Single phase pressure drop in round cylindrical headers of parallel flow MCHXs

This paper presents the investigation of the pressure drop in headers and development of correlation for pressure loss coefficient for single phase flow through round cylindrical headers of parallel MCHXs. The working fluid was compressed air flowing through header with 1 - 20 m/s based on smallest cross section while the velocity through micro-channels was in the range 6 - 30 m/s. The experimental results indicate that the pressure loss coefficient of inlet header is a linear function of the ratio of velocities through micro-channel tube and header, except for the first two micro-channel tubes; the pressure loss coefficient of outlet header is a quadratic function of the ratio of velocities through micro-channel tube and header, and decreases as the velocities through upstream micro-channel tubes increase. Correlations for predicting pressure drop of inlet header and outlet header are developed and agreement for 98% of experimental data is within a ±15 Pa

Investigation of Application of Suction Line Heat Exchanger in R290 Air Conditioner with Small Diameter Copper Tube

R290 is a potential refrigerant replacing R22 because of its zero Ozone Depletion Potential (ODP) and virtually zero Global Warming Potential (GWP). However, R290 is flammable and requires excellent containment to avoid leakage and reduce the risk of fire. The use of small diameter copper tube (5 mm or even smaller) is an effective way to reduce refrigerant charge and thus reduce the risk of fire in the event of a refrigerant leak. However, employing small diameter copper tube will increase pressure drop and consequently reduce system performance. A suction line heat exchanger which employs the low temperature refrigerant in suction line to cool down the refrigerant before expansion value is a potential solution to improve system performance because R290 has low discharge temperature compared with HFC refrigerants (e.g. R22, R410A). This paper presents an investigation of application of a suction line heat exchanger in an R290 air conditioner with small diameter copper tube. A theoretical analysis is proposed at first to investigate the effect of the suction line heat exchanger on capacity and system energy efficiency under variable evaporating and cooling temperatures. A prototype R290 air conditioner with and without a suction line heat exchanger is tested in order to explore the effect of a suction line heat exchanger on system performance and refrigerant charge in real working conditions. Finally, a refrigerant circuit solution for heat pump air conditioners is proposed. The results of theoretical analysis indicate that the capacity and system energy efficiency increase linearly with the heat exchange of the suctionline heat exchanger, and the suction line heat exchanger can improves capacity by up to 12% and system energy efficiency by up to 4% under both cooling and heating modes. This is because the suction line heat exchanger increases the sub-cooling but has less impact on compressor power due to good thermal properties of R290. Further, the evaporating and condensation temperature have insignificant impact on the performance of suction line heat exchanger. The experimental results show that the suction line heat exchanger improves the cooling capacity and system efficiency by 5.3% and 4.5%, respectively. These results agree well with that of the system analysis. The sub-cooling temperature increases 10.2o C and the discharge temperature increases 25.4o C. Further, the suction line heat exchanger reduces the refrigerant charge by as much as 6%. This is because suction line heat exchanger increases the discharge temperature, and thus the superheat region of the condenser increases resulting in less refrigerant in the condenser. Overall, the use of a suction line heat exchanger in a system with small diameter copper tube improves the performance of R290 and more importantly reduces the refrigerant charge

Influence of Oil on Heat Transfer Characteristics of R410A Flow Boiling in Conventional and Small Size Microfin Tubes

Compact heat exchangers for refrigeration and air-conditioning systems are beneficial to reduce cost, charge inventory and leakage of refrigerant, and to improve energy efficiency and safety. Using small diameter microfin tubes is one way to decrease the size of heat exchangers. Currently, small diameter micofin tubes with outside diameter (O.D.) of 5.0 mm and 4.0 mm O.D. begin to be applied in newly developed R410A air conditioners instead of conventional size tubes (e.g. 7.0 mm O.D. microfin tubes). With the decrease of the tube diameter, the pressure drop becomes much larger, resulting in the decrease of the heat exchanger performance. In order to avoid such performance decrease, the heat exchanger should be redesign based on clearly understanding the difference of the heat transfer characteristics between conventional size microfin tubes and small diameter micofin tubes. Therefore, the heat transfer characteristics of R410A flow boiling inside both conventional size microfin tubes and small diameter microfin tubes should be known. Under real working conditions of R410A air conditioner, some amount of oil inevitably circulates with the refrigerant and has a significant impact on refrigerant evaporation heat transfer characteristics (Shen and Groll, 2005; Thome, 1996). Therefore, the influence of oil on heat transfer characteristics of R410A flow boiling inside microfin tubes with different diameters covering from conventional size to small size should be investigated. Experiments of R410A-oil mixture flow boiling inside microfin tubes with different outside diameters of 4.0~7.0 mm were performed. The experimental results show that, for 7.0 mm microfin tube, the influence factor of oil on the heat transfer characteristics are larger than 1.0 under the conditions of low vapor qualities (xr,o \u3c 0.4), presenting the enhancement effect of oil on heat transfer characteristics; with the increase of vapor quality, the enhancement becomes smaller, and is smaller than 1.0 under the conditions of low vapor qualities (xr,o \u3e 0.65), showing the deterioration effect of oil on heat transfer characteristics. As the tube diameter decreases from 7.0 mm to 4.0~5.0 mm, the deterioration effect of oil is weakened, especially at intermediate and high vapor qualities; for 4.0-5.0 mm tubes, the presence of oil shows the enhancement effect on heat transfer characteristics under the conditions of intermittent vapor quality (0.4 \u3c xr,o \u3c 0.8), which is not the same as the deterioration effect for 7.0 mm tubes. The comparison of heat transfer coefficient for two 5.0 mm microfin tubes with different fin structures shows that, larger fin height and contact area of liquid with tube wall may enhance the heat transfer for oil-free R410A, but result in smaller enhancement effect of oil at low vapor qualities and smaller deterioration effect of oil at intermediate and high vapor qualities. Based on the experimental data for conventional and small size microfin tubes, a general heat transfer correlation for R410A-oil mixture flow boiling inside microfin tubes was developed, and it agrees with 94% of the experimental data of R410A-oil mixture in 4.0 mm ~ 7.0 mm microfin tubes within a deviation of ±30%

