•A PCM based double-spiral coiled heat exchanger was numerically investigated.•The heat exchanger could be applied in an air source heat pump for defrosting.•Heat storage and release modeling ...processes were both experimentally validated.•The mean average temperature differences are acceptable, at only −0.71–1.64 °C.•Potential uses and limitations of the modeling work are also further discussed.
There were very limited modeling studies are around a phase change material based double-spiral coiled heat exchanger, due to the difficulties of model development and the lack of mature simulation tools. When a phase change material based double-spiral coiled heat exchanger applied in an air source heat pump for defrosting, its model development becomes more complicated. To solve this fundamental problem, a mathematical model for such heat exchanger was developed, as well as the numerical solution method proposed. To improve the convergence and the speed of numerical simulation of the phase change material’s heat transfer process, both of special treatments in space and time were conducted. According to the circular shape boundary, an implicit solution method was used to reduce the number of grids so that computation became convenient. Heat storage and release processes of this model were finally experimentally validated in a specially built heat pump system during frosting and defrosting, respectively. The deviation conditions are demonstrated acceptable with the mean average temperature differences at the small range of −0.71–1.64 °C. Potential uses and limitations of the modeling work are also further discussed. Contribution of this work are meaningful for the optimization of heat pump and any other systems, where applied a phase change material based heat exchanger.
Refrigerant maldistribution severely deteriorates the heat transfer performance of a microchannel evaporator. Compared with the refrigerant distribution among flat tubes along the header, refrigerant ...distribution among microchannels in the flat tube in the airflow direction has barely been paid attention. In this paper, a heat transfer mathematical model of a microchannel evaporator’s flat tube composed of vertically placed parallel microchannels in the airflow direction was developed. The Refrigerant distribution among the microchannels was evaluated and its influence on heat transfer between air and refrigerant was analyzed. The results showed that the refrigerant distribution and heat transfer performance between air and refrigerant were interrelated and interacted with each other. The temperature of the air leaving the microchannel evaporator changed along the microchannel because of uneven refrigerant distribution among the microchannels, and the air temperature difference between air leaving out of the bottom and the top of the evaporator was approximately 2.13 °C. Ignoring the heat transfer from adjacent microchannels will lead to a small heat transfer deviation for the flat tube; thus, heat transfer among microchannels can be neglected.
•A PCM based solar-assisted air source heat pump system is proposed.•Performance of the system in cooling/heating mode is improved substantially.•Both the ambient temperature and the chilling water ...flow rate have a significant effect on the system.•Researchers should focus more on the overall performance of the system rather than a specific parameter or a single component.
A phase change material (PCM) based solar-assisted air source heat pump (PCM-SAHP) system consisting of an air source heat pump (ASHP), PCM unit and solar thermal collector is proposed. The purpose of the system is to enhance the performance of the ASHPs at low ambient temperatures, and to eliminate the discrepancy between the demand and supply side of cooling/heating energy. The paper is organized as follows: In the first section, the design of the system is compared against other analogue systems. Then, an experimental study on the PCM-SAHP system is provided. The improvements to the system are observed for both the cooling and heating modes. The experimental findings indicate that in the cooling mode the ambient temperature still has a significant effect on the operating performance of the system. In contrast, it is found that the effect of the chilling water flow rate (through the PCM unit) on the system performance is relatively subtle. The authors use an equivalent coefficient to analyze the heat transfer status in the PCM unit. The results of the experiment suggest that researchers should focus more on the overall performance of the system rather than on the heat transfer coefficient of a single component.
