to the decrease of the performance of the ASHP system during RCD. In order to improve the performance of RCD, this study proposes an ASHP defrosting system with coupled liquid-storage gas-liquid separator. An obvious advantage of the proposed system is retarding frosting, reducing defrosting time and increasing the heating capacity and coefficient of performance (COP) of the ASHP system thus reducing the number of defrosting cycles. And the proposed system can improve indoor thermal comfort during defrosting. Experiments show that compared with the original system, the proposed system delays frosting by 30 min and reduces the defrosting time by 74 s (22.7%), improving the heating capacity and COP by 6.78% and 12.8% respectively. Moreover, the proposed system with the coupled liquid-storage gas-liquid separator is economically friendly, which further benefits its practical applications.
查看更多>>摘要：The strawberry (Fragalia ananassa) is commonly used in industry and is thus often frozen; however, drip loss tends to occur following freezing and factors that may reduce drip loss remain unclear. In this study, the role that cell structure, particularly pectin and the cell membrane, has on the drip loss that occurs in different strawberry cultivars was investigated. The relationship between the cell structure in fresh or frozen strawberries and drip loss in thawed strawberries were investigated in ten Japanese strawberry cultivars, with results showing different amounts of drip loss in the various strawberry cultivars. The amount of HCl soluble pectin fraction (HSP) in frozen strawberries was found to correlate with drip loss when thawed at 4 °C. Higher amounts of HSP in the alcohol insoluble solid (AIS) of frozen strawberries showed a higher water holding capacity (WHC). It is therefore suggested that the WHC of the pectin chains in frozen strawberries, which may be adjusted by the neutral sugars in pectin, has the potential of regulating drip leakage during thawing.
查看更多>>摘要：The impact of refrigerants with high global warming potential (GWP) on climate change necessitates the use of alternative refrigerants in cold storage systems. Designed cold storage units using low-GWP alternative refrigerants to R404A, i.e., R407A and R407F, are experimentally studied respectively in this study under year-round working conditions (ambient temperature of 32, 25, 15 and 5 °C). The year-round performance of the units in terms of energy saving and safety under three super-heating degrees (SHs) are also investigated. In each working condition, the performance of cold storage unit using R404A, R407A and R407F is compared in terms of the cooling capacity, energy efficiency ratio, sub-cooling degree, condensing temperature, evaporating temperature and discharge temperature (Td). The results show that the annual energy effect ratio of the unit with R407A and R407F is higher than that of R404A, however the Td of R407F is at an unfavorable level for safety at high ambient temperatures. The annual power consumption, life-cycle cost, total equivalent warming impact and life cycle climate performance of R407A are lower than those of R404A by 2.5%, 2.2%, 7.9% and 7.9%, respectively, and those of R407F are lower than those of R404A by 4.6%, 4.2%, 11% and 14.5%, respectively, indicating R407F is a better low-GWP alternative refrigerant for cold storage systems.
NSTL
Elsevier