Accelerating-particle-deposition Method for Quickly Evaluating Long-term Performance of Fin-and-tube Heat Exchangers

Fin-and-tube heat exchanger is the most commonly used heat exchanger type in air-conditioning systems. In the actual operation of air-conditioning systems, the dust particles involved in the air may partly deposit and form particulate fouling on fins and tubes when the dusty air flows through the heat exchangers. The deposited particles may gradually block the passageway of air flow and occupy the heat transfer area, which results in the continuous increase of air side thermal resistance and the significant deterioration of the heat transfer capacity of heat exchangers during the long-term operation. In order to quickly evaluate the long-term performance of fin-and-tube heat exchangers, an accelerating-particle-deposition method, which is capable of implementing the particle deposition process on the long-running heat exchangers in a short time, is proposed in this study. The idea of the accelerating-particle-deposition method is to employ high concentration dusty air flow through heat exchangers in the accelerated test, and to quickly form the particulate fouling with the same weight as that on long-running heat exchangers under the actual operating environment with low particle concentration. The accelerating factor, which is defined as the ratio of the actual running time to the accelerated testing time, is calculated based on the deposition weight of dust particles. The deposition weight is calculated by the relationship of the impact frequency and deposition probability of dust particles with the particle concentration of dusty air. An experimental apparatus for accelerating the particle deposition process and testing the heat transfer capacity of fin-and-tube heat exchangers is designed. The predicted long-term performances of heat exchangers based on the proposed accelerating-particle-deposition method are compared with the actual performance data of heat exchangers after 5-8 years’ operation published by China Quality Certification Center. The comparison results show that, the predicted results agree well with the actual operation data, and the mean deviation of the heat transfer capacity is within 10%

Event-triggered consensus control for discrete-time stochastic multi-agent systems: The input-to-state stability in probability

This paper is concerned with the event-triggered consensus control problem for a class of discrete-time stochastic multi-agent systems with state-dependent noises. A novel definition of consensus in probability is proposed to better describe the dynamics of the consensus process of the addressed stochastic multiagent systems. The measurement output available for the controller is not only from the individual agent but also from its neighboring ones according to the given topology. An event-triggered mechanism is adopted with hope to reduce the communication burden, where the control input on each agent is updated only when a certain triggering condition is violated. The purpose of the problem under consideration is to design both the output feedback controller and the threshold of the triggering condition such that the closed-loop system achieves the desired consensus in probability. First of all, a theoretical framework is established for analyzing the so-called input-to-state stability in probability (ISSiP) for general discretetime nonlinear stochastic systems. Within such a theoretical framework, some sufficient conditions on event-triggered control protocol are derived under which the consensus in probability is reached. Furthermore, both the controller parameter and the triggering threshold are obtained in terms of the solution to certain matrix inequalities involving the topology information and the desired consensus probability. Finally, a simulation example is utilized to illustrate the usefulness of the proposed control protocol.Royal Society of the UK, the National Natural Science Foundation of China under Grants 61329301, 61203139 and 61473076, the Hujiang Foundation of China under Grants C14002 and D15009, the Shanghai Rising- Star Program of China under Grant 13QA1400100, the ShuGuang project of Shanghai Municipal Education Commission and Shanghai Education Development Foundation under Grant 13SG34, the Fundamental Research Funds for the Central Universities, DHU Distinguished Young Professor Program, and the Alexander von Humboldt Foundation of German