Combined Vacuum Research Articles

Performance Study of Eductor with Finite Secondary Source for Membrane Distillation

This is an experimental work performed to identify the influence of direct contact condensation inside an eductor. The fluid used in the experiments is water in two different phases: liquid and vapor, for primary and secondary flows, respectively. This study was conducted in an attempt to establish the suitability of an eductor as a combined vacuum generator and condenser for membrane desalination applications. The pressure and temperature measurements at critical points in the flow paths have been summarized to identify the influence of primary flow on secondary fluid saturation and condensation. In addition, the mechanism of phase change has been explained through the photography of fluid flow in a two-dimensional eductor. A consistent oscillation of the gas-liquid interface was observed during steady-state operations of the eductor. This work also contributes to the validation of future computational research. It will provide a baseline for computational thermal fluid analysis related to the mixing of condensing and non-condensing flow. In general, the research encompasses the practical operational scenario and provides information on the heat and mass transfer of direct contact condensation with a finite secondary source.

Release of free-state ions from fulvic acid-heavy metal complexes via VUV/H2O2 photolysis: Photodegradation of fulvic acids and recovery of Cd2+ and Pb2+ stripping voltammetry currents

Fulvic acid (FA), a ubiquitous organic matter in the environment, can enhance the mobility and bioavailability of Cd2+ and Pb2+ through competitive complexation to form FA-heavy metal ions (FA-HMIs) complexes with excellent solubility. Because FA-HMIs are electrochemically inactive, square wave anodic stripping voltammetry (SWASV) cannot accurately detect the content of bioavailable Cd2+ and Pb2+ in soils and sediments. This study ostensibly aimed to efficiently recover SWASV signals of Cd2+ and Pb2+ in FA-HMIs by disrupting FA-HMIs complexes using the combined vacuum ultraviolet and H2O2 (VUV/H2O2) process. Essentially, this study explored the photodegradation behavior and photolysis by-products of FA and their effects on the conversion of FA-HMIs complexes to free-state Cd2+ and Pb2+ using multiple characterization techniques, as well as revealed the complexation mechanism of FA with Cd2+ and Pb2+. Results showed that reactive groups such as carboxyl and hydroxyl endowed FA with the ability to complex Cd2+ and Pb2+. After FA-HMIs underwent VUV/H2O2 photolysis for 9 min at 125 mg/L of H2O2, FA was decomposed into small molecular organics while removing its functional groups, which released the free-state Cd2+ and Pb2+ and recovered their SWSAV signals. However, prolonged photolytic mineralization of FA to inorganic anions formed precipitates with Cd2+ and Pb2+, thereby decreasing their SWSAV signals. Moreover, the VUV/H2O2 photolysis significantly improved the SWASV detection accuracy toward the Cd2+ and Pb2+ in real soil and sediment samples, verifying its practicality.

Effect of freezing temperature on dewatering, consolidation properties, and microstructure of landfill sludge

There is currently a considerable amount of landfill sludge (LS) accumulating worldwide, threatening the surrounding environment and land safety. It is urgent to dewater and reduce LS. Chemical conditioning is the most common treatment for LS, which can pollute the environment and limit resource exploitation. Therefore, a more environmentally friendly and efficient freeze-thaw combined vacuum preloading method is proposed. Experimental studies were conducted to investigate the influence of different freezing temperatures on the dewatering properties, compression and consolidation features, and the mechanism of microstructural change in LS. The results show that the freezing temperature has an important influence on the dewatering, compression, vacuum drainage, consolidation, and microstructure characteristics of LS. The compressibility of LS does not improve when the freezing rate is too high. Freeze-thaw cycles can improve sludge's permeability and consolidation properties by one to two orders of magnitude, and a suitable freezing temperature can significantly improve sludge's permeability and consolidation properties. After vacuum drainage and consolidation, the maximum volume reduction ratio of sludge can reach 55.3%, and the water content of LS can be reduced from 86% to 66%. The distribution of large pores and mesopores in LS increases as the freezing temperature decreases, reaching a maximum of −15 °C, which can substantially improve permeability, drainage, and consolidation efficiency.

