Investiagtion On Thermophysical Properties And Flow Distribution Of Microchannel Condenser

With compactactness, high-efficiencey, less refrigerant charge and totally aluminum rather than copper, microchannel heat exchanger is one of the most promising heat exchangers in future. However, the microchannel heat exchanger previously used for automobile air conditioner must be redesigned as operating condition, system match and frost and defrost change for household air conditioner. The present research was supported by the cooperation project in industry, education and research of Guangdong province and Ministry of Education of P.R.China (Grant No.2007A090302115) and (Grant No.2007Z41) to investigate thermophysical properties and flow distribution of microchannel condenser.According to s-NTU and finite control volume, a steady distributed mathematical model was developed to simulate heat transfer and pressure drop of air and refrigerant through louver fin and microchannel, and the simulated result agreed well with experiment. It is the key to the overall performance of microchannel condenser whether the flowing area of tubes and passes matches with refrigerant condensation.15 configuration schemes with different pass and tube setup were compared to obtain the optimum configuration for microchannel condenser.A 6kW window type air conditioner prototype with 6-pass microchannel condenser was built and investigated experimentally under T1 standard, T3 standard and T3 maximum operating condition. The result showed that under T1 standard operating condition the new developed prototype performed better than the one with traditional finned copper round tube heat exchanger:254W about 4.27% higher in refrigerating capacity,204W less in input power and 12.32% higher to 2.28 in EER, which revealed superior performance of the prototype. Testing refrigerant temperature distribution instead of mass flow of tubes was proposed to evaluate the thermal performance of microchannel condenser in operating condition. Both near ends of 19 parallel flat tubes were set 38 T-type surface thermocouples to collect the temperature distribution. The experiment showed that the temperature distribution was uneven both in single phase and in two-phase:inlet refrigerant temperature of each horizontal tube in the same pass decreased along refrigerant flow in the vertical header. For T1 standard, T3 standard and T3 maximum operating condition, the maximum inlet temperature differences between the tubes of the same pass were 5.36℃,6.35℃, 8.49℃in the 1st pass while the maximum outlet temperature differences were 10.04℃, 8.72℃,8.18℃in 5th pass.The flow distribution between multiple parallel tubes and header is typically inreversible and difficult to understand especially phase separation involved in two phase flow. Many different factors were summarized as three types:operating condition, structural parameter and the match of the above two. In the assumption of homogeneous flow, a mathematical model based on fluid network theory was developed to predict flow distribution and phase separation in 9 flat tubes and their connecting headers on the second pass of the microchannel condenser. The simulated mass flow rate distribution in 9 tubes is parabolic and approaches to even distribution when inlet quality comes to the median 0.45 from both directions.The field test on GMV-R620W4/A with 16 indoor machine units installed in Hubei Chinese Medical Hospital showed that cooling capacity was uneven for indoor machine. It was supposed to be refrigerant mass flow maldistribution in the multi-connected airconditioning unit's fluid network. Fluid network theory was employed to develop a model by a series of fluid circuits of different refrigerating components including single-phase pipes and two-phase electronic expansive valve (EEV), evaporator, pipes in parallel or series. Then a particular iterating control algorism was developed to overcome the nonlinearity of fluid resistance and to distribute refrigerant mass flow in proportion to corresponding fluid resistance until the pressure of fluid network achieves a balance. It was found that the closer the indoor unit is to the centre of fluid network, the less its mass flow deviates from nominal value, and the units at both two poles of the pipe network are the ones with maximum disequilibrium.