The thermal conductivity of graphene materials rapidly decreases with the increase of thickness, seriously hindering its application potential in high heat flow scenarios. Here, a novel strategy for synthesizing expandable-thickness graphene block (GPB) with high thermal conductivity is proposed via multiscale blending and multistage pressure induction (MBP). The reduction of the in-plane thermal conductivity with the increase of thickness of the proposed GPB-MBP is lowered by more than 94% compared to existing graphene materials, and the maximum thickness of GPB-MBP reaches 12.1 mm, which is more than 10 times that of the existing graphene materials. Compared with the thickest existing graphene material of 1.05 mm, the in-plane thermal conductivity and cross-plane thermal conductivity of GPB-MBP are 4.7-7.4 times and 6.2-6.6 times higher, respectively. The maximum thermal diffusion ability of GPB-MBP reaches 6.3 W/K, which is 8-53 times higher than that of existing graphene materials. Furthermore, the proportional relationship between in-plane thermal conductivity and the logarithm of length was discovered on macroscopic graphene materials for the first time. The new strategy for synthesizing expandable-thickness GPB offers a new approach for high-heat-flux thermal management.

Positive displacement compressors, e.g., reciprocating, rotary, and scroll compressors, are commonly employed in vapor compression refrigeration systems using various refrigerants, and they may operate under a wide range of working conditions. In the simulation-based design of refrigeration systems, a suitable compressor model for fast and stable prediction of compressor performance is extremely important. This study aims to investigate the applicability of existing models in wide working condition ranges, and to recommend suitable ones which could simultaneously satisfy the following three requirements: high accuracy, reliable extrapolation capability, and small amount of data required for model calibration. The most representative compressor models, including eight data-driven models and four semi-empirical ones, are selected from publications, and they are validated using data from experiments conducted by the present authors as well as those from publications. Model validation results show that the top-performing data-driven and semi-empirical models can achieve sufficient accuracy with an average deviation of less than 2%; compared to data-driven models, semi-empirical models can better maintain accuracy and reasonable trends when extrapolating, and require fewer calibration data to achieve the same level of accuracy. In conclusion, among the existing fast performance-prediction compressor models, semi-empirical ones should be preferentially recommended.

The complexity of indoor environments, coupled with the growing demand for enhanced energy efficiency and comfort, is setting higher requirements for indoor environment analysis, which drives the increasing application of computational fluid dynamics (CFD) technology in the field of residential air conditioning. However, due to the structural diversity of residential scenarios, traditional CFD simulation methods are not universally applicable. Owing to high computational complexity and time-consuming nature, traditional methods struggle to balance efficiency and accuracy in simulation analyses. Consequently, there is an urgent need for more optimal simulation models. It is precisely for this reason that significant advancements have been made in fluid dynamics simulation acceleration in recent years. Among these, data-driven CFD surrogate models have garnered considerable attention due to their powerful acceleration capabilities. However, most studies prioritize "speed optimization", yet simulation of indoor environment demands a balance among speed, generalization capability, and reliability, creating a mismatch between research and implementation. This paper has examined existing mainstream approaches, exploring why they exhibit research blind spots in reliability and generalization capability. Proceeding from the mechanism, it further proposes methodologies for evaluating model reliability and generalization capability, alongside an experimental scheme. Additionally, a targeted solution that introduces multimodal feature extraction meta-learning is presented to systematically resolve the challenges. The experimental results demonstrate that the proposed model outperforms conventional algorithms by 30-60% in stability (predictive consistency under +/- 1%similar to +/- 3% input parameter perturbations), 19-24% in detail reconstruction (flow field local fine-grained feature retention), and 12-40% in generalization (accuracy retention on unseen out-of-domain indoor layouts).