Design of a vacuum system for space active hydrogen maser

With the high precision and stability of its frequency signal outputs, active hydrogen maser plays an important role in such fields as timing, satellite navigation, and communication. However, it needs to be lighter so as to be applied in space. We made a research, based on the calculation of the hydrogen flow and the adsorption efficiency of the adsorption unit, on the parameters of the vacuum system and the structural requirements, and designed a combined vacuum pump for the Space Active Hydrogen Maser (SAHM). This vacuum pump consists of a getter pump and a small ion pump, the total mass of which is about 5 kg. The pumping speed will be about 474 L/s by computation, when an amount, 2.5 MPa L, of hydrogen has been adsorbed by getters. Theoretically, the total source hydrogen inflow in lifetime is not higher than 20% of the total capacity getter pump, thus the design should amply meet the requirements of the SAHM vacuum system, and is of great significance for future SAHM applications.

Liquid metal incorporated graphene oxide films with enhanced through-plane thermal conductivity and flame resistance

With the recent upsurge in electronic and telecommunications industries, there is an extensive demand for thermal interface materials (TIMs) with integrated high thermal conductivity and flame resistance to ensure the performance, lifetime, and safety of electronic devices. Traditional polymer-based TIMs have failed to meet this demand owing to their inherent weaknesses like the utilization of rigid fillers, low through-plane conductivity, and poor thermal stability. Recently, graphene-based films have attracted considerable attention to use as TIMs due to pristine graphene's excellent in-plane thermal conductivity (2000–5000 Wm−1K−1). However, unsatisfactory through-plane thermal conductivity (<0.1 Wm−1K−1), resulting from the horizontally stacked graphene sheets, remains a major challenge in designing graphene-based TIMs for practical applications. Herein, we design a liquid metal (Galinstan) incorporated graphene oxide (GO) films with improved through-plane thermal conductivity and flame resistance via a combined solution mixing and vacuum filtration method. Gallium from the liquid metal Galinstan reacts with GO and reduces it while constructing heat transfer pathways along the through-plane direction. The resultant composite film demonstrates a through-plane thermal conductivity of 0.42 Wm−1K−1, which is much better than graphene oxide films (0.01 Wm−1K−1) and previous similar studies. Moreover, it has excellent thermal stability and can withstand an alcohol flame for 30 s, proving its potential to meet the ever-increasing demand for TIMs.

Physiotherapy in the rehabilitation of patients with urolithiasis with ureteral stones after extracorporeal shock wave lithotripsy

Urolithiasis currently occupies a leading position among all urological pathologies, accounting for 26 % to 37 %, which elevates this problem to the rank of socially significant issues and determines the high relevance of finding new approaches to the development of rehabilitation programs to use in the early postoperative period after shock wave lithotripsy for the prevention of postoperative complications and relapses of the disease. Objective. To study the features of the influence of combined vacuum interferential therapy and mono-influences with interferential currents on nitrogen metabolism and excretory function of the kidneys in patients with urolithiasis with ureteral stones after extracorporeal shock wave lithotripsy. Material and methods of treatment. The study included 105 male patients with urolithiasis and ureteral stones after extracorporeal shock wave lithotripsy, whose mean age was 47.6 ± 3.8 years. The patients were divided by simple randomization into 3 groups comparable in terms of age, clinical and functional characteristics, and instrumental data: from the 2nd day after ESWL against the background of drug treatment, the patients of the main group 1 (n = 35) were prescribed vacuum interferential therapy on the lumbosacral region and the projection of the ureters with a carrier frequency of 5 kHz, in the frequency range of 80–150 Hz, with a vacuum of 0.3–0.4 bar, 15 min per unit, 6–8 procedures per course; from the 2nd day after ESWL against the background of drug treatment, the patients of the comparison group (n = 35) were prescribed mono-impacts of interferential currents on the lumbosacral region and the projection of the ureters with a carrier frequency of 5 kHz, in the frequency range of 80–150 Hz, 15 min per unit, 6–8 procedures per course; the patients in the control group (n = 35) were prescribed antibacterial and anti-inflammatory drug therapy, which served as a background in the main and the comparison groups. Research results. When studying the indicators of nitrogen metabolism in the blood in the initial state, an increase in the content of uric acid and creatinine was noted against the background of a decrease in the content of potassium. After the course of treatment, the most pronounced dynamics was revealed in the patients of the main group, which manifested in a significant decrease of creatinine by 15.2 % (p ˂ 0.05) and uric acid by 13.6 % (p ˂ 0.05) against the background of an 8 % increase in the content of potassium (р ˂ 0.05) in blood. Although the indicators of nitrogen metabolism improved under the influence of interferential therapy in the patients of the comparison group, the differences were not significant and were regarded only as a positive trend. Prior to the start of treatment, a decrease in pH, daily diuresis, glomerular filtration, and tubular reabsorption against the background of an increase in oxalate salts was found in daily urine. After the course of treatment, the patients in the comparison group received significant positive dynamics in all studied parameters, which manifested in an increase in pH, daily diuresis, glomerular filtration, and tubular reabsorption against the background of a decrease in the level of oxalate salts. In the control group, there was only a slight positive trend. Conclusion. Vacuum interferential therapy has an anti-inflammatory and diuretic effect to a greater extent than interferential therapy, contributing to the normalization of the excretory function of the kidneys and nitrogen metabolism in patients with urolithiasis with ureteral stones after ESWL, and can be recommended for use in wide clinical practice.