This paper reports on an experimental study on the operating performance for a novel reverse-cycle hot gas defrosting method for air source heat pumps (ASHPs). Firstly, a description of both an ...experimental prototype ASHP unit where the novel reverse-cycle hot gas defrosting method may be realized, and its operating modes are presented. This is followed by reporting an experimental set up for testing the defrosting performance of the prototype ASHP unit using the novel thermal energy storage (TES) based reverse-cycle hot gas defrosting method. Finally, experimental results are presented and discussed. The experimental results suggested that the use of the novel reverse-cycle hot gas defrosting method for the experimental ASHP unit was able to help shorten the defrosting time by ∼3 min or 38%, and minimize the risk of shutting down the ASHP unit due to low suction pressure through increasing the compressor’s suction pressure by about 200 kPa, when compared to the use of the traditional standard reverse-cycle hot gas defrosting method. In addition, when the TES based reverse-cycle hot gas defrosting method was used, the mean indoor coil surface temperature during defrosting was about 25 K higher than that when the traditional standard reverse-cycle hot gas defrosting method was used. These results suggested that as compared to the use of the traditional standard reverse-cycle hot gas defrosting method, the use of the novel TES based defrosting method would enable a quicker space heating resumption, impacting less on indoor thermal environment due to defrosting. Hence, the novel TES based defrosting method was proven to be effective in reducing the current problems in ASHPs using standard reverse-cycle hot gas defrosting method, such as a longer defrosting time and a poor system reliability.
The hot-wall heat exchanger (HWHE) has been widely used in thermal engineering fields such as ceiling radiant heating/cooling, refrigerator condenser, solar heat collection, and high-temperature heat ...recovery. However, the numerical simulation normally used for heat transfer prediction in HWHE is usually not as convenient as the analytic solutions in engineering applications. In this paper, a new heat transfer mathematical model of HWHE-based on analytic solutions was developed, which could be much faster to obtain the heat transfer properties of HWHE. The proposed model was validated under four conditions with literature values, which showed that the deviations of heat flux are 2.53%, 0.99%, 2.12%, and 1.96%, indicating its accuracy is satisfied. The model was then used to analyze the thermal property of HWHE. The results show the thermal resistance caused by panel with heat convection and conduction accounts for 96.54% of HWHE thermal resistance, and the thermal resistance caused by heat convection on the surface of panel is 74.43%. The analyzation results also show that adding aluminum foil around pipes could decrease HWHE thermal resistance by 5.11%. Besides, the influence of pipe diameters, pipe distance, pipe heat conductivity, side wall heat conductivity, and convective heat transfer coefficient on the heat transfer performance of HWHE was analyzed. The research in this paper can be used for fast prediction and optimization of heat transfer in HWHE.
The problem of hydraulic imbalance among buildings and building heat users in the traditional heating industry is an important reason for the thermal imbalance of the heating system and leads to a ...serious waste of energy and resources. In the urban heating network, the thermal loss caused by thermal imbalance accounts for about 25% of the energy consumption of the heating, the conventional solution of increasing secondary network circulating water flow is not only limited adjustment effect, and greatly increases the power consumption of circulating water pump, which has a tremendous potential for energy saving. In view of the existing heating network problems, this article introduces the thermodynamic working principle, working characteristics and restriction conditions of working efficiency of the jet pump. And the specific application of the jet pump in the actual project is analyzed in detail, and the operation energy consumption of the injection pump system and the original heating system is compared and analyzed. Statistics show that average power saving rate can reach 20.24%, and average heat saving rate is up to 12.52% or more. which reflects the jet pump technology has great potential of energy saving in the city central heating system.
Endometritis adversely affects the ability of cattle to reproduce and significantly reduces milk production. The is mainly composed of epithelial and stromal cells, and they produce the first immune ...response to invading pathogens. However, most of the epithelial cells are disrupted, and stromal cells are exposed to an inflammatory environment when endometritis occurs, especially postpartum. Many bacteria and toxins start attacking stromal cell due to loss of epithelium, which stimulates Toll-like receptor (TLRs) on stromal cells and causes upregulated expression of cytokines. Understanding the genome-wide characterization of bovine endometritis will be beneficial for prevention and treatment of endometritis. In this study, whole-transcriptomic gene changes in bovine endometrial stromal cells (BESCs) treated with LPS were compared with those treated with PBS (control group) and were analyzed by RNA sequencing. Compared with the control group, a total of 366 differentially expressed genes (DEGs) were identified in the LPS-induced group (234 upregulated and 132 downregulated genes), with an adjusted
< 0.05 by DESeq. Gene Ontology (GO) enrichment analysis revealed that DEGs were most enriched in interleukin-1 receptor binding, regulation of cell activation, and lymphocyte-activated interleukin-12 production. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed DEGs were most enriched in the TNF signaling pathway, Toll-like receptor signaling pathway, cytokine-cytokine receptor interaction, NF-κB signaling pathway, and chemokine signaling pathway. The results of this study unraveled BESCs affected with LPS transcriptome profile alterations, which may have a significant effect on treatment inflammation by comprehending molecular mechanisms and authenticating unique genes related to endometritis.