In refrigerant-injected heat pumps, scroll compressors may operate under vapor injection (VI) or two-phase injection (TPI) conditions. For the design of heat pump systems, establishing a fast prediction model of compressor performance is important; however, existing ones are limited to VI scenarios. This study aims to propose a scroll compressor model for both VI and TPI conditions, simultaneously meeting the requirements of fast calculation speed, accurate prediction, reasonable extrapolation trends and minimal calibration data. In the proposed model, the suction mass flow rate is calculated based on volumetric efficiency; the injection mass flow rate is determined using the orifice equation and corrected with two-phase pressure drop multiplier; the discharge mass flow rate is the sum of suction and injection mass flow rates; the input power is calculated using isentropic compression assumption with isentropic efficiency related to pressure terms; the discharge temperature is calculated based on energy conservation, and all these parameters are explicitly formulated to increase the computation speed. Calorimeter tests are conducted to acquire compressor performance data under both VI and TPI conditions for model validation. The validation results show that the proposed model offers millisecond-level calculation time; the average prediction deviations of suction, injection, discharge mass flow rates and input power from the experimental data are all within 3%; and the average deviation of discharge temperature is within 3 degrees C. Furthermore, the proposed model exhibits physically reasonable parameters variation trends during extrapolation, and requires only 8 sets of VI and 3 sets of TPI data for calibration.

The refrigerant R454B and its compatible lubricating oil PVE are increasingly adopted in air conditioners. To prevent oil starvation in compressors, the amount of oil retention in suction pipes should be limited, and thus a model for predicting oil retention is needed. In this paper, an oil retention model of the R454B-PVE oil mixture in suction pipes is developed, which consists of a general model and 5 fluid-specific sub-models. The general model is built on the momentum conservation equations of the vapor phase and the liquid phase in the pipes, while the 5 fluid-specific sub-models are developed for 5 key parameters based on the experimental data on the oil retention characteristics of the R454B-PVE oil mixtures presented in this research. Model accuracies are validated by comparing the model predictions and the experimental data, and it is shown that the mean absolute deviations of predicted values of the sub-models from experimental ones are <12.0%, while the mean absolute deviations of the oil retention model in horizontal and vertical pipes are 14.3% and 11.7%, respectively.

Load-based dynamic energy efficiency testing is recognized as a next-generation performance evaluation method in the room air conditioner industry. The related standards contain two parts： dynamic energy efficiency testing and dynamic energy efficiency calculation. However， various approaches exist for implementing these in current research. This study aimed to provide technical suggestions for the formulation of dynamic energy efficiency standards for air conditioners in China by summarizing and comparing the results of relevant domestic and international research. The virtual building load method was adopted for the dynamic energy efficiency testing method， and three load rates were used： the rated load， an intermediate load， and the minimum load. The indoor temperature was updated using a single-node model of the heat and moisture capacitance values. To determine the dynamic energy efficiency， the dynamic annual performance factor could be calculated by interpolating the load and energy efficiency. A simplified linear model was used for the load， and the model of the operating time was based on the time statistics for annual operation. Some key parameters （e.g.， the heat capacitance， moisture capacitance， and outdoor temperature bin distribution） still need to be determined based on domestic conditions. Future work could focus on improving the repeatability and could include the use of big data analysis to determine the outdoor temperature bin distribution， zoning based on climate regions， and the introduction of carbon emission evaluation metrics.

Electronic expansion valves (EEVs) are widely used in room air-conditioners for refrigerant flow regulation, but often generate abnormal flow-induced noise during horizontal or vertical inlet flow. A built-in guide sleeve can suppress such noise only under horizontal inlet flow, but fails to mitigate it effectively under vertical inlet flow. This study addresses this issue by weakening the fluid–structure interaction between the refrigerant and EEV components. Three sequential research steps are implemented: identifying the most relevant vibration type, designing a dedicated structure to suppress this vibration type, and optimizing the structural geometric parameters. The valve needle vibration is identified as the most relevant vibration type since the inlet refrigerant impinges on the end face of valve needle at a large deflection angle and causes non-uniform refrigerant impact forces around the valve needle. Accordingly, a step-shaped guide sleeve structure is proposed to replace the conventional trumpet-shaped guide sleeve for vibration suppression, which redirects the inlet refrigerant to flow parallel to the axis of valve needle and enhances the uniformity of the refrigerant impact forces. Furthermore, the geometric parameters of the step-shaped guide sleeve are optimized to achieve effective vibration attenuation while preserving the EEV’s inherent throttling performance. Comparative experiments between the original and modified EEVs demonstrate that the abnormal flow-induced noise is completely eliminated, and the maximum sound pressure (a key noise characteristic parameter) decreases from 0.035 Pa to 0.002 Pa, corresponding to a noise reduction rate of 94.3%.