Magnetron sputtering NbSe2 film as lubricant for space current-carrying sliding contact

This study demonstrates that magnetron-sputtered NbSe2 film can be used as a lubricant for space current-carrying sliding contact, which accommodates both metal-like conductivity and MoS2-like lubricity. Deposition at low pressure and low energy is performed to avoid the generation of the interference phase of NbSe3. The composition, microstructure, and properties of the NbSe2 films are further tailored by controlling the sputtering current. At an appropriate current, the film changed from amorphous to crystalline, maintained a dense structure, and exhibited excellent comprehensive properties. Compared to the currently available electrical contact lubricating materials, the NbSe2 film exhibits a significant advantage under the combined vacuum and current-carrying conditions. The friction coefficient decreases from 0.25 to 0.02, the wear life increases more than seven times, and the electric noise reduces approximately 50%.

Settlement Prediction under a Combined Vacuum and Surcharge Preloading in New Reclaimed Land

Rapid settlement assessment in new reclaimed land is important to follow-up engineering construction. Vacuum-surcharge preloading combined with prefabricated vertical drains (PVDs) is a high-efficient and widely used reclaimed soil-reinforced method. This work focuses on micro-macro mechanism of vacuum and surcharge preloading, and the settlement characteristics of subsoil subjected to vacuum-surcharge pressure are discussed. The surcharge preloading is applied to soil particles directly by vertical stress, and the effective stress increases directly by direct contact among particles. However, vacuum pressure increases the effective stress and decreases the pore pressure at the same time. In new reclaimed land, the proposed method is working on the assumption that the new reclaimed land full of water with particles arranging in a loose state of simple cube (SC) way before treatment, while a dense state of terse tetrahedron (TH) way, a stable structure of soil, after treatment. An approach for calculating the final settlement of new reclamation land is derived in two-dimensional (2D) and three-dimensional (3D) perspective, respectively. Meanwhile, this method has been preliminarily applied to two case histories reported in the literature. Results show that 2D estimation method might be more useful for the preliminary assessment of settlement under a combined vacuum and surcharge preloading in new reclaimed land.

Investigating the energy-saving performance of a CdTe-based semi-transparent photovoltaic combined hybrid vacuum glazing window system

Building integrated photovoltaic (BIPV) glazing is currently regarded as a promising building material with a wide range of benefits. Photovoltaic combined vacuum glazing is a relatively new innovative concept in BIPV glazing. On the other hand, photovoltaic combined hybrid vacuum glazing (PVCHVG) is a rarely studied topic in which an air gap exists between vacuum glazing and photovoltaic glazing to form an insulated glazing unit. This paper investigates the overall energy-saving performance of a CdTe-based semi-transparent PVCHVG. A dynamic simulation model was developed and validated with an outdoor experiment to explore the energy-saving performance of the PVCHVG under five different climate conditions in China, and the results were compared with commonly used window systems. The results indicated good insulation properties against both heat loss and heat gain due to the combined action of vacuum glazing and semi-transparent photovoltaic glazing. Compared to clear single-glazing and double-glazing window systems, PVCHVG can save overall energy consumption up to 59.39% and 39.97% in heating-dominated region, and 76.33% and 73.766% in cooling-dominated region, respectively. Furthermore, the PVCHVG window system generated electricity with a good performance ratio and total system efficiency ranging from 85.7% to 85.78% and 7.45%–7.55%, respectively, considering five climate conditions.

