查看更多>>摘要：The heating capacity of air source heat pump increases with the increase of ambient temperature, coupled with the heating demand decreases, resulting in a prolonged partial load operation. The partial load operation of low ambient temperature air source heat pumps applied in cold region is particularly serious. With that also comes an efficiency degradation, taking into consideration that the characterization method of current product standards is not uniform in different countries. In this paper, the mechanism of partial load efficiency degradation was analyzed firstly, the explicit relationship between cycling losses and cyclic time was derived, and a partial load factor characterization method of air source heat pump by coupling start-up loss and stand-by loss was proposed. Secondly, the cycling losses of 3 sets of low ambient temperature ASHPs with different configurations were tested under different ambient conditions by using the intermittent ON-OFF cycles tests method. The magnitude of start-up loss is 2.0%similar to 17.3% for test ASHPs, and the stand-by loss rises sharply from 4.2 to 8.9% for the test ASHPs as the PLR drops from 0.2 to 0.1. The accuracy of the PLF characterization method was verified, and especially that of the load rate below 0.2 was improved.
查看更多>>摘要：In recent years, CO2 has been widely considered as one of the most promising substitutes for the HFCs due to the consideration of both the environmental protection and high efficiency. However, the system performance of a transcritical CO2 cycle is greatly influenced by the discharge pressure, the determination of which has been the most important and hottest issue. Apart from the traditional correlation development through theoretical analysis and experimental data, model-free extreme seeking control method has also been proved to be effective through the simulation investigation in literatures. However, the related experimental investigation is still rare. In this article, we provide the experimental investigation of the ESC method on a transcritical CO2 heat pump water heater. The ESC controller was designed based on the system dynamics and then implemented on the test rig. Experimental investigation was then carried out under the fixed design condition (inlet water temperature at 40 degrees C and air temperature at 0 degrees C) and the fixed off-design condition (inlet water temperature at 45 degrees C and air temperature at 0 degrees C), the system COP operated by the ESC controller was improved by 7.62% after 4167s, and by 8.81% after 4287s respectively. The experimental results under the continuous step change of the inlet water temperature (40-45-40 degrees C) demonstrated the repeatability and the effectiveness of the ESC control under changing working conditions. The static performance of the heat pump water heater was finally explored under the inlet water temperatures of 40 degrees C and 45 degrees C to furtherly demonstrate that the found discharge pressures of the ESC controller were indeed the optimal discharge pressures of the transcritical CO2 heat pump.
查看更多>>摘要：The high-temperature heat pump steam system (HTHPS) is an effective way to replace the calcining boiler's steam. However, the large temperature span of the system leads to colossal exergy loss of refrigerant in expansion valves and other components. Therefore, it is hoped to adopt the dual-flash vaporization process to improve the system's steam production performance. In this paper, three thermodynamic models and exergy models of HTHPS with a double flash evaporation process are established. Furthermore, this paper uses the multivariate simulated annealing algorithm to calculate the optimal COP of the system. Then, by comparing the single-stage compression (SC) system and the quasi-two-stage compression (QTC) system, the influence of the dual-flash vaporization process on HTHPS is analyzed. The results show that under the same environment, the COP of the dual-flash vaporization process compression system is 23.8% 44.54% higher than that of the SC system. With the increase of the system temperature span, the flash steam supplemental process brings the system to improved thermodynamic performance. Moreover, the exergy model of the dual-flash system is closest to the reversed Carnot cycle, when the evaporation temperature is 50 degrees C and the condensing temperature is 110 degrees C.
查看更多>>摘要：Convective cooling/heating experiments were carried out using a device representing a half-layer of a pallet with four crates of tomatoes (var. Cauralina, Solanum Lycopersicum). Air and product temperatures were measured at different positions in the crates, which were exposed to several different ambient conditions in order to study the influence of air velocity, warming/cooling regimes, hole opening area and filming. The experimental results show that the product and air temperatures vary with the distance from the air inlet position. Since airflow rate was varied and controlled using fans equipped with electrical power variation, a correlation between the air velocity and the convective heat transfer coefficient was established. A transient state heat transfer model was developed based on a zonal approach, which considers overall thermo-physical properties for each crate. An aroma model was developed based on three hypothetic first order kinetics. The model takes into account the observation reported in several studies that tomato aroma decreases when the product temperature is lower than 12 degrees C and the aroma is recovered, but only partially, when the product temperature is higher than 12 degrees C. Finally, the thermal model was linked with the aroma model to predict the product temperature and the aroma evolution in two examples of supply chain scenario from producer cold room to consumption point. These numerical results show that the tomato variety and the product time-temperature history in a supply chain exert a significant influence on the aroma evolution.
查看更多>>摘要：With the recent advancements in designs and optimization of gas coolers in CO2 transcritical refrigeration and heat pump systems, the impact of the lubricant on the performance of gas coolers has been gaining increasing attention. The existing literature reviews focus on examining the lubricant effect on the heat transfer performance of gas coolers. However, the studies do not present a detailed discussion and a systematic summary of the flow characteristics. The present paper aims to offer an overall review of the lubricant impact on the flow characteristics of CO2 in gas coolers. The authors hope it will help researchers conduct additional research about practically designing and optimizing gas coolers. This paper reveals the flow mechanism by providing critical discussions on how temperature, lubricant type, lubricant concentration, mass flux, operational pressure, tube diameter, and tube configuration influences the flow characteristics of the CO2-lubricant mixture. Then, the paper summarizes and evaluates the latest developments related to CO2-lubricant mixture in the flow and heat transfer analysis models and pressure drop correlations. Moreover, the current research gap is summarized, and the study proposes development directions. These include measuring thermo-physical properties of CO2-lubricant mixture, examining pressure drop characteristics under different tube configurations, developing universal pressure drop correlations, and studying flow characteristics in vertical tubes.