The unsteady refrigerant flow from a header to its connected parallel tubes in a microchannel evaporator, induced by vapor-liquid interface fluctuations within the header, may lead to non-uniform mass flow distribution among the parallel tubes, thereby severely degrading its heat transfer performance. This study aims to develop a predictive model for unsteady header-to-tube mass flow rate model to improve the flow uniformity of parallel evaporator tubes. In the structural description of headers, all types of headers with various baffle installation positions are unified into a combination of three standard blocks. To accurately quantify the impact of vapor-liquid interface fluctuation on mass flow rates in parallel tubes, the chaotic fluctuation process at the interface is decomposed into three components: the continuous two-phase flow beneath the interface, the continuous vapor flow above the interface, and the discrete droplet flow caused by bubble bursting at the interface. The continuous phase flow is governed by mass and momentum conservation equations, whereas the discrete droplet flow is formulated by the droplet trajectories and the distance from the droplets to tubes. Model validation is conducted using 1872 groups of measured unsteady mass flow data acquired from a vertical header with 12 parallel tubes under various inlet operating conditions. The validation results reveal that the maximum and mean relative deviations between predicted and experimental results are 19.1% and 10.1%, respectively, demonstrating the acceptable accuracy of the proposed model for practical unsteady flow prediction.

The application of stainless steel tubes instead of copper tubes in shell-and-tube heat exchangers for central airconditioning can significantly reduce the price of heat exchangers, but it may bring in the risk of water fouling increase during long-term operation, and thus a comparative investigation on the water fouling characteristics of copper tubes and stainless steel tubes is needed. In this study, an accelerated fouling experimental method is presented to speed up the fouling process on tubes, and the fouling masses, the fouling layer thicknesses as well as the fouling crystal patterns are comparatively analyzed to evaluate the fouling risk of these tubes. In the experiments, the test tube materials include copper and two types of stainless steels (i.e., SUS 304 and SUS 304Cu), water temperatures include 10 degrees C and 60 degrees C to respectively represent refrigeration and heating modes in central air-conditioning, and water flow velocities include 0.5 m/s and 1.6 m/s to respectively represent low and high load modes. The experimental results show that the fouling masses of stainless steel tubes are 23.8%-68.3% lower than those of copper tubes, the fouling layer thicknesses of stainless steel tubes are 23.2%-28.8% lower than those of copper tubes, and the fouling crystal patterns of stainless steel tubes are more unstable than those of copper tubes, which means that stainless steel tubes are less prone to fouling than copper tubes.

Circulating water is used as the working medium in water-based chillers. Salt ions, such as calcium in circulating water, may precipitate during long-term operation, resulting in the attenuation of heat transfer performance. The application of small-diameter tubes in heat exchangers may lead to more prominent fouling problems. This study developed an accelerated fouling method to evaluate water fouling risk. The most typical operating conditions for water-based chillers were selected as the experimental conditions. The test samples included small diameter (5 mm) smooth tubes, with 7 mm smooth tubes selected for the control experiment. The experimental conditions include a circulating water inlet temperature of 60 ℃, a flow rate of 1 m/s, a foulant mass concentration of 800 mg/L, and a test time of 0-400 h. The results revealed that the total fouling mass was 39.5% higher and required fouling time was 17.6% shorter when comparing the 5 mm and 7 mm smooth tubes; the small diameter tubes had a larger total fouling mass and higher fouling rate. The heat transfer coefficients of the 5 mm and 7 mm tubes after fouling decreased by 12.5% and 9.7%, respectively, and the pressure drops increased by 50.6% and 10.4%, respectively, demonstrating a more severe heat transfer performance deterioration of small diameter tubes after fouling. The microscopic observation results of the fouling layer morphology demonstrated that the fouling layer in 5 mm tubes is a form of compact lamellar scaling, which is more difficult to remove compared with the 7 mm tube; therefore, the fouling risk should be considered when promoting the application of small diameter tubes in chillers.