Attenuation of toxicity and occurrence of degradation products of the fungicide tebuconazole after combined vacuum UV and UVC treatment of drinking water

Antifungal azoles are the most frequently used fungicides worldwide and occur as active ingredients in many antifungal pharmaceuticals, biocides, and pesticides. Azole fungicides are frequent environmental contaminants and can affect the quality of surface waters, groundwater, and drinking water. This study examined the potential of combined vacuum UV (185 nm) and UVC (254 nm) irradiation (VUV/UVC) of the azole fungicide tebuconazole and the transformation product 1,2,4-trizole on degradation and changes in ecotoxicity. In vivo ecotoxicity was examined before and after UV treatment using bioassays with test organisms from different trophic levels to integrate changes in biological effect of the parent compound and the degradation products. The test battery included the luminescent bacterium Aliivibrio fischeri, the Gram-positive bacterium Bacillus subtilis, the fungus Fusarium graminearum, the green microalga Raphidocelis subcapitata, and the crustacean Daphnia magna. The combined VUV/UVC treatment of tebuconazole in drinking water efficiently degraded the parent compound at the µg/L-mg/L level and resulted in transformation products with lower toxicity than the parent compound. A direct positive correlation was observed between the applied UV dose (fluence, J/cm2), the disappearance of tebuconazole, and the decrease in ecotoxicity. The combined VUV/UVC process does not require addition of supplementary oxidants or catalysts and our study suggests that VUV/UVC-mediated photolysis of azole fungicides in water can decrease the overall toxicity and represent a potentially environmentally friendly treatment method.

Perspectives on photobiomodulation and combined light-based therapies for rehabilitation of patients after COVID-19 recovery

The post-COVID-19 condition or ‘long COVID’ is a clinical and scientific challenge for society. In this regard, patients after COVID-19 recovery show a vast range of sequels including muscular, articular lesions, neurological, dermatological, and pulmonary issues. These clinical consequences are issues in the present and for the future. In this case, rehabilitation therapies based on photobiomodulation and combined therapies arise as excellent tools to solve it. Herein, we describe and discuss the perspectives on the use of light-based therapies such as photobiomodulation, photodynamic therapy and combined vacuum and laser therapy for rehabilitation of patients who present some sequelae of the COVID-19 infection. We did not intend to produce a comprehensive review; instead we highlight the most important and clinical protocols against these sequels. Moreover, the principles and mechanism of action of each light-based technique proposed were reported and discussed.

Effects of combined ultrasonic and microwave vacuum drying on drying characteristics and physicochemical properties of Tremella fuciformis

This study analyzes the effects of ultrasonic waves on the drying kinetics of Tremella fuciformis during microwave vacuum drying. The physicochemical properties and structural characteristics of T. fuciformis polysaccharides (TFPs) were studied by drying tremella samples using hot air drying (HAD), microwave vacuum drying, ultrasonic pretreatments with microwave vacuum drying (US + MVD), and air-borne ultrasonic pretreatments combined with microwave vacuum drying (USMVD) under acoustic energy densities of 0.14, 0.28, and 0.42 W/mL. The results showed that USMVD and US + MVD accelerated the mass transfer process of T. fuciformis. Compared with HAD treatment, TFP samples obtained by USMVD and US + MVD had a reduced molecular weight to a certain extent, and they had stronger shear thinning ability. In addition, USMVD-TFPs at 0.42 W/mL retained higher total sugar, reducing sugar, and uronic acid, and the degree of reduction in the monosaccharide component content was small.

Microwave vacuum drying of pomegranate peel: Evaluation of specific energy consumption and quality attributes by response surface methodology and artificial neural network