查看更多>>摘要：As the bearer of wet load, the desiccant wheel determines the performance of the rotary wheel dehumidification system. In this study, a novel metal organic framework composite adsorbent desiccant wheel (MOF DW) was prepared by impregnating-spraying method. The complete rotary wheel dehumidification system was built to compare the dehumidification performance of MOF DW and commercially silica gel desiccant wheel (SG DW). The experimental results showed that increasing the rotation speed appropriately improves the utilization ratio of regenerative energy and dehumidification capacity of MOF DW at high ambient humidity ratio. The temperature rise at the inlet and outlet of MOF DW is lower than that of SG DW at low ambient humidity ratios, and the opposite is true at high ambient humidity ratios. In addition, the dehumidification capacity and coefficient of performance of the MOF DW increase by 40%-48% and 13%-19%, respectively, compared with those of the SG DW at the regeneration temperature of 50 degrees C.
查看更多>>摘要：This study experimentally investigated the evaporation heat transfer and two-phase flow characteristics of a vertical upward flow using R1234ze(E) and R32 as test refrigerants in a plate-fin heat exchanger with offset fins. The adiabatic and evaporation flows of R1234ze(E) were visualized using a high-speed camera. Owing to the asymmetry of the inlet and outlet of the channel, a non-uniform velocity profile in the channel-width direction was observed under both adiabatic and evaporation flows. Evaporation flow accompanied by nucleate bubble generation at the lower area of the channel and the occurrence of a dry area at the upper side of the channel were observed. The heat transfer coefficients of R1234ze(E) and R32 were measured, and the effects of mass flux, quality, and heat flux on heat transfer were clarified. The heat transfer was strongly affected by forced convection at higher mass fluxes and quality whereas nucleate boiling dominated at higher heat fluxes and lower quality. The difference in the heat transfer characteristics at local positions in the channel was also investigated by measuring the local wall temperatures. Comparing refrigerants, the heat transfer coefficient of R32 was higher than that of R1234ze(E), regardless of the mass flux and quality, especially at lower mass flux conditions.
Di Nicola, GiovanniKondou, ChiekoTomassetti, Sebastiano
9页
查看更多>>摘要：The triple point temperature of refrigerants is crucial because it serves as a reference for the lower limit of their use temperature in a refrigeration system. In this study, the triple point temperatures for recently proposed refrigerants with low global warming potential were experimentally determined. Ten hydro-fluoro-olefins, two hydro-chloro-fluoro-olefins, and trifluoroiodomethane were investigated. The triple point temperatures of the refrigerants were determined from the obtained cooling and heating curves. To improve measurement accuracy, measurements were conducted on samples obtained from different companies using two different measurement devices. The measured data were found to be in good agreement within the estimated measurement uncertainty.
查看更多>>摘要：In some thermally driven two-phase natural circulation systems, bubble pumps serve as the key driving powers for the cycles. Recently a type of distribute heating bubble pump generator (BPG) is gradually receiving attention due to its compact structure and great potentials to utilize solar energy and low-grade waste heat recovery. The BPG provides a variety of promising features (e.g., passive heat transfer, enhanced reliability), which can benefit the advancing of heat transfer technology. For the primary study, we performed an experiment in a distributed heating BPG. Through utilizing multiple lift tubes and partial visualization configurations, it provides accesses to observe the flow pattern transition and monitor the flow instability, and thus to explore some of the underlying mechanisms affecting BPG performance. Results showed that heat input and immersion height were crucial parameters to enable the operation of distribute heating BPG. With low heat input or high inlet water subcooling level, the flow within the pump was unstable with intermittent flow interruptions. As the heat input increased, the fluid flow became more stable, the vapor generation increased linearly, while the lifted liquid flow rate initially increased then decreased. Correspondingly, the flow pattern at the outlet section of lift tubes gradually changed from slug flow to churn flow, and then to annular flow. The higher of the immersion was, the higher heat input was needed for the flow pattern transition. It was in the churn flow regime at the outlet of lift tubes for the BPG to lift a maximum liquid. At lower immersion level, liquid reflux in the lift tubes was obvious and affected the flow stability as well as the lifting performance. At higher immersion level, the fluid flow was more stable and faster, which lifted more liquid while generated less vapor depending on the inlet subcooling. In general, the BPG showed better performance (both the lifted liquid and vapor generation increased) at smaller inlet subcooling level or lower system pressure. This study highlights the flow pattern evolution and flow stability, which is helpful to the reliable design and effective operation of the distributed heating BPG.
查看更多>>摘要：In this paper, a gray-box dynamic modeling approach for direct-expansion cooling systems is presented. The overall approach incorporates a multi-stage training procedure that consists of 1) identification of component sub-models from quasi-steady-state performance data, 2) system model integration with estimation of refrigerant charge and 3) fine tuning of thermal capacitances of the evaporator and condenser to capture the system dynamic responses. Compared to traditional physics-based models, the proposed modeling approach has advantages including reduced engineering efforts in the model development phase, improved computational efficiency and enhanced prediction accuracy. The modeling method was validated using a 3-ton variable-speed heat pump and proved to be capable of accurately predicting the system transient behaviors over a wide range of operating conditions. The established dynamic model was then applied for control stability analysis, with a specific goal of determining a proper control execution time step. The case study results showed that the stable control execution time step could change significantly, from 3 sec to 19 sec, as the operating conditions and control settings vary, and a proper selection of the execution time step is critical to ensure stable and reliable operations.
NSTL
Elsevier