查看更多>>摘要：The temperature rapid rise phenomenon often happens in the side-by-side frost-free refrigerators during the damper-off cycle, which leads to a decrease in damper-off duration and an increase in energy consumption. A three-dimensional transient model of natural convection during the damper-off cycle is established and experimentally validated to analyze the refrigerating compartment (RC) temperature rapid rise phenomenon. The results show that the violent natural convection airflow driven by the non-uniform temperature distribution will lead to the significant variation of RC temperature. The hot air rises from bottom into top layer along the three passages, which include the front gap (FG) between shelves and door boxes, the side gap (SG) between the rightside wall and door boxes and the rear gap (RG) between shelves and the rear wall. The air enthalpy along FG into the first layer is the dominant factor of the temperature rapid rise, accounting for almost 64.4% of the total inflow air enthalpy. The net air enthalpy flow along FG is larger than other passages in the first 15 s and decreases rapidly as the temperature distribution becomes uniform.
查看更多>>摘要：Due to the implementation of the climate change agreement, negative environmental impacts caused by some traditional refrigerants have been spotlighted. A large number of environmental- friendly refrigerants with nearzero ozone depleting potential (ODP) and low global warming potential (GWP), such as R1234yf, R1233zd(E), R1270, R600a, are synthesized and proposed to replace traditional ones. These refrigerants' boiling heat transfer characteristic plays a crucial role in evaporator design and refrigeration cycle configuration. Thus, this review aimed to bring the state-of-the-art literature review on the boiling heat transfer characteristics of environmentalfriendly refrigerants with ODP close to 0 and GWP below 2000. In addition to describing the effects of heat flux, mass flow, saturation temperature, and mass transfer resistance on the heat transfer coefficient and pressure drop, this review also analyzed the heat transfer mechanism. Moreover, a detailed review of the correlation of boiling heat transfer and pressure drop was provided which might benefit practical applications. Furthermore, prospects and challenges to be addressed were also identified for future research. This paper benchmarks the recent progress on boiling heat transfer characteristics of single and mixed environmental-friendly refrigerants and presents insights into scientific research gaps that need to be addressed.
查看更多>>摘要：In global refrigerated transportation, marine reefers play an extremely important role. Considering the high freight cost and harsh surroundings of the decks during the voyage, reefer containers are always loaded into cabins for safe and economical purpose. However, under severe environment conditions in cabin, the refrigeration efficiency of reefer units was greatly affected, because of the weak ventilation and low heat exchange capacity. In order to find a more scientific and reasonable way to ventilate the cabin of reefer containers, two different ventilation modes, that is, air supply and air exhaust, were experimented and simulated. The temperature distribution was analyzed in a typical reefer cabin with 14.4 m (in length) x 23.2 m (in width) x 16.85 m (in height). Compared with two contrary ventilation modes, the temperature distribution in the cabin at steady state was simulated and analyzed. The results show that, the temperature of the cabin under the air exhaust condition was about 4K lower than that under the air supply condition. But with higher heat dissipation efficiency, the air exhaust condition sacrifices the uniformity of the temperature in the cabin. A relatively uniform temperature distribution and airflow field were obtained under air supply condition. Therefore, considering the heat dissipation effect of the container and the uniformity of airflow distribution, the air supply method was recommended. The two ventilation modes were compared in detail in this paper, and the results provide a reference for ventilation design of reefer container vessels.
查看更多>>摘要：Linear compressor has received continuous attention owing to its high energy conversion efficiency and oil-free operation. The dynamic behavior of a linear compressor is critical to understand its operation mechanism. However, the dynamic behavior of linear compressor cannot be fully visible. In response to this issue, a real-time measurement system is established to analyze the dynamic behavior of linear motor, piston and valve movements and the cylinder pressure. And deprived effects of gas force, i.e., piston offset and leakage are also discussed. The results show that current/displacement curves generate distortion on account of the nonlinear characteristic of gas force. Distortion of current curves (THD is up to 73.69% at 60 Hz) is more obvious due to the minor inductance. The valve appears delay opening, flutter, additional displacement fluctuation and delay closing compared with ideal valve. A high spring stiffness of valve makes slight difference to piston offset. The expansion/compression index in the p-V diagram with a high frequency has apparent increase owing to the variations of duration of heat exchange. The piston offset is nearly constant with the increasing of strokes while piston offset increases with the increment of discharge pressure/frequency. The leakage loss proportion with evaporation temperature at 5 degrees C and condensation temperature at 30 similar to 40 degrees C does not exceed 6%.
查看更多>>摘要：This paper presents an experimental investigation of the effect of R290/R600a refrigerant mixtures on the performance of variable speed hermetic compressors used in household refrigeration. In this context, a small capacity compressor test stand is introduced and four different compositions of R290/R600a mixtures are studied in detail. R290 composition in the refrigerant mixture varies between 40-70% by mass weight. Performance outcomes of the mixtures are compared to baseline refrigerant R600a. The influence of R290 composition in refrigerant mixture over COP, refrigerant mass flow rate and power consumption rates are explored in the present study. Moreover, the effect of compressor speed, evaporation and condensation temperature on the compressor performance are studied via steady-state experimental tests. It has been concluded that increasing the mass weight of R290 in the mixture yields higher power consumption rates, yet it has brought a significant increase in the refrigerant mass flow rate, thus enabling a higher COP. The optimum compressor speed is found to be approximately 2100 rpm for the tested hermetic reciprocating compressor. The COP increases with the increments of evaporation temperature and decrements of the condensation temperature. The COP of the refrigeration cycle is 10-20% greater than that of R600a depending on the composition of the refrigerant mixtures and operating conditions.