Microwave vacuum drying of pomegranate peel was studied through this work and the drying process was modeled with the aid of response surface methodology (RSM) and artificial neural network (ANN) method. The drying experimental runs were performed on varied ranges of microwave power (175, 330, and 485 W) and vacuum pressure (10, 15, and 20 kPa) using a face-centered composite design. The influence of process parameters on five target responses, namely: total hydrolysable tannin (THT), color change (ΔE), maximum temperature evolved (Tmax), effective moisture diffusivity (Deff), and specific energy consumption (SEC) were analyzed. For microwave power and vacuum, the optimal conditions attained with aid of RSM and ANN were 260 W, 15.59 kPa, and 287 W, 19 kPa, respectively. The THT retention, ΔE, Tmax, Deff, SEC determined with RSM and ANN optimized values were 78.51 mg TAE/g of DW, 13.48, 67.02°C, 3.38 × 10−6 m2/s, 20.02 kWh/kg and 90.32 ± 0.35 mg TAE/g of DW, 12.488, 64.83°C, 5.64 × 10−6 m2/s, 20.01 ± 0.33 kWh/kg, respectively. The influence analysis explained that pomegranate peel dried at optimized ANN conditions of microwave power 287 W and vacuum 19 kPa possessed improved physicochemical properties and was efficient in terms of energy consumption compared to peel dried using RSM suggested conditions. Furthermore, the major criterion for evaluation between RSM and ANN tools in this study was the goodness of fit. The value for R2 produced via RSM varied between 0.8354 and 0.9870, whereas subsequent R2 in the case of ANN was considerably greater, varying between 0.9213 and 0.9956. The mean square error value displayed the minimum value in the case of ANN compared to the RSM model. These signified rather greater exactness and predictive capability of the ANN tool. Fourier transform infrared analysis established the existence of peaks between 3,308.92 and 908.89 cm−1 and high-performance liquid chromatography showed that pomegranate peel produced using microwave vacuum drying contains a good amount of gallic acid and tannic acid derivative compounds. Scanning electron microscopy elucidated that the combined microwave vacuum effect resulted in the formation of bigger pores on the peel surface ensuing in elevated extraction of THT at reduced energy consumption. Practical applications Shifting into a bio-economy is unrealistic with no proficient use of agricultural by-products and food waste. Microwave vacuum drying is a novel technique that would be applied to preserve the product bioactive components while drying. This study has established that drying of waste pomegranate peel using microwave vacuum dryer aided in excellent recovery rate of bioactive compounds particularly hydrolysable tannin (strong antioxidant and antimicrobial agent). This promises pomegranate peel powder obtained using microwave vacuum drying to be utilized in functional and nutraceutical food processing industries as a prospective resource of antimicrobial and antioxidant compounds. The findings also identified that the ANN-GA (artificial neural network–genetic algorithm) model would be effective for modeling the microwave vacuum drying method of pomegranate peel with preservation of peel quality. The optimal process values attained using coupled ANN-GA tool could be applied with respect to microwave vacuum-based industrial dryers to achieve the objective of drying at a considerably quicker rate.

Combined surgery and endoluminal vacuum therapy for bronchial fistula and esophageal leakage after surgery for synchronous lung and esophageal cancer.

Combined surgery and endoluminal vacuum therapy for bronchial fistula and esophageal leakage after surgery for synchronous lung and esophageal cancer.

Protective coatings for LPBF Ni-based superalloys using a combination of electrospark deposition and pulsed arc evaporation methods

The combined vacuum technology involving electrospark deposition (ESD) and pulsed arc evaporation (PAE) is used to deposit a double-layer NiAlCrCoHf coating onto a substrate manufactured by a laser powder bed fusion (LPBF) of heat-resistant EP741 nickel superalloy. Thickness of the ESD and PAE layers are 35 and 4 µm, respectively. The energy parameters of ESD ensuring the predominantly epitaxial oriented crystallization of the melt at each successive event of discharge transfer are identified. The top layer of the PAE coating is characterized by a hardness of more than 13 GPa due to a combination of the lamellar and columnar nanostructure. Heating to a temperature of 700 °C causes precipitation of strengthening CrCoNi and Ni2AlHf phases in coating layers. The single-layer ESD coating and the double-layer PAE/ESD coating efficiently protect EP741 nickel alloy against oxidation at 1000 °C. A barrier oxide layer based on Al2O3 with a thickness of <1 µm is formed on the coatings.

Finite Element Analysis of Vertical and Horizontal Drainage Structures under Vacuum Combined Surcharge Preloading

Combined vacuum and surcharge preloading has gradually been widely used because of its advantages of low cost, green environmental protection, and good treatment effect. The conventional prefabricated vertical drain presents obvious defects in vacuum preloading treatment, such as obvious silting, serious bending of the drainage board, large attenuation of vacuum degree of drainage board along the depth, long construction period, and so on, which affect the final reinforcement effect. In this paper, the MIDAS finite element simulation of combined vacuum and surcharge preloading of prefabricated vertical drains (PVDs) and prefabricated horizontal drains (PHDs) is established through the comparative experiment of the engineering field. The comparative experimental study is carried out from the aspects of the vertical settlement, horizontal displacement, and pore water pressure. The results show that under combined vacuum and surcharge preloading, the consolidation effect of soft soil with PHDs is better than that with PVDs. When PHDs are used, the vertical settlement increases by 7.2% compared with PVDs; the horizontal displacement is larger; and the pore water pressure dissipates faster. This is because when the PHDs are adopted, the consolidation direction of the soil is consistent with the direction of the vacuum suction, which is mainly caused by vertical settlement. With the consolidation, the spacing between PHDs is gradually shortened, and the drainage distance is reduced, which can effectively reduce the consolidation time and improve the reinforcement effect of the soil. In addition, the PHDs can move downward uniformly with the soil during the consolidation process and have almost no bending deformation, which makes the vacuum transfer more uniform and effective.