Application of copper tubes with the diameter of 5 mm instead of 7 mm in chillers of central air conditioning systems is the development tendency due to the requirement of copper material saving. However, the heat transfer deterioration by water fouling precipitated in 5 mm tubes may be more serious than that in 7 mm tubes. The purpose of this paper is to comparatively investigate the water fouling characteristics on both 5 mm and 7 mm tubes. An accelerated experimental method of water fouling process was designed to quickly evaluate the fouling degree. Four tube types are tested, i.e. a 7 mm smooth tube, a 7 mm screwed tube, a 5 mm smooth tube and a 5 mm screwed tube, and these tubes are fabricated as flexible coiled tubes with total length of 200 cm. The results show that the water fouling process experiences the initiation, growth and stabilization stages in sequence, and the crystal form of fouling is in the vaterite, aragonite and schistosity patterns respectively. The fouling masses in 5 mm smooth tube and 5 mm screwed tube are respectively 42.1% and 16.9% larger than those in 7 mm smooth tube and 7 mm screwed tube, meaning that 5 mm tubes are more prone to form water fouling compared with 7 mm tubes; while the averaged fouling mass in screwed tubes is 2.8% lower than that in smooth tubes since screwed tubes are benefit to destroy the integrality of fouling layer and enhance the removal of deposited particulates.

To simulate and optimize an enhanced vapor-injection system, it is necessary to develop a vapor-injection scroll compressor model with fast calculation speed, high accuracy, good extrapolation accuracy, and few parameters for computation. However, existing models cannot meet these demands simultaneously. In this study, a physics-based explicit form semi-empirical model of a scroll compressor with vapor injection was developed to predict its mass flow rate, input power, and discharge temperature. In this model, the suction mass flow rate was derived by correcting the pressure ratio using the specific heat ratio and multiplying it by the quadratic function of frequency. The injection mass flow rate was based on the assumption of an isochoric mixing process and obtained by expanding the coefficients. The discharge flow rate was the sum of the suction and injection mass flow rates. The input power was based on the assumption of isentropic compression and corrected by pressure, and the discharge temperature model was based on the heat leakage factor. The model was validated based on experimental data, and the results showed that the model had a calculation speed of milliseconds, and was able to accurately predict the performance of the compressor, with the average deviations of the suction mass flow rate and discharge mass flow rate both within 2%, and the average deviations of the injection mass flow rate, input power, and discharge temperature within 5%, 3%, and 3 ℃, respectively. The model can provide reasonable results outside the range of fitted conditions, and the amount of data required for model fitting has been reduced by more than 50% compared to that of existing models.

Whistling noise is the most serious flowing noise in the indoor electronic expansion valve (EEV) of multi-split air conditioners, and it is caused by resonance between the refrigerant flowing at a specific speed and the EEV chamber with a particular opening. The key for avoiding this noise is to identify the noise-generating speed range for a given EEV opening and keep the refrigerant flow speed outside this range. The purpose of this study is to develop a predictive formula for the noise-generating speed range at a given EEV opening. The basic idea for developing the predictive formula is firstly to establish the functional relation between the whistling noise frequency and the refrigerant flow speed, and then to establish the functional relation between the whistling noise frequency range and the EEV opening, and lastly to combine these two relations to determine the noise-generating speed range for a given EEV opening. The predicted lower and upper limits of the noise-generating speed range are validated through experiments on a typical EEV for room air-conditioners using R410A as refrigerant. It is shown that the deviations of these two limits between predicted results and experimental data are both within 0.02 m/s at various EEV openings, meaning that the accuracy of the developed predictive formula for the noise-generating speed range at a given EEV opening is acceptable in engineering applications.