查看更多>>摘要：The European heat pump market is growing every year as European legislation moves towards the use of natural refrigerants that have less global warming potential than conventional refrigerants. Even though heat pumps are very efficient, the appearance of hard-to-detect faults increases energy consumption. The present study emulated some of the most common faults on a variable-speed heat pump charged with propane. The faults tested were evaporator fouling, compressor valve leakage, liquid line restriction and refrigerant overcharge. The effect of these faults on performance and several heat pump features are described. Evaporator fouling was the fault with least COP degradation and compressor valve leakage with the most (7% and 56% COP reduction, respectively). The results are compared with the results from a comparable study. The heat pump covered the demand generated by nearly all the faults at the expense of increasing the power consumption. The features of each fault are summarized in a chart, which could be used as table for diagnosing faults.
查看更多>>摘要：As one of hydrofluoroolefins, trans-1,1,1,4,4,4-hexafluoro-2-butene (R1336mzz(E)) is treated as a novel working substance for organic Rankine cycles as well as heat pumps for its low GWP, non-flammability, and lower toxicity. The objectives of this study are the viscosity measurement for R1336mzz(E) in liquid, vapor, and supercritical regions by the method of tandem capillary tubes and the development of correlations for the saturated liquid and vapor viscosities. The viscosity measurements for R1336mzz(E) were conducted over the range of temperature from 314 to 394 K for the liquid, 391 to 453 K for the vapor, and 413 to 453 K for the supercritical region, respectively, at pressures from 1.0 to 4.0 MPa. The expanded uncertainties in the viscosity measurement are calculated as 2.26 % for liquid, 2.30 % for vapor, and 2.32 % for supercritical phase using k=2 and 95 % confidence level. The experimental data are correlated with the ECS model; the AAD is 5.04 % for the liquid, 22.19 % for the vapor, and 11.88 % for the supercritical phase, respectively. The saturated viscosities are also estimated by the extrapolation from the measurements to the liquid-vapor saturation conditions. For industrial design and simulation, simple correlations are developed in terms of the saturation temperature to predict the viscosities of the saturated liquid and vapor phases, respectively.
查看更多>>摘要：Helical condenser coiled tube is widely used in refrigeration machines for water heating due to its higher heat transfer, compact structure, high energy efficiency, ease of manufacture and arrangement. In addition, developing and optimizing the helical condenser coil designs are considered a sustainable solution for improving the performance of the system. Based on this vision, a particular difference of this study with other studies performed on similar researches is the influence of different shapes of condenser coil designs on heat transfer and temperature distribution during heating process. Therefore, the main objective of this paper is to improve the thermal performance of a helical coil by modifying the coil shape. Based on the results, the average heat transfer coefficient and average COP of variable pitch coil were increased by 36.48% and 16.17%, respectively, compared with normal coil. Moreover, the obtained results indicated that the water temperature, the water velocity, the heat transfer and the performance of the helical condenser with variable pitch coil were improved in comparison with that of normal coil. However, this investigation could provide some guidance to obtain the optimum coil structure of refrigeration machines for domestic hot water production.
查看更多>>摘要：In this paper, a novel Multi-loop Moving Coil Large Output Volume Linear Compressors (M-MC-L-LC, Max 381.3cc) is proposed. A Co-core Bilateral Reverse Coil(CBRC) and a new Halbach permanent magnet ring array structure are proposed for the first time and applied to the development of prototype. The M-MC-L-LC electromagnetic force calculation equation is derived, the moving offset characteristics under different pressure ratios are studied, and a mathematical model that can reflect the moving offset law is established. Introduction the DC offset control method to solve the moving offset problem. Development the M-MC-L-LC prototype and construction a dynamic test rig for compressors. Experimental results show that increasing DC voltage countervails the moving offset and significantly increases the discharge pressure. The theoretical error of moving offset is within 8% and the mechanical efficiency of prototype is approximately 11.4% higher than that of the conventional compressor. The results of study have theoretical and practical implications for other linear compression moving offset analysis and accurate regulation of refrigeration capacity.
查看更多>>摘要：In this work, we report a detailed study of the thermodynamic performance of Pr0.6Sr0.4MnO3 in a functional magnetic cooling system up to the theoretical limit of its magnetocaloric effect. For this purpose, a onedimensional numerical model of the active magnetic regenerator (AMR) cycle is developed, while the required magnetocaloric properties of the oxide are calculated in the framework of mean field theory. The obtained results are deeply analyzed and then compared with the reference gadolinium metal. Particularly, it is revealed that using Pr0.6Sr0.4MnO3 as refrigerant requires high magnetic fields to generate reasonable temperature spans for practical applications, as it was found that under a magnetic field of 8 T, the studied oxide can generate an AMR temperature span of 33 K. The thermodynamic efficiency was evaluated by directly considering the work arising from the magnetic forces between the Pr0.6Sr0.4MnO3 regenerator and the magnetic field source, as the associated magnetic work is numerically calculated by using Altair Flux 3D software. From the obtained results, it was found that the coefficient of performance can be enhanced by a factor of two when balancing magnetic forces, despite of the moderate magnetocaloric effect shown by Pr0.6Sr0.4MnO3 under the practical fields going from 1 to 2 T. Moreover, the hidden thermodynamic performances of this compound are also pointed out by investigating its magnetocaloric properties in the theoretical limit zone. In this way, our calculations unveil that it is possible to reach cold source temperatures under -22 degrees C even when using helium as heat transfer fluid.
NSTL
Elsevier