Drying of pineapple slices using combined low-pressure superheated steam and vacuum drying

Abstract A method of combining low-pressure superheated steam drying (LPSSD) and vacuum drying (VD) was proposed to improve the dried pineapple quality and increase the drying rate. It was found that the inversion temperature in low-pressure superheated steam drying of pineapple was 85.75 °C in terms of the first falling rate period. The combining drying (LPSSD–VD) reduced the maximum material temperature by 9.5 °C and 0.35 °C, and shortened the drying time by 50 min and 90 min compared with LPSSD and VD at the same drying temperature of 90 °C. The vitamin C retention rate of dried pineapple by LPSSD–VD was 29.33% and 15.94% higher than that of LPSSD and VD, respectively. The color of dried pineapple was also improved. Moreover, the sugar content of dried pineapple can be well controlled to meet the health demand of low sugar and ensure the taste of dried pineapple during LPSSD–VD process.

Case study and numerical simulation of PVD improved soft Bangkok clay with surcharge and vacuum preloading using a modified air-water separation system

This manuscript presents the case study of soft Bangkok clay improvement and simulations using combined vacuum pressures with pre-existing surcharge embankment preloading, including installations of air-tight membranes and horizontal prefabricated drains connected to the top of the PVDs as well as to the vacuum system with a modified air-water separation system. The 17-m-long PVDs were installed from the top of the embankment in a triangular pattern at a spacing of 0.9 m. Monitoring instruments were installed to measure surface settlements, lateral movements, and pore water pressures in the soft clay layer. The subsequent analyses included settlement calculations, settlement predictions using observational methods, flow parameter back-calculations, soil property comparisons before and after improvement, and FEM simulations. The very soft to soft clay was transformed to medium-stiff clay because its undrained shear strengths and maximum past pressures increased, and its water contents, void ratios, and compression indices decreased. The analyses of the degree of consolidation utilized one-dimensional and observational methods as well as a numerical simulation for the prediction of consolidation settlements and comparison to the field data. The results illustrated the effectiveness of the modified vacuum-PVD system. The findings could be used for guidance of similar soft clay improvement projects.

Elliptical cylindrical equivalent model of PVD-assisted consolidation under surcharge combined with vacuum preloading and its application

To consider the shape effect of band prefabricated vertical drain (PVD), an elliptical cylindrical equivalent model of PVD-improved foundation including horizontal and vertical drainage is built, and then the analytical solutions of the excess pore-water pressure and the degree of consolidation under a combined vacuum pressure with time-dependent surcharge load are derived. For the elliptical cylindrical equivalent model, the determinations of parameters related to well resistance and smear effect are discussed. Based on the present solutions, existing commonly-used methods for settlement prediction are remedied and applied in four cases with different loading histories and known parameters. Satisfactory agreement between predictions and measured data verifies the rationality and adoptability of the present methodology to predict settlement. A comprehensive method to predict settlement is presented for practical application.

Effects of hydrogen-rich water combined with vacuum precooling on the senescence and antioxidant capacity of pakchoi (Brassica rapa subsp. Chinensis)

The aim of this work was to study the effect of application of hydrogen-rich water (HRW) combined with vacuum precooling on senescence and antioxidant activity in pakchoi. The results showed that 50% HRW could effectively maintain the high content of chlorophyll and its metabolic derivatives: chlorophyllide a, chlorophyllide b, pheophytin a, and pheophorbide a, which were associated with decreased activities of chlorophyllase, Mg-dechelatase, pheophytinase, and pheophorbide a oxygenase. Additional results indicated that comparing with the groups of vacuum precooling or water + vacuum precooling, treatment with combined HRW and vacuum precooling not only significantly lowered the weight loss rate, but also maintained the contents of chlorophyll and antioxidant substances including total phenols and ascorbic acid, improved activities of antioxidant enzymes containing glutathione reductase, catalase, and superoxide dismutase, and the scavenging rates of the 2,2-diphenyl-1-picrylhydrazyl radical, superoxide anions and hydroxyl radical, and thereby, inhibited accumulation of malonaldehyde. These results suggest that a combination of treatment with HRW and vacuum precooling can be used as an acceptable environmental-friendly technology for delaying the postharvest senescence of pakchoi.