•A control strategy is proposed for the intermittent heating of office buildings.•A physical model for forecasting thermal behavior of the building is developed.•The required heating capacity can be ...limited by extending the warm-up time.•A system with lower thermal inertia has greater energy saving potential.
In order to achieve building energy efficiency and keep the indoor temperature within a defined comfort range during working hours, a predictive control strategy is proposed for intermittent heating of office buildings by predicting the warm-up time on each working day. A physical model for simulating and forecasting thermal behavior of the building is developed using the state space method. Furthermore, a modeling analysis case is given based on a typical office building in Beijing with different types of building insulation and terminal forms. The increasing capacities of heating equipment, transition time, room temperature variation and energy saving rate are simulated and analyzed. Results indicate that when applying an intermittent heating strategy, the required heating capacity for each room is 1.1–1.3 times to that of a traditional continuous heating system. The warm-up time ranges from several minutes to several hours depending on conditions, while cooling down time is much shorter than warm-up time. In addition, we found that heating systems in ordinary buildings with fan coils as terminal devices have faster temperature responses than those in energy efficient buildings with radiators. It is necessary to consider standby heating for bathrooms to keep room temperate above antifreeze value. The heat saving rate for intermittent heating can reach about 20% in a typical week compared with continuous heating, and a system with lower thermal inertia has greater saving potential.
This work aimed to develop a novel chitosan based metal-organic polyhedrons (MOPs)/enzyme hybrid hydrogel with superior antimicrobial properties in wound healing treatment. Hybrid hydrogel was ...prepared by crosslinking glucose oxidase (GOx), vanadium metal-organic polyhedrons (VMOP-2) and chitosan using glutaraldehyde as crosslinker. The formed GVCS hydrogel was characterized by using various techniques, including FTIR, SEM, XPS, TGA and EDX. Data show that GVCS hydrogel was successfully obtained with uniform distribution of GOx and VMOP-2 in the hydrogel structure. Antibacterial tests show that GVCS hydrogel exhibits better bactericidal effect on both gram-negative bacteria (S. aureus) and gram-positive bacteria (E. coli) compared to other hydrogel samples because of its hydroxyl radicals generation capacity in the presence of glucose. MTT assay shows that the hydrogel presents good cell compatibility. In vivo experiments using an infected wound model indicate that GVCS hydrogel can effectively facilitate wound healing. Therefore, chitosan based MOPs/enzyme hybrid hydrogel could be most promising for antibacterial therapy in clinical applications.
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•Frosting characteristics of micro-channel evaporator was experimentally studied.•Structure of micro-channel evaporator influences system frosting performance.•Surface temperature difference of ...micro-channel heat exchanger was analyzed.•Surface temperature difference was proposed to judge system performance state.
Frosting and refrigerant distribution uneven easily are two key problems restricting the wide application of micro-channel heat exchanger in the field of air source heat pump (ASHP). In this paper, frosting experiments were carried out on two kinds of micro-channel heat exchanger in three cases that with different refrigerant distribution characteristics. Frost and temperature distribution characteristics and their influence on the heat pump’s COP and heating capacity was studied. The results show distribution of refrigerant in heat exchanger directly affects the frost and temperature distribution on the surface of micro-channel heat exchanger, and affects the heating capacity and COP of ASHP greatly. Vertical placement of flat tubes and horizontal placement of the headers can be beneficial to the refrigerant distribution compared with that with the headers vertically placed. Flow direction of refrigerant in the heat exchanger must be from bottom to top to avoid deteriorated heat transfer performance and instability of ASHP. Difference between the lowest temperature and largest temperature on heat exchanger surface can be used as an index to judge whether ASHP works in high performance state.
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