The flowing noise of two-phase R410A refrigerant through electronic expansion valves (EEVs) in a multi-split air conditioner is serious when the inlet flow pattern is slug flow, and it can be reduced by installing a flow pattern regulator at the valve inlet so as to adjust the slug flow pattern to a bubble flow pattern. The purpose of this study is to numerically investigate the influence of regulator structural parameters on two-phase flowing noise in the EEV, and to present a method for designing the regulator structural parameters. The structural parameters to be analyzed and optimized include porosity, thickness and hole diameter. The software of FLUENT and Actran is used in the numerical simulations, and the results show that, at the common working condition ranges of multisplit air conditioners, the sound pressure level of flowing noise decreases with the increase of porosity, and it firstly decreases and then increases with the increase of the hole diameter or thickness. The optimal flow pattern regulator with structural parameters of 90 % porosity, 3 mm thickness and 0.6 mm hole diameter was experimentally proved to achieve 5.4 dB reduction of flowing noise comparing to that without the flow pattern regulator, and the effect of installing a flow pattern regulator at the inlet of EEV on the performance of the refrigeration system is within acceptable limits.

Room air-conditioners adopt accumulators to supply vapor refrigerant and liquid refrigerant-lubricating oil mixture to the compressors, and the excessive or insufficient liquid mixture returning to the compressor may respectively lead to liquid slugging or oil starvation, resulting in compressor damage. Optimization design of the accumulator based on accurate simulation is an effective way to appropriately regulate the liquid return rate. The purpose of this paper is to develop a model for accumulator simulation and to validate it by experiments. In the model, the mass and energy conservation equations for both vapor phase and liquid phase within the accumulator cavity are established to reflect the flow processes in two phases; the equations for calculating the refrigerant dissolution and desorption rates are formulated to describe the flow process of vapor-liquid interphase; the pressure balance equations based on flow path divisions are formulated to reflect the flow processes in the inlet and outlet pipes. An experimental rig is built to measure liquid return rate, oil return rate and height of liquid level of the accumulator, and the validation results show that the mean absolute deviations between predicted and experimental data of those three parameters are 6.3 %, 8.7 % and 4.6%, respectively. By using the model, the structural parameters of the oil bleed hole for a room air conditioner accumulator with a rated cooling capacity of 3.5 kW were optimized, and the optimization results indicate the reasonable ranges for the diameter and the height of oil bleed hole are 0.65-0.92 mm and 15-30 mm, respectively.

The two-phase flow pattern of hydrocarbon working fluids on the shell side of a helically baffled heat exchanger for liquefied natural gas determines its heat transfer performance. This study tested the two-phase flow patterns of propane and ethane/propane mixtures on the shell side of a helically baffled heat exchanger using a visualization experimental method. The test results demonstrated that with the increase in vapor quality, the experimental observations sequentially included stratified flow, stratified-spray flow, and spray flow; as the mass flux of propane increased from 20 kg/(m2·s) to 40 kg/(m2·s), the transition vapor quality from stratified flow to stratified-spray flow decreased from 0.7 to 0.3, while the transition vapor quality from stratified-spray flow to spray flow decreased from approximately 1 to 0.7; when the proportion of ethane increased from 0 to 50%, the transition vapor quality from stratified flow to stratified-spray flow increased from 0.30-0.45 to 0.43-0.55, while the transition vapor quality from stratified-spray flow to spray flow increased from 0.69-0.85 to 0.83-close to 1. The existing flow pattern map for water-air mixtures was inadequate for predicting the flow patterns of hydrocarbon working fluids. A new set of flow pattern transition criteria was established with prediction deviations of approximately 6.5%, 5.5%, and 4.2% for the experimental stratified flow, stratified-spray flow, and spray flow, respectively.

Room air conditioners are widely used to control indoor air parameters to user preferred values for thermal comfort, but the existing control methods might be uncomfortable due to changeable user preferences or be highcost due to physiological sensors. The purpose of this study is to develop an adaptive control method for room air conditioners at a low cost. The basic idea is to adopt data mining of operating parameters instead of monitoring by physiological sensors, and the key technology is the control of compressor frequency and indoor unit fan speed based on the application scene of the room air conditioner and the user preferred values of indoor air parameters. During the use of the room air conditioner, the application scene is identified by comparing the probabilities of the room air conditioner being in the sleep scene, work scene, or leisure scene, and the user preferred values are predicted by correcting the group preferred values of users in the same city with the setting records of the user. To ensure the reliability of the control method, the accuracy of application scene identification, user preference prediction, and adaptive control is validated by the data collected from the room air conditioners used in the cities of Shanghai, Guangzhou, Dalian, Wuhan, Chongqing, and Haikou. It is shown that the accuracy of application scene identification, user preferred air temperature prediction and user preferred air velocity prediction is 79 %, 88 %, and 94%, respectively; indoor air temperatures can be controlled within +/- 0.5 degrees C of the set values.

A heat pump dryer usually adopts a centrifugal fan with multi-blades for generating large flow rate of air, but faces the problem of high aerodynamic noise. The application of bionic serrated blades instead of straight-edged blades in fans is a promising way for reducing the aerodynamic noise while keeping the air flow rate. The objective of this study is to develop low-noise and largeair flow rate bionic serrated blades by establishing prediction formulas of air flow rate and aerodynamic noise. The prediction formulas for bionic serrated blades are derived from existing flow rate and noise equations for straight-edged blades, and are extended to bionic serrated blades by maintaining the invariance of the radial projected area between the two types of blades. The relative errors between the predicted values and experimental values for the air flow rate and noise are +4.0 % and +2.5 % respectively. Based on the predicted formulas, the optimal values of serrated parameters are determined for blade design. The effects of designed blades are validated, and the results show that the noise of the fan with the designed blades is reduced by 2.1 dB(A) at the same air flow rate, compared to the original fan with straight-edged blades. The noise reduction and air flow rate maintenance are attributed to the serrated edges of the designed blades breaking down large vortices into uniformly distributed small ones, which mitigates pressure fluctuations and energy loss in the airflow field.

The energy consumption of variable refrigerant flow (VRF) systems can be reduced by adopting the variable evaporation temperature and constant superheat degree (VECS) control strategy instead of constant evaporation temperature and constant superheat degree (CECS) control strategy. However, the constant superheat degree control strategy may weaken the adjustment ability of cooling capacity of indoor units, and result in obvious room temperature fluctuations. In order to decrease room temperature fluctuations and reduce energy consumption simultaneously, a cooperative control strategy of variable evaporation temperature and variable superheat degree (VEVS) is proposed, i.e. one indoor unit is chosen to be controlled by variable evaporation temperature and the rest of the indoor units are controlled by variable superheat degrees. In this control strategy, the target value of evaporation temperature is the lowest value among the upper limits of the evaporation temperatures of all indoor units, and the target values of the superheat degrees of indoor units are predicted according to the cooling demands of rooms. Comparative experiments on room temperature fluctuation and energy consumption among the control strategies of CECS, VECS and VEVS are done. It is shown that both the control strategies of VECS and VEVS achieve smaller room temperature fluctuation and lower energy consumption than those of CECS; compared with the VECS control strategy, the average room temperature fluctuation of the VEVS control strategy is decreased from 1.1 degrees C to 0.5 degrees C due to variable superheat degree, and the energy consumption of the VEVS control strategy is reduced by 4.4 %.

Winter heating of passenger compartments in electric vehicles relies on vehicle air-conditioning systems， in which the air outlet temperature must be rapidly increased to the set value. These systems typically adopt a reverse Carnot heat-pump cycle， which does not achieve sufficient heating capacity in low-temperature environments， thus preventing the air outlet temperature from rapidly reaching the set temperature during cold starts. To ensure that the system provides sufficient heating capacity under these conditions， a hot-gas bypass heat-pump cycle was used to replace the reverse Carnot heat-pump cycle. By bypassing the refrigerant from the high-pressure side to the low-pressure side， the suction pressure of the compressor was increased， ensuring stable operation of the compressor under low-temperature heating conditions. A staged hot-gas-bypass heat-control strategy， with an additional exhaust throttle valve to reduce the time required to establish high pressure， is proposed. A simulation model of a this proposed system， using R290 as the refrigerant， was established， and a test bench was built to calibrate the simulation model. Using this simulation， differences between the hot-gas-bypass heat-pump cycle and the reverse Carnot heat-pump cycle were examined， and the effects of the optimization strategy were verified. Under low-temperature conditions （-25 ℃ and -20 ℃）， the proposed system achieved better heating performance than the reverse Carnot heat pump cycle. Relative to the pre-optimization control strategy， the proposed optimized hot-gas-bypass heat-pump cycle control strategy reduces the time required for the air outlet temperature to reach the target temperature by 36%.